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Magnetic dipoles line up

The interaction of nanoscale magnetic dipoles has been observed for the first time by researchers in Germany. Unexpectedly, the dipoles were seen to form chains, rather than the zigzag pattern expected from simple dipole interaction. The team believes this surprising outcome may be caused by higher-order interactions between the dipoles – a result that may have implications for the development of future hard-disk drives.

To set up their array of nanomagnets, the researchers, led by Hartmut Zabel, a physicist at the Ruhr-Universität Bochum, cut a regular square lattice from a thin magnetic iron-palladium film using electron beam lithography. The lattice was made up of circular islands, each a mere 150 nm in diameter. This alloy was chosen for its soft nature, as well as the potential to tune its Curie temperature – the temperature below which it becomes ferromagnetic – with varying iron concentrations. Ion beam sputtering was then used to remove the unwanted material from between the islands.

Straight lines or zigzags?

Above certain temperatures, thermal movements interfere with the interactions between small-scale magnetic dipoles. The experiments were therefore conducted at low temperatures, at which the orientation of the magnetic islands was based entirely on the interaction between each dipole. This allowed each of the islands to assume the most favourable orientation in terms of energy, a condition called the “ground state”. The researchers were able to observe the resulting pattern of dipole orientations using a photon emission electron microscope at the BESSY II electron synchrotron, at the Helmholtz-Zentrum Geesthach in Berlin. This machine uses X-ray photons to stimulate specific electrons within the target material, which provides information on the orientation of the dipoles being examined.

The team observed that the dipole islands formed a series of parallel and antiparallel chains – with adjacent islands’ north and south poles aligning to form the links in the chain – a result that was not anticipated. With each island having four neighbours to which it could have potentially aligned itself, one might expect a more random-looking zigzag pattern to result – as would be the case in purely simple dipole interactions.

“A magnetic dipole is an ideal mathematical construct working for point-like objects…It is also the dominant interaction in the far field,” says Zabel, adding that in the team’s case, “this type of ordering can only be understood via higher-pole interaction.” He explains that with closely spaced magnetic islands, higher-order relationships – such as quadrupolar and octopolar interactions – must become more important.

Next-generation drives

The results of this study are of great technical relevance to the development of next-generation hard-disk drives. Hard disks store digital data as a pattern of binary digits or bits. Today’s disks are made up of thin ferromagnetic films with opposing directions of magnetization used to encode the two bit states, “0” and “1”. Future disks, however, may use the orientation of tiny magnetic dipoles, similar to the ones examined in this study. To maximize the physical storage density, the ideal disk will use the smallest possible dipoles that can assume the two-bit-state orientations independently of each other. “Switching of the bits should be independent, but with [higher-order] interaction this may not be the case. Therefore it is important to study the magnetic interaction in regular patterns,” Zabel told physicsworld.com.

“[This] study aims at answering an important, fundamental question: how strong is the interaction between neighbouring nanomagnets and what is the nature of their interaction?” explains Axel Enders, an associate professor and nanostructure expert at the University of Nebraska, who was not involved in the work. “It turns out that simple textbook models based on dipolar interactions are insufficient to describe their experimental observations, calling for refinements of theoretical models and, of course, more experiments on the subject.”

While the team has now moved on to other areas of nanomagnetic research, Zabel also notes the great potential for continued study in this area. “There are many parameters to vary,” he comments, citing such possible variables as the island size and periodicity, iron concentration, which affects the Curie temperature, and the symmetry of the base lattice.

The work is published in Physical Review Letters.

Heinrich Rohrer: 1933–2013

The Swiss condensed-matter physicist Heinrich Rohrer who shared the 1986 Nobel Prize for Physics died last week at the age of 79.

Rohrer won the Nobel prize for inventing the scanning tunnelling microscope (STM) at IBM’s Zürich Research Laboratory. Rohrer shared one half of the prize with his IBM colleague Gerd Binnig, while the other half went to Ernst Ruska for his invention of the electron microscope.

Rohrer was born on 6 June 1933 in the small town of Buchs in the Swiss canton of St Gallen. The family moved to Zürich in 1949 and Rohrer studied physics at ETH Zürich where he was taught as an undergraduate by Wolfgang Pauli and Paul Scherrer. He stayed on to do a PhD on the mechanical properties of superconductors and he continued working on superconductors at Rutgers University in the US.

Surface defects

In 1963 Rohrer joined IBM Zürich, where he worked initially on magnetic materials. He encouraged Binning to join the lab in 1978 and the pair studied tiny defects on the surface of silicon – which at the time were hindering the miniaturization of electronic devices. To gain a better understanding of these defects, Rohrer and Binning built the first STM in 1981.

An STM creates an image of the surface of a sample by scanning an atomically sharp tip over its surface. The tip is held less than one nanometre from the surface and a voltage is applied so that electrons can undergo quantum-mechanical tunnelling between tip and surface. The tunnelling current is strongly dependent on the tip–surface separation and this is used in a feedback loop to keep the tip the same distance from the surface. An image is obtained by scanning the tip across the surface to create a topographical map in which individual atoms can be seen.

Important technology

The STM has become an important instrument for surface physics and materials science. A number of related microscopy techniques have since been developed in labs worldwide – including atomic force microscopy.

Rohrer became an IBM Fellow in 1986 and he headed-up the physics department at IBM Zürich in 1986–1988. The Binnig and Rohrer Nanotechnology Center was opened at IBM Zürich in 2011 in honour of the two laureates.

Rohrer died on 16 May in Wollerau, Switzerland.

Is there a difference between climate scepticism and climate denial?

In less than 100 seconds, Simon Buckle tries to disentangle what he believes are two distinctly different approaches to climate science.

Watch more from our 100 Second Science video series.

Flipping spins spread like wildfire

New research into how local heating sets off a chain reaction of magnetic domain reversal could provide important insights into how wildfires spread. That is the conclusion of scientists in the US and Spain, who have pinpointed with greater precision than ever before the conditions under which such a magnetic deflagration will occur. The results could provide insights into runaway chemical reactions and even forest fires.

When a magnetic domain flips from a higher-energy metastable state to a lower-energy stable state, energy is released as heat. If enough energy is released, the temperature of the surrounding domains will rise to the point that they also flip, releasing more heat. This can spark a chain reaction that rapidly causes the magnetization of the entire material to reverse direction.

Magnetic deflagration can, in principle, be observed in any material with a metastable magnetic state. Molecular nanomagnets such as Mn12 acetate are particularly suitable. Each molecule of this crystal has a large-net-spin magnetic moment that is energetically constrained to point in one of two opposite directions. Ordinarily, these two states have the same energy; but if an external magnetic field is applied along the magnetic axis, the antiparallel spins are raised in energy relative to those parallel to the field. However, both the antiparallel and the parallel orientations have a lower energy than an intermediate orientation. This means that spins in the antiparallel direction will not flip unless they are given an activation energy, which can be delivered in the form of heat.

Experiment and theory

In the new work, Andy Kent and scientists at New York University and colleagues at the University of Barcelona, City College of New York and the University of Florida have used experimental results to create a model describing what happens next. They mounted a single crystal of Mn12-acetate cooled to a temperature of 0.4 K, with all its spins aligned in one direction, on an array of uniformly spaced magnetic-field sensors. They then applied a magnetic field in the opposite direction to the polarization of the spins in the crystal, leaving the spins in the metastable antiparallel state. Finally, they applied heat to one end of the crystal.

The energy released when a spin changes from an antiparallel to a parallel orientation depends on the external magnetic field. If the field is weak, the energy is small and the heat simply dissipates. In this case, the spins reverse gradually over a period of about 80 ms as the heat from the applied heat pulse thermally diffuses along the crystal. Above a critical field, however, the energy released by one spin flipping provides enough activation energy to flip adjacent spins. In this case, the magnetic-field sensors detected a wave of spin flips propagating through the material at a constant speed, reversing all the spins in about 100 μs – nearly 1000 times faster than thermal diffusion.

Under control

Based on their observations, the researchers derived a criterion for whether or not such a reversal will ignite. The criterion depends on several properties of the magnetic material: the activation energy of the spin-reversal process; the amount of energy released when a spin reverses; and the rate at which heat diffuses through the material. If heat diffuses away relatively slowly, the energy of adjacent magnetic domains will be raised higher by a neighbouring spin flip. The criterion, the team finds, agrees broadly with previous results from other researchers. Kent explains that, in the new work, the onset of deflagration is more precisely controlled than in previous experiments. “People were able to trigger these processes,” he says, “but not to see this crossover behaviour and how the actual instability is generated. In this work we can control it and that allows us to study it.”

Carley Paulsen of the Neel Institute in Grenoble was involved in the first observation of sudden spin reversal in 1994. He says that although these latest results are “not surprising”, the novel experimental set-up allows new insight into the abrupt crossover from diffusion to deflagration.

In 2005 two of Kent’s co-workers – Myriam Sarachik of City College and George Christou of the University of Florida – were part of a team that argued magnetic deflagration could be understood by analogy to the advancing flame front in a burning chemical. Kent now hopes that the magnetic model may help generate and test models of macroscopic fires and other exothermic chemical reactions that can take off in a similar way. “One thing that makes it really nice from the experimental point of view is that we can study these reactions and then reset the system,” he says. We’re not really burning anything so we don’t have to throw away the ash and start again.”

The research is described in Physical Review Letters.

People, not information, want to be free

Cory Doctorow speaking at the Sense About Science Annual Lecture. (Courtesy: Sense About Science)

By Margaret Harris

I’ve never been a fan of the slogan “Information wants to be free”.  As a journalist and former scientist, I know that the process of creating and disseminating information is definitely not free, and I’m sceptical about the economic alchemy that would supposedly make it that way.  So when I saw that this year’s Sense About Science lecture was entitled “We Get to Choose: How to Demand an Internet That Sets Us Free”, I nearly stayed away.

As it turns out, a more accurate title for the London-based charity’s annual bash would have been “Why Digital Rights Management is Bad and Why You Should Care”, and by the end, the speaker – science-fiction author and blogger Cory Doctorow – had pretty much won me over.

(more…)

Why should we care about climate change?

In less than 100 seconds, Simon Buckle shares his thoughts on a question that can rouse strong feelings.

Watch more from our 100 Second Science video series.

Kepler – it’s not all doom and gloom just yet

By Tushna Commissariat

NASA’s Kepler space telescope. (Courtesy: NASA)

To much general dismay, earlier this month NASA officials announced that their Kepler space telescope had gone into a self-imposed “safe mode”, something that the telescope is programmed to do if one of its primary systems is not fully functional. Although the telescope was then rebooted, it shut down again this week and it seems that all is definitely not well with our favourite exoplanet spotter: the mission collaboration announced that the instrument has suffered a critical failure and may never be fully operational again.

(more…)

‘Ghostly’ 3D images taken without a camera

A simplified 3D imaging system that does not require a conventional camera has been developed by researchers in the UK. The computational imaging technique uses information from single-pixel detectors to create an image, can be used over a range of wavelengths and is cheaper than other 3D methods. The researchers claim that, in addition to taking images, their system could be used as a detector in oil and gas exploration as well as in medical and biological imaging systems.

Most imaging systems – from cameras to the retinas in our eyes – capture images in 2D and then process the information to create a 3D image. While plenty of 3D imaging techniques exist – including stereoscopic, holographic and volumetric imaging – they are expensive and require bulky and specialist equipment, such as lenses and lasers. And despite all this advanced technology, the techniques only work for light at specific wavelengths.

Illuminated imaging

Baoqing Sun, Miles Padgett and others from the University of Glasgow, along with other UK-based colleagues at Cambridge University, set out to create a simple system that can deliver 3D imaging without a camera or any other lenses. Sun told physicsworld.com that the team’s new technique involves using nothing more than a light projector, four single-pixel detectors and a computational imaging technique known as “ghost imaging.”

Computational ghost imaging creates images using “intelligent illumination”. The object to be imaged is lit with a specific, known light pattern (such as a speckle pattern) and the reflected light is detected by a single-pixel photodetector. This device has no spatial resolution and merely collects all the light that is incident on it. The multiple signals of the varying light intensity from the detector are then processed, along with the knowledge of the lighting pattern, to build up the final image. Until now, however, this technique has only been used to build 2D images.

Chequered patterns

In this latest work, the team used a simple light projector to illuminate a polystyrene model of a human head with computer-generated random binary speckle patterns. The light reflected from the head was collected by four single-pixel detectors, which are placed at different angles.

Source images from the four single-pixel detectors from 1000 to one million iterations
Building up the surface features: source images from the four single-pixel detectors from 1000 to one million iterations. The images from each photodetector are reconstructed using an iterative algorithm. The spatial information in each image is identical, however, the apparent illumination source is determined by the location of the relevant photodetector, indicated underneath. No post-processing has been applied to the images. The scale refers to the relative intensity of the images. (Courtesy: Sun et al., University of Glasgow/Science)

Thanks to the random binary speckle patterns – which illuminate the object with a “chequered” pattern – and the varying angles of the detectors, the team was able to see a clear shading profile in the images.

“For each detector, we saw a 2D image that appeared to be illuminated from a different direction, even though we used only one projector to illuminate the head,” says Sun. The individual images were reconstructed using an “iterative algorithm” that the team created. From the shading of the images, the researchers measured the surface reflectivity and the varying depth of surface features could be derived. The 3D head was reconstructed by integrating more than a million iterations of the images. Sun also explains that different speckle patterns are projected one after another and that the greater the number of patterns used, the shaper and more highly resolved the final image.

Across the range

In terms of adapting the new technology to real-world applications, Sun points out that all of the experiments to date have been carried out in a lab. “So if this was to be used outside in bright sunlight, or any such environment, we will have to account for the high noise levels, and this is something we are working on,” he says. But at the same time, he explains that the system works well as a cheap “3D camera” and can be used across a wider wavelength range – from ultraviolet to infrared – compared with normal cameras. This could allow it to be used as a detector in oil and gas exploration, where infrared remote-sensing technologies are used to “see” oil reserves.

The team is also keen to collaborate with other researchers to extend the 3D computational ghost imaging to the terahertz scale, so that it could be used to carry out medical as well as other biological imaging, and this is something the researchers are currently in the process of doing.

The research is published in Science.

What is M-theory?

In less than 100 seconds, Leron Borsten explains how M-theory has the potential to unify the various forms of string theory with the theory of supergravity.

Watch more from our 100 Second Science video series.

BBC radio celebrates 101 years of cosmic rays

By Hamish Johnston

AMS is a modern version version of Hess’s balloon experiments. (Courtesy: NASA)

The BBC’s Melvyn Bragg has lots to talk about. Over the past few months he has chatted about the Icelandic sagas, water, Gnosticism, and much more on his Radio 4 programme In Our Time. So he can be forgiven for missing a centenary and celebrating cosmic rays 101 years after they were discovered by the Austrian physicist Victor Hess.

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