Temperature dependence of magnetic first-order-reversal-curves for hollow Fe3O4 submicron particles

We report results of temperature dependence of first-order-reversal curves (FORCs) for hollow submicron particles with different outer diameter ranging from 400 to 700 nm. At low temperatures below the Verwey transition temperature, Tv, the FORC distribution exhibits a butterfly-like feature, associated with two pronounced FORC peaks, indicating the formation of a vortex structure for hollow Fe3O4 submicron particles. With increasing temperature from T = 10 K, the intensity of the two peaks steeply decreases and the peaks merge at T ∼ 130 K close to Tv. The results suggest a change of stability of the vortex state with temperature and were explained as due to a change of magnetic anisotropy associated with a structural transition at Tv.