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 field. At atmospheric pressure SmM(n2)Ge(2) is ferromagnetic (FM) from T-t1 approximate to 348 K to T-t2 approximate to 196 K, then antiferromagnetic (AFM) to T-t3 approximate to 100 K. The material is again ferromagnetic 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 but 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 temperature in the range 3-75 K, despite the decrease in magnetization by some 16%. The decrease in magnetization is attributed to the collapse of the ordered Sm state near 75 K. The Sm-147 and Sm-149 NMR could, however, only be observed up to 19 K, because of the rapid increase of the transverse relaxation rate with temperature, so we do not have conclusive evidence that the Sm moment has no influence on the AFM-FM transition at T-t3. A remarkable feature of the Sm NMR is that its frequency at a given temperature is independent of pressure whereas the Mn NMR frequency decreases with increasing pressure.