Ac field-switchable magnetic properties of two-dimensional networked nanosize magnets

Abstract

The magnetic properties of interacting magnetic nanoparticles were investigated by examining two kinds of two-dimensional networked single-molecule magnets (SMMs). Through nonlinear magnetic susceptibility measurements, we observed a switchable transformation of the magnetic response upon applying an ac magnetic field to the SMMs. The magnetic properties depending on the measurement time-scale can be understood as a competition of the magnetic interaction between the constituents (i.e., SMMs or SMM aggregates) and the strong Ising nature of the individual constituents. In high-frequency ac fields, the Ising nature of the SMM induces magnetic behavior due to either the aggregate consisting of several SMMs or the individual SMMs, rather than a static response due to magnetic ordering. The magnetic state at the low-frequency limit was investigated through thermoremanent magnetization measurements, and a relaxation of the aggregates in a manner similar to the Griffiths phase was observed in the first material, [Mn4(hmp)4Br2(OMe)2(dcn)2]⋅0.5H2O⋅2THF. The second material, [Mn4(hmp)6(dcn)2](ClO4)2, with strong dipolar interaction between aggregates, exhibited a glass-type system-wide response rather than a relaxation of the aggregates. Our present study on these regularly networked SMM systems demonstrated the existence of a switchable magnetic phenomenon induced by ac magnetic fields. It is difficult to investigate this kind of phenomenon in diffused magnetic nanoparticles with random location and distribution of both particle size and interparticle distance.