K. Devi Chandrasekhar

Spin glass behaviour and extrinsic origin of magnetodielectric effect in non-multiferroic La2NiMnO6 nanoparticles

We report magnetization, dielectric and dc transport properties of La(2)NiMnO(6) nanoparticles. Both dc and ac magnetization measurements indicated a metastable magnetic behaviour with random ferromagnetic and antiferromagnetic interactions below 110 K; critical slow-down, memory and rejuvenation properties signify the spin glass nature. The dc resistivity shows a semiconducting nature but the temperature dependent magnetoresistance (MR) shows a peak at the spin glass transition. The colossal dielectric property and its frequency dependence were interpreted using the Maxwell-Wagner (MW) interfacial polarization model. Impedance analysis along with magnetodielectric (MD) and magnetoresistance (MR) indicates that the observed MD originates from the combined effect of MR and MW interfacial polarization.

Giant magnetocaloric effect in Gd2NiMnO6 and Gd2CoMnO6 ferromagnetic insulators

We have investigated magnetocaloric effect in double perovskite Gd2NiMnO6 (GNMO) and Gd2CoMnO6 (GCMO) samples by magnetic and heat capacity measurements. Ferromagnetic ordering is observed at ~130 K (~112 K) in GNMO (GCMO), while the Gd exchange interactions seem to dominate for T < 20 K. In GCMO, below 50 K, an antiferromagnetic behaviour due to 3d-4f exchnage interaction is observed. A maximum entropy (-{\Delta}SM) and adiabatic temperature change of ~35.5 J Kg-1 K-1 (~24 J Kg-1 K-1) and 10.5 K (6.5 K) is observed in GNMO (GCMO) for a magnetic field change of 7 T at low temperatures. Absence of magnetic and thermal hysteresis and their insulating nature make them promising for low temperature magnetic refrigeration.

Investigation of the intrinsic magnetodielectric effect in La2CoMnO6: role of magnetic disorder

We present a large magnetodielectric (MD) effect of 65 % at 100 kHz with 5 T field in B-site ordered La2CoMnO6 (LCMO) polycrystalline sample. Frequency and temperature dependent impedance and dielectric studies under magnetic field divulge both intrinsic and extrinsic origins for the observed MD effect. The temperature dependent Raman spectroscopy measurement has shown spin-lattice coupling that supports the intrinsic origin of the observed large MD response in LCMO. Extrinsic contributions to MD response mainly originate from disorder and interface effects; here, we signify this by hole carrier (Sr) doping at the A-site of the ordered LCMO sample. The comparison study has disclosed that with the disorder, the intrinsic polarization due to asymmetric hopping decreases significantly, and the disorder induced transport dominates in both MD and magnetoresistance behaviour with close resemblance.

Large magnetodielectric response in Pr0.6Ca0.4MnO3/polyvinylidene fluoride nanocomposites

We have studied the magnetic field effect on low frequency dielectric properties of Pr0.6Ca0.4MnO3/polyvinylidene fluoride nanocomposite with 22.5% volume fraction of Pr0.6Ca0.4MnO3 nanoparticles. A strong magnetodielectric response was observed below 120 K where Pr0.6Ca0.4MnO3 nanoparticles show the magnetic phase transition indicating a direct correlation between magnetism and dielectric properties. A large change in the dielectric permittivity ∼30% has been observed in a magnetic field of 4.6 T with loss as low as 0.17 at 70 K. The observed magnetodielectric response has been attributed to the decrement of polaron activation barrier of Pr0.6Ca0.4MnO3 nanoparticles with the increase in magnetic field.

High dielectric permittivity in semiconducting Pr0.6Ca0.4MnO3 filled polyvinylidene fluoride nanocomposites with low percolation threshold

Semiconducting Pr0.6Ca0.4MnO3 nanoparticles with width of ∼50nm and length ∼80–150nm were prepared by sol-gel method. The dielectric behavior of Pr0.6Ca0.4MnO3, polyvinylidene fluoride composites was investigated by varying the Pr0.6Ca0.4MnO3 volume fraction and frequency. The percolation threshold of the composites was found to be 0.1 and an enhanced effective dielectric permittivity of ∼700 with low dielectric loss of ∼0.3 at 103Hz near the percolation threshold was observed. The low percolation threshold was attributed to the nanosize and nonspherical shape of the filler particles, and the enhanced dielectric properties have been explained by using boundary layer capacitance effect and percolation theory.

Magnetic field induced dielectric relaxation in the strain glass state of Pr0.6Ca0.4MnO3

We present the dielectric and magnetodielectric properties of Pr0.6Ca0.4MnO3 polycrystalline sample. Dielectric permittivity (e′) (and de′/dT) portrays the charge order and other magnetic transitions observed in the magnetization measurement. Dielectric study has revealed a relaxation corresponding to ordering of polarons at ∼60 K that follows Arrhenius behavior both in the presence and absence of magnetic field, and another relaxation was noticed at ∼30 K only under a critical magnetic field (3.2 T) that shows critical slow down of electronic charges obeying power law. Further, the magnetic field induced relaxation shifts to low temperatures with the increase of magnetic field. The observed field induced dielectric relaxation below the reentered charge ordered state is linked with the rapid motions of boundaries of the coexisting phases towards the martensite phase transformation.

Effects of Jahn–Teller distortion on the skyrmion stability of (Cu1−xNix)2OSeO3

We explore the Jahn–Teller distortion of Ni2+ ions on the skyrmion phase of Cu2OSeO3. Polycrystalline (Cu1−xNix)2OSeO3 (0 ≤ x ≤ 0.06) samples were synthesized by solid-state reaction. X-ray diffraction, X-ray absorption near-edge spectroscopy, extended X-ray absorption fine structure spectroscopy, magnetic and magnetodielectric measurements were performed and comprehensively analyzed. These revealed that the Ni2+ ion occupies both the Cu sites almost equally, and the Jahn–Teller distortion of the Ni2+ ion invokes a change in the local trigonal bipyramidal structure, which has a significant influence on the H–T magnetic phase diagram and electrical polarization of (Cu1−xNix)2OSeO3. A large enhancement of the skyrmion area upon Ni-doping can be ascribed to the magnetic anisotropy of the Jahn–Teller active Ni2+ ion in the Cu(I) site. The observed results indicate that chemical doping is a unique way to control and manipulate skyrmion boundaries in Cu2OSeO3.

Synthesis And Magnetic Properties Of La2NiMnO6 Nanoparticles

La 2 NiMnO 6 nanoparticles with varying particle size from 27 to 96 nm were synthesis by the sol gel method. Structural characterization was carried out using the x‐ray diffraction (XRD) and high‐resolution transmission electron microscopy (HRTEM). Magnetization measurements showed predominantly two magnetic transitions that vary with the particle size. The variation of ferromagnetic transition temperatures, large suppression of saturation magnetic moment and the monotonically increase of the saturation magnetization ( M s ) with particle size were attributed to the intrinsic antisite defects and the surface properties of the nanoparticles.