We report the tunability of the exchange bias effect by the first-order metal-insulator transition (known as the Verwey transition) of Fe 3 O 4 in CoO (5 nm)/Fe 3 O 4 (40 nm)/MgO (001) thin film. In the vicinity of the Verwey transition, the exchange bias field is substantially enhanced because of a sharp increase in magnetocrystalline anisotropy constant from high-temperature cubic to lowtemperature monoclinic structure. Moreover, with respect to the Fe 3 O 4 (40 nm)/MgO (001) thin film, the coercivity field of the CoO (5 nm)/Fe 3 O 4 (40 nm)/MgO (001) bilayer is greatly increased for all the temperature range, which would be due to the coupling between Co spins and Fe spins across the interface. Published by AIP Publishing. https://doi.
We report a direct observation of magnetic domain evolution near the Verwey transition (T V ) in Fe 3 O 4 films. We found the stripe domains in the Fe 3 O 4 /Mg 2 TiO 4 film while the irregular domains in the Fe 3 O 4 /MgO film and the similar characters of magnetic domains in the vicinity of T V for both samples: the bigger domain size and the higher contrast of the phase signal below T V and the more disordered domain images at T V . Remarkably, the magnetic behaviors can be well understood and the domain-wall energy and the demagnetizing energy can be calculated from the magnetic domains near T V in the Fe 3 O 4 /Mg 2 TiO 4 film. Our work presents a demonstration of the lowtemperature magnetic domains and gives a new perspective to understand the Verwey transition in Fe 3 O 4 thin films. Published by AIP Publishing. https://doi.
Magnetic vortices in patterned nanocomposite structures have been studied. A double-vortex structure consisting of two vortex cores with opposite chiralities and two edge half-vortices is realized in a micron-size circular disk, which consists of [Co/Pd]7/Ru/Py. A systemic study of the magnetization reversal of a double-vortex state and a single vortex state in an out-of-plane applied field has been performed by means of magnetic force microscopy. It is found that the interlayer exchange and dipolar interactions are critical for nucleation and annihilation of the double-vortex and the single vortex states. Micromagnetic simulations prove that the double-vortex state can stably exist in a patterned nanocomposite structure. Magnetic configurations ranging from the double-vortex and single vortex states to other multidomain states and the in-plane single-domain state are observed. The dependence of the magnetic configuration on the thickness of the soft-magnetic layer and spacer layer is presented in an experimental phase diagram. The results can significantly widen the practical applications of vortices on the three-dimensional spintronic memory and logic devices.
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