| 1. |
- Ahlberg, Martina, et al.
(författare)
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Effect of ferromagnetic proximity on critical behavior
- 2012
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 85:22, s. 224425-1-224425-6
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the magnetic phase transition in amorphous Fe93Zr7(x angstrom)/Co95Zr5(1 angstrom) multilayers, where x = 25, 50, and 75. The extremely thin CoZr layer induces magnetic order at temperatures well above the inherent ordering temperature of Fe93Zr7. The changes in the critical exponent beta, associated with the temperature dependence of the magnetization, imply a crossover from two-to three-dimensional behavior as the FeZr thickness is reduced, consistent with a substantial magnetic induction in the FeZr layers. In addition we determined the exponents delta and gamma, of the critical isotherm and the susceptibility, respectively, and their values confirm the nonuniversal character of the phase transition. Scaling of the results yields an excellent data collapse and is found to hold in the crossover regime where the universality hypothesis is not applicable.
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| 2. |
- Arnalds, Unnar B., et al.
(författare)
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Thermal transitions in nano-patterned XY-magnets
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:4, s. 042409-
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Tidskriftsartikel (refereegranskat)abstract
- We have fabricated ultra-thin disc shaped islands wherein shape anisotropy confines the moment to the island plane, creating XY-like superspins. At low temperatures, the superspins are blocked, and, as the temperature is increased, they undergo a transition into a superparamagnetic state. The onset of this dynamic superspin state scales with the diameter of the islands, and it persists up to a temperature governed by the intrinsic ordering temperature of the island material defining a range in temperature in which dynamic behavior of the magnetic islands can be obtained.
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| 3. |
- Kapaklis, Vassilios, et al.
(författare)
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Melting artificial spin ice
- 2012
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 14, s. 035009-
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Tidskriftsartikel (refereegranskat)abstract
- Artificial spin ice arrays of micromagnetic islands are a means of engineering additional energy scales and frustration into magnetic materials. Here we demonstrate a magnetic phase transition in an artificial square spin ice and use the symmetry of the lattice to verify the presence of excitations far below the ordering temperature. We do this by measuring the temperature dependent magnetisation in different principal directions and comparing with simulations of idealised statistical mechanical models. Our results confirm a dynamical pre-melting of the artificial spin ice structure at a temperature well below the intrinsic ordering temperature of the island material. We thus create a spin ice array that has real thermal dynamics of the artificial spins over an extended temperature range.
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| 4. |
- Kapaklis, Vassilios, et al.
(författare)
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Temperature dependence of the electrical resistivity and electronic structure of amorphous Fe100-xZrx films and multilayers
- 2012
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 24:49, s. 495402-
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Tidskriftsartikel (refereegranskat)abstract
- The electrical resistivity of amorphous Fe100-xZrx metal alloy films and multilayers has been investigated in a wide temperature and composition range. The overall behavior of the resistivity is consistent with bulk measurements, exhibiting prominent semiconductor-like changes at low temperatures. The transition from positive (metallic) to negative temperature coefficient of resistivity behavior is accompanied by minute changes in magnetoresistance and we can therefore rule out magnetic phase changes as being the cause for the observed changes in the resistivity. Using x-ray absorption and emission spectroscopies we are able to probe the unoccupied and occupied electronic densities of states. The corresponding spectra are found to significantly overlap, as expected for a metallic-like electronic structure and the absence of a band gap. Besides a broadening of the x-ray emission lines expected from an amorphous material, remarkably small differences are observed in the electronic structures when changing the amount of Zr. The resistivity data were modeled and agreement with the Mott variable range hopping model was found, indicating localized electronic states due the disordered structure of the Fe100-xZrx alloys.
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| 5. |
- Kapaklis, Vassilios, 1978-, et al.
(författare)
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Violation of Hund’s third rule in structurally disordered ferromagnets
- 2011
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Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 84:2, s. 024411-
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Tidskriftsartikel (refereegranskat)abstract
- Violation of Hund’s third rule caused by structural disorder is observed for the induced magnetic moment of Zr, using X-ray magnetic circular dischroism. The induced spin and orbital magnetic moments are anti-parallel in the crystalline state, but parallel in an amorphous state of the investi- gated Co- and Fe-based materials. First principles calculations are used to provide physical insight into the dependency of the spin-orbit coupling on the interatomic distance and coordination number.
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| 6. |
- Korelis, Panagiotis, et al.
(författare)
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Magnetic and structrural properties of amorphous Fe90Zr10/Al75Zr25 multilayers in the two-dimensional limit.
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The magnetic properties of amorphous Fe90 Zr10 /Al75 Zr25 multilayers have been investigated for Fe90 Zr10 bilayer thicknesses in the range t FeZr = 5-20 Å; aiming at enlightening the influence of reduced magnetic layer thickness on the magnetic order. The Al75 Zr25 bilayer thickness is 40 Å, which is thick enough for the interlayer coupling to be negligible. Both the Curie temperature and the magnetic moment decrease (non-linearly) with decreasing thickness of the Fe90 Zr10 bilayer compared to the bulk values. However, the sample with t FeZr = 5 Å has a different behaviour showing that its thickness is below the critical thickness for ferromagnetic order.
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| 7. |
- Korelis, Panagiotis T., et al.
(författare)
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Highly amorphous Fe90Zr10 thin films, and the influence of crystallites on the magnetism
- 2010
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 519:1, s. 404-409
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Tidskriftsartikel (refereegranskat)abstract
- A method for depositing highly amorphous, iron-rich Fe100-xZrx thin films on to room temperature substrates is presented. The method involves co-depositing Fe and Zr on to an amorphous AlZr layer. Experimental proof that the structures are completely amorphous is given by transmission electron microscopy and polarized neutron reflectometry. The reflectometry measurements also give an indication of the impact that Fe crystallite impurities have on the magnetic structure and properties of amorphous FeZr. The results are consistent with previous investigations on bulk samples, which showed that crystalline impurities make the magnetic structure more non-collinear.
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| 8. |
- Korelis, Panagiotis, 1984-
(författare)
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Uncovering Magnetic Order in Nanostructured Disordered Materials : A Study of Amorphous Magnetic Layered Structures
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The scope of this thesis is the study of the interplay between structure and magnetism in amorphous materials. The investigations focus on the growth of amorphous layers and the study of the influence of structural disorder and reduced physical extension on the magnetic properties of thin films and multilayers. The examined magnetic materials are FeZr alloys, as well as other amorphous transition metal alloys such as CoZr and FeCoZr. Thin films and multilayers of the studied materials were deposited using magnetron sputtering in ultra-high vacuum conditions. Their amorphous structure and layering quality was investigated using X-ray scattering techniques and in several cases with transmission electron microscopy. The chemical composition of the alloys was determined with Rutherford Backscattering Spectrometry. The magnetic properties were investigated using the magneto-optic Kerr effect and SQUID magnetometry, as well as polarized neutron reflectometry and X-ray magnetic circular dicroism measurements. For FeZr alloys deposited as multilayers with Al2O3 as spacer layer, it was found that Fe-rich nanocrystallites, formed at the metal/oxide interfaces, exert large influence on the magnetic properties. The use of AlZr alloys as buffer layers promotes the growth of highly amorphous FeZr layers. FeZr/AlZr multilayers with good layering quality can also be obtained. The influence of the reduced layer thickness on the magnetic moment, Curie temperature and magnetic dimensionality of the magnetic layers is addressed for FeZr/AlZr multilayers. Thin FeZr layers in these structures are found to belong to the 2D XY dimensionality class. The change of the magnetic moment and Curie temperature with reduced FeZr layer thickness is quantified. In addition, the induced magnetic moment in the alloy element Zr was investigated in FeZr and CoZr alloy films. The possibility to imprint a preferred magnetization direction during thin film preparation was demonstrated for FeCoZr layers. Lastly, AlZr alloy films were studied with respect to their oxidation stability at room and elevated temperatures, aiming towards development of materials with passivating properties.
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| 9. |
- Liebig, A., et al.
(författare)
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Experimental realization of amorphous two-dimensional XY magnets
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:2, s. 024430-
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Tidskriftsartikel (refereegranskat)abstract
- The temperature dependence of the magnetization of thin amorphous Fe(89)Zr(11)/Al(78)Zr(22) layerswas investigated. Dimensionality analysis of the ferromagnetic transition of 15 A thick layers yielded critical exponents characteristic of the 2D XY (planar rotor) model. Above the ordering temperature significant polarizability with an applied field is observed, due to the existence of large-scale magnetic correlations, of which the extent and origin have been determined.
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| 10. |
- Liebig, Andreas, et al.
(författare)
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Morphology of amorphous Fe91Zr9/Al2O3 multilayers : Dewetting and crystallization
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:21, s. 214202-1-214202-6
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Tidskriftsartikel (refereegranskat)abstract
- Amorphous Fe91Zr9/Al2O3 multilayers grown by magnetron sputtering have been studied using x-ray reflectometry, x-ray diffraction, Rutherford backscattering spectrometry, and transmission electron microscopy. It could be demonstrated that on the interface between the Fe91Zr9 and the Al2O3, crystalline grains are formed, that for very small repetition thicknesses destroy the periodicity of the multilayers by accumulative roughness. Understanding these effects would enable substantial improvement of the quality of nanolaminated amorphous layers.
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