Exchange Bias Enhancement and Magnetic Proximity Effect in FeVO <inf>4</inf> -Fe <inf>3</inf> O <inf>4</inf> Nanoparticles

We study the behavior of the exchange bias (EB) and the blocking temperature in an antiferromagnetic FeVO 4 –ferrimagnetic Fe 3 O 4 nanocomposite system upon annealing in Ar atmosphere. Surprisingly, the blocking temperature of post-annealed samples increased to ∼ 50 K, more than two-fold compared the Néel temperature (T N = 22 K) of individual FeVO 4 nanoparticles. This significant enhancement of the blocking temperature was accompanied by the corresponding increase of EB, from ∼ 50 Oe in as-prepared samples 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 ∼ 21 K coinciding with the Néel temperature of FeVO 4 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 Fe 3 O 4 particles becomes oxygen deficient, which leads to a modification of the electronic, magnetic and morphological properties of the FeVO 4 /Fe 3 O 4 interface. We associate this unusual behavior with a magnetic proximity effect, in which the ordering temperature of the antiferromagnetic FeVO 4 nanoparticles and the corresponding exchange bias are strongly affected by the adjacent ferrimagnetic Fe 3 O 4 layer.