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Title: Implications of room temperature oxidation on crystal structure and exchange bias effect in Co/CoO nanoparticles

Abstract

In this study, we describe how the exchange bias effect in Co/CoO nanoparticles depends on the size focusing and temperature treatment of precursor Co nanoparticles before oxidation at ambient conditions. By appealing to magnetization, microscopy, neutron and synchrotron x-ray measurements we found that as-synthesized Co nanoparticles readily oxidize in air only after 20 days. The highest exchange bias field of 814 Oe is observed at T = 2K. When the same nanoparticles are centrifuged and annealed at 70 °C in vacuum prior to oxidation, the exchange bias field is increased to 2570 Oe. Annealing of Co nanoparticles in vacuum improves their crystallinity and prevents complete oxidation, so that Co-core/CoO-shell structure is preserved even after 120 days. The crystal structure of CoO shell in both samples is different from its bulk counterpart. Implications of such distorted CoO shells on exchange bias are discussed. Coating of Co nanoparticles with amorphous silica shell makes them resistant to oxidation, but ultimately modifies the crystal structure of both Co core and SiO 2 shell.

Authors:
 [1];  [1];  [2];  [3];  [1];  [1]
  1. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  2. North Carolina State Univ., Raleigh, NC (United States)
  3. Vanderbilt Univ., Nashville, TN (United States)
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1225427
Grant/Contract Number:  
AC05-00OR22725
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Physical Chemistry. C
Additional Journal Information:
Journal Name: Journal of Physical Chemistry. C; Journal ID: ISSN 1932-7447
Publisher:
American Chemical Society
Country of Publication:
United States
Language:
English
Subject:
77 NANOSCIENCE AND NANOTECHNOLOGY

Citation Formats

Feygenson, Mikhail, Formo, Eric V., Freeman, Katherine, Schieber, Natalie P., Gai, Zheng, and Rondinone, Adam J. Implications of room temperature oxidation on crystal structure and exchange bias effect in Co/CoO nanoparticles. United States: N. p., 2015. Web. doi:10.1021/acs.jpcc.5b09046.
Feygenson, Mikhail, Formo, Eric V., Freeman, Katherine, Schieber, Natalie P., Gai, Zheng, & Rondinone, Adam J. Implications of room temperature oxidation on crystal structure and exchange bias effect in Co/CoO nanoparticles. United States. doi:10.1021/acs.jpcc.5b09046.
Feygenson, Mikhail, Formo, Eric V., Freeman, Katherine, Schieber, Natalie P., Gai, Zheng, and Rondinone, Adam J. Mon . "Implications of room temperature oxidation on crystal structure and exchange bias effect in Co/CoO nanoparticles". United States. doi:10.1021/acs.jpcc.5b09046. https://www.osti.gov/servlets/purl/1225427.
@article{osti_1225427,
title = {Implications of room temperature oxidation on crystal structure and exchange bias effect in Co/CoO nanoparticles},
author = {Feygenson, Mikhail and Formo, Eric V. and Freeman, Katherine and Schieber, Natalie P. and Gai, Zheng and Rondinone, Adam J.},
abstractNote = {In this study, we describe how the exchange bias effect in Co/CoO nanoparticles depends on the size focusing and temperature treatment of precursor Co nanoparticles before oxidation at ambient conditions. By appealing to magnetization, microscopy, neutron and synchrotron x-ray measurements we found that as-synthesized Co nanoparticles readily oxidize in air only after 20 days. The highest exchange bias field of 814 Oe is observed at T = 2K. When the same nanoparticles are centrifuged and annealed at 70 °C in vacuum prior to oxidation, the exchange bias field is increased to 2570 Oe. Annealing of Co nanoparticles in vacuum improves their crystallinity and prevents complete oxidation, so that Co-core/CoO-shell structure is preserved even after 120 days. The crystal structure of CoO shell in both samples is different from its bulk counterpart. Implications of such distorted CoO shells on exchange bias are discussed. Coating of Co nanoparticles with amorphous silica shell makes them resistant to oxidation, but ultimately modifies the crystal structure of both Co core and SiO2 shell.},
doi = {10.1021/acs.jpcc.5b09046},
journal = {Journal of Physical Chemistry. C},
issn = {1932-7447},
number = ,
volume = ,
place = {United States},
year = {2015},
month = {11}
}

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