Continuum studies of magnetic polarons and bipolarons in antiferromagnets

We present a simple scaling model of magnetic-polaron and magnetic-bipolaron formation in antiferromagnetic systems. Within this picture, we determine the criteria for an excess electron forming a magnetic polaron and for a pair of excess electrons forming a (triplet) magnetic bipolaron. The dependence of the formation criteria on the dimensionality of the electronic system is specifically addressed. In addition, we consider the effect of the electron-lattice interaction on the size and energetic stability of magnetic polarons and bipolarons as a function of the system's dimensionality. Beyond the formation of isolated magnetic polarons and/or magnetic bipolarons, we consider how a system composed of magnetic polarons within an antiferromagnetic system is driven ferromagnetic as the density of magnetic polarons is increased. Finally, we discuss the large paramagnetic susceptibility associated with the formation of magnetic polarons. The magnetic susceptibility is composed of a Curie-like temperature-dependent component as well as a temperature-independent component. The temperature-dependent term arises from the reorienting of the magnetic polarons under the influence of a magnetic field. The temperature-independent contribution arises from the expansion and contraction of magnetic polarons which are, respectively, aligned parallel or antiparallel with the applied magnetic field.