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

Electric field-driven ion migration can significantly modulate the electric- and magnetic- properties of solids, which creates new opportunities for electromagnetic devices with ultralow power consumption. Earlier works used vertically stacked structures, e.g., ferromagnet/oxide heterostructures, in which the magnetic properties of ferromagnet are controlled by the electric-field-driven oxygen ion migration along the vertical direction [1,2]. However, the existence of an interface obscures the detailed understanding of how the oxygen ion migration occurs and how it affects the electrical and magnetic properties of a ferromagnet. Instead of stacked structures, here we investigate the oxygen ion migration effect in a single ferromagnetic GdOx wire with lateral electric field application [3]. We directly visualize the electric-field-driven lateral motion of oxygen ions by an optical microscope (Fig. 1). The oxygen ions are found to migrate along the electric field direction by thermal activation. By exploiting the electric-field-driven oxygen ion migration, we succeed to increase or decrease the resistivity of GdOx in a controllable way, which allows mimicking the functional behavior of biological synapse (Fig. 2). Large negative magnetoresistance is observed in GdOx wire of which magnitude is significantly enhanced up to 20% by 9 T by oxygen ion control. Our results suggest that the electrical and magnetic properties of a single GdOx can be efficiently controlled by an electric field, which paves the way towards fully functional electromagnetic devices such as artificial synapses.

**References:**

1. Uwe Bauer, Lide Yao and Geoffrey S. D. Beach, Nature Materials, Vol. 14, p.174 (2015) 2. Mantao Huang, Aik Jun Tan and Geoffrey S. D. Beach, Scientific Report, Vol. 7, p.7452 (2017) 3. Jun-Ho Kang, Soogil Lee and Kab-Jin Kim, NPG Asia Materials, Vol. 12, p.44 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ff1482f1cba10df19a5fb53bf372fcf2.jpg)
Fig. 1. Real-time images of the migration of accumulated oxygen ions in a GdOx device.

![](https://assets.underline.io/uploads/markdown_image/1/image/f264f61a5f3af3055a5ccc4d3e50e5df.jpg)
Fig. 2. Longitudinal resistance (Rxx) with repeated switching of the current (I) direction.

 Control of electrical resistance and magnetoresistance by electric field-driven oxygen ion migration in a single GdOx wire

*Session Co-Chair: Xin Fan, University of Denver Division of Natural Sciences and Mathematics*

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Unconventional magnetoresistance induced by sperimagnetism of GdFeCo

Control of electrical resistance and magnetoresistance by electric field-driven oxygen ion migration in a single GdOx wire

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