Muonium quantum diffusion and spin dynamics in solid xenon

We present measurements of the transverse (T ₂ ^–1 ) and longitudinal (T ₁ ^–1 ) spin relaxation rates of muonium (Mu) atoms in solid natural xenon (n-Xe) as well as pure¹³⁶Xe (which has no nuclear moments). The temperature dependences ofT ₂ ^–1 andT ₁ ^–1 in natural Xe belowT 115 K demonstrate the quantum character of Mu diffusion governed by one-phonon interactions. Taking into account both the polaron effect (PE) and the effect of fluctuational preparation of the barrier (FPB) makes it possible to consistently describe Mu diffusion in Xe. Mu spin relaxation in¹³⁶Xe at high temperatures is not due to nuclear hyperfine (NHF) interactions.     

Nuclear magnetic resonance in icosahedral Al-Pd-Mn alloys

²⁷Al and⁵⁵Mn nuclear magnetic resonance shift,K, and²⁷Al spin lattice relaxation time,T ₁, have been measured for the six-dimensional face-centered icosahedral quasicrystals, Al_75-x Pd₁₅Mn_10+x withx=0, 2 and7. The Al₇₅Pd₁₅Mn₁₀ quasicrystal exhibits a temperature independent Knight shiftK and(T ₁ T)^–1=0.022±0.002 (K s)^–1 in a temperature range from room temperature to 5 K because there exist no Mn atoms with local magnetic moment. The replacement of Al with Mn drastically decreases the²⁷AlK, the⁵⁵MnK andT ₁ of²⁷Al, andthe²⁷AlK becomes negative. There is an additional contribution to the spin lattice relaxation time independent of temperature. This is considered to be due to the presence of a localized magnetic moment in the replaced Mn atoms.     

Magnetic behavior of amorphous Sm-Co alloy films

Amorphous films of Sm_100–x Co _x with 70x90 were made by vapor deposition on flat glass substrates kept at 300 K. The films crystallize above 700 K. The film plane is the easy plane with the planar anisotropy of the order of 10⁷ erg cm^–3, indicating a local anisotropy of the order 10⁸ erg cm^–3. With regard to local anisotropy and magnetization, the films appear to correspond to the random anisotropy concept of amorphous magnets. Films evaporated in a magnetic field parallel to the film plane have an induced uniaxial anisotropy of the order of 10⁶ erg cm^–3. Hysteresis loops in fields parallel to the easy axis are perfectly rectangular with coercive fieldsH _c in the range 30 Oe<H _c<3000 Oe, depending on composition, temperature and heat treatment. The magnetization reverses by a thermally activated domain nucleation and growth process having a narrow distribution of time constants. Aging at temperatures below the crystallization temperature reduces the anisotropies andH _c.     

Structural and magnetic properties of rapidly solidified (Ce0.2Fe0.8)1-x Al x ribbons determined by X-ray diffraction and Mössbauer spectrometry

(Ce_0.2Fe_0.8)_1-x Al _x (0 x 0.9) ribbons have been prepared by planar flow casting under an He atmosphere with a linear velocity of 29 m s^-1. Analyses of the ribbons by X-ray diffraction and⁵⁷Fe Mössbauer spectrometry in the temperature range 4–300 K show that all the ribbons are crystalline. With increasingx, the observed phases are Ce(Fe, Al)₂, Ce₂(Fe, Al)₁₇, CeFe_4+y,Al_8-y and the single fcc aluminium phase. For the different phases, the line intensities of the Mössbauer spectra agree with previous results on the preferential substitution sites for aluminium. A coherent hyperfine parameters set was deduced from fitting spectra in the temperature range 4–300 K.     

Photoelectric properties of Pb1−x−y Sn x Ge y Te:In epitaxial films

Pb_1–x–y Sn _x Ge _y Te:In epitaxial films are examined in a wide temperature interval and at various background fluxes. These films have high sensitivity to infrared radiation in the spectral range <20m. The lifetime depends exponentially on temperature and varies from several seconds at T=10 K to 10^–2 s at T=20 K. The two-electron model of Jahn-Teller centers is proposed to explain the results. Multielement photoresistors based on these films are fabricated and D*=1.7×10¹³ cm Hz^1/2 W^–1 at T=25 K is achieved. Noise of the photoresistors is independent of background flux when it varies from 10¹² cm^–2 s^–1 to 10¹⁸ cm^–2 s^–1. As compared with Si:Ga and Ge:Hg photoresistors, the responsitivity is several orders larger at the operating temperature 25–30 K.     

Effect of hydrogen doping on the magnetic properties of Gd2CuO4

We report the results of ac-susceptibility and dc-magnetization measurements for H_yGd₂CuO₄ (0y0.54). It is shown thatH doping lowers the weak ferromagnetic component in the material. The distinct hysteresis loops observed atT=77 K for both non- and hydrogenated samples change its shape withy. The magnetic ordering temperatures T _N ^Cu and T _N ^Gd , as determined from the temperature dependencies of ac-susceptibility, remain unchanged with sample's hydrogenation. This result seems to indicate that extra electrons are not doped onto the Cu-O planes of Gd₂CuO₄. The frequency dependencies ofx(, T) andx(, T) for bothy=0 andy=0.15 samples are analysed., The maximums ofx andx found at about 200K are considered in terms of susceptibility dependence on the spin-lattice relaxation time (). The anomalies in ac-susceptibility found recently in Gd₂CuO₄ atT _a=8 K andT _b=9.5 K decrease significantly withy. Results are discussed in the context of available data on 214T-type compounds.     

Investigation of the behaviour of the impurity atoms in Si by μ− SR-method

The dependence of the residual polarization of negative muons in p-type Si on temperature in the 4.2–270 K range has been investigated. Measurements were carried out in external magnetic field of 0.08 T transverse to the muon spin. The impurity concentration in the sample was 2 · 10¹³ cm^–3. Muon spin relaxation was observed at temperatures below 30 K. The relaxation rate atT=30 K is equal to 0.18±0.08s^–1. The relaxation rate grows with the decrease of temperature and at 4.2 K exceeds 30s^–1. The value of the residual polarization at zero timeP(t=0) is constant within the investigated temperature range.In the rangeT<30 K data on the relaxation rate are well described by the dependence =B·T^–q, whereq=2.75. Power dependence of may evidence the essential role of the phonon mechanism in the relaxation of the electron momentum of the acceptor center.The authors express their gratitude to V.B. Brudanin and I.A. Yutlandov for providing the sample, and to Yu.B. Gurov for advices.     

Mössbauer and magnetization studies on the ferromagnet Fe3−x Ru x Si

⁹⁹Ru and⁵⁷Fe Mössbauer spectroscopic studies were carried out on ternary intermetallic compounds containing ruthenium, Fe_3–x Ru _x Si, within the concentration range 0.1x1.5. Magnetization of the samples was also measured in the temperature range between 4 K and room temperature.⁹⁹Ru Mössbauer spectra ofx=0.5 and 1.0 were fitted satisfactorily with a broad component ofH _hf, the peak positions of which were 340 and 270 kOe, respectively.     

Rate of abstraction of hydrogen atoms from ethane by muonium

Thermal reaction rates for the gas-phase reaction Mu+C₂H₆MuH+C₂H₅ have been measured bySR over the temperature range 510–730 K. The usual Arrhenius expression,k=Aexp(–E _a /RT), fits the data well, giving parametersA=1.0×10^–9 cm³ molecule^–1 s^–1 andE _a =15.35 kcal/mol. The activation energyE _a is 5.5 kcal/mol higher than for the H atom variant of this reaction, indicating a marked difference in reaction dynamics. Preliminary analysis indicates a still greater difference between Mu and H for the corresponding CH₄ reaction.     

Critical thermodynamics of dysprosium ethyl sulphate, a pure dipolar Ising ferromagnet

Using a SQUID magnetometer the magnetization of the Ising ferromagnet dysprosium ethyl sulphate has been measured in the temperature range 0.015–3K and in external fields 0.004–30 Oe. The results atT0.4 K (precritical regime) confirm the pure dipolar nature of the magnetic interactions. Particular interest has been paid to the immediate neighbourhood of the critical pointT _c =118.17 mK andH=0 where the zero-field susceptibility(T) aboveT _c , the spontaneous magnetizationM _s (T), and the magnetization atT _c ,M _c (H), can be described by mean field laws modified by logarithmic singularities. The critical amplitudes, B, andD of, M _s andM _c , respectively, satisfy the relation (2/3B ²)^1/3=D. All these results agree with predictions of (i) the Larkin-Khmel'nitskii theory and (ii) exact solutions of the renormalization group equations for 3-dimensional dipolar Ising ferromagnets.     

Dielectric relaxation of mixed crystals of Rb1-x(NH4)xH2PO4 at microwave frequencies

Measurements of the clamped dielectric constant on mixed single crystals of Rb_1-x(NH₄)_xH₂PO₄,x=0.35, are reported up to 11 GHz. Between 24 and 60 K, the dielectric dispersion can be fitted to a Cole-Cole relaxation, the parameters of which indicate a temperature-dependent distribution of relaxation times consistent with Vogel-Fulcher freezing. Both the audio and the microwave measurements can be scaled up to 100 MHz, with a freezing temperature ofT _o8.4 K. In the GHz range, a relaxation process in addition to the low-frequency freezing mode contributes to the dielectric response.     

Heavy-fermion behavior and spin-glass freezing in Si-stabilized amorphous alloys based on UPt3

U-Pt-Si and U-(Pt, Pd)-Si alloys have been prepared in the amorphous state by splat-cooling. The average nearest-neighbor distance is the same as the U-Pt distance in UPt₃. The low-temperature behavior of these alloys (with composition (U_0.25Pt_0.75)_1–xSi_x, x=0.18, 0.20, 0.25; and (U_0.25(Pt_1–yPd_y)_{0.75 0.8}Si_0.2,y=0.05 and 0.1) has been studied by specific heatC, magnetization and resistivity measurements in the temperature range 1.5 K to 20 K and for magnetic fields up to 6 T. Some samples were investigated down to 0.1 K and the specific heat for one sample was measured in fields up to 22 T. With ₀ =C/T| _T0 between 0.25 and 0.35 J/moleUK², the alloys can be classified as heavy-fermion materials. As opposed to UPt₃, the alloys exhibit spin-glass behavior with a concomitant decrease of ₀. The magnetization exhibits typical spin-glass features (e.g., an irreversibility line). An inverse relationship between ₀ andT _max (temperature ofC/T maximum which is proportional to the spin-glass freezing temperature), irrespective of the Pd content, is observed. The behavior can be qualitatively described by a coexistence of heavy-fermion behavior and spin-glass freezing.     

A study on new phase transitions in Cs2CaCl4·2H2O single crystals by observation of Cs NMR

The ¹³³Cs 1/2→−1/2 spin-lattice relaxation rate, , and the spin-spin relaxation rate, , for a Cs₂CaCl₄·2H₂O single crystal have been measured in function of temperature. The dominant relaxation mechanism of this crystal over the whole temperature range investigated here proceeds via quadrupole interaction. The changes in the ¹³³Cs spin-lattice relaxation rate near 325 K (=T_c1) and 360 K (=T_c2) correspond to phase transitions in the crystal. The change in the spin-lattice relaxation rate at T_c1 is small because the crystal lattice does not change very much during this phase transition. The change in near T_c2 is due to the critical slowing down of the soft mode that typically occurs in structural phase transitions. The temperature dependence of the spin-lattice relaxation rate for this crystal has maximum values at about 240 K, which is attributable to the effect of molecular motion as described by Bloembergen-Purcell-Pound theory. The phase transition temperatures T_c1 and T_c2 obtained from the temperature dependence of the relaxation rate is also clear from data obtained using differential scanning calorimetry. Therefore, we know that previously unreported phase transitions occur at 325 and 360 K.     

Phases and phase transitions in the mixed molecular system (NaCN)1−x(KCN)x

The phase diagram of (NaCN)_1–x(KCN)_x was examined by neutron powder diffraction in the temperature range 5K T300 K. Several non-cubic low-temperature phases were identified for concentrationsx<x _c1=0.15 andxx _c2=0.89. Lattice parameters and ferroelastic deformations were determined from the observed powder patterns. The phase transformations were characterized following the temperature dependence of the appropriate order parameters.     

Diffusion of Au in amorphous Zr x Ni100−x alloys studied by Rutherford Backscattering Spectrometry

The diffusion coefficients (D) of Au in three binary amorphous Zr _x Ni_100–x (x=61, 65, and 67) alloys were measured in the temperature range 549–623 K using the technique of the Rutherford Backscattering Spectrometry (RBS). The D values were found to lie in the range 1.0×10^–21–9.0×10^–20 m²s^–1 for different alloys. The activation energy (Q) was calculated in each case on the basis of an observed Arrhenius temperature dependence of D. The activation energy was found to scale with the crystallization temperature (T _x) of the alloy. Other published measurements for Au diffusion in amorphous Zr-Ni alloys also appear to follow the scaling relation between Q and T _x.     

Heterodiffusion of Cr in molten Fe in the temperature range 1800 to 1970 K

Heterodiffusion of Cr has been studied using the method of thin layer and the radionuclide⁵¹Cr. The diffusion characteristics determined from the experimental results in the temperature range 1800 KT1970 K areD _o=1·59×10^–2 cm²/s andE=22·3±1·6 kcal/mol. The experimental method is discussed in detail and the results are compared with those of other authors.     

Low-temperature specific heat of orientational glasses

This review summarizes specific heat data measured at low temperatures (T<1 K) on orientational glasses. Three species of mixed molecular crystals exhibiting orientational disorder are considered, namely (KBr)_1–x (KCN) _x , (NaCN)_1–x (KCN) _x (Rb)_1–x (NH₄) _x H₂PO₄. For intermediate concentrations of the anisotropic components, glass-like excitations have been observed. It is demonstrated that with respect to thermal properties, orientational disorder leads to the same universal behaviours than for structural disorder, i.e. a specific heat which varies below 1 K and for times 10^–4 s–10 s as:C _p(T,t)T ¹×ln(t). The variation of the glass-like anomaly with compositional disorder is also discussed. It is concluded that the low-energy excitations are related to orientational degrees of freedom which are frozen at low temperatures but still perform reorientations via quantum tunneling motions.     

NMR enhancement and NMR relaxation in a <110> FeSi Néel-type single crystal

⁵⁷Fe NMR measurements with a Néel-type single crystal are described. The behaviour of the magnetic structure was calculated as function of the internal fieldH _i. A clear distinction between wall and domain signals could be made. The corresponding relative enhancement factors _w ^r (H _i) and _D ^r (H _i) have been calculated and are in agreement with the measurements. The relaxation in the 90° Bloch walls is pure exponential. Both ratesT ₁ ^–1 andT ₂ ^–1 are proportional to _w ² (H _i). The measurements make proof, that the 90° walls are folded inside the crystal.K.-H. König and H. Brömer, Verhandl. DPG(VI)22, I, AM-14.11 (1987)     

The theory of neutron scattering from mixed harmonic solids

In the intermediate valent cerium alloys Ce_1–xY_xPd₃, Ce(Pd_1–xRh_x)₃ and Ce(Pd_1–xAg_x)₃ the Gd³⁺-ESR shows a non-linear increase of the linewidth H(T) in the temperature range 4.2KT300K. The deviation from the linear Korringa law can be described by a reduced conduction electron density of states at the Gd site (which is a Ce lattice site) in the energy range E _4f (=width of the Ce 4f states). This supposition allows a determination of E _4f from the H(T)-data. For CePd₃:Gd³⁺ we find E _4f=(650±100) K. E _4f increases with Y- and Rh-concentration and decreases with Ag-concentration.     

1H NMR study of the α phase of Mg2NiHx system

Hydrogen behavior in the α phase of Mg₂NiH_x system was studied by ¹H NMR. ¹H NMR spectra and spin-lattice relaxation times, T₁ and T_1ρ, of Mg₂NiH_0.22 were measured in the temperature range between 100 and 480 K. The drastic change in the linewidth is observed between 170 and 340 K, and ¹H rigid lattice is observed below 170 K, from which it is deduced that the hydrogen atoms are randomly distributed in α-Mg₂NiH_x. The relaxation mechanism for t₁ is the paramagnetic one, while the T_1ρ value is determined partially by hydrogen diffusion. The hydrogen diffusion rate has been determined from the linewidth and the T_1ρ value. The paramagnetic relaxations observed in T₁ and T_1ρ have been discussed relating to the hydrogen diffusion.