We report a correlation between the spin polarization of the tunneling electrons (TSP) and the magnetic moment of amorphous CoFeB alloys. Such a correlation is surprising since the TSP involves s-like electrons close to the Fermi level (EF), while the magnetic moment mainly arises due to all d-electrons below EF. We show that probing the s and d-bands individually provides clear and crucial evidence for such a correlation to exist through s-d hybridization, and demonstrate the tuneability of the electronic and magnetic properties of CoFeB alloys.
We investigate the effect of Al doping on the magnetic properties of the alloy CoFeB. Comparative measurements of the saturation magnetization, the Gilbert damping parameter $α$ and the exchange constant as a function of the annealing temperature for CoFeB and CoFeAlB thin films are presented. Our results reveal a strong reduction of the magnetization for CoFeAlB in comparison to CoFeB. If the prepared CoFeAlB films are amorphous, the damping parameter $α$ is unaffected by the Al doping in comparison to the CoFeB alloy. In contrast, in the case of a crystalline CoFeAlB film, $α$ is found to be…
Spin-orbit torque manifested as an accumulated spin-polarized moment at nonmagnetic normal metal, and ferromagnet interfaces is a promising magnetization switching mechanism for spintronic devices. To fully exploit this in practice, materials with a high spin Hall angle, i.e., a charge-to-spin conversion efficiency, are indispensable. To date, very few approaches have been made to devise new nonmagnetic metal alloys. Moreover, new materials need to be compatible with semiconductor processing. Here we introduce W-Ta and W-V alloys and deploy them at the interface between $β$-W/CoFeB layers. Fir…
The temperature dependence of saturation and spin-flop fields for artificial ferrimagnets (AFi) based on antiparallel coupled CoFeB/Ru/CoFeB trilayers has been investigated in a temperature range between 80K and 600K. The results presented in this paper are relevant for magnetic devices using this system, e.g. magnetic-random access memory based on spin-flop switching. In good accordance to the theory, the saturation field Hsat behaves like Hsat ~ H_0 (T/T_0)/sinh(T/T_0) with a characteristic temperature of T_0 = 150K. Within this model, the Fermi velocity for the Ru layer is of the order of 1…
Voltage control of magnetism and spintronics have been highly desirable, but rarely realized. In this work, we show voltage-controlled spin-orbit torque (SOT) switching in W/CoFeB/MgO films with perpendicular magnetic anisotropy (PMA) with voltage administered through SrTiO3 with a high dielectric constant. We show that a DC voltage can significantly lower PMA by 45%, reduce switching current by 23%, and increase the damping-like torque as revealed by the first and second-harmonic measurements. These are characteristics that are prerequisites for voltage-controlled and voltage-select SOT switc…
We here study magnetic properties of Ni$_{x}$Fe$_{1-x}$ using Augmented space recursion technique coupled with tight-binding linearized muffin tin orbital method. Also the spectral properties of this alloy has been studied here.
Using the fully-relativistic screened Korringa-Kohn-Rostoker method I study the orbital magnetism in the half-metallic Heusler alloys. Orbital moments are almost completely quenched and they are negligible with respect to the spin moments. The change in the atomic-resolved orbital moments can be easily explained in terms of the spin-orbit strength and hybridization effects. Finally I discuss the orbital and spin moments derived from X-ray magnetic circular dichroism experiments.
Extended X-ray Absorption Fine Structure (EXAFS) studies on Ni$_{50}$Mn$_{25+x}$In$_{25-x}$ have been carried out at Ni and Mn K edge as a function of temperature. Thermal evolution of nearest neighbor Ni-Mn and Mn-Mn bond distances in the martensitic phase give a clear evidence of a close relation between structural and magnetic degrees of freedom in these alloys. In particular, the study highlights the role of Ni 3d - Mn 3d hybridization in the magnetism of martensitic phase of these alloys.
We present first principles charge- and spin-selfconsistent electronic structure computations on the Heusler-type disordered alloys Fe$_{3-x}$V$_{x}$X for three different metalloids X=(Si, Ga and Al). In these calculations we use the methodology based on the Korringa-Kohn- Rostoker formalism and the coherent-potential approximation (KKR-CPA), generalized to treat disorder in multi-component complex alloys.
We report a study on the electronic and magnetic properties of a novel Rb$_{x}$Sr$_{1-x}$C alloy. We find that the variation of the lattice parameter as function of Rb composition exhibits a deviation from the Vegard's law of about 0.4 Å. The total energy results predict a ferromagnetic ordering for Sr-rich alloy, and antiferromagnetic ordering for RbC-rich alloy with a total magnetic moment ranging from 2 to 3$μ_\text{B}$ per cation. As for the parent compounds, RbC and SrC, the origin of magnetism arises from the polarization of the carbon $p$-orbitals. From the spin-polarized calculations, …
Magnetism is a key driving force controlling several thermodynamic and kinetic properties of Fe-Cr systems. We present a newly-developed TB model for Fe-Cr, where magnetism is treated beyond the usual collinear approcimation. A major advantage of this model consists in a rather simple fitting procedure. In particular, no specific properties of the binary system is explicitly required in the fitting database. The present model is proved to be accurate and highly transfer-able for electronic, magnetic and energetic properties of a large variety of structural and chemical environments: surfaces, …
We report on a systematic comparative study of the spin Hall efficiency between highly face-centered cubic (fcc)-textured Pt-Al alloy films grown on MgO(001) and poorly-crystallized Pt-Al alloy films grown on SiO$_2$. Using CoFeB as the detector, we show that for Al compositions centering around $x = 25$, mainly L1$_{2}$ ordered Pt$_{100-x}$Al$_x$ alloy films grown on MgO exhibit outstanding charge-spin conversion efficiency. For Pt$_{78}$Al$_{22}$/CoFeB bilayer on MgO, we obtain damping-like spin Hall efficiency as high as $ξ_\textrm{DL} \sim +0.20$ and expect up to seven-fold reduction of po…
The structure, magnetism, magnetic compensation behavior, exchange interaction and electronic structures of Co50-xMn25Ga25+x and Co50-xMn25+xGa25 (x=0-25) alloys have been systematically investigated by both experiments and first-principles calculations. We found that all the samples exhibited body centered cubic structures with a high degree of atomic ordering. With increasing Ga content, the composition dependence of lattice parameters shows a kink point at the middle composition in Co50-xMn25Ga25+x alloys, which can be attributed to the enhanced covalent effect between the Ga and the transi…
Exchange coupling between two magnetic layers through an interlayer is of broad interest for numerous recent applications of nano-magnetic systems. In this Letter we study ferromagnetic exchange coupling through amorphous paramagnetic Fe-Ta alloys. We show that the exchange coupling depends exponentially on spacer thickness and scales with the Fe-Ta susceptibility, which can be tuned via the alloy composition and/or temperature. Such materials are of high interest for the engineering of perpendicularly magnetized CoFeB-MgO based tunnel junctions as it enables ferromagnetic coupling of magnetic…
We here study electronic structure and magnetic properties of bcc Co$_{x}$Fe$_{1-x}$, Cr$_{x}$Fe$_{1-x}$ and Mn$_{x}$Fe$_{1-x}$ in their ferromagnetic phases using Augmented Space Recursion technique coupled with tight-binding linearized muffin tin orbital (TB-LMTO) method. We make a systematic study of magnetic properties like magnetic moment and Curie temperature and try to draw some conclusions on their variation across the row of 3-{\it d} transition metal series in periodic table using arguments based on effects like charge transfer, exchange splitting and hybridization.
We report magnetization and differential thermal analysis measurements as a function of pressure accross the martensitic transition in magnetically superelastic Ni-Mn-In alloys. It is found that the properties of the martensitic transformation are significantly affected by the application of pressure. All transition temperatures shift to higher values with increasing pressure. The largest rate of temperature shift with pressure has been found for Ni$_{50}$Mn$_{34}$In$_{16}$ as a consequence of its small entropy change at the transition. Such a strong pressure dependence of the transition tempe…
The finite-temperature magnetic properties of Fe$_x$Pd$_{1-x}$ and Co$_x$Pt$_{1-x}$ alloys have been investigated. It is shown that the temperature-dependent magnetic behaviour of alloys, composed of originally magnetic and non-magnetic elements, cannot be described properly unless the coupling between magnetic moments at magnetic atoms (Fe,Co) mediated through the interactions with induced magnetic moments of non-magnetic atoms (Pd,Pt) is included. A scheme for the calculation of the Curie temperature ($T_C$) for this type of systems is presented which is based on the extended Heisenberg Hami…
CoPt or FePt equiatomic alloys order according to the tetragonal L10 structure which favors their strong magnetic anisotropy. Conversely magnetism can influence chemical ordering. We present here {\it ab initio} calculations of the stability of the L10 and L12 structures of Co-Pt alloys in their paramagnetic and ferromagnetic states. They show that magnetism strongly reinforces the ordering tendencies in this system. A simple tight-binding analysis allows us to account for this behavior in terms of some pertinent parameters.
We present a study of the dynamic magnetic properties of TiN-buffered epitaxial thin films of the Heusler alloy Fe$_{1.5}$CoGe. Thickness series annealed at different temperatures are prepared and the magnetic damping is measured, a lowest value of $α=2.18\times 10^{-3}$ is obtained. The perpendicular magnetic anisotropy properties in Fe$_{1.5}$CoGe/MgO are also characterized. The evolution of the interfacial perpendicular anisotropy constant $K^{\perp}_{\rm S}$ with the annealing temperature is shown and compared with the widely used CoFeB/MgO interface. A large volume contribution to the per…
We present a combined theoretical and experimental study of two quaternary Heusler alloys CoFeCrGe (CFCG) and CoMnCrAl (CMCA), promising candidates for spintronics applications. Magnetization measurement shows the saturation magnetization and transition temperature to be $3\; μ_B$, $866$ K and $0.9 \; μ_B$, $358$ K for CFCG and CMCA respectively. The magnetization values agree fairly well with our theoretical results and also obey the Slater-Pauling rule, a prerequisite for half metallicity. A striking difference between the two systems is their structure; CFCG crystallizes in fully ordered Y-…
Spintronics is the fast growing field that will play a key role in optimizing power consumption, memory, and processing capabilities of nanoelectronic devices. Heusler alloys are potential candidates for application in spintronics due to their room temperature (RT) half-metallicity, high Curie temperature, low lattice mismatch with most substrates, and strong control on electronic density of states at Fermi level. In this work, we investigate the effect of {substitutional doping and disorder} on the half-metallicity, phase stability, and magnetism of Heusler alloys using density functional the…
A recent experimental study (Phys. Rev. Appl. 9, 024023, 2018) on paramagnetic CeCo$_3$ finds that Magnesium alloying induces a ferromagnetic transition with intrinsic properties large enough for permanent magnet applications. Here we explain these surprising results \textit{via} a first principles study of the electronic structure and magnetism of Magnesium-alloyed CeCo$_3$. We find the origin of this Magnesium-induced ferromagnetic transition to be Stoner physics - the substantial increase in the Fermi-level density-of-states $N(E_F)$ with Mg alloying. Our calculations suggest that both Ce a…
A low-temperature magnetism was revealed in a series of sigma-Fe(100-x)Mo(x) alloys (x=45-53). Its characterization has been done using vibrating sample magnetometry, Mössbauer spectroscopy, and ac magnetic susceptibility. The magnetic ordering temperature was determined to lie in the range of 46 K for x=45 and 22K for x=53, and the ground magnetic state was found to be typical of a spin-glass.
The dynamics of magnetic moments in iron-chromium alloys with different levels of Cr clustering show unusual features resulting from the fact that even in a perfect body-centred cubic structure, magnetic moments experience geometric magnetic frustration resembling that of a spin glass. Due to the long range exchange coupling and configuration randomness, magnetic moments of Cr solutes remain non-collinear at all temperatures. To characterise magnetic properties of Fe-Cr alloys, we explore the temperature dependence of magnetisation, susceptibility, Curie temperature and spin-spin correlations …
Anomalies in the temperature dependences of the recoil-free factor, f, and the average center shift, <CS>, measured by 57-Fe Mossbauer Spectroscopy, were observed for the first time in the archetype of the σ-phase alloys system, Fe-Cr. In both cases the anomaly started at the temperature close to the magnetic ordering temperature, and in both cases it was indicative of lattice vibrations hardening. As no magnetostrictive effects were found, the anomalies seem to be entirely due to a spin-phonon coupling. The observed changes in f and in <CS> were expressed in terms of the underlying changes in…
In 1965 Duwez \textit{et al.} reported having generated an amorphous, stable phase of palladium-silicon in the region 15 to 23 atomic percent (at. \%) silicon. These pioneering efforts have led to the development of solid materials that are now known as Bulk Metallic Glasses (BMG). In 2019 we discovered, computationally, that bulk amorphous Pd becomes magnetic, and so does porous/amorphous Pd. Puzzled by our results we undertook the study of several solid binary systems in the Pd-rich zone; in particular, the study of the glassy metallic alloy $a$-Pd$_{100-c}$Si$_{c}$, for $0 \leq c \leq 22$, …
In a very recent x-ray magnetic circular dichroism experiment concerning with Ru and Rh impurities and metal films on Ag(001) substrate, no local magnetic momen ts were displayed in direct contradiction with previous theoretical works. It is thought that there can be three main reasons for this inconsistency: relaxation, alloying and many-body effects. Some of the above-mentioned systems are studie d by using a first-principles method in which relaxation and alloying are taken into account, even so magnetism is still obtained. For low-coverage systems, hig h magnetic moments in both Ru ($\sim$…
We provide compelling evidence to establish that, contrary to one's elementary guess, the tunneling spin polarization (TSP) of amorphous CoFeB is larger than that of highly textured fcc CoFeB. First principles atomic and electronic structure calculations reveal striking agreement between the measured TSP and the predicted s-electron spin polarization. Given the disordered structure of the ternary alloy, not only do these results strongly endorse our communal understanding of tunneling through AlOx, but they also portray the key concepts that demand primary consideration in such complex systems…
Combining material informatics and high-throughput electronic structure calculations offers the possibility of a rapid characterization of complex magnetic materials. Here we demonstrate that datasets of electronic properties calculated at the ab initio level can be effectively used to identify and understand physical trends in magnetic materials, thus opening new avenues for accelerated materials discovery. Following a data-centric approach, we utilize a database of Heusler alloys calculated at the density functional theory level to identify the ideal ions neighbouring Fe in the $X_2$Fe$Z$ He…
Recent studies have shown that material structures, which lack structural inversion symmetry and have high spin-orbit coupling can exhibit chiral magnetic textures and skyrmions which could be a key component for next generation storage devices. The Dzyaloshinskii-Moriya Interaction (DMI) that stabilizes skyrmions is an anti-symmetric exchange interaction favoring non-collinear orientation of neighboring spins. It has been shown that material systems with high DMI can lead to very efficient domain wall and skyrmion motion by spin-orbit torques. To engineer such devices, it is important to quan…
The changes in the electronic properties of the substitutionally disordered MgC(Ni_{1-x}T_{x})_{3} (T=Fe, Co or Cu) alloys are studied using the atomic sphere formulation of the Korringa-Kohn-Rostoker coherent-potential approximation method (KKR-ASA CPA), while the effects of incipient magnetism in these alloys are studied phenomenologically using Ginzburg-Landau coefficients in conjunction with fixed-spin moment method. We find that the disordered MgC(Ni_{1-x}T_{x})_{3} alloys have a small magnetic moment localized at Fe and Co sites for low concentrations. The overestimation of the calculate…
Generating pure spin currents via the spin Hall effect in heavy metals has been an active topic of research in the last decade. In order to reduce the energy required to efficiently switch neighbouring ferromagnetic layers for applications, one should not only increase the charge- to-spin conversion efficiency but also decrease the longitudinal resistivity of the heavy metal. In this work, we investigate the spin Hall conductivity in W_{1-x}Ta_{x} / CoFeB / MgO (x = 0 - 0.2) using spin torque ferromagnetic resonance measurements. Alloying W with Ta leads to a factor of two change in both the d…
We show that the differences in the magnetic properties of Ni-Pd and Ni-Pt alloys arise mainly due to relativity. In particular, we find that the local magnetic moment of Ni increases with the addition of Pd in Ni-Pd while it decreases with the addition of Pt in Ni-Pt, as found experimentally, only if relativity is present. Our analysis is based on the effects of relativity on (i) the spin-polarized densities of states of Ni, (ii) the splitting of majority and minority spin d-band centers of Ni, and (iii) the separation between s-d band centers of Pd and Pt in Ni-Pd and Ni-Pt alloys.
Using the density functional theory, we perform a full atomic relaxation of the bulk ferrite with 12.5%-concentration of monoatomic interstitial Cr periodically located at the edges of the bcc Fe$_α$ cell. We show that structural relaxation in such artificially engineered alloys leads to significant atomic displacements and results in the formation of novel highly stable configurations with parallel chains of octahedrically arranged Fe. The enhanced magnetic polarization in the low-symmetry metallic state of this type of alloys can be externally controlled by additional inclusion of nonmagneti…
Compositionally complex materials (CCMs) present a potential paradigm shift in the design of magnetic materials. These alloys exhibit long-range structural order coupled with limited or no chemical order. As a result, extreme local environments exist with a large opposing magnetic energy term, which can manifest large changes in the magnetic behavior. In the current work, the magnetic properties of (Cr, Mn, Fe, Ni) alloys are presented. These materials were prepared by room-temperature combinatorial sputtering, resulting in a range of compositions with a single BCC structural phase and no chem…
Temperature-dependent magnetic experiments like pump-probe measurements generated by a pulsed laser have become a crucial technique for switching the magnetization in the picosecond time scale. Apart from having practical implications on the magnetic storage technology, the research field of ultrafast magnetization poses also fundamental physical questions. To correctly describe the time evolution of the atomic magnetic moments under the influence of a temperature-dependent laser pulse, it remains crucial to know if the magnetic material under investigation has magnetic excitation spectrum tha…
Bulk Au-Fe alloys separate into Au-based fcc and Fe-based bcc phases, but L1$_0$ and L1$_2$ orderings were reported in single-phase Au-Fe nanoparticles. Motivated by these observations, we study the structural and ordering energetics in this alloy by combining density functional theory (DFT) calculations with effective Hamiltonian techniques: a cluster expansion with structural filters, and the configuration-dependent lattice deformation model. The phase separation tendency in Au-Fe persists even if the fcc-bcc decomposition is suppressed. The relative stability of disordered bcc and fcc phase…
Sigma-phase intermetallic compound of Fe54Cr46 was investigated using DC and AC magnetic susceptibility techniques. A clear-cut evidence was found that the sample orders magnetically at Tc=23.5 K and its ground magnetic state is constituted by a spin glass. The temperature at which the zero-field cooled magnetization has its maximum decreases with an external magnetic field in line with the Gabay-Toulouse prediction. The temperature at which the AC magnetic susceptibility has its maximum does not depend on frequency which, in the light of the mean-field theory, testifies to very long magnetic …
The electronic properties, exchange interactions, finite-temperature magnetism, and transport properties of random quaternary Heusler Ni$_{2}$MnSn alloys doped with Cu- and Pd-atoms are studied theoretically by means of {\it ab initio} calculations over the entire range of dopant concentrations. While the magnetic moments are only weakly dependent on the alloy composition, the Curie temperatures exhibit strongly non-linear behavior with respect to Cu-doping in contrast with an almost linear concentration dependence in the case of Pd-doping. The present parameter-free theory agrees qualitativel…
The development of quantitative models for radiation damage effects in iron, iron alloys and steels, particularly for the high temperature properties of the alloys, requires understanding of magnetic interactions, which control the phase stability of ferritic-martensitic, ferritic, and austenitic steels. In this work, disordered magnetic configurations of pure iron and Fe-Cr alloys are investigated using Density Functional Theory (DFT) formalism, in the form of constrained non-collinear magnetic calculations, with the objective of creating a database of atomic magnetic moments and forces actin…
We present a detailed experimental study of bulk and powder samples of the Heusler shape memory alloy Ni$_2$MnGa, including zero-field static and dynamic $^{55}$Mn NMR experiments, X-ray powder diffraction and magnetization experiments. The NMR spectra give direct access to the sequence of structural phase transitions in this compound, from the high-T austenitic phase down to the low-T martensitic phase. In addition, a detailed investigation of the so-called rf-enhancement factor provides local information for the magnetic stiffness and restoring fields for each separate coordination, structur…
Enhancing the magnetism and giant anomalous Hall effect in thin Fe-Al films via B2 nanophase growth
Lobachevsky State University of Nizhny Novgorod（下诺夫哥罗德洛巴切夫斯基国立大学）；Institute for Physics of Microstructures, Russian Academy of Sciences（俄罗斯科学院微观结构物理研究所）
The phenomenology of exchange bias effects observed in structurally single-phase alloys and compounds but composed of a variety of coexisting magnetic phases such as ferromagnetic, antiferromagnetic, ferrimagnetic, spin-glass, cluster-glass and disordered magnetic states are reviewed. The investigations on exchange bias effects are discussed in diverse types of alloys and compounds where qualitative and quantitative aspects of magnetism are focused based on macroscopic experimental tools such as magnetization and magnetoresistance measurements. Here, we focus on improvement of fundamental issu…
In general, conventional superconductivity and magnetism are competing phenomena. In some alloys this competition is a function of the concentration of the elements. Here we show that in the palladium alloys Pd$_{1-c}$(H/D/T)$_{c}$ (Pd-ides) the increase in the concentration $c$ of the ides: hydrogen, deuterium, tritium (H/D/T), lowers the predicted magnetism of amorphous palladium ($a$-Pd) gradually, allowing superconductivity to appear for $ c \approx 40\%$. This magnetism explains why superconductivity does not manifest for smaller values of $c$ ($c \leq 40\%$) in these Pd alloys. Also, the…
We have studied the properties of relatively thick (about 120 nm) magnetic composite films grown by pulsed laser deposition method using (GaSb)$_{0.59}$(MnSb)$_{0.41}$ eutectic compound as a target for sputtering. For the studied films we have observed ferromagnetism and anomalous Hall effect above the room temperature, it manifests the presence of spin-polarized carriers. Electron microscopy, atomic and magnetic force microscopy results suggests that films under study have homogenous columnar structure in the bulk while MnSb inclusions accumulate near it's surface. This is in good agreement w…
Deploying an analytical atomistic model of the bulk band structure of the IV-VI semiconductors we connect the spin structure of the topological surface state to the crystal field and spin orbit coupling parameters of the bulk material. While the Dirac-Weyl (or equivalently, Rashba) type topological surface state is often assumed universal, we show that the physics of the surface state is strikingly non-universal. To see this explicitly we calculate the RKKY interaction, which may be viewed as a probe of this surface state spin structure, finding its \emph{qualitative form} depends on the value…
Despite intense research in the field of strongly correlated electron behavior for the past few decades, there has been very little effort to understand this phenomenon in nano particles of the Kondo lattices. In this article, we review the results of our investigation on the fine particles (less than 1 micron) of some of the alloys obtained by high-energy ball-milling to bring out that this synthetic method paves a way to study strong electron correlations in nanocrystals of such alloys. We primarily focus on the alloys of the series, CeRu(2-x)Rh(x)Si2, lying at different positions in Doniach…
