Chemical Solution Deposition of Ordered 2D Arrays of Room-Temperature Ferrimagnetic Cobalt Ferrite Nanodots

Abstract

Over the past decades, the development of nano-scale electronic devices and high-density memory storage media has raised the demand for low-cost fabrication methods of two-dimensional (2D) arrays of magnetic nanostructures. Here, we present a chemical solution deposition methodology to produce 2D arrays of cobalt ferrite (CFO) nanodots on Si substrates. Using thin films of four different self-assembled block copolymers as templates, ordered arrays of nanodots with four different characteristic dimensions were fabricated. The dot sizes and their long-range arrangement were studied with scanning electron microscopy (SEM) and grazing incident small-angle X-ray scattering (GISAXS). The structural evolution during UV/ozone treatment and the following thermal annealing was investigated through monitoring the atomic arrangement with X-ray absorption fine structure spectroscopy (EXAFS) and checking the morphology at each preparation step. The preparation method presented here obtains array types that exhibit thicknesses less than 10 nm and blocking temperatures above room temperature (e.g., 312 K for 20 nm diameter dots). Control over the average dot size allows observing an increase of the blocking temperature with increasing dot diameter. The nanodots present promising properties for room temperature data storage, especially if a better control over their size distribution will be achieved in the future.

Keywords: 2D arrays; block copolymer; chemical solution deposition; cobalt ferrite; ferrimagnetic; nanodot.

Figure 1

Schematic representation of the fabrication process of the cobalt ferrite (CFO) nanodots. Abbreviations: PEO, poly(ethylene oxide); PS, polystyrene; UV/O, ultraviolet/ozone.

Figure 2

Atomic force microscopy (AFM) images of BCP templates from (a–c) SEO-26k; (d–f) SEO-49k; (g–i) SEO-89k; (j–l) SEO-136k. Columns 1, 2 and 3 list the film morphology before solvent vapor annealing (SVA), after SVA in toluene at 50 °C for 2 h, and after SVA in a toluene + water mixed vapor at 50 °C for 1 h, respectively. The scale bars are 100 nm.

Figure 3

Scanning electron microscopy (SEM) images of the ion-loaded SEO-89k templates after UV/O₃ treatment. The treatment durations are 0 min for (a,e); 10 min for (b,f), 30 min for (c,g); and 60 min for (d,h). Images in the first row are from samples before thermal annealing (TA), and in the second row is after TA. The scale bars are 100 nm.

Figure 4

(a) SEM images from CFO nanodots (scale bar 100 nm). The text in the white boxes reports the BCP template name (Table 1) used to prepare the CFO dot assemblies. (b) The corresponding experimental and simulated 2D grazing incident small-angle X-ray scattering (GISAXS) patterns. (c) The corresponding 1D GISAXS patterns along the q_y direction together with the best-fit curves in red. The blue arrow in the 1D pattern of SEO-26k is a visual aid for a broad diffraction peak.

Figure 5

(a) Plane view and (b) cross-sectional transmission electron microscopy (TEM) images of the CFO nanodots obtained from SEO-49k template. The insets are the high-magnification images.

Figure 6

X-ray absorption fine structure spectroscopy (EXAF) signals at (a) Co K-edge and (b) Fe K-edge. The corresponding Fourier transform magnitudes (FT magnitudes) at (c) Co K-edge and (d) Fe K-edge for samples at different preparation stages. All plots share the same sample sequence and color code. The solid red lines are the simulation results. The four dashed vertical lines in (b,d) are used to label the typical peak positions for different shells (see text).

Figure 7

XPS spectra of the (a) Fe 2p and (b) Co 2p core-level regions of CFO nanodots, in which the black open circles indicate the experimental data, the red solid lines are the fitted envelope. The different fitted components are also plotted. A and B sites represent the tetrahedral and octahedral sites in the inverse spinel crystal structure, respectively.

Figure 8

In-plane magnetization curves for nanodots prepared from four different block copolymer templates. All curves were measured at 300 K.

Figure 9

Temperature dependence of the in-plane magnetization for nanodots prepared from (a) SEO-26k, (b) SEO-49k, (c) SEO-89k and (d) SEO-136k. The black and red curves are the field-cooling and zero-field-cooling curves, respectively.