  State tomography allows to characterize quantum states, and was recently
applied to reveal the dynamic magnetization state of a parametrically driven
magnet. The identification of non-classical states, such as squeezed states,
relies on a careful analysis of their emission and their distinction from
thermal and vacuum fluctuations. A technique allowing to detect equilibrium
magnetization fluctuations is a crucial first step in this regard. In this
Letter, we show that inductive magnon noise spectroscopy (iMNS) allows to
characterize 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.
