NMR-STUDY OF THE FERROMAGNETIC PHASES OF SMMN2GE2 AS A FUNCTION OF TEMPERATURE AND PRESSURE

The behavior of the reentrant ferromagnet SmMn2Ge2 has been studied as a function of temperature and pressure using NMR in zero applied fiel d. At atmospheric pressure SmM(n2)Ge(2) is ferromagnetic (FM) from T-t 1 approximate to 348 K to T-t2 approximate to 196 K, then antiferromag netic (AFM) to T-t3 approximate to 100 K. The material is again ferrom agnetic below T-t3. The behavior of T-t3 as a function of pressure has been studied using Mn-55 NMR. The value of dT(t3)/dP at low pressure is found to be about -16 K kbar(-1), in agreement with earlier work bu t its magnitude decreases at higher pressure. The value of T-t3 at 7.8 kbar is 47 K. The Mn-55 NMR frequency increases with increasing tempe rature in the range 3-75 K, despite the decrease in magnetization by s ome 16%. The decrease in magnetization is attributed to the collapse o f the ordered Sm state near 75 K. The Sm-147 and Sm-149 NMR could, how ever, only be observed up to 19 K, because of the rapid increase of th e transverse relaxation rate with temperature, so we do not have concl usive evidence that the Sm moment has no influence on the AFM-FM trans ition at T-t3. A remarkable feature of the Sm NMR is that its frequenc y at a given temperature is independent of pressure whereas the Mn NMR frequency decreases with increasing pressure.