Remanent time decay

FIGURE 22 The time decay of the isothermal remanent magnetizations Im of H0B22C2N and ErB22C2N at 5 and 2 K, respectively. The lines depict the stretched exponential fit ... 

Measurements of the time decay of the remanent magnetization in powdered TDAE-C60 by SQUID showed below Tc the presence of a long tail, which could be described by a stretched exponential function [103]. Similar results were also obtained by ESR time decay measurements [104]. 

Figure 23 Temperature dependence of the relaxation time of the magnetization of the chain compound [Co(hfac)2NITPhOMe] . The empty circles represents the data extracted from a.c. susceptibility while the triangles represents those extracted from the time decay of the remanent magnetization. The line corresponds to the best fit with the Arrhenius law with A = 152( 1)K and ro = 4( l) x 10 s.

The magnetic viscosity S(H) can be determined from the slope of the Af(t)-ln(f) curve and is foimd to vary with tiie field, generally going through a m udmum in the vicinity of the coercivity. The activation volume v is also determined experimentally because both S(H) and Xm(H) can be obtained from the time decay curves and remanence curves, respectively. O Grady et al. (1994) used the above analysis to study the magnetization reversal in (14.3 ATb/85 AFe) as a fimction of the number of bilayers. The time decay curves for a sample with 32 bilayers is shown in fig. 45 and the activation volumes as a function of bilayer number in fig. 46. 

Figure F.4.4. Time decay of the remanent magnetization for FeC particles dispersed in a hydrocarbon oil in the temperature range 1.8-18 K. Dashed and solid lines correspond to the fits with one or two lognormal distributions. (Reproduced with permission from Ref. 194.)...

The measurement of time decay of the remanent magnetization represents one of the most straightforward tools to investigate the dynamical behavior of fine particles and to study the magnetization reversal mechanisms. However, the interpretation of the experimental results is very difficult because of the complexity of actual fine particles systems (presence of size, shape, and interparticle distance distribution, random distribution of easy axes, existence of interparticle interactions, surface effects, etc.). 

F.4.3.2. Time Dependence. For a single particle an exponential decay of the remanent magnetization was predicted by Neel ... 

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