Easy Direction Research Articles

Dynamic magnetoelastic properties of TbxHo0.9−xNd0.1(Fe0.8Co0.2)1.93/epoxy composites

Abstract TbxHo0.9−xNd0.1(Fe0.8Co0.2)1.93/epoxy (0 ⩽ x ⩽ 0.40) composites are fabricated in the presence of a magnetic field. The structural and dynamic magnetoelastic properties are investigated as a function of both magnetic bias field Hbias and frequency f at room temperature. The composites are formed as textured orientation structure of 1–3 type with 〈1 0 0〉 preferred orientation for x ⩽ 0.10 and 〈1 1 1〉-orientation for x ⩾ 0.25. The composites generally possess insignificant eddy-current losses for frequency up to 50 kHz, and their dynamic magnetoelastic properties depend greatly on Hbias. The elastic modulus (E3 H and E3 B) shows a maximum negative ΔE effect, along with a maximum d33, at a relatively low Hbias ~ 80 kA/m, contributed by the maximum motion of non-180° domain-wall. The 1–3 type composite for x ⩾ 0.25 shows an enhanced magnetoelastic effect in comparison with 0 to 3 type one, which can be principally ascribed to its easy magnetization direction (EMD) towards 〈1 1 1〉 axis and the formation of 〈1 1 1〉-texture-oriented structure in the composite. These attractive dynamic magnetoelastic properties, e.g., the low magnetic anisotropy and d33 , max as high as 2.0 nm/A at a low Hbias ~ 80 kA/m, along with the light rare-earth Nd element existing in insulating polymer matrix, would make it a promising magnetostrictive material system.

Thickness dependent uniaxial magnetic anisotropy due to step-edges in (1 1 1)-oriented La0.7Sr0.3MnO3 thin films

Abstract The magnetic anisotropy of films of La0.7Sr0.3MnO3 grown on vicinal (1 1 1)-oriented SrTiO3 substrates are investigated. For temperatures above the tetragonal – cubic structural phase transition temperature of the substrate, a step edge induced uniaxial magnetic anisotropy is found at remanence, with a thickness-driven change in easy axis direction, from perpendicular to the step edges to parallel to the step edges with increasing thickness. The anisotropy constant for the investigated (1 1 1)-oriented samples is of the same magnitude as for previously reported (0 0 1)-oriented samples. The data is discussed in the framework of in-plane rotations of the oxygen octahedra resulting in a uniaxial anisotropy. Furthermore, the magnetic anisotropy is sensitive to the structural phase transition at 105 K of the substrate, and the anisotropy constant increases drastically as the temperature is lowered below 105 K.

Crystal field coefficients for yttrium analogues of rare-earth/transition-metal magnets using density-functional theory in the projector-augmented wave formalism

We present a method of calculating crystal field coefficients of rare-earth/transition-metal (RE-TM) magnets within density-functional theory (DFT). The principal idea of the method is to calculate the crystal field potential of the yttrium analogue (‘Y-analogue’) of the RE-TM magnet, i.e. the material where the lanthanide elements have been substituted with yttrium. The advantage of dealing with Y-analogues is that the methodological and conceptual difficulties associated with treating the highly-localized 4f electrons in DFT are avoided, whilst the nominal valence electronic structure principally responsible for the crystal field is preserved. In order to correctly describe the crystal field potential in the core region of the atoms we use the projector-augmented wave formalism of DFT, which allows the reconstruction of the full charge density and electrostatic potential. The Y-analogue crystal field potentials are combined with radial 4f charge densities obtained in self-interaction-corrected calculations on the lanthanides to obtain crystal field coefficients. We demonstrate our method on a test set of ten materials comprising nine RE-TM magnets and elemental Tb. We show that the calculated easy directions of magnetization agree with experimental observations, including a correct description of the anisotropy within the basal plane of Tb and NdCo5. We further show that the Y-analogue calculations generally agree quantitatively with previous calculations using the open-core approximation to treat the 4f electrons, and argue that our simple approach may be useful for large-scale computational screening of new magnetic materials.

Gd2Te3: an antiferromagnetic semimetal

We report high-precision magnetization (), magnetic susceptibility (), specific heat (Cp (T, H)) and ‘zero-field’ electrical resistivity, , data taken on Gd2Te3 single crystal over wide ranges of temperature and magnetic field (H), with either -axis or -plane. and unambiguously establish that the b-axis is the easy direction of magnetization whereas any direction in the ac-plane is a hard direction. The -type anomaly in ‘zero-field’ specific heat, Cp (T, H = 0), and an abrupt drop in (characteristic of the paramagnetic (PM) — antiferromagnetic (AFM) phase transition) are observed at the Néel temperature, K. and Cp (T,H) clearly demonstrate that shifts to lower temperatures with increasing H irrespective of whether H points in the easy or hard direction. When , the isotherms at temperatures in the range 2.5 K K reveal the existence of a field-induced spin-flop (SF) transition at fields 4.0 T 4.5 T. The first principles electronic band structure and density of states calculations, based on the density functional theory, correctly predict an AFM ground state (stabilized primarily by the 4f Gd3+ – 5p Te2−– 4f Gd3+ superexchange interactions) and the observed semi-metallic behavior for the Gd2Te3 compound. Moreover, these calculations yield the values for the ordered magnetic moment per Gd atom at T = 0, mJ mol−1 K−2 for the Sommerfeld coefficient for the electronic specific heat contribution and K for the Curie–Weiss temperature, respectively. These theoretical estimates conform well with the corresponding experimental values , mJ mol−1 K−2 and K.

Magnetostructural behavior in the non-centrosymmetric compound Nd7Pd3

A systematic study of the physical properties and microscopic magnetism of Nd7Pd3 compound, which in the paramagnetic state crystallizes in the non-centrosymmetric hexagonal Th7Fe3-type structure (hP20–P63mc; with a = 10.1367(1) Å and c = 6.3847(1) Å at 300 K), confirms multiple magnetic ordering transitions that occur upon cooling. Antiferromagnetic transition is observed at TN = 37 K, which is followed by ferromagnetic transformation at TC = 33 K. The first-order magnetic transition at TC is magnetoelastic: it involves a change of crystal symmetry from P63mc to Cmc21 and leads to anisotropic changes of the unit cell parameters. While the antiferromagnetic structure is symmetry allowed in P63mc, the ferromagnetic structure with magnetic moments along the a-direction of the original hexagonal unit cell induces the first order transition to Cmc21. Density functional theory calculations confirm the experimentally observed ground state with the a-axis as the easy magnetization direction.

Tuning the exchange coupling in pulse laser deposited cobalt ferrite thin films by hydrogen reduction

Abstract Pulse laser deposited cobalt ferrite CoFe2O4 (CFO) thin films were transformed to CFO/Co-Fe solid solution hard/soft nanocomposite thin films by annealing the CFO films in hydrogen atmosphere. By controlling the annealing time, the level of transformation between the two phases was controlled thereby the exchange coupling and magnetic properties of the nanocomposite layer. X-ray diffraction patterns showed the formation of (0 0 l) orientation for the CFO hard phase and (0 l l) orientation for bcc Co-Fe solid solution soft phase in all samples. Atomic force microscopy images revealed that by increasing the annealing time, the surface roughness of the films increased. Magnetic measurements demonstrate that the more the annealing time, the higher the saturation magnetization and the lower their coercivity and magnetic anisotropy. Hysteresis loops of the films show that these samples behave like single-phase materials indicating the presence of exchange coupling between two different hard and soft phases. Using X-ray photoelectron spectroscopy (XPS), we determined the cations distribution variation in our samples thus a change in the spinel inversion parameter (y) from 0.70 for the as-deposited CFO to 0.49 for the films reduced in hydrogen for 60 min. The Raman spectroscopy confirms the XPS results in our samples. The present approach may help the development of methods for tuning the saturation magnetization, coercivity and magnetic easy axis direction in magnetic nanocomposite thin films and magnetic oxide-based spintronic devices.

Magnetic Properties of the Fe63.5Ni10Cu1Nb3Si13.5B9 Alloy Nanocrystallized in the Presence of Tensile Stresses

The effect of 10 at % Ni, which was introduced into the classic Finemet Fe73.5Cu1Nb3Si13.5B9 at the expense of the Fe content, on the magnetic properties of the composition has been considered. The alloy was subjected to nanocrystallizing annealing in the presence of tensile stresses and in their absence. It is shown that, similarly to the Ni-free alloy, in the Fe63.5Ni10Cu1Nb3Si13.5B9 alloy subjected to thermomechanical treatment, the magnetic anisotropy with the easy magnetization direction across the ribbon axis (transverse induced magnetic anisotropy) is induced. It was found that the 10 at % Ni addition almost does not affect the value of magnetic anisotropy constant induced under thermomechanical treatment and decelerates the process of magnetic anisotropy inducing at σ ≤ 200 MPa. The Ni-containing alloy subjected to nanocrystallizing annealing (at 520°C) in the presence of tensile stresses and in their absence demonstrates the more than 200-fold increase in the coercive force as the annealing time increases from 1 to 4 h, whereas the coercive force of the Ni-free alloy is almost unchanged. This is likely to be related to the appearance of new structural components in the Ni-alloyed composition upon annealing.

Large magnetoresistance effects in Fe3O4

We investigated the magnetoresistance (MR) of a single crystal of magnetite, Fe3O4. In an effort to distinguish between different contributions to the MR the samples were prepared in two different initial magnetic states, i.e. by either zero-field or by field cooling from room temperature. The different magnetic structures in this sample have a dramatic effect on the magnetoresistance: for initially zero-field-cooled conditions a negative MR of about −20% is observed just below the Verwey transition at K. For decreasing temperature the MR increases, changes sign at ∼78 K and reaches a record positive value of ∼45% at around 50 K. This behavior is completely absent in the field-cooled sample. Magnetization measurements corroborate an alignment of the easy magnetization direction in applied magnetic fields below as a cause of the strong effects observed in both, magnetization and MR. Our results point to a complex interplay of structural and magnetocrystalline effects taking place upon cooling Fe3O4 through .

Magnetic properties and domain structure of ultrathin yttrium iron garnet/Pt bilayers

We report on the structure, magnetization, magnetic anisotropy, and domain morphology of ultrathin yttrium iron garnet (YIG)/Pt films with thickness ranging from 3 to 90 nm. We find that the saturation magnetization is close to the bulk value in the thickest films and decreases towards low thickness with a strong reduction below 10 nm. We characterize the magnetic anisotropy by measuring the transverse spin Hall magnetoresistance as a function of applied field. Our results reveal strong easy plane anisotropy fields of the order of 50-100 mT, which add to the demagnetizing field, as well as weaker in-plane uniaxial anisotropy ranging from 10 to 100 $\mu$T. The in-plane easy axis direction changes with thickness, but presents also significant fluctuations among samples with the same thickness grown on the same substrate. X-ray photoelectron emission microscopy reveals the formation of zigzag magnetic domains in YIG films thicker than 10 nm, which have dimensions larger than several 100 $\mu$m and are separated by achiral N\'{e}el-type domain walls. Smaller domains characterized by interspersed elongated features are found in YIG films thinner than 10 nm.

Zero-field dynamics stabilized by in-plane shape anisotropy in MgO-based spin-torque oscillators

Here, we demonstrate numerically that shape anisotropy in MgO-based spin-torque nano-oscillators consisting of an out-of-plane magnetized free layer and an in-plane polarizer is necessary to stabilize out-of-plane magnetization precession without the need of external magnetic fields. As the in-plane anisotropy is increased, a gradual tilting of the magnetization towards the in-plane easy direction is introduced, favouring zero-field dynamics over static in-plane states. Above a critical value, zero-field dynamics are no longer observed. The optimum ratio of in-plane shape to out-of-plane uniaxial anisotropy, for which large angle out-of-plane zero-field dynamics occur within the widest current range, is reported.

Anisotropic magnetic properties and critical behaviour studies of trigonal Cr5Te8 single crystal

Magnetic and magnetocaloric properties of the single crystalline Cr5Te8 have been studied. Temperature dependent dc and ac magnetization of Cr5Te8, upon cooling, reveals a paramagnetic to ferromagnetic transition at ≈ 220 K (TC). Field dependent magnetization in the ordered state at 2 K reveals strong anisotropy with c-axis as the easy axis of magnetization. A weak itinerant behaviour of Cr-spins is also ascertained from the magnetization studies. Magnetocaloric effect in terms of isothermal magnetic entropy change has been estimated to be ≈-2 J/kg·K near TC for a field change of 70 kOe. The magnetic transition at TC is found to be second order. Critical behaviour analysis along the easy axis direction in Cr5Te8 suggests that this system does not belong to a particular universality class. However, the estimated critical exponent values β = 0.363, γ = 1.399 and δ = 4.83 from modified Arrott plots and scaling of magnetocaloric behaviour satisfactorily obeys the scaling hypothesis wherein the experimental isothermal magnetization curves collapse onto two branches of universal curves below and above TC. Our critical behaviour analyses confirm the quasi-2D weak itinerant ferromagnetic short range type interaction in Cr5Te8.

Anisotropic magnetotransport and magnetic phase diagrams of the antiferromagnetic heavy-fermion superconductor Ce3PdIn11

We report the results of our detailed magnetotransport studies on single crystals of the antiferromagnetic heavy-fermion superconductor Ce$_3$PdIn$_{11}$. Electrical resistivity measurements, carried out in different magnetic field orientations with respect to the crystallographic axes, were mainly aimed to amass further insight about the magnetically ordered state. The results manifest a clear metamagnetic transitions in the ordered phase when the applied field is parallel to the tetragonal $c$ axis, while no similar features are seen for the transverse direction, i.e., with the field confined within the $ab$ plane. This finding elucidates the fact that the $c$ axis is the easy magnetic direction in this system. Based on the electrical transport and heat capacity data obtained for Ce$_3$PdIn$_{11}$, magnetic field -- temperature phase diagrams were constructed, which elucidate fairly enigmatic behaviors in this material featuring the existence of both second- and first-order magnetic phase transitions.

Structural and magnetic properties of Pr5Si3-xGex compounds

The crystal structures, magnetic properties and electronic structures of pseudo-binary compounds Pr5SixGe3-x were investigated. With the increase of the Ge content, structure of Pr5SixGe3-x gradually changes from the tetragonal Cr5B3-type to the hexagonal Mn5Si3-type. For the compounds with the tetragonal structure, the unit cell volumes as well as the magneto-crystalline anisotropy energy (MAE) increase with the Ge content, leading to the diminishing of the magnetization and the maximum magnetic entropy changes -ΔSM. As compared to the binary compound Pr5Si3, the intermetallic compounds with Ge doping show broader working temperature span and considerably large relative cooling power (213.9, and 184.8 J kg−1 at ΔH = 50 kOe for Pr5Si2.5Ge0.5 and Pr5Si2Ge, respectively). For the hexagonal compound Pr5Ge3, a large MAE and an easy magnetization direction close to c axis (polar angle θ ∼7°) were calculated, which is related to the macroscopic magnetic transition behavior at low temperature.

Magnetic Anisotropy and Ferromagnetic Resonance in Nitrogen-Incorporated NiFe2O4Thin Films

The nitrogen-incorporated nickel ferrite (N-NFO) thin films deposited by radio-frequency magnetron sputtering were found to exhibit interesting magnetic properties upon heat treatment. The in-plane (IP) and out-of-plane (OP) magnetizations of the heat-treated N-NFO thin films measured at 20 K were seen not to saturate even at higher fields, indicating a large perpendicular magnetic anisotropy (PMA). The coercive field value was found to decrease upon heat treatment, due to the weakened pinning of domain walls upon decrease in grain boundary volume. The IP magnetization exhibits a tendency to saturate at high fields, whereas the OP magnetization does not, indicative of the existence of easy magnetization direction in the plane of the film. The room temperature ferromagnetic resonance (FMR) spectra in both IP and OP configurations indicated the increase of FMR linewidth upon nitrogenation, revealing the increased distribution of magnetic anisotropy induced by nitrogen in the lattice. An increase in the resonance field value was observed for the N-NFO film grown at 100 W compared with that of NFO in the IP configuration, while a decrease in the value was observed for OP, which is attributed to an enhanced IP magnetic anisotropy. The films grown at 120 and 140 W have shown decrease in a resonance field value in the IP configuration and increase in OP, which is attributed to the increase in thickness-dependent PMA.

Large low-field magnetostriction in Tb0.15Ho0.85−xNdxFe2 Laves compounds

In this letter, we demonstrate that the replacement of Ho with Nd in Tb0.15Ho0.85−xNdxFe2 system not only reduces the cost of the material, but also improves the magnetostrictive properties effectively. Tb0.15Ho0.65Nd0.2Fe2 exhibits a large low-field magnetostriction, which is 50% higher than that of Nd-free compound. The spin reorientation temperature of the system was raised with the introduction of Nd. Meanwhile, the easy magnetization direction of Tb0.15Ho0.65Nd0.2Fe2 is along 〈111〉 axis at room temperature, leading to a rhombohedral symmetry in this compound. This work indicates that Nd can be considered as a low-cost alternative element for the heavy rare earth Ho.

Magneto-volume effect and magnetic properties in DyFe12−xTax compounds

Abstract The structural and magnetic properties of DyFe12−xTax compounds (with x = 0.5, 0.6, 0.65 and 0.7) have been investigated in detail using dc magnetization, differential scanning calorimetry, Mossbauer effect spectroscopy (5–300 K) and high-resolution synchrotron x-ray diffraction (10–700 K). The easy magnetization directions at room temperature are along the c-axis. With decreasing temperature, the magneto-crystalline anisotropy changes from easy axis at room temperature to easy cone at Tsr2 for all compounds, then to easy plane at Tsr1. Both Tsr1 and Tsr2 change significantly over the ranges 158–216 K and 239–259 K with increasing Ta content (x = 0.5–0.7) respectively, while the Curie temperature is found to remain essentially unchanged with a value of ∼540 K. Pronounced magneto volume effects have been detected at the Curie temperature while no clear anomalies are observed around the spin reorientation temperatures. Large spontaneous magnetostriction has been detected below TC for all of the compounds.

Positive and negative magnetoresistances in Co/Cu/Ni spin-valves

We studied the magnetoresistance effects in Co/Cu/Ni spin-valves with magnetic fields along different in-plane directions. When the magnetic easy axis directions of Co and Ni layers are parallel, only typical positive magnetoresistance effects were discovered. Interestingly, a magnetoresistance sign inverse was discovered in the spin-valve with different magnetic easy axis directions between Co and Ni layers. This anomalous behavior may be related to the multi-step magnetization reversal processes in these materials.

Electric-field control of spin orientation of manganocene: An insight into molecule-substrate interactions.

The manipulation of spin orientations in molecular nanomagnets assembled on surfaces is essential for the development of memory devices. These properties are dominated by interactions with the substrate. Here, we show that individual manganocene molecules deposited on Cu(111) exhibit different easy magnetization directions in an applied electric-field due to different contact geometries. Using Hubbard-U corrected density-functional theory to describe strong correlation effects and a non-self-consistent diagonalization method to treat spin-orbit coupling, we demonstrate that the field-induced spin reorientation transition occurs in the standing-up molecule in both high-spin (HS) and low-spin states, while the transition only occurs in the HS state for the flat-lying molecule. We propose plausible mechanisms in terms of charge polarization at the interface as well as modifications of the electronic states near the Fermi level E F. We show that the molecule largely preserves its arrangement of 3d orbitals in the standing configuration due to the "insulating layer" (bridging ligand), whereas direct contact of the Mn ion with the substrate in the lying configuration induces an orbital degeneracy around E F, thus preventing the electrical modulation of magnetic anisotropies.

Evidence of anomalous switching of the in-plane magnetic easy axis with temperature in Fe3O4 film on SrTiO3:Nb by v-MOKE and ferromagnetic resonance.

The evolution of the magnetic anisotropy directions has been studied in a magnetite (Fe3O4) thin film grown by infrared pulsed-laser deposition on SrTiO3(100):Nb substrate. The magnetic easy axes at room temperature are found along the in-plane 〈100〉 film directions, which means a rotation of the easy axis by 45° with respect to the directions typically reported for bulk magnetite and films grown on single-crystal substrates. Moreover, when undergoing the Verwey transition temperature, TV, the easy axis orientation evolves to the 〈110〉 film directions. This anomalous behavior has been demonstrated by measuring first the angular dependence of coercivity and remanence well above and below TV by high-resolution vectorial magneto-optical Kerr effect (v-MOKE). Ferromagnetic resonance (FMR) measurements have additionally proven a well-defined fourfold magnetic anisotropy induced during growth with confirmed easy axis directions along 〈100〉 for T > TV and 〈110〉 for T < TV. These results provide a clear proof of the possibility of tuning magnetic anisotropy in Fe3O4 thin films by proper control on the growth parameters and substrate choice.

Morphological and crystallographic orientation of hematite spindles in an applied magnetic field.

The magnetic response of spindle-shaped hematite (α-Fe2O3) nanoparticles was investigated by simultaneous small-angle and wide-angle X-ray scattering (SAXS/WAXS) experiments. The field-dependent magnetic and nematic order parameters of the magnetic single-domain nanospindles in a static magnetic field are fully described by SAXS simulations of an oriented ellipsoid with the implemented Langevin function. The experimental scattering intensities of the spindle-like particles can be modeled simply by using the geometrical (length, radius, size distribution) and magnetic parameters (strength of magnetic field, magnetic moment) obtained from isotropic SAXS and macroscopic magnetization measurements, respectively. Whereas SAXS gives information on the morphological particle orientation in the applied field, WAXS texture analysis elucidates the atomic scale orientation of the magnetic easy direction in the hematite crystal structure. Our results strongly suggest the tendency for uniaxial anisotropy but indicate significant thermal fluctuations of the particle moments within the hematite basal plane.