Studies On Exchange Bias Effect In Polycrystalline BiFeO₃/FM Systems

Nowadays many people keep their eyes on magneto-electric multiferroics which exhibit simultaneously ferroelectric and magnetic orders with strong coupling between them. From an applicational point of view, this kind of multiferroic materials may finally give a possible way to realize the electrical control of spintronic devices. However, the only multiferroic material people discovered at room temperature till now, BiFeO3(BFO), whose Neel temperature of about643K and Curie temperature of about1143K, is a ferroelectric antiferromagnet(FE-AFM). In order to make full use of the coexistence of antiferromagnetic and ferroelectric orders in spintronic devices, we need to couple BFO with a ferromagnetic material(FM) so as to form a typical exchange bias(EB) system composed of FM/AFM bilayers.So far many groups have published their works on exchange bias in BFO/FM thin films, some different characteristics as compared to traditional EB systems(FeMn/NiFe, etc) were reported, such as temperature dependence, training effect, angular dependence and so on. Nevertheless, most of the research mainly focused their attention on single-crystalline BFO films epitaxially grown on some expensive substrates(e.g., SrTiO3(STO)). Because the fabrication procedure is not only complicate but also costly, it’s unfavorable for further application. In this letter, we first made some single-phase polycrystalline BFO thin films with different thickness by using pulsed laser deposition(PLD). Then combined with magnetron sputtering, polycrystalline BFO/FM(FM=Ni81Fe19, Co) bilayers were finally deposited. We have systematically studied the EB effect, including the temperature dependence, the training effect, the thickness influence of the BFO layer and the angular dependence.1. Studies on temperature dependent exchange bias effect and training behavior in polycrystalline BFO/FM bilayersExtensive studies on the temperature dependent exchange bias effect were carried out in polycrystalline BFO/NiFe and BFO/Co bilayers. In contrast to single-crystalline BFO/FM bilayers, sharp increase of the exchange bias field HE below50K were clearly observed in both of these two bilayers. However, when T is higher than50K, HE increases with T and decreases further when T is larger than230K (for BFO/NiFe) or200K (for BFO/Co), which is similar to those reported in single-crystalline BFO/FM bilayers. After the exploration of magnetic field cooling, the temperature dependent exchange bias can be explained considering two contributions from both the interfacial spin-glass-like frustrated spins and the polycrystalline grains in the BFO layer. Moreover, obvious exchange bias training effect can be observed at both5K and room temperature and the corresponding results can be well fitted based on a recently proposed theoretical model taking into account the energy dissipation of the AFM layer.2. Studies on the BFO thickness and angular dependences on exchange bias effect in polycrystalline BFO/FM bilayersClear exchange bias effects have been observed at room temperature in a series of BFO(t)/NiFe(3.6nm) bilayers, in which the BFO layers are single-phase polycrystalline with t varied from8nm to240nm. With increasing t, both the exchange bias field HE and the coercivity He increase sharply and approach maxima when t is about40nm, which is close to one half the spin cycloidal modulation period (64nm) of the bulk BFO material. The oscillatory variations of He and He with the BFO layer thickness suggest that the cycloidal spin structure may exist in polycrystalline BFO thin films. The angular dependence of the exchange bias exhibits non-collinear unidirectional and uniaxial anisotropies in the BFO(80nm)/Co(4nm) bilayers and the angle between KU and KE is about6°.