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TRANSITION FROM PAULI-PARAMAGNETISM TO
BAND-FERROMAGNETISM IN Ni-FILMS OF A
FEW ATOMIC LAYERS
G. Bergmann
To cite this version:
G.
Bergmann.
TRANSITION
FROM
PAULI-PARAMAGNETISM
TO
JOURNAL DE PHYSIQUE Colloque C6, supplPment au no 8, Tome 39, aotit 1978, page C6-783
TRANSITION FROM PAULI-PARAMAGNETISM TO BAND-FERROMAGNETISM IN
NI-FILMS OF A FEW ATOMIC LAYERS
IFF der KFA JiiZich, 0-5170 JtiZich, Germany.
R6sumB.- Des films de Ni de quelques couches atomiques sont condenses en UHV a 10K sur des films me'talliques de Pb amorphe et Pb, Cu et Bi cristallins. Des mesures de l'effet Hall extraordinaire sont utilisees pour dBterminer l'aimantation et la susceptibilitd des films. Les films de Ni dlune Bpaisseur infBrieure 1 2 couches atomiques ne possSdent pas de moment magnBtique mais montrent une susceptibilitL prononcBe. Entre 2 et 3 couches atomiques de Ni la susceptibilitB diverge et les films plus Bpais ont un moment magnctique. Un film de Ni reprBsente ainsi un systPme, dans lequel l'interactio~~ d'dchange augmente avec l'dpaisseur causant une transition de Pauli para-magndtisme au ferromagngtisme de bande.
Abstract.- Ni films of a few atomic layers are condensed in UHV at 10 K on metallic substrates of amorphous Pb and crystalline Pb, Cu and Bi. Anomalous Hall-effect measurements are used to deter- mine the magnetization and the susceptibility of the films. Ni-films with a thickness smaller than
two atomic layers possess no magnetic moment but show an enhanced susceptibility. Between two and three atomic layers of Ni the susceptibility diverges and thicker films possess a magnetic moment. A thin Ni;film thus represents a system in which the exchange interaction increases with thickness, causing a transition from Pauli-paramagnetism to band-ferromagnetism.
Extremely thin films of ferromagnetic tran- densation temperature supresses the diffusion at the sition metals condensed on top of a non-magnetic surface and into the substrate and any clustering metal /l-5/ experience hybridization of the magne- beside the statistical one.
tic d-states with the conduction electrons of the
substrate. This hybridization may even suppress In figure 1 the Hall curves for a sandwich the magnetism if the film thickness is of the order of Pb,,Bi,,/Ni are plotted. The number by each cur- of a few atomic layers. With the present work we ve gives the coverage of the substrate with Ni in examine experimentally the basic question : at which units of atola.
film thickness do Ni atoms begin to possess a ma- gnetic moment ? How does the occurence of the ma- gnetic moment happen, with a divergence of the sus-
0
ceptibility as the theory of band-magnetism suggests 2 5
30
or by formation of magnetic clusters ?
We detect the magnetic moments of the Ni and
-*'
37their ordering by a rather unconventional but ex-
tremely sensitive method, the anomalous Hall-effect. 4 1
Tkie anomalous Hall-resistivity is proportional to
-18-
5 0
the magnetization of the sample 161. The method is
- Pb,Bi,,lNi
so sensitive that even the paramagnetic behavior -20. 5 6
of 116 of a monolayer of Fe can be easilv detected.
The' evaporation cryostat has a vacuum better Fig. 1 than 10-'
'
torr. In this vacuum we condense firstthe metallic substrate of amorphous Pb,,Biz5 with The original Pb Bi shows a linear Hall curve with
o 7 5 25
a thickness of 250 A. The Ni is sublimized from a negative slope as they are typical for amorphous wire
( m
= 1 mm). The amount of condensed metal is metals / 7 / . The temperature of the film is about measured by a quartz balance. The accuracy of the 10 K. The slope increases progressively with the thickness determination is 1/30 atola. The low con- Nithickness. However, the Hall-curves remain linearand they are temperature independent between 5 k and 20 K up to 2.5 atola Ni. At 3.0 atola the Ni forms magnetic moments which are oriented by the ma- gnetic field-These moments form large clusters (3.7 atola Ni) and approach with increaqing Nithic- kness the typical behaviour of a ferromagnetic film.
Extrapolating the high field part of the Hall-curve to zero field yields the anomalous Hall- resistance of the sandwich, R (0). The latter is
XY
proportional to the magnetic moment of the sand- wich and is plotted in figure 2. In the left part of figure 2 we show the slope of the (linear) Hall- curves. It is porportional to the susceptibility of the Ni and diverges between 2 and 3 atola Ni.
Ni
-
thickness [atola]-
PbPb,Bi,Fig. 2
We performed the same experiments with crys- talline Pb, Cu and the semi-metal Bi as substrates. Again the first 2-3 atola are magnetically passive.
The measurements demonstrate that Ni on top of a magnetic metal prossesses a magnetic moment only if its thickness exceeds 2-3 atola. Below this thickness the Ni is a nearly ferromagnetic metal with an Ahanced susceptibility. The enhancement factor diverges at about 2.5 atola. Above this thickness one obtains a band ferromagnet. Therefore we performed the transition from a Pauli-paramagnet to a band-ferromagnet in thin Ni-films by the chan- ge of the parameter Ni-thickness. The contact of
the first Ni-layer with the metallic substrate yields a hybridization of the Ni-d-states with the conduc- tion electrons of the substrate. This causes a de- localization of the d-wave function. The exchange interaction reduces and the magnetic moment disap- pears. If we add further Ni-planes the influence of the hybridization at the boundaries reduced conti- nuously. The effective exchange interaction increa- ses and at 2-3 Ni-layers the susceptibility diverges as is demonstrated by the Hall-constant of the s,and- wich.
We doubt that this problem of two-dimensional magnetism can be described by the theory of Jaccari- no and Walker /8/. The loss of magnetic moments in the second layer contradicts this theory.
References
/l/ Lee, E.L., Bolduc, P.E. and Violet, C.E., Phys. Rev. Letters
13
(1964) 800/ 2 / Gradmann, U. and Miiller, J., Phys. Stat. Sol. 27 (1968) 313
-
131 Liebermann, L., Clinton, J., Edwards, D.M. and Mathon, J., Phys. Rev. Letters
5
(1970) 232 141 Pierce, D.T. and Siegmann, H.C., Phys. Rev.(1974) 4035
151 Koepke, R. and Bergmann, G., 2 . Physik
B21
(1975) 185/6/ Hurd, C.M. : The Hall-effect in metals and alloys
/7/ Bergmann, G., Phys.Rep. (1976) I59
