Abby Goldman

Mount Holyoke College, South Hadley, Massachusetts, United States

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Publications (10)12.54 Total impact

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    ABSTRACT: The "lone" 6s electron pair often plays a key role in determining the structure and physical properties of compounds containing sixth-row elements in their lower oxidation states: Tl(+), Pb(2+), and Bi(3+) with the [Xe]4f(14)5d(10)6s(2) electronic configuration. The lone pairs on these ions are associated with reduced structural symmetries, including ferroelectric instabilities and other important phenomena. Here we consider the isoelectronic auride Au(-) ion with the [Xe]4f(14)5d(10)6s(2) electronic configuration. Ab initio density functional theory methods are employed to probe the effect of the 6s lone pair in alkali-metal aurides (KAu, RbAu, and CsAu) with the CsCl structure. The dielectric constants, Born effective charges, and structural instabilities suggest that the 6s lone pair on the Au(-) anion is stereochemically inert to minor mechanical and electrical perturbation. Pressures greater than 14 GPa, however, lead to reorganization of the electronic structure of CsAu and activate lone-pair involvement and Au-Au interactions in bonding, resulting in a transformation from the cubic CsCl structure type to an orthorhombic Cmcm structure featuring zigzag Au-Au chains.
    Inorganic Chemistry 07/2013; · 4.59 Impact Factor
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    ABSTRACT: We present simulations of the switching process between clockwise and counterclockwise vortex states in ferromagnetic nanorings in an applied circular field, relevant to potential data storage devices. This circular field can be experimentally generated by passing current through the solid metal tip of an atomic force microscope, which has achieved vortex-to-vortex switching in thicker asymmetric rings [1]. We find that in sufficiently thin rings, the vortex switching process occurs through the nucleation and annihilation of pairs of 360 degree domain walls (DW), with opposite topological indices. The DW with the same circulation as the vortex annihilates first. We can control which DW annihilates first by offsetting the center of our circular field to target a specific DW. Both exchange energy and demagnetization energy must be considered in predicting the energy barrier to DW annihilation. [1] T. Yang, N.R. Pradhan, A. Goldman, A.S. Licht, Y.Li, M. Kemei, M.T. Tuominen, K.E. Aidala. APL, 98, 242505 (2011).
    02/2012;
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    Journal of Applied Physics 01/2012; · 2.21 Impact Factor
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    ABSTRACT: This experimental research investigates a new method of manipulating the magnetic states of ferromagnetic nanorings using a circular magnetic field directed along the ring circumference. This type of azimuthal field can naturally select a vortex magnetization of desired chirality. The understanding of the magnetization switching behavior in an azimuthal field could lead to new designs of practical magnetic data storage devices. Symmetric and asymmetric nanorings made of permalloy are fabricated by a standard technique using electron-beam lithography and e-beam evaporation. Azimuthal fields are generated by passing current through an atomic force microscope tip, which is positioned at the center of the ring. The magnetic field direction and magnitude are controlled by the current. We demonstrate control over switching from an onion state to a vortex state, and also between two vortex states, using magnetic force microscopy to image the resulting magnetic states.
    03/2011;
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    Applied Physics Letters 01/2011; 98:242505. · 3.52 Impact Factor
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    ABSTRACT: CoSeO4 has a structure consisting of edge-sharing chains of Co2 + octahedra which are held together by SeO42 − tetrahedra via shared oxygen atoms at the edges of the octahedra. DC magnetization measurements indicate a transition to an ordered state below 30 K. Powder neutron diffraction refinements suggest an ordered state with two unique antiferromagnetic chains within the unit cell. Isothermal magnetization measurements indicate a temperature-dependent field-induced magnetic transition below the ordering temperature. From neutron diffraction, we find that this corresponds to a realignment of spins from the canted configuration towards the c-axis. The dielectric constant shows a change in slope at the magnetic ordering temperature indicating an interplay between the spin and charge degrees of freedom.
    Journal of Physics Condensed Matter 11/2010; 22(50):506003. · 2.22 Impact Factor
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    ABSTRACT: We investigate the magnetic phase diagram and magnetodielectric properties of CoSeO4, a material which contains chains of edge-sharing octahedral Co(II) connected by tetrahedral SeO4 units. Specific heat measurements in combination with powder neutron diffraction show a transition to long-range magnetic order below 30,. Fits to the high temperature susceptibility give a Curie-Weiss temperature of -35,, indicating the magnetic ground state has a dominant antiferromagnetic ordering. However, magnetization measurements in a field of 100 Oe also indicate the presence of weak ferromagnetism and a field-induced transition in fields larger than 30 kOe. The zero-field magnetic structure consists of Co moments aligning antiparallel to nearest neighbors down the length of the chains. We also present evidence of a linear magnetoelectric response in the ground state magnetic phase.
    03/2010;
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    ABSTRACT: Ferromagnetic nanorings form unique magnetic states that hold tremendous promise for maximizing storage densities. One such state is the vortex state, in which the magnetic field is completely enclosed within the ring, though it is challenging to control the chirality. We study a straightforward method to control the clockwise or counterclockwise chirality using an azimuthal field, as if from a current carrying wire passing through the center of the ring. Our simulations predict the formation of 360 ^o domain walls during switching of 5 nm thick rings in a variety of geometries. The number and location of the domain walls depends on the ring geometry. We explore the reason 360 ^o domain walls form for different widths, asymmetries, and sizes. Experimental implementation is underway to confirm computational predictions. The micromagnetic simulations are performed using OOMMF, Object Oriented Micro Magnetic Framework, a public domain program distributed by NIST to study the evolution of magnetic states with the application of a circular magnetic field.
    03/2010;
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    ABSTRACT: In this work, conduct a systematic study of field evolution of small magnetic rings with the aim of determining stable magnetized states as a function of geometric parameters. Such rings may be a foundation for novel data storage applications. In these experiments, ferromagnetic cobalt and permalloy nanorings, with diameter in range of 300 nm to 800 nm, are fabricated by electron beam lithography, development, thin-film deposition and lift-off. Magnetic force microscopy (MFM) is used to observe magnetic states of nanorings having different geometry. We will discuss the different magnetic configurations that result when an in-plane magnetic field, either homogeneous or inhomogeneous on a scale compared to the dimension of the ring, is used to manipulate the magnetization.
    03/2010;
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    ABSTRACT: Ferromagnetic nanorings form unique magnetic states that hold tremendous potential for maximizing data storage densities. One such state is the closed-flux vortex state, in which the magnetic field is completely enclosed within the ring, thus minimizing the magnetostatic energy, but also keeping the exchange energy low as adjacent magnetic moments are mostly aligned. A natural way to generate this state is through an external azimuthal field, as if from a current carrying wire passing through the center of the ring. We perform micromagnetic simulations to investigate the evolution of magnetic states in an external azimuthal field. For some applied current, the chirality of the ring will reverse, often into an intermediate state that evolves into a perfect vortex at higher current. Thin, wide rings have significantly lower switching currents than thick, narrow rings. We examine the dependence of the switching current and intermediate states on geometric properties such as the diameter, thickness, asymmetry and width of the ring.
    01/2009;