Visualizing the Effect of Oxidation on Magnetic Domain Behavior of Nanoscale Fe₃GeTe₂ for Applications in Spintronics

Abstract

Magnetic van der Waals (vdW) materials offer an opportunity to design heterostructures that will lead to exotic functionalities that arise from interfacial interaction. In addition to coupling to different vdW materials, the naturally oxidized surface layer of a vdW material also forms a heterostructure with its bulk film, giving rise to intriguing phenomena. Here, we directly observe the impact of oxidation on the magnetic domains, namely, magnetic stripe domain and skyrmions, in a nanoscale Fe3GeTe2 flake using cryo Lorentz transmission electron microscopy. After the Fe3GeTe2 is exposed to ambient conditions, partial oxidation leads to an increase in the density of skyrmions even under zero magnetic field. Complete oxidation leads to a loss of the magnetic domain structure. We observe a gradual change in Fe3GeTe2 from single crystal to amorphous as the oxidation increases. The oxidized Fe3GeTe2 primarily consists of iron oxide, which could be antiferromagnetic in nature. We hypothesize that the interfacial interaction between these surface antiferromagnetic oxides and the bulk ferromagnetic Fe3GeTe2, as well as the effect of interfacial roughness, leads to the increase in Néel skyrmion creation. This work opens a path to harness controlled oxidation as a build block to create dense skyrmion lattices without the need for an external magnetic field, leading to potential future applications in spintronic devices.

Harnessing Van der Waals CrPS4 and Surface Oxides for Nonmonotonic Preset Field Induced Exchange Bias in Fe3GeTe2

“…XPS 2p 3/2 and 2p 1/2 peaks suggest that the present oxide phases are Fe 3 O 4 , and Fe 2 O 3 , with both Fe 3+ and Fe 2+ valence states. However, the analysis of the Fe-L3/L2 intensity ratio in EELS, in comparison with ratios obtained from spectra of reference samples, clearly suggests the presence of Fe 3 O 4 . The presence of Fe 3 O 4 is further supported by the observation of a sudden increase in longitudinal resistance in an Fe 3 GeTe 2 flake with an O-FGT layer at around 125 K, indicating a Verwey transition in Fe 3 O 4 (see Figure S6a in the Supporting Information under Section 6).…”

“…When the field is swept with a maximum value greater than H CF at a given temperature preceded by a H PF ≥ H CF , there is no preferred pinning by ferrimagnetic Fe 3 O 4 , resulting in zero EB (refer to Figure S8a–d in Supporting Information under Section 8). In addition to Fe 3 O 4 , previous reports confirmed the presence of an antiferromagnetic FeO phase in the natural O-FGT. , This coexisting FeO phase may have a low T B of around 70 K, which is below the bulk T N = 198 K . The FeO phase is responsible for modulating the EB up to 70 K, where we observed a second minimum corresponding to the T B of FeO (Figure d).…”

“…XPS 2p intensity ratio in EELS, in comparison with ratios obtained from spectra of reference samples, clearly suggests the presence of Fe 3 O 4 . 19 The presence of Fe 3 O 4 is further supported by the observation of a sudden increase in longitudinal resistance in an Fe 3 GeTe 2 flake with an O-FGT layer at around 125 K, indicating a Verwey transition in Fe 3 O 4 (see Figure S6a in the Supporting Information under Section 6). Importantly, the minimum field required to saturate the ferrimagnetic Fe 3 O 4 in the O-FGT at a particular temperature can be considered as the critical preset field (H CF ) for the system.…”

“…The second minimum at 70 K (Figure 1d) further suggests that two different phases in the surface oxide contribute to EB, which is consistent with previous reports. 18,19 However, determining the exact nature of oxide phases is challenging, considering the polycrystalline nature of the surface oxide. Our investigation of EB in CrPS 4 /(O-FGT)/ Fe 3 GeTe 2 helps us unravel the nature of two distinct phases of oxides in O-FGT through their distinct EB fingerprints.…”

“…Recent investigations have highlighted that an air-exposed Fe 3 GeTe 2 crystal naturally forms a thin antiferromagnetic surface oxide layer (O-FGT), resulting in EB in the underlying unoxidized crystal . Analytical techniques, including energy dispersive spectroscopy mapping, X-ray absorption spectroscopy, and electron energy loss spectroscopy have indicated a polycrystalline nature for the oxide layer, suggesting the presence of diverse phases such as FeO, Fe 2 O 3 , and Fe 3 O 4 corresponding to varying Fe 2+ and Fe 3+ oxidation states. , This surface oxide has also been found to enhance EB in CrOCl/Fe 3 GeTe 2 heterostructures . Notably, significant EB has been reported in naturally oxidized and restacked liquid exfoliated Fe 3 GeTe 2 nanoflakes .…”