Magnetic orientation and magnetic anisotropy in paramagnetic layered oxides containing rare-earth ions

The magnetic anisotropies and easy axes of magnetization at room temperature were determined, and the effects of rare-earth (RE) ions were clarified for RE-based cuprates, RE-doped bismuth-based cuprates and RE-doped Bi-based cobaltite regarding the grain orientation by magnetic field. The easy axis, determined from the powder orientation in a static field of 10 T, depended qualitatively on the type of RE ion for all three systems. On the other hand, the magnetization measurement of the c-axis oriented powders, aligned in static or rotating fields, revealed that the type of RE ion strongly affected not only the directions of the easy axis but also the absolute value of magnetic anisotropy, and an appropriate choice of RE ion is required to minimize the magnetic field used for grain orientation. We also studied the possibility of triaxial grain orientation in high-critical-temperature superconductors by a modulated oval magnetic field. In particular, triaxial orientation was attempted in a high-oxygen-pressure phase of orthorhombic RE-based cuprates Y₂Ba₄Cu₇O_y. Although the experiment was performed in epoxy resin, which is not practical, in-plane alignment within 3° was achieved.     

Magnetization reversal in YIG/GGG(111) nanoheterostructures grown by laser molecular beam epitaxy

Abstract

Thin (4–20 nm) yttrium iron garnet (Y₃Fe₅O₁₂, YIG) layers have been grown on gadolinium gallium garnet (Gd₃Ga₅O₁₂, GGG) 111-oriented substrates by laser molecular beam epitaxy in 700–1000 °C growth temperature range. The layers were found to have atomically flat step-and-terrace surface morphology with step height of 1.8 Å characteristic for YIG(111) surface. As the growth temperature is increased from 700 to 1000 °C the terraces become wider and the growth gradually changes from layer by layer to step-flow regime. Crystal structure studied by electron and X-ray diffraction showed that YIG lattice is co-oriented and laterally pseudomorphic to GGG with small rhombohedral distortion present perpendicular to the surface. Measurements of magnetic moment, magneto-optical polar and longitudinal Kerr effect (MOKE), and X-ray magnetic circular dichroism (XMCD) were used for study of magnetization reversal for different orientations of magnetic field. These methods and ferromagnetic resonance studies have shown that in zero magnetic field magnetization lies in the film plane due to both shape and induced anisotropies. Vectorial MOKE studies have revealed the presence of an in-plane easy magnetization axis. In-plane magnetization reversal was shown to occur through combination of reversible rotation and abrupt irreversible magnetization jump, the latter caused by domain wall nucleation and propagation. The field at which the flip takes place depends on the angle between the applied magnetic field and the easy magnetization axis and can be described by the modified Stoner–Wohlfarth model taking into account magnetic field dependence of the domain wall energy. Magnetization curves of individual tetrahedral and octahedral magnetic Fe³⁺ sublattices were studied by XMCD.     

Texturing by cooling a metallic melt in a magnetic field

Abstract

Processing in a magnetic field leads to the texturing of materials along an easy-magnetization axis when a minimum anisotropy energy exists at the processing temperature; the magnetic field can be applied to a particle assembly embedded into a liquid, or to a solid at a high diffusion temperature close to the melting temperature or between the liquidus and the solidus temperatures in a region of partial melting. It has been shown in many experiments that texturing is easy to achieve in congruent and noncongruent compounds by applying the field above the melting temperature T_m or above the liquidus temperature of alloys. Texturing from a melt is successful when the overheating temperature is just a few degrees above T_m and fails when the processing time above T_m is too long or when the overheating temperature is too high; these observations indicate the presence of unmelted crystals above T_m with a size depending on these two variables that act as growth nuclei. A recent model that predicts the existence of unmelted crystals above the melting temperature is used to calculate their radius in a bismuth melt.     

Epitaxial Orientation and Planar Hall Effect for MnAs Films Grown on GaAs(001)

We study the planar Hall effect in high quality thin ferromagnetic films of MnAs grown on GaAs(001) exhibiting hysteresis due to the hindered rotation of the magnetic moment in the plane. The saturation magnetic field H _s, which is necessary to align the magnetization along the hard axis, depends sensitively on the epitaxial orientation of the film. By using out-of-plane magnetic fields directions, we show that H _s is strongest along the direction of the c-axis of the MnAs crystal, thus demonstrating the importance of the crystal field anisotropy for the planar Hall effect.     

Spin-Lattice Coupled Metamagnetism in Frustrated van der Waals Magnet CrOCl (Small 33/2023)

The long-range magnetic ordering in frustrated magnetic systems is stabilized by coupling magnetic moments to various degrees of freedom, for example, by enhancing magnetic anisotropy via lattice distortion. Here, the unconventional spin-lattice coupled metamagnetic properties of atomically-thin CrOCl, a van der Waals antiferromagnet with inherent magnetic frustration rooted in the staggered square lattice, are reported. Using temperature- and angle-dependent tunneling magnetoconductance (TMC), in complementary with magnetic torque and first-principles calculations, the antiferromagnetic (AFM)-to-ferrimagnetic (FiM) metamagnetic transitions (MTs) of few-layer CrOCl are revealed to be triggered by collective magnetic moment flipping rather than the established spin-flop mechanism, when external magnetic field (H) enforces a lattice reconstruction interlocked with the five-fold periodicity of the FiM phase. The spin-lattice coupled MTs are manifested by drastic jumps in TMC, which show anomalous upshifts at the transition thresholds and persist much higher above the AFM Néel temperature. While the MTs exhibit distinctive triaxial anisotropy, reflecting divergent magnetocrystalline anisotropy of the c-axis AFM ground state, the resulting FiM phase has an a-c easy plane in which the magnetization axis is freely rotated by H. At the 2D limit, such a field-tunable FiM phase may provide unique opportunities to explore exotic emergent phenomena and novel spintronics devices.     

Soft magnetic property and magnetization reversal mechanism of Sm doped FeCo thin film for high-frequency application

A series of (Fe_0.67Co_0.33)_1 − xSm_x (0 ≤ x < 0.25) thin films with thickness around 110 nm have been fabricated on silicon(111) substrates by magnetron co-sputtering at ambient condition with a 2.4 kA/m magnetic field applied in the film plane during deposition. With the Sm concentration increasing, FeCo grain size gradually decreases and FeCoSm film eventually becomes amorphous, while the isotropic magnetic property changes to in-plane uniaxial anisotropy as long as Sm is doped. The investigation of the angular dependence of coercivity and switching field indicates that the magnetization reversal mechanism of FeCoSm film is domain-wall depinning and coherent rotation when the applied field is close to the easy axis and hard axis, respectively. The anisotropy field and the resonance frequency of FeCoSm films can be tuned in the range of 15.0-109.5 kA/m and 5.2-11.8 GHz, respectively, by controlling the content of Sm, indicating that FeCoSm films have much potential in high-frequency applications.     

Interactive Effect of Grain Orientation and Grain Size on Magnetic Properties of Fe-78 wt% Ni Ribbons

Fe-78 wt% Ni ribbons were prepared by the melt spinning technique and the interactive contribution of the grain size and grain orientation on the magnetic properties was examined. Heat treatment at 673 K for 1 h followed by furnace cooling was performed to show the annealing impact. At three wheel speeds of 10, 20, and 30 m/s, the saturation magnetization nearly does not change. High wheel speed and heat treatment are inclined to promote the growth of <001> grains. Although the <001> orientation is not the easy axis of magnetization, the improvement of the texture in this direction makes the coercivity decrease, which counteracts the inverse effect of the grain size at high wheel speed. It indicates that for preparing soft magnetic ribbons, the interactive contribution of grain orientation variation and the grain size should be considered.     

In situ observation of magnetic orientation process of feeble magnetic materials under high magnetic fields

Abstract

An in situ microscopic observation of the magnetic orientation process of feeble magnetic fibers was carried out under high magnetic fields of up to 10 T using a scanning laser microscope. In the experiment, carbon fibers and needle-like titania fibers with a length of 1 to 20 μm were used. The fibers were observed to gradually orient their axes parallel to the direction of the magnetic field. The orientation behavior of the sample fibers was evaluated on the basis of the measured duration required for a certain angular variation. As predicted from the theoretical consideration, it was confirmed that the duration required for a certain angular variation normalized by the viscosity of the fluid is described as a function of the fiber length. The results obtained here appear useful for the consideration of the magnetic orientation of materials suspended in a static fluid.     

Small-Angle Neutron Scattering by the Magnetic Microstructure of Nanocrystalline Ferromagnets Near Saturation

The paper presents a theoretical analysis of elastic magnetic small-angle neutron scattering (SANS) due to the nonuniform magnetic microstructure in nanocrystalline ferromagnets. The reaction of the magnetization to the magnetocrystalline and magnetoelastic anisotropy fields is derived using the theory of micromagnetics. In the limit where the scattering volume is a single magnetic domain, and the magnetization is nearly aligned with the direction of the magnetic field, closed form solutions are given for the differential scattering cross-section as a function of the scattering vector and of the magnetic field. These expressions involve an anisotropy field scattering function, that depends only on the Fourier components of the anisotropy field microstructure, not on the applied field, and a micromagnetic response function for SANS, that can be computed from tabulated values of the materials parameters saturation magnetization and exchange stiffness constant or spin wave stiffness constant. Based on these results, it is suggested that the anisotropy field scattering function S_H can be extracted from experimental SANS data. A sum rule for S_H suggests measurement of the volumetric mean square anisotropy field. When magnetocrystalline anisotropy is dominant, then a mean grain size or the grain size distribution may be determined by analysis of S_H.     

Neutron Diffraction Investigation of the DyFe11Ti Magnetic Structure and Its Spin Reorientations

In this article, we report on the magnetic structure of DyFe₁₁Ti and its thermal evolution as probed by neutron powder diffraction. A thermodiffraction technique was used to follow the temperature dependence of the magnetic moments, as well as their orientation. The Dy and Fe moments were coupled to each other in an antiparallel manner to form a ferrimagnet, where the easy magnetization direction at 2 K was the [110] axis in the basal (a, b) plane. This magnetic structure underwent two successive spin reorientation phenomena with increasing temperature. A large Dy magnetic moment of 9.7 Bohr magneton (μ_B) was obtained at low temperatures, and the magnitude decreased rapidly to 7.5μ_B at 200 K. The largest Fe magnetic moment was observed on the Fe 8i position. A ThMn₁₂-type crystal structure was preserved in the studied temperature range, despite the large changes of the magnetic structure. A sharp tilt was observed at the first-order spin reorientation, T_SR1; the angle between the easy magnetization axis and the crystal c axis was reduced from 90° at 2 K to about 20° at 200 K (where c is the easy axis above 200 K); and the Dy and Fe magnetic moments maintained an antiparallel coupling.     

Magnetic and structural phase transitions of MnBi under high magnetic fields

Abstract

High-field x-ray diffraction and magnetization measurements and differential thermal analysis (DTA) were carried out for polycrystalline MnBi with an NiAs-type hexagonal structure to investigate its magnetic and structural phase transitions. The lattice parameter a rapidly decreases below the spin reorientation temperature T_SR(=90 K) in a zero magnetic field. The parameter c decreases gradually with decreasing temperature and exhibits an anomaly in the vicinity of T_SR. By applying a magnetic field of 5 T, the parameter a increases by ～0.05% when T<T_SR and varies smoothly when 8≤T≤300 K. DTA data show that the magnetic phase transition temperature from the ferromagnetic state to the paramagnetic state increases linearly at a rate of 2 KT^?1 with increasing magnetic field up to 14 T.     

Magnetic Field Effects on Surface Morphology and Magnetic Properties of Co–Ni–N Thin Films Prepared by Electrodeposition

The surface morphology and magnetic properties of Co?CNi?CN thin films electrodeposited under an external magnetic field were investigated. The films were electroplated on Al substrates using the same electrodeposition parameters (temperature and pH) for all experiments, with an external magnetic field of 107?Oe applied to the cathode surface. The films were compared with similar samples obtained in the absences of magnetic field. The magneto-induced modifications in the Co?CNi?CN morphology can be explained by the specific local convection of ions at the interface cathode-electrolyte, which promotes changes both in the electrical charge of the double layer and in the thickness of the diffusion layer. From the magnetic measurements, we found that the coercivity varied between H _c =(14÷27)?kA/m depending on the direction of the applied magnetic field and on the sodium nitrate content in the plating bath. It was observed that an induced anisotropy appeared in the Co?CNi?CN films due to the preferential orientation of the easy axis of magnetization in the magnetic field direction. In addition, the Co?CNi?CN alloy films showed good magnetic property, which is considered that not only the smaller grain size of the films, but also more uniform surface of the films than that deposited in absence of applied magnetic field.     

Theory of magnetically polarizable superconductors. II

A microscopic theory of superconductors containing a magnetically polarizable medium is given. By taking into account the Zeeman effect, we derive the crossover behavior from a second- to a first-order phase transition, the magnetization curve in the high-field region, and the magnetization jump at the first-order phase transition temperature. The theory gives a qualitative account of the magnetic behavior, including the remarkable convex curvature of the magnetization curves, observed in the ferromagnetic superconductor ErRh₄B₄ with applied external field parallel to the magnetic easy axis.     

The effect of stress on the magnetic properties of sol–gel-derived Pr0.9Ca0.1MnO3 thin films

The Pr_0.9Ca_0.1MnO₃ (PCMO) thin films prepared by sol–gel method suffered tensile and compressive stress grown on SrTiO₃ and LaAlO₃ substrates. The hysteresis loops at different temperatures show that the coercivity field with tensile stress is larger and the pinning potential of ferromagnetism motion is much stronger. The temperature dependence of the ZFC and FC magnetizations indicates that the stress significantly affects the ferromagnetic (FM) and antiferromagnetic transition temperature of PCMO, and the Curie temperature (T_C) decreases with tensile stress. The films show strong anisotropy properties that the magnetization increases much faster with the magnetic field when H⊥c, but the coercive field and saturation magnetization do not change significantly. In addition, the persistent photoinduced magnetization is investigated, and significant improvement of the FM ordering was observed in low temperature. And the saturation magnetization of each Mn ion is significantly affected by orientation and illumination.

     

NMR Evidence for Spatial Modulations in the Cuprates

Nuclear magnetic resonance (NMR) data on Cu, apical and planar O in La_1.85Sr_0.15CuO₄ are presented. Spin echo double resonance shows that the large Cu magnetic shift distribution is of short-length scale. Analysis of the O data reveals static modulations of the spin susceptibility with a spin–spin correlation function near zero. The Cu shift distribution is found to be of orbital origin. The full planar oxygen spectra show a correlated modulation of the electric field gradient with the spin susceptibility. Similar results on other cuprates indicate universality of these phenomena.     

Controlling Magnetic and Optical Properties of the van der Waals Crystal CrCl3−xBrx via Mixed Halide Chemistry

Magnetic van der Waals (vdW) materials are the centerpiece of atomically thin devices with spintronic and optoelectronic functions. Exploring new chemistry paths to tune their magnetic and optical properties enables significant progress in fabricating heterostructures and ultracompact devices by mechanical exfoliation. The key parameter to sustain ferromagnetism in 2D is magnetic anisotropy—a tendency of spins to align in a certain crystallographic direction known as easy‐axis. In layered materials, two limits of easy‐axis are in‐plane (XY) and out‐of‐plane (Ising). Light polarization and the helicity of topological states can couple to magnetic anisotropy with promising photoluminescence or spin‐orbitronic functions. Here, a unique experiment is designed to control the easy‐axis, the magnetic transition temperature, and the optical gap simultaneously in a series of CrCl_3?xBr_x crystals between CrCl₃ with XY and CrBr₃ with Ising anisotropy. The easy‐axis is controlled between the two limits by varying spin–orbit coupling with the Br content in CrCl_3?x Br_x. The optical gap, magnetic transition temperature, and interlayer spacing are all tuned linearly with x. This is the first report of controlling exchange anisotropy in a layered crystal and the first unveiling of mixed halide chemistry as a powerful technique to produce functional materials for spintronic devices.     

NMR Spin-Lattice Relaxation Rates in Cuprates

An analysis of nuclear spin-lattice relaxation data in the normal state of cuprates that appropriately accounts for the highly anisotropic structures shows no contrasting temperature dependence of the Cu, O, and Y relaxations, which suggests that all nuclei relax by the same mechanism of the spin liquid. To investigate the temperature dependence of this mechanism, the model of fluctuating fields is used in which the rates are expressed in terms of hyperfine interaction energies and an effective correlation time τ _eff characterizing the dynamics of the spin fluid. The former contain the effects of the antiferromagnetic static spin correlations, which cause the hyperfine field constants to be added more coherently at low temperature but incoherently at high temperature. At low temperatures, τ _eff grows linearly with temperature as in ordinary metals. At high temperatures, however, the nuclear spin-lattice relaxation rates in various cuprates unequivocally reflect local moment features. This behavior is similar to that observed for the magnetic transition metals Fe, Co, and Ni, where also some properties show a cross-over from an itinerant behavior of delocalized electrons at low to that of localized moments at high temperatures.     

Broadband microwave absorption in [NiFe/FeMn]n exchange-coupled multilayer films

[NiFe/FeMn]_n exchange-coupled multilayer films have been fabricated on the silicon substrate by magnetron sputtering deposition. The static and dynamic magnetic properties of multilayer films have been investigated with varying numbers of layers. The results show that the linewidth and permeability of imaginary resonance peak are increased with increasing numbers of layers. For the NiFe/FeMn/NiFe sample, the resonance frequency shows a different shift with applying external magnetic field along the direction of easy and hard magnetization axis of the sample, respectively, indicating a different magnetic reversal process in two ferromagnetic layers. It proved that the increase of linewidth was originated from the different interface exchange coupling.     

Peculiarities of the magnetization reversal in heterophase nanocomposite permanent magnets

We have studied the process of magnetization reversal in a thin-film Fe/Sm₂Co₇ exchange coupled bilayer structure under the action of an in-plane external field. An analysis of the local magnetization changes, as measured using the magnetooptical indicator film technique, showed that the magnetization reversal proceeds by inhomogeneous rotation of the magnetic moments in Fe and SmCo layers, both in plane and in the perpendicular direction. It is established that, because of the exchange interaction between layers, the magnetization reversal along the easy axis in the entire structure is determined primarily by the formation of exchange-induced spin helices and domain walls in the magnetically soft layer, whereas the magnetization reversal at an angle of α with respect to the easy axis plays a significant role in the magnetically hard layer and becomes dominating for α=90°.     

Magnetoresistance of UPt3

We have performed measurements of the temperature dependence of the magnetoresistance up to 9 T in bulk single crystals of UPt₃ with the magnetic field along the b-axis, the easy magnetization axis. We have confirmed previous results for transverse magnetoresistance with the current along the c-axis, and report measurements of the longitudinal magnetoresistance with the current along the b-axis. The presence of a linear term in both cases indicates broken orientational symmetry associated with magnetic order. With the current along the c-axis the linear term appears near 5 K, increasing rapidly with decreasing temperature. For current along the b-axis the linear contribution is negative.