Solid State Phenomena Vol. 190

Abstract: We demonstrate the importance of accounting for the complex magnetic ground state and finite temperature magnetic excitations in theoretical simulations of structural and elastic properties of transition metal alloys. Considering Fe₇₂Cr₁₆Ni₁₂ face centered cubic (fcc) alloy, we compare results of first-principles calculations carried out for ferromagnetic and non-magnetic states, as well as for the state with disordered local moments. We show that the latter gives much more accurate description of the elastic properties for paramagnetic alloys. We carry out a determination of the magnetic ground state for fcc Fe-Mn alloys, considering collinear, as well as non-collinear states, and show the sensitively of structural and elastic properties in this system to the detailed alignment between magnetic moments. We therefore conclude that it is essential to develop accurate models of the magnetic state for the predictive description of properties of transition metal alloys.

Abstract: The melt spun ribbons of ferromagnetic shape-memory alloy (FSMA) Ni₅₃Mn₂₄Ga₂₃ have been prepared by rapid quenching. Thermomechanical properties have been studied by multi-point technique and perfect shape memory effect (SME) observed. The magnetic field effect on thermomechanical behavior was studied by placing multi-point press into Bitter magnet. A giant (1.2%) bending strain, due to magnetic field-induced martensitic transformation (magnetic-field-induced SME), has been observed at a constant temperature T= 56 °C in a field of 6 T. At least 80% of martensitic transformation reversibly induced by the external field was observed experimentally. Submicron sized samples of the alloy with thicknesses down to 300 nm have been milled by focused ion beam (FIB) technique. The deformation behavior of these samples was studied by Omniprobe micromanipulator in the vacuum chamber of FIB device. The SME response was tested in situ by heating the samples with a semiconductor laser. Strong two-way SME was observed due to bending strains of the samples under study.

Abstract: At ambient pressure, the Fe_0.49(Rh_1-xPd_x)_0.51 alloys with the Pd concentration within 0 < x < 0.12 exhibit an antiferromagnetic (AF) state below the critical temperature T_t, while above x = 0.12 the alloys have a ferromagnetic (F) order up to the Curie temperature T_C 650 K. The temperature and field dependences of the magnetization in the alloys with x = 0.08; 0.13 were investigated under hydrostatic pressure up to 10 kbar. The application of pressure of about 8.5 kbar is observed to induce the F-AF phase transition in the ferromagnetically ordered compound with x = 0.13. The AF-F-AF phase evolution was revealed with increasing pressure in the AF-ordered alloy with x = 0.08.

Abstract: We report magnetic and magnetocaloric properties of polycrystalline series of the (Ho_0.9Er_0.1)_1-xGd_xCo₂ (x = 0.05, 0.1 and 0.15) solid solutions. These samples were synthesized using high purity rare earth metals and cobalt. X-ray diffraction patterns taken at room temperature reveal that all compounds have the C15 cubic Laves phase structure. Magnetization measurements were carried out using a vibration sample magnetometer with a step motor in fields up to 14 T using a Bitter-type magnet. Heat capacity measurements have been performed in the temperature range of 2-300 K without magnetic field and in a magnetic field of 1 and 2 T. The magnetocaloric effect has been estimated in terms of isothermal magnetic entropy change for all solid solutions in magnetic fields up to 3 T. The effect of increasing Gd amount in (Ho_0.9Er_0.1)_1-xGd_xCo₂ on the magnetic and magnetocaloric properties will be discussed.

Abstract: We report the effect of shorttime vacuum annealing, during 10 minutes at 923 K, 973 K, 1023 K and 1073 K, on magnetostructural properties of as-quenched ribbons of Ni_45.5Mn₄₃In_11.5 Heusler alloy. The martensitic transformation is strongly sensitive to annealing treatments. The martensitic phase starting temperature is significantly shifted from 239 K towards higher temperatures around 370 K. It suffers a break down in two peaks when a field equal or higher than 500 Oe is applied to the as-quenched sample. This effect is not detected in the transformation of annealed ribbons but its signature can be observed at low temperature. Moreover, under high magnetic field up to 30 kOe temperatures associated with both martensitic and reverse transitions do not change for annealed samples, meanwhile the magnetization difference between austenite and martensite increases with the field. Nevertheless, it almost remains unchanged in the as-quenched ribbon.

Abstract: We report on the results of the magnetic and structure properties investigation of Heusler alloy films. Ni-Mn-In thin films were formed by pulsed laser deposition. Stoichiometry was varied and controlled by co-deposition technique. The different deposition conditions and influence of the annealing temperature on the film composition were investigated using Auger electron spectroscopy and Rutherford backscattering spectrometry. The optimal annealing temperature was found to be 620 K. The set of the films deposited on the oxidized Si (100) substrate at room temperature and annealed at 620 K was investigated using X-ray diffractometry at room temperature and Vibrating sample magnetometery at low temperatures. The crystal structure was found to be a mixture of austenitic and martensitic phases at room temperature. Decreasing of Curie temperature from 270 K to 250 K with the decreasing of In concentration from 20 at % to 15 at % was observed.

Abstract: The effect of micro-and nanocrystalline structural state on magnetic properties and magnetic entropy (ΔS_M) of Gd is investigated. The marked influence of severe plastic deformation on ΔS_M is demonstrated.

Abstract: A model for Brayton cooling cycles used in magnetic refrigeration near room temperature was developed. This model was used to calculate a theoretical limit of temperature span and cooling power. The cooling power was calculated for single and double Brayton cooling cycles with Gd as the working body. The obtained results clearly demonstrate the functional ranges of Bryton-cycle refrigerators.

Abstract: Synthesis of nanocomposite based on binary compound and determination of dependence between the values of magnetocaloric effect (MCE) and the grain size of such specimens are the purposes of this work. The results of the investigation in microcrystalline and nanocrystalline samples Y₂₅Fe₇₅ and Y₃₅Fe₆₅ alloys show, that reduction of the grain size down to 80-100 nanometers on nanocrystalline samples of Y₂₅Fe₇₅ and Y₃₅Fe₆₅ alloys leads to increasing of value of MCE by 20-25 % from comparison with values on microcrystalline samples. It can be explained by intergrain exchange interaction in nanocrystalline alloys.

Abstract: The twin boundary motion in Ni-Mn-Ga Heusler alloys has been investigated using Monte Carlo simulations. The Hamiltonian of system includes magnetic and elastic parts and two magnetoelastic terms. It is shown that the twin boundary shifts in a magnetic field at the constant temperature. The spin and strain volume fractions have been obtained at different temperatures.