Tuning the perpendicular magnetic anisotropy of CoCr layers in multilayered systems   The   combination   of   Pt   with   Co   either   in   alloy   or   in   multilayer   form   is   widely   studied   among   the   potential   magnetic   media   for   ultrahigh   density magnetic   recording.   On   the   other   hand   the   combination   of   Co   with   Cr   in   alloy   form   is   currently   providing   commercial   magnetic   media.   In   an   effort   to further   exploit   and   benefit   from   both   systems,   we   fabricated   Co 1-x Cr x /Pt   multilayers   with   two   adjustable   parameters.   The   first   one   is   the   Cr concentration   on   CoCr   layer   (x   =   0,   0.05   0.30),   which   modulates   segregation   effects   on   Co   grains,   thus   tunes   macroscopic   magnetic   features   such as   saturation   magnetization   and   coercive   field.   The   second   one   is   the   small   layer   thickness   (<6   nm)   that   affects   interlayer   coupling,   perpendicular magnetic anisotropy and magnetization enhancement through spin polarization of Pt atoms in a ferromagnetic environment.
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Topics Synthesis of metallic (mainly magnetic) and semiconductor thin films, multilayers and nanoparticles Structural, magnetic, electric, magneto-optic characterization of magnetic nanostructures Activities My research activities include growth, structural and electronic properties of magnetic nanostructures. Since 1995, (during my PhD thesis) I was involved in magnetic multilayer fabrication and electronic properties of them. By   the   end   of   my   PhD   studies   (since   2000)   I   am   involved   in   magnetic   nanoparticle   projects,   participating   in   synthesis   via   physical   and   chemical   methods   and   property characterization. More recently, I also work on magnetic field hyperthermia in colloids and cell cultures and its correlation with structural, morphological and magnetic features. This expertise is exploited in the optimization of collective magnetic features in a wide range of biomedical applications focusing on magnetically activated cancer treatments. Highlights
The effect of composition and structural ordering on the magnetism of FePt nanoparticles Spherical   4   nm   FePt   nanoparticles   were   synthesized   by   the   simultaneous   decomposition   of   Fe(CO) 5    and   the   polyol   reduction   of   Pt(acac)2.   The final   Fe-to-Pt   composition   was   tuned   between   15   -   55   at.%   by   varying   the   ingredient   precursor   ratios.   Structural   ordering   is   promoted   in   all   cases, though   samples   approximating   equiatomic   Fe/Pt   ratios   eventually   transform   to   fct-FePt   phase   while   the   FePt3-phase   is   favored   for   the   Pt-richer samples.   Consequently,   the   magnetic   features   of   the   annealed   nanoparticles   may   be   categorized;   the   hard   magnetic   FePt   region   dominating   for Fe   content   between   40   -   55   at.%   and   the   soft   magnetic   FePt3   region   dominating   in   the   region   20   -   30   at.%   while   Fe   content   less   than   20   at.% results in Pt-richer phases with diminishing ferromagnetic behavior.
Pt-Co Multilayers: Interface Effects at the Monolayer Limit Structural,   magnetic   and   spectroscopic   magneto-optical   features   of   Pt-Co   multilayers   prepared   by   e-beam   evaporation   are   studied.   In   order to   examine   interface   and   induce   magnetism   effects,   the   layer   thickness   was   always   kept   under   4   monolayers   as   to   form   a   hybrid   structure between   alloys   and   the   conventional   multilayers.   X-ray   diffraction   spectra   revealed   the   multilayer   nature   of   the   samples   while   magnetometry measurements   recorded   tuneable   perpendicular   magnetic   anisotropy   and   enhanced   magnetization   values   due   to   the   high   degree   of   Pt- polarization   in   a   ferromagnetic   environment.   The   experimental   technique   that   allows   one   to   study   the   small   induced   magnetism   is   the   X-ray magnetic   circular   dichroism   (XMCD)   which   provides   quantitative   information   on   spin   and   orbital   magnetic   moments   of   the   absorbing   atom   in both   amplitude   and   direction.   XMCD   experiments   were   performed   at   ESRF   on   the   ID12   beamline   at   Pt   L 2,3 -edges   where   enhanced   Pt contribution on the perpendicular anisotropy was evidenced.
Critical radius for exchange bias in naturally oxidized Fe nanoparticles Monodisperse    Fe    nanoparticles    prepared    by    thermal    decomposition    were    readily    oxidized    on    exposure    to    air.    The    resulting nanocrystals    have    been    identified    as    inverse    spinels,    with    the    γ-Fe 2 O 3     as    the    dominant    phase    of    the    small    5    nm    iron    oxide nanocrystals,   while   the   proportion   of   the   Fe 3 O 4    component   gradually   increases   with   particle   size.   The   small   particles’   volume   resulted in   finite   size   effects   i.e   the   magnetization   deviates   from   the   T 3/2    Bloch's   law.   High   field   irreversibility   and   shifted   hysteresis   loops   after field   cooled   processes   have   been   detected,   and   attributed   to   a   low   temperature   surface   spin-glass   layer.   Moreover,   there   is   a   critical diameter, below which the surface spin-glass behaviour and exchange bias effect abruptly disappear.
Αpplication of magnetic nanoparticles as magnetic hyperthermia agents In   this   project   we   study   the   heating   efficiency   of   magnetic   nanoparticles   and   their   application   for   magnetic   hyperthermia   on   cancer   cells.   Firstly, structural,   morphological   and   magnetic   characterization   schemes   were   performed   to   determine   crucial   factors   for   optimizing   their   heating   potential, such   as   size,   polydispersity,   saturation   magnetization,   coercivity.   In   an   effort   to   simulate   the   in   vivo   environment   of   animal   tissue   phantoms   and   study the   thermal   heating   effects   resulting   from   Brownian   motion   and   hysteresis   losses,   nanoparticles   at   various   concentrations   were   embedded   in   aqueous media   of   varying   agar   concentration.   During   the   in   vitro   application   cancer   and   normal   cell   lines   were   incubated   with   magnetic   nanoparticles adequately   functionalized.   The   heating   profile   of   the   particles   as   studied   in   different   concentrations   and   in   correlation   with   their   potential   cytotoxicity. Our   results   revealed   concentration   dependent   cytotoxicity   profile   and   uptake   efficiency   together   with   variable   specific   loss   power   values   yet   with   fast thermal response, opening novel pathways in material selection as hyperthermia agents.
Layer-resolved magnetic moments in Ni/Pt multilayers Ni n /Pt m     multilayers    have    been    prepared,    characterized    via    x-ray    diffraction,    electron    microscopy,    and    magneto-optic    spectroscopy    and    SQUID magnetometry.      Numbers   of   monolayers   in   each   multilayer   period,   n   and   m   ranged   between   2   and   13.   The   structural   studies   have   revealed   negligible interdiffusion   and   excellent   modulation   sequences. The   magnetic   moments   in   Ni n /Pt m    multilayers   are   thoroughly   studied   by   combining   experimental   and   ab initio   theoretical   techniques.   SQUID   magnetometry   probes   the   samples’   magnetizations.   X-ray   magnetic   circular   dichroism   separates   the   contribution   of Ni   and   Pt   and   provides   a   layer-resolved   magnetic   moment   profile   for   the   whole   system.   The   results   are   compared   to   band-structure   calculations.   Induced Pt   magnetic   moments   localized   mostly   at   the   interface   are   revealed.   No   magnetically   “dead”   Ni   layers   are   found.   The   magnetization   per   Ni   volume   is slightly enhanced compared to bulk NiPt alloys.
Ag/Co multilayers: From film growth to GMR sensor The Ag-Co   system   either   in   multilayer   or   in   granular   alloy   form   exhibits   the   GMR   (Giant   MagnetoResistance)   effect.   By   adjusting   the   modulation parameters   an   intermediate   structure   may   be   formed   offering   new   possibilities   for   magnetoelectronic   applications.   This   structure   resides   in   the limit   between   multilayers   and   granular   alloys   and   is   called   granular   multilayer.   The   dependence   of   GMR   values   on   the   individual   layer   thickness and   on   the   total   film   thickness   was   parameterised   and   magnetoresistance   values   of   16%   at   300   K   and   36%   at   30   K   were   achieved.   The   outcome of   this   study   is   the   fabrication   of   a   two-dimension   magnetic   field   sensor   consisting   of   8   specific   elements   forming   a   2x4   array.   The   sensor   is specialised   in   small   magnetic   field   regions   while   its   response   was   found   quite   satisfactory   regarding   its   uniformity   and   repeatability.   The   sensor may be upgraded to larger arrays and to three dimensions in order to fulfil various market needs.
Projects (more info on http://multigr.physics.auth.gr) 1. IKYDA: Greek German collaboration: Tuning magnetism in shape-controlled hybrid nanoparticles for enhanced hyperthermia efficiency and heat-activated drug delivery (2012-2013) 2. Thales: ESF & NSRF Resarch Funding Program:  “Smart” Magnetic nanoparticles probes for magnetic Resonance Imaging(MRI) (2011-2015) 3. RTN network: Synthesis and Orbital Magnetism of core-shell nanoparticles (2004-2008) 4. PENED: Ordered arrays of magnetic nanoparticles for technological applications (2005-2008) 5. NATO Science Program: Novel magnetic nanostructures for sensor fabrication (2004-2006)  6. EPEAEK II: Correlation of structure and Magnetism in novel magnetic nanostructures (2002-2005) 7. Greek-French Collaboration: Novel magnetic nanostructured materials for high-resolution magnetic sensors (2003-2005)  8. RTN Network: Correlation of structure and magnetism in novel nanoscale magnetic particles (2000-2004) 9. HCM Network :Magnetic and Transport properties of novel mesoscopic magnetic thin films and multilayers (1995-1998) 10. EPET II: GMR Sensors for magnetic field recording (1995-1998) 11. GSRT: Magneto-Optic study of artificially modulated magnetic superlattices  (1994-1997) 12. BRITE/EURAM: Multilayered Magnetic Materials: Fundamental and Technological Aspects (1990-1994)