Shaomin Feng

Manifestation of ferroelectromagnetism in multiferroic BiMnO3

Multiferroic BiMnO3 with a highly distorted perovskite structure induced by the stereochemically active 6s2 electron lone pairs of Bi3+ was synthesized at a high pressure of 6 GPa. Magnetization, differential scanning calorimetry, dielectric permittivity, and in situ powder x-ray diffraction as a function of temperature were carried out, respectively. In light of comprehensive evaluation, we can conclude that the synthetic BiMnO3 ceramic displays ferromagnetic and ferroelectric orderings simultaneously, i.e., ferroelectromagnetism below its ferromagnetic Curie temperature TM∼100K.

Determination of the intrinsic ferroelectric polarization in orthorhombic HoMnO3

Whether or not a large ferroelectric polarization P exists in the orthorhombic HoMnO3 with E-type antiferromagnetic spin ordering remains one of the unresolved, challenging issues in the physics of multiferroics. The issue is closely linked to an intriguing experimental difficulty in determining the P of polycrystalline specimens, namely that conventional pyroelectric current measurements performed after a poling procedure under high dc electric fields are subject to large errors due to the problems caused by leakage currents or space charges. To overcome the difficulty, we employed the positive-up negative-down (PUND) method, which uses successively the two positive and two negative electrical pulses, to directly measure electrical hysteresis loops in several polycrystalline HoMnO3 specimens below their Neel temperatures. We found that all the HoMnO3 samples had similar remnant polarization Pr values at each temperature, regardless of their variation in resistivity, dielectric constant and pyroelectric current levels. Moreover, the Pr value of 0.07µCcm 2 at 6K is consistent with the P value obtained from the pyroelectric current measurement performed after a short pulse poling. Our findings suggest that the intrinsic P of polycrystalline HoMnO3 can be determined through the PUND method and P at 0K may reach 0.24µCcm 2 in a single crystalline specimen. This P value is still much smaller than the theoretically predicted one but is one of the largest observed in magnetism induced ferroelectrics.

Intrinsic ferroelectric polarization of orthorhombic manganites with E-type spin order

By directly measuring electrical hysteresis loops using the Positive-Up Negative-Down (PUND) method, we determined accurately the remanent ferroelectric polarization P-r of orthorhombic RMnO3 (R = Ho, Tm, Yb, and Lu) compounds below their E-type spin ordering temperatures. We found that LuMnO3 has the largest P-r of 0.17 mu C/cm(2) at 6 K in the series, the value of which allows us to predict that its single-crystal form can produce a P-r of at least 0.6 mu C/cm(2) at 0 K. Furthermore, at a fixed temperature, P-r decreases systematically with increasing rare earth ion radius from R = Lu to Ho, exhibiting a strong correlation with the variation of the in-plane Mn-O-Mn bond angle and Mn-O distances. Our experimental results suggest that the contribution of the Mn t(2g) orbitals may dominate the ferroelectric polarization.

Structural stability of multiferroic BiFeO3

Multiferroic BiFeO3 was fabricated via a high pressure of 5 GPa at 900 °C. The crystal structure of the BiFeO3 ceramic was determined by X-ray diffraction (XRD) to be a rhombohedral perovskite with a space group of R3c. The temperature dependence of XRDs was collected down to 5 K under ambient pressure that showed no structure phase transition. The structural evolution of BiFeO3 under high pressures up to 56.6 GPa was studied at room temperature using a diamond anvil cell combined with synchrotron radiation XRD. A possible phase transition was proposed at around 10 GPa. The bulk modulus was estimated to be B 0=97.3(7) GPa in the low-pressure range.

Temperature and pressure effects of multiferroic Bi2NiTiO6 compound

Bi2NiTiO6 compound which shows both magnetic (T-M = 58 K) and ferroelectric properties (T-C = 513 K) was synthesized under high pressure of 5 GPa and temperature of 1273 K. The crystal structure, as determined by X-ray powder diffraction and neutron powder diffraction, is a distorted A(B1B2)O-3 type perovskite with space group Pn2(1)a. Structural evolution of multiferroic Bi2NiTiO6 shows that there are two isostructural phase transitions at similar to 2 GPa and similar to 15 GPa under high pressure and at room temperature and indicates that isostructural phase transitions occurred with temperature higher than 823 K under ambient condition. All the isostructural phase transitions come from the Bi ion discontinuous shift, which identifies the phase transition at similar to 15 GPa and at temperature higher than 823 K are the same. Using a modified high-T Birch-Murnaghan equation of state and a thermal-pressure approach, we have derived the thermoelastic parameters of high pressure phase Bi2NiTiO6, including the ambient bulk modulus K-0, temperature derivative of bulk modulus at constant pressure, volumetric thermal expansivity, pressure derivative of thermal expansion, and temperature derivative of bulk modulus at constant volume

Structure transition of multiferroic hexagonal TmMnO3 compound under high pressure

The high-pressure-induced structure transition in multiferroic hexagonal TmMnO3 (h-TmMnO3) has been investigated using an in situ angle-dispersive synchrotron X-ray diffraction technique in a diamond anvil cell. The experimental results show that the phase transition from ambient hexagonal to orthorhombic structure with space group Pbnm begins around 10.2 GPa. The Rietveld refinement method was used to determine the lattice parameters and lattice compressibility of the h-TmMnO3 compound from 0.8 to 28.6 GPa. The pressure evolution of average bond distances and bond angles between the Mn and O atoms in the ab-plane was obtained. The magnetic properties under different pressures as well as their effect on multiferroic properties are discussed using extrapolations from the empirical relation of magnetic order versus rare-earth ionic radius.

Structural stability at high pressure, electronic, and magnetic properties of BaFZnAs: A new candidate of host material of diluted magnetic semiconductors*

The layered semiconductor BaFZnAs with the tetragonal ZrCuSiAs-type structure has been successfully synthesized. Both the in-situ high-pressure synchrotron x-ray diffraction and the high-pressure Raman scattering measurements demonstrate that the structure of BaFZnAs is stable under pressure up to 17.5 GPa at room temperature. The resistivity and the magnetic susceptibility data show that BaFZnAs is a non-magnetic semiconductor. BaFZnAs is recommended as a candidate of the host material of diluted magnetic semiconductor.

THE MULTIFERROIC PROPERTIES OF Bi(Fe1/2Cr1/2)O3 COMPOUND

Dense Bi(Fe1/2Cr1/2)O-3 ceramics with R 3c crystal structure were synthesized by solid-state reaction under high pressure. Transmission electron microscope observations revealed an incommensurable superstructure along direction. Magnetization measurements indicated a transition to a cooperative magnetic state below similar to 130 K. Dielectric properties of Bi(Fe1/2Cr1/2)O-3 showed a dielectric constant anomaly located at similar to 140 K indicating the giant dielectric relaxation in multiferroic Bi(Fe1/2Cr1/2)O-3 compound, which can be explained by the enhanced conductivity and possible Maxwell-Wagner contribution. Large dielectric frequency dispersion was observed at 140-185 K, and was supposed to be a thermal activated intrinsic process.

Electrical Resistivity of Fe‐C Alloy at High Pressure: Effects of Carbon as a Light Element on the Thermal Conductivity of the Earth's Core

We measured the electrical resistivity of iron, Fe99C1, Fe3C, and Fe7C3 up to ~80 GPa using the van der Pauw method in a diamond anvil cell. The electrical resistivity of disordered Fe99C1 at high pressure shows a strong impurity resistivity of carbon. The ferromagnetic-paramagnetic transition in Fe3C and Fe7C3 is associated with the flattening of the resistivity pressure gradient at ~6 GPa. Fe7C3 exhibits the highest electrical resistivity among all iron-light element alloys, and Fe3C and Fe7C3 disobey the Matthiessen’s rule by showing a lower electrical resistivity than a disordered iron-carbon alloy because of chemical ordering. A comparison of the impurity resistivity between silicon, sulfur, nickel, and carbon shows that carbon has an exceedingly stronger alloying effect than other elements. If the chemical ordering observed in Fe-Si system is held true for the Fe-C system, the chemical ordering in Fe7C3 possibly increases the thermal conductivity of the inner core and enlarges the thermal and electrical conductivity gap at the inner-core boundary. Models of the thermal conductivity of liquid Fe70C30 with 8.4 wt % carbon show a low thermal conductivity of 38 Wm 1 K 1 at the pressure-temperature conditions of the topmost outer core. The corresponding heat flow of 6 TW at the core-mantle boundary is notably lower than previous electrical resistivity results on Fe and Fe alloys. The alloying effect of carbon on the electrical and thermal conductivity of iron can thus play a significant role in understanding the heat flux at the core-mantle boundary and the thermal evolution of the core.

The crossover from metal to semi-conductor in Cu-doped LiFeAs system

A series of LiFe1−xCuxAs single crystal with the doping level x = 0 − 0.16 were grown. The resistivity measurement was conducted. The results show that the 10% Cu doping completely suppresses superconducting transition temperature and when the doping level increases to 16%, it presents a semiconducting behavior in the region of 2 K to 300 K.