The electronic and spin states of a series of Co-Fe Prussian blue analogues containing Na(+) ion in the lattice, Na(x)()Co(y)()Fe(CN)(6) x zH(2)O, strongly depended on the atomic composition ratio of Co to Fe (Co/Fe) and temperature. Compounds of Co/Fe = 1.5 and 1.15 consisted mostly of the Fe(III)(t(2g)(5)e(g)(0), LS, S = 1/2)-CN-Co(II)(t(2g)(5)e(g)(2), HS, S = 3/2) site and the Fe(II)(t(2g)(6)e(g)(0), LS, S = 0)-CN-Co(III)(t(2g)(6)e(g)(0), LS, S = 0) site, respectively, over the entire temperature region from 5 to 350 K. Conversely, compounds of Co/Fe = 1.37, 1.32, and 1.26 showed a change in their electronic and spin states depending on the temperature. These compounds consisted mainly of the Fe(III)-CN-Co(II) site (HT phase) around room temperature but turned to the state consisting mainly of the Fe(II)-CN-Co(III) site (LT phase) at low temperatures. This charge-transfer-induced spin transition (CTIST) phenomenon occurred reversibly with a large thermal hysteresis of about 40 K. The CTIST temperature (T(1/2) = (T(1/2) descending + T(1/2) ascending)/2) increased from 200 to 280 K with decreasing Co/Fe from 1.37 to 1.26. Furthermore, by light illumination at 5 K, the LT phase of compounds of Co/Fe = 1.37, 1.32, and 1.26 was converted to the HT phase, and the relaxation temperature from this photoproduced HT phase also strongly depended on the Co/Fe ratio; 145 K for Co/Fe = 1.37, 125 K for Co/Fe = 1.32, and 110 K for Co/Fe = 1.26. All these phenomena are explained by a simple model using potential energy curves of the LT and HT phases. The energy difference of two phases is determined by the ligand field strength around Co(II) ions, which can be controlled by Co/Fe.

The light-induced phase transition between the low-spin (LS) and high-spin (HS) states of some transition-metal ions has been extensively studied in the fields of chemistry and materials science. In a crystalline extended system, magnetically ordering the HS sites of such transition-metal ions by irradiation should lead to spontaneous magnetization. Previous examples of light-induced ordering have typically occurred by means of an intermetallic charge transfer mechanism, inducing a change of valence of the metal centres. Here, we describe the long-range magnetic ordering of the extended Fe(II)(HS) sites in a metal-organic framework caused instead by a light-induced excited spin-state trapping effect. The Fe-Nb-based material behaves as a spin-crossover magnet, in which a strong superexchange interaction (magnetic coupling through non-magnetic elements) between photo-produced Fe(II)(HS) and neighbouring Nb(IV) atoms operates through CN bridges. The magnetic phase transition is observed at 20 K with a coercive field of 240 Oe.

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.

Nanocrystals of iron oxide in a silica matrix exhibiting a giant Hc value of 2.0 T at room temperature are reported. The nanocomposite was obtained by combining reverse‐micelle and sol–gel methods. The nanocrystals of iron oxide are composed of the ϵ‐Fe2O3 phase, with rod‐like particles 100–140 nm long and 20–40 nm wide. The Figure shows the hysteresis curve of the nanocrystals.

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.

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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).

In general, magnetic properties of bulk magnetic materials are independent of the humidity of the environment. To obtain a magnetic material that has humidity-sensitive characteristics, water vapour must penetrate the lattice and act on spin sites. Nanoporous materials composed of metal-assembled complexes may be expected to display some humidity response because materials in this category can show functionalities such as gas storage and molecular recognition. Here, we demonstrate humidity-induced reversible variations in the magnetic properties of cyano-bridged cobalt(II)-manganese(II)-chromium(III) metal assemblies. The observed magnetic humidity response is due to adsorption and desorption of a ligand water molecule on the cobalt ion, which changes cobalt (II) between a 6- and 4-fold coordination geometry and switches the magnetic interaction between ferromagnetic coupling and antiferromagnetic coupling.