Heat capacity, thermal conductivity and magnetocaloric effect in Heusler alloy Ni-=SUB=-47-=/SUB=-Mn-=SUB=-40-=/SUB=-Sn-=SUB=-13-=/SUB=-

Abstract

The results of a study of heat capacity, thermal conductivity and magnetocaloric effect of the polycrystalline Ni47Mn40Sn13 alloy depending on temperature (T=80-350 K) and magnetic field (0-8 T) are given. A considerable difference between the values of a sudden change in heat capacity Delta CP in the heating and cooling mode was found near the magnetostructural martensite--austenite phase transition (MSPT), which is related to influence of latent heat of phase transition. Thermal conductivity in the range of T=80-300 K rises with temperature (dkappa/dT>0) and increases in more than three times. Electronic thermal conductivity in the martensitic phase (T=150 K) is 37% of the total value. An anomalous rise of thermal conductivity Deltakappa=kappa(aust)-kappa(mart)=4.2 Wm K was found in the region of MSPT. The contributions of electrons and phonons to the observed sudden change are 63 and 37% respectively and are conditioned both by a rise of mobility of conduction electrons under a martensite--austenite transition and by an increase of phonons' free path length. The magnetocaloric effect in cyclic magnetic fields with the amplitude of 1.8 T was studied. A dependence of reverse effect value on temperature scanning rate was established. The direct measurements of Delta T in the cyclic magnetic field of 1.2 T show a twofold decrease of the effect amplitude near TC at an increase of cyclic magnetic field frequency from 1 to 30 Hz. Most likely, this is related to magnetic and microstructural heterogeneities which act as an additional thermal dissipation channel. Keywords: Heusler alloys, heat capacity, thermal conductivity, magnetocaloric effect, cyclic magnetic fields.