Ph. D at Ludwig-Maximilians Univ. Munich in 2004. Research interests and expertise in solid state physics - theoretical ab-initio methods for magnetic materials.
... Theoretical description of magnetic Compton scattering and magnetic properties of Cr-chalcoge... more ... Theoretical description of magnetic Compton scattering and magnetic properties of Cr-chalcogenide compounds Diana Benea aus Klausenburg, Rumänien 2004 Page 2. Page 3. ... M ¨unchen, 17. 04. 2004 Diana Benea ..... Unterschrif Dissertation eingereicht am 17. 04. ...
We employ a positron annihilation technique, the spin-polarized two-dimensional angular correlati... more We employ a positron annihilation technique, the spin-polarized two-dimensional angular correlation of annihilation radiation (2D-ACAR), to measure the spin-difference spectra of ferromagnetic nickel. The experimental data are compared with the theoretical results obtained within a combination of the local spin density approximation (LSDA) and the many-body dynamical mean-field theory (DMFT). We find that the self-energy defining the electronic correlations in Ni leads to anisotropic contributions to the momentum distribution. By direct comparison of the theoretical and experimental results we determine the strength of the local electronic interaction U in ferromagnetic Ni as 2.0 ± 0.1 eV.
We compute the Compton profile of Ni using the Local Density Approximation of Density Functional ... more We compute the Compton profile of Ni using the Local Density Approximation of Density Functional Theory supplemented with electronic correlations treated at different levels. The total/magnetic Compton profiles show not only quantitative but also qualitative significant differences depending weather Hubbard corrections are treated at a mean field +U or in a more sophisticated dynamic way. Our aim is to discuss the range and capability of electronic correlations to modify the kinetic energy along specific spatial directions. The second and the fourth order moments of the difference in the Compton profiles are discussed as a function of the strength of local Coulomb interaction U .
ABSTRACT Band structure calculations are reported to describe the effects of Ta for Hf substituti... more ABSTRACT Band structure calculations are reported to describe the effects of Ta for Hf substitution on the electronic structure of the Laves phase of HfFe2. According to experimental results emphasis is put on the investigation of the Hf0.825Ta0.175Fe2 composition in comparison with the reference HfFe2 and TaFe2 compounds. By investigation of the theoretical interatomic exchange interactions constants, we showed the influence of the magnetovolume effect on the magnetic phase stability for one intermediate composition Hf0.825Ta0.175Fe2, chosen in the most interesting part of the magnetic phase diagram with coexistence of ferromagnetic (FM) and antiferromagnetic (AF) behaviour. Evolution of the a lattice parameter is found to drive the change from FM to AF magnetic order. A detailed study of the interatomic Fe-Fe exchange coupling constants and their lattice dependence reveals that the ferromagnetic interaction with the first shell is strongly reduced upon contraction of the a lattice parameter.
Ab-initio fully relativistic band structure calculations of Fe films on Cu (001) substrate are pr... more Ab-initio fully relativistic band structure calculations of Fe films on Cu (001) substrate are presented. The films are modelled as 6Cu/nFe/6Vc slabs, where the number of multilayers (ML) is n=1, 2, 4 and 6. Intermixing between Fe atoms of the thin film and the Cu atoms of the substrate was found to be energetically favourable for 1ML coverage, causing also a decrease of the Fe magnetic moment. The mixing of deposited Fe atoms with the Cu substrate has been studied using the Coherent Potential Approximation (CPA). Various ferromagnetic and ferrimagnetic spin configurations with coupling of layer blocks have been considered, with the in-plane and out-of-plane direction of magnetization. Total energy calculations have been performed and the most stable spin configuration for unrelaxed fcc thin films are shown. While for the 1 and 2 ML Fe/Cu(001) a ferromagnetic out-of-plane spin configuration is the most stable, 4 ML and 6 ML cover regimes exhibit a spin configuration with blocks of l...
The two-photon momentum distribution of annihilating electron-positron pairs in ferromagnetic nic... more The two-photon momentum distribution of annihilating electron-positron pairs in ferromagnetic nickel (Ni) was determined by measuring the spin-polarized two-dimensional angular correlation of annihilation radiation (ACAR). The spectra were compared with theoretical results obtained within LDA+DMFT, a combination of the local density approximation (LDA) and the many-body dynamical mean-field theory (DMFT). The self-energy describing the electronic correlations in Ni is found to make important anisotropic contributions to the momentum distribution which are not present in LDA. Based on a detailed comparison of the theoretical and experimental results the strength of the local electronic interaction U in ferromagnetic Ni is determined as 2.0 ± 0.1 eV.
We discuss the momentum redistribution along nearest and next nearest neighbour bond axes of Fe a... more We discuss the momentum redistribution along nearest and next nearest neighbour bond axes of Fe and Ni, using the Shannon entropy formula. We find that within the combined Density Functional and Dynamical Mean Field Theory weight redistribution takes place towards lower momenta as a function of the local Coulomb parameter U . This effect is more pronounced for Fe than Ni.
A detailed theoretical investigation on the electronic and magnetic properties of tau-phase of Mn... more A detailed theoretical investigation on the electronic and magnetic properties of tau-phase of Mn54Al46 alloys with the CuAu tetragonal type of structure has been performed. All theoretical investigations of the electronic and magnetic properties have been done using the Korringa-Kohn-Rostoker (KKR) band structure method. The disorder in the system has been accounted for by means of the Coherent Potential Approximation (CPA). The spin resolved density of states (DOS) reflects the covalent nature of the interatomic bands. The calculated magnetic moments for the system Mn54-xAl46Znx show a non-linear variation with Zn content ranging between 4.06 µB/unit cell for Mn54Al46 (x = 0) to 4.68 µB /unit cell for Mn50Al46Zn4 (x = 4), suggesting an optimum Zn doping of 4% for Mn, close to the value of 2.9 % determined by experiment. On the other side, the SPR-KKR calculations do not confirm the existence of an optimum Zn for Al substitution which maximizes the magnetic moment of the samples.
Journal of physics. Condensed matter : an Institute of Physics journal, Jan 9, 2014
The electronic structure and magnetic properties of the disordered alloy system fcc-FexNi1-x (fcc... more The electronic structure and magnetic properties of the disordered alloy system fcc-FexNi1-x (fcc: face centered cubic) have been investigated by means of the KKR-CPA (Korringa-Kohn-Rostoker coherent potential approximation) band structure method. To investigate the impact of correlation effects, the calculations have been performed on the basis of the LSDA (local spin density approximation), the LSDA + U as well as the LSDA + DMFT (dynamical mean field theory). It turned out that the inclusion of correlation effects hardly changed the spin magnetic moments and the related hyperfine fields. The spin-orbit induced orbital magnetic moments and hyperfine fields, on the other hand, show a pronounced and element-specific enhancement. These findings are in full accordance with the results of a recent experimental study.
ABSTRACT Magnetic Compton profiles (MCPs) of Ni and Fe along [111] direction have been calculated... more ABSTRACT Magnetic Compton profiles (MCPs) of Ni and Fe along [111] direction have been calculated using a combined Density Functional and many-body theory approach. At the level of the local spin density approximation the theoretical MCPs does not describe correctly the experimental results around the zero momentum transfer. In this work we demonstrate that inclusion of electronic correlations as captured by Dynamical Mean Field Theory (DMFT) improves significantly the agreement between the theoretical and the experimental MCPs. In particular, an energy decomposition of Ni MCPs gives indication of spin polarization and intrinsic nature of Ni 6 eV satellite, a genuine many-body feature.
ABSTRACT The total and magnetically resolved Compton profiles are analyzed within the combined de... more ABSTRACT The total and magnetically resolved Compton profiles are analyzed within the combined density functional and dynamical mean field theory for the transition metal elements Fe and Ni. A rather good agreement between the measured and computed magnetic Compton profiles (MCPs) of Fe and Ni is obtained with the standard Local Spin Density Approximation (LSDA). By including local but dynamic many-body correlations captured by Dynamical Mean Field Theory (DMFT), the calculated magnetic Compton profile is further improved when compared with experiment. The second moment of the difference of the total Compton profiles between LSDA and DMFT, along the same momentum direction, has been used to discuss the strength of electronic correlations in Fe and Ni.
The title compounds were synthesized via the high-temperature (HT) route. The materials are chara... more The title compounds were synthesized via the high-temperature (HT) route. The materials are characterized by Rietveld analysis, magnetic measurements, and electronic band-structure calculations. Two different structural modifications depending on the synthesis conditions are observed: a HT modification with trigonal basic cells (space group: P−3m1) for Cr(1+x)Q2 ((1+x)=1.25, 1.28, 1.34, 1.37, 1.41, 1.43) and a low-temperature (LT) modification with trigonal super-cell
In this work, the structural and magnetic properties of Er1-xZrxFe2 (0.1 ≤ x ≤ 0.4) were investig... more In this work, the structural and magnetic properties of Er1-xZrxFe2 (0.1 ≤ x ≤ 0.4) were investigated. These compounds crystallize in the cubic MgCu2 (C15) structure, the lattice parameters decreasing with Zr content. Electronic structure calculations were performed, showing a good agreement between theory and experiment. All of the samples are ferrimagnetically ordered, presenting compensation points in the M(T) plots. The compensation point values decrease, while the Curie temperatures increase with Zr content. The experimental Fe moments at 5 K decrease with Zr content from 1.70 μB/atom for x = 0.1 to 1.55 μB/atom for x = 0.4. A non-collinear orientation of the magnetic moments was evidenced in these compounds. The magnetocaloric effect was also studied. A modest magnetocaloric effect was found for all of the samples, spreading across a wide temperature range. A maximum |ΔSM| value of 1.19 J kg(-1) K(-1) was found for the sample with x = 0.1 for an applied field change of 0-4 T. Large RCP(ΔS) values were obtained for all of the samples, mainly due to the wide δTFWHM values of the ΔSM(T) curves.
ABSTRACT This paper reports x-ray photoemission spectroscopy results regarding the nucleation and... more ABSTRACT This paper reports x-ray photoemission spectroscopy results regarding the nucleation and growing process of gold nanoparticles (AuNPs) in CaO-SiO2-P2O5 bioactive glasses heat treated at different temperatures. The analysis of the valence band (VB) spectra is discussed in terms of the AuNPs' shape, size, and distribution at the sample surface at different temperatures. Moreover, the theoretical VB photoemission spectra of the investigated samples are presented, and their agreement with experimentally obtained spectra is discussed in detail.
ABSTRACT The structural, magnetic and magnetocaloric properties of several HoFe2-xAlx compounds w... more ABSTRACT The structural, magnetic and magnetocaloric properties of several HoFe2-xAlx compounds were investigated. The compounds in the Fe-rich region (0.36 <= x <= 0.4) crystallize in the cubic MgCu2 (C15) structure, while for the ones in the intermediate region (0.75 <= x <= 1.125) the hexagonal MgZn2 (C14) structure was observed. Electronic structure calculations were performed, showing a good agreement between theory and experiment. The Curie temperatures were found to decrease with Al content. For the Fe-rich compounds, these are close to room temperature, while for the compounds in the intermediate region, transition temperatures are well below 300 K. No magnetic hysteresis was found around the Curie temperature for applied magnetic fields of up to 4 T. All of the investigated compounds undergo a second-order magnetic phase transition at the Curie temperature. A maximum magnetic entropy change value of 7.6 J/kg K was obtained for the sample with x = 1.125, all of the samples displaying rather large RCP values. The possibility of incorporating these materials in magnetic refrigeration devices is discussed.
Measurements of resistivity, Nernst, Seebeck and Hall effects of Bi 2223 epitaxial thin films for... more Measurements of resistivity, Nernst, Seebeck and Hall effects of Bi 2223 epitaxial thin films for magnetic fields between 0 and 5 T and temperature range 5–300 K are reported.
... Theoretical description of magnetic Compton scattering and magnetic properties of Cr-chalcoge... more ... Theoretical description of magnetic Compton scattering and magnetic properties of Cr-chalcogenide compounds Diana Benea aus Klausenburg, Rumänien 2004 Page 2. Page 3. ... M ¨unchen, 17. 04. 2004 Diana Benea ..... Unterschrif Dissertation eingereicht am 17. 04. ...
We employ a positron annihilation technique, the spin-polarized two-dimensional angular correlati... more We employ a positron annihilation technique, the spin-polarized two-dimensional angular correlation of annihilation radiation (2D-ACAR), to measure the spin-difference spectra of ferromagnetic nickel. The experimental data are compared with the theoretical results obtained within a combination of the local spin density approximation (LSDA) and the many-body dynamical mean-field theory (DMFT). We find that the self-energy defining the electronic correlations in Ni leads to anisotropic contributions to the momentum distribution. By direct comparison of the theoretical and experimental results we determine the strength of the local electronic interaction U in ferromagnetic Ni as 2.0 ± 0.1 eV.
We compute the Compton profile of Ni using the Local Density Approximation of Density Functional ... more We compute the Compton profile of Ni using the Local Density Approximation of Density Functional Theory supplemented with electronic correlations treated at different levels. The total/magnetic Compton profiles show not only quantitative but also qualitative significant differences depending weather Hubbard corrections are treated at a mean field +U or in a more sophisticated dynamic way. Our aim is to discuss the range and capability of electronic correlations to modify the kinetic energy along specific spatial directions. The second and the fourth order moments of the difference in the Compton profiles are discussed as a function of the strength of local Coulomb interaction U .
ABSTRACT Band structure calculations are reported to describe the effects of Ta for Hf substituti... more ABSTRACT Band structure calculations are reported to describe the effects of Ta for Hf substitution on the electronic structure of the Laves phase of HfFe2. According to experimental results emphasis is put on the investigation of the Hf0.825Ta0.175Fe2 composition in comparison with the reference HfFe2 and TaFe2 compounds. By investigation of the theoretical interatomic exchange interactions constants, we showed the influence of the magnetovolume effect on the magnetic phase stability for one intermediate composition Hf0.825Ta0.175Fe2, chosen in the most interesting part of the magnetic phase diagram with coexistence of ferromagnetic (FM) and antiferromagnetic (AF) behaviour. Evolution of the a lattice parameter is found to drive the change from FM to AF magnetic order. A detailed study of the interatomic Fe-Fe exchange coupling constants and their lattice dependence reveals that the ferromagnetic interaction with the first shell is strongly reduced upon contraction of the a lattice parameter.
Ab-initio fully relativistic band structure calculations of Fe films on Cu (001) substrate are pr... more Ab-initio fully relativistic band structure calculations of Fe films on Cu (001) substrate are presented. The films are modelled as 6Cu/nFe/6Vc slabs, where the number of multilayers (ML) is n=1, 2, 4 and 6. Intermixing between Fe atoms of the thin film and the Cu atoms of the substrate was found to be energetically favourable for 1ML coverage, causing also a decrease of the Fe magnetic moment. The mixing of deposited Fe atoms with the Cu substrate has been studied using the Coherent Potential Approximation (CPA). Various ferromagnetic and ferrimagnetic spin configurations with coupling of layer blocks have been considered, with the in-plane and out-of-plane direction of magnetization. Total energy calculations have been performed and the most stable spin configuration for unrelaxed fcc thin films are shown. While for the 1 and 2 ML Fe/Cu(001) a ferromagnetic out-of-plane spin configuration is the most stable, 4 ML and 6 ML cover regimes exhibit a spin configuration with blocks of l...
The two-photon momentum distribution of annihilating electron-positron pairs in ferromagnetic nic... more The two-photon momentum distribution of annihilating electron-positron pairs in ferromagnetic nickel (Ni) was determined by measuring the spin-polarized two-dimensional angular correlation of annihilation radiation (ACAR). The spectra were compared with theoretical results obtained within LDA+DMFT, a combination of the local density approximation (LDA) and the many-body dynamical mean-field theory (DMFT). The self-energy describing the electronic correlations in Ni is found to make important anisotropic contributions to the momentum distribution which are not present in LDA. Based on a detailed comparison of the theoretical and experimental results the strength of the local electronic interaction U in ferromagnetic Ni is determined as 2.0 ± 0.1 eV.
We discuss the momentum redistribution along nearest and next nearest neighbour bond axes of Fe a... more We discuss the momentum redistribution along nearest and next nearest neighbour bond axes of Fe and Ni, using the Shannon entropy formula. We find that within the combined Density Functional and Dynamical Mean Field Theory weight redistribution takes place towards lower momenta as a function of the local Coulomb parameter U . This effect is more pronounced for Fe than Ni.
A detailed theoretical investigation on the electronic and magnetic properties of tau-phase of Mn... more A detailed theoretical investigation on the electronic and magnetic properties of tau-phase of Mn54Al46 alloys with the CuAu tetragonal type of structure has been performed. All theoretical investigations of the electronic and magnetic properties have been done using the Korringa-Kohn-Rostoker (KKR) band structure method. The disorder in the system has been accounted for by means of the Coherent Potential Approximation (CPA). The spin resolved density of states (DOS) reflects the covalent nature of the interatomic bands. The calculated magnetic moments for the system Mn54-xAl46Znx show a non-linear variation with Zn content ranging between 4.06 µB/unit cell for Mn54Al46 (x = 0) to 4.68 µB /unit cell for Mn50Al46Zn4 (x = 4), suggesting an optimum Zn doping of 4% for Mn, close to the value of 2.9 % determined by experiment. On the other side, the SPR-KKR calculations do not confirm the existence of an optimum Zn for Al substitution which maximizes the magnetic moment of the samples.
Journal of physics. Condensed matter : an Institute of Physics journal, Jan 9, 2014
The electronic structure and magnetic properties of the disordered alloy system fcc-FexNi1-x (fcc... more The electronic structure and magnetic properties of the disordered alloy system fcc-FexNi1-x (fcc: face centered cubic) have been investigated by means of the KKR-CPA (Korringa-Kohn-Rostoker coherent potential approximation) band structure method. To investigate the impact of correlation effects, the calculations have been performed on the basis of the LSDA (local spin density approximation), the LSDA + U as well as the LSDA + DMFT (dynamical mean field theory). It turned out that the inclusion of correlation effects hardly changed the spin magnetic moments and the related hyperfine fields. The spin-orbit induced orbital magnetic moments and hyperfine fields, on the other hand, show a pronounced and element-specific enhancement. These findings are in full accordance with the results of a recent experimental study.
ABSTRACT Magnetic Compton profiles (MCPs) of Ni and Fe along [111] direction have been calculated... more ABSTRACT Magnetic Compton profiles (MCPs) of Ni and Fe along [111] direction have been calculated using a combined Density Functional and many-body theory approach. At the level of the local spin density approximation the theoretical MCPs does not describe correctly the experimental results around the zero momentum transfer. In this work we demonstrate that inclusion of electronic correlations as captured by Dynamical Mean Field Theory (DMFT) improves significantly the agreement between the theoretical and the experimental MCPs. In particular, an energy decomposition of Ni MCPs gives indication of spin polarization and intrinsic nature of Ni 6 eV satellite, a genuine many-body feature.
ABSTRACT The total and magnetically resolved Compton profiles are analyzed within the combined de... more ABSTRACT The total and magnetically resolved Compton profiles are analyzed within the combined density functional and dynamical mean field theory for the transition metal elements Fe and Ni. A rather good agreement between the measured and computed magnetic Compton profiles (MCPs) of Fe and Ni is obtained with the standard Local Spin Density Approximation (LSDA). By including local but dynamic many-body correlations captured by Dynamical Mean Field Theory (DMFT), the calculated magnetic Compton profile is further improved when compared with experiment. The second moment of the difference of the total Compton profiles between LSDA and DMFT, along the same momentum direction, has been used to discuss the strength of electronic correlations in Fe and Ni.
The title compounds were synthesized via the high-temperature (HT) route. The materials are chara... more The title compounds were synthesized via the high-temperature (HT) route. The materials are characterized by Rietveld analysis, magnetic measurements, and electronic band-structure calculations. Two different structural modifications depending on the synthesis conditions are observed: a HT modification with trigonal basic cells (space group: P−3m1) for Cr(1+x)Q2 ((1+x)=1.25, 1.28, 1.34, 1.37, 1.41, 1.43) and a low-temperature (LT) modification with trigonal super-cell
In this work, the structural and magnetic properties of Er1-xZrxFe2 (0.1 ≤ x ≤ 0.4) were investig... more In this work, the structural and magnetic properties of Er1-xZrxFe2 (0.1 ≤ x ≤ 0.4) were investigated. These compounds crystallize in the cubic MgCu2 (C15) structure, the lattice parameters decreasing with Zr content. Electronic structure calculations were performed, showing a good agreement between theory and experiment. All of the samples are ferrimagnetically ordered, presenting compensation points in the M(T) plots. The compensation point values decrease, while the Curie temperatures increase with Zr content. The experimental Fe moments at 5 K decrease with Zr content from 1.70 μB/atom for x = 0.1 to 1.55 μB/atom for x = 0.4. A non-collinear orientation of the magnetic moments was evidenced in these compounds. The magnetocaloric effect was also studied. A modest magnetocaloric effect was found for all of the samples, spreading across a wide temperature range. A maximum |ΔSM| value of 1.19 J kg(-1) K(-1) was found for the sample with x = 0.1 for an applied field change of 0-4 T. Large RCP(ΔS) values were obtained for all of the samples, mainly due to the wide δTFWHM values of the ΔSM(T) curves.
ABSTRACT This paper reports x-ray photoemission spectroscopy results regarding the nucleation and... more ABSTRACT This paper reports x-ray photoemission spectroscopy results regarding the nucleation and growing process of gold nanoparticles (AuNPs) in CaO-SiO2-P2O5 bioactive glasses heat treated at different temperatures. The analysis of the valence band (VB) spectra is discussed in terms of the AuNPs' shape, size, and distribution at the sample surface at different temperatures. Moreover, the theoretical VB photoemission spectra of the investigated samples are presented, and their agreement with experimentally obtained spectra is discussed in detail.
ABSTRACT The structural, magnetic and magnetocaloric properties of several HoFe2-xAlx compounds w... more ABSTRACT The structural, magnetic and magnetocaloric properties of several HoFe2-xAlx compounds were investigated. The compounds in the Fe-rich region (0.36 <= x <= 0.4) crystallize in the cubic MgCu2 (C15) structure, while for the ones in the intermediate region (0.75 <= x <= 1.125) the hexagonal MgZn2 (C14) structure was observed. Electronic structure calculations were performed, showing a good agreement between theory and experiment. The Curie temperatures were found to decrease with Al content. For the Fe-rich compounds, these are close to room temperature, while for the compounds in the intermediate region, transition temperatures are well below 300 K. No magnetic hysteresis was found around the Curie temperature for applied magnetic fields of up to 4 T. All of the investigated compounds undergo a second-order magnetic phase transition at the Curie temperature. A maximum magnetic entropy change value of 7.6 J/kg K was obtained for the sample with x = 1.125, all of the samples displaying rather large RCP values. The possibility of incorporating these materials in magnetic refrigeration devices is discussed.
Measurements of resistivity, Nernst, Seebeck and Hall effects of Bi 2223 epitaxial thin films for... more Measurements of resistivity, Nernst, Seebeck and Hall effects of Bi 2223 epitaxial thin films for magnetic fields between 0 and 5 T and temperature range 5–300 K are reported.
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