B2 Order Research Articles

Growth of [001]-oriented polycrystalline Heusler alloy thin films using [001]-textured Ag buffer layer on thermally oxidized Si substrate for spintronics applications

To utilize highly spin-polarized Heusler alloys in practical spintronic devices, the realization of highly textured and structurally ordered polycrystalline thin films under limited annealing temperatures (TA) is critical. Compared to the natural [110]-texture of Heusler alloys, the [001]-texture is considered to be favorable for current-perpendicular-to-plane giant magnetoresistance devices due to the reduced lattice misfit with the face-centered-cubic Ag spacer layers. In this study, we fabricated [001]-oriented polycrystalline Co2FeGa0.5Ge0.5 (CFGG) Heusler alloy films epitaxially grown on a [001]-oriented polycrystalline Ag buffer layer on a thermally oxidized Si substrate, and the microstructure of the [001]-oriented Ag/CFGG bilayer film was investigated in detail. The [001]-oriented Ag films were obtained by introducing N2 into Ar during the sputtering process. The [001]-oriented CFGG films exhibited smooth interfaces, B2 ordering, and a high saturation magnetization close to the theoretical value under relatively low annealing at TA = 300 °C, which are critical for industrial applications such as read heads of hard disk drives.

Phase evolution, mechanical properties, and corrosion resistance of Ti2NbVAl0.3Zrx lightweight refractory high-entropy alloys

Herein, a series of Ti2NbVAl0.3Zrx (x = 0.25, 0.5, 0.75, 1) light refractory high entropy alloys were designed and prepared based on the concept of B2 ordering and solid solution strengthening. The Zr0.25, Zr0.5, and Zr0.75 alloys exhibited a disordered BCC + ordered B2 duplex structure, while the Zr1 alloy displayed a BCC single-phase structure. The addition of Zr hinders the formation of B2 ordering, transforming the alloy from a BCC + B2 duplex structure to a BCC single-phase structure. The Zr0.75 alloy has a yield strength of 1022 MPa and a tensile strain of ∼9.4 %. Furthermore, electrochemical tests were conducted in 3.5 wt% NaCl solution revealed that the Ti2NbVAl0.3Zrx LRHEAs showed high resistance to pitting corrosion. Incorporating the Zr element enhanced the polarization resistance and reduced the corrosion rate; however, excessive Zr content led to a shift in micro-galvanic corrosion from the nanoscale to the microscale, resulting in pitting corrosion. The passivation film compositions of the four alloys formed at a potential of 0.7 VSCE were analyzed by X-ray photoelectron spectroscopy, and their corrosion resistance in a chlorine-containing environment was examined in depth.

Enhancing atomic ordering, magnetic and transport properties of Mn2VGa Heusler alloy thin films toward negatively spin-polarized charge injection

Magnetic materials with negative spin polarization have attracted attention for their potential to increase the design freedom of spintronic devices. This study investigated the effects of off-stoichiometry on the atomic ordering, microstructure, and magneto-transport properties in Mn2+xV1−xGa (x = −0.2, 0, +0.2, +0.4) Heusler alloy films, which are predicted to have large negative spin polarization derived from a pseudo band gap in the majority spin channel. The Mn2+xV1−xGa films epitaxially grown on MgO(001) substrates exhibits variations of B2 and L21 order with the Mn concentration. A high-quality L21 ordered film was achieved in the Mn-rich composition (x = +0.2) with B2 and L21 order parameters of 0.97 and 0.86, respectively, and a saturation magnetization of 1.4 μB/f.u., which agrees with the Slater-Pauling rule. Scanning transmission electron microscopy observations showed that B2 and L21 phases coexist in Mn-poor and stoichiometric films, while the L21 phase is dominant in the Mn-rich film with small amounts of Mn–V and Mn–Ga disorders, as revealed by laboratory and anomalous X-ray diffraction. Combined first-principles calculations and anisotropic magnetoresistance analysis confirms that the addition of excess Mn preserves the high spin polarization by suppressing the formation of detrimental antisites of V atoms occupying Mn sites. Therefore, the Mn-rich composition is promising for negatively spin-polarized charge injection in Mn2VGa-based spintronic applications.

Neural network kinetics for exploring diffusion multiplicity and chemical ordering in compositionally complex materials

Diffusion involving atom transport from one location to another governs many important processes and behaviors such as precipitation and phase nucleation. The inherent chemical complexity in compositionally complex materials poses challenges for modeling atomic diffusion and the resulting formation of chemically ordered structures. Here, we introduce a neural network kinetics (NNK) scheme that predicts and simulates diffusion-induced chemical and structural evolution in complex concentrated chemical environments. The framework is grounded on efficient on-lattice structure and chemistry representation combined with artificial neural networks, enabling precise prediction of all path-dependent migration barriers and individual atom jumps. To demonstrate the method, we study the temperature-dependent local chemical ordering in a refractory NbMoTa alloy and reveal a critical temperature at which the B2 order reaches a maximum. The atomic jump randomness map exhibits the highest diffusion heterogeneity (multiplicity) in the vicinity of this characteristic temperature, which is closely related to chemical ordering and B2 structure formation. The scalable NNK framework provides a promising new avenue to exploring diffusion-related properties in the vast compositional space within which extraordinary properties are hidden.

Accelerated structural ordering during liquid metal dealloying and its effect on thermal coarsening of nanoporous refractory alloys

Nanoporous non-noble metals with attractive properties can be fabricated by liquid metal dealloying (LMD). However, further refinement of the ligament size of nanoporous metals produced by LMD is desired because this high-temperature process induces rapid ligament coarsening. Herein, we fabricate nanoporous refractory metals and alloys by LMD. The molybdenum (Mo)-tantalum (Ta) alloy with the smallest ligament size of the specimens contains a B2 ordered structure, whereas the other specimens possess disordered structures. It is proposed that the B2 ordering of the Mo-Ta alloy suppresses ligament coarsening by restricting the surface diffusion of atoms during LMD. The Mo-Ta alloy with B2 ordered structure exists in a previously inaccessible low-temperature region of the related phase diagram, illustrating that LMD can enable access to new materials. Our results indicate that materials with refined ligaments may be obtained by LMD promoting the formation of ordered structures.

Strength of Disordered and Ordered Al‐Containing Refractory High‐Entropy Alloys

Body‐centered‐cubic refractory high‐entropy alloys are promising for high‐temperature structural applications due to their exceptional properties, particularly in terms of yield strength at elevated temperatures. For certain alloy systems, such as Mo–Ti–Cr–Al, both disordered (A2) and ordered (B2) crystal structures are possible. In this particular system, a solid‐state transformation from A2 to B2 during cooling occurs. For Al concentrations above ≈10 at%, B2 order is obtained from the transformation temperature down to room temperature (RT), while A2 is stable above RT below the critical Al content. Herein, two alloys from the Mo–Ti–Cr–Al system close to the transition between A2 and B2 are investigated. Nanoindentation tests reveal that the magnitude of strain rate sensitivity for both alloys is small compared to classical alloys, however, significantly temperature dependent up to the strength plateau temperatures. The yield strength plateau, which is insensitive to the strain rate, is observed at temperatures exceeding 573 K. Modeling of solid solution strengthening reproduces the experimental data in the Al‐lean A2 alloys. However, the observed discontinuous increase of strength in the Al‐rich B2 alloys can only be rationalized by the appearance of B2 order and to no significant other obvious strengthening mechanisms.

Vacancy diffusion barrier spectrum and diffusion correlation in multicomponent alloys

Vacancy diffusion serves a crucial role in many important kinetic behaviors and properties of multicomponent alloys. Essential questions, however, persist regarding how chemical complexity affects diffusion and what unique characteristics, if any, set these alloys apart from traditional metals. Using neural network kinetics model, we study vacancy diffusion in NbMoTa alloy across a broad temperature range (2600 to 800 K). Unlike pure metals, the two key diffusion parameters—diffusion correlation factor f and activation energy ΔGm—are not constant in alloys, but instead substantially decrease with decreasing temperature. This temperature dependence arises from a reduced number of active vacancy jump pathways at lower temperatures, leading to more correlated diffusion. Upon examining vacancy diffusion throughout the entire compositional space of the Nb-Mo-Ta system, we discover that the slowest vacancy diffusion surprisingly occurs in the non-equimolar region, rather than the equimolar concentration where the configurational entropy is highest. The diffusion barrier spectrum, characterizing the diffusion energy landscape, is an intrinsic material characteristic, which controls both f and ΔGm and, thereby, the diffusivity. Finally, we find that the vacancy diffusion rate drops noticeably in the presence of local chemical order in the NbMoTa system, particularly for MoTa alloys with long-range B2 order.

On the Nature of the Decrease in the Rate of the B2 Ordering Process in a Cu–Pd Solid Solution as the Equiatomic Composition Is Approached

On the Nature of the Decrease in the Rate of the B2 Ordering Process in a Cu–Pd Solid Solution as the Equiatomic Composition Is Approached

Modelling of chemical and magnetic order in Ni-Mn-Al shape memory alloys using Monte Carlo simulations

A model for the description of atomic and magnetic ordering in NiMnAl shape memory alloys is presented. The parameters characterizing the energies of the chemical and magnetic interactions were obtained from fitting to experimental data. Monte Carlo simulations based on this model correctly reproduces the critical B2↔L21 ordering temperatures as a function of composition for three pseudobinary composition lines, as well as the nature of the low temperature magnetic transition. A ferromagnetic configuration is predicted for alloys in the stable L21 structure, whereas an antiferromagnetic state is obtained for metastably retained B2 order; this is in agreement with experimental observations. It is verified that atomic ordering is strongly dominated by the interactions between Ni and Al atoms in nearest neighbours. As a consequence, the formation of B2 order can delay the stabilization of the L21 configuration at low temperatures. Since the magnetic interactions change sign with the Mn–Mn distance, the type of stable magnetic configuration depends on atomic ordering.

B2 Ordering and Its Effect on Room Temperature Superelasticity in Mg–Sc Alloy

B2 Ordering and Its Effect on Room Temperature Superelasticity in Mg–Sc Alloy

Effect of B2 ordering on the tensile mechanical properties of refractory AlxNb40Ti40V20−x medium-entropy alloys

Most of refractory high/medium-entropy alloys show promising compressive mechanical properties at elevated temperatures. However, scarce tensile testing data hinder the estimation of the applicability of these alloys for potential high-temperature service. It is also unclear how an ordered structure, which improves high-temperature compressive strength, will affect the tensile performance. This work demonstrated that the effect of B2 ordering on the tensile mechanical properties of new refractory AlxNb40Ti40V20−x (x = 0; 15; 20 at%) medium-entropy alloys was manifold, and it depended on the chemical composition. In an Al15Nb40Ti40V5 alloy having a weak degree of B2 ordering, large uniform room-temperature elongation and moderate high-temperature strength, comparable with a bcc Nb40Ti40V20 alloy, were found. An Al20Nb40Ti40 alloy with the high degree of B2 ordering was brittle at room-temperature and sensitive to testing environment at elevated temperatures. Meantime, it softened notably slower up to 0.4 Tm, albeit losing strength at> 0.4 Tm faster than the bcc Nb40Ti40V20 or weakly B2-ordered Al15Nb40Ti40V5 alloys. Factors responsible for the resulted mechanical properties were thoroughly discussed.

Effects of long/short-range ordering on magnetic properties of Fe-23 wt% Co alloy: A combined experimental and computational study

FeCo and FeCo-based alloys obtained by annealing generally have excellent soft magnetic properties (e.g. high saturation magnetic induction and permeability) induced by B2 ordering. In this work, the effects of long/short-range ordering (LRO/SRO) on magnetic properties of low-cobalt Fe-23 wt% Co alloy were computationally and experimentally studied. Fe3Co ordering and SRO formations in Fe-25 at% Co alloy were predicted by first-principles calculations to lead to the reduction of the atomic volume, significantly weakening the magnetic exchange interaction between Fe and Co atoms. Experimentally, Fe3Co ordering and SRO formations in Fe-23 wt% Co alloy were found to reduce the saturation magnetic polarization Js. As we know, the brittleness of Fe-Co alloys is mainly caused by the long/short-range ordering. Our findings suggest that the increased cooling rates (e.g. water quenching) and quenching temperatures can simultaneously improve the impact toughness and saturated magnetic polarization Js by suppressing the Fe3Co ordering and SRO formations in Fe-23 wt% Co alloy.

L21 ordering of Co2FeSn thin films promoted by high-temperature annealing

We report the effect of annealing on the crystalline ordering and physical properties in thin films of a nodal line semimetal candidate L21-type Co2FeSn. The Co–Fe–Sn films with a composition of Co:Fe:Sn ∼ 2:1:1 were deposited on MgO(001) substrates at a substrate temperature of 150 °C by radio-frequency magnetron sputtering. The as-deposited film showed x-ray diffraction patterns corresponding to the B2 ordering. Annealing at 600 and 700 °C after the deposition resulted in the appearance of the (111) diffraction peak, which is characteristic of the L21 ordering. Although anomalous Hall conductivity and transverse thermoelectric conductivity decreased from those of the as-deposited film with the annealing-induced L21 ordering, the low anomalous Hall conductivity of the 700 °C-annealed film was consistent with the theoretically estimated low value. These results show the significant influence of crystalline ordering on the electrical and thermoelectric transport properties. The annealing process is beneficial for studying the exotic physics arising from topological band features in the L21-ordered Co2FeSn thin films.

Effect of stoichiometry and film thickness on the structural and magnetization dynamics behavior of Co2MnAl thin films cosputtered on Si(1 0 0)

Obtaining low gilbert damping (α) and high spin polarization (P) in Heusler alloy thin films is one of the key requirements from the device application point of view in spintronics. Both α and P are correlated, closely related to the density of states (DOS) at the Fermi level and need to be controlled for engineered material. We investigated the effect of stoichiometry (different Co to Mn atomic percent ratios, viz. 2.21, 4.53, 5.43, and 9.19) and film-thickness {(t) in the range of 24–90 nm} on the structural and magnetization dynamics behavior in co-sputtered Co2MnAl thin films grown over Si(100) substrates. While the stoichiometric (∼50 at.% of Co) films with t ≥ 38 nm were found to exhibit B2 order with low α, the off-stoichiometric films (t ∼ 50 nm) were found to stabilize in partially ordered A2 phase and exhibited higher α. The change in structural ordering due to change in stoichiometry and film-thickness in polycrystalline Co2MnAl (CMA) thin films helped in tuning α in the range of (4.11 ± 0.06) × 10-3 – (11.95 ± 0.28) × 10-3. The minimum value of α ∼ (4.11 ± 0.06) × 10-3 was achieved for stoichiometric CMA (t = 70 nm) thin films which is close to the literature value of epitaxially grown L21 and B2 ordered CMA thin films. The study shows that we can tune and attain low damping in polycrystalline CMA Heusler thin films by controlling the stoichiometry.

Chemical long range ordering in all-d-metal Heusler alloys

Chemical ordering in NiMn-based Heusler alloys with magnetostructural phase transition is crucial for understanding the physics of the phase transition. In the new field of all-d-metal Ni(Co)MnTi Heusler alloys, the experimental determination of chemical order is challenging due to the low difference in scattering power of the different elements. Here, we report a combined approach of neutron and x-ray diffraction for an analysis of chemical order in Ni(Co)MnTi alloys and show that no Heusler-typical L21 order between Ti and Mn is present. Furthermore, Co and Ni atoms do not exhibit order among them; however, the phase transition of Co containing samples can be shifted significantly by changing the degree of B2 order with proper heat treatment. Using first-principles calculations, we reveal how the structural and magnetic sub-systems depend on the degree of B2 disorder.

Substrate dependent reduction of Gilbert damping in annealed Heusler alloy thin films grown on group IV semiconductors

A structural and FMR study is presented for epitaxial thin films of the Heusler alloy Co2FeAl0.5Si0.5 (CFAS) grown on Ge(111) and Si(111) substrates. All films, as-grown and post-annealed, show B2 ordering; full chemical order (L21) is not obtained over the range of anneal temperatures used in this study. As-grown films show a lower Gilbert damping constant, α, when grown on a Si(111) substrate compared to Ge(111). Annealing the films to 450 °C significantly reduces α for CFAS on Ge while increasing α for CFAS on Si. This is related to a substrate dependent competition between improvements in lattice structure and increased interfacial intermixing as a function of anneal temperature. The optimal annealing temperature to minimize α is found to differ by ∼100 K between the two substrates. Above an anneal temperature of 500 °C, films grown on both substrates have increased coercivity, decreased saturation magnetization, and show characteristic two-magnon scattering features.

Co2FeAl Full Heusler Compound Based Spintronic Terahertz Emitter

AbstractTo achieve a large terahertz (THz) amplitude from a spintronic THz emitter (STE), materials with 100% spin polarisation such as Co‐based Heusler compounds as ferromagnetic layer are required. However, these compounds are known to loose their half‐metallicity in the ultrathin film regime, as it is difficult to achieve L21 ordering, which has become a bottleneck for the film growth. Here, the successful deposition using room temperature DC sputtering of the L21 and B2 ordered phases of the Co2FeAl full Heusler compound is reported. Co2FeAl is used as ferromagnetic layer together with highly orientated Pt as nonferromagnetic layer in the Co2FeAl/Pt STE, where an MgO (10 nm) seed layer plays an important role to achieve the L21 and B2 ordering of Co2FeAl. The THz generation in the Co2FeAl/Pt STE is presented, which has a bandwidth of 0.2–4 THz. The THz electric field amplitude is optimized with respect to thickness, orientation, and growth parameters using a thickness dependent model considering the optically induced spin current, superdiffusive spin current, inverse spin Hall effect, and the THz attenuation in the layers. This study, based on the full Heusler Co2FeAl compound opens up a plethora of possibilities in STE research involving full Heusler compounds.

Design and characterization of eutectic refractory high entropy alloys

Eutectic alloys are natural in-situ composites with near-equilibrium structures, impressive strength-ductility trade-off, and perfect castability. In the current study, novel eutectic Al-Cr-Nb-Ti-Zr refractory high entropy alloys (RHEAs) were designed using a CALPHAD (CALculation of PHAse Diagrams) approach. A starting Nb30Ti40Zr30 (at.%) alloy was based on a single-phase bcc solid solution. Alloying with Cr and Al induced the formation of hypoeutectic (Cr20Nb30Ti40Zr10, Al15Cr20Nb15Ti40Zr10, Al23Cr20Nb15Ti32Zr10), eutectic (Al28Cr20Nb15Ti27Zr10), and hypereutectic (Al33Cr20Nb15Ti22Zr10) structures due to the Laves phase precipitation. Al additions also resulted in the B2 ordering of the bcc phase and changing the Laves phase polytype from C15 (fcc) to C14 (hcp). A transfer from the single-phase to eutectic microstructure, connected with the Laves phase amount's growth, increased the alloys' compressive strength and the brittle-to-ductile transition temperature pronouncedly. Quantitative relationships between the Laves phase's volume fraction and the alloys' yield strength at different temperatures were established. Detailed analysis of eutectic alloy after plastic deformation at 800 °C demonstrated the stability of a lamellar B2/C14 Laves phase microstructure due to saving an initial orientation relationship (OR) of (011)B2||(101¯3)C14, [11¯1]B2||[33¯01¯]C14 ensuring a small lattice mismatch. The precipitation of a new phase with a D019 structure having the (011)B2||(101¯3)C14||(02¯21)D019 OR was also revealed. The obtained findings demonstrate new approaches to design RHEAs with (nearly) eutectic structures and a promising balance of mechanical characteristics.

Atomistic simulation of chemical short-range order in HfNbTaZr high entropy alloy based on a newly-developed interatomic potential

Chemical short-range order (CSRO) in high entropy alloys (HEAs) has attracted interests recently and is believed to be capable for tuning their mechanical properties. However, the characterization of CSRO in HEAs through experimental methods remains challenging. In this work, a modified embedded-atom method interatomic potential with good accuracy for studying CSRO in HfNbTaTiZr alloy system was developed. By employing the potential, molecular dynamic/Monte Carlo simulation was performed to investigate the CSRO in HfNbTaZr HEA. The results indicated that Hf-Zr and Nb-Ta atom pairs were preferred in the BCC solid solution of HfNbTaZr, and a new type of CSRO with topological B2 order was predicted, which can help to understand the mechanical properties of HfNbTaZr HEA. It was also found that forming of CSRO was an incubation process for the precipitation in HfNbTaZr, implying the significance of CSRO on the phase stability or precipitation behavior of HEAs. The findings in the present work can help in understanding CSRO and establishing its relationship with precipitates in HEAs, and more topics related to CSRO and phase stability in HfNbTaTiZr alloy system can be further investigated by atomistic simulation.

EXAFS-spectroscopy and thermal neutron diffraction study of the effect of deformation by high pressure torsion on the atomic ordering and magnetic properties of the FeCo alloy

The specific features of B2 atomic ordering and the ordering-dependent structure and magnetic characteristics of the FeCo alloy upon quasi-hydrostatic high-pressure torsion (HPT) in the Bridgman anvils at room and elevated temperatures and subsequent annealing have been studied by thermal neutron diffraction, X-ray diffraction, EXAFS spectroscopy, and magnetometry. An abrupt decrease in the degree of long-range order upon HPT has been revealed by neutron diffraction and EXAFS spectroscopy data exhibiting the formation of an almost ideal B2 ordering of in the nearest coordination spheres of Fe and Co atoms.