Temperature dependence of antiferromagnetic susceptibility in ferritin

Abstract

We show that antiferromagnetic susceptibility in ferritin increases with temperature between 4.2 and 180 K (i.e., below the N\'eel temperature) when taken as the derivative of the magnetization at high fields $(30\ifmmode\times\else\texttimes\fi{}{10}^{4}\text{ }\text{Oe})$. This behavior contrasts with the decrease in temperature previously found, where the susceptibility was determined at lower fields $(5\ifmmode\times\else\texttimes\fi{}{10}^{4}\text{ }\text{Oe})$. At high fields (up to $50\ifmmode\times\else\texttimes\fi{}{10}^{4}\text{ }\text{Oe}$) the temperature dependence of the antiferromagnetic susceptibility in ferritin nanoparticles approaches the normal behavior of bulk antiferromagnets and nanoparticles considering superantiferromagnetism, this latter leading to a better agreement at high field and low temperature. The contrast with the previous results is due to the insufficient field range used $(<5\ifmmode\times\else\texttimes\fi{}{10}^{4}\text{ }\text{Oe})$, not enough to saturate the ferritin uncompensated moment.