Pressure-Induced Magnetic Transition in Manganite (${\mathrm{La}}_{0.75}{\mathrm{Ca}}_{0.25}{\mathrm{MnO}}_3$)

Low temperature Mn $K$-edge x-ray magnetic circular dichroism and x-ray diffraction measurements were carried out to investigate the stability of the ferromagnetic ground state in manganite ${\mathrm{La}}_{0.75}{\mathrm{Ca}}_{0.25}{\mathrm{MnO}}_3$ under nearly uniform compression using diamond anvil cells. The magnetic dichroism signal gradually decreases with pressure and disappears at 23 GPa, and meanwhile a uniaxial compression of ${\mathrm{MnO}}_6$ octahedra along the $b$ axis is observed to continuously increase with pressure and become anomalously large at 23.5 GPa. These changes are attributed to a ferromagnetic-antiferromagnetic transition that is associated with orbital ordering at high pressure.

Figure 1

Normalized XANES and XMCD data for ${\mathrm{La}}_{0.75}{\mathrm{Ca}}_{0.25}{\mathrm{MnO}}_3$ at 13 K collected at 2.0 and 23.0 GPa. The XMCD peak intensity is about 0.1% of the absorption jump.

Figure 2

Normalized XMCD data of manganite ${\mathrm{La}}_{0.75}{\mathrm{Ca}}_{0.25}{\mathrm{MnO}}_3$ collected at 13 K from 1 bar to 23.0 GPa. The inset shows the intensity of overall XMCD strength versus pressure. [“1 bar (Rel)” is taken after pressure release; “1 bar (Amb)” is taken at ambient conditions].

Figure 3

(a) Structure refinement of diffraction patterns at 13.0 GPa 40 K with the program GSAS; (cross: measured pattern; line: calculated pattern; black tick: position of peaks; line below tick: difference between calculated and measured patterns). The inset is a plot of pressure versus volume. (b) The strain for lattice and ${\mathrm{MnO}}_6$ octahedral versus pressure. (For $Pnma$, we set lattice parameter as $b>c>a$.) The dashed line is an estimated pressure dependence of strain, equal to $0.00131\times P+0.005$. $\mathrm{Mn}\mathrm{\text{−}}{\mathrm{O}}_1$ and $\mathrm{Mn}\mathrm{\text{−}}{\mathrm{O}}_{2a}$, $\mathrm{Mn}\mathrm{\text{−}}{\mathrm{O}}_{2b}$ represent the bond length. The $\mathrm{Mn}\mathrm{\text{−}}{\mathrm{O}}_2$ bond length is averaged from $\mathrm{Mn}\mathrm{\text{−}}{\mathrm{O}}_{2a}$ and $\mathrm{Mn}\mathrm{\text{−}}{\mathrm{O}}_{2b}$ bond length within the $ac$ plane (Fig. 4).

Figure 4

(a) ${\mathrm{MnO}}_6$ octahedra at unstrained and strained conditions. The filled $e_g$ orbitals are occupied by electrons, while the opened $e_g$ orbitals are not occupied by electrons; (b) the structure models for $F$-type and $A$-type magnetic ordering. The arrows inside orbitals indicate the spin direction of $d$ electrons.