Impurity conduction and magnetic polarons in antiferromagnetic oxides

Abstract

Low-temperature transport and magnetization measurements for the antiferromagnets SrMnO3 and CaMnO3 identify an impurity band of mobile states separated by energy δ from electrons bound in Coulombic potentials. Very weak electric fields are sufficient to excite bound electrons to the impurity band, increasing the mobile carrier concentration by more than three orders of magnitude. The data argue against the formation of self-trapped magnetic polarons (MPs) predicted by theory, and rather imply that bound MPs become stable only for kBT ≪ δ.PACS numbers: 75.47. Lx, 71.27.+a, 75.50.Ee An electron in a magnetic solid can perturb local moments via exchange interactions between its spin and those of the ions, forming a self-trapped or bound magnetic polaron (MP). Though these concepts were formulated long ago [1], experimental understanding and theoretical development of MP physics have been limited by the relatively small number of materials found to manifest MPs. More recently, renewed interest in the MP problem has been stimulated by studies of carrier-doped antiferromagnetic (AF) manganites [2] and dilute magnetic semiconducting oxides [3]. An important emerging issue for both classes of compounds is the energy position of donor levels (e.g., associated with oxygen vacancies and/or impurities) within the band gap and the contribution of donor-bound charge to MP formation.Perhaps the simplest AF systems for examining such issues are the nominally Mn 4+ compounds, CaMnO 3 (CMO) and SrMnO 3 (SMO) which have a bipartite (Gtype) AF ground state and are free from the complex collective interactions of Jahn-Teller-active Mn 3+ ions that characterize more widely studied LaMnO 3 . They are model systems for MP studies since the couplings between electronic, lattice, and spin degrees of freedom for light electron doping are known [4,5]. Magnetization [6] and scattering [7] studies imply the existence of MPs in the ground state of CMO when electron doped with La, and theory [4,5] predicts these electrons form self-trapped MPs, i.e. those bound solely by magnetic exchange interactions with ionic spins. However, shallow impurity states associated with oxygen vacancies are ubiquitous in oxides, and their influence on the energetics of MP formation has received little attention.Here we report low-temperature transport and magnetic studies on CMO and SMO which reveal surprising features of the donor electronic structure that offer new insight into MP formation in oxides. These compounds are naturally electron doped by low levels of oxygen vacancies (n ∼ 10 18 − 10 19 cm −3 ) so that the conventional picture of donor-bound electrons in small-radius states predicts insulating behavior at low temperatures. Instead, we find that the low-T transport involves an impurity band of mobile electronic states separated by energy δ from electrons bound in Coulombic potentials. Very weak electric fields (F ≤ 50 V/cm) are sufficient to excite bound electrons to the impurity band, increasing the mobile carrier concentration by more than three...

Far-infrared and dc magnetotransport of CaMnO3-CaRuO3superlattices

“…Because the optical gap of CaMnO 3 is ∼ 1.5 eV, 12 the transport is consistent with thermal excitation of carriers from a shallow impurity level below the conduction band, most likely due to oxygen defects. 13,14 Interestingly, the linear extrapolations of the fitted curves cross right at the Néel temperature T N = 125 K. The difference in the activation energies of 17 meV is therefore associated with the formation of magnetic polarons. 18 We note that this energy is in agreement with the activation energy for spin diffusion extracted from a recent nuclear magnetic resonance study of bulk CaMnO 3−x .…”

“…[8][9][10][11] and CaMnO 3 (Refs. [12][13][14][15] and show that a comparison between far-infrared and dc-MR measurements can yield insights into the origin of this behavior. Interest in this system was sparked by the discovery of interface ferromagnetism for temperatures below the Néel temperature of the CaMnO 3 layers.…”

“…These experiments have provided evidence of spin polarization due to magnetic polarons in the CaMnO 3 layers, analogous to those previously observed in bulk CaMnO 3 . 13,14,18 Scattering of charge carriers in the atomically thin CaRuO 3 layers from polaronic defects trapped at structural faults in the superlattice may then act as spin-dependent transport bottlenecks, thus generating a large dc magnetoresistance effect.…”

Ferromagnetism in Mechanically Milled Sn1−x Co x O2 (x=0 to 0.10) Compounds

“…The intrinsic saturation magnetization, M si was determined from the above plots and are found to be 2.9 emu/g, 2.6 emu/g, 2.2 emu/g for x = 0.02, 0.07, and 0.10 samples, respectively, at 290 K. Their corresponding values at 20 K are found to be 5.9 emu/g, 4.9 emu/g, and 3.3 emu/g. The data were analyzed in terms of bound magnetic polaron model (BMP) [35][36][37], i.e., by fitting to the following equation…”

Study of the hydrogenation and re-heating of Co-doped ZnO and In2O3 Nano composites

“…To comprehend the aptness of the BMP model [50][51][52][53], an effort has been made to fit the experiential M-H data to the BMP model given by relation: Co-doped ZnO and In 2 O 3 samples at 300 K. It was discerned that the fitted data diligently trail the investigational data ( Table 1).…”