Coupling of magnetic orders in a 4f metal/oxide system

Antiferromagnetic spintronics is actively explored for use in data storage to enhance robustness, switching speed and packing. A number of archetypal antiferromagnets formed by d-electron (transition) metals are traditionally employed in studies of spin dynamics and control. In contrast, rare-earth antiferromagnets exhibiting 4f-electron magnetism form an emerging class of prospective spintronic materials. In particular, little is known about the potential of elemental rare-earth metals in spintronics. Here, we synthesize films of metallic Eu, an antiferromagnet with a helical magnetic structure, on YSZ and an underlayer of the ferromagnetic semiconductor EuO using molecular beam epitaxy. Their structural quality is established with electron and X-ray diffraction, as well as analytical electron microscopy. Magnetization measurements reveal a peculiar coupling between Eu and EuO magnetic systems leading to negative exchange bias. We demonstrate a strong influence of EuO magnetic state on the electron transport in metallic Eu - a feature appears in the temperature dependence of Eu resistivity around the Curie temperature of EuO. Moreover, magnetic field dependence of the Hall resistivity becomes essentially non-linear; not only the shape of magnetoresistance changes qualitatively but also its sign becomes negative as soon as an ultrathin layer of Eu is in contact with EuO. The results also suggest a magnetic transition in pristine Eu in high fields. The rich physics of Eu and the Eu/EuO structure justifies their exploration with the aim of new spintronic functionalities.