Asma Amjad

University of Central Florida, Orlando, FL, United States

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Publications (6)12.96 Total impact

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    ABSTRACT: A novel water-stable (t1/2 ∼ 6.8 days) mononuclear manganese(IV) complex of a hexacoordinating non-Schiff-base ligand (H4L) with N2O4-donor atoms has been synthesized and characterized crystallographically. High-frequency electron paramagnetic resonance experiments performed on a single crystal reveal a manganese(IV) ion with an S = (3)/2 ground spin state that displays a large single-ion anisotropy, setting the record of mononuclear manganese(IV) complexes reported so far. In addition, spin-echo experiments reveal a spin-spin relaxation time T2 ∼ 500 ns.
    Inorganic Chemistry 05/2014; · 4.59 Impact Factor
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    ABSTRACT: In this work we investigated the magnetic anisotropy of a Mn (IV) monomer via axial and rhombic zero field splitting terms D, E. The d^3 ion sits in an octahedral environment in a P 21/c space group. The complex is studied via single crystal and powder HFEPR over a wide range of frequencies 49GHz to 416GHz and temperatures 2 to 60K. The angle dependence at low temperature and frequency (˜88GHz) reveals a minimum of the resonance field, when the long axis of the crystal is along the magnetic field. The same behavior is observed at higher frequency (˜240GHz). Furthermore, pulse EPR experiments in high frequency quasi-optical spectrometer at low temperature (˜1.487K) a spin echo could be observed and we were able to observe the variation of the T2 times as a function of the magnetic field orientation, and as a function of the temperature.
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    ABSTRACT: We present a detailed experimental and theoretical study of a 1D spin .5ex1 -.1em/ -.15em.25ex2 Co-based molecular chain, trans-[CoCl2(3,5-Br2py)2]. Our results show distinct features that are associated to both intra-molecular interactions (along the chain) and inter-molecular interactions (perpendicular to the direction of chains). The hysteresis observed at low temperature (230mK), indicate presence of 3D ordering attributed to the exchange interactions between the chains. Measurements done at different angles from the chains axial direction (c-axis), restricted within the a-c and b-c planes, reveal uniaxial anisotropy along the c-axis. The experimental data are explained using the mean field approximation, focusing on the behavior of inter-chain interactions and g-tensor anisotropy at the Co sites in the presence of a static magnetic field. A transition between two different relaxation regimes encountered for variable sweep rates will also be discussed.
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    ABSTRACT: Recently, various triangular [MnIII]3 molecules have been extensively studied due to the fact that one can modulate the magnitude and the sign of the inter-ion exchange, thereby giving rise to very simple clusters that constitute some of the cleanest and best examples of so-called single-molecule magnets. However, magnetic and electron paramagnetic resonance (EPR) characterizations of low-spin antiferromagnetic [MnIII]3 complexes have been problematic due to the significant spin frustration that exists for this topology. We show that this frustration is relieved in the highly distorted [NEt4]3Mn3Zn2(salox)3O(N3)8]·MeOH molecule: Susceptibility data suggest a well isolated S = 2 ground state; EPR spectroscopy and high-field torquemetry support this conclusion and further indicate the presence of a very significant zero-field-splitting (zfs) separating the lowest-lying mS = ±2 states from the excited levels within the same S = 2 multiplet. Remarkably, this zfs is sufficient to give rise to magnetic bistability, as evidenced through the observation of low-temperature magnetization hysteresis.
    Physical review. B, Condensed matter 09/2011; 84(9). · 3.77 Impact Factor
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    ABSTRACT: We present a low-temperature study of one-dimensional Co-based molecular chains, trans-[CoCl2(3,5-Br2py)2]. Ac and dc susceptibility experiments show that the cobalt ions tend to form anisotropic ferromagnetic chains, whose properties are dictated by the thermal excitations of 1D domain walls. The observation of anomalies in the hysteresis loops of the sample on increasing the magnetic field sweep rate reveals interesting dynamical effects at the individual chain level. On decreasing the temperature, no evidence for 3D ordering was observed in specific heat measurements, although the magnetization was strongly irreversible below ˜450 mK. Possible sources for this absence of a magnetic phase transition could be the weakness of the interchain interactions, the presence of single-ion anisotropy at skew angles, disordered domains, and lattice defects, etc. These possibilities will be studied both experimentally and theoretically.
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    ABSTRACT: We prepared three structurally related Mn(3)(III)Mn(2)(II) complexes that possess S approximately 1-11 spin ground states as a result of variations in the geometry and identity of mu(2)-eta(1):eta(1) bridging groups. These complexes function as single-molecule magnets yet demonstrate other interesting behavior such as quasi-classical magnetization hysteresis and comparable magnetization reversal barriers (U(eff)).
    Inorganic Chemistry 02/2010; 49(4):1304-6. · 4.59 Impact Factor