| 1. |
- Kanski, Janusz, 1946, et al.
(författare)
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Electronic structure of (Ga,Mn)As revisited
- 2017
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Ingår i: New Journal of Physics. - : Institute of Physics (IOP). - 1367-2630. ; 19:2, s. 1-8
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Tidskriftsartikel (refereegranskat)abstract
- The detailed nature of electronic states mediating ferromagnetic coupling in dilute magnetic semiconductors, specifically (Ga,Mn)As, has been an issue of long debate. Two confronting models have been discussed emphasizing host band vs. impurity band carriers. Using angle resolved photoemission we show that the electronic structure of the (Ga,Mn)As system is significantly modified from that of GaAs throughout the valence band. Close to the Fermi energy, the presence of Mn induces a strong mixing of the bulk bands of GaAs, which results in the appearance of a highly dispersive band in the gap region of GaAs.For Mn concentrations above 1% the band reaches the Fermi level, and can thus host the delocalized holes needed for ferromagnetic coupling. Overall, our data provide a firm evidence of delocalized carriers belonging to the modified host valence band.
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| 2. |
- Nicolai, L., et al.
(författare)
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Bi ultra-thin crystalline films on InAs(111)A and B substrates: a combined core-level and valence-band angle-resolved and dichroic photoemission study
- 2019
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 21:12
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Tidskriftsartikel (refereegranskat)abstract
- The growth of Bi on both the In-terminated (A) face and the As-terminated (B) face of InAs(111) has been investigated by low-energy electron diffraction, scanning tunnelling microscopy, and photoelectron spectroscopy using synchrotron radiation. The changes upon Bi deposition of the In 4d and Bi 5d(5/2) photoelectron signals allow to get a comprehensive picture of the Bi/InAs(111) interface. From the early stage the Bi growth on the A face is epitaxial, contrary to that on the B face that proceeds via the formation of islands. Angle-resolved photoelectron spectra show that the electronic structure of a Bi deposit of approximate to 10 bi-layers on the A face is identical to that of bulk Bi, while more than approximate to 30 bi-layers are needed for the B face. Both bulk and surface electronic states observed are well accounted for by fully relativistic ab initio calculations performed using the one-step model of photoemission. These calculations are used to analyse the dichroic photoemission data recorded in the vicinity of the Fermi level around the (Gamma) over bar point of the Brillouin zone.
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| 3. |
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| 4. |
- Gluba, L., et al.
(författare)
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Band structure evolution and the origin of magnetism in (Ga,Mn) As : From paramagnetic through superparamagnetic to ferromagnetic phase
- 2018
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:11
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Tidskriftsartikel (refereegranskat)abstract
- The high-spectral-resolution optical studies of the energy gap evolution, supplemented with electronic, magnetic, and structural characterization, show that the modification of the GaAs valence band caused by Mn incorporation occurs already for a very low Mn content, much lower than that required to support ferromagnetic spin-spin coupling in (Ga,Mn) As. Only for n-type (Ga,Mn) As with the Mn content below about 0.3% the Mn-related extended states are visible as a feature detached from the valence-band edge and partly occupied with electrons. The combined magnetic and low-temperature photoreflectance studies presented here indicate that the paramagnetic <-> ferromagnetic transformation in p-type (Ga,Mn) As takes place without imposing changes of the unitary character of the valence band with the Fermi level located therein. The whole process is rooted in the nanoscale fluctuations of the local (hole) density of states and the formation of a superparamagnetic-like state. The Fermi level in (Ga,Mn) As is coarsened by the carrier concentration of the itinerant valence band holes and further fine-tuned by the many-body interactions.
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| 5. |
- Gryglas-Borysiewicz, Marta, et al.
(författare)
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Hydrostatic-pressure-induced changes of magnetic anisotropy in (Ga, Mn) As thin films
- 2017
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 29:11
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Tidskriftsartikel (refereegranskat)abstract
- The impact of hydrostatic pressure on magnetic anisotropy energies in (Ga, Mn) As thin films with in-plane and out-of-plane magnetic easy axes predefined by epitaxial strain was investigated. In both types of sample we observed a clear increase in both in-plane and out-of-plane anisotropy parameters with pressure. The out-of-plane anisotropy constant is well reproduced by the mean-field p-d Zener model; however, the changes in uniaxial anisotropy are much larger than expected in the Mn-Mn dimer scenario.
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| 6. |
- Juszynski, P., et al.
(författare)
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Magnetic anisotropy investigations of (Ga,Mn)As with a large epitaxial strain
- 2015
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853. ; 396, s. 48-52
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic properties of 20 nm thick (Ga,Mn)As layer deposited on (Ga,ln)As buffer with very large epitaxial tensile strain are investigated. Gal,In,As buffer with x=30% provides a 2% lattice mismatch, which is an important extension of the mismatch range studied so far (up to 0.5%). Evolution of magnetic anisotropy as a function of temperature is determined by magnetotransport measurements. Additionally, results of direct measurements of magnetization are shown. (C) 2015 Published by Elsevier B.V.
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| 7. |
- Kaleta, Anna, et al.
(författare)
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Enhanced Ferromagnetism in Cylindrically Confined MnAs Nanocrystals Embedded in Wurtzite GaAs Nanowire Shells
- 2019
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Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 19:10, s. 7324-7333
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Tidskriftsartikel (refereegranskat)abstract
- Nearly a 30% increase in the ferromagnetic phase transition temperature has been achieved in strained MnAs nanocrystals embedded in a wurtzite GaAs matrix. Wurtzite GaAs exerts tensile stress on hexagonal MnAs nanocrystals, preventing a hexagonal to orthorhombic structural phase transition, which in bulk MnAs is combined with the magnetic one. This effect results in a remarkable shift of the magneto-structural phase transition temperature from 313 K in the bulk MnAs to above 400 K in the tensely strained MnAs nanocrystals. This finding is corroborated by the state of the art transmission electron microscopy, sensitive magnetometry, and the first-principles calculations. The effect relies on defining a nanotube geometry of molecular beam epitaxy grown core-multishell wurtzite (Ga,In)As/(Ga,Al)As/(Ga,Mn)As/GaAs nanowires, where the MnAs nanocrystals are formed during the thermal-treatment-induced phase separation of wurtzite (Ga,Mn)As into the GaAs-MnAs granular system. Such a unique combination of two types of hexagonal lattices provides a possibility of attaining quasi-hydrostatic tensile strain in MnAs (impossible otherwise), leading to the substantial ferromagnetic phase transition temperature increase in this compound.
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| 8. |
- Kasama, T., et al.
(författare)
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Direct observation of doping incorporation pathways in self-catalytic GaMnAs nanowires
- 2015
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Ingår i: Applied Physics Reviews. - : AIP Publishing. - 1931-9401. ; 118:5
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Tidskriftsartikel (refereegranskat)abstract
- Doping mechanisms of Mn in GaAs nanowires (NWs) that have been grown self-catalytically at 600 degrees C by molecular beam epitaxy (MBE) are investigated using advanced electron microscopy techniques and atom probe tomography, Mn is found to be incorporated primarily in the form of non-magnetic tetragonal Ga0.82Mn0.18 nanocrystals in Ga catalyst droplets at the ends of the NWs, while trace amounts of Mn (22 +/- 4 at. ppm) are also distributed randomly in the NW bodies without forming clusters or precipitates. The nanocrystals are likely to form after switching off the reaction in the MBE chamber, since they are partially embedded in neck regions of the NWs. The Ga0.82Mn0.18 nanocrystals and the low Mn concentration in the NW bodies are insufficient to induce a ferromagnetic phase transition, suggesting that it is difficult to have high Mn contents in GaAs even in 1-D NW growth via the vapor-liquid-solid process. (C) 2015 AIP Publishing LLC.
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| 9. |
- Kret, S., et al.
(författare)
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FIB Method of Sectioning of III-V Core-Multi-Shell Nanowires for Analysis of Core/Sell Interfaces by High Resolution TEM
- 2017
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Ingår i: Acta Physica Polonica. A. - : Polish Physical Society. - 0587-4246 .- 1898-794X. ; 131:5, s. 1332-1335
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Tidskriftsartikel (refereegranskat)abstract
- The core-multishell wurtzite structure (In, Ga) As-(Ga, Al) As-(Ga, Mn) As semiconductor nanowires have been successfully grown on GaAs(111) B substrates using MBE technique. The nanowires cores were grown with gold eutectic catalyser in vapour-liquid-solid growth mode. The double shell overgrowth, on the side facets of nanowires, was performed using lower substrate temperature (about 400 degrees C, and 230 degrees C, for (Ga, Al) As, and (Ga, Mn) As shell growth, respectively). The polytypic ordering, defects, chemistry and geometric perfection of the core and the shells have been analysed at atomic level by advanced transmission electron microscope techniques with the use of axial and longitudinal section of individual nanowires prepared by focused ion beam. High quality cross-sections suitable for quantitative transmission electron microscope analysis have been obtained and enabled analysis of interfaces between the core and the shells with near atomic resolution. All investigated shells are epitaxial without misfit dislocations at the interface. Some of the shells thicknesses are not symmetric, which is due to the shadowing effects of neighbouring nanowires and directional character of the elemental fluxes in the MBE growth process.
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| 10. |
- Kwiatkowski, Adam, et al.
(författare)
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Determining Curie temperature of (Ga,Mn)As samples based on electrical transport measurements: Low Curie temperature case.
- 2016
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 108:24, s. 1-4
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we show that the widely accepted method of the determination of Curie temperature (Tc) in (Ga,Mn)As samples, based on the position of the peak in the temperature derivative of the resistivity, completely fails in the case of non-metallic and low-TC unannealed samples. In this case, we propose an alternative method, also based on electric transport measurements, which exploits temperature dependence of the second derivative of the resistivity upon magnetic field.
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| 11. |
- Kwiatkowski, Adam, et al.
(författare)
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Galvanomagnetic methods of Curie temperature determination in (Ga,Mn)As
- 2018
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier. - 0304-8853 .- 1873-4766. ; 467, s. 120-128
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Tidskriftsartikel (refereegranskat)abstract
- We critically discuss various experimental methods to determine Curie temperature T-C of (Ga,Mn)As thin layers or other conducing magnetic materials by means of electric charge transport measurements. They all base on the influence of sample magnetization on the magnetoresistivity tensor <(rho)overcap>and are an alternative to the method based upon an analysis of the temperature derivative of the sample resistance (Novak a al., 2008). These methods can be applied even when standard SQUID magnetometers are difficult or impossible to use - for example for extremely small samples or in the case of experiments performed at very specific physical conditions, e.g. at high hydrostatic pressure inside the clamp cell. We show that the use of the so called Arrott plot prepared with the use of high magnetic field isotherms rho(xx)(H-0), rho(x)y(H-0) (H-0 - external magnetic field) may lead to substantial (of the order of 10 K) divergence of the obtained T-c values depending on the assumptions which are necessary to make in this case and depending on the direction of a magnetic anisotropy easy axis. We also propose a number of ways how to obtain, basing on low magnetic field isotherms rho(xx)(H-0), rho(xy)(H-0) clear and characteristic features which are closely related to the ferromagnetic-paramagnetic phase transition.
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| 12. |
- Levchenko, K., et al.
(författare)
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Evidence for the homogeneous ferromagnetic phase in (Ga, Mn) (Bi, As) epitaxial layers from muon spin relaxation spectroscopy
- 2019
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Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 9, s. 1-8
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Tidskriftsartikel (refereegranskat)abstract
- Ferromagnetic semiconductor thin layers of the quaternary (Ga,Mn)(Bi,As) and reference, ternary (Ga, Mn) As compounds, epitaxially grown under either compressive or tensile strain, have been characterized from a perspective of structural and magnetization homogeneity. The quality and composition of the layers have been confirmed by secondary-ion mass spectrometry (SIMS). A thorough evaluation of the magnetic properties as a function of temperature and applied magnetic field has been performed by means of SQUID magnetometry and low-energy muon spin relaxation (mu SR) spectroscopy, which enables studying local (on the nanometer scale) magnetic properties of the layers. The results testify that the ferromagnetic order builds up almost homogeneously below the Curie temperature in the full volume fraction of both the (Ga, Mn) As and (Ga, Mn)(Bi, As) layers. Incorporation of a small amount of heavy Bi atoms into (Ga, Mn) As, which distinctly enhances the strength of spin-orbit coupling in the quaternary (Ga, Mn)(Bi, As) layers, does not deteriorate noticeably their magnetic properties.
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| 13. |
- Levchenko, K., et al.
(författare)
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Impact of bismuth incorporation into (Ga,Mn)As thin films on their structural and magnetic properties
- 2015
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Ingår i: Physica Status Solidi C. - : Wiley. - 1610-1642 .- 1862-6351. ; 12:8, s. 1152-1155
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Konferensbidrag (refereegranskat)abstract
- Structural and magnetic properties of thin films of the (Ga,Mn)(Bi,As) quaternary diluted magnetic semiconductor grown by the low-temperature molecular-beam epitaxy technique on GaAs substrates have been investigated. High-resolution X-ray diffraction has been applied to characterize the structural quality and misfit strain in the films. Ferromagnetic Curie temperature and magnetocrystalline anisotropy of the films have been examined by using SQUID magnetometry and low-temperature magneto-transport measurements. Post-growth annealing treatment of the films has been shown to reduce the strain in the films and to enhance their Curie temperature. Significant increase in the magnitude of magneto-transport effects caused by incorporation of a small amount of Bi into the films is interpreted as a result of enhanced spinorbit coupling in the (Ga, Mn)(Bi, As) films. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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| 14. |
- Sadowski, Janusz, et al.
(författare)
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Defect-free SnTe topological crystalline insulator nanowires grown by molecular beam epitaxy on graphene
- 2018
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Ingår i: Nanoscale. - : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 10:44, s. 20772-20778
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Tidskriftsartikel (refereegranskat)abstract
- SnTe topological crystalline insulator nanowires have been grown by molecular beam epitaxy on graphene/SiC substrates. The nanowires have a cubic rock-salt structure, they grow along the [001] crystallographic direction and have four sidewalls consisting of {100} crystal planes known to host metallic surface states with a Dirac dispersion. Thorough high resolution transmission electron microscopy investigations show that the nanowires grow on graphene in the van der Waals epitaxy mode induced when the catalyzing Au nanoparticles mix with Sn delivered from a SnTe flux, providing a liquid Au-Sn alloy. The nanowires are totally free from structural defects, but their {001} sidewalls are prone to oxidation, which points out the necessity of depositing a protective capping layer in view of exploiting the magneto-electric transport phenomena involving charge carriers occupying topologically protected states.
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| 15. |
- Sadowski, Janusz, et al.
(författare)
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Wurtzite (Ga,Mn)As nanowire shells with ferromagnetic properties
- 2017
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Ingår i: Nanoscale. - : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 9:6, s. 2129-2137
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Tidskriftsartikel (refereegranskat)abstract
- (Ga,Mn)As having a wurtzite crystal structure was coherently grown by molecular beam epitaxy on the 1100 side facets of wurtzite (Ga,In)As nanowires and further encapsulated by (Ga,Al)As and low temperature GaAs. For the first time, a truly long-range ferromagnetic magnetic order is observed in non-planar (Ga,Mn)As, which is attributed to a more effective hole confinement in the shell containing Mn by the proper selection/choice of both the core and outer shell materials. © The Royal Society of Chemistry.
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| 16. |
- Sawicki, M., et al.
(författare)
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Cubic anisotropy in (Ga,Mn) As layers : Experiment and theory
- 2018
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:18
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Tidskriftsartikel (refereegranskat)abstract
- Historically, comprehensive studies of dilute ferromagnetic semiconductors, e.g., p-type (Cd,Mn) Te and (Ga,Mn) As, paved the way for a quantitative theoretical description of effects associated with spin-orbit interactions in solids, such as crystalline magnetic anisotropy. In particular, the theory was successful in explaining uniaxial magnetic anisotropies associated with biaxial strain and nonrandom formation of magnetic dimers in epitaxial (Ga,Mn) As layers. However, the situation appears much less settled in the case of the cubic term: the theory predicts switchings of the easy axis between in-plane < 100 > and < 110 > directions as a function of the hole concentration, whereas only the < 100 > orientation has been found experimentally. Here, we report on the observation of such switchings by magnetization and ferromagnetic resonance studies on a series of high-crystalline quality (Ga,Mn) As films. We describe our findings by themean-field p-d Zener model augmented with three new ingredients. The first one is a scattering broadening of the hole density of states, which reduces significantly the amplitude of the alternating carrier-induced contribution. This opens the way for the two other ingredients, namely the so-far disregarded single-ion magnetic anisotropy and disorder-driven nonuniformities of the carrier density, both favoring the < 100 > direction of the apparent easy axis. However, according to our results, when the disorder gets reduced, a switching to the < 110 > orientation is possible in a certain temperature and hole concentration range.
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| 17. |
- Wikberg, J. M., et al.
(författare)
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Evolving Magnetization Dynamics in Mn3-xGa
- 2015
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Ingår i: Springer Proceedings in Physics. - Cham : Springer International Publishing. - 0930-8989. ; 159, s. 23-25, s. 23-25
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Konferensbidrag (refereegranskat)abstract
- Magnetic materials with high magnetic coercivity (Hc), magnetic anisotropy (Ku) and low Gilbert damping (α) are of great importance for future spintronics devices. For instance, for spin-transfer-torque (STT) memory devices low α and high Ku are desired. Such a combination of material properties includes a built in contradiction since both Ku and α are dependent on the spin-orbit interaction (SO); a high Ku material is expected to have a high α. However, recent experimental investigations of Mn3-xGa, a material exhibiting high Curie temperature (Tc) > 700 K and (out-of-plane) perpendicular anisotropy, have shown that such a contradictorily low α and high Ku material exists.
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