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...

The electrical resistivity, Hall coefficient, and thermopower as functions of temperature are reported for lightly electron-doped Ca1−xLaxMnO3(0 ≤ x ≤ 0.10). Unlike the case of hole-doped ferromagnetic manganites, the magnitude and temperature dependence of the Hall mobility (µH ) for these compounds is found to be inconsistent with small-polaron theory. The transport data are better described by the Feynman polaron theory and imply intermediate coupling (α ≃ 5.4) with a band effective mass, m * ∼ 4.3m0, and a polaron mass, mp ∼ 10m0.

Physical properties including magnetic susceptibility, room-temperature electrical resistivity, thermal conductivity, heat capacity, and thermal expansion are reported for high quality single-crystal samples of marokite CaMn 2 O 4. We determined that CaMn 2 O 4 is highly electrically insulating and exhibits long-range antiferromagnetic order below T N = 217.5Ϯ 0.6 K with easy axis along a. Anisotropic thermal expansion, similar to that of crystallographically layered materials, is observed, suggesting that the crystal structure of CaMn 2 O 4 is also assembled from previously undescribed layers. An extensive thermodynamic study of the antiferromagnetic transition was undertaken resulting in a heat-capacity critical exponent ␣ = 0.082Ϯ 0.007 and calculated pressure derivative dT N / dP = 5.154Ϯ 0.174 K / GPa.

The dc magnetization (M ) and electrical resistivity (ρ) as functions of magnetic field and temperature are reported for a series of lightly electron doped Ca1−xLaxMnO3(0 ≤ x ≤ 0.10) specimens for which magnetization [Phys. Rev. B 61, 14319 (2000)] and scattering studies [Phys. Rev. B 68, 134440 (2003)] indicate an inhomogeneous magnetic ground state composed of ferromagnetic (FM) droplets embedded in a G-type antiferromagnetic matrix. A change in the magnetic behavior near x = 0.02 has been suggested to be the signature of a crossover to a long-ranged spin-canted phase. The data reported here provide further detail about this crossover in the magnetization, and additional insight into the origin of this phenomenon through its manifestation in the magnetotransport. In the paramagnetic phase (T ≥ 125 K) we find a magnetoresistance ∆ρ/ρ = −C(M/MS) 2 (MS is the low-T saturation magnetization), as observed in many manganites in the ferromagnetic (FM), colossal magnetoresistance (CMR) region of the phase diagram, but with a value of C that is two orders of magnitude smaller than observed for CMR materials. The doping behavior C(x) follows that of MS(x), indicating that electronic inhomogeneity associated with FM fluctuations occurs well above the magnetic ordering transition. 75.47.Lx, 75.30.Et, I.

The application of weak electric fields ( less, similar 100 V/cm) is found to dramatically enhance the lattice thermal conductivity of the antiferromagnetic insulator CaMnO3 over a broad range of temperature about the Néel ordering point (125 K). The effect is coincident with field-induced detrapping of bound electrons, suggesting that phonon scattering associated with short- and long-ranged antiferromagnetic order is suppressed in the presence of the mobilized charge. This interplay between bound charge and spin-phonon coupling might allow for the reversible control of spin fluctuations using weak external fields.

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