Hyperfine field at La in the intermetallic compound LaMnSi2 measured by PAC using 140Ce nuclear probe

Abstract

Magnetic hyperfine field has been measured in the orthorhombic intermetallic compound LaMnSi2 with PAC spectroscopy using radioactive 140La-140Ce nuclear probes. Magnetization measurement was also carried out in this compound with MPSM-SQUID magnetometer. The LaMnSi2 sample was prepared by melting pure metallic components in stoichiometric proportions in an arc furnace under argon atmosphere. After melting, the sample was annealed at 1000 o C for 60h under helium atmosphere and then quenched in to water. X-ray analysis confirmed the sample to be in a single phase with correct crystal structure expected for LaMnSi2. The radioactive nuclei, 140La (T1/2 = 40h) were produced by direct irradiation of the sample with neutrons in the IEA-R1 nuclear research reactor at IPEN with a flux of ~1013 ncm-2s-1 for about 3-4 min. The irradiated sample was annealed at 1000 o C under helium atmosphere for 24 hours before starting the PAC measurement in order to minimize the effects of eventual radiation damage by fast neutrons during irradiation as well as to allow the decay of the short lived 56Mn (T1/2 = 2.6h) activity also produced in the sample. The PAC measurements were carried out with a six BaF2 detector spectrometer at several temperatures between 10 K and 400 K. The results showed well defined magnetic dipole interaction at all temperatures. Temperature dependence of the hyperfine field, Bhf was found to be anomalous. The magnetization measurement however, showed no anomaly. Therefore, the anomalous behavior of Bhf may be attributed to the interaction where 1f electron of the dilute Ce atom is spin polarized by the magnetic field from Mn sublattice and contributes to the effective hyperfine field. The ferromagnetic transition temperature (TC) of LaMnSi2 was determined to be 400(1) K confirming by magnetic measurement and the earlier results.