Spin arrangements in the double perovskite LaSr_1-xCa_xNiReO₆

Double perovskite oxides are a class of multifunctional materials exhibiting various magnetic orders and electronic states. Their general composition is A₂BB’O₆ and can accommodate almost any of A₂: alkaline earth or lanthanide cations and BB’O₆: transition metals in various oxidation states. Consequently, the structure is flexible to expand, contract and distort with cation exchanges. We use the muon spin rotation, relaxation and resonance (μ⁺SR) technique to study the magnetic properties of LaSr_1-xCa_xNiReO₆ which are determined by geometry and cation exchange. With its unique length and time scales, the implantation of muons in specific crystallographic sites acts as a local magnetic probe of the static and dynamic spin arrangements. In our study we elucidate the type of ordering of the magnetic ground states for the x=0,1 compounds, resulting from two interacting Ni and Re magnetic sub-lattices. For x=0, a glassy state between 75K>T>23K transforms into an incommensurate long-range order below T_c=23K. For x=1, a commensurate long-range order is achieved from a metastable ferrimagnetic ordering at T_c=103K to a stable one at T_xy=50K. A dilute moment state which was identified in both compounds with increasing temperature is eventually dissolved (T>250K) into a paramagnetic state.