The prospect of electric field control of magnetism has motivated extensive research on multiferroic where more than two order parameters coexist. Among multiferroics BiFeO3 is a unique material which has a coupling between ferroelectricity and antiferromagnetism at room temperature, hence is a promising material for new spin-electronics devices. One of the most promising applications is the use of BiFeO3 as an antiferromagnetic layer for an exchange bias. Therefore understanding mechanisms of exchange bias in BiFeO3/ferromagnet could give us important clues to accomplish electric field control exchange bias. In addition an alternative control of exchange bias in BiFeO3/ferromagnet instead of electric field could extend the scope of possible applications. For example light control provides a way for contactless control of high-density spintronic devices.
In the first part of this thesis we have reported an intriguing exchange bias in BiFeO3 thin films on SrTiO3 substrates. While the field cooling temperature (~300 K) is far below the Neel temperature of BiFeO3 (~640 K), we clearly observe the exchange bias (~ ?225 Oe) at 10 K. The exchange bias becomes strong with lowering temperature and depends on the applied magnetic field direction during cooling, irrespective of crystallographic directions. Magnetic force microscopy measurements reveal the existence of a ferri/ferromagnetic material, probably γ-Fe2O3, at the grain boundary of antiferromagnetic BiFeO3, forming a core-shell like structure. Based on the uncompensated spins and strong anisotropy of antiferromagnetic layer, we have explained the observed exchange bias.
In the second part of this thesis, we have investigated the uncompensated spins in exchange-biased BiFeO3/?-Fe2O3 core/shell-like thin films. For both in-plane and out-of-plane directions of the films, clear exchange bias appears and coercive field asymmetrically changes with respect to the annealing temperature and the cycling of magnetic field. These results can be explained by the different characters of two types of uncompensated spins. The uncompensated spins at antiferromagnetic domain-walls (rough interface) are weakly (strongly) affected by the external magnetic field. The control of uncompensated spins at domain-walls is crucial in order to realize exchange bias at room temperature along the out-of-plane direction of BiFeO3/?-Fe2O3 core/shell-like thin films.
In the third part of this thesis, we report the facile control of exchange bias in BiFeO3/La2/3Sr1/3MnO3 thin films on an SrTiO3 substrate using light irradiation. Illumination with weak red light (?: 630 nm, intensity: ~1 mW/cm2) reduced the exchange bias field of BFO/LSMO from ?30 Oe in the dark to ?2 Oe with red light. In accompanying the decrease of exchange bias, the resistance of BiFeO3/La2/3Sr1/3MnO3 significantly increased. These results were attributed to the reduction in the hole-doping ratio of La2/3Sr1/3MnO3 and the weakened exchange coupling between Fe and Mn spins at the interface, resulting from photo-injected electrons from the SrTiO3 substrate. Successive turning on/off of red light gives rise to cyclical change of corresponding exchange bias, which should be useful for applications like photo-controlled tunneling magnetoresistance devices.