Research Examples

Individual magnetic nanocrystals

What is the single domain limit? Below a critical size, the formation of domain walls is suppressed and the nanostructures are uniformly magnetized, i.e. in a single domain state. However, it is not a trivial issue to build a consistent picture of the transition to the single domain state, in particular for supported particles as it would be the case in many technological applications. Using PEEM, we discovered a size-dependent transition from a single domain state to a non-collinear spin structure in isotropic nanoparticles with sizes ranging from 25 down to 5 nm supported on a magnetic substrate. By imaging the magnetization orientation of individual iron nanoparticles, it has been possible to clarify the size limits of single-domain formation in a truly nanoscopic system as described in A. Fraile Rodriguez et al., Phys. Rev. Lett. 104, 127201 (2010). At present we study magnetic hysteresis curves of individual particles as well as superparamagnetic relaxation phenomena in pure and alloyed 3d metal nanoparticles.

Magnetization dynamics

In this project we are exploring the interaction of ultrafast laser pulses with spins in magnetic systems. Recently, the non-thermal interaction of a circularly polarized fs laser pulse with the spins of a ferromagnetic system has been explored, thus demonstrating ultrafast switching of a ferromagnetic system. This triggered a fundamentally new area of research with intriguing questions. How can photons interact with spins on the femtosecond timescale? In our study we are pushing this new field of “opto-magnetism” towards the nanoscale.

Magnetic Nanostructures – Artificial Frustrated Spin Systems

In mesoscopic frustrated systems, where ferromagnetic islands are placed on the sites of a honeycomb lattice, it is possible to study how the moments are organized and how the geometry and external stimuli, such as applied magnetic field, affect the behavior. We studied the basic building blocks of the artificial kagome spin ice system by investigating one-, two- and three-ring structures. In an attempt to induce the lowest energy configurations, arrays of the structures were demagnetized by rotating the sample in a magnetic field. We found indications that the ground state can not be achieved in an infinite system using such a demagnetization protocol. Motivated by the work of Castelnovo et al. on magnetic monopoles as quasiparticles in spin-ice system, we have been able to directly observe emergent magnetic monopoles in infinite artificial kagome spin ice arrays, which arise due to the collective behavior of the magnetic moments in the system. We observed how monopole-antimonopole pairs nucleate and separate in a magnetic field, and how their displacement follows a one-dimensional path in such a two-dimensional system, which results from the frustrated geometry.

Instrumentation development

In the last years, we have concentrated our work on smaller instrumentation developments directly related with specific scientific projects:

• On the fly mode of energy scans, enabling a 100 eV scan in 2 minutes
• Tune/detune polarization switching, enabling the polarization switching in 2 – 3 seconds
• Magnetic field sample holder for PEEM, enabling measurement in applied magnetic fields up to 100 Oe and application of magnetic fields of up to 500 Oe
• Sample storage box, now sold by Ferrovac.com
• Set up for applying current pulses and measure in the PEEM during this pulse (this development was lead by the group of M. Klaeui)
• Installation of femtosecond laser to excite samples in-situ in the PEEM
• Broad range of sample holders for applying electrical and magnetic fields for the RESOXS chamber
• Implementation of endstations belonging to users into the SIM beamline, including special beamline interfaces, for example measuring X-FMR in He atmosphere, and software integration of user stations
• Coupling of mass selected cluster source to PEEM

Collaborations (SIM)

• Frithjof Nolting, Laboratory for Condensed Matter Physics, Paul Scherrer Institut
• Laura Heyderman, Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut
• Thomas Jung, Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut
• Mathias Klaeui, EPFL Lausanne & Paul Scherrer Institut
• Peter Derlet, NUM Division, Paul Scherrer Insitut
• National Competence Center in Research NANO on Nanoscience
• National Competence Center in Research MaNEP (Materials with Novel Electronic Properties), where F. Nolting is associated member and U. Staub is full member
• EU FP7 Network IFOX (Interfacing Oxides)
• EU FP7 Network Ultramagnetron (Ultrafast All-Optical Magnetization Reversal for Magnetic Recording and Laser-Controlled Spintronics)
• EU Marie Curie Network Fantomas (Femtosecond Opto-Magnetism and Novel Approaches to Ultrafast Magnetism at the Nanoscale)
• Besides the formal participation in networks, we have close collaborations with individual groups such as D. Pescia (ETHZ), H. Brune (EPFL), Th. Greber (University of Zurich), Th. Rasing and A. Kimel (University of Nijmegen)

Resonant soft x-ray scattering

Here, we use different resonant x-ray techniques, in particular x-ray diffraction in the vicinity of absorption edges, to study the electronic and magnetic properties of novel materials. This technique focuses on periodic electronic and magnetic structures mainly in crystalline materials. The scientific questions addressed are related to the understanding of basic questions such as: How does superconductivity work? Can we manipulate magnetic structures by electric fields and how fast can we do that? What is its basic mechanism of it? How can we manipulate the electronic properties by magnetic or electromagnetic fields? These are related to e.g. the understanding of the colossal magneto resistance effect.

X-ray Absorption Specroscopy at High Magnetic Fields and Low Temperatures

Our main tools are X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at low temperatures and high magnetic fields available at the X-Treme beam line. This technique is able to detect the element specific magnetic moments with sensitivity better than sub-monolayers of materials. We mainly focus on the magnetism of transition metals and rare earths, but the accessibility of oxygen and nitrogen K edges also gives us the possibility to study, e.g., hybridization effects in inorganic materials and molecular orientation on surfaces.

Molecular Magnetism at Interfaces and Quantum Spintronics

In single-molecule magnets (SMMs) the magnetization relaxation is blocked because of strong magnetic anisotropy. As a consequence, the magnetization lifetimes in these remarkably small, nanometer-sized, systems can reach days or months. In addition, SMMs can show interesting quantum effects such as the quantum tunneling of magnetization (QTM). We study the magnetic behavior of SMMs and their mononuclear counterparts, single-ion molecular magnets (SIMs), when they are in contact with different planar surfaces. We are trying to make use of the molecule-surface interaction in order to modify the static and dynamic properties of SMMs and SIMs. We strongly rely on XAS and XMCD, but we are also using scanning tunneling microscopy (STM), which provides sample topography and electronic structure with intramolecular resolution and X-ray photoelectron spectroscopy (XPS). For our measurements we produce the samples in-situ in our ultra-high vacuum preparation chamber. Our equipment allows to prepare atomically flat, clean, metal surfaces, to grow thin films of oxide and other materials and to deposit SMMs and SIMs with sub-monolayer control.

Magnetism at interface in heterostructures

In the interface of heterostructures of two dissimilar materials a variety of new properties can arrive. With x-ray magnetic circular dichroism we can disentangle the magnetism coming from different layers by using the element specificity. Moreover, by combining different probing detection modes it is possible to narrow down if the magnetism extends through the whole thickness or if it is present only at the interface.