Interpretations of palaeomagnetic observations assume that naturally occurring magnetic particles can retain their primary magnetic recording over billions of years. The ability to retain a magnetic recording is inferred from laboratory measurements, where heating causes demagnetization on the order of seconds. The theoretical basis for this inference comes from previous models that assume only the existence of small, uniformly magnetized particles, whereas the carriers of palaeomagnetic signals in rocks are usually larger, non-uniformly magnetized particles, for which there is no empirically complete, thermally-activated model. This study has developed a thermally-activated numerical micromagnetic model that can quantitatively determine the energy barriers between stable states in nonuniform magnetic particles on geological time scales. We examine in detail the thermal stability characteristics of equidimensional cuboctahedral magnetite and find that contrary to previously published theories, such non-uniformly magnetized particles provide greater magnetic stability than their uniformly magnetized counterparts. Hence, non-uniformly magnetized grains, which are commonly the main remanence carrier in meteorites and rocks, can record and retain highfidelity magnetic recordings over billions of years.micromagnetics | paleomagnetism | geomagnetism S ince the 1900s magnetic recordings observed in rocks and meteorites have been studied to understand the evolution of the Earth and the Solar System. The validity of the findings from these studies depends on a theoretical understanding of rock-magnetic recordings provided by Néel (1, 2) and numerous experimental studies, for example, Strangway et. al. (3) and Evans and Wayman (4). The overwhelming evidence from these authors was that stable natural magnetic remanence (NRM) in rocks resides within ultrafine, uniformly magnetized particles, called single domain (SD) particles. Néel's theory (1, 2) for the behavior of thermally-activated SD particles describes a unique relationship between thermal and temporal stability and gave confidence that palaeomagnetic recordings that become unstable (unblocked) only at high temperatures, retain magnetic recordings from the time of their mineral crystallization, possibly as far back in time as four billion years ago.However, in the 1970s and 80s the widespread use of hysteresis parameters to characterize magnetic mineralogy (5) found that the majority of magnetic particles in rocks are not in uniform magnetic states, but are larger in size (80 -1000 nm) and contain complex magnetic states that are not described by either SD theory or the multidomain theory of micron-sized particles (2, 6). The term pseudo-single-domain (PSD) was coined for such particles and much effort was spent in determining the origin of their magnetic fidelity (Dunlop Psark, 1977, Moon and Merrill, 1985). Due to the complexity of the problem it has not been possible to determine the temporal stability of magnetisation in PSD grains on geological time scales fro...

S U M M A R YMagnetic grain-size and coercivity distributions of a superparamagnetic (SP) particle ensemble together determine its frequency dependence of susceptibility (FDS). Investigating the mathematical theory of this dependence leads to a general dispersion relation between real and imaginary parts of the complex susceptibility for SP particle ensembles, which extends the previous treatment by Néel. Using the new theory, it is demonstrated that the inverse problem of determining the combined grain-size and coercivity distribution from FDS measurements is not uniquely solvable. The inversion of the FDS at one temperature can be described by a deconvolution integral, the kernel of which is analytically calculated. The deconvolved FDS corresponds to an energy barrier distribution. Only using a priori assumptions about the relation between particle volume and coercivity it can be interpreted in terms of a volume or grain-size distribution. In order to deconvolve natural rock measurements, a semi-analytical parametric deconvolution method has been developed, which allows to reconstruct the SP grain-size distribution even from relatively noisy data. Dense measurements of FDS at several temperatures can be used to check for the applicability of the above theory. Observed deviations can be interpreted in terms of magnetostatic particle interaction. A quantitative estimate is presented, which allows to determine the average interaction field together with the volume distribution.

[1] Curie point temperatures (T C ) of natural and synthetic magnetic materials are commonly determined in rock magnetism by several measurement methods that can be mutually incompatible and may lead to inconsistent results. Here the common evaluation routines for high-temperature magnetization and magnetic initial susceptibility curves are analyzed and revised based on Landau's theory of second-order phase transitions. It is confirmed that in high-field magnetization curves T C corresponds to the inflection point, below the temperature of maximum curvature or the double-tangent intersection point. At least four different physical processes contribute to the initial magnetic susceptibility near the ordering temperature. They include variation of saturation magnetization, superparamagnetic behavior, magnetization rotation, and magnetic domain wall motion. Because each of these processes may influence the apparent position of T C , initial susceptibility and high-field curves can yield deviating estimates of T C . A new procedure is proposed to efficiently determine the temperature variation of several magnetic parameters on a vibrating-sample magnetometer, by repeatedly measuring quarter-hysteresis loops during a single heating cycle. This procedure takes measurements during the inevitable waiting time necessary for thermal equilibration of the sample, whereby it is not slower than the commonly performed measurements on a Curie balance. However, it returns saturation magnetization, saturation remanence, high-field and low-field slopes, and other parameters as a function of temperature, which provide independent information about T C and other sample properties.Components: 7,400 words, 9 figures.

No abstract