Magnetism of CuCr_2X_4 (X= S and Se) spinels studied with muon spin rotation and relaxation (μSR)

We present muon spin rotation and relaxation (μSR) results for chalcogenide spinels CuCr_2X_4 with X= S and Se. Both compounds are known as ferromagnetic metals with high Curie temperatures. Our μSR and magnetization data show clear signatures for additional magnetic transitions far below the respective Curie temperatures. They can be related to changes in the Cr valence system from the mixed valence between Cr^3+ and Cr^4+ at high temperatures and collinear ferromagnetism to a charge-ordered state at low temperatures with a conical or even more complex structure. Our results demonstrate that the electronic systems and the related spin structures of both compounds are more complex than assumed so far.