Geometric Spin Frustration in Zn_1-xNi_xCr₂O₄ System

At room temperature, the cubic spinel ZnCr₂O₄ is known as the typical geometrically frustrated spin system. Upon cooling this compound undergoes a crystal structure change driven by removing the frustrated degenerated states, accompanying with the magnetic ordering. In the tetragonal phase the c-axis is larger than the a-axis. Another spinel compound NiCr₂O₄ takes the cubic structure above 310 K. Below 310 K NiCr₂O₄ is tetragonally distorted (c/a < 1) due to cooperative Jahn-Teller ordering. In the previous reports, the geometrical frustrated spin fluctuations still exist in NiCr2O4. In this report we investigated how the geometrical frustration changes and how the ratio of the crystal axis, c/a changes in Zn_1-xNi_xCr₂O₄ system by the low temperature x-ray diffraction measurement and the SQUID magnetometer.