Advanced scanning magnetoresistive microscopy as a multifunctional magnetic characterization method

• Main Author: Mitin, Dmitriy
• Other Authors: Technische Universität Chemnitz, Elektrotechnik und Informationstechnik
• Format: Doctoral Thesis
• Language: English
• Published: Universitätsbibliothek Chemnitz 2017
• Subjects: Rastermagnetowiderstandsmikroskopie; SMRM; magnetische Speicherung; HAMR; L10 FePt; austausch gekoppelte Kopmposit; magnetische Vortizes; scanning magnetoresistive microscopy; perpendicular magnetic recording; heat-assisted magnetic recording; L10 FePt alloy films; exchange coupled composite; magnetic vortex; ddc:621; Mikroskopie; Raster; Magnetspeicher; Magnetismus
• Online Access: http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-224986; http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-224986; http://www.qucosa.de/fileadmin/data/qucosa/documents/22498/Dissertation_Dmitriy_Mitin.pdf; http://www.qucosa.de/fileadmin/data/qucosa/documents/22498/signatur.txt.asc

Advanced scanning magnetoresistive microscopy (SMRM) — a robust magnetic imaging and probing technique — is presented. It utilizes conventional recording heads of a hard disk drive as sensors. The spatial resolution of modern tunneling magnetoresistive sensors is nowadays comparable with more commonly used magnetic force microscopes. Important advantages of SMRM are the ability to detect pure magnetic signals directly proportional to the out-of-plane magnetic stray field, negligible sensor stray fields, and the ability to apply local bipolar magnetic field pulses up to 10 kOe with bandwidths from DC up to 1 GHz. The performance assessment of this method and corresponding best practices are discussed in the first section of this work. An application example of SMRM, the study on chemically ordered L10 FePt is presented in a second section. A constructed heater unit of SMRM opens the path to investigate temperature-dependent magnetic properties of the medium by recording and imaging at elevated temperatures. L10 FePt is one of the most promising materials to reach limits in storage density of future magnetic recording devices based on heat-assisted magnetic recording (HAMR). In order to be implemented in an actual recording scheme, the medium Curie temperature should be lowered. This will reduce the power requirements, and hence, wear and tear on a heat source — integrated plasmonic antenna. It is expected that the exchange coupling of FePt to thin Fe layers provides high saturation magnetization and elevated Curie temperature of the composite. The addition of Cu allows adjusting the magnetic properties such as perpendicular magnetic anisotropy, coercivity, saturation magnetization, and Curie temperature. This should lead to a lowering of the switching field of the hard magnetic FeCuPt layer and a reduction of thermally induced recording errors. In this regard, the influence of the Fe layer thickness on the switching behavior of the hard layer was investigated, revealing a strong reduction for Fe layer thicknesses larger than the exchange length of Fe. The recording performance of single-layer and bilayer structures was studied by SMRM roll-off curves and histogram methods at temperatures up to 180 °C In the last section of this work, SMRM advantages are demonstrated by various experiments on a two-dimensional magnetic vortex lattice. Magnetic vortex is a peculiar complex magnetization configuration which typically appears in a soft magnetic structured materials. It consists of two coupled sub-systems: the core, where magnetization vector points perpendicular to the structure plane, and the curling magnetization where magnetic flux is rotating in-plane. The unique properties of a magnetic vortex making it an object of a great research and technological interest for spintronic applications in sensorics or data storage. Manipulation of the vortex core as well as the rotation sense by applying a local field pulse is shown. A spatially resolved switching map reveals a significant "write window" where vortex cores can be addressed correctly. Moreover, the external in-plane magnet extension unit allow analyzing the magnetic vortex rotational sense which is extremely practical for magnetic coupling investigations of magnetic coupling phenomena.