arXiv subjects

Kamini Gautam

Publications and source records attributed to Kamini Gautam.

4 recordsLinked to original sources

arXiv2026

Collinear ferromagnetism with reduced moment length in kagome magnet Nd3Ru4Al12

We determine the magnetic ground state of the kagome lattice magnet Nd3Ru4Al12 by single-crystal neutron diffraction, supported by experiments with polarized neutrons. We identify this material as a collinear ferromagnet ("hex-FM") with uniform moment length mc = 2.1 {\mu}B/Nd and ordering vector Q = 0, in contrast to a previous, seminal report that proposed unequal moment lengths on two Nd sites, here called the "ortho-FM" state. Our analysis of the flipping ratio in polarized neutron scattering is consistent with the hex-FM state. The results provide a microscopic basis for understanding the large fluctuation-induced Hall and Nernst responses near TC = 41 K, as previously reported for Nd3Ru4Al12.

cond-mat.str-el↗

arXiv2025

X-ray free-electron laser observation of giant and anisotropic magnetostriction in $β$-O$_{2}$ at 110 Tesla

In strong magnetic fields beyond 100 T, the significant Zeeman energy competes with the lattice interactions, where a considerable magnetostriction is expected. However, the microscopic observation of the magnetostriction above 100 T has been hindered due to the short pulse duration of $μ$-seconds and the coil's destruction. Here, we report the observation of the giant and anisotropic magnetostriction of $\sim 1$ % at 110 T in the spin-controlled crystal, $β$-O$_{2}$, by combining the single-shot diffraction of x-ray free-electron laser (XFEL) and the newly developed portable 100 T generator (PINK-02). The very soft and anisotropic response of $β$-O$_{2}$ should originate in the competing van der Waals force and exchange interaction, and also the frustration of spin and lattice on the triangular network. The XFEL experiment above 100 T using PINK-02 enables microscopic investigations on materials' properties at high magnetic fields, providing insights into how spins contribute to the stability of crystal structures.

cond-mat.str-el↗

arXiv2025

Visualization of Co 3d high- and low-spin states via valence electron density

Properties of trivalent cobalt oxides are governed by the spin and orbital states of Co3+ ions, which are strongly coupled to their local coordination environments and chemical bonding. However, direct real-space access to the electronic states has remained challenging. Here, we determine the Co 3d states in the quasi-one-dimensional cobalt oxide Ca3Co2O6 by combining synchrotron X-ray diffraction with valence electron density (VED) analysis based on core differential Fourier synthesis. The reconstructed VED reveals distinct anisotropic distributions at two crystallographically inequivalent Co sites with octahedral and trigonal-prismatic coordination geometries. The octahedral site exhibits a characteristic VED consistent with a low-spin configuration, whereas the trigonal-prismatic site shows pronounced anisotropy that cannot be described solely by crystal electric field (CEF) effects. Quantitative analysis demonstrates that this anisotropy originates from the interplay of CEF effects, spin-orbit coupling, and ligand-assisted 3d-4p hybridization, reflecting partially unquenched orbital angular momentum and its role in the Ising magnetism. These results establish a general framework for understanding site-dependent electronic structure and chemical bonding in transition-metal oxides through real-space VED analysis.

cond-mat.str-el↗

arXiv2022

Coexistence of local structural heterogeneities and long-range ferroelectricity in Pb-free (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics

Environmentally benign (1-x)Ba(Ti$_{0.8}$Zr$_{0.2}$)O$_3$-x(Ba$_{0.7}$Ca$_{0.3}$)TiO$_3$ (BZT-BCT) ceramics are promising materials due to their remarkable high piezoresponse [Liu and Ren, Phys. Rev. Lett. \textbf{103}, 257602 (2009)]. In this Letter, by focusing on local and average structure in combination with macroscopic electromechanical and dielectric measurements we demonstrate the structure property relationship in the tetragonal BZT-BCT ceramic. During high-temperature cubic to tetragonal phase transformation, polar nanoregions are manifested through the spontaneous volume ferroelectrostriction at temperatures below $\sim$ 477 K. Temperature-dependent local structural investigations across the Zr K edge extended x-ray absorption fine structure spectroscopy reveal an anomalous collaboration between the ZrO$_{6}$ and TiO$_6$ octahedra. These octahedra compromise their individuality during polarization development. The presence of domains of submicron size embedded inside the macroscopic ferroelectric regions below T$_{m}$, as well as their hierarchical arrangement, is observed by piezo-response force microscopy. Effects of the existence of the structural/polar heterogeneities below T$_{m}$ are observed also when polarizibilities of the poled and the unpoled samples are compared; the poled sample is found to be more susceptible to the electric field. In addition, by using electric field dependent x-ray diffraction studies we also show that this ceramic under field exhibits reduction of tetragonal distortion, which is consistent with earlier reports.

cond-mat.mtrl-sci↗