Structural and physical properties of Sr‐based 3d–5d double perovskites of Sr₂Fe_0.5Hf_1.5O_6−δ oxides

Abstract

Sr‐based 3d–5d double perovskite Sr2Fe0.5Hf1.5O6−δ (SFHO) oxide powders were synthesized via the solid‐state reaction method, and their structural, dielectric, magnetic, optical, and transport properties were investigated. Structural investigations revealed the SFHO powders crystallized in an orthorhombic crystal structure with Pnma space group. Scanning electron microscopy images demonstrated the relatively homogeneous distributions of spherical SFHO powders, and x‐ray energy dispersive spectra gave out the molar ratio of Sr:Fe:Hf equal to 1.94:0.5:1.87, close to the nominal value. XPS spectra confirmed the existence of Sr2+, Fe3+, Hf4+ ions in the SFHO powders, and oxygen appeared in the forms of lattice oxygen and adsorbed oxygen species, respectively. The SFHO ceramics exhibited a relaxor‐like dielectric behavior. Two dielectric abnormities appeared around 518 and 257 K, which were ascribed to the dielectric response of the oxygen vacancies ( activation energy Ea = 0.93 eV and thermal excitation of the electrons with Ea = ∼ 0.02 eV bound weakly to the, respectively. Ferromagnetic behavior was observed in the SFHO powders at 300 and 2 K, and the saturated magnetization at 2 K was 0.67 μB/f.u. Magnetic transition temperature, TC was determined to be 638 K from the temperature dependence of the magnetization under the zero‐field cooling mode. A spin glass‐like behavior was observed around 390 K, which was confirmed by the frequency‐dependent alternating current (ac) susceptibility measurements. The SFHO powders exhibit a butterfly‐like magnetoresistance (MR)–H plot at 2 K, and the MR (2 K, 7 T) is found to be −2.73% owing to the intergranular magneto‐tunneling effect. Temperature dependence of the resistivity of the SFHO ceramics displayed a semiconducting behavior, and the electrical transport properties of the SFHO ceramics were investigated by Mott's variable‐range‐hopping model, thermally activated semiconductor conductivity model, and the adiabatic small polarons hopping model, respectively. UV–vis diffuse‐reflectance spectra of the SFHO powders demonstrated a direct optical bandgap of Eg = 2.62 eV, which was contributed from the electron transfer from the O(2p) orbital to Fe(3d)/Hf(5d) orbitals. A combination of high‐temperature ferrimagnetism and semiconducting nature of the SHFO powders makes them attractive for spintronics and photovoltaics.

Microstructural Characterization and Magnetic, Dielectric, and Transport Properties of Hydrothermal La2FeCrO6 Double Perovskites

“…Negative MR behavior was observed at 5 K and 300 K. It is noticed that in Figure 7e, the MR-H plot at 5 K exhibits a typical butterfly-like shape, and the MR (5 K, 6 T) is measured to be −4.07%. Such MR value is higher than that reported for the semiconducting DP oxides such as Sr 2 CrReO 6 ceramics with MR (4.2 K, 7 T) = −3.0% [60], Sr 2 CrHfO 6 ceramics with MR (2 K, 7 T) = −2.73% [61], and Sr 2 Fe 0.5 Hf 1.5 O 6−δ ceramics with MR (2 K, 7 T) = −2.05% [62]. However, the butterflylike MR-H curve disappeared at 300 K in Figure 7f, which was attributed to the absence of M-H hysteresis during the magnetic domain rotation under the excitation of the magnetic field.…”

Structural, dielectric, magnetic, optical, and electrical properties of double‐perovskite Sr1.5Mn0.5Fe0.5Hf1.5O6 oxides

“…@ 2 K) and wide optical bandgap (E g = 3.56 eV), and the SMHFO ceramics also exhibit butterfly-like magnetoresistance (MR)-H curve at 2 K with MR (2 K, 7 T) of −3.86%. Such MR value is higher than that reported for the semiconducting DP oxides such as Sr 2 CrReO 6 ceramics (MR (4.2 K, 7 T) = −3.0%), 21 Sr 2 CrHfO 6 ceramics ((2 K, 7 T) = −2.73%), 22 and Sr 2 Fe 0.5 Hf 1.5 O 6−δ ((2 K, 7 T) = −2.05%) ceramics. 23 The current work opens the way for the SMHFO oxides to find promising applications in RT spintronic devices.…”

“…where H peak is the magnetic field at which the resistivity ρ reaches the maximum value. It is noticed that the MR-H plot at 2 K exhibits a typical butterfly-like form, and the MR (2 K, 7 T) is determined to be −3.86%, which is slightly higher than that reported for the Sr 2 CrReO 6 ceramics (MR (4.2 K, 7 T) ∼3.0%), 21 Sr 2 CrHfO 6 ceramics ((2 K, 7 T) = −2.73%), 22 and Sr 2 Fe 0.5 Hf 1.5 O 6−δ ((2 K, 7 T) = −2.05%) ceramics. 23 Similarly, the MR-H plot at 300 K also displays a butterfly-like form, and the MR (300 K, 7 T) is −2.15%.…”

“…The activation energy of the weakly bound to the oxygen vacancy in the Sr 2 Fe 0.5 Hf 1.5 O 6−δ ceramics was reported to be about 0.02 eV. 22 As the frequency of the applied electric fields increases, the jumping frequency of electrons trapped in the shallower level created by oxygen vacancy fail to catch up with the field, and thus, their contributions to the dielectric constants and dielectric loss will drop off, as shown in Figure 3A. Figure 4A shows the variation of tan δ with respect to the temperature measured at a series of frequencies.…”