Colossal Reduction in Curie Temperature Due to Finite-Size Effects in CoFe 2 O 4 Nanoparticles Victor Lopez-Dominguez, † Joan Manel Herna ̀ ndez, † Javier Tejada,* ,† and Ronald F. Ziolo ‡ † Dept. de Física Fonamental, Universitat de Barcelona, C. Martí i Franque ́ s 1, Barcelona 08028, Spain ‡ Centro de Investigació n en Química Aplicada, Boulevard Enrique Reyna 140, Saltillo, 25253 Mexico ABSTRACT: In this work, we show the enormous size effect on the ordering transition temperature, T O , in samples of CoFe 2 O 4 nanoparticles with diameters ranging from 1 to 9 nm. Samples were characterized by HRTEM and XRD analyses and show a bimodal particle size distribution centered at 3 nm and around 6 nm for “small” and “large” particles, respectively. The results and concomitant interpretation were derived from studies of the magnetization dependence of the samples on temperature at low and high magnetic fields and relaxation times using both dc and ac fields. The large particles show a typical superparamagnetic behavior with blocking temperatures, T B , around 100 K and a Curie temperature, T C , above room temperature. The small particles, however, show a colossal reduction of their magnetic ordering temperature and display paramagnetic behavior down to ∼10 K. At lower temperatures, these small particles are blocked and show both exchange and anisotropy field values above 5 T. The order of magnitude reduction in T O demonstrates a heretofore unreported magnetic behavior for ultrasmall nanoparticles of CoFe 2 O 4 , suggesting its further study as an advanced material. KEYWORDS: magnetic nanoparticles, Curie temperature M agnetic nanoparticles are of great interest in many different fields of physics, chemistry, and engineering. At low temperatures, they show magnetic relaxation 1 and quantum properties, such as magnetization tunneling, 2-5 and at room temperature, their applications cover fields such as data storage, 2,6 magnetic resonance imaging, 7 magnetic fluids, 8 and biomedicine. 7,9 Therefore, understanding the fundamental behavior of the magnetic properties of these nanoscale objects is very important. For example, we have the case of the formation of many different types of large agglomerates due to their dipole-dipole interactions with very different magnetic properties between them. Another important aspect of magnetic nanoparticles is the control of their size and shape distributions because both the Curie temperature and the height of the anisotropy energy barriers are strongly affected by the size and shape of the particles. 10,11 Among ferrite materials, ferrimagnetic cobalt ferrite nanoparticles with an inverse spinel structure are of high interest due to their ease of synthesis via different methods, 12,13 remarkable chemical stability, very high magnetocrystalline anisotropy, and moderate saturation mag- netization. It is well-known that large magnetic particles split into magnetic domains in order to decrease their magnetostatic energy. The thickness, δ, of the domain wall where the rotation of spins from one domain to another occurs, is given by δ = a(J/D) 1/2 , where J and D are the exchange and anisotropy energies per lattice site, respectively, and a is the lattice spacing. For practical reasons, it is assumed that the total anisotropy energy of the particle can also be expressed as KV, with K being the so-called anisotropy constant, which depends on the magnetic anisotropy field material, and V being the particle volume, l 3 . Thus, a particle of a size l < δ is a sufficient condition for the particle to consist of a single domain. The total energy of a single-domain particle depends on the exchange interaction, the crystal field anisotropy, dipolar forces, and on the shape of the particle. In general, single-domain particles are very complex objects because, for example, the exchange interactions at the surface are different from those in the bulk, and in addition, the magnetic anisotropy at the surface differs from the anisotropy in the bulk as a consequence of the different symmetry in the local arrangement of the atoms. In the case of the cobalt iron oxide particles, which are of interest in this work, the energy of the exchange interaction per atom greatly exceeds the energy of the magnetic anisotropy per atom. Consequently, the effective exchange interaction at all atomic sites is sufficiently large to make the particle uniformly magnetized. In this case, the low-energy dynamics of such particles reduces to a uniform rotation of the total magnetic moment. 1,14 The magnetic properties of nanoparticles are very much influenced by finite-size and surface effects. These effects can produce a large variety of anomalous magnetic properties, including a reduction in the Curie temperature and the Received: June 22, 2012 Revised: November 14, 2012 Article pubs.acs.org/cm © XXXX American Chemical Society A dx.doi.org/10.1021/cm301927z | Chem. Mater. XXXX, XXX, XXX-XXX