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Submitted on 1 Jan 1988
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MOESSBAUER SPECTROSCOPY NEAR THE
FERROMAGNETIC MONOLAYER Fe(110) ON
W(110)
M. Przybylski, U. Gradmann
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Suppl6ment au no 12, Tome 49, d6cembre 1988
MOESSBAUER SPECTROSCOPY NEAR THE FERROMAGNETIC MONOLAYER
Fe(ll0) ON ~ ( 1 1 0 ) ~
M. Przybylski and U. Gradmann
Physikalisches Institut, Technische Universitaet Clausthal, 0-3392 Clausthal-Zellerfeld, F.R.G.
Abstract. - For Fe(ll0)-films on W(110) near the monolayer, coated by Ag, the Curie-temperature Tc was determined using Moessbauer spectroscopy in the thermal scanning mode. If the films were prepared at 500 K, Tc was independent of the thickness between a half and a full monolayer. It represents the microscopic Curie-temperature Tc (1) =282f 3 K
of extended monolayer patches.
Recent interest in ferromagnetic monolayers comes from a general interest in two-dimensional or- dering phenomena and magnetic order near surfaces, from extended band calculations of ultra-thin films in the ground state [I] and from recent experimental de- tection of monolayer ferromagnetism [2-51.
In the present work we analyse the Fe(ll0)- monolayer on W(110), which has the merit to be a thermodynamically stable system [5], in contrast to Fe on Cu(100) [2] and Fe on Ag(100) [3], which are un- stable and for which therefore, the question of layer growth is controversial [6]. Ferromagnetic order in the pseudomorphic monolayer Fe(ll0) on W(110) has been detected recently by Conversion Electron Moessbauer Spectroscopy (CEMS) in situ in UHV [5]. Curie tem- peratures of 295 K for the Ag-coated monolayer and 210 K for the uncoated monolayer were determined by full Moessbauer analysis or by thermal scanning, respectively. Some unconventional properties of the phase transition were reported.
The basic idea of the present work is the follow- ing: if the monolayer is energetically preferred in com- parison with three-dimensional nuclei, and if the film atoms show attractive interaction (which is the case for Fe on W), then a submonolayer film definitely con- sists of monolayer patches,separated more or less by Fe-free substrate areas. If by proper preparation con- ditions these patches are of sufficient size, then this system of large size monolayer patches should show the intrinsic microscopic monolayer properties, inde- pendently of their coverage, that means independently of the mean film thickness. As a mesure of film thick- ness, we commonly use the number of bulk monolay- ers, D. Because of the misfit f ~= -9.4 ~ %, / the ~ pseudomorphic monolayer corresponds to D = 0.82. Accordingly, we expect, for proper preparation condi- tions in the sense discussed above, an independence of magnetic properties, e.g. Tc, on D, for D 5 0.82. Curie
temperatures can be measured relatively easily by the thermal scan method [5]. Using this method, Tc was determined for Ag-coated films with 0.35
5
D5
1.2, with varied preparation conditions. Results are shown in figure 1. The most important result is the strikingly constant value of Tc (1) =282 f 3 K, measured as ex- pected for 0.3< D
<<
0.82, if the films are prepared at an elevated preparation temperature T p =500 K, orannealed a t
TA
=800 K after preparation at 300 K. Ob- viously, the supposed recrystallization t o large mono- layer patches takes place for these conditions. The observation that annealing up to 800 K leaves Tc (1) completely unchanged documents the high degree of stability of this monolayer. T, (1) =282 f 3 K ob- viously is the intrinsic Curie-temperature of the Ag- 7oated monolayer.Fig. l . - Curie-temperatures Tc versus number of bulk monolayers, D, for pure 67~e(110) films on W(110), coated by Ag. T, was determined by either thermal scanning (0,
*,
e ) or by full spect~al analysis (m), respectively, for samples
prepared at 500 K (o), premared at 300 K (e, m ) or prepared at 300 K and annealed at 800 K ( o ), respectively.
'work supported by the Stiftung Volkswagenwerk.
2 ~ e r m a n e n t address: Solid State Physics Department, Academy of Mining and Metallurgy, Krakow, Poland.
CS - 1706 JOURNAL DE PHYSIQUE
Contrarily, for a preparation at Tp =300 K, Tc de- pends strongly on D. Appare'ntly, the monolayer (and double layer) patches are too small as to be con- sidered as independent components. Superparamag- netic relaxation effects and complicated coupling phe- nomena, including both weakly magnetic monolayer patches and strongly magnetic double layer patches, must be considered. Of special meaning is the sam- ple (Tp = 300 K; D = 0.82), which was discussed in detail previously 151. The Curie temperature of this sample, Tc = 295 K, being measured by full spectral analysis 151, is larger than Tc (1) =282f 3 K, taken from the samples prepared at TP = 500 K. We s u p pose that this difference is caused by a small deviation from D = 0.82, connected with the appearance of some double layer patches.
Our conclusions on the size of monolayer and dou- ble layer patches are confirmed by the CEMS-spectra of two films D = 1.1, prepared at 300 K or 500 K, re- spectively, shown in figure 2. In both cases we observe two magnetic components of equal intensity, represent- ing the two layers of double layer patches, and one additional central component, representing the mone layer patches [B]. For Tp = 500 K, this monolayer component is nonmagnetic (at room temperature), as expected from Tc (1) =282*3 K; obviously, the (large size) monolayer patches are independent of the mag- netic double layer patches. Conversely, for Tp =300 K, the central (monolayer) component becomes weakly magnetic; apparently, the monolayer patches are now so small, that they can be magnetized by contact with the neighboring double layer patches.
Fig. 2.
-
CEMS-spectra, measured at room temperature, for D = 1.1 bulk monolayers of 6 7 ~ e ( l l ~ ) on W(110), coated by Ag, prepared at Tp =300 K and Tp = 500 K, respectively.Finally, the question rises whether the peculiarities of the phase transition, which were observed previously [5] for ( D = 0.82; Tp = 300 K)
,
are special features of this special "monolayer" only. They consisted in the rise of a single line below T,, connected with a finitevalue for Bhf of the magnetic component when extrap- olated to T,. In figure 3 it is shown that this is a gen- eral feature, independent of D, a t least for Tp =300 K. For all three samples, a single line component appears below Tc, in a critical region, the width of which in- creases with increasing D. At T,, the mean hyperfine field Bhf only disappears, the hyperfine field Bhf of the magnetic component remains finite (5 f 1.5 Tesla).
Fig. 3. - Parameters of CEMS-spectra of 6 7 ~ e (110)-films on W(110), coated by Ag, near the pseudomorphic mono- layer, D = 0.82. Films with D = 0.62; 0.82 and 1.11, were prepared at 300 K. Single line probability PSiwl, ( m
,
e,
A), magnetic hyperfine fields Bhf of the magnetic compo- nent ( 0 , o, A) and mean hypefine field Bhf ( m ,
*,
A ) ,respectively, are shown versus T. For reproduction of spec- tra compare [5].
The main message of this work is that reliable mea- surements on the magnetic monolayer should be done using thermodynamically stable systems, with samples prepared at elevated temperature (=500 K for Fe(ll0) o n W(110)). The independence of Tc on D forms a useful criterion for microscopic monolayer behavior.
[I] Fu, C. L., Freeman, A. J. and Oguchi, T., Phys. Rev. Lett. 54 (1985) 2700.
[2] Pescia, D., Stampanoni, M., Bona, G. L., Vater- laus, A., Willis, R. F. and Meier, F., Phys. Rev. Lett. 58 (1987) 2126.
[3] Stampanoni, M., Vaterlaus, A., Aeschlimann, M. and Meier, F., Phys. Rev. Lett. 59 (1987) 2483. [4] Kurzawa, R., Kaemper, K. P., Schmitt, W. and
Guentherodt, G., Solid State Commun. 60 (1986) 777.
[5] Przybylski, M. and Gradmann, U., Phys. Rev. Lett. 59 (1987) 1152.
[6] Steigerwald, D. A. and Egelhoff, W. F., Jr., Phys. Rev. Lett; 60 (1988) 2558.
[7] Przybylski, M. and Gradmann, U., J. Appl. Phys.
63 (1988) 3652.
