ties related to the magnetic particle-particle distance and In this work the effective magnetic anisotropy of MnFe 2 O 4 orientation are concerned. nanoparticles has been obtained from the angular dependence of the magnetic resonance field. Two samples of ionic
In powdered samples, composed of nanoscaled magnetic particles, the magnetic anisotropy has been obtained from magnetic fluids, with different mean values for the particle diameter, were analyzed over a large temperature range. the temperature decay of remanence (7), from Mo ยจssbauer spectroscopy (10), and from magnetization curves (12). Moderate particle concentration was used in both samples in order to avoid particle-particle interaction. The values However, remarkable changes in the magnetic behavior have been observed after removing the carrier fluid from a mag-we obtained for the effective magnetic anisotropy are smaller than the reported values for the bulk over the entire range netic fluid sample, even taking into account changes in the particle-particle distance (15). This Communication intro-of temperatures analyzed and are strongly dependent on the particle size.
duces magnetic resonance as a tool for measuring the magnetic anisotropy of isolated nanoscaled magnetic particles The general interest in studying nanostructured materials in suspension as a magnetic fluid and compares the values comes from the novel physical properties which arise as a obtained with the values reported for the bulk ( ). In addiconsequence of the reduction of one or more of its dimention, for the magnetic fluid samples analyzed here, we found sions to atomic scale. The subject is relevant not only from the magnetic anisotropy to be smaller than the bulk values the fundamental point of view, as for instance, the quantum and directly related to the diameter of the particles, over the tunneling of the magnetization in magnetic nanoparticles temperature range studied. In contrast, the magnetic anisot-(1), but also from the technological point of view, as in the ropy reported for magnetic agglomerates is higher than the use of magnetic nanoparticles for magnetic refrigeration (2).
bulk values and inversely related to the diameter of the parti-This work addresses the effects of size reduction on the cles (8, 10, 12). magnetic anisotropy in mesoscopic magnetic systems, which have been reported either in transition metal films (4) or
In order to use magnetic resonance to measure the magnetic anisotropy of an isolated nanoparticle in a straightfor-in magnetic nanoparticles (6-12). In particular, we focus our attention on the magnetic anisotropy of isolated (nonin-ward way, we must prepare our magnetic fluid sample to be as close as possible to an ideal system composed of teracting) spherical nanoparticles of MnFe 2 O 4 .
noninteracting uniaxial magnetic particles dispersed in a A sample composed of nanoscaled magnetic particles is solid nonmagnetic matrix. In addition, the axis of easy magobtained either as a magnetic agglomerate, meaning clusters netization of the particles must be oriented, as perfectly as of magnetic particles, or as single magnetic particles dispossible, parallel to a given direction, in order to fit the persed in a nonmagnetic matrix. Powdered samples are comrequirements of the simple theoretical model discussed beposed of magnetic agglomerates, in which case the particlelow. In our case, such a condition can be reached with the particle interaction usually plays a very important role in magnetic fluid sample having particle concentrations up to determining some of the many properties of such nanopar-3 1 10 16 particle/cm 3 , cooled in an external magnetic field ticle-based systems, as for instance, the onset of low-energy greater than one tesla (15). In our experimental setup, the excitations associated with magnetic agglomerates (13).
sample holder is allowed to rotate around the vertical axis, Magnetic fluids, however, consist of monodomain nanowhere u is the angle between the ''easy'' axis of the sample scaled magnetic particles dispersed as single entities in a and the horizontal external magnetic field (H e ). carrier fluid, although the particles can interact with one another to produce, for instance, a glass-like phase attributed
The resonance frequency v r , i.e., the Larmor precession frequency of the magnetic moment (m) of the particle in the to dipole-dipole interaction (14). Therefore, magnetic fluids represent ideal systems to deal with, as far as the proper-presence of an effective magnetic field (H eff ), is given by 100