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High magnetic field phase diagram and failure of the magnetic Grüneisen scaling in LiFePO4
Werner, J.; Sauerland, S.; Koo, C.; Neef, C.; Pollithy, A.; Skourski, Y.; Klingeler, R.;
We report the magnetic phase diagram of single-crystalline LiFePO4 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 TN in the thermal-expansion coefficient α imply pronounced magnetoelastic coupling. Quantitative analysis yields the magnetic Grüneisen parameter γmag = 6.7(5) × 10−7 mol/J. The positive hydrostatic pressure dependence dTN/dp = 1.46(11) K/GPa is dominated by uniaxial effects along the a axis. Failure of Grüneisen scaling below ≈40K, 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 α in this temperature regime highlights the relevance of structure changes. Upon application of the magnetic field B||b axis, a pronounced jump in the magnetization implies spin reorientation at BSF = 32 T as well as a precursing phase at 29 T and T = 1.5K. In a two-sublattice mean-field model, the saturation field Bsat,b = 64(2) T enables assessing the effective antiferromagnetic exchange interaction Jaf = 2.68(5)meV as well as anisotropies Db = −0.53(4)meV and Dc = 0.44(8)meV.

Publ.-Id: 29549 - Permalink