Origin of negative magnetization phenomena in (Tm1−xMnx)MnO3 : A neutron diffraction study

Abstract

$(\mathrm{T}{\mathrm{m}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{n}}_{x})\mathrm{Mn}{\mathrm{O}}_{3}$ solid solutions were synthesized at a high pressure of 6 GPa and a high temperature of about 1570--1670 K for 2 h for $x=0$, 0.1, 0.2, and 0.3. Magnetic, dielectric, and neutron diffraction measurements revealed that the introduction of magnetic ${\mathrm{Mn}}^{2+}$ cations into the A site leads to an incommensurate spin structure for $x=0.1$ and to a ferrimagnetic structure for $x\ensuremath{\ge}0.2$. Commensurate magnetic structures have a much larger correlation length $(\ensuremath{\sim}400\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$ for $x=0,\ensuremath{\sim}600\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$ for $x=0.3)$ than the incommensurate magnetic structure $(\ensuremath{\sim}12\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$ for $x=0.1)$. The presence of ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Mn}}^{2+}$ (with different sizes) at the A site causes significant microstrain effects along the $a$ direction which are absent for $x=0$ and get stronger with increasing $x$. Magnetic ordering occurs at the N\'eel temperature ${T}_{N}=37\phantom{\rule{0.28em}{0ex}}\mathrm{K}\phantom{\rule{4pt}{0ex}}(x=0.1)$ and at the ferrimagnetic Curie temperatures ${T}_{C}=75\phantom{\rule{0.28em}{0ex}}\mathrm{K}\phantom{\rule{4pt}{0ex}}(x=0.2)$ and ${T}_{C}=104\phantom{\rule{0.28em}{0ex}}\mathrm{K}\phantom{\rule{4pt}{0ex}}(x=0.3)$. Ordering of magnetic Mn moments triggers short-range order (for $x=0.1)$ and long-range order (for $x\ensuremath{\ge}0.2)$ of the ${\mathrm{Tm}}^{3+}$ cations at the same temperature---an unusual situation in perovskite materials with a simple ${\mathrm{GdFeO}}_{3}$-type $Pnma$ structure. For $x=0.1$, long-range IC magnetic order [with propagation vector $\mathbf{k}=({k}_{0},0,0)$ and ${k}_{0}\ensuremath{\approx}0.40]$ of ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ cations at the B site coexists with short-range order of ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Mn}}^{2+}$ moments at the A site. Short-range order is induced at the N\'eel temperature ${T}_{N}=37\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, increases towards an additional specific heat anomaly at $T=4\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, and remains at lower temperature. The ferrimagnetic structure [with propagation vector $\mathbf{k}=(0,\phantom{\rule{0.28em}{0ex}}0,\phantom{\rule{0.28em}{0ex}}0)]$ consists of ferromagnetically ordered ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ cations at the B site which are coupled antiferromagnetically with ordered ${\mathrm{Mn}}^{2+}$ moments at the A site. ${\mathrm{Tm}}^{3+}$ moments adopt a zigzag magnetic structure which contains a macroscopic ferromagnetic moment that aligns with the direction of the ordered ${\mathrm{Mn}}^{2+}$ moments. Towards low temperature, the ordered ${\mathrm{Tm}}^{3+}$ moments strongly increase and overcome the saturated magnetic Mn moments at the B site, and this behavior results in the observation of magnetization reversal or negative magnetization phenomena with a compensation temperature of about 15 K at small magnetic fields in the $x=0.2$ and 0.3 samples. This is a classical mechanism of the magnetization reversal effects for ferrimagnets.