The magnetic vortex lattice (VL) of polycrystalline MgB2 has been investigated by transverse-field muon spin relaxation (TF-μSR). The evolution of the TF-μSR depolarization rate σ, which is proportional to the second moment of the field distribution of the VL, has been studied as a function of temperature and applied magnetic field. The low-temperature value σ exhibits a pronounced peak near Hext=75 mT. This behavior is characteristic of strong-pinning-induced distortions of the VL which put into question the interpretation of the low-field TF-μSR data in terms of the magnetic penetration depth λ(T). An approximately constant value of σ, such as expected for an ideal VL in the London limit, is observed at higher fields of Hext>0.4 T. The TF-μSR data at Hext=0.6 T are analyzed in terms of a two-gap model. We obtain values for the gap size of ∆1=6.0(3) meV [2∆1/kBTc=3.6(2)], ∆2=2.6(2) meV [2∆2/kBTc=1.6(1)], a comparable spectral weight of the two bands, and a zero-temperature value for the magnetic penetration depth of λab~100 nm. In addition, we performed μSR measurements in zero external field. We obtain evidence that the muon site (at low temperature) is located on a ring surrounding the center of the boron hexagon. Muon diffusion sets in already at rather low temperature of T>10 K. The nuclear magnetic moments can account for the observed relaxation rate and no evidence for electronic magnetic moments has been obtained.
Physical Review B
- Pub Date:
- March 2002
- alloys and binary compounds;
- Muon spin rotation and relaxation;
- Condensed Matter - Superconductivity
- 15 pages, 4 figures