The relaxation rate of the magnetic moment of a shallow acceptor center as a function of impurity concentration in silicon

A study is made into the temperature dependence of residual polarization of negative muons in crystalline silicon with the concentration of impurity of the n-and p-types ranging from 8.7×10¹³ to 4.1× 10¹⁸ cm^?3. The measurements are performed in a magnetic field of 1000 G transverse to the muon spin, in the temperature range from 4.2 to 300 K. The form of the temperature dependence of the relaxation rate v of the magnetic moment of the μAl⁰ acceptor in silicon is determined. For a nondegenerate semiconductor, the relaxation rate depends on temperature as v ∝ T ^q (q ≈ 3). A variation in the behavior of the temperature dependence and a multiple increase in the relaxation rate are observed in the range of impurity concentration in excess of 10¹⁸ cm^?3. The importance of phonon scattering and spin-exchange scattering of free charge carriers by an acceptor from the standpoint of relaxation of the acceptor magnetic moment is discussed. The constant of hyperfine interaction in an acceptor center formed by an atom of aluminum in silicon is estimated for the first time: |A _hf (Al)/2π| ～ 2.5×10⁶s^?1.     

Nuclear Spin relaxation mediated by Fermi-edge electrons in n-type GaAs

A method based on the optical orientation technique was developed to measure the nuclear-spin lattice relaxation time T ₁ in semiconductors. It was applied to bulk n-type GaAs, where T ₁ was measured after switching off the optical excitation in magnetic fields from 400 to 1200 G at low (< 30 K) temperatures. The spin-lattice relaxation of nuclei in the studied sample with n _D = 9 × 10¹⁶ cm^?3 was found to be determined by hyperfine scattering of itinerant electrons (Korringa mechanism) which predicts invariability of T ₁ with the change in magnetic field and linear dependence of the relaxation rate on temperature. This result extends the experimentally verified applicability of the Korringa relaxation law in degenerate semiconductors, previously studied in strong magnetic fields (several Tesla), to the moderate field range.     

Viscous flow of magnetic vortices and rapid relaxation of magnetization in YBaCuO ceramics

The viscous flow of magnetic vortices in granular YBaCuO ceramics of various granule sizes is investigated using the microwave absorption method (f=80 MHz) in the range of varying magnetic fields H=±1500 Oe and of temperatures T=77 to 95 K. The conditions for the formation and melting of the vortex lattice, as well as the transition of the high-temperature superconductor (HTSC) state from the irreversibility region to the region of the viscous flow of magnetic vortices, are considered. The rapid magnetic relaxation times in the range τ=72 to 111 ms are determined from the magnetization relaxation in the HTSC materials under investigation depending on the granule size d, which varies from 1 to 30 μm. The experimental results on the variation of the relaxation time τ from 77 to 120 ms in a ceramic exposed to fast neutrons with various fluences (Φ=10¹⁶ to 10¹⁹ cm⁻²) are considered. The depinning process is analyzed; it occurs in the form of a thermally assisted flux flow in the range of low activation energies. __________ Translated from Fizika Tverdogo Tela, Vol. 43, No. 6, 2001, pp. 968–973. Original Russian Text Copyright ? 2001 by Chashchin.     

Relaxation phenomena and nuclear magnetic resonance of116In(T 1/2=14s) produced by capture of polarized neutrons in the indium III–V compounds

Polarized¹¹⁶In nuclei have been produced by capture of polarized thermal neutrons in several In compounds. At temperaturesT below 77 °K and magnetic field strengthsH ₀ of several kOe, asymmetries of a few percent of the β^? decay of the¹¹⁶In ground state could be observed in polycrystalline InP, InAs and InSb, thus indicating the nuclear polarization. Nuclear magnetic resonance signals have been measured with the result for the magnetic moment μ _i (¹¹⁶In)=2.7723 (10) nm (uncorrected). β^? decay asymmetry and spin lattice relaxation timeT ₁ have been studied as a function ofH ₀ andT. The effect ofH ₀ is to decouple the hyperfine interaction caused by the capture-γ recoil process. However,H ₀ has no influence uponT ₁, which demonstrates the absence of nuclear relaxation due to paramagnetic impurities.T ₁ is determined by quadrupolar relaxation. A quadrupole momentQ(¹¹⁶In)=0.09 (2) b was calculated by comparison of the¹¹⁶In relaxation rates with those of the stable¹¹⁵In isotope in the same compounds. Above 30 °K the temperature dependence of 1/T ₁ agrees with a recent theoretical investigation. Below 30 °K the relaxation rate shows an anomalous behaviour, which can be explained by resonance modes due to recoil lattice defects.     

Phase diagram of YBa2Cu3O7−y at T<T c according to transverse nuclear relaxation data for Cu(2)

Two peaks are observed at T=35 and 47 K in the transverse relaxation rate for Cu(2) nuclei in YBa₂Cu₃O_7?y . A comparison of the relaxation rates for isotopes ⁶³Cu(2) and ⁶⁵Cu(2) at T=47 K indicates the magnetic nature of relaxation. The enhancement of local magnetic field fluctuations perpendicular to CuO₂ planes at T=47 K is associated with critical fluctuations of orbital currents. The peak at T=35 K is attributed to the emergence of an inhomogeneous superconducting phase. The obtained experimental results and the available data from the literature made it possible to propose a qualitatively new phase diagram of the superconducting state.     

The ratio of the quadrupole moments of the isotopes95Mo and97Mo

The spin-lattice relaxation timesT ₁ of the NMR signals of⁹⁵Mo and⁹⁷Mo in aqueous K₂MoO₄ solutions were determined by the inversion recovery technique. To separate the relaxation rates due to electric quadrupole interaction and due to magnetic dipole interaction, pure H₂O and mixtures of H₂O and D₂O were used as solvent. No dependence of the ratio of the relaxation ratesT ₁(⁹⁵Mo)/T ₁(⁹⁷Mo) on the composition of the solvent was to be detected, i.e. the relaxation due to magnetic dipole interaction may be neglected. From the ratio of the relaxation rates the absolute value of the ratio of the quadrupole moments of the molybdenum isotopes was evaluated: ¦Q(⁹⁷Mo)/Q(⁹⁵Mo)¦ = 11.4 ±0.3.     

Non-equilibrium population and slow relaxation of Fe2+ in57Co:LiNbO3 and57Co:LiTaO3 single crystals

The Fe²⁺ fraction observed in the Mössbauer emission spectra of⁵⁷Co:LiNbO₃ and⁵⁷Co:LiTaO₃ exhibits both slow electronic relaxation and nonthermal populations of them _s sublevels of the⁵A_lg orbital singlet ground state at low temperatures (T<15 K) in high magnetic fields. The relaxation rates depend on temperature and on the angle ? between the magnetic field and the crystallographicc-axis.     

Side-hole to anti-hole conversion in time-resolved spectral hole burning of ruby: Long-lived spectral holes due to ground state level population storage

We report time-resolved transient spectral hole burning of Verneuil-grown 20 ppm and ca. 0.6 ppm ruby (Al₂O₃:Cr³⁺) in zero field and low magnetic fields B∥c at 4 K. The hole-burning spectroscopy of the 20 ppm sample implies relatively rapid cross relaxation in the ⁴A₂ ground state on the ∼1 ms timescale both in zero field and in low magnetic fields, B∥c, up to 0.2 T. In the 0.6 ppm sample, side-hole to anti-hole conversion is observed both in zero field and in low magnetic fields. This conversion is caused by population storage in ⁴A₂ ground state levels. Spin-lattice relaxation, on the 200 ms timescale, is directly observed from the time dependence of the resonant hole and anti holes in B∥c, consistent with a very low cross-relaxation rate. However, in zero field cross relaxation in the ⁴A₂ ground state is still a significant relaxation mechanism for the 0.6 ppm sample resulting in hole decay in ∼50 ms.     

Nuclear spin-lattice relaxation due to non-linear excitations in magnetic chains

A simple classical model for nuclear spin-lattice relaxation due to solitons in XY one-dimensional magnetic chains in an external magnetic field is discussed. The results show that one should expect opposite behavior for the H/T dependence of T^-1₁ for the ferromagnetic and the antiferromagnetic case.     

Magnetism of ferriprotoporphyrin IX monomers and dimers

The SQUID and the ⁵⁷Fe Mössbauer spectroscopy studies of the magnetic properties of monomeric and dimeric forms of iron porphyrin were performed between 2 and 305 K. The effective magnetic relaxation rate of the Fe atoms in iron porphyrin monomers exhibits complex temperature dependence, resulting from the competing spin-spin and spin-lattice relaxation processes. The dimerization of iron porphyrin dramatically speeds up the magnetic relaxation. The Fe-Fe antiferromagnetic exchange coupling constant in Fe-O-Fe dimer is J≈−110 cm⁻¹. The complementary application of SQUID and the Mössbauer spectroscopy is proposed as a new precise quantitative analytical methodology for monitoring of the aggregation process of iron porphyrin.     

Nuclear magnetic relaxation of <Superscript>3</Superscript>He in contact with an aerogel above the Fermi temperature

The spin kinetics of ³He in an aerogel has been studied above the Fermi temperature. The magnetic relaxation times T ₁ and T ₂ of adsorbed, gaseous, and liquid ³He in a 95% silica aerogel at a temperature of 1.5 K have been determined as functions of frequency by means of pulse nuclear magnetic resonance. It has been found that the time T ₁ is linear in frequency in all three cases, whereas T ₂ is independent of frequency. To explain the observed behavior of the longitudinal relaxation rate, a theoretical model of relaxation in the adsorbed layer of ³He taking into account the filamentary structure of the aerogel is proposed.     

Temperature dependence of the anomalous emission line intensities in57Co:LiTaO3-initial population and relaxation

Mössbauer spectra of⁵⁷Co: LiTaO₃ single crystals were recorded in an external longitudinal magnetic field of 6 T at different temperatures between 4.2 K and 150 K. The spectra were taken at two different orientations of the crystallographicc-axis relative to the magnetic field. The line intensities of the Fe³⁺-subspectra show a temperature dependent anomalous population of the three Kramers doublets of the Fe³⁺ spinS=5/2 ground state. A relaxation broadening is observed at higher temperatures, which cannot successfully be reproduced within a relaxation model taking into account only the six lowest lying electronic states.     

Spin relaxation parameters in recombining radical ion pair (diphenylsulfide-d10)+/(p-Terphenyl-d14)− obtained by OD ESR and quantum beats techniques

Parameters of paramagnetic relaxation were determined by OD ESR and quantum beats techniques for a recombining pair of radical ions (DPS-d₁₀)⁺/(PTP-d₁₄)^? inn-hexane, isooctane,cis-decalin, and squalane solutions. TheT ₂ relaxation time determined by quantum beats technique is independent of solvent viscosity and magnetic field strength in the range 170–9600 G. These data are in agreement with the results obtained by OD ESR technique assuming fastT ₁ relaxation for radical cation. Neglecting the contribution of radical anion relaxation, we obtainedT _1c=T _2c?50 ns for (DPS-d₁₀)⁺.     

Temperature dependence of the spin-lattice relaxation time for quadrupole nuclei under conditions of NMR line saturation

The contributions of different mechanisms of nuclear spin-lattice relaxation are experimentally separated for ⁶⁹Ga and ⁷¹Ga nuclei in GaAs crystals (nominally pure and doped with copper and chromium), ²³Na nuclei in a nominally pure NaCl crystal, and ²⁷Al nuclei in nominally pure and lightly chromium-doped Al₂O₃ crystals in the temperature range 80–300 K. The contribution of impurities to spin-lattice relaxation is separated under the condition of additional stationary saturation of the nuclear magnetic resonance (NMR) line in magnetic and electric resonance fields. It is demonstrated that, upon suppression of the impurity mechanism of spin-lattice relaxation, the temperature dependence of the spin-lattice relaxation time T₁ for GaAs and NaCl crystals is described within the model of two-phonon Raman processes in the Debye approximation, whereas the temperature dependence of T₁ for corundum crystals deviates from the theoretical curve for relaxation due to the spin-phonon interaction.     

Hole spin relaxation in Ge quantum dots

Hole spin relaxation in an isolated Ge quantum dot due to interaction with phonons is investigated. Spin relaxation in this case occurs through the mechanism of the modulation of the spin-orbit interaction by lattice vibrations. According to the calculations performed, the spin relaxation time due to direct single-phonon processes for the hole ground state equals 1.4 ms in the magnetic field H = 1 T at the temperature T = 4 K. The dependence of the relaxation time on the magnetic field is described by the power function H^?5. At higher temperatures, a substantial contribution to spin relaxation is made by two-phonon (Raman) processes. Because of this, the spin relaxation time decreases to nanoseconds as the temperature is raised to T = 20 K. Analysis of transition probabilities shows that the third and twelfth excited hole states, which are intermediate in two-step relaxation processes, play the main part in Raman processes.     

Spin autocorrelation of Ni in K2Mn0.975Ni0.025F4 studied by nuclear spin-lattice relaxation

From the nuclear spin-lattice relaxation of the out-of-layer ¹⁹F nuclei in magnetic fields perpendicular to the c-axis the low-frequency component of the autocorrelation function 〈S^z(t)S^z(O)〉 of Ni in ordered K₂Mn_0.975Ni_0.025F₄ is found to be substantially reduced relative to the Mn host. The experimental rates vs temperature are in accord with those for relaxation involving two spin excitations calculated with local Green's functions.     

A pulse-resonance method for the determination of frequency-dependent magnetic susceptibilities in the microwave region

A simple set-up for the determination of non-diagnonal elements of the magnetic susceptibility tensor in the microwave region is given. The change of the magnetization during microwave absorption (MMA) is detected and leads directly to the susceptibilityχ′ _yx (ω). The method is applicable to magnetic systems having longitudinal relaxation times larger than 10^?6 sec. The MMA-method enables one to decide whether the sample is para-, ferro- or antiferromagnetic and from this magnetic transition temperatures can be measured.     

NQR investigations of high-T c superconducting oxides

Nuclear quadrupole resonance (NQR) of⁶³Cu has been performed to study the electronic and magnetic properties of YBa₂Cu₃O_6.1 ceramics, treated in iodine or bromine vapours. An analysis of the NQR shows undistorted CuO₂ planes and the absence of an increase of the oxygen content in chains of Cu(1)-O in halogen treated Y-Ba-Cu-O ceramics.¹³⁹La NQR spin-lattice relaxation measurements in powdered La_2?x CuO_4?y compounds are also reported. The present results indicate that the¹³⁹La nuclear relaxation is dominated by the Cu⁺⁺-Cu⁺⁺ magnetic correlations.     

Magnetic hyperfine interaction of 59Fe in nickel

Nuclear magnetic resonance on oriented nuclei (NMR-ON) on ⁵⁹Fe isotope in Ni was performed. The magnetic hyperfine splitting frequency of was determined to be ν(B ₀?=?0)?=?48.32 (2) MHz. Using the known magnetic moment the magnetic hyperfine field was deduced as B _HF?=???28.32 (5) T. The effective nuclear spin-lattice relaxation time was also measured. The measured value is compared with experimental values of 3d-impurity in nickel host.     

Magnetic properties of CuZn ferrite investigated by Mössbauer spectroscopy

The magnetic properties of the Zn_xCu_1?xFe₂O₄ ferrite system (x = 0 to 1) have been investigated by means of Mössbauer Spectroscopy. Mössbauer spectra of the CuZn ferrite system, taken at room temperature for x = 0.0 to 0.4 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to the Fe³⁺ octahedral ions (B-sites), while for x = 0.5 it shows relaxation behaviour and for x ? 0.6 it exhibits a paramagnetic quadrupole doublet. The systematic dependence of the isomer shifts, quadrupole interactions and nuclear magnetic fields of ⁵⁷Fe³⁺ ions in both tetrahedral and octahedral sites has been determined as a function of zinc content. The variation of nuclear magnetic fields at the A and B sites are explained on the basis of A-B and B-B supertransferred hyperfine interactions. Analysis of the relaxation spectrum observed at x = 0.5 (300 K) suggests that the relaxation mechanism is due to domain wall oscillations.