| 1. |
- Adell, Johan, 1980, et al.
(författare)
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Electron spectroscopic studies of nanowires formed by (GaMn)As growth on GaAs(111)B
- 2011
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Ingår i: Solid State Communications. - : Elsevier BV. - 0038-1098 .- 1879-2766. ; 151:11, s. 850-854
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Tidskriftsartikel (refereegranskat)abstract
- Valence band photoemission with photon energies around the Mn2p excitation threshold has been used to study the development of nanowires catalyzed by MnAs particles. A gradual change in the spectra with increasing nanowire length is observed, such that the resonant photoemission eventually dominates over the Auger decay channel. The change is ascribed to dilution of Mn, showing that Mn is transferred from the MnAs particles into the nanowires. (C) 2011 Elsevier Ltd. All rights reserved.
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| 2. |
- Adell, Johan, et al.
(författare)
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Formation of epitaxial MnBi layers on (Ga,Mn)As
- 2009
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121 .- 2469-9950 .- 2469-9969. ; 80:7
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Tidskriftsartikel (refereegranskat)abstract
- The initial growth of MnBi on MnAs-terminated (GaMn)As is studied by means of synchrotron-based photoelectron spectroscopy. From analysis of surface core-level shifts we conclude that a continued epitaxial MnBi layer is formed, in which the MnAs/MnBi interface occurs between As and Bi atomic planes. The well-defined 1×2 surface reconstruction of the MnAs surface is preserved for up to 2 ML of MnBi before clear surface degradation occurs. The MnBi layer appears to be free from intermixed As.
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| 3. |
- Adell, Johan, 1980, et al.
(författare)
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Thermal diffusion of Mn through GaAs overlayers on (Ga, Mn)As
- 2011
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 23:8
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Tidskriftsartikel (refereegranskat)abstract
- Thermally stimulated diffusion of Mn through thin layers of GaAs has been studied by x-ray photoemission. (Ga, Mn)As samples with 5 at% Mn were capped with 4, 6 and 8 monolayer (ML) GaAs, and Mn diffusing through the GaAs was trapped on the surface by means of amorphous As. It was found that the out-diffusion is completely suppressed for an 8 ML thick GaAs film. The short diffusion length is attributed to an electrostatic barrier formed at the (Ga, Mn)As/GaAs interface.
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| 4. |
- Kanski, Janusz, 1946, et al.
(författare)
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Mn-induced modifications of Ga 3d photoemission from (Ga, Mn)As: evidence for long range effects
- 2012
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Ingår i: Journal of Physics: Condensed Matter. - : IOP Publishing. - 1361-648X .- 0953-8984. ; 24:43, s. 1-435802
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Tidskriftsartikel (refereegranskat)abstract
- Using synchrotron based photoemission, we have investigated the Mn-induced changes in Ga 3d core level spectra from as-grown Ga(1-x)Mn(x)As. Although Mn is located in Ga substitutional sites, and therefore does not have any Ga nearest neighbors, the impact of Mn on the Ga core level spectra is pronounced even at Mn concentrations in the region of 0.5%. The analysis shows that each Mn atom affects a volume corresponding to a sphere with around 1.4 nm diameter.
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| 5. |
- Pal, Prabir, et al.
(författare)
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Pseudogap behavior in Pr0.5Sr0.5MnO3 : A photoemission study
- 2011
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 605:9-10, s. 875-877
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Tidskriftsartikel (refereegranskat)abstract
- The valence band electronic structure of Pr0.5Sr0.5MnO3 has been investigated across its paramagnetic metallic (PMM)-ferromagnetic metallic (FMM)-antiferromagnetic insulator (AFMI) transition. Using surface sensitive high resolution photoemission we have conclusively demonstrated the presence of a pseudogap of magnitude 80 meV in the near Fermi level electronic spectrum in the PMM and FMM phases and finite intensity at the Fermi level in the charge ordering (CO)-AFMI phase. The pseudogap behavior is explained in terms of the strong electron-phonon interaction and the formation of Jahn Teller UT) polarons, indicating the charge localizations. The finite intensity at the Fermi level in the insulating phase showed a lack of charge ordering in the surface of the Pr0.5Sr0.5MnO3 samples.
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| 6. |
- Pal, P., et al.
(författare)
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Resonant photoemission spectroscopy studies of the magnetic phase transitions in Pr(0.5)Sr(0.5)MnO(3)
- 2011
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Ingår i: Physica B: Condensed Matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 406:18, s. 3519-3523
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Tidskriftsartikel (refereegranskat)abstract
- The valence band electronic structure of Pr(0.5)Sr(0.5)MnO(3) has been investigated using resonant photoemission spectroscopies across its paramagnetic metallic (PMM)-ferromagnetic metallic (FMM)-antiferromagnetic insulator (AFMI) transition. Using bulk sensitive soft X-ray photoemission, we have confirmed the presence of a gap in the near Fermi level electronic spectrum in the charge ordering (CO) antiferromagnetic insulator (AFMI) phase. The Fermi level exhibits a rigid-band-like shift of about similar to 0.4 eV across the PMM to AFMI transition. Further, we have observed a similar shift of the Pr 4f states to higher binding energies when the sample goes to the CO-AFMI state. The spectral behavior indicates a strong charge carrier localization associated with a charge-density-wave gap related to the CO. On the other hand, the growth of spectral weight at the Fermi level in the PMM state could well be connected to the microscopic phase-separation model. (C) 2011 Elsevier B.V. All rights reserved.
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| 7. |
- Ulfat, Intikhab, 1966
(författare)
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Annealing Induced Modifications in (GaMn)As: Electron Spectroscopic Studies
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- By incorporating magnetism into semiconductors, it may possibly be viable to enhance the functionality of materials. An exceptionally appealing material in this context is GaAs, which can be doped with Mn atoms. (GaMn)As has fascinated research community as a promising candidate for spintronic application. It is quite appealing due to both its compatibility with existing III-V technology and great progress in improving its magnetic properties. Being fabricated by low temperature molecular beam epitaxy (LT-MBE), due to thermal instability at elevated temperatures, the material contains a high density of various defects compensating Mn acceptors. It is a well-established fact that the ferromagnetic state of (GaMn)As can be stabilized via post growth annealing. Nevertheless, in general, the annealed (GaMn)As layers do not remain useful for further epitaxial overgrowth that might beincluded in multilayer structure. In order to overcome this practical difficulty a procedure was devised in which annealing is carried out under amorphous As. The focus of the present thesis is to study the MBE grown (GaMn)As layers annealed under As capping using synchrotron based spectroscopy. The formation of epitaxial MnAs with a 1x2 reconstruction resulting from the reaction between out-diffusing Mn interstitials and As capping has been reported analysing the As3d core levels. The use of topmost MnAs layer of (GaMn)As annealed under As capping has also been described for depositing successive layers of Bi and Mn resulting in formation of uniform surface layers. Moreover Mn2p emission from annealed and as-grown (GaMn)As layers with varying GaAs overlayer thickness has been examined to describe the thickness dependent mechanism that stops the diffusing of interstitial Mn from reaching the surface.
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| 8. |
- Ulfat, Intikhab, 1966, et al.
(författare)
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As3d core level studies of (GaMn)As annealed under As capping
- 2010
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 604:2, s. 125-128
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Tidskriftsartikel (refereegranskat)abstract
- The surface of a Ga(0.95)Mn(0.05)As layer subjected to low temperature annealing under As capping has been studied by core level photoemission with focus on As3d spectrum. By detailed comparison with the surface of pure GaAs subjected to the same surface treatment, the As spectral component of the reacted surface layer has been identified. The relative intensity of this component is consistent with the notion of an MnAs monolayer terminating the annealed (GaMn)As surface.
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| 9. |
- Ulfat, Intikhab, 1966
(författare)
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Detailed photoemission studies of the prototype diluted magnetic semiconductor (Ga,Mn)As
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Magnetic semiconductors are materials that combine the key features needed in information technology, namely magnetism and controllable charge transport. The prospects of integrating these properties and of using the spin as informa-tion carrier have motivated large efforts to produce such materials for imple-mentation in future spin-based electronics. This has turned out to be a very difficult task, and despite all endeavor, there is still no material produced that could be used in common devices. One of the main hurdles is the low tempera-ture that is needed for the ferromagnetic state: (Ga,Mn)As, the material with the highest transition temperature to the ferromagnetic state, still requires cooling to around 200 K. As the basic principles about the mechanisms underlying ferromagnetism in (Ga,Mn)As are still under debate, there is an obvious need for detailed experimental information that can provide important clues towards an improved understanding. This is the goal of the work in the present thesis. (Ga,Mn)As is prepared by low temperature molecular beam epitaxy (MBE), which is a means of obtaining a material with a composition far outside the thermodynamic equilibrium. Although the solubility of Mn in GaAs is below 0.1%, it is possible to grow layers with concentrations up to 20%. However, such layers are thermo-dynamically unstable, and phase segregation occurs rapidly under heating above 300 °C. At the same time it is established that controlled heating at lower tempe-ratures has very positive effects on the magnetic properties, based on removal of Mn atoms in defect crystallographic positions. The heating is thus a delicate process that requires deeper understanding. One aspect in this context is to clarify why the diffusion of Mn does not take place across the boundary between (Ga,Mn)As and GaAs. This question was addressed in the present work. By studying diffusion through GaAs overlayer films, it was demonstrated that the range of diffusion is about 10 atomic layers, and is limited by the build-up of an electrostatic potential barrier. The thesis is also focused on details in the electronic structure that can be relevant for the magnetic properties. One important question in this context is which electron states mediate the magnetic coupling between the Mn atoms. Having access to a dedicated MBE system directly connected to a high-resolution photoelectron spectrometer at the synchrotron radiation laboratory MAX-lab, the conditions for new detailed investigations of this problem were excellent. One of the main results in this thesis is thus the discovery of a strongly dispersive Mn-induced electron energy band, which is suggested to play the key role in spin alignment. Results of studies of very dilute samples also show that each Mn atom in the (Ga,Mn)As system influences a relatively large volume around it, far beyond the distance between nearest neighbors in the lattice. Furthermore, evidence is provided for a locally nonuniform distribution of Mn atoms. The picture of ferromagnetism in (Ga,Mn)As emerging from the present experiments is consistent with an earlier proposed magnetic polaron model, rather different from the currently most applied theories.
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| 10. |
- Ulfat, Intikhab, 1966, et al.
(författare)
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Electron Spectroscopic Studies of Homogenous (GaMn)As layers
- 2012
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Ingår i: Advanced Materials Research. - 1662-8985 .- 1022-6680. - 9783037853634 ; 463-464, s. 380-384
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Konferensbidrag (refereegranskat)abstract
- By incorporating magnetism into semiconductors, it may possibly be viable to enhance the functionality of materials. An exceptionally important material in this context is GaAs, which can be doped with Mn atoms. (GaMn)As has fascinated research community as a promising candidate for spintronic application. It is quite appealing due to both its compatibility with existing HI-V technology and great progress in improving its magnetic properties. Being fabricated by low temperature molecular beam epitaxy (LT-MBE), due to thermal instability at elevated temperatures, the material contains a high density of various defects compensating Mn acceptors. It is a well-established fact that the ferromagnetic state of (GaMn)As can be stabilized via post growth annealing. Nevertheless, in general, the annealed (GaMn)As layers do not remain useful for further epitaxial overgrowth that might be included in multilayer structure. We present a summary of our investigations regarding the synchrotron-based characterization of (GaMn)As layers grown via molecular beam epitaxy carried out at the Swedish National Facility of Synchrotron Radiation-the MAX-lab aiming at the reduction of the density of Mn interstitial and increase in the content of Mn.
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