Large Magnetic Anisotropy Research Articles

Characteristics of spin-thermoelectric generators enhanced by incorporating liquid phase epitaxial Bi-substituted YIG films as a ferrimagnetic insulator

Liquid phase epitaxial (LPE) Bi-substituted YIG films exhibit a large growth-induced magnetic anisotropy (GIMA) perpendicular to the film surface. The GIMA depends on the melt composition and supercooling in the LPE growth. Our film shows a large Ku value of 0.1 × 104 J/m3 even at a Bi content, x, of 0. We assume that GIMA enhances the spin-orbital coupling (SOC) in the local d-electron spins of Fe3+, and that the SOC-enhanced thermal energy transfer from the phonon system to the spin system increases spin currents in spin-thermoelectric (STE) generation. We discuss the characteristics of STE generators on the basis of data obtained from ferromagnetic resonance measurements.

Trigonally Distorted Hexacoordinate Co(II) Single-Ion Magnets.

By simple reactions involving various cobalt(II) carboxylates (acetate and in situ prepared pivalate and 4-hydroxybenzoate salts) and neocuproine (neo), we were able to prepare three different carboxylate complexes with the general formula [Co(neo)(RCOO)2] (R = –CH3 for 1, (CH3)3C– for 2, and 4OH-C4H6– for 3). The [Co(neo)(RCOO)2] molecules in the crystal structures of 1–3 adopt a rather distorted coordination environment, with the largest trigonal distortion observed for 1, whereas 2 and 3 are similarly distorted from ideal octahedral geometry. The combined theoretical and experimental investigations of magnetic properties revealed that the spin Hamiltonian formalism was not a valid approach and the L-S Hamiltonian had to be used to reveal very large magnetic anisotropies for 1–3. The measurements of AC susceptibility showed that all three compounds exhibited slow-relaxation of magnetization in a weak external static magnetic field, and thus can be classified as field-induced single-ion magnets. It is noteworthy that 1 also exhibits a weak AC signal in a zero-external magnetic field.

Tailoring Interfacial Exchange Anisotropy in Hard-Soft Core-Shell Ferrite Nanoparticles for Magnetic Hyperthermia Applications.

Magnetically hard–soft core-shell ferrite nanoparticles are synthesized using an organometallic decomposition method through seed-mediated growth. Two sets of core-shell nanoparticles (S1 and S2) with different shell (Fe3O4) thicknesses and similar core (CoFe2O4) sizes are obtained by varying the initial quantities of seed nanoparticles of size 6.0 ± 1.0 nm. The nanoparticles synthesized have average sizes of 9.5 ± 1.1 (S1) and 12.2 ± 1.7 (S2) nm with corresponding shell thicknesses of 3.5 and 6.1 nm. Magnetic properties are investigated under field-cooled and zero-field-cooled conditions at several temperatures and field cooling values. Magnetic heating efficiency for magnetic hyperthermia applications is investigated by measuring the specific absorption rate (SAR) in alternating magnetic fields at several field strengths and frequencies. The exchange bias is found to have a nonmonotonic and oscillatory relationship with temperature at all fields. SAR values of both core-shell samples are found to be considerably larger than that of the single-phase bare core particles. The effective anisotropy and SAR values are found to be larger in S2 than those in S1. However, the saturation magnetization displays the opposite behavior. These results are attributed to the occurrence of spin-glass regions at the core-shell interface of different amounts in the two samples. The novel outcome is that the interfacial exchange anisotropy of core-shell nanoparticles can be tailored to produce large effective magnetic anisotropy and thus large SAR values.

Large perpendicular magnetic anisotropy of transition metal dimers driven by polarization switching of a two-dimensional ferroelectric In2Se3 substrate.

Large perpendicular magnetic anisotropy (MA) is highly desirable for realizing atomic-scale magnetic data storage which represents the ultimate limit of the density of magnetic recording. In this work, we study the MA of transition metal dimers Co-Os, Co-Co and Os-Os adsorbed on two-dimensional ferroelectric In2Se3 (In2Se3-CoOs, In2Se3-OsCo, In2Se3-CoCo and In2Se3-OsOs) using first-principles calculations. We find that the Co-Os dimer in In2Se3-CoOs has a total magnetic anisotropy energy (MAE) of ∼40 meV. The MAE arising from the Os atom in In2Se3-CoOs is up to ∼60 meV. Such large MAE is attributed to the high spin-orbit coupling constant and the onefold coordination of the Os atom. In addition, perpendicular MA can be enhanced in In2Se3-CoOs and induced in In2Se3-OsCo, In2Se3-CoCo and In2Se3-OsOs by the ferroelectric polarization reversal of In2Se3. We demonstrate that the enlargement of exchange splitting of dxy/dx2-y2 and dxz/dyz orbitals for Os atoms in In2Se3-OsOs, Co atom in In2Se3-CoOs and Os and Co atoms in In2Se3-OsCo is responsible for the increase of MAE; while, for the upper Co atom in In2Se3-CoCo and the Os atom in In2Se3-CoOs, the energy rise of the dz2 orbital owing to the change of the crystal field effect by the reversal of ferroelectric polarization results in the increase of MAE.

Robust Dirac spin gapless semiconductors in a two-dimensional oxalate based organic honeycomb-kagome lattice.

Two-dimensional (2D) ferromagnetic materials with intrinsic and robust spin-polarized Dirac cones are of great interest in exploring exciting physics and in realizing spintronic devices. Using comprehensive ab initio calculations, herein we reveal a family of 2D oxalate-based metal-organic frameworks (MOFs) that possess the desired characteristics. We propose that these 2D oxalate-based MOFs may be assembled by oxalate ions (C2O42-) and two homo-transition metal atoms. We demonstrate that 2D MOFs of Ni2(C2O4)3 and Re2(C2O4)3 are intrinsic Dirac spin gapless semiconductors with linear band dispersion, low energy dissipation and high electron carrier velocity. As robust ferromagnets, they also possess large magnetic moments, large perpendicular magnetic anisotropy, and high Curie temperatures, e.g. 208 K for Ni2(C2O4)3. In particular, spin-orbit coupling triggers a topologically nontrivial band gap of 143 meV in Re2(C2O4)3, which is promising to realize the quantum anomalous Hall effect at high temperatures.

Machine learning assisted hierarchical filtering: a strategy for designing magnets with large moment and anisotropy energy

Machine learning models are developed to hierarchically screen materials so that the ones with large magnetization and magnetic anisotropy energy are selected.

Modulation of the magnetic dynamics of pentagonal-bipyramidal Co(II) complexes by fine-tuning the coordination microenvironment.

Four air-stable mononuclear Co(II) complexes, with the formulas [Co(dapbh)(H2O)(CH3OH)] (1), [Co(Hdapbh)(N3)(CH3OH)]·(CH3OH) (2), [Co(H2aapbh)(CH3OH)2]·(NO3)2 (3) and [Co(H2bapbh)(H2O)(NO3)]·(NO3) (4), have been synthesized and structurally characterized by single crystal X-ray diffraction. In all of the complexes, the Co(II) centers constrained by the rigid pentadentate ligand H2LR with two protonable hydrogens adopt a heptacoordinated pentagonal-bipyramidal geometry. The combined analyses of magnetic data and ab initio calculations unveil large easy-plane magnetic anisotropies for these complexes (D = +37.338, +37.273, +41.138 and +41.139 cm-1 for 1-4, respectively), which indicate that the chemical alterations of the equatorial ligand and the ligand field strength in the axial positions synergistically fine-tune the magnitude of the D values. Magnetic investigations demonstrate the field-induced single-ion magnetic behavior in complexes 1, 2 and 4 with diverse energy barriers (Ueff) of 12.25 for 1, 44.15 for 2 and 48.72 K for 4, corresponding to the geometrical distortion of the heptacoordinated Co(II) ion. That is, the greatest deviation from the ideal D5h symmetry in 4 is responsible for the highest barrier.

Large easy-axis magnetic anisotropy in a series of trigonal prismatic mononuclear cobalt(ii) complexes with zero-field hidden single-molecule magnet behaviour: the important role of the distortion of the coordination sphere and intermolecular interactions in the slow relaxation

A N6-tripodal ligand allows the preparation of a family of prismatic mononuclear cationic CoIIcomplexes with SMM behavior. Their axial anisotropy, magnetic relaxation and hysteresis width are modulated by the anion effects on the crystal structure.

A dysprosium single molecule magnet outperforming current pseudocontact shift agents.

A common criterion for designing performant single molecule magnets and pseudocontact shift tags is a large magnetic anisotropy. In this article we present a dysprosium complex chemically designed to exhibit strong easy-axis type magnetic anisotropy that is preserved in dichloromethane solution at room temperature. Our detailed theoretical and experimental studies on the magnetic properties allowed explaining several features typical of highly performant SMMs. Moreover, the NMR characterization shows remarkably large chemical shifts, outperforming the current state-of-the art PCS tags.

Spontaneous magnetic merons in a half-metallic Mn2I3Br3 monolayer with easy-plane anisotropy.

In recent years, great effort has been made in the study of two-dimensional (2D) van der Waals ferromagnets that can stabilize peculiar chiral spin textures, such as magnetic skyrmions and merons. Here, by first-principles calculations and micromagnetic simulations, we systematically investigate the in-plane magnetic anisotropy, Dzyaloshinskii-Moriya interaction (DMI) and magnetic merons in a Mn2I3Br3 monolayer. Mn2I3Br3 exhibits half-metallic behavior with a large band gap (∼2.7 eV) for spin-down electrons, but is gapless for spin-up ones. In addition, unlike most 2D ferromagnets with an off-plane magnetic easy axis and negligible DMI, the magnetic easy axis of Mn2I3Br3 is in-plane, with a large magnetic anisotropy energy of -13.2 meV and a strong DMI of 4.6 meV, which are mainly induced by the strong spin-orbital coupling of I atoms, microscopically. In particular, spontaneous magnetic merons, stabilized by the DMI, can exist in a wide magnetic field range (0-6 T). Our work not only provides important guidelines for the investigation of the DMI and merons in half-metallic materials, but also demonstrates the Mn2I3Br3 monolayer as an ideal platform to explore the deep physics of magnetic merons and as a promising candidate for magnetic storage devices, as well as spin filters.

Manipulating the electronic structure and physical properties in monolayer Mo2I3Br3via strain and doping

The large magnetic anisotropy energy (MAE) and strain-tunable optical absorption coefficient of the monolayer Janus Mo2I3Br3 open up more opportunities for few-layer van der Waals crystals in spintronics and optoelectronic devices.

Morphology induced large magnetic anisotropy in obliquely grown nanostructured thin film on nanopatterned substrate

The artificial tailoring of magnetic anisotropy by manipulating surface and interface morphology is attracting widespread interest for its application in spintronic and magnetic memory devices. Here oblique angle deposition on a nanopatterned rippled substrate is presented as a novel route of inducing large in-plane uniaxial magnetic anisotropy (UMA) in magnetic thin films. For this purpose, Cobalt films and rippled SiO2 substrates have been taken as a model system for the present study. Here, nanopatterned substrates are prepared by low energy ion beam erosion (IBE), above which films are deposited obliquely along and normal to the ripple directions. A clear anisotropy in the growth behavior has been observed due to the inhomogeneous in-plane organization of adatoms in the form of columns. The increased shadowing effect in the films deposited obliquely normal to the direction of the ripple patterns causes preferential coalescence of the columns along the substrate ripples, resulting in stronger in-plane UMA in the film. This peculiarity in magnetic behavior is addressed by considering the morphological anisotropy governed by enhanced shadowing effect, the shape anisotropy and the dipolar interactions among the magnetostatically coupled ripple structure.

Induction of large magnetic anisotropy energy and formation of multiple Dirac states in SrIrO3 films: Role of correlation and spin–orbit coupling

The 5d transition metal oxides, in particular iridates, host novel electronic and magnetic phases due to the interplay between onsite Coulomb repulsion (U) and spin–orbit coupling (SOC). The reduced dimensionality brings another degree of freedom to increase the functionality of these systems. By taking the example of ultrathin films of SrIrO3, theoretically we demonstrate that confinement led localization can introduce large magnetic anisotropy energy (MAE) in the range of 2–7 meV/Ir, which is one to two order higher than that of the traditional MAE compounds formed out of transition metals and their multilayers. Furthermore, in the weak correlation limit, tailored terminations can yield multiple Dirac states across a large energy window of 2 eV around the Fermi energy, which is rare phenomena in correlated oxides and upon experimental realization it will give rise to unique transport properties with excitation and doping.

Tuning the magnetic anisotropy of transition-metal atoms on two-dimensional In2Se3 substrate via ferroelectric polarization switching

Exploring large and tunable magnetic anisotropy (MA) in a single magnetic atom is of merit for information storage due to the highest information data density. In this work, we studied the MA of transition-metal atoms X (X = Co, Fe, Ir, Mo, Nb, Ta, Ti, V, W) adsorbed on the surface of two-dimensional (2D) ferroelectric (FE) In2Se3 by first-principles calculations. We find that when the ferroelectric polarization of the In2Se3 is switched, the strengths of the MA for Co, Ir, Nb and Ti single atoms can be changed, while the directions of the MA for Fe, Mo, Ta, V and W single atoms are reversed. Calculated electronic structure and orbital-resolved magnetic anisotropy energy suggest that the variations in the occupation of spin-minority state and the exchange splitting of d orbitals at Fermi level by switching FE polarization are responsible for the change of the strength and direction of the MA on X atoms.

Ultrafast spin wave propagation in thick magnetic insulator films with perpendicular magnetic anisotropy

Magnetostatic forward volume spin-waves (MFVSWs) are magnetic moments propagated perpendicular to the film plane and offer reciprocity configurations, which is suitable for spin-wave logic devices with low power consumption. However, owing to the strong shape anisotropy field, magnetic films are easily magnetized in the film plane. Fabrication of thick magnetic films with low Gilbert damping and large perpendicular magnetization anisotropy was an unsolved problem. The promising materials with low damping and easy to modify properties are ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$-based single-crystal films. High quality $({\mathrm{Y}}_{1.26}{\mathrm{Bi}}_{0.18}{\mathrm{Lu}}_{0.96}{\mathrm{Ca}}_{0.6})({\mathrm{Fe}}_{4.4}{\mathrm{Ge}}_{0.6}){\mathrm{O}}_{12}$ micrometer-thick films with strong anisotropy field up to 1862 Oe, narrow out-of-plane ferromagnetic resonance linewidth as low as 2 Oe, and low Gilbert damping approximately $(7.06\ifmmode\pm\else\textpm\fi{}0.15)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ grow on the gadolinium gallium garnet substrate successfully. The garnet film shows a MFVSW group velocity of 2.25 km/s, relaxation time 50 ns, and decay length up to $115\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$, paving the road towards ultralow power dissipation magnonic devices (microwave devices) based on magnetic insulators.

Large magnetic anisotropy in Co–Fe–Ni–N ordered structures: a first-principles study

Material design of promising rare-earth free permanent magnet requires tailoring and controlling the intrinsic magnetic properties namely large saturation magnetization μ 0 M s, giant uniaxial magnetic anisotropy K u, and high Curie temperature T C. Based on first-principles electronic structure calculations, we present a detailed analysis for the intrinsic magnetic properties of Co x Fe1−x Ni and Co x Fe1−x NiN0.25 ordered structures. We predict an enhanced structural stability with improved K u ranging from 1.53–2.29 MJ m−3 for Co x Fe1−x NiN0.25 ordered structures, with the exception of CoNiN0.25 having planar anisotropy. Detailed analysis of the predicted large K u, based on perturbation theory and electronic structure calculations, is attributed to the cumulative effect of contribution from the increased tetragonal distortion and induced orbital distortion from the simultaneous Co substitution and interstitial N-doping. By tailoring the K u, we may create efficient and affordable PMs, bridging the gap between commonly used ferrite and high-performance Nd–Fe–B magnets.

A Ferroelectric Metallomesogen Exhibiting Field-Induced Slow Magnetic Relaxation.

Magnetoelectric (ME) materials exhibiting coupled electric and magnetic properties are of significant interest because of their potential use in memory storage devices, new sensors, or low-consumption devices. Herein, we report a new category of ME material that shows liquid crystal (LC), ferroelectric (FE), and field-induced single molecule magnet (SMM) behaviors. Co(II) complex incorporating alkyl chains of type [Co(3C16 -bzimpy)2 ](BF4 )2 (1; 3C16 -bzimpy=2,2'-(4-hexadecyloxy-2,6-diyl)bis(1-hexadecyl-1H-benzo[d]imidazole)) displayed a chiral smectic C mesophase in the temperature range 321 K-458 K, in which distinct FE behavior was observed, with a remnant polarization (88.3 nC cm-2 ). Complex 1 also exhibited field-induced slow magnetic relaxation behavior that reflects the large magnetic anisotropy of the Co(II) center. Furthermore, the dielectric property of 1 was able to be tuned by an external magnetic field occurring from both spin-lattice coupling and molecular orientational variation. Clearly, this multifunctional compound, combining LC, FE, and SMM properties, represents an entry to the development of a range of next-generation ME materials.

Large stress-induced anisotropy in soft magnetic films for synthetic spin valves

We obtain a large in-plane magnetic uniaxial anisotropy in the soft magnetic Fe60Co20B20 (FeCoB) thin films prepared by rotational sputtering. The anisotropy field (Ha) of 75 to 175 Oe was found in the films with wide ranged thickness from 2.5 to 100 nm, which was attributed to the magnetoelastic energy in association with anisotropic tensile stress. This stress-induced anisotropy has outstanding thermal stability that survives up to 350 °C in the annealing process. The similar large uniaxial anisotropy can be realized in other soft magnetic thin films, such as Fe, Co, Ni, FeCo, and NiFe, with the same synthesis technique. The anisotropic FeCoB film was used as a free layer in a synthetic spin valve. A linear resistance change against external field with a range wider than ±100 Oe together with a significantly reduced coercivity of ∼1.1 Oe (∼8.5 Oe in the case with isotropic free layer) was observed in the transfer curve. The results of this work not only confirm the feasibility of films with large stress-induced magnetic anisotropy as a functional layer in spin-valve devices but also demonstrate a simple synthesis route to induce the magnetic anisotropy, which provides an additional control parameter for the spintronic device design.

DyOCl: A rare-earth based two-dimensional van der Waals material with strong magnetic anisotropy

Comparing with the widely known transitional metal based van der Waals (vdW) materials, rare-earth based ones are rarely explored in the research of intrinsic two-dimensional (2D) magnetism. In this work, we report the physical properties of DyOCl, a rare-earth based vdW magnetic insulator with a direct band gap of $\ensuremath{\sim}5.72\phantom{\rule{3.33333pt}{0ex}}\text{eV}$. The magnetic order of bulk DyOCl is determined by neutron scattering as the $A$-type antiferromagnetic structure below the N\'eel temperature ${T}_{N}=10\phantom{\rule{3.33333pt}{0ex}}\mathrm{K}$. The large magnetic moment near 10.1 ${\ensuremath{\mu}}_{B}$/Dy lies parallel to the $a$-axis with strong uniaxial magnetic anisotropy. At $2\phantom{\rule{3.33333pt}{0ex}}\text{K}$, a moderate magnetic field ($\ensuremath{\sim}2\phantom{\rule{3.33333pt}{0ex}}T$) applied along the easy axis generates spin-flip transitions and polarizes DyOCl to a ferromagnetic state. Density functional theory calculations reveal an extremely large magnetic anisotropy energy ($\ensuremath{-}5850\phantom{\rule{3.33333pt}{0ex}}\ensuremath{\mu}\text{eV}/Dy$) for DyOCl, indicating the great potential to realize magnetism in the 2D limit. Furthermore, the mechanical exfoliation of bulk DyOCl single crystals down to seven layers is demonstrated. Our findings suggest DyOCl is a promising material playground to investigate 2D $f$-electron magnetism and spintronic applications at the nanoscale.

Strain Engineering of Magnetic Anisotropy in Epitaxial Films of Cobalt Ferrite (Adv. Mater. Interfaces 23/2021)

Strain Engineering of Magnetic Anisotropy In article number 2101034, Hiroshige Onoda, Hideto Yanagihara, and co-workers demonstrate that the strain engineering technique is a promising way to induce large magnetic anisotropy without using heavy metals or rare earth elements. The spins (red arrows) of Co ions (light blue) on the octahedral sublattice of the spinel structure change their preferred direction in response to the epitaxial stress of the cobalt ferrite thin film due to the change in the orbital moment.