We report the magnetic phase diagram of single-crystalline ${\mathrm{LiFePO}}_4$ in magnetic fields up to 58 T and present a detailed study of magnetoelastic coupling by means of high-resolution capacitance dilatometry. Large anomalies at $T_{\mathrm{N}}$ in the thermal-expansion coefficient $\alpha$ imply pronounced magnetoelastic coupling. Quantitative analysis yields the magnetic Grüneisen parameter ${\gamma }_{\mathrm{mag}}=6.7\left(5\right)\times {10}^{-7}$ mol/J. The positive hydrostatic pressure dependence $d{T}_{\mathrm{N}}/dp=1.46\left(11\right)$ K/GPa is dominated by uniaxial effects along the $a$ axis. Failure of Grüneisen scaling below $\approx 40\mathrm{K}$, i.e., below the peak temperature in the magnetoelectric coupling coefficient [7], implies several competing degrees of freedom. A broad and strongly magnetic field dependent anomaly in $\alpha$ in this temperature regime highlights the relevance of structure changes. Upon application of the magnetic field $B\left|\right|b$ axis, a pronounced jump in the magnetization implies spin reorientation at $B_{\mathrm{SF}}=32\mathrm{T}$ as well as a precursing phase at 29 T and $T=1.5\mathrm{K}$. In a two-sublattice mean-field model, the saturation field $B_{\mathrm{sat},\mathrm{b}}=64\left(2\right)\mathrm{T}$ enables assessing the effective antiferromagnetic exchange interaction $J_{\mathrm{af}}=2.68\left(5\right)\mathrm{meV}$ as well as anisotropies $D_{\mathrm{b}}=-0.53\left(4\right)\mathrm{meV}$ and $D_{\mathrm{c}}=0.44\left(8\right)\mathrm{meV}$.

• Received 28 February 2019
• Revised 8 May 2019

DOI:https://doi.org/10.1103/PhysRevB.99.214432