Anomalous magnetic properties of brain tissue at low temperature: The 50 K anomaly

A low-coercivity phase, identified as magnetite and/or maghemite, is the main iron oxide in brain tissue. Measurement of susceptibility as a function of temperature (χ-T) in brain tissue samples does not show a Verwey transition but instead shows a perturbation around 50 K. A susceptibility anomaly has been reported at this temperature in several studies of multidomain magnetite crystals, which, however, also display a Verwey transition. We have investigated the magnetic characteristics of this 50 K anomaly further in brain and tumor tissue. The magnetic measurements consist of χ-T curves, measured after cooling in zero field (ZFC) or in a field (FC), as well as hysteresis loops. The 50 K anomaly is expressed as a bump in χ-T curves over a 20 K temperature range, with a peak between 44 and 58 K. The magnetic intensity of the samples is weak; however, the anomaly signal is an order of magnitude larger than known effects related to the magnetic ordering of oxygen at 4.3 K. A phase transition, or magnetic ordering of another phase, does not seem a likely explanation, because both the ZFC and FC curves follow the perturbation, rather than showing a bifurcation at peak susceptibility. This explanation also precludes magnetic blocking of a superparamagnetic component. Hysteresis loops at temperature of the peak perturbation show a splitting of the descending and ascending limbs at the maximum starting field. The magnetic behavior observed in these experiments is consistent with a change in electron activity.