Researchers are attracted to antiferromagnets due to their high frequency dynamical properties with a possibility of developing terahertz frequency range spintronics devices. We use magnetic multilayer heterostructures consisting of successive ferromagnetic layers separated by a non-magnetic metal of suitable thickness to have an antiferromagnetic coupling. These structures are called synthetic antiferromagnets (S-AFM). We use permalloy as a ferromagnetic material and ruthenium as a non-magnetic metal to prepare tetralayer synthetic antiferromagnets. We theoretically predict and experimentally verify the existence of two types of magnon-magnon couplings for antiferromagnetic magnons localized on surface and interior layers of tetralayer synthetic antiferromagnet. Macrospin modeling and micromagnetic simulations are used to show that the dynamic part of Rudermanâ€“Kittelâ€“Kasuyaâ€“Yosida (RKKY) interaction is responsible for the self-hybridization of acoustic and optical magnons localized on surface and interior layers due to an accompanying field-like torque. Likewise, surface optical magnons hybridize with interior acoustic magnons by spin pumping, which creates antidamping torque on neighboring ferromagnetic layer. It is reflected in the double-gap in magnon spectrum, that is, an additional magnon-magnon interaction is observed. Thus, interlayer spin pumping is a magnon-magnon interaction control mechanism in in S-AFMs that can be used to help develop energy efficient and high-speed spintronics devices. Research at University of Michigan was supported from NSF CAREER grant DMR-1847847. Work at Oakland University was funded by NSF under award number ECCS-1941426, and that at University of Delaware was supported from NSF grant DMR-2011824.

Hybridizing antiferromagnetic magnons with spin pumping and exchange fields

The characterization and analysis of magnetization dynamics and magnetic damping of antiferromagnetic magnons is important in the field of antiferromagnetic
spintronics. This is primarily because antiferromagnets have magnetic resonances in the THz range as compared to ferromagnetic resonance which tends to occur in the GHz range. In this
work, we employ frequency-domain THz spectroscopy to explore the antiferromagnetic resonance frequency associated with monocrystalline NiO [1]. Specifically, a continuous-wave THz
spectroscopy system is used to measure the antiferromagnetic resonance of three NiO cuts (111), (110), and (100). To find the absorption peak, a reference measurement is taken in addition
to the NiO measurement. Raw data from the reference and NiO can be visually compared to see two effects: the THz system instrument response and Fabry-Perot interference pattern.
During analysis, the instrument response and Fabry-Perot pattern are subtracted so that a clear absorption peak at the resonant frequency of NiO can be observed. An antiferromagnetic
resonance, near 1 THz, is seen in each cut of NiO. Our results demonstrate the utility of working in the frequency domain to characterize antiferromagnetic materials that will be used
eventually for ultrafast antiferromagnetic spintronic device technologies.<br>

**References:**<br>

[1] Moriyama, Takahiro, et al., Physical Review Materials 3.5, 051402 (2019)

Exploring antiferromagnetic resonances of NiO with frequency domain THz spectroscopy

The interplay between electronic transport and the antiferromagnetic order has attracted a surge of interest. Recent studies have shown that a moderate change in the spin orientation of a collinear antiferromagnet may significantly affect the electronic band structure and subsequently manifest itself in transport properties [1]. Among numerous electrical probes to read out such magnetic order, unidirectional magnetoresistance (UMR), where the resistance changes under the reversal of the current direction, can provide rich insights into the transport properties of spin-orbit coupled systems. However, UMR has never been observed in antiferromagnets before, given the absence of intrinsic spin-dependent scattering. Here, we report that a sizable UMR can emerge in an antiferromagnetic system, specifically in a collinear-antiferromagnet/heavy-metal bilayer. The observed UMR in the antiferromagnetic phase of a FeRh/Pt bilayer evolves nonlinearly with increasing magnetic field, persists to large values at high magnetic field and, most notably, exhibits a sign change at a critical field, which is in stark contrast with the UMRs observed in ferromagnetic and non-magnetic systems [2-3]. We attribute this UMR to the strong effective magnetic field due to spin canting in FeRh. Our results can motivate the growing fields of non-centrosymmetric and topological systems, and suggest a route to the development of tunable antiferromagnet-based spintronics devices.<br/>This research was primarily supported by the NSF through the University of Illinois at Urbana-Champaign Materials Research Science and Engineering Center DMR-1720633 and was carried out in part in the Materials Research Laboratory Central Research Facilities, University of Illinois. Thin-film growth at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Materials Science and Engineering Division.

![](https://assets.underline.io/uploads/markdown_image/1/image/756eaf87bcd78eaae92bc5c59ee591d2.jpg)

Fig. 1: UMR in the antiferromagnetic phase of FeRh [110]/Pt (T = 10 K), as a function of field B at various current densities J.

![](https://assets.underline.io/uploads/markdown_image/1/image/71626e5a0aa513b3c7c5af64e05003bc.jpg)

Fig 2: Generation of a nonlinear charge current transverse to the applied B field (top left), which increases considerably in the presence of canting (bottom right).

Observation of unidirectional magnetoresistance in collinear-antiferromagnet/heavy-metal bilayers

Synthetic antiferromagnets (S-AFMs) have recently been shown to be ideal systems to search for Exceptional Points (EPs) [1]. In a S-AFM, EPs are the point at which the optical and acoustic magnons coalesce and are indistinguishable from one another. EPs can be reached by varying parameters, such as the exchange coupling between magnetic layers. Once an EP is reached, only a single magnon branch exists in the magnon energy spectrum, and this makes EPs an attractive prospect for controlling magnons in magnetic microstructures. Recently, we have computationally calculated and visualized EPs in S-AFMs using micromagnetic simulations [2]. We determined that dipolar interactions play a pivotal role in determining if and where EPs occur in the parameter space used to describe the S-AFM. Our results are both qualitatively and quantitatively different compared to macrospin models [3], where dipolar interactions are not considered. We are now investigating how the macrospin limit can be recovered in micromagnetic simulations if spacer layers are built into the simulation, or if the shape of the micromagnetic object is changed. By recovering the macrospin limit within micromagnetic simulations, we will demonstrate new geometric parameters that can be used to control EPs and the magnon energy spectrum within S-AFMs.<br/><br/>Work at Oakland University was supported by U.S. National Science Foundation under Award No. ECCS-1941426.

![](https://assets.underline.io/uploads/markdown_image/1/image/824acc503fd69f0ff6f80f24f5badeb6.jpg)

FIG. 1. (a) Illustration of two micromagnetic layers in antiferromagnetic alignment. A nonmagnetic spacer layer is represented by the grey section. (b) Magnon spectra shown when simulating the antiferromagnetic state with ferromagnetic interlayer exchange stiffness. No clear presence of an EP exists in the antiferromagnetic state before it becomes unstable due to the FM exchange stiffness.

Adjusting dipolar interactions to control exceptional points in synthetic antiferromagnets

Hybrid magnonic systems host a variety of characteristic quantum phenomena such as the magnetically-induced transparency (MIT) and Purcell effect, which are considered useful for future coherent quantum information processing [1-2]. Here, we experimentally demonstrate a tunable MIT effect in the YIG/Py magnon-magnon coupled system via changing the magnetic field orientations. By probing the magneto-optical Kerr and Faraday effects of Py and YIG, we identify clear features of MIT spectra induced by the mode hybridization between the ferromagnetic resonance of Py and the perpendicular standing spin-wave of YIG. By changing the external magnetic field orientation, we observe a tunable coupling strength between the YIG's spin-wave modes and the Py's uniform mode, upon the application of an out-of-plane magnetic field. Our findings show high promise for investigating tunable coherent phenomena with hybrid magnonic platforms.

Facile Magneto-optic Detection of Tunable Magnetically-Induced Transparency Spectra in Magnon-Magnon Coupled Bilayers

Weyl Semimetals for Spintronics

GOC: Magnetization Dynamics III

technical paper

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Symposium 1: Weyl Semimetals for Spintronics

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Presentations

Hybridizing antiferromagnetic magnons with spin pumping and exchange fields

Exploring antiferromagnetic resonances of NiO with frequency domain THz spectroscopy

Observation of unidirectional magnetoresistance in collinear-antiferromagnet/heavy-metal bilayers

Adjusting dipolar interactions to control exceptional points in synthetic antiferromagnets

Facile Magneto-optic Detection of Tunable Magnetically-Induced Transparency Spectra in Magnon-Magnon Coupled Bilayers

Weyl Semimetals for Spintronics

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