Inhomogeneous magnetism in La-dopedCaMnO3.I. Mesoscopic phase separation due to lattice-coupled ferromagnetic interactions

Abstract

A detailed investigation of mesoscopic magnetic and crystallographic phase separation in Ca1−xLaxMnO3, 0.00 ≤ x ≤ 0.20, is reported. Neutron powder diffraction and DC-magnetization techniques have been used to isolate the different roles played by electrons doped into the eg level as a function of their concentration x. The presence of multiple low-temperature magnetic and crystallographic phases within individual polycrystalline samples is argued to be an intrinsic feature of the system that follows from the shifting balance between competing FM and AFM interactions as a function of temperature. FM double-exchange interactions associated with doped eg electrons are favored over competing AFM interactions at higher temperatures, and couple more strongly with the lattice via orbital polarization. These FM interactions thereby play a privileged role, even at low eg electron concentrations, by virtue of structural modifications induced above the AFM transition temperatures.

Ferromagnetism in a New Manganese‐Related Brownmillerite: La0.5Sr0.5MnO2.5

“…[8,9] Magnetic interactions in the above system depend on the value of x, with the Mn 3 + -OMn 4 + exchange interaction being ferromagnetic, since the e g itinerant electron is exchanged between Mn 3 + and Mn 4 + by a double-exchange mechanism, [10] whereas Mn 3 + -O-Mn 3 + and Mn 4 + -O-Mn 4 + couplings due to superexchange interactions of opposite sign give rise, generally, to antiferromagnetic structures. [11][12][13] Particular attention has been focused on the behaviour around x = 0.5 in the magnetic phase diagram of La 1Àx D x MnO 3 , where, when D is calcium, the ground state changes from a ferromagnetic to an antiferromagnetic insulator and is associated with a charge-ordering state [14][15][16] in which doped Mn 4 + holes are electrostatically locked into a periodic array and are not able to become involved in any conduction mechanism. [17] However, when D is strontium the phase diagram reveals that the competition between the ferromagnetic double-exchange and antiferromagnetic charge-ordering instability gives rise to a latticecoupled first-order phase transition induced by a relatively low magnetic field.…”

Low-temperature permittivity of insulating perovskite manganites

“…1,2,3,4,5,6 Detailed neutron diffraction studies 6 of Ca 1−x La x MnO 3 (x ≤ 0.2) indicate that the heterogeneity of this system is intrinsic, associated with an extremely fine balance between competing ferromagnetic (FM) double-exchange and antiferromagnetic (AF) superexchange interactions.…”

Polaron transport in the paramagnetic phase of electron-doped manganites

“…Antiferromagnetic (AF) superexchange interactions are dominant and dictate a G-type AF ground state for CaMnO 3 below T N = 125 K. Electron doping 6,7,8,9 via substitution of trivalent ions for Ca induces a weak ferromagnetic (FM) moment associated with an inhomogeneous magnetic state. 11 Transport properties in the paramagnetic phase of electron-doped manganites differ substantially from those of hole-doped FM compositions and consensus about the conduction mechanism is lacking. Both the resistivity and thermopower are thermally activated in hole-doped compounds in a manner consistent with a thermally activated mobility and small polaron theory.…”