Inductive magnon noise spectroscopy

TL;DR

This paper introduces inductive magnon noise spectroscopy (iMNS), a broadband technique to characterize thermal magnetization fluctuations in ferromagnetic films, enabling non-invasive analysis of equilibrium magnetic states through microwave emission spectrum analysis.

Contribution

The work presents a novel inductive method for broadband detection of equilibrium magnetization fluctuations, with a comprehensive model and experimental validation showing excellent agreement with ferromagnetic resonance data.

Findings

iMNS can detect microwave emission from thermal magnetization fluctuations.

The model accurately predicts the system's response, including cable thermalization effects.

The technique provides broadband access to equilibrium magnetic properties.

Abstract

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.