51 research outputs found
Skyrmion lattice and critical behavior of Fe<inf>0.6</inf>Co<inf>0.4</inf>Si: A bulk magnetization study
No full textUnique properties of nickel nanoparticles
No full textIntroduction: Magnetic nanoparticles (MNPs) are very attractive for applications in magnetic resonance imaging, MRI, magnetic fluids, catalysts, magnetic recording media, rechargeable batteries, optoelectronics, conducting paints, magnetic hyperthermia and other biomedical applications (1). MNPs dispersed in a liquid medium tend to agglomerate due to van der Waals or other attractive forces prevailing on surface. This necessitates them to coat with an appropriate capping agent. The ligand molecules bound to nanoparticle (NP) surface not only control the growth of the particles during synthesis, but also prevent them from aggregation and decides the future of how the NPs form, agglomerate and respond to a given situation. To understand some of these characteristics, we prepared nickel NPs and demonstrate formation of nanolattice, quantum size effect (QSE) and surface plasmon resonance (SPR).
Methods: We prepared the monodispersed Ni NPs by thermal decomposition methods using trioctylphosphine (TOP) or combinations with triphenylphosphine (TPP) and or oleylamine (OA) (1,2); polyol method produces polydispersed NPs (3). They were characterized comprehensively using varieties of experimental techniques such as XRD, XPS, TEM, EDX, SAXS, Raman, FTIR, UV-Visible, dynamic light scattering (Zeta potential and hydrodynamic particle size), and EXAFS. Physical properties studies include electrical resistivity, thermopower, and heat capacity.
Results & Discussions: TEM images prove monodispersed NP formation, and HCP nanolattice that was confirmed from SAXS pattern analysis (1,2). Ni atoms form FCC and HCP lattices inside the nanolattices up to ~6 nm, above which only FCC phase exist (4). Heat capacity data show rare QSE effect (5). Particle size and dielectric environment influence dispersion behavior and SPR (6). The thermopower gradually turns positive at 25 nm below 75 K (7). Electrical resistivity is anomalous at nanoscale (3).
Conclusions: Ni NPs exhibit anomalous electrical resistivity. They switch sign of thermopower from negative to positive as particle size drops. There exists FCC phase only in the atomic structure above ~6 nm but mixed FCC and HCP phases below ~6 nm. Particles size and dielectric environment influence dispersion behavior and SPR. QSE was observed in the heat capacity. Monodispersed Ni NPs form natural nanolattice.
Keywords: nanolattice, quantum size effect, surface plasmon resonance, thermoelectricity
Acknowledgment: The author gratefully acknowledges all the coauthors listed in the references cited herein below.
References
G. S. Okram, et al. Trioctylphosphine as self-assembly inducer. Faraday Discussions. 2015; 181: 211 – 223.
J. Singh, N. Kaurav, N. P. Lalla and G. S. Okram. Naturally self-assembled nickel nanolattice. J. Mater. Chem. C 2014; 2: 8918-8924.
G. S. Okram, A. Soni, R. Rawat. Anomalous electrical transport behavior in nanocrystalline nickel. Nanotechnology 2008; 19, 185711.
Tarachand, et al., G. S. Okram. Size-induced structural phase transition at ~6.0 nm from mixed fcc-hcp to purely fcc structure in monodispersed nickel nanoparticles. J. Phys. Chemistry C 2016; 120: 28354–28362.
J. Singh, Tarachand, S. S. Samatham, D. Venkateshwarlu, N. Kaurav, V. Ganesan and G. S. Okram. Quantum size effect on the heat capacity of nickel nanolattice Appl. Phys. Lett. 2017; 111: 201904-1-4.
V. Sharma, et al. Influence of Particle Size and Dielectric Environment on Dispersion Behavior and Surface Plasmon in Nickel Nanoparticles. Phys. Chem. Chem. Phys. 2017; 19: 14096-14106.
A. Soni and G. S. Okram. Size Dependent Thermopower in Nanocrystalline Nickel. Appl. Phys. Lett. 2009; 95, 013101
Critical behavior, universal magnetocaloric, and magnetoresistance scaling of MnSi
No full textWe report the critical behavior of a B20 cubic compound MnSi, known by its helical, conical and skyrmion phases in low fields, in its magnetic field-induced ferromagnetic (high-field) phase by means of magnetic-entropy (magnetoresistance) methods. The evaluated critical exponents are beta = 0.25 +/- 0.02(0.19 +/- 0.04), gamma = 1.29 +/- 0.27 (1.32 +/- 0.16), and delta = 6.18 +/- 0.28 (7.79 +/- 0.52). The self-consistency of the newly adopted methods is established by comparing the thus obtained exponents with those estimated using the modified Arrott's plot method and neutron diffraction. The field controlling parameter is n similar to 0.5 and its deviation from the mean-field model exponent confirm the weak first-order and field-induced ferromagnetic behavior. A direct proportional relation between magnetic entropy and magnetoresistance infers itinerant magnetic nature. The collapse of high-field data onto the more generalized magnetic-entropy master curve confirms the field-induced first-to second-order phase transition and thereby tricritical phenomenon in MnSi
Spin-flop quasi-first order phase transition and putative tricritical point in Gd3Co
No full textMagnetic nature of Gd3Co is investigated using detailed measurements of temperature and field dependent magnetization. The antiferromagnetic phase is field-instable due to prevailing ferromagnetic exchange correlations above Neel temperature T-N similar to 130 K. Below T-N, with gradually increasing magnetic fields, the compound undergoes a quasi-first order phase transition from AFM to spin-flop over region and eventually acquires ferromagnetic phase in higher fields. Further the point at which the quasi-first order transition ends and second order transition sets in is the tricritical point, T-TCP similar to 125: 6 K, H-TCP similar to 4: 4 kOe. (C) 2017 Elsevier B.V. All rights reserved
Critical exponents and universal magnetic behavior of noncentrosymmetric Fe<sub>0.6</sub>Co<sub>0.4</sub>Si
No full textUnfolding magnetic, electrical and universal magneto-resistance scaling behavior of Ni2Mn1−xCoxIn
No full textWe uncover the magnetic, electrical transport properties and universal magneto-resistance scaling nature of cubic Ni2Mn1−xCoxIn with nominal compositions of x = 0.05 and 0.1. Positive Weiss temperature, magnetic saturation in relatively low fields, non-hysteretic and non-coercive signatures confirm soft-ferromagnetic behavior of the alloys. Formation of random ferromagnetic clusters in the paramagnetic state for x = 0.1 is noticed through Griffith’s phase signatures in inverse susceptibility. Rhodes–Wohlfarth analysis confirms the itinerant magnetic nature. These alloys exhibit metallic character with Fermi liquid behavior at low temperatures. Relatively smaller negative magneto-resistance indicates the suppression of itinerant spin fluctuations and incoherent scattering of conduction electrons. Second order magnetic phase transition is inferred from a well-scaled normalized magneto-resistance independent of magnetic field strength. Our study stimulates interest to unravel the ground state physical and magnetic properties of substituted (non-magnetic/magnetic for Mn Ni and In) soft-ferromagnetic Ni2MnIn
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