This thesis presents my mostly unpublished experimental work related to two major contributions with the design of: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.
Stockholm: KTH , 1999. , x, 121 p.