Magnetic X-Ray Spectroscopy

We are intensively using the so called XMCD effect for an analysis of complex magnetic systems. This technique provides an element specific way to determine quantitatively spin and orbital magnetic moments of every atomic species, even in complex magnetic systems. Since the magnetic cross-section can be very distinct very low concentrations and some percent of  one monolayer can be addressed. Interesting systems are high performance magnets, d0 magnetism, or exchange bias systems, where the latter have shown unexpected pinned pure orbital magnetic moments at the interface between a ferromagnet and the antiferromagnet. 

If we utilize scattering phenomena we gain additional spatial resolution. Using X-ray Resonant Magnetic Reflectivity (XRMR) we are able to additionally investigate the interface modified magnetism and/or electronic structure. Further details could be found in our recent review article.

In the department we use high-perfomance SQUID magnetometer, MFM and highly sensitive MOKE microscopy. For a better understanding of micromagnetic phenomena we developed two new ways to measure so called First Order Reversal Curves (FORC). Utilizing a decent Nonomoke3 system, we were able to measure high resolution FORC in a few minutes.  As an example, we analyse in combination with high resolution STXM based magnetic microscopy the complex magnetization reversal behaviour of Fe based magnonic lattices.

X-Ray Magnetic Circular Dichroism (XMCD)

X-ray Magnetic Circular Dichroism (XMCD)

XMCD is one of the key methods for microscopic investigations of magnetic systems, which has been pioneered by the Gisela Schütz in 1987 [G. Schütz et al. PRL 58,(1987) 737]. It is based on the magnetization dependent variation of the X-ray absorption coefficient of circular polarized X-rays present at a vicinity of an absorption edge (see Fig. 1). Is allows in an element specific way the determination of spin and orbital magnetic moments. This provides the determination of the microscopic magnetism of each element present in complex magnetic systems, which is necessary to understand modern solid state magnetic and technological relevant phenomena in a very deep way.

Recent scientific results have been performed at magnetic oxide systems, like the ferromagnetic CrO2, halfmetallic Magnetite Fe3O4, Cobaltates, colossal magnetoresistive Manganates, and FePt/FeOx based functional nanoparticles.

In addition methodical developments are an important part of the scientific work in our group. A fast switching 2T magnet systems has been developed, which is constantly improved and optimized for ultra high sensitivity and ultimate signal to noise ratio. For example, in the ZnO:5%Co system, perfectly paramagnetic Co magnetic moments have been quantitatively investigated, despite the very small moments and the dilution based weak signal. A new 8T system is now in use, which allows fast flipping of 7T field in about 10s. This system is stationary positioned at the ANKA/WERA beamline at the KIT in Karlsruhe.

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High Performance Permanent Magnets

Due to the high demand on cheap and powerful magnets necessary for future electro-mobility and high efficient generators, the search for new and better permanent magnets is one important part in this department. In addition we also want to understand microscopically how and why permanent magnets are operating. Here the magnetization loops of MnBi are shown, where a total unsual increase in the coercive field appears with increase temperature. Our goal is to increase the hard magnetic properties, as coercive field and saturation magnetization, and to understand this from a microscopic point of view using XMCD and related methods.

MnBi Hysteresis loops (from  Chen et al. Scripta Materialia 107 (2015) 131)

Y.-C. Chen, G. Gregori, A. Leineweber, F. Qu, C.-C. Chen, T. Tietze, H. Kronmüller, G. Schütz, and E. Goering, "Unique high-temperature performance of highly consensed MnBi permanent magnets," Scripta Materialia 107, 131-135 (2015).

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Mysterious magnetism of ZnO

Detailed investigations have been performed at the diluted magnetic semiconductor system ZnO:5%Co. This system shows no ferromagnetism at the Co ions, which was completely unexpected. The ferromagnetism (FM) observed in this system is therefore not understood at all. Even the microscopic source of the FM is not clear a all. Recent results suggest oxygen vacancies as a source for FM (summary).

Structure of ZnO:5%Co.

In the last decade ZnO nanomaterials have shown unexpected ferromagnetism like behavior, with about no temperature dependence, indication extraordinary high Curie temperatures. This is totally unexpected, because of the filled (empty) shell configurations of ZnO, which should provide only diamagnetic behavior. Usually thin films have been investigated with nanocrystalline grain structure and only very small and therefore ambiguous results have been published. We could produce bulk like ferromagnetic samples, just by pressing pure ZnO powder to compacted pellets. We also found proof for the true magnetism using muon spin rotation (µSR) ( for details  see Scientific Reports 5, Article number: 8871 (2015)).

a) Temperature dependent magnetization after careful subtracting the single crystal based diamagnetism (inset shows raw data). b) Ferromagnetic behaviour after compaction (from Chen et al. APL 103, 162406 (2013)).
Y.-C. Chen, Z. Wang, A. Leineweber, J. Baier, T. Tietze, F. Phillipp, G. Schütz, and E. Goering, "Effect of surface configurations on the room-temperature magnetism of pure ZnO," Journal of Materials Chemistry C 4 (19), 4166-4175 (2016).
T. Tietze, P. Audehm, Y.-C. Chen, G. Schütz, B. B. Straumal, S. G. Protasova, A. A. Mazilkin, P. B. Straumal, T. Prokscha, H. Luetkens, Z. Salman, A. Suter, B. Baretzky, K. Fink, W. Wenzel, D Danilov, and E. Goering, "Interfacial dominated ferromagnetism in nanograined ZnO: a μSR and DFT study," Scientific Reports 5, 8871-8876 (2015).
Y.-C. Chen, E. Goering, L. Jeurgens, Z. Wang, F Phillipp, J. Baier, T. Tietze, and G. Schütz, "Unexpected room-temperature ferromagnetism in bulk ZnO," Applied Physics Letters (103), 162405 (2013).

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Exchange Bias and pinned magnetic moments

Every magnetic hard disk utilizes the effect of exchange bias (EB) in it’s read sensor. EB appears in systems where an antiferromagnet (here FeMn) is in direct contact with a ferromagnet (here Co). Without going into details, this phenomenon is investigated for many years without a detailed understanding and a satisfactory  physical model. 

In the task solving this issue, we performed x-ray resonant reflectometry (XRMR) and XMCD to find the uncompensated magnetic moments in the AF and to distinguish between pinned and rotatable uncompensated moments. We could clearly identify pinned and rotatable uncompensated moments, but the pinned ones are of unexpected purely orbital character. We found a model to describe this phenomenon by a local breaking of the spin-orbit-interaction between the spin and orbital moments of Fe. This loaded spin orbit spring provides a new and unexpected source for anisotropy energy. After this observation similar mechanisms should be also in present in many other magnetic systems (For details see Scientific Reports 6, Article number: 25517 (2016)).

Left: Fe XMCD of the rotatable and pinned moments. Right: Atomistic model for the pinning mechanism, based on crystal field interaction and breaking of the local spin-orbit-coupling spring.  (see Scientific Reports 6, 25517 (2016))

P. Audehm, M. Schmidt, S. Brück, T. Tietze, J. Gräfe, S. Macke, G. Schütz, and E. Goering, "Pinned orbital moments - A new contribution to magnetic anisotropy," Scientific Reports 6, 25517 (2016).
M. Schmidt, J. Gräfe, P. Audehm, F. Phillipp, G. Schütz, and E. Goering, "Preparation and characterisation of epitaxial Pt/Cu/FeMn/Co thin films on (100)-oriented MgO single crystals," Physica Status Solidi A 212 (10), 2114-2123 (2015).

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X-ray resonant magnetic reflectivity (XRMR)

With the XRMR method we can obtain chemical and magnetic depth profiles, with element specific sensitivity. For this purpose we use our own “ERNSt” reflectometer. This allows the investigation of interface magnetism and interface related valence and roughness phenomena. 

XRMR is based on conventional X-ray reflectivity. Here the reflectivity of the X-rays as a function of the angle of incidence (more precisely of the momentum transfer along the surface normal) is modulated by the interference of different partial beams, reflected at each interface present in layered systems. In the conventional reflectivity, the scattering provides information of the optical density profile, i.e. the thickness of the layers and the interface roughness. Utilizing XMCD transfers this reflectivity measurement into its resonant magnetic counterpart: X-ray resonant magnetic reflectivity (XRMR). In analogy this extracts the magnetic profile and magnetization variations at the interfaces. Therefore, we have developed a dedicated UHV system for XRMR investigations. This method requires intensive numerical reflectivity simulations. For this purpose a state of the art software package ReMagX has been developed, which is constantly improved. This software allows the investigation of complex magnetic layered systems.

We have investigated the induced magnetic profile in an antiferromagnet of a so called Exchange bias system. This phenomenon is not understood microscopically so far – even after 50 years of scientific work – but one of the key ingredients of every modern hard disc reading head. Figure 4 shows the XRMR derived magnetic profile of Mn (in MnPd) in contact with iron. We also could distinguish between the fraction of the rotatable and the pinned magnetic moment, which have shown unexpected strength and position dependencies.

S. Macke and E. Goering, "Magnetic reflectometry of heterostructures (Topical Review)," Journal of Physics: Condensed Matter 26 (36), 363201 (2014).
S. Brück, S. Bauknecht, B. Ludescher, E. Goering, and G. Schütz, "An advanced magnetic reflectometer," Review of Scientific Instruments 79, 083109 (2008).
S. Brück, G. Schütz, E. Goering, X. Ji, and K. M. Krishnan, "Uncompensated moments in the MnPd/Fe exchange bias system," Physical Review Letters 101, 126402 (2008).

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