Enhanced Magnon-Photon Coupling at the Angular Momentum Compensation Point of Ferrimagnets Jaechul Shim , 1,2 Seok-Jong Kim , 3 Se Kwon Kim , 4,5,* and Kyung-Jin Lee 1,3,† 1 Department of Materials Science and Engineering, Korea University, Seoul 02841, Korea 2 Semiconductor R&D Center, Samsung Electronics Co. Ltd., Hwaseong, Gyeonggi 18448, Korea 3 KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Korea 4 Department of Physics, KAIST, Daejeon 34141, Korea 5 Department of Physics and Astronomy, University of Missouri, Columbia, Missouri 65211, USA (Received 6 March 2020; accepted 18 June 2020; published 10 July 2020) We theoretically show that the coupling between magnons in an antiferromagnetically coupled ferrimagnet and microwave photons in a cavity is largely enhanced at the angular momentum compensation point (T A ) when T A is distinct from the magnetization compensation point. The origin of the enhanced magnon-photon coupling at T A is identified as the antiferromagnetic spin dynamics combined with a finite magnetization. Moreover, we show that strong magnon-photon coupling can be achieved at high excitation frequency in a ferrimagnet, which is challenging to achieve for a ferromagnet due to low magnon frequency and for an antiferromagnet due to weak magnon-photon coupling. Our results will invigorate research on magnon-photon coupling by proposing ferrimagnets as a versatile platform that offers advantages of both ferromagnets and antiferromagnets. DOI: 10.1103/PhysRevLett.125.027205 Introduction.—Quantum information technology [1] involves process, storage, transmission, and transduction of quantum information. Several physical systems have been examined to fulfill the functional tasks [2–5]. By selectively combining proper physical platforms, various hybrid quantum systems have also been designed to build a multifunctional quantum information processing system [6,7]. Photons in a microwave cavity mediate qubits from one physical constituent to another [6]. For storage and transduction of qubits, the spin ensemble [8,9] is an adequate candidate because spins allow longer coherence time than other physical quantities due to limited dissipa- tive channel to environment. A strong coupling of the spin ensemble to other compo- nents in hybrid quantum system is essential for suppressing the loss of quantum information during the process. As the coupling strength is enhanced by a factor of ffiffiffiffi N p , where N is the spin density [10], a strong magnon-photon coupling in magnetic materials was theoretically predicted [11] and experimentally confirmed [12], leading to recent intensive studies on magnon-photon coupling in magnetic materials with various aspects [13–34], including single magnon detection [18], magnon dressed state [23], spin-information transfer between two magnets through cavity photons [17,24], and attractive level crossing [26–28,31]. For widespread application, not only strong magnon- photon coupling but also high excitation frequency (i.e., high magnon frequency) is required. In particular, the latter allows us to expand the frequency window where the trans- duction of quantum information occurs. Most studies so far have focused on the coupling between ferromagnetic mag- nons and microwave photons. We note that although yttrium iron garnet (YIG), a ferrimagnetic insulator with antiferro- magnetically coupled Fe moments, has been widely used for the magnon-photon coupling, its magnon dynamics is well described by ferromagnetic magnon theories [12–31] because it is far from the compensation condition. In this respect, YIG can be considered as a ferromagnet (FM) with a reduced moment. For ferromagnets, the magnon-photon coupling strength is limited below a few hundred megahertz at the magnon frequency in gigahertz ranges [12–31]. On the other hand, recent studies found that antiferromagnetic magnons also couple to microwave photons [32–34]. For antiferro- magnets (AFMs), the magnon frequency is much higher than for ferromagnets because of the manifestation of antiferro- magnetic exchange interaction [35–37]. The magnon-photon coupling is, however, much weaker for antiferromagnets than for ferromagnets [34] because of net zero magnetic moment in equilibrium. Therefore, it has remained challenging to achieve both high excitation frequency and strong magnon- photon coupling in ferromagnets or antiferromagnets. In this Letter, we theoretically show that this challenge can be overcome by employing a class of ferrimagnets (FIMs). The necessary condition is that two inequivalent spin moments with different Land´ e-g factors are coupled antiferromagnetically, which is satisfied in some rare-earth (RE) transition-metal (TM) ferrimagnets. This condition allows for a finite net magnetic moment, thus a finite Zeeman coupling, at the angular momentum compensation point T A where the net spin density vanishes. The nature of PHYSICAL REVIEW LETTERS 125, 027205 (2020) 0031-9007=20=125(2)=027205(6) 027205-1 © 2020 American Physical Society