Thermoinduced Magnetization in Nanoparticles of Antiferromagnetic Materials

We show that there is a thermoinduced contribution to the magnetic moment of nanoparticles of antiferromagnetic materials. It arises from thermal excitations of the uniform spin-precession mode, and it has the unusual property that its magnitude increases with increasing temperature. This has the consequence that antiferromagnetism is nonexistent in nanoparticles at finite temperatures and it explains magnetic anomalies, which recently have been reported in a number of studies of nanoparticles of antiferromagnetic materials.

Figure 1

The magnetic moment of 4 nm ferrihydrite particles as a function of temperature. The experimental data, shown by bold circles, are those obtained by Seehra et al. [8] from magnetization measurements. The solid line is a linear fit to the data in accordance with Eq. (4).

Figure 2

Schematic illustration of thermoinduced magnetization in nanoparticles of antiferromagnetic materials. ${\mathit{M}}_{sA}$ and ${\mathit{M}}_{sB}$ are the instantaneous sublattice magnetization vectors. ${\mathit{M}}_A$and ${\mathit{M}}_B$ are the sublattice magnetization vectors after averaging over the precession in a state with precession angles ${\theta }_A$ and ${\theta }_B$. $\mathit{M}={\mathit{M}}_A+{\mathit{M}}_B$ is the net magnetization. For clarity, the difference between ${\theta }_A$ and ${\theta }_B$ is exaggerated.