Frustration is defined as a competition between interactions such that not all of them can be satisfied. Geometrical frustration, which arises from the topology of a well-ordered structure become a topic of considerable interest since it can be induced and tuned in a controlled way. In particular, geometrical frustration among spins in magnetic materials can lead to exotic low temperature states, of great interest for fundamental and applied studies. In the specific, we are interested in the so called “artificial spin ices” (ASIs), which is a class of lithographically created arrays of interacting ferromagnetic nanometre-scale islands. From a different viewpoint, ASIs form a metamaterial where the properties are designed in and arise due to the engineering of the mesoscale properties (the size, shape, and placement of the islands). As such, they offer broad scientific and technological perspectives: implementation of statistical mechanics models of theory, model systems for the study of out-of-equilibrium thermodynamics, and prototypes for physical systems that store and process information in unconventional ways, (complex networks and neuromorphic computers). Recently, we demonstrate a method for the optical manipulation of ASIs by the activation of thermal fluctuations (TB) induced by heating the nanostructures with a laser beam. Depending on the wavelength and polarization of the electromagnetic radiation the heating is achieved via excitation of localized plasmon resonances (LSPs), i.e., coherent oscillations of quasi-free electrons in a metal nanostructure. This photo-thermal induced activation can be repeated as many times as desired, with a spatial resolution down to the micron-square and the heating/cooling speed can be varied at will. This new approach combining plasmonics and magnetic frustration opens the pathway to the systematic experimental study of fundamentsl properties of ASIs as well as to their application in opto-activated logic nanodevices.

In this project, we will direct our studies towards the design and fabrication of ASIs of various metamaterial geometries using hybrid structures comprising noble metals, for the efficient excitation of LSPs, together with ferromagnetic nanoelements with sizes and shapes designed to tuning the energy (temperature) for activate their thermal fluctuation. We will investigate the novel magnetic phases that would emerge from the ground states of frustrated lattices via magnetic imaging, as well as their collective dynamics excited by a pulsed electromagnetic radiation. To these purposes we will use the magnetic force and magneto-optical microscopy tools available at nanoGUNE.

Figure: Left, upper panel: possible vertices configurations in a square ASI; vertex configurations of type T1 are the lowest energy (ground state); vertex configurations T2-T4 have higher energy (vertex excitations). Left. lower panel:  Scannig electrom microscopy image of sample implementing a square ASI (a), followed by a magnetic force microscopy image after heating (c); the inset (b) shows an area with ground-state ordering; image (c) elaborated to show ground-state ordering regions (T1 vertex type, white contrast) are separated by lines of vertex excitations (T2 and T3 vertex types, blue and red contrasts) (d). Right: temperature increase of an Au nanostructure due to the excitation os a LSP with an electromagnetic radiation at 800 nm of wavelength.

References and reading list:

• R. F. Wang et al., Nature 439, 303 (2006)

• J. P. Morgan, A. Stein, S. Langridge,  and C. H. Marrows, Nature Phys. 7,  75 (2011)

• J. M. Porro, A. Bedoya-Pinto, A. Berger, and P. Vavassori, New J. Phys. 15, 055012 (2013)

• E. Nikulina, O. Idigoras, P. Vavassori, A. Chuvilin, and A. Berger, Appl. Phys. Lett. 100, 142401 (2012).

• T. Verduci, C. Rufo, A. Berger, V. Metlushko, B. Ilic, and P. Vavassori, Appl. Phys. Lett. 99, 092501 (2011).

SHORT DESCRIPTION OF THE GROUP:

The Nanomagnetism Group at CIC nanoGUNE is conducting world-class basic and applied research in the field of magnetism in nano-scale structures. The Group staff has a longstanding expertise and proven track record in fundamental and applied aspects of nano-magnetism, and specifically in the use of magneto-optical methods. The main scientific topics pursued by the Nanomagnetism Group are:

• understanding magnetism and magnetic phenomena on very small length and very fast time scales in systems with competing interactions by means of experiments and theory
• development of advanced methodologies and tooling for magnetic materials characterization at the nanometer-length scale and the picosecond-timescale (especially magneto-optics)
• design, fabrication and characterization of novel nanometer-scale magnetic structures, meta-magnetic materials, thin films and multilayers
• novel concepts for applied magnetic nano-scale materials

More info: http://www.nanogune.eu/en/research/nanomagnetism 

APPLICATION:

If you are a master student and you are interested in this project, please get in touch with the scientist in charge: Paolo Vavassori (p.vavassori@nanogune.eu)

To apply for a master scholarship fill in the form below and follow the instructions and recomendations of the general call open until 30 June 2020.