Yi Qi Lin

Relaxorlike dielectric behavior and weak ferromagnetism in YFeO3 ceramics

Dielectric characteristics of YFeO3 antiferromagnetic ceramics were evaluated over broad temperature and frequency ranges. Two dielectric relaxations were observed at low and high temperatures, respectively, and a dielectric constant step was detected between them. The low temperature dielectric relaxation was an intrinsic thermally activated process following the Arrhenius law with the activation energy very close to that for electronic ferroelectrics, while the high temperature dielectric relaxation was related to the point defect since it could be significantly suppressed by O2 annealing. M-H hysteresis loop was detected at room temperature, and this indicated the weak ferromagnetism in YFeO3 ceramics.

Dielectric relaxation and polaronic conduction in double perovskite La2MgMnO6

The dielectric and conductive characteristics of double perovskite La2MgMnO6 were investigated together with the mixed-valence structure. The magnetic characterization and x-ray photoemission spectroscopy analysis showed the co-presence of Mn3+ and Mn4+. A Debye type dielectric relaxation with an activation energy of 0.33 eV was observed in La2MgMnO6 ceramics. The relaxorlike dielectric characteristic was attributed to the dipolar effects coming from charge carrier hopping between Mn3+ and Mn4+. The dc conductivity of La2MgMnO6 was well fitted using adiabatic small polaron theory, and also correlated with the mixed-valence structure of Mn3+/Mn4+.

CoFe2O4∕Pb(Zr052Ti0.48)O3 disk-ring magnetoelectric composite structures

Magnetoelectric composite structures of CFO (CoFe2O4)-disk/PZT (Pb(Zr052Ti0.48)O3)-ring and PZT-disk/CFO-ring were prepared and evaluated, where the magnetostrictive and piezoelectric phases were coupled through normal stresses. Under bias magnetic fields around 3400 Oe, the peak magnetoelectric coefficients reached 18.1 and 102.6 mV cm−1 Oe−1 at 1 kHz for CFO-disk/PZT-ring and PZT-disk/CFO-ring structures, respectively, which were much higher than that of the corresponding layered composite. Prediction for magnetoelectric coefficients at low frequency was conducted, and the improved interfacial mechanical coupling through normal stresses was responsible for the enhanced magnetoelectric properties of the present composite structures. Moreover, the frequency dependence of the magnetoelectric coefficient was investigated, and the resonant magnetoelectric coefficients were up to 4120 and 6430 mV cm−1 Oe−1 for CFO-disk/PZT-ring and PZT-disk/CFO-ring composite structures, respectively.

Structure, magnetic, and dielectric characteristics of Ln2NiMnO6 (Ln = Nd and Sm) ceramics

In the present work, the structure, magnetic, and dielectric characteristics of Nd2NiMnO6 and Sm2NiMnO6 have been investigated with comparison to those for La2NiMnO6. The magnetic and dielectric properties of the present ceramics vary with decreasing lanthanide ionic radius (RLn). Both Nd2NiMnO6 and Sm2NiMnO6 are ferromagnetism with the monoclinic symmetry (space group P21/n). The Curie temperature TC of Nd2NiMnO6 and Sm2NiMnO6 is 194 K and 156 K, respectively, and it decreases with decreasing 〈Ni-O-Mn〉 bond angle which depends on RLn. Meanwhile, the dielectric constant e′ monotonically decreases with decreasing RLn. The similar variation tendency of magnetic and dielectric characteristics with RLn indicates that the structural origins for the magnetic and dielectric response should be closely linked with each other.

Room temperature multiferroic Ba4Bi2Fe2Nb8O30: Structural, dielectric, and magnetic properties

A room temperature multiferroic compound Ba4Bi2Fe2Nb8O30 was synthesized using a solid state reaction technique. Rietveld analysis of x-ray diffraction data shows that Ba4Bi2Fe2Nb8O30 has a tetragonal (space group P4bm) tungsten bronze structure. In this structure, the Fe3+ and Nb5+ statistically occupy the octahedral center while the Ba2+ ions and the Bi3+ ions occupy the pentagonal channels and the square channels, respectively. Diffuse dielectric peaks with strong frequency dispersion in the temperature range from 150 to 300 K can be attributed to the random distribution of Fe3+ and Nb5+ at B sites. Magnetic hysteresis loop at room temperature is also obtained, which suggests that Ba4Bi2Fe2Nb8O30 is a room temperature multiferroic compound. The coupling between the relaxor behavior and magnetic ordering is verified by observing an increase of magnetization near the maximum dielectric constant temperature.

Dielectric relaxation mechanisms of BiMn2O5 ceramics

Dielectric properties of multiferroic BiMn2O5 ceramics were evaluated over broad temperature and frequency ranges. Two Debye-type dielectric relaxations were observed at low temperatures (130–250 K) and high temperatures (200–450 K), respectively. The low temperature relaxation with an activation energy of 0.18 eV was attributed to charge carrier hopping process between Mn3+ and Mn4+. The high temperature dielectric relaxation with an activation energy of 0.38 eV, which is similar to the activation energy of conductivity, was associated with oxygen vacancies related defect complex. The dielectric response at high temperatures was significantly suppressed and the dc conductivity increased after oxygen annealing.

Structure, magnetic, and dielectric properties of La2Ni(Mn1-xTix)O6 ceramics

In the present work, the effects of Ti-substitution for Mn upon the structure, magnetic and dielectric properties in La2NiMnO6 ceramics have been investigated. Ti-substitution for Mn strongly affects the crystallographic, magnetic, and dielectric properties of La2NiMnO6 double perovskite ceramics. Refinements of the x-ray diffraction data show that all compositions investigated here have the monoclinic perovskite structure in space group P21/n with a partially ordered arrangement of Ni and Mn/Ti cations. The magnetic nature of the present ceramics with x up to 0.8 is ferromagnetic (FM), while that is antiferromagnetic for La2NiTiO6. The Curie temperature and the spontaneous magnetization, or FM component, decrease with increasing x. These ferromagnetic states are attributed to the Ni2+-O-Mn4+ superexchange interaction. The dielectric constant monotonically decreases and the relaxor-like behavior is suppressed with x. The similar variation tendency of magnetic and dielectric properties with increasing x ca...

Contribution of Electron Hopping on Colossal Dielectric Response of Bi-Substituted LaMnO3 Ceramics

The dielectric properties and conductivity of semiconducting Bi-substituted LaMnO 3 ceramics were investigated. It was found that the electron hopping between Mn 3 + and Mn 4 + contributed to the colossal dielectric response of Bi-substituted LaMnO 3 ceramics. A dielectric relaxation with the same activation energy of conductivity was observed in the temperature range from 120 K to 240 K. Due to the effects of the lone-pair electron of Bi 3 + , the activation energy of the dielectric relaxation in La 1 − x BixMnO 3 ceramics increased with increasing amount of Bi3 +.

Frequency-dependent magnetoelectric coefficient in a magnetostrictive?piezoelectric composite as a complex quantity

The frequency dependence of the magnetoelectric effect in a magnetostrictive–piezoelectric layered composite was predicted and discussed theoretically by taking the elastic, magnetic, piezomagnetic, dielectric and piezoelectric losses of the constituent magnetostrictive and piezoelectric phases into account. The magnetoelectric coefficient was proven to be a complex quantity, and the complexity was caused by the above losses. The imaginary part of the magnetoelectric coefficient was small at low frequencies far from the resonant frequency. However, it became very large and strong frequency dispersion was observed around the resonant frequency. Moreover, the relationship between the real and imaginary parts of the magnetoelectric coefficient could be described by a Cole–Cole plot around the resonant frequency. The predicted frequency-dependent characteristics of the complex mangetoelectric coefficient were consistent with the recent experimental results.

EFFECTS OF ORDERING DOMAIN STRUCTURE ON DIELECTRIC PROPERTIES OF DOUBLE PEROVSKITE La 2 NiMnO 6

Two sets of La2NiMnO6 ceramics with different B-site ordering degree were prepared and evaluated. The valence states of Ni2+ and Mn4+ were confirmed, and the B-site ordering degree was estimated qualitatively using magnetic and Raman analysis. The ordering of Ni2+ and Mn4+ was the primary origin for the relaxor-like behavior of the present ceramics. The ordering degree of B-site ions greatly affected the dielectric properties, i.e., more ordered sample indicated higher dielectric constant. Meanwhile, the grain boundary layer capacitor (GBLC) effect was also activated but limited at the high temperatures and low frequencies.