Nuclear Magnetic Resonance Force Microscopy (NMRFM) is a unique quantum
microscopy technique, which combines the three-dimensional imaging capabilities of
magnetic resonance imaging (MRI) with the high sensitivity and resolution of atomic
force microscopy (AFM). It has potential applications in many different fields. This
novel scanning probe instrument holds potential for atomic-scale resolution.
MgB2 is a classic example of two-band superconductor. However, the behavior
of these two bands below the superconducting transition temperature is not well
understood yet. Also, the anisotropic relaxation times of single crystal MgB2 have
not been measured because it is not yet possible to grow large enough MgB2 single
crystals for conventional NMR. Using our homemade NMRFM probe, we have
set out to measure the relaxation times of micron size MgB2 single crystals to anix
swer several questions relating to the anisotropy, multiband behavior, and coherence
effects in this unusual superconductor.
The goal of a second project is to study the effects of doping on the critical
current of MgB2 superconducting wires. Ti-sheathed MgB2 wires doped with nanosize
crystalline-SiC up to a concentration of 15 wt% SiC have been fabricated, and
the effects of the SiC doping on the critical current density (Jc) and other superconducting
properties studied. In contrast with the previously reported results, our
measurements show that SiC doping decreases Jc over almost the whole field range
from 0 to 7.3 tesla at all temperatures. Furthermore, it is found that the degradation
of Jc becomes stronger at higher SiC doping levels. Our results indicate
that these negative effects on Jc could be attributed to the absence of significant
effective pinning centers (mainly Mg2Si) due to the high chemical stability of the
crystalline-SiC particles.
The principle goal of a third project, the study of magnetic semiconductors,
is to investigate magnetic properties of Mn-implanted GeC thin films. 20 keV energy
Mn ions were implanted in two samples: 1) bulk Ge (100) and 2) a 250 nm thick
epitaxial GeC film, grown on a Si (100) wafer by UHV chemical vapor deposition using
a mixture of germane (GeH4) and methylgermane (CH3GeH3) gases. A SQUID
magnetometer study shows granular ferromagnetism in both samples. While the
Curie temperature for both samples is about 180 K, the in-plane saturated magnetic
moment per unit area for the first sample is about 2.2×10−5
emu/cm2 and that
for the second sample is about 3.0 × 10−5
emu/cm2
. The external field necessary to
saturate the magnetic moment is also larger for the second sample. These results
show clear enhancement of magnetic properties of the Mn-implanted GeC thin film
over the identically implanted Ge layer due to the presence of a small amount of
non-magnetic element carbon.Physic