MAGNETIC-PROPERTIES AND PHASE-TRANSITIONS OF IRON BORACITES, FE3B7O13X (X = CL, BR OR I)

Magnetic measurements for Fe-X (X=Cl, Br and I) boracites in the tempe rature range of 3.6-300K have been performed. Temperature dependence o f the magnetization and magnetic susceptibility for all compounds show s an antiferromagnetic order with Neel temperatures of 11.5K, 18.5K an d 31K, respectively. The Curie-Weiss law holds for the temperature dep endence of the magnetic susceptibility up to room temperature. The eff ective 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 held was obtained, the 4K value of the coercive field of Fe-Cl being approximately 5 times smaller than that of Fe-Br borac ite and 25 times smaller than that of Fe-I boracite. For Fe-I the mm21 ' double left right arrow ml' double left right arrow 3ml' phase trans itions have been evidenced by magnetic measurements, as a small therma l hysteresis in the temperature dependence of the reciprocal magnetiza tion.