Abstract
We have studied the magnetic properties of ultrathin magnetite $({\mathrm{Fe}}_{3}{\mathrm{O}}_{4})$ films and explained them in relation to the antiphase domain structure of the films. The ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ films contain a high density of antiphase boundaries (APBs), at which an antiferromagnetic (AF) exchange interaction can be present. Films with a thickness below 5 nm show superparamagnetic behavior. In this paper, we show that this is not due to a frustration of the AF exchange interactions as has previously been reported, but to a strongly reduced AF exchange coupling at the APBs. The total barrier height for the fluctuation of the magnetic moments then depends on the area of the APB. This is in accordance with our observation that thicker films can also become superparamagnetic when their structural domain size is small enough, which can be achieved by preparing the films at lower temperatures. The thickness dependence of the saturation magnetization has been related to a change in APB density.
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