Magnetisation obtained from the magnetic tomography data at the MISTRAL beamline.

Researchers from the Physics Department of the Universidad de Oviedo, CINN-CSIC and HZB, in collaboration with ALBA, have explored the magnetic configuration of ferrimagnetic structures often employed to build modern spintronic devices and magnetic recording media. At the MISTRAL beamline of ALBA, using vector magnetic tomography, a magnetic trilayer fabricated at Oviedo was characterized.

These findings will permit to generate precise physical models describing the magnetic behaviour of this type of systems and control and exploit them for the design of spintronics and magnetic storage devices.

Modern spintronic devices and magnetic recording media often consist of complex magnetic structures. These structures are designed by precisely adjusting magnetic interactions as exchange, anisotropies and magnetostatics to achieve specific characteristics.

A collaboration between researchers from the Physics Department of the Universidad de Oviedo, the Centro de Investigación en Nanomateriales y Nanotecnología (Oviedo), the Helmholtz Zentrum Berlin für Materialien und Energie (Berlin) and the ALBA Synchrotron have investigated a combination of ferrimagnetic structures, often employed to build this type of devices. 

At the MISTRAL beamline of ALBA, a dichroic vector magnetic tomography of the device was performed and it revealed details of complex magnetisation configurations of the sample. The importance of synchrotron light lies in the fact that this information is currently impossible to evidence with other techniques when studying magnetic thin films.

The ability to characterize the configuration of the magnetisation in complex structures with competing magnetic interactions will permit to generate precise physical models describing the magnetic behaviour of these systems. Thus, experts will be able to control and exploit them for the design of spintronics and magnetic storage devices.

Ferrimagnetic trilayer thin film

The trilayer was fabricated with DC sputtering, a physical process in which the atoms of a solid material are vaporised by bombarding it with energetic ions, and each layer had a thickness of 80nm.

The central layer is a NdCo amorphous alloy that has perpendicular magnetic anisotropy. The black thin arrow indicates the magnetisation of the Co atoms, while the wide arrow indicates the magnetisation of the layer. The top layer is a GdCo amorphous alloy with relatively weak in-plane anisotropy. Co and Gd are antiferromagnetically coupled, as indicated by the black and white arrows, and the composition of the layer is such that the Gd magnetisation dominates over the Co resulting in a net magnetisation pointing down, as indicated by the wide red arrow. On the contrary, the bottom layer is dominated by the Co, resulting in a net magnetisation with the same sense than the central layer.

Sketch of the magnetic trilayer with competing magnetic interactions.

The magnetisation configuration of the top and bottom layers across their thickness will be the result of three competing interactions:

  1. The exchange interaction at the interfaces, which is in fact dominated by that of the Co atoms and favours the ferromagnetic coupling of Co. This is indicated in Figure 1, where the three Co magnetisations have the same direction (thin black arrows).
  2. The perpendicular uniaxial anisotropy in the central layer against in-plane anisotropies in the top and bottom layers.
  3. The magnetostatics that tends to eliminate discontinuities in the magnetisation and favours parallel alignment of the magnetisation at the interfaces by creating closure domains.

In addition to this already complex scenario, the central layer at remanence develops stripe domains with alternating signs of the normal magnetisation, which are imprinted in the top and bottom layers by the exchange interaction.

Magnetic tomography with synchrotron light

In recent years, several experimental tools have been capable to visualize the 3D magnetisation of magnetic structures. Lorentz microscopy [1] in very thin samples and X-ray dichroic tomography [2, 3, 4] have produced good results.

Following this track, researchers investigated at the MISTRAL beamline the magnetic configuration of the sample sketched in Figure 1 by tomographic X-ray magnetic dichroism in transmission mode.

The main figure displays the magnetisation, obtained from the magnetic tomography data, of the top layer in a plane located near the external surface of the sample. The z axis is normal to the layers and to the plane of the figure, and the colour indicates the sign of magnetisation on the y-axis (my).

The figure shows four magnetic domains in a chess board arrangement that have magnetisation approximately parallel to the y axis with opposite senses. As indicated by the arrows, the magnetisation has a relatively large in-plane component in the x-y plane, which evidences the dominance of magnetostatics in the terminating surface.

(a) Zoom of Figure 2 showing a part of the wall separating domains MD1 and MD4. (b) and (c) Magnetisation along the z and y axis (mz and my) in the cross-section plane indicated by the dashed line in (a).

The figure illustrates the continuity of magnetisation between the central and bottom layers due to the Co dominant character of the magnetisation in the bottom ferrimagnetic layer (middle-to-bottom region in the cross sections). However, there is a clear discontinuity between the top GdCo layer and the central NdCo one, as the Gd moment is the one dominating the magnetisation behaviour in the top layer. Thanks to the volume resolution of the tomography method, it is possible to observe the existence of two magnetic singularities at this interface, a vortex and an antivortex, inside the orange and red circles.

With the collaboration of Fundación Española para la Ciencia y la Tecnología. The ALBA Synchrotron is part of the UCCs of the FECYT and has received support through the FCT-20-15798 project.