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

Nanocomposites made of magnetic spinel oxide Co1+yAl2-yO4 particles dispersed in an amorphous SiO2 matrix were synthesized by a sol-gel technique. Samples with various mass fractions of Co1+yAl2-yO4 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 Co1+yAl2-yO4 in SiO2, showed very similar Curie-Weiss paramagnetic behavior with antiferromagnetically interacting Co ions (theta a parts per thousand -100 K). Other samples, with lower x, showed complex behavior, spin-glass magnetic freezing at the lowest temperatures and lower absolute value of theta and consequently lower exchange constant