Browsing by Author "R.N. Bhowmik"

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Experimental study of multiple magnetic transitions in micrometer and nano-grain sized Ni3TeO6-type oxide
(American Institute of Physics Inc., 2020) I. Panneer Muthuselvam; K. Saranya; R. Sankar; R.N. Bhowmik; L. Kavitha
We report the experimental results of dc and ac magnetic susceptibility (χdc and χac) and specific heat (CP) measurements of the powdered Ni3TeO6-type oxide. Upon decreasing the grain size from micrometer (bulk) to nanometer range, the magnetic measurements have indicated multiple transitions, viz., a cusp at paramagnetic to ferrimagnetic (FIM) transition temperature (TC) ∼78 K, an antiferromagnetic (AFM) transition temperature (TN) ∼52 K, and a re-entrant spin glass (RSG) transition at Tm2 ∼7.49 K. The ferrimagnetic transition temperature TC is found to be highly sensitive to dc magnetic fields, where superposition of a small dc field (10 Oe) in the ac susceptibility measurement or the application of a dc magnetic field of up to 100 Oe in the dc magnetic measurement is enough to suppress the observed TC at 78 K. The CP(T) measurement also revealed a RSG state at lower temperatures for the sample with nano-sized grains. The absence of any sharp peak at TC in the CP(T) curves implies the absence of a true long-range FIM spin order in the samples. The coexistence of a RSG state with multiple magnetic transitions is understood as an effect of competitive AFM and FIM phases in the Ni3TeO6-type structure. © 2020 Author(s).
Magnetic spin order in the honeycomb structured Pb6 Co9(TeO6)5 compound
(American Physical Society, 2021) I. Panneer Muthuselvam; K. Saranya; Deepa Kasinathan; R.N. Bhowmik; R. Sankar; Namasivayam Dhenadhayalan; G.J. Shu; Wei-Tin Chen; L. Kavitha; King-Chuen Lin
We present a comprehensive experimental and theoretical study of the structural, electronic, magnetic, and thermodynamic properties of a Pb6Co9(TeO6)5 single crystal. The Pb6Co9(TeO6)5 crystal has shown a unique type of magnetic spin-lattice coupling, in which the lattice structure consists of four different Co ions sites with distorted octahedral coordinations. The x-ray photoelectron spectroscopy (XPS) results confirmed the oxidation states of Pb, Co, Te, and O elements in the sample. Moreover, XPS spectra revealed the adsorbed oxygen in the defect/vacancy sites of the lattice structure. The dc magnetization measurements exhibited a complex magnetic behavior with ferrimagnetic (FIM) transition with Curie temperature TC at ∼21 K. At lower magnetic fields H, the zero-field-cooled and field-cooled curves showed a broad hump at ∼10.8 K and a shoulder peak at ∼6.2 K, which are suppressed at higher magnetic fields. The ac susceptibility data indicated spin-glass-like features. The heat capacity CP measurements confirmed the FIM transition at TC at ∼21 K, but without any trace of additional peaks at lower temperatures. The estimated Curie-Weiss constant θCW showed a peculiar field-dependent behavior along the H∥c direction of the single crystal, where θCW is less field dependent for the H⊥c direction. A large coercivity (13 kOe) is observed at 2 K for H∥c, whereas the magnetization curve of the single crystal is dominated by an antiferromagnetic feature for H⊥c. The behaviors indicate the anisotropy nature of the exchange interactions in the compound. The local spin density approximation + U total energy calculations were performed for various collinear spin configurations of a classical Heisenberg model in order to obtain the magnetic exchange interactions Ji at different distances for different neighbors. © 2021 American Physical Society.