Scientists find ordered magnetic patterns in disordered magnetic materials

A group of scientists working at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) has demonstrated that a particular property known as "chirality" - which may be used to transmit and store data in new ways - is present in nanometer-thick samples of multilayered materials with disordered structures.

While most electronic devices rely on the flow of charge from electrons, the scientific community is eagerly looking to revolutionize electronics by designing materials and methods to control other intrinsic electronic properties, such as their orbits and rotations around atoms, which may be thought of as compass needles pointing in different directions.

Scientists hope that these properties could lead to faster, smaller, more reliable data storage by facilitating spintronics - one aspect of which is the use of spin currents to manipulate domain and domain walls. Devices powered by spintronics can generate less heat and require less power than conventional devices.

In the latest study, published 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 the chirality, or handedness, of the transition region - called the domain wall - between neighboring magnetic domains with opposite spins.

The scientists wanted to control chirality - similar to the right or left hand - to control the magnetic domains and transfer zeros and fingers like traditional computer memory.

The sample consists of an amorphous alloy of gadolinium and cobalt sandwiched between ultrathin layers of platinum and iridium, which are known to strongly affect neighboring spins.

Modern computer circuits typically use silicon wafers based on a crystalline form of silicon that has a regularly ordered structure. In this latest study, the material samples used in the experiments were amorphous or non-crystalline, meaning that their atomic structure was disordered.

Experiments have shown that the magnetic properties of these domain walls have significant chirality and may flip to the opposite position. This flip mechanism is a key enabling technique in the field of spintronics and mutation studies based on electron spin properties.

The science team worked to determine the correct thickness, concentration and layering of elements, among other factors, to optimize this chiral effect.

"Now we've shown that we can have chiral magnetism in amorphous thin films, which no one has shown before," said the study's lead author, Robert Strubel, a postdoctoral researcher in Berkeley Lab's Materials Science Division. The success of the experiment opens up the possibility of controlling certain properties of the domain walls, such as chirality, temperature, and the chiral properties of materials converted with light, he said.

Streubel noted that despite the disordered structure, amorphous materials could also be fabricated to overcome some of the limitations of crystalline materials for spintronic applications. "We want to study these more complex materials that are easier to fabricate, especially for industrial applications."

The team acquired a unique high-resolution electron microscopy technique at Berkeley Lab's Molecular Foundry and conducted experiments in the so-called Lorentzian viewing mode to image the magnetic properties of material samples. They combined these results with ALS's X-ray technique, called magnetic circular dichroism spectroscopy, to confirm nanoscale magnetic hand signatures in the samples.

The Lorentz microscopy technique employed by Molecular Foundry's National Center for Electron Microscopy provides the tens of nanometers of resolution needed to resolve the properties of magnetic domains called spin textures.

"The high spatial resolution of this instrument allows us to see the chirality in the domain walls - we navigated the entire stack of material," said Peter Fischer, co-leader of the study and a senior scientist-scientist in the lab's Materials Science Division.

Fischer notes that increasingly precise, high-resolution experimental techniques - such as the use of electron beams and X-rays - now allow scientists to explore complex materials that lack well-defined structures.

"We're now looking at new types of probes," he says, and are drilling down to smaller scales. "New materials and discoveries often come at the interface of materials, which is why we ask: what happens when you put one layer on top of another? And how does that affect the spin texture, which is the magnetic landscape of the direction of rotation of the material? "

Fischer says the ultimate research tool is coming with next-generation electron and X-ray probes that will allow scientists to directly observe the magnetic transitions occurring at material interfaces in femtoseconds with atomic resolution (the time scale of a trillionth of a second.)

"So our next step is to get into the chiral dynamics of these domain walls in amorphous systems: imaging these domain walls as they move and observing how the atoms are assembled together," he said.Streubel added: "In almost