Marie Yoshikiyo

Hard magnetic ferrite with a gigantic coercivity and high frequency millimetre wave rotation

Magnetic ferrites such as Fe3O4 and Fe2O3 are extensively used in a range of applications because they are inexpensive and chemically stable. Here we show that rhodium-substituted ε-Fe2O3, ε-RhxFe2−xO3 nanomagnets prepared by a nanoscale chemical synthesis using mesoporous silica as a template, exhibit a huge coercive field (Hc) of 27 kOe at room temperature. Furthermore, a crystallographically oriented sample recorded an Hc value of 31 kOe, which is the largest value among metal-oxide-based magnets and is comparable to those of rare-earth magnets. In addition, ε-RhxFe2−xO3 shows high frequency millimetre wave absorption up to 209 GHz. ε-Rh0.14Fe1.86O3 exhibits a rotation of the polarization plane of the propagated millimetre wave at 220 GHz, which is one of the promising carrier frequencies (the window of air) for millimetre wave wireless communications.

Nanometer-size hard magnetic ferrite exhibiting high optical-transparency and nonlinear optical-magnetoelectric effect

Development of nanometer-sized magnetic particles exhibiting a large coercive field (Hc) is in high demand for densification of magnetic recording. Herein, we report a single-nanosize (i.e., less than ten nanometers across) hard magnetic ferrite. This magnetic ferrite is composed of ε-Fe2O3, with a sufficiently high Hc value for magnetic recording systems and a remarkably high magnetic anisotropy constant of 7.7 × 106 erg cm−3. For example, 8.2-nm nanoparticles have an Hc value of 5.2 kOe at room temperature. A colloidal solution of these nanoparticles possesses a light orange color due to a wide band gap of 2.9 eV (430 nm), indicating a possibility of transparent magnetic pigments. Additionally, we have observed magnetization-induced second harmonic generation (MSHG). The nonlinear optical-magnetoelectric effect of the present polar magnetic nanocrystal was quite strong. These findings have been demonstrated in a simple iron oxide, which is highly significant from the viewpoints of economic cost and mass production.

Zeta-Fe2O3 – A new stable polymorph in iron(III) oxide family

Iron(III) oxide shows a polymorphism, characteristic of existence of phases with the same chemical composition but distinct crystal structures and, hence, physical properties. Four crystalline phases of iron(III) oxide have previously been identified: α-Fe2O3 (hematite), β-Fe2O3, γ-Fe2O3 (maghemite), and ε-Fe2O3. All four iron(III) oxide phases easily undergo various phase transformations in response to heating or pressure treatment, usually forming hexagonal α-Fe2O3, which is the most thermodynamically stable Fe2O3 polymorph under ambient conditions. Here, from synchrotron X-ray diffraction experiments, we report the formation of a new iron(III) oxide polymorph that we have termed ζ-Fe2O3 and which evolved during pressure treatment of cubic β-Fe2O3 ( space group) at pressures above 30 GPa. Importantly, ζ-Fe2O3 is maintained after pressure release and represents the first monoclinic Fe2O3 polymorph (I2/a space group) that is stable at atmospheric pressure and room temperature. ζ-Fe2O3 behaves as an antiferromagnet with a Néel transition temperature of ~69 K. The complex mechanism of pressure-induced transformation of β-Fe2O3, involving also the formation of Rh2O3-II-type Fe2O3 and post-perovskite-Fe2O3 structure, is suggested and discussed with respect to a bimodal size distribution of precursor nanoparticles.

External stimulation-controllable heat-storage ceramics

Commonly available heat-storage materials cannot usually store the energy for a prolonged period. If a solid material could conserve the accumulated thermal energy, then its heat-storage application potential is considerably widened. Here we report a phase transition material that can conserve the latent heat energy in a wide temperature range, T<530 K and release the heat energy on the application of pressure. This material is stripe-type lambda-trititanium pentoxide, λ-Ti3O5, which exhibits a solid–solid phase transition to beta-trititanium pentoxide, β-Ti3O5. The pressure for conversion is extremely small, only 600 bar (60 MPa) at ambient temperature, and the accumulated heat energy is surprisingly large (230 kJ L−1). Conversely, the pressure-produced beta-trititanium pentoxide transforms to lambda-trititanium pentoxide by heat, light or electric current. That is, the present system exhibits pressure-and-heat, pressure-and-light and pressure-and-current reversible phase transitions. The material may be useful for heat storage, as well as in sensor and switching memory device applications.

High-frequency millimeter wave absorption of indium-substituted ε-Fe2O3 spherical nanoparticles (invited)

In this work, we prepared indium-substituted e-iron oxide (e-InxFe2−xO3) spherical nanoparticles by a combination method of reverse-micelle and sol-gel techniques. The powder X-ray diffraction pattern with Rietveld analysis shows that e-InxFe2−xO3 has an orthorhombic crystal structure (space group: Pna21), and the In3+ ions mainly replace the Fe3+ ions at B site among the four nonequivalent Fe3+ sites (A–D sites). The magnetic measurements show that the coercive field (Hc) at 300 K decreases with increasing x, i.e., Hc = 21.9 kOe (x = 0), 12.2 kOe (x = 0.04), 11.6 kOe (x = 0.09), 7.8 kOe (x = 0.13), and 5.9 kOe (x = 0.18). Millimeter wave absorption was measured by terahertz time-domain spectroscopy, and the decrease of resonance frequency (fr) is observed, i.e., fr = 182 GHz (x = 0), 160 GHz (x = 0.04), 143 GHz (x = 0.09), 123 GHz (x = 0.13), and 110 GHz (x = 0.18). This decrease in the fr value is understood by the decrease of magnetic anisotropy, which is caused by the replacement of Fe3+ (S = 5/2) wit...

Mesoscopic bar magnet based on ε-Fe2O3 hard ferrite.

Ferrite magnets have a long history. They are used in motors, magnetic fluids, drug delivery systems, etc. Herein we report a mesoscopic ferrite bar magnet based on rod-shaped ε-Fe2O3 with a large coercive field (>25 kOe). The ε-Fe2O3–based bar magnet is a single crystal with a single magnetic domain along the longitudinal direction. A wide frequency range spectroscopic study shows that the crystallographic a-axis of ε-Fe2O3, which corresponds to the longitudinal direction of the bar magnet, plays an important role in linear and non-linear magneto-optical transitions, phonon modes, and the magnon (Kittel mode). Due to its multiferroic property, a magnetic-responsive non-linear optical sheet is manufactured as an application using an ε-Fe2O3–based bar magnet, resin, and polyethylene terephthalate. Furthermore, from the viewpoint of the large coercive field property, we demonstrate that a mesoscopic ε-Fe2O3 bar magnet can be used as a magnetic force microscopy probe.

Multimetal-Substituted Epsilon-Iron Oxide ϵ-Ga0.31Ti0.05Co0.05Fe1.59O3 for Next-Generation Magnetic Recording Tape in the Big-Data Era

From the viewpoints of large capacity, long-term guarantee, and low cost, interest in magnetic recording tapes has undergone a revival as an archive storage media for big data. Herein, we prepared a new series of metal-substituted ϵ-Fe2 O3 , ϵ-Ga(III) 0.31 Ti(IV) 0.05 Co(II) 0.05 Fe(III) 1.59 O3 , nanoparticles with an average size of 18 nm. Ga, Ti, and Co cations tune the magnetic properties of ϵ-Fe2 O3 to the specifications demanded for a magnetic recording tape. The coercive field was tuned to 2.7 kOe by introduction of single-ion anisotropy on Co(II) (S=3/2) along the c-axis. The saturation magnetization was increased by 44 % with Ga(III) (S=0) and Ti(IV) (S=0) substitution through the enhancement of positive sublattice magnetizations. The magnetic tape media was fabricated using an actual production line and showed a very sharp signal response and a remarkably high signal-to-noise ratio compared to the currently used magnetic tape.

The phase transition of ɛ-InxFe2−xO3 nanomagnets with a large thermal hysteresis loop (invited)

A large thermal hysteresis loop was observed in the phase transition on rod-shaped ɛ-InxFe2−xO3 (x ∼ 0.04) nanomagnets. The width of the thermal hysteresis loop, ΔT, increased with increasing rod length (l), i.e., ΔT = 6 K (l = 25 nm), 14 K (40 nm), 25 K (80 nm), and 47 K (170 nm). The observed ΔT value of 47 K is one of the largest values among insulating ferromagnetic materials. The thermal hysteresis loops were analyzed by the Slichter and Drickamer model, and the results showed that the transition enthalpy and entropy do not change. However, the elastic interaction parameter between the transition sites increases with an increasing l value. Maybe the correlation length of a propagating phonon due to elastic interaction competes with the rod length of the samples, causing the rod-length dependence of the thermal hysteresis loop.

Magnetic ground state of nanosized β-Fe2O3 and its remarkable electronic features

To date, iron oxides have been extensively investigated for promising high applicability in various scientific and industrial fields. In general, several forms can be distinguished with respect to their crystal structure, which drives their specific physical (in particular, magnetic) properties. In this study, the pure β-Fe2O3 phase, prepared in a nanoparticle form by a solid-state synthetic strategy, was investigated by employing 57Fe Mossbauer spectroscopy, magnetization measurements, transmission electron microscopy, X-ray powder diffraction, heat capacity measurements, and cyclic voltammetry. It is revealed that below the Neel transition temperature, β-Fe2O3 behaves as a canted antiferromagnet with a small net magnetic moment. For further possible utilization in photoelectrochemical applications, an estimation of the β-Fe2O3 band gap by cyclic voltammetry was performed, which was measured to be ∼2.2 eV.

Large Coercive Field of 45 kOe in a Magnetic Film Based on Metal-Substituted ε-Iron Oxide

Magnetic ferrites are stable, sustainable, and economical. Consequently, they have been used in various fields. The development of large coercive field (large Hc) magnetic ferrites is a very important but challenging issue to accelerate the spread of use and to expand practical applications. In this study, we prepared a rhodium-substituted ε-iron oxide film and observed a remarkably large Hc value of 35 kOe at room temperature. This is the largest value among magnetic ferrites to date. Such a large-Hc ferrite is expected to greatly expand the application of magnetic ferrites. Furthermore, when the temperature dependence of the magnetic properties was measured, an even larger Hc value of 45 kOe was recorded at 200 K. Such large Hc values are much larger than those of conventional hard magnetic ferrites.