R. H. Kodama

Surface spin disorder in ferrite nanoparticles (invited)

Anomalous magnetic properties of organic coated NiFe2O4 nanoparticles have been reported previously (Berkowitz et al.).5 These properties included low magnetization with a large differential susceptibility at high fields and shifted hysteresis loops after field cooling, while Mossbauer spectra indicated that all of the material was magnetically ordered. In the present study, we find that the lack of saturation in high fields is accompanied by irreversibility (i.e., hysteresis loops are open) up to 160 kOe. In addition, the particles exhibit time dependent magnetization in 70 kOe applied field. The high field irreversibility and the loop shift both vanish above 50 K. We propose a model of the magnetization within these particles consisting of ferrimagnetically aligned core spins and a spin- glass-like surface layer. We find that qualitative features of this model are reproduced by a numerical calculation of the spin distribution. The implications of this model for possible macroscopic quantum tunneling in ...

Anomalous properties of magnetic nanoparticles

Abstract Nanoparticles of ferrimagnetic NiFe2O4 and antiferromagnetic NiO exhibit a variety of anomalous magnetic properties. The lower coordination of surface spins is responsible in both cases for the observed behavior. This conclusion is supported by calculations of field-dependent spin distributions in these nanoparticles.

Role of interfacial uncompensated antiferromagnetic spins in unidirectional anisotropy in Ni81Fe19/CoO bilayers (invited)

The uncompensated spins on the surfaces of antiferromagnetic CoO films exhibit a thermoremanent magnetization after field cooling from T>TN that has the same temperature dependence as the exchange field of Ni81Fe19/CoO bilayers after field cooling. This suggests that these interfacial uncompensated spins are responsible for unidirectional anisotropy. A model based on a calculation of the density of these interfacial uncompensated spins predicts the correct magnitude of the exchange field, as well as the observed inverse dependence on average interfacial grain size.

Low‐temperature magnetic relaxation of organic coated NiFe2O4 particles

We report measurements of relaxation rate of the remanent magnetization versus temperature for NiFe2O4 particles coated with the surfactant oleic acid. It has been reported that the organic coating induces a strong surface anisotropy resulting in surface spin canting and anomalous hysteresis properties at low temperature as compared to the bulk material. We have measured, at temperatures down to 400 mK, relaxation of remanent magnetization which extrapolates to a nonzero value at zero temperature. Macroscopic quantum tunneling at these temperatures has been predicted and observed only for materials with high magnetocrystalline anisotropy, so it is particularly interesting to observe it in NiFe2O4, which has a relatively low bulk anisotropy. It is suggested that the strong surface anisotropy induced by the organic coating may be responsible for tunneling in these particles.

Spin-flop tendencies in exchange-biased Co/CoO thin films

In order to study the antiferromagnetic (AFM) spin structure near the interface of exchange-biased bilayers, polarized neutron diffraction measurements were performed on a series of (111) Co (7.5 nm)/CoO (X nm) and CoO (X nm) thin films where X=20, 40, and 100 nm. In these samples, field cooling through the Neel temperature of the AFM increases the component of the CoO moment perpendicular to the applied field, relative to the parallel component. The subsequent application of a 500 Oe field perpendicular to the cooling direction rotates both the Co and CoO moments. Experiments on CoO films without Co showed a smaller difference between the parallel and perpendicular CoO moments in response to cooling and applied fields. Exchange coupling between the Co and CoO layers is apparently responsible for the increased projection of the AFM moments perpendicular to the cooling field.

High field irreversibility in NiFe2O4 nanoparticles (abstract)

Several anomalous magnetic properties of organic coated NiFe2O4 nanoparticles have been reported previously by Berkowitz et al. These properties included low magnetization with a large differential susceptibility at high fields, shifted hysteresis loops after field cooling below 50 K, and Mossbauer spectra indicating a fraction of the material being magnetically ordered but not responsive to applied fields. It was suggested that a ‘‘strongly anisotropic phase’’ was present on the particles surface, and that the organic coating was responsible for producing high local anisotropy fields on the surface atoms. The present study extends the original work in several ways. We find that the lack of saturation in high fields is accompanied by irreversibility up to 20 T, in some cases. We have confirmed the previously reported behavior, in addition to observing similar behavior in samples prepared without the organic surfactant. This implies that we are observing a finite size effect. We have recently reported time...

Spark Erosion for Production of Nanocrystalline Materials

We are currently undertaking a research program to exploit spark erosion for producing nanocrystalline alloys, intermetallics, superconducting, semiconducting and magnetic materials. Therefore we are exploring ways of tuning the various parameters in order to produce uniformly small particles of well defined structure and composition. This report summarizes some productive recent developments.

Fine Particle Synthesis by Spark Erosion

The technique of spark erosion can be used to prepare magnetlc fine particles of almost any composition in sizes from a few nm to tens of µm. The spark erosion method is described, including several recent improvements designed to permit the production of ultrafine particles without contamination.