Magneto-optical Kerr-effect measurements under pulsed magnetic fields over 40 T using a compact sample fixture

TL;DR

This work presents a compact, non-metallic ferrule-based sample fixture integrated with an all-fiber, loop-less Sagnac interferometer and a phase-resolved numerical lock-in analysis to enable polar magneto-optical Kerr effect measurements in ms-scale pulsed magnetic fields up to 43 T. By extracting the Kerr signal from time-resolved data and correcting for linear Faraday backgrounds, the method achieves ~0.1 mrad Kerr-angle resolution and demonstrates measurable Kerr signals in Fe, Ni, and Fe$_3$O$_4$ at both room temperature and 77 K. The approach uses a phase-aware analysis with optimal modulation depth and a robust, plug-and-play setup suitable for high-field facilities, enabling investigations of magnetic properties in extreme conditions and for thin films or non-transparent materials. Overall, this technique broadens the experimental accessibility of MOKE under extreme magnetic fields and supports high-throughput exploration of field-induced magnetic phenomena.

Abstract

The magnetic field is one of the most fundamental control parameters in materials science. A pulsed magnetic-field apparatus can generate high magnetic fields that are inaccessible by conventional DC-field magnets. One important issue is that measurement techniques compatible with pulsed fields are rather limited due to short pulse duration and large electromagnetic or mechanical noise originating from field pulses. The magneto-optical Kerr effect (MOKE), the change in the state of light polarization upon reflection from magnetic materials, has the potential to become a powerful tool for investigation of magnetic properties of a wide range of materials including non-transparent materials or thin films in pulsed fields. Nevertheless, since the MOKE response is typically very small, MOKE measurements under pulsed fields are quite challenging. Here, we present a new method to measure polar MOKE under high pulsed magnetic fields of 2-ms pulse width. The keys of this new technique are a ferrule-based compact sample-fiber fixture and a phase-resolved numerical lock-in analysis, combined with the high-resolution optical apparatus based on an all-fiber loop-less Sagnac interferometer. We succeeded in measuring MOKE signals from various ferromagnetic or ferrimagnetic samples above 40 T and down to 77 K, significantly extending the limits of previously reported pulse-field MOKE measurements. Our apparatus is simple enough to be compatible with larger-scale experiments in pulse-field facilities, thus becoming a new promising tool to optically investigate material properties in pulsed fields.

Figures (12)

• Figure 1: (a) Schematic of the central part of the sample fixture composed of a ferrule-embedded gradient-index (GI) lens focuser, 1/4 waveplate, ferrule sleeve, and polyimide spacer. The spacer thickness is chosen to be 50µm, such that the sample surface is placed at the focal position of the GI lens simply by sandwiching the sample with two ferrules. The diameter of the ferrules is 2.5 mm and the outer diameter of the ferrule sleeve is 3.2mm. (b) Photo of the whole fixture. The ferrule focuser and the blank ferrule are fixed using a ferrule sleeve.
• Figure 2: Measurement setup for the magneto-optical Kerr effect under pulsed magnetic fields. Sets of voltage recorded by the oscilloscope are analyzed using the numerical lock-in analysis. When we use the 6-mm coil, the field pick-up coil and the Kerr fixture cannot be simultaneously inserted to the bore. Thus we first run calibration with the pick-up coil in order to obtain the field-current relation. In subsequent MOKE measurements without the pick-up coil, the field value is calculated from the magnitude of the current.
• Figure 3: Typical voltage signals of (a) current and (b) field sensors recorded by the oscilloscope (blue curves; right vertical axes) for the 10-mm bore coil at room temperature. The current and field values evaluated by numerical integration of the sensor voltages are shown with purple curves (left vertical axes) in (a) and (b), respectively. (c) Time dependence of the magneto-optical rotation signal measured with Fe evaluated using numerical lock-in technique based on Eq. \ref{['eq: thetaK_new-relation']}.
• Figure 4: Results of pulse-field MOKE for various materials at room temperature plotted as a function of magnetic field. The optical power of the light source was 0.8mW. (a) Total measured MO rotation $\theta_{\mathrm {tot}}$ containing the Kerr effect from the sample and the background Faraday effects. Here, $\theta_{\mathrm {tot}}$ was evaluated using Eq. \ref{['eq: thetaK_new-relation']}. (b) Kerr angle $\theta_{\mathrm {K}}$ from the samples obtained by subtracting the field-linear background contribution. For both panels, curves with blue-like colors, orange-like colors, and green represent data for Fe, Ni, and Au, respectively. For Ni and Fe, data obtained with different charging voltages are shown.
• Figure 5: Time-series data of a pulse-field MOKE experiment over 40-T. The field was generated using the 6-mm coil placed in liquid nitrogen. (a) Voltage signal from the current sensor, recorded by the oscilloscope. (b) Current calculated by numerically integrating the sensor voltage. The corresponding field obtained using a calibrated field-current ratio is shown on the right vertical axis. (c) Time dependence of the magneto-optical rotation signal (including both the sample Kerr rotation and background contribution) measured with Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$ using Eq. \ref{['eq:Kerr_conventional']}.