J. Krishna Murthy

Giant zero field cooled spontaneous exchange bias effect in phase separated La1.5Sr0.5CoMnO6

We report a giant zero field cooled exchange bias (ZEB) effect (~0.65 T) in La1.5Sr0.5CoMnO6 sample. Magnetic study has revealed a reentrant spin glass ~90 K, phase separation to spin glass and ferromagnetic phases below 50 K and canted antiferromagnetic transition ~10 K. A small conventional exchange bias (CEB) is established with the advent of spontaneous phase separation down to 10 K. Giant ZEB and enhanced CEB effects are found only below 10 K and are attributed to the large unidirectional anisotropy at the interface of isothermally field induced ferromagnetic phase and canted antiferromagnetic background.

Magnetodielectric behavior in La2CoMnO6 nanoparticles

We have investigated magnetic, dielectric and magnetodielectric properties of La2CoMnO6 nanoparticles prepared by sol-gel method. Magnetization measurements revealed two distinct ferromagnetic transitions at 218 K and 135 K that can be assigned to ordered and disordered magnetic phases of the La2CoMnO6 nanoparticles. Two dielectric relaxations culminating around the magnetic transitions were observed with a maximum magnetodielectric response reaching 10% and 8% at the respective relaxation peaks measured at 100 kHz under 5 T magnetic field. The dc electrical resistivity followed an insulating behavior and showed a negative magnetoresistance; there was no noticeable anomaly in resistivity or magnetoresistance near the magnetic ordering temperatures. Complex impedance analysis revealed a clear intrinsic contribution to the magnetodielectric response; however, extrinsic contribution due to Maxwell-Wagner effect combined with magnetoresistance property dominated the magnetodielectric effect at high temperatures.

Multicaloric effect in multiferroic Y2CoMnO6

We have investigated multiple caloric effects in multiferroic Y2CoMnO6. The polycrystalline sample prepared by the solid state method has shown a ferromagnetic Curie temperature ~75 K with a second order phase transition; a maximum magneto entropy change of ~ 7.3 J kg−1 K−1 with a reasonable relative cooling power ~220 J kg−1 is found without thermal and magnetic hysteresis loss. An electric field driven entropy change of ~ 0.26 J m−3 K−1 was obtained using the Maxwells relation and estimated magnetically induced a total temperature change of ~5.45 K around the Curie temperature that confirms the multicaloric effect in the sample.

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.

Antisite disorder driven spontaneous exchange bias effect in La2-xSrxCoMnO6 (0<x<1)

Doping at the rare-earth site by divalent alkaline-earth ions in perovskite lattice has witnessed a variety of magnetic and electronic orders with spatially correlated charge, spin and orbital degrees of freedom. Here, we report an antisite disorder driven spontaneous exchange bias effect as a result of hole carrier (Sr(2+)) doping in La(2-x)Sr(x)CoMnO6 (0  <  x  <  1) double perovskites. X-ray diffraction and Raman spectroscopy have evidenced an increase in disorder with the increase of Sr content up to x  =  0.5 and thereby a decrease from x  =  0.5 to 1. X-ray absorption spectroscopy has revealed that only Co is present in the mixed valence of Co(2+) and Co(3+) states with Sr doping to compensate the charge neutrality. Magnetotransport is strongly correlated with the increase of antisite disorder. The antisite disorder at the B-site interrupts the long-range ferromagnetic order by introducing various magnetic interactions and instigates reentrant glassy dynamics, phase separation and canted type antiferromagnetic behavior with the decrease of temperature. This leads to a novel magnetic microstructure with unidirectional anisotropy that causes a spontaneous exchange bias effect that can be tuned with the amount of antisite disorder.

Metamagnetic behaviour and effect of field cooling on sharp magnetization jumps in multiferroic Y2CoMnO6

We present sharp magnetization jumps and field-induced irreversibility in magneti- zation in multiferroic Y2CoMnO6. The appearance of magnetic relaxation and field sweep rate dependence of magnetization jumps resembles the martensite-like scenario and suggests the coexis- tence of E*-type antiferromagnetic and ferromagnetic phases at low temperatures. In Y2CoMnO6, the critical field required for the sharp jump can be increased or decreased depending on the mag- nitude and direction of the cooling field; this is remarkably different from manganites or other metamagnetic materials where the critical field increases irrespective of the direction of the applied field cooling. The cooling field dependence on the sharp magnetization jumps has been described by considering exchange pinning mechanism at the interface, like in the exchange bias model. Copyright c EPLA, 2014

Reentrant cluster glass behavior in La2CoMnO6 nanoparticles

Magnetic study on La2CoMnO6 nanoparticles revealed multiple magnetic transitions at 218 K, 135 K and below 38 K and the nature of the low temperature transition was unclear [J Appl Phys 111, 024102 2012]. Presence of mixed valance states of Co and Mn has been confirmed from the XPS measurement and its presence along with antisite disorder affects in reducing the saturation magnetization of the nanoparticles. The zero field cooled and field cooled bifurcation in dc magnetization, relaxation in zero field cooled magnetization and large enhancement in coercive field below the glassy temperature has been discussed. Frequency dependence of ac susceptibility using power law has revealed cluster glass behavior. Further, the dc field superimposed on ac susceptibility and absence of memory effect in ac susceptibility has suggested the existence of non interacting clusters comprising of competing interactions below 38 K. Competing magnetic interactions due to the presence of mixed valances and antisite disorder found to establish a reentered cluster glassy state in the nanoparticles.

Magnetic ordering and conduction mechanism of different electroactive regions in Lu2NiMnO6

The magnetodielectric response of a double perovskite oxide Lu2NiMnO6 (LNMO) synthesised by the sol–gel process has been investigated. The Rietveld refinement of the X-ray diffraction data indicates that the room-temperature crystal structure of LNMO is monoclinic with the space group P21/n, which contains an ordered array of alternate MnO6 and NiO6 octahedra. X-ray photoelectron spectroscopy studies confirm the mix valence state of Mn (4+ and 3+) and Ni (2+ and 3+). The optical band gap (Eg = 1.56 eV) obtained from the UV-Visible absorption spectrum suggests that LNMO is a semiconductor. The field cooled and zero-field cooled measurements show the ferromagnetic behaviour of the sample with the transition temperature (Tc) = 45 K and a saturation magnetization of 5.2 μB/f.u. is observed at 2.5 K. The temperature and frequency dependent dielectric measurements reveal colossal values of the dielectric constant, which are interpreted by the Maxwell-Wagner interfacial polarization. The temperature dependence o...

Strain induced ferromagnetism and large magnetoresistance of epitaxial La1.5Sr0.5CoMnO6 thin films

In this study, the structural, magnetic, and magneto-transport properties of La1.5Sr0.5CoMnO6 (LSCMO) thin films deposited on a SrTiO3 (001) substrate were investigated. A normal θ/2θ x-ray diffraction, rocking curve, ϕ-scan, and reciprocal space mapping data showed that prepared LSCMO thin films are single phase and highly strained with epitaxial nature. Temperature vs. magnetization of LSCMO films exhibits strain-induced ferromagnetic ordering with TC ∼ 165 K. In contrast to the bulk samples, there was no exchange bias and canted type antiferromagnetic and spin glass behavior in films having thickness (t) ≤ 26 nm. Temperature dependent resistivity data were explained using Schnakenbergs model and the polaron hopping conduction process. The slope change in resistivity and magnetoresistance maximum (∼65%) around TC indicates the existence of a weak double exchange mechanism between the mixed valence states of transition metal ions. Suppression of spin dependent scattering with the magnetic field is attri...