R.C. Sahoo

Giant spontaneous exchange bias effect in Sm1.5Ca0.5CoMnO6 perovskite

We report here on the enormously large value of the spontaneous exchange bias (SEB) of ~5.1 kOe in the Sm1.5Ca0.5CoMnO6 bulk double perovskite system after zero-field cooling from an unmagnetized state. Depending on the path used for measuring the magnetic hysteresis loop, P-type or N-type, the EB field varies significantly. Dc and ac magnetization reveals a super-spin-glass-like (SSG) state below 19 K. The EB effect has been observed below the glassy transition temperature. The conventional exchange bias (CEB) as well as the SEB increase with a decrease in temperature and show monotonic variation. All these interesting features can be explained through the multimagnetic phase of double perovskite and the onset of unidirectional anisotropy driven by interface exchange coupling between the SSG, turned isothermally into a field-induced superferromagnetic shell and antiferromagnetic core.

Evidence of cluster-glass-like state at low temperature in anti-site disordered La1.5Ca0.5CoMnO6 double perovskite

We have investigated structural and magnetic properties having cluster-glass (CG) like ordering at low temperature in a polycrystalline La1.5Ca0.5CoMnO6 double perovskite sample. In this anti-site disordered double perovskite, ferromagnetic (FM) ordering is observed at ∼157u2009K. From temperature variation linear ac susceptibility measurements, a CG state is clearly observed below a freezing temperature of ∼50.1u2009K, the origin of which can be best explained through (1) structural disorder and (2) consequence of the coexistence and competition of antiferromagnetic and FM interactions. The linear complex ac susceptibility data near the freezing temperature are best fitted to a critical slowing down model characterized by the dynamic exponents zυu2009∼u20094.95(5) and τ0u2009∼u20091.23 × 10−7u2009s. These values are very similar to what is generally observed in a CG system. The glassy transition temperature versus Hdc23 behaviour follows the Almeida-Thouless line. The time dependent slow relaxation dynamics and aging effect have al...

Strain modulated large magnetocaloric effect in Sm0.55Sr0.45MnO3 epitaxial films

Epitaxial Sm0.55Sr0.45MnO3 thin films were deposited on LAO (001), LSAT (001), and STO (001) single crystalline substrates by pulsed laser deposition technique to investigate the correlation between the substrate induced film lattice strain and magnetocaloric effect (MCE). The film on LAO substrate (S_LAO), which is under compressive strain, undergoes ferromagnetic → paramagnetic transition at TCu2009∼u2009165u2009K. The films on STO (S_STO) and LSAT (S_LSAT) substrates are under tensile strain and have TCu2009∼u2009120u2009K and 130u2009K, respectively. At Tu2009<u2009TC, the zero field cooled and field cooled magnetization curves of all the films show huge bifurcation. In case of S_STO and S_LSAT films, hysteresis is also observed between field cooled cooling and warming cycle in magnetization versus temperature measurement at low magnetic field similar to first order-like magnetic phase transition. No signature of first order magnetic phase transition has been observed in the case of S_LAO film. Most interestingly, both normal (i.e., neg...

A comparative study of structural, magnetic, dielectric behaviors and impedance spectroscopy for bulk and nanometric double perovskite Sm2CoMnO6

A comparative study regarding the structural, magnetic, dielectric and conductivity behavior and impedance spectroscopy of double perovskite Sm2CoMnO6 (SCMO) with different grain sizes of nearly 35 nm (SCMO_N) and 1 μm (SCMO_B) synthesized by a chemical sol-gel route has been performed in detail. The high-resolution recorded XRD pattern confirms the crystalline phase with monoclinic symmetry (space group P21/n) in SCMO_B. The XRD pattern of SCMO_N discloses the coexistence of both monoclinic and orthorhombic (Pnma) phases in the system. The magnetic measurements reveal that, in SCMO_N, the increase of the disordered phase (orthorhombic phase) results in an increase in the Curie temperature and a decrease in the magnetic moment. SCMO_B shows ferromagnetic behavior with two magnetic transitions, whereas the absence of a high-temperature magnetic transition reveals the disorder nature of the SCMO_N sample. The value of effective paramagnetic moment peff (5.75 μB) of SCMO_B is comparable to the calculated value of the spin-only moment for intermediate-spin Co3+ and high-spin Mn3+, as well as high-spin Mn2+ and Co4+, whereas the observed reduced peff (5.5 μB) value is attributed to the spin-only moment due to the increased number of high-spin Mn3+ and low-spin Co3+ in SCMO_N. SCMO_B exhibits a high dielectric constant with prominent high-frequency dielectric dispersion as compared to SCMO_N. The impedance spectroscopy studies disclose different conduction processes at the grain and grain boundary where the grain boundary obeys Arrhenius behavior and the grains follow the variable range hopping (VRH) model. SCMO_N shows high grain and grain boundary resistance as compared to SCMO_B. The grain and grain boundary capacitance show an anomaly near the dielectric relaxation. The nature of the temperature-dependant exponent s in the ac conductivity predicts that the overlapping large polaron tunneling mechanism (QLPT) is the dominant conduction mechanism for both the SCMO_B and SCMO_N samples.

Influence of magnetic frustration and structural disorder on magnetocaloric effect and magneto-transport properties in La1.5Ca0.5CoMnO6 double perovskite

The rare existence of a magnetocaloric effect, a Griffith phase, and frustrated magnetism in the antisite disorder compound La1.5Ca0.5CoMnO6 have been investigated in detail in this work. The nature of the observed Griffith phase (at TGu2009∼u2009226u2009K) can be best understood in terms of ferromagnetic (FM) entities within the globally paramagnetic network above the Curie temperature. From the isothermal magnetization measurement around Curie temperature (TCu2009∼u2009157u2009K) and cluster glass transition temperature (Tgu2009∼u200951u2009K), we have determined the maximum entropy change (−ΔSM) as ∼2.2u2009J/kg K and ∼1.2u2009J/kg K, respectively, for a magnetic field variation of 7u2009T. Interestingly, a sudden drop of resistivity curve at Tg, associated with magnetic frustration or magnetic disorder, can be related to the ferromagnetic (FM) phases with antiferromagnetic antiphase boundaries, giving rise to a large negative magnetoresistance (∼67%) at 45u2009K.

Role of Gd spin ordering on magnetocaloric effect and ferromagnetism in Sr substituted Gd2CoMnO6 double perovskite

The phenomenon of magnetocaloric effect occurring around the ordering temperatures for Gd2−xSrxCoMnO6 (xu2009=u20090 and 0.5) has been investigated in order to disclose the effects of Sr substitution on the individual collinear Gd spin order. It is shown that 25% Sr substitution of Gd ions significantly suppresses the independent spin order, leading to enhancement of ferromagnetism. As a result, magnetic entropy change (−ΔSM) increases from +0.9 J kg−1u2009K−1 to +3.7u2009Ju2009kg−1u2009K−1 at Curie temperature for a change in magnetic field (ΔH) = 6.9u2009T. On the other hand, the −ΔSM changes sign at Neel temperature due to the effect of Sr substitution. We have also observed noticeably large −ΔSM (∼+25.84u2009Ju2009kg−1u2009K−1) and (∼+21.01u2009Ju2009kg−1u2009K−1) for ΔH = 6.9u2009T at 4f-4f negative coupling temperature (T < 10u2009K) of xu2009=u20090 and xu2009=u20090.5 samples, respectively.The phenomenon of magnetocaloric effect occurring around the ordering temperatures for Gd2−xSrxCoMnO6 (xu2009=u20090 and 0.5) has been investigated in order to disclose the effects of Sr substitution on the individual collinear Gd spin order. It is shown that 25% Sr substitution of Gd ions significantly suppresses the independent spin order, leading to enhancement of ferromagnetism. As a result, magnetic entropy change (−ΔSM) increases from +0.9 J kg−1u2009K−1 to +3.7u2009Ju2009kg−1u2009K−1 at Curie temperature for a change in magnetic field (ΔH) = 6.9u2009T. On the other hand, the −ΔSM changes sign at Neel temperature due to the effect of Sr substitution. We have also observed noticeably large −ΔSM (∼+25.84u2009Ju2009kg−1u2009K−1) and (∼+21.01u2009Ju2009kg−1u2009K−1) for ΔH = 6.9u2009T at 4f-4f negative coupling temperature (T < 10u2009K) of xu2009=u20090 and xu2009=u20090.5 samples, respectively.