van der Waals (vdW) magnetic materials provide an ideal platform to study low-dimensional magnetism. However, observations of magnetic characteristics of these layered materials truly distinguishing them from conventional magnetic thin film systems have been mostly lacking. In an effort to investigate magnetic properties unique to vdW magnetic materials, we examine the exchange bias effect, a magnetic phenomenon emerging at the ferromagnetic-antiferromagnetic interface. Exchange bias is observed in the naturally oxidized vdW ferromagnet Fe 3 GeTe 2 , owing to an antiferromagnetic ordering in the surface oxide layer. Interestingly, the magnitude and thickness dependence of the effect is unlike those expected in typical thin-film systems. We propose a possible mechanism for this behavior, based on the weak interlayer magnetic coupling inherent to vdW magnets, demonstrating the distinct properties of these materials. Furthermore, the robust and sizable exchange bias for vdW magnets persisting up to relatively high temperatures presents a significant advance for realizing practical two-dimensional spintronics.

The perpendicular magnetic anisotropy (PMA) properties of Pt/Co/MgO trilayers are investigated as a function of the MgO sputtering power (PMgO) and its thickness (tMgO), both of which are important parameters affecting the degree of oxygen interpenetration into Co during sputtering. A strong PMA is achieved at small values of PMgO and tMgO, where the oxygen interpenetration into Co is expected to be small. The range of oxygen interpenetration is relatively extended in such a way that it affects both the Pt/Co and Co/MgO interfaces. The PMA properties of as-deposited samples are improved by post-annealing for temperatures up to 400 °C examined in this study, probably due to the diffusion of the interpenetrated oxygen atoms toward the Co/MgO interface. In a structure of Pt/Co (0.6 nm)/MgO (2 nm), which is fabricated at PMgO = 50 W and then annealed at 400 °C, a huge saturation field is achieved (over 40 kOe) indicating a very strong PMA. Between the two interfaces of Pt/Co and Co/MgO, the PMA is mainly due to the former in the as-deposited state, but the contribution of the latter increases with the increase in the annealing temperature.

In Pt/Co/MgO trilayers, the contribution to the total perpendicular magnetic anisotropy from the bottom Pt/Co interface is significantly greater than that from the top Co/MgO interface at intermediate annealing temperatures. However, the relative contribution is reversed at the highest annealing temperature examined in this study, 400 °C. These results indicate that as the annealing temperature increases, the quality of the Pt/Co interface deteriorates owing to the intermixing of Pt and Co, whereas the quality of the Co/MgO interface improves as oxygen atoms, which are interpenetrated into the Co layer during the deposition of MgO, are diffused toward the Co/MgO interface.

We experimentally investigate the correlation between interfacial Dzyaloshinskii–Moriya interaction (iDMI) and interfacial perpendicular magnetic anisotropy (iPMA) of Pt/Co/MgO stacks with varying annealing temperature. We observe that iDMI of whole stack decreases as iPMA of whole stack increases, unexpectedly. However, when we separate contributions from the Pt/Co and Co/MgO interfaces, we find that at the Pt/Co interface, the iDMI increases with increasing iPMA, whereas at the Co/MgO interface, no meaningful correlation between the two interfacial energies is observed. Therefore, the correlation between iDMI and iPMA of whole stack is a consequence of different correlations at different interfaces.

No abstract