Negative dielectric constant (NDC) materials have the potential for unique optical and microwave applications and their use can greatly reduce the complexity of device architecture. The observation of temperature‐dependent NDC above 503 K in the perovskite PrMnO3 is reported which obeys the classical Drude theory. This is confirmed through impedance spectroscopy, and the principle behind the associated negative dielectric loss is explained using the Axelrod mechanism.

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Polycrystalline insulating ferromagnetic double perovskite La2CoMnO6 possessing a monoclinic structure and a high Curie temperature (TC = 222 K) was rapidly synthesized (∼30 min) by irradiating a stoichiometric mixture of oxides with microwaves. The sample exhibits negative magnetostriction (λpar), i.e., contraction of length along the magnetic field direction in the ferromagnetic state. At 10 K, λpar does not show saturation up to a magnetic field of 50 kOe, where it reaches 610 × 10−6, which is one of the highest values of magnetostriction found so far among perovskite oxides with 3d ions. The magnitude of λpar decreases monotonically as the temperature increases and becomes negligible above TC. The giant magnetostriction in this double perovskite is suggested to originate from large spin–orbit coupling associated with Co2+ (d7) cation. The obtained magnetostriction value is comparable to λpar = 630 × 10−6 in an identical composition obtained through solid-state reaction over several days in a conventional furnace, which indicates the advantages of microwave-assisted synthesis in saving reaction time and electric power without deteriorating physical properties.

A single phase polycrystalline La0.5Ba0.5CoO3- d sample possessing cubic structure (space group Pm[Formula: see text]m) was synthesized by microwave irradiation within 20 minutes of processing time and its structural, magnetic, electrical, and magnetostrictive properties were investigated. While the temperature dependence of field-cooled magnetization ( M) in a field of H = 0.5 kOe indicates the onset of ferromagnetic transition at T C = 177 K, irreversibility between the zero field-cooled and field cooled M( T) persists even at H = 3 kOe. M( H) at 10 K does not saturate at the maximum available field and has a much smaller value (0.83  μB/Co in a field of 50 kOe) than 1.9  μB/Co expected for spin only contribution from intermediate Co3+ and Co4+ spins. Resistivity shows insulating behavior down to 10 K and only a small magnetoresistance (∼ -2% for H = 70 kOe) occurs around T C. All these results suggest a magnetically heterogeneous ground state with weakly interacting ferromagnetic clusters coexisting with a non-ferromagnetic phase. The length of the sample expands in the direction of the applied magnetic field (positive magnetostriction) and does not show saturation even at 50 kOe. The magnetostriction has a maximum value ( λ par = 265 x 10-6) at 10 K and it decreases with increasing temperature. The smaller value of λ par compared to the available data on La0.5Sr0.5CoO3 ( λ par = 900 x 10-6) suggests that the non-ferromagnetic matrix is most likely antiferromagnetic and it restrains the field-induced expansion of ferromagnetic clusters in the microwave synthesized La0.5Ba0.5CoO3- d sample.