Research: Dynamics and Confinement of Magnetic Nanoparticles

There is much interest in the properties of materials when their physical size becomes very small, approaching the spacing between atoms. Much of this interest is driven by technological considerations, such as the need to increase the density of magnetic storage media, advances in miniaturization of electronic circuit components, and by the need for new medical therapies and probes. For these reasons, the availability of nanoscale materials creates a novel venue for exploring the middle ground between atomic and bulk behaviors, particularly the effects of finite size on many body phenomena such as magnetism or superconductivity. A second research project in our group explores the properties of magnetic nanoparticles, especially in constrained spatial environments and also in metallic hosts.

Much is known about nanoparticles suspended in non-conducting hosts. For an individual moment-bearing particle of volume V, magnetic anisotropy K induces easy magnetic axes, and the magnetization state can only be modified in zero field by thermal activation over an energy barrier EB~KV. The moment is considered blocked, or quasi-static, if the measurement time is less than the thermal activation time. Otherwise, the moment fluctuates freely among its magnetization states and is considered superparamagnetic. For a dilute suspension of magnetic particles, weak interparticle interactions can simply be included in the effective energy barrier which separates the magnetization states of the individual particles. However, as the particle concentration is increased, interactions increasingly dominate, and ultimately one can only consider the states of the system magnetization, which can be visualized qualitatively as minima in a free energy landscape. In this case, the random anisotropy and the variation in interparticle distances lead to spin glass behavior: i.e. short range spatial correlations, accompanied by the progressive slowing down and freezing of dynamical scales. Indeed, spin glass phenomena such as field cooled/zero field cooled hysteresis, strong dependences on measuring time and previous sample history are typical of concentrated magnetic nanoparticle assemblies. Much experimental and theoretical effort has been directed towards understanding the crossover from single particle behavior to collective behavior with reduced temperature and increased particle concentration.

Images of Co nanoparticles dried on mica surfaces, obtained by Atomic Force Microscopy (AFM). Dilute solutions are highly monodisperse, and the partial crystallization is enabled by the relatively low surface tension of the solvent.

We synthesize bulk quantities of magnetic nanoparticles through a chemical process first developed by Alivisatos, involving the injection of molecular precursors into hot surfactants. This process yields suspensions of surfactant coated nanoparticles with sizes which range from 4-12 nm. We have assessed the particle size distribution using a combination of dynamical light scattering and small angle neutron scattering measurements, complemented by atomic force microscopy (AFM) measurements. Above are images of the nanoparticles obtained by performing atomic force microscopy (AFM) measurements in tapping mode on ensembles of nanoparticles prepared in several ways. We find that the nanoparticles are highly monodisperse, and resistant to permanent aggregation in the suspension even at high concentrations. Partial crystallization can be induced by slow drying of moderately concentrated suspensions involving a low surface tension solvent.

Contour plots of scattered neutron intensity from a dried powder of 11 nm Co nanoparticles at 300 K (top), 200 K (middle) and 100 K( bottom). Data obtained on the DCS time of flight spectrometer at NCNR.
We have recently carried out inelastic neutron scattering measurements on a dried powder of 11 nm diameter Co nanoparticles using the Disc Chopper Spectrometer (DCS) at the NIST Center for Neutron Research (see the figure at right). Contrary to the scenario described above, we find that there is strong quasielastic scattering present at high temperatures, but that the fluctuations slow, and the scattering becomes increasingly elastic as the temperature is lowered to ~200 K. This suggests an approximate energy scale on which the individual particles are blocked or frozen, presumably by interparticle interactions. As the temperature is lowered further, we observe a continuous growth in the elastic scattering near q=2 A-1 which only saturates below ~50 K. This result is unexpected, as it indicates that the moments of the individual nanoparticles themselves are still growing even when the nanoparticles are frozen together. We believe that Co nanoparticles are not
Zero field cooled magnetization of the dried powder of 11 nm Co nanoparticles.
truly superparamagnetic, at least at this size. Our results indicate that the formation of a static and spontaneous magnetization within the nanoparticle only occurs below ~50 K, and the suppression from the bulk Curie temperature of Co (1388 K) is a dramatic manifestation of finite size effects. A clear ferromagnetic ordering peak is observed in the zero field cooled magnetization, while the broad high temperature peak corresponds to the blocking of the nanoparticles, since hysteresis and separation of field cooled and zero field cooled magnetizations are only observed for T<200 K.

We have carried out small angle neutron scattering experiments on bulk solutions of Co nanoparticles, and an example of our results is shown at the right. The single particle peak at the nanoparticle diameter of 5 nm is clearly visible, as is the enhancement of long range spatial correlations, presumed magnetic, as the magnetic blocking temperature is approached. These experiments are aimed at assessing

SANS data collected on a suspension of Co nanoparticles in toluene. Data obtained on the SAND instrument at IPNS. Note the dramatic growth of small angle scattering with reduced temperature, signaling the onset of interparticle correlations.
both the length and time scales on which correlations persist, and the extent to which magnetic blocking can be considered a dynamical freezing phenomenon. Unlike a conventional spin glass, magnetic nanoparticle assemblies have internal dynamics, and it is thought that the thermodynamic potential depends on both the intraparticle energy scales, modified by interactions among the particles, especially at high concentrations and low temperatures.

In our experimental program, we aim to explore the extent to which individual magnetic nanoparticles resemble atomic spins, albeit of large and tunable magnitude. We are carrying out a number of experiments aimed at exploring different aspects of this issue, including the effects of particle size and concentration, as well as shape on the development of static correlations.

Much can be deduced about the internal structure of a moment from the nature of magnetic order which it experiences, the associated critical phenomena, and the excitations of the paramagnetic and ordered states. In these experiments, we will construct artificial low dimensional magnets using one dimensional templates, where the role of atomic spins is played by

AFM image of Co nanoparticles filling pores of an alumina matrix. The particles are 11 nm in diameter.
moment-bearing nanoparticles. One such system which we have synthesized is depicted in the figure at the left, which is an atomic force microscope (AFM) image of 11 nm diameter Co nanoparticles filling the 200 nm diameter cylindrical pore of a porous alumina substrate. We might expect that the interaction of the nanoparticles with the interior surface of the pore would lead to enhanced anisotropy in the magnetic properties. We have measured the magnetization of
this system, with the field both parallel and perpendicular to the pore axis (see figure at right). Surprisingly, the degree of anisotropy is very slight, with the saturation moment enhanced less than 50% when the field is along the pore axis. We believe that the relatively small role for the surface anisotropy can be rationalized by noting that relatively few nanoparticles are actually in contact with the pore wall, suggesting the importance of repeating this experiment with smaller pore sizes and larger nanoparticles.



Stony Brook University/
Brookhaven National Laboratory