Nanofabrication of two‐dimensional arrays of magnetite particles for fundamental rock magnetic studies

Abstract

[1] Magnetic measurements of samples with precisely controlled magnetic mineralogy, grain size, and interparticle spacing are needed to provide crucial experimental rock magnetic underpinning for paleomagnetic studies. We report a novel nanofabrication method for producing two-dimensional arrays of cylindrical synthetic magnetite particles with well-defined composition, particle size, and interparticle spacing. The samples are fabricated by writing dot arrays with electron beam lithography, transferring these patterns into sputtered Fe thin films by reactive ion etching in a CO/NH 3 plasma, and oxidizing the resulting Fe particles in a controlled atmosphere to form magnetite. Scanning electron microscopy and transmission electron microscopy have been used to monitor the fabrication process and to determine the particle geometry. The particle sizes of our samples range between 100 nm and 265 nm with center-to-center spacings between 180 nm and 310 nm. Low-temperature magnetic remanence data confirm the stoichiometry of the magnetite. We present magnetic hysteresis data and first-order reversal curve diagrams for our samples and compare these with previously published data from other synthetic and natural magnetite samples. The ability to independently control particle size and interparticle spacing of magnetite grains makes our synthetic samples ideal for studying the influence of magnetostatic interactions on the paleomagnetic recording fidelity of naturally occurring magnetite in rocks.

Room- and low-temperature magnetic properties of 2-D magnetite particle arrays

“…The Verwey transition temperatures are a little lower than the value for ‘perfect’ magnetite at 125 K (Walz 2002); however, the transition temperature is highly sensitive to the presence of stress, lattice vacancies or substitution of Fe by other cations such as Al or Ti. In this study (Table 1), the values are in general agreement with Verwey transition temperatures determined for magnetite samples produced by EBL (King & Williams 2000; Krása et al 2009). Krása et al (2009) reported extimates for DK0011, DK0023 and DK0024–2 of 115, 115 and 114 K, respectively, compared to 110–113, 114 and 100 K in this study.…”

“…The initial FC SIRM is larger than the ZFC SIRM for all the samples (Figs 8a–c). On warming, the curves demagnetize with kinks in the demagnetization curves at the Verwey transition (∼110–120 K), and as observed by Krása et al (2009), there is an additional slight kink at ∼50 K in many of the samples, which was more pronounced in the in the FC data than the ZFC data (Figs 8a–c). Recently, Özdemir & Dunlop (2010) found for two powdered interacting magnetite samples with mean sizes of 37 and 220 nm, a similar 50 K kink in their ‘surface oxidized’ samples for ZFC SIRM warming curves, but not in their reduced samples.…”

“…Its measured coercive force was 23 mT (Table 1). This compares to a value for H C of 17.1 mT reported by Krása et al (2009).…”

“…This provided two sets of hysteresis data for each sample. All the measurements reported by Krása et al (2009) were measured in‐plane.…”

Does size matter? Statistical limits of paleomagnetic field reconstruction from small rock specimens

“…Krása et al [] produced two‐dimensional arrays of magnetite particles of precisely controlled sizes and interparticle spacings over areas of 3 × 10 −6 to 2 × 10 −5 m 2 . They and Muxworthy et al [] analyzed the fidelity of the magnetic recording of these samples.…”