Magnetic Multilayers and Interfaces - Mossbauer Spectroscopy - page 256

Velocity (mm/s)

Fig. 5.9 Temperature dependence of surface hyperfine field in a-FeOOH [ 14 ]

temperature dependence of local magnetization, since the molecular field must be
decreased due to the reduced number of neighboring magnetic atoms. If a surface-
selectively enriched sample is available, the Mössbauer spectroscopy can be used
to study the surface (interface) of non-metallic materials. The Curie temperature of
bcc-Fe is very high (1,043 K) and therefore measurements at elevated tempera-
tures are required for the study of surface (interface) magnetization in metallic
cases as a function of temperature. On the other hand, there are several Fe com-
pounds whose T C or T N are at moderate temperatures. a-FeOOH (Goethite) is an
antiferromagnetic material and the T N is about 400 K, which is an appropriate
value to study the temperature dependence of surface magnetization. By a wet
method, precipitates of a-FeOOH were synthesized in an aqueous solution using
pure 56 Fe isotope. According to a transmission microscope observation (TEM), the
shape of each particle was a needle type. By a succeeding chemical process, the
surface of the particles was coated with extremely thin 57 Fe layers. If the coating is
ideally uniform, the nominal thickness of 57 Fe layer corresponds to 0.1 nm. The
Mössbauer absorption spectra of the surface-enriched a-FeOOH sample were
observed at various temperatures below room temperature [ 14 ]. In Fig. 5.9 , the
observed temperature dependence of the hyperfine field at the surface of a-FeOOH
is shown in comparison with that of a bulk sample. The hyperfine field at zero
temperature is almost the same as the bulk value, but the temperature dependence
is considerably different, decreasing much faster with increase of temperature.
This behavior is qualitatively accounted for by assuming a primitive molecular
field approximation. Here we assume a surface magnetic spin to be S = 5/2 and
treat as an impurity site with a reduced exchange field. The Néel temperature and