On the sharpness of the interfaces in metallic multilayers

Abstract

We show that the three most relevant magnetic properties (magnetic moment, critical temperature, and interlayer exchange coupling) of metallic multilayers can be reproduced with good accuracy by first principles theory, provided that the picture of atomically sharp interfaces is abandoned and one allows instead for both interface alloying and interface roughness. The interface of a metallic multilayer (exemplified by the Fe͞V system) is demonstrated to, at best, have interdiffusion essentially over two to three atomic layers on each side of the interface. Our conclusions are the result of combining experimental work with theoretical modeling, and we argue that this approach is the best avenue to obtain accurate information about the interface quality of metallic multilayers.T he study of physical properties of multilayers (structural, elastic, magnetic, and transport) is one of the most prosperous and rich branches of materials science today (1-3). By growing different atomic layers on top of each other, new phenomena have been observed, e.g., a reduced critical temperature for magnetic ordering, an oscillating interlayer exchange coupling (4), and giant magneto resistance (5). There are several ways in which these properties are used in nanotechnology, using for instance the giant magneto resistance effect (4, 5) in data storage and computer memories. Single-crystal multilayers represent the highest obtainable degree of perfection, with respect to interface and crystalline quality. This type of multilayer is often referred to as a superlattice.Typically, superlattices are grown by molecular beam epitaxy or sputtering. The growth temperature has to be sufficiently high to ensure large surface mobility and thereby enable layer-bylayer growth (6). At the same time, the temperature must be sufficiently low to prevent the metals from interdiffusing and thereby forming alloys. This delicate balance between surface and bulk mobility is of uttermost importance because, as we shall see, all properties that are special for multilayers (interlayer exchange coupling, magnetic moments, and critical temperatures) depend on the interface quality. Although it has been difficult to assess the interface quality in multilayers, it is safe to conclude that an ideal layer-by-layer growth is never obtained.In magnetic superlattices, interface roughness and interface mixing (7) are the most important types of imperfections. These types are schematically illustrated in Fig. 1. In the figure we specify the geometrical extent of the roughness and mixing via the parameters ⌫ T and ⌫ C , respectively. The mixing is assumed to follow a normal distribution with a width specified by ⌫ C . For the roughness we assume that it is composed of terraces with large lateral extent, such that a multilayer of formal composition, e.g., Fe 3 V 7 , may be thought to be composed of multilayers Fe 2 V 8 , Fe 3 V 7 , and Fe 4 V 6 , with a distribution specified by ⌫ T . For details of how the structure of the interface was constructed mathematically, please...

First principle calculations of core-level binding energy and Auger kinetic energy shifts in metallic solids

“…Used together with Green's functions techniques, this method provides a fast computation alternative compared with the explicit construction of interfaces, atom by atom, in supercell slabs [51]. It can be especially useful if the goal is to capture the general trends over many systems.…”

Sample Preserving Deep Interface Characterization Technique

“…In order to reduce the complexity of the problem into a single characterization parameter which determines the main contribution to the core-level shift average we made the following assumptions about the interface roughness and structure of our samples: (i) the effects on the average core-level shifts from island structures at the interfaces in the sample are small; (ii) the main effect on the average core-level shift is due to intermixing of Cu and Ni atoms around the interfaces; and (iii) the spatial extent of the diffusion at an A=B interface may be described by a Gaussian distribution function with standard deviation ÿ. This analysis has been successful in previous studies on magnetic properties [1] and makes a comparison between measurements and calculations realizable.The layer-dependent concentration profile around one A=B interface is obtained as an integral over the Gaussian …”

“…Moreover, while other techniques such as scanning tunneling microscopy and atomic force microscopy provide local structural information, these methods are generally limited to surface analysis and cannot reliable describe buried interface qualities [10,11]. Surface sensitivity is also strongly associated with photoelectron spectroscopy due to the short mean free path of electrons with kinetic energies in the range of 50 -500 eV [12].In this Letter we propose a new nondestructive characterization technique to analyze the interfacial quality of layered structures based on previous theoretical predictions [1,13]. The method relies upon accurate measurements of chemical shifts in core levels [14,15] by means of highkinetic-energy photoelectron spectroscopy.…”

“…In this Letter we propose a new nondestructive characterization technique to analyze the interfacial quality of layered structures based on previous theoretical predictions [1,13]. The method relies upon accurate measurements of chemical shifts in core levels [14,15] by means of highkinetic-energy photoelectron spectroscopy.…”

“…DOI: 10.1103/PhysRevLett.97.266106 PACS numbers: 68.35.Ct, 02.70.Hm, 61.10.Ht Thin film nanodevices such as multilayers and superlattices represent numerous new opportunities in materials science. Their unique properties arise from the introduction of interfaces and the emergence of finite size effects due to quantum interference [1][2][3][4][5][6]. The interfaces represent tunable parameters, where interface composition or film thickness and geometry can be used to alter various magnetic, optical, and mechanical properties.…”