Ingo P. Krug

Alteration of exchange anisotropy and magnetoresistance in Co/Cu/Co/FeMn spin valves by ion bombardment

The dose dependence of the exchange bias field and magnetoresistance by 10 keV-He ion bombardment in an applied magnetic field were investigated for Co/Cu/Co/FeMn top spin valves. The exchange bias field Heb of the pinned Co layer can be enhanced, reversed, and reduced similarly to an exchange biased bilayer system. Additionally, upon ion bombardment, the free Co layer couples increasingly strong to the pinned Co layer until they act magnetically as one single pinned layer. The magnetoresistance decreases exponentially with increasing ion dose, caused by a higher sheet resistance due to increasing defect density and increasing coupling between the free and the pinned ferromagnetic layer. A phenomenological model is extended to describe the ion bombardment-induced changes of Heb of ferromagnet/antiferromagnet exchange biased bilayers by including saturation effects. This model is compared to available experimental data.The dose dependence of the exchange bias field and magnetoresistance by 10 keV-He ion bombardment in an applied magnetic field were investigated for Co/Cu/Co/FeMn top spin valves. The exchange bias field Heb of the pinned Co layer can be enhanced, reversed, and reduced similarly to an exchange biased bilayer system. Additionally, upon ion bombardment, the free Co layer couples increasingly strong to the pinned Co layer until they act magnetically as one single pinned layer. The magnetoresistance decreases exponentially with increasing ion dose, caused by a higher sheet resistance due to increasing defect density and increasing coupling between the free and the pinned ferromagnetic layer. A phenomenological model is extended to describe the ion bombardment-induced changes of Heb of ferromagnet/antiferromagnet exchange biased bilayers by including saturation effects. This model is compared to available experimental data.

Functional electronic inversion layers at ferroelectric domain walls

Ferroelectric domain walls hold great promise as functional two-dimensional materials because of their unusual electronic properties. Particularly intriguing are the so-called charged walls where a polarity mismatch causes local, diverging electrostatic potentials requiring charge compensation and hence a change in the electronic structure. These walls can exhibit significantly enhanced conductivity and serve as a circuit path. The development of all-domain-wall devices, however, also requires walls with controllable output to emulate electronic nano-components such as diodes and transistors. Here we demonstrate electric-field control of the electronic transport at ferroelectric domain walls. We reversibly switch from resistive to conductive behaviour at charged walls in semiconducting ErMnO3. We relate the transition to the formation-and eventual activation-of an inversion layer that acts as the channel for the charge transport. The findings provide new insight into the domain-wall physics in ferroelectrics and foreshadow the possibility to design elementary digital devices for all-domain-wall circuitry.

Imaging and characterization of conducting ferroelectric domain walls by photoemission electron microscopy

High-resolution X-ray photoemission electron microscopy (X-PEEM) is a well-established method for imaging ferroelectric domain structures. Here, we expand the scope of application of X-PEEM and demonstrate its capability for imaging and investigating domain walls in ferroelectrics with high spatial resolution. Using ErMnO3 as test system, we show that ferroelectric domain walls can be visualized based on photo-induced charging effects and local variations in their electronic conductance can be mapped by analyzing the energy distribution of photoelectrons. Our results open the door for non-destructive, contact-free, and element-specific studies of the electronic and chemical structure at domain walls in ferroelectrics.

Néel walls between tailored parallel-stripe domains in IrMn/CoFe exchange bias layers

Tailored parallel-stripe magnetic domains with antiparallel magnetizations in adjacent domains along the long stripe axis have been fabricated in an IrMn/CoFe Exchange Bias thin film system by 10 keV He+-ion bombardment induced magnetic patterning. Domain walls between these domains are of Neel type and asymmetric as they separate domains of different anisotropies. X-ray magnetic circular dichroism asymmetry images were obtained by x-ray photoelectron emission microscopy at the Co/Fe L3 edges at the synchrotron radiation source BESSY II. They revealed Neel-wall tail widths of 1 μm in agreement with the results of a model that was modified in order to describe such walls. Similarly obtained domain core widths show a discrepancy to values estimated from the model, but could be explained by experimental broadening. The rotation senses in adjacent walls were determined, yielding unwinding domain walls with non-interacting walls in this layer system.

Characterization of magnetic force microscopy probe tip remagnetization for measurements in external in-plane magnetic fields

A quantitative analysis of magnetic force microscopy (MFM) images taken in external in-plane magnetic fields is difficult because of the influence of the magnetic field on the magnetization state of the magnetic probe tip. We prepared calibration samples by ion bombardment induced magnetic patterning with a topographically flat magnetic pattern magnetically stable in a certain external magnetic field range for a quantitative characterization of the MFM probe tip magnetization in point-dipole approximation.

Time-resolved magnetic imaging in an aberration-corrected, energy-filtered photoemission electron microscope.

We report on the implementation and usage of a synchrotron-based time-resolving operation mode in an aberration-corrected, energy-filtered photoemission electron microscope. The setup consists of a new type of sample holder, which enables fast magnetization reversal of the sample by sub-ns pulses of up to 10 mT. Within the sample holder current pulses are generated by a fast avalanche photo diode and transformed into magnetic fields by means of a microstrip line. For more efficient use of the synchrotron time structure, we developed an electrostatic deflection gating mechanism capable of beam blanking within a few nanoseconds. This allows us to operate the setup in the hybrid bunch mode of the storage ring facility, selecting one or several bright singular light pulses which are temporally well-separated from the normal high-intensity multibunch pulse pattern.

Tuning the orbital ordering in La0.7Sr0.3MnO3 thin films in all-oxide hybrids

This work presents a systematic characterization including x-ray diffractometry, SQUID magnetometry and x-ray absorption spectroscopy on all-oxide ferromagnetic/ferroelectric heterosystem BaTiO3/La0.7Sr0.3MnO3/SrTiO3(001) fabricated by pulsed-laser deposition. The key aspect of this study is to provide accurate information about differences in electronic and structural properties in LSMO thin films as a function of the oxygen pressure during BTO growth. X-ray absorption spectroscopy experiments at the Mn L 3,2 edge have revealed the conservation of mixed valency configuration of the LSMO films near the interface while tuning the oxygen pressure of overlayer BTO growth. The existing ions favor an in-plane e g orbital ordering, reducing the in-plane strain and promoting room temperature ferromagntism in the LSMO film. Furthermore, diffraction experiments showed that the out-of plane lattice parameter of BTO reduces with increasing oxygen pressure consistent with the x-ray linear dichrosim at Ti edge showing less tetragonal symmetry, although the chemical environment of the Ti ions was not changed notably. We demonstrated a way to control magnetic properties and orbital ordering in LSMO thin films while optimizing the ferroelectric properties of BTO overlayer films, which are promising results in terms of magnetoelectric applications using functional heterosystems.

Microscopic analysis of the composition driven spin-reorientation transition in NixPd1−x/Cu(001)

The spin-reorientation transition (SRT) in epitaxial NixPd1-x/Cu(001) is studied by photoemission microscopy utilizing the X-ray magnetic circular dichroism effect at the Ni L2,3 edge. In a composition/thickness wedged geometry, a composition driven SRT could be observed between 37 ML and 60 ML, and 0 and 38% of Pd. Microspectroscopy in combination with azimuthal sample rotation confirms a magnetization preference changing from the [001] to an in-plane easy axis. At this increased thickness, the domain patterns arrange comparable to SRTs in ultrathin films. The images document domains equivalent to a canted state SRT, at which an additional effect of in-plane anisotropies could be identified.