Evolution of magnetic properties in the normal spinel solid solution Mg _1xCu _xCr ₂O ₄

We examine the evolution of magnetic properties in the normal spinel oxides Mg _1xCu _xCr ₂O ₄ using magnetization and heat capacity measurements. The end-member compounds of the solid solution series have been studied in some detail because of their very interesting magnetic behavior. MgCr ₂O ₄ is a highly frustrated system that undergoes a first-order structural transition at its antiferromagnetic ordering temperature. CuCr ₂O ₄ is tetragonal at room temperature as a result of JahnTeller active tetrahedral Cu ²⁺ and undergoes a magnetic transition at 135K. Substitution of magnetic cations for diamagnetic Mg ²⁺ on the tetrahedral A site in the compositional series Mg _1xCu _xCr ₂O ₄ dramatically affects magnetic behavior. In the composition range 0x0.3, the compounds are antiferromagnetic. A sharp peak observed at 12.5K in the heat capacity of MgCr ₂O ₄ corresponding to a magnetically driven first-order structural transition is suppressed even for small x. Uncompensated magnetismwith open magnetization loopsdevelops for samples in the x range 0.43x1. Multiple magnetic ordering temperatures and large coercive fields emerge in the intermediate composition range 0.43x0.47. The Néel temperature increases with increasing x across the series while the value of the CurieWeiss _CW decreases. A magnetic temperaturecomposition phase diagram of the solid solution series is presented.