Hyuk J. Choi

Quantum-well states in copper thin films

A standard exercise in elementary quantum mechanics is to describe the properties of an electron confined in a potential well. The solutions of Schrödingers equation are electron standing waves—or ‘quantum-well’ states—characterized by the quantum number n, the number of half-wavelengths that span the well. Quantum-well states can be experimentally realized in a thin film, which confines the motion of the electrons in the direction normal to the film: for layered semiconductor quantum wells, the aforementioned quantization condition provides (with the inclusion of boundary phases) a good description of the quantum-well states. The presence of such states in layered metallic nanostructures isbelieved to underlie many intriguing phenomena, such as the oscillatory magnetic coupling of two ferromagnetic layers across anon-magnetic layer, and giant magnetoresistance. But our understanding of the properties of the quantum-well states in metallic structures is still limited. Here we report photoemission experiments that reveal the spatial variation of the quantum-well wavefunction within a thin copper film. Our results confirm an earlier proposal that the amplitude of electron waves confined in a metallic thin film is modulated by an envelope function (of longer wavelength), which plays a key role in determining the energetics of the quantum-well states.

Step-induced magnetic anisotropy in Co/stepped Cu(001) as a function of step density and Cu step decoration

The step-induced in-plane uniaxial magnetic anisotropy of fcc Co/stepped Cu(001) was investigated using a curved substrate to provide a continuous range of vicinal angles from 0° to 6°. The anisotropy strength was found to depend linearly on the step density, indicating that the biaxial strain does not make a significant contribution to the step-induced anisotropy. Using a side growth geometry to decorate the Co step edges with Cu adsorbates, we observed that the step-induced anisotropy strength approaches zero at roughly 0.7 atomic rows of Cu, independent of step density.

MAGNETIC PROPERTIES OF ULTRATHIN FE FILMS GROWN ON STEPPED W(001) AND PD(001) SUBSTRATES

In both Fe/W(001) and Fe/Pd(001) systems, the atomic steps induce an in-plane uniaxial magnetic anisotropy with the easy magnetization axis perpendicular to the step edges. The strength of the step-induced anisotropy was found to have a power law dependence on the step density: a quadratic dependence in the Fe/W system but a linear dependence in the Fe/Pd system. In addition, the Curie temperature is found to be higher on the stepped surface in the Fe/Pd system as compared to the flat surface. The enhancement of the Curie temperature is attributed to the step-induced Pd moments which is supported by the increased surface magneto-optic Kerr effect signal on the stepped surface. No such enhancement of either Curie temperature or magnetic moment was observed in the Fe/W system.

Modification of the magnetic properties of Fe/Cr(001) by controlling the compensation of a vicinal Cr(001) surface

The degree of compensation of a normally uncompensated Cr(001) surface is controlled by using a curved substrate with steps parallel to the [100] direction. In this way, the degree of frustration caused by steps at the interface between an Fe overlayer and the Cr substrate can be systematically varied. Previous work on flat Cr(001) at temperatures below the Cr ordering temperature (311 K) has identified a critical Fe thickness of ∼35–38 A, below which the Fe films display a reduced remanence. For our curved Cr substrate, below this critical Fe thickness three phases are observed for low ( ∼5°) miscut angle respectively: (i) multidomain; (ii) single domain with magnetization perpendicular to the step edges; and (iii) single domain with magnetization parallel to the step edges. At the same temperature, for Fe films above the critical thickness, region (i) disappears and only regions (ii) and (iii) remain. In a second experiment, the adsorption of submonolayer Au on the Fe is ...

Role of film roughness and interdiffusion in the formation of nonferromagnetic fcc Fe in the Fe/Co(100) system

The fcc Fe/Cu(100) and Fe/Co(100) systems are characterized by a wide range of magnetic and structural phases. In particular, a nonferromagnetic fcc phase with a live layer has been observed for room temperature growth Fe films in the ∼5–11 ML thickness range. This nonferromagnetic phase is not present for low temperature (∼120 K) grown films even when the film temperature is raised to room temperature. Annealing the film at 475 K, however, will recover the room temperature phase. Two effects that could account for these features are interdiffusion and surface smoothening. In order to determine which of these effects is responsible for the occurrence of the nonferromagnetic phase, we performed experiments on the Fe/Co(100) system to separate these two effects. An artificially roughened sample grown at room temperature exhibits a ferromagnetic phase only. A sample grown at low temperature in which the first few layers are alloyed to simulate interdiffusion also shows the ferromagnetic phase. Experiments in...