@article { ISI:000249655500007, title = {Magnetic transition and large magnetocaloric effect associated with surface spin disorder in Co and CocoreAgshell nanoparticles}, journal = {Journal of Physical Chemistry C}, volume = {111}, number = {38}, year = {2007}, month = {SEP}, pages = {14060-14066}, publisher = {AMER CHEMICAL SOC}, type = {Article}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

We report a reversible, large magnetocaloric effect in the vicinity of a low-temperature magnetic transition in Co and CocoreAgshell nanoparticles synthesized using a wet chemical method. The as-synthesized assembly of the particles shows a sharp low-temperature peak in the zero-field-cooled (ZFC) magnetization well below the blocking transition temperature, and this feature is associated with the surface spin disorder. Co nanoparticles show a large increase in the magnetic entropy at around 15 K with a peak value of nearly 2.25 J/K center dot kg for an applied field of 30 kOe. A similar trend is also observed in the silver-coated Co particles. These are some of the largest NICE values observed in nanoparticles to date. The features are ascribed to the low-temperature spin-glass-like freezing transition associated with the surface spins in the shell region that is distinct from the behavior of core spins. Our studies reveal that manipulating the surface anisotropy in core-shell nanoparticles has the potential to lead to a large NICE effect and thus prove to be useful for magnetic refrigeration.

}, issn = {1932-7447}, doi = {10.1021/jp073274i}, author = {Poddar, P. and Srinath, S. and Gass, James and Bhagavatula L. V. Prasad and Srikanth, H.} } @article { ISI:000260776800046, title = {Static and dynamic magnetic properties of Co nanoparticles}, journal = {Journal of Nanoscience and Nanotechnology}, volume = {8}, number = {8}, year = {2008}, note = {International Conference on Advanced Nano-Materials, Indian Inst Technol, Phys Dept, Bombay, INDIA, JAN 08-10, 2007}, month = {AUG}, pages = {4086-4091}, publisher = {AMER SCIENTIFIC PUBLISHERS}, address = {25650 NORTH LEWIS WAY, STEVENSON RANCH, CA 91381-1439 USA}, abstract = {

Co nanoparticles have been synthesized using wet-chemical methods. As-synthesized particles show a sharp low temperature peak in zero-field cooled (ZFC) magnetization well below the blocking transition temperature and this feature is associated with surface spin disorder. We have investigated the dynamic magnetic properties of Co using ac susceptibility and resonant RF transverse susceptibility (TS). We also studied the memory and relaxation effects in these nanoparticle systems. From these measurements we show a typical blocking behavior of an assembly of superparamagnetic nanoparticles with a wide distribution of blocking temperatures. The transverse susceptibility measurements on these particles show the presence of anisotropy even above the blocking temperature. The role of surface anisotropy and the size distribution of the particles on the observed memory and magnetic relaxation effects are discussed.

}, keywords = {Ac Susceptibility, Co Nanoparticles, Magnetic relaxation, Transverse Susceptibility}, issn = {1533-4880}, doi = {10.1166/jnn.2008.AN06}, author = {Srinath, S. and Poddar, P. and Sidhaye, Deepti S. and Bhagavatula L. V. Prasad and Gass, James and Srikanth, H.} } @article { ISI:000263733200011, title = {Preparation of nearly monodisperse nickel nanoparticles by a facile solution based methodology and their ordered assemblies}, journal = {Journal of Physical Chemistry C}, volume = {113}, number = {9}, year = {2009}, month = {MAR}, pages = {3426-3429}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

Nickel nanoparticles have been synthesized by a simple wet chemical reduction technique in the presence of a surfactant sodium dodecyl sulfate (SDS) and a capping agent oleic acid. Varying the concentration of the surfactant and the capping agent has been found to critically govern the nature of the nanoparticles prepared. It is observed that at an optimum concentration of SDS and oleic acid, nearly monodisperse Ni nanoparticles are obtained that form highly ordered hexagonally close-packed structures on electron microscopy grids by a simple drop coating procedure. It is also shown that the presence of oleic acid is necessary for the stability of the Ni nanoparticles synthesized. Structural and magnetic characterizations of the nanoparticles are also presented.

}, issn = {1932-7447}, doi = {10.1021/jp807542w}, author = {Sidhaye, Deepti S. and Bala, Tanushree and Srinath, S. and Srikanth, H. and Poddar, Pankaj and Sastry, Murali and Bhagavatula L. V. Prasad} } @article { ISI:000277834300107, title = {Field dependence of the magnetocaloric effect in core-shell nanoparticles}, journal = {Journal of Applied Physics}, volume = {107}, number = {9}, year = {2010}, note = {11th Joint MMM-Intermag Conference, Washington, DC, JAN 18-22, 2010}, month = {MAY}, pages = {09A902}, publisher = {AMER INST PHYSICS}, address = {CIRCULATION \& FULFILLMENT DIV, 2 HUNTINGTON QUADRANGLE, STE 1 N O 1, MELVILLE, NY 11747-4501 USA}, abstract = {

The field dependence of the magnetic entropy change peak at the low temperature surface spin freezing transition in chemically synthesized, monodispersed Co, Co-Ag, and Ni-Ag core-shell nanoparticles is studied, with the aim of gaining insight into the critical exponents of this transition. It is evidenced that although the magnitude of the peak entropy change and position of the peak can be tuned by changing the composition and nature (metallic or organic) of the shell and surfactant layers, the characteristics of the spin freezing transition are not altered. The field dependence of the refrigerant capacity also confirms this finding. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3335514]

}, issn = {0021-8979}, doi = {10.1063/1.3335514}, author = {Franco, V. and Conde, A. and Sidhaye, Deepti and Bhagavatula L. V. Prasad and Poddar, P. and Srinath, S. and Phan, M. H. and Srikanth, H.} }