Spin-driven critical fields in a spinel series based on the matrix ZnCr₂Se₄

Influence of temperature and magnetic field on critical fields in the frustrated antiferromagnet and semiconductor ZnCr₂Se₄ doped with Ga, In and Ce is considered. The first critical field, H_c1, connected with metamagnetic transition remains unchanged with temperature while the second critical field, H_c2, corresponding to the break-down of the helical spin arrangement drops rapidly with temperature. The sharpness of the H_c1 and H_c2 peaks is reduced when the content of the non-magnetic Ga³⁺ and In³⁺ ions in the octahedral positions increases. This phenomenon seems to be connected with the elongation of the interatomic distances leading to the occurrence of the low spin state of the Cr³⁺ ions in the t_2g orbital.