A. B. Lazarev

Study of condensed nitrogen by μSR method

The temperature dependences of parameters of the muon spin relaxation in liquid and crystalline nitrogen have been studied. It has been established that in condensed nitrogen there takes place a fast depolarization of muons. An anomalous behaviour of the amplitude and phase of muon precession is found in the vicinity of the orientation phase transition in solid nitrogen. It has been shown that muon spin relaxation parameters in nitrogen do not change at reduction of the oxygen impurity content from 0.7·10−4 to 10−6. The fast depolarization of muons in condensed nitrogen is apparently due to the formation of muonium atoms. To explain the phenomena observed, a model of the muonium chemical reaction is proposed. The initial phase of the muon precession has been measured as a function of the perpendicular magnetic field to determine the state of short-lived muonium in nitrogen. It has been determined that muonium in nitrogen is in an excited state. Consideration of the nuclear hyperfine interaction of muonium in condensed nitrogen makes it possible to give a qualitative explanation for the temperature dependence of the initial amplitude of the muon precession.

Magnetism in disordered magnetic Fe82−xNixCr18

The zero field μSR-method has been used to study the magnetism in the disordered magnetic alloy Fe82−xNixCr18 near the three-critical, pointx=25. The dynamic and static local field distributions are analyzed. The difference between spin-glass states obtained either from the paramagnetic or after the double transition is discussed.

The μSR investigations on the phasotron at Dubna: The present and the future

After conversion of the LNP JINR phasotron new opportunities have been opened for μSR-research. A new round of investigations has begun since 1987. In this paper we present the summary of the results achieved and give some prospects for future investigations.

Antiferromagnetic solid oxygen studied by positive muons

We present a zero magnetic field muon spin rotation study ofα-O2 (antiferromagnetic phase of solid oxygen) in the temperature range of 10–24 K. Static magnetic order has been observed below theα-β transition temperatureBαβ=23.8 K. The temperature dependence of the muon precession frequency exhibits behavior characteristic of a two-dimensional Heisenberg spin-1 system with the anisotropy parameterα ∼ 10−2 quite similar to that of antiferromagnetic phase of the high-temperature superconductor parent compounds. A unique local field at the muon site has been determined to beB0=1.27(5) kG at low temperatures.

The comparative study of irreversibility effects in Nb foil and high-temperature superconducting ceramics by μSR

We present the results of investigations of superconducting niobium and high temperature ceramical superconductor La1.9Sr0.1CuO4 by the μSR technique. The experiments with the niobium sample confirm high reliability of the μSR-technique in determining such characteristics of type II superconductors asTc,Hc1,Hc2, the magnetic field penetration depth λ, and the critical current densityJc. The analysis of the field dependencies of the distribution width and mean value of the magnetic fields on the muon when the samples are magnetized is carried out. A qualitative difference in the behaviour of the magnetic field distribution width in Nb and LaSrCuO is revealed. Whilst the niobium data are well described in the frame of the critical state model, application of a similar approach to the high-Tc superconductor does not give a satisfactory description of our experimental results.

The μSR study of the relaxation of Ho and Er magnetic moments in high-Tc 1-2-3 compounds

High temperature superconductors HoBa2Cu3O7−δ (Tc ≅93 K), Ho0.5Y0.5Ba2Cu3O7−δ (Tc≅93 K) and ErBa2Cu3O7−δ (Tc≅95 K) were investigated by the zero-field μSR-technique. The muon spin depolarisation rate connected with the fluctuation frequency of rare-earth ion magnetic moments was measured. It was found that the samples with holmium show a fast increase of the muon spin depolarisation rate at temperatures below 20 K, while in ErBa2Cu3O7−δ the depolarisation rate remains low in the whole temperature region studied (4.2 K-270 K). The sharp difference between the behaviours of the muon spin depolarisation rate may be explained by the difference between the ground state of Ho3+ and Er3+ ions in the crystalline field of the lattice.

Penetration depth and pinning effects in high-Tc superconductors La-Sr-Cu-O and (Er, Ho)-Ba-Cu-O studies by μSR

We report the results of μSR investigations of the ceramic samples La2-xSrxCuO4-σ (x=0.1, 0.15, 0.25) and ReBa2Cu3O7-σ (Re=Er, Ho, Y0.5Ho0.5) in the external magnetic field 0–800 Oe. The measurements were performed by the ZFC and FC methods. The irreversibility effects were studied at several temperatures by measuring the mean value and the width of the magnetic field distribution on the muon in the step by step procedure of increasing and subsequent decreasing of the external field. The temperature dependences of the magnetic penetration depth perpendicular to the basal plane λ⊥ were obtained. For the lanthanum sample with 0.15 of Sr its value at the zero temperature is λ⊥ (0)=2400 Å, for Er-Ba-Cu-O λ⊥ (0)=1600 Å.

The μSR-study of the London penetration depth in the high-Tc compounds Bi2Sr2Ca1−xYxCu2O8+δ

Abstract The substitution of the trivalent Y for the bivalent Ca in Bi 2 Sr 2 Ca 1− x Y x Cu 2 O 8+ δ changes the charge carrier density in this compound. We have performed the muon spin rotation measurements of the London penetration depth λ(T) in the high-T c superconductor Bi 2 Sr 2 Ca 1− x Y x Cu 2 O 8+ δ for x=0.0−0.45. The yttrium doping leads to a decrease in T c and to an increase in λ(0). In all investigated samples the penetration depth revealed the temperature dependence which can be explained within the framework of conventional BCS theory in the weak coupling limit.

Fluctuation of rare-earth atom magnetic moments in superconducting ceramics (Ho,Er)-Ba-Cu-O studied by μSR

Zero field muon spin relaxation measurements of the ceramic high-Tc superconductors HoBa2Cu3O7-σ, Oc≃93 K) and ErBa2Cu3O7-σ (Tc≃95 K) reveal the sharp difference between the behaviours of the muon spin relaxation rate at temperatures below 20K. This fact may be explained by assumption that the frequency of fluctuations of Er magnetic moments exceeds by far the fluctuation frequency of Ho in 1-2-3 compounds.

The μSR-investigation of the magnetic and superconducting properties of the compound La2−xSrxCuO4 .

The ceramical system La2-xSrx CuO4 was investigated in the temperature range 4.4–270 K. At the concentrations of Sr x=0.0 and 0.01 the formation of the antiferromagnetic state was observed. The results of the investigations of the magnetic ordering in the fluctuating formation region are given. Investigation of the sample with x=0.07 at the temperature T < 13K confirmed the existence of the spin glass state. The values of the magnetic penetration depth at different temperatures for the superconducting samples are given. The influence of the pinning on the magnetic field distribution in the mixed state of the superconductor is discussed.