The size of nanoparticles can have a profound effect on their coercivity. As the particle size decreases, the coercivity initially increases due to the increased difficulty for domain walls to move in smaller particles. However, at a critical size, particles become single-domain and the coercivity reaches a maximum. Further reduction in size leads to a decrease in coercivity due to the onset of superparamagnetism, where thermal fluctuations can randomize the magnetization direction.
