Antiferromagnetic materials with non-collinear spin structures (NC-AFMs) offer significant potential for spintronic applications[1]. They have vanishingly small magnetization and ultrafast spin dynamics. In addition, they allow for the presence of important counterparts to the effects considered to be exclusive to materials with net magnetization, such as the anomalous Hall effect (AHE)[2], magneto-optical Kerr effect[3] or anomalous Nernst effect (ANE)[4]. One of the promising candidates of the NC-AFM class are Mn antiperovskites, with a typical representative being Mn3NiN. In this contribution we present ANE measurements on epitaxial thin films of Mn3NiN prepared on SrTiO3 substrate with spin structure lying in the (111) plane. Previous theoretical predictions suggest that despite the modest spin-orbit coupling, the magnitude of the ANE in Mn3NiN ought to exceed that of Mn3Sn NC-AFM due to the Berry phase boost[5]. We implement two complementary methods to detect the ANE response of the Mn3NiN films. First, the global Nernst response is measured using a micro-patterned Hall bar device with platinum resistive heaters generating an in-plane thermal gradient. The measured anomalous Nernst data (Fig.1(a)) are in good agreement with the AHE measurements[5], indicating a similar physical origin of the two effects. Second, to add a spatial resolution, scanning thermal gradient microscopy[4] is used. A laser beam incident on a micro-patterned device induces thermal gradients. The method reveals the sample’s domain structure. We find a signature of an ANE component that we can control with the external field due to the unsaturated moment present lying in the (111) plane, different from the previously measured Mn3Sn. This small moment provides a means for manipulating magnetic order, and even writing magnetic domain-like structures (Fig.1(b)). These two methods provide fruitful insight into the magnetic behaviour of NC-AFMs.

![](https://assets.underline.io/uploads/markdown_image/1/image/e3391f086f9d4953ae3f5db220e7247b.jpg)

Fig.1. Anomalous Nernst effect in Mn3NiN. (a) Global ANE as a function of the out-of-plane magnetic field at different base temperatures. (b) Scanning thermal gradient microscopy at T = 100 K. The domain-like structure was written by high-intensity laser beam and 500 mT in-plane magnetic field.

Anomalous Nernst effect in thin films of non-collinear antiferromagnet Mn3NiN

We identified and exploited a novel mechanism in an emerging branch of spintronics based on transport phenomena governed by crystal and magnetic symmetries, and its interplay with the antiferromagnetic order [1]. In this talk, we will focus on the origin of the subsequent experimental observation of a large spontaneous Hall effect in the Mn5Si3 antiferromagnet, arising mostly by time-reversal symmetry breaking. These findings could prove essential for the development of robust large effects, vital in new device concepts. More specifically, the Zeeman effect is among the cornerstones of quantum physics that contributed to the development of spintronics. To date, the Zeeman spin-splitting of atomic energy levels or bands in crystals has been associated with the presence of a magnetic dipole. Countering the established physical picture [2-4], we identify an antiferromagnet with a band-structure comprised of separated crystal-momentum valleys with opposite Zeeman spin-splittings, which allows for their probing and exploiting by analogous electron-transport phenomena as used for the Zeeman effect in the presence of the magnetic dipole. In this talk we focus on one of the most prominent consequences of spin splitting – the spontanous Hall effect. We experimentally demonstrated its presence in Mn5Si3 an antiferromagnet with separated Zeeman spin-split valleys [1]. We carefully disentangled various contributions to the measured transversal signal and we show that the signal is dominated by a robust spontaneous (present at zero magnetic field) Hall contribution with broad coercivity, which can only be attributed to the antiferromagnetic Zeeman effect. We further present structural, magnetization and magneto-transport results in a sample series and discuss the role of epitaxial growth on our results.

**References:**

[1] H. Reichlova, R. L. Seeger et al, under review. arXiv:2012.15651 [2] L. Néel. Magnetism and Local Molecular Field. Science 174, 985-992 (1971). [3] N. J. Ghimire et al. Large anomalous Hall effect in the chiral-lattice antiferromagnet CoNb3S6. Nat. Com. 9, 3280 (2018). [4] C. Sürgers, et al. Large topological Hall effect in the non-collinear phase of an antiferromagnet. Nat. Com. 5, 3400 (2014).

**Images:**

 Spontaneous Hall effect in the Mn5Si3 antiferromagnet, due to antiferromagnetic Zeeman spin-splitting

The ferromagnetic Heusler compound Co2MnGa is a promising Weyl semimetal with the Fermi energy in the vicinity of the Weyl nodes. Bulk Co2MnGa recently attracted attention because of an exceptionally large anomalous Nernst effect [1,2] and exotic surface states [3]. Simultaneously, Co2MnGa thin films were successfully prepared and showed interesting transport properties [4]. Beside others, the non-trivial topology of the band structure of Weyl semimetals leads to unexpected magneto-thermoelectic transport phenomena which will be discussed in this talk. We report on our observation of a record large anomalous Nernst coefficient in Co2MnGa thin films - up to -3µV/K [5, 6], and discuss the procedure for the quantitative determination of the anomalous Nernst coefficient. Several samples with comparable saturated magnetization were studied, yielding significantly different anomalous Nernst responses. We conclude that the microscopic origin of the anomalous Nernst effect in Co2MnGa is complex and contains contributions from the intrinsic Berry phase and surface states. We further employ a series of Co2MnGa thin films to study the validity of the Mott relation by measuring all four transport coefficients - the longitudinal resistivity, anomalous Hall, Seebeck and anomalous Nernst coefficients [6]. We show that the measured anomalous Nernst signal can be connected to the remaining three magneto-thermal transport coefficients in the whole sample series. This is strong evidence that the Mott relation holds also in Weyl semimetals, where the intrinsic Berry curvature dominates the transport. In the last part of the talk we focus on another magneto-transport phenomena and its thermal counterpart – anisotropic magnetoresistance and anisotropic magneto-thermopower. We show that due to high epitaxial quality of the Co2MnGa films both non-crystalline and crystalline contributions to the anisotropic magnetoresistance can be measured. Interestingly, we do not observe the same crystalline contributions in the magneto-thermopower measurements [7].References: [1] S. N. Guin et al., NPG Asia Materials 11, 16 (2019) [2] A. Sakai et al., Nat. Phys. 13, 1119-1124 (2018) [3] I. Belopolski et al., Science 365, 1278-1281 (2019) [4] A. Markou et al., PRB 100, 054422 (2019) [5] H. Reichlova et al., APL 113, 212405 (2018) [6] G. H. Park, H.R. et al. PRB 101, 060406(R) (2020) [7] P. Ritzinger, H.R. et al., in preparation

Magneto-Thermal Transport in Weyl Semimetal Co2MnGa Thin Films INVITED

**Session chair:** Romain Lebrun, Unité Mixte de Physique CNRS/Thales



GOP - Antiferromagnetic Spintronics II

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FG: Antiferromagnetic Spintronics I

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Presentations

Anomalous Nernst effect in thin films of non-collinear antiferromagnet Mn3NiN

Spontaneous Hall effect in the Mn5Si3 antiferromagnet, due to antiferromagnetic Zeeman spin-splitting

Magneto-Thermal Transport in Weyl Semimetal Co2MnGa Thin Films INVITED

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