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J. Phys. Chem. B 2004, 108, 18222-18227
Effects of Interparticle Interactions upon the Magnetic Properties of CoFe2O4 and MnFe2O4 Nanocrystals Christy R. Vestal, Qing Song, and Z. John Zhang* School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 ReceiVed: August 6, 2004; In Final Form: September 11, 2004
The interparticle distance has been systematically varied by controlling the concentration of 8-nm CoFe2O4 or MnFe2O4 spinel ferrite nanocrystals dispersed in eicosane. For both nanoparticulate systems, the blocking temperature decreased with increasing interparticle distance through dilution of nanocrystals, which suggests a decrease in the anisotropy energy barrier EA. The blocking temperature for MnFe2O4 nanoparticles decreased continuously with decreasing nanocrystal concentration in eicosane. However, the blocking temperature for CoFe2O4 remained constant until ∼15% (wt %) concentration, below which the blocking temperature decreased sharply. The reduced remanence (MR/MS) decreased with decreasing interparticle distance for MnFe2O4, while it increased for CoFe2O4. The differences in magnetic response upon varying interparticle distance between the two systems are attributed to the strength of the dipole interactions. The behavior of the CoFe2O4 nanoparticulate system is consistent with the notion of nanoparticulate cluster formation.
Introduction Interest in magnetic nanoparticles has greatly increased in the past few years due to their importance in understanding the fundamentals in magnetism and their wide range of applications such as high-density information storage, ferrofluid technology, magnetically guided drug delivery, and magnetic resonance imaging (MRI) enhancement.1-6 A better understanding of the fundamental properties of magnetic nanoparticles certainly is crucial to such applications, especially the superparamagnetic relaxation in nanoscale materials. The superparamagnetic state occurs when the anisotropy energy EA of nanoparticles is overcome thermally. This energy has been empirically defined for noninteracting nanoparticles by Stoner and Wohlfarth as
EA ) KV sin2 θ
(1)
where K is the anisotropy energy constant, V is the volume of the nanoparticle, and θ is the angle between the magnetization and the easy axis of nanoparticles.7 The blocking temperature, TB, represents the threshold of thermal activation and can be used as an indication for the transition to the superparamagnetic state. At temperatures above TB the magnetic moment of each nanoparticle fluctuates rapidly with no preferred orientation and the assembly of nanoparticles behaves as a common paramagnetic material. Modulating K through the use of crystal chemistry can control the energy barrier.8 The effects of size and magnetocrystalline anisotropy K upon the magnetic properties have been reported in various magnetic nanoparticulate systems over the past decade.6,8-11 In addition to crystalline anisotropy, other factors contribute to the observed magnetic properties of nanoparticles. For example, the effect of surface-bound ligands and their chemistry upon the magnetic properties of manganese ferrite nanoparticles have been recently demonstrated.12 The particle-particle magnetic interactions could contribute to magnetic anisotropy as well and consequently change the magnetic properties of nanoparticles. * To whom correspondence should be addressed.
Understanding the interparticle interactions is of fundamental interest and also is vital to certain applications of magnetic nanoparticles such as high-density data storage and ferrofluids. Interparticle interactions may arise from dipole-dipole interactions between nanoparticles or the exchange interactions occurring between the magnetic ions at the surface of neighboring particles. When a capping surfactant such as oleic acid is used, the increased spacing between particles results in negligible exchange interactions and the primary interaction is considered from the dipole-dipole coupling.13 The energy associated with dipole-dipole interactions (Ed-d) is given by
Ed-d ) -(µomo2)/(4πl3)
(2)
where µo is the permeability, mo is the magnetic moment, and l is the particle-particle separation. The dipole-dipole energy term modifies the anisotropy energy barrier EA for magnetization reversal and is claimed to introduce local minima in the energy barrier.9 Although it is generally understood that particleparticle interactions affect the magnetic properties of nanoparticles, correlating the effects of interparticle interactions with observed magnetic properties has still remained a challenge. Three primary theoretical models have been developed over the years for understanding the effects of interparticle interactions: the Shtrikman-Wohlfarth (SW) model in 1981, the Dormann-Bessais-Fiorani (DBF) model in 1998, and the Mørup-Tronc model in 1994.9,14,15 Both the SW and DBF models show that increasing interparticle interactions will lead to an increase in the energy barrier, EA, by
EA ) KV sin2 θ + Bi
(3)
where Bi is the energy term from interaction, whose form varies according to the respective model and subsequent revisions.15 However, the MT model indicates that increased interparticle interactions should decrease the energy barrier.14 Experimentally, the effects of interparticle interactions are normally determined by varying the interparticle interactions
10.1021/jp0464526 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/03/2004
Magnetic Properties of CoFe2O4 and MnFe2O4 Nanocrystals through using a nonmagnetic matrix to dilute ferrofluids, which are usually based on the suspended magnetite (Fe3O4) particles. A survey of the literature finds discrepancies in the magnetization trends even for materials with the same composition! For example, most studies show that the blocking temperature and coercivity decrease with dilution, i.e., weaker interparticle interactions due to the increase of particle-particle separation.9,14,16-21 These findings collaborate the SW and DBF models that decreasing interparticle interactions result in a reduction of the anisotropy energy barrier for magnetization reversal. On the other hand, other magnetic properties show mixed results from the experiments. For instance, magnetic relaxation from SQUID measurements has shown that Ne´el’s relaxation time τ increases with increasing interactions, which is consistent with the SW and DBF models.9,14,15 However, the relaxation from Mo¨ssbauer studies indicates that τ decreases with increasing interactions, which is consistent with the MT model.9,14,15 These conflicting results are very puzzling since from the fundamentals of magnetism SQUID and Mo¨ssbauer studies should provide consistent results. Other important magnetic properties such as the reduced remanence (MR/MS) also have shown inconsistent effects from varying particle interactions. Several experimental reports indicate that MR/MS decreases with increasing interactions,13,19,22,23 which has been suggested by Monte Carlo simulations.24,25 Nevertheless, other experimental studies find that MR/MS can increase with increasing particle interactions.26,27 Monte Carlo simulations by Kechrakos et al. also suggest that MR/MS can increase or decrease depending upon the strength of interparticle interaction.25 For a better understanding of the effects from interparticle interactions, the experimental systems need to be expanded beyond magnetite particulate systems derived from ferrofluids. CoFe2O4 and MnFe2O4 belong to the same spinel ferrite materials family as magnetite does. Since the magnetic properties show distinct characteristics for each nanoparticulate system, CoFe2O4 and MnFe2O4 nanocrystals could provide fresh insight to the interparticle interactions. Herein we report magnetic studies on the interparticle interaction in eicosane using monodisperse CoFe2O4 and MnFe2O4 nanocrystals with a size distribution less than 7%. Specifically, the effects of interparticle interaction upon the blocking temperature and the reduced remanence have been studied. It is clear that even for highquality nanocrystals from the same magnetic ferrite family differences in magnetic behavior are clearly present. Apparently, it is very unwise to assume that the behavior of differing types of nanoparticulate systems would follow the same trends as that of the magnetite particles, which so far have been commonly used as the prototype for all interparticle interaction behavior. Experimental Section CoFe2O4 and MnFe2O4 spinel ferrite nanoparticles with a size of ∼8 nm were prepared by a combination of non-hydrolytic and seed-mediated growth processes, using the complexes of metal-acetylacetonate and derivatives.28,29 Figure 1 is the transmission electron microgram of CoFe2O4 nanoparticles. Advantages of this synthesis method include a very narrow size distribution of
