TEMPERATURE DEPENDENCE OF MAGNETIC CORRELATIONS WITHIN A MAGNETITE NANOPARTICLE ASSEMBLY

Kathryn Krycka, Wangchun Chen, Julie Borchers, Mark Laver, Brian Maranville, Thomas Gentile

NIST Center for Neutron Research, Gaithersburg, MD, USA

Charles Hogg, Ryan Booth, Sara Majetich

Carnegie Mellon University, Pittsburgh, PA, USA

Yumi Ijiri, Benjamin Breslauer

Oberlin College, Oberlin, OH, USA

Single domain, ferromagnetic nanoparticles are prominently featured for biomedical and data storage applications, but they also provide a means to probe the fundamental dynamics of magnetic interparticle correlations.  Small angle neutron scattering (SANS) is well suited for nanoparticle study because it covers length scales from single particles up to long-range interactions and samples a whole ensemble at once, features inaccessible with local probes such as TEM.  Additionally, polarization analysis of the scattered neutrons allows unambiguous separation of the nuclear and magnetic scattering.

Incomplete polarization of the neutron beam, however, is an experimental reality that must be accounted for.  Efficiencies arising from the use of an FeSi supermirror polarizer and a 3He analyzer range from 75 to 88%.  Using the formalism of Moon, Riste, and Koehler [2] we have implemented and further developed an algorithm to extract all four polarization-corrected spin scattering cross-sections (i.e. ↑↑,↑↓,↓↓, and ↓↑). The procedure was conducted on a pixel-by-pixel basis from position sensitive detector scans so as to fully retain the two dimensional scattering information.   Additionally, the 3He polarization decays during the period of data collection so we have also explicitly included its time dependence.

Applying this algorithm we successfully isolated and directly probed the temperature-dependent magnetic evolution of interparticle correlations from 7 nm diameter magnetite nanoparticles with an average edge-to-edge separation of 2.5 nm.  Magnetic correlation lengths were found to decrease not only upon surpassing the bulk magnetic blocking temperature of 65 K, but continued to shift to increasingly shorter lengths with increasing temperature up to 300 K.  Structural (nuclear) scattering did not significantly vary, as expected.  At all temperatures saturation from an applied field of 1.3 Tesla produced the longest range magnetic correlations, beyond a size measurable with our set-up.  This indicates that even at the highest temperature of 300 K (well above bulk blocking temperature) interparticle magnetic correlations were non-negligible.  While the polarized and unpolarized SANS data yield similar trends, our spin analyzed data clearly demonstrates the benefit of incorporating polarization analysis.

[1] J. Am. Chem. Soc. 2002, 124, 8204-8205.
[2] Phys. Rev. 1969, 181, 920-931.

Category: Physics

Author: Kathryn Krycka

Mentor:  Julie Borchers

Division: 856

Laboratory:  NIST Center for Neutron Research

Room E130-1, Building 235, Mail Stop 6102

Phone: (301) 975-8685

Fax: (301) 921-9847

Email: kathryn.krycka@nist.gov

Not Sigma Xi member