Heike Herper - Academia.edu (original) (raw)

Papers by Heike Herper

Physical review. B, Condensed matter, 1993

Fixed-spin-moment band-structure calculations of Fe3Ni show that the face-centered-cubic structur... more Fixed-spin-moment band-structure calculations of Fe3Ni show that the face-centered-cubic structure is unstable with respect to small tetragonal distortions. Using the Bain transformation for the crossover from the fcc to the bcc structure, we show that the ground state of Fe3Ni corresponds, in agreement with experiment, to the bcc structure, which is by 1.75 mRy/atom lower in energy than the fcc

The European Physical Journal D, 2001

Physical Review B, 2002

The interlayer exchange coupling and the perpendicular magnetoresistance of Fe/Si/Fe systems have... more The interlayer exchange coupling and the perpendicular magnetoresistance of Fe/Si/Fe systems have been investigated within the fully relativistic screened Korringa-Kohn-Rostoker method and the Kubo-Greenwood equation considering interdiffusion effects, i.e., inhomogeneous Fe-Si alloy formation at the interfaces. It is shown that the experimentally observed strong antiferromagnetic interlayer exchange coupling is caused by the formation of Fe-Si alloys at the interface. Furthermore, our calculations give evidence that the small magnetoresistance, which has been observed experimentally in Fe/Si/Fe trilayers has a similar origin. The results presented here give no evidence for a direct connection between the magnetoresistance and interlayer exchange coupling in Fe/Si/Fe systems.

MRS Proceedings, 2002

The transport properties of Fe(001)/Cr/Fe(001) trilayers are discussed with respect to the influe... more The transport properties of Fe(001)/Cr/Fe(001) trilayers are discussed with respect to the influence of transition metal impurities in form of layers. We are able to show that the periodicity of the giant magnetoresistance is directly influenced by the interlayer exchange coupling (IEC). Furthermore, it is observed that the behavior of the IEC strongly depends on whether an impurity overlayer of Mn or V is used. It turns out that the size of the GMR is only little effected by 3d-transition metal impurities, which is in agreement with the experimental findings. The electronic and magnetic properties of the trilayers have been investigated within the fully relativistic, spin-polarized SKKR method and the LDA. The transport properties of the Fe/Cr/Fe systems have been derived from the fully relativistic spin-polarized Kubo-Greenwood equation.

Philosophical Magazine B, 2000

Fe and its alloys in the fcc structure reveal a variety of phases which, depending on the composi... more Fe and its alloys in the fcc structure reveal a variety of phases which, depending on the composition and temperature, show Invar and anti-Invar effects as well as martensitic transformations and shape memory effects. The variety of phases and the involvement of magnetic order on the structural stability will be discussed on the basis of ab initio calculations.

Springer Tracts in Modern Physics, 2012

ABSTRACT Heusler alloys have attracted huge interest due to their outstanding magnetic properties... more ABSTRACT Heusler alloys have attracted huge interest due to their outstanding magnetic properties which make them suitable for various applications. Bulk Heusler alloys are applied in magnetic shape memory and magneto-caloric devices. Here, we focus on thin films of Heusler alloys as needed for magneto-electronic applications. Especially, Co- or Fe-based systems such as Co_2MnSi are known to be half-metallic or have at least a high spin polarization and a high Curie temperature making them appropriate as spintronic materials. In the present paper the influence of composition, disorder, and structural deformation on the magnetic properties is discussed from experimental as well as theoretical point of view. Since the quality of a spintronic device crucially depends on the interplay between the ferromagnet and the substrate the influence of different substrates (GaAs, MgO) on the electronic and magnetic properties is studied.

Physical Review B, 2001

The electronic structure, magnetic properties, and perpendicular electric transport in bcc Fe/ZnS... more The electronic structure, magnetic properties, and perpendicular electric transport in bcc Fe/ZnSe/Fe͑100͒ heterostructures with Zn and Se termination are discussed in terms of the fully relativistic spin-polarized version of the screened Korringa-Kohn-Rostoker method and the Kubo-Greenwood equation. It is found that different ͑Zn or Se͒ terminations cause substantial differences in the interlayer exchange coupling, the magnetic anisotropy, and, most prominently, in the magnetoresistance. The most important result, however, is that the difference between the sheet resistances in the parallel and the antiparallel configuration becomes constant with increasing spacer thickness. This constant value depends on the type of termination and is essentially determined by the interfaces.

Disorder and Strain-Induced Complexity in Functional Materials, 2012

ABSTRACT The structural and magnetic order are the decisive elements which vastly determine the p... more ABSTRACT The structural and magnetic order are the decisive elements which vastly determine the properties of smart ternary intermetallics such as X2YZ Heusler alloys. Here, X and Y are transition metal elements and Z is an element from the III-V group. In order to give a precise prescription of the possibilities to optimize the magnetic shape memory and magnetocaloric effects of these alloys, we use density functional theory calculations. In particular, we outline how one may find new intermetallics which show higher Curie and martensite transformation temperatures when compared with the prototypical magnetic shape-memory alloy Ni2MnGa. Higher operation temperatures are needed for technological applications at elevated temperatures.

Materials Science Forum, 2009

Page 1. Fundamental Aspects of Magnetic Shape Memory Alloys: Insights from ab initio and Monte Ca... more Page 1. Fundamental Aspects of Magnetic Shape Memory Alloys: Insights from ab initio and Monte Carlo Studies Peter Entel 1,a , Markus E. Gruner 1 , Antje Dannenberg 1 , Mario Siewert 1 , Sanjeev K. Nayak 1 , Heike C. Herper 1, and Vasiliy D. Buchelnikov2 ...

MRS Proceedings, 2010

ABSTRACT

The Journal of Physical Chemistry A, 2014

To shed light on the metal 3d electronic structure of manganese phthalocyanine, so far controvers... more To shed light on the metal 3d electronic structure of manganese phthalocyanine, so far controversial, we performed photoelectron measurements both in the gas phase and as thin film. With the purpose of explaining the experimental results,three different electronic configurations close in energy to one another were studied by means of density functional theory. The comparison between the calculated valence band density of states and the measured spectra revealed that in the gas phase the molecules exhibit a mixed electronic configuration, while in the thin film, manganese phthalocyanine finds itself in the theoretically computed ground state, namely, the b1(2g)e3(g)a1(1g)b0(1g) electronic configuration.

Physical Review Letters, 2011

One of the primary objectives in molecular nano-spintronics is to manipulate the spin states of o... more One of the primary objectives in molecular nano-spintronics is to manipulate the spin states of organic molecules with a d-electron center, by suitable external means. In this letter, we demonstrate by first principles density functional calculations, as well as second order perturbation thoery, that a strain induced change of the spin state, from S=1 → S=2, takes place for an iron porphyrin (FeP) molecule deposited at a divacancy site in a graphene lattice. The process is reversible in a sense that the application of tensile or compressive strains in the graphene lattice can stabilize FeP in different spin states, each with a unique saturation moment and easy axis orientation. The effect is brought about by a change in Fe-N bond length in FeP, which influences the molecular level diagram as well as the interaction between the C atoms of the graphene layer and the molecular orbitals of FeP.

Physical Review B, 2009

We investigated the magnetic as well as the structural properties of Fe 3 Si films on GaAs͑001͒-͑... more We investigated the magnetic as well as the structural properties of Fe 3 Si films on GaAs͑001͒-͑4 ϫ 6͒, GaAs͑001͒-͑2 ϫ 2͒, and MgO͑001͒ by x-ray magnetic circular dichroism ͑XMCD͒ and Mössbauer spectroscopy. From the XMCD spectra we determine averaged magnetic moments of 1.3-1.6 B per Fe atom on the different substrates by a standard sum-rule analysis. In addition, XMCD spectra have been calculated by using the multiple-scattering Korringa-Kohn-Rostoker method which allows the site-specific discussion of the x-ray spectra. The Mössbauer spectra show a highly ordered and stoichiometric growth of Fe 3 Si on MgO while the growth on both GaAs substrates is strongly perturbed, probably due to diffusion of substrate atoms into the Fe 3 Si film. Therefore, we have studied the influence of Ga or As impurities on the magnetic properties of Fe 3 Si by calculations using coherent-potential approximation within the Korringa-Kohn-Rostoker method. For selected impurity concentrations additional supercell calculations have been performed using a pseudopotential code ͑VASP͒.

Physical Review B, 2014

ABSTRACT The adsorption of FePc on Co(001) and c(2×2)O/Co(001) was studied by means of density fu... more ABSTRACT The adsorption of FePc on Co(001) and c(2×2)O/Co(001) was studied by means of density functional theory calculations, taking into account the long range van der Waals dispersion forces. Several high symmetry adsorption sites were analyzed, together with two possible orientations of the molecules. For the adsorption of FePc on the bare surface the on-top-of Co position, rotated by 45∘ relative to the substrate orientation, is most stable, whereas on the surface covered by an O adlayer the on-top-of O position is preferred. This has strong impact on the magnetic coupling but leaves the spin state of S =1 unaltered. The total energies of the studied adsorption sites on the bare metal differ by at least 0.75 eV and are characterized by a strong hybridization of the carbon atoms in the peripheral benzenic rings with the Co atoms beneath. In the presence of the O adlayer the various sites are closer in energy, which turns out to be related to the screening of the ferromagnetic film by the oxygen atoms.

$Research paper thumbnail of First-principles and Monte Carlo study of magnetostructural transition and magnetocaloric properties of Ni_{2+x}Mn_{1−x}Ga$

Physical Review B, 2010

First-principles calculations of magnetic exchange parameters of the austenitic and martensitic p... more First-principles calculations of magnetic exchange parameters of the austenitic and martensitic phases of Ni-Mn-Ga allow one to characterize this Heusler system across the phase diagram in agreement with experimental trends. The ab initio investigations have been combined with Monte Carlo simulations for a detailed description of the magnetic, martensitic, and magnetocaloric properties of Ni 2+x Mn 1−x Ga ͑0.18Յ x Յ 0.27͒ Heusler alloys, which undergo a first-order magnetostructural phase transition. For these alloys, the calculated temperature dependence of the magnetic and lattice contributions to the total specific heat as well as the evaluation of the isothermal magnetic entropy ⌬S mag ͑T , H ext ͒ and adiabatic temperature ⌬T͑T , H ext ͒ changes around the magnetostructural transition in an external magnetic field agree fairly well with the experimental data. In particular, results for ⌬S mag ͑T , H ext ͒ and ⌬T͑T , H ext ͒ may be used to speculate about designing new magnetic Heusler alloys with better magnetocaloric properties, i.e., larger ⌬T͑T , H ext ͒ values.

physica status solidi (b), 2005

PACS 75.47.De, 75.70.Cn The influence of Al and Si impurity layers on the giant magnetoresistance... more PACS 75.47.De, 75.70.Cn The influence of Al and Si impurity layers on the giant magnetoresistance (GMR) and the magnetic properties of Fe/V/Fe(110) trilayers is investigated. The calculations are performed by employing the spinpolarized Kubo-Greenwood approach and the screened Korringa-Kohn -Rostoker method for layered systems. All calculations are carried out with a fully-relativistic version. Therefore, we are able to consider also anisotropic magnetoresistance effects, which are common in Fe/V systems. We find that the AMR always makes a tiny contribution to the resistivity in alike multilayers so that the magnetoresistance is entirely due to the GMR. A reduction of the GMR due to the Al and Si impurities is observed for current in-plane (CIP) and perpendicular (CPP) geometry. However, in the case of CIP geometry the influence of the impurities decreases with increasing V layer thickness, whereas in the CPP case the difference alternates between 0 and 7%.