† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant No. U1504107) and the Doctoral Scientific Research Foundation (Grant No. qd15214).
Both the band filling effect and Fe/Mo disorder have a close correlation with the physical properties of the double perovskite Ca2FeMoO6. Two series of Ca2FeMoO6 and Nd0.3Ca1.7FeMoO6 ceramics sintered at (1050 °C, 1200 °C, and 1300 °C) were specially designed to comparatively investigate the band-filling effect and Fe/Mo disorder on the physical properties of Ca2FeMoO6. The x-ray diffraction indicates that Fe/Mo disorder is sensitive to the sintering temperature. The magnetization behavior is mainly controlled by the Fe/Mo disorder not by the band filling effect, manifested by a close correlation of saturated magnetization (Ms) with the Fe/Mo disorder. Interestingly, magnetoresistance (MR) property of the same composition is dominantly contributed by the grain boundary strength, which can be expressed by the macroscopic resistivity values. However, the band filling effect caused by the Nd-substitution can decrease the spin polarization, and thus suppress the MR performance fundamentally. Contrary to the MR response, the Curie temperature (TC) shows an obvious optimization due to the band filling effect, which increases the carrier density near the Fermi level responsible for the ferromagnetic coupling interaction strengthen. Maybe, our work can provoke further research interests into the correlation of the band-filling effects and Fe/Mo disorder with the physical properties of other Fe/Mo-based double perovskites.
As a representative of the strongly correlated systems, double perovskite Sr2FeMoO6 (SFMO) has been paid a great deal of attention. Because it has a considerable high low-field magnetoresistance (LFMR) behavior, a half-metal nature with 100% spin-polarization, and a high TC well above room temperature.[1–4] Those functional properties will make FeMo-based double perovskites systems (A2FeMoO6, A: Ca/Sr/Ba) become promising candidates from both fundamental investigations and potential technological applications in spintronics and magnetic storage devices operated at room temperature.[1,2,5]
In an ideal A2FeMoO6 (A: Ca/Sr/Ba) structure, the electron configuration can be originally addressed as the localized spin-up electrons of the 3d5 (Fe3+) and the itinerant spin-down electron of the 4d1 (Mo5+) shared by the Fe3+(t2g ↓ )−O(2p)−Mo5+(t2g ↓) subband.[5,6] Localized five electrons of Fe3+ (3d5, S = 5/2) antiferromagnetically couple with one electron contributed by Mo5+ (4d1, S = 1/2), resulting in a net magnetization of 4μB/f.u.[7,8] However, a much lower magnetization is commonly reported in the experiments since the inevitable Fe/Mo anti-site defects (ASD) occurs, i.e., Fe occupies Mo positions wrongly and vice versa.[7,9–11] ASD has a close correlation with magnetization. Additionally, theoretical works predicate that ASD significantly influences the electron spin polarization near Fermi level and even can destroy the half-metal property when exceeding a certain concentration.[12,13] Hence, controlling ASD content is crucial for investigating the physical property in FeMo-based double perovskites. The itinerant electron at conduction band is suggested to mediate the ferromagnetic interaction between neighbor Fe cations by a double-exchange-like mechanism.[14–16] Indeed, substituting the divalent A2+ ions in A2FeMoO6 (A: Sr2+/Ba2+) with the trivalent ions (such as La3+, Nd3+), a significantly improved TC can be observed due to the increased electron density.[6,17–20] Although these observations did disclose a positive correlation between ferromagnetic couplings strength and the carrier density near Fermi level, it cannot show the unique contribution of the band filling effect. In those electron-doped systems, the ionic sizes of the Sr2+/Ba2+ (1.26/1.42 Å) strongly differ from the substituting cations La3+/Nd3+ (1.16/1.109 Å), electron dopings not only provide the carrier into the conduction band but also trigger a rotation of (Fe, Mo)O6 octahedra.[6,17–19] The rotation will change the Fe–O–Mo band angle, even reduce the crystal symmetry and consequently modify the carrier density at the Fermi level.[6,17–19] In order to provide a relatively clean research environment, we select Nd0.3Ca1.7FeMoO6 as the research object because of the well-matched ionic sizes of Nd3+ (1.109 Å) and Ca2+ (1.12 Å), which can avoid the steric effects effect as far as possible. Through the above analysis, both the band filling effect caused by the electron doping and ASD significantly function on the physical properties of the FeMo-based double perovskites. Therefore, it is interesting to comparatively investigate two effects on physical properties.
In this work, three groups of single-phased pristine Ca2FeMoO6 (CFMO) and Nd0.3Ca1.7FeMoO6 (NCFMO) ceramics were prepared at various sintering temperature (1050 °C, 1200 °C, and 1300 °C). The corresponding crystal structure, magnetization, resistivity, magnetoresistance, and the TC were systematically and comparatively investigated.
For clarity and simplicity, the CFMO ceramics were synthesized at various sintering temperature (1050 °C, 1200 °C, 1300 °C), which were labeled as C1, C3, C5. At the same sintering temperature, the NCFMO ceramics were also prepared, which were denoted as C2, C4, C6, respectively. Each group of (C1, C2), (C3, C4), (C5, C6) has the same sintering temperature. All the above-mentioned ceramics were prepared via the solid-state reaction process. Frist, the dried raw materials of Nd2O3, CaCO3, Fe2O3, and MoO3 (≥ 99.5%) were weighted, mixed homogeneously by balling milling in ethanol for 24 h, and the slurry was dried. The dried powders were calcined at 800 °C in air for 10 h. Then, the calcined mixtures were ball milled in ethanol for another 24 h, dried and pressed into small thin disks (10 mm × 1 mm). Finally, all the disks were sintered at three different temperatures (1050 °C, 1200 °C, 1300 °C) for 8 h in a reducing atmosphere of a mixed gas of 10% H2/90% Ar.
The structures of the samples were confirmed by x-ray diffraction (XRD) patterns (XRD, Bruker D8 Discover). The microstructures were investigated through a field emission scanning electron microscope (FESEM, Zeiss SUPRA 40). The magnetic and transport data were obtained through a superconducting vibrating sample magnetometer (VSM) and a physical property measurements system (PPMS Quantum Design, 2001NUGC).
The normalized XRD patterns of all samples are shown in Fig.
The M–H curves of C1–C6 ceramics measured at 50 K are present in Figs.
Generally, prepared process drastically affect the macroscopic structures of the ceramics, such as grain sizes, grain density, the connectivity between grains, grain boundary numbers, grain boundary strength and so on, which can strongly influence on the electrical transport, magnetoresistance, chemical states and other physical properties. Hence, the surface structures of C1–C6 are analyzed by SEM images as shown in Figs.
The temperature-dependent resistivity (ρ–T) without and with a magnetic field of 2 T, and the temperature-dependent magnetoresistance (MR-T) curves for all the ceramics are shown in Figs.
The temperature-dependent MR behaviors under a magnetic field of 2 T have been investigated in Fig.
The field-cooling (200 Oe, 1 Oe = 79.5775 A·m−1) magnetization-temperature (M–T) curves of C1–C6 are shown in Figs.
Two series of CFMO (C1, C3, C5) and NCFMO (C2, C4, C6) ceramics sintered at (1050 °C, 1200 °C, 1300 °C) have been specially designed to comparatively investigate the band-filling effect and Fe/Mo ASD on the physical properties of CFMO. The x-ray diffraction indicates that Fe/Mo ASD can be effectively tuned by controlling the sintering temperature. A consistent varied trend of the Ms with the Fe/Mo ASD manifests that the magnetization behavior is mainly controlled by the Fe/Mo ASD not by the band filling effect. Interestingly, magnetoresistance (MR) property of a given composition is dominantly contributed by the grain boundary strength, which can be expressed by the macroscopic resistivity values. However, the band filling effect caused by the Nd-substitution can decrease the spin polarization, and thus suppress MR performance fundamentally. Compared to the maternal CFMO ceramics, a considerable improved TC can be observed in NCFMO ceramics since the band filling effect can increase the carrier density near Fermi level, which is responsible for the ferromagnetic coupling interaction strength. Maybe, our work can provoke further investigations interest into the correlation of the band-filling effects and Fe/Mo disorder with the physical properties of other Fe/Mo-based double perovskites.
[1] | |
[2] | |
[3] | |
[4] | |
[5] | |
[6] | |
[7] | |
[8] | |
[9] | |
[10] | |
[11] | |
[12] | |
[13] | |
[14] | |
[15] | |
[16] | |
[17] | |
[18] | |
[19] | |
[20] | |
[21] | |
[22] | |
[23] | |
[24] | |
[25] | |
[26] | |
[27] | |
[28] | |
[29] | |
[30] | |
[31] | |
[32] | |
[33] | |
[34] | |
[35] | |
[36] |