Blocking of Magnetic Long Range Order Between the Monolayer and the Double Layer of Fe(110) on W(110)
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ABSTRACT In extension of a recent study on submonolayer magnetism of Fe(l 10) on W(l 10) [I], we observed the interplay of morphology and magnetic order in Fe(1 10)-films prepared on W(1 10) at 300 K, in a range of coverages 0 between the pseudomorphic monolayer (0 = 1) and the pseudomorphic double layer (0 = 2), using a combination of STM, SPLEED, CEMS and TOM. Whereas the Curie-temperatures of the monolayer and the double layer are given by Tc(ML) = 230K and TC(DL) = 450K, respectively, we observe in an interval of 1.20< 0 < 1.48 ML a gap of magnetic long range order, for temperatures down to 115K. In CEMS, we observe superparamagnetic fluctuations for T > TC(ML), but magnetic short range order for T < TC(ML). The surprising blocking of long range order in the gap can only be explained from a quasi-antiferromagnetic indirect coupling between double-layer islands
1. INTRODUCTION Ultrathin films of Fe(110) on W(110) in the monolayer regime have been used widely as model systems for monolayer magnetism [2, 3, 4, 5, 6, 7,], because of the thermodynamic stability of the monolayer [4]. However', even in this structurally well
defined model system, dramatic influences of the micro-morphology on the magnetic properties are observed. The combination of STM imaging with magnetic measurements provides the opportunity to obtain new detailed insight in the relation between magnetism and morphology. This has been shown recently in a study of submonolayer magnetism of Fe( 10) on W( 10) using STM in combination with spin polarized electron diffraction (SPLEED) [1]. In extending these studies beyond the monolayer, we were surprised to find, for films prepared at RT, an interval of coverages 1.20 1.7 one observes substantial 3rd layer contributions and the incipient relaxation of the misfit by dislocations. In this paper, we 157
Mat. Res. Soc. Symp. Proc. Vol. 384 @1995 Materials Research Society
Figure 1: STM images of Fe-films prepared on W(110) at 300K. Coverages 0 in units of pseudomorphic monolayers are indicated. The easy axis [1101 is at right angles with the axis of the elongated islands. focus on the range e •< 1.6 where the films were pseudomorphic with the W-substrate (strained by 10.4% in the plane to accomodate the misfit fFeW = - 9.4%), and composed of monolayer and double layer patches.
3. SPIN POLARIZED ELECTRON DIFFRACTION (SPLEED) Our main magnetic probe was SPLEED. We used scattering geometries as described previously [1], with the quantization axis of the electron spin polarization along the easy axis [ITO] of the magnetic film. We measure the exchange asymmetry A., [1], the properly normalized difference in electron reflectivities for spin parallel or antiparallel to [IT0]. Aex is roughly proportional to the magnetization [8]. External fields up to 2000e could be applied along [11'0], generated by currents through the W-substrate. SPLEED was possible in fields up to 20e. Immediately below Tc, we then obtained, even in these low fields, standard easy axis loops, with coercivities down to 0. lOe, and perfe
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