We report on large negative magnetoresistance observed in ferromagnetic thiospinel compound CuCrZrS4. The electrical resistivity increased with decreasing temperature according to the exp(T0/T)1/2,exp(T0/T)1/2, an expression derived from variable range hopping with strong electron–electron interaction. The resistivity under a magnetic field was expressed by the same form with the characteristic temperature T0 decreasing with increasing magnetic field. Magnetoresistance ratio ρ(T,0)/ρ(T,H) is 1.5 for H=90 kOe at 100 K and increases divergently with decreasing temperature reaching 80 at 16 K. Results of magnetization measurements are also presented. A possible mechanism of the large magnetoresistance is discussed.

Effects of X-ray irradiation on the crystal structure and the electrical resistance were examined at low temperatures for the insulating phase of spinel compound CuIr2S4. We found that the resistance decreases by more than five decades by irradiation at 8.5 K. The structural change from triclinic to tetragonal was observed at the same time. The X-ray-induced conductance is deduced to result from the destruction of Ir4+ dimers formed in the insulating phase. Slow relaxation of the resistance in the X-ray-induced state is also reported.