Magnetic state ofLa1.36Sr1.64Mn2O7probed by magnetic force microscopy

Abstract

We have investigated the ferromagnetic (FM) domain structure of a single-crystal bilayered manganite ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ $(x=0.32)$ by using low-temperature magnetic force microscopy. We observed that below $65\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the FM domains form stable treelike patterns with out-of-plane magnetization. With increasing temperature, the FM domain patterns gradually change in the form of domain wall motion. Above $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the FM domain patterns change more and more with each temperature step. The magnetization changes from the out-of-plane to an in-plane direction around $88\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The in-plane FM domains almost completely disappear near the Curie temperature of this sample $({T}_{C}\ensuremath{\approx}110\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, where the resistivity exhibits a sharp increase. We also observed large changes in the magnetic structures upon thermal cycling. We concluded that the formation of FM domains at low temperatures $(T<80\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ is determined by the energy associated with surface magnetic free poles and domain walls. At high temperatures $(80\phantom{\rule{0.3em}{0ex}}\mathrm{K}<T<{T}_{C})$, the two-dimensional FM fluctuations in the basal plane may also play an important role in forming the domain structures. The evolution of the FM domain patterns with temperature coincides with the change in resistivity.