L. Zu

Enhanced low temperature thermoelectric performance of Ag-doped BiCuSeO

We investigated the physical properties of the silver doped layered oxyselenides BiCu1–xAgxSeO (x = 0–0.4), which crystallize in an unusual intergrowth structure with [Cu2Se2]2– and [Bi2O2]2+ layers. The total thermal conductivity is decreased because the heavier Ag doping in BiCuSeO lattice decreased the lattice thermal conductivity. The undoped BiCuSeO exhibits a semiconducting behavior, and the Ag-doped BiCuSeO performs much improved electrical conductivity. Although Ag-doping causes a decreasing Seebeck coefficient, the significant increase of the electrical conductivity compensates the moderate decrease of the Seebeck coefficient, which leads to the strongly improved power factor values. Finally, the figure of merit is improved and reaches a maximum ∼0.07 at 300 K for the sample BiCu0.7Ag0.3SeO.

Spin-glass behavior and zero-field-cooled exchange bias in a Cr-based antiperovskite compound PdNCr3

We report the synthesis, structure, and magnetic and electrical/thermal transport properties of a Cr-based antiperovskite compound PdNCr3, which crystallizes in MgCNi3-type cubic structure (space group Pmm, No. 221). Interestingly, the spin-glass (SG) behavior, which is confirmed by the corresponding characteristic parameters (the freezing temperature T0 = 61.4(2) K, the dynamical exponent zν = 7.103(3), and the flipping time τ0 = 2.714(2) × 10−11 s), is observed in PdNCr3. Furthermore, the value of the Sommerfeld–Wilson ratio (RW ∼ 1.024(3)) for PdNCr3 is much smaller than those of cluster glass systems (RW > 100) and Kondo cluster glass systems (RW = 20–30), indicating that PdNCr3 is a canonical SG system. Density functional theory calculation shows that the origin of SG in PdNCr3 is attributed to the disordering located N vacancies, which is further confirmed by the measurement of sample PdN0.75Cr3 with more N deficiency. On the other hand, infrequently, the zero-field-cooled exchange bias (ZFC-EB) with an exchange bias field (HE) of about 350 Oe is observed after zero-field cooling from an unmagnetized state in PdNCr3. The values of HE are found to depend strongly on temperature and measuring magnetic field. For PdNCr3, the ferromagnetic unidirectional anisotropy, which is the origin of our ZFC-EB effect, is formed around the ferromagnetic–SG interface isothermally during the initial magnetization process below the blocking temperature. In addition, the training effect of ZFC-EB in PdNCr3 is observed after the zero-field cooling process and has been explained well in terms of the spin configurational relaxation model.

A first-order antiferromagnetic-paramagnetic transition induced by structural transition in GeNCr3

A first-order antiferromagnetic (AFM)-paramagnetic (PM) phase transition has been confirmed by systematic magnetic measurements in GeNCr3, and it does not depend on the applied magnetic fields. Interestingly, a structural transition from space group P-421m to I4/mcm without breaking tetragonal symmetry appears around this AFM-PM transition temperature, which may be the driving force for the AFM-PM transition. Moreover, obvious exothermic and endothermic peaks in calorimetric measurements, a sharp peak of specific heat Cp(T), and a slope change of resistivity ρ(T) are also observed around structural transition temperature, indicating a strong coupling between structure and electrical as well as thermal properties.

Oxygen vacancies-induced metal-insulator transition in La2/3Sr1/3VO3 thin films: Role of the oxygen substrate-to-film transfer

Electrical transport properties of La2/3Sr1/3VO3 (LSVO) thin films grown on LaAlO3 (LAO) and SrTiO3 (STO) substrates have been investigated. It is found that the LSVO/LAO show metal-insulator transition when decreasing the temperature, while the LSVO/STO exhibit metallic behavior. The difference in transport properties of LSVO thin films has been discussed based on the variation of oxygen content and can be attributed to different oxygen substrate-to-film transfer. These results highlight the crucial role of oxygen stoichiometry in determining the physical properties of LSVO and the importance of oxygen-substrate contribution in LSVO thin films.

Observation of the Spin-Glass Behavior in Co-Based Antiperovskite Nitride GeNCo3.

Single-phase antiperovskite nitride GeNCo3 with space group Pm3̅m is successfully synthesized by a solid-gas reaction. The crystal structure, magnetism, specific heat at low temperatures, Hall effect, and electrical and thermal transport properties are widely investigated. Exhilaratingly, a canonical spin-glass (SG) behavior is observed in GeNCo3 with a freezing temperature T0 = 79.43 K, dynamical exponent zν = 6.156, and flipping time τ0 = 5.0 × 10(-12) s. The origin of the SG state in GeNCo3 is likely due to the atomic disorder introduced by the Ge vacancies. This is further proven by the measurements of Ge0.9NCo3 with more Ge deficiencies.

Dielectric relaxation and magnetodielectric response in DyMn0.5Cr0.5O3

We investigate the structural, magnetic, and magnetodielectric properties of DyMn0.5Cr0.5O3. The sample can be indexed with an orthorhombic phase with B site disordered space group Pbnm. The valence state of both Mn and Cr ions are suggested to be +3 based on the results of x-ray photoelectron spectroscopy. Two thermally excited dielectric relaxation at temperatures TN2 < T< 300 K and large magnetodielectric effect (MDC = 20%–30%) due to the disordered arrangement of Mn3+/Cr3+ ions associated with electron hopping between them are observed. The absence of any noticeable magnetoresistance effect (MR < 0.5%) demonstrates that the observed magnetodielectric effect is an intrinsic behavior. These results suggest that DyMn0.5Cr0.5O3 is a magnetodielectric compound, whose dielectric properties are dependence of the applied magnetic field, which exhibits such effects near room temperature and holds great promise for future device applications.

Anomalous Hall effect in tetragonal antiperovskite GeNFe3 with a frustrated ferromagnetic state

We report observed anomalous Hall effect (AHE) behavior in the antiperovskite compound GeNFe3 with a tetragonal symmetry. The anomalous Hall coefficient RH was estimated and exhibits a broad peak near the ferromagnetic (FM)–paramagnetic (PM) phase transition, and reduces to zero in the PM phase. The sign of the ordinary Hall coefficient R0 changes sharply as a function of temperature, which provides evidence for the switching of different domain carriers. The relationship between the RH and ρxx data was also plotted using the scaling law RH = aρxx0 + bρxxγ, and the parameter γ is found to be about 0.99. These results suggest that the mechanism of the AHE for tetragonal GeNFe3 is mainly due to skew scattering. This finding enriches the understanding of the AHE mechanism for antiperovskite compounds, and the unusual mechanism for GeNFe3 is considered to be closely related to the unique tetragonal crystal structure and frustrated FM ground state of antiperovskite compounds.