Tunable ferromagnetism in a 2D material at room temperature

Breakthroughs in next-generation spintronic logic and memory devices could hinge on our ability to control spin behavior in two-dimensional materials—stacks of ultrathin layers held together by relatively weak electrostatic (van der Waals) forces. The reduced dimensionality of these so-called “van der Waals materials” often leads to tunable electronic and magnetic properties, including intrinsic ferromagnetism. However, it remains a challenge to tune this ferromagnetism (e.g. spin orientation, magnetic domain phase, and magnetic long-range order) at ambient temperatures.

In this work, researchers performed a study of Fe3GeTe2, a van der Waals material that consists of Fe3Ge layers alternating with two Te layers. The material’s magnetic properties were characterized using a variety of techniques, including x-ray absorption spectroscopy (XAS) with x-ray magnetic circular dichroism (XMCD) contrast at Beamline 6.3.1 and photoemission electron microscopy (PEEM) at Beamline 11.0.1.

>Read more on the Advanced Light Source (ALS) at LBNL website

Image: PEEM images for unpatterned and patterned Fe3GeTe2 samples at 110 K and 300 K. The unpatterned samples formed stripe domains at 110 K, which disappeared at 300 K. The patterned samples formed out-of-plane stripe domains at 110 K and transitioned to in-plane vortex states at 300 K, demonstrating control over magnetism at room temperature and beyond.

Extremely small magnetic nanostructures with invisibility cloak

Future data storage technology

In novel concepts of magnetic data storage, it is intended to send small magnetic bits back and forth in a chip structure, store them densely packed and read them out later. The magnetic stray field generates problems when trying to generate particularly tiny bits. Now, researchers at the Max Born Institute (MBI), the Massachusetts Institute of Technology (MIT) and DESY were able to put an “invisibility cloak” over the magnetic structures. In this fashion, the magnetic stray field can be reduced, allowing for small yet mobile bits. The results were published in Nature Nanotechnology.

For physicists, magnetism is intimately coupled to rotating motion of electrons in atoms. Orbiting around the atomic nucleus as well as around their own axis, electrons generate the magnetic moment of the atom. The magnetic stray field associated with that magnetic moment is the property we know from e.g. a bar magnet we use to fix notes on pinboard. It is also the magnetic stray field that is used to read the information from a magnetic hard disk drive. In today’s hard disks, a single magnetic bit has a size of about 15 x 45 nanometer, about 1.000.000.000.000 of those would fit on a stamp.

One vision for a novel concept to store data magnetically is to send the magnetic bits back and forth in a memory chip via current pulses, in order to store them at a suitable place in the chip and retrieve them later. Here, the magnetic stray field is a bit of a curse, as it prevents that the bits can be made smaller for even denser packing of the information. On the other hand, the magnetic moment underlying the stray field is required to be able to move the structures around.

>Read more on the PETRA III at DESY website

Credit: MIT, L. Caretta/M. Huang [Source]

Magnetic vortices observed in haematite

Magnetic vortices observed in antiferromagnetic haematite were transferred into ferromagnetic cobalt.

Vortices are common in nature, but their formation can be hampered by long range forces. In work recently published in Nature Materials, an international team of researchers has used mapped X-ray magnetic linear and circular dichroism photoemission electron microscopy to observe magnetic vortices in thin films of antiferromagnetic haematite, and their transfer to an overlaying ferromagnetic sample. Their results suggest that the ferromagnetic vortices may be merons, and indicate that vortex/meron pairs can be manipulated by the application of an in-plane magnetic field, giving rise to large-scale vortex–antivortex annihilation. Ferromagnetic merons can be thought of as topologically protected spin ‘bits’, and could potentially be used for information storage in meron racetrack memory devices, similar to the skyrmion racetrack memory devices currently being considered.

>Read more on the Diamond Light Source website

Image: Graphic outlining the antiferromagnetic rust vortices. The grayscale base layer represents the (locally collinear) magnetic order in the rust layer, and the coloured arrows the magnetic order imprinted into the adjacent Co layer.

Magnetization ratchet in cylindrical nanowires

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

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.

>Read more on the Advanced Light Source website

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

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.

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

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.

>Read more on the Diamond Light Source website

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

Writing and deleting magnets with lasers

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.

>Read more on the Bessy II at HZB website

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

Sending electrons on a rollercoaster ride

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

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.

>Read more on the Diamond Light Source website

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

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

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.

>Read more on the Advanced Light Source website

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

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-Tc 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.

>Read more on the Advanced Photon Source website

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

Enhanced magnetic hybridization of a spinterface

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 Cr4O5 compared 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 1STM 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

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.