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The defect characteristic and magnetic properties of CuFe1-xCrxO2 (x = 0.0–0.4) compounds have been investigated in detail. The results demonstrate that Cr3+ ions enter into the CuFeO2 lattice sites and induce local lattice contraction but do not change the valance state of Cu+ and Fe3+ ions. We calculate the positron annihilation lifetime in the bulk and vacancy defect trapping states by the atomic superposition method in 3 × 3 × 1 CuFeO2 super-cell. Combined with theoretical calculation results, the positron annihilation results indicate that the positrons are mainly annihilated in vacancy clusters in CuFe1-xCrxO2. The substitution of Cr3+ for Fe3+ ions decreases the open volume and increases the concentration of vacancy defects but does not change the integral defect surroundings at annihilation sites. Magnetization measurements manifest that doping with Cr3+ reduces the antiferromagnetic transition temperature and promotes the formation of short-range magnetic correlation. Meanwhile, the enhanced short-range magnetic correlation in Cr3+-doped CuFeO2 is more readily driven by thermal activation energy and a faster dynamic behavior can be seen in time windows. [ABSTRACT FROM AUTHOR]

In this paper, M (Sc3+ and Zr4+)-doped CuFeO2 ceramics were prepared by using a solid-state reaction method. The effects of isovalent Sc3+ and nonisovalent Zr4+ substitution on the structure, defects, optical, and magnetic properties of CuFeO2 ceramics are comparatively investigated. The results demonstrated that all of the obtained samples are well crystallized, and M substitution leads to lattice structure distortion. The average grain sizes reduce with the addition of M. Positron annihilation lifetime results indicate that the vacancy clusters are present in all samples, and the Zr-substituted CFO sample has the maximum open volume and concentration of the defect among those three samples. Furthermore, the bandgap of the M-substituted samples becomes wider due to the broad bandgap of Sc2O3 and ZrO2. M substitution enhances the Urbach energy (EU) values in CFO system, which is in good consistent with the dependences of the defect concentration on the substitution elements. The magnetic measurements reveal that Sc and Zr substitution decrease the magnetic temperatures TN1 and TN2, which may be attributed to the consequence of the magnetic dilution effect and the change in the strength of exchange interaction. In addition, the defects are of major importance for regulating the magnetic properties of CFO ceramics. [ABSTRACT FROM AUTHOR]

The polycrystalline CuFeO2 (CFO) and CuFe1-xMxO2 (M = Ti, Hf, Zr, M-CFO) samples are synthesized by solid-state reaction method. The doping effect of tetravalent nonmagnetic transition metal M4+ ions on the microstructure, defect evolution, and magnetic and magnetocaloric properties of the CFO system are comparatively investigated. The substitution of M4+ for Fe3+ increases lattice parameters, changes the bond length of Fe/Cu-O and bond angle of Fe-O-Fe, and improves the microstructure to some extent. Positron annihilation spectroscopy results indicate that M4+ doping induces the agglomeration of small-sized vacancy defects manifested as the increase in vacancy-type defect size and the suppression of inherent defect concentration. Magnetic measurements show that the antiferromagnetic stability of CFO system is clearly affected by M4+ doping, and Ti4+ doping has a more noticeable impact than Hf4+ and Zr4+ doping. Meanwhile, the coexistence of weak ferromagnetism and antiferromagnetism is detected in all doped samples, while Ti-doped system exhibits extraordinary magnetic properties. Especially for Ti-CFO2 (x = 0.03) sample, the maximum magnetization reaches as high as 11.81 emu/g at 0.5 T, which enhanced one order of magnitude than that of undoped CFO. Isothermal M-H data at different temperatures show that entropy change (ΔSM) and refrigerant capacity (RC) for CFO and M-CFO2 samples are significantly weakening with M4+ substitution in bulk CFO. The maximum ΔSM = 4.79 J·kg−1 K−1 and RC = 12.79 J·kg−1 for undoped CFO are obtained near the transition temperature TN2 = 12 K, with the applied fields up to 6 T. The possible reasons for the above observations are discussed in detail. [ABSTRACT FROM AUTHOR]

GdMnO3 (GMO) ceramics were prepared by solid-state reaction method, the structure, defects, dielectric, and magnetic properties of GMO ceramics sintered at different temperatures have been studied. Characterization by XRD indicates that impurity phase-free GMO can be synthesized at 1350 °C. SEM measurements show that the grain size of GMO increases and the microstructure becomes denser with increasing sintering temperature. Positron annihilation results reveal that the vacancies concentration decreases when sintering temperature increases from 1300 to 1350 °C, and then increases when sintering temperature increases from 1350 to 1400 °C. Dielectric measurements indicate that the GMO ceramic sintered at 1350 °C presents higher dielectric constant and smaller dielectric loss. Magnetic properties measurements show that the magnetic properties of GMO can be modulated by sintering temperature. The relation between structure and properties implies that the cation vacancy defect has remarkable effects on the dielectric and magnetic properties of GMO. [ABSTRACT FROM AUTHOR]

Undoped GdMnO3 (GMO) and Gd0.95Ba0.05MnO3 (GBMO) multiferroic ceramics were prepared by solid state reaction method, and the effects of Ba doping on the structure, morphology, dielectric and magnetic properties of GMO system were investigated. The XRD results show that the two samples yield single phase, and Ba doping induce structure distortion. The SEM images show that Ba doping can inhibit the grain growth. The XPS analysis indicates that more Mn4+ and oxygen vacancies are involved in the Ba doped GMO ceramics. Raman measurements show that Ba substitutes Gd into the lattice of GMO and induces lattice disorder. The dielectric measurements reveal that the Ba doped GMO exhibits the giant dielectric properties, but the dielectric loss of Ba doped samples is higher than that of the undoped samples. Temperature and magnetic field dependent magnetization measurements reveal that the low temperature magnetic behaviors of GMO system can be altered by Ba doping. The paramagnetic-antiferromagnetic transition temperature and magnetization of GMO system are decreased after Ba doping. In addition, Ba doping can also influence the long-range order of Gd moments at about 8 K. The possible reasons for the above results are discussed. [ABSTRACT FROM AUTHOR]

The polycrystalline samples of (1−x)BiFeO3-xCaCu3Ti4O12(x = 0, 0.05, 0.10 and 0.15) were synthesized by solid state reaction method followed by rapid liquid phase sintering. The effects of CaCu3Ti4O12(CCTO) doping on the microstructure, dielectric and magnetic properties of BiFeO3(BFO) ceramics were studied by changing CCTO concentrations. The XRD analysis reveals that that a little amount of CCTO as a solid solution in the BFO phase leads to expanding the lattice, no phase transition has occurred in the doping content range, but CCTO doping induces the crystal structure distortion. Impedance spectroscopy results show that the doped BFO ceramics have a broadband stability of the dielectric constant and dielectric loss on frequency, the CCTO doping can improve the dielectric constant and reduce dielectric loss of BFO. It is also observed that an increase in CCTO doping concentration in BFO can improve the magnetic properties of BFO. [ABSTRACT FROM PUBLISHER]

Effect of bivalent cation Ca-doping on magnetic properties in multiferroic YMnO3 manganites was systemically studied by DC and AC magnetic measurements. Series of samples were prepared by solid-state reaction method with composition Y1− xCa xMnO3 ( x= i/8, i=2,3,...,7). The results show that each of the zero-field-cooling (ZFC) curves has a clear cusp and it shifts to higher temperature with increasing Ca-doping x. From the χ−1– T curve, the temperature of charge-ordering (CO) state TCO is given at about 244.0 K and this indicates that the CO state is dominated by the cooperative Jahn–Teller effect in Y1/2Ca1/2MnO3 with small A-site cation, for which there is a different behavior compared to the other manganites of larger 〈 r A〉. When x=2/8 and 7/8 for Y1− xCa xMnO3, the magnetization curve shows strong irreversibility between ZFC and FC data. The variation of AC susceptibility presents a maximum cusp at the beginning temperature of the bifurcation between ZFC and ZF curve and shifts to higher temperature with increasing measurement frequency for AC susceptibility. These results can be characterized by spin-glass, indicating that much more itinerant eg electrons are promoting the magnetic transition. [ABSTRACT FROM AUTHOR]

A series of multiferroic ceramics CuFe 1- x Eu x O 2 ( x  = 0–0.10) are prepared by traditional solid-state reaction. The effects of Eu doping on the microstructure, vacancy-type defects, and magnetic properties of CuFeO 2 ceramics are investigated systematically by means of X-ray diffraction, Raman spectroscopy, scanning electron microscope, positron annihilation lifetime and physical property measurement system. The results show that no phase transition occurs in the entire range of doping content ( x  = 0–0.10), but the single phase structure is damaged by high Eu content ( x  = 0.04–0.10). Positron annihilation measurements indicate that the local electron density and the vacancy-type defect concentration increase gradually with the increase in Eu content from 0 to 0.08. Furthermore, abnormal changes in lifetime parameters can be found in x  = 0.10 sample induced by the existence of impurity phase in the system. The magnetic measurements reveal that all the samples exhibit two successive magnetic transitions at T  = 15 and 11 K. In x  = 0.02 sample, the coexistence of ferromagnetism and antiferromagnetism can be found, and a maximum saturation magnetization of 11.548 emu/g at 5 kOe is achieved. The possible reasons for the above observations are discussed in detail. [ABSTRACT FROM AUTHOR]

In this study, Cu 1− x Ho x FeO 2 nanoceramics were synthesized by sol-gel method to investigate the influence of the vacancy defects and Fe valence state tuning on the magnetic and dielectric properties of Cu 1− x Ho x FeO 2 nanoceramics. XRD and Raman analysis indicated that Cu 1− x Ho x FeO 2 nanoceramics with single-phase delafossite structure were synthesized by sol-gel method; Ho3+ ions had replaced the Cu ions in CuFeO 2 and resulted in lattice deformation. XPS analysis indicated that Ho3+ substitution could tune the Fe valence state, but did not cause significant change in oxygen vacancy concentration. The results of positron annihilation lifetime spectroscopy revealed that cation vacancies concentration in the interface region could be effectively tailored by Ho3+ substitution. The magnetic measurements indicated that Ho3+ substitution enhanced the magnetic transition temperatures and magnetization of Cu 1− x Ho x FeO 2 samples, which could be mainly attributed to the cation vacancies concentration in the interface region and Fe2+ concentration. Most interestingly, the Cu 1− x Ho x FeO 2 nanoceramics exhibited giant dielectric characteristic, which was attributed to the charge ordering of Fe3+ and Fe2+. [ABSTRACT FROM AUTHOR]

• Vacancies at different lattice positions on the properties of MnWO 4 was studied. • The magnetization and transition temperatures of MnWO 4 were improved by non-stoichiometric. • The temperature range in which AF2 phase existed was enlarged by non-stoichiometric. • The effect of Mn vacancy on the properties of MnWO 4 was higher than that of W vacancy. Non-stoichiometric ceramic samples of Mn 0.97 WO 4 and MnW 0.97 O 4 were prepared by a solid-state reaction to study the effects of vacancies at different lattice positions on the structure and properties of MnWO 4 materials caused by non-stoichiometric. Positron annihilation technology was introduced to study the vacancy characteristics of the MnWO 4 , Mn 0.97 WO 4 , and MnW 0.97 O 4 ceramics. The results indicated that non-stoichiometric did not affect the monoclinic lattice structure of any of the MnWO 4 samples, and had no obvious effect on the lattice parameters of the samples. The analysis of positron annihilation lifetime spectroscopy showed that W vacancies were the main defects in MnW 0.97 O 4 samples, and Mn vacancies were favored in Mn 0.97 WO 4 samples, the vacancies concentration for non-stoichiometric MnWO 4 samples was higher than that of stoichiometric MnWO 4. It was observed that the magnetization and transition temperatures of the non-stoichiometric MnWO 4 system were increased, and the temperature range in which AF2 phase existed was enlarged. This study demonstrated that non-stoichiometric could enhance the magnetic properties of MnWO 4 , the effect of Mn vacancy on the properties of MnWO 4 is higher than that of W vacancy. The intimate correlation between the vacancy defects and magnetic properties provides us strong evidence that the magnetic properties of MnWO 4 can be enhanced by regulating vacancy defects. [ABSTRACT FROM AUTHOR]

Polycrystalline CuFeO 2 (CFO) and CuFe 0.99 R 0.01 O 2 (R –CFO, R = La, Sm, Eu, Gd, Dy, and Ho) were prepared via solid-phase synthesis to investigate the effects of rare-earth (R) ion addition on the microstructures and magnetism. Positron annihilation lifetime spectroscopy (PALS) experimental results demonstrated that the size, concentration, and defect types were altered by R substitution due to the competition between the agglomeration of small-sized defects and the decomposition of vacancy clusters, which resulted in the redistribution of electrons around annihilation sites. The magnetic measurements revealed that the antiferromagnetic stability was influenced by R substitution, which was ascribed to the effect of magnetic moments, variations in exchange interactions, and the partial reduction of spin frustration. The magnetization behaviors of the R –CFO samples were also influenced by doping ions, which was mainly explained by the different magnitudes of the R ions' magnetic moments. Ferromagnetism and antiferromagnetism were present in all of the samples. The local electron density n e was associated with the maximum magnetization (M m). The correlative physical mechanisms are discussed in detail. The results demonstrated that the n e plays an important role in regulating the magnetic properties of CFO ceramics. [ABSTRACT FROM AUTHOR]

• Substitution of Ge4+ for Fe3+ induces lattice perturbation and charge imbalance in CuFeO 2. • Partial Cu+ ions are transformed into Cu2+ after Ge substitution. • The concentration of positron traps and local electron density increase obviously for CuFe 1− x Ge x O 2 (x ≥ 0.05) series. • The stability of antiferromagnetic phase in CuFeO 2 is slightly weakened by Ge doping. • Obvious ferromagnetic behavior appears in CuFe 1− x Ge x O 2 (x ≥ 0.05) series. The microstructure, defect and magnetic properties of Ge-substituted CuFeO 2 multiferroics are studied systematically. The introduction of Ge4+ in Fe3+ sites of CuFeO 2 leads to obvious changes in its crystal structure, micro-morphology and surface chemical status. Positron annihilation results show that the concentration of positron traps and the local electron density increases evidently inside CuFe 1− x Ge x O 2 (x ≥ 0.05) series compare with that of un-doped sample. Interestingly, the magnetic hysteresis loops show the magnetic interactions are changed and obvious ferromagnetism appears in the CuFe 1− x Ge x O 2 (x ≥ 0.05) samples. In addition, the stability of the antiferromagnetic phase of CuFeO 2 is weakened. The magnetic transition temperature T N2 slightly decreases with increasing Ge content. The relation between the microstructure and the magnetic behavior are explored in detail in the present paper. [ABSTRACT FROM AUTHOR]

An investigation on the microstructure, defects, and physical properties for Cu 1- x Li x FeO 2 (x = 0.00–0.08) ceramic samples is here presented. The delafossite structure with R 3 ‾ m space group for all samples is retrieved using Powder X, whereas the grain growth can be effectively inhibited by Li doping. Positron annihilation results demonstrate that the vacancies are formed in all specimens, defect size and concentration can be effectively affected by Li doping. Electric properties measurements show that all the prepared samples exhibit giant dielectric behavior, while the undoped CuFeO 2 sample exhibits maximum value around 3.5 × 104. The internal barrier capacitance model (IBLC) can be used to systematically explain the dielectric mechanism of CuFeO 2 system. Optical analysis shows that an obvious absorption peak occurs at around 225 nm, and a decrease of direct optical band gap with the increase in Li doping content is observed. Magnetic properties measurements reveal that the antiferromagnetic transition temperature has almost no change with the increase in Li content, and that the co-existence of ferromagnetism and antiferromagnetism is observed. The magnetization increases with the Li doping amount, in which the x = 0.08 sample exhibits the maximum magnetization (about 24 emu/g). The evolution of magnetic properties is seen to be related to lattice distortion, vacancy defect, and disturbance of the spin structure induced by Li doping. [ABSTRACT FROM AUTHOR]

• Introducing local ferromagnetic spin configuration and cation vacancies. • The magnetic temperature T N2 shifts to the lower temperature due to Ti doping. • Positron annihilation technique is adopted to investigate the defect feature. • The interplay between magnetic properties and microdefect is studied carefully. CuFe 1− x Ti x O 2 (0 ≤ x ≤ 0.08) ceramics are synthesized by solid state reaction method to investigate the effect of Ti substitution on the microstructure, vacancy-type defects, the local electron density, and magnetic properties. The XRD and Raman spectra results show that all experimental samples possess the characteristic peak of CuFeO 2 ceramics. SEM observations indicate that Ti substitution has obvious influence on the morphologies of experimental ceramics, and the microvoids increases significantly with increasing Ti content. Positron annihilation lifetime and Doppler broadening spectra results indicate that the vacancy concentration increases rapidly and then reaches a higher platform (>35%) as x > 0.01, and Ti substitution does not change the defects species in CuFe 1- x Ti x O 2 ceramics. The electron density decreases with increasing x and then reaches a low plateau (≈5.7) at x = 0.08. The χ - T curves reveal that the T N1 is independent of Ti content, while, the T N2 decreases with increasing x at x = 0.00–0.02 and even disappears in the magnetization curves as x > 0.02. The M-H curves indicate that the coexistence of weak ferromagnetism and antiferromagnetism for all the samples could be found. The possible reasons for the above observations are discussed in detail. [ABSTRACT FROM AUTHOR]

Microstructures, defect characteristics, optoelectronic, and magnetic properties of polycrystalline delafossite Cu 1- x Ca x FeO 2 (x = 0.00–0.08) samples have been systemically investigated. Our investigation results distinctly show that divalent Ca2+ ions can successfully substitute partial Cu+ ions and prompt the grain size and induce the partial Fe3+ valence variation to Fe2+. The positron annihilation lifetime spectra signifies that the cation vacancy clusters exist in all samples, the size and concentration of vacancy defects are redistributed with increasing Ca2+ ions content, while the overall defect environment of CuFeO 2 system is not changed. The Hall-effect measurement results indicate the transition of semiconductor type from p-type to n-type. The UV–Visible absorption spectroscopy results indicate that the calculated band gap of the prepared samples is estimated to be 3.42–3.58 eV, indicating the series can be applied to the transparent conductive oxide materials. The magnetic measurements reveal that the transition temperature T N1 and T N2 are independent of Ca2+ content x , while the coexistence of antiferromagnetism and weak ferromagnetism for all the doped samples are observed. A moderate Ca2+ content, i.e. x = 0.02, can distinctly promote weak ferromagnetism. The above internal mechanism is probably relevant to the evolution of lattice structure and defect characteristics caused by Ca2+ ions doping. • Polycrystalline Cu 1- x Ca x FeO 2 samples have been synthesized successfully by direct solid state reaction. • PALS is employed to reveal electron structure and defect evolution for Cu 1- x Ca x FeO 2 ceramics. • The Cu 1- x Ca x FeO 2 series can be applied to the transparent conductive oxide materials. • A moderate Ca content x can distinctly promote weak ferromagnetism. [ABSTRACT FROM AUTHOR]

• Fe vacancies were introduced into BiFeO 3 by nonstoichiometric Zr substitution. • The microstructral defects were studied by positron annihilation technology. • Nonstoichiometric Zr substitution could effectively enhance the multiferroic properties of BiFeO 3. • Fe vacancies concentration was a key factor that modulated the magnetic properties of BiFeO 3. • Fe2+ concentration was the main source of the leakage current in BiFeO 3. Nonstoichiometric BiFe 1−4 x /3 Zr x O 3 (0 ≤ x ≤ 15%) multiferroic ceramics with Fe vacancies were synthesized by using rapid liquid phase sintering process. It was found that nonstoichiometric Zr substitution induced structure distortion, and an appropriate amount of Zr could remove impurity phases. XPS and PAS studies indicated that there was no obvious change in Bi vacancies concentration, but the Fe vacancies concentration increased with increasing substitution amount of Zr. The Fe2+ concentration first decreased and then increased with the increase in substitution amount of Zr, while the oxygen vacancies concentration decreased with increasing substitution amount of Zr. Nonstoichiometric Zr substitution could effectively enhance the multiferroic behaviors of BiFeO 3. The evolution of structure and properties induced by nonstoichiometric Zr substitution indicated that Fe2+ concentration was the main source of the leakage current, while Fe vacancies concentration was a key factor that modulated the magnetic performances of BiFeO 3. Furthermore, the applied magnetic field dependence of dielectric constant suggested that the magnetodielectric effect increased with increasing Zr substitution amount. The results above imply that nonstoichiometric BiFeO 3 composition with Fe vacancies has great potential for exploiting high quality BiFeO 3 -based ceramics. [ABSTRACT FROM AUTHOR]

• Alkaline earth ions doping affects the defects characteristic in CuFeO 2 ceramics. • CuFeO 2 (Eg = 3.1–3.5 eV) can be applied as transparent conductive materials. • Alkaline earth ions doping can regulate the magnetic properties of CuFeO 2. • The defect concentration and remnant magnetization has opposite change tendency. The polycrystalline samples of undoped CuFeO 2 and Cu 0.97 A 0.03 FeO 2 (A = Ca, Sr, Ba) were prepared by solid state reaction method. The single phase delafossite structure with lattice distortion is observed in all of the prepared samples, and the valence state and concentration of Fe3+ have changed after the substitution of alkaline earth ions (A 2+) for Cu+. The particle size is improved by A 2+ doping, and the largest particle size is observed in the Cu 0.97 Ca 0.03 FeO 2 sample (~9 μm). Positron annihilation spectroscopy results demonstrate that a small amount of A 2+ doping increases the open volume and concentration of vacancy defects, but the overall defect environment is almost unaffected by A 2+ ions doping. The UV–Vis absorption spectroscopy investigations indicate that A 2+ doping decreases the direct optical band gap. The magnetic properties measurements exhibit that the low temperature antiferromagnetic interaction of CuFeO 2 system changes obviously by A 2+ doping, the coexistence of ferromagnetic and antiferromagnetic interactions is observed in all of the prepared samples. The antiferromagnetic transition temperature of the doped samples is decreased compared to that of the undoped sample. It is found that the magnetic properties of CFO are closely associated with lattice structure and defects concentration. [ABSTRACT FROM AUTHOR]