Weak ferromagnetism and electronic properties of half-metallic Co2HfSn and Co2ZrSn Heusler alloys

Abstract

Co-based Heusler alloys are currently exploited in several interesting spintronics applications, such as magnetic electrodes in spin-valve and magnetic tunnel junction devices, due to their half-metallic ferromagnetic properties [1]. Their magnetic and electronic transport behavior at room temperature are intensely studied and tuned to meet several applications demands. In particular, the electronic transport properties display an unusual trend of Seebeck coefficient and electrical conductivity proving the half-metallic ferromagnetic behavior of these compounds [2].In this work, Co2HfSn and Co2ZrSn bulk alloys are synthetized by arc-furnace in which the melt contains pure elements in the appropriate stoichiometric quantities; subsequent, annealing at 800 °C for 6 days is performed to release stress and reach the equilibrium phase.Magnetic properties were investigated in the wide temperature range from 2 to 500 K by SQUID and VSM magnetometers; isothermal hysteresis loops M(H) and magnetization as a function of temperature curves M(T) have been measured. Magnetic results are in excellent agreement with the mean-field model of itinerant ferromagnetism (Stoner model) in the limit of very weak ferromagnetism. The linear relationship between M2 and H/M (i.e. Arrott plot) is maintained at all studied temperatures, both well below and above the Curie temperature (see Fig. 1a). The weak itinerant ferromagnetism model, by means of its temperature-dependent coefficients, is successfully used to determine the Curie temperature resulting at 432 K and 458 K for the Co2HfSn and Co2ZrSn alloy, respectively. In addition, the extrapolation of the experimental M(T) curve at T = 0 K reveals a magnetic moment of 2.0 μB/f.u. 1.81 μB/f.u. for the Co2HfSn and Co2ZrSn samples, respectively.Electric and magnetic experimental results are compared with the ab-initio calculation, which provides the spin-polarized density of states (DOS) confirming that Co2HfSn and Co2ZrSn alloys are half-metallic compounds with a majority-spin channel across the Fermi surface, whereas the minority-spin channel shows a small band gap (see Fig. 1b).  a) Arrott plot for the Co2HfSn sample; b) DOS by ab-initio calculation for the Co2ZrSn sample