Intrinsic and Heat-Induced Exchange Coupling through Amorphous Silicon

Abstract

We show that ferromagnetic films separated by a spacer of amorphous Si are exchange coupled for Si thicknesses ds;~40 A. For 14 A~ds, & 22 A we observe antiferromagnetic coupling. The coupling strength of approximately 5 X 10 6 J/m-' is strongly temperature dependent with a positive temperature coefficient. We suggest that localized electronic defect states in the gap of amorphous Si mediate the exchange interaction. The particular coupling mechanism encountered here also works with noncrystalline ferromagnetic layers. PACS numbers: 75.30.Et, 75.70.Fr, 78.66.Sq The discovery of oscillatory exchange coupling in magnetic multilayers [I] and the subsequent observation that this phenomenon exists for a wide variety of transitionand noble-metal spacer materials [2] have provoked a true renaissance of both theoretical and experimental research in magnetism. The mechanisms thought to be responsible for the coupling phenomena [3] all rely on two main properties of the spacer material: its metallic character and crystalline orientation.One might thus be lead to the conclusion that both are necessary requirements for the existence of exchange coupling in layered systems.However, we have shown that exchange coupling multilayers also exists for a different class of spacer materials: amorphous semiconductors [4] and insulators [5]. In this Letter we report a pronounced temperature dependence of the exchange coupling through amorphous Si. The observed positive temperature coefficient gives clear evidence of semiconducting spacer behavior. New measurements on Fe/a-Si/Fe trilayers confirm that ferromagnetic (FM) as well as antiferromagnetic (AFM) coupling occurs depending on the thickness of the Si layer. Over the entire thickness range up to 40 A we find only one antiferromagnetic region. AFM-coupled trilayers display a very low coupling strength of 5 x 10 6 J/m, which is at least 2 orders of magnitude smaller than the values observed on metallic Fe-silicide multilayers [6]. Thermal activation in general is found to increase the coupling strength in both AFM and FM regions, and in special cases it can even change the sign of the coupling. We wish to strongly emphasize that our earlier observations of lightinduced exchange coupling through a-Si [7] as well as a-SiO [8] were based on an incorrect temperature determination.Extensive investigations revealed that in the above cases all the changes of the coupling upon light irradiation must indeed be attributed to sample heating by the light source. On the other hand, no light effects can be seen when the samples are kept at constant temperature. This observation clearly is in contrast to the recent report by Mattson et al. [9], who have found photosensitive coupling in multilayers with spacers composed of an unknown mixture of Fe silicides.Based on the observed strong temperature dependence we infer that the coupling mechanism encountered with nonmetallic spacers is different from the one commonly seen in metallic multilayers.We propose that localized electronic states in the ga...

Ab Initio Study of Electric Transport and Interlayer Exchange Coupling in Fe-Si-Fe Systems

“…Multilayer systems consisting of Fe/Si or Fe/FeSi are candidates for new electronic devices, because it is possible to change the size of the magnetic coupling by light [Mattson et al (1993)] or heat [Briner and Landolt (1994); Inomata et al (1995); Highmore et al (1995)]. Therefore, it is necessary to understand which features in ‡uence the associated variables.…”

Magnetic coupling in Co/Cr2O3/CrO2 “trilayer” films

“…[2][3][4][5] This coupling sometimes appears to be distinct from the very low temperature tunneling phenomena between two ferromagnets, through a dielectric spacer layer, [6][7][8][9] as the coupling is sometimes oscillatory. [2][3][4][5] There is an increasing body of evidence that impurities, in the insulating layer, will ''dope'' the insulating layer and alter the net polarization of electrons injected into the insulating layer, [10][11][12][13][14][15][16][17] with strong temperature effects, 12,14 most recently demonstrated in Cr 2 O 3 .…”

“…16,[20][21][22] From the outset, i.e., from the initial stages of surface preparation, the core level binding energies indicated that the stable surfaces were Cr 2 O 3 . The native Cr 2 O 3 surface on CrO 2 22 was used as a native insulating spacer layer between CrO 2 and Co, as has been undertaken elsewhere. 19 Samples were fabricated with 20-, 40-, and 60-Å-thick are nominally antiferromagnetic, with Néel temperatures around 297 and 307 K, respectively, by considering the fact that Néel temperature decreases in very thin films, the samples studied in this work appear to behave much like a ferromagnetic/paramagnetic insulator/ferromagnetic trilayer system in the temperature range of 240-400 K. We used in situ longitudinal MOKE to obtain the hysteresis loops, before and after cobalt deposition, as described elsewhere.…”

Features of the magnetic properties of Co/Si/Co thin-film systems

“…The most inter esting experimental investigations of the structural and magnetic properties have been reported for Fe/Si [7⎯11] and Co/Si [10][11][12][13][14][15][16][17][18][19] thin film systems. Analysis of the available data showed that the antiferromagnetic exchange between iron layers in Fe/Si/Fe systems was observed for silicon layer thicknesses within 1.4-1.7 and 1.4-2.2 nm (see, e.g., [20,21]). However, data on the exchange interaction between magnetic layers via nonmagnetic spacers in Co/Si/Co thin film systems are rather contradictory.…”

“…As for the mechanism of exchange coupling between magnetic layers in the presence of a semiconductor spacer, it differs from the case of a metallic interlayer. According to the existing notions [20,21], a possible mechanism of this exchange in Fe,Co/Si/Fe,Co thin film systems is related to the tunneling of polarized electrons via localized defect states which are always present in the semiconductor layer.…”