Nanosized iron oxides still attract significant attention within the scientific community, because of their application-promising properties. Among them, ε-Fe 2 O 3 constitutes a remarkable phase, taking pride in a giant coercive field at room temperature, significant ferromagnetic resonance, and coupled magnetoelectric features that are not observed in any other simple metal oxide phase. In this work, we review basic structural and magnetic characteristics of this extraordinary nanomaterial with an emphasis on questionable and unresolved issues raised during its intense research in the past years. We show how a combination of various experimental techniques brings essential and valuable information, with regard to understanding the physicochemical properties of the ε-polymorph of Fe 2 O 3 , which remained unexplored for a long period of time. In addition, we recapitulate a series of synthetic routes that lead to the formation of ε-Fe 2 O 3 , highlighting their advantages and drawbacks. We also demonstrate how magnetic properties of ε-Fe 2 O 3 can be tuned through the exploitation of various morphologies of ε-Fe 2 O 3 nanosystems, the alignment of ε-Fe 2 O 3 nanoobjects in a supporting matrix, and various degrees of cation substitution. Based on the current knowledge of the scientific community working in the field of ε-Fe 2 O 3 , we finally arrive at two main future challenges: (i) the search for optimal synthetic conditions to prepare single-phase ε-Fe 2 O 3 with a high yield, desired size, morphology, and stability; and (ii) the search for a correct description of the magnetic behavior of ε-Fe 2 O 3 at temperatures below the characteristic magnetic ordering temperature.

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Photoinduced phase-transition materials, such as chalcogenides, spin-crossover complexes, photochromic organic compounds and charge-transfer materials, are of interest because of their application to optical data storage. Here we report a photoreversible metal-semiconductor phase transition at room temperature with a unique phase of Ti(3)O(5), lambda-Ti(3)O(5). lambda-Ti(3)O(5) nanocrystals are made by the combination of reverse-micelle and sol-gel techniques. Thermodynamic analysis suggests that the photoinduced phase transition originates from a particular state of lambda-Ti(3)O(5) trapped at a thermodynamic local energy minimum. Light irradiation causes reversible switching between this trapped state (lambda-Ti(3)O(5)) and the other energy-minimum state (beta-Ti(3)O(5)), both of which are persistent phases. This is the first demonstration of a photorewritable phenomenon at room temperature in a metal oxide. lambda-Ti(3)O(5) satisfies the operation conditions required for a practical optical storage system (operational temperature, writing data by short wavelength light and the appropriate threshold laser power).

Iron oxide (Fe(2)O(3)) has four crystal structures: gamma-, epsilon-, beta-, and alpha-Fe(2)O(3). Until now, routes of the phase transformations among the four Fe(2)O(3) phases have not been clarified because a systematic synthesis that yields all four Fe(2)O(3) phases has yet to be reported. Herein we report the synthesis of a series of Fe(2)O(3) nanoparticles using mesoporous SiO(2). The crystal structures of the Fe(2)O(3) nanoparticles change in the order of gamma-Fe(2)O(3) --> epsilon-Fe(2)O(3) --> beta-Fe(2)O(3) --> alpha-Fe(2)O(3) as the particle size increases. Threshold sizes were estimated as gamma --> epsilon at 8 nm, epsilon --> beta at 30 nm, and beta --> alpha at 50 nm in the synthesis using FeSO(4) as a precursor. The phase transformations among the four Fe(2)O(3) phases have been observed for the first time.

Millimeter waves (30-300 GHz) are starting to be used in next generation high-speed wireless communications. To avoid electromagnetic interference in this wireless communication, finding a suitable electromagnetic wave absorber in the millimeter wave range is an urgent matter. In this work, we prepared a high-performance millimeter wave absorber composed of a series of aluminum-substituted epsilon-iron oxide, epsilon-Al(x)Fe(2-x)O(3), nanomagnets (0 < or = x < or = 0.40) with a particle size between 25 and 50 nm. The materials in this series have an orthorhombic crystal structure in the Pna2(1) space group, which has four nonequivalent Fe sites and Al ion that predominantly occupies the tetrahedral [FeO(4)] site. The field-cooled magnetization curves showed that the T(C) values were 448, 480, and 500 K for x = 0.40, 0.21, and 0, respectively. The magnetization versus external magnetic field showed that the coercive field H(c) values at 300 K were 10.2, 14.9, and 22.5 kOe for x = 0.40, 0.21, and 0, respectively. The millimeter wave absorption properties were measured at room temperature by terahertz time domain spectroscopy. The frequencies of the absorption peaks for x = 0.40, 0.30, 0.21, 0.09, 0.06, and 0 were observed at 112, 125, 145, 162, 172, and 182 GHz, respectively. These absorptions are due to the natural resonance achieved by the large magnetic anisotropies in this series. Such frequencies are the highest ones for magnetic materials. Because aluminum is the third most abundant atom, aluminum-substituted epsilon-iron oxide is very economical, and thus these materials are advantageous for industrial applications.