Low-frequency spectra of dislocation paths in NaCl crystals in the electron spin resonance scheme

It is shown that the criterion of dislocation mobility resonance in NaCl crystals in the electron spin resonance scheme in the Earth’s magnetic field depends on both the sample orientation in this field and the location of impurity atoms in the dislocation core. The measured low-frequency spectrum of dislocation paths makes it possible to resolve a series of peaks corresponding to different positions of a Ca impurity that correlate with the results from computer simulations of the dislocation core structure.     

Resonant dislocation motions in NaCl crystals under EPR conditions in the Earth’s magnetic field with a radio-frequency pump field

Three resonant peaks l(ν) of dislocation paths, appearing in NaCl crystals under simultaneous exposure to the Earth’s magnetic field B Earth and an ac pump field $$\tilde B$$ have been observed in the radio-frequency range (ν ∼ 1 MHz). Each peak corresponds to a specific experimental geometry. Resonance frequencies correspond to ordinary EPR conditions in which, however, instead of the total Earth’s field, its projections onto particular crystallographic directions play part. These directions are probably related to orientations of magnetic moments at impurity centers in dislocation cores. A physical model describing well the positions all three EPR peaks l(ν) is proposed. The peak height l m is proportional to the squared amplitude of the pump field and the time t of sample exposure to crossed fields: $$l_m \propto \tilde B_m^2 t$$ . The threshold amplitude $$\tilde B_m^{th} \sim 1 \mu {\rm T}$$ of the pump field is detected, below which the effect rapidly disappears. The time of spin-lattice relaxation of radical pairs on dislocations is estimated by the threshold field $$\tilde B_m^{th}$$ level.     

Radio-frequency paramagnetic resonance spectra, detected from dislocation displacement in NaCl single crystals

Results are reported here of a study of the resonance effect of a constant magnetic field and a variable magnetic field crossed with it on the rate of macroplastic deformation and motion of edge dislocations in NaCl crystals. The frequencies of the variable magnetic field at which the maximum variations in the plasticity of the crystals are observed correspond to the resonant frequencies for transitions between the Zeeman sublevels in paramagnetic complexes of point defects and complexes consisting of a point defect and a dislocation. Analysis of the radio-frequency spectra obtained enabled us to establish the role of intercrystal reactions in the formation of the mechanical properties of the crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 1778–1784 (October 1999)     

In-situ NMR study of dislocation motion in Ca-doped NaCl crystals

Using a fast multi-window NMR technique, we have measured in-situ the mean jump width x of mobile dislocations during plastic deformation in a series of NaCl single crystals with varying Ca⁺⁺ content. Aside from immobile forest dislocations, the Ca⁺⁺ impurities form additional obstacles for the moving dislocations thus lowering x. We found that the Ca⁺⁺-related obstacles exhibit a pronounced non-random distribution which results in a corresponding broad distribution of x. We show that the data can be evaluated by means of an appropriate distribution function g(1/x) with an uncommon dependence of the observed fitting parameters on the Ca⁺⁺ content. As expected, quenching of a sample leads to a more uniform distribution of the Ca⁺⁺-related obstacles resulting in a corresponding narrowing of g(1/x).     

Changes in the microhardness and dielectric permittivity of TGS crystals after their exposure to a static magnetic field or ultralow crossed fields in the EPR scheme

Changes are found in the microhardness and dielectric permittivity of triglycine sulfate (TGS) crystals after magnetic treatment in a permanent magnetic field or in the scheme of electron paramagnetic resonance (EPR) using the Earth’s magnetic field. The temporal kinetics of microhardness reduction and the rise in dielectric permittivity (as along with their recovery) are nearly identical. The effects disappear when the crystal’s axis of symmetry is parallel to the static magnetic field in both variants of exposure.     

Resonance magnetoplasticity in the EPR scheme under ultralow magnetic fields

Resonance relaxation displacements of dislocations in NaCl crystals exposed to crossed ultralow magnetic fields (static field B = 26–261 μT and radiofrequency field B ～ 3 μT) in the electron paramagnetic resonance scheme are studied. The effect is also observed in the magnetic field of the Earth when crossed with a pulsed pump field with a resonance duration of ～0.5 μs. Changes in the microhardness of crystals of ZnO, triglycine sulfate, and potassium hydrogen phthalate after their exposure to the magnetic field of the Earth and the orthogonally directed pump field are found out.     

Effect of dislocation density on the ageing of NaCl ∶ BaCl2 crystals

The ageing of NaCl BaCl₂ crystals deformed to different degrees has been investigated by means of electric conductivity measurements at 550°C. It has been found that the precipitation of BaCl₂ out of the lattice takes place mainly on edge dislocations. In the earliest stage of the process a Cottrell cloud is formed around the dislocations and precipitation nuclei are formed as the concentration increases. The results are in good quantitative agreement with the simple Cottrell theory.     

The magnetic properties of Eu nanoclusters formed in NaCl crystals during plastic deformation and aggregation

Plastic deformation of aged crystals and diffusion-controlled aggregation of Eu²⁺ ions in freshly quenched NaCl: Eu crystals causes the formation of complexes bound by exchange interaction. The solution of large-sized precipitates by quenching followed by impurity aggregation results in the nucleation of magnetically sensitive clusters comprising few atoms. A constant magnetic field with induction 5 T increases the probability of the transition of magnetically sensitive clusters from the high-to low-spin metastable state and induces changes in their atomic structure.     

Effect of a magnetic field on the yield point of NaCl crystals

A constant magnetic field is found to have a substantial effect on the macroplasticity of NaCl crystals when they are being actively strained at a constant rate during magnetic treatment. We have measured the dependence of the yield point σ _y on the magnetic induction B=0–0.48 T and the strain rate . It is shown that this magnetic effect has a threshold character and is observed only for B>B _c , where B _c grows with increasing as . The lower the strain rate , the larger the relative decrease in the yield point σ _y (B)/σ _y (0) at fixed field B>B _c . At small enough strain rates the threshold field B _c ceases to depend on and goes constant. A theoretical model is proposed which is in good agreement with the observed regularities. The model is based on the competition between thermally activated and magnetically stimulated depinning of dislocations from paramagnetic impurity centers. Zh. éksp. Teor. Fiz. 115, 951–958 (March 1999)     

An EPR study of the orientational transition in smectic liquid crystals in a magnetic field

Spin probe EPR spectroscopy is used to detect an orientational transition in the smectic mesophase in 4-n-octyl-4'-cyanobiphenyl (8CB) liquid crystal under the action of an intense magnetic field (5.87 T).     

Influence of light and magnetic field on dislocation mobility in ionic crystals

We discuss the role of exchange forces between paramagnetic centers in the bulk and on dislocationsd as a determinant of the plastic properties of diamagnetic crystals. It is shown that the movement of individual dislocations may be used as a measure of the intensity of mechanically active radical chemical reactions with a sensitivity to changes in the spin state at the level of one hundred atoms. G. R. Derzhavin University, Tambov. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 31–36, July, 1998.     

MgF2单晶中位错缺陷的ESR谱——含有130多条谱线

在室温条件下的激光晶体MgF2单晶中，实验发现含有130多条峰的电子自旋共振(ESR)波谱。两个样品分别取自MgF2单晶生长放肩的尖锥部位和MgF2:Co晶体.两个样品都没有经过任何辐照处理。两个样品具有相同的各向异性谱，说明掺入的Co2+离子引发了与MgF2单晶放肩部位相同的位错缺陷，产生了相同的多核固体自由基。这些顺磁固体自由基稳定且寿命长，产生的ESR信号是各向异性的。经初步计算拟合，谱线是由三种不同的多核自由基产生的。当磁场方向与晶体的[100]或[010]方向平行时，样品的ESR信号出现在磁场从0.2292特斯拉（T）到0.4654T的0.2362T范围内（相当于能带宽度为0.233eV）。最窄的线宽DH约为0.00128特斯拉，DH相当于相邻的能级差,是非常小的，仅有1.85×10-7eV 或1.46×10-3cm-1。这一事实表明其基态简并度是相当高的，在不太高的直流磁场下几乎是一个由准连续的能级组成的能带。这有可能成为可调谐的固体激光介质的新基点。     

On the relation between thermally activated and magnetically stimulated processes during dislocation movement in InSb crystals in a magnetic field

It is shown that, in contrast to conventional mobility of dislocations in InSb crystals, which is characterized by a thermally activated temperature dependence of velocity (v ∝ exp(-U/kT), where U is the activation energy), relaxation displacements of dislocation in the same crystals in a magnetic field in the absence of external load are described by a more complex temperature dependence. The V(T) dependence in the temperature range 120–250°C studied here exhibits a tendency to linearization in the ln v vs. 1/T coordinates only in its low-temperature part and rapidly attains saturation upon an increase in temperature. The observed decrease in the thermal sensitivity of relaxation mobility of dislocations in the magnetic field upon heating is interpreted in the framework of the model describing the detachment of a dislocation from a point defect as a sequence of two processes: (i) magnetically stimulated lowering of the barrier, U → U′ (over time τ_sp of the spin evolution in the system) and (ii) expectation of thermal fluctuation (over a time τ_th ∝ exp(U′/kT)). Thus, at low temperatures, we have , and the total time before detachment amounts to τ_th + τ_sp ≈ τ_th. On the contrary, at high temperatures, we have , and τ_th + τ_sp ≈ τ_sp (i.e., the motion becomes athermal). It is shown that this model correctly describes the results of measurements and makes it possible to separate the effects. In particular, it is found that the barrier height decreases from the activation energy U = 0.8 eV under a load of 10 MPa to U′ = 0.25 eV in a magnetic field of B = 0.8 T.     

In-situ study of deformation-enhanced atomic diffusion in NaCl by nuclear magnetic resonance

Abstract

Enhancements in atomic diffusion during deformation are investigated by ²³Na rotating frame nuclear spin relaxation measurements on pure NaCl single crystals. The enhanced diffusion is shown to arise from deformation-induced excess concentration of vacancies. Evaluation of the data yields the concentration of these vacancies as a function of temperature and strain rate.     

Plasticization of NaCl crystals under the combined action of short mechanical and magnetic pulses

The dynamic yield stress of NaCl crystals is found to decrease under loading with a mechanical pulse of submicrosecond duration initiated by an electron beam preceded by a pulse of a vortex electromagnetic field (the time delay is 10^?6 s). This effect becomes more pronounced as the intensity of loading increases.     

EPR studies of changes in the charge state of Cr over a cross section of dislocation pipes in ZnS crystals

Short-term high-temperature annealing of ZnS crystals in a zinc atmosphere is shown to cause rapid Zn diffusion through dislocation pipes along growth-dislocation lines. As a result, the impurity ions of divalent chromium localized in Cottrell atmospheres outside Read cylinders become singly ionized. Plastic deformation of such ZnS crystals or the passing of an electric current through them under a voltage higher than a certain threshold value leads to a decrease in the number of univalent chromium ions. This decrease can be explained by an increase in the radius of Read cylinders as growth dislocations leave Cottrell atmospheres and by an increase in the linear density of their electric charge.     

The influence of thermal processing on the relative mobility of edge and screw dislocations in NaCl crystals

The effect of thermal processing on the mobility of dislocations is investigated in NaCl crystals doped with impurities of various types—high-solubility impurities (Ca²⁺) and low-solubility impurities (Pb²⁺). The results obtained after aging and thermal processing indicate that the type of impurity and its state (e.g., level of aggregation) in the crystal have a strong effect on the relative mobility of edge and screw dislocations, and also on the parameters of double transverse slip. Fiz. Tverd. Tela (St. Petersburg) 41, 1041–1043 (June 1999)