Mössbauer Studies of Spin-Orbit Coupling in LiCo $_{0.99}$ ${}^{57}$ Fe

The polycrystalline LiCo_0.99⁵⁷Fe_0.01PO₄ was synthesized by solid-state reaction method. The crystal structure was determined to be orthorhombic with a space group of Pnma by x-ray diffractometer (XRD). The temperature dependence of the magnetization was investigated by superconducting quantum interference device (SQUID) magnetometer. Although LiCo_0.99⁵⁷Fe_0.01PO₄ showed a typical anti-ferromagnetic behavior at temperatures below T_N = 23 K, a rapid increase in magnetization was observed at temperatures below 9 K. The microscopic magnetic properties of LiCo _0.99⁵⁷Fe_0.01PO₄ were characterized by ⁵⁷Fe external field Mössbauer spectroscopy. At temperatures below 9 K, the magnetic hyperfine field (H_hf) showed a rapid increase, while the electric quadrupole splitting (ΔE_Q) decreased rapidly. The H_hf and ΔE_Q under external field of 4.8 T changed significantly compared to the value without external magnetic field. These are originated from orbital angular moment contribution by spin-orbit coupling at temperatures below 9 K, while orbital angular moment is quenched by the crystalline field due to distorted CoO₆(FeO₆) asymmetric structure at temperatures above 9 K.