Magnetism of manganites, semiconductors and spin glasses

Sammanfattning: Magnetic and electrical properties of selected compounds containing manganese (Mn) are investigated by SQUID magnetometry and transport measurements.(Ga,Mn)As is a magnetic semiconductor obtained from GaAs by substituting Ga3+ for Mn2+. Mn acts in the alloy as a magnetic impurity, as well as a hole dopant. A carrier mediated ferromagnetic interaction is observed in (Ga,Mn)As single layers, as well as in (Ga,Mn)As/GaAs superlattices. The magnetic and electrical properties of these structures are controlled by the amount of holes, and thus by the amount of compensating defects such as AsGa antisites. Magnetic inhomogeneity appears for thin layers as well as for layers containing large concentration of Manganese.In non magnetic metallic elements containing a small amount of manganese impurities, a magnetic interaction develops, oscillating in sign with the distance between Mn atoms. Due to random distribution of manganese in a Ag(Mn) alloy, competing ferromagnetic and antiferromagnetic interaction appears, yielding magnetic frustration and the appearance of a spin glass phase at low temperature. These disordered systems show aging, chaos and memory phenomena, which are investigated in the three dimensional Ag(Mn) and Fe0.5Mn0.5TiO3 spin glasses using time dependent magnetization measurements.Perovskite manganites of type (R3+1-xA2+x)MnO3 show colossal magnetoresistive e_ects (CMR). For an optimum doping x, a ferromagnetic order is established, and large changes of their electrical resistance with an applied magnetic field are observed; a magnetoresistance which can be tailored by adding oriented grain boundaries in thin films of these materials. The Manganese appears in the system as Mn3+ and Mn4+, and both ferromagnetic and antiferromagnetic interaction is mediated by the charge carriers along the Mn-O-Mn bonds of the perovskite structure. Depending on the cations forming the manganite, and their relative amount, glassy dynamics may appear, yielding aging and memory features similar to those observed in spin glasses.