Critical thickness for magnetism in LAO/STO
LaAlO3 and SrTiO3
are two non-magnetic insulating materials. After growing at least 4 unit
cells of LAO on a STO substrate, the polar/nonpolar interface exhibits a
number of interesting properties including a high mobility 2D
conductivity, superconductivity below 100 mK, magnetism and an electric
field-controlled metal-insulator and superconductor-insulator transition.
Tuning the magnetism in LAO/STO
We are trying to learn more about the microscopic nature of this
magnetism by tuning it in different ways. For example we found that local
strain manipulates the ferromagnetic patches in LAO/STO. The images below
show repeated scans of a small group of ferromagnetic patches that change
as a result of local strain we apply with the tip of the SQUID's chip.
Modulated superfluid density in twinned high Tc superconductors
Several physical properties like the local strain, the bond angle, and
the magnetic order may change on twin boundaries in orthorhombic crystals.
These properties are known to affect superconductivity in bulk
measurements. We use scanning SQUID to image the local magnetometry and
susceptibility on the surface of twinned superconductors. We observe
increased diamagnetic susceptibility in underdoped, but not overdoped,
single crystals of the pnictide superconductor Ba(Fe1-xCox)2As2 ,
consistent with enhanced superfluid density on twin boundaries.
Interesting information is also acquired by following the vortex behavior.
Individual vortices avoid pinning on or crossing the twin boundaries, and
prefer to travel parallel to them. These results help us connect the
magnetic properties with the local changes in the crystal.
Magnetic imaging of individual magnetotactic bacteria
Biotechnology represent a new and exciting application of SQUID microscopy. Several biomedical applications, such as bio-separation, MRI and drug delivery use nanomagnets. The magnetic properties of nanomagnets are usually measured in large groups, which can be problematic due to the large dispersion of their properties. Magnetotactic bacteria are a group of bacteria that naturally grow magnetic particles, which magnetically align along a chain and result in alignment of the bacteria with earth's magnetic field (like a compass needle). We use scanning SQUID to detect magnetotactic bacteria and measure their moment properties and their response to small fields on an individual basis. We observe large dispersion in their magnetic properties.
Dynamics of single vortices on grain boundaries in YBCO thin films
Above the critical current vortices travel in a type-II superconductor
allowing for dissipation. Using scanning Hall probe microscopy, we have
detected the hopping of individual vortices between pinning sites along
grain boundaries in YBCO thin ?lms. The hopping frequency increased with
the applied current, which drives the vortices faster. Detecting the
motion of individual vortices allowed us to probe the current-voltage
(I-V) characteristics of the grain boundary with voltage sensitivity below
a femtovolt. We found a very sharp onset of dissipation that shows
essentially no dependence on temperature or grain boundary angle.
Electronic properties of complex oxide interfaces
Magnetic properties of carbon nanotube coils
Collaborators: Ernesto Joselevich
Modulation of superfluid density on grain boundaries in high temperature superconductors
Applications for high Tc superconductivity are very difficult to achieve. The reason is rooted in the fundamental physics of high-Tc superconductors. The very short coherence length, just a few angstroms, makes Josephson junctions likely to appear in these materials. Recently we found stronger superconductivity near twin boundaries, rather than the weaker superconductivity expected from a weak link. We plan on investigating the strength of superconductivity using the most common type of naturally occurring weak link: grain boundaries.
We probe the strength of superconductivity by measuring the local magnetic susceptibility (response to magnetic field). Superconductors expel magnetic fields in proportion to the local density of the cooper pairs, which are the elements of the superfluid.
Single vortex dynamics on grain boundaries
Scanning SQUID study of individual protein templated magnetic nanocrystals
Collaborators: Tanya Prozorov, Ames.