Magnetoplastic effect in irradiated NaCl and LiF crystals

The effect of low x-ray irradiation doses (≈10² rad) on the magnetoplastic effect — the detachment of dislocations from paramagnetic centers under the action of an external magnetic field B — in alkali-halide crystals has been investigated. The measurements were performed on LiF crystals and three types of NaCl crystals, differing in impurity content. The dependence of the mean free path l of the dislocations on the rotational frequency ν of a sample in a magnetic field was especially sensitive to low irradiation doses. In unirradiated crystals this dependence is a single-step dependence and is characterized by a critical frequency ν _c ∝B ² above which the magnetoplastic effect is not observed. The frequency ν _c depends only on the type of paramagnetic centers, and not on their density. Even the lowest irradiation dose employed (<100 rad) leads to a sharp restructuring of the dependence l(ν), converting it into a two-step dependence (for edge dislocations) with an additional critical frequency ν _c2, that is insensitive to the irradiation dose, and that corresponds to the appearance of magnetically sensitive stoppers of a new type under irradiation. The initial critical frequency ν _c1, as a rule, also varies with the dose, reflecting the change in state of the impurity complexes (Ca in NaCl and Mg in LiF). Specifically, it is shown for NaCl(Ca) crystals that as the irradiation dose increases, the frequency ν _c1 increases, gradually approaching the value ν _c2, so that by the time the dose is ≈300 rad, the dependence l(ν) once again becomes a single-step dependence, dropping sharply only for ν⩾ν _c2. It is shown that the addition of a small number of Ni atoms to a NaCl crystal makes the Ca complexes radiation resistant, and the critical frequency ν _c1 corresponding to them initially equals ν _c2 for crystals with no Ni. The recombination kinetics of radiation defects in the case in which the samples are irradiated under a tungsten lamp was investigated. A possible physical model of the observed dependences is discussed. Zh. éksp. Teor. Fiz. 111, 615–626 (February 1997)     

Magnetoplastic effect in NaNO2 ferroelectric crystals

This paper reports on a laser interferometric study of the effect of a dc magnetic field (MF) on the rate of plastic deformation (creep) $\dot \varepsilon$ of NaNO₂ ferroelectric crystals under compression. It is established that the application of a dc MF to a loaded specimen results in an increase in the creep rate and that removal of the MF brings about a decrease in $\dot \varepsilon$ . Subjecting an unloaded specimen to a dc MF beforehand also affects its strain rate under the subsequent loading. The observed magnetoplastic effect is most clearly pronounced within a certain $\dot \varepsilon$ interval, and the magnitude of this effect for the NaNO₂ ferroelectric is several times larger than that for LiF crystals.     

Magnetoplastic effect and spin-lattice relaxation in a dislocation-paramagnetic-center system

A magnetic induction threshold B _c above which the magnetoplastic effect — depinning of dislocations from paramagnetic pinning centers — can be observed in samples placed in a magnetic field is predicted and observed in Al, NaCl, and LiF crystals. The existence of a threshold is associated with the fact that for B<B _c the spin-lattice relaxation time τ_sl in a dislocation-paramagnetic-center system is less than the time required for spin evolution in a magnetic field resulting in the removal of the spin forbiddenness of an electronic transition that “switches off” the dislocation-pinning-center interaction. It is shown that the threshold field B _c is sensitive to temperature and x-ray irradiation of the samples. A new method for measuring the spin-lattice relaxation time in paramagnetic centers on dislocations is proposed on the basis of the data obtained. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 628–633 (25 April 1996)     

Magnetoplastic effect in twinning of bismuth crystals under a concentrated load

Twinning of bismuth crystals under a concentrated load is found to be partly suppressed by a constant magnetic field. The main mechanisms of the influence of a constant homogeneous magnetic field on the twinning of bismuth single crystals subjected to long-term concentrated loading is studied. It is revealed that the length and the number of wedge twins at an indentation decrease in the magnetic field. This suggests a decrease in the mobility of partial twinning dislocations and in the intensity of the nucleation of wedge twin interlayers in a constant magnetic field. Application of the magnetic field increases the width of twins at the mouth. No anisotropy of the magnetoplastic effect is observed upon twinning.     

Magnetoplastic effect in InSb

We have observed dislocation motion in InSb semiconductor crystals under the action of a static magnetic field in the absence of a mechanical load. The dependence of the average dislocation travel distance and of the relative number of diverging and converging half-loops on the magnetic induction and the “magnetic treatment” time is obtained. The activation energy of the motion of diverging dislocations in a magnetic field in the temperature range 120–250°C is estimated. Possible reasons for the observed phenomenon are discussed. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 4, 298–302 (25 August 1999)     

Dielectric effect and electrical relaxation in mica crystals

Results are given of an investigation of the dielectric state and electrical relaxation in crystals of dioctahedral muscovite mica. By comparing experimental and theoretical data for the spectra of thermostimulated depolarization (TSD) currents, the temperature dependences of the conductivity during linear heating log = f(1/T) and results of a study of the transfer of the electrical relief during contact under pressure between polarized and nonpolarized crystals, a conclusion is drawn concerning the drift mechanism of relaxation of the dielectric state in mica.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 99–101, October, 1991.     

Muon spin relaxation in a spin glass CuMn observed in finite longitudinal magnetic fields

Muon spin depolarization in a spin glass CuMn (1.1 at.%, T_g = 10.8 K) has been observed in longitudinal external magnetic fields H_L = 0–640 Oe. The depolarization rate did not depend on H_L at T > T_g, reflecting a fast dynamical fluctuation of Mn moments. In contrast, a remarkable field dependence was observed below T_g. Static and dynamical local fields on μ⁺ from Mn moments were found to coexist at T ～ 0.9 T_g. The results suggested a rapid change of spin dynamics around T_g.     

Electric manipulation of spin relaxation using the spin Hall effect

Using the spin Hall effect, magnetization relaxation in a Ni_{81}Fe_{19}/Pt film is manipulated electrically. An electric current applied to the Pt layer exerts spin torque on the entire magnetization of the Ni81Fe19 layer via the macroscopic spin transfer induced by the spin Hall effect and modulates the magnetization relaxation in the Ni81Fe19 layer. This method allows us to tune the magnetization dynamics regardless of the film size without applying electric currents directly to the magnetic layer.     

Electron spin relaxation of copper(II) ions in diamagnetic crystals

The electron spin-lattice and spin-spin phase relaxation measurements of Cu²⁺ ions in various crystals are reviewed and discussed. Examples of the Debye temperature determination from a wide temperature range measurements of the spin-lattice relaxation time T₁ are shown. An influence of the Jahn-Teller dynamics on T₁ is presented. The phase relaxation described by the phase memory time T_M is affected by temperature due to the spin packet width modulation by molecular motions. The T_M is anisotropic in crystals and can be different for different hyperfine lines of an EPR spectrum.     

Intraband radioluminescence of LiF crystals

Spectra of high-energy electrons are calculated in the electron-hole ionization-passive region of lithium fluoride crystals for the conditions of intense irradiation by short pulses of accelerated electrons. The yield of intraband radioluminescence of these crystals is estimated in comparison with the yield of this kind of luminescence of more extensively studied NaCl crystals. The calculations demonstrate that the yield of radioluminescence determined by electron transitions in the conduction band of LiF crystals is two orders of magnitude weaker than the yield of analogous luminescence of NaCl crystals. This is explained, first, by special features of the energy band structure and, second, by the form of the energy dependence of the density of states in the conduction band of LiF crystals. The yield of hole-type intraband radioluminescence is estimated for various assumed relations between the widths of the valence and the forbidden bands.     

A generalized Kubo-Toyabe formula for muon spin relaxation in crystals with uniaxial symmetry

The Kubo-Toyabe semiclassical formula, describing the time development of the polarization of a particle in zero external field at a lattice site with cubic local environment, is generalized for uniaxial site symmetry. The relaxation function and, in particular, its first moments and long time asymptotics obtained in a closed form depend on the angle between polarization and the crystalc-axis and are shown to vary sensitively with the asymmetry of the field distribution at the particular muon site. Besides the exact uniaxial variant of the Kubo-Toyabe relaxation function, an approximate simple interpolation formula is also derived, which is correct for both short times and in its long time asymptotics. The two parameters (, ₁) in the uniaxial formulae can be determined by using the observed values of the second momentM ₂ for two different crystal orientations.     

Short-lived primary radiation defects in LiF crystals

The spectral and kinetic parameters of electron-pulse-initiated transient absorption and emission of LiF crystals were studied using pulsed spectrometry with a nanosecond time resolution. The measurements were performed in the spectral region of 6 eV, the temperature range of 11–150 K, and within 10^?8–10 s after the termination of an electron pulse. It is shown that the electron-pulse irradiation not only gives rise to F, V _k , and H centers in the LiF crystal but also to certain short-lived defects of two types that differ in the spectral positions of the absorptive and radiative transitions, the lifetime, and the temperature dependence of the production efficiency. Defects of type I feature absorptive transitions at 5.5 and 5.1 eV and a radiative transition at 5.8 eV, whereas the absorptive transitions at 5.3 and 4.75 eV and a radiative transition at 4.4 eV are characteristic of type-II defects. It is found that a variation in the ratio between the concentrations of the different types of short-lived centers in the range of 11–150 K does not affect the quantum efficiency of the F centers. It is assumed that the observed centers are self-trapped excitons of various types.     

Dual nature of RN-centers in LiF crystals

An alternative model of the N_c-center as a complex defect consisting of the F ₂ ⁺ -center perturbed by the F_L defect (where F_L is Li⁰ in the F-center) is confirmed by the methods of optical spectroscopy and X-ray diffraction analysis of LiF crystals. Upon selective optical excitation, these centers are decomposed into F₂ and F_L defects.     

Muon spin relaxation for Ar+NO2 systems in transverse and longitudinal magnetic fields

Surface muons produced in UT-MSL were introduced into argon gas of 4.0±0.2 atm with NO₂ (0–30 ppm), and muonium signals were detected in the presence of a transverse (1.7–3.4 G) and a longitudinal magnetic field (0–3.5 kG) at 295±1 K. The cross section for the transverse relaxation was (11.0±1.0)×10⁻¹⁶ cm². The relaxation rates in different longitudinal magnetic fields show that the rate does not follow the conventional equation which assumes that the relaxation occurs mainly by spin-exchange interaction. Similar measurements were performed for the Mu+O₂ system. These findings indicate that chemical reactions contribute to these relaxation rates.     

Impurity cathodoluminescence of oxygen-containing LiF crystals

Cathodoluminescence of oxygen-containing LiF crystals (LiF-O, LiF-O,OH, LiF-WO₃) is studied by pulsed spectrometry with nanosecond resolution upon excitation of crystals by a single electronbeam pulse at 15 K.     

Nonmonotonic creep in LiF crystals containing Mg

Precision measurements of the creep rate using interferometric recording of the process in LiF crystals containing 0.002 to 0.03 wt % Mg using interferometric recording of the process show that nonmonotonic episodes in the form of alternating segments with relatively high and low creep rates periodically appear against a background of overall creep attenuation. The accumulation of creep strain takes place mostly on the segments with increased rates, which causes kinks to appear in the creep curve that are noticeable when shifts are resolved at a level of fractional microns. Measurements were made of the height of steps L based on “rate-strain increment” curves for stresses smaller and closer to the yield point. Increasing the stresses causes the value of L to fall off, while increasing the Mg content causes it to increase. It is proposed that the nonmonotonic behavior of creep is connected with structural nonuniformity of the material, which is determined by the impurity content and the nonuniformity of the strain process itself. For small strains in the neighborhood of the yielding area, where strain comes about as a result of a broadening of a pre-existing slip band, the impurity atoms enhance the nonuniformity of the strain (L increases). As the strain (stress) increases, L decreases, which corresponds to a smoothing out of the nonuniformities. It is emphasized that the step-like character of the strain accumulation is a general property. Fiz. Tverd. Tela (St. Petersburg) 40, 690–693 (April 1998)     

Cooling overall spin temperature: protein NMR experiments optimized for longitudinal relaxation effects

In experiments performed on protonated proteins at high fields, 80% of the NMR spectrometer time is spent waiting for the ¹H atoms to recover their polarization after recording the free induction decay. Selective excitation of a fraction of the protons in a large molecule has previously been shown to lead to faster longitudinal relaxation for the selected protons [K. Pervushin, B. Vögeli, A. Eletsky, Longitudinal ¹H relaxation optimization in TROSY NMR spectroscopy, J. Am. Chem. Soc. 124 (2002) 12898–12902; P. Schanda, B. Brutscher, Very fast two-dimensional NMR spectroscopy for real-time investigation of dynamic events in proteins on the time scale of seconds, J. Am. Chem. Soc. 127 (2005) 8014–8015; H.S. Attreya, T. Szyperski, G-matrix Fourier transform NMR spectroscopy for complete protein resonance assignment, Proc. Natl. Acad. Sci. USA 101 (2004) 9642–9647]. The pool of non-selected protons acts as a “thermal bath” and spin-diffusion processes (“flip-flop” transitions) channel the excess energy from the excited pool to the non-selected protons in regions of the molecule where other relaxation processes can dissipate the excess energy. We present here a sensitivity enhanced HSQC sequence (COST-HSQC), based on one selective E-BURP pulse, which can be used on protonated ¹⁵N enriched proteins (with or without ¹³C isotopic enrichment). This experiment is compared to a gradient sensitivity enhanced HSQC with a water flip-back pulse (the water flip-back pulse quenches the spin diffusion between ¹H^N and ¹H^α spins). This experiment is shown to have significant advantages in some circumstances. Some observed limitations, namely sample overheating with short recovery delays and complex longitudinal relaxation behaviour are discussed and analysed.     

Metastable F2 center states in LiF crystals

Using time-resolved pulsed absorption and luminescence spectroscopy we have studied the creation ofF ₂ centers during electron pulses of nanosecond duration in the temperature range 80 to 300 K. We show that the formation ofF ₂ centers in long-lived metastable states is thermally activated and competes withF ₂ center relaxation to the singlet state. It is shown that the S-T crossing in the set of states which describe theFC₂ center lies above the¹ _u level. We propose that when the crystal is excited with the lowest energy quanta which produce a photocurrent, the formation of tripletF ₂ centers takes place by localization of a conduction electron on anF ₂ ⁺ center.State Academy of Architecture and Construction, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 17–22, November, 1994.