Based on the fitted hyperfine parameters, the intensity ratios of the pure α-iron phase (sextet 1) was used to perform the MOMS analysis. The correspondingI2/I3 ratios were plotted versus ϕ and fitted by equation (3.18). I2/I3 of the sextets 2 and 3, having a reduced number of neighbors, were set manually to the same value as obtained for sextet 1. An example of the experimental results is given in Figure 3.10 b) where the symbols are the measured ratios and the solid line is the fit.
3.3 Comparison of MOMS and MOKE
Before MOMS is used for the analysis of magnetic anisotropy, several of its important properties in comparison with MOKE will be illustrated.
In CEMS the hyperfine field of only one isotope (in our case 57Fe) is taken into account. The measurement depth can be adjusted by changing the deposition or implantation depth of the probe isotope. As the conversion electrons are absorbed in the top layer, the signal intensity decreases exponentially with increasing depth and CEMS is consequently restricted to the upper-most 150 nm of the sample. The MOKE method only analyzes the properties of the illuminated sample area and the hysteresis loop of this spot is measured by sweeping an external magnetic saturation field (see section 2.1.1). The restriction to the illuminated area limits the measure-ment depth to the absorption length lα of the laser in the film material. lα is given by the reciprocal value of the absorption coefficientα:
lα = λlaser
4πk = 1
α. (3.19)
λlaser is the wavelength of the incident laser light and k is the extinction coefficient of the re-fractive index n = n+ik. For λlaser = 632.8 nm and, consequently, kF e = 3.07 [Joh74], the absorption length is lα ≈ 16.4 nm. Therefore MOKE is only sensitive to the very surface of the specimen. Nevertheless, a 75 nm thick film is supposed to have a uniform domain structure from the surface to the substrate, because it is in the range of the ”thin film limit” [Hub00].
In addition to these differences in the measurement volumes, the method of measuring the mag-netic anisotropy is also different. MOMS is sensitive to the static spin distribution of the sample.
Due to the Fermi contact interaction (see section 2.1.3), the direction of highest spin intensity in a magnetically anisotropic specimen is in 180◦ symmetry to the easy axis of magnetization. By means of MOKE, full hysteresis loops are measured and the information about the anisotropy is extracted from the change of the magnetization state.
To illustrate the different properties of MOMS and MOKE, measurements were performed on two samples, both 10×7 mm2 in size. One was showing strong magnetic anisotropy, the other was nearly isotropic. The first sample, called 2-S (see section 4.2.3), was a natural iron film of 55 nm thickness, capped with a 15 nm57Fe marker layer and another 5 nmnatFe layer to prevent the57Fe from oxidation. The natFe was deposited by electron-beam evaporation and the 57Fe by an effusion-cell, and this film was in its as-deposited state. The second sample was a pure
57Fe film (sample P), 78 nm thick and irradiated with 1×1016 Xe+/cm2 at room-temperature with an external magnetic field of 104 Oe applied parallel to the long axis of the sample.
The different layer structures of both films led to different results of the MOKE analysis, as
0
Figure 3.11: MOKE polar plots ofMr/Ms and Hc for samples: a) 2-S (as-deposited) and b) P as-deposited (open squares) and implanted (solid circles).
illustrated in Figure 3.11. While 2-S (a) shows a completely isotropic coercivity and relative remanence after deposition, P (b) is strongly anisotropic when as-deposited (open squares) or ion irradiated (solid circles).
After the MOKE analysis a saturation field (300 Oe) was applied in the direction χ relative to the long axis of the samples. This external field was then released to leave the specimens in remanent magnetization state. After this treatment, MOMS (α= 45◦) was measured for various angles ϕ. This procedure - sample magnetization and MOMS measurement - was repeated for various anglesχ. According to the discussion at the beginning of this section, an orientation of the preferred hyperfine field direction along the remanence axis χ is expected for the isotropic sample 2-S. This sample shows no free energy minimum for the in-plane magnetization direction.
In the anisotropic specimen P the spins should be aligned along the easy axis of magnetization, independent of the direction of the applied magnetic field χ.
The MOMS results of these measurements are presented in Fig. 3.12 and the fit parameters are summarized in Table 3.3. The measured values of ψa agree in nearly all the cases with the expectations. For sample 2-S the measured value always agrees with the angle χ, the only exception being the magnetization direction χ = 100◦. The values ca ≈ 0.75(2), cb ≈ 0.18(2) and cop≈0.07(2), show only small variations for differentχ.
For sample P in all the cases ψa = 0◦, as it was expected. At the angles χ = 70◦ - 90◦ the ratioca/cb is significantly smaller than at the other angles, as shown in Table 3.3. The MOKE hysteresis loops are nearly square-shaped with high remanence at angles between 85◦ and 95◦. To sum up, the expected results are observed with only one exception in specimen 2-S.
A similar way of measuring the easy axis was proposed by Richter and Woods [Ric91, Woo02], who used a vibrating sample magnetometer (VSM) for this measurement.
3.3 Comparison of MOMS and MOKE
Figure 3.12: MOMS graphs for samples 2-S and P, being in remanence state in different directions χ. In sample 2-S (a) ψa follows the remanence direction χ, while P (b) always shows a parallel alignment of spins and easy axis direction.
MOMS fit Exp.
Table 3.3: Parameters of theχ-dependent MOMS measurement of samples 2-S and P. The values in the last two columns show the comparison of the out of plane component of the hyperfine field as resulting from the fit and as measured by CEMS with perpendicular incidence.