ALBA Synchrotron
The vector tomography method developed at MISTRAL beamline of the ALBA Synchrotron enables to visualize with nanometric resolution the orientation of the magnetization in magnetic singularities located in magnetic films or multilayers. After 10 years of research, it is reported the first observation of hyperbolic Bloch points, attractive entities for magnetic information transport.
About 70% of all the digitally stored data in the world are located in magnetic bits on disks that have to rotate to reach the location of movable reading sensors. This storage technology, thirty years old, consumes energy and dissipates heat at undesired levels. The search for more efficient methods has been, and still is, an active field of investigation.
Instead of movable parts as disks that require electrical motors, one aims to move the magnetic domains in magnetic ultra-thin films by applying electrical currents or other excitations reducing the operating powers by orders of magnitude. Within this general approach, known as spintronics, the magnetic domains and the walls that separate domains with opposite magnetization are very important actors.
The magnetic structure of the domain walls historically classified in Bloch and Neel types, includes singularities namely skyrmions, merons and Bloch points among others, that have only been observed in recent years. The structure of the magnetization in these singularities may confer them enough energetic stability to be considered as possible dynamic entities for spintronic-based magnetic memories.
As their sizes are nanometric and their magnetic conformation is in general complicated, state of the art microscopy methods are required to visualize them. At the MISTRAL beamline of the ALBA Synchrotron this topic has been investigated since already ten years and progressively the accuracy of the description of magnetic entities has been improved. The experimental method, known as vector magnetic tomography, allows to visualize with nanometric resolution the orientation of the magnetization in magnetic singularities located in magnetic films or multilayers. It is based on the angular dependence of the dichroic magnetic absorption (different X ray absorption for right and left handed circularly polarized photons).
An example of this synchrotron light-based technique is the most recent published work done by scientists of the Physics department of the Oviedo University and ALBA scientists. The sample, a trilayer of GdCo/NdCo/GdCo, was grown by co-sputtering on Si-N membranes in Oviedo and it was engineered for displaying ferro and antiferromagnetic couplings as sketched in panels a and b of the next figure.
Magnetic configuration of the trilayer. Panel a, large arrows, shows the net magnetization of each layer. The two in the top are antiferromagnetically coupled (magnetizations in opposite senses) whereas the bottom pair are ferromagnetic coupled (magnetizations in the same sense). The double arrows indicate easy anisotropy axis in each layer. Aint Top and Aint Bot indicate the effective exchange coupling at the interfaces. Panel b shows the individual magnetic moments of Co (black) and Gd (purple). Note that the different proportions of Gd and Co in the top and bottom layers results in net magnetization pointing down at the top layer and pointing up at the bottom layers.
Careful analysis of the magnetic tomograms gave the result depicted in the next image that is completely experimentally determined.
Several selected Bloch points in the trilayer located at the top or bottom interfaces. The z axis is perpendicular to the film. At the bottom interface with parallel coupling, Bloch points are circulating (BP1 and BP2) whereas hyperbolic Bloch points nucleate at the top interface that has antiferromagnetic coupling.
Bloch points 3 and 4 have the magnetization pointing radially inwards at the equatorial plane and outwards at the north/south poles resulting in a hyperbolic configuration. To the best of our knowledge this is the first reported observation of hyperbolic Bloch points.
These results illustrate the strength of the vector tomography method developed at MISTRAL beamline and the variety and complexity of Bloch points in engineered interfaces.
Reference: Javier Hermosa-Muñoz, Aurelio Hierro-Rodríguez, Andrea Sorrentino, José I. Martín, Luis M. Alvarez Prado, Eva Pereiro, Carlos Quirós, María Vélez & Salvador Ferrer. Hyperbolic Bloch points in ferrimagnetic exchange spring. Physics Vol. 61, June 24, 10771.