V. D. Buchelnikov

Magnetocaloric effect in ribbon samples of Heusler alloys Ni–Mn–M (M=In,Sn)

Direct measurements of the magnetocaloric effect in samples of rapidly quenched ribbons of Mn50Ni40In10 and Ni50Mn37Sn13 Heusler alloys with potential applications in magnetic refrigeration technology are carried out. The measurements were made by a precise method based on the measurement of the oscillation amplitude of the temperature in the sample while is subjected to a modulated magnetic field. In the studied compositions both direct and inverse magnetocaloric effects associated with magnetic (paramagnet-ferromagnet-antiferromagnet) and structural (austenite-martensite) phase transitions are found. Additional inverse magnetocaloric effects of small value are observed around the ferromagnetic transitions.

First-principles calculation of the instability leading to giant inverse magnetocaloric effects

The structural and magnetic properties of functional Ni-Mn-Z (Z=Ga, In, Sn) Heusler alloys are studied by first-principles and Monte Carlo methods. The ab initio calculations give a basic understanding of the underlying physics which is associated with the strong competition of ferro- and antiferromagnetic interactions with increasing chemical disorder. The resulting d-electron orbital dependent magnetic ordering is the driving mechanism of magnetostructural instability which is accompanied by a drop of magnetization governing the size of the magnetocaloric effect. The thermodynamic properties are calculated by using the ab initio magnetic exchange coupling constants in finite-temperature Monte Carlo simulations, which are used to accurately reproduce the experimental entropy and adiabatic temperature changes across the magnetostructural transition.

Monte Carlo and first-principles approaches for single crystal and polycrystalline Ni2MnGa Heusler alloys

The magnetization behaviour of the ferromagnetic shape memory Heusler Ni2MnGa alloy under applied magnetic fields is studied using first-principles and Monte Carlo (MC) calculations. Calculations were carried out for single-crystal and polycrystalline structures with magnetic domains. In the multi-domain approach, the stochastic competition between the magnetic anisotropy field and the external magnetic field is taken into account by introducing a probability factor. By constructing a complex Hamiltonian model with ab initio input parameters, we can predict the temperature dependence of the magnetization in Heusler alloys for low and high magnetic fields by means of MC simulations. The theoretical iso-field magnetization curves are in good agreement with experimental data.

Structural and magnetic phase transitions in shape memory alloys Ni2+xMn1-xGa

Abstract The Heusler-type alloys Ni 2 + X Mn 1 − X Ga exhibit well-defined shape memory properties in a ferromagnetic state. The change of composition X moves martensitic transition temperature T M and the Curie point T C towards each other. To study this behavior, the measurements of AC magnetic susceptibility and DC resistivity were performed. The correspondence of the experimental data with calculated T—X phase diagram is discussed.

New Heusler alloys with a metamagnetostructural phase transition

Investigations of new ferromagnetic shape-memory Ni-Mn-Z Heusler alloys (Z = In, Sn, Sb) are reviewed. Experimental data are described and explained on the assumption that these alloys undergo a phase transition from the ferromagnetic to the antiferromagnetic state (metamagnetic transition). The results of theoretical studies of the phase diagrams of these alloys are considered with regard to the possible change in the character of magnetic ordering (from ferromagnetic to antiferromagnetic) and interaction of the structural martensitic transformation with the metamagnetic transition.

Coupled magnetoelastic and electromagnetic waves in uniaxial crystals having spiral magnetic structure

Abstract Spectrum of coupled magnetoelastic and electromagnetic wave propagating along the axis of the spiral in uniaxial crystals having spiral magnetic structure is theoretically investigated. It is shown, that in these magnetic materials there is resonance interaction of spin waves with elastic and electromagnetic waves near two values of the wave number k ≈ 0 and ≈ q ; q is the quasi-momentum of the spiral. In this case forbidden frequency bands arise in the spectrum of coupled waves. Spectrum of oscillations having right- and left-hand circular polarization appears to be nondegenerated.

Electronic and magnetic properties of the Co2-based Heusler compounds under pressure: first-principles and Monte Carlo studies

Structural, magnetic and electronic properties of stoichiometric Co2 YZ Heusler alloys (Y = Cr, Fe, Mn and Z = Al, Si, Ge) have been studied by means of ab initio calculations and Monte Carlo simulations. The investigations were performed in dependence on different levels of approximations in DFT (FP and ASA modes, as well as GGA and GGA + U schemes) and external pressure. It is shown that in the case of the GGA scheme the half-metallic behavior is clearly observed for compounds containing Cr and Mn transition metals, while Co2FeZ alloys demonstrate the pseudo half-metallic behavior. It is demonstrated that an applied pressure and an account of Coulomb repulsion (U) lead to the stabilization of the half-metallic nature for Co2 YZ alloys. The strongest ferromagnetic inter-sublattice (Co–Y) interactions together with intra-sublattice (Co–Co and Y–Y) interactions explain the high values of the Curie temperature obtained by Monte Carlo simulations using the Heisenberg model. It is observed that a decrease in valence electrons of Y atoms (i.e. Fe substitution by Mn and Cr) leads to the weakening of the exchange interactions and to the reduction of the Curie temperature. Besides, in the case of the FP mode Curie temperatures were found in a good agreement with available experimental and theoretical data, where the latter were obtained by applying the empirical relation between the Curie temperature and the total magnetic moment.

First principles investigation of structural and magnetic properties of Ni–Co–Mn–In Heusler alloys

The structural and magnetic properties of Co-doped Ni–Mn–In Heusler alloys are investigated in the framework of density functional theory and the Monte-Carlo method. The first principles calculations of the equilibrium properties (crystalline structure and magnetic ordering) indicate that the ferromagnetic state is favorable in the austenite structure, whereas the ferrimagnetic order is stable in the martensite structure. The strong competition of ferro- and antiferromagnetic exchange interactions arise from the austenite-martensite crossover. The temperature dependences of total magnetization are calculated by means of the proposed Hamiltonian model with the ab initio magnetic exchange couplings and magnetostructural interrelation. The calculated properties are in good agreement with the experimental ones.

Monte Carlo modeling of exchange bias effect in Ni 50 Mn 25+x Sb 25-x Heusler alloys

A theoretical model for investigation of an exchange bias effect in Heusler Ni50Mn25+xSb25-x alloys by Monte Carlo simulations is presented. It is shown that the exchange bias effect of Heusler Ni 50Mn25+xSb25-x alloys is observed in the composition range 7.5 < x < 15 at the temperature T = 13 K. For case of Ni 50Mn37.5Sb12.5 alloy, we have obtained the exchange bias field HEB = 0.05 T at 4.6 K and a blocking temperature TB = 46.7 K. Theoretical results are in good qualitatively agreement with experimental data.

Direct and inverse magnetocaloric effect in Ni1.81Mn1.64In0.55, Ni1.73Mn1.80In0.47, and Ni1.72Mn1.51In0.49Co0.28 Heusler alloys

The magnetic and magnetocaloric properties of Ni1.81Mn1.64In0.55, Ni1.73Mn1.80In0.47, and Ni1.72Mn1.51In0.49Co0.28 Heusler alloys are investigated. The magnetocaloric effect (MCE) is experimentally measured using the direct method with the help of different protocols. The phase transformation temperatures and the latent heat of metamagnetostructural transitions are determined from the temperature dependences of magnetization and via differential scanning calorimetry. In the case of Ni1.81Mn1.64In0.55 and Ni1.73Mn1.80In0.47 alloys, the temperature shift of martensitic transformation induced by external magnetic field is found. It is demonstrated that, in the Ni1.72Mn1.51In0.49Co0.28 alloy, added Co atoms raise the temperature of structural and magnetic phase changes and enhance the MCE. In addition, it is revealed that the MCE depends on measurement protocols near the first-order phase transition.