Effects Of Magnetic Materials Research Articles

Thermomagnetic recording of highly Bi-substituted iron garnet film using scanning laser for spatial light modulation

Spatial light modulations (SLM) utilizing the magneto-optical (MO) effect of magnetic materials are expected to offer fast switching and small pixel sizes as small as the wavelength of the light. However, the small MO effect is a major issue. In this paper, we report a thermomagnetic recording of highly bismuth-substituted garnet film, known for large Faraday effects. Y0.5Bi2.5Fe4GaO12 (Bi,Ga:YIG) film with a Faraday rotation of −4.66 degrees was used as an MO medium. A laser scanning thermomagnetic recording system using a Galvanometer mirror was developed and the size and quality of recorded magnetic domains were investigated. The smallest recorded magnetic domain diameter was 0.62 μm with a small standard deviation of 0.09 μm. Line patterns with a width of 1 μm can be recorded in this film. We found that Bi,Ga:YIG films have potential as a material for SLMs with fast switching, submicron pixel size, and large MO effect.

Indirect Exchange Interaction Leads to Large Lattice Contribution to Magnetocaloric Entropy Change.

Materials with a large magnetocaloric response are highly desirable for magnetic cooling applications. It is suggested that a strong spin-lattice coupling tends to generate a large magnetocaloric effect, but no microscopic mechanism has been proposed. In this Letter, we use spin-lattice dynamics simulation to examine the lattice contribution to the magnetocaloric entropy change in bcc iron (Fe) and hcp gadolinium (Gd) with exchange interaction parameters determined from abinitio calculations. We find that indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction in hcp Gd leads to longer-range spin-lattice coupling and more strongly influences the low-frequency long-wavelength phonons. This results in a higher lattice contribution toward the total magnetocaloric entropy change as compared to bcc Fe with short-range direct exchange interactions. Our analysis provides a framework for understanding the magnetocaloric effect in magnetic materials with strong spin-lattice couplings. Our finding suggests that long-range indirect RKKY-type exchange gives rise to a larger lattice contribution to the magnetocaloric entropy change and is, thus, beneficial for magnetocaloric materials.

Surface induced electronic Berry curvature in bulk Berry curvature free materials

In recent years it has become clear that electronic Berry curvature (BC) is a key concept to understand and predict physical properties of crystalline materials. A wealth of interesting Hall-type responses in charge, spin and heat transport are caused by the BC associated to electronic bands inside a solid: anomalous Hall effects in magnetic materials, and various nonlinear Hall and Nernst effects in non-magnetic systems that lack inversion symmetry. However, for the largest class of known materials –non-magnetic ones with inversion symmetry– electronic BC is strictly zero. Here we show that precisely for these bulk BC-free materials, a finite BC can emerge at their surfaces and interfaces. This immediately activates certain surfaces in producing Hall-type transport responses. We demonstrate this by first principles calculations of the BC at bismuth, mercury-telluride (HgTe) and rhodium surfaces of various symmetries, revealing the presence of a surface Berry curvature dipole and associated quantum nonlinear Hall effects at a number of these. This opens up a plethora of materials to explore and harness the physical effects emerging from the electronic Berry curvature associated exclusively to their boundaries.

Basic formulation and first-principles implementation of nonlinear magneto-optical effects

First-principles calculation of nonlinear magneto-optical effects has become an indispensable tool to reveal the geometric and topological nature of electronic states and to understand light-matter interactions. While intriguingly rich physics could emerge in magnetic materials, further methodological developments are required to deal with time-reversal symmetry breaking, due to the degeneracy and gauge problems caused by symmetry and the low-frequency divergence problem in the existing calculation formalism. Here we present a gauge-covariant and low-frequency convergent formalism for the first-principles computation. Remarkably, this formalism generally works for both non-magnetic and magnetic materials with or without band degeneracy. Reliability and capability of our method are demonstrated by studying example materials (i.e., bilayers of MnBi$_2$Te$_4$ and CrI$_3$) and comparing with published results. Moreover, an importance correction term that ensures gauge covariance of degenerate states is derived, whose influence on physical responses is systematically checked. Our method enables computation of nonlinear magneto-optical effects in magnetic materials and paves the way for exploring rich physics created by the interplay of light and magnetism.

A Theory for Anisotropic Magnetoresistance in Materials with Two Vector Order Parameters

Anisotropic magnetoresistance (AMR) and related planar Hall resistance (PHR) are ubiquitous phenomena of magnetic materials. Although the universal angular dependences of AMR and PHR in magnetic polycrystalline materials with one order parameter are well known, no similar universal relation for other class of magnetic materials are known to date. Here a general theory of galvanomagnetic effects in magnetic materials is presented with two vector order parameters, such as magnetic single crystals with a dominated crystalline axis or polycrystalline non-collinear ferrimagnetic materials. It is shown that AMR and PHR have a universal angular dependence. In general, both longitudinal and transverse resistivity are non-reciprocal in the absence of inversion symmetry: Resistivity takes different values when the current is reversed. Different from simple magnetic polycrystalline materials where AMR and PHR have the same magnitude, and π/4 out of phase, the magnitudes of AMR and PHR of materials with two vector order parameters are not the same in general, and the phase difference is not π/4. Instead of π periodicity of the usual AMR and PHR, the periodicities of materials with two order parameters are 2π.

Progress of microscopic thermoelectric effects studied by micro- and nano-thermometric techniques

Heat dissipation is one of the most serious problems in modern integrated electronics with the continuously decreasing devices size. Large portion of the consumed power is inevitably dissipated in the form of waste heat which not only restricts the device energy-efficiency performance itself, but also leads to severe environment problems and energy crisis. Thermoelectric Seebeck effect is a green energy-recycling method, while thermoelectric Peltier effect can be employed for heat management by actively cooling overheated devices, where passive cooling by heat conduction is not sufficiently enough. However, the technological applications of thermoelectricity are limited so far by their very low conversion efficiencies and lack of deep understanding of thermoelectricity in microscopic levels. Probing and managing the thermoelectricity is therefore fundamentally important particularly in nanoscale. In this short review, we will first briefly introduce the microscopic techniques for studying nanoscale thermoelectricity, focusing mainly on scanning thermal microscopy (SThM). SThM is a powerful tool for mapping the lattice heat with nanometer spatial resolution and hence detecting the nanoscale thermal transport and dissipation processes. Then we will review recent experiments utilizing these techniques to investigate thermoelectricity in various nanomaterial systems including both (two-material) heterojunctions and (single-material) homojunctions with tailored Seebeck coefficients, and also spin Seebeck and Peltier effects in magnetic materials. Next, we will provide a perspective on the promising applications of our recently developed Scanning Noise Microscope (SNoiM) for directly probing the non-equilibrium transporting hot charges (instead of lattice heat) in thermoelectric devices. SNoiM together with SThM are expected to be able to provide more complete and comprehensive understanding to the microscopic mechanisms in thermoelectrics. Finally, we make a conclusion and outlook on the future development of microscopic studies in thermoelectrics.

Transverse thermoelectric generation using magnetic materials

The transverse thermoelectric effect refers to the conversion of a temperature gradient into a transverse charge current, or vice versa, which appears in a conductor under a magnetic field or in a magnetic material with spontaneous magnetization. Among such phenomena, the anomalous Nernst effect in magnetic materials has been receiving increasing attention from the viewpoints of fundamental physics and thermoelectric applications owing to the rapid development of spin caloritronics and topological materials science. In this research trend, a conceptually different transverse thermoelectric conversion phenomenon appearing in thermoelectric/magnetic hybrid materials has been demonstrated, enabling the generation of a large transverse thermopower. Here, we review the recent progress in fundamental and applied studies on the transverse thermoelectric generation using magnetic materials. We anticipate that this perspective will further stimulate research activities on the transverse thermoelectric generation and lead to the development of next-generation thermal energy harvesting and heat-flux sensing technologies.

Speed enhancement of magnetic logic-memory device by insulator-to-metal transition

Complementary metal-oxide-semiconductor logic circuits used in conventional computers require frequent communication with external nonvolatile memory, causing the memory wall problem. Recently reported magnetic logic with reconfigurable logic operation and built-in nonvolatile memory can potentially bridge this gap. However, its high-frequency performance is not well studied. Here, we first perform experimental and theoretical investigation on the switching time of magnetic logic-memory devices combining magnetic units and negative differential resistance (NDR) of semiconductors. It is found that the switching time of S-type NDR (transistor circuits) in logic operations is ∼300 ns and determined by the transistor's internal turn-on properties. We then propose a magnetic logic-memory device by coupling the anomalous Hall effect in magnetic materials and the insulator-to-metal transition in VO2. Our device realizes reliable output (output ratio > 1000%), a low work magnetic field (<20 mT), and excellent high-frequency performance (switching time = 1–10 ns).

Experimental Observation of Exchange-Driven Chiral Effects in Curvilinear Magnetism.

The main origin of the chiral symmetry breaking and, thus, for the magnetochiral effects in magnetic materials is associated with an antisymmetric exchange interaction, the intrinsic Dzyaloshinskii-Moriya interaction (DMI). Recently, numerous inspiring theoretical works predict that the bending of a thin film to a curved surface is often sufficient to induce similar chiral effects. However, these originate from the exchange or magnetostatic interactions and can stabilize noncollinear magnetic structures or influence spin-wave propagation. Here, we demonstrate that curvature-induced chiral effects are experimentally observable rather than theoretical abstraction and are present even in conventional soft ferromagnetic materials. We show that, by measuring the depinning field of domain walls in the simplest possible curve, aflat parabolic stripe, the effective exchange-driven DMI interaction constant can be quantified. Remarkably, its value can be as high as the interfacial DMI constant for thin films and can be tuned by the parabola's curvature.

Interpretation of experimental evidence of the topological Hall effect

The topological Hall effect in magnetic materials is considered the ultimate trademark of the skyrmion phase. The phenomenon is identified by distinct non-monotonic features in the Hall effect signal presumed to be the evidence of the topological origin. It is demonstrated here that similar features, unrelated to the skyrmion physics, arise in heterogeneous ferromagnets when components of the material exhibit the extraordinary Hall effect with opposite polarities. Relevance of this mechanism to the published data is discussed.

전동기에 미치는 자성재료의 영향

전동기에 미치는 자성재료의 영향

Enhanced magneto-electric effect in manganite tricolor superlattice with artificially broken symmetry**Project supported by the National Natural Science Foundation of China (Grant No. 61471301).

The magneto-electric effect in magnetic materials has been widely investigated, but obtaining an enhanced magneto-electric effect is challenging. In this study, tricolor superlattices composed of manganese oxides—Pr0.9Ca0.1MnO3, La0.9Sr0.1MnO3, and La0.9Sb0.1MnO3—on (001)-oriented Nb:SrTiO3 substrates with broken space-inversion and time-reversal symmetries are designed. Regarding the electric polarization in the hysteresis loops of the superlattices at different external magnetic fields, both coercive electric field Ec and remnant polarization intensity Pr clearly show strong magnetic-field dependences. At low temperatures (< 120 K), a considerable magneto-electric effect in the well-defined tricolor superlattice is observed that is absent in the single compounds. Both maxima of the magneto-electric coupling coefficients ΔEc and ΔPr appear at 30 K. The magnetic dependence of the dielectric constant further supports the magneto-electric effect. Moreover, a dependence of the magneto-electric effect on the periodicity of the superlattices with various structures is observed, which indicates the importance of interfaces. Our experimental results verify previous theoretical results regarding magneto-electric interactions, thereby paving the way for the design and development of novel magneto-electric devices based on manganite ferromagnets.

Beyond a phenomenological description of magnetostriction

Magnetostriction, the strain induced by a change in magnetization, is a universal effect in magnetic materials. Owing to the difficulty in unraveling its microscopic origin, it has been largely treated phenomenologically. Here, we show how the source of magnetostriction—the underlying magnetoelastic stress—can be separated in the time domain, opening the door for an atomistic understanding. X-ray and electron diffraction are used to separate the sub-picosecond spin and lattice responses of FePt nanoparticles. Following excitation with a 50-fs laser pulse, time-resolved X-ray diffraction demonstrates that magnetic order is lost within the nanoparticles with a time constant of 146 fs. Ultrafast electron diffraction reveals that this demagnetization is followed by an anisotropic, three-dimensional lattice motion. Analysis of the size, speed, and symmetry of the lattice motion, together with ab initio calculations accounting for the stresses due to electrons and phonons, allow us to reveal the magnetoelastic stress generated by demagnetization.

Potential of thermoelectric power generation using anomalous Nernst effect in magnetic materials

This article introduces the concept and advantage of thermoelectric power generation (TEG) using anomalous Nernst effect (ANE). The three-dimensionality of ANE can largely simplify a thermopile structure and realize TEG systems using heat sources with a non-flat surface. The improvement of ZT can be expected because of the orthogonal relationship between thermal and electric conductivities. The calculations of an achievable electric power predicted that an improvement of thermopower by one order of magnitude would open up a usage of practical applications.

Improvement of Magnetic Force Microscope Performance by Tuning the Coating Material of Sensor Tip

Effects of magnetic material, coating thickness, and tip radius on the magnetic force microscope (MFM) spatial resolution and the switching field have been investigated. MFM tips are prepared by coating soft and hard magnetic materials on non-magnetic Si-base tips. MFM spatial resolutions better than 8 nm are obtained by optimizing the coating layer thickness at around 20 nm on the Si tips with top radius of 3 – 5 nm. The switching field can be increased greater than 1 kOe by coating a high-Ku magnetic material and by increasing the coating thickness. The tips are successfully applied to the observations of magnetic thin films which include magnetization structures less than 20 nm in length.

Theory and Simulation of Magnetic Materials: Physics at Phase Frontiers

The combination of theory and simulation is necessary in the investigation of properties of complex systems where each method alone cannot do the task properly. Theory needs simulation to test ideas and to check approximations. Simulation needs theory for modeling and for understanding results coming out from computers. In this review, we give recent examples to illustrate this necessary combination in a few domains of interest such as frustrated spin systems, surface magnetism, spin transport and melting. Frustrated spin systems have been intensively studied for more than 30 years. Surface effects in magnetic materials have been widely investigated also in the last three decades. These fields are closely related to each other and their spectacular development is due to numerous applications. We confine ourselves to theoretical developments and numerical simulations on these subjects with emphasis on spectacular effects occurring at frontiers of different phases.

Effect of Magnetic Materials on the In-Vessel Magnetic Configuration in KSTAR

KSTAR has a nonlinear magnetic material, INCOLOY® alloy 908 (Incoloy), in toroidal field and poloidal field (PF) coil systems. The effect of Incoloy on the magnetic configuration for the plasma initiation was investigated with systematic magnetic field measurements, finite element model (FEM) calculations, and in situ measurements of the magnetic properties. The profile of the vertical field near the field-null center was measured with a vertically movable electron beam (e-beam) probe and Hall sensor arrays in addition to pickup coils in the vacuum vessel. The measured profiles of the additional fields from Incoloy in the PF coils are in good agreement with the FEM calculations. In a typical KSTAR startup configuration, the effect of Incoloy is significant. First, it degrades the connection length significantly due to an additional vertical field in the field-null region, and second, it changes the radial and vertical stabilities by modifying the radial gradient of the vertical field. Initial up-down asymmetry measurements of the vertical fields showed very small static error fields from the PF coils. Calculations suggest that the main sources of the measured downshift of the plasma column are asymmetric eddy currents in the cryostat.

Improvement of Magnetic Force Microscope Resolution and Application to High-Density Recording Media

Magnetic force microscope (MFM) tips are prepared by coating magnetic materials on nonmagnetic Si tips with 4 nm radius. The effects of magnetic material and coating thickness on the MFM resolution and the switching field are investigated. MFM resolutions better than 8 nm have been confirmed with tips coated with soft magnetic or hard magnetic materials with optimized thicknesses. The switching field varies in a wide range 0.1-3.0 kOe depending on the coating material and the coating thickness (10-80 nm). High-resolution MFM tips are applied to the observations of magnetization structures of perpendicular and bit-patterned media samples. Magnetization structures of less than 20 nm in scale are clearly observed.

Fabrication of Magnetic Tips for High-Resolution Magnetic Force Microscopy

Effects of magnetic material, coating thickness, and tip radius on magnetic force microscope (MFM) spatial resolution have been systematically investigated. MFM tips are prepared by using an UHV sputtering system by coating magnetic materials on non-magnetic Si tips employing targets of Ni, Ni-Fe, Co, Fe, Fe-B, and Fe-Pd. MFM spatial resolutions better than 9 nm have been confirmed by employing magnetic tips coated with high magnetic moment materials with optimized thicknesses.

Structure and Magnetic Properties of CoPt, CoPd, FePt, and FePd Alloy Thin Films Formed on MgO(111) Substrates

CoPt, CoPd, FePt, and FePd alloy thin films are deposited on MgO(111) single-crystal substrates by using a radio-frequency magnetron sputtering system. The effects of magnetic material and substrate temperature on ordered phase formation are investigated. CoPt, FePt, and FePd epitaxial films with the close-packed plane parallel to the substrate surface are obtained at substrate temperatures ranging between room temperature (RT) and 600 °C. CoPd films grow epitaxially at temperatures higher than 200 °C, whereas CoPd amorphous films are formed below the temperature. Metastable L11 ordered phase formation is recognized for the CoPt films prepared at temperatures ranging between RT and 400 °C. As the temperature increases from RT to 300 °C, the order degree of L11 phase increases. With further increasing the temperature, the degree declines. The CoPt films formed above 500 °C are with A1 disordered structure. L10 ordered FePt and FePd films with the c-axis 55 ° canted from the perpendicular direction are obtained at 600 °C, whereas A1 disordered FePt and FePd films are formed below the temperature. No ordered phase is formed in CoPd film. The L11 ordered CoPt films show perpendicular magnetic anisotropies. As the order degree of L11 phase increases, perpendicular anisotropy is enhanced. On the contrary, the L10 ordered FePt and FePd films have in-plane anisotropies. The magnetic properties are influenced by the kind of ordered phase and the order degree.