Exploiting B Site Disorder for Phase Control in the Manganites

Disorder on the active d element site is usually very disruptive for conduction and long range order in perovskite transition metal oxides. However, in the background of phase competition such ‘B site’ dopants also act to promote one ordered phase at the expense of another. This occurs either through valence change of the transition metal or via creation of ‘defects’ in the parent magnetic state. We provide a framework for understanding the complex variety of phenomena observed in B site doped manganites and identify the key parameters that control the physics. Using a spatially resolved analysis of B ions in various manganite phases we explain the existing data and predict new situations where highly polarisable phase separated states can be created. Correlated electron systems like the cuprates and manganites involve competition between various long range ordered phases [1,2]. The interplay of this phase competition with weak disorder underlies phenomena like cluster coexistence, percolative transport, and colossal response. The nature of disorder seems to be crucial for these effects, as observed in the manganites [2], and a low concentration of impurities on the active d element site is an effective trigger for phase separation [3–12] and the associated percolative effects. The results of such substitution depend on the reference state and the chemical nature of the impurity. For manganites, with rare earth (RE) and alkaline earth (AE) combination RE1−xAExMnO3, several intriguing results exist for Mn site doping. (i) Magnetic dopants like Cr [3–7], Co or Ni [8] (but not Fe) on the Mn site in a x = 0.5 charge ordered insulating (CO-I) manganite promote a percolative ferromagnetic metal (FM-M), while non magnetic dopants of the same valence do not. (ii) The orbital ordered A type antiferromagnet (AF) at x = 0 is destabilised in favour of a ferromagnetic state [9] by both magnetic and non magnetic dopants. (iii) In contrast to the cases above, where charge-orbital order is suppressed, doping Fe on a ferromagnetic metal [10, 11] at x ∼ 0.4 promotes a charge ordered insulating state! On spatial imaging most of these systems reveal phase separation (PS) and many of them also exhibit enormous magnetoresistance. It is vital to uncover the organising principle behind this diversity of effects, if we are to exploit B site disorder as a tool for phase control. There is unfortunately no microscopic model, let alone a theory, for randomly located B dopants in the manganites. In this paper we write down the first detailed model for B impurities in a manganite host, and study the effect of these dopants in a variety of manganite phases using a real space Monte Carlo technique. Our principal results are the following: (i) We discover that the following hierarchy of effects arise in all B doping cases: (a) change of the effective valence on the Mn sites, (b) percolation of the metallic phase through impurity free regions, and (c) ‘reconstruction’ of the background magnetism and charge order by magnetic dopants. (ii) By exploring the prominent manganite states, and different B dopants, we are able to explain most of the outstanding experimental results. (iii) We suggest a new experiment to test out an unexplored insulator-metal transition driven by B site disorder. (iv) We demonstrate how B impurity locations determine the percolation pattern and may allow atomic level control of current paths in a material. The simplest classification of B site dopants is in terms of their valence in the manganite host. Among the usual dopants Zn, Mg, and Co, are divalent, i.e, in a 2+ state, Ni, Cr, Fe, Sc, and Al are trivalent, while Ru, Sn, and Ti are tetravalent. Some elements can exist in multiple valence states, e.g, Ni can also be +2, and Ru can be +5, but that will not affect our qualitative arguments. The p-1 ar X iv :0 71 0. 22 78 v3 [ co nd -m at .s tr -e l] 2 1 N ov 2 00 8 Kalpataru Pradhan, Anamitra Mukherjee and Pinaki Majumdar valence, α, of the dopant affects the effective carrier density on the Mn sites through the charge neutrality requirement on the compound RE 1−xAE 2+ x Mn 3+ν 1−ηB α ηO 2− 3 , where η is the % of B site doping, and we write the Mn valence as 3 + ν. This yields ν(η, α, x) = (x + η(3 − α))/(1 − η). The effective eg electron count on Mn is n = 1− ν, modified from n0 = 1 − x at η = 0. This change of effective carrier density can itself drive phase change as we will see later. Secondly, dopants with same valence can have different effects depending on their magnetic character. Non magnetic dopants only affect the Mn valence, while those with partially filled d shells can have magnetic coupling to the neighbouring Mn moments. Experiments suggest that Cr has strong AF coupling [13] to the Mn ions, Ni couples ferromagnetically [13], while Fe, despite its magnetic d configuration, couples rather weakly. Based on the inputs above we construct the following model for manganites with B site dopants: Htot = Href + Himp +Hcoup, with