Integral and local AC-susceptibility

Abstract: This thesis presents mymostly unpublished experimental workrelated to twomajor contributions with the design of:    A high precision and versatilebulk average(orintegral) AC-susceptometer and its application to investigationsof exchange anisotropy in Co multilayers.    A novelLocal Susceptibilitymeasurement technique exploiting the narrow gap ininductive ring-heads in magnetic recording technology.Representative applications of this instrument are presentedas well.Integral AC-susceptibilitymeasurementsEven though AC-susceptibility is a well-established andmatured experimental technique, there are many new challengesfor improvements and modifications, in particular for theinvestigation of low dimensional systems. Thus, we haveconstructed and built a versatile and highly sensitiveAC-susceptometer. Some of the design concepts presented in thisthesis, which are new or not normally found in literatureare:    A time-adjusted background-correction scheme, whichreduces the error from non-simultaneous measurement of sampleand background.    A monitor-coil, which serves as a software-controlledphase-shift determination at any time during ameasurement.    A balanced current source, which enables the primary andsecondaries to a have a common potential that will reduce thebackground signal.    A software, which allows any measurement (e.g. a standardwarming scan for the determination of the temperaturedependence of the susceptibility) to besegmented into anarbitrary number of individually specified combinations ofmeasuring field-strength and frequency.Our instrument has provided the basis for considerablenumber of publications (18) in recent years. It may be pointedout that the sensitivity of this home-built susceptometer, 10-9emu, is perhaps the best to be reported inliterature, and compares competitively with those for a SQUIDmagnetometer.The versatility of the instrument is demonstrated with a newapproach for the study of exchange anisotropy of thincobalt/cobalt-oxide bilayers, Paper 1-3. In this method, wedetermine the exchange anisotropy values with only a smallexciting field, and no complete flux reversal process isinvolved. The values obtained for the exchange anisotropy inour studies are found to be much closer to the expectedtheoretical values.LocalSusceptibility measurementsAC-susceptibility is conventionally thought of as anintegralorbulk averagedmeasurement. With an inductive combinedwrite/read head, we have constructed and built an instrument,which enables us to measure the Local Susceptibility. Theinstrument scans magnetically responsive surfaces and imagesthe in-plane uniaxial susceptibility, including both itsmagnitude and orientation, on a length-scale determined by thedimensions of the gap (approaching submicrometer level) of theemployed head, Paper 4-5.The basic design and the appropriate derivations forquantitative analysis of the data are presented. In order todemonstrate the capabilities of the instrument we presentinvestigations on a variety of materials, which include hard aswell as soft ferromagnets and a study on melt-texturedhigh-temperature superconductors. It is important to point outthat many, if not most, of the results discussed are notpossible with any other hitherto known technique. Inparticular, we may point out the capability to obtainin-planeproperties for superconducting surfaces, whichis perhaps a unique information obtainable from our LocalSusceptibility probe technique.