Disordered magnets

Kumra, S., Giedd, J.N., Vaituzis, A.C., Jacobsen, L.K., McKenna, K., Bedwell, J., Hamburger, S., Nelson, J.F., Lenane, M., and Rapoport, J.L. (2000) Childhood-onset psychotic disorders magnetic resonance imaging of volumetric differences in brain structure. Am J Psychiatry 157 1467-1474. 

Shibayama, Y., H. Sato, T. Enoki, and M. Endo. 2000. Disordered magnetism at the metal-insulator threshold in nano-graphite-based carbon material. Phys. Rev. Lett. 84 1744-1747. 

While some disordered magnetic systems do display spin-glass behavior, those that are not geometrically frustrated can behave in a fundamentally different way. In... 

For continuous muon beam data, there is also a short time after the muon stop t = 0) for which the data are distorted and cannot be used. Often called initial dead time , this period is usually less than 30 ns, and with some care in setting up the counters and electronics can be reduced to as little as 5 ns. While often of little consequence in many p.SR experiments, the size of the dead time limits the maximum relaxation rate that can be measured, and disordered magnetic states with rare-earth moments often generate muon relaxation times of less than 10 ns, so initial dead time is an important consideration in the experiments described in this review. 

In this section we discuss some disordered magnetic systems that do not fit well under any of the subject headings of the other sections in this review. am-DyAg (sect. 8.3.1) is the only amorphous f-moment material studied with [iSR to date, as the pyrochlores (sect. 8.3.2) are the only perfectly fhistrated f-magnets studied with p,SR. Quasicrystals (sect. 8.3.3) must stand somewhat separately because they have atomic structure that is neither amorphous nor merely disordered-crystalline. For a discussion of spin-glass-like behavior in Ce, Yb, and U strongly correlated electron materials, see sect. 9. 

At temperatures far below Tg, where the moments may be static, and it would then be possible to measure the field distribution without the complication of spin dynamics, ZF-p,SR is of little use in these (and in many other dense-moment disordered magnetic) materials because most of the information wanted is lost in the dead time. There is then, however, some opportunity for LF-p,SR. If a significant fraction of the moments have fluctuation rates of less than a few per microsecond (and a 1/3 tail, when seen, indicates there is such a fraction), and if the longitudinal field that can be... 

The study of HF and related materials is a major field of present-day condensed matter research and a vast collection of data employing all available techniques and probes exists (see, for example, Grewe and Steglich 1991). p,SR is a small but important part of this endeavor. One speciality of p,SR, its sensitivity to very small magnetic moments and to rather low frequency fluctuations (i.e., the limit to —> 0), makes it particularly useful here. Some HF intermetallics possess moments of the order of 10 Ub or less (yet nonzero) and others show spin fluctuations in the MHz region down to mK temperatures. In addition, often a disordered magnetic state is the ground state. We have already outlined the power of [xSR in this field (see sect. 8). 

In summary, the [iSR spectral shape reveals that a strongly disordered magnetic state has been formed. In contrast, Echizen et al. (2000) have concluded from bulk magnetic. 

The Gaussian shape of the additional relaxation indicates the development of a strongly disordered magnetic state with moments around 0.03 jUg- The authors note that the onset of this relaxation corresponds to the appearance of a broad and sizeable anomaly in specific heat (Furuno et al. 1988) and to the development of flie dependence of x on magnetic history. 

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