1. 
 Brucas, R., et al.
(författare)

Magnetic anisotropy and evolution of groundstate domain structures in bcc Fe81Ni19/Co(001) superlattices
 2008

Ingår i: Physical Review B. Condensed Matter and Materials Physics.  The American Physical Society.  10980121. ; 78:2, s. 024421

Tidskriftsartikel (refereegranskat)abstract
 The magnetic anisotropy and evolution of striped magnetic domain structures in bcc Fe81Ni19/Co(001) superlattices with the total thickness ranging from 85 to 1370 nm has been studied by magnetooptical Kerr effect and magnetic force microscopy. At a thickness of about 85 nm [25 bilayers (BL)] the domains appear as stripe domains, typical for perpendicular anisotropy films, with the weak cubic anisotropy of the inplane magnetization component stabilizing the stripe direction. The magnetic domain period strongly depends on the thickness of the superlattice. As the thickness increases, the equilibrium magnetization orients at oblique angles with respect to the film plane and continuously varies with the thickness from inplane to outofplane. We first apply a simple phenomenological model which correctly predicts the transition from inplane to outofplane magnetization as well as increasing domain period and saturation field with increasing BL number. The results indicate the presence of partial fluxclosure domains at the film surface with the tilt angle continuously varying with the superlattice thickness. By solving a linearized Landau–Lifshitz equation together with Maxwell’s equations in magnetostatic approximation for samples consisting of up to 1000 individual layers, we calculate the spinwave dispersion and determine the stability conditions for the saturated ferromagnetic state. From these results the dependence of the saturation field on the number of layers is inferred and agrees well with the experiment. The uniaxial bulk anisotropy is attributed to distortions along the c axis and the results further show evidence for the presence of an easyplane interface anisotropy in these samples.


2. 
 Eriksson, Olle, et al.
(författare)

Ordering in diluted magnetic semiconductors : A magnetic percolation phenomenon (invited)
 2007

Ingår i: Journal of Applied Physics.  00218979. ; 101:9, s. 09H114

Tidskriftsartikel (refereegranskat)abstract
 We report on first principles calculations that describe the electronic structure, magnetic exchange interactions, and ordering temperatures of diluted magnetic semiconductors. The calculated interatomic exchange couplings are used in a Heisenberg Hamiltonian, and ordering temperatures are calculated with the use of Monte Carlo simulations. The accuracy of the method is analyzed by comparing observed and calculated ordering temperatures of several Mn and Cr doped IIIV and IIVI semiconductors. The effect of magnetic percolation is discussed along with clustering phenomena and the effect of strong electronelectron interaction.


3. 
 Haferman, H., et al.
(författare)

Competing anisotropies in bcc Fe81Ni19/Co(001) superlattices
 2009

Ingår i: Applied Physics Letters.  00036951. ; 94:7, s. 0731020731023

Tidskriftsartikel (refereegranskat)abstract
 Amagnetization reorientation transition has been observed in Fe81Ni19/Co(001) superlattices bymeans of magnetooptical Kerr effect and magnetic force microscopy measurements.The transition is driven by the variation of the interfacedensity. Firstprinciples calculations are combined with a linear stability analysisof the Landau–Lifshitz equation to clarify the mechanism that drivesthe transition. We are able to identify it as beingdriven by competing interface inplane and uniaxial bulk outofplane anisotropies.The origin of the bulk anisotropy is attributed to tetragonaldistortions experimentally observed in these superlattices.


4. 
 Iusan, Diana, et al.
(författare)

Effect of diffusion and alloying on the magnetic and transport properties of Fe/V/Fe trilayers
 2007

Ingår i: Physical Review B. Condensed Matter and Materials Physics.  10980121. ; 75:2, s. 024412

Tidskriftsartikel (refereegranskat)abstract
 The magnetic and transport properties of the Fe/V/Fe(001) trilayers were studied using the selfconsistent Green's function technique based on the tightbinding linear muffintin orbital method in the atomicsphere approximation. The coherent potential approximation was used to describe the effects of interdiffusion and alloying at the interfaces on the properties of the semiinfinite bcc Fe(001)/mFe/nV/mFe/Fe(001) trilayers. The electric conductance was calculated using the KuboLandauer formalism, in the currentperpendiculartoplane geometry. It is shown that a dipole moment is created at the Fe/V interface due to the charge transfer from vanadium to iron, and a small induced magnetic moment is present in the first vanadium layer and is antiparallel to that of iron. The interlayer exchange coupling shows rapid oscillations for small spacer thicknesses, and the interdiffusion and alloying at the interface stabilize the ferromagnetic coupling. Moreover, the interdiffusion reduces the vanadiuminduced magnetic moment and increases the iron magnetic moment at the interface. The giant magnetoresistance (GMR) ratio presents damped oscillations as a function of the vanadium spacer thickness. The interdiffusion and the presence of Mn impurities at the interface reduce considerably the GMR ratio and produce results that are in agreement with experimental data.


5. 
 Iusan, Diana, et al.
(författare)

Effects of strain on ferroelectric polarization and magnetism in orthorhombic HoMnO3
 2013

Ingår i: Physical Review B. Condensed Matter and Materials Physics.  10980121. ; 87:1, s. 014403

Tidskriftsartikel (refereegranskat)abstract
 We explore how the ferroelectric polarization of antiferromagnetic Etype orthorhombic HoMnO3 can be increased, by investigating the effects of inplane strain on both the magnetic properties and the ferroelectric polarization, using combined density functional theory calculations and a model Hamiltonian technique. Our results show that the net polarization is strongly enhanced under compressive strain, due to an increase of the electronic contribution to the polarization. In contrast, the ionic contribution is found to decrease. We identify the electronlattice coupling, due to JahnTeller (JT) distortions, and its response to strain, to be responsible for the observed behavior. The JTinduced orbital ordering of occupied Mne(g)(1) electrons in alternating 3x(2)r(2)/3y(2)r(2) orbital states in the unstrained structure, changes under inplane compressive strain to a mixture with x(2)z(2)/y(2)Z(2) states. The asymmetric hopping of e(g) electrons between Mn ions along zigzag spin chains (typical of the AFME spin configuration) is therefore enhanced under strain, explaining the large value of the polarization. Using a degenerate doubleexchange model including electronphonon interaction, we reproduce the change in the orbital ordering pattern. In this picture, the orbital ordering change is related to a change of the Berry phase of the e(g) electrons. This causes an increase of the electronic contribution to the polarization.


6. 
 Iusan, Diana, et al.
(författare)

Electronic structure and chemical and magnetic interactions in ZnO doped with Co and Al : Experiments and ab initio densityfunctional calculations
 2008

Ingår i: Physical Review B. Condensed Matter and Materials Physics.  10980121. ; 78:8, s. 085319

Tidskriftsartikel (refereegranskat)abstract
 We present results of electronic structure and magnetization measurements of Co:ZnO and Co:ZnO codoped with Al thinfilm samples fabricated by solutionbased methods together with ab initio electronic structure calculations. Electronic structure measurements indicate that the Co states lie close to the valenceband edge with pinning of the Fermi level primarily due to native defects yielding a heavily ndoped material. The findings in the electronic structure measurements are corroborated by results from theoretical calculations. We find that it is necessary to go beyond the localdensity approximation to achieve agreement with experiments. Moreover, the theoretical calculations indicate a tendency for the formation of Co clusters, giving rise to an antiferromagnetic exchange interaction between the Co atoms. The magnetization measurements are well in line with the theoretical predictions, showing a dominating superparamagnetic behavior arising from small antiferromagnetic clusters containing uncompensated spins.


7. 
 Iusan, Diana, et al.
(författare)

Influence of defects on the magnetism of Mndoped ZnO
 2007

Ingår i: Journal of Applied Physics.  00218979. ; 101:9, s. 09H101

Tidskriftsartikel (refereegranskat)abstract
 The properties of dilute magnetic semiconductors are usually strongly influenced by the defects present in the system. Ab initio calculations may provide valuable insight for the microscopic understanding of the interactions with defects. Here, we present studies of Mndoped ZnO in the presence of several defects by a combined approach of ab initio electronic structure calculations using KorringaKohnRostokercoherent potential approximation and Monte Carlo simulations (MCSs). Electronic structure and magnetic interactions have similar trends for wurtzite and zincblende crystal structures. A weak antiferromagnetic interaction has been found for 5% Mn doping in defectfree ZnO. Defects such as O vacancies and Zn interstitials lead to antiferromagnetic interactions between the Mn atoms, while Zn vacancies and oxygen substitution by nitrogen yield ferromagnetic interactions. As the concentration of Mn is low and the exchange interactions are short ranged, MCSs show small values of Curie temperatures (not more than 50 K). However, for a few cases with codoping of Mn and defects, we obtained higher Curie temperatures (around 130 K). Estimates of the Curie temperatures, assuming an average separation of the Mn atoms in the meanfield solution of Heisenberg model, are in very good agreement with the results obtained from MCS.


8. 
 Iusan, Diana, et al.
(författare)

Microscopic picture of Co clustering in ZnO
 2009

Ingår i: Physical Review B. Condensed Matter and Materials Physics.  The American Physical Society.  10980121. ; 79:12, s. 125202

Tidskriftsartikel (refereegranskat)abstract
 Density functional theory was applied to study the chemical and magnetic interactions between Co atoms doped in ZnO. It was found that the Co impurities tend to form nanoclusters and the interactions between these atoms are antiferromagnetic within the local spindensity approximation (LSDA)+Hubbard U approach. The extracted interatomic exchange parameters agree reasonably well with recent experimental results. We have analyzed and compared the electronic structure obtained using the LSDA and LSDA+U approaches and found that the LSDA+U gives the most reasonable result, highlighting the importance of shortranged antiferromagnetic interactions due to superexchange.


9. 
 Iusan, Diana, et al.
(författare)

Role of defects on the magnetic interactions in Mndoped ZnO
 2007

Ingår i: Physica status solidi. A, Applied research.  00318965. ; 204:1, s. 5360

Tidskriftsartikel (refereegranskat)abstract
 Oxide based diluted magnetic semiconductors are highly controversial from the point of view of intrinsic ferromagnetism brought in by doping with transition metal ions. By abinitio KorringaKohnRostokerCoherentPotentialApproximation (KKRCPA) calculations in the framework of density functional theory, we have obtained the electronic structure and magnetic exchange interaction parameters of Mndoped ZnO in the presence and absence of several types of defects. A weak antiferromagnetic exchange interaction is observed in pure Mndoped ZnO in the dilute limit with an increase in the strength of interactions with increasing concentration of Mn. In the presence of donor defects, such as oxygen vacancies and interstitial Zn, the interactions remain antiferromagnetic whereas in case of acceptor defects like Zn vacancies and N substitution of O, ferromagnetic interactions are observed. We have used the abinitio exchange parameters in MonteCarlo simulations for calculating finite temperature properties. Due to the short ranged behavior of interactions and disorder effects, the Curie temperatures calculated from Monte Carlo simulations show moderate values, not exceeding 50 K when the Mn content is 5%.


10. 
 Iusan, Diana, et al.
(författare)

Theoretical study of the magnetism of Mndoped ZnO with and without defects
 2006

Ingår i: Physical Review B. Condensed Matter and Materials Physics.  10980121. ; 74:23, s. 235208

Tidskriftsartikel (refereegranskat)abstract
 We calculate the exchange interaction parameters of a classical Heisenberg Hamiltonian for Mndoped ZnO (Mn concentration between 5% and 20%) by an ab initio KorringaKohnRostoker coherentpotentialapproximation method in the framework of density functional theory. A weak antiferromagnetic exchange interaction is observed in pure Mndoped ZnO in the dilute limit with an increase in the strength of interactions with increasing concentration of Mn. In the presence of donor defects, such as oxygen vacancies and interstitial Zn, the interactions remain antiferromagnetic, whereas in case of acceptor defects like Zn vacancies and N substitution of O, ferromagnetic interactions are observed. Due to the shortranged character of interactions and disorder effects, the Curie temperatures calculated from Monte Carlo simulations yield low values (∼45 K). However, in a few combinations of Mn and defect concentrations, the calculated Curie temperature can be as high as 135 K. If clustering of Mn atoms on a zincblende lattice is taken into account, the MnMn spin correlations within a cluster are found to persist up to 600 K. Finally, we have shown that a modified meanfield theory, which we refer to as the “average mean field” estimate, yields values of the ordering temperature that are in good agreement with Monte Carlo simulations.

