Temperature dependence of the magnetization reversal in Co(fcc)–BN–Co(poly hcp) structures

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Abstract
The magnetic properties of multilayer structures with two magnetic layers of the same metal (Co) but with different crystallographic structures separated by an insulating BN layer have been studied. These structures were prepared on Si (001) substrates by a combination of molecular beam epitaxy (metallic layers) and electron cyclotron resonance-assisted sputtering (BN layer). An fcc Co single-crystal layer (60 Å) was first stabilized by growing it on a copper fcc buffer layer and subsequently a polycrystalline Co layer (70 Å) with hcp structure was grown on top of the insulating BN layer. A CoO antiferromagnetic layer, formed adjacent to this hcp Co layer, significantly influenced the magnetic behavior of the polycrystalline hcp Co layer. The magnetic hysteresis loops for these structures were measured at temperatures ranging from 5 to 350 K with the magnetic field applied along the easy (110) in-plane axis of the fcc Co. A very sharp flipping of the magnetization was found for the fcc Co layer with a nearly temperature-independent coercive field that increased from 14 mT below 100 K to 16 mT at 300 K. In contrast, the magnetization reversal in the hcp Co layer was smoother and its coercivity varied significantly with temperature depending on the strength of the exchange coupling with the adjacent CoO layer. At 5 K the coercivity was greater than 0.2 T and decreased with increasing temperature, becoming essentially zero above room temperature. When cooling in a magnetic field, an exchange offset was observed below 150 K that increased to about 0.1 T at 5 K.