Abstract
Magnetic properties and hyperfine parameters of the cummingtonites (FexMg1-x)7Si8O22(OH)2 have been examined using susceptibility, magnetisation measurements and Mossbauer spectroscopy. The magnetic ordering temperatures of three cummingtonite samples with x=0.95 (grunerite sample), x=0.49 and x=0.36 have been measured by Mossbauer spectroscopy to be 47+or-1 K, 6.5+or-1 K and 3.5+or-0.5 K, respectively. The magnetic measurements performed on the grunerite sample indicate a magnetic ordering temperature of 44+or-0.5 K, a positive value of the paramagnetic Curie temperature theta p=+47 K and a metamagnetic transition at a critical magnetic field Hc of about 1.4 T accompanied by a small hysteresis. From the analysis of these data, one can deduce the values of the effective magnetic coupling, between the Fe2+ moments on adjacent ribbons z'J' and of the Fe2+ moments inside one ribbon zJ; the ratio z'J'/zJ is about 0.04 which reflects the strong ferromagnetic coupling of moments within each ribbon, and a weaker antiferromagnetic coupling between adjacent ribbons is characteristic of low-dimensional (here 1D) magnetic systems. The Mossbauer spectrum of the grunerite sample recorded at 14 K can be fitted with four different hyperfine magnetic fields Hhf equal to 63, 114, 153 and 245 kOe (with positive values of Vzz) which can be assigned to Fe2+ in the four non-equivalent M1, M2, M3 and M4 sites, respectively. The substantial difference between the Hhf values can be related to the symmetry of the local crystal field connected with the more or less pronounced distortion of the octahedral sites and also to a lower extent to covalency effects.
Published Version
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