Nanomagnets in semiconductors and semiconductor interfaces

Cooperative magnetism at reduced dimensions is a challenging field not only from the viewpoint of basic research, but also in prospect for possible new applications. The project aim is to investigate the magnetic phase diagram and evolution of magnetic interactions, investigating (i) superlattices of two-dimensional (2D) magnetic semiconductor layers, (ii) irregular arrangements of small magnetic granules embedded randomly in diamagnetic semiconductors or of irregular 2D magnetic islands at semiconductor interfaces, and (iii) of regular arrays of superparamagnetic dots on semiconductor surfaces. The rich variety of magnetic phases of low-dimensional (layers and islands of) antiferromagnetic EuTe and metamagnetic EuSe has already been calculated in the former work of the applicant. In this project the predicted magnetic phases of EuTe/PbTe- and EuSe/EuTe-superlattices should be measured using ultrasensitive Josephson (SQUID) magnetometry. Imperfections like islands and fractional layers at the interfaces between differently magnetically ordered layers as they exist in a superlattice structure act as superpara-magnetic particles. Their mutual interactions and the crystal field and shape anisotropy influence most of the fundamental micromagnetic properties (magnetization profile, hysteresis loops, metastable states) and can be studied systematically by altering either the shape/size of or the spatial arrangement of magnetic particles. Thus the interparticle interaction of superparamagnetic particles (particle size 10-100 nm) can be intentionally modified, applying lithographic techniques to fabricate regular (1000x1000) dot arrays of circularly shaped magnetic nano-particles of different spacings on a semiconductor surface. Furthermore irregular superparamagnetic islands are studied which are caused by the fractional growth of monolayers (monolayer fluctuations) or by ferromagnetic precipitations in implanted (nonmagnetic) semiconductors. Various magnetometric techniques (susceptibility, field- and zero-field cooled magnetization, thermal modulation) are applied taking into account a wide range of magnetic fields (0 - 7 T) and temperatures (1.6 - 300 K). For future applications the exploration of the magnetic order and magnetic interactions of low-dimensional (natural and artificially tailored) magnetic objects is very important to establish a possible link between magnetism, electronics and optics.

The magnetic behavior of condensed matter is of great interest since ancient time, but its complexity is not yet fully understood. Research areas like solid state physics, chemistry and novel technologies have now achieved the maturity to design materials and structures in very small dimensions for outstanding magnetic properties. Effects like giant magnetoresistance and ultrahigh density magnetic data storage have revolutionized the communication industry. The basis for this achievements is a thorough knowledge of the fundamental interactions in a magnetic body and their evolution if the dimensions of the magnetic object are reduced to the nanometer scale. The project has contributed to a deeper understanding of the rich magnetic phase diagram of metamagnets, artificially fabricated multilayers and ferromagnetic semiconductors by comparing computer simulations with measurements of the magnetization at varying magnetic fields and (low) temperatures. The samples were grown at Linz University and in the USA. The most sustainable improvement of infrastructure was the installation of an ultrasensitive Josephson (SQUID) magnetometer, at the moment state of the art and outstanding in Austria. Magnetometry is an integrating analytical tool which bridges the nanoscopic world of the magnetic atoms with the macroscopic magnetic behavior represented by hysteresis loops and temperature scans. Magneto-optic investigations are more local probes, since optical excitations are selective to the internal electronic transitions of the solid and its constituents. Thus magneto-optic Kerr reflection spectra (measured at Bayreuth University) have been compared with magnetometric data to extract essential parameters and to reveal subtle differences between both experimental methods. Interfaces between magnetic and nonmagnetic species act as an amplifier of magnetic interactions, hence magnetic nano-composites (particles, wires in a matrix) with a high surface to volume ratio offer new unforeseen properties. By the electrochemical deposition of nickel wires in a porous silicon template a novel heterogeneous ferromagnetic semiconductor has been successfully fabricated with ingenious properties, which offers prospects for application as novel high-field magnetoresistive sensors. Nanoscopic magnetic precipitations in steel under severe plastic deformation have been also monitored in order to demonstrate that sensitive magnetometry could be an interesting method for investigating functional materials in industry. The close cooperation with the Austrian Center of Electron Microscopy at TU Graz has strongly stimulated the progress of our work and is gratefully acknowledged.