Nanocrystalline Nickel Ferrite, NiFe2O4: Mechanosynthesis, Nonequilibrium Cation Distribution, Canted Spin Arrangement, and Magnetic Behavior

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• 作者：Vladimir <img src="http ; //pubs.acs.org/images/entities/Scaron.gif" border="0">epelá ; k ; Ingo Bergmann ; Armin Feldhoff ; Paul Heitjans ; Frank Krumeich ; Dirk Menzel ; Fred J. Litterst ; Stewart J. Cam
• 刊名：Journal of Physical Chemistry C
• 出版年：2007
• 出版时间：April 5, 2007
• 年：2007
• 卷：111
• 期：13
• 页码：5026 - 5033
• 全文大小：545K
• 年卷期：v.111,no.13(April 5, 2007)
• ISSN：1932-7455

文摘

Nickel ferrite (NiFe₂O₄) nanoparticles with an average crystallite size of about 8.6 nm were prepared bymechanochemical synthesis (mechanosynthesis). In-field Mössbauer spectroscopy and high-resolution TEMstudies revealed a nonuniform structure of mechanosynthesized NiFe₂O₄ nanoparticles consisting of an orderedcore surrounded by a disordered grain boundary (surface) region. The inner core of a NiFe₂O₄ nanoparticleis considered to possess a fully inverse spinel structure with a Néel-type collinear spin alignment, whereasthe surface shell is found to be structurally and magnetically disordered due to the nearly random distributionof cations and the canted spin arrangement. As a consequence of frustrated superexchange interactions in thesurface shell, the mechanosynthesized NiFe₂O₄ exhibits a reduced nonsaturating magnetization, an enhancedcoercivity, and a shifted hysteresis loop. The study also demonstrates that one can tailor the magnetic propertiesof mechanosynthesized NiFe₂O₄ particles by suitably controlling their size. The thickness of the surface shellof about 1 nm estimated from size-dependent magnetization measurements is found to be in good agreementwith that obtained from high-resolution TEM and Mössbauer experiments. On heating above 673 K, themechanosynthesized NiFe₂O₄ relaxes to a structural and magnetic state that is similar to the bulk one.