Superparamagnetic Behavior in Noninteracting NiFe 2 O 4 Nanoparticles Grown in SiO 2 Matrix Subarna Mitra and Kalyan Mandal Magnetism Laboratory, S. N. Bose National Center for Basic Sciences, Salt Lake, Kolkata, India Inverse spinel NiFe 2 O 4 nanoparticles (2.5 nm–25 nm) in SiO 2 matrix were prepared by sol–gel method. The phase and particle size of the samples were determined by X-ray diffraction and transmission electron microscope. Magnetic properties and Mössbauer spectra of the samples were studied with different ferrite particle sizes. Superparamagnetic behavior was observed at room temperature when the average particle size was less than 6 nm and also for larger particles at higher temperatures. The Mössbauer spectra changed from two-peaks to six-peaks pattern with the increase in particles size and decrease in temperature. Two sets of sextets in the Mössbauer spectra indicate the presence of iron ion in the tetrahedral as well as in octahedral sites. The Langevin function used to explain paramagnetism is found to describe well the magnetic behavior of these samples consisting of single domain particles in the superparamagnetic state. Keywords Anisotropy; Blocking temperature; Cation distribution; Coercivity; Hysterisis; Langevin function; Mössbauer spectra; Nanoparticles; Nickel ferrite; Quadrupole splitting; Single domain; Sol–gel; Spinel structure; SQUID; Superparamagnetism; TEM. Introduction Nanostructured materials exhibit unusual physical and chemical properties, significantly different from those of conventional bulk materials, due to their extremely small size or large surface area. So their preparation and characterization have attracted increasing attention in the past decade [1–6]. Spinel ferrites have been studied during last several decades from both scientific and technological points of view [7]. They are widely used in high-frequency electronic devices, taking advantage of their high initial permeability and resistivity. Spinel ferrites exhibit a rich variety of magnetic properties, reflecting the flexibility of spinel structure in accommodating a wide range of cations. Among the various topics of research on spinel ferrites, the magnetic properties of ultrafine ferrite particles have recently attracted considerable attention [8–10]. In this work, noninteracting NiFe 2 O 4 nanoparticles in SiO 2 matrix were prepared by sol–gel method [11, 12]. In these samples, the ratio of NiFe 2 O 4 to SiO 2 was 30:70 by weight and 15:85 by volume to reduce the interaction between the ferrite particles. The size and temperature dependent magnetic properties of these embedded ferrite nanoparticles were investigated in detail. Experimental NiFe 2 O 4 ferrite nanoparticles in SiO 2 matrix were prepared by the sol–gel method [12, 18]. The weighed amount of nitrate salts Ni(NO 3  2 · 6H 2 O and Fe(NO 3  3 · 9H 2 O were dissolved in distilled water and the solution was added to Tetraethoxy Orthosilicate Received February 10, 2006; Accepted September 30, 2006 Address correspondence to Kalyan Mandal, Magnetism Laboratory, S. N. Bose National Center for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700 098, India; E-mail: kalyan@bose.res.in [Si(OC 2 H 5  4 ] in such a way that in the final sample the ratio of the ferrite to SiO 2 becomes 30:70 by weight and 15:85 by volume. The pH of the solution was maintained at 2.0 as the isoelectric point of silica is close to that pH value [11]. The mixture was then dried slowly to form gel. The gel was annealed at different temperatures and the samples are denoted throughout the latter section as Sample A (700  C, 1 hr), Sample B (700  C, 1.5 hr), Sample C (700  C, 2 hr), Sample D (800  C, 1 hr), Sample E (800  C, 2 hr), Sample F (900  C, 2 hr), Sample G (1000  C, 2 hr), Sample H (1000  C, 6 hr), and Sample I (1000  C, 24 hr) as mentioned in Table 1. An X-ray diffractometer (XRD) (Philips, PW 1710) with CuK  radiation was used to identify the phase of the samples. The particle size was estimated by XRD as well as by a JEM-200-CX Transmission Electron Microscope (TEM). The Mössbauer spectra of the samples have been recorded at different temperatures down to 20 K in transmission geometry using CMTE (Model 250) constant acceleration type drive with a 5 mCi 57 Co -ray source in Rh matrix. The magnetic properties of the samples were measured by a SQUID (MPMS, Quantum Design) and by a vibrating sample magnetometer. Results and discussion Figure 1 shows the representative XRD spectra of Samples A, D, F, H, and I. The particle sizes of the Samples A to I are calculated in the range 2.5 nm to 25.0 nm and are listed in Table 1. Figure 2 shows (a) the TEM micrograph of particle size distribution of sample D and (b) the high-resolution lattice fringes of the same sample. The Variation of coercivity with particle sizes is shown in Fig. 3. Magnetic hysteresis loops of the samples A and H are plotted respectively in Figs. 4(a) and 4(b) at temperatures 10 K, 50 K, 100 K and 300 K. M vs. H data of sample A, along with the theoretical fitting of Langevin function are shown in Fig. 5 at temperatures 100 K, 150 K,