Monolayer-resolved detection of magnetic hyperfine fields at Cu/Ni(111) interfaces

Abstract

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...

Static Magnetic Hyperfine Fields in Magnetically Polarized Pd

“…The absolute value of B hf at 111 Cd in the Pd adlayer is tentatively even stronger as compared with the value in the topmost layer of Ni, indicating that the magnetic polarization is rather strong in the isoelectric Pd layer with direct contact to Ni. At a comparable position in the Ni/Cu system B hf is zero [20]. The strong polarization in the adlayer propagates the polarization into the second Pd layer with a smaller, although still measurable value.…”

Coordination-Number Dependence of Magnetic Hyperfine Fields atC111don Ni Surfaces

“…One way to measure magnetic properties on the atomic scale is offered by the use of radioactive probe atoms and the observation of their interaction with the immediate environment. Applying hyperfine-interaction techniques, monolayer-resolved studies are possible [2,3] because of the short range of the hyperfine interaction, where essentially only the nearest neighbors (quantitatively expressed in the coordination number NN) contribute to the interaction. Nuclear methods, in particular, the perturbed angular correlation spectroscopy (PAC), have an extremely high sensitivity.…”

“…A distinction of the different fractions is obtained by the observation of their increase and decrease with increasing temperature. By this we follow a well established procedure for the same PAC probe with measured EFGs on similar surfaces, e.g., Ni(111) [2,12]; Cu(111), Cu(001) [8]; Pd(111) [14], Pd(001) [15]. At room temperature, the probes are incorporated into steps or even into the terraces.…”

Magnetic properties of thin Ni films measured by a dc SQUID-based magnetic microscope

“…15 It has been suggested that the magnetic moment decreases only in the first Ni layer of a single crystal covered by Cu layers. 16 Nevertheless, a large discrepancy still exist between the experimental data 17 and the theoretical estimates of the magnetic moment in the few monolayer limit. 14,15 It has been observed that the orientation of magnetization in epitaxial Ni films depends on the film thickness.…”