Quantifying the amplitudes of ultrafast magnetization fluctuations in Sm$_{0.7}$Er$_{0.3}$FeO$_{3}$ using femtosecond noise correlation spectroscopy

TL;DR

This paper demonstrates femtosecond noise correlation spectroscopy (FemNoC) as a method to directly measure ultrafast magnetization fluctuations in a magnetic material, providing quantitative insights into spin dynamics relevant for spintronics.

Contribution

The study introduces a quantitative approach linking FemNoC signals to magnetization fluctuation amplitudes, with three calibration protocols for accurate measurement.

Findings

Quantitative measurement of ultrafast magnetization fluctuation variance.

Calibration protocols for FemNoC accuracy.

Demonstration of FemNoC in a complex magnetic oxide.

Abstract

Spin fluctuations are an important issue for the design and operation of future spintronic devices. Femtosecond noise correlation spectroscopy (FemNoC) was recently applied to detect ultrafast magnetization fluctuations. FemNoC gives direct access to the spontaneous fluctuations of the magnetization in magnetically ordered materials. In FemNoC experiments, the magnetic fluctuations are imprinted on the polarization state of two independent femtosecond probe pulses upon transmission through a magnetic sample. Using a subharmonic demodulation scheme, the cross-correlation of the signals from both pulse trains is calculated. Here, we quantitatively link the FemNoC output signal to an optical polarization rotation, and then in turn to the magnitude of the inherent spin fluctuations. To this end, three different calibration protocols are presented and compared in accuracy. Ultimately, we quantitatively determine both the variance of optical polarization noise in rad$^2$, and that of the ultrafast magnetization fluctuations in (A/m)$^2$.