The superparamagnetism of Yucca Mountain Tuff

Yucca Mountain Tuff contains small titanomagnetite grains with narrow size distributions in the superparamagnetic range [Schlinger et al., 1988]. Magnetic measurements on three samples (comprising hysteresis loops at low and ambient temperatures, acquisition and demagnetization of isothermal remanent magnetization, thermal demagnetization of the saturation remanence and of a low-temperature thermoremanence, and frequency- and temperature-dependent susceptibilities) allow evaluation of the magnetic properties in terms of Neel's [1949] single-domain theory. Precise grain volume distributions have been obtained by applying the blocking volume concept to thermal demagnetization results. In contrast, an attempt to derive mean particle volumes by fitting a Langevin curve to the room temperature magnetization curves fails, probably because the precondition for the Langevin function, KV/kT << 1, is not met. It is only for the sample with the smallest grains and in weak fields (< 20 mT) that a Langevin fit provides a reasonable volume estimate. There is good agreement between the experimental results and the calculated frequency and temperature dependence of susceptibility, thus verifying that Neel's theory is sufficient for the magnetic description of single-domain assemblages spanning the superparamagnetic/stable single-domain boundary. However, some deviations between modeled and measured susceptibilities exist, and the physical causes may include size-dependent anisotropy, nonuniform magnetizations, and also an uncertain preexponential time "constant" tau(o). While tau(o) = 10(-11) s gives the best fit for the sample with the largest grains, tau(o) = 10(-9) s is more reasonable for the others. Thus tau(o) may indeed be size- and temperature-dependent as predicted by Brown [1959]. The commonly cited parameter chi(fd) (frequency dependence of susceptibility) reaches 30% at room temperature (RT) for one sample with a blocking temperatures just below RT, while chi(fd) = 0 at RT for a superparamagnetic sample with smaller grains. These results thus exemplify that chi(fd) is not limited to 15%, as a number of studies suggest, and that chi(fd) = 0 must not be taken to imply the absence of superparamagnetic grains.