Spin dynamics in single-molecule magnets combining surface acoustic waves and high-frequency electron paramagnetic resonance

We report an experimental technique that integrates high-frequency surface acoustic waves (SAWs) with high-frequency electron paramagnetic resonance (HFEPR) spectroscopy in order to measure spin dynamics on fast time scales in single-molecule magnets. After the system is driven out of equilibrium by triggering magnetic avalanches, or simply by heating with short SAW pulses, the evolution of the spin populations within fixed energy levels is measured using HFEPR spectroscopy.

Figure 1

Double-well potential diagram illustrating how spins relax during an avalanche. Spins in the right well relax to the ground state by emitting phonons, which result in heating of the system. This heating feeds back into the avalanche by exciting spins in the left well.

Figure 2

(Color online) Evolution of the reflected microwave amplitude during an avalanche triggered via a SAW pulse. The EPR signals are presented in arbitrary units with a reference level of zero microwave absorption. In (a), $f=270\mathrm{GHz}$, $H=1.1\mathrm{T}$, and $m_s=-10$ to $-9$; in (b), $f=237\mathrm{GHz}$, $H=0.58\mathrm{T}$, and $m_s=-9$ to $-8$; in (c) $f=320\mathrm{GHz}$, $H=0.54\mathrm{T}$, and $m_s=10$ to 9; and in (d), $f=268\mathrm{GHz}$, $H=0.54\mathrm{T}$, and $m_s=9$ to 8. In (c) and (d), the dotted traces correspond to microwave absorption during a SAW pulse after the occurrence of the avalanche.

Figure 3

(Color online) Time evolution of the EPR signal induced by SAW pulses at different temperatures from $T=2.5\mathrm{K}$ to $T=8\mathrm{K}$. The magnetic field is $H=6120\mathrm{Oe}$, corresponding to a frequency difference, $f=269\mathrm{GHz}$, between $m_s=9$ and $m_s=8$. The pulse duration is $5\mathrm{ms}$ and the beginning corresponds to $t=0$. The dashed line corresponds to a simulation of the data.