Nanocrystalline Nickel Ferrite, NiFe 2 O 4 : Mechanosynthesis, Nonequilibrium Cation Distribution, Canted Spin Arrangement, and Magnetic Behavior Vladimir S ˇ epela ´ k,* ,²,‡ Ingo Bergmann, ² Armin Feldhoff, § Paul Heitjans, § Frank Krumeich, | Dirk Menzel, ⊥ Fred J. Litterst, ⊥ Stewart J. Campbell, # and Klaus D. Becker ² Institute of Physical and Theoretical Chemistry, Braunschweig UniVersity of Technology, Hans-Sommer-Strasse 10, D-38106 Braunschweig, Germany, Institute of Physical Chemistry and Electrochemistry, Leibniz UniVersity of HannoVer, Callinstrasse 3-3A, D-30167 HannoVer, Germany, Laboratory of Inorganic Chemistry, Swiss Federal Institute of Technology Zu ¨rich, Ho ¨nggerberg HCI-G105, CH-8093 Zurich, Switzerland, and Institute of Condensed Matter Physics, Braunschweig UniVersity of Technology, Mendelssohnstrasse 3, D-38106 Braunschweig, Germany ReceiVed: NoVember 16, 2006; In Final Form: December 29, 2006 Nickel ferrite (NiFe 2 O 4 ) nanoparticles with an average crystallite size of about 8.6 nm were prepared by mechanochemical synthesis (mechanosynthesis). In-field Mo ¨ssbauer spectroscopy and high-resolution TEM studies revealed a nonuniform structure of mechanosynthesized NiFe 2 O 4 nanoparticles consisting of an ordered core surrounded by a disordered grain boundary (surface) region. The inner core of a NiFe 2 O 4 nanoparticle is considered to possess a fully inverse spinel structure with a Ne ´el-type collinear spin alignment, whereas the surface shell is found to be structurally and magnetically disordered due to the nearly random distribution of cations and the canted spin arrangement. As a consequence of frustrated superexchange interactions in the surface shell, the mechanosynthesized NiFe 2 O 4 exhibits a reduced nonsaturating magnetization, an enhanced coercivity, and a shifted hysteresis loop. The study also demonstrates that one can tailor the magnetic properties of mechanosynthesized NiFe 2 O 4 particles by suitably controlling their size. The thickness of the surface shell of about 1 nm estimated from size-dependent magnetization measurements is found to be in good agreement with that obtained from high-resolution TEM and Mo ¨ssbauer experiments. On heating above 673 K, the mechanosynthesized NiFe 2 O 4 relaxes to a structural and magnetic state that is similar to the bulk one. Introduction Interest in nanosized spinel ferrites of the type MFe 2 O 4 (M is a divalent metal cation) has greatly increased in the past few years due to their importance in understanding the fundamentals in nanomagnetism 1a and their wide range of applications such as high-density data storage, ferrofluid technology, sensor technology, spintronics, magnetocaloric refrigeration, hetero- geneous catalysis, magnetically guided drug delivery, and magnetic resonance imaging. 1 To emphasize the site occupancy at the atomic level, the structural formula of 2-3 spinel ferrites may be written as (M 1-λ 2+ Fe λ 3+ )[M λ 2+ Fe 2-λ 3+ ]O 4 , where paren- theses and square brackets denote cation sites of tetrahedral (A) and octahedral [B] coordination, respectively. λ represents the so-called degree of inversion defined as the fraction of the (A) sites occupied by Fe 3+ cations. Nickel ferrite, NiFe 2 O 4 , as a soft magnetic n-type semicon- ducting material, is an important member of the spinel family with a fully inverse structure (λ ) 1) in the bulk state. 2 Its preparation by the classical solid-state reaction requires a number of stages, including homogenization of the powder precursors, compaction of the reactants, and finally prolonged heat treatment at considerably elevated temperatures. 3 One goal of modern ferrite research and development has been to identify simpler processing schemes that do not rely upon high-temperature treatments for inducing solid-state reactions. Several techniques have already been used to produce NiFe 2 O 4 nanoparticles, including hydrothermal reactions, 4 coprecipitation, 5 combustion synthesis, 6 thermal decomposition, 7 the sol-gel method, 8 microwave processing, 9 electrospinning, 10 the reverse micelle technique, 11 the plasma deposition method, 12 the radio frequency thermal plasma torch technique, 13 the pulsed wire discharge, 14 sonochemical synthesis, 15 and high-energy milling. 2,16 The latter method can deliver nanocrystalline ferrites (and oxides in general) either by particle size reduction of bulk material to the nanometer scale without changes in its chemical composi- tion 2,16,17 or by inducing a heterogeneous solid-state chemical reaction between the ferrite precursors, i.e., by the mechanically induced formation reaction (mechanosynthesis). 18 In this article, we will report on the single-step synthesis of nanocrystalline NiFe 2 O 4 via high-energy milling of binary oxide precursors (R- Fe 2 O 3 and NiO) at room-temperature. Although the mechano- synthesis of NiFe 2 O 4 has already been reported in a few papers, 19 to the best of our knowledge there is no report in the literature on the defect state or the disordered structure of NiFe 2 O 4 prepared by the nonconventional mechanochemical route. Note that, in the above-mentioned papers, only the formation of mechanosynthesized NiFe 2 O 4 has been established by X-ray diffraction and/or Mo ¨ssbauer spectroscopy. In this article, we * To whom correspondence should be addressed. Tel.: +49-531- 3917387. Fax: +49-531-3917305. E-mail: v.sepelak@tu-bs.de. ² Institute of Physical and Theoretical Chemistry, Braunschweig Uni- versity of Technology. ‡ On leave from the Slovak Academy of Sciences, Kos ˇice, Slovakia. § Leibniz University of Hannover. | Swiss Federal Institute of Technology Zu ¨rich. ⊥ Institute of Condensed Matter Physics, Braunschweig University of Technology. # The University of New South Wales, Australian Defence Force Academy, Canberra, Australia. 5026 J. Phys. Chem. C 2007, 111, 5026-5033 10.1021/jp067620s CCC: $37.00 © 2007 American Chemical Society Published on Web 03/09/2007