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Smart Alloys Surface Patterning for Properties Tuning

Periodic Reporting for period 1 - SASPAT (Smart Alloys Surface Patterning for Properties Tuning)

Reporting period: 2017-10-01 to 2019-09-30

Nowadays it is the current technological advances what pushes forward the limits of applied scientific research, being this even more emphasized in materials science research. In particular, active materials have become an attractive field of study, both from the fundamental point of view of their peculiar structures and properties, and from the applications in new products, where industry constantly requires the implementation of new materials to improve their performances. Among active materials, a particular class of them is attracting the attention of excellence research over the last decades: the so-called Shape Memory Alloys (SMAs). These materials are metal alloys that undergo phase transitions (resulting in large mechanical deformations) induced by changing the temperature and/or applying a stress on them. The phenomenon of the transformation itself is known as the shape memory effect and relies on the Martensitic Transformation. This transformation is accompanied by large recoverable strains, as the material will recover its original form by the application of the pertinent stimulus (temperature or stress). This makes the SMAs perfect candidates for a wide variety of applications ranging from bioengineering or biomedical actuators, to their implementation in a number of industries such as robotics, aerospace or automotive. Magnetic SMAs present a large magnetic-field-induced strain (MFIS), where the application of a magnetic field deforms the material: these are the so-called Magnetic Shape Memory Alloys (MSMAs). Hence, the actuation of the material can be produced also upon the application of a magnetic field, being a contactless reaction, and that the response happens in the milliseconds timescale. Well-known typical MSMA alloys are Ni2MnGa and non-stoichiometric derived compounds, presenting deformations up to 10% of their dimensions upon the application of moderate magnetic fields in the range of few kG.
The SASPAT project presents two main goals: (a) to train the talented young researcher Dr. Jose M. Porro, in the attractive growing field of Magnetic Shape Memory Alloys (MSMAs), where the host and partner institutions have developed their expertise and knowledge; and (b) the design, development and optimisation of smart patterned MSMA surfaces for applications in industrial sectors such as microfluidics and optical microswitching. The aim of the project is to exploit the effect of the martensitic transformation on the patterned MSMAs to fabricate tunable smart surfaces. The training program includes learning the processes for growing single crystals and epitaxial thin films made of MSMAs, the patterning of the surfaces using top-down and bottom-up lithography techniques, and the structural and magnetic characterization of the patterned surfaces to explore and optimize their optical response and wettability upon the application of the corresponding stimulus.
The project was divided into different well-defined work packages, from which the following results have been achieved:

• Thin films made of MSMAs with martensitic transformation temperatures of around 320 K (47ºC) and magnetic ordering temperatures of around 350 K (77ºC) were fabricated. The epitaxiality of these MSMA thin films was demonstrated, showing also well-defined crystalline phases both in the martensite and austenite phases. An interesting superferromagnetic-like state was found in a sample with a particular composition.

• MSMA single crystals with martensitic transformation temperatures of around 370 K (97ºC) and magnetic ordering temperatures of around 430 K (157ºC) were produced. These had lattice parameters so that their ratio (c/a) is similar to that obtained in MSMAs that show MFIS of around 12% shown in literature. Therefore, although direct measurements of the deformation induced by magnetic fields have not yet been performed, the fabricated single crystals are very likely to show MFIS higher than the proposed value. Hence, the grown single crystals are very promising candidates to show magnetic actuation at high temperatures.

• The influence of the atomic and magnetic ordering in the structural and magnetic properties of polycrystalline and single crystal MSMAs was studied. For this, several proposals to perform powder neutron diffraction, single crystal neutron diffraction and polarized single crystal neutron diffraction experiments were prepared and were awarded beamtime at the ILL institute (Grenoble, France). The results from the experiments demonstrated the correlation between the magnetic properties of the alloys and the atomic site occupancies of the different elements within the crystal lattice of the alloys. X-ray magnetic circular dichroism experiments were performed at the Spring-8 synchrotron (Hyogo, Japan), and the results obtained from the experiment allowed mapping the single element site specific magnetic spin densities of one of the MSMA single crystals.

• Patterning of epitaxial MSMA thin films was achieved using hard spheres colloidal lithography. Antidot lattices of MSMA thin films have been fabricated, and their properties are still under investigation. Patterning of single crystal MSMAs is being currently implemented at the time of writing the present report. Laser ablation techniques are being used for this purpose, allowing fast lithography times compared to other proposed patterning techniques. This objective is still ongoing.
The progress beyond the state of the art obtained within the SASPAT project so far is summarized in the section above this, and consisted in: (i) obtaining epitaxial MSMA thin films with actuation temperatures above 50°C and magnetic transition temperatures around 80°C, with theoretical magnetic field induced deformations of around 12% of their volume. A first-time realization of a superferromagnetic-like state in epitaxial MSMA thin films was observed by means of AC susceptibility measurements; (ii) MSMA single crystals with actuation and magnetic transition temperatures 20°C above currently reported ones were obtained, also showing theoretical magnetic field induced deformations above 12% of their volume; (iii) patterned epitaxial MSMA thin films are being currently obtained, while the patterning of single crystal MSMA surfaces is also being implemented. This part of the project is still ongoing.
The planning and implementation of the SASPAT project has undoubtedly had a clear impact in the current and future career prospects of the fellow. From a purely scientific point of view, the fellow has taken advantage of a multidisciplinary and collaborative environment. The network of collaborators of the fellow has been greatly extended not only due to the natural interactions created from the local environment between the different groups composing BCMaterials and the University of the Basque Country, but also through the different experiments undertaken in large scale facilities. All the undertaken activities have had a significant impact on his career, since all the incorporated competencies form an outstanding basis for establishing himself as an independent researcher. The successful implementation of the project, together with all the expertise gained by the fellow has proven excellent and crucial to his research career.
MSMA epitaxial thin film pattened into an antidot lattice shape.
Evolution of the atomic site occupancies as a function of the Fe% in a series of NiMnGaCoCu alloys