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.
|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.|
|Zero field cooled magnetization of the dried powder of 11 nm Co nanoparticles.|
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.|
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.|