5.5 Steps, ripening and interdiffusion	179

pressed Ag(111). Other Ostwald ripening experiments of a similar nature have yielded Q5 0.766 0.04 eV and ES 5 0.226 0.01 eV for adatoms on Cu(111) (Giesen & Ibach 1999). These authors and their co-workers have also observed step ¯uctuations, and rapid decay processes when multilayer islands coalesce, and have recently invoked electronic mechanisms in explanation. Thus, as we have seen in the previous two sections, research on atomistic processes at metal surfaces has encountered the need for elec- tronic structure calculations in the search for complete explanation. The background needed for this understanding is given in section 6.1.

5.5.3Interdiffusion in magnetic multilayers

Magnetic multilayers are typically formed by interspersing a magnetic metal (Fe, Ni, Co, Cr, etc.) with a non-magnetic spacer, often a noble metal (Cu, Ag, Au, Pt etc.). The sequence Co/Cu/Co . . . for example has a giant magnetoresistance whose properties are controlled by the various layer thicknesses and perfection; there are many such systems, whose properties have been extensively reviewed in the last few years, as discussed later in sections 6.3 and 8.3. In this section we concentrate on the growth mode, taking Fe/Ag/Fe(110) as the example.

As described in section 5.4.1, Ag/Fe(110) is a typical SK growth system, with two layers before islands form, the ®rst of which has the c531 structure, which has a nominal coverage of 0.8ML, and the second is close to a compact Ag(111) layer. Auger amplitudes from this structure have been measured (Noro et al. 1995, Venables et al. 1996, Venables & Persaud 1997); there is nothing unusual about the Ag/Fe(110) interface. However, deposition of Fe on thin ®lms of Ag/Fe(110) results in some interdiVusion, the extent of which depends on the Ag ®lm thickness, deposition and annealing conditions. An example is shown in ®gure 5.21, where the ratio of Ag/Fe AES intensities is plotted against Fe coverage, and is compared with a layer growth calculation (Persaud et al. 1998).

The lower curves are calculated assuming no surface segregation, the two curves re¯ecting some uncertainty in the correct inelastic mean free path for the Auger electrons. For deposits of under 1 ML at room temperature, the data follow this layer growth curve, more or less. But between 1 and 2 ML, there is clearly some segregation, where the calculation assumes that all of the ®rst 0.8 ML Ag has moved to the surface; this is clearly not a bad approximation. But annealing to around 250°C results in more segregation, and deposition at 250°C results in almost complete segregation. Results for other Ag layer thicknesses show a similar trend: interdiVusion at the ML level proceeds even at room temperature, and there is long range interdiVusion already at a few hundred degrees Celsius.

From the arguments given in section 5.1.1, we can see that metal deposition systems should follow the island growth mode, if the surface energy of the deposit (Fe ,2.9 J/m2) is greater than that of the substrate (Ag ,1.2 J/m2); surface energy values are

discussed and tabulated in section 6.1.4. Thus islands of the strongly bound material, Fe, once formed, could lower their energy by allowing themselves to be coated with a thin skin of Ag substrate material! This corresponds to a curious form of

180 5 Surface processes in epitaxial growth

			SUBSTRATE: 1.8ML Ag/Fe(110)								1.4		SUBSTRATE: 5ML Ag/Fe(110)
						RT deposition										RT deposition
						250oC deposition										250 oC deposition
						Annealed to 250oC					1.2					RT dep, anneal 250 oC
	0.6					Annealed to 300oC											"		300 oC
																250 dep, ann 250oC
Intensities											1.0						"	300oC
											0.8
	0.4
Fe
					Calculation
to
					(parameter p = 0.8 ML)						0.6
ofAg
Ratio											0.4					Calculation
	0.2															(parameter p = 0.8 ML)
		Calculation										Calculation
		(No segregation)									0.2	(No segregation)
		(a)										(b)
	0.0										0.0
	0	1	2	3	4	5	6	7	8	9	0	1	2	3	4	5	6	7	8	9

Fe coverage (ML)

Fe coverage (ML)

Figure 5.21. AES deposition curves for Fe/1.8 ML Ag/Fe(110), showing (a) rearrangement of the Ag layer between 1 and 2ML, and segregation on annealing, or during deposition at elevated temperature. The Fe/5 ML Ag/Fe(110) curves (b) show rather less complete segregation. The parameter p is the amount interchanged with the surface in the model (Persaud et al. 1998, reproduced with permission).

interdiVusion, in which islands or layers, rather than single atoms, bury themselves in (i.e. burrow into) the substrate. At low temperatures this will not happen, because the substrate atoms will not diVuse. However, STM studies of surface steps on noble metals have shown that steps can move quite rapidly, even at room temperature (Poensgen et al. 1992) and burying of deposited clusters has been observed by in situ TEM at elevated temperatures (Zimmerman et al. 1999). It is now clear that the diYculties various groups have experienced in producing well-de®ned thin ®lms of magnetic metals on noble metal surfaces are related to eVects of this nature. Such magnetic metals generally have higher surface energies than the substrates; they also undergo structural phase changes with increasing thickness; the magnetic features are discussed in sections 6.3 and 8.3.

One case which has been studied by STM is Ni/Ag(111) (Meyer & Behm 1995). Here Ni can both diVuse by hopping over the surface, or, at higher temperature, can exchange with a silver atom. This immobile Ni atom now acts as a nucleus for further growth of Ni clusters. In a ®xed temperature deposition, this corresponds to creation of nuclei at a rate proportional to the adatom concentration; if the Ni±Ni bond is strong enough, then i50. Similar cases are Fe, Co and Ni/Au(111), with a complex