M. Patra

Coexistence of superparamagnetic and superspin glass behaviors in Co50Ni50 nanoparticles embedded in the amorphous SiO2 host

We investigate the magnetic properties of the nanoparticles of Co50Ni50 alloy embedded in the amorphous SiO2 host with volume fractions, ϕ=5% and 10%. The static and dynamic aspects of the magnetic properties are investigated by temperature and time dependence of low-field dc magnetization and frequency dependence of ac susceptibilities. The experimental results and analyses suggest that the dilution of the Co50Ni50 nanoparticles belongs to the crossover regime from superparamagnetic to superspin glass states, which are distinguished from the atomic canonical spin glasses.

Multifunctionality attributed to the self-doping in polycrystalline La0.9MnO3 : Coexistence of large magnetoresistance and magnetocaloric effect

We report the multifunctionality near room temperature attributed to the self-doping in polycrystalline La0.9MnO3. The peak in the temperature dependence of magnetoresistance (MR) and change of magnetic entropy (ΔSM) are observed at 254 K with −MR=55% at 50 kOe and −SM=4.9 J/kg K at 20 kOe where the Curie temperature and metal-insulator transition coexist at 254 K associated with a first order transition. The correlation between resistivity and ΔSM is observed in a limited region around TC. Here ΔSM is close to the largest reported values in manganites, suggesting that the material is interesting for multiple applications.

Anomalous magnetic field dependence of magnetocaloric effect at low temperature in Pr0.52Sr0.48MnO3 single crystal

We report the magnetocaloric effect (MCE) in a Pr0.52Sr0.48MnO3 single crystal. A peak in the temperature dependence of magnetic entropy change (ΔSM) with a fairly large negative value (≈3.8 J/kg K) is observed at 275 K close to Curie temperature. MCE is spread over a wide temperature range resulting in a considerable refrigerant capacity (≈293 J/kg). At low temperature the sign of ΔSM changes, below which anomalous field dependence of ΔSM is observed indicating the coexistence of ferromagnetic and antiferromagnetic interactions. Interplay between the interactions is strongly influenced by direction and magnitude of applied magnetic field in the ordered state.

Spin-glass like features in cluster-glass compounds La1−δMn0.7Fe0.3O3

The static and dynamic features of magnetization are investigated using dc magnetization and ac susceptibility measurements on La1−δMn0.7Fe0.3O3 for δ = 0 and 0.13. The results indicate that short range ferromagnetic clusters coexist with the glassy magnetic state. The analysis on the frequency dependence of spin freezing temperature (Tf) using the Vogel–Fulcher law and the dynamical scaling behaviour near Tf indicates that the spin freezing temperature behaves like a spin-glass transition temperature for both the compounds. The cluster-glass states of both the compounds exhibit similar characteristic features.

Grain size effect on the magnetic cluster-glass properties of La0.88Sr0.12CoO3.

We report the grain size effect of hole-doped cobaltite, La(0.88)Sr(0.12)CoO(3), where average sizes are varied from ∼35 to ∼240 nm. The bulk compound is a cluster-glass (CG) compound composed of short range ferromagnetic (FM) clusters embedded in the spin-glass (SG) matrix at low temperature. The short range FM clusters are still retained in the nanocrystalline compound with average size ∼35 nm which are associated with the SG component, displaying CG-like spin dynamics at low temperature. The exchange bias (EB) effect manifested by the shifts in the hysteresis loop is observed due to the field cooling where EB effect is weakened systematically with decreasing grain size. The decrease in the fraction of the FM component is found to be correlated with the weakening of the EB effect with decreasing grain size. Interestingly, the signature of the EB phenomenon due to the field-cooled effect is also evidenced in the temperature as well as the time dependence of resistivity. The grain interior phase separation scenario around the FM/SG interface region has been proposed to interpret the experimental results.

Reply to comment on 'Particle size dependent exchange bias and cluster-glass states in LaMn0.7Fe0.3O3 '

In reply to the comment by Geshev we emphasize that loop shift in the compounds is not a simplified phenomenon of minor loop effect of a ferromagnet rather, it is a genuine signature of exchange bias effect. The estimate of anisotropy field and the plot of exchange bias field at 5 K with the maximum field used for the measurement of hysteresis loop, in addition to the previously reported results such as temperature dependence of exchange bias field, training effect, etc, confirm the exchange bias effect.

Particle size dependent exchange bias and cluster-glass states in LaMn0.7Fe0.3O3

The cluster-glass compound LaMn0.7Fe0.3O3 was synthesized with average particle sizes ~20, ~90, and ~300 nm. We observed a shift of the magnetic hysteresis loop in the field axis while the sample was cooled in an external magnetic field. The systematic shift of the hysteresis loops and the cooling field dependence of the shift indicated the phenomenon of exchange bias. The exchange bias field was found to be strongly dependent on the particle size, where the exchange bias field decreased considerably with an increase of particle size, and the weak effect of exchange bias was observed for particles with size ~300 nm. A cluster-glass state with short range ferromagnetic clusters embedded in the spin-glass like host has been proposed, where the average size of the ferromagnetic cluster increases with particle size.

Spin polarized tunneling magnetoresistance in the self-doped manganite La0.9MnO3

We observe the tunneling magnetoresistance (TMR) at low temperature (T) and low field attributed to the self-doping in polycrystalline La0.9MnO3. The occurrence of TMR is suggested due to the tunneling through the grain boundary (GB) region. Tunneling component of the magnetoresistance ascribed to the disordered magnetic GB region follows the Curie–Weiss-like T dependence. The nonlinear current-voltage curves at low T indicate that the inelastic tunneling via localized states involved with the GB effect is dominant where at low temperature the number of inelastic channels and the fraction of charge carriers entering in each inelastic channel change with temperature.

Exchange bias effect involved with tunneling magnetoresistance in polycrystalline La0.88Sr0.12CoO3

We report the exchange bias (EB) effect along with tunneling magnetoresistance (MR) in polycrystalline La0.88Sr0.12CoO3. Analogous to the shift in the magnetic hysteresis loop along the field (H)-axis a shift is clearly observed in the MR-H curve when the sample is cooled in a static magnetic field. Training effect (TE) is a significant manifestation of EB effect which describes the decrease of EB effect when sample is successively field-cycled at a particular temperature. We observe TE in the shift of the MR-H curve which could be interpreted by the spin configurational relaxation model. A strong field-cooled (FC) effect in the temperature as well as time dependence of resistivity is observed below spin freezing temperature. The unusual MR results measured in FC mode are interpreted in terms of intragranular interface effect between short-range ferromagnetic clusters and spin-glass matrix giving rise to the EB effect. EB effect in MR has been observed in bilayer or multilayer films which has not yet seen in a polycrystalline compound. EB effect involved with tunneling MR and semiconducting transport property attributed to the intragranular intrinsic nanostructure is promising for the spintronic applications.

Exchange bias effect and intragranular magnetoresistance in Nd0.84Sr0.16CoO3

Electrical transport properties as a function of magnetic field and time have been investigated in polycrystalline, Nd(0.84)Sr(0.16)CoO(3). A strong exchange bias (EB) effect is observed associated with the fairly large intragranular magnetoresistance (MR). The EB effect observed in the MR curve is compared with the EB effect manifested in the magnetic hysteresis loop. A training effect, described as the decrease of EB effect when the sample is successively field cycled at a particular temperature, has been observed in the shift of the MR curve. The training effect could be analyzed by successful models. The EB effect, MR and a considerable time dependence in MR are attributed to the intrinsic nanostructure giving rise to varieties of magnetic interfaces in the grain interior.