Ferrimagnetic Garnet Targets for Exotic Spin-Dependent Force Searches

We describe target fabrication and characterization for a search for exotic spin-dependent interactions between neutrons and electrons. These interactions could be mediated by axion-like particles, which are leading candidates for dark matter. We have conducted experiments measuring the spin rotation of polarized cold neutrons in targets of terbium iron garnet [1]. This material is a ferrimagnet that exhibits orbital compensation of the magnetism associated with electron spins. At a temperature near 250K, the compensation is expected to be complete, leaving a target with net spin excess but no magnetism: a non-zero neutron spin rotation at this temperature would be a signal of new physics. The spin excess - a key parameter for our exotic force search - is extracted with the help of a molecular field model. This model, after adjusting for sample purity and incomplete magnetic saturation, is in good agreement with neutron spin rotation and SQUID magnetometry data. However the remaining spin excess uncertainty limits the sensitivity of our experiments to exotic forces. We discuss plans for direct measurement of the spin excess, as well as improvements in sample purity.

[1] C. D. Hughes, et al., J. Magn. Magn. Mater. 629, 173273 (2025).