Modern microscopy techniques indicate that the electrical switching of magnetic domains in multi-ferroic materials is possible. However, the application of such functionality in a real device has yet to be proven. In this work we fabricated an all-oxide spin valve with the ferroelectric anti-ferromagnet BiFeO3 (BFO) as the pinning layer. The multi-layered heterostructure was grown epitaxially on a (0 0 1) SrTiO3 substrate and magneto-resistance was achieved at room temperature, which was switchable magnetically in a similar way to conventional metallic spin valves. Some key physical and material issues for building up such a novel device were addressed, in particular the hetero-epitaxy-induced strain effects on the electrical and magnetic properties of each layer and the establishment of exchange bias between BFO and an oxide ferrimagnet, e.g. Zn0.7Ni0.3Fe 2O4 (ZNFO). The strains caused a significant increase in the coercivity but a decrease in the saturation magnetization of the ferrimagnet used. The former is particularly undesirable because it increases the required switching field. The all-oxide architecture allowed the spin valve to be field annealed from a temperature above the high Néel point of BFO (∼660 K), after which a very large exchange bias field (Hex) was achieved at 5 K and kept at a decent value at room temperature. The Hex-T curve did not follow the widely observed (1 - T/TN)β temperature dependence, but could be explained by the random field model with one-dimensional (1-D) anti-ferromagnetic sublattice magnetization derived from the spin wave theory. Based on the observed 1-D spin wave behavior and the geometric arrangements of the paramagnetic ions at the (0 0 1) surface we propose an atomic model in which only a part of the spin along the diagonal lines in the BFO (0 0 1) surface was strongly exchange coupled with ZNFO.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Polymers and Plastics
- Metals and Alloys