Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Integral and local AC-susceptibility: instrumentation and applications
KTH, Superseded Departments, Physics.ORCID iD: 0000-0003-2170-0076
1999 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This thesis presents my mostly unpublished experimental work related to two major contributions with the design of:

A high precision and versatile bulk average(or integral) AC-susceptometer and its application to investigations of exchange anisotropy in Co multilayers.

A novel Local Susceptibility measurement technique exploiting the narrow gap in inductive ring-heads in magnetic recording technology. Representative applications of this instrument are presented as well.

Integral AC-susceptibility measurements

Even though AC-susceptibility is a well-established and matured experimental technique, there are many new challenges for improvements and modifications, in particular for the investigation of low dimensional systems. Thus, we have constructed and built a versatile and highly sensitive AC-susceptometer. Some of the design concepts presented in this thesis, which are new or not normally found in literature are:

A time-adjusted background-correction scheme, which reduces the error from non-simultaneous measurement of sample and background.

A monitor-coil, which serves as a software-controlled phase-shift determination at any time during a measurement.

A balanced current source, which enables the primary and secondary to a have a common potential that will reduce the background signal.

Software, which allows any measurement (e.g. a standard warming scan for the determination of the temperature dependence of the susceptibility) to be segmented into an arbitrary number of individually specified combinations of measuring field-strength and frequency.

Our instrument has provided the basis for considerable number of publications (18) in recent years. It may be pointed out that the sensitivity of this home-built susceptometer, 10-9emu, is perhaps the best to be reported in literature, and compares competitively with those for a SQUID magnetometer.

The versatility of the instrument is demonstrated with a new approach for the study of exchange anisotropy of thin cobalt/cobalt-oxide bilayers, Paper 1-3. In this method, we determine the exchange anisotropy values with only a small exciting field, and no complete flux reversal process is involved. The values obtained for the exchange anisotropy in our studies are found to be much closer to the expected theoretical values.

Local Susceptibility measurements

AC-susceptibility is conventionally thought of as an integral or bulk averaged measurement. With an inductive combined write/read head, we have constructed and built an instrument, which enables us to measure the Local Susceptibility. The instrument scans magnetically responsive surfaces and images the in-plane uniaxial susceptibility, including both its magnitude and orientation, on a length-scale determined by the dimensions of the gap (approaching submicrometer level) of the employed head, Paper 4-5.

The basic design and the appropriate derivations for quantitative analysis of the data are presented. In order to demonstrate the capabilities of the instrument we present investigations on a variety of materials, which include hard as well as soft ferromagnets and a study on melt-textured high-temperature superconductors. It is important to point out that many, if not most, of the results discussed are not possible with any other hitherto known technique. In particular, we may point out the capability to obtain in-plane properties for superconducting surfaces, which is perhaps a unique information obtainable from our Local Susceptibility probe technique.

 

Place, publisher, year, edition, pages
Stockholm: KTH , 1999. , x, 121 p.
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-2786ISBN: 992-952269-7 OAI: oai:DiVA.org:kth-2786DiVA: diva2:8499
Public defence
1999-04-30, 00:00
Note
QC 20100813Available from: 2000-01-01 Created: 2000-01-01 Last updated: 2010-08-13Bibliographically approved
List of papers
1. Determination of exchange anisotropy by means of ac susceptometry in Co/CoO bilayers
Open this publication in new window or tab >>Determination of exchange anisotropy by means of ac susceptometry in Co/CoO bilayers
1997 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 81, no 8, 5003-5005 p.Article in journal (Refereed) Published
Abstract [en]

The strength, or stiffness K-ud, of the magnetic unidirectional anisotropy in Co (90-300 Angstrom)/CoO bilayers has been studied with ac susceptometry. We develop a model to describe the ac susceptibility which indicates that the stiffness K-ud decreases linearly with temperature as (1 - T/T-crit). As the temperature increases and K-ud decreases, a rotation of the magnetization to a uniaxial easy direction is observed. The values of K-ud determined by this small amplitude field oscillation technique are found to be larger than the values obtained when evaluated by the usual hysteresis loop technique. We observe that the exchange field, H-ex, could be greatly enhanced and its deterioration after repeated magnetization reversals significantly reduced by partially capping the Co surface prior to oxygen exposure.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-24093 (URN)A1997WV53700100 ()
Note
QC 20100813Available from: 2010-08-13 Created: 2010-08-13 Last updated: 2017-12-12Bibliographically approved
2. Measurements of the ferromagnetic/antiferromagnetic interfacial exchange energy in CO/CoO and Fe/FeF2 layers
Open this publication in new window or tab >>Measurements of the ferromagnetic/antiferromagnetic interfacial exchange energy in CO/CoO and Fe/FeF2 layers
Show others...
1998 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 83, no 11, 6893-6895 p.Article in journal (Refereed) Published
Abstract [en]

Two measurement techniques, both relying on reversible rotations of the magnetization, have been used to determine the magnitude of the interfacial exchange energy (IEE) between ferromagnetic and antiferromagnetic (F/AF) layers. One technique is to use the anisotropic magnetoresistance to determine rotations of the magnetization away from the unidirectional easy axis, where the rotation is accomplished by applying external magnetic fields less than the effective F/AF exchange field. The second technique uses measurements of the ac susceptibility as a function of the angle between the ac field and the unidirectional exchange field. Both of the reversible process techniques result in values of the IEE larger (by as much as a factor of 10 in Co/CoO bilayers) than the traditional irreversible technique of measuring a shift in the hysteresis loop. The ac susceptibility technique was also used to measure one Fe/FeF2 bilayer. For this sample, the IEE values obtained by reversible and irreversible methods are equivalent.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-24094 (URN)000077795500222 ()
Note
Tidigare titel: "Measurements of the ferromagnetic/antiferromagnetic interfacial exchange coupling energy in CO/CoO and Fe/FeF2 layers" QC 20100813Available from: 2010-08-13 Created: 2010-08-13 Last updated: 2017-12-12Bibliographically approved
3. Competing anistropies in exchange biased Co/CoO bilayers
Open this publication in new window or tab >>Competing anistropies in exchange biased Co/CoO bilayers
(English)Article in journal (Other academic) Submitted
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-24097 (URN)
Note
QS 20120327Available from: 2010-08-13 Created: 2010-08-13 Last updated: 2012-03-27Bibliographically approved
4. Mapping local susceptibility using a scanning coaxial write/read head
Open this publication in new window or tab >>Mapping local susceptibility using a scanning coaxial write/read head
1998 (English)In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 16, no 4, 2687-2692 p.Article in journal (Refereed) Published
Abstract [en]

A technique to study local susceptibility of magnetic material is introduced. We describe a unique probe based on a pair of write/read heads, to measure and model both the susceptibility components at any point on a surface. With this approach it is possible to map the strength and direction of the local in-plane uniaxial susceptibility at surfaces. The usefulness of our technique is validated by measurements on oriented gamma-Fe2O3 in a credit card. In addition, we present studies on an amorphous soft ferromagnet containing different defect structures to reveal the micromagnetic structural details around defects on a surface. Our approach has the distinct advantage of determining the local susceptibility and anisotropy on a length scale not possible by using other techniques available today.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-24095 (URN)000074852700105 ()
Note
QC 20100813Available from: 2010-08-13 Created: 2010-08-13 Last updated: 2017-12-12Bibliographically approved
5. Mapping local in-plane uniaxial suspectibility of melt-textured superconductors
Open this publication in new window or tab >>Mapping local in-plane uniaxial suspectibility of melt-textured superconductors
(English)Article in journal (Other academic) Submitted
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-24098 (URN)
Note
QC 20100813Available from: 2010-08-13 Created: 2010-08-13 Last updated: 2010-08-13Bibliographically approved

Open Access in DiVA

No full text

Authority records BETA

Ström, Valter

Search in DiVA

By author/editor
Ström, Valter
By organisation
Physics
Condensed Matter Physics

Search outside of DiVA

GoogleGoogle Scholar

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 939 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf