Spin canting across core/shell Fe3O4/MnxFe3−xO4 nanoparticles

Abstract

Magnetic nanoparticles (MNPs) have become increasingly important in biomedical applications like magnetic imaging and hyperthermia based cancer treatment. Understanding their magnetic spin configurations is important for optimizing these applications. The measured magnetization of MNPs can be significantly lower than bulk counterparts, often due to canted spins. This has previously been presumed to be a surface effect, where reduced exchange allows spins closest to the nanoparticle surface to deviate locally from collinear structures. We demonstrate that intraparticle effects can induce spin canting throughout a MNP via the Dzyaloshinskii-Moriya interaction (DMI). We study ~7.4 nm diameter, core/shell Fe3O4/MnxFe3−xO4 MNPs with a 0.5 nm Mn-ferrite shell. Mössbauer spectroscopy, x-ray absorption spectroscopy and x-ray magnetic circular dichroism are used to determine chemical structure of core and shell. Polarized small angle neutron scattering shows parallel and perpendicular magnetic correlations, suggesting multiparticle coherent spin canting in an applied field. Atomistic simulations reveal the underlying mechanism of the observed spin canting. These show that strong DMI can lead to magnetic frustration within the shell and cause canting of the net particle moment. These results illuminate how core/shell nanoparticle systems can be engineered for spin canting across the whole of the particle, rather than solely at the surface.

Spin waves across three-dimensional, close-packed nanoparticles

“…, which is in close agreement to the magnetic saturation versus temperature curve of figure 1(c) produced from SQUID magnetometry measurements (see footnote 6) with a blocking temperature is about 30 K [26]. The specific temperatures of 190 and 300 K are chosen in order to normalize inelastic magnetic data acquired at these temperatures (discussed later).…”

“…Such magnetic softening could in principle promote longer ranged inter-particle magnetic coupling. However, due to differences in precursor decomposition temperatures, the particles were found to be comprised of magnetite for the innermost 6.4 nm diameter and manganese ferrite out to their surfaces [26]. The Verwey transision was not observed for the nanoparticle system based on field-cooled and zero-field cooled hysteresis measurements [26], consistent with a general trend for Verwey suppression as a function of decreasing magnetite nanoparticle size [27].…”

“…Location  - -( ) ( ) determined from the ratio of exchange stiffness to Dzyaloshinskii-Moriya interaction strength [26,54] does not fit the data.…”

“…However, due to differences in precursor decomposition temperatures, the particles were found to be comprised of magnetite for the innermost 6.4 nm diameter and manganese ferrite out to their surfaces [26]. The Verwey transision was not observed for the nanoparticle system based on field-cooled and zero-field cooled hysteresis measurements [26], consistent with a general trend for Verwey suppression as a function of decreasing magnetite nanoparticle size [27]. Alternating toluene and ethanol rinses were used to remove excess oleic acid and reduce incoherent hydrogen scattering, leaving a thin, protective oleic acid coat of 0.3 nm or less in thickness surrounding each nanoparticle [28].…”

“…A similar sample of magnetite core|manganese ferrite shell nanoparticles with better inter-particle ordering, but smaller sample volume and a slightly thinner manganese ferrite shell of 0.5 nm [26], was used to obtain the temperature dependence of the magnetization (M) and to estimate the inter-particle magnetic correlation length in remanence. For this sample, SANS polarization analysis of the neutron spin before and after scattering from the sample allows the structural and magnetic scattering to be unambiguously separated [28,[33][34][35].…”