Magnetotransport and Multiferroic Properties of Perovskite Rare-earth Manganites

This work details the studies of several perovskite rare earth manganite (RMnO3) systems, focusing on the effects of both A-site and B-site doping of TbMnO3 and the magnetotransport behavior of La1-xSrxMnO3:insulator composites. In pure TbMnO3 synthesized by solution route, it was revealed that the magnetic properties could be modulated by structural modifications resulting from different synthesis methods. In bulk Tb0.67Ho0.33MnO3, signatures of a weak ferromagnetic moment were detected in the magnetization measurements, and an anomaly due to Ho3+-Mn3+ exchange striction was observed in the polarization measurement. These results signify that both the ionic radius and the magnetic properties of the dopant affect the multiferroic properties in this system. Signatures of both long-range magnetic ordering as well as spin-glass-like behavior were observed in the DC and AC susceptibility data of Tb1-xMxMnO3 (M = Ca, Sr). This suggests the importance of the A-site ionic-size mismatch in determining the magnetic interactions. In bulk TbMn1-xCrxO3, the DC and AC susceptibility measurements and neutron diffraction revealed competition between several different spin configurations. A magnetic phase diagram for TbMn1-xCrxO3 is proposed. Composite thin films of La0.67Sr0.33MnO3:ZnO and La0.67Sr0.33MnO3:MgO were grown using solution routes. A shift in the ferromagnetic transition to lower temperatures in the composite films indicates strain effects or doping at the interfaces. Disorder at the interfaces is presumed to result in spin-polarized tunneling at the grain boundaries. A significant enhancement of magnetoresistance, particularly at low applied magnetic fields (≤ 0.5 T), was observed in all composite films, making these promising for magnetic field sensing applications

Ultra-low-field magneto-elastocaloric cooling in a multiferroic composite device

The advent of caloric materials for magnetocaloric, electrocaloric, and elastocaloric cooling is changing the landscape of solid state cooling technologies with potentials for high-efficiency and environmentally friendly residential and commercial cooling and heat-pumping applications. Given that caloric materials are ferroic materials that undergo first (or second) order phase transitions near room temperature, they open up intriguing possibilities for multiferroic devices with hitherto unexplored functionalities coupling their thermal properties with different fields (magnetic, electric, and stress) through composite configurations. Here we demonstrate a magneto-elastocaloric effect with ultra-low magnetic field (0.16 T) in a compact geometry to generate a cooling temperature change as large as 4 K using a magnetostriction/superelastic alloy composite. Such composite systems can be used to circumvent shortcomings of existing technologies such as the need for high-stress actuation mechanism for elastocaloric materials and the high magnetic field requirement of magnetocaloric materials, while enabling new applications such as compact remote cooling devices

Effect of sub-micron grains and defect-dipole interactions on dielectric properties of iron, cobalt, and copper doped barium titanate ceramics

Introduction: Dilutely doped ferroelectric materials are of interest, as engineering these materials by introducing point defects via doping often leads to unique behavior not otherwise achievable in the undoped material. For example, B-site doping with transition metals in barium titanate (BaTiO3, or BTO) creates defect dipoles via oxygen vacancies leading enhanced polarization, strain, and the ability to tune dielectric properties. Though defect dipoles should lead to dielectric property enhancements, the effect of grain size in polycrystalline ferroelectrics such as BTO plays a significant role in those properties as well.Methods: Herein, doped BTO with 1.0% copper (Cu), iron (Fe), or cobalt (Co) was synthesized using traditional solid-state processing to observe the contribution of both defect-dipole formation and grain size on the ferroelectric and dielectric properties.Results and discussion: 1.0% Cu doped BTO showed the highest polarization and strain (9.3 μC/cm2 and 0.1%, respectively) of the three doped BTO samples. While some results, such as the aforementioned electrical properties of the 1.0% Cu doped BTO can be explained by the strong chemical driving force of the Cu atoms to form defect dipoles with oxygen vacancies and copper’s consistent +2 valency leading to stable defect-dipole formation (versus the readily mixed valency states of Fe and Co at +2/+3), other properties cannot. For instance, all three Tc values should fall below that of undoped BTO (typically 120°C–135°C), but the Tc of 1.0% Cu BTO actually exceeds that range (139.4°C). Data presented on the average grain size and distribution of grain sizes provides insight allowing us to decouple the effect of defect dipoles and the effect of grain size on properties such as Tc, where the 1.0% Cu BTO was shown to possess the largest overall grains, leading to its increase in Tc.Conclusion/future work: Overall, the 1% Cu BTO possessed the highest polarization, strain, and Tc and is a promising dopant for engineering the performance of the material. This work emphasizes the challenge of extricating one effect (such as defect-dipole formation) from another (grain size modification) inherent to doping polycrystalline BTO

Simultaneous large optical and piezoelectric effects induced by domain reconfiguration related to ferroelectric phase transitions

Electrical switching of ferroelectric domains and subsequent domain wall motion promotes strong piezoelectric activity; however, light scatters at refractive index discontinuities such as those found at domain wall boundaries. Thus, simultaneously achieving large piezoelectric effect and high optical transmissivity is generally deemed infeasible. Here, it is demonstrated that the ferroelectric domains in perovskite Pb(In1/2Nb1/2)O3 Pb(Mg1/3Nb2/3)O3-PbTiO3 domain-engineered crystals can be manipulated by electrical field and mechanical stress to reversibly and repeatably, with small hysteresis, transform the opaque poly-domain structure into a highly transparent mono-domain state. This control of optical properties can be achieved at very low electric fields (less than 1.5 kV cm−1) and is accompanied by a large (>10000 pm V−1) piezoelectric coefficient that is superior to that of linear state-of-the-art materials by a factor of three or more. The coexistence of tunable optical transmissivity and high piezoelectricity paves the way for a new class of photonic devices

Acoustic Energy Harvesting of Piezoelectric Ceramic Composites

Acoustic energy is an often overlooked but increasingly prevalent source of ambient energy that could be scavenged to power a wide range of devices. Piezoelectric materials are often used, but the tradeoff between acoustic impedance matching and the amount of ceramic piezoelectric material as the active material has not previously been investigated. In this work, commercially available 1–3 dice and fill composites with various fill factors (25%, 45%, and 65% of Pb(Zr,Ti)O3) and different acoustic impedance values were tested using an impedance tube and then modeled using a KLM equivalent circuit model. As expected, a higher amount of ceramic material resulted in a higher acoustic absorption coefficient. Experimentally, the highest fill factor with the highest piezoelectric coefficient also resulted in larger output power at all dB levels, reaching a maximum of 115 nW (84 nW/cm3) at 111 dBSPL for the 65% fill sample. In the model, the 25% fill factor with the best acoustic impedance matching shows the highest expected output power instead, but this discrepancy is most likely due to a lowered piezoelectric coefficient during testing due to the clamping conditions

Ultra-low-field magneto-elastocaloric cooling in a multiferroic composite device

The advent of caloric materials for magnetocaloric, electrocaloric, and elastocaloric cooling is changing the landscape of solid state cooling technologies with potentials for high-efficiency and environmentally friendly residential and commercial cooling and heat-pumping applications. Given that caloric materials are ferroic materials that undergo first (or second) order phase transitions near room temperature, they open up intriguing possibilities for multiferroic devices with hitherto unexplored functionalities coupling their thermal properties with different fields (magnetic, electric, and stress) through composite configurations. Here we demonstrate a magneto-elastocaloric effect with ultra-low magnetic field (0.16 T) in a compact geometry to generate a cooling temperature change as large as 4 K using a magnetostriction/superelastic alloy composite. Such composite systems can be used to circumvent shortcomings of existing technologies such as the need for high-stress actuation mechanism for elastocaloric materials and the high magnetic field requirement of magnetocaloric materials, while enabling new applications such as compact remote cooling devices.</p