Structural Coherence and Magnetic Coupling in Fe/Si
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ABSTRACT A number of studies have inferred the presence of an Fe-silicide in Fe/Si multilayers. Our transmission electron diffraction data provide direct evidence for the presence of an Fe-silicide. Despite similarities in structural coherence and saturation magnetization behavior for Fe/Si and Fe/{FeSi}, direct evidence for Fe-silicide only occurs for the Fe/{FeSi} multilayers.
INTRODUCTION The properties of Fe layers separated by Si spacer layers are a topic of current interest. Prior to the early 1990's studies had involved thick Si layers, 35A or greater, with varying Fe layers thicknesses and a consistent pattern of properties had emerged. Fe layers >20A were isolated crystalline units having a reduced magnetization and there was no magnetic coupling between these layers. As the layer thickness dropped below 20A these Fe layers became amorphous and near a thickness of 12A they ceased to be magnetic [1]. Recently a number of studies have found quite different properties when the spacer layer is thin. Toscano, et al. found evidence of an oscillating magnetic coupling in evaporated FeSiFe trilayers when the Si layer thicknesses were in the range of 7 to 30A [2]. A group at Argonne reported both a non-oscillating, antiferromagnetic coupling and a structural coherence between Fe layers for sputtered Fe/Si multilayers with thin Si spacer layers [3]. The magnetic coupling was a maximum for 14A Si layers and structural coherence was lost as the Si layer thickness increased above 17A. The Argonne group subsequently reported antiferromagnetic coupling through a thicker spacer layer and structural coherence up to spacer layers of 40A when the spacer was a mixture of Fe and Si [4]. There are independent reports that this magnetic coupling can be modified optically [3,5]. More recently, Inomata, et al. studied electron transport in sputtered Fe/Si multilayers and reported a magnetoresistance (MR) that is negative and has a significant change in its temperature dependence as the Si layer exceeds 15A [6]. They also claim a change in coupling from ferromagnetic to antiferromagnetic at room temperature for their samples. The structure of these thin Si spacer layer samples is not well determined. Toscano, et al. explain the magnetic coupling in the FeSiFe trilayers by claiming that the Si layer is an amorphous semiconductor [2]. The Argonne group use a number of properties to infer that the coupling and structural coherence are related to the formation of Fe-silicides [3,4]. Inomata, et al. use the temperature dependence of magnetic properties to infer that their Si spacer layers are a narrow gap Fe-silicide, e-FeSi, when the Si layer is less than 15A and is a combination of this silicide and amorphous Si when the Si spacer thickness is greater than 15A [6]. In an earlier study we used transmission electron diffraction (TED) in an attempt to obtain direct evidence for Fe-silicides in sputtered Fe/Si and found no such evidence [1]. In the present paper we report the extension of our TED study to Fe/{FeSi} multilayers: samples f
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