Showing papers on "Magnetization published in 2021"

Electrically tunable correlated and topological states in twisted monolayer–bilayer graphene

Abstract: Twisted van der Waals heterostructures with flat electronic bands have recently emerged as a platform for realizing correlated and topological states with a high degree of control and tunability. In graphene-based moire heterostructures, the correlated phase diagram and band topology depend on the number of graphene layers and the details of the external environment from the encapsulating crystals. Here, we report that the system of twisted monolayer–bilayer graphene (tMBG) hosts a variety of correlated metallic and insulating states, as well as topological magnetic states. Because of its low symmetry, the phase diagram of tMBG approximates that of twisted bilayer graphene when an applied perpendicular electric field points from the bilayer towards the monolayer graphene, or twisted double bilayer graphene when the field is reversed. In the former case, we observe correlated states that undergo an orbitally driven insulating transition above a critical perpendicular magnetic field. In the latter case, we observe the emergence of electrically tunable ferromagnetism at one-quarter filling of the conduction band, and an associated anomalous Hall effect. The direction of the magnetization can be switched by electrostatic doping at zero magnetic field. Our results establish tMBG as a tunable platform for investigating correlated and topological states. Stacking a monolayer and bilayer of graphene, with a small twist angle between them, creates a tunable platform where the physics of both twisted bilayer graphene and twisted double bilayer graphene can be realized.

Electrically tunable correlated and topological states in twisted monolayer–bilayer graphene

Abstract: Twisted van der Waals heterostructures with flat electronic bands have recently emerged as a platform for realizing correlated and topological states with a high degree of control and tunability. In graphene-based moire heterostructures, the correlated phase diagram and band topology depend on the number of graphene layers and the details of the external environment from the encapsulating crystals. Here, we report that the system of twisted monolayer–bilayer graphene (tMBG) hosts a variety of correlated metallic and insulating states, as well as topological magnetic states. Because of its low symmetry, the phase diagram of tMBG approximates that of twisted bilayer graphene when an applied perpendicular electric field points from the bilayer towards the monolayer graphene, or twisted double bilayer graphene when the field is reversed. In the former case, we observe correlated states that undergo an orbitally driven insulating transition above a critical perpendicular magnetic field. In the latter case, we observe the emergence of electrically tunable ferromagnetism at one-quarter filling of the conduction band, and an associated anomalous Hall effect. The direction of the magnetization can be switched by electrostatic doping at zero magnetic field. Our results establish tMBG as a tunable platform for investigating correlated and topological states. Stacking a monolayer and bilayer of graphene, with a small twist angle between them, creates a tunable platform where the physics of both twisted bilayer graphene and twisted double bilayer graphene can be realized.

Direct observation of two-dimensional magnons in atomically thin CrI3

Abstract: Magnons are collective spin excitations in crystals with long-range magnetic order. The emergent van der Waals magnets1–3 provide a highly tunable platform to explore magnetic excitations in the two-dimensional limit with intriguing properties, manifesting from their honeycomb lattice structure and switchable magnetic configurations. Here, we report the direct observation of two-dimensional magnons through magneto-Raman spectroscopy with optical selection rules determined by the interplay between crystal symmetry, layer number and magnetic states in atomically thin CrI3. In monolayers, we observe an acoustic magnon mode at ~0.3 meV. It has strict cross-circularly polarized selection rules locked to the magnetization direction that originates from the conservation of angular momentum of photons and magnons dictated by three-fold rotational symmetry4. Additionally, we reveal optical magnon modes at ~17 meV. This mode is Raman silent in monolayers, but optically active in bilayers and bulk due to a relaxation of the parity criterion resulting from the layer index. In the layered antiferromagnetic states, we directly resolve two degenerate optical magnon modes with opposite angular momentum and conjugate optical selection rules. From these measurements, we quantitatively extract the spin-wave gap, magnetic anisotropy and intralayer and interlayer exchange constants, and establish two-dimensional magnets as a new platform for exploring magnon physics. Magnons are collective excitations that dictate many of a magnet’s low-temperature properties. By means of Raman scattering, the magnon spectra of CrI3 are measured in the monolayer limit.

Ni substitution effect on the structure, magnetization, resistivity and permeability of zinc ferrites

Abstract: Zn1−xNixFe2O4 ferrites up to x = 1.0 with Δx = 0.2 have been synthesized via solid state reactions and the sol–gel autocombustion technique with step-by-step co-firing. Data on the chemical composition and the surface morphology of the samples have been obtained using a scanning electron microscope. An X-ray powder diffractometer has been used to establish the phase purity and to determine the unit cell parameters. It has been found that the obtained samples had a spinel structure with the Fdm (No. 227) space group. The unit cell parameters decrease with increasing nickel concentration. The a unit cell parameter decreases almost linearly from ∼8.443 A for x = 0.0 down to ∼8.337 A for x = 1.0. The V unit cell volume decreases almost linearly from ∼601.72 A3 for x = 0.0 down to ∼579.52 A3 for x = 1.0. The magnetic characteristics of the obtained samples are determined and discussed. The Curie point of obtained samples varies in the range of 803.5–572.7 K. The maximum spontaneous magnetization of ∼74.6 emu g−1 at room temperature was fixed for the solid solution with x = 0.6. Ac-resistivity drops by more than 3 orders of magnitude in the frequency range 1–106 Hz. The composition with x = 0.6 has the minimum ac-resistivity of 5.3 kOm cm at a frequency of 106 Hz. The maximum value of the (μ′) real part of ∼11.2 and (μ′′) imaginary part of ∼5.2 of the permeability in the frequency range of 50 MHz–10 GHz is observed for the composition with x = 0.4. The composite samples for the microwave study were prepared by mixing of the ferrite powders with molten paraffin wax. The volume fraction of the ferrite filler in the composites was 25%. The largest value of the (μ′) real part of ∼3 and (μ′′) imaginary part of ∼0.63 of permeability is found for the x = 0.4 composite. The formation of the composite significantly reduces permeability.

Observation of the antiferromagnetic spin Hall effect

Abstract: The discovery of the spin Hall effect1 enabled the efficient generation and manipulation of the spin current. More recently, the magnetic spin Hall effect2,3 was observed in non-collinear antiferromagnets, where the spin conservation is broken due to the non-collinear spin configuration. This provides a unique opportunity to control the spin current and relevant device performance with controllable magnetization. Here, we report a magnetic spin Hall effect in a collinear antiferromagnet, Mn2Au. The spin currents are generated at two spin sublattices with broken spatial symmetry, and the antiparallel antiferromagnetic moments play an important role. Therefore, we term this effect the ‘antiferromagnetic spin Hall effect’. The out-of-plane spins from the antiferromagnetic spin Hall effect are favourable for the efficient switching of perpendicular magnetized devices, which is required for high-density applications. The antiferromagnetic spin Hall effect adds another twist to the atomic-level control of spin currents via the antiferromagnetic spin structure. A magnetic spin Hall effect is reported in the collinear antiferromagnet Mn2Au.

Layer Hall effect in a 2D topological axion antiferromagnet

Abstract: Whereas ferromagnets have been known and used for millennia, antiferromagnets were only discovered in the 1930s1. At large scale, because of the absence of global magnetization, antiferromagnets may seem to behave like any non-magnetic material. At the microscopic level, however, the opposite alignment of spins forms a rich internal structure. In topological antiferromagnets, this internal structure leads to the possibility that the property known as the Berry phase can acquire distinct spatial textures2,3. Here we study this possibility in an antiferromagnetic axion insulator-even-layered, two-dimensional MnBi2Te4-in which spatial degrees of freedom correspond to different layers. We observe a type of Hall effect-the layer Hall effect-in which electrons from the top and bottom layers spontaneously deflect in opposite directions. Specifically, under zero electric field, even-layered MnBi2Te4 shows no anomalous Hall effect. However, applying an electric field leads to the emergence of a large, layer-polarized anomalous Hall effect of about 0.5e2/h (where e is the electron charge and h is Planck's constant). This layer Hall effect uncovers an unusual layer-locked Berry curvature, which serves to characterize the axion insulator state. Moreover, we find that the layer-locked Berry curvature can be manipulated by the axion field formed from the dot product of the electric and magnetic field vectors. Our results offer new pathways to detect and manipulate the internal spatial structure of fully compensated topological antiferromagnets4-9. The layer-locked Berry curvature represents a first step towards spatial engineering of the Berry phase through effects such as layer-specific moire potential.

Creation and observation of Hopfions in magnetic multilayer systems

Abstract: Among topological solitons, magnetic skyrmions are two-dimensional particle-like objects with a continuous winding of the magnetization, and magnetic Hopfions are three-dimensional objects that can be formed from a closed loop of twisted skyrmion strings. Theoretical models suggest that magnetic Hopfions can be stabilized in frustrated or chiral magnetic systems, and target skymions can be transformed into Hopfions by adapting their perpendicular magnetic anisotropy, but their experimental verification has been elusive so far. Here, we present an experimental study of magnetic Hopfions that are created in Ir/Co/Pt multilayers shaped into nanoscale disks, known to host target skyrmions. To characterize three-dimensional spin textures that distinguish Hopfions from target skyrmions magnetic images are recorded with surface-sensitive X-ray photoemission electron microscopy and bulk-sensitive soft X-ray transmission microscopy using element-specific X-ray magnetic circular dichroism effects as magnetic contrast. These results could stimulate further investigations of Hopfions and their potential application in three-dimensional spintronics devices.

Mn-Rich MnSb2Te4: A Topological Insulator with Magnetic Gap Closing at High Curie Temperatures of 45–50 K

Abstract: Ferromagnetic topological insulators exhibit the quantum anomalous Hall effect, which is potentially useful for high-precision metrology, edge channel spintronics, and topological qubits. The stable 2+ state of Mn enables intrinsic magnetic topological insulators. MnBi2Te4 is, however, antiferromagnetic with 25 K Neel temperature and is strongly n-doped. In this work, p-type MnSb2Te4, previously considered topologically trivial, is shown to be a ferromagnetic topological insulator for a few percent Mn excess. i) Ferromagnetic hysteresis with record Curie temperature of 45-50 K, ii) out-of-plane magnetic anisotropy, iii) a 2D Dirac cone with the Dirac point close to the Fermi level, iv) out-of-plane spin polarization as revealed by photoelectron spectroscopy, and v) a magnetically induced bandgap closing at the Curie temperature, demonstrated by scanning tunneling spectroscopy (STS), are shown. Moreover, a critical exponent of the magnetization beta approximate to 1 is found, indicating the vicinity of a quantum critical point. Ab initio calculations reveal that Mn-Sb site exchange provides the ferromagnetic interlayer coupling and the slight excess of Mn nearly doubles the Curie temperature. Remaining deviations from the ferromagnetic order open the inverted bulk bandgap and render MnSb2Te4 a robust topological insulator and new benchmark for magnetic topological insulators.

Anomalous transport due to Weyl fermions in the chiral antiferromagnets Mn3X, X = Sn, Ge.

Abstract: The recent discoveries of strikingly large zero-field Hall and Nernst effects in antiferromagnets Mn3X (X = Sn, Ge) have brought the study of magnetic topological states to the forefront of condensed matter research and technological innovation. These effects are considered fingerprints of Weyl nodes residing near the Fermi energy, promoting Mn3X (X = Sn, Ge) as a fascinating platform to explore the elusive magnetic Weyl fermions. In this review, we provide recent updates on the insights drawn from experimental and theoretical studies of Mn3X (X = Sn, Ge) by combining previous reports with our new, comprehensive set of transport measurements of high-quality Mn3Sn and Mn3Ge single crystals. In particular, we report magnetotransport signatures specific to chiral anomalies in Mn3Ge and planar Hall effect in Mn3Sn, which have not yet been found in earlier studies. The results summarized here indicate the essential role of magnetic Weyl fermions in producing the large transverse responses in the absence of magnetization.

Field-free spin-orbit torque-induced switching of perpendicular magnetization in a ferrimagnetic layer with a vertical composition gradient

Abstract: Current-induced spin-orbit torques (SOTs) are of interest for fast and energy-efficient manipulation of magnetic order in spintronic devices. To be deterministic, however, switching of perpendicularly magnetized materials by SOT requires a mechanism for in-plane symmetry breaking. Existing methods to do so involve the application of an in-plane bias magnetic field, or incorporation of in-plane structural asymmetry in the device, both of which can be difficult to implement in practical applications. Here, we report bias-field-free SOT switching in a single perpendicular CoTb layer with an engineered vertical composition gradient. The vertical structural inversion asymmetry induces strong intrinsic SOTs and a gradient-driven Dzyaloshinskii–Moriya interaction (g-DMI), which breaks the in-plane symmetry during the switching process. Micromagnetic simulations are in agreement with experimental results, and elucidate the role of g-DMI in the deterministic switching processes. This bias-field-free switching scheme for perpendicular ferrimagnets with g-DMI provides a strategy for efficient and compact SOT device design. Switching of ferrimagnets by current-induced spin-orbit torque is promising for spintronics, due to their high-speed dynamics and small macroscopic magnetization. Switching of perpendicularly magnetized materials, however, requires a bias field for symmetry breaking. Here, Zheng et al demonstrate field-free current-induced switching of perpendicular ferrimagnets, using a compositional gradient-driven Dzyaloshinskii–Moriya interaction.

Chiral-spin rotation of non-collinear antiferromagnet by spin–orbit torque

Abstract: Electrical manipulation of magnetic materials by current-induced spin torque constitutes the basis of spintronics. Here, we show an unconventional response to spin–orbit torque of a non-collinear antiferromagnet Mn3Sn, which has attracted attention owing to its large anomalous Hall effect despite a vanishingly small net magnetization. In epitaxial heavy-metal/Mn3Sn heterostructures, we observe a characteristic fluctuation of the Hall resistance under the application of electric current. This observation is explained by a rotation of the chiral-spin structure of Mn3Sn driven by spin–orbit torque. We find that the variation of the magnitude of anomalous Hall effect fluctuation with sample size correlates with the number of magnetic domains in the Mn3Sn layer. In addition, the dependence of the critical current on Mn3Sn layer thickness reveals that spin–orbit torque generated by small current densities, below 20 MA cm−2, effectively acts on the chiral-spin structure even in Mn3Sn layers that are thicker than 20 nm. The results provide additional pathways for electrical manipulation of magnetic materials. Current-induced rotation in epitaxial films of the non-collinear antiferromagnet Mn3Sn is investigated.

Magnetic domains and domain wall pinning in atomically thin CrBr3 revealed by nanoscale imaging.

Abstract: The emergence of atomically thin van der Waals magnets provides a new platform for the studies of two-dimensional magnetism and its applications. However, the widely used measurement methods in recent studies cannot provide quantitative information of the magnetization nor achieve nanoscale spatial resolution. These capabilities are essential to explore the rich properties of magnetic domains and spin textures. Here, we employ cryogenic scanning magnetometry using a single-electron spin of a nitrogen-vacancy center in a diamond probe to unambiguously prove the existence of magnetic domains and study their dynamics in atomically thin CrBr3. By controlling the magnetic domain evolution as a function of magnetic field, we find that the pinning effect is a dominant coercivity mechanism and determine the magnetization of a CrBr3 bilayer to be about 26 Bohr magnetons per square nanometer. The high spatial resolution of this technique enables imaging of magnetic domains and allows to locate the sites of defects that pin the domain walls and nucleate the reverse domains. Our work highlights scanning nitrogen-vacancy center magnetometry as a quantitative probe to explore nanoscale features in two-dimensional magnets.

Chiral Symmetry Breaking for Deterministic Switching of Perpendicular Magnetization by Spin-Orbit Torque.

Abstract: Symmetry breaking is a characteristic to determine which branch of a bifurcation system follows upon crossing a critical point. Specifically, in spin-orbit torque (SOT) devices, a fundamental question arises: how can the symmetry of the perpendicular magnetic moment be broken by the in-plane spin polarization? Here, we show that the chiral symmetry breaking by the antisymmetric Dzyaloshinskii-Moriya interaction (DMI) can induce the deterministic SOT switching of the perpendicular magnetization. By introducing a gradient of saturation magnetization or magnetic anisotropy, the dynamic noncollinear spin textures are formed under the current-driven SOT, and thus, the chiral symmetry of these dynamic spin textures is broken by the DMI, resulting in the deterministic magnetization switching. We introduce a strategy to induce an out-of-plane (z) gradient of magnetic properties as a practical solution for the wafer-scale manufacture of SOT devices.

Superconductivity and normal-state properties of kagome metal RbV3Sb5 single crystals

Abstract: We report the discovery of superconductivity and detailed normal-state physical properties of RbV3Sb5 single crystals with V kagome lattice RbV3Sb5 single crystals show a superconducting transition at Tc ~ 092 K Meanwhile, resistivity, magnetization and heat capacity measurements indicate that it exhibits anomalies of properties at T* ~ 102 - 103 K, possibly related to the formation of charge ordering state When T is lower than T*, the Hall coefficient RH undergoes a drastic change and sign reversal from negative to positive, which can be partially explained by the enhanced mobility of hole-type carriers In addition, the results of quantum oscillations show that there are some very small Fermi surfaces with low effective mass, consistent with the existence of multiple highly dispersive Dirac band near the Fermi energy level

Opening Magnetic Hysteresis by Axial Ferromagnetic Coupling: From Mono-Decker to Double-Decker Metallacrown

Abstract: Combining Ising-type magnetic anisotropy with collinear magnetic interactions in single-molecule magnets (SMMs) is a significant synthetic challenge. Herein we report a Dy[15-MCCu -5] (1-Dy) SMM, where a DyIII ion is held in a central pseudo-D5h pocket of a rigid and planar Cu5 metallacrown (MC). Linking two Dy[15-MCCu -5] units with a single hydroxide bridge yields the double-decker {Dy[15-MCCu -5]}2 (2-Dy) SMM where the anisotropy axes of the two DyIII ions are nearly collinear, resulting in magnetic relaxation times for 2-Dy that are approximately 200 000 times slower at 2 K than for 1-Dy in zero external field. Whereas 1-Dy and the YIII -diluted Dy@2-Y analogue do not show remanence in magnetic hysteresis experiments, the hysteresis data for 2-Dy remain open up to 6 K without a sudden drop at zero field. In conjunction with theoretical calculations, these results demonstrate that the axial ferromagnetic Dy-Dy coupling suppresses fast quantum tunneling of magnetization (QTM). The relaxation profiles of both complexes curiously exhibit three distinct exponential regimes, and hold the largest effective energy barriers for any reported d-f SMMs up to 625 cm-1 .

Electrical and thermal generation of spin currents by magnetic bilayer graphene.

Abstract: Ultracompact spintronic devices greatly benefit from the implementation of two-dimensional materials that provide large spin polarization of charge current together with long-distance transfer of spin information. Here spin-transport measurements in bilayer graphene evidence a strong spin–charge coupling due to a large induced exchange interaction by the proximity of an interlayer antiferromagnet (CrSBr). This results in the direct detection of the spin polarization of conductivity (up to 14%) and a spin-dependent Seebeck effect in the magnetic graphene. The efficient electrical and thermal spin–current generation is the most technologically relevant aspect of magnetism in graphene, controlled here by the antiferromagnetic dynamics of CrSBr. The high sensitivity of spin transport in graphene to the magnetization of the outermost layer of the adjacent antiferromagnet, furthermore, enables the read-out of a single magnetic sublattice. The combination of gate-tunable spin-dependent conductivity and Seebeck coefficient with long-distance spin transport in a single two-dimensional material promises ultrathin magnetic memory and sensory devices based on magnetic graphene. Graphene promises long-distance transfer of spin information with concomitant high charge carrier mobility. Proximity coupling of bilayer graphene with the 2D interlayer antiferromagnetic CrSBr now enables active generation of spin currents in graphene both electrically and thermally.

Exploration of crystal structure, magnetic and dielectric properties of titanium-barium hexaferrites

Abstract: The BaFe12-xTixO19 hexaferrites up to x = 3.00 were extended. XRD patterns were Rietveld fitted for P63/mmc (no. 194) space group and the unit cell parameters were defined. The a parameter and V volume of unit cell change non-monotonically with × while the c parameter has linear behavior. The Ti4+ cations substitute the Fe3+ cations in the 2a, 4fVI and 12 k octahedral positions. This is confirmed by the Mossbauer spectroscopy. Ms spontaneous magnetization was determined with the law of approach to saturation from field magnetization at 5 K and 300 K. The e/ real part of the permittivity increases constantly with increasing temperature and decreases with frequency. The main objective of this study is an interpretation of the magnetic and dielectric properties of the titanium-barium hexaferrites which has performed in frame of breakdown of Fe3+ - O2– - Fe3+/2+(Ti4+) indirect superexchange interactions taking into account the positions occupation.

Beyond Solid Solution High‐Entropy Alloys: Tailoring Magnetic Properties via Spinodal Decomposition

Abstract: Since its first emergence in 2004, the high-entropy alloy (HEA) concept has aimed at stabilizing single- or dual-phase multi-element solid solutions through high mixing entropy. Here, this strategy is changed and renders such massive solid solutions metastable, to trigger spinodal decomposition for improving the alloys’ magnetic properties. The motivation for starting from a HEA for this approach is to provide the chemical degrees of freedom required to tailor spinodal behavior using multiple components. The key idea is to form Fe-Co enriched regions which have an expanded volume (relative to unconstrained Fe-Co), due to coherency constraints imposed by the surrounding HEA matrix. As demonstrated by theory and experiments, this leads to improved magnetic properties of the decomposed alloy relative to the original solid solution matrix. In a prototype magnetic FeCoNiMnCu HEA, it is shown that the modulated structures, achieved by spinodal decomposition, lead to an increase of the Curie temperature by 48% and a simultaneous increase of magnetization by 70% at ambient temperature as compared to the homogenized single-phase reference alloy. The findings thus open a pathway for the development of advanced functional HEAs.

Two-dimensional magneto-photoconductivity in non-van der Waals manganese selenide

Abstract: Deficient intrinsic species and suppressed Curie temperatures (Tc) in two-dimensional (2D) magnets are major barriers for future spintronic applications. As an alternative, delaminating non-van der Waals (vdW) magnets can offset these shortcomings and involve robust bandgaps to explore 2D magneto-photoconductivity at ambient temperature. Herein, non-vdW α-MnSe2 is first delaminated as quasi-2D nanosheets for the study of emerging semiconductor, ferromagnetism and magneto-photoconductivity behaviors. Abundant nonstoichiometric surfaces induce the renormalization of the band structure and open a bandgap of 1.2 eV. The structural optimization strengthens ferromagnetic super–exchange interactions between the nearest-neighbor Mn2+, which enables us to achieve a high Tc of 320 K well above room temperature. The critical fitting of magnetization and transport measurements both verify that it is of quasi-2D nature. The above observations are evidenced by multiple microscopic and macroscopic characterization tools, in line with the prediction of first-principles calculations. Profiting from the negative magnetoresistance effect, the self-powered infrared magneto-photoconductivity performance including a responsivity of 330.4 mA W−1 and a millisecond-level response speed are further demonstrated. Such merits stem from the synergistic modulation of magnetic and light fields on photogenerated carriers. This provides a new strategy to manipulate both charge and spin in 2D non-vdW systems and displays their alluring prospects in magneto-photodetection.

Half-Metallicity and Magnetism in the Full Heusler Alloy Fe 2 MnSn with L21 and XA Stability Ordering Phases

Abstract: In this paper, we study and discuss the ab initio method and Monte Carlo simulations of the Fe2MnSn full Heusler alloy for the both structures: XA and L21. In fact, we have computed the structural, the electronic, the magnetic and the critical magnetic behavior of this alloy. Firstly, we have performed the ab initio calculations using the GGA approximations. The results of the electronic structures show that the full Heusler Fe2MnSn alloy shows a half-metallic character and a 100% spin polarization, only for the XA phase, at the Fermi level. This half-metallic behavior is confirmed by the integral value of the computed total magnetic moment value (6.00 μB). For non-null temperature values, we illustrate the magnetization behavior by using the Monte Carlo simulations (MCS) under the Metropolis algorithm. The magnetizations are illustrated and discussed as a function of the temperature and the exchange coupling interactions. To complete this study, we present and discuss the Monte Carlo results and compared them with the existing experimental values in the literature.

Controllable field-free switching of perpendicular magnetization through bulk spin-orbit torque in symmetry-broken ferromagnetic films.

Abstract: Programmable magnetic field-free manipulation of perpendicular magnetization switching is essential for the development of ultralow-power spintronic devices. However, the magnetization in a centrosymmetric single-layer ferromagnetic film cannot be switched directly by passing an electrical current in itself. Here, we demonstrate a repeatable bulk spin-orbit torque (SOT) switching of the perpendicularly magnetized CoPt alloy single-layer films by introducing a composition gradient in the thickness direction to break the inversion symmetry. Experimental results reveal that the bulk SOT-induced effective field on the domain walls leads to the domain walls motion and magnetization switching. Moreover, magnetic field-free perpendicular magnetization switching caused by SOT and its switching polarity (clockwise or counterclockwise) can be reversibly controlled in the IrMn/Co/Ru/CoPt heterojunctions based on the exchange bias and interlayer exchange coupling. This unique composition gradient approach accompanied with electrically controllable SOT magnetization switching provides a promising strategy to access energy-efficient control of memory and logic devices.

Magnetically driven short-range order can explain anomalous measurements in CrCoNi.

Abstract: The presence, nature, and impact of chemical short-range order in the multi-principal element alloy CrCoNi are all topics of current interest and debate. First-principles calculations reveal that its origins are fundamentally magnetic, involving repulsion between like-spin Co–Cr and Cr–Cr pairs that is complemented by the formation of a magnetically aligned sublattice of second-nearest-neighbor Cr atoms. Ordering models following these principles are found to predict otherwise anomalous experimental measurements concerning both magnetization and atomic volumes across a range of compositions. In addition to demonstrating the impact of magnetic interactions and resulting chemical rearrangement, the possible explanation of experiments would imply that short-range order of this type is far more prevalent than previously realized.

Current-Induced Spin Torques on Single GdFeCo Magnetic Layers.

Abstract: Spintronics exploits spin-orbit coupling (SOC) to generate spin currents, spin torques and, in the absence of inversion symmetry, Rashba and Dzyaloshinskii-Moriya interactions (DMI). The widely used magnetic materials, based on 3d metals such as Fe and Co, possess a small SOC. To circumvent this shortcoming, the common practice has been to utilize the large SOC of nonmagnetic layers of 5d heavy metals (HMs), such as Pt, to generate spin currents by Spin Hall Effect (SHE) and, in turn, exert spin torques on the magnetic layers. Here, we introduce a new class of material architectures, excluding nonmagnetic 5d HMs, for high performance spintronics operations. We demonstrate very strong current-induced torques exerted on single GdFeCo layers due to the combination of large SOC of the Gd 5d states, and inversion symmetry breaking mainly engineered by interfaces. These “self-torques” are enhanced around the magnetization compensation temperature (close to room temperature) and can be tuned by adjusting the spin absorption outside the GdFeCo layer. In other measurements, we determine the very large emission of spin current from GdFeCo. This material platform opens new perspectives to exert “self-torques” on single magnetic layers as well as to generate spin currents from a magnetic layer.

Unravelling the nature of magneto-electric coupling in room temperature multiferroic particulate (PbFe0.5Nb0.5O3)-(Co0.6Zn0.4Fe1.7Mn0.3O4) composites.

Abstract: Multiferroic composites are promising candidates for magnetic field sensors, next-generation low power memory and spintronic devices, as they exhibit much higher magnetoelectric (ME) coupling and coupled ordering parameters compared to the single-phase multiferroics. Hence, the 3-0 type particulate multiferroic composites having general formula (1 - Φ)[PbFe0.5Nb0.5O3]-Φ[Co0.6Zn0.4Fe1.7Mn0.3O4] (Φ = 0.0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, (1 - Φ) PFN-ΦCZFMO) were prepared using a hybrid synthesis technique. Preliminary structural and microstructural analysis were carried out using XRD and FESEM techniques, which suggest the formation of 3-0 type particulate composite without the presence of any impurity phases. The multiferroic behaviour of the composites is studied with polarization versus electric field (P-E) and magnetization versus magnetic field (M-H) characteristics at room temperature. The nature of ME coupling was investigated elaborately by employing the Landau free energy equation along with the magneto-capacitance measurement. This investigation suggests the existence of biquadratic nature of ME coupling (P2M2). The magneto-electric coupling measurement also suggests that strain mediated domain coupling between the ferroelectric and magnetic ordering is responsible for the magneto-electric behaviour. The obtained value of direct ME coefficient 26.78 mV/cm-Oe for Φ = 0.3, found to be higher than the well-known single-phase materials and polycrystalline composites.