Zhao-hua Cheng

Influence of negative lattice expansion and metamagnetic transition on magnetic entropy change in the compound LaFe11.4Si1.6

Magnetization of the compound LaFe11.4Si1.6 with the cubic NaZn13-type structure was measured as functions of temperature and magnetic field around its Curie temperature TC of ∼208 K. It is found that the magnetic phase transition at TC is completely reversible. Magnetic entropy change ΔS, allowing one to estimate the magnetocaloric effect, was determined based on the thermodynamic Maxwell relation. The achieved magnitude of |ΔS| reaches 19.4 J/kg K under a field of 5 T, which exceeds that of most other materials involving a reversible magnetic transition in the corresponding temperature range. The large entropy change is ascribed to the sharp change of magnetization, which is caused by a large negative lattice expansion at the TC. An asymmetrical broadening of |ΔS| peak with increasing field was observed, which is resulted from the field-induced itinerant-electron metamagnetic transition from the paramagnetic to ferromagnetic state above the TC.

Very large magnetic entropy change near room temperature in LaFe11.2Co0.7Si1.1

A very large magnetic entropy change ΔS has been observed in Fe-based cubic NaZn13-type compound LaFe11.2Co0.7Si1.1 near the Curie temperature TC of 274 K. The value of the entropy change is ∼20.3 J/kg K under a magnetic field of 5 T at TC=274 K. It markedly exceeds that of pure Gd at the corresponding temperature range [V. K. Pecharsky & K. A. Gschneidner, Jr., Phys. Rev. Lett. 78, 4494 (1999)]. The great entropy change produced by the sharp change of magnetization is associated with a large negative lattice expansion at TC. The very large magnetic entropy change and low cost suggest that the compound LaFe11.2Co0.7Si1.1 has great potential for applications as magnetic refrigerants near room temperature.

Monolayer PtSe2, a New Semiconducting Transition-Metal-Dichalcogenide, Epitaxially Grown by Direct Selenization of Pt

Single-layer transition-metal dichalcogenides (TMDs) receive significant attention due to their intriguing physical properties for both fundamental research and potential applications in electronics, optoelectronics, spintronics, catalysis, and so on. Here, we demonstrate the epitaxial growth of high-quality single-crystal, monolayer platinum diselenide (PtSe2), a new member of the layered TMDs family, by a single step of direct selenization of a Pt(111) substrate. A combination of atomic-resolution experimental characterizations and first-principle theoretic calculations reveals the atomic structure of the monolayer PtSe2/Pt(111). Angle-resolved photoemission spectroscopy measurements confirm for the first time the semiconducting electronic structure of monolayer PtSe2 (in contrast to its semimetallic bulk counterpart). The photocatalytic activity of monolayer PtSe2 film is evaluated by a methylene-blue photodegradation experiment, demonstrating its practical application as a promising photocatalyst. Moreover, circular polarization calculations predict that monolayer PtSe2 has also potential applications in valleytronics.

Direct measurements of magnetocaloric effect in the first-order system LaFe11.7Si1.3

The magnetocaloric effect was investigated in LaFe11.7Si1.3, which undergoes a first-order transition at ∼188 K from the ferromagnetic to paramagnetic state. The magnetic entropy change upon a field increase from 0 to 5 T is as large as 29 J/kg K (212 mJ/cm3 K). The adiabatic temperature change obtained via direct measurements reaches 4 K under a field change from 0 to 1.4 T. The large values of entropy change and adiabatic temperature change confirmed the large potential of present compound LaFe11.7Si1.3 as a magnetic refrigerant in the corresponding temperature range.

Room temperature giant dielectric tunability effect in bulk LuFe2O4

We report the extreme sensitivity of dielectric permittivity to applied dc bias electric field in bulk LuFe2O4. A small bias field of 50V∕cm can greatly reduce the dielectric permittivity in the vicinity of room temperature, which is in strong contrast to conventional ferroelectric materials where a large electric field of the order of tens of kV/cm is required. This giant dielectric tunability effect within a broad temperature interval around room temperature is very promising for tunable device applications. The possible origins of this giant effect are discussed.

Large magnetic entropy change inLa(Fe,Co)11.83Al1.17

Large magnetic entropy change with comparable magnitude to that of pure Gd has been observed in compounds La(Fe12xCox)11.83Al1.17 (x50.06,0.08) at their Curie temperatures of ;273 K and ;303 K, respectively. These compounds are of a cubic NaZn 13-type structure with soft ferromagnetism. The magnetic entropy change is reversible in the whole experimental temperature range from ;230 to ;330 K. The most interesting feature is that the Curie temperature can be easily tuned by adjusting the substitution of Co for Fe. It is suggested that the present compounds are suitable candidates for magnetic refrigerants in a wide range near room temperature. The calculated DS curve in the molecular field approximation is in a satisfactory agreement with the experimental one.

Cooling field dependence of exchange bias in phase-separated La0.88Sr0.12CoO3

We report the observation of exchange bias phenomena in the hole-doped perovskite cobaltite La0.88Sr0.12CoO3 in which a spontaneous phase separation occurs. When the sample is cooled in a static magnetic field through a freezing temperature, the magnetization hysteresis loops shift to the negative field. Moreover, the exchange bias strongly depends on the cooling field. These results highlight the important role of a glassy interface between the intrinsic inhomogeneous phases in a phase-separated system.

Magnetic properties and magnetic entropy change of LaFe11.5Si1.5Hy interstitial compounds

LaFe11.5Si1.5Hy interstitial compounds have been prepared by hydrogen absorption and subsequent desorption. The Curie temperatures are easily tunable for a wide temperature range from ~ 195 to ~ 340 K by hydrogen content. The maximal magnetic entropy changes, under a magnetic field change of 0?5 T, are as large as 16.8? 20.5 J kg?1 K?1, exceeding that of Gd (|?S| ~ 9.8 J kg?1 K?1 at TC = 293 K) by 70?110% in the vicinity of room temperature. The large magnetic entropy change of all hydrides may be attributed to the first-order itinerant-electron metamagnetic transition confirmed by the Arrott plots. Large |?S|, convenient adjustment of TC and small thermal and magnetic hystereses, make LaFe11.5Si1.5Hy interstitial hydrides promising candidates for magnetic refrigerants in the corresponding temperature range.

Strong interplay between structure and magnetism in the giant magnetocaloric intermetallic compound LaFe11.4Si1.6: a neutron diffraction study

Crystallographic and magnetic structures of the cubic NaZn13-type intermetallic compound LaFe11.4Si1.6 have been studied by means of powder neutron diffraction. Rietveld analysis indicates that Si atoms substitute for Fe atoms randomly on two different Fe sites. All spins in the unit cell are aligned ferromagnetically with the FeI?(8b) moment smaller than the FeII?(96i) one. The long-range ferromagnetic ordering induces a drastic expansion of the lattice and the coexistence of the large and small volume phases near the Curie temperature. Even in the ferromagnetic state, the lattice expansion still correlates strongly with the spontaneous magnetic moment, marked by a large positive magnetovolume coupling constant kC = 1.14 ? 10?8 cm6?emu?2. From the temperature dependence of Fe?Fe bond lengths, we suggest that the Fe?Fe exchange interaction between the clusters (each formed by a central FeI atom and 12 surrounding FeII atoms) plays an important role in the magnetic properties of La(Fe1?xAl/Six)13, as does that within the clusters.

Exceeding natural resonance frequency limit of monodisperse Fe3O4 nanoparticles via superparamagnetic relaxation

Magnetic nanoparticles have attracted much research interest in the past decades due to their potential applications in microwave devices. Here, we adopted a novel technique to tune cut-off frequency exceeding the natural resonance frequency limit of monodisperse Fe3O4 nanoparticles via superparamagnetic relaxation. We observed that the cut-off frequency can be enhanced from 5.3 GHz for Fe3O4 to 6.9 GHz forFe3O4@SiO2 core-shell structure superparamagnetic nanoparticles, which are much higher than the natural resonance frequency of 1.3 GHz for Fe3O4 bulk material. This finding not only provides us a new approach to enhance the resonance frequency beyond the Snoeks limit, but also extend the application for superparamagnetic nanoparticles to microwave devices.