Natalie Frey Huls, Lise-Marie Lacroix, Shouheng Sun, Cindi L. Dennis, and Robert D. Shull


An understanding of the magnetic anisotropy, or the directional dependence of magnetic properties in materials, is crucial to the research and development of a diverse range of applications ranging from thin films for magnetic recording to nanoparticles for MRI contrast enhancement and hyperthermia. Transverse susceptibility the measurement of the magnetic susceptibility (the ratio of magnetization to applied field) in one direction as the external field is swept in a perpendicular direction has been shown to be an excellent tool to directly probe the magnetic anisotropy and switching fields of a variety of materials. Here we discuss the transverse susceptibility measurement system recently built at NIST for the purpose of exploring the magnetic anisotropy in nanoparticles for use in biomedical applications. We focus on what this measurement can teach us about the anisotropic magnetic properties of arrays of iron oxide-based nanoparticles with various physical attributes. We examine the Fe3O4 system (a popular biocompatible material) specifically addressing the transverse susceptibility features as a function of particle size, particle size distribution, and particle dipolar interaction strength achieved by varying the concentration of Fe3O4 in a non-magnetic organic matrix. As an example of the insights transverse susceptibility lends into more complicated nanoparticle systems, we also present data collected on Fe-Fe oxide core-shell nanoparticles, a system that is gaining attention in the magnetic nanoparticle community due to the high moment of the Fe core and the chemical stability afforded by the Fe oxide shell. We discuss the temperature-dependent anisotropy of the core-shell nanoparticles and demonstrate how the core and shell contributions can each be observed. Overall, these results indicate that transverse susceptibility is an extremely useful method for evaluating nanoparticle systems for biomedical applications.