Anisotropic Fe₃O₄/Mn₃O₄ Hybrid Nanocrystals with Unique Magnetic Properties.

Abstract

This work explores novel nanomagnets by site- and facet-selective epitaxy of Mn₃O₄ nanodomains onto colloidal Fe₃O₄ nanoprisms in solution. At 190 °C, the Mn₃O₄ nanodomains epitaxially grow at three vertexes of the Fe₃O₄ nanoprisms in solution and form horns-on-prism hybrid nanocrystals. At 240 °C and in the same reaction solution, the epitaxy occurs on the top facet of the Fe₃O₄ nanoprisms, which results in prism-on-prism hybrid nanocrystals. As the temperature increases from 190 to 240 °C, the ratio between the prism-on-prism and horns-on-prism nanocrystals increases. A possible formation mechanism of Fe₃O₄/Mn₃O₄ hybrid nanocrystals is proposed. Novel magnetic behaviors, such as compensation point, large positive (or negative) exchange bias, and unusual hysteresis loop character (constricted at low field and expanded at high field), have been observed for both types of anisotropic hybrid nanomagnets. These unique magnetic properties are consistent with controlled switch of relative magnetization orientations between Fe₃O₄ and Mn₃O₄ nanodomains from parallel to antiparallel exchange-coupled configurations.