M. Hampele

Investigation of muon states in silicon and germanium by field-quenching and RFμSR

The temperature dependence of the three states of positive muons in the semiconductors with diamond structure (μ+ in diamagnetic statesμd and paramagnetic muonium Mu and Mu*) have been investigated on six Si (pure, B and P doped) and four Ge (ultrapure, CZ-grown undoped, Ga and Sb doped) single crystals by longitudinal field-quenching and radio-frequencyμ+SR. Clear evidence for the transition Mu* →μd is found. The influence of light-induced charge-carriers is shown to be quite different in p- and n-type material.

Investigation of muon-state dynamics in silicon by longitudinal field-quenching and radio-frequency μ+ spin resonance

The two paramagnetic muon states-normal and anomalous muonium-and the diamagnetic muon states have been investigated in different monocrystalline silicon samples (intrinsic, boron-, phosphorus-, or arsenic-doped, float-zone or Czochralski-grown) between 6K and 800K by means of longitudinal field-quenching (LFQ) and radio-frequency μ + spin resonance (RFμSR). The LFQ data can be described consistently by coupled equations of motion for the muonium spin systems if spin exchange processes as well as transitions between different muon states are taken into account. It is shown that the initial formation probabilities of the different muon states, the ionization rate of the anomalous muonium, and the electron spin exchange rates depend strongly on the charge carrier densities. These results are in agreement with the RFμSR data obtained on the same samples if the different time scales of RFμSR and LFQ experiments are taken into account. At temperatures above 300 K both RFμSR and longitudinal relaxation results appear to indicate reversible ionization of normal muonium.

μ− SR in semiconductorsSR in semiconductors

Abstractμ− SR experiments have been performed on Si between room temperature and 6 K. The amplitude of the muon spin precession signal in an applied magnetic field of 0.04 T decreased below 30 K. A zero-field measurement at 6 K revealed a μ− spin precession frequency of 650 MHz. The muonic atom represents an aluminium acceptor in the silicon matrix, its electronic state is responsible for the μSR signal. A possible influence of the γ recoil produced by the X-ray cascade is discussed.

Radio-frequency spin resonance of positive muons in α-iron at high temperatures

Resonant transitions between the Zeeman levels of positive muons implanted into α-iron foils have been observed above the Curie temperature by applying a 17.8 MHz transverse radio-frequency field and varying the longitudinal external field. Resonance signals of free and trapped muons are detected.

Analog integrating detection technique for pulsed muon beams

A new analog integrating detection technique for time resolvedμSR measurements at high intensity pulsed muon beams has been developed. Data acquisition is realized by digitizing the integrated anode signal for eachμ burst by means of a fast transient recorder. Details of setup and performance are described.

μ+ SR studies of antiferromagnetic chromiumSR studies of antiferromagnetic chromium

Abstractμ+ SR measurements have been performed on Cr single crystals at temperatures 60 mK≤T≤295 K in applied magnetic fields 0≤Bappl≤1.5 T. The temperature dependence of the observed precession frequencies and transverse relaxation rates can be explained by the assumption that theμ+ are hopping between adjacent tetrahedral interstices. At temperaturesT≤11 K evidence for an interaction between theμ+ and the spin-density waves in Cr has been found. The directions and magnitudes of the lattice magnetic moments are unaffected by the applied magnetic fields.

Influence of elastic strain onμ+ SR in α-iron single crystalsSR in α-iron single crystals

The spin-precession frequencies and the transverse spin relaxation rates of positive mouns (μ+) have been measured on two elastically strained α-Fe single crystal platelets as well as on an unstrained reference α-Fe crystal at temperatures down to 2.7 K in applied magnetic field 0≤Bappl≤3 T. The drastic effects of the strains may be qualitatively understood in terms of their influence on both the magnetic domain structure and theμ+ energies at the various interstitial sites. This leads to the conclusion that at low temperaturesμ+ in α-Fe occupy configurations related to octahedral interstitials with dipolar fieldBdip=0.70 T.

Pressure cell and combined cryostat/furnace for high‐pressure μSR studies

This paper outlines the design characteristics of a combined cryostat/furnace set‐up dedicated to high‐pressure μSR experiments using a cylindrical high‐pressure cell made of the high‐strength titanium alloy Ti‐6Al‐4V‐ELI. This set‐up provides the possibility to perform μSR measurements on samples with diameter of 12 mm and length of 35 mm in the temperature regime 3.7Eleq TEleq 800 K at hydrostatic pressures up to 800 MPa. Results for high‐pressure μSR experiments on Ni and alpha ‐Fe are presented.

Investigation of low-temperature quantum diffusion in α-iron byμ+ SR experiments on a single-crystal sphereSR experiments on a single-crystal sphere

The longitudinalμ+-spin relaxation rate has been measured on a high-purity spherical α-iron single crystal at temperaturesT down to 20 mK and in applied magnetic fieldsBappl parallel to 〈111〉 up to 3 T. Only above 1 K can the data be satisfactorily described by one rate constantГ. At 1 T≤Bappl≤2 T and 50 mK≤T≤300 mK, oscillations (“wiggles”) were in addition superimposed on the longitudinal relaxation. A qualitative understanding of the measurements may be achieved in terms of the increasing influence of internal stresses onμ+ diffusion as the temperature is lowered.

Dipolar magnetic field and fermi contact field at the μ+ site in α-Fe at very low temperatures

In a longitudinalμ+SR experiment on a high-purityα-Fe single crystal sphere magnetically saturated in a 〈 111 〉 direction damped oscillations (wiggles) were observed in a temperature range 30 mK to 600 mK and in a certain regime of applied magnetic fieldsBappl. Meassurements of the wiggle frequency as a function ofBappl give us directly the Fermi fieldBFermi=(−1.13±0.02)T and the dipolar magnetic field ¦Bdip∥¦=(0.66±0.03)T.Bdip∥ was used to determine the prefactor in the Arrhenius law obeyed by theμ+ hopping rate between 100 K and 1000 K. A comparision with the corresponding values for protons and deuterons suggests diffusion via the adiabatic mechanism.