Anisotropy of SuperconductingMgB2as Seen in Electron Spin Resonance and Magnetization Data

Abstract

We observed the conduction electron spin resonance (CESR) in fine powders of MgB2 both in the superconducting and normal states. The Pauli susceptibility is chi(s) = 2.0 x 10(-5) emu/mole in the temperature range of 450 to 600 K. The spin relaxation rate has an anomalous temperature dependence. The CESR measured below T(c) at several frequencies suggests that MgB2 is a strongly anisotropic superconductor with the upper critical field, H(c2), ranging between 2 and 16 T. The high-field reversible magnetization data of a randomly oriented powder sample are well described assuming that MgB2 is an anisotropic superconductor with H(ab)(c2)/H(c)(c2) approximately 6-9.

MgB2 ultrathin films fabricated by hybrid physical chemical vapor deposition and ion milling

“…29 Nevertheless, we use it to make the rough estimation that our 2.9 nm film has H ⊥ c (0) and Hc // (0) of 8 T and 24 T, respectively, which yields an anisotropy γ = H // c (0)/H ⊥ c (0) = 3.5. This value is less than in polycrystalline MgB 2 films and in single crystal MgB 2 (γ = 6-9) 30 and significantly higher than that for any thicker clean epitaxial films (γ = 1.2). 31 It is interesting to note that, as film thickness decreases, the c lattice parameter increases slightly from the bulk value because of the small buckling of ab planes (as estimated from TEM measurements).…”

Anisotropic superconducting properties ofMgB2single crystals probed by in-plane electrical transport measurements

“…Since crystal structure of MgB 2 consists of alternating Mg and B sheets, electronic anisotropy of this material may be expected. Clear signs of anisotropic nature of MgB 2 has been demonstrated from measurements performed on thin films, 11,12 aligned crystallites 13 and fine powder 14 samples. However, results obtained by various groups give very different estimation of H c2 anisotropy ratio ranging from γ=1.7-2 obtained for thin films 11,12 and aligned crystallites 13 up to γ=6-9 for randomly oriented powder samples.…”

ESR spectrometer with a loop-gap resonator for cw and time resolved studies in a superconducting magnet

“…The small space available for closing the flux inside the brass cap gives rise to a further ∼10 % frequency upshift. The design allows the use of cylindrically shaped samples with diameters up to 8 mm and a height of 5 to 8 mm that are used for the simple pass HF-ESR apparatus with 8 mm stainless steel tubing, such as a loose powder contained in a Teflon sample holder of a metallic sample as RbC 60 [24] or fine grains of MgB 2 cast in epoxy [10].…”

“…The spectra were measured by the currently described spectrometer (a and b) at S and X-band, respectively, as well as on a Bruker ESP300 commercial spectrometer at X-band (c). In the fine powder of the MgB 2 superconductor the conducting particles were separated by a powder of the non-magnetic SnO 2 insulator [10,26]. Measurement in the superconducting magnet requires a magnetic field calibrating g-factor standard.…”