Xiaoping Song

The magneto–elastic coupling effect correlates to the changes of moment and lattice upon magnetic phase transition. Here, we report that, in the pseudo-binary Laves-phase Tb_1-xDy_xCo₂ system (x = 0.0, 0.7, and 1.0), thermal expansion and magnetostriction can probe the ferrimagnetic transitions from cubic to rhombohedral phase (in TbCo₂), from cubic to tetragonal phase (in DyCo₂), and from cubic to rhombohedral then to tetragonal phase (in Tb_0.3Dy_0.7Co₂). Furthermore, a Landau polynomial approach is employed to qualitatively investigate the thermal expansion upon the paramagnetic (cubic) to ferrimagnetic (rhombohedral or tetragonal) transition, and the calculated thermal expansion curves agree with the experimental curves. Our work illustrates the correlation between crystal symmetry, magnetostriction, and thermal expansion in ferrimagnetic Laves-phase alloys and provides a new perspective to investigate ferrimagnetic transitions.

We report the magnetic and magnetostrictive behaviors of the pseudobinary ferrimagnetic spinel oxide system (1−x)CoFe₂O₄–xCoAl₂O₄ [Co(Fe_1−xAl_x)₂O₄], with one end-member being the ferrimagnetic CoFe₂O₄ and the other end-member being CoAl₂O₄ that is paramagnetic above 9.8 K. The temperature spectra of magnetization and magnetic susceptibility were employed to detect the magnetic transition temperatures and to determine the phase diagram of this system. Composition dependent and temperature dependent magnetostrictive behaviors reveal an exotic phase boundary that separates two ferrimagnetic states: At room temperature and under small magnetic fields (∼500 Oe), Fe-rich compositions exhibit negative magnetostriction while the Al-rich compositions exhibit positive magnetostriction though the values are small (x = 0.35, 0.4, 0.45, 0.5) exhibit near-zero magnetostriction and enhanced magnetic susceptibility, which may be promising in the applications for magnetic cores, current sensors, or magnetic shielding materials.

Pt/CeO₂ catalysts with nanoporous structures were prepared by the facile dealloying of melt-spun Al_92−XCe₈Pt_X (X = 0.1; 0.3 and 0.5) ribbons followed by calcination. The phase compositions and structural parameters of the catalysts were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). The specific surface area and pore size distribution were characterized by N₂ adsorption–desorption tests. The catalytic properties were evaluated by a three-way catalyst (TWC) measurement system. The results revealed that the dealloyed samples exhibited a nanorod framework structure. The Pt nanoparticles that formed in situ were supported and highly dispersed on the CeO₂ nanorod surface and had sizes in the range of 2–5 nm. For the catalyst prepared from the melt-spun Al_91.7Ce₈Pt_0.3 ribbons, the 50% CO conversion temperature (T₅₀) was 91 °C, and total CO could be converted when the temperature was increased to 113 °C. An X-ray photoelectron spectroscopy (XPS) test showed that the Pt_0.3/CeO₂ sample had a slightly richer oxygen vacancy; and a H₂ temperature-programmed reduction (H₂-TPR) test demonstrated its superior adsorption ability for reduction gas and high content of active oxygen species. The experiments indicated that the catalytic performance could be retained without any attenuation after 130 h when water and CO₂ were present in the reaction gas. The favorable catalytic activities were attributed to the high specific areas and small pore and Pt particle sizes as well as the strong interactions between the CeO₂ and Pt nanoparticles. The Pt nanoparticles were embedded in the surface of the CeO₂ nanorods, inhibiting growth. Therefore, the catalytic stability and water resistance were excellent.

The morphotropic phase boundary (MPB), which is the boundary separating a tetragonal phase from a rhombohedral phase by varying the composition or mechanical pressure in ferroelectrics, has been studied extensively for decades because it can lead to strong enhancement of piezoelectricity. Recently, a parallel ferromagnetic MPB was experimentally reported in the TbCo₂–DyCo₂ ferromagnetic system and this discovery proposes a new way to develop potential materials with giant magnetostriction. However, the role of magnetic domain switching and spin reorientation near the MPB region is still unclear. For the first time, we combine micromagnetic theory with Monte Carlo simulation to investigate the evolution of magnetic domain structures and the corresponding magnetization properties near the MPB region. It is demonstrated that the magnetic domain structure and the corresponding magnetization properties are determined by the interplay among anisotropy energy, magnetostatic energy and exchange energy. If the anisotropy energy barrier is large compared with the magnetostatic energy barrier and the exchange energy barrier, the MPB region is a T and R mixed structure and magnetic domain switching is the dominant mechanism. If the anisotropy energy barrier is small, the MPB region will also contain M phases and spin reorientation is the dominant mechanism. Our work could provide a guide for the design of advanced ferromagnetic materials with enhanced magnetostriction.

A spontaneous exchange bias (SEB) discovered by Wang et al. [Phys. Rev. Lett. 106 (2011) 077203.] after zero-field cooling (ZFC) has attracted recent attention due to its interesting physics. In this letter, we report a giant SEB tuned by Sb-doping in Ni₅₀Mn₃₈Ga_12-xSb_x Heusler alloys. Such an SEB was switched on below the blocking temperature of approximately 50 K. The maximum exchange bias H_E can arrive at 2930 Oe in a Ni₅₀Mn₃₈Ga₁₀Sb₂ sample after ZFC to 2 K. Further studies showed that this SEB was attributable to interaction of superspin glass (SSG) and antiferromagnetic matix, which was triggered by the crossover of SSG from canonical spin glass to a cluster spin glass. Our results not only explain the underlying physics of SEB, but also provide a way to tune and control the SEB performance.

The crystal structure, magnetization, and spontaneous magnetostriction of ferromagnetic Laves phase GdFe₂ compound have been investigated. High resolution synchrotron x-ray diffraction (XRD) analysis shows that GdFe₂ has a lower cubic symmetry with easy magnetization direction (EMD) along [100] below Curie temperature T _C. The replacement of Gd with a small amount of Tb changes the EMD to [111]. The Curie temperature decreases while the field dependence of the saturation magnetization (M _s) measured in temperature range 5–300 K varies with increasing Tb concentration. Coercivity H _c increases with increasing Tb concentration and decays exponentially as temperature increases. The anisotropy in GdFe₂ is so weak that some of the rare-earth substitution plays an important role in determining the easy direction of magnetization in GdFe₂. The calculated magnetostrictive constant λ ₁₀₀ shows a small value of 37×10⁻⁶. This value agrees well with experimental data 30×10⁻⁶. Under a relatively small magnetic field, GdFe₂ exhibits a V-shaped positive magnetostriction curve. When the field is further increased, the crystal exhibits a negative magnetostriction curve. This phenomenon has been discussed in term of magnetic domain switching. Furthermore, magnetostriction increases with increasing Tb concentration. Our work leads to a simple and unified mesoscopic explanation for magnetostriction in ferromagnets. It may also provide insight for developing novel functional materials.

Body-centered-cubic (bcc) Ni nanowires were successfully synthesized by multistep ac electro-deposition on anodic aluminum oxide templates. bcc-Ni is ferromagnetic with a lattice constant of 0.288 nm, and bcc-Ni to fcc-Ni phase transition occurs under heat treatment. Magnetic measurements suggest that the ferromagnetic transition of bcc-Ni nanowires involves a structure change evolving from bcc to fcc in the magnetic-ordering process. Furthermore, the processes of structure transition in bcc-Ni nanowires under the heating conditions were observed in situ using transmission electron microscopy. In addition, our work also establishes that fcc-Ni nanowires and bulk Ni have only a ferromagnetic transition at the Curie point, involving no crystal structure change.

Novel 3D hollow cage-like α-Fe₂O₃ and Fe microspheres were fabricated by electrospinning followed by an annealing process for the first time.

A lithium-assisted approach has been developed for the exfoliation of pristine graphite, which allows the large-scale preparation of few-layer graphene nanosheets. The process involves an unexpected physical insertion and exfoliation, and the graphene nanosheets prepared by this method reveal undisturbed sp²-hybridized structures. A possible two-step mechanism, which involves the negative charge being trapped around the edges of the graphite layers and a subsequent lithiation process, is proposed to explain the insertion of lithium inside the graphite interlayers. If necessary, the present exfoliation can be repeated and thinner (single or 2–3 layer) graphene can be achieved on a large scale. This simple process provides an efficient process for the exfoliation of pristine graphite, which might promote the future applications of graphene.