Exchange Bias Enhancement and Magnetic Proximity Effect in FeVO4-Fe3O4 Nanoparticles

We study the behavior of the exchange bias (EB) and the blocking temperature in an antiferromagnetic FeVO4-ferrimagnetic Fe3O4 nanocomposite system upon annealing in Ar atmosphere. Surprisingly, the blocking temperature of post-annealed samples increased to similar to 50 K, more than two-fold compared the Neel temperature (T-N = 22 K) of individual FeVO4 nanoparticles. This significant enhancement of the blocking temperature was accompanied by the corresponding increase of EB, from similar to 50 Oe in as-prepared samples to similar to 110 Oe in post-annealed samples. The temperature dependence of EB can be described by two approximately linear regions with different slopes, with an inflection point at T similar to 21 K coinciding with the Neel temperature of FeVO4 nanoparticles. The region above the inflection point with non-zero EB is characterized by a weaker temperature dependence and is expanded well beyond T-N. The x-ray photoemission spectroscopy measurements indicate that the surface of post-annealed Fe3O4 particles becomes oxygen deficient, which leads to a modification of the electronic, magnetic and morphological properties of the FeVO4/Fe3O4 interface. We associate this unusual behavior with a magnetic proximity effect, in which the ordering temperature of the antiferromagnetic FeVO4 nanoparticles and the corresponding exchange bias are strongly affected by the adjacent ferrimagnetic Fe3O4 layer.