Magnetic properties and spin dynamics of CoFeB–SiO$_{2}$/C multilayer granular heterostructures

It has been found that CoFeB–SiO$_{2}$/C and CoFeB–SiO$_{2}$/Bi$_{2}$Te$_{3}$ multilayer heterostructures with a cluster structure of CoFeB layers feature a long-range magnetic order in the entire temperature range from 2 to 300 K. At high temperatures ($T$ = 300 K), CoFeB clusters exhibit magnetic properties characteristic of superparamagnets. At low temperatures ($T$ = 5 K), clusters are ferromagnetic, and the easy magnetization axis is in the film plane. The temperature of the ferromagnetic-to-superparamagnetic state transition of clusters depends on a dielectric interlayer material: the use of Bi$_2$Te$_3$ instead of C as a spacer layer leads to an increase in the transition temperature by a factor of 4 and an increase in the magnetization blocking temperature of CoFeB clusters in a field of 100 Oe by a factor of 3.