Authors: M. J. Martínez-Pérez, B. Müller, J. Lin, L. A. Rodriguez, E. Snoeck, R. Kleiner, J. Sesé and D. Koelle
Nanoscale, 2020, 12, 2587
Abstract: Vortex-mediated magnetization reversal in individual ultra-small (∼100 nm) ferromagnetic particles at low temperatures is studied by nanoSQUID magnetometry. At zero applied bias ﬁeld, the ﬂux-closure magnetic state (vortex) and the quasi uniform conﬁguration are bi-stable. This stems from the extremely small size of the nanoparticles that lies very close to the limit of single-domain formation. The analysis of the temperature-dependent (from 0.3 to 70 K) hysteresis of the magnetization allows us to infer the nature of the ground state magnetization conﬁguration. The latter corresponds to a vortex state as also conﬁrmed by electron holography experiments. Based on the simultaneous analysis of the vortex nucleation and annihilation data, we estimate the magnitude of the energy barriers separating the quasi single-domain and the vortex state and their ﬁeld dependence. For this purpose, we use a modiﬁed power-law scaling of the energy barriers as a function of the applied bias ﬁeld. These studies are essential to test the thermal and temporal stability of ﬂux-closure states stabilized in ultra-small ferromagnets.