Ordered Glasses

The interpretation of optical spectra of solids is even more complicated than for atomic and molecular systems, as it requires a previous understanding of their atomic and electronic structure. Unlike liquids and gases, the basic units of solids (atoms or ions) are periodically arranged in long (crystals) or short (glasses) order. This aspect confers particular characteristics to the spectroscopic techniques used to analyze solids, and gives rise to solid state spectroscopy. This new branch of the spectroscopy has led to the appearance of new spectroscopic techniques, which are increasing day by day. 

The ratio of the intensities of the cross-peaks and the diagonal peaks are related to the glass order parameter qEA... 

Line shape studies of 7Rb and deuteron NMR lines have thrown more light on the frustration in the system. Blinc et al.5 have shown clearly the measurement of second moment can be related to the Edwards-Anderson spin-glass order parameter in the glass phase. 

The Edwards-Anderson spin-glass order parameter is defined as32... 

Y(Mn,Al)2. A means to reduce fhistration is to substitute some of the Mn ions (e.g., Y(Mni jAl t)2) since the strict tetrahedral exchange correlation is then partially broken. Substitution by Al also causes lattice expansion, which leads to some moment stabilization and in turn causes slowing down of spin fluctuations. The system enters a spin glass state at rather elevated temperatures Tg ss 50 K) for x = 0.06 and above (Motoya et al. 1991). pSR measmements on a sample with x = 0.1 clearly confirm spin glass order below Tg and also support the notion of slowed down spin fluctuations by a more Gaussian shape of the muon spin depolarization fimction at T Tg (Cywinski and Rainford 1994). 

Zalar B, Blinc R, Albert W, Petersson J (1997) Discrete nature of the orientational glass ordering in Nal-xKxCN. Phys Rev B 56 R5709-R5712... 

This feature is sketched in fig. 19. Thus, a vector spin system in a field H within the SK model exhibits a phase with spin-glass order transverse to the field, reminiscent of a spin-flop phase of a pure antiferromagnet. Just below the GT line, only weak irreversibilty occurs in the longitudinal component but there is a crossover to strong irreversibility in at a region which goes as like the AT line (indicated by the dashed line in fig. 19). 

Fig. 19. Plot of the Gabay-Toulouse (GT) line (solid line) for an infinite-range vector-spin glass (eq, 27). The low-temperature phase (SG) has nonzero transverse spin-glass ordering. The deAlmeida-Thouless (AT) line (dashed line) strictly no longer occurs but there is a well-defined crossover region which follows a similar curve.

Fig. 20. Magnetic phase diagram for an infinite-range vector-spin glass with nonzero mean, in the exchange distribution. P = paramagnet FM = ferromagnet SG = spin glass. The mixed phase F where ferromagnetism coexists with transverse spin-glass order is separated from FM by a GT line (solid line). The dashed line indicates a crossover region as in fig. 19.

Fig. 21. Magnetic phase diagram for an infinite-range veetor spin glass with uniaxial anisotropy D (Cragg and Sherrington 1982b, Roberts and Bray, 1982). Longitudinal (L) and transverse (T) spin-glass order occurs as well as both together (L -I- T).

Hiraoka et al. (1986) have found the onset of spin-glass order at the dilution concentration (1 —x) =0.11 in similar compounds of the diluted antiferromagnetic system, Eu Ybj Se, by measurements of ac-x Eu-NMR. 

Finally, systems where long-range periodic (FM, AFM) and spin-glass orderings compete exhibit, magnetic properties which obviously are not in accordance with present theories (fig. 18 and 20). In sec. 8 we made a first attempt to classify the large amount of experimental data on this subject which now needs confirmation by theoretical work beyond the SK-mean-field theory. 

Why is the corrosion rate of metallic glass orders of magnitudes lower than the crystalline metal ... 

S. M0rup, Superparamagnetism and spin glass ordering in magnetic nanocomposites. Europhys. Lett. 28, 671-676 (1994)... 

---