Chapter 6: Magnetic Anisotropy and Exchange Bias in Size-distributed Ultrasmall Nanoparticle Systems Check Access

Understanding the thermal dependence of the magnetic properties of nanomaterials is mandatory to enable and develop new applications. There are many parameters that influence such properties and their originated effects. For instance, saturation magnetization m_s and magnetic anisotropy energy $κ$ become size-dependent at the nanoscale. Consequently, thermal variations impose effects on magnetic materials, such as superparamagnetism. We investigated the influence of polydispersity on the thermal dependencies of coercivity H_C(T) and bias field H_exc(T) of ultrasmall core@shell magnetic nanoparticles in compressed powder and ferrofluids. The analysis of the thermal behaviour of these two quantities allows us to shed light on the physical processes involved. By considering the necessary conditions for magnetic coupling and the volume-dependent transition to the superparamagnetic state, we demonstrate that the thermal dependence of exchange bias is due to two different temperature-modulated effects. The first effect is the freezing of interfacial spins and the second is the changing populations of superparamagnetic and blocked nanoparticles. Our results demonstrate that even at very low temperatures, the fraction of nanoparticles subjected to superparamagnetic relaxation has an important effect on the magnetic behaviour of nanoparticle systems, being also responsible for the modulation of coupling and, thus, the exchange bias effect.