1 X-Ray Sources at Large-Scale Facilities
21
1.3.3 Polarization of Synchrotron Radiation
Polarization of light in spectroscopy is a highly valuable tool, as certain selection
rules imply that certain electronic features can be distinguished merely by using
differently polarized X-radiation, for example, in the study of surface absorbates on
crystalline surfaces, or in distinguishing between differently oriented ferromagnetic
or antiferromagnetic domains. Magnetic dichroism using tunable and polarized soft
or hard X-rays at synchrotron sources offers the unique feature of chemical specificity.
Magnetic features down to below the 100-nm scale can be imaged by exploiting the
dependence of the absorption on the polarization of X-rays in ferromagnetic and antiferromagnetic materials. The methods are called X-ray magnetic circular dichroism
(XMCD) and X-ray magnetic linear dichroism (XMLD), respectively, for ferromagnetic and antiferromagnetic materials (see Chap. 4). Imaging down to the nanometre
range is very important for magnetic structures, as at this scale, the influence of
domain boundaries between one magnetic direction and another becomes significant
and new phenomena can occur which would be negligible in larger structures. An
understanding of the energetics of nanomagnetism is thus essential in the drive to
further miniaturize magnetic memory storage devices.
The polarization of undulator radiation can be controlled and varied in so-called
APPLE (Advanced Planar Polarized Light Emitter) undulators. APPLE undulators
have undergone successive refinements (reflected by their generational names of
APPLE-I, II, III and X), based on the flexibility of movements of their magnet
arrays; here, we consider only the basic principles.
Instead of the two Halbach magnet arrays found in ‘normal’ undulators discussed
thus far, APPLE undulators consist of four arrays (two above, two below) that can
be longitudinally shifted relative to each other [A1–A4, Fig. 1.15a] [9]. Each array
consists of a periodic repetition of four different orientations [down (blue ⊗); reverse
(green arrow); up (yellow ); and forward (red arrow), see Fig. 1.15b]. With all array
components aligned longitudinally, normal linear horizontal (LH) radiation as found
in ‘normal’ undulators is produced. Antisymmetric shifts (that is, one array moving
in the positive z-direction, the other in the negative) of A1 and A4 between −λ u /2
and +λ u /2 will maintain linearly polarized radiation but will vary the tilt angle
from linear vertical (LV at −λ u /2), through LH (zero shift) back to LV (+λ u /2).
Conversely, symmetric movements of A1 and A4 by approximately ±λ u /4 will
produce right circularly polarized light (at approximately −λ u /4), or left circularly
polarized light (at +λ u /4). In this last instance, the exact shift in the arrays depends
on the gap size and details of the magnet strengths. Symmetric shifts smaller than
this result in elliptical polarization, that is, radiation with both a linear and circularly
polarized component.
Synchrotron sources provide fluxes and brilliances many orders of magnitude
greater than those from laboratory-based sources. Third-generation facilities were
defined by their use of insertion devices, most importantly undulators. The fourth
generation of synchrotrons, just beginning to come online at the time of writing,
increase brilliances by two orders of magnitude by reducing the horizontal (orbital
21
1.3.3 Polarization of Synchrotron Radiation
Polarization of light in spectroscopy is a highly valuable tool, as certain selection
rules imply that certain electronic features can be distinguished merely by using
differently polarized X-radiation, for example, in the study of surface absorbates on
crystalline surfaces, or in distinguishing between differently oriented ferromagnetic
or antiferromagnetic domains. Magnetic dichroism using tunable and polarized soft
or hard X-rays at synchrotron sources offers the unique feature of chemical specificity.
Magnetic features down to below the 100-nm scale can be imaged by exploiting the
dependence of the absorption on the polarization of X-rays in ferromagnetic and antiferromagnetic materials. The methods are called X-ray magnetic circular dichroism
(XMCD) and X-ray magnetic linear dichroism (XMLD), respectively, for ferromagnetic and antiferromagnetic materials (see Chap. 4). Imaging down to the nanometre
range is very important for magnetic structures, as at this scale, the influence of
domain boundaries between one magnetic direction and another becomes significant
and new phenomena can occur which would be negligible in larger structures. An
understanding of the energetics of nanomagnetism is thus essential in the drive to
further miniaturize magnetic memory storage devices.
The polarization of undulator radiation can be controlled and varied in so-called
APPLE (Advanced Planar Polarized Light Emitter) undulators. APPLE undulators
have undergone successive refinements (reflected by their generational names of
APPLE-I, II, III and X), based on the flexibility of movements of their magnet
arrays; here, we consider only the basic principles.
Instead of the two Halbach magnet arrays found in ‘normal’ undulators discussed
thus far, APPLE undulators consist of four arrays (two above, two below) that can
be longitudinally shifted relative to each other [A1–A4, Fig. 1.15a] [9]. Each array
consists of a periodic repetition of four different orientations [down (blue ⊗); reverse
(green arrow); up (yellow ); and forward (red arrow), see Fig. 1.15b]. With all array
components aligned longitudinally, normal linear horizontal (LH) radiation as found
in ‘normal’ undulators is produced. Antisymmetric shifts (that is, one array moving
in the positive z-direction, the other in the negative) of A1 and A4 between −λ u /2
and +λ u /2 will maintain linearly polarized radiation but will vary the tilt angle
from linear vertical (LV at −λ u /2), through LH (zero shift) back to LV (+λ u /2).
Conversely, symmetric movements of A1 and A4 by approximately ±λ u /4 will
produce right circularly polarized light (at approximately −λ u /4), or left circularly
polarized light (at +λ u /4). In this last instance, the exact shift in the arrays depends
on the gap size and details of the magnet strengths. Symmetric shifts smaller than
this result in elliptical polarization, that is, radiation with both a linear and circularly
polarized component.
Synchrotron sources provide fluxes and brilliances many orders of magnitude
greater than those from laboratory-based sources. Third-generation facilities were
defined by their use of insertion devices, most importantly undulators. The fourth
generation of synchrotrons, just beginning to come online at the time of writing,
increase brilliances by two orders of magnitude by reducing the horizontal (orbital