Microscopic Evidence of Spin State Order and Spin State Phase Separation in Layered Cobaltites $R{\mathrm{BaCo}}_2{\mathrm{O}}_{5.5}$ with $R=$Y, Tb, Dy, and Ho

We report muon-spin relaxation measurements on the magnetic structures of $R{\mathrm{BaCo}}_2{\mathrm{O}}_{5.5}$ with $R=\mathrm{Y}$, Tb, Dy, and Ho. Three different phases, one ferrimagnetic and two antiferromagnetic, are identified below 300 K. They consist of different ordered spin state arrangements of high-, intermediate-, and low-spin ${\mathrm{Co}}^{3+}$ of ${\mathrm{CoO}}_6$ octahedra. Phase separation into well separated regions with different spin state order is observed in the antiferromagnetic phases. The unusual strongly anisotropic magnetoresistance and its onset at the FM-AFM phase boundary is explained.

Figure 1

Characteristic $\mathrm{ZF}-\mu \mathrm{SR}$ time spectra (left panel) and corresponding Fourier spectra (right panel) for temperatures within each of the different magnetic phases of $Y{\mathrm{BaCo}}_2{\mathrm{O}}_{5.49}$.

Figure 2

Muon-spin precession frequencies as a function of normalized temperature $T/T_{\mathrm{C}}$ for $R{\mathrm{BaCo}}_2{\mathrm{O}}_{5+\delta }$ with $R=\mathrm{Y}$, Tb, Dy, and Ho with $\delta \approx 0.5$. The solid lines present the fit or calculation for $Y$ for respective magnetic structures, see text. Pairs of lines which fit with the same power law and belong to the same structure are drawn in the same color. Magnetic reflections above magnetic structures are given for the unit cell $a_p\times 2a_p\times 2a_p$. For the FM phase, the expected frequencies for various magnetic structures [5, 13, 14, 15, 16] have been calculated for $T=0.94T_C$, see text.