Unidirectional anisotropy of the resistivity in 112-type EuBaCo$_2$O$_{5.5}$ cobaltite

It is found that the resistivity of EuBaCo$_2$O$_{5.5}$ depends not only on the magnitude of an applied magnetic field but also on its sign. This phenomenon is new for cobalt and manganese oxides. The symmetric or shifted hysteresis loops (depending on the cooling method) for the resistivity correspond to similar loops for the magnetization. The angular dependence of the resistivity is described by the relation $\Delta\rho\sim\sin\theta$, where the angle $\theta=0$ corresponds to the direction of magnetization, at which the sample has been cooled. The results are interpreted in terms of the exchange interaction between the ferromagnetic (FM) and antiferromagnetic (AFM) phases. At the cyclic variation of temperature or magnetic field, the resistivity of EuBaCo$_2$O$_{5.5}$ increases and does not return to the initial state. The metastable state of EuBaCo$_2$O$_{5.5}$ is related to the kinetic phenomena accompanying the first order FM-AFM phase transition.