The low‐temperature magnetic properties of magnetite are reviewed, and implications for rock magnetism considered. The behaviour of fundamental properties of magnetite at low temperatures near the Verwey transition (Tv ) are documented, and attention is given to various Verwey transition theories. The low‐temperature behaviour of the magnetic energies that control domain structure is reviewed in detail. For the first time in rock magnetic literature, the low‐temperature anomaly in spontaneous magnetization (Ms ) is documented and the differences between the saturation magnetization and Ms near the Verwey transition are discussed. It is argued that the low‐temperature behaviour of the magnetocrystalline anisotropy, and in particular the anomaly at Tv , is most likely to affect multidomain remanence during low‐temperature cycling. For multidomain crystals it is calculated that the large increase in magnetocrystalline anisotropy intensity and reduction in symmetry on cooling through Tv is likely to reduce the stability of closure domains.

There are some fundamental experimental observations of properties of thermoremanent magnetization (TRM) and partial TRM (pTRM) in multidomain (MD) magnetite that cannot be explained by Ntel's theories of TRM. We present experimental results that show (1) that pTRMs are additive at any temperature, (2) that a pTRM acquired in field H between temperatures T1 and T 2 decreases on zero-field cooling below T 2 when normalized by M s (T), (3) that thermal pre-history has a strong effect on the intensity of a pTRM. These results strongly point to reorganization of domain structure during cooling being the dominant controlling factor in TRM acquisition in MD material. We further develop the approach of McClelland and Sugiura [ 1987] where TRM and pTRM are considered to be nonequilibrium states, and change in domain structure with changing temperature provides the driving force to allow a pTRM to shift toward the demagnetized state on zero-field cooling, for example. A random element is essential in such a kinetically controlled system; in this paper we consider the physical mechanism providing this random element to be the variation of direction of the easy axis of magnetization throughout the grain due to local crystal defects, or stress effects due to the domains themselves, for example. Thermally driven domain structure changes then cause essentially random local changes of magnetization, which are governed by kinetic equations. Our model is developed by considering the magnetization of discrete cells within a cubic grain chosen to have reasonably uniform magnetic properties within the cell but probably different between cells, and the model satisfactorily explains our experimental observations. The strong effect of thermal prehistory is ascribed to the existence of a spectrum of local energy minima states, and the behavior of an MD grain is likened to that of a spin glass.grains are adequate to explain experimental observations, at least qualitatively. However, theories of TRM in multidomain (MD) grains grossly•fail to explain experimental observations in some fundamental areas. Ndel [1955] laid the foundations for the present theories of TRM in iron oxides. He pointed out that remanent magnetization exists because of barriers to change in magnetization.He postulated that in large multidomain grains these barriers prevent the movement of domain walls and that they are proportional to the coercive force H c. In small single domain grains, he suggested that barriers are due to anisotropic magnetic properties. In iron oxides which have uniaxial shape anisotropy, there are two antiparallel low energy favourable states with an energy barrier Kv (K is anisotropy constant; v is volume) which has to be surmounted to change state. A particular SD grain will "block" in its contribution to the total TRM when the thermal fluctuations due to the falling temperature no longer exceed a critical value necessary to change the magnetization direction. We will call this the "thermofiuctuational" model, which has sucessfully predicted...

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In this paper, the mechanisms of low‐temperature demagnetization of remanence in multidomain magnetite are considered. New experimental observations of the behaviour of the saturation isothermal remanence, thermoremanence and partial thermoremanence at low temperatures are presented. The results show that there are two main contributions to this low‐temperature demagnetization. The first and predominant contribution (type‐1 demagnetization) is due to ‘kinematic’ domain state reorganization and occurs throughout cooling from room temperature to the Verwey transition, Tv , at 120–124 K. The second contribution arises from the change in anisotropy from cubic to monoclinic at Tv , which changes the overall domain structure of the grain. On warming in zero field, some domain walls will not return to their original positions but will take up a position that leads to a lower net remanence (type‐2 demagnetization). In stoichiometric magnetite, demagnetization does not occur at 130 K at the isotropic point, Tk , contrary to some previous predictions. In non‐stoichiometric magnetite, the influence of the Verwey transition is greatly reduced, and anomalous behaviour is observed at Tk .

The behaviour of grain-growth CRM in SD grains can be predicted using Neel's (1949) theory for the acquisition of TRM. This theoretical approach suggests that the ratio of CRM to TRM will not be constant throughout the grain-size range for either magnetite or haematite. Hence the blocking-temperature spectra for CRM and TRM in an identical set of magnetic grains will be different, and grain-growth CRM can be identified by non-linear palaeointensity plots over certain temperature intervals. It is shown that on thermal demagnetization both CRM and TRM should unblock at the same temperature, Tb, but their magnitudes will be different, because the net fractional alignment for CRM is controlled by the blocking volume and the reaction temperature, while that for TRM is controlled by the final volume and the blocking temperature. CRM/TRM ratios for magnetite and haematite are calculated using the standard relaxation-time equation, and experimental values of spontaneous magnetization as a function of temperature. Calculation of CRM/TRM ratios for actual examples of spectra suggest that grain-growth CRM in both magnetite and haematite can be distinguished from TRM on the basis of a Thellier-Thellier palaeointensity experiment for data that span a temperature interval from room temperature up to at least 450 "C, or for smaller, high-temperature intervals. However, grain-growth CRM cannot be distinguished from TRM if pTRM checks fail below about 400°C, as the CRM/TRM ratio is close to 1 below this temperature. Single-domain CRM grown over laboratory time-scales should always be smaller than a laboratory TRM according to this model, while natural CRM formed over much longer times than available in the laboratory may be as much as twice as strong as TRM. Multidomain grain-growth CRM may always be larger than TRM, due to the difficulty of nucleating domain walls during low-temperature crystal growth.