We experimentally demonstrate the existence of magnetic coupling between two ferromagnets separated by a thin Pt layer. The coupling remains ferromagnetic regardless of the Pt thickness, and exhibits a significant dependence on temperature. Therefore, it cannot be explained by the established mechanisms of magnetic coupling across nonmagnetic spacers. We show that the experimental results are consistent with the presence of magnetism induced in Pt in proximity to ferromagnets, in direct analogy to the well-known proximity effects in superconductivity.PACS numbers: 75.70.Cn, 75.10.Hk For centuries, platinum metal has been highly valued for its luster and rarity. With the technological revolution, Pt has found many applications owing to its high chemical inertness and catalytic properties. Recently, it was shown that one can modify the dynamical magnetic characteristics of ferromagnets (F) by passing an electrical current through Pt/F bilayers [1][2][3]. This phenomenon caused by the spin Hall effect in Pt [4] has generated a significant interest in spin-dependent electronic properties of this material.Spin Hall effect in Pt originates from the spin-orbit interaction of the predominantly d-type conduction electrons. Because of the large density of states of the d-electrons, Pt also almost satisfies the Stoner criterion for the onset of ferromagnetic ordering [5]. Indeed, magnetism has been predicted and experimentally observed in Pt nanoparticles [6][7][8], nanocontacts [9], nanowires [10,11], and thin films [12,13]. These observations raise a question important for spintronic applications: What are the magnetic properties of thin Pt films in heterostructures with magnetic materials?We address this question by studying the magnetic properties of structures that consist of two ferromagnets separated by a Pt layer. We observe ferromagnetic coupling between the magnetic layers that depends both on the Pt thickness d and on temperature T . The coupling decreases with increasing T . It persists up to room temperature RT=295 K for sufficently small d, but vanishes below RT for larger d. These characteristics distinguish this coupling from the well-known Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism of coupling between ferromagnets separated by a nonmagnetic spacer, which oscillates with spacer thickness and is generally independent of temperature [14]. Our analysis indicates that the observed coupling originates from the magnetization induced in Pt at the interfaces with the ferromagnets, suggesting that spin-dependent properties of both Pt and F are mutually affected in Pt/F spintronic heterostructures. The multilayers were deposited at RT on oxidized Si substrates by magnetron sputtering at ultrapure Ar gas pressure of 5 mTorr. The residual gas pressure was 7 × 10 −9 Torr, as verified by a residual gas analyzer. Substrate roughness was less than 0.5 nm, as verified by atomic force microscopy. The thicknesses of the deposited layers were monitored by a quartz crystal sensor, which was independently calibrated to a...

Cavity quantum electrodynamics, which explores the granularity of light by coupling a resonator to a nonlinear emitter [1], has played a foundational role in the development of modern quantum information science and technology. In parallel, the field of condensed matter physics has been revolutionized by the discovery of underlying topological robustness in the face of disorder [2-4], often arising from the breaking of time-reversal symmetry, as in the case of the quantum Hall effect. In this work, we explore for the first time cavity quantum electrodynamics of a transmon qubit in the topological vacuum of a Harper-Hofstadter topological lattice [5]. To achieve this, we assemble a square lattice of niobium superconducting resonators [6] and break time-reversal symmetry by introducing ferrimagnets [7] before coupling the system to a single transmon qubit. We spectroscopically resolve the individual bulk and edge modes of this lattice, detect vacuumstimulated Rabi oscillations between the excited transmon and each mode, and thereby measure the synthetic-vacuum-induced Lamb shift of the transmon. Finally, we demonstrate the ability to employ the transmon to count individual photons [8] within each mode of the topological band structure. This work opens the field of chiral quantum optics experiment [9], suggesting new routes to topological many-body physics [10,11] and offering unique approaches to backscatter-resilient quantum communication.

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