Ferromagnetic Alloys Research Articles

Combining magnetocaloric and elastocaloric effects to achieve a broad refrigeration temperature region in Ni43Mn41Co5Sn11 alloy

Solid state refrigeration is promising to replace the traditional vapor compression refrigeration technology because of the environment friendliness and energy saving. The application of solid state refrigeration requires the refrigerants possess not only favorable caloric effects but also wide refrigeration temperature region. However, the narrow working temperature region is a bottleneck problem for this technology. In this paper, we propose a method by combining the magnetocaloric and elastocaloric effects in one refrigerant to solve this problem. The ferromagnetic shape memory Ni43Mn41Co5Sn11 alloy exhibits large magnetocaloric effect across martensitic transformation and elastocaloric effect above martensitic transformation temperature simultaneously. By combining the successive caloric effects at adjacent temperature region, a broad refrigeration temperature region from 250 to 355 K can be achieved. This work indicates the combination of different caloric effects would be an effective way to widen refrigeration temperature region.

Polarisation-dependent single-pulse ultrafast optical switching of an elementary ferromagnet

The ultimate control of magnetic states of matter at femtosecond (or even faster) timescales defines one of the most pursued paradigm shifts for future information technology. In this context, ultrafast laser pulses developed into extremely valuable stimuli for the all-optical magnetization reversal in ferrimagnetic and ferromagnetic alloys and multilayers, while this remains elusive in elementary ferromagnets. Here we demonstrate that a single laser pulse with sub-picosecond duration can lead to the reversal of the magnetization of bulk nickel, in tandem with the expected demagnetization. As revealed by realistic time-dependent electronic structure simulations, the central mechanism involves ultrafast light-induced torques that act on the magnetization. They are only effective if the laser pulse is circularly polarized on a plane that contains the initial orientation of the magnetization. We map the laser pulse parameter space enabling the magnetization switching and unveil rich intra-atomic orbital-dependent magnetization dynamics featuring transient inter-orbital non-collinear states. Our findings open further perspectives for the efficient implementation of optically-based spintronic devices.

Modeling of Magneto-Mechanical Damping in Ferromagnetic Alloys: a Brief Review

This review is devoted to the analysis of three components of magneto-mechanical damping (MMD), namely, macroeddy, microeddy, and hysteresis components, in ferromagnetic alloys. Theoretical models and equations for all three components of the MMD are discussed. The application of the classical theory of micro- and macroeddy MMD to determine the influence of the frequency of cyclic external mechanical load on the damping ability of the material is shown. A discussion on the role of various factors, such as temperature, strain amplitude, vibration frequency, and external magnetic field, on the magnitude of the MMD is provided.

Superconducting spin-valve effect in heterostructures with ferromagnetic Heusler alloy layers

The transport properties of two types of spin valves are analyzed, in which the Heusler alloy Co2Cr1-xFexAly was used as one of the two ferromagnetic layers in the F1/F2/S structures. The Heusler alloy layer was used: 1) as a weak ferromagnet, in the case of the F2 layer; 2) as a halfmetal, in the case of the F1 layer. In the first case, a large classical effect of the superconducting spin valve Delta Tc was obtained, which was facilitated by a significant triplet contribution to the effect of the superconducting spin valve Delta Tctrip. In the second case, a gigantic effect value Delta Tctrip was found reaching 0.5 K. Keywords: superconductivity, ferromagnetism, thin films, superconducting spin valve.

Structural, electronic and elastic properties of FeCrAs Half-metallic ferromagnetic Half-Heusler Alloy: A First-Principles study

In this work, we employ full potential linearized augmented plane wave approach based on the density functional theory to calculate the structural, electronic, and elastic properties of FeCrAs Half-Heusler compound. For electron exchange, we have considered the most common Generalized Gradient Approximation (PBE-GGA and PBEsol-GGA), as exchange and correlation potential for investigating these properties. The half-metallic ferromagnetic behavior is confirmed by density of states and spin-polarized band structures analysis, which show that the FeCrAs has an indirect band-gap in the spin-down channel and a metallic behaviour in the spin-up channel. Furthermore, the elastic constants (Cij) and the related elastic moduli confirm their stability in the cubic phase and demonstrate their brittle nature. The Debye’s temperature along with compressional, Shear and average elastic wave velocities has also been calculated.[copyright information to be updated in production process]

Electrical resistance of Ni-=SUB=-42+x-=/SUB=-Mn-=SUB=-47-x-=/SUB=-Sn-=SUB=-11-=/SUB=- alloys

The electrical resistance and structure of Ni42+xMn47-xSn11 alloys have been studied; where x=0, 1, 2, 3, 4, At room temperature, Ni42+xMn47-xSn11 alloys are ordered in the cubic L21 Ni2MnSn structure characteristic of Heusler alloys. The phase transition between the cubic austenite phase and martensite with lower symmetry, characteristic of ferromagnetic non-stoichiometric Heusler alloys, is observed at temperatures below room temperature. It was found that the partial replacement of Mn atoms by Ni atoms leads to an increase in the phase transition temperatures and to their shift to room temperature. Keywords: Heusler alloys, electrical resistance, phase transition.

Electrical resistance of Ni-=SUB=-42+x-=/SUB=-Mn-=SUB=-47-x-=/SUB=-Sn-=SUB=-11-=/SUB=- alloys

The electrical resistance and structure of Ni42+xMn47-xSn11 alloys have been studied; where x=0, 1, 2, 3, 4, At room temperature, Ni42+xMn47-xSn11 alloys are ordered in the cubic L21 Ni2MnSn structure characteristic of Heusler alloys. The phase transition between the cubic austenite phase and martensite with lower symmetry, characteristic of ferromagnetic non-stoichiometric Heusler alloys, is observed at temperatures below room temperature. It was found that the partial replacement of Mn atoms by Ni atoms leads to an increase in the phase transition temperatures and to their shift to room temperature. Keywords: Heusler alloys, electrical resistance, phase transition.

Large Anomalous Nernst Angle in Co2MnGa Thin Film

The new trends for anomalous Nernst effect (ANE)-based thermoelectric devices require materials with large ANE values to realize the scalable generation of voltage. Recently, very large ANE values have been observed in single crystals of some novel magnetic materials. However, to allow work to proceed on developing ANE-based devices, these materials need to be produced in thin-film form, and to date, thin films have not achieved the same large ANE values as bulk materials. In this letter, we report a large ANE in a 50 nm thick film of ferromagnetic Heusler alloy Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> MnGa, matching the values achieved in the bulk material. By systematically mapping the thermoelectric transport properties, we extracted an anomalous Nernst angle in the range of 11.5% –14.2% at 300 K.

Analysis of magnetocaloric effect in ferromagnetic shape memory alloy crystals

In the framework of the theory of diffuse martensitic transitions to be based on thermodynamic and kinetic relationships the mechanism of the magnetocaloric (MC) effect in shape memory ferromagnetic alloy crystals is theoretically analysed. The theory adequately describes magnetostructural transformations and MC effect in these crystals at adiabatically applying to them and eliminating from magnetic field. The analysis also includes magneto-anisotropic crystals how to be according to literature data crystals Ni49.5Mn25.4Ga25 alloy. Keywords: magnetocaloric effect, magnetostructural martensite transitions, magneto-anisotropic crystals, isothermal entropy change.

Comparison of Two Approaches for Microstructural Modeling of the Ferromagnetic Alloys Behavior

In this letter, two approaches to describe the behavior of a ferromagnetic alloy in an external magnetic field within the framework of the micromagnetism theory are compared. The first one is based on the minimization of total magnetic energy. Another approach is based on describing the dynamics of the process using the Landau–Lifshitz–Gilbert equation. We have considered the problems of magnetization of a single martensite variant of the Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> MnGa alloy within the framework of the theory of magnetism, in which one of the structural elements is a domain wall of a certain thickness, using bothapproaches. The solutions to these boundary value problems were obtained numerically by the finite-element method using the FeniCS package. The evolution of the magnetic structure was investigated. Magnetization curves were obtained in the external magnetic field directed at different angles to the anisotropy axis.

First-principles investigations of the half-metallic ferromagnetic LaCoTiIn equiatomic quaternary Heusler alloy for spintronics

We have evaluated the structural and mechanical stability, electronic structure, total spin magnetic moment, and Curie temperature of LaCoTiIn Equiatomic Quaternary Heusler Alloy (EQHA) using first-principles studies. The Generalized Gradient Approximation (GGA) and GGA+U schemes have been used as exchange-correlation functional for the above calculations. From the ground state calculation, LaCoTiIn EQHA with a Type-III structure in the ferromagnetic (FM) state is found to be stable. The electronic structure of LaCoTiIn EQHA depicts half-metallic behavior which has metallic overlap in the spin up ([Formula: see text] channel and a semiconductor band gap in the other channel. The spin–orbit coupling of LaCoTiIn has a great influence on the band gap of the material. The computed band gap values for the spin down ([Formula: see text] channel are 0.480 eV and 0.606 eV by using the GGA and GGA+U schemes. The total spin magnetic moment is 1 [Formula: see text], according to the Slater–Pauling rule, [Formula: see text] = ([Formula: see text] - 18) [Formula: see text]. These results obtained can be used as a valuable reference for future research, or they will be used to further motivate the experimental synthesis of the corresponding alloy.

Influence of annealing on the microstructure and magnetic properties of the τ-MnAl alloy deformed by high pressure torsion

Ferromagnetic alloys of the Mn-Al system are promising materials for permanent magnets and details of electric motors. Deformation methods of processing these alloys allow obtaining high magnetic properties, however, they are still not well researched. In the current study the effect of the τ-MnAl alloy structure obtained by severe plastic deformation on the phase composition, structure, and magnetic properties gained during subsequent annealing is investigated. It was shown that a twinned structure is preferable for recrystallization (in terms of recrystallized volume) than a dislocation one. The largest coercive force of 3.7 kOe was reached in the current investigation after deformation by 20 revolutions (ε ≈ 6.83). Subsequent annealing led to a drastic fall in coercive force due to vanishing of defects (dislocations and nanotwins) acting as sites of domain wall pinning.

Acceleration of B2/L21 order-disorder transformation in Ni2MnAl Heusler alloys by in-magnetic-field annealing

Ferromagnetic L21 phase of Ni2MnAl with high order degree is difficult to synthesize because of low L21/B2 order-disorder transformation temperature. It is found that magnetic field accelerated the transformation from the antiferromagnetic disordered B2 to ordered L21 phase. Magnetization increased by in-magnetic-field annealing at 673 K, which indicated the acceleration of the ordering to L21 phase. Magnetic field effects on the B2/L21 order-disorder transformation were discussed based on magnetic-field-induced change of transformation temperature, enhancement of degree of order at annealing temperature, and reduction of critical radius of L21 nuclei. Growth kinetics of L21 phase were evaluated by Johnson-Mehl-Avrami equation, which indicated that 15-T annealing accelerated the growth of L21 phase 5.7 times faster than zero-field annealing. Our results showed that magnetic field can contribute to shortening the annealing time for synthesis of ferromagnetic L21-Ni2MnAl alloy.

Field-dependent nonelectronic contributions to thermal conductivity in a metallic ferromagnet with low Gilbert damping

Heat conduction in metals is typically dominated by electron transport since electrons carry both charge and heat. In magnetic metals magnons, or spin waves, excitations of the magnetic order can be used to transport information. Heat conduction via magnons has been previously shown mostly for insulating magnets with low Gilbert damping and resulting long spin-wave lifetimes where conduction electrons cannot contribute. Here we show that thin films of properly optimized metallic ferromagnetic (FM) alloys show significant nonelectronic contributions to heat conduction, which furthermore depend on the direction of an applied magnetic field. These measurements are enabled by micromachined thermal isolation platforms optimized for thermal conductivity measurements of thin-film systems. Electrical conductivity measurements on exactly the same samples allow application of the Wiedemann-Franz relation, which shows large nonelectronic contributions to thermal conductivity for the cobalt-iron alloy with $25%$ Co. This composition has been shown to have exceptionally low damping for a metallic FM. The thermal conductivity of a 75-nm-thick film of the $25%$ Co alloy changes by more than $20%$ at some temperatures, while a reference sample with $50%$ cobalt that has much higher damping shows no field-direction dependence. Our measurements indicate that applied magnetic fields alter the magnon lifetimes in these films and that these magnons contribute to thermal conductivity in this metallic magnetic alloy with low Gilbert damping.

Core-shell metallic alloy nanopillars-in-dielectric hybrid metamaterials with magneto-plasmonic coupling

Combining plasmonic and magnetic properties, namely magneto-plasmonic coupling, inspires great research interest and the search for magneto-plasmonic nanostructure becomes considerably critical. Here we designed a nanopillar-in-matrix structure with core–shell alloyed nanopillars for both BaTiO3 (BTO)-Au0.5Co0.5 (AuCo) and BTO-Au0.25Cu0.25Co0.25Ni0.25 (AuCuCoNi) hybrid systems, i.e., ferromagnetic alloy cores (e.g., Co or CoNi) with plasmonic shells (e.g., Au or Au/Cu). These core–shell alloy nanopillars are uniformly embedded into a dielectric BTO matrix to form a vertically aligned nanocomposite (VAN) structure. Both hybrid systems present excellent epitaxial quality and interesting multi-functionality, e.g., high magnetic anisotropy, magneto-optical coupling response, tailorable plasmonic resonance wavelength, tunable hyperbolic properties and strong optical anisotropy. These alloyed nanopillars-in-matrix designs provide enormous potential for complex hybrid material designs with multi-functionality and demonstrate strong interface enabled magneto-plasmonic coupling along with plasmonic and magnetic performance.

Magnetoelectric Ring-Type Inductors Tuned by Electric and Magnetic Fields

Devices with tunable inductance (“inductors”) are widely used in control circuits of modern electronics. The typical inductor is an electromagnetic coil with a ferromagnetic core inside. In this letter, a new type of magnetoelectric (ME) inductors with a composite core is manufactured and investigated. The core is made of lead zirconate-titanate piezoceramic and has a ring shape. A layer of amorphous ferromagnetic alloy FeBSiC with high magnetostriction is deposited to the outer or inner surface of the piezoceramic ring. Due to the magnetoelectric effect, the inductance of the devices can be smoothly tuned up to 412% with an electric field applied to the piezoceramic ring and up to 1270% with a magnetic field applied along the axis of the core or in the plane of the core. The features of the electric tuning of inductors containing the core with internal or external ferromagnetic layers are explained.

Flexible Spin Valves: Interlayer Interaction and Deformation Sensitivity

Exchange-coupled spin valves based on ferromagnetic alloys CoFeNi and antiferromagnetic alloy FeMn are obtained on flexible polyimide substrates by magnetron sputtering. The magnetoresistive properties of films and microstrips of spin valves are measured at various degrees of bending deformation of the sample. The behavior of the dependence of the deformation sensitivity of the spin valve on the interaction between the magnetic layers and on the arrangement of the anisotropy axes with respect to the deformation vector is characterized. It is found that the deformation sensitivity decreases with an increase in the interval between the fields of magnetization reversal of the free and fixed layers in the spin valve.

First-principles calculations on the spin anomalous Hall effect of ferromagnetic alloys

The spin anomalous Hall effect (SAHE) in ferromagnetic metals, which can generate a spin-orbit torque to rotate the magnetization of another ferromagnetic layer through a nonmagnetic spacer in magnetic junctions, has attracted much attention. We theoretically investigated the spin anomalous Hall conductivity (SAHC) of the $L{1}_{0}$-type alloys $X\mathrm{Pt}$ ($X=\mathrm{Fe},\mathrm{Co},\mathrm{Ni}$) on the basis of first-principles density functional theory and linear response theory. We found that the SAHC of FePt is much smaller than the anomalous Hall conductivity (AHC), leading to very small polarization for the anomalous Hall effect $\ensuremath{\zeta}=\mathrm{SAHC}/\mathrm{AHC}$ of around 0.1. On the other hand, the SAHC increases with an increasing number of valence electrons (${N}_{\mathrm{v}}$), and CoPt and NiPt show relatively large values of $|\ensuremath{\zeta}|$, greater than 1. The negative contribution of the spin-down-down component of AHC is the origin of the large SAHC and $\ensuremath{\zeta}$ in CoPt and NiPt, which is due to the antibonding states of Pt around the Fermi level in the minority-spin states.

Predictive study of ferromagnetism and antiferromagnetism coexistence in Ba1-xGdxRuO3 induced by Gd-doping

Some ferromagnetic alloys which adopt the perovskite or double-perovskite structure exhibit some remarkable properties, such as electromagnetic effects, charge and orbital ordering, i.e., dielectric and magnetoresistance effects in the same time. These phenomena are related to both electrical conductivity and spin orbit orientation. In order to optimize and explore the structural, magnetic and electronic properties of GdxBa1-xRuO3 alloy, we investigated here the first-principles calculations using the generalized gradient approximation (GGA+U+SO) as implemented in the Wien2K package. The concentration classification of GdxBa1-xRuO3 alloy with (x = 0, 0.125, 0.25, 0.5, 0.875, 1) is given. In this work, we have identified features such transition phases, spin ordered and charge conduction that enable a priori of both crystal structure and magnetic behavior prediction.Our considerable GdxBa1-xRuO3 alloy is a half-metallic in the cubic phase, and, Mott insulator for x=0.875 and semiconductor for x=1 in the orthorhombic phase. The GdxBa1-xRuO3 alloy therefore undergoes a transition between a cubic phase and another orthorhombic at x = 0.5. It is clear that at this point our alloy (Gd0.5Ba0.5RuO3) is at the same time FM and AFM A-type, in another way, we can say that A-AFM and FM configurations coexist in our alloys. In the case of our GdxBa1-xRuO3 alloy, we can see that the total magnetic moment increases linearly with the concentrations "x" since it has passed from 15.99 μB for x = 0 to 39.95 μB for x = 0.5, this is valid in the cubic phase. That is related to a heavily magnetic moment of spin in the Ru atom which increases also linearly with increasing x, while the magnetic moment of Gd decreases slightly. In the orthorhombic phase, its value remains zero regardless of the concentration because we are in an antiferromagnetic (AF) configuration. The collaboration of the 3d-Ru and 2p-O states is suggested to play an important role for the ferromagnetism in the considered alloy. These orbitals were the most regular in the two bands respectively: the conduction band and the valence band in the two phases given here (cubic and orthorhombic). We also note the mixed collaboration of the states 3d-Ba. On the other hand, the contribution of 3d-Gd states was only effective in the band of conduction, at the time when that of the 4f-Gd states was noticed especially in the orthorhombic phase.

Sub-15-fs X-ray pump and X-ray probe experiment for the study of ultrafast magnetization dynamics in ferromagnetic alloys.

In this paper, we present a new setup for the measurement of element-specific ultrafast magnetization dynamics in ferromagnetic thin films with a sub-15-fs time resolution. Our experiment relies on a split and delay approach which allows us to fully exploit the shortest X-rays pulses delivered by X-ray Free Electrons Lasers (close to the attosecond range), in an X-ray pump - X-ray probe geometry. The setup performance is demonstrated by measuring the ultrafast elemental response of Ni and Fe during demagnetization of ferromagnetic Ni and Ni80Fe20 (Permalloy) samples upon resonant excitation at the corresponding absorption edges. The transient demagnetization process is measured in both reflection and transmission geometry using, respectively, the transverse magneto-optical Kerr effect (T-MOKE) and the Faraday effect as probing mechanisms.