PHYSICAL REVIEW APPLIED 13, 034035 (2020) Zero-field Optic Mode Beyond 20 GHz in a Synthetic Antiferromagnet H. J. Waring ,1,* N. A. B. Johansson,1 I. J. Vera-Marun ,2 and T. Thomson 1 1 Nano-Engineering and Spintronic Technologies (NEST), Department of Computer Science, University of Manchester, Manchester, UK 2 Department of Physics and Astronomy, University of Manchester, Manchester, UK (Received 2 December 2019; revised manuscript received 31 January 2020; accepted 3 February 2020; published 13 March 2020) Antiferromagnets have considerable potential as spintronic materials. Their dynamic properties include resonant modes at frequencies higher than can be observed in conventional ferromagnetic materials. An alternative to single-phase antiferromagnets are synthetic antiferromagnets (SAFs), engineered structures of exchange-coupled ferromagnet/nonmagnet/ferromagnet trilayers. SAFs have significant advantages due to the wide-ranging tunability of their magnetic properties and inherent compatibility with current device technologies, such as those used for Spin-transfer-torque magnetic random-access memory production. Here we report the dynamic properties of fully compensated SAFs using broadband ferromagnetic res- onance and demonstrate resonant optic modes in addition to the conventional acoustic (Kittel) mode. These optic modes possess the highest zero-field frequencies observed in SAFs to date with resonances of 18 and 21 GHz at the first and second peaks in antiferromagnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, respectively. In contrast to previous SAF reports that focus only on the first RKKY antiferromagnetic coupling peak, we show that a higher optic mode frequency is obtained for the second antiferromagnetic coupling peak. We ascribe this to the smoother interfaces associated with a thicker non- magnetic layer. This demonstrates the importance of interface quality to achieving high-frequency optic mode dynamics entering the subterahertz range. DOI: 10.1103/PhysRevApplied.13.034035 I. INTRODUCTION in ferromagnets, leading to the possibility of engineer- ing higher-frequency devices than used currently [1,7,8]. Antiferromagnetic (AF) spintronics has recently become Most antiferromagnets found in nature are also dielectrics, a key area of research to enhance the capabilities of which contributes to the elimination of Joule heating spintronic devices [1,2]. They are increasingly poised to [9–11]. However, metallic AFMs and particularly those augment or even replace current ferromagnetic compo- where there is freedom to design the required properties nents across a range of technologies, a remarkable resur- can offer advantages, for example, when there is a need for gence given the description of “interesting and useless” layer thicknesses below 10 nm. bequeathed to this class of materials by Néel [3]. Due Magnetic materials are essential components of high- to their characteristic antiparallel spin ordering, antifer- frequency technologies and devices. The maximum speed romagnets (AFMs) have a low or zero net magnetiza- of operation and frequency range of magnetic materials tion, eliminating the stray field responsible for promoting is determined by the ferromagnetic resonance frequency, thermal activation of reversal and hence offering advan- f [12,13]. To realize gigahertz-range magnetic devices tages in data storage such as magnetic random-access r it is essential to engineer materials that support a much memories (MRAM) [4], Spin-transfer-torque magnetic greater f [14]. The resonant frequency of ferromagneti- random-access memory (STT MRAM) [5], and mag- r cally ordered materials is given by Kittel’s equation [15], netic sensors [6]. Furthermore, their dynamic response can reach towards the terahertz range, orders of magni- γ tudes higher than the low-gigahertz range typically found f A = (H + H )(H + H + 4πM ),(1) r 2π k k s *[email protected] A where fr is the conventional acoustic mode ferromag- Published by the American Physical Society under the terms of netic resonance, γ is the gyromagnetic ratio, H is the the Creative Commons Attribution 4.0 International license. Fur- applied magnetic field, H is the uniaxial anisotropy, and ther distribution of this work must maintain attribution to the k author(s) and the published article’s title, journal citation, and Ms is the saturation magnetization. Resonant frequencies DOI. can be increased through the application of large magnetic 2331-7019/20/13(3)/034035(10) 034035-1 Published by the American Physical Society WARING, JOHANSSON, VERA-MARUN, and THOMSON PHYS. REV. APPLIED 13, 034035 (2020) fields, with notable examples including resonant frequen- configuration where the magnetizations of the FM layers cies nearing 100 GHz with applied fields of around 5 T align antiparallel. In effect, the RKKY interaction creates in Co-Cr-Pt-Ta thin films, as demonstrated by Oates et spin density oscillations in the NM spacer layer as its al.[16]. However, for future device development, it is thickness is altered. The strength and period of the RKKY critical to consider miniaturization and heat management coupling is highly sensitive to the Fermi surface of the NM [9], which makes the application of large magnetic fields metal [33,34]. In particular, it is found that the periodicity technically challenging. It is therefore desirable for mag- of the coupling is given by netic materials to possess a very high self-bias, mean- ing no externally applied magnetic field is required to 1 = ,(2) achieve higher resonance frequencies. Conventional meth- |(1/λ) − (n/d)| ods of accomplishing this goal involve using materials λ with a high-saturation magnetization 4πMs or increasing where is the Fermi wavelength, d is half the lattice Hk. From a material selection point of view, the focus parameter of the spacer material, and n is a positive integer of the community has pivoted toward ferrites such as satisfying the criterion ≥ 2d [35]. CoFe-based materials [17–19]. Enhancements to Hk are The presence of this interlayer coupling has a significant typically achieved through the application of a magnetic impact on the precessional dynamics of the SAF system. field during deposition with potential use of annealing [20], Here the resonance frequency is enhanced by the addition tailoring the nanocrystalline structure of the films using of an effective interlayer coupling term (Hex) in the disper- o chemical stress or oblique sputtering [17,21], use of pre- sion relation that introduces an optic mode resonance (fr ) stressed substrates [22], or lamination and lithographic in the AF configuration [36], shown schematically in Fig. 1 patterning [14]. Exchange-biased systems have also been and given by proposed [23]. The highest frequencies in self-bias sys- γ o tems achieved to date are in the X -band range, around fr = (H + Hex + Hk)(H + Hex + Hk + 4πMs). 10 GHz [17,23]. There are notable difficulties in further tai- 2π (3) loring of Hk using the methods outlined above, such as the application of a dc field during depositions, which induces This optic mode has been studied by several groups due an anisotropy that is difficult to modify and prestressed to its potential to significantly enhance self-biased fer- substrates requiring mechanical flexibility. romagnetic resonances towards low-terahertz frequencies The limitations in achieving further increases of H k [36]. In recent works, notable enhancements to the f fre- to realize higher-frequency magnetic devices have led to r quency were demonstrated by Li et al.[37] who reported the exploitation of AF materials that have faster dynamic a zero-field optic mode resonance of 11.32 GHz. Further, responses than are available in their ferromagnetic coun- SAFs fabricated using a range of ferromagnetic compo- terparts [1,7,24]. Resonance experiments of single-phase nents have been explored including Ni-Fe [38], Co [39,40] antiferromagnets are typically conducted with terahertz- and (Fe,Co)N [41]. Additionally, ferromagnetic compo- range optical or electrical field probing techniques nents created from highly spin-polarized Heusler alloys [8,25–29], with limited reports of antiferromagnets with such as Co FeAl have been reported [42]. dynamics in the range of conventional microwave elec- 2 tronics [30]. Therefore, there is a particular research interest in synthetic antiferromagnetic (SAF) structures, (a) (b) comprised of an exchange-coupled ferromagnet-non- magnet-ferromagnet (FM-NM-FM) multilayers. However, as the interlayer exchange coupling (JIEC) seen in SAFs is smaller than the direct exchange coupling seen in single-phase antiferromagnets, the resonance frequencies are also reduced. This opens up additional possibilities for AFM-based devices, extending the range of frequen- cies that can be generated, and provides a method to tailor the frequency obtained through controlling the coupling interaction [31,32]. The interlayer coupling results from FIG. 1. Magnetic precession of an FM-NM-FM trilayer system the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction due to a perturbation field hrf when (a) NM thickness t1 promotes [33], which for an appropriate choice of spacer layer such a ferromagnetic alignment, and (b) NM thickness t2 promotes an as Ru is responsible for an oscillatory short-range inter- antiferromagnetic alignment. A field H needstobeappliedto(a) action that can promote ferromagnetic or antiferromag- to enable similar high resonant frequencies to that displayed at netic layer alignment depending on spacer layer thickness. zero field in (b). The red-colored layers are ferromagnetic, while Here the nonmagnetic spacer layer is chosen to produce a the blue is the nonmagnetic spacer layer. 034035-2 ZERO-FIELD OPTIC. PHYS. REV. APPLIED 13, 034035 (2020) In this work, we demonstrate that higher frequen- The layer structure of the multilayers, shown in Fig. cies (in excess of 20 GHz) can be obtained for the 2(a), is determined using X-ray reflectivity (XRR) where zero-field optic mode using SAFs with the structure the data are fitted to a Fresnel model using the GENX Co0.2Fe0.6B0.2(5 nm)/Ru(tRu)/Co0.2Fe0.6B0.2(5 nm), where software package (see Appendix B1), with an example tRu is the Ru layer thickness.