Ali C. Basaran

Highly effective superconducting vortex pinning in conformal crystals

We have investigated the vortex dynamics in superconducting thin film devices with non-uniform patterns of artificial pinning centers (APCs). The magneto-transport properties of a conformal crystal and a randomly diluted APC pattern are compared with that of a triangular reference lattice. We have found that in both cases the magneto-resistance below the first matching field of the triangular reference lattice is significantly reduced. For the conformal crystal, the magneto-resistance is below the noise floor indicating highly effective vortex pinning over a wide magnetic field range. Further, we have discovered that for asymmetric patterns the R vs. H curves are mostly symmetric. This implies that the enhanced vortex pinning is due to the commensurability with a stripe in the non-uniform APC pattern and not due to a rearrangement and compression of the whole vortex lattice.

Magnetic field modulated microwave spectroscopy across phase transitions and the search for new superconductors

This article introduces magnetic field modulated microwave spectroscopy (MFMMS) as a unique and high-sensitivity technique for use in the search for new superconductors. MFMMS measures reflected microwave power as a function of temperature. The modulation induced by the external ac magnetic field enables the use of phase locked detection with the consequent sensitivity enhancement. The MFMMS signal across several prototypical structural, magnetic, and electronic transitions is investigated. A literature review on microwave absorption across superconducting transitions is included. We show that MFMMS can be used to detect superconducting transitions selectively with very high sensitivity.

Exchange bias: The antiferromagnetic bulk matters

Using controlled ion bombardment, the contribution of interface and bulk antiferromagnetic spins to exchange bias (EB) is investigated. Several sets of ferromagnetic (FM)/antiferromagnetic (AFM) (Ni/FeF2) bilayers capped with a nonmagnetic and inert Au layer of varying thickness were grown simultaneously. He-ion bombardment was employed to selectively create defects in the EB structure at the FM/AFM interface or in the AFM bulk. Numerical simulations provide the depth profile of the ion damage. Quantitative structural and magnetic characterizations were compared before and after the bombardment revealing the relationship between interfacial and bulk located defects. These studies show that the creation of defects in the bulk of the antiferromagnet crucially affects the magnitude of EB.

Methodology and search for superconductivity in the La–Si–C system

In this paper we describe a methodology for the search for new superconducting materials. This consists of a parallel synthesis of a highly inhomogeneous alloy which covers large areas of the metallurgical phase diagram combined with a fast, microwave-based method which allows non-superconducting portions of the sample to be discarded. Once an inhomogeneous sample containing a minority phase superconductor is identified, we revert to well-known thorough identification methods which include standard physical and structural methods. We show how a systematic structural study helps in avoiding misidentification of new superconducting materials when there are indications from other methods of new discoveries. These ideas are applied to the La?Si?C system which exhibits promising normal state properties which are sometimes correlated with superconductivity. Although this system shows indications for the presence of a new superconducting compound, the careful analysis described here shows that the superconductivity in this system can be attributed to intermediate binary and single phases of the system.

Detection of in-depth helical spin structures by planar Hall effect

We developed a method to determine the magnetic helicity and to study reversal mechanisms in exchange biased nanostructures using Planar Hall Effect (PHE). As a test case, we use an in-depth helical spin configuration that occurs during magnetization reversal in exchange coupled Ni/FeF2 heterostructures. We show the way to induce and determine the sign of the helicity from PHE measurements on a lithographically patterned cross. The helicity sign can be controlled by the angle between the externally applied magnetic field and a well-defined unidirectional anisotropy axis. Furthermore, the PHE signal reveals complex reversal features due to small deviations of the local unidirectional anisotropy axes from the crystallographic easy axis. The simulations using an incomplete domain wall model are in excellent agreement with the experimental data. These studies show that helical spin formations in nanomagnetic systems can be studied using laboratory-based magnetotransport.

Search for Superconductivity in Micrometeorites

We have developed a very sensitive, highly selective, non-destructive technique for screening inhomogeneous materials for the presence of superconductivity. This technique, based on phase sensitive detection of microwave absorption is capable of detecting 10−12 cc of a superconductor embedded in a non-superconducting, non-magnetic matrix. For the first time, we apply this technique to the search for superconductivity in extraterrestrial samples. We tested approximately 65 micrometeorites collected from the water well at the Amundsen-Scott South pole station and compared their spectra with those of eight reference materials. None of these micrometeorites contained superconducting compounds, but we saw the Verwey transition of magnetite in our microwave system. This demonstrates that we are able to detect electro-magnetic phase transitions in extraterrestrial materials at cryogenic temperatures.

Simple transition metal oxides(Conference Presentation)

Hybrid materials allow the engineering of new material properties by creative uses of proximity effects. When two dissimilar materials are in close physical proximity the properties of each one may be radically modified or occasionally a completely new material emerges. In the area of magnetism, controlling the magnetic properties of ferromagnetic thin films without magnetic fields is an on- going challenge with multiple technological implications for low- energy consumption memory and logic devices. Interesting possibilities include ferromagnets in proximity to dissimilar materials such as antiferromagnets or oxides that undergo metal-insulator transitions. The proximity of ferromagnets to antiferromagnets has given rise to the extensively studied Exchange Bias[1]. Our recent investigations in this field have addressed crucial issues regarding the importance of the antiferromagnetic [2-3] and ferromagnetic [4] bulk for the Exchange Bias and the unusual short time dynamics [5]. In a series of recent studies, we have investigated the magnetic properties of different hybrids of ferromagnets (Ni, Co and Fe) and oxides, which undergo metal-insulator and structural phase transitions. Both the static as well as dynamical properties of the ferromagnets are drastically affected. Static properties such as the coercivity, anisotropy and magnetization [6-8] and dynamical properties such as the microwave response are clearly modified by the proximity effect and give raise to interesting perhaps useful properties. Work supported by US-AFOSR and US-DOE

Corrigendum: Methodology and search for superconductivity in the La–Si–C system

IOP P UBLISHING S UPERCONDUCTOR S CIENCE AND T ECHNOLOGY Supercond. Sci. Technol. 25 (2012) 049501 (1pp) doi:10.1088/0953-2048/25/4/049501 Corrigendum: Methodology and search for superconductivity in the La–Si–C system 2011 Supercond. Sci. Technol. 24 075017 J de la Venta 1,2 , Ali C Basaran 1,2,3 , T Grant 4 , A J S Machado 4,5 , M R Suchomel 6 , R T Weber 7 , Z Fisk 4 and Ivan K Schuller 1,2 Department of Physics, University of California San Diego, La Jolla, CA 92093, USA Center for Advanced Nanoscience, University of California San Diego, La Jolla, CA 92093, USA Materials Science and Engineering, University of California San Diego, La Jolla, CA 92093, USA Department of Physics and Astronomy, University of California Irvine, Irvine, CA 92697, USA EEL—University of S˜ao Paulo, SP 12600970, Brazil Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA EPR Division Bruker BioSpin Corporation, Billerica, MA 01821-3931, USA E-mail: jdelaventa@physics.ucsd.edu Received 23 January 2012 Published 14 February 2012 Online at stacks.iop.org/SUST/25/049501 The correct figure 4(c) is presented below. This correction in the abscissa axis does not affect any other results or conclusions of the paper. We gratefully acknowledge Professor I Felner for pointing out this correction. Figure 4. (c) ZFC (−−) and FC (−N−) magnetization curves at 100 Oe for sample 3. c 2012 IOP Publishing Ltd Printed in the UK & the USA