arXiv subjects

S. K. Malik

Publications and source records attributed to S. K. Malik.

At least 37 records · Page 2Linked to original sources

arXiv2006

Magnetic avalanche-like behavior in disordered manganite (Eu0.4La0.1)(Sr0.4Ca0.1)MnO3

The half-doped manganite, (Eu0.4La0.1)(Sr0.4Ca0.1)MnO3, has been found to exhibit sharp step-like metamagnetic transitions below 5 K. The number of magnetic steps increases with decreasing temperature and this number suddenly rises from 3 at 2 K to ~50 at 1.7 K. The self-similar character of the multiple magnetic steps at reduced temperature identifies these steps as avalanche-like transitions. The occurrence of a multitude of magnetic steps at low temperatures, the decreasing effect of magnetic field sweep rate on the step-like metamagnetism, and the decreasing irreversibility of transition in the specific heat suggest that reduced spin-lattice coupling facilitates the transformation of antiferromagnetic to ferromagnetic state via an avalanche-like behavior.

cond-mat.str-el↗

arXiv2005

Magnetic Properties of Ternary Gallides of type RNi4Ga (R = Rare earths)

The magnetic properties of RNi4Ga (R = La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu) compounds have been investigated. These compounds form in a hexagonal CaCu5 type structure with a space group P6/mmm. Compounds with the magnetic rare earths, R = Nd, Sm, Gd, Tb, Dy, Ho, Er and Tm, undergo a ferromagnetic transition at 5 K, 17 K, 20 K, 19 K, 12 K, 3.5 K, 8 K and 6.5 K, respectively. The transition temperatures are smaller compared to their respective parent compounds RNi5. PrNi4Ga is paramagnetic down to 2 K. LaNi4Ga and LuNi4Ga are Pauli paramagnets. All the compounds show thermomagnetic irreversibility in the magnetically ordered state except GdNi4Ga.

cond-mat.str-el↗

arXiv2005

Magnetic and transport properties of Mo substituted La0.67Ba0.33Mn1-xMoxO3 perovskite system

The effect of doping Mo for Mn on the magnetic and transport properties of the colossal magnetoresistance material, La0.67Ba0.33MnO3, has been studied. Compounds of the series La0.67Ba0.33Mn1-xMoxO3 (x=0.0 to 0.1) have been prepared and found to crystallize in the orthorhombic structure (space group Pbnm). Energy Dispersive X-ray Analysis (EDAX) measurements confirm the stoichiometry of all the samples. Magnetotransport and magnetization measurements reveal that the metal-insulator transition temperature (Tp) decreases from 330K for x=0 to 255K for x=0.1. The change in Tp on Mo substitution is relatively much smaller than the corresponding change observed on substitution by other transition elements, such as Ti, Fe, Co, Ni, etc. Further, the ferromagnetic transition temperature (TC) is nearly unchanged by Mo substitution. This is in striking contrast to the large decrease in TC observed with substitution of above-mentioned 3d elements. These unusual magnetic and transport properties of La0.67Ba0.33Mn1-xMoxO3 may be either due to the formation of magnetic pair between Mn and Mo or due to strong Mo(4d)-O(2p) overlap, which in turn, may affect the Mn-Mn interaction via the oxygen atoms

cond-mat.mtrl-sci↗

arXiv2005

Competition between Superconductivity and Charge Density Wave Ordering in the Lu$_5$Ir$_4$(Si$_{1-x}$Ge$_x$)$_{10}$ Alloy System

We have performed bulk measurements such as dc magnetic susceptibility, electrical resistivity and heat capacity on the pseudo-ternary alloys Lu$_5$Ir$_4$(Si$_{1-x}$Ge$_x$)$_{10}$ to study the interplay and competition between superconductivity and the charge density wave (CDW) ordering transition. We track the evolution of the superconducting transition temperature T$_{SC}$ and the CDW ordering temperature T$_{CDW}$ as a function of x (concentration of Ge) ($0.0 \leq x\leq~1.0$). We find that increasing x (increasing disorder) suppresses the T$_{CDW}$ rapidly with the concomitant increase in T$_{SC}$. We present a temperature-concentration (or volume) phase diagram for this system and compare our results with earlier work on substitution at the Lu or Ir site to show how dilution at the Si site presents a different situation from these other works. The heat capacity data in the vicinity of the CDW transition has been analyzed using a model of critical fluctuations in addition to a mean-field contribution and a smooth lattice background. We find that the critical exponents change appreciably with increasing disorder. This analysis suggests that the strong-coupling and non mean-field like CDW transition in the parent compound Lu$_5$Ir$_4$Si$_{10}$ changes to a mean-field like transition with increasing Ge concentration.

cond-mat.str-el↗

arXiv2005

High temperature magnetic ordering in La2RuO5

Magnetic susceptibility, heat capacity and electrical resistivity measurements have been carried out on a new ruthenate, La2RuO5 (monoclinic, space group P21/c) which reveal that this compound is a magnetic semiconductor with a high magnetic ordering temperature of 170K. The entropy associated with the magnetic transition is 8.3 J/mole-K -close to that expected for the low spin (S=1) state of Ru4+ ions. The low temperatures specific heat coefficient g is found to be nearly zero consistent with the semiconducting nature of the compound. The magnetic ordering temperature of La2RuO5 is comparable to the highest known Curie temperature of another ruthenate, namely, metallic SrRuO3, and in both these compounds the nominal charge state of Ru is 4+.

cond-mat.mtrl-sci↗

arXiv2005

Magnetism in C60 Films Induced by Proton Irradiation

It is shown that polycrystalline fullerene thin films on hydrogen passivated Si(111) substrates irradiated by 2 MeV protons displays ferromagnetic-like behavior at 5 K. At 300 K both the pristine and the irradiated film show diamagnetic behavior. Magnetization data in the temperature range 2 - 300 K, in 1 Tesla applied field, for the irradiated film shows much stronger temperature dependence compared to the pristine film. Possible origin of ferromagnetic-like signals in the irradiated films are discussed.

cond-mat.mtrl-sci↗

arXiv2005

The effect of Cu-doping on the magnetic and transport properties of La0.7Sr0.3MnO3

The effects of Cu-doping on the structural, magnetic, and transport properties of La0.7Sr0.3Mn1-xCuxO3 (0 < x < 0.20) have been studied using neutron diffraction, magnetization and magnetoresistance (MR) measurements. All samples show the rhombohedral structure with the R3c space-group from 10K to room temperature (RT). Neutron diffraction data suggest that some of the Cu ions have a Cu3+ state in these compounds. The substitution of Mn by Cu affects the Mn-O bond length and Mn-O-Mn bond angle resulting from the minimization of the distortion of the MnO6 octahedron. Resistivity measurements show that a metal to insulator transition occurs for the x more than 0.15 samples. The x = 0.15 sample shows the highest MR(_80%), which might result from the co-existence of Cu3+/Cu2+ and the dilution effect of Cu-doping on the double exchange interaction.

cond-mat.mtrl-sci↗

arXiv2005

Structure, magnetic and transport properties of Ti-substituted La0.7Sr0.3MnO3

Ti-substituted perovskites, La0.7Sr0.3Mn1-xTixO3, with x between 0 to 0.20, were investigated by neutron diffraction, magnetization, electric resistivity, and magnetoresistance (MR) measurements. All samples show a rhombohedral structure (space group R3c) from 10 K to room temperature. At room temperature, the cell parameters a, c and the unit cell volume increase with increasing Ti content. However, at 10 K, the cell parameter a has a maximum value for x = 0.10, and decreases for x greater than 0.10, while the unit cell volume remains nearly constant for x greater than 0.10. The average (Mn,Ti)-O bond length increases up to x=0.15, and the (Mn,Ti)-O-(Mn,Ti) bond angle decreases with increasing Ti content to its minimum value at x=0.15 at room temperature. Below the Curie temperature T_C, the resistance exhibits metallic behavior for the x _ 0.05 samples. A metal (semiconductor) to insulator transition is observed for the x_ 0.10 samples. A peak in resistivity appears below T_C for all samples, and shifts to a lower temperature as x increases. The substitution of Mn by Ti decreases the 2p-3d hybridization between O and Mn ions, reduces the bandwidth W, and increases the electron-phonon coupling. Therefore, the TC shifts to a lower temperature and the resistivity increases with increasing Ti content. A field-induced shift of the resistivity maximum occurs at x less than or equal to 0.10. The maximum MR effect is about 70% for La0.7Sr0.3Mn0.8Ti0.2O3. The separation of TC and the resistivity maximum temperature Tmax enhances the MR effect in these compounds due to the weak coupling between the magnetic ordering and the resistivity as compared with La0.7Sr0.3MnO3.

cond-mat.mtrl-sci↗

arXiv2005

Magnetocaloric effect in the intermetallic compound DyNi

Magnetic and heat capacity measurements have been carried out on the polycrystalline sample of DyNi which crystallizes in the orthorhombic FeB structure (space group Pnma). This compound is ferromagnetic with a Curie temperature of 59 K. Magnetization-field isotherms at low temperatures shows a step-like behavior characteristic of metamagnetic transitions. The magnetocaloric effect has been measured both in terms of isothermal magnetic entropy change and adiabatic temperature change for various applied magnetic fields. The maximum values of the entropy change and the temperature change are found to be 19 Jkg-1K-1 and 4.5 K, respectively, for a field of 60 kOe. The large magnetocaloric effect is attributed to the field-induced spin-flop metamagnetism occurring in this compound, which has a noncollinear magnetic structure at low fields.

cond-mat.str-el↗

arXiv2005

Heat capacity and magnetoresistance in Dy(Co,Si)2 compounds

Magnetocaloric effect and magnetoresistance have been studied in Dy(Co1-xSix)2 [x=0, 0.075 and 0.15] compounds. Magnetocaloric effect has been calculated in terms of adiabatic temperatue change (Delta Tad) as well as isothermal magnetic entropy change (Delta SM) using the heat capacity data. The maximum values of DeltaSM and DeltaTad for DyCo2 are found to be 11.4 JKg-1K-1 and 5.4 K, respectively. Both DSM and DTad decrease with Si concentration, reaching a value of 5.4 JKg-1K-1 and 3 K, respectively for x=0.15. The maximum magnetoresistance is found to about 32% in DyCo2, which decreases with increase in Si. These variations are explained on the basis of itinerant electron metamagnetism occurring in these compounds.

cond-mat.mtrl-sci↗

arXiv2005

Magnetic and Transport Properties of Ternary Indides of type R2CoIn8 (R = Ce, Pr and Dy)

We have synthesized and investigated the magnetic and transport properties of a series of compounds, R2CoIn8 (R = rare earth). Compounds form in single phase with a tetragonal structure (space group P4/mmm, no. 162). The Ce compound shows heavy fermion behavior. The magnetic susceptibility of Pr2CoIn8 shows a marked deviation from the Curie-Weiss behavior at low temperatures, which is attributed to the crystalline electric field effects. Heat capacity and magnetization measurements show that Dy2CoIn8 undergoes a magnetic transition at 17 K and a second transition near 5 K, the latter of which may be due to spin reorientation. Magnetization of this compound shows two metamagnetic transitions approximately at 3.6 T and 8.3 T.

cond-mat.mtrl-sci↗

arXiv2005

Observation of magnetoelectric behavior at room temperature in Pb(Fe1-xTix)O3

The coexistence of ferroelectric and ferromagnetic properties at room temperature is very rarely observed. We have been successful in converting ferroelectric PbTiO3 into a magnetoelectric material by partly substituting Fe at the Ti site. The Pb(Fe1-xTix)O3 system exhibits ferroelectric and ferromagnetic ordering at room temperature. Even more remarkably, our results demonstrate a coupling between the two order parameters. Hence it could be a futuristic material to provide cost effective and simple path for designing novel electromagnetic devices.

cond-mat.mtrl-sci↗

arXiv2005

Magnetism and Transport in YbMn2Sb2

A new ternary intermetallic compound, namely, YbMn2Sb2, has been synthesized and its magnetic and electrical transport properties have been studied in the temperature range of 2 to 300 K. This compound crystallizes in a trigonal, La2O2S type structure (space group P3bm1, No. 164) and is found to be ferromagnetically ordered at room temperature. The magnetism is attributed to the ordering of Mn sublattice. M5 xray absorption spectrum of YbMn2Sb2 obtained at room temperature suggests that the valency of Yb in this compound is close to 2. Electrical resistivity of this compound is metal like and a positive magnetoresistance of 13 percent is observed at 5 K in an applied field of 9T. Key words Rare earth intermetallics and alloys, Magnetic properties, Xray absorption spectroscopy, Electrical transport.

cond-mat.mtrl-sci↗

arXiv2004

Large magnetoresistance in low temperature metallic region of manganite compounds (La0.7-2xEux)(Ca0.3Srx)MnO3 (0.05<x<0.15)

Magnetoresistance (MR) and magnetization (d.c and a.c) measurements have been carried out on the manganites, (La0.7-2xEux)(Ca0.3Srx)MnO3 (0.05<x<0.15), in the temperature range of 5K-320K. At 5K, an unusually large MR of almost 98% is observed in the x=0.15 sample, nearly up to fields of 4-5 Tesla. This large high-field MR occurs in the metallic region, far below the insulator-metal transition temperature, and does not vary linearly with applied field. The unusual magnetoresistance is explained the light of various possibilities of phase segregation and cluster spin-glass behavior. Keywords: Manganites, High field MR, Phase segregation.

cond-mat.mtrl-sci↗

arXiv2004

Resistivity and Thermoelectric power of NaxCoO2 (x =1.0, 0.7 and 0.6) system

Results of thermo-electric power (S) and electrical resistivity (r) measurements are reported on NaxCoO2 compounds with x = 1.0, 0.7 and 0.6. These are single-phase compounds crystallizing in the hexagonal structure (space group P63/mmc) at room temperature. Thermo-electric power values at 300K (S300K) are, 80mV/K, 39mV/K and 37mV/K for x = 1.0, 0.7 and 0.6 samples, respectively. The samples with x=0.7 and 1.0 are metallic down to 5 K, while the x = 0.6 sample is semiconducting. The value of r300K for x = 1.0 sample is \~0.895 mW-cm and the power factor (S2/r) is = 7.04 x 10-3 W/mK2 which qualifies it as a good thermo-electric material. In x =1.0 sample, S(T) is positive throughout 300-5K temperature range and decreases monotonically to zero as temperature T= 0. In contrast, S(T) of x = 0.7 and 0.6 samples changes sign and shows negative values between 90 K and 16 K before approaching zero as T = 0. Anomalous S(T) behavior of x = 0.6 and 0.7 samples, which are coincidentally the precursor materials to the reported superconductivity in this class of materials, indicates a dramatic change in the electronic structure of these compounds on lowering the Na content.

cond-mat.str-el↗

arXiv2004

Synthesis, structure and magnetization studies on (Fe1-xCrx)2TeO6 tellurate system

The effect of substitution of Fe by Cr on the structural and magnetic properties of Fe2TeO6 in the system (Fe1-xCrx)2TeO6, (0<x<1) has been investigated on polycrystalline samples synthesized by solid state reaction. The lattice parameters are found to obey Vegard's law in the entire composition range. The Neel temperature decreases linearly with increasing Cr content from 230 K for x=0 to 90 K for x=1 and is attributed to the weakening of antiferromagnetic exchange interaction as Cr3+ replaces Fe3+ in Fe2TeO6.

cond-mat.mtrl-sci↗

arXiv2004

Growth and magnetic properties of MnO2−delta nanowire microspheres

We report the synthesis of MnO2−delta microspheres using hydrothermal and conventional chemical reaction methods. The microspheres of MnO2−delta consist of nanowires having a diameter of 20-50 nm and a length of 2-8 micro m. The value of the oxygen vacancy delta estimated from the x-ray photoelectron spectrum (XPS) is 0.3. The magnetization versus temperature curve indicates a magnetic transition at about 13 K. It is found that a parasitic ferromagnetic component is imposed on the antiferromagnetic structure of MnO2−delta, which might result from distortion of the lattice structure due to oxygen vacancies. The magnetic transition temperature TN is about 10 K lower than that of the bulk MnO2 single crystal.

cond-mat.mtrl-sci↗

arXiv2004

Induction of superconductivity in Y0.4Pr0.6Ba2-xSrxCu3O7 system with increasing Sr substitution

Samples of Y0.4Pr0.6Ba2-xSrxCu3O7 have been synthesized by a solid- state reaction route. The samples with x = 0.60 crystallize in an orthorhombic structure (with orthorhombic distortion decreasing with increasing x), while the samples with x=0.80 and 1.0 crystallize in a pseudo tetragonal structure. Resistance (R) measurements as a function of temperature (T) show that x = 0.0 sample is highly semiconducting with R2K/R300K ratio of ~ 65. This ratio decreases to only ~23 and 5 for x=0.20 and x=0.4 samples, respectively. Further, the x=0.60 sample shows onset of a broad superconducting transition temperature (Tconset) at around 20 K without achieving the TcR=0 state down to 2 K. The TcR=0 state is observed in x = 0.80 and 1.00 samples at around 5 and 14 K, respectively. Thermo-electric power (S), also exhibits TcS~0 state at around 11 K and 16 K, respectively for x = 0.8 and 1.00 samples. Thermo electric power at room temperature is positive and decreases with increasing x, indicating enhanced number of mobile holes. These results demonstrate that substitution of Sr at Ba-site systematically induces metallicity and eventually superconductivity in Y0.4Pr0.6Ba2-xSrxCu3O7 system. The results are explained on the basis of reduced Pr(4f) orbital hybridization with O(2p) in supercondu cting Cu-O2 planes, resulting in delocalization of the mobile carriers. The role of decreased Pr/Ba intermixing disorder is also considered.

cond-mat.supr-con↗