Origin of the spin reorientation transitions in (Fe$_{1-x}$Co$_{x}$)$_{2}$B alloys
Kirill D. Belashchenko, Liqin Ke, Markus Däne, Lorin X. Benedict, Tej Nath Lamichhane, Valentin Taufour, Anton Jesche, Sergey L. Bud'ko, Paul C. Canfield, Vladimir P. Antropov
Low-temperature measurements of the magnetocrystalline anisotropy energy $K$ in (Fe$_{1-x}$Co$_{x}$)$_{2}$B alloys are reported, and the origin of this anisotropy is elucidated using a first-principles electronic structure analysis. The calculated concentration dependence $K(x)$ with a maximum near $x=0.3$ and a minimum near $x=0.8$ is in excellent agreement with experiment. This dependence is traced down to spin-orbital selection rules and the filling of electronic bands with increasing electronic concentration. At the optimal Co concentration, $K$ depends strongly on the tetragonality and doubles under a modest 3% increase of the $c/a$ ratio, suggesting that the magnetocrystalline anisotropy can be further enhanced using epitaxial or chemical strain.
Antiferromagnetic MnPt exhibits a spin reorientation transition (SRT) as a function of temperature, and off-stoichiometric Mn-Pt alloys also display SRTs as a function of concentration. The magnetocrystalline anisotropy in these alloys is studied using first-principles calculations based on the coherent potential approximation and the disordered local moment method. The anisotropy is fairly small and sensitive to the variations in composition and temperature due to the cancellation of large contributions from different parts of the Brillouin zone. Concentration and temperature-driven SRTs are …
Three-dimensional (3D) X-ray diffraction methods were used to analyze the evolution of the load-induced rearrangements of monoclinic twin microstructures within bulk nickel-titanium specimens in 3D and across six orders of magnitude in length scales: changes in lattice plane spacings and orientations at the nanoscale, growth and nucleation of martensite twin variants at the microscale, and localization of plastic strain into deformation bands at the macroscale. Portions of the localized deformation bands were reconstructed in situ and in 3D. Analyses of the data elucidate the sequence of twin …
Single Pulse All Optical Helicity Independent Switching (AO-HIS) represents the ability to reverse the magnetic moment of a nanostructure using a femtosecond single laser pulse. It is an ultrafast method to manipulate magnetization without the use of any applied field. Since the first switching experiments carried on GdFeCo ferrimagnetic systems, single pulse AO-HIS has been restricted for a while to Gd-based alloys or Gd/FM bilayers where FM is a ferromagnetic layer. Only recently has AO-HIS been extended to a few other materials: MnRuGa ferrimagnetic Heusler alloys and Tb/Co multilayers with…
We report on the increase in the spin reorientation temperature in HoFe0.5Cr0.5O3 compound by isovalent substitution Cr3+ at the Fe-site and the magnetocapacitance in the HoFeO3 compound. Spin reorientation transition is evident around 50 K and 150 K for the x = 0 and x = 0.5 compounds respectively. The increase in the spin reorientation transition temperature in case of x = 0.5 compound can be attributed to the domination of the Ho3+ to Fe3+ interaction over the Fe3+ to Fe3+ interaction. Decrease in Neel temperature from 643 K (x = 0) to 273 K (x = 0.5) can be ascribed to the decrease in the …
Spin reorientation transitions and other related magnetic phenomena, which owe their origin to the complex interplay between multiple magnetic sublattices, have long attracted scientific attention both from the perspective of fundamental curiosity and technological applications. In this study, combining first principles calculations together with finite temperature Monte Carlo simulations, we explore the possible origins of reorientation transition of Cr spins in NdCrO$_3$. We construct a NdCrO$_3$ specific magnetic model, consisting of symmetric superexchange interactions between magnetic ion…
From electronic-structure-based thermodynamic linear-response, we establish chemical ordering behavior in complex solid solutions versus how Gibbs' space is traversed -- applying it on prototype refractory A2 Ta-Nb-Mo-W high-entropy alloys. Near ideal stoichiometry, this alloy has anomalous, intricate chemical ordering tendencies, with long-ranged chemical interactions that produce competing short-range order (SRO) with a crossover to spinodal segregation. This atypical SRO arises from canonical band behavior that, with alloying, create features near the Fermi-surface (well-defined even with d…
The magnetic anisotropy (MA) of Mo/Au/Co0.9Fe0.1/Au/MgO(0.7 - 3 nm)/Au/Co0.9Fe0.1/Au heterostructure has been investigated at room temperature as a function of MgO layer thickness (tMgO). Our studies show that while the MA of the top layer does not change its character upon variation of tMgO, the uniaxial out-of-plane MA of the bottom one undergoes a spin reorientation transition at tMgO of about 0.8 nm, switching to the regime where the coexistence of in- and out-of-plane magnetization alignments is observed. The magnitudes of the magnetic anisotropy constants have been determined from ferrom…
We present the results of large scale molecular dynamics simulations aimed at understanding the origins of high friction coefficients in pure metals, and their concomitant reduction in alloys and composites. We utilize a series of targeted simulations to demonstrate that different slip mechanisms are active in the two systems, leading to differing frictional behavior. Specifically, we show that in pure metals, sliding occurs along the crystallographic slip planes, whereas in alloys shear is accommodated by grain boundaries. In pure metals, there is significant grain growth induced by the appli…
The influence of long-range dipole interactions on two dimensional magnetism has been studied extensively in the spin-reorientation transition of ferromagnetic ultrathin films. Although there is a great deal of experimental information on the perpendicular domain phase that is stabilized by dipole interactions, the transitions to or from the domain phase are subtle and difficult to characterize experimentally. Magnetic susceptibility measurements show no divergence in the vicinity of the spin-reorientation transition as a function of thickness -- a null result that is difficult to interpret wi…
Understanding the formation of bulk metallic glasses (BMG) in metallic systems and finding a reliable criterion for selection of BMG compositions are among the most important issues in condensed matter physics and material science. Using the results of magnetic susceptibility measurements performed on both amorphous and crystallized Cu-Hf alloys (30-70 at% Cu) we find a correlation between the difference in magnetic susceptibilities of corresponding glassy and crystalline alloys and the variation in the glass forming ability (GFA) in these alloys. Since the same correlation can be inferred fro…
We report perpendicular magnetic anisotropy in the ferromagnetic Heusler alloy Co$_2$MnGa in a MgO/Co$_2$MnGa/Pd trilayer stack for Co$_2$MnGa thicknesses up to 3.5 nm. There is a thickness- and temperature-dependent spin reorientation transition from perpendicular to in-plane magnetic anisotropy which we study through the anomalous Hall effect. From the temperature dependence of the anomalous Hall effect, we observe the expected scaling of $ρ_{xy}^{AHE}$ with $ρ_{xx}$, suggesting the intrinsic and side-jump mechanisms are largely responsible for the anomalous Hall effect in this material.
Alloying magnesium (Mg) with rare earth elements such as yttrium (Y) has been reported to activate the pyramidal <c + a> slip systems and improve the plasticity of Mg at room temperature. However, the origins of such dislocations and their dissociation mechanisms remain poorly understood. Here, we systematically investigate these mechanisms using dispersion-inclusive density functional theory, in combination with molecular dynamics simulations. We find that <c + a> dislocations form more readily on the pyramidal I plane than on the pyramidal II plane in Mg. The addition of Y atoms in Mg facili…
Damage-tolerance can be an elusive characteristic of structural materials requiring both high strength and ductility, properties that are often mutually exclusive. High-entropy alloys are of interest in this regard. Specifically, the single-phase CrMnFeCoNi alloy displays tensile strength levels of ~1 GPa, excellent ductility (~60-70%) and exceptional fracture toughness (KJIc > 200 MPa/m). Here, through the use of in-situ straining in an aberration-corrected transmission electron microscope, we report on the salient atomistic to micro-scale mechanisms underlying the origin of these properties.…
In half-doped Pr0.50A0.50CoO3 metallic perovskites, the spin-lattice coupling brings about distinct magnetostructural transitions for A=Ca and A=Sr at temperatures close to 100 K. However, the ground magnetic properties of Pr0.50Sr0.50CoO3 (PSCO) strongly differ from Pr0.50Ca0.50CoO3 ones, where a partial Pr3+ to Pr4+ valence shift and Co spin transition makes the system insulating below the transition. This work investigates and describes the relationship between the Imma-to-I4/mcm symmetry change [Padilla-Pantoja et al, Inorg. Chem. 53, 12297 (2014)] and the original magnetic behavior of PSC…
We experimentally investigated the magnetic properties of NiCo$_2$O$_4$ epitaxial films known to be conductive oxides with perpendicular magnetic anisotropy (PMA) at room temperature. Both magneto-torque and magnetization measurements at various temperatures provide clear experimental evidence of the spin reorientation transition at which the MA changes from PMA to easy-cone magnetic anisotropy (ECMA) at a certain temperature ($T_{\rm{SR}}$). ECMA was commonly observed in films grown by pulsed laser deposition and reactive radio frequency magnetron sputtering, although $T_{\mathrm{SR}}$ is dep…
The near equiatomic NiTi alloy is the most successful shape memory alloy by a large margin. It is widely and increasingly used in biomedical devices. Yet, despite having a repeatable superelastic effect and excellent shape-memory, NiTi is very far from satisfying the conditions that characterize the most reversible phase transforming materials. Thus, the scientific reasons underlying its vast success present an enigma. In this work, we perform rigorous mathematical derivation and accurate DFT calculation of transformation mechanisms to seek previously unrecognized twin-like defects that we ter…
Anisotropic magnetoresistance (AMR) of Cr2Ge2Te6 (CGT), a layered ferromagnetic insulator, is investigated under an applied hydrostatic pressure up to 2 GPa. The easy axis direction of the magnetization is inferred from the AMR saturation feature in the presence and absence of the applied pressure. At zero applied pressure, the easy axis is along the c-direction or perpendicular to the layer. Upon application of a hydrostatic pressure>1 GPa, the uniaxial anisotropy switches to easy-plane anisotropy which drives the equilibrium magnetization from the c-axis to the ab-plane at zero magnetic fiel…
We have studied the phase volume fraction related magnetoresistance (MR) across the first order martensite transformation (MT) of Ni44Cu2Mn43In11 alloy. Within the metastability of MT, an isothermal application of magnetic field converts the martensite into austenite. The field induced austenite phase fraction (fIA) at any temperature depends on the availability and instability of martensite phase fraction (fM ) at that temperature. This fIA is found to contribute most significantly to the observed giant MR while the contribution from pure martensite and austenite phase fraction is negligible.…
The negative sign of the anomalous Nernst thermopower ($S_\text{ANE}$) observed in Mn-Ga ordered alloys is an attractive property for thermoelectric applications exploiting the anomalous Nernst effect (ANE); however, its origin has not been clarified. In this study, to gain insight into the negative $S_\text{ANE}$, we prepared epitaxial thin films of Mn$_{x}$Ga$_{100-x}$ with $x$ ranging from 56.2 to 71.7, and systematically investigated the structural, magnetic, and transport properties including the anomalous Hall effect (AHE) and the ANE. The measured $S_\text{ANE}$ is negative for all samp…
Heusler alloys exhibiting magnetic and martensitic transitions enable applications like magnetocaloric refrigeration and actuation based on the magnetic shape memory effect. Their outstanding functional properties depend on low hysteresis losses and low actuation fields. These are only achieved if the atomic positions deviate from a tetragonal lattice by periodic displacements. The origin of the so-called modulated structures is the subject of much controversy: They are either explained by phonon softening or adaptive nanotwinning. Here we used large-scale density functional theory calculation…
Magnetic thin film heterostructures have been widely studied for fundamental interests in the emergence of novel phenomena accompanied by the heterointerface formation as well as their promising practical potential. Combining x-ray magnetic circular dichroism with scanning tunneling microscopy, we show for Mn/Fe thin film heterostructures that the interfacial factors dominating electronic and magnetic properties of the entire system dynamically change with the amount of the Mn overlayer. Element specific magnetization curves of the Fe layer exhibit a two-step spin reorientation transition from…
Heat resistant FeCrNi alloys are widely used in the petrochemical industry because they exhibit a unique combination of creep and oxidation resistance at temperatures exceeding 900$^\circ$C. Their creep properties are often optimized by micro-additions of carbide forming elements. In the present work, the influence of Ta micro-additions has been experimentally investigated both on as-cast and aged microstructures to understand the origin of the significant impact of this element on the creep resistance. Calculations with thermocal software were also carried out to support experimental data. It…
We develop a first-principles procedure for the individual evaluation of the intrinsic, side-jump, and skew-scattering contributions to the anomalous Hall conductivity $σ_{xy}$. This method is based on the different microscopic conductive processes of each origin of $σ_{xy}$ in the Kubo--Bastin formula. We also present an approach for implementing this scheme in the tight-binding linear muffin-tin orbital (TB-LMTO) method with the coherent potential approximation (CPA). The validity of this calculation method is demonstrated for disordered FePt and FePd alloys. We find that the estimated value…
Mn-based alloys exhibit unique properties in the spintronics materials possessing perpendicular magnetic anisotropy (PMA) beyond the Fe and Co-based alloys. It is desired to figure out the quantum physics of PMA inherent to Mn-based alloys, which have never been reported. Here, the origin of PMA in ferrimagnetic Mn$_{3-δ}$Ga ordered alloys is investigated to resolve antiparallel-coupled Mn sites using x-ray magnetic circular and linear dichroism (XMCD/XMLD) and a first-principles calculation. We found that the contribution of orbital magnetic moments in PMA is small from XMCD and that the fini…
The boson peak appears in all amorphous solids and is an excess of vibrational states at low frequencies compared to the phonon spectrum of the corresponding crystal. Until recently, the consensus was that it originated from "defects" in the glass. The nature of these defects is still under discussion, but the picture of regions with locally disturbed short-range order and/or decreased elastic constants has gained some traction. Recently, a different theory was proposed: The boson peak was attributed to the first van Hove singularity of crystal lattices which is only smeared out by the disorde…
The binary (simple metal) phase diagram Au-Cd contains a number of intermetallic compounds with various distortions, superlattices and vacancies. To understand the reasons of these structural complexities and their phase stability, we analyze these crystal structures within the nearly free-electron model in the frame of Fermi sphere - Brillouin zone interactions. Examination of the Brillouin-Jones configuration in relation to the nearly-free electron Fermi sphere provides insights for significance of the valence electron energy contribution to the phase stability. Representation of these compl…
High-entropy alloys (HEAs) are solid solutions of multiple elements with equal atomic ratios which present an innovative pathway for de novo alloy engineering. While there exist extensive studies to ascertain the important structural aspects governing their mechanical behaviors, elucidating the underlying deformation mechanisms still remains a challenge. Using atomistic simulations, we probe the particle rearrangements in a yielding, model HEA system to understand the structural origin of its plasticity. We find the plastic deformation is initiated by irreversible topological fluctuations whic…
Magnetocrystalline anisotropy (MCA) in doped Ce$_{2}$Co$_{17}$ and other competing structures was investigated using density functional theory. We confirmed that the MCA contribution from dumbbell Co sites is very negative. Replacing Co dumbbell atoms with a pair of Fe or Mn atoms greatly enhance the uniaxial anisotropy, which agrees quantitatively with experiment, and this enhancement arises from electronic-structure features near the Fermi level, mostly associated with dumbbell sites. With Co dumbbell atoms replaced by other elements, the variation of anisotropy is generally a collective eff…
Microstructural origin of the simultaneous enhancements in strength and ductility of a nitrogen-doped high-entropy alloy
Soochow University（苏州大学）；Research Institute of Aero-Engine, Beihang University（北京航空航天大学发动机研究院）；Center for Advancing Materials Performance from the Nanoscale (CAMP-Nano) and Hysitron Applied Research Center in China (HARCC), and Center for Alloy Innovation and Design (CAID), State Key Lab for Mechanical Behavior of Materials, Xi’an Jiaotong University（西安交通大学材料行为科技重点实验室、纳米尺度材料性能提升中心（CAMP-Nano）、海塞特中国应用研究中心（HARCC）及合金创新与设计中心（CAID））；Max Planck Institute for Sustainable Materials（马普可持续材料研究所）；Institute for Materials Science, University of Stuttgart（斯图加特大学材料科学研究所）；ICAMS, Ruhr-Universität Bochum（波鸿鲁尔大学综合材料与结构科学研究所）；State Key Laboratory of Powder Metallurgy, School of Materials Science and Engineering, Central South University（中南大学材料科学与工程学院粉末冶金国家重点实验室）；Department of Materials, Royal School of Mine, Imperial College London（帝国理工学院皇家矿业学院材料系）
The temperature dependent crystal structure analysis in martensitic phase of Ni_2Mn_(1+x)Sn_(1-x) (x = 0.40 and 0.44) magnetic shape memory alloy is performed using X-ray diffraction. The martensitic phase consists of 4-Layered and 14-Layered orthorhombic structure. The phase fraction of 4-layered and 14-layered orthorhombic structure change with temperature. Interestingly, the magnetic property also changes with temperature and corresponds to structural change temperature. Thus, the exchange coupling constant between original Mn and substituted Mn is calculated, which shows that the 4-layered…
The body centered cubic (BCC) high entropy alloys MoNbTaW and MoNbTaVW show exceptional strength retention up to 1900K. The mechanistic origin of the retained strength is unknown yet is crucial for finding the best alloys across the immense space of BCC HEA compositions. Experiments on Nb-Mo, Fe-Si and Ti-Zr-Nb alloys report decreased mobility of edge dislocations, motivating a theory of strengthening of edge dislocations in BCC alloys. Unlike pure BCC metals and dilute alloys that are controlled by screw dislocation motion at low temperatures, the strength of BCC HEAs can be controlled by edg…
