Thermal Control of Spin Excitations in the Coupled Ising-Chain Material RbCoCl3

We have used neutron spectroscopy to investigate the spin dynamics of the quantum (S = 1/2) antiferromagnetic Ising chains in RbCoCl3. The structure and magnetic interactions in this material conspire to produce two magnetic phase transitions at low temperatures, presenting an ideal opportunity for thermal control of the chain environment. The high-resolution spectra we measure of two-domain-wall excitations therefore characterize precisely both the continuum response of isolated chains and the "Zeeman-ladder" bound states of chains in three different effective staggered fields in one and the same material. We apply an extended Matsubara formalism to obtain a quantitative description of the entire dataset, Monte Carlo simulations to interpret the magnetic order, and finite-temperature density-matrix renormalization-group calculations to fit the spectral features of all three phases.

Scanning Tunneling Microscopy using Superconducting Tips to Probe Absolute Spin Polarization

Matthias Alexander Eltschka

The interplay of superconductivity and magnetism is investigated for systems with dimensions ranging from the mesoscopic to the atomic scale by scanning tunneling microscopy (STM) at millikelvin temperatures and by numerical calculations. Based on geometrically confined superconductors in magnetic fields, a novel STM approach is introduced to quantitatively probe the spin polarization of tunneling electrons. In the first part of this work, the effects of magnetic fields and geometrical confinement are probed for superconducting vanadium STM tips. Due to the unique confinement ranging from the atomic to the mesoscopic scale, the superconducting properties of the STM tips vary considerably from their bulk counterparts. To analyze the experimentally determined magnetic field dependence of several V tips, the superconductivity is numerically calculated for modeled cone geometries with various opening angles. The numerical approach based on a one-dimensional Usadel equation leads to a direct correlation between the opening angle ¿ and the order of the superconducting phase transition. First order phase transitions occur when the opening angle is smaller than a critical value (¿ < ¿c), whilst larger opening angles (¿ > ¿c) result in second order phase transitions. The comparison of experimental findings and numerical results reveals the existence of first and second order quantum phase transitions in the V STM tips. In addition, the numerical calculations also explain experimentally observed broadening effects of the superconducting spectra by the specific tip geometry. In the second part, the superconducting V tips are employed in a novel approach to quantitatively probe the spin polarization of tunneling electrons on the nanoscale. For this purpose, the Meservey-Tedrow-Fulde technique is transferred to STM in order to combine their virtues, such as the quantitative probing capability of the spin polarization, the precise control at the atomic scale and the well-defined vacuum tunnel barrier. To demonstrate the capabilities of the new technique, the local spin structure is resolved for a magnetic Co nanoisland, where spin polarizations ranging from -56% up to 65% were found, depending on the local position. Furthermore, the spin polarization P strongly varies with the tip-to-sample distance z (dP/dz ¿ 10%/Å), which is described by the different decays of the spin-up and spin-down wave functions into the vacuum tunnel barrier. The final part describes the local interaction between isolated magnetic moments and the superconducting ground state. Copper phthalocyanine molecules on the superconducting V(100) surface induce bound states within the superconducting gap due to the magnetic coupling and the Coulomb potentials. Spatially resolved measurements reveal the non-isotropic structure of the spectral weights that is explained by the adsorption site on the 5x1 reconstruction of the V(100) surface. The quasi-particle excitations are not only observed on the magnetic molecule but also occur in its close vicinity. With increasing distance from the molecular structure, the intensities of the bound states decay within the distance x ¿ ±30Å and show periodic oscillations at the same time. Comparing the experimental findings to a one-dimensional model suggests the presence of a complicated scattering potential, which can be simplified by assuming two point scatterers within the molecular structure.

EPFL2015

Transport properties of cluster-assembled magnetic nanostructures

Giulia Di Domenicantonio

The aim of this thesis is to get a deeper understanding of magnetic properties of small clusters when they are embedded in a host material. The study was motivated by the observation of the very peculiar properties that clusters present in the gas phase or when deposited on a substrate. The intriguing perspective is that those properties would be preserved in a solid state sample that could be used for technological applications. Preformed and well characterized clusters are used as building blocks to produce nanostructured films in which they are embedded in a host material in order to preserve them from chemical and thermal degradation when the sample is exposed to ambient conditions. For this purpose an innovative set-up has been constructed that allows the independent control of cluster size, concentration and chemical composition. In particular samples containing small magnetic clusters embedded in a non magnetic matrix have been produced in order to study the evolution of the magnetic behaviour of such cluster assembled materials. These samples have been subsequently studied by means of magnetotransport, in particular by measuring their magneto-resistance and Hall voltage as a function of temperature in magnetic fields of up to 5T. Additionally, a novel measurement protocol detecting the derivative of the resistance with respect to temperature as a function of magnetic field has been used to characterize the samples. Magnetotransport is known to be an important tool for the investigation of the magnetic behaviour as the conduction electrons can be considered as microscopic probes of the state of the sample. However, even though these properties have been studied extensively, a complete and universally accepted theory on the mechanisms giving rise to the observed magneto-resistance in cluster assembled materials is still to be found. In this work, the high quality of the produced samples allows an unequivocal identification of all the aspects that are not correctly described by the theories in use. In particular it is observed that, in the case of small clusters, the effect of magnetic interactions cannot be neglected even at very low concentrations. The consequence is twofold since both the superparamagnetic model of magnetization and the well known (1 – m2) expression for the magneto-resistance fail if a correlation between magnetic moment exists. Furthermore we claim that the Mott hypothesis of two parallel currents adopted for electrical conduction is not sufficient to describe the details of transport in granular systems. In fact the particular differential resistance measurement introduced in this work allows an underlining of the importance of the spin-channel mixing mechanisms that are not taken into account in the Mott picture.

EPFL2008

Anisotropic Cascade of Field-Induced Phase Transitions in the Frustrated Spin-Ladder System BiCu_{2}PO_{6}

Krunoslav Prsa, Henrik Moodysson Rønnow, Christian Rüegg, Shuang Wang

BiCu2PO6 is a frustrated two-leg spin-ladder compound with a spin gap that can be closed with a magnetic field of approximately 20 T. This quantum phase transition and its related phase diagram as a function of magnetic field and temperature (H, T) are investigated up to 60 T by means of specific heat, magnetocaloric effect, magnetization, and magnetostriction measurements. In contrast to other gapped quantum magnets, BiCu2PO6 undergoes a series of unexpected first-and second-order phase transitions when an external magnetic field is applied along the crystallographic c axis. The application of a magnetic field along the b axis induces two second-order phase transitions. We propose that the anisotropy and complex phase diagram result from the interplay between strong geometrical frustration and spin-orbit interaction necessary for the description of this fascinating magnetic system.