A. G. Badalyan

Electron paramagnetic resonance studies of manganese centers in SrTiO3: Non-Kramers Mn3+ ions and spin-spin coupled Mn4+ dimers

An X- and Q-band electron paramagnetic resonance (EPR) study is reported on SrTiO3 single crystals doped with 0.5 at. % MnO. The EPR spectra originating from the S = 2 ground state of the Mn3+ ions are shown to belong to three distinct types of Jahn-Teller centers. The ordering of the oxygen vacancies due to the reduction treatment of the samples and the consequent formation of an oxygen vacancy associated with the Mn3+ centers is explained in terms of the localized charge compensation. The EPR spectra of the SrTiO3:Mn crystals show the presence of next-nearest-neighbor exchange coupled Mn4+ pairs in the 〈110〉 directions.

Manganese oxide nanoparticles in SrTiO3:Mn

Weakly concentrated “as-grown” and reduced SrTiO3:Mn crystals and ceramics are investigated by means of electron paramagnetic resonance (EPR) to get new insights concerning the Mn substitution in the strontium titanate structure. The temperature dependence of Mn2+ and Mn4+ EPR spectra strongly indicates the formation of MnO2 or MnO small particles to be the favored configuration for the Mn insertion in SrTiO3. Only a part of manganese ions incorporates into the crystal lattice in B position as Mn4+ ions.

Pulse-electron paramagnetic resonance of Cr3+ centers in SrTiO3

Electron paramagnetic resonance on chromium doped SrTiO3 samples grown using the Verneuil technique shows the presence of charge-compensated Cr3+-VO as one of the dominant chromium centers. The spin-lattice relaxation processes have been investigated in samples with both isotropic Cr3+ and Cr3+-VO centers in heavily doped SrTiO3. The relaxation of longitudinal magnetization was dominated by the sum of two exponentials with two time constants (i.e., a slow and a fast constant) at liquid-helium temperatures. The results of fitting the temperature variation of T1 suggest that the dominant exponential contribution is related to the spin-phonon relaxation time arising from the local phonon mode.

Electron spin resonance of copper pair centers in crystals with perovskite structure

Copper pair centers, which could be of interest for obtaining quantitative information about exchange interactions in superconductors based on cuprate perovskites, are observed in crystals with the perovskite structure by the ESR method. Such centers are investigated in KTaO3:Cu and K1−xLixTaO3:Cu crystals. A model consisting of a chain of two equivalent Cu2+ ions and three oxygen vacancies, extending along the 〈 100〉 axis, is proposed for the centers. The exchange interaction in the pairs is ferromagnetic.

A noncavity scheme of optical detection of high-frequency magnetic and cyclotron resonances in semiconductors and nanostructures

To perform optical detection of high-frequency magnetic resonance (ODMR) and cyclotron resonance (ODCR) with a spatial resolution, we have proposed a noncavity scheme of an ODMR-ODCR spectrometer using a quasi-optical microwave channel. The efficiency of this scheme for obtaining information on the spin properties of ZnO quantum dots and CdMnTe quantum wells and on the effective masses of carriers in crystalline silicon films with a spatial resolution within a focused laser beam is illustrated.

EPR study of charge compensation of chromium centers in the strontium titanate crystal

Chromium centers and their charge compensation in two single strontium titanate crystals, i.e., SrTiO3: Cr (0.05 at %) and Sr0.9995TiO3: Cr0.0005 grown with strontium deficiency, have been studied by the electron paramagnetic resonance method. The crystals have been investigated both immediately after growth and after oxidation and reduction procedures. Oxidation and reduction are performed by crystal annealing in a corresponding gas atmosphere at high temperature. Chromium centers associated with oxygen vacancy (Cr3+-VO) are detected in the reduced crystals. It is shown that strontium vacancies are formed in the crystal grown with strontium deficiency, which leads to a lowering of the tetragonal symmetry of (Cr3+-VO) and Cr5+ centers to the orthorhombic symmetry. Possible compensation mechanisms for charges of various chromium centers are considered.

Optically detected magnetic resonance of Mn-related excitations in (Cd,Mn)Te quantum wells

Optically detected magnetic resonance (ODMR) was applied to reveal the exchange interaction effects between Mn2+ ions and confined holes in (Cd,Mn)Te quantum wells with 2D hole gas. Two anisotropic ODMR signals with different angular variations were found and ascribed to isolated manganese ions and to exchange-coupled complexes consisting of manganese and holes. It is shown that calculations on the basis of spin Hamiltonian for these systems are in agreement with the experimental data.

Detection of directed electron-hole recombination energy transfer from an ionic crystal matrix to self-assembled nanocrystals

It is found that the energy released in the spin-dependent tunneling recombination of electron-hole pairs and self-trapped excitons in an ionic crystal matrix is directionally transferred to low-dimensional semiconductor structures embedded in the matrix as a result of self-assembled growth. The EPR spectra of electron and hole centers in the matrix crystal are detected by tunneling afterglow and photostimulated luminescence that are excited in the low-dimensional structure.

Magnetic resonance of exchange-coupled copper complexes in perovskite-structure crystals: The potassium tantalate and cuprate superconductors

This paper reports on parallel EPR studies of high-temperature superconductors based on the cuprate perovskites RBa2Cu3O6+x (R=Y, Gd, Nd) and of KTaO3: Cu, which also has a perovskite structure. EPR measurements performed on copper-doped KTaO3 crystals revealed Cu2+-Cu2+ copper pair centers. The copper ions making up pairs are assumed to occupy adjacent tantalum sites. The pair centers are chains consisting of two equivalent Cu2+ ions and three oxygen vacancies aligned in the 〈100〉 direction. The crucial point in the model proposed is the presence of an oxygen vacancy sandwiched between two Cu2+ ions, whereas the outer vacancies do not necessarily occupy neighboring sites. In this structure, complete charge compensation is achieved. Ferromagnetic exchange coupling takes place between the two copper ions. An investigation of the exchange and superhyperfine interactions of copper centers in crystalline potassium tantalate has permitted the estimation of the respective interactions in crystals of the cuprate superconductors which exhibit magnetic resonance signals due to exchange-coupled copper clusters in the case of oxygen deficiency.

EPR and ENDOR in manganese doped SrTiO3: Electric quadrupole interaction and local disorder

Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) studies were performed on heavily manganese doped SrTiO3 crystals. At low temperatures ( T ≈ 6 K), the ENDOR measurements on Mn T i 4 + show the presence of electric field gradient associated with the electrical polarization. The observation of EPR signals of rhombic Mn 2 + complexes remotely compensated in the SrTiO3 lattice is discussed in connection with local disorder in which Mn 2 + appears due to the reduction process.