Disordered B2 Phase Research Articles

Interfacial exchange interactions and magnetism of Ni2MnAl/Fe bilayers

Based on a multi-scale calculations, combining ab-initio methods with spin dynamics simulations, we perform a detailed study of the magnetic behavior of Ni2MnAl/Fe bilayers. Our simulations show that such a bilayer exhibits a small exchange bias effect when the Ni2MnAl Heusler alloy is in a disordered B2 phase. Additionally, we present an effective way to control the magnetic structure of the Ni2MnAl antiferromagnet, in the pseudo-ordered B2-I as well as the disordered B2 phases, via a spin-flop coupling to the Fe layer.

Plastic deformation by upsetting the Ni-Fe-Mn-Ga alloy

The temperatures ductility of the polycrystalline Ni2.19Fe0.04Mn0.77Ga alloy was investigated. The analysis of the temperature dependence of the specific magnetization shows that temperatures of the martensitic and magnetic phase transformations have the following values: MF=325 K, AS=343 K, TC=365 K. The differential scanning calorimetry shows that in the temperature range of 930 to 1070 K in the alloy the phase transition of the ordered L21 phase to disordered B2 phase is observed. The stress-strain curves of cylindrical samples of alloy at temperatures of 773 K, 873 K and 973 K were constructed.

Effect of elemental composition on phase formation during milling of multicomponent equiatomic mixtures

Using mechanochemical synthesis through milling of equiatomic multicomponent mixtures of Cr, Fe, Co, Ni, Al, Ti, Mo, and Nb metals in various combinations, we have synthesized powder alloys with different phase compositions: amorphous phase (AP), AP + BCC phase, AP + BCC phase + MO, and FCC + BCC phases. The FCC phase has been shown to be a Ni-based solid solution. The presence of aluminum in a starting mixture helps to stabilize the BCC phase owing to the formation of a disordered B2 phase. Al dissolves in both the BCC and FCC solid solutions, increasing their lattice parameters. In Al-free starting mixtures, Cr is responsible for the formation of the BCC solid solution. The formation of an AP during milling of multicomponent mixtures is favored by the presence of transition metals with a large atomic radius: Ti, Mo, and Nb.

Spatial distribution of structural disorder in Co2FeSi films studied by anisotropic magnetoresistance

We studied the spatial distribution of the partially disordered B2 phase in Co2FeSi films by anisotropic magnetoresistance measurements. The analysis of our experimental results allows not only for the discrimination between different models for the spatial distribution of structural disorder, but also enables us to estimate the volume fraction occupied by the partially disordered B2 phase within the Co2FeSi films grown on GaAs. In particular, our findings are in accordance with a laterally inhomogeneous distribution of the structural disorder in the topmost part of Co2FeSi films.

Strain-induced structural transformation of single-phase Al–Cu–Fe icosahedral quasicrystal during mechanical milling

A single-phase stable icosahedral quasicrystalline sample of high quality with the composition Al62.5Cu25Fe12.5 was produced by the spray forming technique. The material was further investigated by mechanical milling under an argon atmosphere to avoid oxidation during milling. At the initial stages of milling (within 5 h) a significant broadening of the diffraction peaks was observed, indicating a reduction of crystallite size and the introduction of lattice strain, which can be linked to phason strain of the quasilattice. Line broadening was noticed to increase with increasing milling time and in the material milled for longer time only a few broad diffraction peaks, which can be identified as a nanoscale bcc phase (i.e. disordered B2 phase, a ∼ 2.9 Å), were visible. At this stage the diffraction signals belonging to the quasicrystals were no longer observable, indicating a complete transformation of the quasicrystals into the bcc phase. Finally, the bcc phase formed during milling transformed back to the quasicrystalline phase during subsequent annealing treatment. The microhardness measured on the milled powders was found to decrease with increasing milling time, most likely as a consequence of the increased volume fraction of the ductile bcc phase. Attempts are made to rationalize the structural transformation.

Theoretical investigation of the effects of composition and atomic disordering on the properties of Ni 2Mn(Al 1− xGa x) alloy

The influences of composition and Mn–Ga/Al disordering on the magnetic and elastic properties of the Ni 2Mn(Al 1− x Ga x ) Heusler alloy are investigated by the use of the first-principles exact muffin-tin orbital method in combination with coherent-potential approximation. The transition temperature from the completely disordered B2 phase to the ordered L2 1 phase, estimated by the use of Bragg–Williams–Gorsky approximation, is in excellent agreement with experiments. The site-occupancy in the partially disordered alloy is determined by comparing the free energies of the alloys with different site-occupation configurations. It is found that Mn atoms generally prefer to exchange with Al atoms for the alloy with low degree of disorder and low Ga concentration. With increasing degree of disorder and Ga concentration, Mn may exchange with Ga as well. The total magnetic moment of the completely ordered alloy exhibits a minimum around x = 0.7 due to the competition between the increasing magnetic moments of Mn atoms and the decreasing moments of Ni atoms with increasing x. For the partially disordered alloy, both total and local magnetic moments of Mn and Ni decrease linearly with increasing degree of disorder, and Ga atoms show significant magnetism. The shear moduli C′ of the alloys are calculated with respect to both x and the degree of disorder. The results show that C′ decreases with increasing x and degree of disorder. The dependence on composition and on the degree of disorder of the martensitic transition temperatures is discussed in terms of the calculated C′.

Structural and magnetic properties of Co 2CrAl Heusler alloys prepared by mechanical alloying

Mechanical alloying has been used to produce nanocrystalline samples of Co 2CrAl Heusler alloys. The samples were characterized by using different methods. The results indicate that, it is possible to produce L2 1-Co 2CrAl powders after 15 h of ball-milling. The grain size of 15 h ball milled L2 1-Co 2CrAl Heusler phase, calculated by analyzing the XRD peak broadening using Williamson and Hall approach was 14 nm. The estimated magnetic moment per formula unit is ∼2 μ B. The obtained magnetic moment is significantly smaller than the theoretical value of 2.96 μ B for L2 1 structure. It seems that an atomic disorder from the crystalline L2 1-type ordered state and two-phase separation depresses the ferromagnetic ordering in alloy. Also, the effect of annealing on the structural and magnetic properties of ball milled powders was investigated. Two structures were identified for annealed sample, namely L2 1 and B2. The obtained value for magnetic moment of annealed sample is smaller than the as-milled sample due to the presence of disordered B2 phase and improvement of phase separation.

Nanocrystallization and structural correlation in quasicrystalline and crystalline phases during mechanical milling

Abstract The main objective of the proposed work is to investigate the formation of nanoquasicrystalline and related nanocrystalline phases in Al—Cu—Fe system during mechanical milling at the initial stage of milling. The mechanical milling of a quasicrystalline Al65Cu20Fe15 alloy was performed in a high-energy ball mill (Szegvari attritor) by varying milling time from 2.5 min to 60 min under liquid hexane medium at the speed of 400 rpm with a ball to powder ratio of 40 : 1. X-ray diffraction was carried out for evaluating the lattice strain, lattice parameters and crystallite sizes of the milled sample. It was found that the evolution of monoclinic Al13Fe4 phase from Al65Cu20Fe15 quasicrystalline phase occurred after 2.5 min of milling, which was further confirmed by transmission electron microscopy. After 5 min of ball milling nano size disordered B2 phase (bcc, a = 0.29 nm) was also found. The systematic analysis of the XRD patterns for evolving lattice parameter, sequential strain suggests that the lattice strain during milling has been quite considerable.

Atomic ordering and interlayer diffusion of Co2FeSi films grown on GaAs(001) studied by transmission electron microscopy

The influence of the growth temperature on the atomic ordering and interlayer diffusion of Heusler alloy Co2FeSi films grown on GaAs(001) substrates has been studied using high-resolution transmission electron microscopy. The Co2FeSi∕GaAs(001) films grown below 200°C show a coexistence of the Heusler-type L21 phase and the disordered B2 phase, which can be seen mainly near the interface, due to the low growth temperature. This phase coexistence can affect the spin polarization of the Co2FeSi layer near the interface. On the other hand, the film grown at an elevated temperature of 300°C shows a uniformly atomically ordered L21 phase, indicating that 300°C is approximately the transition temperature to the atomically ordered L21 structure. The elevation of the growth temperature, however, results in the formations of interfacial compounds above 250°C, which could be detrimental to the efficient electrical spin injection.

Magnetic properties of polycrystalline [formula omitted] alloys

We have investigated the magnetic properties of the Heusler phase Co 2 Cr 1 - x Fe x Al in the composition regime ( x = 0.3 - 0.5 ) in the disordered B2 phase. Both bulk and surface static and dynamic magnetic aspects were addressed by employing alternating gradient magnetometry (AGM), magneto-optical Kerr effect (MOKE) and Brillouin light scattering (BLS). All samples show ferromagnetic hysteresis loops and a tendency of increasing saturation magnetization M s with the iron content. With BLS the behavior of bulk spin waves and the Damon Eshbach (DE) surface spin wave mode have been studied. The spectra are typical for opaque bulk ferromagnetic samples with strong exchange. The measured spin wave frequencies as a function of magnetic field are in good agreement with the calculated values. Saturation magnetization and gyromagnetic ratio g have been determined from the field-dependent peak positions of the bulk and the DE modes. The g -factor extracted from the DE mode shows a clear tendency of increase with increasing Fe-content. However, we could not find any peculiarities of the alloy with x = 0.4 , which had been proposed as a Heusler phase on the basis of electronic structure calculations [T. Block, C. Felser, G. Jakob, J. Ensling, B. Mühling, P. Gütlich, R.J. Cava, J. Solid State Chem. 176 (2003) 646].

Effects of ball milling and annealing of an alloy in the Al-Fe-Cu system: Implications for phase equilibria

An icosahedral quasicrystalline alloy in the Al-Fe-Cu system has been mechanically milled in a high-energy ball mill (Szegvari attritor) for 1, 3, 6 and 10 h. Samples were characterized by X-ray diffraction and transmission electron microscopy. The evolution of nanosize crystallites of the disordered B2 phase (bcc; a = 0.29 nm), coexisting with either the parent icosahedral phase or an amorphous phase, occurs during milling. Isothermal heat treatment of milled powder at various temperatures (200, 500, 600, 700, 800 and 850°C) leads in all cases, except at 200°C, to the transformation from disordered B2 and amorphous phases to an ordered B2 phase with a high degree of long-range ordering. The maximum degree of superlattice ordering was found after isothermal treatment at 800oC. The implications of these results are discussed with reference to phase equilibria existing between crystalline and quasicrystalline phases in the Al-Fe-Cu system.

An investigation on the transformation of the icosahedral phase in the Al-Fe-Cu system during mechanical milling and subsequent annealing

Abstract Icosahedral quasicrystalline material in the Al-Fe-Cu system was mechanically milled in an attritor ball mill (Szegvari attritor) for 1, 3, 6 and 10 h in dry air, at a speed of 400 rev min−1 and with a ball-to-powder ratio of 20 to 1. Structural transformations and the consequent phase evolutions during mechanical milling and subsequent heat treatments were studied by X-ray diffraction, differential thermal analysis (DTA) and transmission electron microscopy techniques. After milling for 1 h, the evolution of disordered B2 phase (a=0.29 nm) was observed to coexist with the parent icosahedral phase, whereas a microstructure consisting of nanosized B2 particles distributed in an amorphous matrix was observed after further milling (3–10 h). However, no sharp peak corresponding to the phase transformation was identified in DTA, but the microstructure of the powder milled for 10 h after the DTA experiment was found to transform to a mixture of icosahedral and B2 phases, where B2 appears to be the majo...