Exchange Bias in a Granular System of Antiferromagnetic Nanoparticles with Strong Magnetic Anisotropy Embedded in a Ferromagnetic Matrix

Recently, magnetic properties of nanoparticles have gained high interest from both technological and fundamental research, stemming partially from the applications in high-density magnetic storage media and biomedicine. In order to stabilize the recording units in nanometer scales at finite temperatures, the most challenge is to beat the superparamagnetism. Exchange bias, which represents a shift in the hysteresis loop induced by intimate contact between heterogeneous components, may be used to solve this issue. We present a nanogranular system with an antiferromagnetic-ferromagnetic cores-matrix morphology and study the interfacial-coupling and field-cooling dependence of exchange bias by conducting a modified Monte Carlo Metropolis method. When the interfacial coupling is antiferromagnetic, the competition between Zeeman and interfacial-coupling energies determines the strength and sign of exchange bias. Positive exchange bias appears when the cooling field overcomes the interfacial coupling. Whereas in the system with such a special morphology, the ferromagnetic anisotropy and exchange coupling may also determine the exchange bias behaviors to some extent when the interfacial coupling is antiferromagnetic and the strength of cooling field is intermediate. On the other hand, in the system with ferromagnetic interfacial coupling, exchange bias is negative constantly and linear roughly with not large interfacial coupling but independent of cooling field. The phenomena are clarified on the basis of the theories of surplus magnetization and pinning effect as well as by means of the microscopic spin configurations of system.