Optimized Broadband Cryogenic Ferromagnetic Resonance Spectrometer using a Closed Cycle Refrigerator

TL;DR

Addressing the need for broadband, temperature-tunable FMR at cryogenic temperatures with controlled background, the paper demonstrates a VNA-based CryoFMR using a CCR and a custom GCPW. The method integrates SOLR calibration to reference the measurement plane to the device under test, enabling reliable extraction of $H_{\mathrm{res}}$, $\Delta H$, $\gamma$, and $\alpha$ across $11$--$350\,\mathrm{K}$ and $1$--$20\,\mathrm{GHz}$. The authors validate the system with a $100$-nm YIG thin film, achieving $\alpha = (1.15 \pm 0.06)\times 10^{-3}$ at room temperature and linewidths as small as $5\,\mathrm{Oe}$, along with $0.02$ dB transmittance resolution. The work presents a cost-effective, widely accessible platform for investigating temperature-dependent magnetization dynamics, interfacial spin-transfer phenomena, and anisotropy in thin films and heterostructures.

Abstract

We present a vector network analyzer (VNA) based broadband cryogenic ferromagnetic resonance (FMR) spectrometer, operating up to 20 GHz over a temperature range of 11 to 350 K. A cost effective architecture is implemented through the integration of a closed cycle refrigerator (CCR) and a custom fabricated grounded coplanar waveguide (GCPW), designed for broadband transmission and reliable cryogenic operation. The VNA calibration is performed prior to measurements to account for microwave background and transmission losses, enabling reliable extraction of FMR spectra across the full temperature / frequency range. The sensitivity of the spectrometer is benchmarked using a yttrium iron garnet (YIG) thin film, yielding well resolved resonances with narrow linewidths and high sensitivity.

Figures (7)

• Figure 1: Block diagram of the Broadband CryoFMR spectrometer.
• Figure 2: A picture of the in-house home-built broadband Cryo-FMR system with the CCR vacuum sample chamber placed between the poles of the electromagnet. The vacuum hose and compressor gas lines are kept along one direction to facilitate easy rotation of the electromagnet.
• Figure 3: (a) A picture of the inverted CCR showing the cryogenic RF cable installation, with cable bending near the first- and second-stage junction, (b) Photograph of the copper sample holder with the grounded coplanar waveguide (GCPW) mounted on top, and the sample kept on the signal line in a flip-chip configuration using Kapton tape.
• Figure 4: GCPW transmittance response as a function of frequency (a) with and without vias, (b)Top view of the GCPW showing dimensions in inches; the yellow color represents the copper cladding.
• Figure 5: The GCPW response measured (a) with and without calibration, and (b) at different temperatures following SOLR calibration of the VNA performed at room temperature.