Ju He

Peculiar magnetism of BiFeO3 nanoparticles with size approaching the period of the spiral spin structure.

Size effect of multiferroics is important for its potential applications in new type miniaturized multifunctional devices and thus has been widely studied. However, is there special size effect in the materials with spiral modulated spin structure (such as BiFeO3)? It is still an issue to be investigated. In this report, structural, magnetic and magnetoelectric coupling properties are investigated for sol-gel prepared BiFeO3 nanoparticles with various sizes. It is found that a structural anomaly arises for the particles with size close to the 62 nm period of the spiral modulated spin structure, which induces an obviously increased ferromagnetism. In addition, large magnetoelectric coupling effect is observed in 62 nm BiFeO3 nanoparticles. Our result provides another insight into the size effect of BiFeO3, and also a clue to the magnetic structure at nanoscale.

Simultaneously efficient blue and red light generations in a periodically poled LiTaO3

Generations of efficient blue light at 447 nm and red light at 671 nm were achieved by frequency doubling and tripling of a diode-pumped, Q-switched 1342 nm Nd:YVO4 laser with a periodically poled LiTaO3 (PPLT). The blue light at 447 nm was generated by sum-frequency mixing of the fundamental at 1342 nm with the generated second harmonic at 671 nm. The first-order and third-order reciprocals of the PPLT compensated the phase mismatches of second-harmonic and sum-frequency processes, respectively, making them quasiphase matched. The resulting averaged blue light power of 51 mW and red light power of 207 mW under the averaged fundamental power of 500 mW indicate that the PPLT may be used to construct an all-solid-state blue and red dual wavelength laser.

Efficient generation of red light by frequency doubling in a periodically-poled nearly-stoichiometric LiTaO3 crystal

An efficient generation of red light in a periodically-poled nearly-stoichiometric LiTaO3 (PPSLT) by extracavity single-pass frequency doubling of a diode-pumped, Q-switched Nd:YVO4 laser at 1342nm was realized. An average power of 1.4W of the 671nm red light is obtained at the fundamental power of ∼2.8W with the conversion efficiency of 50%. The high conversion efficiency and steady output of red light indicate that the thick PPSLT is a competitive candidate for frequency conversion in order to construct a compact all-solid-state red laser.

Multiferroic properties and magnetoelectric coupling in Fe/Co co-doped Bi3.25La0.75Ti3O12 ceramics

Polycrystalline Bi3.25La0.75(Ti3−xFex/2Cox/2)O12 (0 ≤ x ≤ 0.5) ceramics are synthesized by the conventional solid state reaction method. Good ferroelectricity and weak ferromagnetism are obtained simultaneously at room temperature when x ≥ 0.2. The ceramics with x = 0.25 possess the best comprehensive properties, including high phase purity, large ferroelectric polarization, great dielectric constant, and also, larger remnant magnetization compared with other samples. Furthermore, significant dielectric anomalies appear near both the ferromagnetic Curie temperature and the spin glass freezing temperature. The possible magnetoelectric coupling mechanism is considered to be the associated binding effect of magnetic interaction on the migration of charged defects, especially oxygen vacancies.

Induction and control of room-temperature ferromagnetism in dilute Fe-doped SrTiO3 ceramics

The magnetic characteristics of dilute Fe-doped SrTiO3 ceramics are studied. The room-temperature ferromagnetism (with transition temperature around 650 K) is successfully realized in Sr0.98Ti0.9Fe0.1O3−δ and Sr0.98Ti0.92Fe0.1O3−δ ceramics. It is found that a fine-tuning of the components could alter the substitution sites of Fe ions and in-turn modulates the magnetism of the material. A systematic analysis reveals that the co-substitution of Fe ions at nonequivalent A and B sites in ABO3 type perovskites is in favor of the ferromagnetism, which could be attributed to the mixed-valence-states of Fe ions and the variation of exchange interactions. This work provides an innovation for the induction and control of ferromagnetism in dilute magnetic materials.

Quasi-Cw ultraviolet generation in a dual-periodic LiTaO3 superlattice by frequency tripling

We propose a dual-periodically domain-inverted structure for realizing an efficient third-harmonic generation at an arbitrary fundamental wavelength. The structure allows to couple two separate optical parametric processes, frequency doubling and frequency adding, through a cascaded quasi-phase-matched interaction, therefore, generate third-harmonic directly from a superlattice sample. As an example, we designed and fabricated such a structure in a LiTaO3 crystal. Using a LD pumped 1.064 mum quasi-cw Nd:YVO4 laser as a fundamental source, considerable ultraviolet at 355 nm was generated from this superlattice by frequency tripling. And green at 532 nm was simultaneously obtained by frequency doubling.

Magnetic and dielectric properties of metamagnetic TbCo0.5Mn0.5O3.07 ceramics

The ceramic TbCo0.5Mn0.5O3.07 of double-perovskite structure was prepared by solid-state reaction. Its crystal structure and magnetic and dielectric properties were investigated by first-principles calculations and experimental observations. TbCo0.5Mn0.5O3.07 possesses a monoclinic structure with P21/n space group. The c axis is the easy-magnetization axis, and it is largely caused by Co2+ anisotropy. The predominant valence states are Mn4+ and Co2+, with a small amount of Co3+ coexisting with Co2+. The ordering of Mn4+ and Co2+ results in ferromagnetic Mn4+–Co2+ interactions. Partial disorder of the B-site creates antiferromagnetic Co2+–O–Co2+ or Mn4+–O–Mn4+ interactions. The origin of metamagnetism is associated with the coexistence of antiferromagnetic and ferromagnetic phases. The magnetic exchange bias is strongly dependent on magnetic field, which is considered to be related to the metamagnetic behavior. The possibility of spin glass behavior is excluded by AC susceptibility measurements. The two observed dielectric relaxations are caused by electrons hopping between Co2+ and Mn4+ and between Co3+ and Mn4+.

Effect of substrates on magnetization of BiFeO3 films

BiFeO3 (BFO) films were fabricated on Pt/Ti/SiO2/Si and single crystalline LiNbO3 (LN) substrates using metal organic decomposition method and annealed in N2/O2. Magnetizations of these films were systematically characterized. It is found that BFO films prepared on Pt/Ti/SiO2/Si substrates exhibit stronger saturation magnetization (MS) than those prepared on LN substrates, and their magnetizations rely more on annealing atmosphere. We consider that both oxide feature of LN substrates and in-plane compressive stress introduced by LN reduce the Fe2+ content in the top BFO films and further stabilize the films against post-treatment. This work provides a valuable guidance for fabricating high quality magnetic oxide films.

Study of mechanical and dielectric spectrum in YFe1-xMnxO3 ceramics

The mechanical spectra of Mn-substituted yttrium orthoferrite YFe1-xMnxO3 (x = 0, 0.2, 0.3, 0.4) ceramics were performed at kilohertz from 100 to 360 K. Two internal friction (IF) peaks are observed around 150 K and 230 K, respectively, and both the IF peaks exhibit frequency dispersion behavior. The IF peak around 150 K is associated with a step increase in the modulus and its mechanical relaxation rate follows the Vogel-Fulcher relation with τ0 = 4.45 × 10−11s, Eα = 0.03 eV, and TVF = 155 K. This IF peak can be explained in terms of a freezing of oxygen vacancies after excluding the possible magnetic spin glass transition. Another IF peak around 230 K presents a relaxation behavior and it follows Arrhenius law. Furthermore, the relaxation behavior was verified by the dielectric spectrum and it can be ascribed to the charge carrier hopping between Fe2+ and Fe3+.

Simultaneous generation of three primary colours using aperiodically poled LiTaO3

Simultaneous generation of efficient red, green and blue (RGB) light was achieved by doubling and tripling a diode-pumped, Q-switched 1342 and 1063 nm Nd:GdVO4 laser using an aperiodically poled LiTaO3 crystal. Quasi-white colour was obtained by mixing the RGB outputs at 115.8 °C. The average powers of the RGB light were 31.3, 18.4 and 3.7 mW, respectively.