Using a highly stoichiometric magnetite, the pressure-induced phase transitions of Fe 3 O 4 have been revisited here by performing Fe K-edge x-ray absorption and magnetic circular dichroism measurements up to P=65 GPa at room temperature and 71 GPa at 20 K. We have observed a structure transition at around 27 GPa from magnetite to a high pressure phase h-Fe 3 O 4 with the loss of the net ordered magnetic moments for both temperature. The orthorhombic CaTi 2 O 4 -type (Bbmm) structure of the high
Chiral magnetism that manifests in the existence of skyrmions or chiral domain walls offers an alternative way for creating anisotropies in magnetic materials that might have large potential for application in future spintronic devices. Here we show experimental evidence for an alternative type of in-plane exchange-bias effect present at room temperature that is created from a chiral 90 • domain wall at the interface of a ferrimagnetic-ferromagnetic Dy-Co/Ni-Fe bilayer system. The chiral interfacial domain wall forms due to the exchange coupling of Ni-Fe and Dy-Co at the interface and the presence of Dzyaloshinskii-Moriya interaction in the Dy-Co layer. As a consequence of the preferred chirality of the interfacial domain wall, the sign of the exchange-bias effect can be reversed by changing the perpendicular orientation of the Dy-Co magnetization. The chirality-created tunable exchange bias in Dy-Co/Ni-Fe is very robust against high in-plane magnetic fields (μ 0 H ≤ 6 T) and does not show any aging effects. Therefore, it overcomes the limitations of conventional exchange-bias systems.
Charge-transfer-induced interfacial ferromagnetism and its impact on the exchange bias effect in La 0.7 Sr 0.3 MnO 3 /NdNiO 3 correlated oxide heterostructures were investigated by soft x-ray absorption and x-ray magnetic circular dichroism spectra in a temperature range from 10 to 300 K. Besides the antiferromagnetic Ni 3+ cations which are naturally part of the NdNiO 3 layer, Ni 2+ ions are formed at the interface due to a charge-transfer mechanism involving the Mn element of the adjacent layer. They exhibit a ferromagnetic behavior due to the exchange coupling to the Mn 4+ ions in the La 0.7 Sr 0.3 MnO 3 layer. This can be seen as detrimental to the strength of the unidirectional anisotropy since a significant part of the interface does not contribute to the pinning of the ferromagnetic layer. By analyzing the line-shape changes of the x-ray absorption at the Ni L 2,3 edges, the metal-insulator transition of the NdNiO 3 layer is resolved in an element-specific manner. This phase transition is initiated at about 120 K, way above the paramagnetic to antiferromagnetic transition of the NdNiO 3 layer which measured to be 50 K. Exchange bias and enhanced coercive fields were observed after field cooling the sample through the Néel temperature of the NdNiO 3 layer. Different from La 0.7 Sr 0.3 MnO 3 /LaNiO 3 , the exchange bias observed in La 0.7 Sr 0.3 MnO 3 /NdNiO 3 is due to the antiferromagnetism of NdNiO 3 and the frustration at the interface. These results suggest that reducing the interfacial orbital hybridization may be used as a tunable parameter for the strength of the exchange bias effect in all-oxide heterostructures which exhibit a charge-transfer mechanism.
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