Hard Magnetization Direction Research Articles

The energy of 180° domain walls of uniaxial crystals with the different magnetocrystalline anisotropy type

The main aim of the present work is to analyze the magnetocrystalline anisotropy (MCA) energy function of uniaxial crystals and to derive analytical expressions for the domain wall (DW) energy taking into account two MCA constants. Thus, the article presents a detailed analysis of the magnetocrystalline anisotropy energy function of uniaxial crystals taking into account two MCA constants (K1, K2). The values of the MCA energy extremes and the position of the easy magnetization directions (EMD) and hard magnetization directions (HMD) were determined. The MCA diagram was plotted in “K1”-“K2” coordinates. Six types of MCA have been found for uniaxial crystals. Two of them are simple with one maximum and one minimum of the EA(θ) function, and four are complex with two absolute and one local extreme for each. It is shown that EA(K1, K2) function has the smallest difference between the maximum and minimum values equal to |K1|/4 and the smallest angle between EMD and HMD equal to π/4 when the K1 + K2 = 0 condition is met. Analytical expressions for the 180° Bloch domain wall energy surface density (γ) were derived for uniaxial crystals with each MCA type. It is found that the γ(K1, K2) function has a minimum, equal to γ=2A|K1| when the relation K1 + K2 = 0 between MCA constants is satisfied. The derived analytical expressions are useful for a detailed spin-reorientation transition analysis. To illustrate this, examples of the application of the obtained results to MCA analyses of real crystals and DW energy calculations are given.

Intrinsic and extrinsic relaxation mechanisms for controlling spin current intensity in Fe 100−x Co x /Ta bilayers

Controlling the damping parameter in metallic ferromagnetic thin films is a key step for spintronic applications in which spin currents are generated by spin pumping. The coexistence of two states with low and high damping constant values would allow to obtain states of high and low spin current intensity, respectively. We have fabricated Fe 100−x Co x /Ta (with nominal x = 0, 15, 20, 25, 30 and 35) bilayers in which the Fe 100−x Co x layers grow epitaxially and the Ta layer is polycrystalline. We have found the coexistence of Gilbert damping and two magnon scattering mechanisms linked to a sign change in the magnetocrystalline anisotropy constant that allows the manipulation of low and high intensity states of the measured inverse spin Hall effect voltage. Bilayers with lower Co concentrations ( x⩽ 25%) present different relaxation mechanisms (isotropic Gilbert damping and two magnon scattering) and an extra ferromagnetic resonance linewidth broadening produced due to mosaicity. Bilayers with Co concentration x> 25% present a dominating Gilbert damping for all directions in the film plane. However, in this concentration range the damping constant is anisotropic and when the magnetic field is applied along the hard magnetization direction α increases ∼420% with respect to the value obtained for the easy magnetization direction. Coexistence of isotropic Gilbert damping and two magnon scattering generated spin currents 2.5 times larger when the field is applied along the hard magnetization axis compared to the value observed in the easy magnetization axis. Thesefindings make the Fe 100−x Co x /Ta system an excellent candidate for spintronic device applications.

Theoretical investigation of the giant magnetocaloric effect in Er0.7Tm0.3Al2

This study presents a theoretical investigation of the giant magnetocaloric effect recently reported in the intermetallic compound Er0.7Tm0.3Al2. Our model Hamiltonian includes the crystalline electrical field, exchange, and Zeeman interactions in both Er3+ and Tm3+ sublattices. A remarkable agreement was achieved between the theoretical calculations and experimental data concerning heat capacity, ΔST and ΔTad (magnetocaloric quantities). Additionally, the analysis of magnetic anisotropy and simulations for the rotating magnetocaloric effect in Er0. 7Tm0.3Al2 were performed. The maximum value for the rotating magnetocaloric effect was predicted around the temperature of spin reorientation transition from the <111>-easy to the <001>-hard magnetization directions.

Controlling Magnetic Anisotropy in Amplitude Expansion of Phase Field Crystal Model

The amplitude expansion for a magnetic phase‐field‐crystal (magnetic APFC) model enables a convenient coarse‐grained description of crystalline structures under the influence of magnetic fields. Considering higher‐order magnetic coupling terms, the possibility of tuning the magnetic anisotropy in these models is demonstrated. This allows reproducing the easy and hard directions of magnetization. Such a result can be achieved without increasing the computational cost, enabling simulations of the manipulation of dislocation networks and microstructures in ferromagnetic materials. As a demonstration, the simulation of the shrinkage of a spherical grain with the magnetic anisotropy of Fe is reported.

On the electronic and magnetic properties of RVO3 (R = Er, Ho, Y, Lu) vanadates: first principles study

In this paper, we investigate the influence of the rare earth element (R) on the physical properties of RVO3 (R = Er, Ho, Y, Lu). For this purpose, a theoretical work is reported in this study using ab initio method with the intent of enhancing our knowledge of the electronic, and magnetic properties of RVO3 (R = Er, Ho, Y, Lu) compounds. The electronic properties of these materials show the presence of Jahn-Teller effect, with a band gap of approximately 0.58eV, 1.57eV, 1.12eV, 1.28eV, for ErVO3, HoVO3, YVO3, LuVO3, respectively. The magnetic contribution of Ho, Er, and V atoms in HoVO3, and ErVO3 compounds was disentangled using X-ray magnetic circular dichroism (XMCD). The magnetic anisotropies of RVO3 with (R = Er, Ho, Y, Lu) are studied and analyzed using spin orbit coupling. We noticed an easy axis of magnetization along the c-direction with strong magneto-crystalline anisotropy energies for the non-magnetic rare earth elements (R = Lu, Y)VO3. For the magnetic rare earth elements, the easy and hard magnetization direction were found to be along b-axis and c-axis, and a-axis and c-axis for HoVO3, and ErVO3, respectively. This means that instead of the standard magnetization demagnetization process, large thermal effects can be obtained by spinning their single crystals in constant magnetic fields. The exchange coupling between (Ho–V) is also found to be greater than the other exchange interactions in HoVO3. Accordingly, a large conventional magnetocaloric could be created in HoVO3, since the Ho Ising moments experience a metamagnetic transition. However, the magnetocaloric effect (MCE) is expected to be lower in ErVO3 due to the weak (Er–V) exchange coupling.

UI3 – 5f-electron magnetic van der Waals material

We grew high-quality single crystals of the 5f electron van der Waals compound UI3 and investigated them by measurements of specific heat and magnetization as function of temperature and magnetic field. UI3 behaves as an antiferromagnet with a first-order magnetic phase transition at the Neél temperature TN ​= ​2.65 ​K. It is characterized by a sharp symmetric specific-heat peak which is gradually shifted to lower temperatures by increasing magnetic field applied along either the a- or b-axis. The behavior in magnetic field reveals orthorhombic magnetocrystalline anisotropy with the hard magnetization direction along the c-axis. The 2-K a- and b-axis magnetization curves exhibit a metamagnetic transition at the critical field μ0Hc ​≈ ​3 ​T and ≈ 1.8 ​T, respectively. In higher fields, when the long-range antiferromagnetism is suppressed by a metamagnetic transition, signs of short-range magnetic ordering of antiferromagnetic correlations in the paramagnetic state show up both in specific heat and magnetization. The anomalous S-shape field dependence of a-axis magnetization well above Hc can be understood as the crossover from the correlated paramagnetic regime to the high-field polarized paramagnet. The magnetic phase diagrams for the magnetic field applied along the a- and b-axis, respectively, designed using the mentioned experimental results are presented. We have also found that the UI3 crystals are easily cleavable which predisposes this material for direct investigation of 5f electron magnetism in the 2D limit on exfoliated atomically thin samples.

First principles calculations to investigate electronic, magnetic anisotropy energy and optical properties of MnCrP half-Heusler alloy

In this current research work, the electronic, magnetic and optical properties of half Heusler alloy MnCrP are calculated and discussed within DFT framework. These calculations are done by using Wien2k code with Perdew–Burke–Ernzerhof Generalized Gradient Approximation and Tran-Blaha modified Becke-Johnson (TB-mBJ) exchange and correlation potentials. Band gap study along with density of states analysis divulge the magnetic nature of MnCrP. The magnetic moments with PBE-GGA and TB-mBJ are 4.426μB and 4.4542μB respectively. It shows the suitability of this material in spintronic device fabrications. The magnetic anisotropy energy of this half Heusler is also determined by the difference of energies in easy and hard magnetization directions. Several optical parameters are calculated, including dielectric constants, absorption parameter, refractive indices, reflectivity, extinction coefficient, energy loss function and conductivity. The imaginary part of the dielectric constant also predicts that the compound MnCrP is optically metallic and suitable for sensor application and optoelectronic device.

Anisotropy of magnetocaloric effect in Fe3GeTe2

We have investigated the magnetocaloric effect (MCE) of an Fe3GeTe2 single crystal near its Curie temperature (Tc) for magnetic field applying along the hard and easy magnetization directions. Magnetic susceptibility measurements indicate that it undergoes a ferromagnetic to paramagnetic phase transition around Tc∼ 200 K with high anisotropy. The maximum values of magnetic entropy change (ΔSM) are estimated for magnetic field from 0–1 T to 0–7 T. Our main observation is that the ΔSM along the easy axis is two to six times larger than that along the hard directions. We propose that magnetocrystalline anisotropy should be taken into consideration in order to maximize the performance of magnetic refrigerants.

Magnetization Reversal in Concave Iron Nano-Superellipses

Square magnetic nanodots can show intentional or undesired shape modifications, resulting in superellipses with concave or convex edges. Some research groups also concentrated on experimentally investigating or simulating concave nano-superellipses, sometimes called magnetic astroids due to their similarity to the mathematical shape of an astroid. Due to the strong impact of shape anisotropy in nanostructures, the magnetization-reversal process including coercive and reversibility fields can be expected to be different in concave or convex superellipses than that in common squares. Here, we present angle-dependent micromagnetic simulations on magnetic nanodots with the shape of concave superellipses. While magnetization reversal occurs via meander states, horseshoe states or the 180° rotation of magnetization for the perfect square, depending on the angle of the external magnetic field, more complicated states occur for superellipses with strong concaveness. Even apparently asymmetric hysteresis loops can be found along the hard magnetization directions, which can be attributed to measuring minor loops since the reversibility fields become much larger than the coercive fields.

Co5/3Nb1/3BO4: A new cobalt oxyborate with a complex magnetic structure

Needle-shape single crystals of Co5/3Nb1/3BO4 warwickite were grown using the flux technique. X-ray diffraction measurements have revealed an orthorhombic structure (Sp. Gr. Pbnm) where the octahedral M1 site is occupied by a mixture of Co2+/Nb5+ ions and the M2 site is exclusively filled by Co2+ ions. Using dc magnetization measurements it was established that the new material undergoes two magnetic transitions: an antiferromagnetic transition at TN1 = 27 K and a ferrimagnetic one at TN2 = 14 K, below which a hysteresis cycle opens. Both magnetic transitions are marked by anomalies in the specific heat. High magnetic anisotropy with c-axis as a hard magnetization direction was detected.

Dependence of exchange bias and coercive field on Cu spacer thickness in oblique deposited Co/Cu/CoO multilayers

Effect of growth-induced uniaxial anisotropy and thickness of non-magnetic Cu spacer between the ferromagnetic/antiferromagnetic Co/CoO multilayers on the exchange bias, coercive field, and magnetic anisotropy have been studied in 10–300 K temperature range. Co(8 nm)/Cu(tCu)/CoO(4 nm) (tCu = 0.6–2.6 nm) multilayers on Si (100) substrates were prepared using the oblique-angle magnetron sputtering deposition to induce growth induced magnetic anisotropy. The top Co layer was allowed to be naturally oxidized to obtain antiferromagnetic CoO. The films show a strong uniaxial in-plane anisotropy determined by ferromagnetic resonance measurements. Magnetization measurements for both easy and hard magnetization directions revealed that the exchange bias and coercive fields decrease by increasing Cu thickness. The existence of strong exchange bias across the thickest Cu layer shows the long-range character of exchange and dipolar interactions at 10 K. Dependence of Cu spacer thickness shows an oscillatory behavior that can be ascribed to the competitions between Ruderman–Kittel–Kasuya–Yosida (RKKY) and dipolar interactions.

Application of DFT for magnetic anisotropy energy calculations of vanadium ferrite VFe2O4

Structural, electronic and magnetic properties of vanadium ferrite are studied using density functional theory with Perdew Burke Ernzerhof Generalized Gradient Approximation (PBE-GGA) type exchange-correlation functional. The calculated lattice constant of VFe2O4 has 0.667% difference with experimental one. The optimized lattice constant is used in self-consistent field calculations to investigate the magnetic and electronic properties. The magnetic properties of VFe2O4 reveals it's half magnetic nature. Furthermore; the magnetic moments, spin-exchange splitting, exchange coefficients, spin-polarized crystal field splitting and edge spin splitting energy of the spinel are also calculated for both spin channels. The large value of magnetic moment (13.999 μB) for VFe2O4 shows it's suitability for fabrication of the spintronic devices. Magnetic anisotropy energy of this spinel is also estimated from the difference of easy and hard directions of magnetization.

Strong magnetic anisotropy and unusual magnetic field reinforced phase in URhSn with a quasi-kagome structure

The physical properties of URhSn with quasi-kagome structure are studied using single-crystalline samples via electrical resistivity, magnetic susceptibility, heat capacity, thermal expansion, and high-field magnetization measurements. Remarkable magnetic anisotropy is found in the ferromagnetic (FM) state below ${T}_{\mathrm{C}}=16\phantom{\rule{4pt}{0ex}}\mathrm{K}$ as well as in the ordered state between ${T}_{\mathrm{C}}$ and ${T}_{\mathrm{O}}=54\phantom{\rule{4pt}{0ex}}\mathrm{K}$, where the easy and hard magnetization directions are the hexagonal [0001] and $[10\overline{1}0]$ axes. In the paramagnetic state, the magnetic susceptibility shows a Curie-Weiss behavior; the Weiss temperatures are positive and negative for [0001] and $[10\overline{1}0]$, respectively, indicating the presence of both FM and antiferromagnetic (AFM) correlations. The entropy release for $5f$ electrons is approximately $R\phantom{\rule{0.16em}{0ex}}\mathrm{ln}3$ at ${T}_{\mathrm{O}}$. The thermal expansion coefficient is strongly anisotropic around ${T}_{\mathrm{O}}$ between the hexagonal basal plane and the [0001] axis, indicating its remarkable anisotropic magnetoelastic response and uniaxial stress dependences. Interestingly, the magnetic field response of the higher-temperature ordered state is unusual: ${T}_{\mathrm{O}}(H)$ increases and the heat-capacity jump is enhanced with the magnetic field for $H\phantom{\rule{0.16em}{0ex}}||\phantom{\rule{4pt}{0ex}}[0001]$. Based on the established thermodynamic evidence for the second-order transition at ${T}_{\mathrm{O}}(H)$, a plausible scenario is the occurrence of a canted AFM ordering or a conical state under magnetic fields, which is stabilized when coupled with field-induced magnetic moments along the [0001] axis. Another possibility is the occurrence of quadrupole ordering at ${T}_{\mathrm{O}}(H)$.

Influence of microstructure and texture evolution on magnetic properties attained by annealing of a cold-rolled Fe-Co-10V semi-hard magnetic alloy

Typically, in processing of Fe-Co-V type magnetic alloys, hot rolled sheets are being severely cold rolled and then annealed at a specific temperature for an appropriate soaking time. In this work, in order to achieve the optimal magnetic properties, effect of various annealing cycles after 86% cold rolling on a 40Fe-50Co-10V (wt%) alloy were examined and the correlation between magnetic properties and microstructure and texture evolution was investigated. In order to supplement extensive microstructure evolution data available from previous publications, texture analysis was carried out using X-ray and electron backscatter diffraction methods and magnetic properties were determined via a vibrating sample magnetometer. It was observed that by increasing the annealing temperature the saturation magnetization decreased due to the formation of paramagnetic (Co,Fe)3V precipitates and austenite, while the coercivity increased in a reverse manner. The impact of austenite on decreasing the saturation magnetization was more pronounced than that of precipitates. Phase boundaries were more effective than grain boundaries in promoting the coercivity. However, nano-size (Co,Fe)3V precipitates inside the ferromagnetic matrix were also important in enhancing the coercivity. Besides formation of the precipitates, variations of the intensity of 〈111〉 direction as hard magnetic direction and intensity of {111}〈112〉 texture component, led to the alteration of magnetic remanence attained at 500–750 °C. Based on application of these alloys as semi hard and hard magnets, the optimal magnetic properties are achieved at a temperature range of 550–650 °C.

Investigation of structural and magnetic properties of PrIr3B2 single crystal

We report the growth of a single crystal of PrIr3B2 by Czochralski pulling method and its structural and anisotropic magnetic properties. At room temperature, PrIr3B2 crystallizes in the hexagonal CeCo3B2-type (P6/mmm) structure. However, a change of crystal symmetry is indicated by a hysteretic transition observed in both the resistivity and heat capacity taking place at ≈ 280 K, which is similar to the observed transition from monoclinic (at room temperature) to hexagonal at 395 K previously reported in CeIr3B2 in literature. The jump in the heat capacity at the transition during cooling is nearly 400 J/mol K. The Curie-Weiss behavior of susceptibility conforms to a trivalent state of the Pr ions, which order antiferromagnetically or ferrimagnetically, as inferred from a relatively broad peak in the susceptibility centered around 10 K. The bulk magnetic transition is further corroborated by an anomaly in the heat capacity at low temperatures. Both the temperature dependence of magnetization, and the isothermal magnetization at 2 K reveal a highly anisotropic magnetic behavior in PrIr3B2 with c-axis [0001] as the hard direction of magnetization. The electrical resistivity shows normal metallic behavior down to TN, below which it shows an upturn suggesting the onset of superzone gap.

Effect of Ferromagnetic Exchange and Magnetic Field Orientation on the Magnetocaloric Effect of Uniaxial Van Vleck Paramagnet

The magnetocaloric effect (MCE), namely, the magnetic entropy change upon the isothermal magnetization of a uniaxial Van Vleck paramagnet with a spin of S = 1 and single-ion easy-plane magnetic anisotropy has been studied in this work. It has been shown that, upon magnetization along the easy plane, the value of normal MCE (decrease in the entropy upon magnetization) can be anomalously high, when the ratio of anisotropy and ferromagnetic exchange parameters of a paramagnet is close to a threshold value corresponding to the transition to the ferromagnetic state. It has been found that, upon magnetization along the hard magnetization direction, i.e., perpendicular to the easy plane, the MCE at low temperatures demonstrates anomalous behavior, namely, the magnetic entropy increases upon magnetization.

Magnetic and magnetotransport properties of single-crystalline R2PdGe6 (R = Pr, Gd and Tb)

Single crystals of R2PdGe6 (R = Pr, Gd and Tb) compounds were grown by the Bi-flux method. Pr2PdGe6 is an antiferromagnetic compound with Néel temperature TN = 15 K, in which a field-induced magnetic transition (spin flip) occurs when a magnetic field is applied along either a or b axis; a small magnetization and hysteresis loop were observed when a field is applied along c axis. Gd2PdGe6 is a collinear antiferromagnetic compound with TN = 37 K along b axis. Tb2PdGe6 is an antiferromagnetic compound with TN = 48 K and its hard magnetization direction is along b axis. The temperature dependences of the resistance of the entire three compounds present inflection points at the respective TN. A large resistance (as well as magnetoresistance) change can be found at the spin flip transition of Pr2PdGe6, but the change is not obvious at the spin flop transition of Gd2PdGe6.

Magnetic Hysteresis in Nanocomposite Films Consisting of a Ferromagnetic AuCo Alloy and Ultrafine Co Particles.

One fundamental requirement in the search for novel magnetic materials is the possibility of predicting and controlling their magnetic anisotropy and hence the overall hysteretic behavior. We have studied the magnetism of Au:Co films (~30 nm thick) with concentration ratios of 2:1, 1:1, and 1:2, grown by magnetron sputtering co-deposition on natively oxidized Si substrates. They consist of a AuCo ferromagnetic alloy in which segregated ultrafine Co particles are dispersed (the fractions of Co in the AuCo alloy and of segregated Co increase with decreasing the Au:Co ratio). We have observed an unexpected hysteretic behavior characterized by in-plane anisotropy and crossed branches in the loops measured along the hard magnetization direction. To elucidate this phenomenon, micromagnetic calculations have been performed for a simplified system composed of two exchange-coupled phases: a AuCo matrix surrounding a Co cluster, which represents an aggregate of particles. The hysteretic features are qualitatively well reproduced provided that the two phases have almost orthogonal anisotropy axes. This requirement can be plausibly fulfilled assuming a dominant magnetoelastic character of the anisotropy in both phases. The achieved conclusions expand the fundamental knowledge on nanocomposite magnetic materials, offering general guidelines for tuning the hysteretic properties of future engineered systems.

XMCD study of the local magnetic and structural properties of microcrystalline NdFeB-based alloys

X-ray Magnetic Circular Dichroism (XMCD) technique was used to investigate local magnetic properties of microcrystalline Nd10.4Zr4.0Fe79.2B6.4 samples, oriented along either easy or hard magnetization direction. The Nd L 2,3 and Fe K edge XMCD spectra were measured at room temperature under a magnetic field of T. A very strong dependence of XMCD spectra on the sample orientation has been observed at the NdL 2,3-edges, whereas the Fe K-edge XMCD spectra are found to be practically isotropic. This result indicates that magnetic anisotropy of NdFeB-based alloys originates from the Nd sublattice. In addition, element selective magnetization curves have been recorded by measuring the intensity of XMCD signals as a function of an applied magnetic field up to T. To find a correlation between local and macroscopic magnetic properties of studied samples we compared these data with magnetization curves, measured by vibrating sample magnetometer up to T. Results are important for understanding the origin of high-coercivity state in NdFeB-based intermetallic compounds.

Martensitic transition, structure and magnetic anisotropy of martensite in Ni-Mn-Sn single crystal

The structural and magnetic properties of Ni50Mn37.5Sn12.5 single crystal were investigated. The alloy undergoes martensitic transformation at 308 K from the austenite phase to the structurally modulated tetragonal 4M martensite phase with lattice parameters a4M = 6.177 Å, c4M = 5.669 Å. The alloy shows 7.9% pre-strain upon uniaxial compression along the 〈001〉 direction, which is near the theoretical 8.2% maximum twinning strain. Magnetization measurements reveal that the magnetic anisotropy of the martensite phase is uniaxial with the easy magnetization axis corresponding to the shortest c axis of the tetragonally distorted unit cell, while its a axis is the hard magnetization direction. The magnetic anisotropy constant Ku saturates at low temperature around 7 × 104 J/m3 and then gradually decreases with increasing temperature as the system approaches the Curie temperature of martensite at 215 K.