Projects

New materials

Currently there is a global search for novel superconducting and magnetic materials (Katsnelson, Irkhin et al. 2008, Hideo, Keiichi et al. 2015). The particular interests lies on making new materials with tunable properties such as magnetization and superconductor´s energy gap. For magnetic materials metallic ferromagnets are the usual choice for applications. This is due to their relatively simple fabrication techniques and great reproducibility. Although these offer good magnetic properties, such as magnetization and coercivity, these are difficult to control. Some alloys do improve this tuning capabilities such as iron-copper and cobalt-copper (Co-Cu) and CuNi  (American Society for, United et al. 1980, Nishizawa and Ishida 1984). However depending of the concentration these may not show very well defined curie temperatures (Jiles 1991).  Other properties such as magnetic domain size also are difficult to control in conventional ferromagnets.Our proposal to tackle this issues is by investigating new materials. The aim of this work is to explore new growth methods in order to produce thin films of such materials. In film growth techniques we have considerable experience. Once this films have been prepared the intention is to characterize its structural magnetic and transport properties. This research has great implications not only in fundamental research but in the development of new electronic devices based on this materials.

Inverse proximity effect in Superconductor/Ferromagnet hybrids

A superconductor is a material which exhibits zero resistance at low temperature. This is the result of electron pair formation, known as Cooper pairs, which are responsible for superconductivity.When a superconductor is placed next to a ferromagnet some Cooper pairs penetrate the ferromagnet inducing superconductivity at the border. Such phenomena is known as direct proximity effect and constitutes a very active area of research today. This effect is responsible for the control of vortex orientation in a F/S/F structure (Patiño, Aprili et al. 2013). Also such structures have the ability to control the superconducting electron spin state; from singlet to triplet (Robinson, Witt et al. 2010). This has given birth to a new field of research called super-spintronics.The possibility of inducing these spin polarized triplets in a superconductor (instead of the ferromagnet) in known as inverse proximity effect. Although already predicted theoretically (Grein, Löfwander et al. 2013), has just been started to be explored experimentally (Kalcheim, Millo et al. 2015). The aim of this project is to investigate spin triplet components in superconductor/ferromagnet systems. For this purpose multilayers of superconducting and ferromagnetic materials will be grown and characterized their properties. The proximity effects will be studied using tunneling measurements which allow an indirect study of the density of states in the superconductor.

Quantum tunneling devices

Quantum tunneling has been the basis of numerous scientific and technological applications. Among the former is scanning tunneling microscopy (STM) where one can image individual atoms at a surface. Regarding technological applications the most famous one are Magnetic tunnel junctions (MTJ) which constitute the basis of the read heads in modern hard drives. MTJ consists of two ferromagnets separated by an insulating material.  These operate by transporting a very small amount of electrons across the structure via quantum tunneling.  When only one insulating material is used we speak of a single tunnel barrier. In the present project we wish to explore novel configurations of tunnel barrier junctions.  This is supported on previous experience on the investigation of simple tunnel junctions done in collaboration with high energy physicist Neelima Kelkar (Patiño and Kelkar 2015). In that occasion the collaboration centered on calculations of the so called dwell time which is related with the time an electron takes to cross the barrier. Current work is being carried out on calculations of current-voltage characteristics. We expect the current to drastically change from low to high values depending on the applied voltage. This has great applications in scanning tunneling microscopy and magnetic tunnel junctions among others.

Magnetic properties of Nano bars and Nano disks

It is well known that continuous magnetic films under applied field opposite to its magnetization break in small magnetic regions (domains) each one pointing in different directions. Little has been done experimentally in fabricating a large number of magnetic micro and nano bars simply using lithography to define the pattern and physical deposition techniques such as sputtering. This techniques offer the possibility of eliminating unwanted effects related to the fabrication method such as preferential growth orientation usually present using other techniques such as electrodeposition (Henry, Ounadjela et al. 2001).Nano disks can be fabricated using colloidal techniques and physical deposition techniques such as sputtering or ebeam (Cowburn 2002, Herreño-Fierro, Patiño et al. 2016). Given its nano scale dimensions, as our resent simulation work indicates, these can hosts single domain and magnetic vortices within the nano disks. Thus this type of structures could find applications as magnetic storage devices replacing current hard disk technology.

References

American Society for, M., et al. (1980). "Bulletin of alloy phase diagrams." Bulletin of alloy phase diagrams.

Cowburn, R. P. (2002). "Magnetic nanodots for device applications." Journal of Magnetism and Magnetic Materials 242–245, Part 1: 505-511.

Grein, R., et al. (2013). "Inverse proximity effect and influence of disorder on triplet supercurrents in strongly spin-polarized ferromagnets." Physical Review B 88(5): 054502.

Henry, Y., et al. (2001). "Magnetic anisotropy and domain patterns in electrodeposited cobalt nanowires." Eur. Phys. J. B 20(1): 35-54.

Herreño-Fierro, C. A. and E. J. Patiño (2015). "Maximization of surface-enhanced transversal magneto-optic Kerr effect in Au/Co/Au thin films." physica status solidi (b) 252(2): 316-322.

Herreño-Fierro, C. A., et al. (2016). "Surface sensitivity of optical and magneto-optical and ellipsometric properties in magnetoplasmonic nanodisks." Applied Physics Letters 108(2): 021109.

Hideo, H., et al. (2015). "Exploration of new superconductors and functional materials, and fabrication of superconducting tapes and wires of iron pnictides." Science and Technology of Advanced Materials 16(3): 033503.

Jiles, D. (1991). Introduction to Magnetism and Magnetic Materials.

Kalcheim, Y., et al. (2015). "Inverse proximity effect at superconductor-ferromagnet interfaces: Evidence for induced triplet pairing in the superconductor." Physical Review B 92(6): 060501.

Katsnelson, M. I., et al. (2008). "Half-metallic ferromagnets: From band structure to many-body effects." Reviews of Modern Physics 80(2): 315-378.

Nishizawa, T. and K. Ishida (1984). "The Co−Cu (Cobalt-Copper) system." Bulletin of Alloy Phase Diagrams 5(2): 161-165.

Patiño, E. J., et al. (2013). "Vortex flipping in superconductor/ferromagnet spin-valve structures." Physical Review B 87(21): 214514.

Patiño, E. J. and N. G. Kelkar (2015). "Experimental determination of tunneling characteristics and dwell times from temperature dependence of Al/Al2O3/Al junctions." Applied Physics Letters 107(25): 253502.

Ramírez, J.-G., et al. (2013). "Ultra-thin filaments revealed by the dielectric response across the metal-insulator transition in VO2." Applied Physics Letters 102(6): 063110.

Robinson, J. W. A., et al. (2010). "Controlled Injection of Spin-Triplet Supercurrents into a Strong Ferromagnet." Science 329(5987): 59-61.