  The inability of systems of interacting objects to satisfy all constraints
simultaneously leads to frustration. A particularly important consequence of
frustration is the ability to access certain protected parts of a system
without disturbing the others. For magnets such "protectorates" have been
inferred from theory and from neutron scattering, but their practical
consequences have been unclear. We show that a magnetic analogue of optical
hole-burning can address these protected spin clusters in a well-known,
geometrically frustrated Heisenberg system, gadolinium gallium garnet. Our
measurements additionally provide a resolution of a famous discrepancy between
the bulk magnetometry and neutron diffraction results for this magnetic
compound.
