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