The spiral beam distinguishes the antiferromagnetic state

April 29, 2023

(Nanowerk News) Using a spiral x-ray beam generated at the Advanced Light Source (ALS), the researchers distinguish between the energetically equivalent (“degenerated”) states in the antiferromagnetic lattice (physical review B, “The antiferromagnetic real space configuration examined by dichroism in a scattered X-ray beam with orbital angular momentum”).

Work demonstrates the potential of these beams to investigate otherwise inaccessible properties, to better understand phenomena of fundamental interest and for applications such as spintronics. (a) In this experiment, circularly polarized x-rays are scattered by an antiferromagnet array with lattice defects, producing a spiral beam with orbital angular momentum (OAM) with positive and negative helicity. (b) The resulting diffraction pattern differs depending on the polarization and helicity of the beam (an effect known as dichroism). Peaks with diffraction order (H) +1 and -1 have positive and negative helicity (ℓ) respectively. Each peak has positive (red) and negative (blue) half circular dichroisms, with an inverted pattern for opposite helicities. (Image: Berkeley Labs)

X-ray beam with a twist

X-ray experiments are an important tool for understanding the electronic and magnetic properties of materials. The polarization (that is, the direction of the oscillating electromagnetic field) of x-rays is often used to investigate anisotropy or chirality. A property of x-rays that has not been used in experiments is their orbital angular momentum (OAM). X-rays with OAM have azimuth-varying phase, which means the phase rotates as the x-ray spreads out. This causes a gradient in the electromagnetic field, which can cause the bent photons to have different interactions with the material.

X-rays with OAM have a helicity of ℓ = ±1, which corresponds to whether the phase rotates clockwise or counterclockwise. Similar to how polarization is used in experiments, OAM can be used to investigate properties of chirality and magnetism, and potentially more exotic properties such as topology. It could also improve the resolution of x-ray imaging and microscopy techniques. In this work, the researchers demonstrate how helicity-dependent effects in resonant x-ray scattering (RXS) can be used to investigate the magnetic configuration of lattice.

Creates a crooked light

One way to create x-ray beams with OAM is by scattering from topological defects. Here, a square lattice of permalloy nanomagnets was synthesized on a silicon substrate. Two additional nanomagnets were inserted in the center to deform the topological edges. (a) Scanning electron microscopy image of the nanomagnet array with topological defects.  (b) The magnetic configuration, measured using a PEEM XMCD, showing the order of the antiferromagnetic lattice (a) Scanning electron microscopy image of the nanomagnet array with topological defects. (b) The magnetic configuration, measured using a PEEM XMCD, showing the antiferromagnetic order of the lattice. (Image: Berkeley Labs)

In ALS Beamline, photoemission electron microscopy (PEEM) with x-ray magnetic circular dichroism (XMCD) is used to image the magnetic configuration. The results showed that the arrangement of the nanomagnets is antiferromagnetic, where the direction of magnetization alternates on the adjacent nanomagnets.

To investigate what OAM rays can reveal about the antiferromagnetic lattice, RXS experiments were carried out with circularly polarized light at ALS Beamline (COSMIC Scattering). Scattering from the nanomagnets generates beams with positive and negative OAM helicities, and circular dichroism was used to compare beams with opposite helicities at different antiferromagnetic peaks.

Helicity-dependent scattering

The researchers found that the circular dichroism has a distinct pattern, which is reversed for beams with the opposite helicity. Furthermore, the antiferromagnetic lattice forms in one of two degenerate ground states, and helicity-dependent circular dichroism can be used to differentiate them. A circular arrangement of six red/blue diffraction patterns, with three showing red spots on top of blue spots, and three showing blue spots on red spots.  In the middle is the words 'Thermal cycling' Changing the antiferromagnetic ground state. When heated to 380 K and cooled back to room temperature, one of two antiferromagnetic ground states randomly forms. Here, room temperature dichroism is shown for each of the six thermal cycles. (Image: Berkeley Labs)

Since the two ground states degenerate, they will form with equal probability if the antiferromagnet is heated and returned to room temperature. To test this, the nanomagnet array was repeatedly heated to 380 K and then cooled. At room temperature, the two configurations emerge with nearly equal probability, as would be expected for random heat transfer between two degenerate ground states.

This was one of the first experiments to show how the helicity of light could be used to study magnetism. Information about the real-space magnetic configuration of lattices is usually not accessible in such experiments, so this work demonstrates the potential for OAM rays to gain information beyond what is usually obtained in other experiments.

Promising future avenues include using OAM rays in traditional antiferromagnet resonance diffraction studies, in domain and defect wall nanodiffraction studies, and, if OAM rays can be used to measure spin-specific sub-lattices, to measure spin currents directly.

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