Showing papers on "Magnetic anisotropy published in 2013"

Structural and magnetic characterization of the intermartensitic phase transition in NiMnSn Heusler alloy ribbons

Abstract: Phase transitions and structural and magnetic properties of rapidly solidified Ni50Mn38Sn12 alloy ribbons have been studied. Ribbon samples crystallize as a single-phase, ten-layered modulated (10M) monoclinic martensite with a columnar-grain microstructure and a magnetic transition temperature of 308 K. By decreasing the temperature, martensite undergoes an intermartensitic phase transition around 195 K. Above room temperature, the high temperature martensite transforms into austenite. Below 100 K, magnetization hysteresis loops shift along the negative H-axis direction, confirming the occurrence of an exchange bias effect. On heating, the thermal dependence of the coercive field HC shows a continuous increase, reaching a maximum value of 1017 Oe around 50 K. Above this temperature, HC declines to zero around 195 K. But above this temperature, it increases again up to 20 Oe falling to zero close to 308 K. The coercivity values measured in both temperature intervals suggest a significant difference in the...

An electrostatic model for the determination of magnetic anisotropy in dysprosium complexes

Abstract: Understanding the anisotropic electronic structure of lanthanide complexes is important in areas as diverse as magnetic resonance imaging, luminescent cell labelling and quantum computing. Here we present an intuitive strategy based on a simple electrostatic method, capable of predicting the magnetic anisotropy of dysprosium(III) complexes, even in low symmetry. The strategy relies only on knowing the X-ray structure of the complex and the well-established observation that, in the absence of high symmetry, the ground state of dysprosium(III) is a doublet quantized along the anisotropy axis with an angular momentum quantum number mJ=±(15)/2. The magnetic anisotropy axis of 14 low-symmetry monometallic dysprosium(III) complexes computed via high-level ab initio calculations are very well reproduced by our electrostatic model. Furthermore, we show that the magnetic anisotropy is equally well predicted in a selection of low-symmetry polymetallic complexes.

Learning from Nature to Improve the Heat Generation of Iron-Oxide Nanoparticles for Magnetic Hyperthermia Applications

Abstract: The performance of magnetic nanoparticles is intimately entwined with their structure, mean size and magnetic anisotropy. Besides, ensembles offer a unique way of engineering the magnetic response by modifying the strength of the dipolar interactions between particles. Here we report on an experimental and theoretical analysis of magnetic hyperthermia, a rapidly developing technique in medical research and oncology. Experimentally, we demonstrate that single-domain cubic iron oxide particles resembling bacterial magnetosomes have superior magnetic heating efficiency compared to spherical particles of similar sizes. Monte Carlo simulations at the atomic level corroborate the larger anisotropy of the cubic particles in comparison with the spherical ones, thus evidencing the beneficial role of surface anisotropy in the improved heating power. Moreover we establish a quantitative link between the particle assembling, the interactions and the heating properties. This knowledge opens new perspectives for improved hyperthermia, an alternative to conventional cancer therapies.

Mononuclear single-molecule magnets: tailoring the magnetic anisotropy of first-row transition-metal complexes.

Abstract: Magnetic anisotropy is the property that confers to the spin a preferred direction that could be not aligned with an external magnetic field. Molecules that exhibit a high degree of magnetic anisotropy can behave as individual nanomagnets in the absence of a magnetic field, due to their predisposition to maintain their inherent spin direction. Until now, it has proved very hard to predict magnetic anisotropy, and as a consequence, most synthetic work has been based on serendipitous processes in the search for large magnetic anisotropy systems. The present work shows how the property can be predicted based on the coordination numbers and electronic structures of paramagnetic centers. Using these indicators, two CoII complexes known from literature have been magnetically characterized and confirm the predicted single-molecule magnet behavior.

Threshold current for switching of a perpendicular magnetic layer induced by spin Hall effect

Abstract: We theoretically investigate the switching of a perpendicular magnetic layer by in-plane charge current due to the spin Hall effect. We find that in the high damping regime, the threshold switching current is independent of the damping constant and is almost linearly proportional to both effective perpendicular magnetic anisotropy field and external in-plane field applied along the current direction. We obtain an analytic expression of the threshold current, in excellent agreement with numerical results. Based on the expression, we find that magnetization switching induced by the spin Hall effect can be practically useful when it is combined with voltage-controlled anisotropy change.

Magnetic anisotropy and spin-parity effect along the series of lanthanide complexes with DOTA.

Abstract: Spotting trends: Upon going from Tb(III) to Yb(III) centers in the complexes of the DOTA(4-) ligand, a reorientation of the easy axis of magnetization from perpendicular to parallel to the Ln-O bond of the apical water molecule is experimentally observed and theoretically predicted (SMM=single-molecule magnet). Only ions with an odd number of electrons show slow relaxation of the magnetization.

Strain-controlled magnetic domain wall propagation in hybrid piezoelectric/ferromagnetic structures

Abstract: The control of magnetic order in nanoscale devices underpins many proposals for integrating spintronics concepts into conventional electronics. A key challenge lies in finding an energy-efficient means of control, as power dissipation remains an important factor limiting future miniaturization of integrated circuits. One promising approach involves magnetoelectric coupling in magnetostrictive/piezoelectric systems, where induced strains can bear directly on the magnetic anisotropy. While such processes have been demonstrated in several multiferroic heterostructures, the incorporation of such complex materials into practical geometries has been lacking. Here we demonstrate the possibility of generating sizeable anisotropy changes, through induced strains driven by applied electric fields, in hybrid piezoelectric/spin-valve nanowires. By combining magneto-optical Kerr effect and magnetoresistance measurements, we show that domain wall propagation fields can be doubled under locally applied strains. These results highlight the prospect of constructing low-power domain wall gates for magnetic logic devices.

A theoretical analysis of chemical bonding, vibronic coupling, and magnetic anisotropy in linear iron(II) complexes with single-molecule magnet behavior

Abstract: The electronic structure and magnetic anisotropy of six complexes of high-spin FeII with linear FeX2 (X = C, N, O) cores, Fe[N(SiMe3)(Dipp)]2 (1), Fe[C(SiMe3)3]2 (2), Fe[N(H)Ar′]2 (3), Fe[N(H)Ar*]2 (4), Fe[O(Ar′)]2 (5), and Fe[N(t-Bu)2]2 (7) [Dipp = C6H3-2,6-Pri2; Ar′ = C6H3-2,6-(C6H3-2,6-Pri2)2; Ar* = C6H3-2,6-(C6H2-2,4,6-Pri2)2; Ar# = C6H3-2,6-(C6H2-2,4,6-Me3)2], and one bent (FeN2) complex, Fe[N(H)Ar#]2 (6), have been studied theoretically using complete active space self-consistent field (CASSCF) wavefunctions in conjunction with N-Electron Valence Perturbation Theory (NEVPT2) and quasidegenerate perturbation theory (QDPT) for the treatment of magnetic field and spin-dependent relativistic effects. Mossbauer studies on compound 2 indicate an internal magnetic field of unprecedented magnitude (151.7 T) at the FeII nucleus. This has been interpreted as arising from first order angular momentum of the 5Δ ground state of FeII center (J. Am. Chem. Soc. 2004, 126, 10206). Using geometries from X-ray structural data, ligand field parameters for the Fe-ligand bonds were extracted using a 1 : 1 mapping of the angular overlap model onto multireference wavefunctions. The results demonstrate that the metal–ligand bonding in these complexes is characterized by: (i) strong 3dz2–4s mixing (in all complexes), (ii) π-bonding anisotropy involving the strong π-donor amide ligands (in 1, 3–4, 6, and 7) and (iii) orbital mixings of the σ–π type for Fe–O bonds (misdirected valence in 5). The interplay of all three effects leads to an appreciable symmetry lowering and splitting of the 5Δ (3dxy, 3dx2−y2) ground state. The strengths of the effects increase in the order 1 < 5 < 7 ∼ 6. However, the differential bonding effects are largely overruled by first-order spin–orbit coupling, which leads to a nearly non-reduced orbital contribution of L = 1 to yield a net magnetic moment of about 6 μB. This unique spin–orbital driven magnetism is significantly modulated by geometric distortion effects: static distortions for the bent complex 6 and dynamic vibronic coupling effects of the Renner–Teller type of increasing strength for the series 1–5.Ab initio calculations based on geometries from X-ray data for 1 and 2 reproduce the magnetic data exceptionally well. Magnetic sublevels and wavefunctions were calculated employing a dynamic Renner–Teller vibronic coupling model with vibronic coupling parameters adjusted from the ab initio results on a small Fe(CH3)2 truncated model complex. The model reproduces the observed reduction of the orbital moments and quantitatively reproduces the magnetic susceptibility data of 3–5 after introduction of the vibronic coupling strength (f) as a single adjustable parameter. Its value varies in a narrow range (f = 0.142 ± 0.015) across the series. The results indicate that the systems are near the borderline of the transition from a static to a dynamic Renner–Teller effect. Renner–Teller vibronic activity is used to explain the large reduction of the spin-reversal barrier Ueff along the series from 1 to 5. Based upon the theoretical analysis, guidelines for generating new single-molecule magnets with enhanced magnetic anisotropies and longer relaxation times are formulated.

Single molecule magnetism in a family of mononuclear β-diketonate lanthanide(III) complexes: rationalization of magnetic anisotropy in complexes of low symmetry

Abstract: The use of an amino-pyridyl substituted β-diketone, N-(2-pyridyl)-ketoacetamide (paaH), has allowed for the isolation of two new families of isostructural mononuclear lanthanide complexes with general formulae: [Ln(paaH*)2(H2O)4][Cl]3·2H2O (Ln = Gd (1), Tb (2), Dy (3), Ho (4), Er (5) and Y (6)) and [Ln(paaH*)2(NO3)2(MeOH)][NO3] (Ln = Tb (7), Dy (8), Ho (9) and Er (10)). The dysprosium members of each family (3 and 8) show interesting slow magnetic relaxation features. Compound 3 displays Single Molecule Magnet (SMM) behaviour in zero DC field with an energy barrier to thermal relaxation of Ea = 177(4) K (123(2) cm−1) with τ0 = 2.5(8) × 10−7 s, while compound 8 shows slow relaxation of the magnetization under an optimum DC field of 0.2 T with an energy barrier to thermal relaxation of Ea = 64 K (44 cm−1) with τ0 = 6.2 × 10−7 s. Ab initio multiconfigurational calculations of the Complete Active Space type have been employed to elucidate the electronic and magnetic structure of the low-lying energy levels of compounds 2–5 and 8. The orientation of the anisotropic magnetic moments for compounds 2–5 are rationalized using a clear and succinct, chemically intuitive method based on the electrostatic repulsion of the aspherical electron density distributions of the lanthanides.

Domain wall tilting in the presence of the Dzyaloshinskii-Moriya interaction in out-of-plane magnetized magnetic nanotracks.

Abstract: We show that the Dzyaloshinskii-Moriya interaction (DMI) can lead to a tilting of the domain wall (DW) surface in perpendicularly magnetized magnetic nanotracks when DW dynamics is driven by an easy axis magnetic field or a spin polarized current. The DW tilting affects the DW dynamics for large DMI and the tilting relaxation time can be very large as it scales with the square of the track width. The results are well explained by an analytical model based on a Lagrangian approach where the DMI and the DW tilting are included. We propose a simple way to estimate the DMI in a magnetic multilayers by measuring the dependence of the DW tilt angle on a transverse static magnetic field. Our results shed light on the current induced DW tilting observed recently in Co/Ni multilayers with inversion asymmetry, and further support the presence of DMI in these systems.

Large zero-field cooled exchange-bias in bulk Mn2PtGa.

Abstract: We report a large exchange-bias effect after zero-field cooling the new tetragonal Heusler compound ${\mathrm{Mn}}_{2}\mathrm{PtGa}$ from the paramagnetic state. The first-principles calculation and the magnetic measurements reveal that ${\mathrm{Mn}}_{2}\mathrm{PtGa}$ orders ferrimagnetically with some ferromagnetic inclusions. We show that ferrimagnetic ordering is essential to isothermally induce the exchange anisotropy needed for the zero-field cooled exchange bias during the virgin magnetization process. The complex magnetic behavior at low temperatures is characterized by the coexistence of a field-induced irreversible magnetic behavior and a spin-glass-like phase. The field-induced irreversibility originates from an unusual first-order ferrimagnetic to antiferromagnetic transition, whereas the spin-glass-like state forms due to the existence of antisite disorder intrinsic to the material.

A method to control magnetism in individual strain-mediated magnetoelectric islands

Abstract: Patterned electrodes on a piezoelectric substrate are demonstrated to produce a localized strain of sufficient magnitude to control the magnetic anisotropy of a Ni island. Strain-induced magnetic anisotropy was measured using the magneto-optical Kerr effect, and the measured shifts in magnetic anisotropy were consistent with strain predicted using linear finite element analysis. This approach overcomes the effect of the substrate clamping the in-plane strain and should be scalable to thin films. This approach represents a key step toward realizing the next generation of strain mediated magneto-electric magnetic random access memory devices with low writing energy and high writing speed.

Atomic-scale magnetism of cobalt-intercalated graphene

Abstract: Using spin-polarized scanning tunneling microscopy and density functional theory, we have studied the structural and magnetic properties of cobalt-intercalated graphene on Ir(111). The cobalt forms monolayer islands being pseudomorphic with the Ir(111) beneath the graphene. The strong bonding between graphene and cobalt leads to a high corrugation within the Moir\'e pattern which arises due to the lattice mismatch between the graphene and the Co on Ir(111). The intercalation regions exhibit an out-of-plane easy axis with an extremely high switching field, which surpasses the significant values reported for uncovered cobalt islands on Ir(111). Within the Moir\'e unit cell of the intercalation regions, we observe a site-dependent variation of the local effective spin polarization. State-of-the-art first-principles calculations show that the origin of this variation is a site-dependent magnetization of the graphene: At top sites the graphene is coupled ferromagnetically to the cobalt underneath, while it is antiferromagnetically coupled at fcc and hcp sites.

Spin switching and magnetization reversal in single-crystal NdFeO 3

Abstract: We report an experimental and computational study of single-crystal NdFeO${}_{3}$, which features two inequivalent magnetic sublattices, namely, Fe and Nd sublattices that are coupled in an antiparallel fashion. This paper reveals that a strong interaction between 3$d$ and 4$f$ electrons of the two sublattices along with a spin-lattice coupling drives an extremely interesting magnetic state that is highly sensitive to the orientation and history of weak magnetic field. The following phenomena are particularly remarkable: (1) sharply contrasting magnetization $M$($T$) along the $a$ and $c$ axes; (2) a first-order spin switching along the $a$ axis below 29 K when the system is zero-field-cooled; and (3) a progressive magnetization reversal when the system is field-cooled. The intriguing magnetic behavior is captured in our first-principles density functional theory calculations.

Exchange couplings in magnetic films

Abstract: Recent advances in the study of exchange couplings in magnetic films are introduced. To provide a comprehensive understanding of exchange coupling, we have designed different bilayers, trilayers and multilayers, such as anisotropic hard-/soft-magnetic multilayer films, ferromagnetic/antiferromagnetic/ferromagnetic trilayers, [Pt/Co]/NiFe/NiO heterostructures, Co/NiO and Co/NiO/Fe trilayers on an anodic aluminum oxide (AAO) template. The exchange-coupling interaction between soft- and hard-magnetic phases, interlayer and interfacial exchange couplings and magnetic and magnetotransport properties in these magnetic films have been investigated in detail by adjusting the magnetic anisotropy of ferromagnetic layers and by changing the thickness of the spacer layer, ferromagnetic layer, and antiferromagnetic layer. Some particular physical phenomena have been observed and explained.

Magnetite–Maghemite Nanoparticles in the 5–15 nm Range: Correlating the Core–Shell Composition and the Surface Structure to the Magnetic Properties. A Total Scattering Study.

Abstract: Very small superparamagnetic iron oxide nanoparticles were characterized by innovative synchrotron X-ray total scattering methods and Debye function analysis. Using the information from both Bragg and diffuse scattering, size-dependent core–shell magnetite–maghemite compositions and full size (number- and mass-based) distributions were derived within a coherent approach. The magnetite core radii in 10 nm sized NPs well match the magnetic domain sizes and show a clear correlation to the saturation magnetization values, while the oxidized shells seem to be magnetically silent. Very broad superstructure peaks likely produced by the polycrystalline nature of the surface layers were experimentally detected in room temperature oxidized samples. Effective magnetic anisotropy constants, derived by taking the knowledge of the full size-distributions into account, show an inverse dependence on the NPs size, witnessing a major surface contribution. Finally, an additional amorphous component was uncovered within the ...

Adatoms and clusters of 3d transition metals on graphene: electronic and magnetic configurations.

Abstract: We investigate the electronic and magnetic properties of single Fe, Co, and Ni atoms and clusters on monolayer graphene (MLG) on SiC(0001) by means of scanning tunneling microscopy (STM), x-ray absorption spectroscopy, x-ray magnetic circular dichroism (XMCD), and ab initio calculations. STM reveals different adsorption sites for Ni and Co adatoms. XMCD proves Fe and Co adatoms to be paramagnetic and to exhibit an out-of-plane easy axis in agreement with theory. In contrast, we experimentally find a nonmagnetic ground state for Ni monomers while an increasing cluster size leads to sizeable magnetic moments. These observations are well reproduced by our calculations and reveal the importance of hybridization effects and intra-atomic charge transfer for the properties of adatoms and clusters on MLG.

Origin of the magnetic anisotropy in heptacoordinate Ni(II) and Co(II) complexes.

Abstract: The nature and magnitude of the magnetic anisotropy of heptacoordinate mononuclear Ni(II) and Co(II) complexes were investigated by a combination of experiment and ab initio calculations. The zero-field splitting (ZFS) parameters D of [Ni(H(2)DAPBH)(H(2)O)(2)](NO(3))(2)⋅2 H(2)O (1) and [Co(H(2)DAPBH)(H(2)O)(NO(3))](NO(3)) [2; H(2)DAPBH = 2,6-diacetylpyridine bis- (benzoyl hydrazone)] were determined by means of magnetization measurements and high-field high-frequency EPR spectroscopy. The negative D value, and hence an easy axis of magnetization, found for the Ni(II) complex indicates stabilization of the highest M(S) value of the S = 1 ground spin state, while a large and positive D value, and hence an easy plane of magnetization, found for Co(II) indicates stabilization of the M(S) = ±1/2 sublevels of the S = 3/2 spin state. Ab initio calculations were performed to rationalize the magnitude and the sign of D, by elucidating the chemical parameters that govern the magnitude of the anisotropy in these complexes. The negative D value for the Ni(II) complex is due largely to a first excited triplet state that is close in energy to the ground state. This relatively small energy gap between the ground and the first excited state is the result of a small energy difference between the d(xy) and d(x(2)-y(2)) orbitals owing to the pseudo-pentagonal-bipyramidal symmetry of the complex. For Co(II), all of the excited states contribute to a positive D value, which accounts for the large magnitude of the anisotropy for this complex.

Damping of CoxFe80−xB20 ultrathin films with perpendicular magnetic anisotropy

Abstract: We use vector network analyzer ferromagnetic resonance to study the perpendicularly magnetized CoFeB films. We report the dependence of the anisotropy, the g-factor, and the damping upon the Fe-Co compositional ratio in the amorphous and crystalline states. The damping and the anisotropy increase upon crystallization but vary little with composition on the Fe-rich side. At high cobalt content, the anisotropy lowers while the damping and the sample inhomogeneity increase. The compositional dependences seem to extrapolate from the properties of bulk CoFe alloys, with differences that can be understood from the correlated impacts of spin-orbit interaction on anisotropy, g-factor, and damping.

Quadratic Scaling of Intrinsic Gilbert Damping with Spin-Orbital Coupling in L 1 0 FePdPt Films: Experiments and Ab Initio Calculations

Abstract: The dependence of the intrinsic Gilbert damping parameter α(0) on the spin-orbital coupling strength ξ is investigated in L1(0) ordered FePd(1-x) Pt(x) films by time-resolved magneto-optical Kerr effect measurements and spin-dependent ab initio calculations. Continuous tuning of α(0) over more than one order of magnitude is realized by changing the Pt/Pd concentration ratio showing that α(0) is proportional to ξ(2) as changes of other leading parameters are found to be negligible. The perpendicular magnetic anisotropy is shown to have a similar variation trend with x. The present results may facilitate the design and fabrication of new magnetic alloys with large perpendicular magnetic anisotropy and tailored damping properties.

Giant Ising-type magnetic anisotropy in trigonal bipyramidal Ni(II) complexes: experiment and theory.

Abstract: This paper reports the experimental and theoretical investigations of two trigonal bipyramidal Ni(II) complexes, [Ni(Me6tren)Cl](ClO4) (1) and [Ni(Me6tren)Br](Br) (2). High-field, high-frequency electron paramagnetic resonance spectroscopy performed on a single crystal of 1 shows a giant uniaxial magnetic anisotropy with an experimental Dexpt value (energy difference between the Ms = ± 1 and Ms = 0 components of the ground spin state S = 1) estimated to be between −120 and −180 cm–1. The theoretical study shows that, for an ideally trigonal Ni(II) complex, the orbital degeneracy leads to a first-order spin–orbit coupling that results in a splitting of the Ms = ± 1 and Ms = 0 components of approximately −600 cm–1. Despite the Jahn–Teller distortion that removes the ground term degeneracy and reduces the effects of the first-order spin–orbit interaction, the D value remains very large. A good agreement between theoretical and experimental results (theoretical Dtheor between −100 and −200 cm–1) is obtained.

Magnetic Poles Determinations and Robustness of Memory Effect upon Solubilization in a DyIII-Based Single Ion Magnet

Abstract: The [Dy(tta)3(L)] complex behaves as a single ion magnet both in its crystalline phase and in solution. Experimental and theoretical magnetic anisotropy axes perfectly match and lie along the most electro-negative atoms of the coordination sphere. Both VSM and MCD measurements highlight the robustness of the complex, with persistence of the memory effect even in solution up to 4 K.

Yafet–Kittel-type magnetic order in Zn-substituted cobalt ferrite nanoparticles with uniaxial anisotropy

Abstract: Zn-substituted cobalt ferrite (Zn x Co1−x Fe2O4 with 0.0 ≤ x ≤ 1.0) nanoparticles coated with triethylene glycol (TREG) were prepared by the hydrothermal technique. The effect of Zn substitution on temperature-dependent magnetic properties of the TREG-coated Zn x Co1−x Fe2O4 nanoparticles has been investigated in the temperature range of 10–400 K and in magnetic fields up to 9 T. The structural, morphological, and magnetic properties of TREG-coated Zn x Co1−x Fe2O4 NPs were examined using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, transmission electron microscopy (TEM), and vibrating sample magnetometry (VSM). The average crystallite size estimated from X-ray line profile fitting was found to be in the range of 7.0–10 nm. The lattice constant determined using the Nelson–Riley extrapolation method continuously increases with the increase in Zn2+ content, obeying Vegard’s law. TEM analysis revealed that the synthesized particles were nearly monodisperse, roughly spherical shaped nanoparticles in the size range of 9.0–15 nm. FT-IR spectra confirm that TREG is successfully coated on the surface of nanoparticles (NPs). The substitution of non-magnetic Zn2+ ions for magnetic Co2+ ions substantially changes the magnetic properties of the TREG-coated Zn x Co1−x Fe2O4 NPs. The saturation magnetization and the experimental magnetic moment are observed to initially increase (up to x = 0.2), which is explained by Neel’s collinear two-sublattice model, and then continuously decrease with further increase in Zn content x. This decrease obeys the three-sublattice model suggested by Yafet–Kittel (Y–K). While the Y–K angle is zero for the CoFe2O4 NPs coated with TREG, it increases gradually with increasing Zn concentrations and extrapolates to 82.36° for ZnFe2O4 NPs coated with TREG. The increase in spin canting angles (Y–K angles) suggests the existence of triangular (or canted) spin arrangements in all the samples (except for the samples with x = 0.0) under consideration in this work. From the computation of Y–K angles for the TREG-coated Zn x Co1−x Fe2O4 NPs, it can be concluded that all the zinc-doped cobalt ferrite nanoparticles (for x > 0.0) have a Y–K-type magnetic order, while the pure cobalt ferrite nanoparticles (x = 0.0) have a Neel-type magnetic order. Zero field cooled (ZFC) and field cooled (FC) measurement results further verify that the samples with 0.6 ≤ x ≤ 1.0 have superparamagnetic behavior at room temperature, which shows weak interaction between magnetic particles. The blocking temperatures obtained from ZFC–FC curves decrease as a function of Zn concentration. It was found that the effective magnetic anisotropy, the coercivity, and remanence magnetization continuously decrease with increasing Zn concentration. Lower reduced remanent magnetization (M r/M s) values (<0.5) suggest that all the samples have uniaxial anisotropy. Ferromagnetic resonance (FMR) measurement shows that the FMR spectra of all the samples have broad linewidth because of the magnetic nanoparticles with randomly distributed anisotropy axes, and the decrease in the internal field conversely leads to the increase in the resonance field with respect to increasing Zn concentration.

Rashba Spin-Orbit Anisotropy and the Electric Field Control of Magnetism

Abstract: The control of the magnetism of ultra-thin ferromagnetic layers using an electric field rather than a current, if large enough, would lead to many technologically important applications. To date, while it is usually assumed the changes in the magnetic anisotropy, leading to such a control, arises from surface charge doping of the magnetic layer, a number of key experiments cannot be understood within such a scenario. Much studied is the fact that, for non-magnetic metals or semi-conductors, a large surface electric field gives rise to a Rashba spin-orbit coupling which leads to a spin-splitting of the conduction electrons. For a magnet, this splitting is modified by the exchange field resulting in a large magnetic anisotropy energy via the Dzyaloshinskii-Moriya mechanism. This different, yet traditional, path to an electrically induced anisotropy energy can explain the electric field, thickness, and material dependence reported in many experiments.

Stabilizing the magnetic moment of single holmium atoms by symmetry

Abstract: Single magnetic atoms on non-magnetic surfaces have magnetic moments that are usually destabilized within a microsecond, too speedily to be useful, but here the magnetic moments of single holmium atoms on a highly conductive metallic substrate can reach lifetimes of the order of minutes. The magnetic moments of individual magnetic atoms are attractive components for both memory and quantum computing applications. But interactions between such atoms and the substrates on which they are mounted tend to destabilize the magnetic moments, giving them lifetimes of typically less than a few milliseconds. Toshio Miyamachi and colleagues have now identified a system consisting of single atoms of the lanthanide series rare earth element holmium on a highly conductive surface, in which intrinsic symmetries related to the properties of both the atom and the substrate combine to minimize these destabilizing interactions. As a result, the magnetic moments of the atoms can achieve lifetimes of several minutes. Single magnetic atoms, and assemblies of such atoms, on non-magnetic surfaces have recently attracted attention owing to their potential use in high-density magnetic data storage and as a platform for quantum computing1,2,3,4,5,6,7,8. A fundamental problem resulting from their quantum mechanical nature is that the localized magnetic moments of these atoms are easily destabilized by interactions with electrons, nuclear spins and lattice vibrations of the substrate3,4,5. Even when large magnetic fields are applied to stabilize the magnetic moment, the observed lifetimes remain rather short5,6 (less than a microsecond). Several routes for stabilizing the magnetic moment against fluctuations have been suggested, such as using thin insulating layers between the magnetic atom and the substrate to suppress the interactions with the substrate’s conduction electrons2,3,5, or coupling several magnetic moments together to reduce their quantum mechanical fluctuations7,8. Here we show that the magnetic moments of single holmium atoms on a highly conductive metallic substrate can reach lifetimes of the order of minutes. The necessary decoupling from the thermal bath of electrons, nuclear spins and lattice vibrations is achieved by a remarkable combination of several symmetries intrinsic to the system: time reversal symmetry, the internal symmetries of the total angular momentum and the point symmetry of the local environment of the magnetic atom.

Control of the magnetism and magnetic anisotropy of a single-molecule magnet with an electric field.

Abstract: Through systematic density functional calculations, the mechanism of the substrate induced spin reorientation transition in FePc/O-Cu(110) is explained in terms of charge transfer and rearrangement of Fe-3d orbitals. Moreover, we find giant magnetoelectric effects in this system, manifested by the sensitive dependence of its magnetic moment and magnetic anisotropy energy on an external electric field. In particular, the direction of magnetization of FePc/O-Cu(110) is switchable between in-plane and perpendicular axes, simply by applying an external electric field of 0.5 eV/A along the surface normal.

Slow magnetic relaxation in Co(III)-Co(II) mixed-valence dinuclear complexes with a Co(II)O5X (X = Cl, Br, NO3) distorted-octahedral coordination sphere.

Abstract: The reaction of the multisite coordination ligand (LH4) with CoX2·nH2O in the presence of tetrabutylammonium hydroxide affords a series of homometallic dinuclear mixed-valence complexes, [Co(III)Co(II)(LH2)2(X)(H2O)](H2O)m (1, X = Cl and m = 4; 2, X = Br and m = 4; 3, X = NO3 and m = 3). All of the complexes have been structurally characterized by X-ray crystallography. Both cobalt ions in these dinuclear complexes are present in a distorted-octahedral geometry. Detailed magnetic studies on 1-3 have been carried out. M vs H data at different temperatures can be fitted with S = 3/2, the best fit leading to D(3/2) = -7.4 cm(-1), |E/D| < 1 × 10(-3), and g = 2.32 for 1 and D(3/2) = -9.7 cm(-1), |E/D| <1 × 10(-4), and g = 2.52 for 2. In contrast to 1 and 2, M vs H data at different temperatures suggest that compound 3 has comparatively little magnetic anisotropy. In accordance with the large negative D values observed for compounds 1 and 2, they are single-molecule magnets (SMMs) and exhibit slow relaxation of magnetization at low temperatures under an applied magnetic field of 1000 Oe with the following energy barriers: 7.9 cm(-1) (τo = 6.1 × 10(-6) s) for 1 and 14.5 cm(-1) (τo = 1.0 × 10(-6) s) for 2. Complex 3 does not show any SMM behavior, as expected from its small magnetic anisotropy. The τo values observed for 1 and 2 are much larger than expected for a SMM, strongly suggesting that the quantum pathway of relaxation at very low temperatures is not fully suppressed by the effects of the applied field.

Anatomy of perpendicular magnetic anisotropy in Fe/MgO magnetic tunnel junctions: First-principles insight

Abstract: Using first-principles calculations, we elucidate microscopic mechanisms of perpendicular magnetic anisotropy (PMA) in Fe/MgO magnetic tunnel junctions through evaluation of orbital and layer resolved contributions into the total anisotropy value. It is demonstrated that the origin of the large PMA values is far beyond simply considering the hybridization between Fe-$3d$ and O-$2p$ orbitals at the interface between the metal and the insulator. Onsite projected analysis shows that the anisotropy energy is not localized at the interface but it rather propagates into the bulk showing an attenuating oscillatory behavior which depends on orbital character of contributing states and interfacial conditions. Furthermore, it is found in most situations that states with ${d}_{yz(xz)}$ and ${d}_{{z}^{2}}$ character tend always to maintain the PMA while those with ${d}_{xy}$ and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ character tend to favor the in-plane anisotropy. It is also found that while MgO thickness has no influence on PMA, the calculated perpendicular magnetic anisotropy oscillates as a function of Fe thickness with a period of 2 ML and reaches a maximum value of 3.6 mJ/m${}^{2}$.

Enhanced interface perpendicular magnetic anisotropy in Ta|CoFeB|MgO using nitrogen doped Ta underlayers

Abstract: We show that the magnetic characteristics of Ta|CoFeB|MgO magnetic heterostructures are strongly influenced by doping the Ta underlayer with nitrogen. In particular, the saturation magnetization drops upon doping the Ta underlayer, suggesting that the doped underlayer acts as a boron diffusion barrier. In addition, the thickness of the magnetic dead layer decreases with increasing nitrogen doping. Surprisingly, the interface magnetic anisotropy increases to ∼1.8 erg/cm2 when an optimum amount of nitrogen is introduced into the Ta underlayer. These results show that nitrogen doped Ta serves as a good underlayer for spintronic applications including magnetic tunnel junctions and domain wall devices.