Magnetic Polarizations of Electrons at Dislocations in Alkali Halides

A new ferromagnet has been discovered in the alkali halides. The temperature dependence of the permanent magnetic moment agrees with the Bloch spin-wave theory, $M=M_0[1-{\left(\frac{T}{T_c}\right)}^{\frac{3}{2}}]$, between 2 and 290°K. A calculation of the Curie temperature $T_c$ gave (462±1)°K for two samples of NaCl and 545°K for a sample of KCl. The magnitude of the magnetization $M_0$ at 0°K can be accounted for by ${10}^{14}$ electrons per ${\mathrm{cm}}^3$. We believe all these electrons are trapped along dislocations and are responsible for the magnetic behavior described. These very pure single crystals of alkali halides contain a few parts per million of ferric ions, but they typically have some ${10}^6$ dislocations per ${\mathrm{cm}}^2$ which arise during growth from the melt. The experiments on the permanent magnetic moment show first an increase with soft x irradiation and then a decrease until the moment vanishes. The interpretation is that x irradiation excites electrons to the conduction band where they move until trapped to a depth of some 0.25 eV at dislocations. The number of electrons at first is small and the electrons may have parallel spins (Hund's rule), but as the number increases the ferromagnetic state is destroyed.