Showing papers on "Magnetic anisotropy published in 2017"
TL;DR: Xu et al. as mentioned in this paper used magneto-optical Kerr effect microscopy to show that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation.
Abstract: Magneto-optical Kerr effect microscopy is used to show that monolayer chromium triiodide is an Ising ferromagnet with out-of-plane spin orientation. The question of what happens to the properties of a material when it is thinned down to atomic-scale thickness has for a long time been a largely hypothetical one. In the past decade, new experimental methods have made it possible to isolate and measure a range of two-dimensional structures, enabling many theoretical predictions to be tested. But it has been a particular challenge to observe intrinsic magnetic effects, which could shed light on the longstanding fundamental question of whether intrinsic long-range magnetic order can robustly exist in two dimensions. In this issue of Nature, two groups address this challenge and report ferromagnetism in atomically thin crystals. Xiang Zhang and colleagues measured atomic layers of Cr2Ge2Te6 and observed ferromagnetic ordering with a transition temperature that, unusually, can be controlled using small magnetic fields. Xiaodong Xu and colleagues measured atomic layers of CrI3 and observed ferromagnetic ordering that, remarkably, was suppressed in double layers of CrI3, but restored in triple layers. The two studies demonstrate a platform with which to test fundamental properties of purely two-dimensional magnets. Since the discovery of graphene1, the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent additions include semiconductors with spin–valley coupling2, Ising superconductors3,4,5 that can be tuned into a quantum metal6, possible Mott insulators with tunable charge-density waves7, and topological semimetals with edge transport8,9. However, no two-dimensional crystal with intrinsic magnetism has yet been discovered10,11,12,13,14; such a crystal would be useful in many technologies from sensing to data storage15. Theoretically, magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temperatures by the Mermin–Wagner theorem16. Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism. Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 kelvin is only slightly lower than that of the bulk crystal, 61 kelvin, which is consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase, highlighting thickness-dependent physical properties typical of van der Waals crystals17,18,19. Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect20, whereas in trilayer CrI3 the interlayer ferromagnetism observed in the bulk crystal is restored. This work creates opportunities for studying magnetism by harnessing the unusual features of atomically thin materials, such as electrical control for realizing magnetoelectronics12, and van der Waals engineering to produce interface phenomena15.
3,802 citations
TL;DR: In this article, the authors study the origin of magnetic anisotropy, a necessary ingredient to have magnetic order in two dimensions, combining two levels of modeling, density functional calculations and spin model Hamiltonians.
Abstract: The observation of ferromagnetic order in a monolayer of CrI$_3$ has been recently reported, with a Curie temperature of 45 Kelvin and off-plane easy axis Here we study the origin of magnetic anisotropy, a necessary ingredient to have magnetic order in two dimensions, combining two levels of modeling, density functional calculations and spin model Hamiltonians We find two different contributions to the magnetic anisotropy of the material, both favoring off-plane magnetization and contributing to open a gap in the spin wave spectrum First, ferromagnetic super-exchange across the $\simeq $ 90 degree Cr-I-Cr bonds, are anisotropic, due to the spin orbit interaction of the ligand I atoms Second, a much smaller contribution that comes from the single ion anisotropy of the $S=3/2$ Cr atom Our results permit to establish the XXZ Hamiltonian, with a very small single ion anisotropy, as the adequate spin model for this system Using spin wave theory we estimate the Curie temperature and we highlight the essential role played by the gap that magnetic anisotropy induces on the magnon spectrum
516 citations
TL;DR: In this paper, a review of magnetic magnetic anisotropy at magnetic metal/oxide interfaces is presented, along with some applications of this interfacial PMA in STT-MRAM.
Abstract: Spin electronics is a rapidly expanding field stimulated by a strong synergy between breakthrough basic research discoveries and industrial applications in the fields of magnetic recording, magnetic field sensors, nonvolatile memories [magnetic random access memories (MRAM) and especially spin-transfer-torque MRAM (STT-MRAM)]. In addition to the discovery of several physical phenomena (giant magnetoresistance, tunnel magnetoresistance, spin-transfer torque, spin-orbit torque, spin Hall effect, spin Seebeck effect, etc.), outstanding progress has been made on the growth and nanopatterning of magnetic multilayered films and nanostructures in which these phenomena are observed. Magnetic anisotropy is usually observed in materials that have large spin-orbit interactions. However, in 2002 perpendicular magnetic anisotropy (PMA) was discovered to exist at magnetic
metal/oxide interfaces [for instance Co(Fe)/alumina]. Surprisingly, this PMA is observed in systems where spin-orbit interactions are quite weak, but its amplitude is remarkably large—comparable to that measured at Co/Pt interfaces, a reference for large interfacial anisotropy (anisotropy ∼1.4 erg/cm2 = 1.4 mJ/m2). Actually, this PMA was found to be very common at magnetic metal/oxide interfaces since it has been observed with a large variety of amorphous or crystalline oxides, including AlOx, MgO, TaOx, HfOx, etc. This PMA is thought to be the result of electronic hybridization between the oxygen and the magnetic transition metal orbit across the interface, a hypothesis supported by ab initio calculations. Interest in this phenomenon was sparked in 2010 when it was demonstrated that the PMA at magnetic transition metal/oxide interfaces could be used to build out-of-plane magnetized magnetic tunnel junctions for STT-MRAM cells. In these systems, the PMA at the CoFeB/MgO interface can be used to simultaneously obtain good memory retention, thanks to the large PMA amplitude, and a low write current, thanks to a relatively weak Gilbert damping. These two requirements for memories tend to be difficult to reconcile since they rely on the same spin-orbit coupling. PMA-based approaches have now become ubiquitous in the designs for perpendicular STT-MRAM, and major microelectronics companies are actively working on their development with the first goal of addressing embedded FLASH and static random access memory-type of applications. Scalability of STT-MRAM devices based on this interfacial PMA is expected to soon exceed the 20-nm nodes. Several very active new fields of research also rely on interfacial PMA at magnetic metal/oxide interfaces, including spin-orbit torques associated with Rashba or spin Hall effects, record high speed domain wall propagation in buffer/magnetic metal/oxide-based magnetic wires, and voltage-based control of anisotropy. This review deals with PMA at magnetic metal/oxide interfaces from its discovery, by examining the diversity of systems in which it has been observed and the physicochemical methods through which the key roles played by the electronic hybridization at the metal/oxide interface were elucidated. The physical origins of the phenomenon are also covered and how these are supported by ab initio calculations is dealt with. Finally, some examples of applications of this interfacial PMA in STT-MRAM are listed along with the various emerging research topics taking advantage of this PMA.
515 citations
TL;DR: It is reported that two-coordinate cobalt imido complexes featuring highly covalent Co═N cores exhibit slow relaxation of magnetization under zero direct-current field with a high effective relaxation barrier up to 413 cm-1, a new record for transition metal based SMMs.
Abstract: The pursuit of single-molecule magnets (SMMs) with better performance urges new molecular design that can endow SMMs larger magnetic anisotropy. Here we report that two-coordinate cobalt imido complexes featuring highly covalent Co═N cores exhibit slow relaxation of magnetization under zero direct-current field with a high effective relaxation barrier up to 413 cm-1, a new record for transition metal based SMMs. Two theoretical models were carried out to investigate the anisotropy of these complexes: single-ion model and Co-N coupling model. The former indicates that the pseudo linear ligand field helps to preserve the first-order orbital momentum, while the latter suggests that the strong ferromagnetic interaction between Co and N makes the [CoN]+ fragment a pseudo single paramagnetic ion, and that the excellent performance of these cobalt imido SMMs is attributed to the inherent large magnetic anisotropy of the [CoN]+ core with |MJ = ± 7/2⟩ ground Kramers doublet.
303 citations
TL;DR: It is shown that, in the presence of phonon dissipation, the relevant energy scale for the spin relaxation is given by the lower-lying phonon modes interacting with the local spins, which opens a channel for spin reversal at energies lower than that set by the magnetic anisotropy.
Abstract: The use of single molecule magnets in mainstream electronics requires their magnetic moment to be stable over long times. One can achieve such a goal by designing compounds with spin-reversal barriers exceeding room temperature, namely with large uniaxial anisotropies. Such strategy, however, has been defeated by several recent experiments demonstrating under-barrier relaxation at high temperature, a behaviour today unexplained. Here we propose spin-phonon coupling to be responsible for such anomaly. With a combination of electronic structure theory and master equations we show that, in the presence of phonon dissipation, the relevant energy scale for the spin relaxation is given by the lower-lying phonon modes interacting with the local spins. These open a channel for spin reversal at energies lower than that set by the magnetic anisotropy, producing fast under-barrier spin relaxation. Our findings rationalize a significant body of experimental work and suggest a possible strategy for engineering room temperature single molecule magnets.
301 citations
TL;DR: Reducing the coordination number of the metal centers appears to increase axial magnetic anisotropy, giving rise to larger magnetic relaxation barriers and 100-s magnetic blocking temperatures of up to 20 K, as observed for the N23– radical-bridged dilanthanide complexes.
Abstract: Increasing the operating temperatures of single-molecule magnets-molecules that can retain magnetic polarization in the absence of an applied field-has potential implications toward information storage and computing, and may also inform the development of new bulk magnets. Progress toward these goals relies upon the development of synthetic chemistry enabling enhancement of the thermal barrier to reversal of the magnetic moment, while suppressing alternative relaxation processes. Herein, we show that pairing the axial magnetic anisotropy enforced by tetramethylcyclopentadienyl (CpMe4H) capping ligands with strong magnetic exchange coupling provided by an N23- radical bridging ligand results in a series of dilanthanide complexes exhibiting exceptionally large magnetic hysteresis loops that persist to high temperatures. Significantly, reducing the coordination number of the metal centers appears to increase axial magnetic anisotropy, giving rise to larger magnetic relaxation barriers and 100-s magnetic blocking temperatures of up to 20 K, as observed for the complex [K(crypt-222)][(CpMe4H2Tb)2(μ-[Formula: see text])].
237 citations
TL;DR: This work image the three-dimensional magnetic structure in the vicinity of the Bloch points, which until now has been accessible only through micromagnetic simulations, and identifies two possible magnetization configurations: a circulating magnetization structure and a twisted state that appears to correspond to an ‘anti-Bloch point’.
Abstract: Techniques exist for imaging the magnetization patterns of magnetic thin films and at the surfaces of magnets, but here hard-X-ray tomography is used to image the three-dimensional magnetic structure within a micrometre-sized magnet in the vicinity of Bloch points. Techniques have long existed for imaging the two-dimensional magnetization patterns of thin-film magnets, but the three-dimensional complexities of magnetization structure within the body of a magnet is not so amenable to direct investigation. Claire Donnelly et al. have made substantial progress in lifting this veil by harnessing hard-X-ray tomography to determine the inner magnetic structure of micrometre-sized magnets. The properties of current X-ray sources limit the spatial resolution to about 100 nanometres, but it is anticipated that future instrumental developments could greatly improve on this. In soft ferromagnetic materials, the smoothly varying magnetization leads to the formation of fundamental patterns such as domains, vortices and domain walls1. These have been studied extensively in thin films of thicknesses up to around 200 nanometres, in which the magnetization is accessible with current transmission imaging methods that make use of electrons or soft X-rays. In thicker samples, however, in which the magnetization structure varies throughout the thickness and is intrinsically three dimensional, determining the complex magnetic structure directly still represents a challenge1,3. We have developed hard-X-ray vector nanotomography with which to determine the three-dimensional magnetic configuration at the nanoscale within micrometre-sized samples. We imaged the structure of the magnetization within a soft magnetic pillar of diameter 5 micrometres with a spatial resolution of 100 nanometres and, within the bulk, observed a complex magnetic configuration that consists of vortices and antivortices that form cross-tie walls and vortex walls along intersecting planes. At the intersections of these structures, magnetic singularities—Bloch points—occur. These were predicted more than fifty years ago4 but have so far not been directly observed. Here we image the three-dimensional magnetic structure in the vicinity of the Bloch points, which until now has been accessible only through micromagnetic simulations, and identify two possible magnetization configurations: a circulating magnetization structure5 and a twisted state that appears to correspond to an ‘anti-Bloch point’. Our imaging method enables the nanoscale study of topological magnetic structures6 in systems with sizes of the order of tens of micrometres. Knowledge of internal nanomagnetic textures is critical for understanding macroscopic magnetic properties and for designing bulk magnets for technological applications7.
235 citations
TL;DR: In this article, the first observation of skyrmionic magnetic bubbles with variable topological spin textures formed at room temperature in a frustrated kagome Fe3 Sn2 magnet with uniaxial magnetic anisotropy is reported.
Abstract: The quest for materials hosting topologically protected skyrmionic spin textures continues to be fueled by the promise of novel devices. Although many materials have demonstrated the existence of such spin textures, major challenges remain to be addressed before devices based on magnetic skyrmions can be realized. For example, being able to create and manipulate skyrmionic spin textures at room temperature is of great importance for further technological applications because they can adapt to various external stimuli acting as information carriers in spintronic devices. Here, the first observation of skyrmionic magnetic bubbles with variable topological spin textures formed at room temperature in a frustrated kagome Fe3 Sn2 magnet with uniaxial magnetic anisotropy is reported. The magnetization dynamics are investigated using in situ Lorentz transmission electron microscopy, revealing that the transformation between different magnetic bubbles and domains is via the motion of Bloch lines driven by an applied external magnetic field. These results demonstrate that Fe3 Sn2 facilitates a unique magnetic control of topological spin textures at room temperature, making it a promising candidate for further skyrmion-based spintronic devices.
191 citations
TL;DR: In this paper, the authors demonstrate that a bi-layer of a heavy metal (Pt) and a Bi-axial antiferromagnetic (AFM) dielectric (NiO) can be a source of a coherent THz signal.
Abstract: The development of compact and tunable room temperature sources of coherent THz-frequency signals would open a way for numerous new applications. The existing approaches to THz-frequency generation based on superconductor Josephson junctions (JJ), free electron lasers, and quantum cascades require cryogenic temperatures or/and complex setups, preventing the miniaturization and wide use of these devices. We demonstrate theoretically that a bi-layer of a heavy metal (Pt) and a bi-axial antiferromagnetic (AFM) dielectric (NiO) can be a source of a coherent THz signal. A spin-current flowing from a DC-current-driven Pt layer and polarized along the hard AFM anisotropy axis excites a non-uniform in time precession of magnetizations sublattices in the AFM, due to the presence of a weak easy-plane AFM anisotropy. The frequency of the AFM oscillations varies in the range of 0.1-2.0 THz with the driving current in the Pt layer from 108 A/cm2 to 109 A/cm2. The THz-frequency signal from the AFM with the amplitude exceeding 1 V/cm is picked up by the inverse spin-Hall effect in Pt. The operation of a room-temperature AFM THz-frequency oscillator is similar to that of a cryogenic JJ oscillator, with the energy of the easy-plane magnetic anisotropy playing the role of the Josephson energy.
186 citations
TL;DR: The effects of the transverse ligands on the magnetic and electronic structure of 1-Dy were investigated through ab initio methods, eliciting significant magnetic axiality, even in the fourth Kramers doublet, thus demonstrating the potential of rigid diamide ligands in the design of new SIMs with defined magneticAxiality.
Abstract: DyIII single-ion magnets (SIMs) with strong axial donors and weak equatorial ligands are attractive model systems with which to harness the maximum magnetic anisotropy of DyIII ions. Utilizing a rigid ferrocene diamide ligand (NNTBS), a DyIII SIM, (NNTBS)DyI(THF)2, 1-Dy (NNTBS = fc(NHSitBuMe2)2, fc = 1,1′-ferrocenediyl), composed of a near linear arrangement of donor atoms, exhibits a large energy barrier to spin reversal (770.8 K) and magnetic blocking (14 K). The effects of the transverse ligands on the magnetic and electronic structure of 1-Dy were investigated through ab initio methods, eliciting significant magnetic axiality, even in the fourth Kramers doublet, thus demonstrating the potential of rigid diamide ligands in the design of new SIMs with defined magnetic axiality.
178 citations
TL;DR: A family of three magnetic two-dimensional materials with half-metallic band structures, FeCl2, FeBr2, and FeI2, are predicted, which should make these materials useful for 2D spin valves and other spintronic applications.
Abstract: Through a systematic search of all layered bulk compounds combined with density functional calculations employing hybrid exchange-correlation functionals, we predict a family of three magnetic two-dimensional (2D) materials with half-metallic band structures. The 2D materials, FeCl2, FeBr2, and FeI2, are all sufficiently stable to be exfoliated from bulk layered compounds. The Fe2+ ions in these materials are in a high-spin octahedral d6 configuration leading to a large magnetic moment of 4 μB. Calculations of the magnetic anisotropy show an easy-plane for the magnetic moment. A classical XY model with nearest neighbor coupling estimates critical temperatures, Tc, for the Berezinskii–Kosterlitz–Thouless transition ranging from 122 K for FeI2 to 210 K for FeBr2. The quantum confinement of these 2D materials results in unusually large spin gaps, ranging from 4.0 eV for FeI2 to 6.4 eV for FeCl2, which should defend against spin current leakage even at small device length scales. Their purely spin-polarized c...
14 Sep 2017
TL;DR: In this paper, the authors reported the wafer-scale growth of 2D ferromagnetic thin films of Fe3GeTe2 via molecular beam epitaxy, and their exotic magnetic properties can be manipulated via the Fe composition and the interface coupling with antiferromagnetic MnTe.
Abstract: Recently, layered two-dimensional ferromagnetic materials (2D FMs) have attracted a great deal of interest for developing low-dimensional magnetic and spintronic devices. Mechanically exfoliated 2D FMs were discovered to possess ferromagnetism down to monolayer. It is therefore of great importance to investigate the distinct magnetic properties at low dimensionality. Here, we report the wafer-scale growth of 2D ferromagnetic thin films of Fe3GeTe2 via molecular beam epitaxy, and their exotic magnetic properties can be manipulated via the Fe composition and the interface coupling with antiferromagnetic MnTe. A 2D layer-by-layer growth mode has been achieved by in situ reflection high-energy electron diffraction oscillations, yielding a well-defined interlayer distance of 0.82 nm along {002} surface. The magnetic easy axis is oriented along c-axis with a Curie temperature of 216.4 K. Remarkably, the Curie temperature can be enhanced when raising the Fe composition. Upon coupling with MnTe, the coercive field dramatically increases 50% from 0.65 to 0.94 Tesla. The large-scale layer-by-layer growth and controllable magnetic properties make Fe3GeTe2 a promising candidate for spintronic applications. It also opens up unprecedented opportunities to explore rich physics when coupled with other 2D superconductors and topological matters. Molecular beam epitaxy enables wafer-scale growth of Fe3GeTe2, an atomically thin ferromagnetic compound. A team led by Faxian Xiu at Fudan University demonstrated layer-by-layer growth of large-area, 8 nm-thick films of Fe3GeTe2 on sapphire and GaAs substrates in a high-vacuum molecular beam epitaxy system. The measured Curie temperature of 216.4 K was found to vary systematically with the Fe composition, indicating that Fe doping is a viable route to achieving tailored ferromagnetic ternary compounds with tunable Curie temperature. Furthermore, upon coupling Fe3GeTe2 with antiferromagnetic MnTe, the magnetic properties of the former could be enhanced owing to the exchange interaction from the ferromagnetic/antiferromagnetic superlattice interface. As a result, the coercive field increased by 50% with respect to bare Fe3GeTe2. These results highlight that Fe3GeTe2 and its heterostructures are promising candidates for spintronic devices.
TL;DR: This work shows, by synchrotron X-ray absorption spectroscopy, electric field induction of magnetic dipole moments in a platinum monatomic layer placed on ferromagnetic iron, and indicates that electric quadrupole induction produces magnetic dipoles moments and provides a large magnetic anisotropy change.
Abstract: Electric fields at interfaces exhibit useful phenomena, such as switching functions in transistors, through electron accumulations and/or electric dipole inductions. We find one potentially unique situation in a metal–dielectric interface in which the electric field is atomically inhomogeneous because of the strong electrostatic screening effect in metals. Such electric fields enable us to access electric quadrupoles of the electron shell. Here we show, by synchrotron X-ray absorption spectroscopy, electric field induction of magnetic dipole moments in a platinum monatomic layer placed on ferromagnetic iron. Our theoretical analysis indicates that electric quadrupole induction produces magnetic dipole moments and provides a large magnetic anisotropy change. In contrast with the inability of current designs to offer ultrahigh-density memory devices using electric-field-induced spin control, our findings enable a material design showing more than ten times larger anisotropy energy change for such a use and highlight a path in electric-field control of condensed matter. Electric field control of magnetization is usually weak and this hampers its application for the ultralow-power-consumption spintronic devices. Here, the authors demonstrate a mechanism to enhance the control of magnetic anisotropy by voltage-induced electric quadrupole in a metal–dielectric interface.
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TL;DR: It is demonstrated that Fe3 Sn2 facilitates a unique magnetic control of topological spin textures at room temperature, making it a promising candidate for further skyrmion-based spintronic devices.
Abstract: Various and spontaneous magnetic skyrmionic bubbles are experimentally observed for the first time, at room temperature in a frustrated kagome magnet Fe3Sn2 with unixial magnetic anisotropy. The magnetization dynamics were investigated using in-situ Lorentz transmission electron microscopy, revealing that the transformation between different magnetic bubbles and domains are via the motion of Bloch lines driven by applied external magnetic field. The results demonstrate that Fe3Sn2 facilitates a unique magnetic control of topological spin textures at room temperature, making it a promising candidate for further skyrmion-based spintronic devices.
TL;DR: This work exploits tunability to enhance the strength of the exchange field by a factor of 10x at low temperatures, when compared to a permalloy/CoO heterostructure.
Abstract: Entropy-stabilized materials are stabilized by the configurational entropy of the constituents, rather than the enthalpy of formation of the compound. A unique benefit to entropy-stabilized materials is the increased solubility of elements, which opens a broad compositional space, with subsequent local chemical and structural disorder resulting from different atomic sizes and preferred coordinations of the constituents. Known entropy-stabilized oxides contain magnetically interesting constituents, however, the magnetic properties of the multi-component oxide have yet to be investigated. Here we examine the role of disorder and composition on the exchange anisotropy of permalloy/(Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O heterostructures. Anisotropic magnetic exchange and the presence of a critical blocking temperature indicates that the magnetic order of the entropy-stabilized oxides considered here is antiferromagnetic. Changing the composition of the oxide tunes the disorder, exchange field and magnetic anisotropy. Here, we exploit this tunability to enhance the strength of the exchange field by a factor of 10x at low temperatures, when compared to a permalloy/CoO heterostructure. Significant deviations from the rule of mixtures are observed in the structural and magnetic parameters, indicating that the crystal is dominated by configurational entropy. Our results reveal that the unique characteristics of entropy-stabilized materials can be utilized and tailored to engineer magnetic functional phenomena in oxide thin films.
TL;DR: In this article, the magnetic properties of hybrid epoxy-based composite materials with magnetic barium ferrite (BaM) and conductive carbon (multi-walled carbon nanotubes and graphite nanoplatelets) fillers were evaluated.
Abstract: The aim of this study was to evaluate the magnetic properties of hybrid epoxy-based composite materials with magnetic barium ferrite (BaM) and conductive carbon (multi-walled carbon nanotubes and graphite nanoplatelets) fillers. This study was performed in order to fabricate composites with uniform and aligned magnetic components of the filler. The magnetization field dependences of the composites and BaM powder were determined and analyzed using a vibrating magnetometer. Of additional concern was the filler distribution characterization by optical investigation. Coercivity of BaM/epoxy composites increases more than in 4 times in comparison with BaM powder. Composites with aligned filler reveal an anisotropy of the magnetization relative to the alignment axis. However, BaM particles retain their uniform distribution in aligned fibers, which is confirmed by a magnetic remanence-to-saturation magnetization (M(0)/M
max) ratio of 0.5.
TL;DR: In this article, the impact of different insertion layers (Ta, Pt, and Mg) at the CoFeB|MgO interface on voltage-controlled magnetic anisotropy (VCMA) effect and other magnetic properties was studied.
Abstract: We studied the impact of different insertion layers (Ta, Pt, and Mg) at the CoFeB|MgO interface on voltage-controlled magnetic anisotropy (VCMA) effect and other magnetic properties. Inserting a very thin Mg layer of 0.1–0.3 nm yielded a VCMA coefficient of 100 fJ/V-m, more than 3 times higher than the average values of around 30 fJ/V-m reported in Ta|CoFeB|MgO-based structures. Ta and Pt insertion layers also showed a small improvement, yielding VCMA coefficients around 40 fJ/V-m. Electrical, magnetic, and X-ray diffraction results reveal that a Mg insertion layer of around 1.2 nm gives rise to the highest perpendicular magnetic anisotropy, saturation magnetization, as well as the best CoFe and MgO crystallinity. Other Mg insertion thicknesses give rise to either under- or over-oxidation of the CoFe|MgO interface; a strong over-oxidation of the CoFe layer leads to the maximum VCMA effect. These results show that precise control over the Mg insertion thickness and CoFe oxidation level at the CoFeB|MgO int...
TL;DR: In this paper, liquid phase epitaxy (LPE) technique was used to grow yttrium iron garnet (YIG) films with thicknesses of ≈100 nm and surface roughness as low as 0.3 nm.
Abstract: Using a liquid phase epitaxy (LPE) technique (1 1 1) yttrium iron garnet (YIG) films with thicknesses of ≈100 nm and surface roughnesses as low as 0.3 nm have been grown on (1 1 1) gadolinium gallium garnet (GGG) substrates as a basic material for spin-wave propagation experiments in microstructured waveguides. The continuously strained films exhibit nearly perfect crystallinity without significant mosaicity and with effective lattice misfits of and below. The film/substrate interface is extremely sharp without broad interdiffusion layer formation. All LPE films exhibit a nearly bulk-like saturation magnetization of () Gs and an 'easy cone' anisotropy type with extremely small in-plane coercive fields <0.2 Oe. There is a rather weak in-plane magnetic anisotropy with a pronounced six-fold symmetry observed for the saturation field <1.5 Oe. No significant out-of-plane anisotropy is observed, but a weak dependence of the effective magnetization on the lattice misfit is detected. The narrowest ferromagnetic resonance linewidth is determined to be 1.4 Oe @ 6.5 GHz which is the lowest value reported so far for YIG films of 100 nm thicknesses and below. The Gilbert damping coefficient for investigated LPE films is estimated to be close to .
TL;DR: This study suggests that various skyrmion lattices can emerge at low temperatures, where the skyrMions exhibit distinct topological nature and high sensitivity to the local magnetic anisotropy arising from the underlying chemical lattice.
Abstract: Magnetic skyrmions exhibit particle-like properties owing to the topology of their swirling spin texture, providing opportunities to study crystallization of topological particles. However, they mostly end up with a triangular lattice, and thus, the packing degree of freedom in the skyrmion particles has been overlooked so far. We report a structural transition of the skyrmion lattice in MnSi. By use of small-angle neutron scattering, we explore a metastable skyrmion state spreading over a wide temperature and magnetic field region, after thermal quenching. The quenched skyrmions undergo a triangular-to-square lattice transition with decreasing magnetic field at low temperatures. Our study suggests that various skyrmion lattices can emerge at low temperatures, where the skyrmions exhibit distinct topological nature and high sensitivity to the local magnetic anisotropy arising from the underlying chemical lattice.
TL;DR: In this article, robust PMA in ultrathin thulium iron garnet (TmIG) films of high structural quality down to a thickness of 5.6 nm is demonstrated.
Abstract: With recent developments in the field of spintronics, ferromagnetic insulator (FMI) thin films have emerged as an important component of spintronic devices. Ferrimagnetic yttrium iron garnet in particular is an excellent insulator with low Gilbert damping and a Curie temperature well above room temperature, and has been incorporated into heterostructures that exhibit a plethora of spintronic phenomena including spin pumping, spin Seebeck, and proximity effects. However, it has been a challenge to develop high quality sub-10 nm thickness FMI garnet films with perpendicular magnetic anisotropy (PMA) and PMA garnet/heavy metal heterostructures to facilitate advances in spin-current and anomalous Hall phenomena. Here, robust PMA in ultrathin thulium iron garnet (TmIG) films of high structural quality down to a thickness of 5.6 nm are demonstrated, which retain a saturation magnetization close to bulk. It is shown that TmIG/Pt bilayers exhibit a large spin Hall magnetoresistance (SMR) and SMR-driven anomalous Hall effect, which indicates efficient spin transmission across the TmIG/Pt interface. These measurements are used to quantify the interfacial spin mixing conductance in TmIG/Pt and the temperature-dependent PMA of the TmIG thin film.
TL;DR: In this article, the physical insights obtained by scanning tunneling microscopy into single magnetic adsorbates on the $s$-wave superconductor lead (Pb) are discussed.
Abstract: In superconductors, magnetic impurities induce a pair-breaking potential for Cooper pairs, which locally affects the Bogoliubov quasiparticles and gives rise to Yu-Shiba-Rusinov (YSR or Shiba, in short) bound states in the density of states (DoS). These states carry information on the magnetic coupling strength of the impurity with the superconductor, which determines the many-body ground state properties of the system. Recently, the interest in Shiba physics was boosted by the prediction of topological superconductivity and Majorana modes in magnetically coupled chains and arrays of Shiba impurities. Here, we review the physical insights obtained by scanning tunneling microscopy into single magnetic adsorbates on the $s$-wave superconductor lead (Pb). We explore the tunneling processes into Shiba states, show how magnetic anisotropy affects many-body excitations, and determine the crossing of the many-body groundstate through a quantum phase transition. Finally, we discuss the coupling of impurities into dimers and chains and their relation to Majorana physics.
TL;DR: In this article, the effect of compressive and tensile strain on the perpendicular magnetic anisotropy of light element heterostructures of Co films, Co/graphene, and Co/BN was investigated.
Abstract: Perpendicular magnetic tunnel junctions in the next-generation magnetic memory using current induced magnetization switching will likely rely on a material design that can enhance the perpendicular magnetic anisotropy of heterojunctions containing only light elements Using first-principles calculations, we investigated the effect of compressive and tensile strain on the perpendicular magnetic anisotropy of light element heterostructures of Co films, Co/graphene, and Co/BN We found that the perpendicular magnetic anisotropy of Co/graphene is greatly enhanced compared to the Co films, while that of Co/BN is reduced compared to the Co films In addition, tensile strain can further enhance perpendicular magnetic anisotropy of Co/graphene and Co/BN heterojunctions by 485% and 808%, respectively, compared to the unstrained systems A density of state analysis, combined with layer and orbital magnetic anisotropy contributions obtained from a second-order perturbation theory of the spin-orbit coupling, reveals that the tensile strain effect arises from the increase of the hybridization between same spin ${d}_{xy}$ and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ states of the surface Co film Our results suggest that strain engineering is an effective approach to enhance the perpendicular magnetic anisotropy of light element heterostructures
TL;DR: In this article, an iridium-doped ultrathin iron film was used to improve the voltage-controlled magnetic anisotropy (VCMA) and the perpendicular magnetic aisotropic (PMA) properties of spintronic devices.
Abstract: Voltage control of spin enables both a zero standby power and ultralow active power consumption in spintronic devices, such as magnetoresistive random-access memory devices. A practical approach to achieve voltage control is the electrical modulation of the spin–orbit interaction at the interface between 3d-transition-ferromagnetic-metal and dielectric layers in a magnetic tunnel junction (MTJ). However, we need to initiate a new guideline for materials design to improve both the voltage-controlled magnetic anisotropy (VCMA) and perpendicular magnetic anisotropy (PMA). Here we report that atomic-scale doping of iridium in an ultrathin Fe layer is highly effective to improving these properties in Fe/MgO-based MTJs. A large interfacial PMA energy, Ki,0, of up to 3.7 mJ m−2 was obtained, which was 1.8 times greater than that of the pure Fe/MgO interface. Moreover, iridium doping yielded a huge VCMA coefficient (up to 320 fJ Vm−1) as well as high-speed response. First-principles calculations revealed that Ir atoms dispersed within the Fe layer play a considerable role in enhancing Ki,0 and the VCMA coefficient. These results demonstrate the efficacy of heavy-metal doping in ferromagnetic layers as an advanced approach to develop high-density voltage-driven spintronic devices. Researchers from Japan's AIST demonstrated a new approach to reduce the energy consumption of spintronic devices. Magnetic random-access memory requires approximately 10,000 times more energy to record data than to safely maintain it — a discrepancy that arises due to the wastefulness of electric-current-based switching of magnetic bits. Takayuki Nozaki and colleagues now report a device that enables us to write magnetic memory using electric fields, a more energy-efficient control mechanism. The team introduced an iridium-doped ultrathin iron film in magnesium oxide-based magnetic tunnel junctions, and found that the heavy-metal dopants provoked a strong voltage-controlled magnetic anisotropy change with high-speed response. Physical role of heavy metal dopants was unveiled by first-principles calculations. The developed technique can lead to a new type of non-volatile memory with ultra-low energy consumption. Highly efficient voltage control of magnetic anisotropy has been demonstrated utlizing an ultrathin Ir-doped Fe layer in MgO-based magnetic tunnel junctions. Ir adoms are dispersed inside the ultrathin Fe layer through the interdiffusion process. Large spin–orbit interaction of Ir atoms having proximity-induced magnetism is attributed to the enhancement of the voltage-controlled magnetic anisotropy (VCMA) effect. High speed response of the VCMA effect was also confirmed by voltage-induced ferromagnetic resonance. The achieved properties first satisfy the required specification for the new type of magnetoresistive random access memory (MRAM) driven by voltage.
TL;DR: Electrical flipping of magnetization between the out-of-plane and the in-plane directions in (Co/Pt)3/(011) Pb(Mg1/3Nb2/3)O3-PbTiO3 multiferroic heterostructures through a voltage-controllable spin reorientation transition (SRT) enables the realization of power efficient PMA magnetoelectric and spintronic devices.
Abstract: One of the central challenges in realizing multiferroics-based magnetoelectric memories is to switch perpendicular magnetic anisotropy (PMA) with a control voltage. In this study, we demonstrate electrical flipping of magnetization between the out-of-plane and the in-plane directions in (Co/Pt)3/(011) Pb(Mg1/3Nb2/3)O3–PbTiO3 multiferroic heterostructures through a voltage-controllable spin reorientation transition (SRT). The SRT onset temperature can be dramatically suppressed at least 200 K by applying an electric field, accompanied by a giant electric-field-induced effective magnetic anisotropy field (ΔHeff) up to 1100 Oe at 100 K. In comparison with conventional strain-mediated magnetoelastic coupling that provides a ΔHeff of only 110 Oe, that enormous effective field is mainly related to the interface effect of electric field modification of spin–orbit coupling from Co/Pt interfacial hybridization via strain. Moreover, electric field control of SRT is also achieved at room temperature, resulting in a ...
TL;DR: In this paper, a simple model of orbital populations was exploited to elucidate a number of features typical for the interface MCA, and the effect of the electric field on it, for $3d$ transition-metal thin films used in magnetic tunnel junctions.
Abstract: Voltage-controlled magnetic anisotropy (VCMA) is an efficient way to manipulate the magnetization states in nanomagnets and is promising for low-power spintronic applications. The underlying physical mechanism for VCMA is known to involve a change in the $d$ orbital occupation on the transition-metal interface atoms with an applied electric field. However, a simple qualitative picture of how this occupation controls the magnetocrystalline anisotropy (MCA) and even why in certain cases the MCA has the opposite sign remains elusive. In this paper, we exploit a simple model of orbital populations to elucidate a number of features typical for the interface MCA, and the effect of the electric field on it, for $3d$ transition-metal thin films used in magnetic tunnel junctions. We find that in all considered cases, including the Fe(001) surface, clean $\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{x}(001)/\mathrm{MgO}$ interface, and oxidized Fe(001)/MgO interface, the effects of alloying and the electric field enhance the MCA energy with electron depletion, which is largely explained by the occupancy of the minority-spin ${d}_{xz,yz}$ orbitals. However, the hole-doped Fe(001) exhibits an inverse VCMA in which the MCA enhancement is achieved when electrons are accumulated at the Fe (001)/MgO interface with the applied electric field. In this regime, we predict a significantly enhanced VCMA that exceeds 1 pJ/Vm. Realizing this regime experimentally may be favorable for the practical purpose of voltage-driven magnetization reversal.
Journal Article•
TL;DR: This work shows a unique ionic-liquid-gating system for strong interfacial magnetoelectric coupling with many practical advantages, paving the way toward ion-liquid -gating spintronic/electronic devices.
Abstract: Ionic-liquid gating on a functional thin film with a low voltage has drawn a lot of attention due to rich chemical, electronic, and magnetic phenomena at the interface. Here, a key challenge in quantitative determination of voltage-controlled magnetic anisotropy (VCMA) in Au/[DEME]+[TFSI]−/Co field-effect transistor heterostructures is addressed. The magnetic anisotropy change as response to the gating voltage is precisely detected by in situ electron spin resonance measurements. A reversible change of magnetic anisotropy up to 219 Oe is achieved with a low gating voltage of 1.5 V at room temperature, corresponding to a record high VCMA coefficient of ≈146 Oe V−1. Two gating effects, the electrostatic doping and electrochemical reaction, are distinguished at various gating voltage regions, as confirmed by X-ray photoelectron spectroscopy and atomic force microscopy experiments. This work shows a unique ionic-liquid-gating system for strong interfacial magnetoelectric coupling with many practical advantages, paving the way toward ion-liquid-gating spintronic/electronic devices.
TL;DR: In this paper, the authors studied spin-wave excitations in a circular ferromagnetic nanodot in different inhomogeneous, topologically nontrivial magnetization states, specifically, vortex and skyrmion states.
Abstract: We study spin-wave excitations in a circular ferromagnetic nanodot in different inhomogeneous, topologically nontrivial magnetization states, specifically, vortex and skyrmion states. Gradual change in the strength of the out-of-plane magnetic anisotropy and the Dzyaloshinskii-Moriya exchange interaction leads to continuous phase transitions between different stable magnetic configurations and allows for mapping of dynamic spin modes in and between the vortex, Bloch-type skyrmion, and N\'eel-type skyrmion states. Our study elucidates the connections between gyrotropic modes, azimuthal spin waves, and breathing modes in these various stable magnetization states and helps us to understand the rich spin excitation spectrum on the skyrmion background.
TL;DR: In this article, it was shown that the square symmetry of 2D metal-organic frameworks (MOFs) made from octaamino-substituted phthalocyanines and square planar Ni2+ ions, which enable strong conjugation of π electrons, has a critical impact on the magnetic properties of the lattice.
Abstract: We use first-principles calculations to show that the square symmetry of two-dimensional (2D) metal–organic frameworks (MOFs) made from octaamino-substituted phthalocyanines and square planar Ni2+ ions, which enable strong conjugation of π electrons, has a critical impact on the magnetic properties of the lattice. In particular, we predict the unexpected emergence of a rare high-temperature ferromagnetic half-metallic ground state in one case. Among charge neutral MOFs made from (2,3,9,10,16,17,23,24)-octaiminophthalocyanine (OIPc) metallated with divalent first-row transition metal ions (M-OIPc; M = Cr2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) and connected through square planar Ni-bisphenylenediimine moieties, NiMn-OIPc exhibits a half-metallic and ferromagnetic ground state with a large exchange energy resulting from the unique strong hybridization between the d/π orbitals of Mn, the Pc ring, and the Ni-bisphenylenediimine nodes. Notably, we show that for NiMn-OIPc there is a considerable difference between the ferromagnetic ordering temperature (Tc) predicted by a 2D Ising model, which exceeds 600 K, and a Tc of 170 K predicted by our more realistic Monte Carlo simulation that includes magnetic anisotropy. Critically, our simulations adopt two spin models that incorporate magnetic anisotropy in the form of exchange anisotropy and single-ion anisotropy. We further show that in the bulk, 2D layers of NiMn-OIPc adopt a slipped-parallel stacking configuration, and exhibit interlayer magnetic coupling that is sensitive to the relative in-plane displacement between adjacent layers. These results highlight the critical role of magnetic anisotropy in modeling the properties of 2D magnetic systems. More generally, it demonstrates that strong hybridization between open-shell ions and delocalized aromatic π systems with appropriate symmetry, combined with large magnetic anisotropy, will be an effective design strategy to realize ferromagnetic 2D MOFs with high Tc.
TL;DR: In this paper, the authors investigated how the spin-wave transmission across a curved magnonic waveguide can be modified through rotation of an externally delivered uniform magnetic field in a particular direction.
Abstract: Using a combination of experiment and simulation, the authors have investigated comprehensively how the spin-wave transmission across a curved magnonic waveguide is modified through rotation of an externally delivered uniform magnetic field. They reveal that, while the spin-wave transmission is reasonable when the bend is magnetized along an axis of symmetry, the transmission can be substantially improved (by a factor of 2) through just rotating the magnetic field by 15\ifmmode^\circ\else\textdegree\fi{} in a particular direction. To explain their results, the authors study the landscape of the internal magnetic field spanning the structure, and discuss how it can be tuned in order to optimally distribute the flow of anisotropic spin waves across networks of magnonic waveguides.