Helical spin order

Nagamiya, T., 1967, Helical spin ordering-I theory of helical spin configurations, Seitz, F., D. Turnbull, and H. Ehrenreich, eds.. Solid State Physics (Academic Press, New York and London), Vol. 20, pp. 306-411. 

Helical spin order Consider a one dimensional spin chain with sizeable first (7i) and second (J2) neighbour interactions. In the mean-field approximation, the energy of the system is given by... 

Fig. 11. Fourier transform exchange J q) for the c-axis direction in Dy in both ferromagnetic (78 K) and helical (98 K) temperature regions. The peak near q = 0.2 corresponds to the peak in the generalized susceptibility x(9) and is a signature of the incommensurate helical 4f spin ordering occurring with the same wave vector. Note that the peak remains, even below consistent with a transition driven not by the exchange but by the magnetostriction. (After Nicklow 1971.)...

The finding by Lecomte et al. (1986b) is consistent with results from neutron-diffraction measurements on YDy 3% by Rathmann and Touborg (1977) which reveal helical AFM ordering of the Dy moments below = 9 K (again similar to the YTb and YGd alloys). In contrast, Baberschke et al. (1984) measure on a similar sample, YDy 3%, differences in the ZFC- and FC-magnetization (see fig. 71) below Tf = 9.2 K and field dependences of the remanences (IRM and TRM), similar to features in spin glasses. 

De Gennes and Saint James (1963) explained the temperature variation of the spin periodicity in the HAFM structure near by the effects of conduction electrons scattering by the 4f spin disorder. EUiott and Wedgewood (1964) considered the influence of superzone boundaries introduced by the helical structure into the energy spectrum of the conduction free electrons. They showed that with an increase of the 4f spin order the value of Q should decrease and the first-order HAFM-FM transition should occur. Miwa (1965) in his model took into account both these factors. On the base of Miwa s model Umebayashi et al. (1968) obtained the following ejqiression for the pressure dependence of (p... 

Fig. 5. Illustration of a helical spin structure. Ordering in each plane is ferromagnetic but the moment direction from plane to plane changes through a constant turn angle W resulting in overall antiferromagnetic behavior (after Taylor ).

A helical arrangement within columns was also found for other metal 3-diketonate complexes provided with chiral side chains (32) by Serrano and co-workers.35,36 These compounds form rectangular columnar mesophases with helical order within the columns. A spin-coated sample of 32 showed a positive exciton-splitted signal in the CD spectra, which was interpreted as a left-handed (M) helix. Annealing of the film resulted in much higher optical activities and a shift of the absorption maxima. The observed optical changes clearly point to a chiral organization of the columns in the mesophase. 

Spin-lattice relaxation times and 13C chemical shifts were used to study conformational changes of poly-L-lysine, which undergoes a coil-helix transition in a pH range from 9 to 11. In order to adopt a stable helical structure, a minimum number of residues for the formation of hydrogen bonds between the C = 0 and NH backbone groups is necessary therefore for the polypeptide dodecalysine no helix formation was observed. Comparison of the pH-dependences of the 13C chemical shifts of the carbons of poly-L-lysine and (L-Lys)12 shows very similar values for both compounds therefore downfield shifts of the a, / and peptide carbonyl carbons can only be correlated with caution with helix formation and are mainly due to deprotonation effects. On the other hand, a sharp decrease of the 7] values of the carbonyl and some of the side chain carbons is indicative for helix formation [854]. 

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