Materials with anisotropic structures have a wide variety of uses in both optical and magnetic systems. So far, however, both their rarity and brittleness have prevented their widespread application. In his research, Professor Miguel Arranz at the University of Castilla-La Mancha in Spain uses advanced techniques to produce these structures artificially. His team’s innovations have now produced robust materials displaying the unique properties of both optical and magnetic birefringence: offering potential improvements to technologies including magnetic sensors, and microscopy on ultra-small scales.
When light passes through a transparent medium, it will be slowed down to varying degrees, depending on the refractive index of the material. In the majority of cases, this value remains the same, regardless of the paths taken by light passing through a material: a property named ‘isotropy.’ Intriguingly, however, there are exceptions to this rule in a small group of more advanced crystals. Named ‘anisotropic’ materials, the intricate arrangements of their crystal lattices mean that their refractive indices can vary, depending on the direction of travel of the light entering them. In turn, this means that light can be slowed down to different degrees within the same material.
“This is the origin of the ‘birefringence’ effect, which was first shown in calcite crystals,” explains Prof Arranz. “Since then, this phenomenon has been largely applied in many optical devices, including polarisers, beam splitters and wave plates.” Despite such valuable and widely varied applications, the use of anisotropic materials in practical research has remained limited so far. Not only are the materials rare and expensive: their delicate crystal structures also make them prone to shattering. In his research, Prof Arranz explores advanced techniques for fabricating more robust anisotropic materials, with the aim of making them more practical and affordable in real-world applications.
Through several previous studies, Prof Arranz and his colleagues have shown how anisotropic materials can be created artificially, by altering the structures of more conventional isotropic materials in a number of possible ways. “We can induce anisotropy either in their crystalline structures, the alignment of their microscopic constituents, or in their macroscopic shapes,” he describes. “This last case is particularly common in 2D systems, whose surfaces can be modified along a defined direction, breaking the initial isotropy to show the birefringence effect.”
To do this, Prof Arranz’s team uses cutting-edge techniques to fabricate ‘laser-induced periodic surface structures’ (LIPSS), which form patterns using the properties of polarised light. Since light is a transverse wave, the electrical field, E, of a natural light beam will oscillate in all directions perpendicular to their path of travel. In polarised light, on the other hand, its electrical field in these waves is forced to oscillate in just one perpendicular direction. When 2D materials are irradiated by an ultra-short pulse train of polarised laser beams, this means that an interference fringe is obtained onto their surface, whose periodicity is in the order of the laser wavelength and its symmetry axis is parallel to the E direction. The end result is a material featuring LIPSS, which can be fine-tuned to display enhanced properties including friction and surface wetting. Yet for Prof Arranz, the optical properties of these structures are particularly interesting.
Anisotropy in rippled materials
Under the right conditions, Prof Arranz and his team have shown how LIPSS can be used to fabricate robust anisotropic materials, starting from entirely isotropic 2D surfaces. “The resulting surface consists of a regular array of ripples, whose symmetry axis runs parallel to the polarisation direction of the incident laser beam,” he says. “Later, the light transmitted across that undulated 2D system travels with two different speeds, as its E aligns normal or parallel to the ripples.”
This effect occurs since light is an electromagnetic wave: containing both transverse electric and magnetic waves, which oscillate both perpendicular, and perfectly in sync with each other. In turn, the electric and magnetic field strengths of these waves oscillate in the same direction as their respective waves. This means that depending on whether a wave’s electric field is parallel or perpendicular to the direction of a material’s LIPSS ripples, its refractive index can vary. Through their experiments, Prof Arranz’s team have now artificially induced this property in several materials: from 2D glass plates to more flexible polymer foils. So far, this has enabled the researchers to fabricate anisotropic materials cheaply and reliably, and with a wide variety of desirable properties. However, optical waves are not the only subject of their focus.
Related effects in magnetic materials
Magnetic materials acquire their properties through the overall alignment of their constituent atoms, whose orbiting electrons create countless tiny loops of electrical current. Since magnetic fields must always be generated perpendicular to electrical currents, a field forms along the direction of this alignment. The flip side of this effect is that the alignment of these current loops must also be altered when external magnetic fields are applied to the materials. This means that when an external field alternates in direction over time, the magnetisation of a 2D material will be driven to alternate itself.
In most magnetic materials, these alternations will occur regardless of the orientation of the external field – a key example of isotropy. In anisotropic magnetic materials, however, different microscopic mechanisms are found to reverse their magnetisation, depending on the relative orientation between the external field and the crystal structure. “Accordingly, an electromagnetic wave will interact differently with the surface magnetisation when traversing the magnetised media,” Dr Arranz adds. “That yields the occurrence of two different transmission velocities for the emerging light: the so-called ‘magnetic birefringence effect.’” Where conventional birefringence arises from an in-built anisotropy in a material’s refractive index, this more subtle effect arises from a more adjustable anisotropy in the dominant mechanism responsible for magnetisation reversal of 2D materials.
Exploring different techniques
Like its optical counterpart, magnetic anisotropy can be fabricated by starting from isotropic metallic materials, and rearranging their molecular structures using advanced techniques. Yet because these materials are not transparent and strongly reflective, LIPSS cannot be applied to them directly. Instead, ripples must sometimes be created using sculpting beams of charged ions. Through a technique named ‘sputtering at grazing incidence,’ ions can set off cascades of collisions between atoms comprising the surface layers of a 2D material. By precisely controlling the energies, masses, and approach angles of their ion beams, researchers can finely tune the molecular structures and pattern the surface of those films at nanometer scale.
Through their experiments, Prof Arranz and his colleagues have clearly demonstrated two different techniques for generating magnetic birefringence. “In the first case, anisotropy was induced in a polymeric substrate using LIPSS. Magnetic alloy films were then deposited on top of the undulated polymeric foils, inducing magnetic birefringence,” he describes. This technique bypassed the lack of transparency and flexibility of the alloys, enabling the team to produce ripples as a secondary effect. “In the second case, cobalt surfaces grown on glass plates are eroded with an argon ion beam, creating a rippled structure,” Prof Arranz continues. Using advanced mathematical calculations to guide the sputtering of this beam, the researchers could generate their desired effects more directly.
In each case, the team clearly demonstrated how magnetic birefringence can be realised through relatively simple fabrication techniques. Prof Arranz envisages a number of important applications for these techniques in the near future. Among these are magnetometers: sensors which measure the strengths and directions of external magnetic fields. By incorporating anisotropic magnetic materials into these devices, researchers could measure the orientations of external fields far more accurately, offering new advances in applications ranging from mechanical stress measurement to metal detecting.
In addition, magnetic birefringence could improve the capabilities of transmission optical microscopy, having in mind the high sensitivity of the E polarisation plane to detect any anisotropy source ruling the magnetisation reversal, in either the surface or the bulk of a 2D magnetic media. For instance, the existence of a tiny thickness gradient or inhomogeneities in the magnetic layer are enough to create different mechanisms to switch the magnetisation there.
At the same point, that would yield a measurable magnetic birefringence effect in the transmitted light, by simply changing the direction of the external magnetic field. Thus, recording that magneto-optical effect all over the 2D sample, we could obtain an intensity scan, enabling us to show a high resolution image of its surface or internal structure. Prof Arranz and his colleagues will now continue to explore these effects and the techniques for generating them in even more detail. With further improvements, their innovations could soon be adopted by researchers worldwide.
- Colino, J.M., Arranz, M.A. (2011). Control of surface ripple amplitude in ion beam sputtered polycrystalline cobalt films. Applied Surface Science, 257(9), p.4432-4438.
- Arranz, M.A., Colino, J.M., Palomares, F.J. (2014). On the limits of uniaxial magnetic anisotropy tuning by a ripple surface pattern. Journal of Applied Physics, 115(18), p.183906.
- Arranz, M.A., Colino, J.M. (2015). Angular tuning of the magnetic birefringence in rippled cobalt films. Applied Physics Letters, 106(25), p.253102.
- Arranz, M.A., Colino, J.M. (2016). Magnetooptical Voigt effect in rippled polycrystalline Co films. Journal of Physics D: Applied Physics, 49(40), p.405306.
Prof Miguel A. Arranz’s current research interest lies in magnetic anisotropy and magneto-optical effects in two-dimensional systems.
Spanish Ministry of Economy and Competitiveness, MINECO (MAT2006-06242 and MAT2015-65295-R) and Junta de Comunidades de Castilla-La Mancha (No. PPII10-0054-1318).
- Dr Elena H. Sánchez and Prof Dr José M. Colino, Institute of Nanoscience, Nanotechnology
and Molecular Materials, Universidad Castilla-La Mancha, 45071 Toledo, Spain
- Dr Esther Rebollar and Dr Marta Castillejo, Institute of Physical-Chemistry Rocasolano, Consejo Superior de Investigaciones Científicas, 28006 Madrid, Spain
Miguel A. Arranz received his PhD in 1994 from Universidad Autónoma from Madrid, Spain. He became Associate Professor at Universidad Castilla-La Mancha, Ciudad Real, Spain in 2003.
Departamento de Física Aplicada
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W: Investigación – Sitio Personal Miguel Angel Arranz (uclm.es)