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Fig. 5. Powder XRD patterns of the iron oxide nanocubes (recorded with CuKa x-ray source (background eliminated).

Fig. 6. Hysteresis loop of maghemite nanocubes measured at 300 K and 5 K.

Fig. 7. Temperature dependence of the magnetization M for maghemite nanocubes in applied fields of 50 Oe under FC (•) and ZFC (○) conditions.

Fig. 8. Scanning electron micrograph of mesocrystals of maghemite nanocubes formed on the mica surface by slow drying of a highly concentrated toluene-based dispersion (8.4 ´ 10¹⁴ particles per milliliter) of maghemite nanocubes under a constant magnetic field. (Scale bar: 1 mm.)

Fig. 9. Low-magnification TEM image of thick, three-dimensional mesocrystals of maghemite nanocubes formed on a carbon-coated Cu grid by slow drying of a highly concentrated toluene-based dispersion (8.4 ´ 10¹⁴ particles per milliliter) of maghemite nanocubes under a constant magnetic field. (Scale bar for both the image and the Inset: 1 mm.)

SI Text

The XRD pattern of the cubic nanocrystals is shown in SI Fig. 5. The XRD pattern of the cubic iron oxide nanocrystals can be assigned to the 110, 220, 311, 400, 422, 511, and 440 reflexes of a spinel structure (Fd-3m), characteristic for both maghemite (g-Fe₂O₃, JCPDS no. 39-1346, S.G. P4₁32, a = 8.3515 Å) and magnetite (Fe₃O₄, JCPDS no. 19-0629, S.G. Fd-3m, a = 8.396 Å).

The magnetic characterization was carried out in the temperature range of 5 K to 300 K. For the zero field cooled (ZFC) measurements, the sample was cooled down from room temperature to 5 K in the absence of an external magnetic field, and the magnetic data were acquired during the warming run in a constant external field. In the field cooled measurements (FC), the sample was initially cooled down to 5 K in the presence of a magnetic field and the FC data were recorded during the warm-up cycle in the same magnetic field.

SI Fig. 6 shows the magnetic response of the maghemite nanocubes at 300 K and at 5 K. The nanocubes show a strongly nonlinear behavior with no hysteresis at room temperature whereas there is a pronounced hysteresis loop at 5 K. This behavior is typical of nanosized particles that are superparamagnetic (SP) at room temperature and develop a finite magnetic coercivity below the spin freezing (or blocking) temperature. The magnetization value M for the nanocubes was 71 emm/g.

The low field temperature dependence of the magnetization M(T) in the ZFC state and FC state were measured in a 50 Oe magnetic field (shown in SI Fig. 7). The peak temperature (T_B) in the ZFC state is observed at 129 K, which is the typical magnetic behavior of a SP material with a characteristic spin freezing (or blocking) temperature T_B, below which the spins are successively locked into their sites and do not contribute to the overall magnetization. The magnetization decreases with increasing temperature above T_B as expected.

It is possible to produce micrometer-sized, freestanding mesocrystals by placing several drops of a highly concentrated (8.4 ´ 10¹⁴ particles per milliliter) toluene-based dispersion of maghemite nanocubes onto a substrate and subjecting the drying dispersion to a constant magnetic field for at least 2 weeks. SI Fig. 8 shows a scanning electron micrograph (obtained with a Philips XL 30 ESEM) of maghemite nanocube mesocrystals formed on a freshly cleaved mica surface, and SI Fig. 9 shows a low-magnification transmission electron microscope (TEM) image of mesocrystals formed on a carbon-coated Cu grid.