Anisotropic magnetoresistance in antiferromagnetic La$_{1-x}$Sr$_{x}$MnO$_{3}$ films

Anisotropic magnetoresistance (AMR) is commonly used to probe magnetocrystalline anisotropy in conducting ferromagnets. Here, we have measured AMR in antiferromagnetic thin films of La$_{1-x}$Sr$_{x}$MnO$_{3}$ near $x$=0.5. These epitaxial films were grown using ozone-assisted oxide MBE. They exhibit $A$-type magnetic order where the spins are aligned ferromagnetically within the planes and coupled antiferromagnetically between neighboring planes. The transport within these films is presumed to be highly two-dimensional as a result of $d_{x}2_{-y}2$ orbital occupancy. Upon cooling below the magnetic ordering (N\'{e}el) temperature, the AMR signal is found to be modulated with a periodicity of $\pi $, while at lower temperatures, a $\pi $/2 periodicity emerges. The amplitude of the $\pi $/2 modulation increases as the temperature is lowered. We shall discuss the data and possible explanations will be presented.