Double-perovskite Samples Research Articles

Correlation between the exchange bias effect and antisite disorder in Sr2−xLaxCoNbO6 (x=0, 0.2)

We unravel the effect of La substitution and hence antisite disorders on the exchange bias (EB) mechanism in Sr2−xLaxCoNbO6 (x=0, 0.2) double-perovskite samples using the detailed analysis of the field cooled magnetization isotherms (M−H) and training effect. The field dependence of the freezing temperature deviates from both Gabay-Toulouse and de Almeida-Thouless lines and the analysis suggests that the x=0 sample follows a different universality class with moderate anisotropy in the frozen spins. Interestingly, we find that the EB effect is significantly suppressed in the x=0.2 sample due to increase [decrease] in the size of ferromagnetic (FM) [cluster glass (CG)] domain, which reduces the effective disordered interface responsible for the EB. The changes in fraction of FM, antiferromagnetic, and CG-like interactions with the La substitution and applied magnetic field are found to be crucial in governing the EB effect in these samples. Furthermore, the training effect measurements show the unequal shift in the left and right branches of the M−H loops and their different evolution with the field cycles (n). The analysis reveals that the rotatable spins relax approximately one order of magnitude faster than the frozen spins at the disordered interface. We find a possible correlation between the observed EB effect and the antisite disorders in these samples. Published by the American Physical Society 2024

Structural, dielectric and magnetic study of double perovskite La2CoMnO6

Polycrystalline double perovskite La2CoMnO6 sample was prepared via solid state reaction route and its structural, dielectric and magnetic properties were investigated. XRD confirmed the phase formation without any impurity. Rietveld refinement analysis suggested the presence of monoclinic (P21/n, ordered) as well as rhombohedral and orthorhombic (R3c &Pbnm, disordered) structures. Microstructural analysis using FESEM confirmed that average grain size lies in micrometer range. Dielectric constant (log ε′) and loss tangent (tanδ) both exhibited large values at lower frequencies as well as in higher temperature range. Nyquist impedance plot proposed NTCR (negative temperature coefficient of resistance) nature of double perovskite sample. DC conductivity showed increasing trend with increase in temperature and also, the conductivity is of the order of 10−2 S/m, which represent semiconducting behavior of prepared sample. Also, the analysis suggested the co-existence of thermal activation (TA), small polaron hopping (SPH) and variable range hopping (VRH) conduction mechanisms. Temperature dependent magnetic measurements (M-T) suggested that sample exhibit single magnetic transition at 226 K. Field dependent magnetic measurements (M-H) confirmed the ferromagnetic nature of the prepared sample. M-H loop is not well saturated even at 60 kOe which suggested the presence of anti-site disorder. Multi-domain magnetic structure of the prepared sample suggests its suitability for memory device applications.

Qualitative study of structural phase transition in nickel doped La2CoTi(1−x)NixO6 double perovskite

La2CoTi(1−x)NixO6 (x = 0, 0.2 and 0.6) (A2BB′(1−x)B′′(x)O6) double perovskite samples in bulk pellet form were prepared successfully by using sol-gel method at a sintering temperature of 1350 °C. From the Rietveld refinement approach, crystal structures of prepared samples were investigated using X-ray powder diffraction data, wherein, a structural phase transition occurred from monoclinic (P21/n) to orthorhombic (Pbnm) to rhombohedral (R3̅c) as the nickel content is increased. X-ray photoelectron spectroscopy study reveals the presence of mixed oxidation state of cobalt ions (Co2+/Co3+) which changes significantly as the nickel concentration is increased to 20% and hence affects the bond length with the structure distortion from monoclinic to orthorhombic with symmetry Pbnm. In 60% nickel doped compound, there occurs a random distribution of Co2+/Co3+ and Ni3+ among the B′ and B′′ site with added repulsions between trivalent cobalt cations seeming in the structure due to Ti4+ located at antisite B site. Consequently, to maintain charge neutrality, distorted rhombohedral structure was observed. Tolerance factor was also estimated for all the prepared bulk compounds to further confirm the formation and structural transitions in the prepared compounds.

Probing the effect of Pr doping on the magnetic and magnetocaloric properties of [formula omitted] double perovskites

We report the possible variations in the magnetocaloric properties of Ba2−xPrxFeMoO6 (0.0 ≤ x ≤ 0.4) induced by the controlled substitution of rare earth element Praseodymium (Pr) at the Barium-site in the lattice. Pellets of the double perovskite samples prepared via the conventional solid state reaction method were thoroughly investigated using X-ray diffraction followed by Rietveld analyses that established a cubic crystal structure (Fm-3m) in these samples. The magnetization of the samples was measured as a function of temperature that indicated a single ferromagnetic to paramagnetic phase transition in these materials as the temperature was increased. The Pr doping did not change the crystalline structure but resulted in non-monotonic evolution of the Curie temperature (TC) and decreased magnetization. A second order of magnetic phase transition in these materials was confirmed by the Arrott plots analyses as well as the ‘universal master curve method’ which indicated the reversible nature of magnetization plus lower hysteresis associated with these materials. Though the Pr doping did not enhance the magnetic entropy change and the relative cooling power yet the near room temperature TC, chemically stable structure, moderate magnetization and tunable magnetocaloric properties of these materials make them attractive for exploration in numerous applications in spintronics and magnetic refrigeration technology.

Structural, electrical and magneto-electric characteristics of double perovskite: BiCaFeCeO6

A double perovskite sample of (Ca, Ce) modified bismuth ferrite (i.e., BiCaFeCeO6) has been synthesized by a solid state reaction method. A single-phase characteristic of the material was carried out by analyzing basic X-ray crystal structure. The micrograph of the sintered sample was recorded at room temperature by using scanning electron microscope. The image of the micrograph shows the homogeneous distribution of with different grain size and shape. It shows that a high-density sample can be fabricated in the above experimental condition. This prepared complex electronic system (BiCaFeCeO6) exhibits multiferroic characteristics that has experimentally been supported by detailed analysis of capacitive, impedance, magnetic and other related behavior of the material Attempts have been made to study the (a) effect of microstructures containing grains, grain boundaries and electrodes in impedance and capacitive characteristics, (b) relationship between properties and crystal structure and (c) nature of relaxation mechanism of the prepared samples. Detailed impedance analysis with experimental data, collected at different frequency (103–106 Hz) and temperatures, has provided many important characteristics of the material. The variation of conductivity in alternating current of the material at different temperatures follows Jonsher's universal power law. Some multiferroic characteristics (dielectric constant, tangent loss, conductivity, magneto-electric) of bismuth ferrite have been tailored (decrease/increase) by multiple doping of (Ca, Ce) which will be useful to explore the possibility for applications.

Magnetic field at Ce impurities in La sites of La0.5Ba0.5MnO3 double perovskites

Due to its rich variety of electromagnetic properties, such as a colossal magnetoresistance, charge and orbital ordering, and metal-insulator transition, the magnetic behavior in La0.5Ba0.5MnO3 double perovskite compounds has been investigated by several techniques, however more experimental data, especially from atomic resolution techniques, are still necessary to understand such complex behavior. In this paper, we have used a nuclear and short-range technique, the Perturbed Angular Correlation (PAC) spectroscopy, to investigate the magnetic hyperfine interaction at the 140Ce and 111Cd probe nuclei as impurities occupying La sites. This double perovskite samples were synthesized by Sol-Gel route. The crystal structure was determined by X-ray diffraction and the analyses showed that this method produced perovskite oxides with cubic structure in Pm-3m space group and the homogeneity was determined by Transmission Electron Microscopy (TEM). The local properties investigated by PAC spectroscopy revealed a ferromagnetic transition temperature above 300 K and a pure antiferromagnetic interaction below 100 K. Moreover, it also indicates an anomalous behavior of the temperature dependence of magnetic hyperfine field at La sites measured with 140Ce probe nuclei, which can be ascribed to the contribution of 4f band of Ce to Magnetic Hyperfine Field (Bhf) at low temperatures due to the increase in its localized character.

Preparation and Characterization of non-stoichiometric SrBaFexMo2-xO6 (1.0 ≤ x ≤ 1.5) double perovskite

A series of non-stoichiometric amounts of SrBaFexMo2−xO6 (x = 1.0, 1.2, 1.3, 1.4 and 1.5) (SBFMO) double perovskite samples have been prepared by conventional sol-gel method. The samples were subjected to X-ray diffraction studies and are found to crystallize in cubic structure with a space group . Using X-ray diffraction data, the lattice parameters and the unit cell volume were determined and found to decrease with increase in content of Fe. Porous nature of materials has been studied using Scanning Electron Microscopy technique (SEM). Energy Dispersive X-ray Spectroscopy (EDS) of the present samples showed that Sr, Ba, Fe, Mo and O elements are present in the samples and non-existence of other impurities elements. The absorption bands observed in the range 400-1000 cm−1 of Fourier Transform Infrared (FTIR) spectra indicates the presence of FeO6 and MoO6 octahedra and also confirms the formation of the double perovskite phase.

Magnetic properties and B-site cation ordering in (Sr0.5Ba0.5)2FeSbO6 perovskite

We investigate the dependence of magnetic properties on the B-site cation ordering for a series of (Sr0.5Ba0.5)2FeSbO6 double-perovskite samples for which the Fe/Sb long-range ordering ranges from nearly nonexisting to essentially complete. Also the size of crystallographically ordered domains varies among the samples. All the samples are antiferromagnetic at low temperatures independent of the level of ordering, indicating multiple co-existing exchange and/or superexchange interactions. Nevertheless, a clear trend is seen for the Néel temperature and the effective magnetic moment both increasing with increasing degree of order and the size of the ordered domains.

Determination of Hund's coupling in 5d oxides using resonant inelastic x-ray scattering

We report resonant inelastic X-ray scattering (RIXS) measurements on ordered double perovskite samples containing Re$^{5+}$ and Ir$^{5+}$ with $5d^2$ and $5d^4$ electronic configurations respectively. In particular, the observed RIXS spectra of Ba$_2$YReO$_6$ and Sr$_2$MIrO$_6$ (M=Y, Gd) show sharp intra-t$_{2g}$ transitions, which can be quantitatively understood using a minimal `atomic' Hamiltonian incorporating spin-orbit coupling ($\lambda$) and Hund's coupling ($J_H$). Our analysis yields $\lambda=0.38(2)$eV with $J_H=0.26(2)$eV for Re$^{5+}$, and $\lambda=0.42(2)$eV with $J_H=0.25(4)$eV for Ir$^{5+}$. Our results provide the first sharp estimates for the Hund's coupling in $5d$ oxides, and suggest that it should be treated on equal footing with spin-orbit interaction in multi-orbital $5d$ transition metal compounds.

Evidence of cluster-glass-like state at low temperature in anti-site disordered La1.5Ca0.5CoMnO6 double perovskite

We have investigated structural and magnetic properties having cluster-glass (CG) like ordering at low temperature in a polycrystalline La1.5Ca0.5CoMnO6 double perovskite sample. In this anti-site disordered double perovskite, ferromagnetic (FM) ordering is observed at ∼157 K. From temperature variation linear ac susceptibility measurements, a CG state is clearly observed below a freezing temperature of ∼50.1 K, the origin of which can be best explained through (1) structural disorder and (2) consequence of the coexistence and competition of antiferromagnetic and FM interactions. The linear complex ac susceptibility data near the freezing temperature are best fitted to a critical slowing down model characterized by the dynamic exponents zυ ∼ 4.95(5) and τ0 ∼ 1.23 × 10−7 s. These values are very similar to what is generally observed in a CG system. The glassy transition temperature versus Hdc23 behaviour follows the Almeida-Thouless line. The time dependent slow relaxation dynamics and aging effect have also been investigated to confirm a CG-like state formation in this compound.

Reversible magnetocaloric response in Sr2FeMoO6 double pervoskite

In the present work, polycrystalline Sr2FeMoO6 double perovskite sample has been synthesized by solid state reaction. The magnetization-temperature curve shows the transition temperature at 358K, which is lower than normal; this is attributed to the anti-site defects and oxygen vacancies present in the sample. A low maximum magnetic entropy change−∆SMmax, and the relative cooling power equal to 0.09Jkg−1K−1 and 8Jkg−1 for ΔH=1T, respectively was observed. On the other hand, Arrott plots and mean field theory analysis reveals the second order of ferromagnetic transition at T=TC. The phenomenon of reversible magnetocaloric effect makes this compound interesting for exploration as a future magnetic refrigerant.

Large Room-Temperature Tunneling Magnetoresistance of “Bulrush-Like” Double Perovskite Ba$_{2}$FeMoO$_{6}$

We conducted magnetic and transport measurements on two Ba2 FeMoO6 double perovskite samples prepared by wet chemistry and solid-state reaction methods, respectively. The one synthesized by the latter is in the form of isotropic grains. The bulrush-like Ba2FeMoO6 sample fabricated by wet chemistry is high in aspect ratio and Fe/Mo ordering. Utilizing the high shape anisotropy of the sample, we obtained a material with electric anisotropy where the long axis of the sample is almost parallel to the plane of the pellet. With carrier transport perpendicular to the plane, there was an appreciable increase in room-temperature tunneling magnetoresistance (TMR) response (from 22% to 31% at 10 kOe) while the concentration of Fe/Mo disorder remained unchanged. Experimental results indicate that the TMR value could be greatly enhanced by adjusting the nature and amount of grain boundary (GB) insulating barriers

Cationic Order in Double Perovskite Oxide, Sr2Fe1−x Sc x ReO6 (x = 0.05, 0.1)

We have synthesized by sol-gel method the following polycrystalline double perovskite samples: Sr2Fe1−x ScxReO6 (x = 0, 0.05, 0.1). The results of the Rietveld refinements presented single double perovskite phases with orthorhombic symmetry for the system Sr2Fe1−x ScxReO6, the differences in atomic radii between Fe3+ and Sc3+ cause a lowering in symmetry with respect to the parent Sr2FeReO6 tetragonal compound. The Curie temperatures are found at about 426 and 436 (±5) K for Sr2Fe0.9Sc0.1ReO6 and Sr2Fe0.9Sc0.05ReO6, respectively. The Mossbauer spectra measured at 77 K show complex hyperfine structures resulting from different magnetic contributions at Fe3+ sites; the average hyperfine field is estimated 50 T and the isomer shift at 0.5 mm/s. At room temperature an intermediate valence state for Fe is also observed.

57Fe M�ssbauer spectroscopy investigation of the FeII/III mixed-valence state and the B-site order in double perovskite A2FeMoO6

B-site ordered double-perovskite samples A 2 FeMoO 6 , with A=Ca, Sr or Ba were synthesized in a reducing atmosphere using an encapsulation technique. The samples were characterized by 57 Fe Mössbauer spectroscopy . The valence of Fe was found to be in an intermediate II/III state, being close to the midpoint for A=Sr and slightly shifted towards II for Ba. The degree of B-site ordering was found to be more than 95%. Using various synthesis temperatures the ordering could be tuned. Fe atoms replacing Mo at the antisite and adjacent to it were identified in the Mössbauer data. Presence of antiphase boundaries was also seen.

Formula omitted] Mössbauer spectroscopy investigation of the FeII/III mixed-valence state and the B-site order in double perovskite A2FeMoO6

B-site ordered double-perovskite samples A2FeMoO6, with A=Ca, Sr or Ba were synthesized in a reducing atmosphere using an encapsulation technique. The samples were characterized by 57Fe Mössbauer spectroscopy. The valence of Fe was found to be in an intermediate II/III state, being close to the midpoint for A=Sr and slightly shifted towards II for Ba. The degree of B-site ordering was found to be more than 95%. Using various synthesis temperatures the ordering could be tuned. Fe atoms replacing Mo at the antisite and adjacent to it were identified in the Mössbauer data. Presence of antiphase boundaries was also seen.

Ferromagnetic behaviour in the LaKMnMoO 6 double perovskite

The structural and magnetic properties of a LaKMnMoO 6 double perovskite sample have been investigated. The powder sample has been prepared using a conventional solid state reaction at 1100 °C. Structural studies at room temperature show that our sample crystallizes in an orthorhombic structure with the P222 space group. Measurements of the magnetization versus temperature on the LaKMnMoO 6 sample show a magnetic transition from paramagnetic to ferromagnetic state with decreasing temperature. The Curie temperature T C is found to be 180 K. The critical exponent γ, defined by M sp= M sp (0) (1− T/ T C) γ and deduced from field dependent magnetization measurements at several temperatures below T C, is found to be 0.33. The moment per Mn ion, deduced from the Curie constant, C, is found to be 5.68 μ B which is slightly smaller than the Mn 2+ moment equal to 5.90 μ B.

Magnetoresistance effect in the fluctuating-valence BaSmFe2O5+w system

The occurrence of negative magnetoresistance (MR) in semiconductive BaSmFe2O5+w double-perovskite samples is demonstrated. A peak in the MR value was observed at the Verwey–type transition temperature. The transition signifies the charge separation of the Fe2.5+ fluctuating mixed valence state into high-spin Fe2+ and Fe3+. The samples were ferrimagnetic with a Curie temperature of ∼710 K. Upon oxidizing/reducing the samples the size of the MR peak and the temperature at which the peak occurred varied. The largest MR value observed was 1.4% at 7 T.

Iron mixed-valence compounds, BaSm(Cu 0.5+ xFe 0.5− x) 2O 5+ δ: I: Synthesis and chemical characterization

Oxygen-deficient BaSm(Cu 0.5+ x Fe 0.5− x ) 2O 5+ δ ( x=−0.5, −0.45, −0.4, −0.1 and 0.0) double-perovskite samples have been successfully obtained by means of a novel synthesis technique utilizing an Fe/FeO getter encapsulated together with the starting material mixture in an evacuated silica-glass tube. The reducing conditions during the synthesis correspond to an oxygen partial pressure of ∼7.6×10 −16 atm. From X-ray diffraction (XRD), transmission electron micrographs (TEM) and energy-dispersive X-ray (EDX) analyses, the samples have been confirmed to be of essentially single phase. By post-annealing the as-synthesized samples under O 2 or H 2/Ar mixed gas, the amount of excess oxygen was controlled. Approaching δ≈0 required prolonged annealing periods under reducing conditions. Samples having −0.4< x<−0.1 did not form as a single-phase material under the present synthesis conditions.

Iron mixed-valence compounds, BaSm(Cu 0.5+ xFe 0.5− x) 2O 5+ δ: II: The Fe 2.5+ state and magnetic properties

Recently, a fluctuating mixed-valence state formally denoted as Fe 2.5+ was observed in BaSmFe 2O 5+ δ double-perovskite samples, having δ<0.5 [J. Lindén, P. Karen, A. Kjekshus, J. Miettinen, T. Pietari, M. Karppinen, Phys. Rev. B 60 (1999) 15251.]. Upon cooling such samples to the Verwey-type transition temperature ( T V≈200 K), a mixing-separation transition of the valence state occurs. In order to study in detail the formation of the mixed-valence state BaSm(Cu 0.5+ x Fe 0.5− x ) 2O 5+ δ , samples with a certain amount of Fe (0–10% and 40–50%) replaced by Cu have been characterized by 57Fe Mössbauer spectroscopy and magnetization measurements. All samples were found to be magnetically ordered at room temperature. The amount of Fe entering the Fe 2.5+ state decreased rapidly as the Cu doping level was increased, as judged by the Mössbauer spectra. Increasing the Cu concentration also led to an increase of T V.

Magnetic properties, oxygen content and metal valences in BaRE(Cu 0.5Fe 0.5) 2O 5+ δ with RE=Lu, Yb, Y, Eu, Sm, Nd and Pr

Oxygen-deficient double-perovskite samples of BaRE(Cu 0.5Fe 0.5) 2O 5+ δ (RE=Lu, Yb, Y, Eu, Sm, Nd and Pr; 0≤ δ≤0.45) have been characterized for their magnetic properties. For the samples with RE=Lu–Sm, the oxygen content could be raised above δ≈0 only by means of high-pressure heat treatment. In the case of the larger rare earth elements, i.e. Nd and Pr, excess oxygen was easily introduced under normal pressure as well. The (Cu 0.5Fe 0.5)O 2 planes were found to exhibit antiferromagnetism at or below room temperature and weak spontaneous magnetization below ∼80 K. The Néel temperature decreases with increasing amount of excess oxygen. Based on the charge-neutrality principle and the 57Fe Mössbauer data, the average valences of Fe and Cu were deduced. The ratio of six-coordinated Fe 3+ to Fe 5+ depends on both the choice of RE and the sample preparation process. The excess oxygen introduced into the RE layer upon the high-pressure heat treatment increases not only the valence of Fe but also the valence of Cu. The behavior of an observed susceptibility anomaly around 80 K was studied upon varying δ.