Superconductivity in iron-based layered compounds

The featured article this time is advanced online publication in Nature on Superconductivity in FeAs-based layered compounds by Takahashi et al.

This is the first publication in Science/Nature from what I am sure many to follow on this topic, with a few new publications per day appearing on arxiv.

By starting with LaOFeAs compound and doping F at oxygen sites, the

superconducting temperature of 26K is reached under atmospheric pressure. This Tc can be increased up to 43K by applying pressure (maximum of 43K at

4GPa, with the higher pressure decreasing the Tc). This is the highest

non-cuprate Tc observed, and there is already some evidence that the pairing mechanism is non-conventional.

3D Neutron Microscopy

This week we highlight a paper in Nature Physics by Nikolay Kardjilov and co-authors “Three-dimensional imaging of magnetic fields with polarized neutrons”.

3D tomography and microscopy with x-rays is nothing new. Neutrons, however, provide the advantage of strong scattering from magnetic spins – but microscopy with neutrons is limited due to lack of focusing optics, low brightness and monochromacity of neutron sources.

Kardjilov and co-authors present a new technique based on observing the rotation of spin polarization of neutrons as they travel through magnetic material. The result is a 3D view of local magnetization with 100-micron spatial resolution. This technique requires highly polarized and monochromatic neutron beams.

Spintronics and magnetic data storage review

Related to 2007 Nobel Prize in Physics that was awarded last month for discovery of Giant Magnetoresistance (GMR) effect, there is an excellent review in a recent Nature Materials on Spintronics and its use in magnetic data storage devices, by a group that includes a new Nobel Laureate, Albert Fert (Chappert et al., Nature Materials 6, 813 – 823 (2007)).

The review covers fundamentals of GMR, spin-valve heads, magnetic tunnel junctions, magnetic random access memory (MRAM), and nanomagnetism.

Magnets and Earthquakes

Karin Dahmen of Urbana gave a wonderful talk at KITP on similarity between Berkhausen avalanches in magnetic materials and earthquakes. Crackling noise you hear when you pour milk onto the rice crispies, the noise due to switching of magnetic domains, and the avalanches/earthquakes have a lot in common.

KITP archive also has another talk on the same fascinating topic by Karin from 2005.

Secrets of universal random close packing, plus wedging the phase diagrams

Two quick highlights – one from soft matter and another from hard condensed matter.

First highlight is a PRL with an intriguing title ” Why is Random Close Packing Reproducible?” by Randall Kamien and Andrea Liu.

In very simple terms, the random close packing – jammed configuration where each sphere is confined by its neighbours – is invariably at .64 of volume filling. The ordered hexagonally close packed can achieve higher ratios, of about 0.74, and one would think that the special number of 0.64 can be derived and/or proven mathematically – not so.

Kamien and Liu show that 0.64 is the number for which the number of available configuration states disappears (goes to zero).

The second highlight is another PRL by Oleg Krupin, Eli Rotenberg and Steve Kevan “Controlling the Magnetic Ground State in Cr1-xVx Films”.

Here they used a very clever “wedge” design by making a wedge of CrV material, where the thickness varies along one direction and composition (V doping) varies along the perpendicular direction. This allows mapping out the magnetic phase diagram (C-SDW, I-SDW AFM phase vs. Paramagnetic phase) as a function of temperature, thickness of the film and V composition without having to change the samples.

Persistence of memory

Persistence of memory – how does a magnet “remember” it’s state? Does it remember what it used to look like if you cycle the magnetic field? It appears the answer is yes, and disorder is the driving force behind the “persistence” factor.

Mike Pierce and collaborators have studied CoPt magnetic films using soft x-rays tuned to Co L2-3 adsorption edge, where circular dichroism provides a significant contrast between different orientations of magnetisation in the film. The speckle resulting in small-angle x-ray scattering pattern codes the information about specific magnetic domain orientation within the sample. By cycling the magnetic field and comparing the speckles one can calculate the “persistence” of memory or correlation of magnetic domain configurations between the cycles, as the fingerprint-like domains are forced to nucleate, grow and disappear.

Nanopatterned magnets, zeptoliter pipette redux

June’07 issue of Physics today features an illuminating review by Chien, Zhu & Zhu of topic of Nanopatterned magnets.

Much of it relates to the work done by nanomagnetism group at Argonne. Basic idea is that in >100nm pancake-flat (thickness ~10 nm) ferromagnetic disks often minimize magnetostatic energy by forming the whirl-like close-loop magnetization patterns known as “vortices” (not to be confused with all kinds of other vortices, such as type-II superconductors). Nanosized or more symmetric ferromagnetic particles typically form single-domain magnetization state, which is less interesting. But vortices can have some interesting dynamic properties. In elliptical magnetic pancakes equilibrium states call for two or more vortices – and even in the case of just two vortices their interactions can get quite complicated.

The same June issue of Physics Today features a review of zeptoliter pipette article mentioned on this site previously (here and here), which also mentions yours truly by name. Friso sounds skeptical, and his criticism may very well be valid – however the role of surface segregation is not easy to address in these type of nanoscopic systems.