Magnetic Nanoparticle Metrology

Summary:

Colloidal cobalt nanoparticles possess strong magnetic moments and unique catalytic properties, and have applications in information storage, energy, and medicine. Currently these nanoparticles are commercially available. However, due to the reactive nature of cobalt upon exposure to air, a systematic approach is needed to fully characterize these nanoparticles during their growth and their aging after synthesis. We have synthesized monodispersed, colloidal cobalt nanoparticles through thermo-decomposition and demonstrated the formation of magnetic field-induced assemblies of cobalt nanoparticles in colloids. These colloids can be used to monitor the magnetic behavior of these nanocrystals in frozen and liquid carrier, which is particularly critical for the investigations of their interactions and their magnetic field-induced assemblies.

The shape effect of these nanoparticles has also been investigated. Cobalt nanocubes have been synthesized and their magnetic behavior has been compared with their spherical counterparts. One striking difference is the relaxation behavior of the two different shapes upon the removal of the external magnetic fields. As the strength of the external field is gradually reduced to zero, the magnetization of the spherical nanoparticle also decreases gradually. For the cubic particles in the columns, on the other hand, their magnetization decreases in a much slower fashion until they rearrange their magnetic moments into small circular groups, resulting in a sudden drop in their magnetization.

We have also combined transmission electron microscopy and magnetic measurements to probe the growth and aging of these colloidal cobalt nanoparticles. During the growth, cobalt cluster complexes have been detected. As the colloid ages in air, cobalt oxide grows inhomogeneously onto the surface of these colloidal Co nanoparticles, resulting in a drop in the magnetic moment and an increase in the hysteresis loop shift and coercivity. The combination of our results on the colloidal cobalt nanoparticles provides a detailed understanding of the underlying physics which dominates their growth, reactivity and ageing, all critically important for key application in energy and medicine.

 

Major Accomplishments: 

• Synthesized and characterized monodisperse magnetic properties of cobalt nanoparticles

• Monitored the effects of oxidation and aging on the magnetic moment of cobalt nanoparticles

 

Associated Publications

• G. Cheng, D. Romero, G. Fraser, and A.R. Hight Walker, "Magnetic-Field-Induced Assemblies of Cobalt Nanoparticles," Langmuir, 21 12055 (2005). (December 20 Cover Article)

• G. Cheng, C.L. Dennis, R.D. Shull, and A.R. Hight Walker, "Influence of the Collodial Environment on the Magnetic Behavior of Cobalt Nanoparticles," Langmuir, 23 11740 (2007).

• C.L. Dennis, G. Cheng, K.A. Baler, B.B. Maranville, A.R. Hight Walker and R.D.Shull, "The Influence of Temperature on the Magnetic Behavior of Colloidal Cobalt Nanoparticles ," IEEE Transactions 43 2448 (2007).

• G.J. Cheng, and A.R. Hight Walker, "Synthesis and characterization of cobalt/gold bimetallic nanoparticles," J. Mag. Mag. Mater. 311 31 (2007).

• G.J. Cheng, and A.R. Hight Walker, "Dipolar Chains Formed by Chemically Synthesized Cobalt Nanocubes," J. Mag. Mag. Mater. 321 1351 (2009).

• G. Cheng, C.L. Dennis, R.D. Shull, and A.R. Hight Walker, "Probing the Growth and Aging of Cobalt Nanocrystals," Crystal Growth Design, 9 3714 (2009).