Magnetic vortex formation in hollow Fe3O4 submicron particles studied using first-order reversal curves

Abstract We report results of first-order reversal curves (FORCs) measurements for hollow Fe3O4 submicron spherical particles, varying particle size from 400 to 700 nm. For all the samples, the FORC diagrams exhibit a butterfly-like feature of distribution peaks, being characteristic to the vortex formation. With increasing temperature from 10 K nucleation and annihilation fields of a vortex sharply increase and become almost constant above the Verwey transition temperature ( T v ∼ 115 K). This reflects temperature-dependent magnetocrystalline anisotropy, associated with a structural transition at T v . Above T v the field range at which a vortex state is stable increases with increasing particle size, indicating that an increasing contribution of demagnetizing fields dominates a vortex stability for larger hollow particles.