Magnetic and thermal properties at the initial stage of formation of spin-glass-type states in the system Li_0.5Fe_2.5–xGa_xO₄ (x=0.9, 1.0)

Complex investigations of thermal and magnetic properties of polycrystalline samples of diluted ferrimagnetic spinels Li_0.5Fe_2.5–xGa_xO₄ with a concentration x=0.9 and 1.0 of nonmagnetic Ga³⁺ ions show that at the initial concentration stage of the formation of spin-glass (SG)-type disordered states, when nonzero spontaneous magnetization for longitudinal spin components coexists with long-range SG order for transverse components, the low-energy excitations are spin waves with a quadratic energy-momentum relation and are responsible for the violation of the long-range SG order at the freezing temperature T_f. The existence of an uncorrelated disorder in the magnetic subsystem at T>T_f leads to the emergence of other types of modes (in addition to the spin waves) in the excitation spectrum. This is manifested in the form of peaks on the temperature dependences of the magnetic contributions to the heat capacity.