Magnetocaloric and colossal magnetoresistance effects of the layered perovskite La 1.4 Sr 1.6 Mn 2 O 7 compound have been studied.A broad peak of magnetic entropy change （-△S M） is found above the Curie temperature...Magnetocaloric and colossal magnetoresistance effects of the layered perovskite La 1.4 Sr 1.6 Mn 2 O 7 compound have been studied.A broad peak of magnetic entropy change （-△S M） is found above the Curie temperature （T C =120 K）,which can be associated with the existence of two-dimensional short range ferromagnetic order.Additionally,the curvilinear shape of-△S M for layered perovskite is quite different from that of the Ln 1-x A x MnO 3 probably arising from magnetocrystalline anisotropy.At the same time,a wide peak of colossal magnetoresitance effect near T C is found in the layered provskite La 1.4 Sr 1.6 Mn 2 O 7.展开更多
We review colossal magnetoresistance in single phase manganites, as related to the field sensitive spin-charge interactions and phase separation; the rectifying property and negative/positive magnetoresistance in mang...We review colossal magnetoresistance in single phase manganites, as related to the field sensitive spin-charge interactions and phase separation; the rectifying property and negative/positive magnetoresistance in manganite/Nb：SrTio3 p-n junctions in relation to the special interface electronic structure; magnetoelectric coupling in manganite/ferroelectric structures that takes advantage of strain, carrier density, and magnetic field sensitivity; tunneling magnetoresistance in tunnel junctions with dielectric, ferroelectric, and organic semiconductor spacers using the fully spin polarized nature of manganites; and the effect of particle size on magnetic properties in manganite nanoparticles.展开更多
Bi doped SrTiO3 ceramics with Sr deficiency,i.e.Sr1-1.5xBixTiO3(x=0,0.01,0.05,0.1),were prepared via conventional solid-state reaction route.A colossal permittivity(CP)over 104 with low dielectric loss less than 0.05w...Bi doped SrTiO3 ceramics with Sr deficiency,i.e.Sr1-1.5xBixTiO3(x=0,0.01,0.05,0.1),were prepared via conventional solid-state reaction route.A colossal permittivity(CP)over 104 with low dielectric loss less than 0.05wasobtained in x=0.05 Sr1-1.5xBixTiO3 ceramics.In addition,the dielectric constant is maintained at a value greater than 104 in the range of 102-105 Hz and almost frequency independent.Phase structure analysis and density functional theory calculations suggest that the Bi·Sr-V"Sr-Bi·Sr defect complex with hole-pinned defect-dipoles maybe responsible for the high-performance CP properties.This work gives a new way to achieve high performance CP materials in ABO3 perovskite ceramics.展开更多
Ferromagnetic semiconductor Ga_(1–x)Mn_(x)As_(1–y)P_(y) thin films go through a metal–insulator transition at low temperature where electrical conduction becomes driven by hopping of charge carriers.In this regime,...Ferromagnetic semiconductor Ga_(1–x)Mn_(x)As_(1–y)P_(y) thin films go through a metal–insulator transition at low temperature where electrical conduction becomes driven by hopping of charge carriers.In this regime,we report a colossal negative magnetoresistance(CNMR)coexisting with a saturated magnetic moment,unlike in the traditional magnetic semiconductor Ga_(1–x)Mn_(x)As.By analyzing the temperature dependence of the resistivity at fixed magnetic field,we demonstrate that the CNMR can be consistently described by the field dependence of the localization length,which relates to a field dependent mobility edge.This dependence is likely due to the random environment of Mn atoms in Ga_(1-x)Mn_(x)As_(1-y)P_(y) which causes a random spatial distribution of the mobility that is suppressed by an increasing magnetic field.展开更多
Donor-acceptor co-doped rutile TiO_(2) ceramics with colossal permittivity(CP)have been extensively investigated in recent years due to their potential applications in modern microelectronics.In addition to CP and low...Donor-acceptor co-doped rutile TiO_(2) ceramics with colossal permittivity(CP)have been extensively investigated in recent years due to their potential applications in modern microelectronics.In addition to CP and low dielectric loss,voltage stability is an essential property for CP materials utilized in high-power and high-energy density storage devices.Unfortunately,the voltage stability of CP materials based on codoped TiO_(2) does not catch enough attention.Here,we propose a strategy to enhance the voltage stability of co-doped TiO_(2),where different ionic defect clusters are formed by two acceptor ions with different radii to localize free carriers and result in high performance CP materials.The(Ta+Al+La)co-doped TiO_(2) ceramic with suitable La/Al ratio exhibits colossal permittivity with excellent temperature stability as well as outstanding dc bias stability.The density functional theory analysis suggests that La^(3+)Al^(3+)V_(0)Ti^(3+)defect clusters and Ta^(5+)-Al^(3+)pairs are responsible for the excellent dielectric properties in(Ta+Al+La)co-doped TiO_(2).The results and mechanisms presented in this work open up a feasible route to design high performance CP materials via defect engineering.展开更多
We report the study of magnetic and transport properties of polycrystalline and single crystal Na(Zn,Mn)Sb,a new member of“111”type of diluted magnetic materials.The material crystallizes into Cu2Sb-type structure w...We report the study of magnetic and transport properties of polycrystalline and single crystal Na(Zn,Mn)Sb,a new member of“111”type of diluted magnetic materials.The material crystallizes into Cu2Sb-type structure which is isostructural to“111”type Fe-based superconductors.With suitable carrier and spin doping,the Na(Zn,Mn)Sb establishes spin-glass ordering with freezing temperature(Tf)below 15 K.Despite lack of long-range ferromagnetic ordering,Na(Zn,Mn)Sb single crystal still shows sizeable anomalous Hall effect below Tf.Carrier concentration determined by Hall effect measurements is over 1019 cm-3.More significantly,we observe colossal negative magnetoresistance(MR≡[ρ(H)−ρ(0)]/ρ(0))of-94%in the single crystal sample.展开更多
The extraordinary colossal magnetoresistance (CMR) behavior in Mn-site doped system La_(0.67)Ca_(0.33)Mn_(1-x)Cr_xO_3 (0.00x0.15) was reported. It was found that the substitution with Cr on Mn sites introduces an addi...The extraordinary colossal magnetoresistance (CMR) behavior in Mn-site doped system La_(0.67)Ca_(0.33)Mn_(1-x)Cr_xO_3 (0.00x0.15) was reported. It was found that the substitution with Cr on Mn sites introduces an additional bump in zero-field resistivity. With increasing Cr content, this additional bump grows up drastically while the original resistivity peak associated with magnetic order transition diminishes gradually. Under the applied magnetic field, both bumps of resistivity are deeply compressed, which leads to the appearance of two peaks in CMR response. As a result, the temperature range of CMR response is significantly broadened, spanned from the lowest to near room temperature. These results suggest that Mn-site element substitution could be a potent way of tuning CMR response.展开更多
The representative sample La0.58Dy0.09Ca0.33MnO3 of Dy doped La0.67Ca0.33MnO3 rare-earth manganites was investigated. The most important effect of Dy doping is to introduce the magnetoimpurity and form the spin cluste...The representative sample La0.58Dy0.09Ca0.33MnO3 of Dy doped La0.67Ca0.33MnO3 rare-earth manganites was investigated. The most important effect of Dy doping is to introduce the magnetoimpurity and form the spin clusters which induce dramatically large CMR in La0.58Dy0.09Ca0.33MnO3. The fitting results of field-induced resistivity decrease to the Brillouin function indicate that the CMR is caused by the spin dependent hopping between spin clusters. It is the magnetic field that reduces the size of spin clusters and induces a field-induced irreversible CMR behaviour.展开更多
Systematical investigations of zero-field resistivity, magnetoresistance and magnetization were performed for a typical manganese compound La2/3Ca1/3MnO3. It is argued that the common origin for insulator-metal and pa...Systematical investigations of zero-field resistivity, magnetoresistance and magnetization were performed for a typical manganese compound La2/3Ca1/3MnO3. It is argued that the common origin for insulator-metal and paramagenetic ferromagnetic-transitions as well as colossal magnetoresistance is due to the formation of ferromagnetic clusters in the paramagnetic background. The transition to metallic state is resulted from percolation of ferromagnetic metallic clusters, while the colossal magnetoresistance is due to the application of magnetic field, which accelerates the growth of ferromagnetic metallic clusters and causes the shift of the onset temperature for the metallic percolation to higher temperature. Based on the random resistor network model, the zero-field resistivity versus temperature dependence is simulated by using experimental parameters, and experimental data well agree with those in whole temperature range, giving a strong support to our approach.展开更多
A phenomenological model based on phase separation between ferromagnetic metallic and paramagnetic insulating domains was applied to analyze the electrical transport and colossal magnetoresistance for mixed-valent man...A phenomenological model based on phase separation between ferromagnetic metallic and paramagnetic insulating domains was applied to analyze the electrical transport and colossal magnetoresistance for mixed-valent manganites of RE_(2/3)AE_(1/3)MnO_3. The results show that the model can yield results in agreement with experimental observations in these manganites. The present approach provides a simple picture to visualize the reason that the temperature dependence of resistance (with and without applied magnetic fields) in these compounds has the peculiar shape, without invoking any complicated concept.展开更多
During the last 10 years, a lot of interests have been devoted to the so-called CDC （colossal dielectric constant） materials. The first materials exhibiting this behavior were the perovskite-based ceramics on the CC...During the last 10 years, a lot of interests have been devoted to the so-called CDC （colossal dielectric constant） materials. The first materials exhibiting this behavior were the perovskite-based ceramics on the CCTO （CaCuaTi4012） composition. Relative dielectric permittivity can attain values up to （or even larger than） 105. Nevertheless, their dielectric losses are too high, the lower values ranging 10%, in a narrow frequency range, thus limiting their applications. The underlying physical mechanisms at the origin of the CDC are still under study. The analysis of broadband impedance spectroscopy measurements leads most of the authors to propose an interracial polarization mechanism （at the electrodes or at internal barriers）, there is a limited number of complementary electrical characterization techniques, which, up to now, comfort the proposed interracial polarization mechanisms. In the present work, I-V and time-domain polarization are used to characterize these materials. One of the main results is the observation of a non-symmetrical response of these materials related to the direction of the polarization. These results are observed for both macroscopic level on bulk polycrystalline material and within individual grains of the same samples. These results do not fit current accepted models for polarization for CDC materials.展开更多
The poor temperature stability of the BaTiO_(3) ceramic has always been the main problem limiting their application.This situation has been improved but sacrifices the intrinsic polarization,which significantly reduce...The poor temperature stability of the BaTiO_(3) ceramic has always been the main problem limiting their application.This situation has been improved but sacrifices the intrinsic polarization,which significantly reduces the dielectric constant.In this work,the mechanism of multiple polarization was creatively introduced,and the temperature stability and dielectric properties of BaTiO_(3)-based ceramics are simultaneously enhanced.In particular,the Ba_(0.9925)Bi_(0.005)Ti_(0.995)Ca_(0.005)O_(2.995)(BBTC0.5)ceramic sample achieved excellent temperature stability(-14.8%to 8.85%)over an ultra-wide temperature range(-47 to 400℃)and exhibited colossal permittivity(27,125,25℃,1 kHz)and low dielectric loss(0.07,25℃,1 kHz).The dielectric properties,complex impedance spectra combined with XPS results indicate that the defective dipole clusters(Ti^(3+)-V_(O)-Ti^(3+),Bi_(Ba) and Ca″_(Ti)-V_(O))along with surface effects lead to colossal permittivity effect.More importantly,SEM images show the presence of the second phase at grain boundaries,which prevent the carriers within the grains from accumulating at the grain boundaries.As a result,the dielectric loss was reduced and the temperature stability was further extended.This strategy breaks the traditional limitation of single/noncomprehensive enhancement by singlepolarization mechanism,and is of great theoretical and practical significance to promote the research and application of high-performance BaTiO_(3)-based ceramic materials.展开更多
Colossal permittivity(CP)materials,particularly co-doped TiO_(2) ceramics,have garnered significant attention for their potential in high-performance ceramic capacitors.However,understanding the origin of CP remains a...Colossal permittivity(CP)materials,particularly co-doped TiO_(2) ceramics,have garnered significant attention for their potential in high-performance ceramic capacitors.However,understanding the origin of CP remains a challenge,with the role of doping ratios between acceptor and donor ions largely underexplored.This study addresses this gap by systematically investigating the effects of Ga^(3+)concentrations on the microstructure and CP of Ga_(y)Nb_(0.025)Ti_(0.975-y)O_(2),prepared via the solid-state reaction method.The sintered ceramics exhibited a dense rutile TiO_(2) phase with increasing grain sizes and oxygen vacancies.Notably,CP values as high as 10^(5) were achieved at Ga^(3+)/Nb^(5+)ratio<1.0.Optimal dielectric properties were observed at Ga^(3+)/Nb^(5+)=1.0,yielding a CP of 6.4×10^(4) and a loss tangent<0.03,surpassing the performance of many existing CP materials.Impedance spectroscopy revealed distinct electrical heterogeneity,with conductive grains and highly resistive grain boundaries with activation energies>1.0 eV.Ceramics with 5%Ga^(3+) doping showed diminished CP due to the absence of semiconducting grains.The findings suggest that CP originates from the internal barrier layer capacitor.This study not only elucidates the crucial role of doping ratios in tailoring CP but also establishes a pathway for developing advanced dielectric materials with superior performance for ceramic capacitors.展开更多
Molecular-based polar structures are among most intriguing candidates for the generation of mechani-cal functions such as the ferroelectric.These struc-tures undergo a symmetry switching upon heating and generally yie...Molecular-based polar structures are among most intriguing candidates for the generation of mechani-cal functions such as the ferroelectric.These struc-tures undergo a symmetry switching upon heating and generally yield anisotropic lattice variation.Vol-umetric negative thermal expansion(NTE)remains challenging to observe,most of which exhibits rela-tively small magnitudes as well.Herein,we realize colossal volumetric NTE in metal-organic frame-works(MOFs)via novel polar-nonpolar structure transition.Intriguingly,a colossal volumetric NTE(α_(v)=−302.45×10^(−6)K^(−1))was achieved from 320-380 K in CAU-10-CH_(3)due to polar-nonpolar transi-tion(space group from I41md to I41/amd),much larger than most NTE MOFs.Variable-temperature crystallographic analysis reveals that the aluminum-oxygen octahedra turns from low-symmetry to high symmetry upon heating and thus causes a“spring”compression of the octahedral helical chain,thus leading to colossal NTE.Functional-group modifica-tions further identify the excellent tunability of such transition.The strongest uniaxial NTE,to date,was simultaneously found in CAU-10-H along the c direction(α_(c)=−577.22×10^(−6)K^(−1)),whereas the CAU-10-NH_(2)contributes to a sluggish phase transi-tion.This work provides the first example of tailoring a colossal NTE via polar-nonpolar transition in frame-works.展开更多
Dielectric materials are essential in modern electronics,serving as the backbone of numerous components across a wide array of electronic devices[1,2].As technology advances,the demand for materials with high permitti...Dielectric materials are essential in modern electronics,serving as the backbone of numerous components across a wide array of electronic devices[1,2].As technology advances,the demand for materials with high permittivity,low dielectric loss,and thermal stability continues to rise.Traditional strategies to enhance permittivity often involve mechanisms such as phase transitions in ferroelectrics or interfacial polarization in boundary layer capacitor(IBLC)systems.However,each comes with trade-offs.展开更多
Copper calcium titanate(CaCu_(3)Ti_(4)O_(12),CCTO)ceramics with colossal permittivity have gained widespread concern because of their potential application in modern electronic devices with miniaturization and integra...Copper calcium titanate(CaCu_(3)Ti_(4)O_(12),CCTO)ceramics with colossal permittivity have gained widespread concern because of their potential application in modern electronic devices with miniaturization and integration.However,the extent of grain and grain boundary contribution to the colossal permittivity of CCTO-based ceramics based on the internal barrier layer capacitor(IBLC)model is still in debate.This affects their electrical performance optimization and real-world applications.In this study,a series of novel lead-free colossal permittivity ceramic s,xLiCuNb_(3)O_(9-)(1-x)CaCu_(3)Ti_(4)O_(12)(LCNOCCTO),were designed and prepared using a solid-phase reaction approach.The colossal permittivity response mechanism of LCNO-CCTO ceramics was further explored by performing the complex impedance spectrum and analyzing the activation energy from the grain and grain boundary contribution viewpoint.As a result,the LCNO-CCTO ceramics present the cubic perovskite structure with the space groups of■.All the LCNOCCTO ceramics exhibit the significantly enhanced colossal permittivity(10^(5))response,and the ceramic with x=0.15 shows the highest permittivity of about 4.64×10^(5)(20 Hz,room temperature)accompanied by a lower grain resistance of 9.61Ωand larger grain activation energy of 0.21 eV.The enhanced colossal permittivity response is primarily attributed to the great electrical response inside grains of LCNO-CCTO ceramics,resulting from a smaller grain resistance.Also importantly,the high-frequency dielectric relaxation characteristics are improved by incorporating the LCNO into CCTO ceramics as an ion form.Accordingly,the LCNO-CCTO ceramics show a suppressed high-frequency dielectric loss.These results can provide a thorough knowledge and useful optimization strategy for developing high-performance colossal permittivity materials.展开更多
A series of Sr_(1-1.5x)Lu_(x)TiO_(3)(x=0,0.005,0.01,0.015,and 0.02)ceramics was sintered under an air at-mosphere through the solid-state reaction method.The results show that doping with Lu^(3+)consid-erably enhances...A series of Sr_(1-1.5x)Lu_(x)TiO_(3)(x=0,0.005,0.01,0.015,and 0.02)ceramics was sintered under an air at-mosphere through the solid-state reaction method.The results show that doping with Lu^(3+)consid-erably enhances material permittivity.The ceramic with x=0.01 exhibits a colossal permittivity(CP)of~101000 with a tanδof~0.16 at a frequency of 1 kHz,demonstrating enhanced stability over a wide temperature(30-300℃)and frequency(102-106 Hz)range.Based on the analysis of dielectric relaxation,X-ray photoelectron spectroscopy(XPS),and the universal dielectric response law,the CP effect is primarily due to the formation of defect dipoles,which are correlated with the presence of oxygen vacancies,such as Ti^(3+)-V_(O)^(¨)-Ti^(3+),V_(Sr)″-V_(O)^(¨),LuTi′-V_(O)^(¨)-Ti^(3+),and Lu_(Sr)·-Lu_(Ti)′-V_(O)^(¨)-Ti^(3+).These defect dipoles serve to pin electrons,limiting long-range transitions,and enhancing local po-larization.Doping with Lu^(3+)also induces a secondary Lu_(2)Ti_(2)O_(7)phase,which was characterized by X-ray diffraction(XRD)and energy-dispersive X-ray spectroscopy(EDS).The results generated in this study can inform the development and application of new CP materials based on SrTiO_(3).展开更多
基金supported by the National Natural Science Foundation of China (Nos. 50331040 and 60171043)
文摘Magnetocaloric and colossal magnetoresistance effects of the layered perovskite La 1.4 Sr 1.6 Mn 2 O 7 compound have been studied.A broad peak of magnetic entropy change （-△S M） is found above the Curie temperature （T C =120 K）,which can be associated with the existence of two-dimensional short range ferromagnetic order.Additionally,the curvilinear shape of-△S M for layered perovskite is quite different from that of the Ln 1-x A x MnO 3 probably arising from magnetocrystalline anisotropy.At the same time,a wide peak of colossal magnetoresitance effect near T C is found in the layered provskite La 1.4 Sr 1.6 Mn 2 O 7.
基金supported by the National Natural Science Foundation of Chinathe National Basic Research Program of China(Grant Nos.2012CB922003,2011CBA00102,and 2009CB929502)
文摘We review colossal magnetoresistance in single phase manganites, as related to the field sensitive spin-charge interactions and phase separation; the rectifying property and negative/positive magnetoresistance in manganite/Nb：SrTio3 p-n junctions in relation to the special interface electronic structure; magnetoelectric coupling in manganite/ferroelectric structures that takes advantage of strain, carrier density, and magnetic field sensitivity; tunneling magnetoresistance in tunnel junctions with dielectric, ferroelectric, and organic semiconductor spacers using the fully spin polarized nature of manganites; and the effect of particle size on magnetic properties in manganite nanoparticles.
基金This work was supported by the National Natural Science Foundation of China[Grant Nos.51677033,51802061,51702069].
文摘Bi doped SrTiO3 ceramics with Sr deficiency,i.e.Sr1-1.5xBixTiO3(x=0,0.01,0.05,0.1),were prepared via conventional solid-state reaction route.A colossal permittivity(CP)over 104 with low dielectric loss less than 0.05wasobtained in x=0.05 Sr1-1.5xBixTiO3 ceramics.In addition,the dielectric constant is maintained at a value greater than 104 in the range of 102-105 Hz and almost frequency independent.Phase structure analysis and density functional theory calculations suggest that the Bi·Sr-V"Sr-Bi·Sr defect complex with hole-pinned defect-dipoles maybe responsible for the high-performance CP properties.This work gives a new way to achieve high performance CP materials in ABO3 perovskite ceramics.
基金This work was supported by the National Science Foundation Grant No.DMR 1905277.
文摘Ferromagnetic semiconductor Ga_(1–x)Mn_(x)As_(1–y)P_(y) thin films go through a metal–insulator transition at low temperature where electrical conduction becomes driven by hopping of charge carriers.In this regime,we report a colossal negative magnetoresistance(CNMR)coexisting with a saturated magnetic moment,unlike in the traditional magnetic semiconductor Ga_(1–x)Mn_(x)As.By analyzing the temperature dependence of the resistivity at fixed magnetic field,we demonstrate that the CNMR can be consistently described by the field dependence of the localization length,which relates to a field dependent mobility edge.This dependence is likely due to the random environment of Mn atoms in Ga_(1-x)Mn_(x)As_(1-y)P_(y) which causes a random spatial distribution of the mobility that is suppressed by an increasing magnetic field.
基金financially supported by the Fundamental Research Foundation for University of Heilongjiang Province(No.2018-KYYWF-1628)the National Natural Science Foundation of China(Nos.51471057 and 51677033)。
文摘Donor-acceptor co-doped rutile TiO_(2) ceramics with colossal permittivity(CP)have been extensively investigated in recent years due to their potential applications in modern microelectronics.In addition to CP and low dielectric loss,voltage stability is an essential property for CP materials utilized in high-power and high-energy density storage devices.Unfortunately,the voltage stability of CP materials based on codoped TiO_(2) does not catch enough attention.Here,we propose a strategy to enhance the voltage stability of co-doped TiO_(2),where different ionic defect clusters are formed by two acceptor ions with different radii to localize free carriers and result in high performance CP materials.The(Ta+Al+La)co-doped TiO_(2) ceramic with suitable La/Al ratio exhibits colossal permittivity with excellent temperature stability as well as outstanding dc bias stability.The density functional theory analysis suggests that La^(3+)Al^(3+)V_(0)Ti^(3+)defect clusters and Ta^(5+)-Al^(3+)pairs are responsible for the excellent dielectric properties in(Ta+Al+La)co-doped TiO_(2).The results and mechanisms presented in this work open up a feasible route to design high performance CP materials via defect engineering.
基金financially supported by the Ministry of Science and Technology(MOST)NSF of China through the research projects(2018YFA03057001,11820101003)+2 种基金CAS Project for Young Scientists in Basic Research(YSBR-030)support of Beijing Nova program(2020133)the Youth Innovation Promotion Association of CAS(2020007).
文摘We report the study of magnetic and transport properties of polycrystalline and single crystal Na(Zn,Mn)Sb,a new member of“111”type of diluted magnetic materials.The material crystallizes into Cu2Sb-type structure which is isostructural to“111”type Fe-based superconductors.With suitable carrier and spin doping,the Na(Zn,Mn)Sb establishes spin-glass ordering with freezing temperature(Tf)below 15 K.Despite lack of long-range ferromagnetic ordering,Na(Zn,Mn)Sb single crystal still shows sizeable anomalous Hall effect below Tf.Carrier concentration determined by Hall effect measurements is over 1019 cm-3.More significantly,we observe colossal negative magnetoresistance(MR≡[ρ(H)−ρ(0)]/ρ(0))of-94%in the single crystal sample.
文摘The extraordinary colossal magnetoresistance (CMR) behavior in Mn-site doped system La_(0.67)Ca_(0.33)Mn_(1-x)Cr_xO_3 (0.00x0.15) was reported. It was found that the substitution with Cr on Mn sites introduces an additional bump in zero-field resistivity. With increasing Cr content, this additional bump grows up drastically while the original resistivity peak associated with magnetic order transition diminishes gradually. Under the applied magnetic field, both bumps of resistivity are deeply compressed, which leads to the appearance of two peaks in CMR response. As a result, the temperature range of CMR response is significantly broadened, spanned from the lowest to near room temperature. These results suggest that Mn-site element substitution could be a potent way of tuning CMR response.
文摘The representative sample La0.58Dy0.09Ca0.33MnO3 of Dy doped La0.67Ca0.33MnO3 rare-earth manganites was investigated. The most important effect of Dy doping is to introduce the magnetoimpurity and form the spin clusters which induce dramatically large CMR in La0.58Dy0.09Ca0.33MnO3. The fitting results of field-induced resistivity decrease to the Brillouin function indicate that the CMR is caused by the spin dependent hopping between spin clusters. It is the magnetic field that reduces the size of spin clusters and induces a field-induced irreversible CMR behaviour.
基金Funded by the National "973" Project of China (No.2006CB921606)the Foundation from the Ministry of the National Education (Nos. 309020and 20060487011)
文摘Systematical investigations of zero-field resistivity, magnetoresistance and magnetization were performed for a typical manganese compound La2/3Ca1/3MnO3. It is argued that the common origin for insulator-metal and paramagenetic ferromagnetic-transitions as well as colossal magnetoresistance is due to the formation of ferromagnetic clusters in the paramagnetic background. The transition to metallic state is resulted from percolation of ferromagnetic metallic clusters, while the colossal magnetoresistance is due to the application of magnetic field, which accelerates the growth of ferromagnetic metallic clusters and causes the shift of the onset temperature for the metallic percolation to higher temperature. Based on the random resistor network model, the zero-field resistivity versus temperature dependence is simulated by using experimental parameters, and experimental data well agree with those in whole temperature range, giving a strong support to our approach.
文摘A phenomenological model based on phase separation between ferromagnetic metallic and paramagnetic insulating domains was applied to analyze the electrical transport and colossal magnetoresistance for mixed-valent manganites of RE_(2/3)AE_(1/3)MnO_3. The results show that the model can yield results in agreement with experimental observations in these manganites. The present approach provides a simple picture to visualize the reason that the temperature dependence of resistance (with and without applied magnetic fields) in these compounds has the peculiar shape, without invoking any complicated concept.
文摘During the last 10 years, a lot of interests have been devoted to the so-called CDC （colossal dielectric constant） materials. The first materials exhibiting this behavior were the perovskite-based ceramics on the CCTO （CaCuaTi4012） composition. Relative dielectric permittivity can attain values up to （or even larger than） 105. Nevertheless, their dielectric losses are too high, the lower values ranging 10%, in a narrow frequency range, thus limiting their applications. The underlying physical mechanisms at the origin of the CDC are still under study. The analysis of broadband impedance spectroscopy measurements leads most of the authors to propose an interracial polarization mechanism （at the electrodes or at internal barriers）, there is a limited number of complementary electrical characterization techniques, which, up to now, comfort the proposed interracial polarization mechanisms. In the present work, I-V and time-domain polarization are used to characterize these materials. One of the main results is the observation of a non-symmetrical response of these materials related to the direction of the polarization. These results are observed for both macroscopic level on bulk polycrystalline material and within individual grains of the same samples. These results do not fit current accepted models for polarization for CDC materials.
基金supported by the Natural Science Foundation of Hebei province(E2020201021 and E2023201019)Hebei Province Innovation Capability Enhancement Plan Project(22567620H)+2 种基金Industry-University-Research Cooperation Project of Colleges and Universities in Hebei Province(CXZX2025016)Industry-University-Research Cooperation Major Projects of Shijiazhuang City(241130477A)Research Innovation Team of College of Chemistry and Environmental Science of Hebei University(hxkytd2102).
文摘The poor temperature stability of the BaTiO_(3) ceramic has always been the main problem limiting their application.This situation has been improved but sacrifices the intrinsic polarization,which significantly reduces the dielectric constant.In this work,the mechanism of multiple polarization was creatively introduced,and the temperature stability and dielectric properties of BaTiO_(3)-based ceramics are simultaneously enhanced.In particular,the Ba_(0.9925)Bi_(0.005)Ti_(0.995)Ca_(0.005)O_(2.995)(BBTC0.5)ceramic sample achieved excellent temperature stability(-14.8%to 8.85%)over an ultra-wide temperature range(-47 to 400℃)and exhibited colossal permittivity(27,125,25℃,1 kHz)and low dielectric loss(0.07,25℃,1 kHz).The dielectric properties,complex impedance spectra combined with XPS results indicate that the defective dipole clusters(Ti^(3+)-V_(O)-Ti^(3+),Bi_(Ba) and Ca″_(Ti)-V_(O))along with surface effects lead to colossal permittivity effect.More importantly,SEM images show the presence of the second phase at grain boundaries,which prevent the carriers within the grains from accumulating at the grain boundaries.As a result,the dielectric loss was reduced and the temperature stability was further extended.This strategy breaks the traditional limitation of single/noncomprehensive enhancement by singlepolarization mechanism,and is of great theoretical and practical significance to promote the research and application of high-performance BaTiO_(3)-based ceramic materials.
基金funded by the National Science,Research,and Innovation Fund(NSRF)and the Fundamental Fund of Khon Kaen Universitypartially supported by the Research of Khon Kaen Universitythe Thailand Graduate Institute of Science and Technology(TGIST)for his Ph.D.scholarship[Grant Number SCA-CO-2558-1033-TH].
文摘Colossal permittivity(CP)materials,particularly co-doped TiO_(2) ceramics,have garnered significant attention for their potential in high-performance ceramic capacitors.However,understanding the origin of CP remains a challenge,with the role of doping ratios between acceptor and donor ions largely underexplored.This study addresses this gap by systematically investigating the effects of Ga^(3+)concentrations on the microstructure and CP of Ga_(y)Nb_(0.025)Ti_(0.975-y)O_(2),prepared via the solid-state reaction method.The sintered ceramics exhibited a dense rutile TiO_(2) phase with increasing grain sizes and oxygen vacancies.Notably,CP values as high as 10^(5) were achieved at Ga^(3+)/Nb^(5+)ratio<1.0.Optimal dielectric properties were observed at Ga^(3+)/Nb^(5+)=1.0,yielding a CP of 6.4×10^(4) and a loss tangent<0.03,surpassing the performance of many existing CP materials.Impedance spectroscopy revealed distinct electrical heterogeneity,with conductive grains and highly resistive grain boundaries with activation energies>1.0 eV.Ceramics with 5%Ga^(3+) doping showed diminished CP due to the absence of semiconducting grains.The findings suggest that CP originates from the internal barrier layer capacitor.This study not only elucidates the crucial role of doping ratios in tailoring CP but also establishes a pathway for developing advanced dielectric materials with superior performance for ceramic capacitors.
基金supported by the National Key R&D Program of China(grant no.2020YFA0406202)the National Natural Science Foundation of China(grant nos.22090042,22275015,21731001,21971009)+1 种基金the Program for Changjiang Scholars,the Innovative Research Team in University(grant no.IRT1207)the Program of Introducing Talents of Discipline to Universities(grant no.B14003).
文摘Molecular-based polar structures are among most intriguing candidates for the generation of mechani-cal functions such as the ferroelectric.These struc-tures undergo a symmetry switching upon heating and generally yield anisotropic lattice variation.Vol-umetric negative thermal expansion(NTE)remains challenging to observe,most of which exhibits rela-tively small magnitudes as well.Herein,we realize colossal volumetric NTE in metal-organic frame-works(MOFs)via novel polar-nonpolar structure transition.Intriguingly,a colossal volumetric NTE(α_(v)=−302.45×10^(−6)K^(−1))was achieved from 320-380 K in CAU-10-CH_(3)due to polar-nonpolar transi-tion(space group from I41md to I41/amd),much larger than most NTE MOFs.Variable-temperature crystallographic analysis reveals that the aluminum-oxygen octahedra turns from low-symmetry to high symmetry upon heating and thus causes a“spring”compression of the octahedral helical chain,thus leading to colossal NTE.Functional-group modifica-tions further identify the excellent tunability of such transition.The strongest uniaxial NTE,to date,was simultaneously found in CAU-10-H along the c direction(α_(c)=−577.22×10^(−6)K^(−1)),whereas the CAU-10-NH_(2)contributes to a sluggish phase transi-tion.This work provides the first example of tailoring a colossal NTE via polar-nonpolar transition in frame-works.
文摘Dielectric materials are essential in modern electronics,serving as the backbone of numerous components across a wide array of electronic devices[1,2].As technology advances,the demand for materials with high permittivity,low dielectric loss,and thermal stability continues to rise.Traditional strategies to enhance permittivity often involve mechanisms such as phase transitions in ferroelectrics or interfacial polarization in boundary layer capacitor(IBLC)systems.However,each comes with trade-offs.
基金financially supported by the National Natural Science Foundation of China(No.52202136)the Natural Science Foundation of Jiangxi Province(No.20232BAB204017)+2 种基金the State Key Laboratory of Electrical Insulation and Power Equipment(No.EIPE24203)Guangdong Provincial Key Laboratory of Electronic Functional Materials and Devices(No.EFMD2024002Z)the Innovation Special Foundation Project for Graduate Students of Nanchang Hangkong University(No.YC2024019)
文摘Copper calcium titanate(CaCu_(3)Ti_(4)O_(12),CCTO)ceramics with colossal permittivity have gained widespread concern because of their potential application in modern electronic devices with miniaturization and integration.However,the extent of grain and grain boundary contribution to the colossal permittivity of CCTO-based ceramics based on the internal barrier layer capacitor(IBLC)model is still in debate.This affects their electrical performance optimization and real-world applications.In this study,a series of novel lead-free colossal permittivity ceramic s,xLiCuNb_(3)O_(9-)(1-x)CaCu_(3)Ti_(4)O_(12)(LCNOCCTO),were designed and prepared using a solid-phase reaction approach.The colossal permittivity response mechanism of LCNO-CCTO ceramics was further explored by performing the complex impedance spectrum and analyzing the activation energy from the grain and grain boundary contribution viewpoint.As a result,the LCNO-CCTO ceramics present the cubic perovskite structure with the space groups of■.All the LCNOCCTO ceramics exhibit the significantly enhanced colossal permittivity(10^(5))response,and the ceramic with x=0.15 shows the highest permittivity of about 4.64×10^(5)(20 Hz,room temperature)accompanied by a lower grain resistance of 9.61Ωand larger grain activation energy of 0.21 eV.The enhanced colossal permittivity response is primarily attributed to the great electrical response inside grains of LCNO-CCTO ceramics,resulting from a smaller grain resistance.Also importantly,the high-frequency dielectric relaxation characteristics are improved by incorporating the LCNO into CCTO ceramics as an ion form.Accordingly,the LCNO-CCTO ceramics show a suppressed high-frequency dielectric loss.These results can provide a thorough knowledge and useful optimization strategy for developing high-performance colossal permittivity materials.
基金Project supported by the fund of the Major Science and Technology Programs of Yunnan Province(202102AB080008)the Science and Technology Program of Yunnan Precious Metal Laboratory(YPML-2022050205,YPML-2022050219)。
文摘A series of Sr_(1-1.5x)Lu_(x)TiO_(3)(x=0,0.005,0.01,0.015,and 0.02)ceramics was sintered under an air at-mosphere through the solid-state reaction method.The results show that doping with Lu^(3+)consid-erably enhances material permittivity.The ceramic with x=0.01 exhibits a colossal permittivity(CP)of~101000 with a tanδof~0.16 at a frequency of 1 kHz,demonstrating enhanced stability over a wide temperature(30-300℃)and frequency(102-106 Hz)range.Based on the analysis of dielectric relaxation,X-ray photoelectron spectroscopy(XPS),and the universal dielectric response law,the CP effect is primarily due to the formation of defect dipoles,which are correlated with the presence of oxygen vacancies,such as Ti^(3+)-V_(O)^(¨)-Ti^(3+),V_(Sr)″-V_(O)^(¨),LuTi′-V_(O)^(¨)-Ti^(3+),and Lu_(Sr)·-Lu_(Ti)′-V_(O)^(¨)-Ti^(3+).These defect dipoles serve to pin electrons,limiting long-range transitions,and enhancing local po-larization.Doping with Lu^(3+)also induces a secondary Lu_(2)Ti_(2)O_(7)phase,which was characterized by X-ray diffraction(XRD)and energy-dispersive X-ray spectroscopy(EDS).The results generated in this study can inform the development and application of new CP materials based on SrTiO_(3).
