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Register a new userPressure dependence of Curie temperature and resistivity in complex Heusler alloys
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Using first-principles electronic structure calculations, we have studied the dependence of the Curie temperature on external hydrostatic pressure for random Ni2MnSn Heusler alloys doped with Cu and Pd atoms, over the entire range of dopant concentrations. The Curie temperatures are calculated by applying random-phase approximation to the Heisenberg Hamiltonian whose parameters are determined using the linear response and multiple scattering methods, based on density-functional theory. In (Ni1-x,Pdx)2MnSn alloys, the Curie temperature is found to increase with applied pressure over the whole concentration range. The crossover from the increase to the decrease of the Curie temperature with pressure takes place for Cu concentrations larger than about 70% in (Ni1-x,Cux)2MnSn Heusler alloys. The results for the reference Ni2MnSn Heusler alloy agree well with a previous theoretical study of E. Sasioglu, L. M. Sandratskii and P. Bruno Phys. Rev. B 71 214412 (2005) and also reasonably well with available experimental data. Results for the spin-disorder-induced part of the resistivity in (Ni1-x,Pdx)2MnSn Heusler alloys, calculated by using the disordered local moment model, are also presented. Finally, a qualitative understanding of the results, based on Andersons superexchange interaction and Stearns model of the indirect exchange interaction between localized and itinerant d electrons, is provided.
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Evolution of magnetism in single crystals of the van der Waals compound VI3 in external pressure up to 7.3 GPa studied by measuring magnetization and ac magnetic susceptibility is reported. Four magnetic phase transitions, at T1 = 54.5 K, T2 = 53 K, TC = 49.5 K, and TFM = 26 K, respectively have been observed at ambient pressure. The first two have been attributed to the onset of ferromagnetism in specific crystal-surface layers. The bulk ferromagnetism is characterized by the magnetic ordering transition at Curie temperature TC and the transition between two different ferromagnetic phases TFM, accompanied by a structure transition from monoclinic to triclinic symmetry upon cooling. The pressure effects on magnetic parameters were studied with three independent techniques. TC was found to be almost unaffected by pressures up to 0.6 GPa whereas TFM increases rapidly with increasing pressure and reaches TC at a triple point at ~ 0.85 GPa. At higher pressures, only one magnetic phase transition is observed moving to higher temperatures with increasing pressure to reach 99 K at 7.3 GPa. In contrast, the low-temperature bulk magnetization is dramatically reduced by applying pressure (by more than 50% at 2.5 GPa) suggesting a possible pressure-induced reduction of vanadium magnetic moment. We discussed these results in light of recent theoretical studies to analyze exchange interactions and provide how to increase the Curie temperature of VI3.
We provide a macroscopic theory and experimental results for magnetic resonances of antiferromagnetically-coupled ferrimagnets. Our theory, which interpolates the dynamics of antiferromagnets and ferromagnets smoothly, can describe ferrimagnetic resonances across the angular momentum compensation point. We also present experimental results for spin-torque induced ferrimagnetic resonance at several temperatures. The spectral analysis based on our theory reveals that the Gilbert damping parameter, which has been considered to be strongly temperature dependent, is insensitive to temperature. We envision that our work will facilitate further investigation of ferrimagnetic dynamics by providing a theoretical framework suitable for a broad range of temperatures.
Recently, the observation of atomic thin film SnTe with a Curie temperature (Tc) higher than that of the bulk (Chang et. al., Science 353, 274 (2016)) has boosted the research on two-dimensional (2D) ferroic materials tremendously. However, the origin of such phenomenon has yet been thoroughly investigated, which hinder the understanding and design of novel materials with ferroic orders at 2D limit. By using the density functional theory, we investigated the structural and ferroelectrical properties of 2D SnTe, to reveal the thickness dependence. The calculated results demonstrate that the 2D SnTe automatically transform into periodical bilayer structure, resulting from the surface effect. Moreover, based on the double-well potential and atomic distortion analysis, we found the Tc of the 2D SnTe is higher than the bulk counterpart, and more surprisingly, the Tc exhibits an unusual non-monotonous dependence of thickness, featuring a pronounced atomic distortion and Curie temperature maximum at 8 atomic-layers. In addition, this non-monotonous dependence is sensitive to the external strain and it can be tuned easily by the external compressive strain.
Ni-Mn-In magnetic shape-memory Heusler alloys exhibit generally a large thermal hysteresis at their first-order martensitic phase transition which hinder a technological application in magnetic refrigeration. By optimizing the Cu content in Ni$_2$Cu$_x$Mn$_{1.4-x}$In$_{0.6}$, we obtained a thermal hysteresis of the martensitic phase transition in Ni$_{2}$Cu$_{0.2}$Mn$_{1.2}$In$_{0.6}$ of only 6 K. We can explain this very small hysteresis by an almost perfect habit plane at the interface of martensite and austenite phases. Application of hydrostatic pressure does not reduce the hysteresis further, but shifts the martensitic transition close to room temperature. The isothermal entropy change does not depend on warming or cooling protocols and is pressure independent. Experiments in pulsed-magnetic fields on Ni$_{2}$Cu$_{0.2}$Mn$_{1.2}$In$_{0.6}$ find a reversible magnetocaloric effect with a maximum adiabatic temperature change of -13 K.
A series of sigma-phase Fe_{100-x}V_x samples with 34.4 < x < 59.0 were investigated by neutron and X-ray diffraction and Mossbauer spectroscopy (MS) techniques. The first two methods were used for verification of the transformation from alpha to sigma phase and they also permitted to determine lattice parameters of the unit cell. With MS the Debye temperature, T_D, was evaluated from the temperature dependence of the centre shift, <CS>, assuming its entire temperature dependence originates from the second-order Doppler shift. To our best knowledge, it is the first ever-reported study on T_D in sigma-FeV alloys. Both attice parameters i.e. a and c were revealed to linearly increase with x. T_D shows, however, a non-monotonic behaviour as a function of composition with its extreme values between 425K for x=40 and 600K for x=59. A local maximum of 525K was found to exist at x=43.
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