arXiv subjects

A. K. Nigam

Publications and source records attributed to A. K. Nigam.

At least 73 records · Page 4Linked to original sources

arXiv2010

Magnetic, magnetocaloric and neutron diffraction studies on TbNi5-xMx (M = Co and Fe) compounds

The effect of substitution of Co and Fe for Ni in TbNi5 on the structural, magnetic and magneto-thermal properties has been investigated. Considerable enhancement of Curie temperature is observed with Fe substitution, whereas the increase is nominal in the case of Co. Neutron diffraction measurements reveal the redistribution of moments and site preference of substitutional ions in Ni 2c and 3g sites. In TbNi4Fe, both Ni and Fe as well as Tb are found to carry moment while in the case of TbNi4Co, mainly Tb carries the moment. Magnetocaloric behavior has been investigated from the magnetization and the heat capacity measurements. The magnetic and magnetocaloric properties are found to be strongly correlated in these compounds.

cond-mat.str-el↗

arXiv2010

Structure, Transport and Magnetic properties in La$_{2x}$Sr$_{2-2x}$Co$_{2x}$Ru$_{2-2x}$O$_{6}$

The perovskite solid solutions of the type La$_{2x}$Sr$_{2-2x}$Co$_{2x}$Ru$_{2-2x}$O$_{6}$ with 0.25 $\leq$ x $ \leq $ 0.75 have been investigated for their structural, magnetic and transport properties. All the compounds crystallize in double perovskite structure. The magnetization measurements indicate a complex magnetic ground state with strong competition between ferromagnetic and antiferromagnetic interactions. Resistivity of the compounds is in confirmation with hopping conduction behaviour though differences are noted especially for $x$ = 0.4 and 0.6. Most importantly, low field (50Oe) magnetization measurements display negative magnetization during the zero field cooled cycle. X-ray photoelectron spectroscopy measurements indicate presence of Co$^{2+}$/Co$^{3+}$ and Ru$^{4+}$/Ru$^{5+}$ redox couples in all compositions except $x$ = 0.5. Presence of magnetic ions like Ru$^{4+}$ and Co$^{3+}$ gives rise to additional ferromagnetic (Ru-rich) and antiferromagnetic sublattices and also explains the observed negative magnetization.

cond-mat.str-el↗

arXiv2010

Effect of B-site Dopants on Magnetic and Transport Properties of LaSrCoRuO$_6$

Effect of Co, Ru and Cu substitution at B and B' sites on the magnetic and transport properties of LaSrCoRuO$_6$ have been investigated. All the doped compositions crystallize in the monoclinic structure in the space group $P2_1/n$ indicating a double perovskite structure. While the magnetization and conductivity increase in Co and Ru doped compounds, antiferromagnetism is seen to strengthen in the Cu doped samples. These results are explained on the basis of a competition between linear Co-O-Ru-O-Co and perpendicular Co-O-O-Co antiferromagnetic interactions and due to formation of Ru-O-Ru ferromagnetic networks.

cond-mat.str-el↗

arXiv2010

Supression of electron correlations in the superconducting alloys of Rh$_{17-x}$Ir$_x$S$_{15}$

We have studied the effect of Iridium doping (Rh$_{17-x}$Ir$_{x}$S$_{15}$) in the Rhodium sites of the strongly correlated superconductor Rh$_{17}$S$_{15}$. Even at low levels of doping (x = 1 and 2) we see a drastic change in the superconducting properties as compared to those of the undoped system. We deduce that there is a reduction in the density of states at the Fermi level from reduced Pauli susceptibility and Sommerfeld coefficient in the doped samples. Moreover, the second magnetization peak in the isothermal magnetization scan (`fishtail') which was very prominent in the magnetization data of the undoped crystal is suppressed in the doped samples. The temperature dependence of resistivity of the doped crystals show a remarkably different behavior from that of the undoped crystal with the appearance of a minima at lower temperatures, the position of which is fairly constant at different fields. Our data supports the notion that Iridium, which is a bigger atom than Rhodium expands the lattice thereby, reduces the electron correlations that existed due to the interaction between closer lying Rhodium atoms in the undoped system.

cond-mat.supr-con↗

arXiv2010

Field induced changes across magnetic compensation in Pr(1-x)Gd(x)Al(2) alloys

The magnetic compensation phenomenon has been explored in the Pr(1-x)Gd(x)Al(2) series. The contributions from Pr and Gd moments compensate each other at a specific temperature in the ordered state (below (T(c)). At high fields, the magnetic reorientation (with respect to the external field direction) of the Pr and Gd moments appears as a minimum in the thermomagnetic response. We demonstrate several interesting attributes related with the magnetic reorienation phenomenon, viz., oscillatory behavior of the magneto-resistance, sign change of the anamalous Hall resistivity, fingerprints of field induced changes in the specific heat and ac-susceptibility data.

cond-mat.str-el↗

arXiv2010

Disorder Induced Negative Magnetization in LaSrCoRuO6

This paper reports effect of thermally induced disorder on the magnetic properties of LaSrCoRuO6 double perovskite. While the ordered sample is antiferromagnetic, the disordered sample exhibits negative values of magnetization measured in low applied fields. Isothermal magnetization on this sample shows hysteresis due to presence of ferromagnetic interactions. Based on neutron diffraction and X-ray Absorption Fine Structure (XAFS) studies, these results have been interpreted to be due disorder in site occupancy of Co and Ru leading to octahedral distortions and formation of Ru-O-Ru ferromagnetic linkages. Below 150K these ferromagnetic Ru spins polarize the Co spins in a direction opposite to that of the applied field resulting in observed negative magnetization.

cond-mat.str-el↗

arXiv2010

Observation of spontaneous magnetization jump and field-induced irreversibility in Nd5Ge3

We report the observation of spontaneous and ultra-sharp jumps in the low temperature magnetization isotherms of polycrystalline Nd5Ge3. Field-induced and ultra-sharp jumps are also seen in resistivity and heat capacity data. These jumps are accompanied by field-induced irreversibilities. The consistency seen in these three data clearly shows that the spin, electronic and lattice states are strongly coupled. Time-induced growth of the ferromagnetic phase is observed at a constant field and temperature, implying the metastability of the magnetic phase in the low field region. Various experimental findings point towards a strong field-induced magneto-structural irreversibility in this compound.

cond-mat.str-el↗

arXiv2010

Observation of re-entrant spin glass behavior in (Ce1-xErx)Fe2 compounds

Clear experimental evidence of re-entrant spin glass state has been revealed in Er doped CeFe2 compounds. The zero field cooled - field cooled bifurcation in dc magnetization, frequency dependence of freezing temperature, relaxation in zero field cooled magnetization and presence of large remanence confirm the spin glass state in these compounds. Frequency dependence is found to follow the critical slowing down mechanism. The random substitution of Er and the change in the valence state of Ce along with an enhancement of the ferromagnetic component in the Fe sublattice seem to be responsible for the spin glass state. Using detailed experimental protocols, we also prove that the low temperature state in these compounds is not a magnetic glass. The absence of exchange bias gives an indication that there is no coexistence of ferromagnetism and spin glass state in these compounds. The RSG state is found to be associated with the randomly magnetized clusters instead of atomic level randomness.

cond-mat.str-el↗

arXiv2010

On the question of metallicity and ferromagnetism in charge ordered insulating Nd0.5Sr0.5MnO3 nanoparticles

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 and discuss the overwhelming evidence from experiments on nano and bulk manganites as well as the absence of correlation between size reduction and pressure effects on manganites. We propose a phenomenological understanding on the basis of enhanced surface disorder to explain the appearance of weak ferromagnetism and metallicity in nanoparticles of Nd0.5Sr0.5MnO3. We also provide supportive evidence from an ab-initio electron structure calculation and a recent numerical simulation and argue that the mechanism is universal in all nanosize charge ordered manganites.

cond-mat.str-el↗

arXiv2010

Pressure induced magnetic and magnetocaloric properties in NiCoMnSb Heusler alloy

The effect of pressure on the magnetic and the magnetocaloric properties around the martensitic transformation temperature in NiCoMnSb Heusler alloy has been studied. The martensitic transition temperature has significantly shifted to higher temperatures with pressure, whereas the trend is opposite with the application of applied magnetic field. The maximum magnetic entropy change around the martensitic transition temperature for Ni45Co5Mn38Sb12 is 41.4 J/kg K at the ambient pressure, whereas it is 33 J/kg K at 8.5 kbar. We find that by adjusting the Co concentration and applying suitable pressure, NiCoMnSb system can be tuned to achieve giant magnetocaloric effect spread over a large temperature span around the room temperature, thereby making it a potential magnetic refrigerant material for applications.

cond-mat.str-el↗

arXiv2010

Irreversibility of field-induced magnetostructural transition in NiCoMnSb shape memory alloy revealed by magnetization, transport and heat capacity studies

The effects of magnetic field on the martensitic transition have been studied in Ni45Co5Mn38Sb12. We find a large field-induced irreversibility in this system, as revealed by the field dependence of resistivity, magnetization, and heat capacity data. At the critical temperature, the field-induced conversion of the martensitic to austenite phase is not reversible under any field variation. At this temperature any energy fluctuation induces nucleation and growth of the equilibrium austenite phase at the expense of the metastable martensitic phase and gets arrested. All these three measurements completely rule out the coexistence of austenite and martensitic phases in the irreversibility regime.

cond-mat.str-el↗

arXiv2010

Observation of enhanced exchange bias behavior in NiCoMnSb Heusler alloys

We report the observation of large exchange bias in Ni50-xCoxMn38Sb12 Heusler alloys with x=0, 2, 3, 4, 5, which is attributed to the coexistence of ferromagnetic and antiferromagnetic phases in the martensitic phase. The phase coexistence is possibly due to the supercooling of the high temperature ferromagnetic phase and the predominant antiferromagnetic component in the martensitic phase. The presence of exchange bias is well supported by the observation of training effect. The exchange bias field increases with Co concentration. The maximum value of 480 Oe at T=3K is observed in x=5 after field cooling in 50 kOe, which is almost double the highest value reported so far in any Heusler alloy system. Increase in the antiferromagnetic coupling after Co substitution is found to be responsible for the increase in the exchange bias.

cond-mat.str-el↗

arXiv2010

Magnetic and the magnetocaloric properties of Ce1-xRxFe2 and Ce(Fe1-xMx)2 compounds

We have studied selected rare earth doped and transition metal doped CeFe2 compounds by examining their structural, magnetic and magneto-thermal properties. With substitution of Ce by 5 and 10% Gd and 10% Ho, the Curie temperature can be tuned to the range of 267-318 K. Localization of Ce 4f electronic state with rare earth substitutions is attributed for the enhancement of Curie temperature. On the other hand, with Ga and Al substitution at the Fe site, system undergoes paramagnetic to ferromagnetic transition and then to an antiferromagnetic phase on cooling. The magnetocaloric effect across the transitions has been studied from both magnetization isotherms and heat capacity data. It is shown that by choosing the appropriate dopant and its concentration, the magnetocaloric effect around room temperature can be tuned.

cond-mat.str-el↗

arXiv2010

Magnetostructural transition in Ce(Fe0.975Ga0.025)2 compound

The magnetic and magnetostructural properties of the polycrystalline Ce(Fe0.975Ga0.025)2 have been investigated as a function of temperature and magnetic field. In Ce(Fe0.975Ga0.025)2 the magnetic transition from antiferromagnetic (AFM) to ferromagnetic state (FM) is accompanied by a structural transformation from rhombohedral to cubic structure. Phase coexistence is present during both the temperature and field driven transformations from the AFM to FM phase.

cond-mat.str-el↗

arXiv2010

Magnetization jumps and relaxation effect in doped CeFe2

For the first time, we find that the dynamic antiferromagnetic phase present in CeFe2 gets stabilized with Ga and Si substitutions. We find that phenomena such as strain-induced first order jumps in the magnetization curves, asymmetry between the M-H curves during the increasing and decreasing field cycles, the envelope curve being inside the virgin curve, occur in these compounds. Temperature and time dependences of magnetization show that the compounds possess glassy behavior at low temperatures. Multi-step magnetization behavior, unusual relaxation effect, thermal and magnetic history dependence, which are signatures of the martensitic scenario due to the strong magneto-structural coupling, are found to be present in this system. We also show that one can induce the magnetization steps with the help of appropriate measurement protocol. Detailed magnetization relaxation studies have been carried out to understand the dynamics of magnetic phase transition.

cond-mat.str-el↗

arXiv2010

Metastable magnetization behavior in magnetocaloric R6Co1.67Si3 (R=Tb and Nd) compounds

Magnetic field and time induced steps have been observed in the recently discovered ternary silicide R6Co1.67Si3. Huge relaxation steps are observed across different loops in the low temperature magnetization isotherms. Giant relaxation present in this system indicates the existence of incubation time to get the saturated moment at a certain field. Measurement protocol sensitive magnetization behavior observed in this system may arise from the strong magnetostructural coupling and/or magnetic frustration. Electrical resistivity and magnetoresistance also reflect the magnetic state of the compound. Magnetocaloric effect is found to be large at temperatures close to the magnetic transition temperature.

cond-mat.str-el↗

arXiv2010

Temperature and magnetic field induced structural transformation in Si doped CeFe2: in-field x-ray diffraction study

Using x-ray powder diffraction technique at various temperatures and applied magnetic fields, we have studied the magnetostructural properties of Ce(Fe0.95Si0.05)2. The x-ray diffraction data establish quantitative relationships between bulk magnetization and the evolution of structurally distinct phases with magnetic field and temperature, and confirm the distinct features of first order phase transition like supercooling and superheating, metastability, and phase co-existence of different structural polymorphs. We observe the lattice volume mismatch across the structural phase transition, which appears to be the cause for the step behavior of the magnetization isotherms at low temperatures. The present study shows that the lattice distortion has to be treated explicitly, like spin, along with the effects of lattice-spin coupling to account for the magnetization behavior of this system. This structure template can resolve the issue of kinetics in this material as observed in different time scale measurements and with different experimental protocols.

cond-mat.str-el↗

arXiv2010

Stabilization of antiferromagnetism in CeFe2 alloys: Effects of chemical and hydrostatic pressure

Effects of Al, Mn and Sb dopings in CeFe2 and effect of applied pressure have been investigated. Al doping gives rise to the FM-AFM transition and a reduction in the magnetic moment and TC values, clearly indicating the growth of the AFM component. Mn and Sb dopings only cause a reduction in TC value. It is found that in general external pressure enhances the antiferromagnetism in both the pure and the doped alloys. Enhancement of the Ce 4f- Fe 3d hybridization as a result of dopings and with the external pressure may be the reason for the stabilization of antiferromagnetism in these alloys.

cond-mat.str-el↗