Magnetic properties of Co1+yAl2−yO4–SiO2 nanocomposites synthesized by sol–gel method

Nanocomposites made of magnetic spinel oxide Co 1+y Al 2−y O 4 particles dispersed in an amorphous SiO 2 matrix were synthesized by a sol–gel technique. Samples with various mass fractions of Co 1+y Al 2−y O 4 in composite x , and with various Co concentrations in spinel y were studied. The grain sizes were below 100 nm with a large size distribution. The paper represents thorough investigation of magnetic properties, but special attention was given to peculiar effects and low temperature magnetic frozen states. Magnetic measurements reveal multiple magnetic ground states at the lowest temperature. The frequency dependent ac susceptibility measurements indicate spin-glass behavior of these ground states at low temperatures. The samples, with higher concentration x of Co 1+y Al 2−y O 4 in SiO 2 , showed very similar Curie–Weiss paramagnetic behavior with antiferromagnetically interacting Co ions ( θ ≈ −100 K). Other samples, with lower x , showed complex behavior, spin-glass magnetic freezing at the lowest temperatures and lower absolute value of θ and consequently lower exchange constant.