Browsing by Author "Mishra, Esha"

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    Direct observation of the magnetic anisotropy of an Fe(II) spin crossover molecular thin film
    (IOPP, 2023-07) Dale, Ashley S.; Yazdani, Saeed; Ekanayaka, Thinlini K.; Mishra, Esha; Hu, Yuchen; Dowben, Peter A.; Freeland, John W.; Zhang, Jian; Cheng, Ruihua; Physics, School of Science
    In this work, we provide clear evidence of magnetic anisotropy in the local orbital moment of a molecular thin film based on the SCO complex [Fe(H2B(pz)2)2(bipy)] (pz = pyrazol−1−yl, bipy = 2,2'−bipyridine). Field dependent x-ray magnetic circular dichroism measurements indicate that the magnetic easy axis for the orbital moment is along the surface normal direction. Along with the presence of a critical field, our observation points to the existence of an anisotropic energy barrier in the high-spin state. The estimated nonzero coupling constant of ∼2.47 × 10−5 eV molecule−1 indicates that the observed magnetocrystalline anisotropy is mostly due to spin–orbit coupling. The spin- and orbital-component anisotropies are determined to be 30.9 and 5.04 meV molecule−1, respectively. Furthermore, the estimated g factor in the range of 2.2–2.45 is consistent with the expected values. This work has paved the way for an understanding of the spin-state-switching mechanism in the presence of magnetic perturbations.
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    Evidence of dynamical effects and critical field in a cobalt spin crossover complex
    (Royal Society of Chemistry, 2022-01) Ekanayaka, Thilini K.; Wang, Ping; Yazdani, Saeed; Phillips, Jared Paul; Mishra, Esha; Dale, Ashley S.; N'Diaye, Alpha T.; Klewe, Christoph; Shafer, Padraic; Freeland, John; Streubel, Robert; Wampler, James Paris; Zapf, Vivien; Cheng, Ruihua; Shatruk, Michael; Dowben, Peter A.; Physics, School of Science
    The [Co(SQ)2(4-CN-py)2] complex exhibits dynamical effects over a wide range of temperature. The orbital moment, determined by X-ray magnetic circular dichroism (XMCD) with decreasing applied magnetic field, indicates a nonzero critical field for net alignment of magnetic moments, an effect not seen with the spin moment of [Co(SQ)2(4-CN-py)2].
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    Magnetic Field Perturbations to a Soft X-ray-Activated Fe (II) Molecular Spin State Transition
    (MDPI, 2021-10) Hao, Guanhua; N’Diaye, Alpha T.; Ekanayaka, Thilini K.; Dale, Ashley S.; Jiang, Xuanyuan; Mishra, Esha; Mellinger, Corbyn; Yazdani, Saeed; Freeland, John W.; Zhang, Jian; Cheng, Ruihua; Xu, Xiaoshan; Dowben, Peter A.; Physics, School of Science
    The X-ray-induced spin crossover transition of an Fe (II) molecular thin film in the presence and absence of a magnetic field has been investigated. The thermal activation energy barrier in the soft X-ray activation of the spin crossover transition for [Fe{H2B(pz)2}2(bipy)] molecular thin films is reduced in the presence of an applied magnetic field, as measured through X-ray absorption spectroscopy at various temperatures. The influence of a 1.8 T magnetic field is sufficient to cause deviations from the expected exponential spin state transition behavior which is measured in the field free case. We find that orbital moment diminishes with increasing temperature, relative to the spin moment in the vicinity of room temperature.
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    Probing the unpaired Fe spins across the spin crossover of a coordination polymer
    (RSC, 2021) Ekanayaka, Thilini K.; Kurz, Hannah; Dale, Ashley S.; Hao, Guanhua; Mosey, Aaron; Mishra, Esha; N'Diaye, Alpha T.; Cheng, Ruihua; Weber, Birgit; Dowben, Peter A.; Physics, School of Science
    For the spin crossover coordination polymer [Fe(L1)(bipy)]n (where L1 is a N2O22− coordinating Schiff base-like ligand bearing a phenazine fluorophore and bipy = 4,4′-bipyridine), there is compelling additional evidence of a spin state transition. Both Fe 2p X-ray absorption and X-ray core level photoemission spectroscopies confirm that a spin crossover takes place, as observed by magnetometry. Yet the details of the temperature dependent changes of the spin state inferred from both X-ray absorption and X-ray core level photoemission, differ from magnetometry, particularly with regard to the apparent critical transition temperatures and the cooperative nature of the curve progression in general. Comparing the experimental spin crossover data to Ising model simulations, a transition activation energy in the region of 160 to 175 meV is indicated, along with a nonzero exchange J. Overall, the implication is that there may be perturbations to the bistability of spin states, that are measurement dependent or that the surface differs from the bulk with regard to the cooperative effects observed upon spin transition.