| Exchange bias is a phenomenon observed at interfaces between ferromagnetic (F) and antiferromagnetic (AF) materials. Exchange coupling between spins in the two materials results in a unidirectional anisotropy which manifests itself in experiments as a shift of the magnetization hysteresis loop of the ferromagnet. Despite substantial research efforts, the mechanism of this interfacial phenomenon has yet to be fully understood. The amount of shift serves as a measure of the effect called the exchange bias field, or briefly, exchange bias.; We have studied the exchange bias in a series of thin F/AF bilayers grown by ozone-assisted molecular beam epitaxy. Calcium doped lanthanum manganite perovskites La(1-x)Ca xMnO3 of appropriate doping levels x were used to realize the bilayers. The films were grown on (100)-oriented closely lattice-matched perovskite SrTiO3 substrates. Through detailed structural characterization, we show that our bilayers resemble very much a model system, on which the simple theoretical approach of Meiklejohn and Bean can be tested.; In our series of bilayers, the thickness of the AF layer has been varied, while that of the F layer has been kept constant. We have studied the exchange bias as a function of the AF layer thickness The observed dependence is similar to that exhibited by exchange-biased systems based on other materials.; The exchange bias effect has been successfully utilized in the magnetic recording industry. From the applications standpoint, it is interesting to know the critical AF thickness of an exchange-biased system---a value, below which the effect vanishes. From the results of our investigation, we determine the critical AF thickness.; Within the framework of the simple model of Meiklejohn and Bean for the exchange bias, we have used the AF thickness dependence and the critical value to estimate the magnetocrystalline anisotropy of the antiferromagnetic material, a quantity not previously reported for AF manganites. |
