Amorphous ferromagnetism and re-entrant magnetic glassiness in single-crystalline Sm2Mo2O7

We discuss the magnetic properties of a Sm2Mo2O7 single crystal as investigated by means of different experimental techniques. In the literature, a conventional itinerant ferromagnetic state is reported for the Mo4+ sublattice below ∼78 K. However, our results of dc magnetometry, muon spin spectroscopy (μ+SR), and high-harmonics magnetic ac susceptibility unambiguously evidence highly disordered conditions in this phase, in spite of the crystalline and chemical order. This disordered magnetic state shares several common features with amorphous ferromagnetic alloys. This scenario for Sm2Mo2O7 is supported by the anomalously high values of the critical exponents, as mainly deduced by a scaling analysis of our dc magnetization data and confirmed by the other techniques. Moreover, μ+SR detects a significant static magnetic disorder at the microscopic scale. At the same time, the critical divergence of the third-harmonic component of the ac magnetic susceptibility around ∼78 K leads to additional evidence towards the glassy nature of this magnetic phase. Finally, the longitudinal relaxation of the μ+ spin polarization (also supported by results of ac susceptibility) evidences re-entrant glassy features similar to amorphous ferromagnets.