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Critical Phenomena and Exchange Coupling in Magnetic Heterostructures
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2012. , 58 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 902
Keyword [en]
phase transitions, critical behavior, 2D XY model, MOKE, magnetic thin films and superlattices, amorphous multilayers, magnetic proximity effects
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-168310ISBN: 978-91-554-8282-4 (print)OAI: oai:DiVA.org:uu-168310DiVA: diva2:503635
Public defence
2012-03-30, Å80101, Ångströmslaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2012-03-09 Created: 2012-02-08 Last updated: 2012-03-29Bibliographically approved
List of papers
1. Influence of boundaries on magnetic ordering in Fe/V superlattices
Open this publication in new window or tab >>Influence of boundaries on magnetic ordering in Fe/V superlattices
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2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 21, 214429- p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-136258 (URN)10.1103/PhysRevB.81.214429 (DOI)000278936900004 ()
Available from: 2010-12-11 Created: 2010-12-11 Last updated: 2017-12-11Bibliographically approved
2. Two-dimensional XY-like amorphous Co68Fe24Zr8/Al70Zr30 multilayers
Open this publication in new window or tab >>Two-dimensional XY-like amorphous Co68Fe24Zr8/Al70Zr30 multilayers
2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 22, 224404- p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-155916 (URN)10.1103/PhysRevB.83.224404 (DOI)000291727600006 ()
Available from: 2011-07-05 Created: 2011-07-04 Last updated: 2017-12-11Bibliographically approved
3. Experimental realization of amorphous two-dimensional XY magnets
Open this publication in new window or tab >>Experimental realization of amorphous two-dimensional XY magnets
2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 2, 024430- p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-156953 (URN)10.1103/PhysRevB.84.024430 (DOI)000292873700002 ()
Available from: 2011-08-20 Created: 2011-08-11 Last updated: 2017-12-08Bibliographically approved
4. Temperature dependence of magnetic properties in weakly exchange coupled Fe/V superlattices
Open this publication in new window or tab >>Temperature dependence of magnetic properties in weakly exchange coupled Fe/V superlattices
2013 (English)In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 341, 142-147 p.Article in journal (Refereed) Published
Abstract [en]

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).

Keyword
Magnetic superlattice, Magnetic phase transition, Exchange interaction
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-168308 (URN)10.1016/j.jmmm.2013.04.058 (DOI)000320583800026 ()
Available from: 2012-02-16 Created: 2012-02-08 Last updated: 2017-12-07Bibliographically approved
5. Effect of ferromagnetic proximity on critical behavior
Open this publication in new window or tab >>Effect of ferromagnetic proximity on critical behavior
2012 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 85, no 22, 224425-1-224425-6 p.Article in journal (Refereed) Published
Abstract [en]

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. 

National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-168796 (URN)10.1103/PhysRevB.85.224425 (DOI)000305532200002 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Available from: 2012-02-16 Created: 2012-02-15 Last updated: 2017-12-07Bibliographically approved
6. Element specific magnetization in Fe/Pd quantum well structures
Open this publication in new window or tab >>Element specific magnetization in Fe/Pd quantum well structures
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-168443 (URN)
Available from: 2012-02-16 Created: 2012-02-10 Last updated: 2012-08-01

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