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Sökning: WFRF:(Hjörvarsson Björgvin) > Ahlberg Martina

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1.
  • Ahlberg, Martina, 1980- (författare)
  • Critical Phenomena and Exchange Coupling in Magnetic Heterostructures
  • 2012
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The continuous phase transition in thin magnetic films and superlattices has been studied using the magneto-optical Kerr effect (MOKE) and polarized neutron scattering (PNR).  It has been shown that the critical behavior of amorphous thin films belonging to the 2D XY universality class can be described within the same theory as crystalline sample. This means that quenched disorder only serves as a marginal perturbation in systems with this symmetry. The connection between interlayer exchange coupling and the observed critical behavior in Fe/V superlattices was explored. The results prove that the origin of unusually high values of the exponent β can be traced to a position dependence of the magnetization at elevated temperatures. The magnetization of the outermost layers within the superlattice shows a more pronounced decrease at lower temperatures, compared to the inner layers, which in turn have a more abrupt decrease in the vicinity of the critical temperature. This translates to a high exponent, especially when the layers are probed by a technique where more weight is given to the layers close to the surface, e.g.MOKE.  The interlayer exchange coupling as a function of spacer thickness and temperature was also studied in its own right. The data was compared to the literature, and a dependence on the thickness of the magnetic layers was concluded. The phase transition in amorphous FeZr/CoZr multilayers, where the magnetization emanates from ferromagnetic proximity effects, was investigated. Even though the determined exponents of the zero-field magnetization, the susceptibility and the critical isotherm did not correspond to any universality class, scaling plots displayed an excellent data collapse. Samples consisting of Fe δ-layers (0.3-1.4 monolayers) embedded in Pd were studied using element-specific resonant x-ray magnetic scattering. The magnetization of the two constituents showed distinctly different temperature dependences.
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2.
  • Ahlberg, Martina, et al. (författare)
  • Effect of ferromagnetic proximity on critical behavior
  • 2012
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 85:22, s. 224425-1-224425-6
  • 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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3.
  • Ahlberg, Martina, et al. (författare)
  • Influence of boundaries on magnetic ordering in Fe/V superlattices
  • 2010
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:21, s. 214429-
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the role of surface boundaries on the magnetic properties of [Fe/V](n) superlattice structures, with n=2-10. Using the magneto-optical Kerr effect and polarized neutron reflectivity measurements, we examine the evolution of both the total and the layer-resolved magnetizations as a function of temperature. By varying n, we observe a large shift in the transition temperatures T-c and a substantial change in the total magnetization critical exponent beta. In particular, the thicker samples exhibit nonuniversal exponent values. By resolving the magnetization as a function of position within the superlattice, we show that this behavior arises from contributions of the surfaces. Furthermore, we attribute the large shift in T-c to long-ranged interactions present in the superlattice.
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4.
  • Ahlberg, Martina, et al. (författare)
  • Reversed interface effects in amorphous FeZr/AlZr multilayers
  • 2014
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90:18, s. 184403-
  • Tidskriftsartikel (refereegranskat)abstract
    • We report an anomalous enhancement of the critical temperature (T-c) when the thickness (d) of the magnetic layer is decreased from 60 to 20 angstrom in amorphous FeZr/AlZr multilayers. Further reduction of the thickness causes a decrease of T-c, as expected by finite-size scaling, while the magnetic moment decreases monotonically for all values of d. The increase of the critical temperature is attributed to a reversed interface effect where local changes in the nearest-neighbor distance and coordination number gives a higher effective magnetic coupling at the interfaces compared to the interior of the layer. We have successfully described the results within a model where such interface effects are combined with finite-size scaling.
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5.
  • Ahlberg, Martina, et al. (författare)
  • Temperature dependence of magnetic properties in weakly exchange coupled Fe/V superlattices
  • 2013
  • Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 341, s. 142-147
  • Tidskriftsartikel (refereegranskat)abstract
    • We use Fe(3)/V(x)Fe(3)/V(x) superlattices, x = 16-27 monolayers (ML) to explore the interlayer exchange coupling (IEC) as a function of both spacer layer thickness and temperature. Fe/V is a common model system for studies addressing the IEC, but the behavior in the weakly exchange coupled regime, complemented with the temperature dependence of the magnetic properties, have remained unexplored. We observe clear regions with ferro- and antiferromagnetic coupling, which is manifested in oscillations in the saturation field, the remanence and the critical temperature. The oscillation in the interlayer coupling (J') has a period of 5.8(1) ML. This is approximate to 0.8 ML smaller than observed for Fe(7)/V(x) superlattices which illustrates the influence of the magnetic layer thickness on the interlayer coupling. The temperature dependence of the remanent magnetization was proven to be significantly affected by the strength of the ferromagnetic coupling, leading to high values of the effective critical exponent beta(eff).
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6.
  • Ahlberg, Martina, et al. (författare)
  • Two-dimensional XY-like amorphous Co68Fe24Zr8/Al70Zr30 multilayers
  • 2011
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:22, s. 224404-
  • Tidskriftsartikel (refereegranskat)abstract
    • We present an experimental realization of a magnetic two-dimensional XY system using amorphous materials. The classification of the dimensionality is based on the critical behavior of amorphous Co68Fe24Zr8(d)/Al70O30 (20 angstrom) multilayers, where d = 11 - 16 angstrom. Analysis of the remanent magnetization, the magnetic isotherms, the initial susceptibility, and the magnetic correlation length shows that the magnetic phase transition can be described by the 2D XY model. The samples are not paramagnetic above the critical temperature but are characterized by local magnetic order manifested in the field and temperature dependence of the magnetization. Furthermore, an average spin-spin interaction length of 8.1 angstrom was estimated using the thickness dependence of the Curie temperature.
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7.
  • Arnalds, Unnar B., et al. (författare)
  • Thermal transitions in nano-patterned XY-magnets
  • 2014
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:4, s. 042409-
  • 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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8.
  • Arnalds, Unnar, et al. (författare)
  • Thermalized ground state of artificial kagome spin ice building blocks
  • 2012
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 101:11, s. 112404-
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • We present a direct magnetic imaging study on the thermal macrospin ordering of artificial kagome spin ice building blocks. Using photoemission electron microscopy, employing x-ray magnetic circular dichroism, we are able to resolve the single domain magnetic nature of the macrospins and determine the states of the combined building block structures. The nano-patterning and material selection allows thermally activated magnetization reversal for the macrospins to occur. The ordering of the macrospins is dominated by the ground state, consistent with a thermal ground state ordering. This work paves the way for the realization of extended artificial spin ice structures exhibiting experimentally accessible thermal behavior
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9.
  • Hase, Thomas P. A., et al. (författare)
  • Proximity effects on dimensionality and magnetic ordering in Pd/Fe/Pd trialyers
  • 2014
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90:10, s. 104403-
  • Tidskriftsartikel (refereegranskat)abstract
    • The element-specific magnetization and ordering in trilayers consisting of 0.3-1.4 monolayer (ML) thick Fe layers embedded in Pd(001) has been determined using x-ray resonant magnetic scattering. The proximity to Fe induces a large moment in the Pd which extends similar to 2 nm from the interfaces. The magnetization as a function of temperature is found to differ significantly for the Fe and Pd sublattices: The Pd signal resembles the results obtained by magneto-optical techniques with an apparent three-dimensional (3D) to two-dimensional (2D) transition in spatial dimensionality for Fe thickness below similar to 1 ML. In stark contrast, the Fe data exhibits a 2D behavior. No ferromagnetic signal is obtained from Fe below the 2D percolation limit in Fe coverage (similar to 0.7 ML), while Pd shows a ferromagnetic response for all samples. The results are attributed to the temperature dependence of the susceptibility of Pd and a profound local anisotropy of submonolayered Fe.
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10.
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