Magnetic properties of silica coated spindle-type hematite particles

Abstract

Magnetic properties of particles are generally determined from randomly oriented ensembles and the influence of the particle orientation on the magnetic response is neglected. Here, we report on the magnetic characterization of anisotropic spindle-type hematite particles. The easy axis of magnetization is within the basal plane of hematite, which is oriented perpendicular to the spindle axis. Two standard synthesis routes are compared and the effects of silica coating and particle orientation on the magnetic properties are investigated. Depending on the synthesis route we find fundamentally different magnetic behavior compatible with either single domain particles or superparamagnetic sub-units. Furthermore, we show that silica coating reduces the mean blocking temperature to nearly room temperature. The mechanical stress induced by the silica coating appears to reduce the magnetic coupling between the sub-units.

Morphology and Orientational Behavior of Silica-Coated Spindle-Type Hematite Particles in a Magnetic Field Probed by Small-Angle X-ray Scattering

“…This behavior can be attributed to superparamagnetic relaxation and is in perfect agreement with a previous study. 16 The superparamagnetic relaxation at room temperature for the silica-coated particles is unexpected, since it is normally observed for particles with diameter <30 nm only. 23 The investigated particles have formed by aggregation of subunits with a typical size of 10 nm, as suggested by Ocaña et al 9 This indicates that these subunits set the length scale relevant for the magnetic properties and, in particular, for the superparamagnetic relaxation.…”

“…The existence of pores is confirmed by TEM images (see inset of Figure 2) and is in agreement with observations made on similar particles. [15][16][17] The porosity reduces the effective scattering length density F c . The values for the scattering length densities F water ) 0.946 × 10 -3 nm -2 and F hem ) 4.16 × 10 -3 nm -2 were calculated based on the densities d water ) 1.0 g/cm 3 and d hem ) 5.26 g/cm 3 and were used as fixed parameters.…”

“…18 The magnetic moment can switch rather easily in the basal plane due to the small magnetic anisotropy energy within the plane compared to the energy required to tilt μ b out of the basal plane. 18 It has been shown 16 that the synthesized spindle-type hematite particles are monocrystalline with the c-axis collinear to the major particle axis a b, and μ b is perpendicular to a b. Thus, in a magnetic field B b , the particle orientation a b is only restricted to the plane perpendicular to B b , and the particles are free to rotate around the direction of B b without increasing E pot .…”

Exchange bias in a nanocrystalline hematite/permalloy thin film investigated with polarized neutron reflectometry

“…[1][2][3] A thorough understanding of bulk hematite's exotic magnetic properties such as anisotropic super-exchange, 4,5 low anisotropy, 6 and unusual spin structure 7 was already achieved by the mid-1960s. 1 More recently, there has been renewed interest in hematite systems at nanometer dimensions for applications in biomedical nanoparticles, 8,9 spin valves, [10][11][12] and chemical sensors. 13 Based on the work of Dzyaloshinsky 4 and Moriya, 5 the magnetic structure of hematite can be explained due to anisotropic super-exchange of the Dzyaloshinsky-Moriya (DM) form, 4,5 together with nearly zero net-uniaxial anisotropy due to the cancellation of the first-order single ion and dipolar anisotropies.…”

Hybrid magnetic iron oxide nanoparticles with tunable field-directed self-assembly

“…S3 in ESI †). We found that as the conversion time increases, the saturation magnetization (SM) of these hybrid particles steeply increases from a lower value of around SM ≈ 0.12 Am 2 kg −1 for pure hematite particles 42,44 until it levels off at a value SM ≈ 72 Am 2 kg −1 , which corresponds to the expected value for pure maghemite ( Fig. 1(f )).…”