Tuning the structural and magnetic properties of Sr 2 FeReO 6 by substituting Fe and Re with valence invariant metals

In the course of this work the effect of metal substitution on the structural and magnetic properties of the double perovskites Sr2MM’O6 (M = Fe, substituted by Cr, Zn and Ga; M’ = Re, substituted by Sb) was explored by means of X-ray diffraction, magnetic measurements, band structure calculations, Mößbauer spectroscopy and conductivity measurements. The focus of this study was the determination of (i) the kind and structural boundary conditions of the magnetic interaction between the M and M’ cations and (ii) the conditions for the principal application of double perovskites as spintronic materials by means of the band model approach. Strong correlations between the electronic, structural and magnetic properties have been found during the study of the double perovskites Sr2Fe1-xMxReO6 (0 < x < 1, M = Zn, Cr). The interplay between van Hove-singularity and Fermi level plays a crucial role for the magnetic properties. Substitution of Fe by Cr in Sr2FeReO6 leads to a non-monotonic behaviour of the saturation magnetization (MS) and an enhancement for substitution levels up to 10 %. The Curie temperatures (TC) monotonically increase from 401 to 616 K. In contrast, Zn substitution leads to a continuous decrease of MS and TC. The diamagnetic dilution of the Fe-sublattice by Zn leads to a transition from an itinerant ferrimagnetic to a localized ferromagnetic material. Thus, Zn substitution inhibits the long-range ferromagnetic interaction within the Fe-sublattice and preserves the long-range ferromagnetic interaction within the Re-sublattice. Superimposed on the electronic effects is the structural influence which can be explained by size effects modelled by the tolerance factor t. In the case of Cr substitution, a tetragonal – cubic transformation for x > 0.4 is observed. For Zn substituted samples the tetragonal distortion linearly increases with increasing Zn content. In order to elucidate the nature of the magnetic interaction between the M and M’ cations, Fe and Re were substituted by the valence invariant main group metals Ga and Sb, respectively. X-ray diffraction reveals Sr2FeRe1-xSbxO6 (0 < x < 0.9) to crystallize without antisite disorder in the tetragonal distorted perovskite structure (space group I4/mmm). The ferrimagnetic behaviour of the parent compound Sr2FeReO6 changes to antiferromagnetic upon Sb substitution as determined by magnetic susceptibility measurements. Samples up to a doping level of 0.3 are ferrimagnetic, while Sb contents higher than 0.6 result in an overall antiferromagnetic behaviour. 57Fe Mößbauer results show a coexistence of ferri- and antiferromagnetic clusters within the same perovskite-type crystal structure in the Sb substitution range 0.3 < x < 0.8, whereas Sr2FeReO6 and Sr2FeRe0.9Sb0.1O6 are “purely” ferrimagnetic and Sr2FeRe0.1Sb0.9O6 contains antiferromagnetically ordered Fe sites only. Consequently, a replacement of the Re atoms by a nonmagnetic main group element such as Sb blocks the double exchange pathways Fe–O–Re(Sb)–O–Fe along the crystallographic axis of the perovskite unit cell and destroys the itinerant magnetism of the parent compound. The structural and magnetic characterization of Sr2Fe1-xGaxReO6 (0 < x < 0.7) exhibit a Ga/Re antisite disorder which is unexpected because the parent compound Sr2FeReO6 shows no Fe/Re antisite disorder. This antisite disorder strongly depends on the Ga content of the sample. Although the X-ray data do not hint at a phase separation, sample inhomogeneities caused by a demixing are observed by a combination of magnetic characterization and Mößbauer spectroscopy. The 57Fe Mößbauer data suggest the formation of two types of clusters, ferrimagnetic Fe- and paramagnetic Ga-based ones. Below 20 % Ga content, Ga statistically dilutes the Fe–O–Re–O–Fe double exchange pathways. Cluster formation begins at x = 0.2, for 0.2 < x < 0.4 the paramagnetic Ga-based clusters do not contain any Fe. Fe containing Ga-based clusters which can be detected by Mößbauer spectroscopy firstly appear for x = 0.4.

Mode coupling for precise measurements of the electronic g-factor of hydrogen-like ions in Penning traps

The g-factor is a constant which connects the magnetic moment $vec{mu}$ of a charged particle, of charge q and mass m, with its angular momentum $vec{J}$. Thus, the magnetic moment can be writen $ vec{mu}_J=g_Jfrac{q}{2m}vec{J}$. The g-factor for a free particle of spin s=1/2 should take the value g=2. But due to quantum electro-dynamical effects it deviates from this value by a small amount, the so called g-factor anomaly $a_e$, which is of the order of $10^{-3}$ for the free electron. This deviation is even bigger if the electron is exposed to high electric fields. Therefore highly charged ions, where electric field strength gets values on the order of $10^{13}-10^{16}$V/cm at the position of the bound electron, are an interesting field of investigations to test QED-calculations. In previous experiments [H"aff00,Ver04] using a single hydrogen-like ion confined in a Penning trap an accuracy of few parts in $10^{-9}$ was obtained. In the present work a new method for precise measurement of magnetic the electronic g-factor of hydrogen-like ions is discussed. Due to the unavoidable magnetic field inhomogeneity in a Penning trap, a very important contribution to the systematic uncertainty in the previous measurements arose from the elevated energy of the ion required for the measurement of its motional frequencies. Then it was necessary to extrapolate the result to vanishing energies. In the new method the energy in the cyclotron degree of freedom is reduced to the minimum attainable energy. This method consist in measuring the reduced cyclotron frequency $nu_{+}$ indirectly by coupling the axial to the reduced cyclotron motion by irradiation of the radio frequency $nu_{coup}=nu_{+}-nu_{ax}+delta$ where $delta$ is, in principle, an unknown detuning that can be obtained from the knowledge of the coupling process. Then the only unknown parameter is the desired value of $nu_+$. As a test, a measurement with, for simplicity, artificially increased axial energy was performed yielding the result $g_{exp}=2.000~047~020~8(24)(44)$. This is in perfect agreement with both the theoretical result $g_{theo}=2.000~047~020~2(6)$ and the previous experimental result $g_{exp1}=2.000~047~025~4(15)(44).$ In the experimental results the second error-bar is due to the uncertainty in the accepted value for the electron's mass. Thus, with the new method a higher accuracy in the g-factor could lead by comparison to the theoretical value to an improved value of the electron's mass. [H"af00] H. H"affner et al., Phys. Rev. Lett. 85 (2000) 5308 [Ver04] J. Verd'u et al., Phys. Rev. Lett. 92 (2004) 093002-1

Quantum effects and dynamics in hydrogen-bonded systems: a first-principles approach to spectroscopic experiments

Computer simulations have become an important tool in physics. Especially systems in the solid state have been investigated extensively with the help of modern computational methods. This thesis focuses on the simulation of hydrogen-bonded systems, using quantum chemical methods combined with molecular dynamics (MD) simulations. MD simulations are carried out for investigating the energetics and structure of a system under conditions that include physical parameters such as temperature and pressure. Ab initio quantum chemical methods have proven to be capable of predicting spectroscopic quantities. The combination of these two features still represents a methodological challenge. Furthermore, conventional MD simulations consider the nuclei as classical particles. Not only motional effects, but also the quantum nature of the nuclei are expected to influence the properties of a molecular system. This work aims at a more realistic description of properties that are accessible via NMR experiments. With the help of the path integral formalism the quantum nature of the nuclei has been incorporated and its influence on the NMR parameters explored. The effect on both the NMR chemical shift and the Nuclear Quadrupole Coupling Constants (NQCC) is presented for intra- and intermolecular hydrogen bonds. The second part of this thesis presents the computation of electric field gradients within the Gaussian and Augmented Plane Waves (GAPW) framework, that allows for all-electron calculations in periodic systems. This recent development improves the accuracy of many calculations compared to the pseudopotential approximation, which treats the core electrons as part of an effective potential. In combination with MD simulations of water, the NMR longitudinal relaxation times for 17O and 2H have been obtained. The results show a considerable agreement with the experiment. Finally, an implementation of the calculation of the stress tensor into the quantum chemical program suite CP2K is presented. This enables MD simulations under constant pressure conditions, which is demonstrated with a series of liquid water simulations, that sheds light on the influence of the exchange-correlation functional used on the density of the simulated liquid.