The magnetic behavior of the bordering Ni and Cu monolayers at the interface of ultrathin Cu films on Ni(lll) has been investigated. m In probes have been used, utilizing perturbed yy angular correlations, to study strength, orientation, and temperature dependence of the magnetic hyperfine field at the probe nuclear site. This magnetic hyperfine field is strongly reduced to about 50% in the first Ni layer covered by Cu; it vanishes in the adjacent Cu layer. The results are compared to hyperfine fields at free Ni(l 11) surfaces, to magnetometry measurements, and to theoretical predictions.PACS numbers: 75.70.Cn, 76.80.+yThe magnetic properties of ferromagnets covered by nonmagnetic materials and the magnetic polarization of these coverages has become a matter of considerable interest in both experiment and theory. The magnetic interaction is generally present only over a short range at the interface between magnetic and nonmagnetic materials. Therefore, detailed investigations require experimental methods with monolayer resolution. Interfaces buried below very thin overlayers can be studied by spinand element-sensitive surface methods 1 like spin-resolved Auger electron or photoelectron spectroscopy. However, if the interface is covered by a thicker film, only hyperfine techniques can be utilized.So far, only Mossbauer spectroscopy at 57 Fe probe atoms has been used to study interface magnetism with monolayer resolution. The magnetic hyperfine field Z?hf at Fe(l 10) surfaces is increased by Ag coverage. 2 Similarly, the interfaces of 4-monolayer (ML) Fe films sandwiched in Au show enlarged hyperfine fields as compared to the central layers. 3 The perturbed yy angular correlation (PAC) technique, which-like Mossbauer spectroscopy-is based on the hyperfine interaction of probe nuclei with extranuclear electromagnetic fields, has a high potential for surface 4 and interface 5 studies. There are certain aspects which make the PAC method extremely favorable as a complementary approach to study interface magnetism with monolayer resolution. In this Letter we shall report on the first application of PAC to magnetism at interfaces.In contrast to the Mossbauer experiments performed so far, where about 1 ML of 57 Fe probes is needed, in PAC the concentration of probes is reduced by orders of magnitude to about 10 ~4 ML. This, on the other hand, makes it possible to study individual monolayers at surfaces, interfaces, and thin films of many materials, since they can be labeled with radioactive PAC probes acting as isolated observers. In addition, the superior sensitivity of PAC to structural information via the electric-field gradient permits the determination of probe sites. 6 For the present PAC experiments we utilize the isomeric nuclear state (/=y, ^1/2 = 84 ns) of the 171-245-keV yy cascade in m Cd, which is populated by electron capture of m In (/1/2 =2.8 d). The detection of y\ selects a subgroup of states within the isomeric nuclear level, resulting in an anisotropic 72 emission in delayed coincidence with respect to...