-
[show abstract]
[hide abstract]
ABSTRACT: We report a study of the electronic properties of the ferromagnetic semiconductor (Ga,Mn)As using magnetic linear dichroism in the angular dependence of Mn 2p photoemission under hard x-ray excitation. Bulk plasmon loss satellites demonstrate that the probed Mn ions are incorporated deep within the GaAs lattice, while the observed large dichroism indicates that the spectra originate from ferromagnetic substitutional Mn. Simulations of the spectra using an Anderson impurity model show that the ferromagnetic Mn 3d electrons of substitutional Mn in (Ga,Mn)As are intermediate between localized and delocalized.
Physical Review Letters 11/2011; 107(19):197601. · 7.37 Impact Factor
-
A. Casiraghi,
A. W. Rushforth,
M Wang, N. R. S. Farley,
P. Wadley,
J. L. Hall,
C. R. Staddon,
K. W. Edmonds,
R. P. Campion,
C.T. Foxon,
B. L. Gallagher
[show abstract]
[hide abstract]
ABSTRACT: We have investigated the effects of post growth low temperature annealing on the magnetic, electrical and structural properties of (Ga_0.94,Mn_0.06)(As_0.9,P_0.1) layers grown by molecular beam epitaxy. By controlling the annealing time we are able to tune the magnetic anisotropy between an easy axis in the plane for the as-grown samples, to an easy axis perpendicular to the plane for fully annealed samples. The increase of the carrier density, as a result of annealing, is found to be the primary reason for the change in magnetic anisotropy, in qualitative agreement with theoretical predictions. Comment: 13 pages, 3 figures, submitted to Applied Physics Letters
06/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: An x-ray method is described for the structure characterization of semiordered GaN nanorods. In contrast to other works based on synchrotron radiation, the method uses a standard x-ray laboratory equipment so that it is suitable for a rapid characterization of the nanorods in a technological laboratory. The method uses a grazing-incidence diffraction setup and it makes it possible to determine the mean size of the rods and their angular twist with respect to the crystalline substrate. The applicability of the method is demonstrated on a series of GaN nanorod samples and the parameters of the nanorods are compared with the results of scanning electron microscopy.
Journal of Applied Physics 12/2008; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We present an experimental investigation of the magnetic, electrical, and structural properties of Ga <sub>0.94</sub> Mn <sub>0.06</sub> As <sub>1-y</sub> P <sub>y</sub> layers grown by molecular beam epitaxy on GaAs substrates for y≤0.3 . X-ray diffraction measurements reveal that the layers are under tensile strain, which gives rise to a magnetic easy axis perpendicular to the plane of the layers. The strength of the magnetic anisotropy and the coercive field increases as the phosphorous concentration is increased. The resistivity of all samples shows metallic behavior with the resistivity increasing as y increases. These materials will be useful for studies of micromagnetic phenomena requiring metallic ferromagnetic material with perpendicular magnetic anisotropy.
Journal of Applied Physics 11/2008; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A variety of x-ray techniques is employed to investigate sol-gel-synthesized Co- and Mn-doped ZnO films. X-ray diffraction (XRD) shows that the dominant structure is the c-axis orientated wurtzite ZnO. Dopant K-edge extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge structure (XANES) indicate that the Co and Mn ions are fourfold co-ordinated with valency 2+/3+ and 3+/4+, respectively. Surface-sensitive dopant L-edge XANES and x-ray magnetic circular dichroism (XMCD) for the Co dopant are well described by atomic multiplet calculations for tetrahedral Co2+, while for Mn the data indicate that the symmetry is lowered due to Jahn–Teller distortion. The XMCD show conclusively that both Co- and Mn-doped ZnO films are purely paramagnetic.
New Journal of Physics 05/2008; 10(5):055012. · 4.18 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We demonstrate sensitivity of the Mn 3d valence states to strain in the ferromagnetic semiconductors (Ga,Mn)As and (Al,Ga,Mn)As, using x-ray magnetic circular dichroism (XMCD). The spectral shape of the Mn $L_{2,3}$ XMCD is dependent on the orientation of the magnetization, and features with cubic and uniaxial dependence are distinguished. Reversing the strain reverses the sign of the uniaxial anisotropy of the Mn $L_3$ pre-peak which is ascribed to transitions from the Mn 2p core level to p-d hybridized valence band hole states. With increasing carrier localization, the $L_3$ pre-peak intensity increases, indicating an increasing 3d character of the hybridized holes. Comment: 4 pages plus 2 figures, accepted for publication in Physical Review B
02/2008;
-
A. A. Freeman,
K. W. Edmonds,
G van der Laan,
R. P. Campion, N. R. S. Farley,
A. W. Rushforth,
T. K. Johal,
C.T. Foxon,
B. L. Gallagher,
A Rogalev,
F Wilhelm
[show abstract]
[hide abstract]
ABSTRACT: The element-specific technique of x-ray magnetic circular dichroism (XMCD) is used to directly determine the magnitude and character of the valence band orbital magnetic moments in (III,Mn)As ferromagnetic semiconductors. A distinct dichroism is observed at the As K absorption edge, yielding an As 4p orbital magnetic moment of around -0.1 Bohr magnetons per valence band hole. This is strongly influenced by strain, indicating its crucial influence on the magnetic anisotropy. The dichroism at the Ga K edge is much weaker. The K edge XMCD signals for Mn and As both have positive sign, which indicates the important contribution of Mn 4p states to the Mn K edge spectra. Comment: 5 pages, 2 figures
01/2008;
-
[show abstract]
[hide abstract]
ABSTRACT: An ultrahigh vacuum chamber that enables the in situ growth of thin films and multilayers by magnetron sputtering techniques is described. Following film preparation, x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements are performed by utilizing an in vacuum electromagnet. XMCD measurements on sputtered thin films of Fe and Co yield spin and orbital moments that are consistent with those obtained previously on films measured in transmission geometry and grown in situ by evaporation methods. Thin films of FeN prepared by reactive sputtering are also examined and reveal an apparent enhancement in the orbital moment for low N content samples. The advantages of producing samples for in situ XAS and XMCD studies by magnetron sputtering are discussed.
Review of Scientific Instruments 07/2006; 77(7):073903-073903-6. · 1.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Hugely anisotropic Mn L2,3 x-ray magnetic linear dichroism (XMLD) is reported for the ferromagnetic semiconductor (Ga,Mn)As. The sign, magnitude, and line shape of the dichroism changes with azimuthal rotation of the samples between the in-plane ⟨110⟩ and ⟨100⟩ directions. The strong anisotropy is confirmed by multiplet calculations for a Mn 3d5 configuration where charge anisotropy and magnetocrystalline anisotropy are absent. This allows a precise determination of the Mn 3d crystal-field splitting, and establishes the anisotropy in the XMLD as a general feature for localized moment systems with crystal-field symmetry.
Phys. Rev. B. 06/2006; 73(23).
-
[show abstract]
[hide abstract]
ABSTRACT: Remarkably anisotropic Mn L2,3 x-ray magnetic circular dichroism spectra from the ferromagnetic semiconductor (Ga,Mn)As are reported. States with cubic and uniaxial symmetry are distinguished by careful analysis of the angle dependence of the spectra. The multiplet structures with cubic symmetry are qualitatively reproduced by calculations for an atomiclike d5 configuration in tetrahedral environment, and show zero anisotropy in the orbital and spin moments within the experimental uncertainty. However, hybridization with the host valence bands is reflected by the presence of a preedge feature with a uniaxial anisotropy and a marked dependence on the hole density.
Physical Review Letters 04/2006; 96(11):117207. · 7.37 Impact Factor
-
T. Jungwirth,
J. Masek,
K. Y. Wang,
K. W. Edmonds,
M. Sawicki,
M. Polini,
Jairo Sinova,
A. H. MacDonald,
R. P. Campion,
L. X. Zhao, N. R. S. Farley,
T. K. Johal,
G van der Laan,
C.T. Foxon,
B. L. Gallagher
[show abstract]
[hide abstract]
ABSTRACT: We report on a comprehensive study of the ferromagnetic moment per Mn atom in (Ga,Mn)As ferromagnetic semiconductors. Theoretical discussion is based on microscopic calculations and on an effective model of Mn local moments antiferromagnetically coupled to valence band hole spins. The validity of the effective model over the range of doping studied is assessed by comparing with microscopic tight-binding/coherent-potential approximation calculations. Using the virtual crystal k.p model for hole states, we evaluate the zero-temperature mean-field contributions to the magnetization from the hole kinetic and exchange energies, and magnetization suppression due to quantum uctuations of Mn moment orientations around their mean-field ground state values. Experimental low-temperature ferromagnetic moments per Mn are obtained by superconducting quantum interference device and x-ray magnetic circular dichroism measurements in a series of (Ga,Mn)As semiconductors with nominal Mn doping ranging from ~2% to 8%. Hall measurements in as-grown and annealed samples are used to estimate the number of uncompensated substitutional Mn moments. Based on our comparison between experiment and theory we conclude that all these Mn moments in high quality (Ga,Mn)As materials have nearly parallel ground state alignment. Comment: 11 pages, 11 figures, submitted to Phys. Rev. B
08/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: We demonstrate that carefully prepared (Ga,Mn)As films can show large magnetic moments per atom across a wide range of Mn concentrations, indicating almost full participation of the Mn in the ferromagnetism. Applying sum rules to Mn L2,3 x-ray magnetic circular dichroism (XMCD) spectra yields a magnetic moment per Mn of around 4.5μB, including a small positive orbital moment. We also present direct evidence for antiferromagnetic coupling between interstitial and substitutional Mn in unannealed (Ga,Mn)As. The Mn L2,3 x-ray absorption line shapes display no sizeable site or concentration dependence, but in unannealed (Ga,Mn)As the XMCD signal is significantly smaller, and increases linearly under high magnetic fields.
Phys. Rev. B. 02/2005; 71(6).
-
[show abstract]
[hide abstract]
ABSTRACT: Using As2 as an arsenic source together with other appropriate growth and annealing procedures, we have improved the electrical and magnetic properties of GaMnAs films and obtained Curie temperatures as high as 159 K. For Mn concentrations up to 7%, we have demonstrated that the compensation previously observed in such films is mainly due to Mn interstitials, which can be removed by using appropriate annealing procedures for thin films. For the Mn concentrations >7%, we suggest that additional AsGa defects also cause compensation, which cannot be removed by our present annealing procedure.
Journal of Materials Science Materials in Electronics 12/2004; 15(11):727-731. · 1.08 Impact Factor
-
M. Sawicki,
K. Y. Wang,
K. W. Edmonds,
R. P. Campion,
C. R. Staddon, N. R. S. Farley,
C.T. Foxon,
E. Papis,
E. Kaminska,
A. Piotrowska,
T. Dietl,
B. L. Gallagher
[show abstract]
[hide abstract]
ABSTRACT: We show, by SQUID magnetometry, that in (Ga,Mn)As films the in-plane uniaxial magnetic easy axis is consistently associated with particular crystallographic directions and that it can be rotated from the [-110] direction to the [110] direction by low temperature annealing. We show that this behavior is hole-density-dependent and does not originate from surface anisotropy. The presence of uniaxial anisotropy as well its dependence on the hole-concentration and temperature can be explained in terms of the p-d Zener model of the ferromagnetism assuming a small trigonal distortion. Comment: 4 pages, 6 Postscript figures, uses revtex4
10/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: We present Mn L<sub>3,2</sub> x-ray absorption measurements from two dilute magnetic semiconductor systems: p-type (Ga,Mn)As, which is ferromagnetic with T<sub>C</sub>=140 K ; and n-type (Ga,Mn)N, in which the magnetic impurities are predominantly paramagnetic. After removing a Mn-rich oxide surface layer by chemical etching, the Mn L<sub>3,2</sub> spectra from (Ga,Mn)As appear less localized than in previous reports, which is ascribed to screening due to p–d hybridization. Our results suggest that previous studies may have been influenced by the quality of the surface. In contrast, in the (Ga,Mn)N film the Mn ground state is closer to pure d<sup>5</sup>. © 2004 American Institute of Physics.
Journal of Applied Physics 07/2004; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We have identified a Mn-rich layer on the surface on (Ga,Mn)As thin films which significantly influences soft x-ray absorption measurements. The Mn L<sub>3,2</sub> x-ray absorption spectra of the untreated films show a strong multiplet structure, consistent with earlier observations and characteristic of MnO. After removal of the surface layer, the multiplet structure is less pronounced and the spectrum is shifted to ∼0.5 eV lower photon energy. Comparison with calculated spectra imply a localized Mn ground state for the untreated sample and a hybridized ground state after etching. In addition, a large x-ray magnetic circular dichroism is observed at the Mn L<sub>3,2</sub> edge in the etched film. These results may explain several peculiarities of previously reported x-ray absorption studies from (Ga,Mn)As. © 2004 American Institute of Physics.
Applied Physics Letters 06/2004; · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on measurements of the hole density, ferromagnetic transition temperature TC, and magnetization in a series of as-grown and annealed (Ga,Mn)As samples. Estimating the fraction of incorporated Mn occupying interstitial and substitutional sites allows a direct comparison of the predictions of mean field theory with experiment, and a determination of the magnetic moment per substitutional Mn. The saturation of TC at high Mn concentration is consistent with the mean field prediction. The estimated magnetic moment per Mn is close to the expected 5μB for all samples studied if an antiferromagnetic coupling between interstitial and substitutional Mn is assumed. © 2004 American Institute of Physics.
Journal of Applied Physics 05/2004; 95(11):6512-6514. · 2.17 Impact Factor
-
K W Edmonds,
P Bogusławski,
K Y Wang,
R P Campion,
S N Novikov, N R S Farley,
B L Gallagher,
C T Foxon,
M Sawicki,
T Dietl,
M Buongiorno Nardelli,
J Bernholc
[show abstract]
[hide abstract]
ABSTRACT: We present a combined theoretical and experimental study of the ferromagnetic semiconductor (Ga,Mn)As which explains the remarkably large changes observed on low-temperature annealing. Careful control of the annealing conditions allows us to obtain samples with ferromagnetic transition temperatures up to 159 K. Ab initio calculations, in situ Auger spectroscopy, and resistivity measurements during annealing show that the observed changes are due to out diffusion of Mn interstitials towards the surface, governed by an energy barrier of 0.7-0.8 eV. Electric fields induced by Mn acceptors have a significant effect on the diffusion.
Physical Review Letters 02/2004; 92(3):037201. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A novel sol-gel route to c-axis orientated undoped and Co, Fe, Mn and V doped ZnO films is reported. Sols were prepared from a hydrated zinc acetate precursor and dimethyl formamide (DMF) solvent. Films were spin-coated on to hydrophilic sapphire substrates then dried, annealed and post-annealed, producing almost purely uniaxial ZnO crystallites and a high degree of long-range structural order. Specific orientation of hexagonal crystallites is demonstrated both perpendicular and parallel to the substrate surface. Cobalt doping resulted in the formation of columnar ZnO nanocrystals. Vanadium doped films formed the spinel oxide ZnAl2O4, resulting from the reaction between ZnO and the sapphire substrate. Structural, optical and morphological characterisation demonstrated the high quality of the films.
08/2003;
-
[show abstract]
[hide abstract]
ABSTRACT: We show that by annealing Ga1−xMnxAs thin films at temperatures significantly lower than in previous studies, and monitoring the resistivity during growth, an unprecedented high Curie temperature TC and conductivity can be obtained. TC is unambiguously determined to be 118 K for Mn concentration x = 0.05, 140 K for x = 0.06, and 120 K for x = 0.08. We also identify a clear correlation between TC and the room temperature conductivity. The results indicate that Curie temperatures significantly in excess of the current values are achievable with improvements in growth and post-growth annealing conditions. © 2002 American Institute of Physics.
Applied Physics Letters 12/2002; 81(26):4991-4993. · 3.84 Impact Factor
