Terms Of Magnetocrystalline Anisotropy Research Articles

Chemical synthesis of Mn–Zn magnetic ferrite nanoparticles: Effect of secondary phase on extrinsic magnetic properties of Mn–Zn ferrite nanoparticles

This paper presents a comprehensive investigation of the magnetic properties of co-precipitated MnxZn1–xFe2O4 nanoferrites, where x takes the values of 0.5, 0.6, and 0.7. X-ray diffraction patterns indicated the presence of spinel phase. However, the ferrite composition x = 0.6 contains little trace of secondary phase α-Fe2O3. The occurrence of cation redistribution in the current ferrite samples can be concluded from the deviation of oxygen position parameter (U) to that of optimum value (0.375). The bond angles facilitate the reduction of the A–B interaction and the increase of the B–B interaction. With the substitution of Mn2+ ions, the saturation magnetization (Ms) exhibits both drops and increases whereas the coercivity (Hc) consistently declines. The ferrite composition x = 0.7 exhibits the maximum value of Ms and is equal to 27.9 emu/g. The blocking temperature (TB) rises in proportion to the degree of Mn2+ ion doping. The observation of quadruple doublets in the Mӧssbauer spectra indicates that the ferrite nanoparticles exist in single domain state exhibiting the quadruple interactions. The range of isomer shift (δ) 0.199–0.267 mm/s indicates the existence of exclusively high spin Fe3+ ions. Two paramagnetic doublets are anticipated to exist as a result of the core and shell of present ferrite nanoparticles. In the core-shell morphology of nanoparticles, the core is characterized by larger quadruple splitting (Δ) while the shell is characterized by smaller Δ. The results are integrated in terms of magnetocrystalline anisotropy and secondary phase presuming the core-shell structure of nanoparticles.

Deep learning approach for image classification of magnetic phases in chiral magnets

Chiral magnets have been vastly studied in the last years due to their potential applications as information carrying devices. In this work, it was studied the influence of the magnetocrystalline anisotropy term on the magnetic properties of two-dimensional chiral magnets, by means of Monte Carlo simulations. On the obtained magnetic arrangements, a Deep Learning algorithm based on computer vision techniques, was used for the phase recognition in each sample and their later classification. In this way, the low-temperature phase diagrams were built for a given set of anisotropy constants. It was observed that the skyrmion phase can be stabilized by tuning the magnetocrystalline anisotropy constant; in particular, the skyrmion region in the low-temperature magnetic phase diagrams can be increased by using an easy-plane anisotropy term. The machine learning algorithm described in this work can be easily extended to real-world applications, in the analysis and classification of magnetic force or Lorentz transmission electron microscopy images.

Predicting new iron garnet thin films with perpendicular magnetic anisotropy

Magnetic iron garnets are insulators with low Gilbert damping with many applications in spintronics. Many emerging spintronic applications require perpendicular magnetic anisotropy (PMA) although garnets have only a few PMA types (i.e. terbium and samarium garnet). More and stable PMA garnet options are needed for investigating new spintronic phenomena. In this study, we predict 20 new epitaxial magnetic iron garnet film/substrate pairs with stable PMA at room temperature. The effective anisotropy energies of 10 different garnet films that are lattice-matched to 5 different commercially available garnet substrates (total 50 film/substrate pairs) have been calculated using shape, magnetoelastic and magnetocrystalline anisotropy terms. Strain type, tensile or compressive depending on substrate choice, as well as the sign and the magnitude of the magnetostriction constants of garnets determine if a garnet film may possess PMA. We show the conditions in which Samarium, Gadolinium, Terbium, Holmium, Dysprosium and Thulium garnets may possess PMA on the investigated garnet substrate types. New PMA garnet films with tunable saturation moment and field may improve spin-orbit torque memory and compensated magnonic thin film devices.

Fine-tuning of canted magnetization in stepped Fe films through thickness variation, Au capping, and quantum confinement

Author(s): Daobrowski, M; Cinal, M; Schmid, AK; Kirschner, J; Przybylski, M | Abstract: We present a joint experimental and theoretical study that demonstrates how to efficiently control a canted state of magnetization in Fe films grown on Ag(001) vicinal surface and precisely characterize it with the magneto-optical Kerr effect. It is shown that by employing different mechanisms to tune the magnetization tilting angle, any magnetization orientation within the plane perpendicular to the step edges can be achieved. In particular, increasing the Fe film thickness leads to continuous rotation of the magnetization easy axis toward the film surface and the sense of this rotation in uncovered films is opposite to that in films covered with Au. Another tuning mechanism is provided by oscillatory changes of the tilting angle at low temperatures due to formation of quantum well states in Fe films. The observed canting of magnetization is explained within a phenomenological model by an interplay of the shape anisotropy and two magnetocrystalline anisotropy terms, perpendicular and step-induced anisotropies, which results in an effective uniaxial magnetic anisotropy. The fitted thickness dependencies of the anisotropy constants accurately reproduce experimental variations of the tilting angle with both Fe and Au thicknesses as well as transient changes of the magnetization orientation in ultrathin Fe films upon submonolayer Au coverage, observed with spin-polarized low-energy electron microscopy.

Unveiling a Scaling and Universal Behavior for the Magnetocaloric Effect in Cubic Crystal Structures: A Monte Carlo Simulation

The magnetocaloric effect and the universal character for the magnetic entropy change regarding the cubic crystal structures (SC, BCC, FCC) were investigated, in a qualitative way, using Monte Carlo simulations. A classical Heisenberg Hamiltonian with nearest neighbors, and next nearest neighbors interactions was implemented. In order to compute the critical temperature of the system depending on the coordination number, it was calculated the dependence of the magnetization and magnetic susceptibility as a function of temperature. Magnetic field dependence on the magnetization for isothermal processes was performed considering a magnetocrystalline anisotropy term. In this way, the magnetic entropy change (ΔSm) was computed. Results show that the rescaled ΔSm as well as the exponent (n) characterizing the field dependence of the magnetic entropy change curves, collapse onto a single curve for the studied crystal structures. By this reason, it can be assured that ΔSm exhibits a universal behavior regarding the strength and contribution of the magnetic exchange energy to the total magnetic energy.

A Comparative Study of Magnetic Properties of Large Diameter Co Nanowires and Nanotubes.

A comparative study of the magnetic properties of the arrays of Co nanowires and nanotubes with large external diameters (180 nm) has been carried out. The nanowires/nanotubes were grown by electrodeposition into the self-assembled pores of anodic alumina membranes. The experimental study of their magnetic behavior was focused on the angular dependence of hysteresis loops and their parameters. In both nanowire and nanotube arrays, from the analysis of experimental data, effective longitudinal magnetic anisotropy is concluded, which is stronger in the case of the nanotube array. In addition, the extremely small remanence observed for all loops indicates the important role played by magnetostatic interactions. Micromagnetic simulations were first performed considering intrinsic shape and magnetocrystalline anisotropy terms, together with an effective easy-plane anisotropy to account for those magnetostatic interactions. A qualitative agreement between experiments and simulations is found despite the complexity introduced by the intrinsic and extrinsic array properties (i.e., large diameters, grain structure, and array configuration). In addition, simulations were also carried out for individual nanowire/nanotube with a particular emphasis to understand their differences at the remanence, due to pure geometry contribution.

Extended exchange interactions stabilize long-period magnetic structures in Cr1∕3NbS2

The topologically protected, chiral soliton lattice is a unique state of matter offering intriguing functionality, and it may serve as a robust platform for storing and transporting information in future spintronic devices. While the monoaxial chiral magnet Cr1∕3NbS2 is known to host this exotic state in an applied magnetic field, its detailed microscopic origin has remained a matter of debate. Here, we work towards addressing this open question by measuring the spin wave spectrum of Cr1∕3NbS2 over the entire Brillouin zone with inelastic neutron scattering. The well-defined spin wave modes allow us to determine the values of several microscopic interactions for this system. The experimental data are well-explained by a Heisenberg Hamiltonian with exchange constants up to the third nearest neighbor and an easy plane magnetocrystalline anisotropy term. Our work shows that both the second and third nearest neighbor exchange interactions contribute to the formation of the helimagnetic and chiral soliton lattice states in this robust three-dimensional magnet.

Effect of boron content on structure and magnetic properties in CoFe2O4 spinel nanocrystals

We study the effect of boron content on the structural and magnetic properties of CoFe2O4 spinel nanocrystallines synthesized by sol-gel method. The crystal structure and phase identification of samples are studied by using X-ray diffraction experiment and Rietveld analysis. Rietveld refinement results reveal that all samples have cubic symmetry with space group Fd3m. The cationic distributions are obtained from Rietveld refinement that boron ions are settled into both tetrahedral and octahedral sites in spinel lattice. The crystallite sizes of samples are found in a range of 47–67 nm that is in the limit of single domain in such structure. All samples show ferromagnetic nature and magnetic transition was not seen in the temperature range of 5–400 K. The magnetic domains are pinned with adding boron ions into the CoFe2O4 spinel structure at low temperatures. Thus, an increment in the propagation field (Hp) and temperature (Tp) by boron content in CoFe2O4 structure is observed. In addition, the saturation magnetization (Ms) normalized by crystal size increases with increasing boron concentration. The temperature dependence of magnetic properties of the samples taken by experimental data are confirmed with the Neel-Arhenius model by adding thermal dependence of magnetocrystalline anisotropy term. The results indicate that boron-doping into the spinel structure enhances ferromagnetic coupling and suppresses super-exchange interaction between tetrahedral (X) and octahedral (Y) sites.

Temperature-induced magnetization reversal and ultra-fast magnetic switch at low field in SmFeO3

We have found that SmFeO3, a family of rare-earth orthoferrites, exhibits temperature-induced magnetization reversal below the critical low-temperature along the a-axis of Pbnm symmetry. First-principles calculations demonstrate that this negative magnetization is mainly attributed to the competition between two magnetocrystalline anisotropy terms in the inequivalent magnetic sites of Sm3+ and Fe3+. Interestingly, the room temperature magnetization-field curve further shows an ultra-fast magnetization switch in the low-magnetic field, suggesting a potential applicability to high-speed magnetic switching devices.

Structurally Tailored Hexagonal Ferroelectricity and Multiferroism in Epitaxial YbFeO3 Thin-Film Heterostructures

Multiferroics have received a great deal of attention because of their fascinating physics of order-parameter cross-couplings and their potential for enabling new device paradigms. Considering the rareness of multiferroic materials, we have been exploring the possibility of artificially imposing ferroelectricity by structurally tailoring antiferromagnets in thin-film forms. YbFeO(3) (YbFO hereafter), a family of centrosymmetric rare-earth orthoferrites, is known to be nonferroelectric (space group Pnma). Here we report that a YbFO thin-film heterostructure fabricated by adopting a hexagonal template surprisingly exhibits nonferroelastic ferroelectricity with the Curie temperature of 470 K. The observed ferroelectricity is further characterized by an extraordinary two-step polarization decay, accompanied by a pronounced magnetocapacitance effect near the lower decay temperature, ~225 K. According to first-principles calculations, the hexagonal P6(3)/mmc-P6(3)mc-P6(3)cm consecutive transitions are primarily responsible for the observed two-step polarization decay, and the ferroelectricity originates from the c-axis-oriented asymmetric Yb 5d(z(2))-O 2p(z) orbital hybridization. Temperature-dependent magnetization curves further reveal an interesting phenomenon of spontaneous magnetization reversal at 83 K, which is attributed to the competition between two distinct magnetocrystalline anisotropy terms, Fe 3d and Yb 4f moments.

Micromagnetic studies of three-dimensional pyramidal shell structures

We present a systematic numerical analysis of the magnetic properties of pyramidal-shaped core-shell structures in a size range below 400 nm. These are three-dimensional structures consisting of a ferromagnetic shell which is grown on top of a non-magnetic core. The standard micromagnetic model without the magnetocrystalline anisotropy term is used to describe the properties of the shell. We vary the thickness of the shell between the limiting cases of an ultra-thin shell and a conventional pyramid and delineate different stable magnetic configurations. We find different kinds of single-domain states, which predominantly occur at smaller system sizes. In analogy to equivalent states in thin square films we term these onion, flower, C and S states. At larger system sizes, we also observe two types of vortex states, which we refer to as symmetric and asymmetric vortex states. For a classification of the observed states, we derive a phase diagram that specifies the magnetic ground state as a function of structure size and shell thickness. The transitions between different ground states can be understood qualitatively. We address the issue of metastability by investigating the stability of all occurring configurations for different shell thicknesses. For selected geometries and directions hysteresis measurements are analysed and discussed. We observe that the magnetic behaviour changes distinctively in the limit of ultra-thin shells. The study has been motivated by the recent progress made in the growth of faceted core-shell structures.

Magnetocrystalline anisotropy of Fe-Si alloys on MgO(001)

Experimental investigations on magnetic anisotropy energies of epitaxial ${\text{Fe}}_{100\ensuremath{-}x}{\text{Si}}_{x}$ thin films on MgO(001) have been extended to low Si concentration. We separated different contributions and found that the strain-induced magnetocrystalline anisotropy term favors an in-plane easy axis for the ${\text{Fe}}_{94.5}{\text{Si}}_{5.5}$ sample, but an out-of-plane easy axis for the ${\text{Fe}}_{75}{\text{Si}}_{25}$ sample. First principles calculations using the highly precise full potential linearized augmented plane wave method indicated that this results from the sign change in magnetoelastic coupling coefficient in the composition range. Analysis in electronic structures provides clear insights for the understanding of magnetic anisotropy in these films.

Interface Induced Uniaxial Magnetic Anisotropy in Amorphous CoFeB Films on AlGaAs(001)

We demonstrate an isolated magnetic interface anisotropy in amorphous CoFeB films on (Al)GaAs(001), similar to that in epitaxial films but without a magnetocrystalline anisotropy term. The direction of the easy axis corresponds to that due to the interfacial interaction proposed for epitaxial films. We show that the anisotropy is determined by the relative orbital component of the atomic magnetic moments. Charge transfer is ruled out as the origin of the interface anisotropy, and it is postulated that the spin-orbit interaction in the semiconductor is crucial in determining the magnetic anisotropy.

Influence of hydrogen insertion on the magnetic properties of the RFe11Ti phases

Insertion of hydrogen atoms within the crystal structure induces a significant modification of the physical properties. A moderate unit cell increase and a large increase of the Curie temperature is found. Other properties such as the magnetocrystalline anisotropy are also affected by H insertion within the lattice. Combining magnetisation measurements, a.c. susceptibility and thermomagnetic analysis, the magnetic phase diagrams of the RFe11TiH compounds are established. The competition between the magnetocrystalline anisotropy terms leads to spin reorientation phenomena in RFe11TiH with R=Er or Ho. For R=Tb and Dy the spin reorientation transitions observed in the RFe11Ti compounds have disappeared.

磁石材料における希土類元素の役割

Recent high-performance permanent magnets consist of rare-earth elements and transition metals. In this article, a role of rare-earth elements in permanent magnet materials is explained in terms of magnetocrystalline anisotropy in rare-earth intermetallics. A Coulomb interaction between aspherically distributed 4f-charge of trivalent rare-earth ions and neighboring localized charges has been shown to be a major source of such large magnetic anisotropy. A relation between the magnetic anisotropy and coercivity is briefly described by using a coherent rotation model of magnetic moment. High field magnetization curves in a series of Nd2Fe14B-type single crystal samples are introduced in order to explain why the Nd-Fe-B magnets show the highest energy product value. Finally, a concept of "anisotropy-magnetization specialization" has been given, which is useful to understand the high-performance mechanism in this system.

Magnetic properties ofR6Fe13−xM1+xcompounds and their hydrides

The magnetic properties of various ${R}_{6}{\mathrm{Fe}}_{13\ensuremath{-}x}{M}_{1+x}$ compounds $(R=\mathrm{La}$, Nd, Gd, and Dy) crystallizing in the ${\mathrm{La}}_{6}{\mathrm{Co}}_{11}{\mathrm{Ga}}_{3}$ structure have been investigated by magnetic measurements and x-ray diffraction. It is shown that all ${\mathrm{Nd}}_{6}{\mathrm{Fe}}_{13}M$ compounds with $M=\mathrm{Au}$, Ag, Cu, Si, and Ga order antiferromagnetically around 415 K and evidence is provided that the frequently reported, increase of the magnetization at lower temperature is due to impurities. High-field measurements made at 4.2 K on ${\mathrm{Nd}}_{6\ensuremath{-}x}{\mathrm{Dy}}_{x}{\mathrm{Fe}}_{12.7}{\mathrm{Ga}}_{1.3}$ compounds with $x=0.0$, 0.2, 0.5, and 1.0 show that the $R\ensuremath{-}\mathrm{Fe}$ coupling is not yet broken at 35 T. A large hysteresis in the field dependence of the magnetization is present in all compounds including ${\mathrm{La}}_{6}$${\mathrm{Fe}}_{11}$${\mathrm{Al}}_{3}$, indicating the role of the Fe-sublattice anisotropy. A theoretical model for the field dependence of the magnetization is constructed, based on local minimization of the free energy. By taking into account the second- and fourth-order magnetocrystalline anisotropy terms, the magnetization behavior of the compounds, including the large hysteresis, can be explained excellently. A structure of ferromagnetically ordered Fe sheets coupling antiferromagnetically to each other is proposed to explain the experimental data. The hydrides of these compounds are all ferromagnetic (light $R)$ or ferrimagnetic (heavy $R)$ and have an easy magnetization direction along the $c$ axis due to the Fe-sublattice anisotropy. Their ordering temperatures are near 450 K, just slightly above the N\'eel temperature of the parent compounds, which can be understood from the proposed spin structure.

Synthesis and characterization of hard magnetic materials: PrFe10.5V1.5Nx

The PrFe10.5V1.5 intermetallics and their nitrides were successfully synthesized. In terms of magnetocrystalline anisotropy, PrFe10.5V1.5Nx are characteristics of an easy axis from 0 K to Curie temperature, with an anisotropy field up to 152.9 kOe at 1.5 K and 108.4 kOe at room temperature. In combination with a high Curie temperature of 820 K and a large saturation magnetization of 157.46 emu/g at 1.5 K and 142.77 emu/g at room temperature, these nitrides are favorable for permanent magnet applications. As a preliminary attempt, magnetic powders based on PrFe10.5V1.5Nx were obtained with a maximum energy product of 16.0 and 28.8 MGOe at room temperature and 1.5 K, respectively.

Effect of Antisymmetric Exchange Interaction on the Magnetization and Resonance in Antiferromagnets

The magnetization curves and the resonance frequencies are calculated for two types of antiferromagnets with an antisymmetric exchange interaction of the form ${\mathbf{D}}_{\mathrm{ij}}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathbf{M}}_{i}\ifmmode\times\else\texttimes\fi{}{\mathbf{M}}_{j}$ and a uniaxial magnetocrystalline anisotropy term $K {sin}^{2}\ensuremath{\theta}$. In one type of these antiferromagnets, the D vector is perpendicular to the easy axis. This is presumably the case in the orthoferrites. For this case, we have calculated the resonance frequency as a function of an external field applied parallel to the easy axis. In the second type of these antiferromagnets the D vector lies parallel to the easy axis. This is the situation in hematite below the Morin temperature. For this case, we have calculated the resonance frequency as a function of an external field applied in the plane perpendicular to the easy axis. In both cases we obtained two resonance modes, and the lower mode was found to have a frequency ${W}_{\ensuremath{-}}=0$ for a characteristic critical field, at which the antiferromagnetic axis becomes perpendicular to the easy axis.

Temperature Dependence of Ferromagnetic Uniaxial Anisotropy Coefficients

The problem of a spin one Weiss molecular field Hamiltonian with uniaxial anisotropy is solved exactly. The free energy contains magnetocrystalline anisotropy terms of all even order Legendre polynomials. The P2 coefficient varies as the third power of the magnetization rather well even for fairly large anisotropy, and over the entire temperature range. The higher order terms can be of both signs, and all change sign as the temperature is increased.