Measurement of the Isolated Magnetic Susceptibility.

The isolated susceptibility may be defined as a (non-thermodynamic) average over the canonical ensemble, but while it has often been discussed in the literature, it has not been clearly measured. In this presentation we demonstrate an unambiguous measurement of the isolated magnetic susceptibility at avoided nuclear-electronic level crossings in a dilute spin ice system, containing well-separated holmium ions. The measurement is performed with ac suscpetibility up to frequencies of 2 MHz. We show that the isolated suscpetibility quantifies the superposition of quasi-classical spin states at these points, and is a direct measure of state concurrence and state population. We also demonstrate other materials showing direct evidence of isolated magnetic suscpetibiltiy and discuss implications of these observations.