Nucleation and Growth Mechanism Causes Switching of Exchange Bias in Double-Superlattice System
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ature (9–21 K) using electrochemically etched tungsten wires for analyzing tips, in order to record the diffusion of single copper adatoms on the close-packed Cu(111) substrate. They prepared their samples by evaporating ~0.01 monolayer of copper at 15 K, a temperature at which single copper atoms are mobile, on Cu(111) substrates that were previously cleaned by several sputter and anneal cycles. The researchers analyzed STM images showing two Cu adatoms separated by distances of up to 70 Å. More than 65,000 Cu spacing distances from a total of 3400 images were analyzed. The researchers established an oscillatory behavior of the potential energy, with a periodicity of λ F/2, and an envelope that decays as ~1/d2 for large separation d. The method used to determine the interaction potential was to extract it from the measured pair distribution, obtained from the timedependence of the distance between two adatoms, correcting for geometrical effects inherent to the measuring process. The discrepancies between their experimental results and the previous theoretical descriptions of the phenomenon will help to re-evaluate the assumptions made as well as neglected terms in theoretically describing the potential energy between adatoms, and will also help to understand the growth of Cu on Cu(111) at low temperatures. CLAUDIU MUNTELE
sequence [Fe(50 Å)/Cr(20 Å)]F 5/[Fe(14 Å)/ Cr(11 Å)]AF20. A 20-Å layer of Cr was sandwiched between the F and AF superlattices to provide a ferromagnetic intersuperlattice coupling. An artificial exchange bias (uniaxial anisotropy) was built into the system by epitaxially growing the sample onto a single-crystal MgO(110) substrate. Magneto-optic Kerr effect measurements were performed around two critical values (-406 Oe and -447 Oe) of the turning field in the field loop, Hmin. The hysteresis loop obtained for the -406 Oe measurements is narrow, with a bias around -38.5 Oe; the magnetization in the F superlattice reverts to its original orientation at -33.6 Oe. However, with Hmin at -447 Oe, the F superlattice magnetization does not revert to its original orientation until 40.7 Oe, indicating that the AF superlattice has reversed its direction. Polarized neutron reflectivity measurements confirm the reversal of the AF superlattice. The fact that the bias direction switches at a value of -447 Oe, which
is much lower than the 14-kOe field required to saturate the AF superlattice, or the 2-kOe field needed to initiate spinflop transitions, indicates that a different mechanism is at work. A nucleation and growth scenario is consistent with these results; further work is under way to determine the field-dependence of the magnetic layer structure more precisely. TIM PALUCKA
Thin Films of α-Al2O3 Result from the Use of an Alternative Anhydrous Solution
Thin films of crystalline α-Al2O3 are obtained through an innovative sol-gel process developed by Naoufal Bahlawane and Tadahiko Watanabe at the Kyushu National Industrial Research Institute in Japan. This process has the advantage of reducing the tra
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