MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Materials Synthesis and Characterization 15
interatomic spacings (measured in ) are needed. Synchrotron and neutron
sources usually have tunable wavelengths as well as high intensity. For
many purposes, however, laboratory scale X-ray diffraction is often adequate.
2. 2. 2. 1 Neutron diffraction
Neutrons, having a magnetic moment, are sensitive to the magnetic
structure as well as the atomic structure. For example, an antiferromagnet
which has a larger magnetic unit cell than the atomic cell, will cause extra
neutron diffraction peaks not seen in X-ray diffraction. Polarized neutrons
are also sensitive to the orientation of these magnetic moments. Since
magnetic measurements only give the net moment, which is often zero for
an antiferromagnet, neutron diffraction is far superior in determining
magnetic structures. Inelastic neutron diffraction can provide further
information concerning the structure dynamics. For instance phonon and
magnon (spin wave) dispersions can be measured.
2.2.2.2 X-ray diffraction
X-ray diffraction (XRD) can be used both to quickly determine which
phases are present in an unknown sample and to perform a detailed
structural investigation. The difference lies more in the sample preparation
and data analysis than the measurement apparatus. Although synchrotron
source x-ray diffraction experiments are quicker, much of what is desired can
be learned from a standard laboratory experiment.
X-rays scatter off the periodic arrays of atoms in a crystal lattice. The
scattered x-rays produce a pattern unique for a particular substance. This
pattern can be used for either phase identification or it can be analyzed to
determine the position of the atoms in the cell. The orientation of the crystal
with respect to the incoming x-rays determines the orientation of the
diffracted x-rays. So, either a single crystal must be precisely oriented to detect