Originally it was believed that the presence of heavy-mass charge carriers at low temperatures in some special-rare-earth or actinide compounds was simply the results of a suppression of magnetic order in these materials. Various experiments reveral, however, that magnetic order may occur from a heavy-electron state or that a heavy-electron state may also develop within a magnetically ordered matrix. It turned out that pure compounds witout any sign of a cooperative phase transition down to very low emperatures are rare but examples are known where microscopic experimental probes give evidence for strong magnetic correlations involving moments of much reduced magnitude (=< 0.1 mu _B) in such cases. It appears that elecrtronic and magnetic inhomogeneities both in real and reciprocal space occur which are not simply the result of chemical inhomogenities. Long range magnetic order among strongly reduced magnetic moments seems to be a particular feature of some heavy-electron materials. Other examples show, that disorder may lead to a suppression of cooperative phase transitions and both macroscopic and microscopic physical properties indicate that conservative model calculations are not sufficient to describe the experimental observations. The main difficulty is to find a suitable theoretical approach that considers the various interactions of similar strength on an equal footing. Different examples of these various features are demonstrated and discussed.
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