Magnetic cellular automata and the formation of glassy and magnetic structures from a chain of magnetic particles

Abstract

We show that magnetic materials made of chains of small magnetic particles display many unusual properties. This is associated mainly with a variety of stable different magnetic structures which can arise there. In particular, there arises a magnetic glass, which may be characterized by a whole set of hysteresis loops and by a large variety of Barkhausen jumps arising in the returned branches of the hysteresis loops. We consider in detail a simple example of such a system---a chain of magnetic nanoparticles. To describe such a single chain first we use numerical micromagnetic simulations. On the basis of these simulations, with the use of a perturbation theory, we derive an analytical model which is an anisotropic Heisenberg model. This is a Heisenberg model with an additional anisotropy term arising due to the shape of the particles. Such a term also arises naturally in some classical magnetic materials such as ${\mathrm{Mn}}_{2}\mathrm{Ni}$ chains. We describe all possible stable states of the system as well as transitions between the states induced by magnetic field. Each of these transitions is arising $a\phantom{\rule{0.3em}{0ex}}la$ the spin flop transition. It may be displayed and detected in experiments as a Barkhausen jump in a hysteresis loop. The series of described spin flop transitions will lead to the formation of different types of returned branches in hysteresis loops. We present exact analytical and numerical results describing the energy spectrum and the magnetization of such systems. The results may be used in the design of nanomaterials as well as for magnetic random access memory and magnetic quantum cellular automata elements.