Inducing Magnetism in Wide Band Gap Hosts
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Inducing Magnetism in Wide Band Gap Hosts Ram Seshadri, Aditi S. Risbud, and Gavin Lawes1 Materials Department and Materials Research Laboratory University of California, Santa Barbara, CA 93106, USA 1 Department of Physics and Astronomy Wayne State University, Detroit, MI 48202, USA ABSTRACT We have used precursor routes to prepare magnetic transition metal ion (tM) substituted wurtzite ZnO powders with up to 15% tM substitution (tM = Co and Mn) on the cation site. Careful magnetic studies reveal these samples show no cooperative magnetic ordering, and certainly no ferromagnetism. Instead, the nearest-neighbor coupling is actually antiferromagnetic. Modeling of the temperature dependence of the magnetic susceptibility indicates the difficulty in inducing ferromagnetism, in keeping with the results of density functional calculations. The alternate strategy of inducing dilute ferrimagnetism in wide band gap spinel hosts with two cation sites has been more successful; dilute magnets based on tM substitution in spinel ZnGa2 O4 seem promising, displaying magnetic hysteresis in nearly transparent samples. INTRODUCTION Dietl et al.[1] have suggested that the substitution of magnetic transition metal ions on cation sites in hole doped wide band gap semiconductors such as ZnO and GaN would give rise to ferromagnetic materials with Curie temperatures of the order of the room temperature. This suggestion has resulted in a number of reports of ferromagnetism in Co and Mn-substituted zinc oxides, particularly in thin film samples. A number of experiments on Mn-substituted ZnO,[2, 3, 4] as well as ZnO substituted with other transition metals[4, 5, 6] have been reported. A recent report on room temperature ferromagnetism in bulk Mn:ZnO prepared using low temperature techniques[7] has also appeared. Ferromagnetism has also been observed in Co:ZnO semiconducting quantum dots.[8] Following these reports, we have developed a precursor route to bulk samples of Co and Mn-substituted zinc oxides in order to characterize the intrinsic magnetic behavior.[9, 10] Since antiferromagnetic interactions are generally stronger in insulating oxides, we have devised a strategy to exploit such interactions in a diluted system. Fe-substituted ZnGa2 O4 is a robust dilute ferrimagnet, showing spontaneous magnetic ordering with hysteresis below about 100 K. EXPERIMENTAL DETAILS We have prepared bulk samples of Zn1−x tMx O (tM = Mn, Co) with x ranging from 0.02 to 0.15. It has been found previously that substitution above these levels leads to the development of a spinel phase related to Co3 O4 when tM = Co[9] so we confine our experiments to smaller values of x. To obtain bulk samples, a single-source crystalline precursor is useful in order to ensure random atomic scale mixing of Zn2+ and Mn2+ /Co2+ ions. The use of oxalate precursors allows the removal of carbon as CO and CO2 , leaving a phase-pure oxide product. Preparation and
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0.5 x = 0.01 x = 0.02 x = 0.03 x = 0.05 x = 0.10 x = 0.15
30000
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