Exchange bias and exchange spring in CoFe2O4/FeO/CoFe nanocomposites

Abstract

A series of CoFe2O4/FeO/CoFe nanocomposites was prepared by reducing cobalt ferrite nanoparticles under the mixed gas of Ar (95%) and H2 (5%) at 450 °C for different times, and the effects of exchange bias and exchange spring have been observed. X-ray diffraction and Mössbauer spectroscopy demonstrate CoFe2O4/FeO/CoFe nanocomposites are obtained when the reduction time exceeds 0.5 h, and the concentrations of CoFe2O4 and FeO decrease, accompanied by the increase of CoFe on extending the reduction time. The field emission scanning electron microscope and transmission electron microscope images show the spherical CoFe2O4, FeO, and CoFe nanoparticles with the average sizes about 17.0 nm are randomly distributed with a certain degree of agglomeration. Magnetic measurements indicate there is a strong exchange bias behavior between the CoFe2O4 and FeO nanoparticles in the sample reduced for 0.5 h with an exchange bias field being 1161.4 Oe at 50 K, and a good exchange spring effect between CoFe2O4 and CoFe phases in the sample reduced for 1.5 h with saturation magnetization, remnant magnetization, and coercivity at 300 K being 121.33 emu/g, 41.90 emu/g, and 802.4 Oe. These results are important not only for understanding the exchange-coupling interaction in the reduction process of cobalt ferrite but also for their application in spin-valve devices.