The Mössbauer effect and magnetic phase transitions

Abstract

The Mossbauer effect provides a direct method for identifying the spin axis in magnetic crystals and observing magnetic phase transitions. The order of the transition may be inferred from the Mossbauer spectrum. Phase changes can occur as a function of temperature (e.g. when the anisotropy fieldBA changes sign) or as a function of applied magnetic field. In an antiferromagnet a field ≃(2BEBA)1/2 along the spin axis whereBE is the exchange field causes the spin-flop transition which is normally first order (sharp) whereas the transition to the paramagnetic phase which occurs at higher fields≃2BE is second order (continuous). In quasi-one-dimensional crystals Mossbauer spectra show that the spin-flop transition is first order locally but occurs over a range of fields throughout the crystal, so that the first order character is masked in a conventional magnetization measurement. In fields applied at a finite angle>BA/2BE to the spin axis the transition becomes second order, i.e. a continuous rotation of the spins occurs. In canted antiferromagnets (or weak ferromagnets) the spin-flop transition is also continuous; in addition a “screw” re-orientation may be induced by fields applied perpendicular to the spin axis and arises from antisymmetric exchange. For crystals with lowTN the hyperfine field changes when a magnetic field is applied and has a minimum at a phase transition; this may be used to map out the magnetic phase diagram.