SPONTANEOUS MAGNETIC-ORDERING IN CERH3B2 OBSERVED BY ZERO-FIELD MUON-SPIN RELAXATION

Muon-spin relaxation measurements have been made on polycrystalline Ce Rh3B2 in zero magnetic field. Well above the Curie temperature (T-C co ngruent to 120 K) muon relaxation is described by a Gaussian rate (sim ilar to 0.1 mu s(-1)) indicative of static and random nuclear B-11 dip ole moments. Near Te the relaxation rate increases dramatically due to slowing down of fluctuating Ce magnetic moments. Oscillations are obs erved in the muon spin polarization below T-C, which is unambiguous ev idence for the onset of local magnetic ordering. The magnitude of the ordered field initially increases with decreasing temperature and atta ins a maximum value of 13 mT near 50 K. The general shape of the field versus temperature curve resembles the magnetization curve, as expect ed for an ordinary ferromagnet. However, further decrease in temperatu re reduces the field to 11 mT at 12 K, which we attribute to magnetic ordering of Rh moments. Below 50 K these moments appear to order antif erromagnetically with Ce moments thus producing an effective field at the muon site that is reduced in magnitude. The muon-spin relaxation d ata unambiguously demonstrate the onset of magnetic ordering in CeRh3B 2 in zero magnetic field and contradicts previous neutron scattering r esults which suggested the ordering was field induced.