Spin fluctuations and frustrated magnetism in multiferroic FeVO4

We report V-51 nuclear magnetic resonance (NMR) studies on single crystals of the multiferroic material FeVO4. The high-temperature Knight shift shows Curie-Weiss behavior K-51 = a/(T + theta), with a large Weiss constant theta approximate to 116 K. However, the V-51 spectrum shows no ordering near these temperatures, splitting instead into two peaks below 65 K, which suggests only short-ranged magnetic order on the NMR time scale. Two magnetic transitions are identified from peaks in the spin-lattice relaxation rate 1/T-51(1) at temperatures T-N1 approximate to 19 K and T-N2 approximate to 13 K, which are lower than the estimates obtained from polycrystalline samples. In the low-temperature incommensurate spiral state, the maximum ordered moment is estimated as 1.95 mu(B)/Fe, or 1/3 of the local moment. Strong low-energy spin fluctuations are also indicated by the unconventional power-law temperature dependence 1/T-51(1) proportional to T-2. The largeWeiss constant, short-range magnetic correlations far above T-N1, small ordered moment, significant low-energy spin fluctuations, and incommensurate ordered phases all provide explicit evidence for strong magnetic frustration in FeVO4.