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Comparison of NiBr2 growth on HOPG and NbSe2
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School of Science |
Master's thesis
Electronic archive copy is available via Aalto Thesis Database.
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en
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42
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Abstract
Since the discovery of graphene, research in two-dimensional (2D) materials has intensified as their properties at the atomic scale differ from the bulk. Among these, 2D multiferroics, particularly those that show magnetoelectric coupling, have shown great potential for memory devices and next-generation spintronics because of the unique coexistence of magnetic and electric ordering. After the experimental realization of multiferroicity in monolayer (ML) NiI2, efforts have been directed towards the exploration of new ML multiferroic materials with similar or enhanced properties. In particular, NiBr2 has recently gained interest as it has a similar spiral magnetic structure. However, most of the research has focused on finding its multiferroic behaviour, while comparing it on different substrates remains limited. Yet, understanding the choice of substrate is very important for controlling and using its multiferroic behaviour.
In this thesis, the aim was not only to investigate the multiferroic behavior of NiBr2, but also to grow it on two different substrates, highly oriented pyrolytic graphite (HOPG) and niobium diselenide (NbSe2). The goal was to determine whether the choice of substrate influence the multiferroic behavior of ML NiBr2. Low-temperature scanning tunneling microscopy (LT-STM) was used to characterize the samples and the growth of ML was done by using molecular beam epitaxy (MBE).
On HOPG, we found ML growth around step edges and observed stripe patterns at 2.9 K, which are associated with multiferroicity. We also studied the temperature dependence of these stripes. On the other hand, on NbSe2, we observed a full ML along with triangular bilayer (BL) growth of NiBr2. We did not detect any stripes, even when cooling down the system to 2.9 K, but a strong moire pattern was observed. These observations suggest that the choice of substrate plays an important role in stabilizing or suppressing multiferroic behaviour in NiBr2.