Magnetic and transport properties of nonequilibrium Eu-Fe thin films

We synthesized nonequilibrium Eu_xFe_1-x (x = 0-0.95) alloy thin films by coevaporation and studied their structural, magnetic and transport properties using x-ray diffraction (XRD), Mössbauer spectroscopy, a SQUID and a DC four-probe method. The XRD and Mössbauer spectroscopy suggested that Fe atoms supersaturate in the bcc-Eu phase up to approximately 30 at.% Fe. On the other hand, these methods gave no indication of an extension in the solubility limit of Eu in the bcc-Fe phase. These nonequilibrium films possess the following properties: in the Fe-rich side, they exhibit a ferrimagnetic behaviour characterized by compensation temperatures at which magnetization is nonzero. In the Eu-rich side, the magnetization curve at 4.2 K exhibits a small, irreversible kink at about 8 T. At the same applied field, the magnetoresistance ratio (MR) changes irreversibly from positive to negative values, while subsequent runs yield only reversible, decreasing MR curves. These observations on Eu-rich films suggest that the dissolved Fe atoms disturb the long-range helical order of the Eu spins, and that the spin structure changes irreversibly at about 8 T. At present, it remains unclear whether the above magnetic and transport behaviours are caused by the magnetic moments of Fe or simply by a structural disorder.