The following sections will deal with the incident polarised electrons interacting with the bound electrons in a single crystal magnetic Fe sample, as the magnetic Fe sample is anisotropic, special attention must be paid to sample preparation and orientation. The single crystal magnetic Fe sample was commercially purchased from the University of Aarhus, and was grown on a NaCl [111] surface normal crystal by evaporation depo- sition. This growth mechanism is well known to produce an iron sample with a [110] surface normal through the Nishiyama-Wasserman mechanism [154]. The experiments performed in the EMS laboratory examine directions that are perpendicular to the surface normal, and the growth mechanism allows for two crystal directions to be parallel to the surface along the long edge of the NaCl [111] substrate (Fig. 6.3).
<111> y-axis
<100> y-axis
<111> y-axis
Na or Cl atom, Fe atom
a)
b)
c)
Figure 6.3: The three possible Fe[110] surface normal growth mechanisms due to the three-fold
symmetry of the NaCl[111] surface normal substrate. There are slight lattice mismatches which are nullied by the misalignments in the Fe growth, known as Nishiyama-Wasserman growth [154]. a) and c) result in an experimental y-axis orientation of Feh111i, whilst b) results in an experimental y-axis orientation of Feh100i. Both the growth orientations are possible.
In Fig. 6.3, the y-axis corresponds to the measured direction in the EMS spectrometer, which is also the axis of magnetisation. The Feh100i (y-axis) growth mechanism (Fig.
6.3: b), has a small lattice mismatch (x-axis≈3 %, y-axis≈1 %) which is nullied by a combination of crystal strain at the interface, and a slight out of plane tilt in the direction of the Fe[110] surface normal. The Feh111i(y-axis) growth mechanism (Fig. 6.3: a and c) is a little more strained (x-axis≈1 %, y-axis≈12 %) than the Feh100igrowth, and involves
a slight rotation around the [110] surface normal, so the resulting experimental y-axis is not precisely orientated along the Feh111idirection. The Feh111i y-axis orientation is
still possible because of the out of plane tilt incorporated in the Nishiyama-Wasserman growth.
a) Fe <111>
b) Fe <100>
c)
Fe <100>
minimised diffraction
Figure 6.4:Diffraction images taken at different points across the same magnetised Fe sample. a)
The diffraction pattern indicates ah111iy-axis arrangement, as shown by the characteristich222i
diffraction spots (°) b) The diffraction pattern indicates a primarilyh100i y-axis arrangement as shown by the characteristich110iandh211idiffraction spots (°). c) By slightly rotating the
sample about the measurement direction, diffraction is minimised, the transmission experiments were performed in this conguration.
The sample was then magnetised along the y-axis (100) orientation prior to being placed inside the experimental chamber and thinned in the same manner as the other single crystal samples (see Chap. 4). The sample was magnetised, by an iron core electromagnet with an internal magnetic eld of 2 T, that had a 15 mm air gap in which the sample was placed. The magnetic eld was obtained by applying 10 A across three solenoids that were in series, which varied from 160-300 turns. Applying the magnetic eld for 30 seconds proved more than long enough to magnetise the Fe sample. The sample was mounted onto a specially made copper sample holder and the magnetisation of the Fe sample was
measured with a magnetometer both before and after magnetisation and before and after experimental measurements were performed. Prior to magnetisation the sample produced a net zero magnetic eld, after magnetisation a eld on the order of 20 nT was measured approximately 1 cm from the sample. After the experiment was performed the magnetic eld of the Fe sample was again measured and was similar to the magnetic eld measured prior to the experiment
High energy electron diffraction patterns were taken in situ on the prepared magnetic Fe sample (Fig. 6.4) to examine the samples orientation. By scanning across the sample and measuring high energy electron diffraction images, it can be seen that there are re- gions where the y-axis is orientated along Feh111i(Fig. 6.4: a), and regions where the y-axis is primarily orientated alongh100i(Fig. 6.4: b). The magnetic Fe sample was po- sitioned so that the y-axis was in ah100iorientation and then rotated about the y-axis by
≈3◦, to minimise the amount of diffraction (Fig. 6.4: c). The effect of diffraction on the Compton prole could in principle be nullied by tilting the sample in combination with rotation to setup a two-beam case [155], although there was no ability to tilt the sample in the EMS spectrometer.
The transmission mode experiments in the following sections were performed on the sample position which produced the diffraction image shown in Fig. 6.4: c). The re- sults are expected to be primarily for a magnetised Feh100isample, but any translational
movement of the sample through vibrations in the building could introduce magnetic Fe
h111iresults into the measurement.
Spin-polarised electron-energy-loss-spectroscopy (SPEELS) and spin polarised EMS measurements would be performed along the y-axis of the magnetised Fe sample (Fig. 6.5: b), which corresponds to ah100i(Γ-H, Fig. 6.5: a) orientation. Magnetic electron- Compton proles are measured along the momentum transfer direction, which is in the xz-plan (Fig. 6.5) of the experiment at 45◦ to the surface normal, and corresponds to a
Γ
<010>
<001>
<100>
<101>
<111
>
<110>
N
N
H
H
H
P
<011> <100> <011> <002>a)
b)
y
z
x
Figure 6.5: a) The rst Brillouin zone boundary for a body centred cubic (BCC) structure, show-
ing the high symmetry (100, 110, and 111) directions with the corresponding high symmetry crystal points (H, N and P) respectively. b) The sample orientation as positioned in the spectrome- ter, with the incident electron beam coming in along the surface normalh011i, the y-axis measured in an EMS experimenth011i, and the momentum transfer vectorh002iof which the projection is measured in an electron-Compton scattering experiment.