Effect of a weak magnetic field on the state of structural defects and the plasticity of ionic crystals

The aim of this work is to examine the influence of a weak (on the energy scale) magnetic field on the state of dislocations and point defects in ionic crystals. It is found that complex point defects existing in a metastable state are sensitive to a magnetic field B∼1 T. The contributions are identified, and the kinetics of various types of reactions within the structural defects and between them leading to plastification of the crystals in a magnetic field are determined. The effect of light on the sensitivity of the point defects to a magnetic field is described, and the spectral characteristics of this effect are determined. A resonant effect of the combined action of a weak constant magnetic field and a high-frequency magnetic field on the dislocation mobility is found to occur when these fields satisfy the conditions of electron paramagnetic resonance. Zh. éksp. Teor. Fiz. 115, 605–623 (February 1999)     

Electron paramagnetic resonance in a subsystem of structural defects as a factor in the plasticization of NaCl crystals

A change in the macroplastic deformation rate and an increase in the travel distances of edge dislocations are observed in NaCl single crystals placed in crossed dc and rf magnetic fields. The magnetic-field frequencies at which softening maxima are observed correspond to the resonance frequencies of transitions between Zeeman sublevels in paramagnetic complexes of structural defects. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 400–405 (10 September 1998)     

Selective effect of a weak DC magnetic field on triglycine sulfate crystals

The effect of the selective influence of a dc magnetic field on the characteristics of nominally pure triglycine sulfate crystals was detected for the first time. A short (minutes) exposure to a weak magnetic field B₀=0.08±0.01 T caused long-term (hundreds of hours) changes in the spontaneous polarization, coercive field, Curie temperature, and permittivity of the crystal at the Curie point. The effect is selective in nature presumably because of the participation of hydrogen bond protons in the spin-dependent processes of the transformation of defect complexes in real crystals.     

Reversible and irreversible magnetic-field-induced changes in the plastic properties of NaCl crystals

It is established that a weak magnetic field with induction B∼1 T gives rise to irreversible changes in NaCl crystals without freshly introduced dislocations, while a strong magnetic field with induction 16<B<35 T gives rise predominantly to reversible changes. It is inferred that there exist two different channels whereby a magnetic field influences the state of point defects. Fiz. Tverd. Tela (St. Petersburg) 40, 2065–2068 (November 1998)     

Splitting of NMR signals in parallel fields in the easy-plane antiferromagnet FeBO3

Experimental observations are reported of the splitting of NMR lines of ⁵⁷Fe into two absorption peaks in a static magnetic field H ₀ parallel to a variable field H ₁ in the basis plane. The field dependence of the intensity and the variation in the resonance frequencies of the absorption peaks with H ₀ are studied. These results can be used to explain some features of the layered domain structure of iron borate. Fiz. Tverd. Tela (St. Petersburg) 41, 290–292 (February 1999)     

The effect of magnetic and electric fields on the state of point defects in single-crystal NaCl

It is established that exposing crystals to a pulsed electric field with an amplitude of ∼10³ kV/m creates metastable states of the point defects, while a magnetic field with an induction of 7 T causes them to relax. Fiz. Tverd. Tela (St. Petersburg) 40, 2184–2188 (December 1998)     

Influence of structural deformations on the magnetic properties of Jahn-Teller complexes of divalent copper in CaxSr1−x F2 mixed crystals

Crystals of variable constituency Ca_xSr_1−x F₂:Cu (0⩽x⩽0.05, x≅0.5, 1.0⩾x⩾0.95) are investigated by EPR and x-ray structural analysis. The electron-Zeeman, intrinsic hyperfine, and ligand hyperfine interaction parameters are determined; they characterize the magnetic properties of the copper paramagnetic complexes formed in the crystals. Models of the molecular structures of these complexes are discussed, along with the influence of structural deformations and vibronic interactions on their magnetic properties. Fiz. Tverd. Tela (St. Petersburg) 40, 445–451 (March 1998)     

Magnetosensitive defects generated during plastic straining of NaCl:Eu crystals

The excitation of an equilibrium subsystem of point defects in plastically strained NaCl:Eu crystals has been studied. It is established that the macroplastic straining of crystals containing three-dimensional EuCl₂ precipitates leads to a decrease in the amount of these precipitates, accompanied by the appearance of new emission centers of two types. In the easy glide stage, the plastic straining is additionally accompanied by a decrease in the fraction of flat EuCl₂ precipitates and an increase in the content of nonequilibrium small-size complexes of impurity-vacancy dipoles sensitive to an external magnetic field. In thoroughly annealed crystals with the point defect subsystem in a nearly equilibrium state, individual dislocations exhibit displacement caused by the action of a pulsed magnetic field and by the joint action of constant and microwave magnetic fields under the electron paramagnetic resonance conditions.     

In situ investigation of the effect of a magnetic field on the mobility of dislocations in deformed KCl:Ca single crystals

This paper presents the results of an experimental investigation into the mobility of edge dislocations in KCl:Ca single crystals and the effect of a static magnetic field of 0.3 T on the dislocation mobility. The experiments on the effect of a magnetic field on the dislocation mobility were carried out with the use of a high-resolution (1 ms) method that permits in situ measurements of the sample dipole moment induced by the motion of charged dislocations as the crystal is being deformed. It is found that the starting stress is reduced in a magnetic field and the activation volume for overcoming of point defects by the dislocations is increased. It is further found that the magnetic field increases the rate of motion of the dislocations at the initial stage of deformation (to the point of dislocation multiplication) but has no effect on the mobility in the multiplication stage. Fiz. Tverd. Tela (St. Petersburg) 39, 630–633 (April 1997)     

Dislocations used to probe the defect state of an ionic crystal lattice excited by a pulsed magnetic field

This paper presents the results of an investigation into the effect of a pulsed magnetic field on the state of linear and point defects in ionic crystals. For different amplitudes (1–7 T) and pulse lengths (3×10⁻⁵ to 10² s) of the pulsed field the kinetics of the transformation of defects into a new state and their relaxation after the field is turned off are studied in the temperature range 77–400 K. It is found that the relaxation of the states of point defects is mainly through recombination, and the change of state of the dislocations and of the point defects contribute nonadditively to the change in the dislocation mobility. The exposure of the crystal to a magnetic field leads to an increase in the dislocation mobility when the sample is mechanically stressed and to a decrease in the dislocation displacement with a second field pulse. Fiz. Tverd. Tela (St. Petersburg) 39, 634–639 (April 1997)     

Dynamic self-organization of magnetic domains in amorphous films with perpendicular anisotropy

Dynamic self-organization of magnetic domains is observed in amorphous gadolinium-cobalt films in narrow temperature intervals on both sides of the magnetic compensation point. Spiral dynamic domains form in a limited range of ac magnetic field amplitudes and frequencies. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 688–692 (10 December 1997)     

Magnetic field effect on the photoluminescence of an Eu impurity during its aggregation in NaCl crystals

It is found that a magnetic field up to 20 T affects photoluminescence absorption spectra in NaCl:Eu crystals in the process of impurity aggregation into complexes. The spectral changes are irreversible and are observed after quenching the crystals at the early stages of the formation of small metastable complexes containing impurity-vacancy Eu²⁺-V dipoles. Emporal correlations revealed between the occurrence of the magnetoplastic effect and a change in the optical properties of the crystals in a magnetic field.     

Softening of ionic crystals as a result of a change in the spin states of structural defects under paramagnetic resonance conditions

The purpose of this work was to investigate the plastic properties of Ca-or Eu-doped NaCl and KCl single crystals in crossed constant and microwave magnetic waves under paramagnetic resonance conditions. It was found that when the photon energy of the microwave field equals the Zeeman splitting of the electronic spin sublevels, resonance softening of the crystals, manifested as an increase in the free path of individual dislocations and the macroplastic flow velocity as well as a decrease of the microhardness of the crystals, is observed. It was established that metastable Ca-and Eu-impurity complexes, which are also sensitive to the constant magnetic field in the absence of the microwave field, as well as complexes formed by dislocations and point defects are responsible for resonance softening.     

Magnetically stimulated hardening of NaCl(Pb) crystals

Dislocation motion in NaCl(Pb) crystals under a pulsed mechanical load with and without a magnetic field is investigated. It is found that the dislocation mobility decreases when these crystals are deformed in a magnetic field. It is inferred that the observed magnetically stimulated hardening of NaCl(Pb) is due to a characteristic feature of spin-dependent electronic transitions in the dislocation-lead impurity system which increase the barrier for dislocation motion. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 3, 226–228 (10 August 1999)     

Paradoxes of the influence of small Ni impurity additions in a NaCl crystal on the kinetics of its magnetoplasticity

A comparative study of magnetoplasticity in two types of NaCl crystals differing in impurity content only by a small Ni addition (0.06 ppm) in one of them, NaCl(Ni), has been carried out. Two methods of sample magnetic exposure were used: in a constant field B = 0–0.6 T and in crossed fields in the EPR scheme—the Earth’s field B_Earth (50 μT) and a variable pumping field \(\tilde B( \sim 1 \mu T)\) at frequencies ν ~ 1 MHz. In the experiments in the EPR scheme, the change of the field orientation from \(\tilde B \bot B_{Earth}\) to \(\left. {\tilde B} \right\|B_{Earth}\) led to almost complete suppression of the effect in the NaCl(Ni) crystals and reduced only slightly (approximately by 20%) the height of the resonance peak of dislocation mean paths in the crystals without Ni, with the amplitude of the mean paths in NaCl(Ni) in the orientation \(\tilde B \bot B_{Earth}\) having been appreciably lower than that in NaCl. In contrast, upon exposure to a constant magnetic field, a more intense effect was observed in the crystal with Ni. The threshold pumping field amplitude \(\tilde B\), below which the effect is absent under resonance conditions, for the NaCl(Ni) crystals turned out to be a factor of 5 smaller than that for NaCl, while the thresholds of a constant magnetic field coincide for both types of crystals. All these differences are discussed in detail and interpreted.     

Formation of bound magnetic polarons in Cd1−x MnxTe crystals. Contribution of the average exchange field and thermodynamic fluctuations of the magnetization to the binding energy

It is shown that the temperature dependence of the binding energy of exciton-impurity complexes in the semimagnetic semiconductors Cd_1−x Mn_xTe (x≈0.05) is described well in the donor-electron model with an effective characteristic magnetic-polaron energy. The contributions of the average exchange field and of thermodynamic fluctuations of the magnetization to the binding energy and their variation with temperature with and without a weak magnetic field (H⩽3.5 kOe) are determined. How doping with scandium, vanadium, and cobalt effects the appearance of the magnetic-polaron in the experimental crystals was studied. Fiz. Tverd. Tela (St. Petersburg) 39, 527–535 (March 1997)     

Bromine metallization studied by X-ray absorption spectroscopy

The experimental data carried out by M?ssbauer and magnetic resonances investigations of the structural phase transitions in K₂ZnCl₄ crystals are discussed by a simple electrostatic model, calculating, the lattice contributions to the local electric potential V(r), electric field intensity E(r) and electric field gradient tensor, (r) and taking into account both the fractional electric point charges and rigid lattice approximations. The validity of the model is proved by a good fit of the computing results and experimental data of quadrupole splitting parameters at K sites obtained by ³⁹K-NMR methods in high temperature incommensurate phase ( Pnam symmetry). The experimental results obtained by M?ssbauer and EPR methods in commensurate phase (Pna2₁ symmetry) of iron and copper doped K₂ZnCl₄ crystals are explained by relaxing the rigid lattice approximation. The insertion of probe ions appear to be done on not-exactly-Zn²⁺ site. Received 3 February 1999 and Received in final form 4 May 1999     

Resonance magnetoplasticity in ultralow magnetic fields

Resonance relaxation displacements of dislocations in NaCl crystals placed in crossed static and alternating ultralow magnetic fields in the electron paramagnetic resonance scheme are discussed. The Earth’s magnetic field B_Earth ≈ 50μT and other fields in the range of 26–261 μT are used as the static field. New strongly anisotropic properties of the effect have been revealed. Frequency spectra including numerous peaks of paths at low pump frequencies beginning with 10 kHz, as well as the quartet of equidistant peaks at high frequencies (~1.4 MHz at B=B_Earth), have been measured. The effect is also observed in the pulsed pump field with a resonance duration of ~0.5 μs. Resonance changes have been detected in the microhardness of ZnO, triglycine sulfate, and potassium hydrogen phthalate crystals after their exposure in the Earth’s magnetic field in the same electron paramagnetic resonance scheme.     

Effect of stabilization processes on NMR signals from 180° domain boundaries in cubic ferrite-garnet crystals

Numerical methods were used to study the effect of stabilization processes on the line shape of NMR signals from 180° domain walls in cubic ferrite-garnet crystals, taking into account the structural change of the boundaries as they shift quasistatically in a magnetic field. The dependences of the amplitude of absorption maxima on the induced anisotropy constants and the amplitude of the external magnetic field are analyzed. Fiz. Tverd. Tela (St. Petersburg) 40, 694–698 (April 1998)     

The influence of a magnetic field on the inelastic properties of LiF crystals

The effect of weak magnetic fields (0.1–0.8 T) on the internal friction and Young’s-modulus defect of LiF crystals is investigated over a range of relative strain amplitudes ɛ ₀ from 10⁻⁶ to 10⁻⁴ at frequencies of 40 and 80 kHz. Experiments with these fields show that the internal friction increases and the effective elastic modulus decreases, indicating an increase in the plasticity of the samples. Plots are obtained of the internal friction versus the magnitude of the magnetic field at various values of the strain amplitude ɛ ₀. Fiz. Tverd. Tela (St. Petersburg) 41, 1035–1040 (June 1999)