Crystal structure and magnetic properties of the unique Jahn-Teller system154Sm0.6Sr0.4MnO3

Structural neutron diffraction investigations as well as data on resistance, linear susceptibility, and second harmonic of magnetization are presented for ${}^{154}{\mathrm{Sm}}_{0.6}{\mathrm{Sr}}_{0.4}{\mathrm{MnO}}_{3}$ manganite. The structural studies reveal the unique Jahn-Teller (JT) character of this compound which is established to exhibit a $\mathrm{Pbmn}$ space group. Unusual large coherent JT distortions are found to develop below ${T}_{\mathrm{JT}}\ensuremath{\approx}180 \mathrm{K}$ and remain even in a metallic ferromagnetic phase below the Curie point ${T}_{c}\ensuremath{\approx}120 \mathrm{K}$. In the paramagnetic region the resistance is consistent with polaron hopping. A field hysteresis of the second harmonic is observed above ${T}_{c}.$ This evidences the existence of the macroscopic ferromagnetic regions in the paramagnetic matrix. In contrast, temperature dependence of the linear susceptibility reveals a plateau at $Tg{T}_{c}.$ These conflicting peculiarities of the magnetic properties can be explained by assuming the ferromagnetic behavior to be associated with the antiferromagnetic regions possessing a weak ferromagnetism. The ordered regions appear as a result of first order transition at $T\ensuremath{\approx}160 \mathrm{K},$ destroy at ${T}_{*}\ensuremath{\approx}137.5 \mathrm{K}$ and reveal a characteristic three-critical behavior for $\stackrel{\ensuremath{\rightarrow}}{T}{T}_{*}+0$. Change in a balance between the ferromagnetic double exchange hopping of ${e}_{g}$ electrons and the antiferromagnetic interactions of ${t}_{2g}$ spins, which caused by the cooperative JT effect, and the charge ordering correlations are supposed to account for the anomalous magnetic behavior above ${T}_{c}.$