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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).

Investigation of magnetic, electronic- and magneto-transport properties of Nd 0.5 Sr 0.5 Co (1− y ) Mn y O 3 has been carried out with y =0–1. These studies, in effect, simultaneously reveal the effect of Mn doping on a ferromagnetic-metallic system Nd 0.5 Sr 0.5 CoO 3 as well as the effect of Co doping on the antiferromagnetic-insulating system Nd 0.5 Sr 0.5 MnO 3 . Our linear/nonlinear ac susceptibility and dc magnetization measurements indicate that in both the cases ferromagnetic–paramagnetic transition temperature decreases and resistivity increases. The magnetoresistance of the samples in the lowest temperature which is nearly zero for Nd 0.5 Sr 0.5 CoO 3 , has been found to increase monotonically with the increase of Mn substitution. This effect is attributed to the phase separation scenario. Upon increasing the concentration of Co in Nd 0.5 Sr 0.5 MnO 3 no indication of ferromagnetic–metallic phase is observed.

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.

The effect of doping of rare earth Pr3+ ion as a replacement of Sm3+ in Sm0.5Sr0.5MnO3 is investigated. Temperature dependent dc and ac magnetic susceptibility, resistivity, magnetoresistance measurements on chemically synthesized (Sm0.5−xPrx)Sr0.5MnO3 show various unusual features with doping level x=0.15. The frequency independent ferromagnetic to paramagnetic transition at higher temperature (∼191 K) followed by a frequency dependent reentrant magnetic transition at lower temperature (∼31 K) has been observed. The nature of this frequency dependent reentrant magnetic transition is described by a critical slowing down model of spin glasses. From non-linear ac susceptibility measurements it has been confirmed that the finite size ferromagnetic clusters are formed as a consequence of intrinsic phase separation, and undergo spin glass-like freezing below a certain temperature. There is an unusual observation of a 2nd harmonic peak in the non-linear ac susceptibility around this reentrant magnetic transition at low temperature (∼31 K). Arrott plots at 10 and 30 K confirm the existence of glassy ferromagnetism below this low temperature reentrant transition. Electronic- and magneto-transport measurements show a strong magnetic field—temperature history dependence and strong irreversibility with respect to the sweeping of magnetic field. These results are attributed to the effect of phase separation and kinetic arrest of the electronic phase in this phase separated manganite at low temperatures.

The phenomenon of destabilization of antiferromagnetic insulating state into a ferromagnetic metallic one in Nd 0.5 Sr 0.5 MnO 3 with the variation of particle/grain size is critically investigated. Based on our neutron diffraction study, magnetic and transport experiments, we observe ferromagnetism and metallic behavior in Nd 0.5 Sr 0.5 MnO 3 (∼40 nm grain size) as against A-type antiferromagnetic order in the sample with the largest grain size (∼800 nm). The latter shows a systematic change in the lattice parameters with temperature, and an antiferromagnetic ground state similar to that of a bulk system. Interestingly, the sample with the smallest grain sizes exhibits insignificant structural changes (compared to the largest grain size sample) but a complete change in the magnetic state (ferromagnetic behavior) as revealed from the neutron diffraction study. Magnetic measurements also confirm a ferromagnetic state in the small-grained sample. Electronic transport measurements exhibit a metal–insulator transition in this sample. The effects are primarily attributed to enhanced surface disorder.

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.

An experimental study of p-silicon (Si)/La 0.7 Sr 0.3 MnO 3 (LSMO) junction in which the LSMO and silicon are separated by a thin interfacial silicon dioxide (SiO 2 ) layer has been fabricated. Two type of SiO 2 layer has been discussed here – one is native oxide and another one is thermal oxide. The thermal SiO 2 film is grown on Si by annealing it at 800 °C in oxygen atmosphere. The LSMO film of about 64 nm thick has been grown on SiO 2 at 800 °C substrate temperature in 10 −1 mbar oxygen pressure by Pulsed Laser Deposition technique. The LSMO/SiO 2 /Si heterostructure exhibits MOS diode-like behavior with all type of possible current flow mechanisms (such as thermionic emission, tunneling, recombination degeneration, etc.) through the heterojunction. The high field Fowler-Nordheim [ln(J FN /E 2 ) vs 1/E] plot at different temperatures confirms that the dominating transport mechanism across the heterostructure is tunneling. The junction resistance changes under magnetic field and the junction magnetoresistance is found to be ∼31% with 1T applied magnetic field at room temperature at a bias voltage of 2.2 V. The capacitor-voltage characteristics confirm the presence of trap charges.

Epitaxial films of ZnO doped with magnetic ion Fe and, in some cases, with 1% Al show clear evidence of room temperature ferromagnetic ordering. The Al doped optimized samples with carrier concentration n c ∼8.0×10 20 cm −3 show about 3 times enhanced saturation magnetization (0.58 μ B /Fe 2+ ) than the one with n c ∼3.0×10 20 cm −3 (0.18 μ B /Fe 2+ ). A clear correlation between the magnetization per transition metal ion and the ratio of the number of carriers to the number of donors have been found as is expected for carrier-induced room temperature ferromagnetism. The transport mechanism of the electrons in all the DMS films at low temperature range has been identified with the Efros's variable range hopping due to the electron–electron Coulomb interaction.

Half-doped Sm0.5Ca0.5MnO3 nanoparticles with varying particle size down to ~14 nm are synthesized by chemical "pyrophoric reaction" route and their electronic transport and magnetotransport behaviors are investigated in detail in order to understand the nanodimensional size effect on the charge ordering in manganites. The experimental result shows that reduction of particle size (having same chemical composition) down to nanometer scale destabilizes the charge ordering. Weak ferromagnetism and metal–insulator transition are observed in the sample having lowest particle size (14 nm) as a consequence of such suppression of charge ordering. The observed phenomena have been attributed to enhanced surface pressure in the nanoparticles.

The mixed ferrite nanometric Co – Zn ferrites [ Co 1-x Zn x Fe 2 O 4] with x = 0, x = 0.35, x = 0.5, x = 0.65, and x = 1 having particle size down to ~20 nm, synthesized through chemical sol–gel route have been found to have higher Curie temperature and higher resistivity as compared to the ones derived from conventional processes. Studies were made to estimate the effect of Co concentration (x) on these mixed ferrite nanometric Co – Zn ferrites having a AB2O4 -type cubic inverse spinel structure. Investigation of magnetic properties of all Co – Zn nanoferrites with varying Co content has been made using a VSM at room temperature. The saturation magnetization and coercive field have been observed to be highest for x = 0.35 and x = 0, respectively. The magneto-impedance characteristics and the complex impedance spectroscopy [Z(ω,T) = Z′+iZ″] have also been investigated in detail for all the samples at different frequencies.

Epitaxial films of ZnO doped with magnetic ion Fe and, in some cases, with 1% Al show clear evidence of room temperature ferromagnetic ordering but containing huge amount of paramagnetic moment in it. The total ferromagnetic and paramagnetic contributions have been extracted from the low temperature SQUID measurements. A clear correlation between the magnetization per transition metal ion and the ratio of the number of carriers and number of donors have been found in these films and established the theory of carrier induced ferromagnetism. The experimental data has been best explained through the modification of electronic structure of oxide semiconductors with impurity states.

The phenomenon of magnetocaloric effect occurring around the ordering temperatures for Gd2−xSrxCoMnO6 (x = 0 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−1 K−1 to +3.7 J kg−1 K−1 at Curie temperature for a change in magnetic field (ΔH) = 6.9 T. 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 (∼+25.84 J kg−1 K−1) and (∼+21.01 J kg−1 K−1) for ΔH = 6.9 T at 4f-4f negative coupling temperature (T < 10 K) of x = 0 and x = 0.5 samples, respectively.

We have investigated magnetization, magneto-transport, and complex impedance spectroscopy of structurally characterized multiferroic composites x La 0.7 Sr 0.3 MnO 3 (LSMO)–(1-x) ErMnO 3 (EMO) (0≤x≤1). Para- to ferromagnetic Curie temperatures for this series of composites indicate almost complete immiscibility within the mixture of those two compounds of LSMO and EMO having nearly identical chemical formula. Magnetization as a function of magnetic field curves at T=300 K exhibit an appreciable enhancement of coercive field of LSMO in the composites with increase in EMO content. This feature has been attributed to the increasing grain boundary pinning center of LSMO grains at their interfaces in the composites. Magneto-transport measurements reveal a considerable decrease in spin polarized tunneling transport with increase in EMO content in the composites. This feature is understood as a consequence of increasing grain boundary pinning centers of magnetic domains of LSMO grains in the composites. Room temperature complex impedance spectroscopy studies, i.e., -Z′′ versus Z′ curves (Nyquist plots) for EMO, x=0.1, x=0.2, and LSMO samples reveal that relaxation time of both bulk and grain boundary increases with increase in EMO content in the composites. Slow relaxation processes of conduction electrons at both bulk and grain boundary occurs due to the proximity of highly resistive insulating EMO in the composites.

We have investigated the magnetic and optical properties of chemically low temperature-synthesized Zn1– x Fe x O (x = 0.05 and 0.10) diluted magnetic semiconducting nanoparticles (∼7 nm). Observed magnetic behaviour of x = 0.05 samples showed that the net magnetic interaction was antiferromagnetic-like, a feature established by Curie–Weiss fit, concave Arrott–Belov–Kouvel (ABK) plots with the absence of spontaneous magnetization even at 5 K and stretched exponential-type time-dependent magnetization behaviour. Optimization of the Fe(x) dopant concentration in Zn1– x Fe x O gave the most favourable room-temperature ferromagnetism for x = 0.10, as supported by finite coercive field (∼94.4 Oe) and remanent magnetization (0.011 µB/Fe ion) from strong hysteretic magnetization vs. magnetic-field curves at room temperature. The Curie temperature of the x = 0.10 sample was estimated at ∼388 K. The existence of a room-temperature ferromagnetic phase was further established by the convex nature of the ABK plots wit...

The observation of giant positive junction magnetoresistance (JMR) in our cobalt ferrite (CFO)/p-Si heterojunction has been reported here. The pulsed laser deposition technique has been used for fabrication of the heterojunction. The junction confirms a very good rectifying magnetic diode like behavior at low temperature, whereas at high temperatures the junction shows nonlinear I–V characteristics. The magnetic field-dependent magnetoresistance (MR) of both CFO film and across the heterojunction have been studied in detail. The CFO film shows negative MR behavior, whereas the junction shows large positive JMR behavior throughout the temperature range. The spin lifetime (142 ps) and spin diffusion length (331 nm) have been estimated of the heterostructure at 10 K. The highest JMR value (~1600 %) has been observed at 10 K, and it gradually decreases at higher temperature range. The origin of positive JMR in our heterojunction has been best explained by the standard spin injection theory.

We report here microstructural, magnetic and optical properties of high quality epitaxial thin films of diluted magnetic semiconductor Zn1 − xMnxO (0.01 ≤ × ≤ 0.25) grown on (0001) sapphire substrate (α-Al2O3) by pulsed laser deposition technique. Seven (01-10) planes of ZnMnO film are found to match with six (-12-10) planes of sapphire substrate through excellent domain matching epitaxy by 30° rotations of ZnMnO unit cell about c-axis with respect to the sapphire unit cell. From high resolution transmission electron micrograph image the misfit dislocations are observed at the interface of sapphire and ZnMnO film growth plane. The insulating Zn0.82 Mn0.18 O epitaxial thin film showed ferromagnetic behavior (hysteretic) with coercive field ∼ 6.2 mT, and a maximum saturation moment of 0.42 μB/Mn+2 ion at a field of 0.5 T at the lowest attainable temperature (T = 10 K). The strong concave behavior of magnetization as a function of the temperature curves in this strongly insulating ferromagnetic diluted magnetic semiconductor film has been best explained through non-mean-field polaron-percolation-theory. The increase of band gap from bulk ZnO with dopant concentration (x), observed from absorbance spectra, has been attributed to the sp-d spin — exchange interaction between the band electrons and localized d electrons of Mn+2 ions of Zn1 − xMnxO films in the presence of tetrahedral crystal field interaction.

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 TG ∼ 226 K) 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 (TC ∼ 157 K) and cluster glass transition temperature (Tg ∼ 51 K), we have determined the maximum entropy change (−ΔSM) as ∼2.2 J/kg K and ∼1.2 J/kg K, respectively, for a magnetic field variation of 7 T. 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 45 K.

Single phase multiferroic Bi1-xNdxFe1-yCoyO3 (x=0, 0.10, 0.15, 0.20; y = 0, 0.10) nanoparticles (average particle size 20-80 nm) were synthesized by novel chemical sol-gel route. The detailed microstructural analysis has been performed using HRXRD, HRTEM and FESEM techniques. The Rietveld refinement of the XRD data reveals that the samples are well crystalline with distorted rhombohedral structure having R3c space group without having any iron rich impurity phases. A noticeably large value of dielectric constant (880) and low tan δ loss (0.04) are observed for the sample Bi0.9Nd0.1Fe0.9Co0.1O3 at 100 Hz at room temperature. An anomaly near the Neel temperature in dielectric measurement has been observed for the samples with x = 0.10 and y = 0.0, 0.10. This anomaly has also been found in temperature dependent dielectric loss curve. A spectacular enhancement of Ms (7.26 emu/gm) and coercivity Hc (1.75 T) has been observed for Bi0.9Nd0.1Fe0.9Co0.1O3 nanoceramics revealing ferromagnetic ordering at the ground state (5 K). We have also observed well saturated P-E Hystersis loops with a remanent polarization (Pr) of 0.2 µC/ cm2 and coercive field Ec of 5 kV/cm for Bi0.9Nd0.1Fe0.9Co0.1O3 nanoceramics.

Fabrication of a low cost and high precision thermoelectric power (S) setup is described here. It is made with spring loaded sample holder which hold the sample between two Cu blocks. Measurements range is in between ∼ few µV/K to ∼ few hundred µV/K. The data acquisitions are fully automated through LabVIEW software.

In this article, microstructural, magnetic, magneto-transport, electronic transport properties of Pr1-xSrxMnO3 system have been systematically studied in nanometric dimension over the range 0.2 ≤ x ≤ 0.4 by preparing the compound through pyrophoric reaction route. XRD diffraction measurements indicate that all the samples are in orthorhombic Pbnm space group. Electronic and magneto-transport properties of these materials have been thoroughly investigated in nanometric grain size modulation. These Sr doped compounds at x=0.3 exhibit more ferromagnetic metallic behavior at zero field and negative magneto resistance in low temperature. The different magnetic phase was found using ac magnetic susceptibility measurement. Magneto –transport nature is explained using spin polarization tunneling model.

Nanocrystalline nickel particles were embedded in amorphous alumina and crystalline TiN matrices using a pulsed laser deposition process to investigate the effect of texturing on magnetic properties of nickel nanocrystallites. The crystalline quality of both the matrix and magnetic particles was investigated by cross-sectional high-resolution transmission electron microscopy. The embedded Ni nanocrystals were found to be epitaxial in the case of the TiN matrix and polycrystalline in Al2O3 amorphous matrix. The Ni nanocrystals on TiN/Si grow epitaxially because the TiN acting as a template grows epitaxially on Si substrate via domain epitaxy. On the other hand, Ni nanocrystals in the Al2O3 matrix are polycrystalline because of the amorphous nature of the alumina matrix. Magnetization versus temperature measurements have shown that the blocking temperature, above which the samples lose magnetization–field (M–H) hysteretic behavior, of the Ni–TiN sample (approximately 190 K) is significantly higher than that of Ni–Al2O3 sample (approximately 30 K) with a similar size distribution of embedded magnetic particles. A comparison of the values of coercivity (Hc) of the two samples, measured from M–H data, indicates that epitaxial Ni nanocrystals also exhibit significantly higher coercivity than polycrystalline Ni particles in amorphous alumina matrix. The high values of TB and Hc of Ni–TiN samples with respect to TB of N–A12O3 samples are believed to be associated with preferred alignment of nanocrystallites.

The study explores the distribution pattern of tal palm in the rural landscape of Bangladesh along with their age-gradation, production, marketing and income from the sale of palm products. Field data were collected by interviewing 100 farmers from five villages of Jhalakhati district. Farmers were categorized into five categories based on the extent of their total landholdings. Twenty households (four from each category) were selected randomly from each village and the farmers interviewed and asked about their tal trees, their management, tapping, yields, processing and marketing the produce and prices. Field visits to the trees were made. Different parcels of land (e.g. homesteads, roadside, canal banks, pond banks and crop fields) characterize the rural landscape. Palms of all age classes occurred in each and every type. Considerable differences between the households in strategies for marketing and selling of palm products were encountered. Indigenous techniques in tapping and gur processing,...

We report the dielectric characteristics of sol-gel derived nanoparticles of half doped ferromagnetic double perovskite LaSrCoMnO6 (LSCMO) over broad temperature and frequency ranges. The diffuse dielectric peak with strong frequency dispersion, Vogel-Fulcher functional relation like relaxation behaviour etc. confirm the relaxor type dielectric nature of these magnetic nanoparticles. The chemical and compositional disorder in the LSCMO nanoparticles, the Co/Mn site disorder, and the surface disorder are attributed to the relaxor like behaviour by the formation of local polar nano-regions. The observed magnetodielectric effect is attributed to the phenomena of magnetic field controlled electric relaxation and magnetoresistance.

We have developed a novel thin film processing method based upon pulsed laser deposition to process nanocrystalline magnetic materials with accurate size and interface control. Using this method, single domain nanocrystalline Fe and Ni particles in 5–10 nm size range embedded in amorphous as well as crystalline alumina have been produced. Magnetization measurements of these layered thin films as function of field and temperature were carried out using a superconducting quantum interference device magnetometer. The size of Fe and Ni nanodots measured using transmission electron microscopy and calculated using magnetic data are in excellent agreement with each other.

Very high-resolution electrical resistivity data have been presented for Zr-rich ( Cu 0.36 Zr 0.64)1-x Al x and ( Ni 0.5 Zr 0.5)1-x Al x(0≤x≤0.2) non-magnetic transition-metal-based amorphous metallic glasses in the temperature range of 1.2–300 K. All the alloys are found to fall into the strong–scattering regime (ρ300>165μΩ· cm ) with negative temperature coefficient of resistance even till 300 K. With the addition of Al in both the systems, the value ρ300 increases because of the enhancement of the degree of disorder. In the low-temperature range, they show a tendency towards superconductivity with trasition temperature Tcc

We report detailed field dependent electronic- (ρ-T) and magneto- transport (MR-H) studies of La1−xSmxSr0.1MnO3 (0.1≤x≤0.5) nanoparticles. Doping induced disorder at La site is observed in field dependent ρ-T measurements of the sample. At low doping side, nice metal to insulator transition (MIT) peak appears in ρ-T data whereas with increasing of Sm+3 contents, metallic behavior is suppressed under the insulating background although a weak signature of MIT is found. Anomalous resistive nature of the samples with increasing of x can be explained in such a way that doping at nonmagnetic La site with magnetic Sm+3 ion induces an extra magnetic coupling in the system which changes the long range ferromagnetic ordering to spin glass/cluster glass state in antiferromagnetic background. The field dependent magneto resistance (MR) mechanism at different temperatures is investigated using spin polarized tunneling model of conduction electrons between two adjacent grains at the grain boundaries. For the sample of ...

The half metallic ferromagnet Co2CrAl full Huesler alloy was successfully prepared by arc melting process. The electrical and magnetic properties of Co2CrAl alloy have been studied in the temperature range of 5 – 300 K. The ferromagnetic Curie temperature Tc of the same alloy has been observed at 329.8 K. The alloy shows semiconducting like electronic transport behavior throughout the studied temperature range. The origin of the semiconducting behavior of Co2CrAl alloy can be best explained by the localization of conduction electrons and the presence of an energy gap in the electronic spectrum near the Fermi level EF.

An experimental study of p−La0.7Ca0.3MnO3/SrTiO3/n−Si heterostructure in which La0.7Ca0.3MnO3 (LCMO) and Si are separated by a thin interfacial SrTiO3 (STO) layer with typical thickness ∼ 15 nm, has been in situ fabricated with the pulsed laser deposition technique. The junction exhibits good rectifying diode like behavior over the temperature range of 10 - 300 K. The heterostructure also exhibits metal-oxide-semiconductor like behavior with all type of possible current flow mechanisms through the heterojunction. The junction magnetoresistance (JMR) (∼ 30% at 300 K) properties of p-LCMO/STO/n-Si heterostructure have been studied over the temperature range of 100-300 K. The JMR is positive and strongly depends on temperature at an applied forward bias voltage of 3 V. The relation between JMR and external magnetic field is found to be Δρ/ρ≈ α Hβ type, having both α and β temperature dependent. We attribute the emergence of positive JMR to the quantum mechanical tunneling transport mechanism across the heter...

We have studied the evolution of the transport and magnetic properties with holmium doping for . We observe a systematic decrease in the ferromagnetic transition temperature , the metal - insulator transition temperature and the rhombohedral cell volume with increasing holmium concentration. For the concentration with x = 0.4, we observe a metallic spin-glass phase with the spin-glass freezing temperature , and a large magnetoresistance ratio at low temperature, which indicates that a proportion of the spins are in a frustrated state. The high-field magnetization of the holmium-doped samples is much less than the magnetization that would be expected if the were in a ferromagnetic state, showing that there is substantial canting in the manganese lattice. We believe that the ferromagnetic component in the spin-glass phase for x = 0.4 crosses the percolation threshold, giving rise to metallicity in the sample.

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).

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.

In this article, we report that the charge/orbital order state of bulk antiferromagnetic Sm0.5Ca0.5MnO3 is suppressed and confirms the appearance of weak ferromagnetism below 65 K followed by a low temperature spin glass (SG) like transition at 41 K in its nano metric counterpart. Exchange anisotropy effect has been observed in the nanomanganites and can be tuned by the strength of the cooling magnetic field (Hcool). The values of exchange bias fields (HE), coercivity (HC), remanence asymmetry (ME) and magnetic coercivity (MC) are found to strongly depend on cooling magnetic field and temperature. HE increases with increasing Hcool but for larger Hcool, HE tends to decrease due to the growth of finite size ferromagnetic clusters. A proposed phenomenological core–shell type model is attributed to an exchange coupling between the spin-glass like shell (surrounding) and antiferromagnetic core of Sm0.5Ca0.5MnO3 nanomanganites mainly on the basis of uncompensated surface spins. Results suggest that the intrinsic phase inhomogeneity due to the surface effects of the nanostructured manganites may cause exchange anisotropy, which is of special interests for potential application in multifunctional spintronic devices.

Highly textured Nanocrystilline nickel dots were embedded in crystalline TiN matrix using pulsed laser deposition technique. The crystalline quality and magnetic properties of these highly textured Ni nanodots were investigated in details. The blocking temperature of the Ni particles are found to be ∼190 K. The electrical resistivity (ρ) of these self assembled Ni particles embedded in TiN was measured in fields of 0 – 4 Tesla between 4.2 – 65 K. At low temperature ρ(T) shows-√T dependence below the temperature (Tmin) where the resistivity minimum is observed and the behavior is best explained through quantum interference effect. High magnetoresistance is observed in Ni/TiN metal.

We investigated the electronic transport, magnetic, and magneto-transport properties of perovskite Pr0.6Sr0.4MnO3 (PSMO) manganite in nanometric grain size modulations in the range of 32-50 nm. Structural property confirms all samples are in single phase well crystallined with Pbnm space group. In the electronic transport nanometric sample shows a metal-insulator transition varying from 215 K to 275 K. The low field magnetoresistance (LFMR) (upto 1 Tesla) of the lowest nanometric sample is found maximum (∼55%) at 10 K compared to other nanometric samples. However, in nanometric dimension, paramagnetic to ferromagnetic transition temperatures are increased rather than its reported bulk counterpart value (∼300 K). The micro-structural, ac-susceptibility, electronic-, magneto-transport properties in nano manganites is analyzed on the basis of enhanced surface disorder at the grain surface in order to explain the appearance of metallicity, LFMR and modified magnetic properties.

The zinc doped cobalt ferrite (Co0.65Zn0.35Fe2O4) thin film has been fabricated on p-Si substrate (100) using pulsed laser deposition technique. The Current-Voltage characteristics without and with external 6 T magnetic field have been measured at different temperatures in the range of 5-300 K across the junction (CZFO/p-Si). The junction shows good rectifying behavior throughout the temperature range. The magnetic field dependent junction magnetoresistance (JMR) behavior of the heterojunction (CZFO/p-Si) have been studied at a constant temperature. The heterojunction shows positive JMR throughout the temperature range. The highest positive JMR observed at low temperature 5 K and decreases at higher temperatures.

The epitaxial Ni nanoparticles embedded in metallic TiN matrix [TiN(Ni)] on p-type (001) Si heterojunction [TiN(Ni)/(001)p-Si] has been fabricated using pulsed laser deposition technique. The I-V characteriatics of TiN(Ni)/p-Si heterojunction have been studied at different temperatures in the range of 100-300 K. The heterojunction shows reasonably good diode like rectifying behavior in this temperature range. The ideality factor, reverse saturation current, series resistance and turn-on voltages have been estimated at different operating temperatures. The magnetoresistance (MR) properties of TiN(Ni) film and the junction MR of TiN(Ni)/p-Si heterojunction have been studied up to a magnetic field of 8 Tesla. The junction MR attains a peak with high positive junction MR value at 250 K. The origin of high value of junction MR has been best explained using standard spin injection theory.

The design and operation of a low cost and precise thermoelectric power (S) setup is described here. It is made simple to load and hold the sample between two Cu blocks using a spring loaded plunger arrangement. The useable range of measurements is from a ∼ few μV/K to a few hundred μV/K. The versatile nature of the setup is also demonstrated employing it in high magnetic field environment up to 8 T. S(T) data acquisition are fully automated employing a computer, LabVIEW software and IEEE interfacing GPIB card (NI).

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 ∼157 K. From temperature variation linear ac susceptibility measurements, a CG state is clearly observed below a freezing temperature of ∼50.1 K, 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 zυ ∼ 4.95(5) and τ0 ∼ 1.23 × 10−7 s. 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...

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.

The heterojunction of nanocrystalline Ni nanoparticles embedded in crystalline TiN matrix grown on p-Si substrate exhibits diode like excellent rectifying behavior in the temperature range of 100-300 K. The I-V characteristics with and without external magnetic field have been studied in the temperature range of 100-300 K. The negative junction magnetoresistance (JMR) has been observed at 100 and 150 K, and positive JMR at 200, 250 and 300 K. The appearance of JMR in these temperatures range is best explained by standard spin injection theory. The negative MR (up to 8 tesla magnetic field) of the film was observed in the temperature range of 100-300 K.

The effect of doping of rare earth Pr3+ ion replacing Sm3+ in Sm0.5Sr0.5MnO3 is investigated in details. Measurements of linear and non linear ac magnetic susceptibility, resistivity, magnetoresistance on chemically synthesized (Sm0.5-xPrx)Sr0.5MnO3 shows various interesting features with doping level x=0.15. Here we observe the frequency independent FM-PM transition at higher temperature followed by a frequency dependent re-entered magnetic transition at lower temperature through complex ac susceptibility measurements. We have ascribed our observation to the formation of finite size ferromagnetic clusters which are formed as a consequence of intrinsic phase separation and undergo Spin glass-like freezing below certain temperature in this manganite. The magnetoresistance of the samples also show strong irreversibility with respect to sweeping of the field between highest positive and negative values. All these experimental results have been attributed to phase separation effect and kinetic arrest of elect...

The magnetoresistance (MR) of the films have been measured at different temperatures to investigate the sp-d exchange interaction between the conducting s-electron spins and the d-electron spins localized at the magnetic Fe impurities in 5% Fe and 1% Al doped ZnO. The unusual behavior of MR observed in this study have been attributed to the relative magneto-transport phenomenon arising from combined effects of spin orbit coupling and a broken symmetry of ferromagnetically ordered states in Dilute magnetic semiconductor systems.

We report on x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) studies of the paramagnetic (Mn,Co)-co-doped ZnO and ferromagnetic (Fe,Co)-co-doped ZnO nano-particles. Both the surface-sensitive total-electron-yield mode and the bulk-sensitive total-fluorescence-yield mode have been employed to extract the valence and spin states of the surface and inner core regions of the nano-particles. XAS spectra reveal that significant part of the doped Mn and Co atoms are found in the trivalent and tetravalent state in particular in the surface region while majority of Fe atoms are found in the trivalent state both in the inner core region and surface region. The XMCD spectra show that the Fe$^{3+}$ ions in the surface region give rise to the ferromagnetism while both the Co and Mn ions in the surface region show only paramagnetic behaviors. The transition-metal atoms in the inner core region do not show magnetic signals, meaning that they are antiferromagnetically coupled. The present result combined with the previous results on transition-metal-doped ZnO nano-particles and nano-wires suggest that doped holes, probably due to Zn vacancy formation at the surfaces of the nano-particles and nano-wires, rather than doped electrons are involved in the occurrence of ferromagnetism in these systems., Comment: Proceedings of "XAFS theory and nanoparticles"

The intermediate band-width manganite La0.5Ca0.5MnO3 has a CE type charge, orbital and antiferromagnetic order below 195 K owing to strong electronic correlation and commensurability effect. We have critically investigated in details all the possible mechanisms behind the destabilization of charge/orbital ordering in La0.5Ca0.5MnO3 with the reduction of grain size in the nanometric regime as evidenced from our various experimental results. We have tried to found out a universal scenario to explain such behavior in nanometric manganites in general. This study shows that the room temperature Pnma perovskite structure, correlated with the lattice distortion, Mn-O distances and octahedral tilts, is practically unaffected with the reduction of particle size. However, in these nanocrystalline La0.5Ca0.5MnO3, the stable charge ordered state with antiferromagnetic ordering becomes significantly suppressed and FM ordering spontaneously appears below T(c) = 260 K as compared to its bulk counterpart. From strong experimental evidences on nano and bulk manganites, we revisit this issue and also discuss the absence of correlation between size reduction and pressure effects on manganites. We argue from a phenomenological analysis that the above observations could be best attributed to enhanced surface disorder in the nanosized half-doped manganite grains which is likely to be general in all nano size charge ordered manganites.

The room temperature multiferroic nanocomposite materials of ferrite‐ferroelectric ceramics viz. cobalt‐zinc ferrite (i.e. Co0.65Zn0.35Fe2O4) and lead zirconate titanate (i.e. PbZr0.52Ti0.48O3) sintered at three different temperatures were prepared by sol‐gel technique. Magnetization versus magnetic field measurements show typical hysteresis loops, attesting room temperature ferrimagnetic behavior of the composites. The ferrimagnetic Curie temperature is nearly 230 °C which is slightly decreased from pure Co‐Zn ferrite. The dielectric constant increases with increasing particle size. The ferroelectric Tc decreases gradually but the transition becomes increasingly diffuse with decrease in particle size. The polarization versus electric field (P‐E) measurement exhibits a clear saturating nature of hysteresis loop which proves the excellent ferroelectricity of the nanocomposite.