Rakhim R. Rakhimov

Magnetic properties of La0.6Sr0.4MnO3 thin films on SrTiO3 and buffered Si substrates with varying thickness

La0.60Sr0.40MnO3 (LSMO) thin films of varying thickness from 12 to 55 nm were deposited using the pulsed-laser deposition technique onto single-crystalline SrTiO3 (STO) and STO-buffered Si substrates. The Tc of LSMO films grown on STO-buffered Si substrates decreases faster than films directly grown on STO with decreasing film thickness. The LSMO/STO film with thickness of 55 nm shows Tc at about 360 K, which is close to the bulk value, whereas Tc LSMO film on STO-buffered Si film of similar thickness is reduced to 320 K. This difference is attributed to the strain and interfacial disorders in LSMO film on STO/Si. The film surface morphology is influenced by the film thickness. Oxygenation of LSMO films on STO-buffered Si affects the Tc minimally but improved the overall magnetization of the films due to better oxygenation, which is also the case for postannealing the sample at elevated temperatures. The thermomagnetic history effects observed in LSMO films of STO-buffered Si indicate the presence of inho...

Integration of epitaxial colossal magnetoresistive films onto Si(100) using SrTiO3 as a template layer

We report on the integration of epitaxial colossal magnetoresistive La0.67Ba0.33MnO films on Si(100) semiconductor using SrTiO3 template layer by pulsed-laser deposition. X-ray diffraction reveals the superior quality of the manganite film that grows epitaxially on heteroepitaxially grown SrTiO3 template layer on Si substrate. The epitaxial films demonstrate remarkable surface morphology, magnetic transition and hysteresis, magnetoresistance, and ferromagnetic resonance, illustrating the ferromagnetic nature of the film and possible device applications at room temperature.

Spin and valence states of manganese ions in manganese-doped yttrium orthoaluminate

Abstract Two paramagnetic charge states of manganese with total electron spin of S=5/2 (Mn2+) and S=3/2 (Mn4+) were identified by electron paramagnetic resonance (EPR) method in Mn doped YAlO3. In addition to the allowed ΔMS=1 spin transitions, low field forbidden ΔMS=2 transitions were observed for both Mn4+ and Mn2+ and ΔMS=3 transitions for Mn2+. Relative intensities of forbidden transitions increase with a decrease of temperature and decrease with an increase of Mn concentration. The effect is due to the increase of spin–lattice and/or spin–spin relaxation times at low temperature and the decrease of spin–spin relaxation time at higher dopant concentration. Optically induced disproportionation reaction 2Mn4+→Mn3++Mn5+ in YAlO3 was studied by EPR.

Synthesis and magnetic characterizations of manganite-based composite nanoparticles for biomedical applications

We report chemically synthesized highly crystalline lanthanum strontium manganite (LaSrMnO3) and Eu-doped Y2O3 and their composites. The synthesis yields nanoparticles of size 30–40nm. Magnetic measurements performed on nanoparticles and composites show magnetic transition at about 370K with a superparamagnetic behavior at room temperature. The ferromagnetic resonance studies of the nanoparticles show large linewidth due to surface strains. The composite nanoparticles also display luminescent behavior when irradiated with ultraviolet light. The manganites as well their composite with the luminescent nanoparticles may be very useful for biomedical applications.

Ferromagnetic properties of epitaxial manganite films on SrTiO3∕Si heterostructures

We report on the magnetic properties of epitaxial La0.7Ba0.3MnO3 and La0.7Sr0.3MnO3 films on Si (100) and Si (111) substrates using SrTiO3 template layer, which demonstrate magnetic and electrical properties at and above room temperature. The magnetization data show magnetic transition and magnetic hysteresis at and above room room temperature. The films show well-defined magnetic domains. The ferromagnetic resonance studies show anisotropic effects related to ferromagnetic properties of films. The smaller grain size of about 20nm in manganite films on SrTiO3∕Si may be one of the reasons to minimize the strain effect through strain relaxation at the interface between SrTiO3 and manganites through the formation of three-dimensonal islands.

Ferromagnetism in nanocrystalline epitaxial Co:TiO2 thin films

We report on the observation of remarkable room-temperature ferromagnetism in nanocrystalline epitaxial Co:TiO2 films grown on sapphire (0001) substrates by a pulsed-laser deposition technique using high-density targets. The films were characterized by x-ray measurements, atomic force microscopy, micro-Raman, electron-paramagnetic resonance, and magnetization studies. The films exhibit three-dimensional islandlike growth that contains nanocrystalline particles. Our experimental results suggest that the remarkable ferromagnetism in Co:TiO2 films is controlled either by the interstitial Co2+ ions or small clusters, which are mainly present at the interface and on the surface of the films. Our work clearly indicates that Co interstitials and nanoclusters cause room-temperature ferromagnetism in Co-doped TiO2.

Microwave response near zero magnetic field in transition-metal-doped silicate glasses

A sharp microwave response centered at zero magnetic field was observed in manganese- and iron-doped Na2O–CaO–MgO–SiO2 glasses with phase opposite to the normal Mn+2 and Fe+3 paramagnetic signals. The response can be described as magneto-induced microwave conductivity in the dielectric glass that derives from spin-dependent charge migration within the first coordination sphere of the paramagnetic ion. In contrast to the spin-polarized tunneling in ferromagnets between different valence states of metals, the observed effect is due to spin-dependent tunneling that occurs in the vicinity of manganese or iron in diluted paramagnetic systems.

Electron paramagnetic resonance and quantum-mechanical analysis of binuclear niobium clusters in lithium–niobium phosphate glasses

Electron paramagnetic resonance (EPR) spectra of Nb4+ ions in Li2O–Nb2O5–P2O5 glasses with different composition of oxide components have been investigated. The EPR spectrum shape analysis of Nb4+ (electron configuration 4d1, electron spin S=1/2) reveals the formation of triplet niobium binuclear complex (total electron spin S=1) in glasses. The amount of Nb4+ ions in glasses reversibly changes with temperature and is explained via the mechanism of electron hopping between niobium ions in clusters. The dependence of the amount of Nb4+ ions upon Li2O content has a maximal character, which implies that small amounts of Li+ ions stabilize the Nb4+ pairs, but cause their disproportionation at higher concentrations of Li+ ions in the glass. Quantum mechanical analysis of electronic and spin states of binuclear niobium clusters has been performed on model binuclear complexes, (HO)3Nb–O–Nb(OH)3, [(HO)3Nb–O–Nb(OH)3]Li+, and [(HO)3Nb–O–Nb(OH)3](Li+)2 that exhibit the reversible disproportionation reaction Nb4+–O–N...

Ferromagnetic resonance studies in ZnMnO dilute ferromagnetic semiconductors

We report on the ferromagnetic resonance studies on ZnMnO films grown by the pulsed-laser deposition technique. ZnMnO films were annealed in different atmospheres. The films grown and annealed in oxygen demonstrate ferromagnetic behavior at room temperature and below. However, annealing in either nitrogen or argon deteriorates the ferromagnetic response of the films. Further annealing the films in oxygen recovers the ferromagnetic response. Our results suggest that oxygen plays a major role for controlling the ferromagnetic properties in ZnMnO films.

Spin dynamics of the triplet Cr4+ in the vicinity of energy level anti-crossing

Abstract EPR spectra of the triplet Cr 4+ in Y 2 SiO 5 were investigated. Rhombic symmetry of zero-field splitting leads to three electron spin terms T X , T Y , and T Z in the absence of the external magnetic field. We have observed the magnetically induced anti-crossing of the terms. In the vicinity of energy level anti-crossing the linewidths and positions of the resonance lines depend on temperature, which is explained by the dynamic Jahn–Teller effect. The temperature dependence of spin relaxation rates is described by the direct spin–lattice relaxation process limited to electron–phonon interactions, where the phonon frequencies are close to the applied microwave frequency.