Spin-glass and non–spin-glass features of a geometrically frustrated magnet

We use neutron scattering to show that the low-temperature, short-range ordered spin configuration in the geometrically frustrated magnet $\rm SrCr_{9\it p}Ga_{12-9\it p}O_{19}$ (p=0.92(5)) is composed of small groups of spins whose dipole moments cancel. The local magnetic fluctuation spectrum, $\chi^{\prime\prime} (\omega )$, vanishes approximately in proportion to ω for $\hbar\omega < 0.5\;{\rm meV}$, distinguishing this magnet from conventional spin glasses which display a featureless continuum of excited states. We argue that this behavior results from the absence of local, low-energy excitations in the zero-spin clusters from which the frozen spin configuration is composed