Tag: magnetic domains

Extreme Domain Wall Speeds Observed in Ferromagnets

Extreme Domain Wall Speeds Observed in Ferromagnets

The manipulation of magnetic domains is of paramount interest because of its potential applications in spintronics and next generation technologies for mass storage. In current storage devices, such as hard disk drives, information is processed using either magnetic fields or spin currents. However, existing technologies are limited in speed, not only due to engineering factors but also because of fundamental limits in driving domain walls at high speed. The motion of domains driven using conventional magnetic field and spin currents is limited to a speed of about 0.5 km/s, due to the phenomenon named Walker breakdown. Above this threshold speed, domains become unstable and develop different spin dynamics. Interestingly, recent theoretical investigations have predicted that speeds exceeding 10 km/s are achievable in ferromagnetic materials when driven by an optical laser pulse.

In this work, we optically excited a CoFe/Ni multilayer sample and measured the ultrafast response of magnetic domains using small angle X-ray scattering at Diffraction and Projection Imaging (DiProI) beamline of the FERMI free electron laser. 

Read more on Elettra website

Image: Experimental schematic and evolution of labyrinthine domain pattern as a function of delay time. (a) Optical pump – EUV magnetic scattering probe experimental setup with highlighted an MFM image of the domain sample pattern. The white arrow indicates the preferential direction of the linear texture of the domain pattern. Magnetic diffraction scattering on the CCD is fitted with a 2D phenomenological model, from which we separate the ring and lobe components. (b) Isolated isotropic (ring) and anisotropic (lobes) fit components with arrows indicating the radius (qR, qL) and full-width half maximum (ΓR, ΓL) of scattering. Time-resolved (a) amplitude (AR), (b) ring radius (qR), and (c) width (ΓR) obtained from the fit of the isotropic scattering (ring). Delay curves are plotted for a range of measured fluence values from 0.8 to 13.4 mJ/cm2. The scattering amplitude, which is proportional to magnetization, decays immediately following laser excitation indicating demagnetization which recovers on picosecond timescales. The ring radius (qR) and width (ΓR) of the isotropic scattering approximate the average real-space domain size and correlation length of the labyrinthine domains, respectively.

Revealing the thermal heat dance of magnetic domains

Revealing the thermal heat dance of magnetic domains

Scientists invented a new way of tracking electronic properties inside materials, and used it to visualize magnetic domains in a previously unseen way.

Everyone knows that holding two magnets together will lead to one of two results: they stick together, or they push each other apart. From this perspective, magnetism seems simple, but scientists have struggled for decades to really understand how magnetism behaves on the smallest scales. On the near-atomic level, magnetism is made of many ever-shifting kingdoms—called magnetic domains—that create the magnetic properties of the material. While scientists know these domains exist, they are still looking for the reasons behind this behavior.

Now, a collaboration led by scientists from the U.S. Department of Energy’s Brookhaven National Laboratory, Helmholtz-Zentrum Berlin (HZB), the Massachusetts Institute of Technology (MIT), and the Max Born Institute (MBI) published a study in Nature in which they used a novel analysis technique—called coherent correlation imaging (CCI)—to image the evolution of magnetic domains in time and space without any previous knowledge. The scientists could not see the “dance of the domains” during the measurement but only afterward, when they used the recorded data to “rewind the tape.”

The “movie” of the domains shows how the boundaries of these domains shift back and forth in some areas but stay constant in others. The researchers attribute this behavior to a property of the material called “pinning.” While pinning is a known property of magnetic materials, the team could directly image for the first time how a network of pinning sites affects the motion of interconnected domain walls.

“Many details about the changes in magnetic materials are only accessible through direct imaging, which we couldn’t do until now. It’s basically a dream come true for studying magnetic motion in materials,” said Wen Hu, scientist at the National Synchrotron Light Source II (NSLS-II) and co-corresponding author of the study.

Read more on the Brookhaven National Laboratory website

Image: The image shows the areas where the borders of magnetic domains accumulate over time. It is similar to a photo of a traffic intersection taken at night with a long exposure time. In such a photo, we would see brighter areas along the paths that most cars’ headlights traveled. Here we see brighter areas where most domain walls come together.