Magnetic properties of the olivine series: Fe2SiO4-Mg2SiO4; Mn2SiO4-Mg2Sio4; Fe2SiO4-Mn2SiO4

Abstract

The magnetic susceptibility of the three olivine series Fe2xMg2(1-x)Si04, x=0.10, 0.25, 0.60, 0.80, 1.00; Mn2xMg2(1-x) SiO4, x=0.20, 0.40, 0.60, 0.80, 1.00; Fe2xMn2(1-x)SiO4, x=0.10, 0.30, 0.50, 0.80, 0.90; has been measured from 4.3°K to 300°K. The apparatus employed for these measurements was specially designed for the purpose, and employs a digital voltmeter-digital printer combination in conjunction with a Cahn automatically recording electrobalance. All system components are synchronized by a control circuit, providing complete automatic operation. The inverse susceptibility-temperature curves have the characteristic shape of antiferromagnets for all samples measured. The room temperature molar susceptibility of the Fe-Mg series varies from 2.11 x 10^-2 for x= 1.00 to 0.228 x 10^-2 for x=0.10. For this series, theta varies from 140°K at x=1.00 to 6°K at x=0.00, while T_N is 25°K for x=1.00 and was not observed for x=0.25, 0.10. A second transition temperature was observed far x=1.00, 0.80, 0.60. The effective moment of the Fe++ ion was found to vary from 6.1 Bohr magnetons at x=1.00 to 5.3 Bohr magnetons, at x=0.10. The molar susceptibility of the Mn-Mg compounds was found to vary from 2.00 x 10^-2 at x= 1.00 to 0.491 x 10^-2 at x=0.20, while theta was found to vary from 146°K at x=1.00 to 41°K at x=0.20. The Neel temperature for this series was found to vary from 80°K at x=1.00 to 20°K at x= 0.40, and was not observed for x=0.20. A second transition was observed for x=1.00, 0.80, 0.60. The effective moment of the Mn++ ion was found to be essentially constant with an average value of 5.84 Bohr magnetons. With the exception of the x=1/2 compound, all members of the Fe-Mn series exhibited parameters similar to those of pure manganese and pure iron orthosilicate. The x=1/2 compound exhibited a theta of 118 °K, a T_N of 40 °K, and no second transition was observed. The experimental results are in reasonable agreement with the molecular field theory as developed in this work, for reasonable variations of interaction parameters with cell dimensions, if the Fe++ ion prefers inversion sites rather than mirror sites, and if the Fe-O-Mn superexchange interaction is about 25 per cent of the Fe-O-Fe or Mn-O-Mn superexchange interaction, while the Fe-Mn direct exchange interaction is about 140 per cent of the Fe-Fe or Mn-Mn direct exchange interaction.