Cascade of Magnetic Field Induced Spin Transitions in ${\mathrm{LaCoO}}_3$

We present magnetization and magnetostriction studies of ${\mathrm{LaCoO}}_3$ in magnetic fields approaching 100 T. In contrast with expectations from single-ion models, the data reveal two distinct first-order transitions and well-defined magnetization plateaus. The magnetization at the higher plateau is only about half the saturation value expected for spin-1 ${\mathrm{Co}}^{3+}$ ions. These findings strongly suggest collective behavior induced by interactions between different electronic configurations of ${\mathrm{Co}}^{3+}$ ions. We propose a model that predicts crystalline spin textures and a cascade of four magnetic phase transitions at high fields, of which the first two account for the experimental data.

Figure 1

(a) Magnetization of ${\mathrm{LaCoO}}_3$ in units ${\mu }_B/\mathrm{Co}$ as a function of ${\mu }_{0}H$ at $T=1.7\mathrm{K}$. The up and down arrows indicate transitions for increasing and decreasing fields respectively. Two plateau regions can be seen. The inset shows an expanded view of the first transition between 55 and 63 T for $T=1.6$ (bottom), 4.0, 10.0, 20.0, 25.0, 27.0, 28.0, 28.5, 29.0, 30.0, 40, and 55 (top) K. (b) Magnetostriction of LCO, measured using a high sensitivity optical strain gauge, as a function of ${\mu }_{0}H$. The inset shows $\chi =dM/dH$ measured in a pulsed single turn coil (140 T in $\sim 2\mu \mathrm{s}$). The two transitions are merged into a single peak in the very rapidly changing field. No further transitions are found below 140 T.

Figure 2

Phase diagram for LCO based on the results shown in Fig. 1a. Blue (dashed) and red (dotted) lines denote rising and falling fields respectively while black symbols connected by the solid curve represent the phase boundary obtained as an average of the rising and falling field values at a given $T$. SSC1 and SSC2 denote spin state crystalline one and two respectively. The inset shows low lying energy states as a function of applied field for ${\mathrm{Co}}^{3+}$ in LCO based on a single ion description [17].

Figure 3

(a) The red (thin) curve is the predicted normalized magnetization curve $m({\mu }_{0}H)$ for LCO based on the spin model (3). A cascade of four transitions is shown at fields $H_{c1}$ to $H_{c4}$. The scaled green (thin) curve is the measured $m({\mu }_{0}H)$ shown in Fig. 1a. The right hand axis gives values of the pseudospin variable $⟨\sigma ⟩$ used in the model. (b) Predicted spin state crystalline structures for each magnetization plateau in panel (a). The blue (larger) and red (smaller) balls denote ${\mathrm{Co}}^{3+}$ ions in the $S=0$ and $S_z=+1$ states respectively. The magnetic Co ions form body-centered (bcc) and a face-centered (fcc) cubic structures in the $m=1/4$ and $m=1/2$ plateau regions.