Magnetic properties and phase transitions of iron boracites, Fe3B7O13X (X = Cl, Br or I)

Abstract Magnetic measurements for Fe-X (X = C1, Br and I) boracites in the temperature range of 3.6—300K have been performed. Temperature dependence of the magnetization and magnetic susceptibility for all compounds shows an antiferromagnetic order with Neel temperatures of 11.5K, 18.5K and 31K, respectively. The Curie-Weiss law holds for the temperature dependence of the magnetic susceptibility up to room temperature. The effective magnetic moment obtained from the inverse susceptibility slope, for all compounds, is higher than but close to the spin only value of the Fe2+ ion. From hysteresis measurements the temperature dependence of the coercive field was obtained, the 4K value of the coercive field of Fe-Cl being approximately 5 times smaller than that of Fe-Br boracite and 25 times smaller than that of Fe-I boracite. For Fe-I the mm21′ ⇔ ml' ⇔ 3ml' phase transitions have been evidenced by magnetic measurements, as a small thermal hysteresis in the temperature dependence of the reciprocal magnetization.