# Synopsis: Unmasking the True Spin Relaxation Time

#### Spin lifetime in small ensembles of electron spins measured by magnetic resonance force microscopy

K. C. Fong, M. R. Herman, P. Banerjee, D. V. Pelekhov, and P. C. Hammel

Published December 8, 2011

In a typical electron spin resonance experiment, an ac magnetic field rotates the spins in a sample and induction electronics track their return to equilibrium. In comparison to this “classic” technique, magnetic resonance force microscopy (MFRM) is sensitive to a much smaller volume of spins—less than a hundred, in some cases. Instead of inductors, MFRMs measure the small change in oscillation frequency of a nanoscale magnetic tip as it is brought into contact with the molecular or atomic spins on a surface.

The so-called spin-lattice relaxation time—a measure of the spins’ interaction with their environment—can be extracted from these frequency shifts, which makes the technique useful for studying magnetic inhomogeneities in superconductors or spin qubits. But the measured spin relaxation time can vary with the power of the ac field or the distance between the oscillating tip and the surface. In a paper appearing in Physical Review B, Kin Chung Fong at the California Institute of Technology and his colleagues map out these effects in MRFM experiments, and show they can be removed to reveal the true local spin dynamics in a small ensemble of spins.

Fong et al. use a home-built MRFM to study spins localized at oxygen vacancies in silica. Their measurement probes a cubic volume $12$ nanometers on a side, equivalent to a few hundred spins. Fong et al. achieve experimental conditions that allow their measurement to track the intrinsic spin-lattice relaxation time, suggesting the technique could study spin dynamics in other nanoscale materials. – Jessica Thomas