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1.
Magnetoplastic effect in irradiated NaCl and LiF crystals 总被引:2,自引:0,他引:2
V. I. Al’shitz E. V. Darinskaya O. L. Kazakova 《Journal of Experimental and Theoretical Physics》1997,84(2):338-344
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) 相似文献
2.
Changes in individual groups of paramagnetic centers after reduction and reductive butylation of Polish flame coal (70.8 wt.%
C) were studied by electron paramagnetic resonance (EPR) spectroscopy. The modern method of reductive butylation of coal in
a potassium-liquid ammonia system was used. This process increases the solubility of coal in organic solvents. Microwave saturation
of EPR spectra was applied to test the spin-lattice relaxation in coal. The measured EPR spectra were a superposition of broad
(ΔB
pp, 0.42–0.49 mT) and narrow (ΔB
pp, 0.09–0.13 mT) Lorentz lines. Paramagnetic centers located in simple and multiring aromatic structures were responsible for
the broad and narrow lines, respectively. Microwave saturation indicates that slow and fast spin-lattice relaxation processes
are characteristic for these two types of structures in the original coal. A decrease of the microwave power saturation of
the broad Lorentz line after a single reduction of coal was observed. It increased for both 4 times reduced coal and reductively
butylated coal. As the result of multiple reduction and butylation, spin-lattice relaxation processes in simple coal aromatic
units were fastened. The narrow Lorentz lines of both 4 times reduced and reductively butylated coal were saturated and the
spin-lattice relaxation time increased. 相似文献
3.
M. G. Tyazhlov A. I. Filin A. V. Larionov V. D. Kulakovskii D. R. Yakovlev A. Waag G. Landwehr 《Journal of Experimental and Theoretical Physics》1997,85(4):784-796
Spin relaxation of Mn ions in a Cd0.97Mn0.03Te/Cd0.75Mg0.25Te quantum well with photogenerated quasi-two-dimensional electron-hole plasma at liquid helium temperatures in an external
magnetic field has been investigated. Heating of Mn ions by photogenerated carriers due to spin and energy exchange between
the hot electron-hole plasma and Mn ions through direct sd-interaction between electron and Mn spins has been detected. This process has a short characteristic time of about 4 ns,
which leads to appreciable heating of the Mn spin subsystem in about 0.5 ns. Even under uniform excitation of a dense electron-hole
plasma, the Mn heating is spatially nonuniform, and leads to formation of spin domains in the quantum well magnetic subsystem.
The relaxation time of spin domains after pulsed excitation is measured to be about 70 ns. Energy relaxation of excitons in
the random exchange potential due to spin domains results from exciton diffusion in magnetic field B=14 T with a characteristic time of 1 to 4 ns. The relaxation time decreases with decreasing optical pump power, which indicates
smaller dimensions of spin domains. In weak magnetic fields (B=2 T) a slow down in the exciton diffusion to 15 ns has been detected. This slow down is due to exciton binding to neutral
donors (formation of bound excitons) and smaller spin domain amplitudes in low magnetic fields. The optically determined spin-lattice
relaxation time of Mn ions in a magnetic field of 14 T is 270±10 and 16±7 ns for Mn concentrations of 3% and 12%, respectively.
Zh. éksp. Teor. Fiz. 112, 1440–1463 (October 1997) 相似文献
4.
New features of the dependence of the average travel distance l of dislocations on the magnetic field B have been found in an investigation of the magnetostimulated dislocation mobility in LiF crystals: A transition has been
found from ordinary proportionality l∝B
2 to saturation l≈ const in high fields B. It is shown that the experimental points can be described satisfactorily by the theoretical dependence l∝ [(B
0/B)2+1]−1 (B
0≈0.8 T), typical for the mechanism of longitudinal spin relaxation in a system of radical pairs, which are supposedly formed
when dislocation nuclei interact with paramagnetic impurity centers. According to the theory, the level of the field B
0 is determined by the characteristic frequency of the oscillations of the internal fields in the lattice, which for B
0∼1 T is of the order of 1011s−1, which corresponds to the typical frequency of characteristic oscillations of dislocation segments between pinning centers,
which, naturally, does not depend on temperature. This in turn explains the fact that the measured values of B
0 are the same at 293 K and 77 K.
Pis’ma Zh. éksp. Teor. Fiz. 70, No. 11, 749–753 (10 December 1999) 相似文献
5.
V. G. Orlov G. S. Sergeev Tetsuo Asaji E. A. Kravchenko Yu. F. Kargin 《Journal of Experimental and Theoretical Physics》2010,110(2):296-300
The nuclear quadrupole spin-lattice relaxation was studied in the range 4.2–300 K for single crystals of Bi4Ge3O12 doped with minor amounts (the tenth fractions of mol%) of paramagnetic atoms of Cr, Nd, and Gd. Unusual spin dynamic features
were recently found for these crystals at room temperature: a dramatic (up to 8-fold) increase in the effective nuclear quadrupole
spin-spin relaxation time T
2* occurred upon doping the pure Bi4Ge3O12 sample. Unlike T
2*, the effective spin-lattice relaxation time T
1* at room temperature differs insignificantly for both doped and pure samples. But at lower temperatures, the samples exhibit
considerably different behavior of the spin-lattice relaxation with temperature, which is caused by different contributions
to the relaxation process of the dopant paramagnetic atoms. The distinctive maximum in the temperature dependence of the spin-lattice
relaxation time for the Nd-doped crystal is shown to result from the crystal electric field effects. 相似文献
6.
R. L. Lichti T. R. Adams D. W. Cooke R. S. Kwok C. Boekema J. C. Lam D. E. Farrell N. Bansal 《Hyperfine Interactions》1991,63(1-4):73-79
Muon spin relaxation (μSR) data taken at LAMPF on ac-axis oriented fine powder sample of YBa2Cu3O7−δ (YBCO) embedded in epoxy are analyzed for relaxation rate anisotropy. Clear differences beyond simple magnetic field penetration
depth anisotropy are observed forB‖c andB
c. The low-temperature anisotropy ratio is consistent with oriented ceramic data. Small crystallite size and anisotropic flux
pinning characteristics are suggested as the fundamental cause of the additional effects. 相似文献
7.
S. A. Lekomtsev K. N. Mikhalev A. Yu. Yakubovskiĭ A. R. Kaul 《Journal of Experimental and Theoretical Physics》2006,102(4):671-676
The spin-lattice and spin-spin relaxation times of 139La are measured in manganite LaMnO3. Analysis of the frequency dependence of the spin-lattice relaxation rate in the paramagnetic temperature range shows that this quantity is determined by magnetic fluctuations. The magnitude of the fluctuating field is estimated. It is shown that the correlation time for spin fluctuations varies with temperature in accordance with the Arrhenius law. The high value of the spin-spin relaxation rate in the paramagnetic region can be due to strong anisotropy of fluctuating magnetic fields at La nuclei. 相似文献
8.
G. G. Fedoruk 《Physics of the Solid State》2004,46(9):1631-1636
The transient nutation of states dressed by a microwave field in a two-level system (E′1 centers in a quartz crystal) is observed in pulsed electron paramagnetic resonance (EPR) in the course of an additional pulse of a linearly polarized radio-frequency (rf) field that has an amplitude 2B2 and is applied parallel to the static magnetic field. It is shown that, when the frequency of the rf field coincides with the frequency of nutation of the bare spin system, the signal of this nutation is modulated by the nutation of dressed states at the frequency ω2=γB2, where γ is the electron gyromagnetic ratio. The decay time of nutation of dressed states is considerably (no less than four times) longer than that of bare states of E′1 centers due to spin-spin relaxation and correlates with the spin-lattice relaxation time in the rotating coordinate system. 相似文献
9.
Selective single and double quantum excitation pulsed NMRON has been utilized to obtain rotation patterns, free induction
decays, and spin echoes in antiferromagnetic54Mn−MnCl2·4H2O and the quasi-2 dimensional ferromagnet54Mn−Mn(COOCH3)2·4H2O. The pulsed technique is well suited to observing fast spin-lattice relaxation and T1 values down to 100 ms have been measured in these systems. These short times may make magnetic insulators viable hosts for
on-line experiments. The dependence on field and temperature of the54Mn spin-lattice relaxation time T1 has been investigated and a T1 minimum at high fields observed in both systems. A spin-spin relaxation time T2≈50 μs is measured in54Mn−MnCl2·4H2O. The observation of NMRON in the paramagnetic phase of MnCl2·4H2O allows the resonant frequencies in the antiferromagnetic and paramagnetic phases to be compared, yielding a value for the
zero point spin deviation in the former phase. 相似文献
10.
Si dangling bond centers in aerosol particles of amorphous hydrogenated silicon formed by thermal decomposition of SiH4 in Ar were studied by pulsed electron paramagnetic resonance. The hole-burning and inversion-recovery experiments demonstrate that large-scale rapid spectral diffusion takes place in the samples with high spin concentration. Correlation times τc of the spectral diffusion and spin-lattice relaxation timesT 1 were obtained in the temperature range between 77 and 290 K. Above 130 K, τc andT 1 are proportional one to the other. The unusual feature of this spectral diffusion is that the shape of the central part of the spectral hole does not change when the delay time increases. The other paramagnetic centers previously investigated showed a remarkable change of the hole shape which was induced by modulation of dipolar interaction due to spin flips. It is suggested that the observed anomaly in the Si dangling bond centers arises due to cooperative spin flips. 相似文献
11.
V. I. Al’shits N. N. Bekkauer A. E. Smirnov A. A. Urusovskaya 《Journal of Experimental and Theoretical Physics》1999,88(3):523-526
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) 相似文献
12.
Pulsed NMRON, CW NMRON and thermal NMR-NO methods have been utilized to study54Mn-MnCl2 · 4H2O. The54Mn spin-lattice relaxation timeT
1 in zero applied field has been measured between 35 and 90 mK in the antiferromagnetic phase. Above 65 mK the dominant relaxation
mechanism is a Raman process with the electronic magnons, but at lower temperatures a direct process takes over. NMRON has
been observed for the first time in the paramagnetic phase, and a line width of 300 kHz, with both homogeneous and inhomogeneous
contributions, is observed. In the antiferromagnetic phase the line width is 35 kHz, and there are also homogeneous and inhomogeneous
contributions. The dependence ofT
1 for the54Mn spins on field and temperature was studied in the paramagnetic phase. AT
1 minimum centred atB
0=2.64 T was observed. The hyperfine parameter <54
AS>/h=−513.6(3) MHz in the paramagnetic phase, and comparison with the value in the antiferromagnetic phase gives 0.013(1) for
the zero point spin deviation. 相似文献
13.
B. Z. Malkin M. V. Vanyunin M. J. Graf J. Lago F. Borsa A. Lascialfari A. M. Tkachuk B. Barbara 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,66(2):155-163
Temperature and magnetic field dependences of the 19F
nuclear spin-lattice relaxation in a single crystal of LiYF4 doped
with holmium are described by an approach based on a detailed consideration
of the magnetic dipole-dipole interactions between nuclei and impurity
paramagnetic ions and nuclear spin diffusion processes. The observed
non-exponential long time recovery of the nuclear magnetization after
saturation at intermediate temperatures is in agreement with predictions of
the spin-diffusion theory in a case of the diffusion limited relaxation. At
avoided level crossings in the spectrum of electron-nuclear states of Ho3
+ ions, rates of nuclear spin-lattice relaxation increase due to
quasi-resonant energy exchange between nuclei and paramagnetic ions in
contrast to the predominant role played by electronic cross-relaxation
processes in the low-frequency ac-susceptibility. 相似文献
14.
Nuclear magnetic resonance of cobalt metal was investigated in the paramagnetic and ferromagnetic states and in the critical region below Tc. The Knight shift and spin lattice relaxation times were measured in the paramagnetic phase in the solid and liquid states from 1578 K to 1825 K. The resonant frequency, spin-lattice and spin-spin relaxation times were measured in the ferromagnetic phase from room temperature to 1385 K. The main part of (T1T)-1 results from fluctuating orbital moments in both phases except near Tc where this process forms the background for critical spin relaxation. The critical exponents for T-11 and for the magnetization in the ferromagnetic state were found to be n' = 0.96 ± 0.07 and β = 0.308 ± 0.012, respectively. 相似文献
15.
It is found experimentally that the threshold magnetic field B
c for the magnetoplastic effect, i.e., the field at which the depinning of dislocations from paramagnetic impurities in an
external magnetic field begins to be observed, increases with increasing concentration C of Ca impurity in NaCl crystals in the range C=(0.5–100) ppm. It is shown that the dependence B
c(C) exhibits a distinct tendency toward saturation. The physical interpretation of the observed dependence rests on the notion
that as the impurity concentration C increases, the average size of the impurity complexes increases and, accordingly, the local atomic configuration around the
impurity atoms changes according to a definite pattern. In particular, the average number
of cation vacancies among the nearest neighbors increases from 1 to 6 as the number N of Ca atoms in the complex increases, and this trend, in turn, should cause the thermal vibration amplitude of the Ca atoms
to increase. In other words, the phenomenon in question appears to be physically analogous in its microscopic mechanisms to
the previously observed increase of B
c with increasing temperature. The proposed interpretation is further supported by good correlation of the experimental dependence
B
c(C) with the calculated function
.
Fiz. Tverd. Tela (St. Petersburg) 40, 81–84 (January 1998) 相似文献
16.
Korb JP Diakova G Goddard Y Bryant RG 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2007,186(2):176-181
Proton spin-lattice relaxation by paramagnetic centers may be dramatically enhanced if the paramagnetic center is rotationally immobilized in the magnetic field. The details of the relaxation mechanism are different from those appropriate to solutions of paramagnetic relaxation agents. We report here large enhancements in the proton spin-lattice relaxation rate constants associated with organic radicals when the radical system is rigidly connected with a rotationally immobilized macromolecular matrix such as a dry protein or a cross-linked protein gel. The paramagnetic contribution to the protein-proton population is direct and distributed internally among the protein protons by efficient spin diffusion. In the case of a cross-linked-protein gel, the paramagnetic effects are carried to the water spins indirectly by chemical exchange mechanisms involving water molecule exchange with rare long-lived water molecule binding sites on the immobilized protein and proton exchange. The dramatic increase in the efficiency of spin relaxation by organic radicals compared with metal systems at low magnetic field strengths results because the electron relaxation time of the radical is orders of magnitude larger than that for metal systems. This gain in relaxation efficiency provides completely new opportunities for the design of spin-lattice relaxation based contrast agents in magnetic imaging and also provides new ways to examine intramolecular protein dynamics. 相似文献
17.
N. G. Fazleyev 《Hyperfine Interactions》1990,60(1-4):691-694
The magnetic resonance lineshape of paramagnetic ion-nuclei in metals is calculated using the temperature Green functions
method and is analyzed for limiting cases of fast and slow spin lattice relaxation of localized moments. The longitudinal
spin lattice relaxation rate for paramagnetic ion-nuclei in type II superconductors due to the hyperfine coupling with local
moments is calculated. The influence of the fluctuation coupling of electrons on relaxation of paramagnetic ion-nuclei in
“dirty” type II superconductors is investigated in magnetic field slightly above the upper critical field Hc2. 相似文献
18.
V. A. Atsarkin V. V. Demidov O. A. Ryabushkin G. A. Vasneva 《Applied magnetic resonance》1994,7(2-3):195-208
The experimental works described are performed by the authors over last two decades by means of the LFD EPR technique. The essence of this method is low-frequency detection of the longitudinal spin magnetization while the magnetic resonance is excited by a strong microwave field. The first kind of LFD EPR is the enchanced longitudinal susceptibility effect (ELSE) which has been elaborated and applied to study spin thermodynamics in solids since 1972. Various applications of ELSE are described such as direct measuring of the spin-spin interaction temperatureT ss in the course of resonance saturation, spin-lattice and cross relaxation, dynamic nuclear polarization etc. Another version of LFD EPR was employed to study electron spin-lattice relaxation of paramagnetic centers in high-temperature superconductors (HTSC). Recent experimental data are presented on the electron spin-lattice relaxation of Cu2+ ions in YBa2Cu3O6+x at various temperatures andx values. 相似文献
19.
Zero field μSR has been used to probe rare earth spin dynamics in the magnetic superconductors, Y1−x
Er
x
Ni2B2C. The muon spin relaxation function is stretched exponential, exp (−(λt)β), in form, as usually found for spin glass systems above the glass temperature. However, the Y1−x
Er
x
Ni2B2C compounds show no evidence of coexisting superconducting and static spin glass ground states even at concentrations below
the critical value (x=0.6) for long range antiferromagnetic order. The temperature dependence of both the muon spin relaxation rate λ and the exponent
β suggests that Er spin dynamics change significantly at the superconducting transition temperature.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
20.
E. Torikai K. Nagamine K. Nishiyama E. Hirose Y. Ikedo T. Hashimoto P. Birrer I. Tanaka H. Kojima S. Srinivas T.P. Das S. Maekawa K. Yamada S. Hosoya Y. Endoh 《Hyperfine Interactions》1997,105(1-4):175-179
The paramagnetic (μ-O) state formed at the oxygen site in high Tc LaSrCuO was used to probe an interaction between the localized moment of the paramagnetic electron and the conduction electrons
via zero field μ- spin relaxation. The enhanced relaxation rates consistently observed in the superconducting state of various Sr concentrations
are explained as an effect of spin‐pairing in the high Tc supercurrent.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献