Magnetic Phase Transitions in Substituted Spinels of Zn1−x Cu χ Cr2 Se4; 0.0 ≤ x ≤ 0.3

Abstract

Magnetic and crystallographic properties have been studied by neutron powder diffraction and measurements of magnetization and magnetization hysteresis-loops for substituted spinels of Zn1−xCuxCr2Se4 with 0.0≤x≤0.3. It is found that the Zn0.85Cu0.15Cr2Se4 spinel has two magnetic phase transitions at 23.0 K (Néel temperature; T N) and 410 K (Curie temperature; T C) and that the Zn0.70Cu0.30Cr2Se4 spinel has magnetic transitions at 24.5 K (T N) and 415 K (T C) on heating. The low-temperature magnetic phase transition is from a spiral antiferromagnet to a ferromagnet, and the high-temperature magnetic phase transition is from a ferromagnet to a paramagnet, while ZnCr2Se4 shows a magnetic phase transition only from a spiral antiferromagnet to a paramagnet at about 21.0 K. From neutron powder diffraction, it is also found that the spinels of Zn1−x Cu x Cr2Se4; 0.0 ≤ x ≤ 0.3. show satellite-like magnetic reflection having indexes (h ± Q, k, l) with Q = 0.470 below T N and short-range order of spins (spin glass-like) above T N. The incommensurate antiferromagnetic phase below T N results from a spiral long-range order of the spins of Cr3+. The intermediate ferromagnetic phase between T N and T C is related not to the spiral spin order but to double-exchange magnetic interaction among Cr3+ and Cr4+ mediated by current carriers, positive holes, which is made by the substitution of Zn2+ ions with Cu1+ ions in Zn1−x Cu x Cr2Se4.