Bulk Spin Torque-Driven Perpendicular Magnetization Switching in L10 FePt Single Layer

Due to its inherent superior perpendicular magnetocrystalline anisotropy, the FePt in L1₀ phase enables magnetic storage and memory devices with ultrahigh capacity. However, reversing the FePt magnetic state, and therefore encoding information, has proven to be extremely difficult. Here, it is demonstrated that an electric current can exert a large spin torque on an L1₀ FePt magnet, ultimately leading to reversible magnetization switching. The spin torque monotonically increases with increasing FePt thickness, exhibiting a bulk characteristic. Meanwhile, the spin torque effective fields and switching efficiency increase as the FePt approaches higher chemical ordering with stronger spin–orbit coupling. The symmetry breaking that generates spin torque within L1₀ FePt is shown to arise from an inherent structural gradient along the film normal direction. By artificially reversing the structural gradient, an opposite spin torque effect in L1₀ FePt is demonstrated. At last, the role of the disorder gradient in generating a substantial torque in a single ferromagnet is supported by theoretical calculations. These results will push forward the frontier of material systems for generating spin torques and will have a transformative impact on magnetic storage and spin memory devices with simple architecture, ultrahigh density, and readily application.     

Effect of AlN layer thickness on structure and magnetic properties of FePt/AlN multilayers

FePt (50 nm) and [FePt(xnm)/AlN(1, 2, 3 nm)]₁₀ (x=2, 3 nm) films were prepared by RF magnetron sputtering technique, then were annealed at 550 °C for 30 min. This work investigates the effect of AlN layer thickness on structure and magnetic properties of FePt/AlN multilayers. Superlattice (0 0 1) peaks can be found in the grazing incidence X-ray diffraction of FePt and [FePt (3 nm)/AlN (1, 2, 3 nm)]₁₀ films, which indicate that the FCC phase has been partially transformed into ordered L1₀ phase. Compared with the single layer FePt film, superlattice (0 0 1) peaks of FePt/AlN multilayers are weak and wide, which indicates that the introducing of AlN hinders the growth of FePt particle, and also shows the introducing of AlN is not beneficial to the transformation from FCC phase to L1₀ phase. In addition, the low-angle XRD spectra show the layered structure of FePt/AlN has been broken after annealing. The coercivities, particle size, intergrain exchange interactions of FePt/AlN films are decreased with increasing AlN layer thickness.     

Exchange coupled nanocomposite hard magnetic alloys

Abstract

The processing, structures and phase constitutions and the magnetic properties of nanocomposite hard magnetic alloys are reviewed. The emphasis is on rare earth (RE)–iron–boron alloys in which the hard magnetic phase RE₂Fe₁₄B is intermixed with one or more soft magnetic phases. Processing–structure–property relationships are the principal focus, in particular, the role of the hard and soft nanocrystallite dimensions in promoting intergrain ferromagnetic exchange coupling and the consequent enhancement of remanent magnetisation and the technologically important maximum energy density. The powder processing, chill block melt spinning, mechanical alloying and thin film deposition routes to develop nanocrystalline and nanocomposite structures are reviewed. The coercivity mechanism in ultrafine grained alloys and the influence of crystallite dimensions are discussed, as are the effects on intrinsic and extrinsic properties of RE substitutions, replacement of iron by other transition metals and enrichment of the boron content. Exchange enhancements in Sm–Co based nanocomposite bulk alloys and in nanoscale FePt/α-Fe composite thin films are briefly considered, together with thin film materials involving exchange coupling between ferromagnetic and antiferromagnetic phases, in core–shell type structures of transition metal compounds surrounded by oxides and in mechanically alloyed materials. The processing and magnetic properties of bonded magnets based on nanocrystalline/nanocomposite REFeB alloys are discussed. The possibility of producing anisotropic hard/soft composites with properties approaching the theoretical maximum is considered and the extent to which this goal has been realised for fully dense alloys identified.     

Microstructure evolution and mechanical properties of Nb-alloyed Cu–Zr–Al–Ni large-sized metallic glass composites

The effects of Nb on the microstructures and mechanical properties of large-sized (Cu_0.47Zr_0.47Al_0.06)_99???xNi₁Nb_x (x?=?0, 0.5, 1, 2?at.-%) bulk metallic glass composites were investigated. It is verified that the liquidus temperature (T_l) of the Nb-added alloys decreases to cause the increase of glass-forming ability (GFA). The addition of Nb adjusts the distribution and the volume fraction of B2-CuZr phase in the Cu–Zr–Al–Ni large-sized composites by changing the GFA of the alloys. The mechanical properties of the composites strongly depend on the volume fraction and distribution of B2-CuZr phase in the glassy matrix. The alloy with 0.5?at.-% Nb addition exhibits the high mechanical properties, which should be attributed to the uniform distribution and the proper volume fraction of B2-CuZr phase in the glassy matrix.     

Microstructure and Mechanical Properties of Al25 − xCr25 + 0.5xFe25Ni25 + 0.5x (x = 19, 17, 15 at%) Multi‐Component Alloys

A series of Al_{25 ? x}Cr_{25 + 0.5x}Fe₂₅Ni_{25 + 0.5x} (x = 19, 17, 15 at%) multi‐component alloys are prepared by arc‐melting and rapid solidification of copper molds. The technique of thermal‐mechanical processing is further applied to the master alloys to improve their mechanical properties. These alloys consist of face‐centered cubic (FCC) and body‐centered cubic (BCC) structure. The volume fraction of the BCC phase increases as Al content increase and Cr and Ni contents decrease, accompanied with a microstructural evolution from dendritic structure to lamella‐like structure. Due to the increase of volume fraction of BCC phase, the master alloys exhibit an increased strength and a declined ductility as Al content increases. The rapid solidified alloys have more BCC phase compared with the master alloys, which enhances the strength and decreases the ductility. After homogenization, hot‐rolling, and annealing at 1000 °C, the Al₈Cr_33.5Fe₂₅Ni_33.5 alloy displays excellent combination of strength (yield strength is ～635 MPa and fracture strength is ～1155 MPa) and ductility (tension strain is ～11%).

     

High temperature magnetic properties of mechanically alloyed Fe–Zr powder

We report the preparation and characterization of amorphous/non-equilibrium solid solution Fe_100 − xZr_x (x = 20–35) alloys by mechanical alloying process. The microstructure and magnetic properties of milled powders have been studied as a function of Zr substitution. The effective magnetic moment of as-milled powders decreases as concentration of Zr is increased. Thermomagnetization measurements confirmed that the Fe₈₀Zr₂₀ sample exhibits two clear magnetic phase transitions due to the co-existence of an amorphous phase and a Fe rich non-equilibrium solid solution. All the other samples exhibiting an amorphous structure showed a single magnetic phase transition with Curie temperature of ~ 570 °C,which did not vary much with different composition. The Curie temperature of the mechanically alloyed powders is noticeably higher than those of melt-spun amorphous ribbons.     

Effect of substitution of In3+ ions on the electrical and magnetic properties and mössbauer study of Mg0.9Mn0.1In x Fe2−x O4 ferrites

The effect of substitution of diamagnetic In³⁺ ions on the electrical and magnetic properties of Mg-Mn ferrites was studied in the ferrite series Mg_0.9Mn_0.1 In _x Fe_2–x O₄ wherex varied from 0–0.8 in steps of 0.1. The incorporation of In³⁺ ions in place of Fe³⁺ ions resulted in an increase of lattice parameter owing to the larger size of the substituted ions, and an increase of d.c. resistivity owing to reduction of Verwey's hopping mechanism. It also resulted in improvement of saturation magnetization and produced a marked increase in the value of initial permeability, thus upgrading the bulk magnetic properties of these ferrites. These bulk magnetic properties improve due to substitution of diamagnetic In³⁺ ions,x, up to 0.5 only, whereas they deteriorate for a higher content of In³⁺ ions. The variations of saturation magnetization have been explained on the basis of modified cation distribution and their magnetic interactions. A large increase in the value of initial permeability has been attributed to its dependence onM _s and magnetocrystalline anisotropy constant. A comparison of bulk magnetic properties with the inferences drawn from Mössbauer studies of these samples shows a similar trend. It is concluded that the substitution of In³⁺ ions,x, up to 0.5 in Mg-Mn ferrites results in the production of a hyperfine field at A as well as B sites, followed by ferromagnetic relaxation and paramagnetic transition for higher concentrations of In³⁺ ions. These variations have been explained on the basis of the effect produced by In³⁺ ions on the magnetic interactions, supertransferred hyperfine fields and domain-wall oscillations.     

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.     

Potential 2D Materials with Phase Transitions: Structure,Synthesis, and Device Applications

Layered materials with phase transitions, such as charge density wave (CDW) and magnetic and dipole ordering, have potential to be exfoliated into monolayers and few‐layers and then become a large and important subfamily of two‐dimensional (2D) materials. Benefitting from enriched physical properties from the collective interactions, long‐range ordering, and related phase transitions, as well as the atomic thickness yet having nondangling bonds on the surface, 2D phase‐transition materials have vast potential for use in new‐concept and functional devices. Here, potential 2D phase‐transition materials with CDWs and magnetic and dipole ordering, including transition metal dichalcogenides, transition metal halides, metal thio/selenophosphates, chromium silicon/germanium tellurides, and more, are introduced. The structures and experimental phase‐transition properties are summarized for the bulk materials and some of the obtained monolayers. In addition, recent experimental progress on the synthesis and measurement of monolayers, such as 1T‐TaS₂, CrI₃, and Cr₂Ge₂Te₆, is reviewed.     

Magnetic properties of the Fe–Nd–Al alloys prepared by suction casting

The magnetic properties of the Fe _x Nd_{90 − x} Al₁₀ (x = 30, 40, 43, 50, 53) alloys prepared by suction casting were investigated. The intrinsic coercivity (_iH_c) decreases significantly with the substitution of Fe for Nd. The Fe₅₃Nd₃₇Al₁₀ (x = 53) alloy shows soft magnetic behavior, which contains Fe-rich (Fe₇₅Nd₁₈Al₇) crystalline phase and Nd-rich (Fe₂₈Nd₅₉Al₁₃) amorphous phase. The Nd-rich amorphous phase in the Fe₅₃Nd₃₇Al₁₀ alloy exhibits soft magnetic behavior. However, the Nd-rich bulk amorphous alloy of the same composition prepared by suction casting shows hard magnetic behavior. The interesting phenomenon is discussed.     

Magnetic behaviour and microstructural properties of amorphous and crystallized Y3−x GdxFe5O12 prepared by the amorphous citrate process (x=0, 0.5 and 3)

The systems Y_3–x GdxFe₅O₁₂ withx=0, 0.5 and 3 have been synthesized by the amorphous citrate process. Their structural and magnetic properties have been studied for various heat-treatment schedules. The systems withx=0, 0.5 and 3 are amorphous when heat treated up to 650, 600 and 500° C, respectively, and the results of amorphous, as well as crystallized, samples are discussed. The volume fraction of the garnet phase is found to increase with higher heat-treatment temperature. Besides the garnet phase, two other phases appear to grow, the volume fraction of the spinel phase increasing with increase in gadolinium concentration. Microstructral properties have been further correlated with the magnetic properties.     

Microstructures and mechanical properties of as‐cast titanium–zirconium–molybdenum ternary alloys

In this study titanium–zirconium–molybdenum alloys (Ti₅₀Zr₅₀)_100‐xMo_x (xMo; x = 0 at.%, 1 at.%, 3 at.%, 5 at.% or 7 at.%) were investigated, focusing on the effect of molybdenum addition on their microstructures and mechanical properties. Transmission electron microscopy observations revealed that the binary Ti₅₀Zr₅₀ alloy was composed entirely of an acicular hexagonal structure of the α’ phase. When the molybdenum content was 1 at.%, the alloy was composed of β and ω phases. However, when 3 at.% or more molybdenum was added, only the equiaxed, retained β phase was observed. Tensile tests at room temperature indicated that the mechanical properties of the 1Mo alloy were inferior owing to the embrittlement effects of the ω phase and the difficulty of dislocation motion through the ω phase. Our research suggested that the 5Mo alloy had excellent ductility (16.5 %) as well as adequate strength (780 MPa). The improved mechanical properties were attributed to the enhanced stability of the β phase and the disappearance of the ω phase.     

Effect of annealing in external magnetic field on the microstructure and magnetic properties of FePt films

We have studied the influence of annealing in an external magnetic field on the microstructure and magnetic properties of a multilayer Si/Fe(2 nm)/Fe₅₀Pt₅₀(20 nm)/Pt(2 nm) structure synthesized by means of sequential RF magnetron sputtering of the components. The magnetic field was oriented perpendicular to layers of the structure. It is established that annealing in the external magnetic field leads to the formation of predominant (001) texture in the multilayer structure with L1₀-FePt phase. Thus, a method of obtaining multilayer structures based on FePt films required for the perpendicular magnetic recording has been developed.     

Correlation between microstructure and magnetic properties in mechanically alloyed nanogranular Fe100 − xAlx

Structural and magnetic properties of Fe_{100 − x}Al_x powders, mechanically alloyed for a fixed time of 10 h, is presented. Although Mössbauer studies show that the magnetic phase is a minimum, bulk magnetic properties like area of the M–H loop and coercivity are maximum in the Al-rich x = 66 (Fe–66 at.% Al) system. Quadrupole splitting and isomer shift of the non-magnetic phase in this system are identical to that of stoichiometric FeAl₂. The unexpected trend of the bulk magnetic parameters in the x = 66 composition is explained in terms of the enhanced intergranular exchange coupling, mediated by the Fe atoms in the non-magnetic phase.     

Composition and phase structure dependence of mechanical and magnetic properties for AlCoCuFeNix high entropy alloys

In this study, the effects of composition and phase constitution on the mechanical properties and magnetic performance of AlCoCuFeNi_x (x = 0.5, 0.8, 1.0, 1.5, 2.0, 3.0 in molar ratio) high entropy alloys (HEAs) were investigated. The results show that Ni element could lead to the evolution from face centered cubic (FCC), body centered cubic (BCC) and ordered BCC coexisting phase structure to a single FCC phase. The change of phase constitution enhances the plasticity but reduces the hardness and strength. One of the interesting points is the excellent soft magnetic properties of AlCoCuFeNi_x HEAs. Soft magnetic performance is dependent on composition and phase transition. AlCoCuFeNi_1.5 alloy, achieving a better balance of mechanical and magnetic properties, could be applied as structure materials and soft magnetic materials (SMMs). High Curie temperature (>900 K) and strong phase stability below 1350 K of AlCoCuFeNi_0.5 alloy confirm its practicability in a high-temperature environment. Atomic size difference (δ) is utilized as the critical parameter to explain the lattice strain and phase transformation induced by Ni addition.     

Nano-structure formation of Fe-Pt perpendicular magnetic recording media co-deposited with MgO, Al2O3 and SiO2 additives

Perpendicular magnetic recording media samples were prepared by sputter deposition on sapphire with a layer sequence of MgO seed-layer/Cr under-layer/FeSi soft magnetic under-layer/MgO intermediate layer/FePt-oxide recording layer. The effects of MgO, Al₂O₃ and SiO₂ additives on the morphology and orientation of the FePt layer were investigated by transmission electron microscopy. The samples exhibited (001) orientation of the L1₀ FePt phase with the mutual orientations of sapphire substrate//MgO(100)[001]//Cr(100)[11¯0]//FeSi(100)[11¯0]//MgO(100)[001]//FePt(001)[100]. The morphology of the FePt films varied due to the co-deposited oxides: The FePt layers were continuous and segmented by stacking faults aligned at 54° to the surface. Films with SiO₂ addition, beside the oriented columnar FePt grains, exhibited a fraction of misoriented crystallites due to random repeated nucleation. Al₂O₃ addition resulted in a layered structure, i.e. an initial continuous epitaxial FePt layer covered by a secondary layer of FePt-Al₂O₃ composite. Both components (FePt and MgO) of the MgO-added samples were grown epitaxially on the MgO intermediate layer, so that a nano-composite of intercalated (001) FePt and (001) MgO was formed.The revealed microstructures and formation mechanisms may facilitate the improvement of the structural and magnetic properties of the FePt-oxide composite perpendicular magnetic recording media.     

Granular L10 FePt nanocomposite films fabricated by rapid thermal annealing

Granular (FePt)_100−x-(NiO)_x nanocomposite thin films with x = 0 − 42 vol% were fabricated on a natural-oxidized Si(100) substrate. It is found that both the coercivity and FePt domain size decrease with increasing NiO content for the (FePt)_100−x-(NiO)_x films. When the FePt-NiO composite film with NiO content of 10.4 vol% is post-annealed at 750 °C with a high heating ramp rate of 100 °C/s, the in-plane coercivity (H_c//) and perpendicular coercivity (H_c⊥) of the FePt films are 513 and 430 kA/m, respectively. On the other hand, we used conductive atomic force microscope (CAFM) to confirm that the NiO compound is distributed at boundary of FePt particles that will constrain the domain size of FePt and decrease the exchange coupling interactions between FePt magnetic particles.     

Onset of hard magnetic L10 FePt phase with (001) texture

The perpendicular anisotropic magnetic properties of in-situ deposited FePt/Pt/Cr trilayer films were elucidated as functions of the deposition temperature and the sputtering rate of the FePt magnetic layer. Ordered L1₀ FePt thin films with perpendicular anisotropy and a (001) texture can be developed at a temperature as low as 300 °C with the sputtering of a FePt layer at a low rate. The larger Pt(001)[100] lattice induced an expansion of the FePt a- and b-axis, leading to the contraction of the FePt c-axis, enabling the epitaxial growth of the L1₀ FePt(001) texture to occur. A low rate of sputtering of the FePt thin film promotes the formation of the magnetically hard FePt(001) texture on the surface of the Pt(001) buffer layer at low temperature, while the high sputtering rate of FePt layer suppresses the phase transformation.     

Low temperature ordering of FePt films by in-situ heating deposition plus post deposition annealing

FePt thin films with 40 nm thickness were prepared on thermally oxidized Si (001) substrates by dc magnetron sputtering at the nominal growth temperature 375 °C. The effects of annealing on microstructure and magnetic properties of FePt thin films were investigated. The as-deposited FePt thin films show soft magnetic properties. After the as-deposited FePt thin films were annealed at various temperatures and furnace cooled, it is found that the ordering temperature of L1₀ FePt phase could be reduced to 350 °C. For FePt thin films annealed at 350 °C, the in-plane and out-of-plane coercivities of the films increased to 510 and 543 kA/m, respectively, and the films had hard magnetic properties. A highly (001) orientation was obtained, when FePt thin films were annealed at 600 °C. And the hysteresis loops of FePt thin films annealed at 600 °C show out-of-plane magnetic anisotropy.     

Magnetic Properties of CuTi<Subscript>2−2<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript>2<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>4</Subscript> Materials

The magnetic properties of semi-conductor CuTi_2−2x Cr_2x S₄ systems have been studied. The nearest neighbour J ₁(x) and the next-neighbour super-exchange J ₂(x) interactions are evaluated by using the mean field theory for this spinel CuTi_2−2x Cr_2x S₄ system. The magnetic energy, the intraplanar and the interplanar interactions are deduced. The high-temperature series expansion (HTSE) combined with the Padé approximants method (PA) is applied to this spinel system to determinate the magnetic phase diagrams, i.e. T _C versus dilution x. The critical exponents associated with the magnetic susceptibility (γ) and with the correlation length (ν) are deduced in order phase.