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Start of funding 01.01.2015
Orbital engineering of nanoparticles: fromdiamagnetic solar cell to ferromagnetic spin-polarized material
Dr. Amitesh Paul
Technische Universität München
Physik-Department E21 - Experimental physics
Prof. Dr. Matt Law
University of California, Irvine
School of Physical Sciences
The project aims at the synthesis of pure ferromagnetic CoS2, pure diamagnetic FeS2, and Co1-xFexS2 nanoparticle alloys and to investigate their particle-size-dependent structural, electronic, and magnetic properties with the primary aim to reach a large spin polarization (P=100% for x=0.25 in theory). The mentioned materials will be prepared in the form of nanocrystals in solution (in the group of Prof. Law at UC-Irvine) and the desired half-metallicity is expected to be achieved by shifting the Fermi level and DOS through Fe doping. The group will use a suite of techniques e.g., TEM, SEM, XRD, AFM, SAXS, SANS, GISANS, XRR, magnetometry, x-ray spectroscopy (XAS, XMCD, .... etc.) for structure and magnetic-property determination (in the group of Dr. A. Paul at TUM).
Final report:
We have synthesized cobalt iron pyrite (CoxFe1-xS2) nanocrystals and investigated their structure, morphology, and magnetic properties. Using X-ray diffraction (XRD), small-angle-X-ray-scattering (SAXS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), we show that there is a systematic variation in the lattice constant, primary grain size, and aggregate size with increasing cobalt content. We find a reasonable agreement of lattice constant a with Vegard’s law which is evidence for alloying with x close to the nominal ones. TEM-based energy dispersive spectroscopy (EDS) confirmed these compositions. Each nanoparticle is a collection of a few NCs whose number remain similar over a wide range of x. Magnetically, the cobalt iron pyrites show interesting regimes of competing exchange and dipolar interactions with increasing Fe content, which is investigated using DC magnetization and AC susceptibility at various temperatures and frequencies. Below x = 0.5, the system remains mostly paramagnetic. With an increase in x, for x = 0.5, the nanocrystals remain largely isolated as they show non-interacting SPM-like behavior, i.e. absence of magnetic dipole interactions between the embedded AF nanoclusters. Here, the exchange interaction is not strong enough to suppress the SPM behavior. However, dipolar interactions result in a collective state at larger x. For x = 0.625, the collective state possesses SSG-like behavior along with embedded FM characteristics. In samples of magnetic nanoparticles, exchange interactions are often important and can be sufficiently strong to suppress the SPM relaxation. We have constructed a phase diagram as function of temperature and Co fraction and explained the terms of competition between the various dipolar, exchange, inter- and intracluster interactions. Functional n-doped iron pyrite with tuning possibilities of NC size and shape has opened up new prospects in the field of spin polarized ferromagnetism. In the future, improvements in the NC quality, detailed structural analysis, and analysis of functional properties will be addressed.