AmericanChemicalSociety.com

Report on PRF 46160-G10

PI: Gavin Lawes (Wayne State University)

The research activities conducted under the auspices of this grant over the past year have led to a more complete understanding of how magnetic order develops in nanoparticles and of the correlations between the magnetic properties of nanoparticles and their hydrodynamic response in solution.  Both of these characteristics are expected to be crucial in exploring magnetodielectric coupling in ferrofluids and nanoparticle composites.  The most significant results arising from this research is summarized in the following.

The development of magnetic order in nanoparticles

It is well known that the magnetic properties of nanomaterials differ significantly from bulk compounds.  These differences are typically believed to arise from finite size effects, including the absence of magnetic domains in nanoparticles.  However, some of our recent work suggests that the microscopic development of magnetic order may also be profoundly different in nanoparticles than in bulk compounds.  This result would have important ramifications for the development of nanomagnetic devices, particularly those based on materials having complex magnetic structures.

1. Suppression of spiral magnetic phases in Mn₃O₄ nanoparticles. We had previously established that bulk Mn₃O₄ exhibited significant magnetodielectric effects associated with the development of complex magnetic structures at temperatures below the ferrimagnetic ordering transition.  Our magnetic and thermodynamic studies on Mn₃O₄ nanoparticles provide strong evidence that these lower temperature magnetic transitions are absent, which eliminates the magnetodielectric coupling observed in bulk samples.  We tentatively attribute this suppression to the random magnetic fields produced by spins frozen on the surface of the nanoparticles.  This result is important, as it suggests that the nanoscale properties of some magnetodielectric materials may be markedly different than bulk samples, which may hamper the incorporation of these systems into devices.

2. Magnetism in MnAs nanoparticles.  Bulk MnAs is known to exhibit a ferromagnetic-paramagnetic phase transition coincident with a structural phase transition.  In a collaborative project with Prof. S. Brock (WSU Chemistry), we found that the MnAs crystal structure associated with the higher temperature paramagnetic phase can actually be stabilized at low temperatures in nanoparticles.  Surprisingly, these nanoparticles exhibit ferromagnetic properties that are very similar to those found in MnAs nanoparticles having the ferromagnetic crystal structure.  This study illustrates the complex interplay between magnetism and structure as well as highlighting exciting possibilities for stabilizing metastable crystal structures in nanoparticles.

Magnetohydrodynamics

One of the goals of this project was to investigate magnetodielectric effects in magnetic nanoparticles embedded in an insulating matrix.  Suspending magnetic nanoparticles in a dielectric carrier liquid provides the additional possibility of exploring the effects of nanoparticle distribution on their capacitive response, as this distribution can be controlled by applying an external magnetic field.  We have conducted a number of studies designed to clarify the magnetohydrodynamics of ferrofluids in order to develop a framework for analyzing the interplay of the frequency dependent hydrodynamic and dielectric responses.  

1. Pattern formation investigated by light scattering. Working with colleagues at Kettering University, we have conducted an extensive investigation of the hydrodynamic response of ferrofluids, focusing specifically on probing the dynamics of pattern formation in an external magnetic field.  By measuring time dependent light scattering patterns, we have characterized the formation of chains of nanoparticles when an external field is applied to the ferrofluid.  We also find evidence that the dielectric response of the ferrofluid is weakly modified by the presence of these nanoparticle chains.  By further exploring this effect, we plan to elucidate the mechanisms for this spin-charge coupling to determine how to optimize the response.

2. Loss mechanisms in ferrofluids. We have done extensive studies focused on clarifying the mechanisms of energy loss in magnetic ferrofluids.  These involve the coupling of external ac magnetic and electric fields to the ferrofluid.  Magnetic losses can be attributed primarily to Neel or Brownian relaxation in the individual nanoparticles, while the origins of the dielectric losses are less clear, but may be related to chain formation and hopping conduction.  This issue of losses in dielectric ferrofluids is particularly relevant for understanding how these materials can be incorporated into applications designed to reduce energy consumption.

Impact

The research conducted for this project has led to a number of significant results concerning mechanisms for controlling the magnetic properties and magnetodielectric coupling in nanoparticles and the magnetohydrodynamics in ferrofluids.  Additionally, this project has played an integral role in three M.S. degrees and one Ph.D. degree awarded during the course of this grant, and has provided support for a number of undergraduate students, including under-represented minorities.  The scientific accomplishments in the past year are outlined in the following publications.

Publications arising from PRF 46160-G10

(2008-2009 academic year only)

1. Y. Shen, Y.-C. Cheng, G. Lawes, J. Neelavalli, C. Sudakar, R. Tackett, H.P. Ramnath, E. M. Haacke, Magnetic Resonance Materials in Physics, Biology, and Medicine 21, 345 (2008).

2. C. Rablau, P.P. Vaishanava, C. Sudakar, R. Tackett, G. Lawes, R. Naik, Phys. Rev. E 78, 051502 (2008).

3. R. Regmi, R. Tackett, G. Lawes,  Journal of Magnetism and Magnetic Materials 321, 2296 (2009).

4. Keerthi Senevirathne, Ronald Tackett, Parashu Ram Kharel, Gavin Lawes, Kanchana Somaskandan, Stephanie L. Brock, ACS Nano 3, 1129 (2009).

5. Elayaraja Muthuswamy, Parashu Ram Kharel, Gavin Lawes, Stephanie L Brock, ACS Nano 3, 2383 (2009).