Preface: phys. stat. sol. (c) 4/12

Advanced electro-magnetic machines and systems require new materials with improved properties. Heterogeneous 3D nanomodified soft magnetic materials could be efficiently applied. Multistage technology of iron particle surface nanomodification by sequential oxidation and Si-organic coatings will be reported. The thickness of layers is 0.5-5 nm. Compaction and annealing are the final steps of magnetic parts and components shaping. The soft magnetic composite material shows the features: resistivity is controlled by insulating coating thickness and equals up to ρ =10-4 Ω⋅m for metallic state and ρ =104 Ω⋅m for insulator state, maximum magnetic permeability is μm = 2500 and μm = 300 respectively, induction is up to Bm=2.1 T. These properties of composite soft magnetic material allow applying for transformers, throttles, stator-rotor of high-efficient and powerful electric machines in 10 kHz–1MGz frequency range. For microsystems and microcomponents application, good opportunity to improve their reliability is the use of nanocomposite materials. Electroplating technology of nanocomposite magnetic materials into the ultra-thick micromolds will be presented. Co-deposition of the soft magnetic alloys with inert hard nanoparticles allows obtaining materials with magnetic permeability up to μm=104, magnetic induction of Bs=(0.62–1.3) T. Such LIGA-like technology will be applied in MEMS to produce high reliable devices with advanced physical properties.

Chemical surface modification for the preparation of MgO cladding to enhance magnetic properties of Fe-Si-Nb-B-Cu nanocrystalline soft magnetic composites

Electron holography, which visualizes magnetic and/or electric fields in materials on the nanometre scale, is a powerful tool for the study of fundamental issues in physics as well as the characterization of advanced materials. This paper presents an overview of recent electron holography studies on advanced magnetic materials, which include hard magnetic materials (both nucleation-type and pinning-type magnets), soft magnetic materials (both classical alloys and recently developed nanostructured materials), magnetic recording materials (Co–CoO tape and other related topics) and magnetic functional materials (ferromagnetic shape memory alloys and colossal magnetoresistive manganites).

Soft-hard magnetic composites have attracted much attention as magnetic materials for permanent magnets due to high coercivity and saturation magnetization in various fields such as motor, storage device and magnetic actuators. Hard magnetic materials with low saturation magnetization improve magnetic properties through exchange coupling effect with soft magnetic materials. Rare earth metals, which are used as magnetic materials in permanent magnetic, have issued such as high cost due to resource scarcity. Transition metals have been used for fabricating magnetic material with high magnetic properties due to low cost and abundant in the resource. Spinel magnetic materials with transition metals have excellent chemical, thermal stability, large magnetocrystalline anisotropy and magnetic properties. Composite magnetic materials in one dimensional (1D) morphology are effective in improving coercivity. In this work, soft-hard composite fiber has been fabricated by electrospinning method using iron oxide as soft magnetic material and cobalt ferrite as hard magnetic material. The change in the oxidation state of iron oxide can be controlled with sintering atmosphere and influences on the magnetic properties. In comparing with simple CoFe2O4, the fabricated Fe3O4–CoFe2O4 exhibits excellent saturation magnetization of 133.63 emu g−1 and enhanced coercivity of 2746.9 Oe.

The paper presents three new aspects to the design and analysis of electrical machines in general, and the manufacture of small electrical machines which utilize new powder-metallurgical, soft-magnetic-core material in particular. First, a comparison of the Maxwell-stress and Poynting-vector methods of determining power-flow conditions in electrical machines is made. Calculations are based on three-dimensional (3D) field data, which include the full harmonic spectra due to excitation and load conditions, conductor slotting, and winding factors. The method employed to obtain the 3D field data is based on a new computed tomography technique in which the spatial distribution of the fields are accurately obtained in the air gap of the machine. The machines studied in the investigation are characterized by the use of soft magnetic powder-metallurgical core materials used in their construction. A detailed appraisal of the properties of these new powder-metallurgical materials and examples of their use in small electrical drives will be given in the paper. The method of force calculation adopted, together with the use of new soft magnetic materials, has resulted in a significant advance for the manufacture of small electrical machines.

<p indent="0mm">The third-generation semiconductor (also known as the “wide energy gap semiconductor”, WBG) has been widely adopted and applied in strategic emerging industries. As the digital age unfolds, computing ability has become an important indicator of a country’s comprehensive national strength. The physical foundation for high computing power requires AI hardware to enable high energy density, high integration, and miniaturization. Given its key role in circuit energy conversion, the inductor must not only exhibit high energy density but also maintain excellent electrical and magnetic properties, posing significant challenges in minimizing losses during high-frequency operation. Therefore, it is imperative to develop new inductors and soft magnetic materials for high-power density and high-frequency applications. Recently, Fe-based amorphous and nano-crystalline alloys have attracted great attention in scientific research and industry because of their high saturated magnetic flux density and low core loss. Under high-frequency operating conditions, they are expected to bridge the performance gap between traditional silicon steel and soft magnetic ferrite materials in terms of high-power density and low core loss. Given their unique preparation methods, atomic structures, high strength, and hardness, it is necessary to develop specific processing and heat treatment methods tailored to amorphous and nano-crystalline alloys for industrial applications. This paper reviews the progress of Fe-based amorphous/nano-crystalline soft magnetic materials, including their development, preparation, processing, and industrial applications. The challenges and future development trends of amorphous/nano-crystalline soft magnetic powders and powder cores in potential industrial applications are discussed.

Soft magnetic materials have shown promise in diverse applications due to their fast response, remote actuation, and large penetration range for various conditions. Herein, a new soft magnetic composite material capable of reprogramming its magnetization profile without changing intrinsic magnetic properties of embedded magnetic particles or the molecular property of base material is reported. This composite contains magnetic microspheres in an elastomeric matrix, and the magnetic microspheres are composed of ferromagnetic microparticles encapsulated with oligomeric-PEG. By controlling the encapsulating polymer phase transition, the magnetization profiles of the magnetic composite can be rewritten by physically realigning the ferromagnetic particles. Diverse magnetic actuators with reprogrammable magnetization profiles are developed to demonstrate the complete reprogramming of complex magnetization profile.

Abstract

Compositionally graded materials are attractive since the properties can be optimized according to the spatial requirement. We report the use of spark plasma sintering for the processing of compositionally graded magnetic materials. Magnetic, mechanical and electrical properties were screened using high throughput characterization techniques. A significant change in all these properties, e.g., saturation magnetization (Ms) ranging from 57 to 224 emu/g, Curie temperature (Tc) ranging from 358 to 1116 °C, and Vickers hardness ranging from 91 to 195 HV across the length of a single cylindrical sample was observed.Introduction: Elemental Fe, Co, Ni exhibit ferromagnetic behavior, and their alloys such as Fe-Co-2V (hiperco) and Ni-21Fe (permalloy) are well established commercially available soft magnetic materials. Fe-Co-2V exhibits an attractive combination of high saturation magnetization, high Curie temperature, high yield strength, and is very useful for high temperature magnetic applications. On the other hand, Permalloys exhibit low coercivity, high permeability, and moderate saturation magnetizations and are of high interest for transformers, electric motors and other electromagnetic devices. Magnetic components of a power electronic convertors functioning at high frequency can be &gt;50 % of total weight of the system[1]. It has been demonstrated that graded properties of soft magnetic materials are beneficial for next generation power convertors/generators[2, 3]. Therefore, developing magnetic components with spatially optimized properties is of high interest[1].This work demonstrates the use of spark plasma sintering (SPS) for the accelerated preparation of a compositionally graded magnetic materials. Fe/Co/Ni/NiFe/FeCoV have been prepared using gas atomized spherical powders of Fe, Co, Ni, Ni-21Fe and Fe-Co-2V. This graded SPS technique produces a dense bulk metallic sample with discrete changes in composition, resulting in tunable, spatially optimized structural and functional properties.Experiment: Gas atomized prealloyed Fe-Co-2V and Ni-21Fe, and elemental Co, Fe and Ni, from Tosoh SMD Inc. were used as starting powders. The purity of all the powders is &gt;99.9%. The 5 compositions were loaded layer by layer in a graphite die. Sintering of compositionally graded Fe/Co/Ni/NiFe/FeCoV soft magnetic materials was performed using a Spark Plasma Sintering (SPS) equipment (Fuji Electronic Industrial, SPS-211LX) at a vacuum level below 8 Pa. The sample was vertically cut in two sections, one section was annealed at 800 °C for 4 h. The crystal structure of graded samples was determined by X-ray diffraction (XRD) using a Bruker D8 discover diffractometer (CuKa radiation). Elemental mapping was performed by an energy dispersive X-ray spectrometer attached to the SEM. The magnetic properties were measured using a physical property measurement system (PPMS) (EverCool-II, Quantum Design), equipped with a vibrating sample magnetometer. The microhardness of the graded samples was measured by a Vickers hardness tester (Future-Tech).Results and conclusion: Saturation magnetization (Ms), Curie temperature (Tc) and Vickers hardness (HV) of a typical compositionally graded Fe-Co-Ni-NiFe-FeCoV sample are shown in figure 1. Both ends (Fe and FeCoV) of the sample exhibit Ms of &gt;200 emu/g, while the central region, which is Ni, exhibits the minimum Ms of ~57 emu/g. The Curie temperature of the compositionally graded sample measured by employing a permanent magnet placed near the TGA pan. The Co region exhibits the highest Tc, with the Tc following the sequence of Co &gt; FeCoV &gt; Fe &gt; NiFe &gt; Ni. These Ms and Tc values closely matched with the values reported in earlier references. A significant change in mechanical properties across the sample was observed with a maximum Vickers hardness of 195 HV in the FeCoV region. Thus, SPS processed samples with spatially varying compositions were developed, relevant to the urgent need for magnetic components with spatially optimized properties.Acknowledgement: This work is supported by the AME Programmatic Fund by the Agency for Science, Technology and Research, Singapore under Grant No. A1898b0043. **

The Technical Committee of the IEEE Magnetics Society has selected 7 research topics to develop their roadmaps, where major developments should be listed alongside expected timelines; (i) hard disk drives, (ii) magnetic random access memories, (iii) domain-wall devices, (iv) permanent magnets, (v) sensors and actuators, (vi) magnetic materials and (vii) organic devices. Among them, magnetic materials for spintronic devices have been surveyed as the first exercise. In this roadmap exercise, we have targeted magnetic tunnel and spin-valve junctions as spintronic devices. These can be used for example as a cell for a magnetic random access memory and spin-torque oscillator in their vertical form as well as a spin transistor and a spin Hall device in their lateral form. In these devices, the critical role of magnetic materials is to inject spin-polarised electrons efficiently into a non-magnet. We have accordingly identified 2 key properties to be achieved by developing new magnetic materials for future spintronic devices: (1) Half-metallicity at room temperature (RT); (2) Perpendicular anisotropy in nano-scale devices at RT. For the first property, 5 major magnetic materials are selected for their evaluation for future magnetic/spintronic device applications: Heusler alloys, ferrites, rutiles, perovskites and dilute magnetic semiconductors. These alloys have been reported or predicted to be half-metallic ferromagnets at RT. They possess a bandgap at the Fermi level EF only for its minority spins, achieving 100% spin polarisation at EF. We have also evaluated L10-alloys and D022-Mn-alloys for the development of a perpendicularly anisotropic ferromagnet with large spin polarisation. We have listed several key milestones for each material on their functionality improvements, property achievements, device implementations and interdisciplinary applications within 35 years time scale.

Soft-magnetic fluxguide materials

The Faraday effect, as well as the magneto-optical Kerr effect (MOKE), dates back to the 1800s when it was noticed that optical phenomena can be influenced by a magnetic field. Although distinct, the two effects are commonly treated together. Furthermore, given their versatility they have resulted in applications that span over a century while involving a variety of magnetic materials. Magneto-optical recording and MOKE systems are just two examples of such applications, enjoying widespread use at different points in time, and continuing to be currently available in some technological form. As such, MOKE systems are nowadays sophisticated instruments that are prevalently employed in the characterization of thin magnetic films of a certain type. These MOKE systems are based on polarization changes experienced by light beams traveling under a magnetic field. Thus, magnetic–optic interactions allow us to gain some valuable information about, for instance, magnetization orientation or magnetization reversal. For this reason, we need an understanding of the basis of these effects where magnetic and optical phenomena coexist, as well as the type of materials to which such a magnetization direction analysis can be applied. This insight enables us to uncover important contemporary issues in magnetism, such as spin direction in different domains, spin reversal, or the highly desired spin reorientation transition. The latter is a significant effect that occurs in magnetic thin films that have a strong perpendicular magnetic anisotropy, belonging to a class of materials that are today's promising candidates for ultrahigh-density recording media. Nevertheless, before expanding on the subject of today's uses of magneto-optical phenomena, it is equally noteworthy to elaborate on time-honored subjects such as magnetic recording and optical readout by the Faraday–Kerr effect.It is a known fact that magnetic materials with intermediate values for magnetic anisotropies have historically been employed as magnetic recording media. As such, traditional magnetic tapes are made of ferromagnetic solid solutions of \(\gamma -{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}\mbox{ \textendash }{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}\), a substance that usually presents itself in the form of acicular particles with certain magnetic characteristics. Aside from these characteristics being measurable with MOKE systems, this unique acicular shape gives them a uniaxial magnetic anisotropy and consequently a good coercivity which is strongly dependent on the ratio of ferrous to ferric ions. It was noticed that higher coercivity values can be obtained when the ratio \({\mathrm{Fe}}^{2+}/{\mathrm{Fe}}^{3+}\) is greater than 0.04; however these better values are also due to the type of binder in which the particles were dispersed, such as paraffin or epoxy resin. Nevertheless, some of the first magnetic recording materials to which it was possible to apply optical readout by the Faraday–Kerr effect were intermetallic compounds such as MnBi. As such, the Faraday–Kerr effect was initially used to study magnetic domains in MnBi films [1]. MnBi has a hexagonal nickel arsenide crystal structure with a very high magnetic anisotropy constant where the direction of easy magnetization is along the c-axis [2]. Due to the method of preparation, this c-axis was oriented normal to the surface with a spread of only 15∘. This detail was particularly important in early experiments of optical measurement, as the domain magnetization vectors needed to have components along the direction of propagation of light. Combined with the fact that the optical setup was made such that domains of either polarity appeared significantly dark, while those of opposite polarity remained noticeably light, the idea of using this high contrast for magnetic recording and optical readout emerged. In fact, the first reported MO recording was in 1957 [3].Since those early days of magneto-optical recording, the field of magneto-optical applications has expanded, and at the same time shifted toward uses that include in situ vectorial analyses of magnetization measurements in various geometries at different light scattering planes. As such, we are encountering these days a highly developed area of magneto-optical instruments allowing not only precise investigations at different radiation wavelengths, but also high-throughput screening and optimization of magnetic materials with applications beyond magnetic recording.

<sec><p indent="0mm">Magnetic random-access memory (MRAM) has emerged as a leading candidate for next-generation storage technology, offering high speed, high density, non-volatility, low power consumption, and radiation resistance. MRAM has evolved through three generations: field-driven toggle MRAM, spin-transfer torque (STT)-MRAM, and the experimental spin-orbit torque (SOT)-MRAM. The current commercial MRAM products primarily rely on STT-MRAM, which is based on Ta/CoFeB/MgO multilayers with the interfacial perpendicular magnetic anisotropy (PMA). However, STT-MRAM performance deteriorates at sub<sc>-10 nm</sc> size due to thermal instability. In contrast, the tetragonal Mn-based binary alloys exhibit stable bulk PMA and enhanced properties for sub<sc>-10 nm</sc> tunnel junction devices. Previous research has demonstrated the potential of these alloys for high-density MRAM, with magnetization switching achieved via SOT without the need for external magnetic fields. Among Mn-based binary alloys, <italic>L</italic>1₀-MnAl stands out due to its strong PMA, high spin polarization, and low magnetic damping, making it a promising material for next-generation MRAM. On the other side, the <italic>B</italic>2-CoGa alloy serves as both an ideal buffer layer and a spin current source for the epitaxial growth of PMA <italic>L</italic>1₀-MnAl thin films on GaAs substrates. Our study demonstrates that doping <italic>B</italic>2-CoGa with Pt significantly enhances the SOT efficiency in (CoGa)_{1−<italic>x</italic>}Pt<italic>_x</italic>/<italic>L</italic>1₀-MnAl bilayers while preserving the excellent PMA properties of <italic>L</italic>1₀-MnAl. </sec><sec> Experimental results show that Pt doping up to <italic>x</italic> = 0.1 keeps the PMA characteristics while simultaneously reducing the critical current density required for magnetization switching from <sc>4.63×10⁷ A/cm²</sc> to <sc>2.59×10⁷ A/cm².</sc> SOT efficiency increases from 0.042 to 0.080. This enhancement is attributed to the increased resistivity of the (CoGa)_{1−<italic>x</italic>}Pt<italic>_x</italic> layer, which does not compromise its effective spin Hall conductivity. The optimized bilayer, with <italic>x</italic> = 0.075, supports magnetic tunnel junctions with lateral size as small as <sc>8.09 nm,</sc> reducing power consumption by 37%. The SOT effect utilizes spin currents generated by phenomena such as the spin Hall effect to switch local magnetization in adjacent ferromagnetic layers. The damping-like SOT efficiency (<italic>ξ</italic>_DL), a crucial parameter for reducing device power consumption, is influenced by both spin Hall conductivity and resistivity of the material. The introduction of Pt impurities into (CoGa)_{1−<italic>x</italic>}Pt<italic>_x</italic> bilayers optimizes this balance, significantly enhancing <italic>ξ</italic>_DL. Compared to traditional heavy metals used as buffer layers, the (CoGa)_{1−<italic>x</italic>}Pt<italic>_x</italic> layer provides better lattice matching with Mn-based binary alloys, preserving their crystalline quality and PMA. This work systematically investigates the structural, magnetic, and SOT-driven magnetization switching properties of epitaxially grown (CoGa)_{1−<italic>x</italic>}Pt<italic>_x</italic>/<italic>L</italic>1₀-MnAl bilayers, focusing on the dependence of magnetization switching on Pt concentration. Deterministic SOT-induced switching is achieved for <italic>x</italic> ≤ 0.1, with optimal PMA and SOT performance observed at <italic>x</italic> = 0.075. These results demonstrate the feasibility of incorporating Pt-doped CoGa layers into MRAM designs to enhance performance and scalability. This study provides valuable insights into improving SOT efficiency in Mn-based MRAM structures by incorporating Pt-doped CoGa buffer layers. These findings contribute to the development of high-density, high-performance MRAM devices with reduced power consumption. </sec>

A potential approach to extend the current areal density of magnetic recording beyond 300 Gb/in/sup 2/ is physical patterning of the media. Among all the techniques that have been employed to-pattern the continuous media into nanometer scale discrete tracks and bits, the focused ion beam (FIB) nanofabrication is one of the most straightforward techniques that does not require the use of various types of masks and molds. There has been much interest in nano-patterning of hard and soft magnetic materials and perpendicular recording media using the FIB, but very few of them has been focused on the milling processes. In this work, the FIB patterning of commercial post-sputtering longitudinal media is demonstrated. Special efforts have been put on optimizing the milling process through controlling the magnification, milling depth, and the ion beam scan mode. It was found that the parallel milling mode was effective in reducing the re-deposition at small depth and thus led to smaller nano-structures. The FIB processed nanostructures have been characterized using AFM and MFM. Patterns with square islands of 50 nm and spacing of 10 nm have been obtained. It is interesting to note that, in both serial and parallel milling modes at large depth, the re-deposition has accumulated at the junctions of the original dots and formed new dots. As dots formed by re-deposition have already lost their magnetic properties, they should be suppressed as much as possible. In the contrast to this, the re-deposition was almost eliminated in the parallel-milling mode at small depth. It can be seen clearly from the MFM observation that the in-plane magnetization has been pertained for the patterned structures at dimensions as small as 50 nm and single domain structures were observed. The structures obtained in this work can be used to study magnetic recording at contact mode and at very high recording density.

The importance of newly developed permanent magnetic materials in many electromechanical,magnetomechanical and electronic applications is attributed to the drastic improvement inmicrostructure related properties, such as the remanence, the magnetic energy densityproduct and the coercive field. The influence of the microstructure on the magneticproperties of the magnets will be discussed, where special emphasis is laid on rare earthpermanent magnets. Highest performance, anisotropic Nd–Fe–B magnets withJr>1.5 T,(BH)max>450 kJ m−3 and JHc>750 kA m−1, which are produced by the powder metallurgy route, show a strong influence of compositionand processing parameters on the magnetic properties. The magnetic properties ofSm(Co,Cu,Fe,Zr)z sintered magnets, which are used nowadays for high temperature applications between 300 and500 °C,are determined by the cellular precipitation microstructure, which is developed during a complexheat treatment and by the microchemistry. Special hard magnetic powder materials, such asSm2Fe17N3 and nanocrystalline,composite Nd2Fe14B /(α-Fe,Fe3B) materials have been developed especially for usage in bonded magnetic materials, whichshow the strongest annual increase in the production of permanent magnets. Thephenomenon of the enhancement of remanence, occurring in single phase and compositeNd2Fe14B based magnets with isotropic grain alignment, is attributed to intergrain exchangeinteractions.