Sujoy Roy

Interplay of structure and transport properties of sodium-doped lanthanum manganite

The crystal structure, magnetic and electrical transport properties of the sodium-doped lanthanum manganites La1-xNaxMnO3 (0.07≤x≤0.40) have been studied in detail using x-ray powder diffraction, atomic absorption spectroscopy, a SQUID (superconducting quantum interference device) magnetometer and the four-probe resistivity measurement technique. A rhombohedrally distorted perovskite structure has been observed in the range 0.07≤x≤0.20. Both the lattice parameter and unit-cell volume decrease with increase in the Na content. A ferromagnetic-to-paramagnetic phase transition associated with a metal-insulator transition is observed for all the La1-xNaxMnO3 compounds. There is a systematic change in both the Mn-O-Mn bond angle and the tolerance factor with Na content. The compositional variation of the magnetic and metal-insulator transition temperatures is explained as due to the distortion of the MnO6 octahedron and increase in the tolerance factor that controls the hopping interaction. In the metallic region a ρ~AT2 behaviour is observed due to the magnon excitation effect. The resistivity shows a field-dependent minimum at low temperature that has been explained as due to the intergrain transport phenomenon.

Size Induced Variations in Structural and Magnetic Properties of Double Exchange La0.8Sr0.2MnO3−δ Nano-Ferromagnet

A detailed study on the influence of particle size varied from 8 nm to 53 nm on the structural and magnetic properties of La0.8Sr0.2MnO3−δ has been done. The unit cell volume increases and the microstrain in the compound shows peak formation as the particle size decreases. Nano particles of La0.8Sr0.2MnO3−δ exhibit superparamagnetism whose blocking temperature has a nonlinear and logarithmic decreasing tendency as function of particle size and applied magnetic field, respectively. Evidence of formation of a magnetically dead layer at the surface has been found and the ratio of the thickness of the dead layer to the particle size increases exponentially with particle size. The coercivity of the nanoparicles increases manifold as particle size varies from 53 nm to 21 nm. In the single domain region the coercivity exhibits a d−1.125 behavior. The temperature dependence of the saturation magnetization shows strong collective excitation due to the spin wave that varies as Tα with α>αbulk of 3/2. Thus the spin ...

Colossal magnetoresistance in Ce doped manganese oxides

In this study we investigate the effects of Ce doping in R1−xAxMnO3 (R=La, Ce, and A=Sr, Ce) on the magnetic and transport properties of this system. For La1−xCexMnO3 (LCMO), an increase in Ce concentration is accompanied by an increase in TC from 225 to 236 K, as well as an increase in the electrical resistivity. An extremely high resistivity is observed in the new system Ce1−xSrxMnO3 (CSMO) which becomes insulating below its Curie temperature of 43 K. A maximum magnetoresistance (MR) ratio of 40% for CSMO and 53% for LCMO is observed. A larger change in resistivity is seen to correspond to an increase in the Ce concentration, however this is offset by an overall resistivity increase which keeps the MR ratio low. The high resistivity may be due to unreacted oxides in the samples. If true, the amount of impurity appears to be proportional to the Ce doping. If this impurity level can be reduced, a significant colossal magnetoresistance effect could be exhibited by these systems.

Charge Transport and Colossal Magnetoresistance Phenomenon in La1−xZrxMnO3

In this study we have investigated the magnetic and electrical transport properties of Zr doped lanthanum manganite perovskite. The structural, magnetic, and transport properties of the Zr doped compounds were determined using x-ray diffraction, dc magnetic susceptibility, and a four probe method for electrical resistivity and magnetoresistance measurements in the temperature range of 5–400 K. The structure of the compounds was found to be rhombohedral. The magnetization versus temperature curves show ferromagnetic regions with the magnetic transition temperatures getting saturated for x⩾0.07 compounds. The resistivity curves show decreasing resistivity with increasing Zr content in the compound. The resistivity of the compounds is very high and is explained as due to the localization tendency of the electrons. The metal–insulator transition temperature shows a compositional dependence and has additional contributions apart from magnetism. The results are explained by the double exchange interaction and M...

Crystal structure, magnetic properties and electronic structure of the MnBi intermetallic compound

The low-temperature phase of the MnBi alloy has a coercivity μ0Hc of 2.0 T at 400 K and exhibits a positive temperature coefficient from 0 to 400 K. In the higher temperature range it shows a much higher coercivity than that of the NdFeB magnets, which suggests that it has considerable potential as a permanent magnet for use at high temperatures. In the temperature range from 30 to 150 K, the Mn atom is found to change its spin direction from a perpendicular to a parallel orientation with respect to the c axis. The anisotropy field increases with increasing temperature which gives rise to a higher coercivity at the higher temperatures. The maximum energy product (BH)max of the magnet is 7.7 and 4.6 MG Oe at room temperature and 400 K, respectively. The electronic structure of MnBi indicates that the Mn atom possesses a magnetic moment of 3.6μB, and that the Bi atom has a magnetic moment of −0.15μB which is due to the s–d and p–d hybridization between Bi and Mn atoms. We have also investigated the volume dependence of the magnetic moments of Mn and Bi. The results indicate that an increase in the intra-atomic exchange splitting due to the cell volume expansion leads to a large magnetic moment for the Mn atom. The Mn magnetic moment attains a value of 4.6μB at a volume expansion rate of ΔV/V ≈ 100%.

Synthesis of alkyl sulfonate/alcohol-protected γ -Fe2O3 nanocrystals with narrow size distributions

Highly crystalline γ -Fe2O3 nanoparticles with narrow size distributions that are coated with 1-undecanesulfonic acid were synthesized via two distinct approaches using oxidation and site-exchange reactions. However, similar nanocrystals protected with 1-octanol could only be achieved via the site-exchange method, while the oxidation approach led to Fe2O3 nanoparticles of poor crystallinity and size uniformity. Our magnetization measurements confirmed the superparamagnetic nature of our Fe 2O3 nanoparticle products and the effects of the coating materials on magnetization properties.  2003 Elsevier Science (USA). All rights reserved.

Magnetic behavior of mechanically Milled FeNi-CoO nanocomposites

The effects of crystallite size and ball-mill-induced defects on the magnetic properties of ferromagnetic (FM) FeNi-antiferromagnetic (AFM) CoO (Neel Temperature=290 K) nanocomposites are assessed by magnetic measurements. Asymmetric zero-field-cooled (ZFC) hysteresis loops were observed for samples milled for 30h and longer. The field-cooled (FC) hysteresis loops were asymmetric and shifted in the direction opposite to the cooling field. The enhancement of coercivity and squareness ratio (M/sub R//M/sub S/), along with a presence of a loop shift after field cooling indicates the presence of an exchange bias effect between the FM and AFM phases of the composite. The exchange bias field extracted from the 5 K FC measurement shows continuous enhancement with milling times up to 30 hours and a reduction upon prolonged milling. The temperature dependent exchange bias field measurement shows that the exchange bias field reduces to zero at temperature T<200 K<T/sub N/ (290 K) of CoO. Thus, in agreement with thin film systems, exchange bias properties can be obtained in FM-AFM fine powder nanocomposites upon mechanical milling. Further, the present study has the potential implication of improving magnetic properties of hard magnetic materials upon milling with antiferromagnetic materials.

Study of the colossal magnetoresistance properties of the compound La1-xSrxAyMn1-yO3 (A = Cr, Re)

We report on the variation of the magnetoresistance property of La1-xSrxMnO3 whose Mn sites are doped with chromium and rhenium. Two different series of compounds of the formula La0.7Sr0.3CryMn1-yO3 (y = 0.05, 0.10, 0.15, 0.20, 0.30, 0.40 and 0.50) and La1-xSrxRe0.10Mn0.90O3 (x = 0.12, 0.20 and 0.30) are prepared. A detailed study has been carried out to determine the structural, magnetic and transport properties of the chromium and rhenium doped compounds using the x-ray diffraction, dc magnetic susceptibility, four probe method for electrical resistance and magnetoresistance measurement techniques. X-ray diffraction shows the formation of a homogeneous compound with the rhombohedral (R-3c space group) structure within the Cr concentration of 0.05<y<0.15 for the La0.7Sr0.3CryMn1-yO3 and 0.12<x<0.30 for the La1-xSrxRe0.10Mn0.90O3 compounds. The magnetization, magnetic transition temperature and the metal-insulator transition temperature decrease due to the Cr (Re) substitution. The magnetization curves broaden and the overall resistivity of the compounds increases with increasing Cr or decreasing Sr content except for the La0.88Sr0.12Re0.10Mn0.90O3 compound, whose resistivity decreases and which shows metallic behaviour below the transition temperature. The resistivity and the magnetization show a scaling dependence on each other. It is shown that the variation in the magnetic and transport properties of the substituted compounds can be ascribed to a compositional dependence of the Mn3+-Mn4+ ferromagnetic interaction as a result of the antiferromagnetic interaction of Cr3+-O-Cr3+ ions and to the distortion of the crystal lattice. The magnetoresistance shows an increasing trend due to the increased resistivity of the compounds and is explained on the basis of percolation of the charge carriers through the ferromagnetic clusters present in the insulating antiferromagetic compounds and scattering effect at the grain boundaries.

Magnetic phase transitions in (Tb,Y)Mn2M2 (M=Ge and Si) systems

The temperature- and field-induced magnetic phase transitions in pseudoternary systems Tb1−xYxMn2Ge2 and Tb1−xYxMn2Si2, which are characterized by a separate magnetic ordering of the Mn and Tb subsystems, have been studied on strongly textured samples by magnetization and thermal expansion measurements. In the concentration region 0.4⩽x⩽1.0, where the Mn moments are ordered only, the magnetic structure is antiferromagnetic. The magnetic ordering in the Tb subsystem is accompanied by a transformation of the Mn magnetic structure. As a result, the compounds show a spontaneous magnetization. This transition is of a first-order type. In both systems the transition temperature for the Tb subsystem TC(Tb) decreases linearly with increasing Y concentration and goes to zero at about x=0.40. Canted phases and field-induced first-order transitions into the collinear phase have been observed in both systems below this temperature. It is concluded that the f–d exchange interaction is the primary factor governing the ...

Magnetic and electronic transport properties of YbxCa1−xMnO3 compounds

The polycrystalline YbxCa1−xMnO3 (x=0.1,0.2) were studied by x-ray powder diffraction, magnetic, and electrical resistivity measurements. The YbxCa1−xMnO3 crystallizes in an orthogonally distorted perovskite structure, and shows the ferromagnetic ordering with TC more than 110 K. However, the field dependence of magnetization and electrical resistivity exhibits very complicated behavior. A magnetic field induced insulator–metal transition has been found in the Yb0.2Ca0.8MnO3 compound. In addition, the large asymmetry in magnetization and resistivity hysteresis loops has been observed in this compound at 10 K, which might be due to the charge ordering and magnetocrystalline anisotropy.