Magnetic structure of Cu_0.3Zn_0.7Cr₂Se ₄

Ferromagnetic spinel system Cu_1-xZn_xCr ₂Se₄ has been studied by neutron diffraction, NMR and magnetization measurements on both polycrystalline and single crystal samples. The magnetic state are discussed mainly for x = 0.7: At low temperature, the system is essentially in the conically ordered state, and the uniform magnetization M and the modulation vector Q_m are along (100) in zero magnetic field but not necessarily parallel to each other. All Cr moments within a plane perpendicular to Q_m are parallel, where the conical angle θ_c of the Cr moment is ∼50° and the pitch δφ of the azimuthal angles φ is estimated to be ∼38°. The width of the random distribution of δφ around the average value increases rather significantly with increasing temperature T, while the correlation length of the ferromagnetic component does not exhibit significant change up to room temperature. When the external magnetic field is applied, the conical axis of each domain rotates toward the field direction without changing the modulation vector Q_m. Brief discussion on the relationship between the non-trivial magnetic structure and the behavior of the anomalous Hall resistivity is also given.