Magnetic and Structural Properties of the Maghemite Core-Shell Nanoparticles at Elevated Temperatures

Iron oxide-based nanoparticles are of great interest for various biomedical applications, such as targeted drug delivery, magnetic resonance imaging, magnetic separation, hyperthermia treatment etc. due to their small size,... [ view full abstract ]

Iron oxide-based nanoparticles are of great interest for various biomedical applications, such as targeted drug delivery, magnetic resonance imaging, magnetic separation, hyperthermia treatment etc. due to their small size, non-toxicity and biocompatibility. The most common structural modifications of iron oxide are maghemite (γ-Fe₂O₃), magnetite (Fe₃O₄), hematite (α-Fe₂O₃), wuestite (FeO), β-Fe₂O₃ and ε-Fe₂O₃. At high temperature structural transformations between these phases are possible that change in part their magnetic properties. Such transformations in nanosized particles can be different from those of bulk material. The aim of this study was to identify the main features of changes of magnetic and structural properties of core-shell maghemite nanoparticles at elevated temperature. 

Composite nanoparticles («magnetic core – polymer shell») grown by homogeneous nucleation with average core diameter of ~ 8 nm [1] were used for investigation. Temperature dependence of magnetization during heating and cooling in air in the range from RT to 650^oC was measured by vibrating sample magnetometer. The phase composition and structure parameters of the nanoparticles’ core were studied in temperature range from RT to 910^oC using synchrotron X-ray diffraction.

Fig.1. Thermal behaviour of nanoparticles’ core unit cell at the first heating (a) and temperature dependence of sample’s magnetization in external magnetic field H = 3940 Oe (b).

The studied nanoparticles possess superparamagnetic properties at room temperature and undergo irreversible changes of magnetic properties while being heated to ~ 250^oC. According to diffraction data maghemite cores contained before heating up to 20 w.% of ammonium chloride as a impurity from fabrication process. At temperature about ~ 250^oC atypical structural phase transition from maghemite to magnetite was identified. After heating to ~ 700^oC sample undergoes the second-order phase transition from magnetite to wuestite. It is supposed that these irreversible transformations occur due to reducing action of impurity and/or polymer shell presence in composite nanoparticles at elevated temperatures.

This work is supported by project DB/KMON of Ministry of Education and Science of Ukraine. Authors are grateful to Dr. O.S. Zaichenko and Dr. N.Ye. Mitina for synthesis of maghemite core-shell nanoparticles.

[1] P. Demchenko, N. Nedelko, N. Mitina et al., J. Magn. Mat. 379 (2015) 28-38.