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with magnitudes of 5% to the 3D grid nodes. Synthetic data were calculated for the checkerboard model. Then we added random errors to the synthetic data and inverted them with the same algorithm that we used for the observed data. The inverted image of the checkerboard suggests where the resolution is good and where it is poor. The checkerboard resolution tests and other synthetic tests we conducted showed that both the high-velocity Tonga slab and the low-velocity back arc and mantle wedge were reliably resolved and that there was no trade-off between them. 12. Y. Tatsumi, J. Geophys. Res. 94, 4697 (1 989); J. H. Davies and D. J. Stevenson, ibid. 97, 2037 (1992). 13. Y. Zhang and T. Tanimoto, Nature 355,45 (1 992); T. Tanimoto and D. J. Stevenson, J. Geophys. Res. 99, 4549 (1 994) 14. Y. Shen and D. W. Forsvth, J. Geo~hys. Res. 100, 221 1 (1 995). 15. Y. Xu and D. Wiens, ibid., in press. To invert regional wave forms we used a nonlinear inversion method that adopts a reflectivity formalism (28) to compute the partial derivatives. This method allows the entire regional distance (400-to 1500-km range) wave form to be inverted from P wave arrival to surface waves at frequencies between 0.01 and 0.055 Hz. Broadband seismograms from earthquakes of 10 to 240 km deep that propagate almost entirely within one of the tectonic regions of the southwest Pacific were used in the wave form inversion. Parameter variances and resolution tests suggest that the results are well constrained to depths of about 200 km. The S wave velocities from the wave form inversion and the P wave velocities from the tomography would not necessarily show the same structure. The larger total heterogeneity from the wave form inversion may result from a greater effect of partial melt beneath the Lau back arc on S wave velocity than on P wave velocity (9, 29).

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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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