A real space description of magnetic field induced melting in the charge ordered manganites: I. The clean limit

Abstract

We study the melting of charge order in the half doped manganites using a model that incorporates double exchange, antiferromagnetic superexchange, and Jahn-Teller coupling between electrons and phonons. We primarily use a real space Monte Carlo technique to study the phase diagram in terms of applied field (h) and temperature (T ), exploring the melting of charge order with increasing h and its recovery on decreasing h. We observe hysteresis in this response, and discover that the 'field melted' high conductance state can be spatially inhomogeneous even without extrinsic disorder. The hysteretic response plays out in the background of field driven equilibrium phase separation. Our results, exploring h, T , and the electronic parameter space, are backed up by analysis of simpler limiting cases and a Landau framework for the field response. This paper focuses on our results in the 'clean' systems, a companion paper studies the effect of cation disorder on the melting phenomena. c , discussed later) is small. The associated energy scale gµ B h c k B T CO , where T CO is the zero field melting temperature (g is the gyromagnetic ratio and µ B the Bohr magneton). The smallness of h c is attributed to the small energy difference between the CE-CO-I and FM-M states. (ii) The field induced transition is seen to be first order and is accompanied by hysteresis since there are competing metastable minima, particularly for materials close to the CE-CO-I -FM-M phase boundary. (iii) The field induced FM-M is believed to be homogeneous, and is arXiv:0811.3563v4 [cond-mat.str-el]

A real space description of magnetic field induced melting in the charge ordered manganites: II. The disordered case

“…(a) shows the data for the Ca family and (b) for the Sr family. As defined in the companion paper 27 , the CO melting fields are defined as the magnetic field values at which the CO volume fraction changes by a large amount abruptly. There is a concomitant sharp drop in the resistivity (ρ) and also formation of a FM magnetic state.…”

“…We now study the field response of the CE-CO-I state to magnetic field sweeps in presence of disorder. We track the magnetic field induced phase transition between the CE-CO-I and the FM-M as it takes place in the disordered background and contrast it with what we found in the clean case 27 . Fig.4 and 5 show this evolution of the zero field CE-CO-I state (for λ/t = 1.55) as we sweep the magnetic Fig.…”

“…Theoretical results 38-41 on clean system and our companion paper 27 set the stage for the present work on dis-order effects. Our goal is to understand the strikingly different response of similar bandwidth charge ordered manganites to magnetic fields [42][43][44] , to capture the disorder induced rounding of melting transition and to explain the real space nature of the field melted state in presence of disorder.…”

“…These issues are particularly important in understanding pump probe and optical excitation experiments [22][23][24][25][26] apart from thermal and magnetic field cycling induced phase transitions. In our companion paper 27 we have addressed issues of this nature for a clean system. This paper discusses disorder effects.…”

“…Given the intense activity in the field of manganites in the past decade, many aspects such as colossal magnetoresistance, multiferroic properties, doping, temperature and disorder effects have been extensively studied. We refer to the companion paper 27 on the clean field melting problem for details of the existing literature in this field. Here we focus on half doped manganites.…”