The effect of antisite disorder on magnetic and exchange bias properties of Gd-substituted Y2CoMnO6double perovskite

Combining experimental investigations and first-principles density functional theory (DFT) calculations, we report physical and magnetic properties of Gd-substituted Y₂CoMnO₆double perovskite, which are strongly influenced by antisite-disorder-driven spin configurations. On Gd doping, Co and Mn ions are present in mixed-valence (Co³⁺, Co²⁺, Mn³⁺and Mn⁴⁺) states. Multiple magnetic transitions have been observed: (i) paramagnetic to ferromagnetic transition is found to occur atT_C= 95.5 K, (ii) antiferromagnetic transition atT_N= 47 K is driven by3d-4fpolarization and antisite disorder present in the sample, (iii) change in magnetization belowT⩽20 K, primarily originating from Gd ordering, as revealed from our DFT calculations. AC susceptibility measurement confirms the absence of any spin-glass or cluster-glass phases in this material. A significantly large exchange bias effect (HEB= 1.07 kOe) is found to occur below 47 K due to interfaces of FM and AFM clusters created by antisite-disorder.

Aqueous Mg-Ion Supercapacitor and Bi-Functional Electrocatalyst Based on MgTiO₃ Nanoparticles

Supercapacitor and hydrogen-based fuel cells are cheap and environmental-friendly next-generation energy storage devices that are intended to replace Lithium-ion batteries. Metal oxide nanostructures having perovskite crystal structure have been found to exhibit unique electrochemical properties owing to its unique electronic band structure and multiple redox-active ions. Herein, MgTiO₃ nanoparticles (MTO-1) were synthesized by wet-chemical sol-gel technique with an average particle size of 50-55 nm, which exhibited superior supercapacitor performance of capacitance (C) = 25 F/g (at 0.25 A/g), energy density (E_D) = 17 Wh/kg, power density (P_D) = 275 W/kg and 82.41% capacitance retention (after 1000 cycles). Aqueous 1 M Mg(ClO₄)₂ solution was used as the electrolyte. MTO-1 revealed an overpotential () = 1.329 V and Tafel slope (b) = 374 mV/dec towards Oxygen Evolution Reaction (OER) electrocatalyst and exhibited = 0.914 V and b = 301.4 mV/dec towards Hydrogen Evolution Reaction (HER) electrocatalyst, both in presence of alkaline 1 M KOH solution, making these MgTiO₃ nanoparticles very promising for potential use in various technologically important electrochemical applications.

Molten Salt Synthesized MgNiO₂ Micro/Nano-Particles for High Energy Density Supercapacitor and Electrocatalyst for Hydrogen Evolution Reaction in Alkaline Medium

In recent years, solid solutions have shown promising results as functional materials for different applications. These materials have tunable physiochemical properties and electronic properties, and are being intensively studied for next generation electrochemical charge storage as well as noble metal free low cost electrocatalyts. In the present work, Magnesium Nickel Oxide (MgNiO₂) solid solution is prepared by molten salt synthesis. MgNiO₂ particles having octahedron shaped morphology with size of 550 nm with an agglomerative behavior was observed through morphological studies. Raman studies revealed presence of three two-phonon modes as well as two one-phonon modes, which confirm the phase purity of MgNiO₂ sample. MgNiO₂ particles behaved as a promising supercapacitor candidate by exhibiting a large specific capacitance of 76 F/g. It also revealed electrochemical stability over an expansive potential range under the presence of 0.5 mol L^-1Sodium Sulfate (Na₂SO₄) electrolyte, having a high energy density of nearly 51 Wh/kg with a power density of nearly 825 w/kg. Further, MgNiO₂ particle showed improved electrocatalytic potential towards Hydrogen Evolution Reaction (HER) in 1 mol L^-1 Potassium Hydroxide (KOH) alkaline medium, by demonstrating an overpotential of 0.636 V with a Tafel slope of 0.22205 v/dec. Based on these observed promising results, it can be conclusively inferred that MgNiO₂ solid solution is a potential candidate for environmental friendly high voltage supercapacitor and HER electrocatalyst applications.

Tuning of multi-magnetic phase and exchange bias effect by antisite disorder in Ca-doped La2CoMnO6double perovskites

The exchange bias effect at the magnetic interfaces and multi-magnetic phases strongly depends on the antisite disorder (ASD) driven spin configuration in the double perovskite systems. The percentage of ASD in double perovskites is extensively accepted as a key for designing diverse new nanospintronics with tailored functionalities. In this regards, we have investigated such ASD driven phenomena in Ca²⁺doped bulk and polycrystalline La_2-xCa_xCoMnO₆(0 ⩽x⩽ 1) series of samples. The structural and Raman studies provide evidence of an increase in the disorder due to the increment of Ca concentration in the parent compound (x= 0). The enhancement of disorder in the doped system induces various magnetic orderings, magnetic frustration and cluster glass-like behavior, which have been confirmed from AC and DC magnetic studies and neutron diffraction studies. As a result, significantly large exchange bias effects, namely zero-field cooled (spontaneous) and field-cooled (conventional) exchange bias, are found. These results reveal the tuning of ASD by doping, which plays an active role in the spin configuration at the magnetic interfaces.

Impact of Gd Doping on the Microstructural-, Electronic-, Magnetotransport, and Magnetic Properties of Pr0.6Gd0.1Sr0.3MnO₃ Nanomanganite

Gadolinium doping effects on structural-, electronic transport and magnetic properties of Pr_0.6Gd_0.1Sr_0.3MnO₃ (PGSMO) nanomanganite (40-65 nm) have been investigated. We have considered the core-shell structure of our PGSMO nanoparticles, which can explain its intriguing magnetic and transport properties. It is reported 65 nm sample exhibits higher metal to insulator transition temperature, T_P (∼215 K) and less resistive behavior compare to its nano counterparts. Enhanced low-field magnetoresistance (LFMR) is observed with a reduction of particle size; however maximum change in magnetoresistance (MR), ∼72% is recorded for 65 nm sample around T_P at 60 kOe field. Low-temperature magnetization data reveals the existence of glassy phase in the compound. It is revealed that substitution of optimal Gd doping at A site on nanometric scale promotes exotic magnetic and transport properties on nanoscale without adding any external physical perturbation or introducing any significant lattice distortion.

Low Temperature Electrical Spin Injection from Highly Spin Polarized Co₂CrAl Heusler Alloy into p-Si

The low temperature spin accumulation in p-Si using Co2CrAl/SiO2 tunnel junction has been investigated in detail. The heterojunction has been fabricated using electron beam evaporation (EBE) technique. The 3-terminal contacts in Hanle geometry has been made for spin transport measurements. The electrical transport properties have been investigated at different isothermal conditions in the temperature range of 10-300 K. The current-voltage characteristics of the junction shows excellent rectifying magnetic diode like behaviour in lower temperature range (below 200 K). At higher temperature, the junction shows nonlinear behaviour without rectifying characteristics. We have observed spin accumulation signal in p-Si semiconductor using SiO2/Co2CrAl tunnel junction in the low temperature regime (30-100 K). Hence the highly spin polarized Full Heusler alloys compounds, like Co2CrAl etc., are very attractive and can act as efficient tunnel device for spin injection in the area of spintronics devices in near future. The estimated spin life time is τ = 54 pS and spin diffusion length inside p-Si is LSD = 289 nm at 30 K for this heterostructure.

Exchange bias effect in nanostructured magnetic oxides

The exchange bias phenomenon observed in nanostructured magnetic oxides with the combination of a variety of magnetic phases such as ferromagnetic, antiferromagnetic, ferrimagnetic, cluster glass, spin glass and disordered magnetic state are reviewed. The systems of different types of magnetic oxides are investigated here. The exchange bias phenomenon has been discussed based on macroscopic experimental tools such as magnetization and magnetoresistance measurements. Some of the applications are also discussed. Some of the factors controlling exchange bias phenomenon and some unsolved issues are also highlighted.

A Griffiths-like phase in antiferromagnetic R0.5Eu0.5MnO3 (R = Pr, Nd, Sm)

We report a Griffiths-like phase in isovalent doped rare-earth manganites R(0.5)Eu(0.5)MnO(3) (R = Pr, Nd, Sm). Rietveld refinement of the structural data reveals strong orthorhombic and Jahn-Teller distortions. The dc and ac magnetic studies nicely demonstrate aspects of Griffiths phase-like behaviour. The presence of short range ferromagnetically correlated spin clusters is observed above the antiferromagnetic transition temperature. A disorder driven phase inhomogeneity arising from strong structural distortion is the possible origin of this behaviour.

Superparamagnetic state by linear and non-linear AC magnetic susceptibility in Mn0.5Zn0.5Fe2O4 ferrites nanoparticles

The Mn0.5Zn0.5Fe2O4 nanoparticles has been synthesized using citrate-gel-precursor method. The direct mixing of nitrates and acetates yields homogeneous nanoparticles. Phase formation and crystal structure of the synthesized powder were examined through the X-ray diffraction (XRD). Fourier transform infrared (FTIR) spectra of the sample confirm the spinel structure. The average particle size was determined by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). The average particle size is found to be about 13 nm. Superparamagnetic-like nature of the nanoparticles of Mn0.5Zn0.5Fe2O4 has been revealed through various dc and linear and non-linear ac magnetization measurements. However, the nanoparticles do not behave like ideal non-interacting superparamagnets. The magnetic particle size is found to be about 8 nm with saturation magnetization about 18.1 emu/g. The blocking temperature (T(B)) of the nanoparticle assembly is found to be about 150 K as observed from dc and ac magnetization measurements. The frequency dependence of the blocking temperature (T(B)) is found to follow Vogel-Fulcher law. The associated characteristic time tau0 is found to be 10(-5) s. This value is different from that generally found for non-interacting superparamagnetic (SPM) systems (tau0 = 10(-9)-10(-10) s).

Sign reversal of junction magnetoresistance in p-La0.7Ca0.3MnO3/SiO2/n-Si heterostructure: a possibility in spintronics application

We have fabricated a p-La0.7Ca0.3MnO3/SiO2/n-Si heterostructure, consisting of a p-type manganite (La0.7Ca0.3MnO3) and n-type Si with a interfacial layer of SiO2 with typical thickness of about 9 nm using pulsed laser deposition technique. The junction exhibits rectifying behavior over the temperature range of 10-300 K with rectification factor 52 at room temperature. Investigation on the electrical properties of p-La0.7Ca0.3MnO3/SiO2/n-Si heterostructure exhibits nonlinear J-V characteristics in a wide temperature range. A crossover from negative to positive junction magnetoresistance (JMR) is observed in p-La0.7Ca0.3MnO3/SiO2/n-Si heterostructure in current perpendicular to film plane (CPP) geometry. The temperature dependent sign of junction magnetoresistance of the heterojunction has been investigated carefully in details. It is found that the junction exhibits the positive junction magnetoresistance when the temperature is greater than the ferromagnetic to paramagnetic transition temperature (Tc) of the top highly spin-polarized half-metallic ferromagnetic La0.7Ca0.3MnO3 manganite film layer. The relation between junction magnetoresistance and external magnetic field is found to be of (delta rho/rho approximately equal alphaHbeta) type having both alpha and beta temperature dependent. We attribute the emergence of negative JMR at lower temperature (< Tc) and positive JMR at higher temperature (> Tc) to the quantum mechanical tunneling transport mechanism across the heterojunction. Our results might be very useful to fabricate artificial devices using the manganite-based heterojunction grown on single crystalline n-Si (100) in spintronics device applications.

Size-dependent electronic-transport mechanism and sign reversal of magnetoresistance in Nd0.5Sr0.5CoO3

A detailed investigation of electronic-transport properties of Nd(0.5)Sr(0.5)CoO(3) has been carried out as a function of grain size ranging from micrometre order down to an average size of 28 nm. Interestingly, we observe a size induced metal-insulator transition in the lowest grain-size sample while the bulk-like sample is metallic in the whole measured temperature regime. An analysis of the temperature dependent resistivity in the metallic regime reveals that the electron-electron interaction is the dominating mechanism while other processes like electron-magnon and electron-phonon scatterings are also likely to be present. The fascinating observation of enhanced low temperature upturn and minimum in resistivity on reduction of grain size is found due to electron-electron interaction (quantum interference effect). This effect is attributed to enhanced disorder on reduction of grain size. Interestingly, we observe a cross over from positive to negative magnetoresistance in the low temperature regime as the grain size is reduced. This observed sign reversal is attributed to enhanced phase separation on decreasing the grain size of the cobaltite.

Observation of Griffiths phase in antiferromagnetic La(0.32)Eu(0.68)MnO3

We report the Griffiths phase (GP) scenario for isovalent doping in antiferromagnetic La(0.32)Eu(0.68)MnO(3). Rietveld refinement of structural data displays strong crystal structural distortion. The dc and ac magnetic studies nicely demonstrate unambiguous aspects of robust GP. The presence of ferromagnetically correlated spin clusters is found above the transition temperature, T(N). The disorder-driven phase inhomogeneity is correlated to strong structural distortion, giving rise to the GP. This is an unique example in manganite where double exchange interaction does not play any role for the observed phenomenon.

Metallicity and ferromagnetism in nanosystem of charge ordered Nd0.5Sr0.5MnO3

The fascinating phenomenon of destabilization of charge/orbital order in Nd0.5Sr0.5MnO3 with the reduction of grain size is critically investigated. Based on our magnetic and transport experiments followed by a theoretical analysis, we analyze various possible mechanisms and try to delineate a universal scenario behind this phenomenon. We revisit this issue carefully and discuss various evidences from experiments in nano and bulk manganites on the absence of correlation between size reduction and pressure effects on manganites. We propose a phenomenological model based on enhanced surface disorder to explain the appearance of weak ferromagnetism and metallicity in nanosize Nd0.5Sr0.5MnO3 system. All evidence seems to suggest that the transport is mediated through the surface via enhanced density of states in the nanometric grains. We provide theoretical support for this by performing an ab-initio electronic structure calculation as well as from a recent numerical simulation and argue that the mechanism is likely to be general in all nanosize charge ordered manganites.

Evidence of electronic phase arrest and glassy ferromagnetic behaviour in (Nd(0.4)Gd(0.3))Sr(0.3)MnO3 manganite: comparative study between bulk and nanometric samples

The effect of the doping of rare earth Gd(3+) ions replacing Nd(3+) in Nd(0.7)Sr(0.3)MnO(3) is investigated in detail. Measurements of resistivity, magnetoresistance, magnetization, and linear and nonlinear ac magnetic susceptibility on chemically synthesized (Nd(0.7-x)Gd(x))Sr(0.3)MnO(3) show various interesting features with doping level x=0.3. A comparative study has been carried out between a bulk and a nanometric sample (grain size ∼60 nm) synthesized from the same as a prepared powder to maintain an identical stoichiometry. The resistivity of the samples shows strong dependence on the magnetic field-temperature history. The magnetoresistance of the samples also shows strong irreversibility with respect to sweeping of the field between the highest positive and negative values. Moreover, the resistivity is found to increase with time after field cooling and then switching off the field. All these phenomena have been attributed to phase separation effects and the arrest of phases in the samples. Furthermore, the bulk sample displays a spin glass like behaviour as evident from the frequency dependence of linear ac magnetic susceptibility and critical divergence of the nonlinear ac magnetic susceptibility. The experimentally obtained characteristic time τ(0) after dynamical scaling analysis of the frequency dependence of the ac susceptibility is found to be 10(-17) s which implies that the system is different from a canonical spin glass. An unusual frequency dependence of the second harmonic of ac susceptibility around the magnetic transition temperature led us to designate the magnetic state of the sample to be glassy ferromagnetic. On reduction of the grain size low field magnetoresistance and phase arrest phenomena are found to enhance but the glassy state is observed to be destabilized in the nanometric sample.

Microstructural, Electrical-, Magneto-Transport Properties of Grain Size Modulated Nanocrystalline Nd0.67Sr0.33MnO3 CMR Manganites

We have investigated the effect of nanometric grain sizes on Microstructural, electrical-, magneto-transport and magnetic behaviors in nanocrystalline Nd0.67Sr0.33MnO3 CMR manganites. Three nanocrystalline powders of Nd0.67Sr0.33MnO3 were synthesized through chemical route “Pyrophoric Reaction Process” and calcined for 5 hrs at calcinations temperature (TCal = 650°C, 750°C, and 850°C). XRD patterns indicate that all the synthesized powders have pseudo-cubic perovskite structure without any secondary impurity phase. Using Debye Scherrer formula we calculated the crystallites size for three nanocrystalline Nd0.67Sr0.33MnO3 powders (∼ 30, 40, and 54 nm for TCal = 650°C, 750°C, and 850°C respectively). TEM micrographs show that the average particle sizes are in nanometric regime (ψ ∼ 30–50 nm). In AC susceptibility and resistivity measurement we observed that there is an almost constant Curie temperature (TC) has value around 240 K and gradual decrease of metal-insulator transition temperature (TP) (from 200-129 K) with decrease of TCal. The magneto resistance of ultra fine nanoparticles increases with grain sizes. Highest magnetoresistance observed ∼ 24 for Nd0.67Sr0.33MnO3 with TCal = 850°C. Experimental results revels, the effect of nanometric grain sizes has an important impact in magnetic properties and magneto-transport behaviors.

Size-induced metallic state in nanoparticles of ferromagnetic insulating Nd0.8Sr0.2MnO3

A detailed investigation of the electronic- and magneto-transport properties of Nd(0.8)Sr(0.2)MnO(3) with the variation of grain size (down to 42 nm) is presented here. Interestingly, we observe that the ferromagnetic insulating state is suppressed and a metallic state is stabilized as the grain size of the sample is reduced. As a result, a metal-insulator transition is observed in this low doped manganite which is insulating in nature in its bulk form. Destabilization of polaronic order in the ferromagnetic insulating state due to enhanced surface disorder on grain size reduction is attributed to this effect. A phenomenological model is proposed to represent the concept of destabilization of polaron formation in the surface region of the nanograins. Resistivity and magnetoresistance data are carefully analysed employing different suitable models. Electrical third harmonic resistance is measured to directly probe the electrical nonlinearity in the samples.

Temperature dependence of phonon modes in nanocrystalline La0.67ca0.33MnO3 as observed by infrared spectroscopy

Low temperature infrared absorption measurements have been carried out on nanocrystalline La0.67Ca0.33MnO3 powder across the magnetic and the insulator-to-metal phase transitions. Interesting changes are observed in the temperature dependence of the mid-IR polaronic background and the stretching mode of the MnO, octahedron. Upon cooling below 250 K, the overall absorbance increases, but, unlike in the case of microcrystalline LCMO, the stretching mode is not completely screened even at 5 K. The temperature dependence of the stretching mode parameters points to a much reduced phonon-polaron coupling as compared to that in the bulk material.

Effect of nanometric grain size on electronic-transport, magneto-transport and magnetic properties of La(0.7)Ba(0.3)MnO(3) nanoparticles

We have investigated the effect of nanometric grain size on electronic-transport, magneto-transport and magnetic properties of single-phase La(0.7)Ba(0.3)MnO(3) (LBMO) nanoparticles having an average grain size in the nanometric regime (21-35 nm). We have observed that both the metal-insulator transition temperature (T(P)) and para-ferromagnetic transition temperature (T(C)) shift to lower temperature with a decrease in average grain size. For the entire series of samples, a distinct minima in resistivity at a temperature (T(min)) followed by an upturn at a very low temperature (≤47 K) is observed. We have attributed the steeper low temperature (∼47 K) resistivity upturn in the smaller grain size sample than that in the larger grain size sample below T(min) to the increased value of charging energy (E(C)). E(C) has been estimated to be 1.3, 0.56 and 0.04 K for an average grain size of 21, 25 and 30 nm, respectively. Magneto-transport measurements show that the magnitude of low field MR (LFMR) varies with average grain size. In order to investigate the MR behavior of LBMO nanoparticles, we have analyzed our data in the light of a phenomenological model, based on spin-polarized transport of conduction electrons at the grain boundaries. Magneto-transport measurements show that the magnitude of low field MR (LFMR), as well as of high field MR (HFMR), remains constant up to sufficiently high temperature (∼50 K) and then drops sharply with temperature. We found that this strange temperature dependence of MR is decided predominantly by the nature of the temperature response of the surface magnetization (M(S)) of nanosized magnetic particles.