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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...

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Specific heat, low-and high-field magnetic susceptibility, thermal expansion, and magnetostriction measurements have been made on single crystals of CeBi and CeSb in order to investigate the low-temperature magnetic phase transitions of these compounds. We have observed structural transitions at the respective N~el temperatures. The maximum tetragonal distortions below T N are of the order of 10-3 in both cases. Specific heat results confirm the unusually small overall splitting of the ground-state multiplet. Applications of external magnetic fields induce a variety of interesting effects and provide insight into the magnetic phase diagram of these substances. The results are discussed qualitatively.

Using spin-polarized secondary and Auger electron spectroscopy we find an induced magnetization in epitaxially grown V adlayers on Fe ͑100͒. Spin-dependent attenuation of the secondary electrons is quantitatively treated, following a model by Siegmann, to determine the adlayer magnetization. The first monolayer of V has a negative magnetic moment of Ϫ0.3Ϯ0.08 B per atom and thus couples antiferromagnetically to the Fe substrate. Subsequent V layers exhibit a positive magnetization. The reduction of the magnetization of Fe at the interface is found to be small. We determine an upper bound ͑M bulk ϪM interface ͒/M bulk Ͻ0.2 of the relative demagnetization of Fe upon V adsorption.Induced magnetic ordering in ''nonmagnetic'' thin films deposited on the surface of a ferromagnet has attracted considerable interest. A series of recent studies include Cr, Mn, V, as well as Ru, epitaxially grown on Fe ͑100͒. An induced magnetic moment unambiguously is found which in the coverage range of the first monolayer ͑ML͒ is oriented antiparallel to the magnetization of the substrate for the 3d-metal adlayers 1-5 and parallel for Ru. 6 For thicker adlayers an antiferromagnetic arrangement of adjacent ferromagnetic ͑100͒ sheets of about one monolayer thickness has been reported to occur in Cr ͑Ref. 2͒ and Mn, 3 whereas for V ͑Ref. 5͒ and Ru ͑Ref. 6͒ the moments basically are confined to the layers at the interface. Comparison of these experimental observations with computational predictions is a real challenge, however, if quantitative measurements of the induced magnetic moments are available. The only one in these systems to our knowledge is on Cr/Fe ͑100͒ by Turtur and Bayreuther 4 using alternating gradient magnetometry. It is the purpose of the present study to provide a further quantitative analysis of an induced magnetization in an adlayer on a surface of a ferromagnet. As a test system we choose V/Fe ͑100͒. Among the 3d metals V points towards less filled d bands and thus is an interesting candidate for studying effects of proximity to a ferromagnetic surface and altered atomic coordination.V at V/Fe interfaces of slabs and multilayers is predicted to carry an induced magnetic moment antiparallel to the Fe magnetization, 7-9 accompanied by a sizeable demagnetization of the Fe atoms at the interface. Computations of induced magnetizations in further V layers away from the interface also exist 7,8 and yield decreasing magnetic moments with increasing distance from the Fe interface. An induced moment in V antiparallel to the Fe magnetization is inferred from NMR measurements on ͑110͒ oriented multilayers. 10 Recently Walker et al. have found antiferromagnetic coupling of the first V monolayer to the Fe substrate and parallel alignment of the second one. 5 A number of Mössbauer studies on ͑110͒ oriented multilayers are claimed to confirm the calculated reduction of the Fe magnetization at the interface. 11 In this paper we present a quantitative study of thin V layers deposited on a bulk Fe ͑100͒ substrate. We use the combinatio...