Inductive noise spectroscopy of thermally excited magnons

Abstract

State tomography allows quantum states to be characterized, and was recently applied to reveal the dynamic magnetization state of a parametrically driven magnet. The identification of nonclassical states, such as squeezed states, relies on a careful analysis of their emission and their distinction from thermal and vacuum fluctuations. A technique allowing detection of equilibrium magnetization fluctuations is a crucial first step in this regard. In this paper, we show that inductive magnon noise spectroscopy (iMNS) allows characterization of the thermal magnetization fluctuations of a ferromagnetic thin film in a broadband coplanar waveguide-based scheme. Relative to a cold microwave background, the microwaves emitted by the equilibrium magnetization fluctuations can be detected via spectrum analysis. We provide a comprehensive picture of our microwave system by quantitatively modeling its response, including the thermalizing influence of the cables. The model allows for direct comparison to low-power broadband ferromagnetic resonance measurements with excellent agreement, corroborating the equilibrium character of the iMNS measurement by probing the linear response of the equilibrium state. Our work thus demonstrates broadband access to the equilibrium properties of magnetization fluctuations using a purely inductive approach.