Mechanism of magnetization reversal in bulk and nanoparticles of magnetite

The process by which magnetic moments switch directions is crucial for understanding the performance of magnetic storage materials and in biomedical applications such as magnetic particle imaging and magnetic hyperthermia. Here, we utilize the geodesic nudged elastic band method to identify the atomic-level minimum energy path for a field-free reorientation of magnetic moments in bulk magnetite and single-domain cuboidal magnetite nanoparticles terminated by <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mo>{</a:mo><a:mn>100</a:mn><a:mo>}</a:mo></a:mrow></a:math> surfaces. We show that this magnetization reversal involves three successive elementary rotations of magnetic moments in distinct <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mo>{</b:mo><b:mn>110</b:mn><b:mo>}</b:mo></b:mrow></b:math> planes. For iron-rich terminated nanoparticles, this energy barrier depends on the degree of surface spin anisotropy. However, no such effect was observed for oxygen-rich terminations. The former effect is particularly pronounced in nanoparticles with large surface-to-volume ratio and diminishes as particle size increases.