arXiv subjects

Boris Rakvin

Publications and source records attributed to Boris Rakvin.

7 recordsLinked to original sources

arXiv2018

Embedded electron spin decoherence as indicator of the matrix material structure

In this work the problem of characterizing matrix material structure from embedded electron spin decoherence is studied both theoretically and experimentally. Theoretical calculation using nuclear spin bath model and cluster correlation expansion method shows that the positions of decoherence time scale extremums among single crystal orientations of the matrix material coincide with those of the nearest neighbour proton dipolar couplings. This finding is confirmed by single crystal pulsed EPR experiment performed on $γ$-irradiated malonic acid (MA). Electron spin decoherence decay profile in polycrystalline matrix material is obtained from the orientation dependence as an average over sampled orientations on a Fibonacci grid. In addition, it is pointed out theoretically that a further removal of crystal ordering in the nuclear spin bath can reduce decoherence time scale from the polycrystalline value. This prediction is verified experimentally by the Hahn echo time decay scale in a new amorphous polymorph of MA, obtained for the first time by mechanical milling. Thus the embedded electron spin decoherence can be viewed as a quantitative indicator for studying structures and/or structure changes of the matrix material.

cond-mat.mtrl-sci↗

arXiv2014

Magnetic anisotropy of spin tetramer system SeCuO$_3$ studied by torque magnetometry and ESR spectroscopy

We present an experimental study of macroscopic and microscopic magnetic anisotropy of a spin tetramer system SeCuO$_3$ using torque magnetometry and ESR spectroscopy. Large rotation of macroscopic magnetic axes with temperature observed from torque magnetometry agrees reasonably well with the rotation of the $\textbf{g}$ tensor above $T \gtrsim 50$~K. Below 50~K, the $\textbf{g}$ tensor is temperature independent, while macroscopic magnetic axes continue to rotate. Additionally, the susceptibility anisotropy has a temperature dependence which cannot be reconciled with the isotropic Heisenberg model of interactions between spins. ESR linewidth analysis shows that anisotropic exchange interaction must be present in SeCuO$_3$. These findings strongly support the presence of anisotropic exchange interactions in the Hamiltonian of the studied system. Below $T_N=8$~K, the system enters a long - range antiferromagnetically ordered state with easy axis along the $<\bar{1} 0 1>^*$ direction. Small but significant rotation of magnetic axes is also observed in the antiferromagnetically ordered state suggesting strong spin-lattice coupling in this system.

cond-mat.str-el↗

arXiv2012

Single crystals of DPPH grown from diethyl ether and carbon disulfide solutions - Crystal structures, IR, EPR and magnetization studies

Single crystals of the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) obtained from diethyl ether (ether) and carbon disulfide (CS2) were characterized by the X-ray diffraction, IR, EPR and SQUID magnetization techniques. The X-ray structural analysis and IR spectra showed that the DPPH form crystallized from ether (DPPH1) is solvent free, whereas that one obtained from CS2 (DPPH2) is a solvate of the composition 4DPPH.CS2. Principal values of the g-tensor were estimated by the X-band EPR spectroscopy at room and low (10 K) temperatures. Magnetization studies revealed the presence of antiferromagnetically coupled dimers in both types of crystals. However, the way of dimerization as well as the strength of exchange couplings are different in the two DPPH samples, which is in accord with their crystal structures. The obtained results improved parameters accuracy and enabled better understanding of properties of DPPH as a standard sample in the EPR spectrometry.

cond-mat.mtrl-sci↗

arXiv2012

Study of the local field distribution on a single-molecule magnet-by a single paramagnetic crystal; a DPPH crystal on the surface of an Mn12-acetate crystal

The local magnetic field distribution on the subsurface of a single-molecule magnet crystal, SMM, above blocking temperature (T >> Tb) detected for a very short time interval (~ 10-10 s), has been investigated. Electron Paramagnetic Resonance (EPR) spectroscopy using a local paramagnetic probe was employed as a simple alternative detection method. An SMM crystal of [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O (Mn12-acetate) and a crystal of 2,2- diphenyl-1-picrylhydrazyl (DPPH) as the paramagnetic probe were chosen for this study. The EPR spectra of DPPH deposited on Mn12-acetate show additional broadening and shifting in the magnetic field in comparison to the spectra of the DPPH in the absence of the SMM crystal. The additional broadening of the DPPH linewidth was considered in terms of the two dominant electron spin interactions (dipolar and exchange) and the local magnetic field distribution on the crystal surface. The temperature dependence of the linewidth of the Gaussian distribution of local fields at the SMM surface was extrapolated for the low temperature interval (70-5 K).

cond-mat.mtrl-sci↗

arXiv2010

Low temperature electron-spin relaxation in the crystalline and glassy states of solid ethanol

X-band electron paramagnetic resonance (EPR) spectroscopy was used to study the spectral properties of a nitroxide spin probe in ethanol glass and crystalline ethanol, at 5 - 11.5 K. The different anisotropy of molecular packing in the two host matrices was evidenced by different rigid limit values for maximal hyperfine splitting in the signal of the spin probe. The significantly shorter phase memory time, , for the spin probe dissolved in crystalline ethanol, as compared to ethanol glass, was discussed in terms of contribution from spectral diffusion. The effect of low-frequency dynamics was manifested in the temperature dependence of and in the difference between the data measured at different spectral positions. This phenomenon was addressed within the framework of the slow-motional isotropic diffusion model [S. Lee, and S. Z. Tang, Phys. Rev. B 31, 1308 (1985)] predicting the spin probe dynamics within the millisecond range, at very low temperatures. The shorter spin-lattice relaxation time of the spin probe in ethanol glass was interpreted in terms of enhanced energy exchange between the spin system and the lattice in the glass matrix due to boson peak excitations.

cond-mat.soft↗

arXiv2010

Electron spin-lattice relaxation in solid ethanol: the effect of nitroxyl radical hydrogen bonding and matrix disorder

The electron spin-lattice relaxation of TEMPO and TEMPONE was measured at temperatures between 5 and 80 K in crystalline and glassy ethanol using X-band electron paramagnetic resonance spectroscopy. The experimental data at the lowest temperatures studied were explained in terms of electron-nuclear dipolar interaction between the paramagnetic center and the localized excitations, whereas at higher temperatures low-frequency vibrational modes from the host matrix and Raman processes should be considered. The strong impact of hydrogen bonding between the dopant molecule and ethanol host on the spin relaxation was observed in ethanol glass whereas in crystalline ethanol both paramagnetic guest molecules behaved similarly.

physics.chem-ph↗

arXiv2008

EPR and magnetization studies on single crystals of a heterometallic (Cu^{II}} and Cr^{III}) complex: zero-field splitting determination

Magnetic properties of single crystals of the heterometallic complex [Cu(bpy)$_3$]$_2$[Cr(C$_2$O$_4$)$_3$]NO$_3\cdot $9H$_2$O (bpy = 2,2'-bipyridine) have been investigated. From the recorded EPR spectra, the spin-Hamiltonian parameters have been determined. The magnetization measurements have shown magnetic anisotropy at low temperatures, which has been analysed as a result of the zero-field splitting of the Cr$^{\mathrm{III}}$ ion. By fitting the exactly derived magnetization expression to the measured magnetization data, the axial zero-field splitting parameter, D, has been calculated. Comparing to the EPR measurements, it has been confirmed that $D$ can be determined from the measurements of the macroscopic magnetization on the single crystals.

cond-mat.mtrl-sci↗