Tunable Magnon Photon Coupling Using a BLIG ((LuBi)<inf>3</inf>Fe<inf>5</inf>O<inf>12</inf>) Film and a Split Ring Resonator

Abstract

Split ring resonators (SRR) are a common metamaterial with good microwave absorption, ease of fabrication, low weight and they can be hybridized with complementary metal-oxide-semiconductor (CMOS) platforms. The planar SRR structure also allows coupling of microwave photons and magnons. The photon-magnon coupling strength needs to be greater than the mean energy loss in each subsystem, and occurs when both sub-systems are close to resonance. In a ferromagnetic system, the magnon bands can be controlled by an externally applied magnetic field or electric current. 3D structures of YIG (Yttrium iron garnet) films were first used to study magnon polariton coupling [1], [2]. Recently, the coupling of SRR and FMR (ferro magnetic resonance) modes have been observed in a planar YIG film placed inside an SRR cavity [3] and an enhanced coupling is experimentally demonstrated in planar YIG kept in an inverted SRR [4]. The photonmagnon coupling strength of a planar YIG film was improved by using with a notch filter on a stub line [5]. Microwave photons also allow for excitation of the higher order spin-wave (SW) modes, which contribute to stronger photon-magnon coupling. Spin waves can be excited in in-plane configurations namely backward volume spin waves and the magnetostatic surface SW. These ferrites find tremendous applications in transport of spin waves, realization of SW based logic and microwave devices etc, [6]. Unlike a 25 μm YIG used in [3], we use a bismuth lutetium iron garnet (BLIG) epitaxial film of thickness 7.9 μm, grown over gadolinium gallium garnet substrate. BLIG films are known to have better Faraday coefficients, than YIG, which could influence the photon-magnon coupling. A SRR of dimensions as shown in Fig. 1 was simulated in HFSS and resonant frequencies of 2.5 GHz, 4.9 GHz and 7.5 GHz were obtained. We fabricated the SRR on a Roger substrate (e ~ 3.66), with copper lines of thickness 35 μm, ensuring a 50Ω termination. The characterization setup consists of a 20 GHz RF signal generator (Hittite), an RF circulator and a spectrum analyzer (Rhode & Schwarz). An electromagnet and a Gauss meter are used to apply the static in-plane magnetic field, in a direction perpendicular to the microstrip line. We used the flip chip method for placing the BLIG film in the cavity on top of a microstrip line, and excited spin waves at ferromagnetic resonance [3]. The RF current flowing through the microstrip line also excites microwave photons in the SRR. A static applied magnetic field, greater than 200 Oe, saturates the BLIG film while the resonant fields of the SRR cause the magnetization to precess and excite SWs. We place the BLIG film to cover the SRR and a part of the microstrip line, and vary the excitation frequency from 2 to 5 GHz and the applied field from 200 Oe to 750 Oe. The power spectral density was measured on the spectrum analyzer. In post processing, we obtain the transmission (S 21 ) characteristics, and plot it against applied magnetic field, to obtain the spectra shown in Fig. 2(a). We observe an SRR mode at 3.17 GHz, which does not change with the field variations. Along with that, we also observe FMR modes whose resonant frequency varies linearly with the applied field. For a field of 435 Oe, we observe the appearance of a small dip at 2.75 GHz, which keeps shifting to the right as we increase the field. In Fig 2(b), we plot the frequencies corresponding to all the dips as observed in Fig. 2(a) apart from those corresponding to SRR dips. We observe a splitting of the spin wave mode resulting from a strong coupling between the microwave photons and magnons. The anti-crossing of the dispersion curves is a consequence of the coupled oscillation of the magnon-photon pair. We are in the process of studying this coupling in other spin wave systems such as magnonic crystals and aim to contribute to the state of the art in the performance of planar microwave devices.