Magnetic excitation temperature

The interactions also suppress the collective magnetic excitations at low temperatures. Figure 6.17 shows the temperature dependence of the magnetic hyperfine... 

At some critical temperature, a structural distortion to dimerized chains occurs together with the opening of a gap in the magnetic excitations. Such a distortion is attributable to a 2kF spin-Peierls transition, just as in MEM(TCNQ)2 (cf. Section III.A.7). A 2kF transition results into chain tetramerization for p = 5 and into chain dimerization for p = 1, as also shown in Fig. 5. For instance, the spin-Peierls transition takes place at T sp = 395 K in KTCNQ and at TsP = 381 K in RbTCNQ [67] (a second form, called RbTCNQ II, also exists for the Rb salt [69], but it is not considered here). By analogy with the case of polymers, the dimerized phase at low temperature is also called a bond-ordered-wave (BOW) phase [47]. 

It is reasonable to expect that both the thermal stability and magnetic ordering temperatures in hybrid systems may be improved by synthesizing materials with extended M-O-M arrays. In other words, as the connectivity between transition metals through oxygen increases, hybrid metal oxides should assume properties closer to transition metal oxides and further from purely organic polymers. In this section, we focus exclusively on transition metal systems, since their parent oxides have the most exciting properties [139]. 

The magnitude of the spontaneous magnetization M in ordered magnets is temperature dependent due to spin wave excitations. Spin wave frequencies are so fast (THz) that they are fully motional-narrowed in jxSR. All one will observe is the expectation value of the internal field which is coupled (but not necessarily directly proportional) to the expectation value of M T). For this reason one calls the of an ordered magnet a quasistatic field. The spontaneous precession vanishes at a second-order magnetic transition point and reaches a saturation value for T —> 0. An example is shown in fig. 27. 

Figure 16 shows the temperature dependence of the magnetic excitation of NdBg near 95 cm together with the phonon density of states near 170cm (Pofahl et al. 1986). At 300 K the peak at 95 cm is the center of two transitions Tg Tg and the The excitation Ff - F (24meV= 190cm ) is... 

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