Giant resistance change across the phase transition in spin crossover molecules

The electronic origin of a large resistance change in nanoscale junctions incorporating spin crossover molecules is demonstrated theoretically by using a combination of density functional theory and the non-equilibrium Green's functions method for quantum transport. At the spin crossover phase transition there is a drastic change in the electronic gap between the frontier molecular orbitals. As a consequence, when the molecule is incorporated in a two terminal device, the current increases by up to four orders of magnitude in response to the spin change. This is equivalent to a magnetoresistance effect in excess of 3,000 %. Since the typical phase transition critical temperature for spin crossover compounds can be extended to well above room temperature, spin crossover molecules appear as the ideal candidate for implementing spin devices at the molecular level

Calculated magnetic X-ray circular dichroism of half-metal CrO

In a non-polar geometry for non-cubic ferromagnetic materials, it is shown that the X-ray absorption dichroism spectrum is composed of a non-magnetic contribution (natural dichroism) emanating from the reduced crystal symmetry and a magnetic contribution from spin-orbit coupling and spin polarization. Their computation at the $L_{2, 3}$ edges of the half-metal CrO2 chromium site, illustrate the good agreement with experiment. This is found only when both dichroic contributions are taken into account. Moreover, X-ray magnetic dichroism sum rules can be applied directly to extract the spin and orbital magnetic moments only after removing the natural dichroism from the full spectrum

Magnetic anisotropy Berry's phase

By considering the intrinsic anisotropy, present in almost all magnetic systems, as a perturbation to the usual Zeeman term, we show that the spin-spin dipolar interaction also known as zero-field splitting (ZFS) leads to an extra geometrical phase in addition to the conventional Berry's phase. Furthermore, we suggest some ways to observe the energy shift in electron paramagnetic resonance spectra due to Berry's phase and how we can separate it from the conventional Zeeman Berry's phase. One of the authors (MM) dedicates this work to the memory of his mother, Djabou Zoulikha, who died on 3 February 2019

Electrostatic spin crossover effect in polar magnetic molecules

The magnetic configuration of a nanostructure can be altered by an external magnetic field, by spin-transfer torque or by its magnetoelastic response. Here, we explore an alternative route, namely the possibility of switching the sign of the exchange coupling between two magnetic centres by means of an electric potential. This general effect, which we name electrostatic spin crossover, occurs in insulating molecules with super-exchange magnetic interaction and inversion symmetry breaking. As an example we present the case of a family of di-cobaltocene-based molecules. The critical fields for switching, calculated from first principles, are of the order of 1 V nm-1 and can be achieved in two-terminal devices. More crucially, such critical fields can be engineered with an appropriate choice of substituents to add to the basic di-cobaltocene unit. This suggests that an easy chemical strategy for achieving the synthesis of suitable molecules is possible