Enhanced RCP and large inverse magnetocaloric effect of CoFe2_2O4_4 nanoparticles synthesized by auto-combustion method

This work focuses on the microstructure, magnetic properties and magnetocaloric effect of CoFe$_2$O$_4$ (CFO) nanoparticles elaborated by sol-gel auto combustion method. The XRD investigation indicates that CFO is crystallized in a cubic spinel structure and the SEM micrograph shows a fine quasi-spherical with an average grain sizes of 160 nm. The temperature dependence of the Raman spectra reveals the ferromagnetic to paramagnetic (FM-PM) transition started from 723 K and the magnetization versus temperature measurements shows the Curie temperature located at T$_{\rm C}$ = 785 K. Large value of magnetocaloric temperature change of $\Delta$T =11.2 K with a high RCP of 687.56 J Kg$^{-1}$ are achieved indirectly via the Maxwell approach making our CFO nanopowder suitable candidate for both environmentally friendly magnetic refrigeration and medical applications at ambient temperature

Magnetism in multiferroic Pb5Cr3F19

We report on the first observation of a magnetic transition in multiferroic Pb5Cr3F19. The system undergoes a ferroelectric transition at TC = 545 K and probably an antiferromagnetic transition around TN = 11 K. Between 50 K and TN, the system is not paramagnetic but shows the presence of correlated spin clusters, indicating a superparamagnetic or spin glass state. The observable changes of the magnetic properties ongoing through ferroelectric phase transition show a magnetoelectric coupling that may be important for new devices

Room-temperature magnetoelectric effect in lead-free multiferroic (1−x)(1-x) Ba0.95_{0.95}Ca0.05_{0.05}Ti0.89_{0.89}Sn0.11_{0.11}O3_3-(x)(x)CoFe2_2O4_4 particulate composites

Multiferroic particulate composites $(1-x)$ Ba$_{0.95}$Ca$_{0.05}$Ti$_{0.89}$Sn$_{0.11}$O$_3$-$(x)$CoFe$_2$O$_4$ with ($x$ = 0.1, 0.2, 0.3, 0.4 and 0.5) have been prepared by mechanical mixing of the calcined and milled individual ferroic phases. X-ray diffraction and Raman spectroscopy analysis confirmed the formation of both perovskite Ba$_{0.95}$Ca$_{0.05}$Ti$_{0.89}$Sn$_{0.11}$O$_3$ (BCTSn) and spinel CoFe$_2$O$_4$ (CFO) phases without the presence of additional phases. The morphological properties of the composites were provided by using Field Emission Scanning Electron Microscopy. The BCTSn-CFO composites exhibit multiferroic behavior at room temperature, as evidenced by ferroelectric and ferromagnetic hysteresis loops. The magnetoelectric (ME) coupling was measured under a magnetic field up to 10 kOe and the maximum ME response found to be 0.1 mV /cm/ Oe for the composition 0.7 BCTSn-0.3 CFO exhibiting a high degree of pseudo-cubicity and large density

The strain-induced transitions of the piezoelectric, pyroelectric and electrocaloric properties of the CuInP2_2S6_6 films

The low-dimensional ferroelectrics, ferrielectrics and antiferroelectrics are of urgent scientific interest due to their unusual polar, piezoelectric, electrocaloric and pyroelectric properties. The strain engineering and strain control of the ferroelectric properties of layered 2D Van der Waals materials, such as CuInP$_2$(S,Se)$_6$ monolayers, thin films and nanoflakes, are of fundamental interest and especially promising for their advanced applications in nanoscale nonvolatile memories, energy conversion and storage, nano-coolers and sensors. Here, we study the polar, piezoelectric, electrocaloric and pyroelectric properties of thin strained films of a ferrielectric CuInP$_2$S$_6$ covered by semiconducting electrodes and reveal an unusually strong effect of a mismatch strain on these properties. In particular, the sign of the mismatch strain and its magnitude determine the complicated behavior of piezoelectric, electrocaloric and pyroelectric responses. The strain effect on these properties is opposite, i.e., "anomalous", in comparison with many other ferroelectric films, for which the out-of-plane remanent polarization, piezoelectric, electrocaloric and pyroelectric responses increase strongly for tensile strains and decrease or vanish for compressive strains.Comment: 16 pages, 5 figures, to be presented at the VI Lithuanian-Polish Meeting on Physics of Ferroelectric

Magnetoelectric coupling in multiferroic CFO/BCTSn core shell nanofibers elaborated by co-axial electrospinning method

Multiferroic CoFe2O4-Ba0.95Ca0.05Ti0.89Sn0.11O3 core-shell nanofibers (CFO@BCTSn NFs) were synthesized by a sol-gel co-axial electrospinning technique. The scanning electron microscope and transmission electron microscope were used to check nanofibers' core-shell structure/configuration. X-ray diffraction and a high-resolution transmission electron microscope were used to confirm the spinel structure of CFO and the perovskite structure of BCTSn. The magnetic character of the resultant CFO@BCTSn NFs was determined by SQUID magnetometry. The piezoelectricity was verified using piezo-response force microscopy, which revealed an entirely covered ferroelectric shell outline, in accordance with SEM and TEM observations. The magnetoelectric (ME) coefficient was measured as a function of the applied external DC magnetic field. The maximum ME coefficient obtained for the CFO@BCTSn NFs was 346 mV cm-1 Oe-1. The high magnetoelectric coupling suggests that CFO@BCTSn NFs could be a promising candidate for magnetic field sensor and magnetoelectric device applications

Electron and hole trapping in Eu- or Eu,Hf-doped LuPO4 and YPO4 tracked by EPR and TSL spectroscopy

EPR spectroscopy in X- and Q-bands was employed to trace charge carrier trapping upon exposure to X-rays of LuPO4:Eu, LuPO4:Eu,Hf and YPO4:Eu,Hf flux-grown single crystals, as well as LuPO4:Eu sintered ceramics. These data were complemented with thermoluminescence (TL) studies on the same compositions. EPR spectra proved that all the single crystals contained Pb impurity incorporated from the flux. Upon exposure to X-rays the concentration of Pb3+ changed proving that Pb participated in carrier trapping. The ionizing radiation caused also partial conversion of Eu3+ into Eu2+ and in the (Eu,Hf) doubly doped crystals also Hf4+ was transformed into Hf3+ upon exposure to X-rays. In the LuPO4:Eu sintered ceramics X-rays transformed Eu3+ into Eu2+ and the strong EPR signal of a hole trapped at O2- in the vicinity of a defect (presumably Lu3+-vacancy) was recorded. Some contribution from the PO42- molecular ion could also be observed. However, in crystals the EPR signal from the latter was much more intense compared to the former. In ceramics, the trapped hole was found to hop between the O2- ions and only below similar to 70 K it localized at just one of them. Complementary measurements of TL of the samples showed meaningful differences between the glow curves of the crystals and of the ceramics. The TL glow curve of the latter contained only one band peaking around 120 degrees C which is related to destruction of the trapped hole being mainly the O- center. All single crystals showed multi-peak TL glow curves covering the range of 30-500 degrees C. They were found to result mainly from a hole released from the PO42- molecular complex and to some extent also from the O- center. The results relative to EPR and TL spectroscopy proved that in orthophosphates Eu3+ is an effective electron trapping center while holes may be trapped at impurity ions, like Pb2+, or localized at oxygen ions possibly stabilized by nearby defects, e.g. Lu-vacancie

New composites based on liquid crystalline elastomers and electroactive nanomaterials

The insertion of inorganic nanoparticles and nanowires in a liquid crystalline elastomeric environment is here investigated. The combination of ferroelectric and conductive properties of the nanomaterials with the thermo-mechanical and shape memory response of liquid single crystal elastomers based on polysiloxane is indeed very promising for new technological applications, such as electroactive actuators. In this work the preparation and physical-chemical properties of new composites are presented and discussed in comparison with those of standard liquid single crystal elastomers (LSCEs). In particular, we are reporting the preliminary results of new composites including either lead titanate nanoparticles or molibdene oxide nanowires, having different electric and conductive properties