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It is demonstrated that the low temperature nuclear orientation technique can be used to determine the axes of quantization in a multiaxial ordered ensemble of nuclear spins. Expressions for the anisotropic ?-ray intensity from particular geometries are derived for the case in which each subsystem has its own axial symmetry. The determination of the atomic magnetic structure in antiferromagnets is discussed and the results of recent experiments on helically ordered166mHo-Ho are presented. In these experiments, the angular variation of the intensities of some of the more intense ?-rays were measured and compared to theory. A value of 80.4(1)°, not including systematic errors, was obtained for the semi-cone angle of the helix formed by the atomic magnets.

Nuclear magnetic resonance of oriented54Mn nuclei in antiferromagnetic MnCl2 · 4H2O has been observed. The first two lines of the sextuplet split by quadrupole interaction are at frequencies 500.4 and 514.7 MHz, implying a hyperfine field of 643(5) kG. The stronger line at 500.4 MHz has a half-width at half maximum of 60 kHz and is shifted downward and split in frequency on application of a magnetic field. The nuclear spin-lattice relaxation time is dependent both on the applied field and the size and/or quality of the crystal.

Abstract: Pulsed NMRON, CW NMRON and thermal NMR-NO methods have been utilized to study54Mn-MnCl2 4H2O. The54Mn spin-lattice relaxation timeT 1 in zero applied field has been measured between 35 and 90 mK in the antiferromagnetic phase. Above 65 mK the dominant relaxation mechanism is a Raman process with the electronic magnons, but at lower temperatures 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 ofT 1 for the54Mn spins on field and temperature was studied in the paramagnetic phase. AT 1 minimum centred atB 0=2.64 T was observed. The hyperfine parameter <54 AS>/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.

Abstract: The first observation of pulsed nuclear magnetic resonance of oriented nuclei in an insulating magnetic material is reported. The system studied was the ordered antiferromagnet54Mn−MnCl2.4H2O. The response of the54Mn spins to a single pulse of variable length, the free induction decay, and Hahn spin-echo have all been observed. A discussion of the transverse decoherent relaxation timeT 2 in MnCl2.4H2O is given.

The effect on the most intense M=3→2 NMRON line of54MnMnCl2. 4H2O resulting from a magnetic field B0 applied along the crystalline c- and b-axes is observed. Possible mechanisms for these results are discussed.

Abstract: We report on nuclear spin-lattice relaxation studies on54Mn-MnCl2.4H2O and 2-dimensional54Mn-Mn(COOCH3)2.4H2O. In the former crystal, we find that at the lowest temperatures a direct process, normally thought of as forbidden, becomes the mechanism for relaxation. In lower dimensional systems the relaxation might be expected to be relatively fast and indeed in the acetate the relaxation timeT 1 is ≈1000 s in an applied field of 0.2 T for the Mn2 site and is much shorter at lower fields. In 1-dimensional54Mn-(CH3)4NMnCl3(TMMC) a significant γ-ray anisotropy is observed on cooling indicating thatT 1 is also short in this system. The shortness ofT 1 in the lower dimensional systems suggests that they may be suitable hosts for on-line experiments.