Investigation of the reaction 35Cl(τ, α)34Cl at Eτ = 15 MeV

The ³⁵Cl(τ, α)³⁴C reaction has been used to study the properties of ³⁴Cl levels up to an excitation energy of 5 MeV. Angular distributions of 37 levels were measured with a split-pole magnetic spectrograph, at a bombarding energy of 15 MeV. New levels have been found at 3847, 3964, 4206, 4321 and 4715 keV, all ± 10 keV. There is a strongly excited multiplet at E_x = 5.0 MeV with components at 4939 ± 11, 4958 ± 11, 4971 ± 11, 4998 ± 12 and 5010 ± 13 keV. A DWBA analysis of the α-particle angular distributions yielded l_n values and spectroscopic factors. New spin and parity assignments were obtained. The T = 1 character of the levels at E_x = 4.21 and 4.72 MeV has been determined. Experimental spectroscopic factors are compared with results from recent many-particle shell-model calculations.     

Resonant structure observed in the 3He(τ, γ)6Be capture reaction

The ³He(τ,γ)⁶Be capture reaction has been studied for ³He bombarding energies from 12 to 27, MeV. Transitions to the first excited state in ⁶Be(T = 1, 2⁺) are readily seen. Transitions to the ground state in ⁶Be (T = 1, 0⁺) are very weak and their presence could not be ascertained. The 90° excitation function for these transitions shows a broad maximum centered at E_τ = 23 ±1 MeV. This is interpreted as a resonance in the compound nucleus ⁶Be at E_x = 23.0±0.5 MeV with a configuration other than ³He+³He. These results are compared with other experimental work as well as with theoretical predictions.     

Pressure effects on the magnetic transition temperatures of NiS2

Both antiferromagnetic (T_N) and weak ferromagnetic (T_c) transition temperatures of NiS₂ have been measured under pressure up to 18.4 kbar. Values of dT_N/dP=(0.9 ± 0.1) deg/kbar and dT_c/dP=(0.4 ± 0.1) deg/kbar are obtained. The present results allow estimates of the magnetic Grüneisen constant γ_m = - d ln T_N/d ln V of (26±5) and of the entropy change of (1.4±0.5) J/mol·deg at the first order transition temperature T_c.     

Magnetic transitions in K2MnCl4·2H2O

Results of (dM/dH) measurements on tetrahedral K₂MnCl₄·2H₂O as a function of temperature and magnetic field, are presented. An antiferromagnetic transition along the tetragonal axis is observed at T_N = (3.05±0.05) K. The H-T magnetic phase diagram was completely determined, and shows the usual characteristics of that of a low anisotropy antiferromagnet. The T = 0 critical fields are compatible with the values H_E = (29.2±0.3) kOe and H_A = (5.9±0.6) kOe for the exchange and anisotropy fields.     

Spectroscopic investigation of rare-earth chromium borates RCr3(BO3)4 (R = Nd, Sm)

The optical absorption spectra of single crystals of neodymium and samarium chromium borates are studied. A magnetic phase transition in NdCr₃(BO₃)₄ (at T _c = 8 ± 1 K) and SmCr₃(BO₃)₄ (at T _c = 5 ± 1 K) is observed. The effective magnetic field acting on the Nd³⁺ ion from the side of the ordered magnetic subsystem of chromium is estimated.     

Precision lifetime measurements with the coincident high-velocity DSA method in inverse reactions

Mean lives of low-lying states of ¹⁸O, ³⁰Si, ³³S and ³⁴S have been measured with the Dopplershift attenuation method by heavy-ion bombardment of ²H and ³H targets. The γ-ray Doppler patterns are observed with a large Ge(Li) detector at 0° in coincidence with protons around 180°, so that the experiments are performed with high-velocity, mono-energetic and unidirectional beams of nuclei in well-defined excited states. Interpretation of the γ-ray patterns measured with Mg, Al, Cu and Ag as slowing-down material, with accurately known experimental stopping powers yields consistent results with small errors: ¹⁸O, τ_nm (1.98 MeV) = 2790 ± 110 fs; ³⁰Si, τ_m(2.24) = 351 ± 19 fs and τ_m(3.50) = 89 ± 8 fs; ³³S, τ_m(0.84) = 1650 ± 50 fs and τ_m(2.31) = 206 ± 8 fs; ³⁴S, τ_m(2.13) = 470 + 20 fs and τ_m(3.30) = 196 ± 13 fs. The deduced transition strengths are compared with results from large-scale shell-model calculations.     

Neutron diffraction study of magnetic ordering in Tm0.5Eu0.5Se

A single crystal of the magnetic semiconductor Tm_0.5Eu_0.5Se was studied by means of neutron diffraction in the temperature range from 1.8 to 293 K. Long-range magnetic order is detected at temperatures below T_c = (18.5±1) K. The measured ferromagnetic moment component of (2.12±0.05) μ_B per rare-earth ion at saturation in zero external magnetic field indicates approximately antiparallel alignment of Tm moment and Eu spin (mutual angle 134°). The experimentally determined neutron magnetic form factor confirms the divalent state of both Tm and Eu in Tm_0.5Eu_0.5Se.     

Measurement of the D0, D+ and Ds+ meson lifetimes at √s = 10.58 GeV

We have measured the lifetimes of the D⁰, D⁺ and D_s⁺ mesons with data from the CLEO detector. We find τ_D⁰ = (5.0 ± 0.7 ± 0.4) × 10⁻¹³s, τ_D⁺ = (11.4 ± 1.6 ± 0.7) × 10⁻¹³s and τ_Ds⁺ = (4.7 ± 2.2 ± 0.5) × 10⁻¹³s, giving lifetime ratios τ_D⁺/τ_D⁰ = 2.3 ± 0.5 and τ_Ds⁺/τ_D⁰ = 0.9 ± 0.5.     

Magnetostructural phase transitions in NiO and MnO: Neutron diffraction data

Structural and magnetic phase transitions in NiO and MnO antiferromagnets have been studied by high-precision neutron diffraction. The experiments have been performed on a high-resolution Fourier diffractometer (pulsed reactor IBR-2), which has the record resolution for the interplanar distance and a high intensity in the region of large interplanar distances; as a result, the characteristics of both transitions have been determined simultaneously. It has been shown that the structural and magnetic transitions in MnO occur synchronously and their temperatures coincide within the experimental errors: T_str ≈ T_mag ≈ (119 ± 1) K. The measurements for NiO have been performed with powders with different average sizes of crystallites (~1500 nm and ~138 nm). It has been found that the transition temperatures differ by ~50 K: T_str = (471 ± 3) K, T_mag = (523 ± 2) K. It has been argued that a unified mechanism of the “unsplit” magnetic and structural phase transition at a temperature of T_mag is implemented in MnO and NiO. Deviation from this scenario in the behavior of NiO is explained by the quantitative difference—a weak coupling between the magnetic and secondary structural order parameters.     

High-velocity DSA measurements in the 1–20 ps lifetime region: Comparison between DSA and RD

Mean lives in the range 1–20 ps of low-lying states of ¹³C, ¹⁶N, ²⁰O and ³⁶Cl have been measured with the Doppler-shift attenuation method by heavy-ion bombardment of ²H and ³H targets. The recoils are slowed down in Mg, Al, Cu, Ag and Au. The γ-ray patterns are observed with a large Ge(Li) detector at 0° in coincidence with protons; for ¹³C the patterns are measured in singles with a Compton-suppression spectrometer. Analysis of the γ-ray patterns with ⁴He-scaled stopping power data of Northcliffe and Schilling yields the following results: ¹³C, τ_m(3.85 MeV) = 12.6 ± 0.3ps; ¹⁶N, τ_m(0.40 MeV) = 5.1 ± 0.3 ps; ²⁰O, τ_m(1.67 MeV) = 9.8 ± 0.7 ps; ³⁶Cl, τ_m(0.79 MeV) = 19.9 ± 1.7 ps, τ_m(1.16 MeV) = 9.2 ± 0.6 ps and τ_m(1.60 MeV) = 0.94 ± 0.06 ps. A comparison with results obtained with the recoil-distance method shows agreement to about 10 %, with a slight tendency to somewhat longer lifetimes for the recoil-distance technique. The above stopping power is also used to reanalyze our previously published measurements. The new mean lives differ less than 4.5 % from the previous results.     

Helical magnetic structure in CrB2

A cycloidal magnetic structure was discovered in CrB₂ by neutron diffraction using a thin single crystal plate prepared with natural boron. The propagation vector was determined to be τ = 0.285 τ₁₁₀. The magnetic moments of a magnitude of 0.5 ± 0.1μ_B turn in the a?c plane.     

The ferromagnetic monolayer Fe(110) on W(110)

Ferromagnetic order in the pseudomorphic monolayer Fe(110) on W(110) was analyzed experimentally using Conversion Electron Mössbauer Spectroscopy (CEMS) and Torsion Oscillation Magnetometry (TOM). The monolayer is thermodynamically stable, crystallizes to large monolayer patches at elevated temperatures and therefore forms an excellent approximation to the ideal monolayer structure. It is ferromagnetic below a Curie-temperatureT _c,mono, which is given by (282±3) K for the Ag-coated layer, (290±10) K for coating by Cu, Ag or Au and ≈210 K for the free monolayer. For the Ag-coated monolayer, ground state hyperfine fieldB _hf (0)=(11.9±0.3) T and magnetic moment per atom μ=2.53 μ_B could be determined, in fair agreement with theoretical predictions. Unusual properties of the phase transition are detected by the combination of both experimental techniques. Strong magnetic anisotropies, which are essential for ferromagnetic order, are determined by CEMS.     

Predictions for anomalous τ+τ−γ production at LEP 1

We calculate distributions for τ⁺τ^?γ production at LEP 1 taking into account a potentially existing anomalous magnetic moment a _τ of the τ lepton. The existing upper limits for ¦a _τ¦ are known from the dependence of the decay Z ⁰ → τ⁺τ^?γ on a _τ ² and are of the order of (1–5)%. We show that such limits are also sensitive to linear terms in a _τ, which are of equal importance at ¦a _τ¦ ～ (1–2)% and dominate below this value. Contributions from an electric dipole moment dτ do not interfere with the electromagnetic vertex or with the anomalous magnetic moment. Appropriate formulae are derived.     

Magnetic properties and peculiarity of magnetic states in dilute antiferromagnets Mn1−x Zn x F2

The dependence of magnetic moment and susceptibility on temperature, magnetic field and frequency of some single crystals Mn_1?x Zn _x F₂ (x≈x _e=0.75—percolation limit) were experimentally investigated. Our experiments show that (Bazhan and Petrov 1984; Cowleyet al 1984; Villain 1984) in these crystals the nonequilibrium magnetic state of spinglass type with finite correlation length appears as temperature decreasesT<T in weak magnetic fields. This state is determined by fluctuation magnetic moments √nμ (wheren is the number of magnetic ions, corresponding to finite correlation length andμ the magnetic moment Mn⁺¹). In the experiments in low magnetic fields and frequencies there are no peculiarities in the magnetic susceptibility temperature dependence atT≠T _f. At temperaturesT>T _f andT<T _f magnetic susceptibility is determined by 1 $$\chi \left( {T > T_f } \right) = \frac{{N\left\langle \mu \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \frac{N}{n}\frac{{\left\langle {\sqrt n \mu } \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \chi \left( {T< T_f } \right)$$ . In strong magnetic fields and large frequencies there are peculiarities in thex(T) dependence atT=T _f. AtT<T _f and strong magnetic fieldsX(T)=x ₀ andT<T _f and at large frequenciesx(T)=x ₀+α/T. The dependences of magnetic susceptibility on the frequency are determined by the magnetic system relaxation. Calculations and comparison with experiments show that the relaxation of the investigated magnetic systems atT<T _f follows the relaxation lawM(t)=M(0) exp[?(t/τ) ^r ], suggested in Palmeret al (1984) for spin-glasses relaxation taking into account the time relaxation distributionτ ₀....τ _max in the system and its ‘hierarchically’ dynamics.     

The heat capacity of the layer compound tetrachlorobis (methylammonium) manganese—II (CH3NH3)2MnCl4, from 10 to 300k

The heat capacity of the layer compound, tetrachlorobis (methylammonium) manganese II, (CH₃NH₃)₂MnCl₄, has been measured over the range 10K <T<300K. In this region, two structural phase transitions have been observed previously by other techniques: one transition is from a monoclinic low temperature (MLT) phase to a tetragonal low temperature (TLT) phase, and the other is from TLT to an orthorhombic room temperature (ORT) phase. The present experiments have shown that the lower transition (MLT→TLT) occurs at T = 94.37±0.05K with ΔH_t = 727±5 J mol^?1 and ΔS_t = 7.76±0.05 J K^?1 mol^?1, and the upper transition (TLT→ORT) takes place at T = 257.02±0.07K with ΔH_t = 116±1J mol^?1 and ΔS_t = 0.451±0.004 J K^?1mol^?1. These results are discussed in the light of recent measurements on (CH₃NH₃)₂CdCl₄, and also with regard to a recent theoretical model of the structural phase transitions in compounds of this type.In addition to the structural phase transitions, (CH₃NH₃)₂MnCl₄ also undergoes magnetic ordering at T < 150K. The magnetic component to the heat capacity, as deduced from a corresponding states comparison of the heat capacity of the present compound with that of the Cd compound, is shown to be consistent with the behaviour expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

Neutron thermometry applied to magnetization and spin-lattice relaxation measurements on silver nuclei

We have studied spin-lattice relaxation in silver metal at temperatures and magnetic fields beyond the validity of the normal Korringa relation τ ₁ = κ/T _c using transmission of polarized and unpolarized neutrons. For the Korringa constant of ¹⁰⁹Ag we obtained κ = (10.0± 0.3) sK. Unpolarized neutrons were also used to measure the critical value of entropy S for nuclear antiferromagnetic ordering and to determine the induced magnetization M in the ordered state. The nuclear susceptibility X = μ ₀ M/B = 0.36 ± 0.01 was found to be constant, independent of S and B (B _c ≈ 100 μT).     

Spektroskopische Untersuchungen an DyAsO4

DyAsO₄ undergoes a crystallographic phase transition atT _D=11.2K which is induced by a cooperative Jahn-Teller-effect. As deduced from the optical absorption spectra the distance between the two lowest lying Kramers doublets of the Dy³⁺ ion is increased from (6.1±0.5) cm^?1 aboveT _D to (25.0±0.5) cm^?1 at 4.2 K. BelowT _D the splitting factor of the lowest doublet becomes nearly uniaxial with a maximum value ofg _b =17.5±1.0 along the crystallographicb-axis. AtT _N=2.44 K the crystals order antiferromagnetically. The absorption lines of Er³⁺ ions in DyAsO₄ show already a splitting immediately belowT _D which is explained by magnetic short range ordering of the Dy³⁺ ions in the temperature rangeT _{N D} .     

Influence of Ni-concentration on the magnetic structure in Tb(Cu,Ni)2 system

A powder sample of orthorhombic Tb(Cu_0.7Ni_0.3)₂ has been studied by neutron diffraction at T = 4.2 K and above the Curie temperature. It is found that this compound is ferromagnetically ordered at 4.2 K. The magnetic moment of Tb in this compound is (9.2 ± 0.2)μ_B, with components (2.2 ± 0.2, 0, 8.9 ± 0.1)μ_B. The influence of the Ni-concentration on the magnetic structure is discussed in the whole Tb(Cu, Ni)₂ system.     

TSC study of electric dipole relaxations in chlorapatite

Electric dipole relaxations in chlorapatite, Ca₅(PO₄)₃Cl, have been studied with the fractional polarization mode of the thermally stimulated currents (TSC) method. Fifty-one of the fifty-seven sets of data obtained in the range 10–443°K fell naturally into four groups yielding compensation temperatures T_C of T_C1, = 202°C, T_C2: = 202°C, T_C3 = 420°C and T_C4= 644°C, with estimated error < 10°C, and characteristic relaxation times τ_C of τ_C1 = 1.3 × 10^?7s, τ_C2 = 3.2 × 10^?6s, τ_C3 = 8.8 × 10^?5s and τ_C4 = 2.3 × 10^?4s. Atomic-scale physical models involving Cl^? ion motion are offered for the 202°C compensation at the temperature of the reported monoclinic-to-hexagonal phase transition and for the 420°C compensation, at which temperature the Cl^? ions individually are thought to have enough thermal energy to maintain the hexagonal form dynamically.     

A Mössbauer study of Pb4Fe3O8Cl,a three-layer magnet

The study of Mössbauer spectra from Pb₄Fe₃O₈Cl allows the observation of crystallographic sites associated with two kinds of magnetic layers present in the structure. Magnetic order appears simultaneously in the two sites below T_N at 602 K. The following features are observed (i) magnetic moments in a plane of easy magnetization perpendicular to the fourfold axis, (ii) antiferromagnetic behavior, as evidenced by the rotation of the magnetic moments under the effect of an external field, (iii) slightly different critical exponents β (0.311 ± 0.003 and 0.295 ± 0.003) for the two magnetic layers.