A. Belenchuk

Doping of interfaces in (La0.7Sr0.3MnO3)1−x:(MgO)x composite films

Composite thin films of (La0.7Sr0.3MnO3)1−x:(MgO)x (x=0–0.5) were grown on Al2O3 (0001) substrates by a metalorganic aerosol deposition technique. A columnar growth of the films with the predominance of (111)- and (110)-orientation was observed. Pure films (x=0) show a Curie temperature of TC=362 K, a metallic behavior accompanied with a low residual resistivity ρ∼10−4 Ω cm at T=4.2 K and a very small low-field magnetoresistance. Low amounts of MgO doping, x=0.05, result in a totally different electrical transport behavior which is a pronounced low-field magnetoresistance MR=25% at T=4.2 K. The MgO was found to be located at the interfaces between the grains thus building tunneling barriers and enhancing spin polarized tunneling similar to a system with vertical artificial tunnel junctions.

A-Site Ordering versus Electronic Inhomogeneity in Colossally Magnetoresistive Manganite Films

Epitaxial La(3/4)Ca(1/4)MnO3/MgO(100) (LCMO) thin film shows an unusual rhombohedral (R-3c) structure with a new perovskite superstructure at room temperature due to the CE-type ordering of La and Ca with modulation vector q=1/4[011]. A-site ordered film was found to be electronically homogeneous down to the 1 nm scale as revealed by scanning tunnelling microscopy/spectroscopy. In contrast, orthorhombic and A-site disordered LCMO demonstrate a mesoscopic phase separation far below the Curie temperature (TC). Unique La/Ca ordering compensates the cation mismatch stress within one supercell, a(S) approximately 1.55 nm, and enhances the electronic homogeneity. The phase separation does not seem to be a unique mechanism for the colossal magnetoresistance (CMR) as very large CMR approximately 500% was also observed in A-site ordered films.

Giant negative photoconductivity in La0.7Ca0.3MnO3 thin films

The increase of the resistance up to two orders of magnitude under laser illumination (λ=760 nm) was observed in La0.7Ca0.3MnO3 (LCMO) epitaxial thin films in ferromagnetic state. Optical absorption also increases by 10–15 % and the magnetic second-harmonic generation signal decreases down to zero under the irradiation. The light induced changes are reversible with characteristic relaxation times τ∼1–30 s. Magnetic field, B=4 T, suppresses the photoconductivity and decreases its relaxation time. Photoinduced effects are caused by the injection of a large number of extra carriers, which change the (antiferromagnetic) AFM/FM phase balance in LCMO, favoring the insulating AFM state.

Interface-controlled magnetism and transport of ultrathin manganite films

We report ferromagnetic, TC=240 K, and metallic, TMI=250 K, behaviors of a three unit cell thick interface engineered lanthanum manganite film, grown by metalorganic aerosol deposition technique on SrTiO3(100) substrates. Atomically resolved electron microscopy and chemical analysis show that ultrathin manganite films start to grow with La-O layer on a strongly Mn/Ti-intermixed interface, engineered by an additional deposition of 2 u.c. of Sr-Mn-O. Such interface engineering results in a hole-doped manganite layer and stabilizes ferromagnetism and metallic conductivity down to the thickness of d = 3 u.c. The films with d = 8 u.c. demonstrate a bulk-like transport behavior with TMI∼TC=310−330 K.