Magnetization depinning transition, anisotropic magnetoresistance and inplane anisotropy in two polytypes of La2/3Sr1/3MnO3La2/3Sr1/3MnO3 epitaxial films

The isothermal magnetoresistance [R(θ)][R(θ)] of [0 0 1] and [1 1 0] epitaxial films of La2/3Sr1/3MnO3La2/3Sr1/3MnO3 measured as a function of the angle θθ between current (I⇒) and magnetic field (H⇒), both in the plane of the film, is measured at several temperatures between 10 and 300 K. The magnetic easy axis of these polytypes is intimately related to the orientation of Mn–O–Mn bonds with respect to the crystallographic axis on the plane of the substrate and energy equivalence of some of these axes. The magnetization vector (M⇒) of the [0 0 1] and [1 1 0] type films is pinned along the [1 1 0] and [0 0 1] directions, respectively, at low fields. A magnetization orientation phase transition (MRPT) which manifests itself as a discontinuity and hysteresis in R(ψ)R(ψ) where ψψ is the angle between H⇒ and the easy axis for the H⇒ below a critical value H⇒* has been established. The boundary of the pinned and depinned phase on the H–T plane has been established. The highly robust pinning of magnetization seen in [1 1 0] films is related to their uniquely defined easy axis. The isothermal resistance R⊥R⊥ and R∥R∥ for I⇒⊥H⇒ and I⇒∥H⇒, respectively, for both polytypes follows the inequality R⊥>R∥R⊥>R∥ for all ranges of fields (0⩽H⩽3500Oe) and temperatures (10–300 K). A full fledged analysis of the rotational magnetoresistance is carried out in the framework of Döring theory for MR in single crystal samples. Strong deviations from the predicted angular dependence are seen in the irreversible regime of magnetization.