Efeitos da pressão nas propriedades estruturais e magnéticas de compostos multifuncionais de composição nominal Ni50Mn35In15

Abstract

Here we report the use hydrostatic pressure to tune the structural properties of the Heusler compounds and then to verify the effects of the lattice contraction on their magnetic properties. Two polycrystalline compounds with nominal composition of Ni50Mn35In15, but with different crystalline structures at room temperature, were studied. X-ray diffraction (XRD) analysis by Rietveld method showed that the first Ni50Mn35In15 samples, A133, present two crystalline phases at ambient pressure and temperature conditions, cubic (space group Fm3തm) and monoclinic modulated (space group I2/m) structural phase, while the second sample, A139, present only the cubic (space group Fm3തm) phase. XRD measurements as a function of temperature have shown that the second phase of sample A133 (monoclinic modulated phase) is responsible for the martensitic phase of the sample which stabilizes when the temperature is lower than 250 K. In contrast, for the sample A139 no structural change was observed with temperature variation. Magnetization measurements as a function of temperature and magnetic field showed that the compound A133 shows a magnetic transition from paramagnetic (PM) to ferromagnetic (FM) in the austenitic phase at 311 K, followed by a first order magnetic-structural transition of the FM austenite phase for an antiferromagnetic (AFM) martensitic phase between 290 K and 276 K, and the new magnetic transition from the AFM state to FM in the martensitic phase at 194 K. While for the A139 compound only a second-order magnetic transition was observed, from the PM to FM state around 316 K. With the increase of the magnetic field the magnetostructural transition temperatures and the thermal hysteresis decrease in the sample A133, while for the A139 sample the magnetization increases considerably while the magnetic transition temperature is shifted to higher temperatures. The application of pressure in these two materials showed different effects, while for A133 compound the increase of pressure causes the cubic phase to stabilize in 10 GPa, for the A139 sample we observe the appearance of the martensitic phase for pressures higher than 6 GPa. Measurements performed during decompression showed that A133 had structural shape memory, while for A139 sample the process was completely reversible. In order to further understanding the impact of the structural changes on magnetic properties of the studied materials, we developed a new diamond pressure cell to perform magnetization experiments on the commercial equipment MPMS. The cell was them commissioned and showed the efficiency for magnetization experiment under high pressure (up to 10 GPa). This new instrumentation will certainly help on the understanding of the magnetic properties of these materials as well of the other similar compounds. In general, the results obtained indicates that the crystalline structure of the Heusler Ni50Mn35In15 compounds can be altered by applying pressure, which should also induces changes in the macroscopic properties of these materials and may increase their potential for technological applications.