V. Moshnyaga

Spin polarization in half-metals probed by femtosecond spin excitation

Knowledge of the spin polarization is of fundamental importance for the use of a material in spintronics applications. Here, we used femtosecond optical excitation of half-metals to distinguish between half-metallic and metallic properties. Because the direct energy transfer by Elliot-Yafet scattering is blocked in a half-metal, the demagnetization time is a measure for the degree of half-metallicity. We propose that this characteristic enables us vice versa to establish a novel and fast characterization tool for this highly important material class used in spin-electronic devices. The technique has been applied to a variety of materials where the spin polarization at the Fermi level ranges from 45 to 98%: Ni, Co(2)MnSi, Fe(3)O(4), La(0.66)Sr(0.33)MnO(3) and CrO(2).

Intrinsic inhomogeneities in manganite thin films investigated with scanning tunneling spectroscopy

Thin films of La0.7Sr0.3MnO3 on MgO show a metal insulator transition and colossal magnetoresistance. The shape of this transition can be explained by intrinsic spatial inhomogeneities, which give rise to a domain structure of conducting and insulating domains at the submicrometer scale. These domains then undergo a percolation transition. The tunneling conductance and tunneling gap measured by scanning tunneling spectroscopy were used to distinguish and visualize these domains.

Preparation of rare-earth manganite-oxide thin films by metalorganic aerosol deposition technique

A chemical deposition technique based on the use of solutions of metal-chelate coordination compounds has been applied to prepare rare-earth–manganite-oxide thin films. La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3 thin films have been grown epitaxially on MgO(100) substrates and characterized by structural (x-ray diffraction analysis, atomic force microscopy) and magnetotransport (T=4.2–300 K, and B=0–5 T) measurements.

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.

Fabrication of high quality plan-view TEM specimens using the focused ion beam

We describe a technique using a focused ion beam instrument to fabricate high quality plan-view specimens for transmission electron microscopy studies. The technique is simple, site-specific and is capable of fabricating multiple large, >100 μm(2) electron transparent windows within epitaxially grown thin films. A film of La0.67Sr0.33MnO3 is used to demonstrate the technique and its structural and functional properties are surveyed by high resolution imaging, electron spectroscopy, atomic force microscopy and Lorentz electron microscopy. The window is demonstrated to have good thickness uniformity and a low defect density that does not impair the films Curie temperature. The technique will enable the study of in-plane structural and functional properties of a variety of epitaxial thin film systems.

Boson peak in overdoped manganites La1−xCaxMnO3

In the charge-ordered phase of strongly doped manganites La

Direct imaging of lattice-strain-induced stripe phases in an optimally doped manganite film

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Exchange bias in La0.7Sr0.3MnO3/SrMnO3/La0.7Sr0.3MnO3 trilayers

Ca