Helical antiferromagnet

Below the Neel temperature, the beta phase is in an incommensurate helical antiferromagnetic state in which the helical turn angle varies from 24.0 at the Curie temperature to 42.9° at the Neel temperature (Greenhough et al., 1981). However, various turn angles are commensurate with the basal plane of the hexagonal crystal structure and nine such transitions have been definitely identified and the results are summarized in Part 14.13. Using sensitive calorimetry, Astrbm and Benediktsson (1988) were able to confirm only two of these transitions at 166 and 173 K and measured the enthalpies of transition to be 2 1 J/mol in both cases, but this is cmisidered to be within experimental noise and therefore these transitimis were not considered in the evaluation of the thermodynamic data. 

The beta phase below the Neel temperature at 180 K exists in an incommensurate helical antiferromagnetic state in which the helical turn angle varies from 24.0° at the Curie temperature to 42.9° at the Neel temperature (Green-hough et al., 1981). However, various turn angles are commensurate with the... 

The transformation at the Curie temperature involves the formation of an incommensurate helical antiferromagnetic state up to the lower Neel... 

Lee (1964) and Landry (1967) considered the pressure influence on the turn angle

helical antiferromagnetic spin structure in the framework of the model proposed by Herpin and Meriel (1961) and F.nz (1960). The free energy of the crystal was assumed to be ... 

Palmer (1975) proposed that there are two possible types of spiral spin domains in the helical antiferromagnetic state of the lanthanide metals with an unique anisotropy axis —... 

This allows one to propose that it has three magnetic phases, double ferromagnetism below Tc = 217K, a helical antiferromagnetic state n = 205 K and a ferromagnetic state below 200 K. 

The behavior of the elastic constants c and 033, and Yoimg s moduli <, and Et, of the lanthanide metals in the helical antiferromagnetic phase has been studied most. The temperature dependencies of these values reveal pecuUarities not only at the transition to the magnetically ordered state, 7n, but also at the temperature of the transition to the phase with the ferromagnetic component, 7c (see Scott 1978). 

Fig. 26. Magnetic phase diagram of dysprosium obtained from elastic constant C33 measurements in a magnetic field (Isci and Palmer 1978). Ferromagnetic (FM), paramagnetic (PM), helical antiferromagnetic (HAFM) and fan phases are shown.

The field d endencies of Ea and Ei, were measured for a Tbo sDyo.s single crystal by Kataev et al. (1989c). Like Tb and Dy this alloy is a helical antiferromagnet with an easy basal plane (Bykhover et al. 1990). The field behavior of Ei, and Ea in the HAFM phase is analogous to that observed for C33 in Dy and Tb by Isci and Palmer (1978) and files et al. (1984) (see fig. 28). The magnetic phase diagram H T) of Tbo.5Dyo,5 constructed... 

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