Mikhail Vaganov

Abstract:

Magnetoactive elastomers may display magnetic hysteresis, even if they are filled only with magnetically soft particles. In this work we discuss application of the first-order reversal curve (FORC) analysis to identify and study hysteresis of that kind. To obtain theoretical magnetization curves and FORC diagrams for the composites in question, a simple model comprising two magnetically soft particles embedded in an elastic environment is proposed. Elucidation of the features present in FORC diagrams is conducted using computer simulation. Despite the simplicity of the model, it allows one to explain the origin of diagonal ridge patterns, which turn up in the FORC diagrams of elastomer composites with magnetically soft filler, and to quantify the experimental results. As the ridge pattern of the diagrams is inherently exclusive to such types of materials, it could be useful for their diagnostics.

Abstract:

The Stoner-Wohlfarth model of a single-domain grain is applied to a complex situation: magnetization of a solid multigrain particle embedded in an elastic medium. In this situation, application of a magnetic field establishes a specific magnetomechanical process: polarization and switching of individual grains change the net energy of the particle and, as a result, make it rotate as a whole relative to the matrix. Because of that coupling, the magnetic hysteresis loop of a particle composed of highly coercive grains progressively shrinks with the increase of the matrix compliance. The effect is studied theoretically by numerical simulations on a particle comprising several hundred magnetically uniaxial grains with randomly oriented easy axes. The results of the model are discussed with regard to magnetic measurements performed on dispersions of spherical NdFeB microparticles in PDMS matrices of varied stiffness.

Abstract:

This thesis details three studies performed with the aim of deepening our understanding of how nuclear and electronic spins can be manipulated such that they might be used in quantum information processing.

I start by discussing the possibility of using pulses of static electric fields to coherently control qudits implemented on molecular magnets. The success of this control depends on the level of spin-electric coupling (SEC) which reflects how the respective Hamiltonian changes with the application of an electric field. I present our research on a family of Mn(II)-containing molecules in which the systematic control of SEC is realised by varying the coordination environment of their spin centre. Their trigonal bipyramidal molecular structure with C₃ symmetry leads to a significant molecular electric dipole moment. Due to this, as well as high polarisability of the ligands, an applied electric field induces enhanced structural distortions. This gives rise to significant experimentally observed SEC, which is further rationalised by wavefunction theoretical calculations.

I then discuss the SEC in a molecular magnet [Yb (trensal)], which similarly possesses C₃ symmetry, but instead of manganese, this molecule contains a rare-earth ion of ytterbium (III). At cryogenic temperatures, [Yb (trensal)] can be described by an effective spin-1/2 Hamiltonian. However, our study shows that the significant values of SEC exhibited by [Yb (trensal)] can be only explained if the Hamiltonian is additionally equipped with the extended Stevens operators. The unique property of [Yb (trensal)] is that it demonstrates linear SEC even when the E-field is oriented perpendicularly to the C₃-axis of the molecule, and that this perpendicular SEC is of the same order of magnitude as the parallel effect.

In the third study, I show how, by using electron-nuclear double resonance, we implement a logical qubit encoded on four states of an I = 3/2 nuclear spin hyperfine-coupled to an S = 1/2 electron spin qubit. The encoding protects against the dominant decoherence mechanism in such systems – fluctuations of the quantizing magnetic field. We explore the dynamics of the encoded state both under a controlled application of the fluctuation and under natural decoherence processes. Our results confirm the potential of these proposals for practical, implementable, fault-tolerant quantum memories.