Effect of Specimen Geometry on Quantitative EDS Analysis with Four-Quadrant Super-X Detectors

Abstract

Confirming theoretical predictions more than 25 years old, we show that Co slabs are indirectly ex-changed coupled via thin Cu layers with a coupling that alternates back and forth between antiferro-magnetic and ferromagnetic. Four oscillations are observed with a period of = lo A. Moreover, the an-tiferromagnetically coupled Co/Cu superlattices exhibit extraordinarily large saturation magnetoresis-tances at 300 K of more than 65%. PACS numbers: 75.50.Rr, 72. 15.Gd, 75.70.Cn Magnetic coupling between isolated 3d transition-metal ions in paramagnetic host metals via the spin po-larization of the conduction electrons is well estab-lished. ' In particular, various properties of spin glasses such as CuMn can be accounted for by an exchange cou-pling that oscillates in sign from ferromagnetic to anti-ferromagnetic depending on the separation of the mag-netic ions. The period of the oscillation is very short and for simple nearly-free-electron metals is of the order of the Fermi wavelength. However, in contrast, the na-ture of the exchange coupling between ferromagnetic slabs separated by paramagnetic transition metals has long been controversial. Both recent experiments and studies dating back to the 1960s have showed ferromag-netic coupling decaying monotonically with increasing separation of the magnetic slabs for a wide range of sys-tems. While early models purported to explain this nonoscillatory decay, later theories show rather generally that an oscillatory decay of the RKKY (Ref. 7) form is expected. Recently, we observed oscillating magnetic coupling between Co layers separated by thin Ru layers and Fe or Co layers separated by Cr layers. Ru, howev-er, is a relatively complex 4d transition metal and Cr, one of the most unusual transition metals with a compli-cated antiferromagnetic spin-density-wave ground state. ' In this paper we present evidence for an oscilla-tory indirect magnetic exchange interaction in one of the most nearly-free-electron-like transition metals, copper. This is directly manifested as giant oscillations in the saturation magnetoresistance of Co/Cu superlattice structures as the Cu layer thickness is varied. Indeed these structures show the largest saturation magne-toresistance values yet found in any ferromagnetic sys-tem, attaining values of more than 65% at room temper-ature. While the observed oscillation period is short at only =10 A. , nevertheless, the period is about twice as long as the Fermi wavelength of copper and thus surpris-ingly inconsistent with the expected period within the simplest RKKY model. The structures were prepared in a dc magnetron sputtering system at an argon pressure of 3.25 mTorr at a deposition rate of 2 A/sec. The films were grown on chemically etched Si(100) and Si(111) wafers at =40 C. Series of up to twenty structures at a time were grown via computerized control of shutters. The films are polycrystalline with grain sizes of about 200 A as determined from scanning-tunneling-microscopy im-ages. Auger depth profiles of representative samples showed no significant (1 at. %) oxygen or carbon con-taminants within the film structure, although as expected oxygen was found at the silicon/film interface. The layer thicknesses were primarily determined by using a surface profilometer to measure the thickness of nominally 1000-A single-layer samples of the film constituents prepared at the same time. The superlattice periods determined in this way were in good agreement (=10%) with those found from x-ray-diA'raction studies. There is the possibility that strain in the film structures will sys-tematically modify the actual thickness of the layers in the superlat tice from the nominal values. X-ray-diA'raction data show that both the Cu and thin ((20 A) Co layers are fcc and are predominantly (111) tex-tured. Copper-based superlattice structures have been exten-sively studied in the past. '" Recently, evidence for an-tiferromagnetic coupling in single-crystal fcc (100) Co/ Cu/Co superlattices' ' grown on Cu(001) and bcc (001) Fe/Cu/Fe trilayers' grown on Ag(001) has been found for Cu layers 5-8 and 10 monolayers thick, re-spectively. In each case a single antiferromagnetic re-gion was observed with evidence for a crossover from ' or to a ferromagnetic ' region. We have prepared exten-sive sets of both Fe/Cu and Co/Cu superlattices for a wide range of Fe, Co, and Cu layer thicknesses. For the Fe/Cu structures we find antiferromagnetic coupling but only for structures containing ultrathin Fe layers a few angstrom thick. In contrast, the Co/Cu structures show evidence for antiferromagnetic coupling for Co layer thicknesses ranging from just 2.5 to more than 200 A. For Co/Cu superlattices grown on 50-A-thick Cu buAer layers with =104 Cu spacer layers, we find magnetic hysteresis loops similar to those reported for comparable single-crystal Co/Cu superlattices. ' In particular, there is a large remanent magnetization in zero field which we