Chemistry Reference
In-Depth Information
the magnetic structure in the Mössbauer pattern will collapse at a temperature
much lower than the Curie temperature. If electron spins oscillate very rapidly, a
nucleus does not perceive any magnetic field. In usual magnetic materials at higher
than Curie temperature, it is the case. On the other hand, a paramagnetic spin
system may exhibit a hyperfine structure if the relaxation time of electron spin is
long enough. Such a situation can be realized in insulating systems where the
concentration of magnetic spins is low and interactions among them are negligibly
weak. A typical example is Al
2
O
3
including 1 % Fe
2
O
3
. The paramagnetic
hyperfine structure has different features and is able to be distinguished from that
of magnetically ordered state.
The direction of magnetization can be speculated from the intensity ratio of the
six lines in a magnetically split Mössbauer pattern. In the case of ideally thin
absorbers, the relation between the intensity ratio and the angle between the
gamma-ray beam and direction of the magnetic field (i.e. the direction of mag-
netization) h is as follows: If the intensity ratio of 6 lines is denoted as 3:X:1:1:X:3,
the dependence of X on h is expressed as 4sin
2
h/(1 ? cos
2
h). For a powder sample
where h is randomly distributed, the ratio of 6 lines becomes 3:2:1:1:2:3. If the
magnetization is oriented in the direction of the gamma ray beam, an intensity
ratio of 3:0:1:1:0:3 would be obtained. If the magnetization is perpendicularly
oriented to the gamma ray beam, it becomes 3:4:1:1:4:3. In case of thin film
samples, the direction of spontaneous magnetization against the film plane is
estimated by observing the value of X. Examples of spectra in Fig.
5.1
showed that
the value X is close to 4, indicating the direction of magnetization is close to the
film plane. If a strong magnetic field enough for saturation is applied in the
perpendicular direction to the sample plane (parallel to the gamma ray beam) but
the observed X is non-zero, the spin structure is suggested to be non-collinear.
57
Fe Studies on Magnetic Multilayers
5.3
5.3.1 Fe/Mg Multilayers
Studies on metallic multilayers with artificially designed superstructures have been
actively carried out since 1980s. By alternately depositing two metallic elements
with controlling the layer thicknesses in the accuracy of atomic layers, nanoscale
superstructures are constructed. In order to control the layer thickness satisfacto-
rily, the deposition rate must be rather slow and therefore the atmospheric vacuum
must be sufficiently high. Techniques for ultrahigh vacuum and controlling the
layer thickness are necessary conditions to prepare artificial nanoscale super-
structures. Concerning the constituents of multilayers, a variety of combinations
have been attempted and the establishment of artificial superstructures with the
wavelength of a few nm has been confirmed in many cases. Obtained structures are
not always epitaxial but can be non-epitaxial and occasionally amorphous layers
