Hyperfine Interactions 77 (1993)203-214 203 NMRON studies of the antiferromagnetic and paramagnetic phases of 54Mn-MnC12 • 4H20 M. Le Gros, A. Kotlicki and B.G. Turrell Department of Physics, Universityof British Columbia, Vancouver, B.C., Canada V6TIZ1 Received 14January 1993 Pulsed NMRON, CW NMRON and thermal NMR-NO methods have been utilized to study 54Mn-MnCI2 •4H20. The 54Mnspin-lattice relaxation time 7'1 in zero applied field has been measured between 35 and 90 mK in the antiferromagnetic phase. Above 65 mK the domi- nant relaxation mechanism is a Raman process with the electronic magnons, but at lower tem- peratures a direct process takes over. NMRON has been observed for the first time in the paramagnetic phase, and a line width of 300 kHz, with both homogeneous and inhomogeneous contributions, is observed. In the antiferromagnetic phase the line width is 35 kHz, and there are also homogeneous and inhomogeneous contributions. The dependence of TI for the ~4Mn spins on field and temperature was studied in the paramagnetic phase. A 7"1minimum centred at B0 = 2.64 T was observed. The hyperfine parameter (54AS>/h = -513.6(3) MHz in the paramagnetic phase, and comparison with the value in the antiferromagnetic phase gives 0.013(1) for the zero point spin deviation. 1. Introduction In previous papers [1-4] we have discussed the application of pulsed NMR on oriented nuclei (NMRON) in insulating magnetic materials. Here we discuss in detail an investigation of 54Mn-MnC12 • 4H20 primarily utilizing this technique, but also complementing the study by performing some measurements using CW NMRON and NMR-TDNO (NMR thermally detected by nuclear orientation) [5]. In particular we have been able to perform the first NMRON measurements in the paramagnetic phase. Manganese chloride tetrahydrate has been the subject of detailed nuclear orien- tation (NO) [6] and continuous wave (CW) NMRON studies [7-9]. The relative simplicity of its crystal preparation and the easy accessibility of its phases (antifer- romagnetic, spin-flop and paramagnetic) by applying modest magnetic fields have made it an excellent system for developing CW, pulsed and thermal NMR- TDNO in magnetic insulators. MnC12 • 4H20 is a four sublattice, easy-axis antiferromagnet with biaxial single © J.C. Baltzer AG, SciencePublishers