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INTERFACE MAGNETISM OF EPITAXIAL Fe FILMS ON Sb BY MÖSSBAUER EFFECT
S. Hine, T. Shigematsu, T. Shinjo, T. Takada
To cite this version:
S. Hine, T. Shigematsu, T. Shinjo, T. Takada. INTERFACE MAGNETISM OF EPITAXIAL Fe
FILMS ON Sb BY MÖSSBAUER EFFECT. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-
84-C2-85. �10.1051/jphyscol:1979229�. �jpa-00218627�
JOURNAL DE PHYSIQUE Colloque C2, supplPment
au n o
3, Tome 40,mars
1979, Page C2-84INTERFACE MAGNETISM O F EPITAXIAL Fe FILMS ON Sb
BY MOSSBAUER
EFFECTS. Hine, T. Shigematsu, T. Shinio and T. Takada
I n s t i t u t e for ChemicaZ Research, Kyoto University U j i , Kyoto-fu, 621, J a p m
R6sumd.- Les spectres Gssbauer de films dpitaxiques de Fe( l l I) d16paisseur 48, 32, 24, 16 et 81 en sandwiches avec des couches de Sb ont dt& obtenus P 4.2K et 300K. Les champs hyperfins sont plus faibles et plus largement distribuds pour les films les plus minces. Le champ hyperfin moyen pour les atomes superficiels de fer est environ de 320 kOe P 4.2K. Les rapports d'intensitb (3:4:1:1:4:3) sont inddpendants de l'dpaisseur des films et suggsrent que les moments mag6tiques dans les plans Fe(lll) sont parallsles P la surface. I1 n'y a pas de pics paramagnstiques P 4.2K, ce qui signifie que la "couche morte" magndtiquement parlant n'existe pas dans les plans Fe(ll1).
Abstract .-MEssbauer spectra of epitaxial thin Fe( 1 l I) films (48, 32, 24, 16, and 81) sandwiched with Sb(0001) layers are obtained at 4.2K and 300K. The hyperfine fields at 4.2K are reduced with the decrease of the film thickness and their distribution is increased.The average hyperfine field of the Fe surface layers is about 320 kOe at 4.2K. The intensity ratio of 3:4:1:1:4:3 independent of the film thickness at 4.2K suggests that the magnetic moments in an Fe(lll) plane are completely oriented surface parallel. There is no paramagentic peak at 4.2K in the spectra, which means that the magnetically "dead layers" do not exist in the Fe(ll1) plane.
1. Introduction.- The authors have investigated the surface magnetic properties of ferromagnetic metals, Fe and Ni, by means of the Gssbauer spec- troscopy /I-3/. In order to enhance the surface characteristics, ultra thin Fe(or Ni) films sandwi- ched with MgF2 or MgO were used as samples for the
~sssbauer absorption experiments. All of the samples already reported in the previous papers /I-3/ con- sisted of polycrystalline materials. It is impor- tant to study the characteristic magnetic proper- ties depending on a crystal plane. This paper presents the information on the surface magnetism of the epitaxially grown bcc Fe(lll) plane.
2. Sample preparation.- Evaporated Sb films on my- lar substrates at room temperature have good textu- re; the Sb(0001) plane is parallel to the film plane. Very thin Fe film evaporated on such Sb film is growing epitaxially. The crystallographic rela- tion in the multi-layer films is Sb(OOOI)//Fe(lll)//
Sb(0001) /4/. Multi-layer epitaxial films were pre- pared by alternate depositions of Sb and Fecenrichad in ''~e up to 50%) onto a mylar substrate (10 cm square, apartby25cm from the evaporation source) at room temperature with an evaporation rate of 0.1 to 0.5i/s and 0.01 to 0.5;/s respectively, in an oil- free UHV system with an electron beam gun as a hea- ting device. The vacuum during deposition was better than 1x10-'torr. The quartz crystal oscillator was used as a thickness monitor. The average thicknes- ses of Fe films were 48, 32, 24, 16, and 81. The alternate deposition process was repeated for seve-
ral times to gain enough thickness for the MEssbauer measurement. The thickness of the Sb layers was thicker than 300i. These multi-layer samples were fully satisfied with the epitaxial relation above- mentioned.
3. Results and Discussion.- The Gssbauer absorption spectra at 300K and 4.2K for thin Fe(ll1) films sandwiched with Sb(0001) layers are shown in figure 1 and figure 2, respectively.
The spectra for the Fe films correspond to the bcc Fe, but it is clearly shown that as the film thickness becomes thinner the fraction whose hyper- fine field is smaller than the bulk's one is gra- dually increasing. This tendency for the hyperfine field is opposite to the case of MgF2-, MgO- and Ag-coated Fe films /I, 3, 6/, but is similar to that of Cu-coated Fe film / 7 / . These anomalous parts are not corresponding to the intermetallic compounds such as FeSb and FeSbl /5/.
It is suggested that the anomalous parts of these spectra reflect the information from the sur- face layers of the Fe(ll1) plane. At 4.2K, the ave- rage hyperfine field of the anomalous parts is about 320 kOe which is 6 % less than the bulk value. The spectrum for the 8i Fe film ,shows the asymmetric six-finger pattern, whose asymmetry would be ascri- bed to the existence of an electric field gradient in the surface layers of the Fe film. It seems that the main contribution to the line broadening is the distribution of the hyperfine field.
For the 8 1 Fe film, the minimum positions for
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979229
each line at 300K have shifted considerably, but at 4.2K those are the same as the bulk bcc Fe.
-5 0 5
Velocity ( m m
/s
1Fig. 1 : Mijssbauer spectra at 300K of Sb-coated Fe(lll) films.
- 5 0 5
Velocity ( mm/s
Fig. 2 : ~Gssbauer spectra at 4.2K of Sb-coated Fe(l1l) films.
It is concluded that the 81 Fe film was subjected to the thermal agitation at 300K due to the limited di- mension. The magnetically "dead layers" that based upon the concept of no d-spin polarizetion in the surface layers 181 do not exist from the fact that no ~aramagnetic peak is shown at 4.2K. It is empiri- cally known that the magnetic moment is roughly pro- portional to the hyperfine field. The magnetic mo- ment in the surface of Fe(ll1) plane contact with Sb is smaller than the bulk Fe as well as the case of the Cu-coated Fe film 171.
The intensity ratio provides the information on the orientation of magnetic moments. The ratio of 3:4:1:1:4:3 means that the magnetic moments are completely oriented normal to the gamma ray. All of the spectra in figure 2, reveal that the intensity ratio remains 3:4:1:1:4:3 independent of the film thickness. The intensity ratio of the 81 Fe film coated with MgO was nearly 3:0:1:1:0:3, which is ascribed to the surface anisotropy 131. In contrast the magnetic moments in the Fe(1ll) film sandwiched with Sb are completely aligned in the film plane.
This is consistent with the shape anisotropy of thin films.
Ihe hyperfine field is affected by the number of the magnetic near neighbours and their configu- rations. Therefore the hyperfine field at a surface site may depend on the crystal plane. It is of in- terest whether the well-defined surface makes a difference in the ~Gssbauer spectra compared with polycrystalline samples. However, the present spec- tra for epitaxially grown Fe(lll) films on Sb(0001) showed the distribution of the hyperfine field si- milar to the case of polycrystalline samples 11-31 and the hyperfine field of Fe at the top surface layer was not clearly distinguished.
References
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