Skyrmions and antiskyrmions are topologically protected spin structures with opposite vorticities. Particularly in coexisting phases, these two types of magnetic quasiparticles may show fascinating physics and potential for spintronic devices. While skyrmions are observed in a wide range of materials until now antiskyrmions were exclusive to materials with D2d symmetry [1]. In our recent work, we have shown first and second-order antiskyrmions stabilized by magnetic dipole-dipole interaction in Fe/Gd-based multilayers [2]. Using Lorentz transmission electron microscopy imaging, we observed coexisting first and second-order antiskyrmions, Bloch skyrmions, and type-2 bubbles (Fig. 1) and determine the range of material properties and magnetic fields where the different spin objects form and dissipate. Phase diagrams of the spin objects were created in dependence on the magnetic out-of-plane field, temperature, saturation magnetization, and uniaxial magnetic anisotropy. We performed micromagnetic simulations to obtain more insight into the studied system and conclude that low values of saturation magnetization and uniaxial magnetic anisotropy lead to the existence of this zoo of different spin objects and that they are primarily stabilized by dipole-dipole interaction. Further, we investigated the nucleation process of antiskyrmions experimentally and theoretically revealing the necessity of a crossing point of three magnetic stripe domains to form an isolated antiskyrmion with an out-of-plane magnetic field. The previous unobserved second-order antiskyrmions and the disclosed coexistence of antiskyrmions and skyrmions potentially reachable for a whole range of different materials provide great potential for further studies on quasi-particle interactions, spin dynamics as well as for possible future applications in spintronics.

![](https://assets.underline.io/uploads/markdown_image/1/image/50c16dd6974021e442524cdd7ed4e0a3.jpg)

Fig. 1. Underfocused LTEM image of [Fe/Ir/Gd]2/[Fe/Gd]76/[Fe/Ir/Gd]2 showing five different coexisting spin objects (a). Bloch skyrmion with counter- (b) and with clockwise rotation (c), type-2 bubble (d), first-order antiskyrmion (e), and second-order antiskyrmion (f). Zoomed-in spin objects (b-f, I) and simulated LTEM contrast (b-f, II) caused by the theoretical spin textures (b-f, III).

Dipolar-Stabilized First and Second-Order Antiskyrmions in Ferrimagnetic Multilayers

The longitudinal spin Seebeck effect (LSSE) is an efficient method for generating incoherent magnon excitations by applying a vertical temperature gradient across a magnetic material. To avoid the contamination of the thermally generated “pure” spin current by the electronic degrees of freedom, magnetic insulators are exclusively preferred for LSSE. Although the ferrimagnetic insulator Y3Fe5O12 (YIG) is known as the hallmark system for LSSE because of its ultra-low Gilbert damping,[1] other members of the insulating iron garnet family, including the compensated ferrimagnet Gd3Fe5O12 (GdIG), have recently attracted considerable interest due to their unique compensated spin configurations.[2] In particular, the fascinating observation of the change in sign of the LSSE signal in a GdIG/Pt bilayer around the magnetic compensation temperature due to reorientation of sublattice magnetizations has motived us to address to an emerging fundamental question: “Is there a general trend for the temperature dependence of LSSE in compensated ferrimagnets?” Therefore, we have performed a comprehensive investigation of LSSE in GGG/GdIG(t)/Pt(5nm) bilayers with 5 different thicknesses ranging between 31 and 272 nm. We find that all the GdIG films possess an in-plane magnetic easy axis, and the compensation temperature decreases from 270 to 220 K with decreasing film-thickness from 272 to 31 nm. Using a proposed rescaling method, we have demonstrated, for the first time, a “universal scaling” behavior for the temperature dependence of LSSE signal for our GdIG films with different compensation temperatures, which, we believe, holds true for other compensated ferrimagnetic systems as well. In addition, we have performed the LSSE study on a 31 nm GdIG film grown on lattice-mismatched GSGG substrate that exhibits an out-of-plane magnetic easy axis and observed a clear distinction in the magnetic field dependent LSSE signal, relative to the 31 nm GGG/GdIG film showing an in-plane magnetic easy axis. This highlights a strong correlation between the LSSE signal and the magnetic anisotropy in compensated ferrimagnets. Our findings will pave the way to explore LSSE in other novel compensated ferrimagnets.

Universal scaling of the temperature dependence of longitudinal spin Seebeck effect in compensated ferrimagnets

THz radiation in the frequency range from 0.3 to 30 THz bridges the gap between electronic and optical frequencies. It has been used for probing and driving fundamental resonances in gaseous, liquid, and solid materials. However, it is still challenging to generate broadband THz radiation with sufficient power in a convenient way. Recently, a new type of THz emitter has been discovered, which is based on the inverse spin Hall effect [1, 2]. These “spintronic” THz emitters typically consist of ferromagnetic (FM)/nonmagnetic metal bilayers, which are easy to prepare. In previous studies we have shown that bilayers consisting of ferrimagnetic (FI) Tb(Gd)-Fe alloys can be utilized as well as efficient spintronic THz emitters [3,4]. We find that the THz emission amplitude closely follows the in-plane magnetization of the Fe sublattice. In a further study, we have utilized the magnetic compensation temperature of a FI layer to control the THz emission solely by temperature [5]. This is enabled by coupling two ferrimagnetic layers and depending on the relative alignment of the Fe moments in the two layers, the spintronic emitter system can be either in a high- or in a low-amplitude terahertz emitting state. This approach was extended to spin valve systems with a pinned and a free FM Fe layer, decoupled from each other by either a W or a Pt layer. In this case the THz emission amplitude can be controlled by small external magnetic fields [6] and allows for fast switching, which opens a new route for a controllable and efficient type of spintronic terahertz emitter system.

**References:**

[1] T. Kampfrath et al., Nature Nanotechnology 8, 256 (2013). [2] T. Seifert et al., Nature Photonics 10, 483 (2016). [3] R. Schneider et al., ACS Photonics 5, 3936 (2018). [4] R. Schneider et al., Appl. Phys. Lett. 115, 152401 (2019). [5] M. Fix et al., Appl. Phys. Lett. 116, 012402 (2020). [6] M. Fix et al., Appl. Phys. Lett. 117, 132407 (2020).

**Images:**

 Ferrimagnetic thin films systems for spintronic THz emitters

**Session chair:** Max Hirschberger, The University of Tokyo



GOF - Skyrmions, Antiskyrmions and Topology

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Dipolar-Stabilized First and Second-Order Antiskyrmions in Ferrimagnetic Multilayers

Universal scaling of the temperature dependence of longitudinal spin Seebeck effect in compensated ferrimagnets

Ferrimagnetic thin films systems for spintronic THz emitters

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