Mössbauer Studies of Spin-Orbit Coupling in LiCo$_{0.99}$ ${}^{57}$ Fe$_{0.01}$ PO$_{4}$

The polycrystalline LiCo
_0.99
⁵⁷
Fe
_0.01
PO
₄
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
_0.99
⁵⁷
Fe
_0.01
PO
₄
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
_0.99
⁵⁷
Fe
_0.01
PO
₄
were characterized by
⁵⁷
Fe external field Mössbauer spectroscopy. At temperatures below 9 K, the magnetic hyperfine field (
H
_hf
) showed a rapid increase, while the electric quadrupole splitting (Δ
E
_Q
) decreased rapidly. The
H
_hf
and Δ
E
_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 CoO
₆
(FeO
₆
) asymmetric structure at temperatures above 9 K.