Modulated spin structures

The rather well-defined spin precession frequencies in CePdSn and CePtSn (especially the single frequency in the upper AFM state of CePtSn) are inconsistent with an incommensurately modulated spin structure. Dipolar sum calculations were carried out for the muon stopping site mentioned earlier and the spin structure of the upper AFM state of CePtSn as derived by Kadowaki et al. (1993), in order to elucidate the problem somewhat further (Kalvius et al. 1995b). The calculations generated four fi eld distributions of file type discussed in sect. 3.7 (see eq. 51) ... 

TmS was found (by neutron diffraction) to order in a complicated structure, elose, however, to the (111 > type II ordering claimed by Koehler et al. (1979). Magnetization measurements by Peyrard et al. (1983) confirmed trivalency of the Tm ion. Peyrard et al. (1983) and Lassailly et al. (1983) found a modulated spin structure by neutron diffraction with a it-vector close to (j,, ) and a moment amplitude of 3.4/iB- The newest available neutron data (Lassailly et al. 1986) show that in TmS at very low temperature we actually encounter a Ak modulated structure with the spins along <100>,... 

This indicates a modulated spin structure. The maximum is around 300 T corresponding to a moment of 1.5 /in quite in contrast to the low moment of NpSnj. From high-pressure data it was concluded that NpSuj is a Kondo lattice system. Kondo behavior may also be present in Npln3 as suggested by recent resistivity data (Gal et al. 1992). 

The applications of solid-state C-NMR spectra for the study of polymorphs and solvates can go beyond evaluations of resonance band positions and make use of additional spectral characteristics. For instance, studies of relaxation times of furosemide polymorphs were used to show the presence of more molecular mobility and disorder in Form II, while the structure of Form I was judged to be more rigid and uniformly ordered [158]. The analysis of the solid-state C-NMR spectra of (li ,3 j-3-/ -thioanisoyl)-l,2,2-trimethylcyclopentanecarboxylic acid was facilitated by the 7-modulated spin-echo technique, which was used to deduce the number of protons bound to each carbon atom [159]. Differences in the dipolar dephasing behavior between the two polymorphs of ( )- ra 5-3,4-dichloro-A/-methyl-7V-[ 1,2,3,4-tetrahydro-5-methoxy-2-(pyrrolidin-1 -yl)]naphth-1 -yl-benzeneacetamide were noted and ascribed to motional modulation of the carbon-hydro-... 

Second, the positions and llneshapes of resonances arising from potentially mobile parts of the peptide (e,g, side chains) have revealed dynamical aspects of the solid-state structures of peptides. The analysis of molecular motions is simplified In the solid state by the absence of overall molecular tumbling, which modulates spin interactions and leads to complex frequency -dependent spectral responses. In particular, signals arising from aromatic ring side chains are well separated from other resonances, and may be interpreted in terms of reorientation models of these side chains. Such ring dynamics are of great importance in protein structures, and studies with model peptides can help elucidate fundamental aspects of these processes. 

While the SPT experiment has obvious utility, it is cumbersome to use unless the selective nature of the experiment is specifically being exploited for structure elucidation purposes. A group of experiments that may be categorized as J-modulated spin echo experiments allows the simultaneous investigation of the entire spectrum. As the name of this group of experiments implies, they utilize a spin echo of the type shown in Eq. (1), over which a scalar coupling driven process (using jcH most commonly) is superimposed. 

The antiferromagnetic spin structure of YbNiAl was determined by neutron dif action experiments. Below Tn = 2.9K a sinusoidal modulated structure with an amplitude of /4=1.9(2))1b is observed (Ehlers et al. 1997). The magnetic propagation vector is m = [0.779(1)00], and the ytterbium magnetic moments lie perpendicular to the hexagonal c-axis. These investigations are paralleled by a detailed study of Al nuclear spin-lattice relaxation measurements (Fay et al. 1997). 

Fig. 2. Schematic description of the spin structures in the heavy rare earth metals. The initial high-T ordering, in all except the ferromagnet Gd, is to an incommensurate periodic spin order with propagation direction along the c-axis. CAM (c-axis modulation) is a structure in which the c-axis components of the moment are sinusoidally modulated which evolves into the APD (quasi anti-phase domain) in Er in which a gradual squaring-up of the c-axis moment occurs with additional order appearing in the basal plane. The temperatures of the transitions are from magnetization and neutron diffraction data. (After Legvold 1980.)...

Erbium exhibits a complex magnetic behaviour, with at least four obserrable characteristic temperatures below rN = 85K, the moments order in a sinusoidal c-axis modulated (CAM) structure at rH=54K, there appears a component perpendicular to the c-axis resulting in a helicoidal structure this intermediate phase exhibits a sequence of lock-in transitions of commensurate phases (spin-slip structures) with wave vectors fm = f, 4 observed by iX-ray scattering (Gibbs et al. 1986), the... 

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