Asymmetries were observed across a monolayer of dimethylanilinoaza[C 60 ]fullerene, (DMA-NC 60 , 1) sandwiched between gold electrodes of relatively large size (0.265 mm 2 ). Two modes of behavior are observed: (1) a sigmoidal and slightly asymmetric behavior, bespeaking of a moderate unimolecular rectifier (rectification ratio of about 2), and (2) above a threshold voltage V 1 (≈0.6 to 1.0 V), a dramatic increase of current to 0.3 to 1 A (as high as 1.36 × 10 7 electrons molecule -1 s -1 at 1.5 V), followed by ohmic behavior from V 1 to a relatively smaller negative bias V 2 (≈ -0.5 V to -0.6 V). At more negative potentials (e.g., at -1.5 V) the current is very small (a few µA). This high asymmetry in current persists for between 10 and 20 cycles of voltage scan. This increased, but ohmic conductivity is probably due to defects that grow at domain boundaries, since this behavior is not seen when very small electrodes (1 µm 2 area) are used. The defects could be stalagmitic filaments of gold which grow from the bottom electrode above V 1 but are broken at the negative bias V 2 , or else they could be due to some unknown electrochemical couple. This device is vaguely reminiscent of Zener diodes or varistors: if operated between, say, + 2 V and -2 V, it is a super-rectifier, with a rectification ratio of up to 20 000 at 1.5 V.

We have used molecular-beam epitaxy to grow high-quality pseudomorphic Ni and Co1Ni9 films on Cu(001). From temperature-dependent surface magneto-optic Kerr effect measurements of these films, we have determined the finite-size scaling behavior of the Curie temperature of ultrathin films for a thickness range of n=2.5–16 monolayers (ML). The film thickness dependent Curie temperature for each of these ferromagnetic thin-film systems, TC(n), is described by a finite-size scaling formula: [TC(∞) − TC(n)]/TC(n) = [(n − n′)/n0]−1/ν, where TC(∞) is the bulk Curie temperature, n0=2.5±0.5 ML for Co films and 3.5±0.4 ML for Ni and Co1Ni9 films is the microscopic length scale, and ν=0.76±0.08 is the bulk correlation length exponent. An interesting result is that TC(n) extrapolates to zero in the single mononolayer limit, n′=1.

Angle-resolved photoemission is used to determine the energy bands of permalloy (Ni 0.8 Fe 0.2) and compare them to Ni, Co, and Cu. The energy and momentum resolution ͑Ϸ0.01 eV and Ϸ0.01 Å Ϫ1 ͒ is high enough to resolve the magnetically split bands at the Fermi level that are responsible for spin-dependent conductivity and tunneling. For the ⌺ 1 band we find the magnetic exchange splittings ␦E ex ϭ0.27 eV ͑0.23 eV for Ni͒, ␦k ex ϭ0.16Ϯ0.02 Å Ϫ1 (0.12Ϯ0.01 Å Ϫ1 for Ni͒, the Fermi velocity v F↑ ϭ(0.22Ϯ0.02)10 6 m/s (0.28ϫ10 6 m/s for Ni, 0.33ϫ10 6 m/s for fcc Co͒, and the widths ␦k ↑ р0.11 Å Ϫ1 and ␦k ↓ ϭ0.24 Å Ϫ1. Compared to Ni, permalloy features a 27% larger magnetic splitting of the Fermi surface and an extremely short mean free path of 4-8 Å for minority spins.

The magnetization reversal in four arrays of micron-size circular holes ͑antidots͒ in a Permalloy film has been studied by means of quantitative magneto-optic Kerr vector magnetometry and magnetic force microscopy. The primitive antidot meshes of the arrays investigated here can be classified as square, rectangular, hexagonal, and oblique. The vector magnetometry data show that the hole arrays induce a magnetic anisotropy completely different from that of the unpatterned film, with new hard axes along the directions connecting nearest neighboring holes. Also the coercive field is strongly affected by the pattern. The results of the vector magnetometry analysis indicate that the reversal process takes place through a collective and periodic domain nucleation and expansion process. The domain structure in the remanent state has been investigated by magnetic force microscopy imaging. The images display well-defined domain structures, which are periodic and commensurate with the holes array.