Eur. Phys. J. B 77, 87-100 (2010)
https://doi.org/10.1140/epjb/e2010-00262-7

Frustration driven magnetic states of A-site spinels probed by μSR

¹ Physics Department, Technische Universität München, 85747 Garching, Germany
² Experimental Physics V, Center for Electronic Correlations and Magnetism, Universität Augsburg, 86159 Augsburg, Germany
³ Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden
⁴ IPKM, Technische Universität Braunschweig, 38106 Braunschweig, Germany
⁵ Institute of Applied Physics, Academy of Sciences, 2028 Chisinau, Republic of Moldova

Corresponding author: ^a vladimir.tsurkan@physik.uni-augsburg.de

Received: 13 May 2010
Published online: 16 September 2010

Abstract

The normal A-site spinels MnAl₂O₄, FeAl₂O₄, CoAl₂O₄, as well as related mixed (Mn_0.5Fe_0.5Al₂O₄) and partially inverted (Fe_1.4Al_1.6O₄) spinels have been studied by μSR. The magnetic ions are subject to magnetic frustration by competing interactions. In all materials and at all temperatures the μSR spectra consist of two signals suggesting a bimodal distribution of the fluctuation rates of magnetic moments. A characteristic temperature T_M is found in each compound, representing either a magnetic phase transition into a long-range ordered state (MnAl₂O₄, Fe_1.4Al_1.6O₄) or the formation of a spin liquid phase (FeAl₂O₄, CoAl₂O₄, Mn_0.5Fe_0.5Al₂O₄). The magnetic ground state of MnAl₂O₄ shows coexistence of antiferromagnetic and spin liquid phases. In FeAl₂O₄ and CoAl₂O₄ long-range order is suppressed altogether, the ground state can be characterized as a fast relaxing spin liquid coexisting with a small fraction of paramagnetic spins. The partial replacement of Mn by Fe in Mn_0.5Fe_0.5Al₂O₄ prevents long-range order and leads to a spin liquid state in the low temperature limit. The partial occupancy of B-sites by magnetic ions in Fe_1.4Al_1.6O₄ strengthens the exchange coupling, allowing the formation of long-range magnetic order at a rather high temperature (~100 K). Magnetic phase diagrams are presented demonstrating that for the studied compounds the magnetic properties are determined by the degree of frustration.