| 1. |
- Ahlberg, Martina, 1980-
(författare)
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Critical Phenomena and Exchange Coupling in Magnetic Heterostructures
- 2012
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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)
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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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| 3. |
- Ahlberg, Martina, et al.
(författare)
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Influence of boundaries on magnetic ordering in Fe/V superlattices
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:21, s. 214429-
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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)
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Two-dimensional XY-like amorphous Co68Fe24Zr8/Al70Zr30 multilayers
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:22, s. 224404-
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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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| 5. |
- Andersson, Gabriella, et al.
(författare)
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Effects of strain on magnetic anisotropy in Fe and Co based heterostructures
- 2008
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Ingår i: Phase Transitions. - : Informa UK Limited. - 0141-1594 .- 1029-0338. ; 81:7 & 8, s. 679-701
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Tidskriftsartikel (refereegranskat)abstract
- The magnetic ordering in transition metals is strongly affected by any strain in the lattice, due to the close connection between strain and anisotropy. This article reviews recent developments, both experimental and theoretical, in the field of strain-induced magnetic anisotropies, with focus on films and multilayers based on Fe and Co. It is demonstrated that the reorientation of preferred magnetization directions can be controlled through both the growth direction and the strain state, which is caused by interleaving with suitable spacers, as well as by tuning the chemical composition if the magnetic material is an alloy. Both in-plane and out-of-plane easy directions can be forced upon the films and superlattices in this manner. Some methods for local structural manipulation are also reviewed.
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| 6. |
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| 7. |
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| 8. |
- Andersson, Gabriella, et al.
(författare)
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Influence of compressive biaxial strain on the hydrogen uptake of ultra-thin, single crystal vanadium layers
- 1997
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Ingår i: Physical Review B. - 0163-1829. ; 55:3, s. 1774-1781
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the influence of biaxial compressive strain on the thermodynamic propertied of hydrogen in thin, single-crystal vanadium layers (0.9, 1.6, and 2.0 nm). At H/V<0.1 (atomic ratio), the host-mediated H-H interaction was found to be attractive. For H/V>0.1, the interaction was found to be extremely weak in the 0.9-nm-thick layer and repulsive in the 1.6- and 2.0-nm-thick V layers. The repulsive interaction was found to increase with increasing layer thicknesses. These results are unexpected as the interaction is attractive in both bulk V and in thin vanadium layers under biaxial tensile strain for HV<0.5. Both the strained and unstrained vanadium lattices are found to have roughly the same hydrogen affinity at H/V=0.1 (0.3 eV/H). No ordered bulklike beta phase was observed at any concentration (0
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| 9. |
- Arnalds, Unnar B., et al.
(författare)
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Magnetic structure and diffracted magneto-optics of patterned amorphous multilayers
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:14, s. 144434-1-144434-8
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Tidskriftsartikel (refereegranskat)abstract
- We present magneto-optical Kerr effect measurements of patterned arrays of Co68Fe24Zr8 / Al2O3 amorphous multilayers. The multilayers were patterned in two dimensions into two different arrangements of circular and ellipsoidal islands. Magnetization loops were recorded in a longitudinal geometry using both the specularly reflected beam as well as diffracted beams scattered off the patterned films. The magnetization of the patterned structures is significantly different from the magnetization of a continuous multilayer owing to the lateral confinement of the pattern and the introduction of additional dipolar coupling between the layers at the edges of the islands. By investigating the magnetic response at the different diffraction orders from the two different configurations of islands we are able to observe the magnetization at different length scales and determine the magnetic response of the circular and ellipsoidal islands individually.
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| 10. |
- Björck, Matts, et al.
(författare)
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The effect of strain and interfaces on the orbital moment in Fe/V superlattices
- 2007
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 313:1, s. 230-235
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Tidskriftsartikel (refereegranskat)abstract
- The dependence of the Fe orbital moment on strain and interfaces in Fe/V superlattices has been investigated by X-ray magnetic circular dichroism (XMCD). The orbital moment was determined to be lower at the interfaces than in the bulk, which we attribute to Fe–V hybridization. An enhancement of the orbital moment with increasing strain in the Fe layers was observed. This enhancement is attributed to an unquenching of the orbital moment. Consequently, the orbital moment of Fe in Fe/V is concluded to be influenced by two competing parameters. It is lowered by increasing interface density, and enhanced by increasing strain.
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