Since the discovery of graphene, a great number of studies have demonstrated exotic phenomena based on electrical properties of two-dimensional (2D) materials. Although 2D material systems particularly have a van der Waals (vdW) gap, it has been mostly neglected to understand experimentally observed longitudinal and Hall conductivities, usually denoted by σxx and σxy, respectively. To include phenomena related to vertical transport related to the vdW gaps, we develop a simulation tool on time-dependent voltage distribution, which can reproduce real situations in vdW-based devices. For that, we firstly propose a simulation model to account for not only the conventional fixed resistivity, but also a variable resistivity depending on interlayer voltage differences. We finally visualize the current distribution as a function of time under different voltage bias with our simulation. The developed simulation based on the resistance model can provide a good template to understand the dynamics on electrical properties of 2D-based systems.

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/6f4e5779f983a3f7a6d51bc78baf5e53.png) 

Figure 1. The schematic image of the simulation and current density distribution obtained by the simulation.

Simulation on time dependent voltage distribution regarding van der Waals tunneling gap

Magnetic domain wall (DW) motion has been received much attention owing to its potential for applications in next-generation computing devices, such as memory [1],
logic [2], and neuromorphic devices [3]. In such devices, it is essential to manipulate the domain wall position in a controllable way. The extrinsic pinning sites such as ‘notch’ have
conventionally been used for the reliable positioning of DW. However, such extrinsic pinnings are not relevant in device applications because the extrinsic treatments cause the damage to
the sample. Furthermore, the pinning position is not reconfigurable because the extrinsic treatments change the properties of sample permanently.
In this study, we present a novel method to control the DW position in a damage-free and reconfigurable way. We utilize the inter-domain wall interaction in synthetic antiferromagnets
(SAF). The SAF is composed of two magnetic layers separated by non-magnetic spacer. In this system, domain wall at the bottom layer (‘bottom-DW’) feels the dipolar field from the DW
at the top layer (‘top-DW’). As the dipolar field produces the repulsive force to the DW, the bottom-DW can be effectively pinned by the top-DW. Therefore, the top-DW can act as an
effective pinning site for the bottom-DW motion. We demonstrated this idea experimentally that the bottom-DW can be pinned by top-DW for both the field- and current-driven DW
motion. Furthermore, we found that the position of pinning site can be freely changed by changing the position of top-DW. Our work demonstrates the damage-free and reconfigurable
pinning site for the DW motion, which could provide a novel functionality for DW-motion-based spintronics applications. 

**References** [1] S. Parkin, et al. Science Vol. 320, 5873, p.190 (2008).
[2] Z. Luo, et al. Nature Vol. 579, p.214 (2020).
[3] T. Jin, et al. J. Phys. D: Appl. Phys. Vol. 52, 44, p.445001 (2019).

Reconfigurable domain wall pinning in synthetic antiferromagnets

In this presentation, microscopic structures and magnetic properties of the Fe5−xGeTe2 single crystal, recently discovered as a promising van der Waals (vdW) ferromagnet, are introduced. Our study demonstrate that the √3 × √3 superstructures have two different phases due to the symmetry of Fe(1) ordering. Intriguingly, the observed √3 × √3 ordering breaks the inversion symmetry of crystal, resulting in substantial antisymmetric exchange interaction. The temperature dependence of magnetization reveals a sharp magnetic anomaly suggesting helical magnetism of the Fe5−xGeTe2 due to its non‐centrosymmetricity. Analytical study also supports that the observed ordering can give rise to the helimagnetism. The work will provide essential information to understand the complex magnetic properties of the new vdW ferromagnet, Fe5−xGeTe2 for future topology‐based spin devices.

Helimagnetism of the Fe5GeTe2 crystal

Rrare earth (RE) - transition metal (TM) ferrimagnetic alloys have been attracting much attentions due to the unconventional spin dynamics phenomena occuring at two critical temperatures, one is called magnetization compensation temperature, TM, and the other one is called angular momentum compensation temperature, TA. For examples, a fast domain-wall motion was detected near the TA of a GdFeCo alloy [1], the skyrmion Hall effect disappeared at TA of a GdFeCo alloy [2] and a highly efficiet spin-orbit torque was measured near the TM of CoTb alloy [3]. These reports make ferrimagnets more promising candidates for spintronic applications. However, the transports of RE-TM ferrimagnets have not been updated for decades. Especially, one of the most significant questions still remain is whether the transport of RE-TM ferrimagnets are solely associated with either the RE sub-moment or the TM sub-moment, or with both sub-moments. There were some controversies on assigning a dominant role to one of the two sub-moments [4, 5] or both sub-moments [6] in the early studies, but most of recent works have assumed that the TM plays a dominant role in the transport of ferrimagnets [7]. However, there has been no universally accepted consensus on this issue, mainly because of lack of experimental scheme to identify the transport of RE-TM ferrimagnets. In this work, we provide a supporting analysis on the opinion that both submoments are significantly involved to the transport by exploring longitudinal and transverse resistivities of GdFeCo. The longitudinal resistivity, ρxx, which reflects the change of anisotropic magnetoresistance (AMR), is found to depend on the magnetic field, B, and temperature, T, in a distinct way: the ρxx increases with increasing B for T &lt; TM, while it decreases for T &gt; TM, and the slope of ρxx – B curve increases as the temperature decreases. On the other hand, the transverse resistivity, ρxy, which exhibits conventional anomalous Hall effect (AHE), has little dependence on the external magnetic field. The observed field-and temperature-dependence of the magnetoresistance of GdFeCo qualitatively differ from those of ferromagnetic transition metals caused by the electron-magnon scattering mechanism. These unconventional longitudinal and transverse magnetoresistances can be explained by the antiferromagnetic nature of the magnetic coupling and the ensuing sperimagnetism of GdFeCo. Fe and Gd moments in GdFeCo are coupled antiferromagnetically but their directions are spatially dispersed due to the weak exchange interactions [8] (Fig. 1(a)). When a magnetic field is applied to sperimagnetic GdFeCo, the cone angle of each sub-moment either increases or decreases depending on the relative direction between the sub-magnetic moment and the external field (Fig. 1(b)). This field-induced opposite change of the two cone angles can explain the observed magnetoresistances as shown in Fig 2. Our results suggest that the unconventional resistance change of GdFeCo is, contrary to the recent reports [6, 7], caused by both the RE and the TM sub-moments.

**References:**

[1] K. J. Kim, S. K. Kim and Y. Hirata, Nat. Mater. 16, 1187 (2017) [2] Y. Hirata, D. H. Kim and S. K. Kim, Nat. Nanotechnol. 14, 232 (2019) [3] S. G. Je, J. C. Rojas-Sánchez and T. H. Pham, Appl. Phys. Lett. 112, 062401 (2018) [4] A. Ogawa, T. Katayama and M. Hirano, Jpn. J. Appl. Phys. 15, 87 (1976) [5] Y. Mimura, N. Imamura, and Y. Kushiro, J. Appl. Phys. 47, 3371 (1976) [6] T. R. McGuire, R. J. Gambino and R. C. Taylor, J. Appl. Phys. 48, 2965 (1977) [7] T. Okuno, K. J. Kim and T. Tono, Appl. Phys. Express 9, 073001 (2016) [8] M. Mansuripur and M. F. Ruane, IEEE Trans. Magn. MAG-22, 33 (1986)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/18ed05aee1587cb676144fd52e613656.jpg)
Fig. 1 (a) Sperimagnetism of GdFeCo. (b) Cone angles of Fe(θFe) and Gd(θGd) submoments and their opposite behavior on external B-field.

![](https://assets.underline.io/uploads/markdown_image/1/image/12be328050e6ee3254a830a963e93cbb.jpg)
Fig. 2 (a) External B-field dependent change of longitudinal resistivity, △ρ = ρxx(B)-ρxx(0), at various temepratures. (b) The slope of the longitudinal resistivity change at various temperatures in (a). The white dots are representing the absolute value of black ones.

 Unconventional magnetoresistance induced by sperimagnetism of GdFeCo

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Simulation on time dependent voltage distribution regarding van der Waals tunneling gap

Reconfigurable domain wall pinning in synthetic antiferromagnets

Helimagnetism of the Fe5GeTe2 crystal

Unconventional magnetoresistance induced by sperimagnetism of GdFeCo

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