Leigh Weston | Searcharxiv

arXiv2017

Near-perfect spin filtering and negative differential resistance in an Fe(II)S complex

Density functional theory and nonequilibrium Green's function calculations have been used to explore spin-resolved transport through the high-spin state of an iron(II)sulfur single molecular magnet. Our results show that this molecule exhibits near-perfect spin filtering, where the spin-filtering efficiency is above 99%, as well as significant negative differential resistance centered at a low bias voltage. The rise in the spin-up conductivity up to the bias voltage of 0.4 V is dominated by a conductive lowest unoccupied molecular orbital, and this is accompanied by a slight increase in the magnetic moment of the Fe atom. The subsequent drop in the spin-up conductivity is because the conductive channel moves to the highest occupied molecular orbital which has a lower conductance contribution. This is accompanied by a drop in the magnetic moment of the Fe atom. These two exceptional properties, and the fact that the onset of negative differential resistance occurs at low bias voltage, suggests the potential of the molecule in nanoelectronic and nanospintronic applications.

cond-mat.mtrl-sci↗

arXiv2016

Multiferroic crossover in perovskite oxides

Recently, the perovskite BiCoO$_3$ has been shown experimentally to be isostructural with PbTiO$_3$, while simultaneously the $d^6$ Co$^{3+}$ ion has a high spin ground state with $C$-type antiferromagnetic ordering. Using hybrid density functional calculations, we investigate the atomic, electronic and magnetic structure of BiCoO$_3$ to elucidate the origin of the multiferroic state. To begin with, we perform a qualitative trend sudy of the role of $d$ electrons in affecting the tendency for perovskite materials to exhibit a ferroelectric distortion; this work initially explores a qualitative trend study in artificial cubic and tetragonal LaBO$_3$ perovskites. We choose La as the A-cation so as to remove the effects of Bi $6s$ hybridization. Through first-principles calculations of the LaBO$_3$ series, where B is a $d^0 - d^8$ cation from the $3d$-block, the trend study reveals that increasing the $d$ orbital occupation initially removes the tendency for a polar distortion, as expected. However, for high spin $d^5-d^7$ and $d^8$ cations a strong ferroelectric instability is recovered. We explained this effect in terms of the pseudo Jahn-Teller theory for ferroelectricity. It is shown that, in some cases, unpaired electron spins actually drive ferroelectricity, rather than inhibit it, which represents a shift in the understanding of how ferroelectricity and magnetism interact in perovskite oxides. It follows, that for the case of BiCoO$_3$, the Co$^{3+}$ ion plays a major role in the ferroelectric lattice instability. Importantly, the ferroelectric polarization is greatly enhanced when the Co$^{3+}$ ion is in the high spin state, when compared to the nonmagnetic, low spin state, and a large coupling of the electrical and magnetic polarization is present. Importantly, it is demonstrated that the ground spin state is switched by reducing the internal ferroelectric polarization.

cond-mat.mtrl-sci↗