Local Magnetic Field at a Positive Muon in Ferromagnetic Cobalt

Abstract

The local magnetic field at a stopped positive muon has been measured in single crystals of Co as a function of temperature between 4 and 1100 K and of external magnetic field between 0 and 2 kG. The measurements demonstrate the importance of a dipolar contribution to the local field and give a strong indication that the muon occupies the octahedral interstitial site.In this Letter, the results of a muon spin rotation (MSR) experiment performed at the Swiss Institute for Nuclear Research (SIN) on single crystals of ferromagnetic cobalt are reported. The MSR technique enables one to measure the magnitude and direction of the local magnetic field at the site of the stopped muon. Two possible sites in the Co lattice were considered: the octahedral and tetrahedral interstitial sites. The hyperfine field arising from the contact interaction of the conduction electrons with the muon was computed from the measured local field by taking account of the field contribution from the dipolar interaction of the muon with the neighboring Co ions. The temperature dependence of the local magnetic field B^ is most consistently understood by assuming that the muon occupies the octahedral interstitial site. A similar analysis allows one to predict correctly the observed external-field dependence of the local field.The MSR technique has been described in detail elsewhere. 1 Briefly, it consists of stopping a polarized beam of positive muons in the sample to be investigated and observing the timedependent angular distribution of decay positrons. Because the positron emission is correlated with the muon spin direction, one may observe the precession of the muon in its local field. A typical precession pattern is shown in Fig. 1. From the precession frequency and the known gyromagnetic ratio, the magnitude of the local field is determined. The sign of the local field is determined from the initial phase of the precession observed by a counter perpendicular to the initial muon polarization. A damping of the precession signal describable by a dephasing time T 2 is generally observed.The temperature-dependent measurements of the local field were made at zero external field on an approximately ellipsoidal single crystal. The sample was cooled in a He flow cryostat and heated in an evacuated oven by noninductive electrical heating elements. Measurements were also made at room temperature in various external fields on a crystal which had been machined TIME (;JS FIG. 1. A MSR time histogram taken with a cobalt single crystal at 4 K in zero external field. 1644

Biophysical mechanisms of phase contrast in gradient echo MRI

“…The notion of a sphere of Lorentz is often used to estimate the local magnetic field at the nucleus of interest in the isotropic and homogeneous medium placed in the external homogeneous magnetic field B 0 [see for example (27)]. With this approach, the sources of this local magnetic field are considered as magnetic dipoles and are usually separated into nearby and distant by artificially selected spherical Lorentzian boundary around the nucleus.…”

Effet Mössbauer et techniques complémentaires - I. — La spectroscopie du muon positif

“…Par contre lorsqu'elle n'est pas nulle, son estimation est importante [22] car elle donne la possibilité de déterminer le site du Jl+. Prenons le cas du cobalt étudié en détail par deux groupes [23,24]. La figure 6 montre les signaux de précession observés à différentes températures.…”

“…7. -Variation en fonction de la température du champ Bu et du champ Bhf dans le cobalt [23]. Une transition de phase cristallographique (hex -c.f.c.)…”

“…[Thermal variation of B03BC and Bh in cobalt [23]. A crystallographic phase transition (hex -f.c.c.)…”

Advances in Muon Spin Rotation

“…The Jl+ depolarization rate has been measured in a number of samples of iron. Figure 36 shows a compilation of the results obtained by different groups (33,35,(95)(96)(97).…”