arXiv subjects

Arpita Sen

Publications and source records attributed to Arpita Sen.

3 recordsLinked to original sources

arXiv2025

Site-selective polar compensation of Mott electrons in a double perovskite heterointerface

Double perovskite oxides (DPOs) with two transition metal ions ($A_2$$BB^\prime$O$_6$) offer a fascinating platform for exploring exotic physics and practical applications. Studying these DPOs as ultrathin epitaxial thin films on single crystalline substrates can add another dimension to engineering electronic, magnetic, and topological phenomena. Understanding the consequence of polarity mismatch between the substrate and the DPO would be the first step towards this broad goal. We investigate this by studying the interface between a prototypical insulating DPO Nd$_2$NiMnO$_6$ and a wide-band gap insulator SrTiO$_3$. The interface is found to be insulating in nature. By combining several experimental techniques and density functional theory, we establish a site-selective charge compensation process that occurs explicitly at the Mn site of the film, leaving the Ni sites inert. We further demonstrate that such surprising selectivity, which cannot be explained by existing mechanisms of polarity compensation, is directly associated with their electronic correlation energy scales. This study establishes the crucial role of Mott physics in polar compensation process and paves the way for designer doping strategies in complex oxides.

cond-mat.mtrl-sci↗

arXiv2020

The Exchange Bias effect in pure Co2C nanoparticles

We study the low temperature magnetic properties of nanoparticles of pure transition metal carbide, viz., Co2C, with an average particle diameter of $40 \pm 10$ nm. These Co2C nanoparticles are ferromagnetic (FM) up to room temperature with blocking temperatures above room temperature. The coercive field shows abrupt deviation from the Kneller law below 50 K. In this low temperature regime the magnetization hysteresis loop shows shifts due to exchange bias (EB) effect, with an exchange field of ~ 250 Oe. Analysis of training of the EB effect and ac and dc magnetic measurements suggest that EB arises in the nanoparticles due to a core-shell structure with a FM core and a cluster glass shell. The shell contains uncompensated spins, some of which are freely rotatable while some are frozen. DFT calculations of structural and magnetic properties of small Co2C clusters of diameter of few Angstroms confirm a core-shell structure, where the structurally ordered core has uniform magnetic moment distribution and the structurally disordered shell has non-uniform moment distribution.

cond-mat.str-el↗

arXiv2019

Localized spin waves at low temperatures in a Cobalt Carbide nanocomposite

We study magnetic, transport and thermal properties of Cobalt carbide nanocomposite with a mixture of Co2C and Co3C phases in 1:1 ratio, with an average particle diameter of 40$\pm 15$ nm. We show that the behavior of the nanocomposite is completely different from that of either Co3C or Co2C. We observed that with decreasing temperature the saturation magnetization MS(T) increases, however, below 100 K, there is a steep rise. A detail analysis shows the increase in MS(T) down to 100 K is explained via the surface spin freezing model. However, below 100 K the steep increase in MS(T) is explained by a finite size effect related to a confinement of spin waves within the nano particles. The measurement of heat capacity shows broad peak at 100 K along with presence of another anomaly at a lower temperature 43 K(=Tex). Resistance measurement in the nanocomposite shows metallic behavior at high T with an unusual anomaly appearing at Tex, which is near the T regime where MS(T) begins to increase steeply. A measurement of the temperature gradients across the sample thickness indicates an abrupt change in thermal conductivity at Tex which suggests a phase transition at Tex. Our results are explained in terms of a transformation from a magnetically coupled state with a continuous spectrum of spin waves into a magnetically decoupled state below 100 K with confined spin waves.

cond-mat.str-el↗