A. Ney

Programmable computing with a single magnetoresistive element.

The development of transistor-based integrated circuits for modern computing is a story of great success. However, the proved concept for enhancing computational power by continuous miniaturization is approaching its fundamental limits. Alternative approaches consider logic elements that are reconfigurable at run-time to overcome the rigid architecture of the present hardware systems. Implementation of parallel algorithms on such ‘chameleon’ processors has the potential to yield a dramatic increase of computational speed, competitive with that of supercomputers. Owing to their functional flexibility, ‘chameleon’ processors can be readily optimized with respect to any computer application. In conventional microprocessors, information must be transferred to a memory to prevent it from getting lost, because electrically processed information is volatile. Therefore the computational performance can be improved if the logic gate is additionally capable of storing the output. Here we describe a simple hardware concept for a programmable logic element that is based on a single magnetic random access memory (MRAM) cell. It combines the inherent advantage of a non-volatile output with flexible functionality which can be selected at run-time to operate as an AND, OR, NAND or NOR gate.

Ferromagnetism and colossal magnetic moment in Gd-focused ion-beam-implanted GaN

The structural and the magnetic properties of Gd-focused ion-beam-implanted GaN layers are studied. Gd3+ ions were uniformly implanted in molecular beam epitaxy grown GaN layers at room temperature with an energy of 300keV at doses ranging from 2.4×1011to1.0×1015cm−2 which corresponds to an average Gd concentration range of 2.4×1016–1.0×1020cm−3. The implanted samples were not subjected to any annealing treatment. No secondary phase related to Gd was detected by x-ray diffraction in these layers. Magnetic characterization with superconducting quantum interference device reveals a colossal magnetic moment of Gd and ferromagnetism with an order temperature above room temperature similar to that found in epitaxially grown Gd-doped GaN layers. The effective magnetic moment per Gd atom in these samples is, however, found to be an order of magnitude larger than that found in epitaxially grown layers for a given Gd concentration which indicates that the defects play an important role in giving rise to this effect.

Advanced spectroscopic synchrotron techniques to unravel the intrinsic properties of dilute magnetic oxides: the case of Co:ZnO

The use of synchrotron-based spectroscopy has revolutionized the way we look at matter. X-ray absorption spectroscopy (XAS) using linear and circular polarized light offers a powerful toolbox of element-specific structural, electronic and magnetic probes that is especially well suited for complex materials containing several elements. We use the specific example of Zn1−xCoxO (Co:ZnO) to demonstrate the usefulness of combining these XAS techniques to unravel its intrinsic properties. We demonstrate that as long as phase separation or excessive defect formation is absent, Co:ZnO is paramagnetic. We can establish quantitative thresholds based on four reliable quality indicators using XAS; samples that show ferromagnet-like behaviour fail to meet these quality indicators, and complementary experimental techniques indeed prove phase separation. Careful analysis of XAS spectra is shown to provide quantitative information on the presence and type of dilute secondary phases in a highly sensitive, non-destructive manner.

Sensitive SQUID magnetometry for studying nanomagnetism

The superconducting quantum interference device (SQUID) magnetometer is one of the most sensitive experimental techniques to magnetically characterize samples with high sensitivity. Here we present a detailed discussion of possible artifacts and pitfalls characteristic for commercial SQUID magnetometers. This includes intrinsic artifacts which stem from the inherent design of the magnetometer as well as potential issues due to the user. We provide some guidelines on how to avoid and correct these, which is of particular importance when the proper magnetization of nanoscale objects will be established in cases where its response is dwarfed by that of the substrate it comes with, a situation frequently found in the field of nanomagnetism.

Element specific investigations of the structural and magnetic properties of Gd:GaN

The authors present element specific measurements of the x-ray linear dichroism and the x-ray magnetic circular dichroism (XMCD) on Gd:GaN samples. They can show that the majority of the Gd dopant atoms goes to substitutional Ga sites and that a small XMCD is detectable for Gd. There are significant deviations of the magnetic hysteresis recorded for Gd compared to superconducting quantum interference device measurements. Our measurements show that the magnetic signal from the Gd dopant atom itself is rather small highlighting the role of magnetic contributions of the GaN host crystal.

Irradiation enhanced paramagnetism on graphene nanoflakes

We have studied the magnetization of vertically aligned graphene nanoflakes irradiated with nitrogen ions of 100 KeV energy and doses in the range 1011–1017 ions/cm2. The non-irradiated graphene nanoflakes show a paramagnetic contribution, which is increased progressively by ion irradiation at low doses up to 1015/cm2. However, further increase on implantation dose reduces the magnetic moment which coincides with the onset of amorphization as verified by both Raman and x-ray photoelectron spectroscopic data. Overall, our results demonstrate the absence of ferromagnetism on either implanted or unimplanted samples from room temperature down to a temperature of 5 K.

Understanding the submicron domain structure of MnAs thin films on GaAs(001): Magnetic force microscopy measurements and simulations

Over a wide temperature range of 30 °C around room temperature, MnAs films on GaAs(001) semiconductor substrates break up into ordered arrays of submicron-sized ferromagnetic α and paramagnetic β wires. Both the hard and the easy axis of magnetization (perpendicular to the wires) lie in the film plane and a large variety of complex domain patterns are found in micromagnetic investigations with magnetic force microscopy (MFM). A systematic analysis of the domain configurations is given and the most likely configurations are identified through MFM contrast simulations.

Element-Specific Magnetic Hysteresis of Individual 18 nm Fe Nanocubes

Correlating the electronic structure and magnetic response with the morphology and crystal structure of the same single ferromagnetic nanoparticle has been up to now an unresolved challenge. Here, we present measurements of the element-specific electronic structure and magnetic response as a function of magnetic field amplitude and orientation for chemically synthesized single Fe nanocubes with 18 nm edge length. Magnetic states and interactions of monomers, dimers, and trimers are analyzed by X-ray photoemission electron microscopy for different particle arrangements. The element-specific electronic structure can be probed and correlated with the changes of magnetic properties. This approach opens new possibilities for a deeper understanding of the collective response of magnetic nanohybrids in multifunctional materials and in nanomagnetic colloidal suspensions used in biomedical and engineering technologies.

Structural, magnetic, and optical properties of Co- and Gd-implanted ZnO(0001) substrates

ZnO(0001) substrates were ion implanted with 100 keV of Co and 300 keV of Gd at different fluences ranging from 5×1013–1×1015/cm2. The resulting Co:ZnO and Gd:ZnO samples were analyzed with respect to their structural, magnetic, and optical properties. The effect of annealing at 350 °C on the structure and the resulting magnetic and optical properties were investigated as well. For Co:ZnO hardly any changes were observable, neither in the structural nor in the magnetic properties, even though the existence of substitutional Co2+ in the ZnO lattice could be shown by means of low temperature photoluminescence especially for Zn-annealed samples. For the much larger Gd ion the implantation leads to a changed crystal structure, which leads to a ferromagneticlike behavior for higher implantation doses, which could even be enhanced by annealing in vacuum. Ferromagnetic behavior in annealed Gd:ZnO is corroborated by ferromagnetic resonance measurements at low temperatures. The distinct behavior of Gd- and Co-impl...

Structural, chemical and magnetic properties of secondary phases in Co-doped ZnO

We have utilized a comprehensive set of experimental techniques such as transmission electron microscopy (TEM) and synchrotron-based x-ray absorption spectroscopy (XAS) and the respective x-ray linear dichroism and x-ray magnetic circular dichroism to characterize the correlation of structural, chemical and magnetic properties of Co-doped ZnO samples. It can be established on a quantitative basis that the superparamagnetic (SPM) behavior observed by integral superconducting quantum interference device magnetometry is not an intrinsic property of the material but stems from precipitations of metallic Co. Their presence is revealed by TEM as well as XAS. Annealing procedures for these SPM samples were also studied, and the observed changes in the magnetic properties found to be due to a chemical reduction or oxidation of the metallic Co species.