Ferromagnetism and Colossal Magnetoresistance from the Coexistence of Comparable Charge and Spin Density Orders

We report a complete multicomponent mean-field-theory for the coexistence and competition of charge ordering (CO), antiferromagnetic (AFM) and ferromagnetic (FM) spin ordering in the presence of a uniform magnetic field. Doping the AFM or CO state always generates a ferromagnetic component. Itinerant FM, AFM and CO, necessarily coexist and compete in a particle-hole asymmetric system. Melting of large AFM-CO orders by small magnetic fields and the related phenomenon of Colossal Magnetoresistance (CMR) may arise whenever the CO and AFM order parameters have similar magnitude and momentum structure. Hole doping favors FM metallic states and CMR while electron doping favors AFM-CO states in agreement with the phase diagram of perovskite manganites. PACS: 75.10.Lp, 75.30.Vn [email protected] 1 The pervovskite manganites (La, Pr)1−x(Ca, Sr, Ba)xMnO3, in the doping region x ≈ 0.2− 0.4 exhibit a transition to a ferromagnetic (FM) ground state which is accompanied by a large drop of the resistivity [1]. This transition can be tuned by the application of a magnetic field producing negative “Colossal Mangetoresistance” (CMR) [2]. Despite the intense experimental and theoretical efforts, many fundamental issues are still under debate including the physical origin of the CMR phenomenon. Ferromagnetism in these materials is usually attributed to the double exchange mechanism [3, 4], in which the lattice degrees of freedom [5, 6] might also be involved. However, the CMR phenomenon could be more general since it has also been observed in pyrochlore manganites [7], where double exchange and Jahn-Teller effects on the transport can be safely excluded [8, 9]. One of the most puzzling aspects of perovskite manganites is that the hole doped (x < 0.5) and the electron doped (x > 0.5) compounds behave very differently. In the intermediate doping region x ≈ 0.5 there is a kind of boundary between the hole doped regime where the metallic ferromagnetic phases and CMR take place and the electron doped regime where essentially there are phases of coexisting charge and spin ordering. Understanding the physics in this intermediate region x ≈ 0.5 ± ε appears crucial, and much of the recent experimental activity has focused on it [10, 11, 12, 13, 14, 15, 16, 17] reporting some additional puzzling facts. The coexistence of the AFM charge ordered (AFM-CO) state with FM metallic state has been established [16, 13, 14, 15]. Apparently a small part of the carriers remains metallic in the AFM-CO regime, and has been reported that even in the hole doped regime the carriers are separated into a part that is metallic and a part that is still charge ordered [16, 18, 19]. Microscopic theoretical models would also support a spatial separation of FM and AFM-CO phases [20]. Even more puzzling is the fact that the AFM-CO state near the half-filling boundary can be melt by the application of magnetic fields of few Teslas despite the fact that the CO gap is very large (≈ 0.5eV ) and would correspond to several hundreds of Teslas [16, 11, 10]. The melting of the AFM-CO state appears to progress through the increase of the number of carriers which are FM metallic [16]. The above experimental findings suggest that CO, AFM and FM coexist and compete