System Bits: March 11

Colored diamonds: a superconductor’s best friend
Researchers from the University of California, Berkeley and Ben-Gurion University of the Negev in Israel and UCLA have figured out that colored diamonds can measure the tiny magnetic fields in high-temperature superconductors, providing a new tool to probe these much ballyhooed but poorly understood materials.

Diamond sensors will give us measurements that will be useful in understanding the physics of high temperature superconductors, which, despite the fact that their discoverers won a 1987 Nobel Prize, are still not understood, the researchers said.

High-temperature superconductors are exotic mixes of materials like yttrium or bismuth that, when chilled to around 180 degrees Fahrenheit above absolute zero (-280ºF), lose all resistance to electricity, whereas low-temperature superconductors must be chilled to several degrees above absolute zero. When discovered 28 years ago, scientists predicted we would soon have room-temperature superconductors for lossless electrical transmission or magnetically levitated trains – but that never happened.

This new probe may shed light on high-temperature superconductors and help theoreticians crack this open question. The color centers in diamonds have the unique property of exhibiting quantum behavior, whereas most other solids at room temperature do not. This was quite surprising, and part of the reason that these new sensors have such a high potential, the researchers said.

Nitrogen-vacancy centers have been considered for use in probing for cracks in metals, such as bridge structures or jet engine blades, for homeland security applications, as sensitive rotation sensors, and perhaps even as building blocks for quantum computers.

The crystal lattice of a pure diamond is pure carbon (black balls), but when a nitrogen atom replaces one carbon and an adjacent carbon is kicked out, the ‘nitrogen-vacancy center’ becomes a sensitive magnetic field sensor. (Source: UC Berkeley)

The diamond sensors combine high sensitivity with the potential for high spatial resolution, and since they operate at higher temperatures than their competitors – superconducting quantum interference device, or SQUID, magnetometers – they turn out to be good for studying high temperature superconductors. Although several techniques already exist for magnetic probing of superconducting materials, there is a need for new methods which will offer better performance, the researchers noted.

They’ve used the sensor to detect tiny magnetic vortices, which appear and disappear as the material becomes superconducting and may be a key to understanding how these materials become superconducting at high temperatures.

Now that they have proved it is possible to probe high-temperatures superconductors, they plan to build more sensitive and higher-resolution sensors on a chip to study the structure of an individual magnetic vortex in hopes of discovering something new that cannot be seen with other technologies.

Modifying magnetic properties with electricity
Offering hope for use in ultra-fast computer hard drives in the future, scientists from ETH Zurich and PSI have found evidence that the magnetic properties of particular materials can be modified extremely quickly.

In most materials, magnetic order can be redirected only by applying an external magnetic field. In the case of a special class of materials, there is a second option: multiferroic magnetic properties can be changed by an applied voltage. Multiferroics are currently the subject of intense research in physics, an interest which stems from possible future uses in computer storage media.

In today’s devices data is currently written onto a computer’s hard drive using a mechanical magnetic head but in the future multiferroic hard drives could be inscribed electrically at a much faster rate. However currently only the basic physical properties of this relatively new class of materials is being studied. Until now, there has been no experimental evidence that the magnetic order of multiferroics can be changed quickly enough to compete with the hard drives of today with the fastest measured change using electric field was in the thousandths-of-a-second range. In comparison: using a magnetic head, data can already be saved on a hard drive a million times faster, the researchers said.

In the experiment, the scientists stimulated a terbium manganite crystal (depicted on the right) with a low frequency light pulse (red) and measured the excitation with x-radiation (blue). (Source: ETH Zurich)

They have now found experimental evidence that the ordering of the magnetic moments in multiferroics can also respond much more quickly to voltage – in less than a billionth of a second, i.e. one thousand times faster than the speed at which data can be written on a hard drive today.