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| 2. |
- Andersson, Gabriella, et al.
(författare)
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Perpendicular magnetocrystalline anisotropy in tetragonally distorted Fe-Co alloys
- 2006
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 96:3
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Tidskriftsartikel (refereegranskat)abstract
- We report on the experimental realization of tetragonal Fe-Co alloys as a constituent of Fe(0.36)Co(0.64)/Pt superlattices with huge perpendicular magnetocrystalline anisotropy energy, reaching 210 mu eV/atom, and a saturation magnetization of 2.5 mu(B)/atom at 40 K, in qualitative agreement with theoretical predictions. At room temperature the corresponding values 150 mu eV/atom and 2.2 mu(B)/atom are achieved. This suggests that Fe-Co alloys with carefully chosen combinations of composition and distortion are good candidates for high-density perpendicular storage materials.
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| 3. |
- Bergman, Anders, et al.
(författare)
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Magnetic properties of Fe/Co(001) superlattices from first-principles theory
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 74:17
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Tidskriftsartikel (refereegranskat)abstract
- The magnetic properties of Fe/Co(001) superlattices have been studied using fully-relativistic first-principles theories. The average magnetic moment shows a behavior similar to bulk Fe-Co alloys, i.e., an enhanced magnetic moment for low Co concentrations, as described by the Slater-Pauling curve. The maximum of the magnetization curve, however, is lowered and shifted towards the Fe-rich compositions. The increased average magnetic moment for the Fe-rich superlattices, compared to bulk Fe, is due to an enhancement of the Fe spin moment close to the Fe-Co interface. The orbital moments were found to be of the same size as in bulk. The effect of interface roughness on the magnetic properties was investigated, and it was found that-despite local fluctuations due to the varying coordination-the average magnetic moment is only slightly affected. From a mapping of first-principles interactions onto the screened generalized perturbation method, we calculate the temperatures for when Fe/Co superlattices break up into an alloy configuration. Furthermore, the tetragonal distortion of the superlattice structure was found to only have a minor effect on the magnetic moments. Also, the calculated easy axis of magnetization is in the film plane for all compositions studied. It lies along the [100] direction for Fe-rich superlattices and along the [110] direction for Co-rich compositions. The transition of the easy axis occurs around a Co concentration of 50%.
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| 4. |
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| 5. |
- Burkert, Till, et al.
(författare)
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Giant Magnetic Anisotropy in Tetragonal FeCo Alloys
- 2004
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 93:2, s. 027203-
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Tidskriftsartikel (refereegranskat)abstract
- The magnetic anisotropy energy (MAE) of hcp Gd was calculated from first principles using the full-potential linear muffin-tin orbital method. It was found that the principal contributions to the MAE are the dipole-dipole interaction between the localized 4f spins and the spin-orbit interaction of the valence band states. The dipole contribution has the form 1/2K(1)(d)(1-cos 2 theta), where theta is the angle between the magnetization direction and the c axis. The contribution of the spin-orbit interaction is shown to arise from the polarization of the conduction band that becomes exchange split due to exchange interaction with the localized 4f electrons. We argue that this leads to significant contributions from higher order anisotropy constants. An imposed reduced 4f moment leads to a repopulation of the electronic states at the Fermi level and a reduced exchange splitting of the valence states, which we demonstrate leads to a modification of the MAE. This modification is in qualitative agreement with the observed temperature dependence of the MAE. In addition, the dependence of the MAE on the c/a ratio has been studied.
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| 6. |
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| 7. |
- Burkert, Till, et al.
(författare)
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Magnetic anisotropy of L1(0) FePt and Fe1-xMnxPt
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:13, s. 134411-
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Tidskriftsartikel (refereegranskat)abstract
- The uniaxial magnetic anisotropy energy (MAE) of L1(0) FePt and Fe1-xMnxPt, x=0-0.25, was studied from first principles using two fully relativistic computational methods, the full-potential linear muffin-tin orbitals method and the exact muffin-tin orbitals method. It was found that the large MAE of 2.8 meV/f.u. is caused by a delicate interaction between the Fe and Pt atoms, where the large spin-orbit coupling of the Pt site and the hybridization between Fe 3d and Pt 5d states is crucial. The effect of random order on the MAE was modeled by mutual alloying of the sublattices within the coherent potential approximation (CPA), and a strong dependence of the MAE on the degree of chemical long-range order was found. The alloying of FePt with Mn was investigated with the virtual crystal approximation and the CPA as well as supercell calculations. The MAE increases up to 33% within the concentration range studied here, an effect that is attributed to band filling. Furthermore, the dependence of the MAE on the structural properties was studied.
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| 8. |
- Burkert, Till, et al.
(författare)
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Magnetic ordering in a weakly coupled Fe/V(001) superlattice
- 2002
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Ingår i: Physical Review B. - 0163-1829. ; 66:22, s. 220402(R)-
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Tidskriftsartikel (refereegranskat)abstract
- A large shift of the ordering temperature is observed in an Fe3/V13(001) superlattice upon altering the interlayer coupling J′. The modification of J′ is accomplished by temporary alloying of the V spacer layers with hydrogen. A phase with short-range magnetic order is found in a wide temperature range between the paramagnetic phase and the onset of long-range order. It is interpreted as two-dimensional-XY behavior and is attributed to the absence of an in-plane magnetic anisotropy and the vanishing of the interlayer exchange coupling close to the ordering temperature.
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| 9. |
- Burkert, Till, 1972-
(författare)
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Materials for Magnetic Recording Applications
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the first part of this work, the influence of hydrogen on the structural and magnetic properties of Fe/V(001) superlattices was studied. The local structure of the vanadium-hydride layers was determined by extended x-ray absorption fine structure (EXAFS) measurements. The magnetic ordering in a weakly coupled Fe/V(001) superlattice was investigated using the magneto-optical Kerr effect (MOKE). The interlayer exchange coupling is weakened upon alloying with hydrogen and a phase with short-range magnetic order was observed. The second part is concerned with first-principles calculations of magnetic materials, with a focus on magnetic recording applications. The uniaxial magnetic anisotropy energy (MAE) of Fe, Co, and Ni was calculated for tetragonal and trigonal structures. Based on an analysis of the electronic states of tetragonal Fe and Co at the center of the Brillouin zone, tetragonal Fe-Co alloys were proposed as a material that combines a large uniaxial MAE with a large saturation magnetization. This was confirmed by experimental studies on (Fe,Co)/Pt superlattices. The large uniaxial MAE of L10 FePt is caused by the large spin-orbit interaction on the Pt sites in connection with a strong hybridization between Fe and Pt. Furthermore, it was shown that the uniaxial MAE can be increased by alloying the Fe sublattice with Mn. The combination of the high-moment rare-earth (RE) metals with the high-TC 3d transition metals in RE/Cr/Fe multilayers (RE = Gd, Tb, Dy) gives rise to a strong ferromagnetic effective exchange interaction between the Fe layers and the RE layer. The MAE of hcp Gd was found to have two principal contributions, namely the dipole interaction of the large localized 4f spins and the band electron magnetic anisotropy due to the spin-orbit interaction. The peculiar temperature dependence of the easy axis of magnetization was reproduced on a qualitative level.
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| 10. |
- Colarieti-Tosti, Massimiliano, et al.
(författare)
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Theory of the temperature dependence of the easy axis of magnetization in hcp Gd
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 72:9, s. 094423-
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Tidskriftsartikel (refereegranskat)abstract
- The magnetic anisotropy energy (MAE) of hcp Gd was calculated from first principles using the full-potential linear muffin-tin orbital method. It was found that the principal contributions to the MAE are the dipole-dipole interaction between the localized 4f spins and the spin-orbit interaction of the valence band states. The dipole contribution has the form 1/2K(1)(d)(1-cos 2 theta), where theta is the angle between the magnetization direction and the c axis. The contribution of the spin-orbit interaction is shown to arise from the polarization of the conduction band that becomes exchange split due to exchange interaction with the localized 4f electrons. We argue that this leads to significant contributions from higher order anisotropy constants. An imposed reduced 4f moment leads to a repopulation of the electronic states at the Fermi level and a reduced exchange splitting of the valence states, which we demonstrate leads to a modification of the MAE. This modification is in qualitative agreement with the observed temperature dependence of the MAE. In addition, the dependence of the MAE on the c/a ratio has been studied.
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