Degree Of L21 Ordering Research Articles

High L21-atomic ordering and spin-polarization in Co2MnZ (Z = Ge, Sn) Heusler thin films with low-temperature annealing process

Although an enhanced magnetoresistance (MR) has been observed in many Co2-based Heusler alloys by promoting their structural ordering from B2 to L21 by post-annealing at higher temperatures (Tann > 500 °C), it is desirable to search for other Heusler alloys that crystallize in L21-order below 300 °C, as the maximum Tann is restricted for processing devices. For Co2MnZ (Z = Ge, Sn) Heusler alloys, an L21-order is expected to appear even in the as-deposited state or by a low-temperature annealing process due to their very high L21 to B2-order transition temperature (>1500 K). Here, epitaxial Co2MnZ films were grown on MgO (001) substrate at room temperature (RT) and post-annealed at Tann = 200–500 °C. Interestingly, as-sputtered films exhibit L21-ordering, which improves systematically upon increasing Tann. The spin-polarization of electric current (β) was estimated at RT using nonlocal spin-valve (NLSV) devices by measuring the spin-accumulation signal in a copper (Cu) channel. It was found that at Tann = 300 °C, the β value of Co2MnGe films is higher (∼0.65) than that of Co2FeGe0.5Ga0.5 films due to a higher degree of L21-order, which makes the Co2MnGe alloy a promising ferromagnetic electrode for spintronic device applications.

Superelastic behavior and elastocaloric effect in a Ni51.5Fe21.5Ga27.0 ferromagnetic shape memory single crystal under compression

Ni51.5Fe21.5Ga27.0 single crystals have been subjected to different heat treatments resulting in a different degree of L21 ordering. Superelastic response has been measured at different temperatures in compression mode. The mechanical behavior strongly depends on axis orientation. In the [001] direction, perfect superelasticity over a wide range of temperatures is found. For the [110] orientation, the material fails by brittle fracture short above austenite transformation finish temperature, Af. A linear dependence of the critical stress with temperature has been found in agreement with Clausius-Clapeyron equation. The slope does not significantly change with the degree of order, but it is notably affected by the crystal orientation. The microstructure of the samples after mechanical tests has been studied by transmission electron microscopy. The superelastic cycling produces dislocations with a Burgers vector that suggests local microplastic deformation of the martensitic phase. Finally, the adiabatic temperature change has been used to chacterize the elastocaloric effect in this alloy. The adiabatic cooling is found to be larger in the [110] than in the [001] orientation at 240 K. However, the brittleness of [110] samples avoid testing the adiabatic temperature change at room temperature. The adiabatic cooling in [001] orientation decreases systematically with temperature, which is related to decrease of the strain and entropy change of transformation.

Effects of composition and crystallographic ordering on the ferromagnetic transition in Ni[sbnd]Co[sbnd]Mn[sbnd]In magnetic shape memory alloys

In the present study, the effects of composition and the level of crystallographic ordering were studied in Ni45Co5Mn50-XInX magnetic shape memory alloys with X = 13.55, 13.71 and 13.92, in order to reveal the relationship between the Curie temperature (TC) of the austenite phase and the degree of L21 ordering. Various samples were prepared using secondary heat treatments at different temperatures below the order-disorder transition temperature and for different durations, after solution heat treatment, in order to control the degree of ordering. TCs were determined using calorimetric and magnetometry measurements. The relationship between the TC and the degree of order was used to predict the evolution of the order during selected heat treatments. It was shown that the TC of austenite exhibits a monotonous non-linear dependence on the fraction of the L21 phase. This relationship was corroborated via ab initio numerical calculations within a mean field approximation. Dependence of the crystallographic order parameter on temperature and duration of secondary heat treatments showed that ordering kinetics is fast, the total degree of order depends only on the annealing temperature and cooling rate for long enough treatments, and maximum achievable degree of order is limited by the low thermal energy available for atomic diffusion at low temperatures. Finally, small changes in composition were observed to have a significant impact on martensitic transformation temperatures, and a significant but smaller effect on the ferromagnetic transition in these systems.

Thickness dependent structural, magnetic and magneto-dynamic properties of Mn rich Ni-Mn-Sn films

Off-stoichiometric Mn-rich Ni-Mn-Sn films of different thicknesses were deposited on Si (100) substrate by dc magnetron sputtering at ambient temperature and then annealed ex situ at 550 °C for 1 h under high vacuum. X-ray diffraction patterns revealed that as deposited films were amorphous, whereas the annealed films exhibited room temperature cubic structure with space group Fm3¯m (space group 225) with high degree of L21 ordering. Magnetic study of the annealed film disclosed room temperature ferromagnetic order with easy axis of magnetization along the plane of the film. Magneto-dynamic study of the annealed films was carried out using a micro strip-ferromagnetic resonance (FMR) spectrometer. The deduced values of saturation magnetization from frequency dependence of the resonance field of the FMR spectra were in close agreement with those obtained from dc magnetization measurement. The analysis of frequency variation of linewidth shows the presence of low Gilbert damping constant of 0.007 for the 500 nm thick film. Gilbert damping constant was found to increase slightly with film thickness. The polar angle variation FMR study for 1500 nm film revealed the presence of high effective perpendicular anisotropy (K1 ≈ −9.0 ± 0.5 × 105 erg/cm3) and easy magnetization along its plane.

Enhancement of L21 order and spin-polarization of Heusler alloy Co2MnSi thin film by Ag alloying

We report enhancement of L21 order and the resultant improvement of spin polarization of Co2MnSi (CMS) Heusler alloy thin films by a small amount of Ag alloying (xAg⩽4.5 atomic (at.) %). The degree of L21 order in CMS increases with xAg(=2.7at.% and 4.5at.%), which shows a positive correlation with the magnitude of negative anisotropic magnetoresistance ratio, suggesting the enhancement of spin polarization. Since the benefit of Ag-doping was confirmed even at low annealing temperature around 350°C, this method could be useful for processing spintronic devices using Heusler alloys at relatively low annealing or substrate temperature.

Increasing the achievable state of order in Ni-based Heusler alloys via quenched-in vacancies

The ability to tune the degree of L21 order is of utmost importance for the magneto-mechanical properties of Ni-Mn-based Heusler alloys, e.g., the appearance of a martensitic phase in the Ni2MnAl system. Here, differential scanning calorimetry is established as a convenient tool for determining the state of order by way of its effect on the magnetic transition temperature, and it is used for studying the low-temperature ordering kinetics in Ni2MnAl. A significant acceleration of ordering kinetics due to excess vacancies retained after high-temperature quenching is demonstrated. Using this effect, equilibrium of order could be attained at temperatures as low as 623 K, where ordering under equilibrium vacancy concentration would take unpractically long.

The effect of heat treatments on Ni43Mn42Co4Sn11 meta-magnetic shape memory alloys for magnetic refrigeration

The inverse magnetocaloric effect (MCE) in bulk polycrystalline and melt-spun ribbons of the Ni43Mn42Co4Sn11 meta-magnetic shape memory alloy (MSMA) is investigated. The influence of several material properties on the MCE and relative cooling power (RCP) are discussed and the property combinations for optimum MCE and RCP identified for a given thermodynamic framework. These include a small slope of magnetic field vs. martensitic transformation temperature phase diagram, a narrow transformation range, low transformation thermal hysteresis and a large change in magnetization on martensitic transformation, which results in low levels of applied magnetic fields desired for repeated MCE on field cycling. The thermo-magnetic responses of the samples were measured before and after heat treatments. The heat-treated ribbons produced the most favorable MCE by exhibiting the highest magnetization change and smallest elastic energy storage through the transformation. This was attributed to the specific microstructural features, including grain size to thickness ratio and degree of L21 ordering. In addition, issues in the literature in determining RCP for MSMAs are discussed, and a new method to find RCP is proposed and implemented. Completely reversible magnetic-field-induced martensitic transformation cycles were used to investigate hysteresis losses relative to actual refrigeration cycles, whereby the RCP was calculated using the defined thermodynamic framework and indirectly measured entropy changes. The annealed ribbons exhibited the high RCP level of 242Jkg−1 under the applied field of 7T compared with a theoretical maximum of 343Jkg−1. Similar values of RCP in other MSMAs can be achievable if microstructural elastic energy storage and hysteresis loss are minimized during the transformation with the help of annealing treatments.

Fabrication of High-Quality Co2FeSi/SiO x N y /Si(100) Tunnel Contacts Using Radical-Oxynitridation-Formed SiO x N y Barrier for Si-Based Spin Transistors

A high-quality Co2FeSi (CFS)/SiO x N y /Si tunnel junction was fabricated, in which the SiO x N y barrier layer was formed by radical oxynitridation of an Si(100) substrate and the CFS electrode was formed by silicidation of an Fe/Co/amorphous-Si multilayer deposited on the barrier layer. The ultrathin SiO x N y barrier layer completely blocked diffusion of Co and Fe atoms into the Si substrate during rapid thermal annealing (RTA) for the silicidation. X-ray diffraction investigations clarified that the CFS film on the ultrathin SiO x N y barrier layer exhibited a highly (110)-oriented texture structure and that the film had the L21 structure with a high degree of L21 order. High resolution cross-sectional transmission electron microscopy investigations revealed that the CFS/SiO x N y interface was atomically flat and that the crystal lattice of the CFS film was directly grown on the SiO x N y surface without degradation of the crystallinity at the interface.

Study of yield stress anomaly of Fe2MnAl single crystal by in situ TEM straining

L21-structured Fe59Mn17Al24 shows a yield stress anomaly with a peak yield stress at 700 K. The aim of the work reported here was to determine the dislocation mechanisms involved in this anomalous behaviour by performing in situ straining on Fe59Mn17Al24 single crystals in a transmission electron microscope at 300–900 K. Cross-slip of ⟨111⟩ dislocations was frequently found to occur between {110} and {211} planes at all temperatures tested. At 300 K, dislocations were four-fold dissociated and the partials moved together under stress. At 700–800 K, partial dislocations with a Burgers vector of a/2⟨111⟩ dominated the deformation. These partial dislocations moved independently in the ordered matrix in a jerky manner, with rapid motion between long periods of locking. X-ray diffraction measurements showed that the degree of L21-order slightly decreased with increasing temperature in the range 300–700 K, and dropped rapidly above 700 K. At 900 K, the material was B2-ordered. The increased yield stress at elevated temperatures is primarily attributed to the uncoupling of a/2⟨111⟩ partial dislocations trailing shear-induced anti-phase boundaries.

Quadratic magneto-optical Kerr effect in Co2MnSi

Quadratic magneto-optical Kerr effects (QMOKE) are investigated in epitaxial Co2MnSi thin films as a function of the post-deposition annealing temperature. We determine the amplitude of the QMOKE signal for different annealing temperatures, which provide various degrees of L21 crystal ordering, as manifested by X-ray diffraction. We observe that QMOKE is significantly present only when the L21 ordering phase is also present, and that the QMOKE signal increases with the higher degree of L21 ordering. Additionally, we notice that the linear magneto-optical Kerr effect (LMOKE) decreases with higher annealing temperature. A comparison of the linear and quadratic MOKE contributions shows the amplitudes are of similar magnitude. From these results, we conclude that the presence of QMOKE is linked to the higher degree of L21 ordering in Co2MnSi.

Size correlated long and short range order of ternary Co2FeGa Heusler nanoparticles

The long and short range order of chemically prepared Co2FeGa Heusler nanoparticles with various sizes are determined by x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) spectroscopy. Specifically, EXAFS fittings reveal the size dependent crystal structure and short range order of the Heusler type Co2FeGa nanoparticles. With decreasing particle size, the degree of L21 order in the nanoparticles decreases and the probability of B2 disorder increases simultaneously. The consequences of antisite disorder on the size correlated structure of Co2FeGa nanoparticles are also discussed.

Analysis of L21-ordering in full-Heusler Co2FeSi alloy thin films formed by rapid thermal annealing

The authors developed a new analysis approach for evaluation of atomic ordering in full-Heusler alloys, which is extension of the commonly used Webster model. Our model can give accurate physical formalism for the degree of atomic ordering in the L21 structure, including correction with respect to the fully disordered A2 structure, i.e., the model can directly evaluate the degree of L21 ordering under a lower ordering structure than the complete B2-ordering structure. The proposed model was applied to full-Heusler Co2FeSi alloy thin films formed by rapid thermal annealing. The film formed at TA=800 °C showed a high degree of L21 ordering of 83% under a high degree of B2 ordering of 97%.

Enhancement of spin-asymmetry by L21-ordering in Co2MnSi/Cr/Co2MnSi current-perpendicular-to-plane magnetoresistance devices

Co 2 MnSi / Cr / Co 2 MnSi (001)-fully epitaxial current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices were fabricated via an UHV magnetron sputtering system. The relationship between the degree of chemical ordering in Co2MnSi (CMS) and the CPP-GMR characteristics was investigated systematically against the annealing temperature of the devices. X-ray diffraction profiles and reflection high-energy electron diffraction images indicated that annealing improved L21-ordering. The MR ratio also increased upon annealing and the maximum MR ratio of 5.2% and ΔRA of 6.5 mΩ μm2 were achieved by annealing at 400 °C. These results indicate that promoting the degree of L21-ordering in CMS enhances the bulk and/or interface spin-asymmetry coefficients.

Preparation and characterization of highly L21-ordered full-Heusler alloy Co2FeAl0.5Si0.5 thin films for spintronics device applications

We report the investigation of structure and magnetic properties of full-Heusler alloy Co2FeAl0.5Si0.5 (CFAS) thin films grown on MgO-buffered MgO (001) substrates through magnetron sputtering. It was found that single-crystal CFAS thin films with high degree of L21 ordering and sufficiently flat surface could be obtained after postdeposition annealing. All the films show a distinct uniaxial magnetic anisotropy with the easy axis of magnetization along the in-plane [110] direction. These results indicate that the use of the MgO buffer for CFAS is a promising approach for achieving a higher tunnel magnetoresistance ratio, and thus for spintronics device applications.

High spin polarization in a two phase quaternary Heusler alloy Co2MnAl1−XSnX

We have investigated the structure and the spin polarization of Co2MnAl, Co2MnSn, and Co2MnAl0.5Sn0.5 alloys. The structures of the Co2MnAl and Co2MnSn alloys that were annealed for four days at 450°C were B2 and L21, respectively. Co2MnAl0.5Sn0.5 had a dendrite microstructure that was formed by the phase separation into Al-enriched B2 and Sn-enriched L21 phases during solidification. The saturation magnetization values of Co2MnAl, Co2MnAl0.5Sn0.5, and Co2MnSn were 3.66μB, 4.3μB, and 4.9μB, respectively, which are slightly lower than the theoretical values of 4μB, 4.5μB, and 5μB due to the low degree of L21 ordering. The spin polarization P measured by the point contact Andreev reflection technique were 0.56 for B2-Co2MnAl, 0.6 for L21-Co2MnSn, and interestingly, a higher value of 0.63 for the two phase Co2MnAl0.5Sn0.5 alloy.

Effects of ordering type and degree on monoclinic distortion of 18R-type martensite in Cu-Zn-Al alloys

In order to clarify the relationship between the extent of the distortion from the ideal structure and the nature of ordering (type, degree), a rigid hard sphere model for the M18R structure is developed by introducing the modified Bragg-Williams-Gorski-type order parameters. The ide- ally close-packed 9R or 18R structures are produced from the disordered or D03 ordered parent phases, while the modified 9R and 18R structures are produced from the B2 ordered and L21 ordered parent phases, respectively. The extent of the distortion in the close-packed plane in- creases with increasing degree of B2 order or with decreasing degree of L21 order. Related experiments based on the variation of X-ray diffraction pattern for the stabilization of Cu- Zn-Al shape memory alloy have confirmed the validity of the theoretical development.