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Magnetism of Thin Films and Heterostructures

English title Magnetism of Thin Films and Heterostructures
Applicant Hug Hans Josef
Number 130519
Funding scheme Project funding (Div. I-III)
Research institution Departement Physik Universität Basel
Institution of higher education University of Basel - BS
Main discipline Condensed Matter Physics
Start/End 01.04.2010 - 31.05.2012
Approved amount 321'295.00
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Keywords (5)

magnetic thin film; magnetic heterostructure; exchange-bias; magnetic force microscopy; data storage

Lay Summary (English)

Lead
Lay summary
Various sensor and storage technologies rely on magnetic thin-film components with a unidirectional anisotropy. To obtain it, exchange bias (EB) in adjacent antiferromagnet (AF) / ferromagnet (F) structures is used. EB is not fully understood. It is generally believed that EB implies the presence of pinned uncompensated spins pinUCS in the AF layer that are coupled to the F layer. An obstacle to understanding the EB effect is that only a subset of the UCS (those pinned, and coupled to the F) are responsible for the EB. The experimental method and preparation may affect these subsets in distinct ways and an interpretation of UCS measurements must take this into account.In our previous work we have advantageously combined MFM with magnetometry to demonstrate the co-existence of pinned UCS that are parallel and antiparallel to the cooling field in metallic (IrMn) and oxidic (CoO) EB systems. We further conclude that the EB effect is mainly a result of pinned interfacial UCS, which are antiparallel to the FM spins. In addition we have studied the evolution of ferromagnetic domains over a pattern of pinned UCS and have substantially enhanced the exchange bias effect by decoupling the grains of the AF layer.Within this project various thin film magnetic heterostructures will be sputter-deposited onto flat and microstructured substrates. A first workpackage is devoted to the study of the switching field distribution of particulate media. New exchange-bias systems exhibiting an exchange-bias field above 1T will be developed. A second workpackage is devoted to the development of MFM measurement methods for exchange-biased samples with in-plane magnetic anisotropy, to independently map the rotating and pinned UCS, to study the role of the UCS pinned in the bulk of the AF, and to map the blocking temperature of single AF grains. The results are expected to contribute to a better microscopic understanding of the exchange bias effect and the role of the different subsets of UCS. New materials with a considerably larger exchange bias effects will be developed on the basis of the microscopic undestanding.
Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants

Employees

Publications

Publication
Engineering the ferromagnetic domain size for optimized imaging of the pinned uncompensated spins in exchange-biased samples by magnetic force microscopy
Joshi NR, Ozer S, Ashworth TV, Stickar PG, Romer S, Marioni MA, Hug HJ (2011), Engineering the ferromagnetic domain size for optimized imaging of the pinned uncompensated spins in exchange-biased samples by magnetic force microscopy, in APPLIED PHYSICS LETTERS, 98(8), 082502-1-082502-3.
Exchange Bias and Domain Evolution at 10 nm Scales
Schmid I, Marioni MA, Kappenberger P, Romer S, Parlinska-Wojtan M, Hug HJ, Hellwig O, Carey MJ, Fullerton EE (2010), Exchange Bias and Domain Evolution at 10 nm Scales, in PHYSICAL REVIEW LETTERS, 105(19), 197201-1-197201-4.

Scientific events

Active participation

Title Type of contribution Title of article or contribution Date Place Persons involved
JEMS 2012 10.09.2012 Parma, Italy
DPG 2012 28.03.2012 Berlin, Germany
Seminar London Center for Nanotechnology 07.03.2012 London
Seminar IFW Dresden 06.03.2012 Dresden, Germany
Seminar University of Regensburg 14.02.2012 Regensburg, Germany
56th Conference Magn Magn Mat 30.10.2011 Scottsdale, Arizona, USA
DPG March 2011 14.03.2011 Dresden, Germany
55th MMM 2010 08.11.2010 Atlanta, USA
ICNM-2010 28.09.2010 Istanbul, Turkey
ICSM 2010 26.04.2010 Antalya, Turkey
11. Joint MMM 2010-Intermag 18.01.2010 Washington DC, USA


Associated projects

Number Title Start Funding scheme
117970 Magnetism of Thin Films and Heterostructures 01.10.2007 Project funding (Div. I-III)
117970 Magnetism of Thin Films and Heterostructures 01.10.2007 Project funding (Div. I-III)

Abstract

The phenomenon of exchange bias occurs when an antiferromagnetic (AF) material is grown in contact with a ferromagnetic (F) material and was first discovered in Co/CoO particles by Meiklejohn and Bean [1] in 1956. Since its discovery there have been numerous theoretical and experimental attempts to explain the effect and in particular to predict the magnitude in the shift in the hysteresis loop, which results from the unidirectional anisotropy induced in such systems when the AF layer is cooled through its Néel temperature. It is commonly agreed that the EB-effect arises from pinned uncompensated spins at the F/AF interface or in the inside of the AF film. Although excellent experimental techniques have been developed to measure, 2-dimensionally map, depth-profile these UCS and their distribution and density inside the AF, the situation remained partially unclear, presumably because most techniques require models with too many unknowns to fit the experimental results. A clear understanding of important phenomena related to the exchange bias effect is still not available. Since our first publication on this topic in 2003 [16] we have introduced high resolution low temperature magnetic force microscope to image pinned UCS with unmatched later resolution and sensitivity. Thanks to our thorough understanding of the image contrast in MFM and the methods previously developed to calibrate the MFM tip, we are able to deconvolute the stray field on the sample’s surface (or the planar UCS density at the F/AF-interface) from measured frequency shift data. These quantitative MFM methods gives my Basel laboratory a world-wide unique position to quantitatively analyze magnetic heterostructures with excellent spatial resolution. Since the beginning of 2004 a magnetism group was established at my Empa laboratories. Various sputter-coating methods ideally suited for the fabrication of complicated magnetic heterostructures have been implemented. Many state-of-the art structural, chemical and magnetic characterization methods are available. This situation at the University of Basel and at Empa has given a unique opportunity to design experiments and samples to perfectly match the best-abilities of a specific analytical method to address a well-posed scientific question.The work proposed here is a continuation of what has been achieve in the SNF project 200021-117970 on the same topic. In this proposal the research is structured into 3 workpackages. A first one devote to the exploration of the exchange-bias effect for heat assisted magnetic recording (HAMR) (task 1). In a second task the classical AF-materials that are the basis for an uncountable number of different magneto-electronic applications will be replaced by rare-earth/transition-metal (RE/TM) alloys. These materials can become antiferromagnetic, when the RE and TM magnetic moments that are coupled antiferromagnetically become the same size. In contrast the “classical” AF that are forced to generate a small number of pinned uncompensated spins when interfaced to a ferromagnetic (F) layer, the RE/TM-type AF are expected to couple all their interfacial spins perfectly with the F-layer spins. Hence an super-exchange bias effects is expected to occur that is orders of magnitude larger than what is observed with the “classical” AF-materials. Note that RE/TM-systems may also be explorered for HAMR. Workpackage 2 is devoted to the exploration of magnetic force microscopy measurement methods to measure various unresolved issues of EB-systems. Again most experiments proposed here are world-wide unique and - to my best knowledge - will presently not been attempted in any other laboratory of the world. Among the key experiments are the mapping of rotating UCS and the study of the role of the AF-bulk for the EB-effect. Workpackage 3 then serves to exploit and dissiminate our results.
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