magnetism – Page 2 – Lightsources.org

Magnetization ratchet in cylindrical nanowires

2018/06/262018/07/11

A team of researchers from Materials Science Institute of Madrid (CSIC), University of Barcelona and ALBA Synchrotron reported on magnetization ratchet effect observed for the first time in cylindrical magnetic nanowires (magnetic cylinders with diameters of 120nm and lengths of over 20µm).

These nanowires are considered as building blocks for future 3D (vertical) electronic and information storage devices as well as for applications in biological sensing and medicine. The experiments have been carried out at the CIRCE beamline of the ALBA Synchrotron. The results are published in ACS Nano.

The magnetic ratchet effect, which represents a linear or rotary motion of domain walls in only one direction preventing it in the opposite one, originates in the asymmetric energy barrier or pinning sites. Up to now it has been achieved only in limited number of lithographically engineered planar nanostructures. The aim of the experiment was to design and prove the one-directional propagation of magnetic domain walls in cylindrical nanowires.

>Read more on the ALBA website

Image: (extract) Unidirectional propagation of magnetization as seen in micromagnetic simulations and XMCD-PEEM experiments. See entire image here.

Scientists find ordered magnetic patterns in disordered magnetic material

2018/06/082018/06/11

Study led by Berkeley Lab scientists relies on high-resolution microscopy techniques to confirm nanoscale magnetic features.

A team of scientists working at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has confirmed a special property known as “chirality” – which potentially could be exploited to transmit and store data in a new way – in nanometers-thick samples of multilayer materials that have a disordered structure.

While most electronic devices rely on the flow of electrons’ charge, the scientific community is feverishly searching for new ways to revolutionize electronics by designing materials and methods to control other inherent electron traits, such as their orbits around atoms and their spin, which can be thought of as a compass needle tuned to face in different directions.

These properties, scientists hope, can enable faster, smaller, and more reliable data storage by facilitating spintronics – one facet of which is the use of spin current to manipulate domains and domain walls. Spintronics-driven devices could generate less heat and require less power than conventional devices.

In the latest study, detailed in the May 23 online edition of the journal Advanced Materials, scientists working at Berkeley Lab’s Molecular Foundry and Advanced Light Source (ALS) confirmed a chirality, or handedness, in the transition regions – called domain walls – between neighboring magnetic domains that have opposite spins.

Scientists hope to control chirality – analogous to right-handedness or left-handedness – to control magnetic domains and convey zeros and ones as in conventional computer memory.

Image: (extract, here original image)The top row shows electron phase, the second row shows magnetic induction, and the bottom row shows schematics for the simulated phase of different magnetic domain features in multilayer material samples. The first column is for a symmetric thin-film material and the second column is for an asymmetric thin film containing gadolinium and cobalt. The scale bars are 200 nanometers (billionths of a meter). The dashed lines indicate domain walls and the arrows indicate the chirality or “handedness.” The underlying images in the top two rows were producing using a technique at Berkeley Lab’s Molecular Foundry known as Lorentz microscopy.
Credit: Berkeley Lab

Ferromagnetic and antiferromagnetic coupling of spin molecular interfaces

2018/05/282018/05/31

Researchers from the physics department of the Università “La Sapienza” in Rome, Centro S3 of Modena and ALBA, have demonstrated that magnetic coupling of metal-organic molecules to a magnetic substrate mediated by a graphene layer can be tuned in strength and direction by choosing the symmetry of the molecular orbitals that is largely preserved thanks to the graphene layer. The results have been published in the journal Nano Letters.

Paramagnetic molecules become potential building blocks in spintronics when their magnetic moments are stabilized against thermal fluctuations, for example, by a controlled interaction with a magnetic substrate. Spin molecular interfaces with preserved magnetic activity and exhibiting magnetic remanence at room temperature (RT) can open the route to engineer highly spin-polarized, nanoscale current sources. The need to fully control the organic spin interface and the tuning of ferromagnetic (FM) or antiferromagnetic (AFM) coupling to achieve a stable conductance has motivated a vast experimental interest.

>Read more on the ALBA website

Image: Figure 1: a,b) Antiferromagnetic/Ferromagnetic coupling as deduced by element-specific hysteresis loops of a FePc and CuPc (respectively) to a Cobalt layer with perpendicular magnetic anisotropy intercalated below graphene. c,d) orbital-porjection of the spin-density for the FePc and CoPc interface reflecting the different symmetry of the molecular orbitals involved in the ferromagnetic and antiferromagnetic interaction.

The power supplies giving Diamond a boost

2018/05/242018/05/25

The electrons that produce Diamond’s ultra-bright light whizz round the storage ring fast enough to travel around the entire world 7.5 times in a single second. But they don’t start out life super speedy, and they need a huge energy boost to get them ready for work!

Diamond’s electrons are generated in the injection system, where they are produced by a glowing filament (just like a dim light bulb) and accelerated to ninety thousand electron volts (90 keV). From there, a linear accelerator (linac) takes over, accelerating the electrons to a hundred million electron volts (100 MeV, or 0.1 GeV).

That’s not fast enough though, so the electrons from the linac are fed into the booster ring, where they’re are accelerated to 3 GeV by passing through an RF cavity millions of times. It’s like microwaving the electrons to get them to accelerate, which is not an easy task. The electrons want to travel in a straight line, and have to be forced to bend around the ring by dipole bending magnets. As the energy of the electrons increases, it gets harder to keep them moving around the booster ring, and the bending magnets need more power.

Image: Members of the Power Supply team working in the Booster Supply Hall.

Writing and deleting magnets with lasers

2018/04/182018/04/19

**Scientists * have found a way to write and delete magnets in an alloy using a laser beam – a surprising effect.**

* at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) together with colleagues from the Helmholtz-Zentrum Berlin (HZB) and the University of Virginia in Charlottesville, USA

The reversibility of the process opens up new possibilities in the fields of material processing, optical technology, and data storage.
Researchers of the HZDR, an independent German research laboratory, studied an alloy of iron and aluminum. It is interesting as a prototype material because subtle changes to its atomic arrangement can completely transform its magnetic behavior. “The alloy possesses a highly ordered structure, with layers of iron atoms that are separated by aluminum atomic layers. When a laser beam destroys this order, the iron atoms are brought closer together and begin to behave like magnets,” says HZDR physicist Rantej Bali.

Bali and his team prepared a thin film of the alloy on top of transparent magnesia through which a laser beam was shone on the film. When they, together with researchers of the HZB, directed a well-focused laser beam with a pulse of 100 femtoseconds (a femtosecond is a millionth of a billionth of a second) at the alloy, a ferromagnetic area was formed. Shooting laser pulses at the same area again – this time at reduced laser intensity – was then used to delete the magnet.

Image: Laser light for writing and erasing information – a strong laser pulse disrupts the arrangement of atoms in an alloy and creates magnetic structures (left). A second, weaker, laser pulse allows the atoms to return to their original lattice sites (right). (Find the entire image here)
Credit: Sander Münster / HZDR

Toward control of spin states for molecular electronics

2018/04/172018/04/19

The results represent a major step toward the goal of programmable, nanoscale molecular electronics for high-speed, low-power, logic and memory applications.

To squeeze more information into smaller spaces, we will need to downsize from microchips to the nanoscale: the molecular level. Compared to bulk silicon, organic molecules have many advantages when used as building blocks for memory and logic components. They can be implemented as flexible thin films, they can be easily printed, and their potential switching speed is high, while their power requirements are low. But the field of molecular spintronics is still very young, and before its promise can be realized, scientists need a fuller understanding of the fundamental physics in play.

A molecule with crossover appeal

In the molecule studied here—[Fe{H₂B(pz)₂}₂(bipy)]—the spin state is determined by the configuration of the central metal’s outer electrons (i.e., the Fe d-orbital electrons). The presence of the surrounding organic ligands splits the Fe d orbitals. If the splitting is large, the electrons will pair up in the lower orbitals (a low-spin state). If the splitting is small, the electrons can spread out over both levels (a high-spin state). For some classes of molecules, transitions from low- to high-spin states (and vice versa) can be triggered. This “spin crossover” phenomenon is a promising functionality that may be suitable for application in molecular spintronic devices.

>Read more on the ALS website

Image: The molecule studied in this work is a metal–organic coordination complex, i.e., a molecule with a transition metal (iron) at the center, surrounded by organic compounds (“ligands”). The molecular formula is [Fe{H₂B(pz)₂}₂(bipy)], where pz = pyrazol-1-yl and bipy = 2,2′-bipyridine. This picture shows a ball-and-stick model (see more here)

Sending electrons on a rollercoaster ride

2018/04/092018/04/26

A first-of-its-kind x-ray instrument for frontier research with high-brightness x-rays is now in operation at Argonne National Laboratory. The new device utilizes a unique superconducting technology that speeds electrons on a path much like that of a rollercoaster.

The insertion device (ID), called a Helical Superconducting Undulator (HSCU), was designed at the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility at DOE’s Argonne National Laboratory. The device has three primary advantages over other types of IDs for producing high-brightness x-rays: (1) it generates a stronger magnetic field than other IDs; (2) it allows researchers to select a single energy from the x-ray beam without using any x-ray optics; and (3) it produces an x-ray beam with circular polarization. Argonne developed the helical undulator with $2 million in funding from the DOE Office of Science.

>Read more on the Advanced Photon Source website

Image: Matthew Kasa and Susan Bettenhausen of the Advanced Photon Source (APS) Accelerator Division Magnetic Devices Group put the finishing touches on installation of the Helical Superconducting Undulator in Sector 7 of the APS storage ring.

Antiferromagnets as a new kind of information storage technology

2018/03/122018/03/16

Magnetic materials have been used for storing information for more than half a century, from the first magnetic tapes to modern data servers. These technologies have in common the usage of ferromagnets, producing magnetic fields which are easily measurable. Researchers at the University of Nottingham are working with Diamond Light Source to develop new technologies based on a different class of magnetic material: an antiferromagnet, which does not produce a magnetic field, but which has a hidden magnetic order that can be used to store the ones and zeros of information.

Looking at the atomic scale, each atom is like a small magnetic compass, having a small magnetic moment. In a ferromagnet, once the information is written, all those atomic moments remain oriented in the same direction. In antiferromagnets, each magnetic moment aligns exactly opposite to its neighbours, effectively cancelling them out (Figure 1). This arrangement has some important advantages for memory applications: magnetic bits do not interact with each other, so can be packed more closely; they do not interact with external magnetic fields; their resonant frequencies, which determine the speed that information can be written, is typically 1000 times larger than in ferromagnets. Antiferromagnets can therefore be useful, but how would you store and read information in a material whose total magnetic moment is always zero? Dr Peter Wadley, a researcher at the University of Nottingham, and Sonka Reimers, a joint Nottingham and Diamond PhD student, are trying to answer that question in their search for new technologies for information storage and processing.

Figure: Schematic of magnetic moment orientation for binary information storage using (left) a ferromagnet. Full image here.

Precise layer growth in a superlattice controls electron coupling and magnetism

2018/03/022018/03/12

Two-dimensional (2-D) crystalline films often exhibit interesting physical characteristics, such as unusual magnetic or electric properties. By layering together distinct crystalline thin films, a so-called “superlattice” is formed. Due to their close proximity, the distinct layers of a superlattice may significantly affect the properties of other layers. In this research, single 2-D layers of strontium iridium oxide were sandwiched between either one, two, or three layers of strontium titanium oxide to form three distinct superlattices. Researchers then used x-ray scattering at the U.S. Department of Energy’s Advanced Photon Source (APS) to probe the magnetic structure of each superlattice. The x-ray data revealed that the number of layers of the titanium-based material produced a dramatic difference in the magnetic behavior of the iridium-based layer. These findings are especially significant because the iridium compound is one of the perovskites, a class of materials known for their unique electric, magnetic, optical, and other properties that have proven useful in sensor and energy-related devices, and which are being intensively investigated for their application towards improved electronics and other technologies.

>Read more on the Advance Photon Source website
Image: Fig. 1. Illustration of superlattices. Panel (a) shows the Sr2IrO4 crystalline superlattice, with alternating layers of SrIrO3 and SrO. The SrIrO3 layers are perovskites, depicted as diamond-like shapes formed by six oxygen atoms; inside each diamond is a gold-colored iridium ion (cation), while pink strontium ions lay near the diamond’s ends. The SrIrO3 layers are separated by non-perovskite (inert) SrO layers, depicted as pink bars. Panel (b) shows the more-recently developed SrIrO3/SrTiO3 superlattice used for this research. Three distinct SrIrO3/SrTiO3 superlattices were created, having 1, 2, or 3 layers of inert SrTiO3 layers (colored green) separating the active SrIrO3 layers. Bold green boxes highlight the number of inert SrTiO3 layers in the three distinct lattices. While both SrIrO3 (gold diamonds) and SrTiO3 (green diamonds) are perovskites, the green-colored SrTiO3 layers buffer the active SrIrO3 layers. (The entire image is visible here)

Tuning magnetic frustration in a dipolar trident lattice

2018/02/212018/02/22

Frustrated interactions are key to a wide range of phenomena, from protein folding and magnetic memory to fundamental studies of emergent exotic states.

Geometrical frustration and “spin ice”

When bar magnets are brought together, opposite poles will attract and like poles will repel, and the magnets will arrange themselves accordingly, to minimize energy. However, if the magnets are constrained to a lattice structure where each one has just two possible orientations, some magnets could end up geometrically “frustrated,” with neither orientation being lower in energy than the other. The system becomes what’s known as a “spin ice,” analogous to water ice, which retains intrinsic randomness (residual entropy) even at absolute zero.

Systems incorporating geometrical frustration are fascinating because their hard-to-predict behavior is key to a wide range of phenomena, from protein folding and magnetic memory to the emergence of exotic states of matter. For example, the emergence of magnetic monopole–like excitations in spin ice raises the intriguing possibility of “magnetic-charge” circuitry based on currents of magnetic monopole excitations.

Image: (extract, entire image here) Magnetic scattering patterns calculated from XMCD data for various lattice parameters. While relatively sharp peaks indicative of long-range order are seen in (a) and (c), the diffuse patterns in (b) indicate highly disordered configurations.

An energy-resolution record for resonant inelastic x-ray scattering

2018/02/152018/02/21

Resonant inelastic x-ray scattering (RIXS) is a powerful technique for studying electronic excitations in a wide variety of new and complex materials, offering momentum- and energy-resolution and potentially even analysis of scattered polarization. Since its inception in the 1990s, the development of RIXS instrumentation and scientific subjects have benefited from a closely intertwined evolution; improvements in energy resolution and throughput, spurred by specific scientific cases, have in turn made new subjects of study feasible. In the continued quest for substantially improved energy resolution, a novel prototype RIXS flat-crystal spectrometer was recently tested at X-ray Science Division beamline 27-ID-B at the U.S. Department of Energy’s Advanced Photon Source (APS). The spectrometer established a new record resolution for RIXS below 10 meV, together with a promise to do even better soon.

Early RIXS work was aimed at the study of charge transfer excitations in transition metal oxides (TMO), including the high-T_c superconducting Cuprates, where electronic excitations could be observed at a few eV. As the understanding of strongly correlated electron systems progressed, orbital degrees of freedom came into focus: in many Mott insulators, transitions between the active d-orbitals, the “dd excitations”, were hot topics and could reliably be observed with the then state-of-the-art resolution of 100-200 meV. Magnetism and magnetic ordering are central questions in the study of correlated electron systems. For example, the layered perovskite Iridates showing strikingly similar magnetic exchange interactions as the Cuprates, implying that unconventional superconductivity might be found here, to the intriguing assertion that magnetic properties of honeycomb Iridates might point to a quantum spin liquid as ground state of this material, the spectrum of novel, exotic properties uncovered or anticipated promise a treasure trove of scientific discoveries. In the late 2000s, RIXS was established as a probe of magnetic excitations. However, spectral features associated with magnetic excitations (“magnons”) lie at a fraction of an eV or even in the sub-10meV regime. A significant advance in energy resolution is needed to attack such subjects with RIXS.

Figure: Schematic rendering of the new flat-crystal RIXS spectrometer.

Enhanced magnetic hybridization of a spinterface

2018/01/172018/01/24

Interfaces between organic semiconductors and ferromagnetic metals offer intriguing opportunities in the rapidly developing field of organic spintronics. Understanding and controlling the spin-polarized electronic states at the interface is the key toward a reliable exploitation of this kind of systems. It is indeed important to master and reliably reproduce the chemical reactions responsible of the spin-polarization at the interface.

Here we propose an approach consisting in the insertion of an ultrathin, two-dimensional Cr₄O₅ magnetic oxide layer at the interface between a C₆₀ fullerene organic semiconductor and a Fe(001) ferromagnetic metal to both maximize the spin polarization and to overcome the reproducibility issues usually present in case of direct interface between metallic layer and organic semiconductor.
C60 fullerene showed a greater surface diffusivity when growing on Cr₄O₅compared to the Fe(001) case. From the first stages of surface coverage, C₆₀ tends to form islands rather than isolated molecules, leading to a well-ordered growth at higher thicknesses (Figure 1, above).

>Read more on the Elettra website

Figure 1. STM image 200 x 200 nm² of the surface of a C₆₀/ Cr₄O₅/Fe(001) sample with a fullerene coverage of about 0.5 ML. The image was taken at room temperature with ΔV= 1.7 V, I = 400 pA.

“X-ray streaking” allows ultrafast processes to be followed using a single pulse of light

2017/12/222018/01/15

Grazing light for rapid events

An international team of scientists has developed a new experimental method at the FLASH X-ray laser which allows the sequence of events involved in a process to be observed using a single, ultrashort pulse of light from FLASH. Their method is called “X-ray streaking” and enables researchers to observe ultrafast processes continuously, instead of being confined to taking snapshots at discrete intervals using separate X-ray pulses. Apart from the extreme brightness of the FLASH beam, the scientists also made use of an X-ray lens which they introduced into the beamline in a particular configuration, so as to capture a chronological sequence of events using a single X-ray pulse. To demonstrate the functionality of X-ray streaking, they observed the ultrafast demagnetisation of cobalt.

The invention of X-ray lasers has considerably boosted the study of the dynamics of matter. Pump-probe experiments allow artificially induced (“pumped”) processes and reactions to be photographed (“probed”) using an extremely short X-ray pulse at predetermined intervals. Ideally, these photographs, taken with different time delays, can then be assembled to create a film showing the sequence of events during an ultrafast process with a temporal resolution of the order of femtoseconds. One limitation of this otherwise promising experimental technique is, however, that the experiment has to be conducted all over again for each time delay. This means that before each observation, the process of interest must be triggered using the same starting conditions and it must run through the same sequence of events – both of which rule out extreme experimental conditions.

>Read more on the FLASH website

Image Caption: (a,b) Raw images from the reflection and reference detectors respectively. Both the images for the pumped and the un-pumped event are acquired using a single x-ray pulse. (c) Transient reflectivity image (as defined in the text) calculated from the images shown in (a,b). (d) Reshaped transient reflectivity image after calibration of the time window. Article published in Scientific Reports.

Topological insulator gap in graphene contacted with Pb

2017/12/112018/01/17

Up to now the proposed modifications do not allow to introduce graphene to existing electronic devices.

Graphene is the most promising two dimensional material for nanoelectronic applications featuring the relativistic-like electronic spectrum. Contact of graphene with various materials and its functionalization allows to manipulate the electronic structure, e.g. to change the conductivity type and band gap creation. The latter is of great interest due to the requirements for graphene transistor realisation. Furthermore, graphene contact with heavy/magnetic metals results in the lifting of the spin degeneracy of the Dirac cone, opening the spintronics field for its applications. However, up to now the proposed modifications do not allow to introduce graphene to existing electronic devices.

>Read more on the Elettra website.

Image: a) Sketch of the studied system, the Pb atoms presented by yellow spheres; b) ARPES image of graphene/Pb/Pt(111) in the region of K point, taken as a sum of two spectra with p-and s-polarization of light; c) schematic spin structure of the graphene states in the case of large “intrinsic” spin-orbit interaction d) ARPES mapping of the system in two orthogonal k-directions near the K point of graphene.

Electrical hiding of magnetic information

2017/12/012018/01/15

Results have been published in Sientific Reports.

Researchers have proved the ability of peculiar magnetic materials to hide magnetic information and reveal it under certain conditions and at room temperature.

Since the 1950’s, magnetic materials have been used to store all kinds of information. Magnetically stored information is convenient because it is easily accessible using very well-known magnetic data reading procedures. However, sensitive information must be carefully stored to ensure confidentiality; thus easy access becomes a bad instead of a good feature. The optimal way to prevent unauthorised information access is to make it invisible.

>Read more on the ALBA website

Antiferromagnetic dysprosium reveals magnetic switching with less energy

2017/11/062017/11/13

HZB scientists have identified a mechanism with which it may be possible to develop a form of magnetic storage that is faster and more energy-efficient.

They compared how different forms of magnetic ordering in the rare-earth metal named dysprosium react to a short laser pulse. They discovered that the magnetic orientation can be altered much faster and with considerably less energy if the magnetic moments of the individual atoms do not all point in the same direction (ferromagnetism), but instead point are rotated against each other (anti-ferromagnetism). The study was published in Physical Review letters on 6. November 2017 and on the cover of the print edition.

Dysprosium is not only the atomic element with the strongest magnetic moments, but it also possesses another interesting property: its magnetic moments point either all the same direction (ferromagnetism) or are tilted against each other, depending on the temperature. This makes it possible to investigate in the very same sample how differently oriented magnetic moments behave when they are excited by an external energy pulse.

Image: A short laser pulse pertubates magnetic order in dysprosium. This happens much faster if the sample had a antiferromagnetic order (left) compared to ferromagnetic order (right). Credit: HZB