We use SMOKE to measure the hysteresis loops in the polar and the longitudinal direction during the initial deposition of Fe on Pt(111). Fig. 4-10 (a) shows the evolutions of both polar and longitudinal Kerr hysteresis loops versus the thickness of Fe films. In Fig. 4-10 (a), all the loops were measured at room temperature and the maximum applied magnetic field we used was 300 Oe. No Kerr signal is observed when the thickness of Fe (dFe) ≤ 1.0 ML, even the sample was cooled down to 165 K. We also measured the Kerr signal with a maximum applied magnetic field of 900 Oe, there still no Kerr signal could be observed. The Kerr hysteresis loops measure at 165 K of 1 ML Fe/Pt(111) are shown in Fig. 4-11. It is possible that the Curie temperature is below 165 K or the coercivity is higher than 900 Oe for 1.0 ML Fe/Pt(111). The other possible reason is that the 1.0 ML Fe/Pt(111) system is not ferromagnetic. Both the polar and the longitudinal Kerr hysteresis loops can be obtained when dFe = 1.5 ML. Since the coercivity (HC) of the polar hysteresis loop is much greater than the in-plane one, the easy axis of the magnetization is in the in-plane direction. Only longitudinal Kerr hysteresis loop can be observed when dFe ≥ 2.0 ML, i.e., the easy axis is in the in-plane direction. The thickness dependences of the saturated polar and longitudinal Kerr intensities for Fe/Pt(111) are shown in Fig. 4-10 (b). The longitudinal Kerr intensities increase with the thickness of Fe films when dFe ≥ 1.5 ML. The evolution of longitudinal coercivitiy vs Fe coverage is shown in Fig. 4-10 (c). The HC of the longitudinal hysteresis loop is very low in this system, it reaches a saturated value of 32 Oe when the thickness of Fe is higher than 2.0 ML.
Figure 4-10: (a) The polar and longitudinal Kerr hysteresis loops at different coverage of Fe (dFe) thin films on Pt(111) at room temperature; (b) The evolutions of the polar and longitudinal Kerr intensities for Fe/Pt(111) correspond to different coverage of Fe. (c) The evolution of longitudinal coercivity versus Fe coverage.
Figure 4-11: The polar and longitudinal Kerr hysteresis loops of 1 ML Fe/Pt(111) at the different low temperature. The maximum of applied magnetic field is 900 Oe.
Our results have some differences with the Nahm’s report [73]. They did not observe any hysteresis loops when Fe thickness is less than 4.1 ML. At dFe = 4.1 ML, only polar hysteresis loop with a coercivity of 50 Oe can be observed. When dFe > 4.1 ML, the easy axis of the magnetization is in the in-plane direction. The in-plane coercivity is only about 9 Oe for dFe = 5.1 ML. The reason why these differences occurred may be due to the different methods of depositing Fe films. The method they used is e-beam heating while the one we used is thermal evaporation. Our deposition rate is lower than theirs, thus our Fe atoms have enough time to diffuse on the surface to form an ordered arrangement. Indeed, the satellites of LEED pattern surrounding each integer spots were observed when dFe > 1.0 ML. This is the evidence that Fe atoms are in an ordered state on Pt(111) surface. D. Repetto et al. have reported the similar results to our but they detect a spin reorientation transition (SRT) from in-plane to out-of-plane when dFe is reduced below a critical coverage of dcrit ≈ 2.8 ML. Such a SRT is found for many thin-film/substrate systems and is commonly ascribed to the dominant role of the interface anisotropy [74].
Experimentally, Fe films deposited on non-magnetic substrate is a well-studied system which shows a complex correlation between magnetic and structural properties depending on deposition temperature and preparation procedure [74-78]. Now we study
the magnetic property of Fe/Pt(111) which Fe films are grown at 180 K (low temperature;
LT). Liquid nitrogen is used to cool the sample continuously and control the temperature at 180 K in the deposition process and the MOKE measurements are taken at LT. The evolutions of both polar and longitudinal Kerr hysteresis loops versus the thickness of Fe films growth on Pt(111) at LT are shown in Fig. 4-12 (a). For Fe coverage below dFe= 0.75 ML no Kerr signals can be obtained in the MOKE experiments at 180 K. For a film growing from monolayer islands this value is typically 0.65 ML [79]. This result is consistent with the assumption that hysteresis appears when the film has passed the percolation threshold. In the LT growth, part of the deposited Fe is present in the second layer therefore the onset of hysteresis to higher coverages. It will be studied by means of LEED in the following section. Only polar Kerr hysteresis loop is detected when the thickness of Fe is 0.75 ML. The results show that the LT-grown Fe films are ferromagnetic for thicknesses greater than 0.75 ML. The longitudinal Kerr hysteresis loop is irregular with increasing Fe thickness up to 1.0 ML. For Fe thicknesses ≥1.3 ML, only longitudinal Kerr hysteresis loop can be detected which the ultrathin Fe films on Pt(111) are present in-plane easy axis. Clearly visible is a spin reorientation transition (SRT) from out-of-plane to in-plane when dFe is increased to a critical coverage of dcrit = 1.3 ML.
Such a SRT is found for many thin-film/substrate systems and is commonly ascribed to the dominant role of the interface anisotropy [80-82]. Fig. 4-12 (b) shows the remanence Kerr intensity of Fe films on Pt(111) as a function of thickness for LT growth, giving a result consistent with the finding in Refs. [74, 76]. Fig. 4-12 (c) shows HC of Fe films on Pt(111) as a function of dFe for LT growth. The saturation of longitudinal HC = 70 Oe at LT is larger than 35 Oe at room temperature. HC decreases as the temperature increases.
This result is in agreement with that the relation between the coercive force and the temperature of a thin film can be described by the mathematical formula:
)
Figure 4-12: (a) The polar and longitudinal Kerr hysteresis loops at different coverage of Fe thin films on Pt(111) at 180 K; (b) The evolutions of the polar and longitudinal Kerr intensities for Fe/Pt(111) correspond to different coverage of Fe (dFe) at LT. (c) The evolution of the polar and longitudinal coercivity versus Fe coverage at LT.
Fig. 4-13 shows LEED I-V spectrum of (0,0) beam intensity versus energy curves for different thickness Fe films deposited on Pt(111) at 180 K. We have calculated the interlayer distance as a function of the film thickness from Eq. 4-3. The results are shown in Fig. 4-14. For LT-deposited films, the LEED I-V results show a linear increase of the interlayer distance, starting from about 1 ML Fe, and a value 15% larger than the substrate interplanar distance is observed for a 2 ML thick Fe film. The interlayer distance shows a pronounced behavior for the Fe films grown at room temperature. We have detected the LEED I-V spectrum of (0,0) beam intensity for dFe > 2 ML, but the
relative variation of the (0,0) beam intensity versus energy is desultory. The results indicate that Fe atoms deposited on Pt(111) at 180 K is 3-D islands which have irregular, ramified shape and grow in height with increasing Fe coverage. Compared with RT growth, a high island density in Fe films on Pt(111) substrates is observed, which is caused by the reduced mobility of Fe adatoms at low temperature. This structural evolution resembles the evolution of the lateral spacing with increasing Fe thickness, shows in Fig. 4-14, and it is evidence of a structural transformation within the Fe films during the film growth for a low temperature. The MOKE and LEED I-V results, we suggest that the structure of the Fe film changes from fcc-like to a bcc-like starting from 1 ML for low temperature growth. The similar structural transformation starts from 2 ML for the Fe films grown on Pt(111) at room temperature. The phenomenon also observes for the growth of Fe on Cu(100) at low temperature [84].
Figure 4-13: LEED I-V spectrum of the (0,0) beam curves for the clean surface of the Pt(111) substrate, and for different thickness Fe films on Pt(111) deposited at 180 K, respectively.
Figure 4-14: The Vertical interlayer distances versus thickness of Fe films deposited on Pt(111) at 180 K. The value of horizontal dashed line is 2.27 Å for the interlayer distance of the Pt(111).
Compared with RT growth, a high island density in Fe films on Pt(111) substrates is observed, which is caused by the reduced mobility of Fe adatoms at low temperature. The polar magnetization shows a strong temperature dependence in the investigated temperature range. In RT grown Fe/Pt(111) gives the magnetoelastic anisotropy due to the larger strain. It is a contribution to the in-plane anisotropy. It is often assumed that a different surface roughness can modify the surface magnetoelastic and magnetocrystalline anisotropy energy. A judge of the roughness-induced contribution to the perpendicular anisotropy can be obtained considering theoretical models by Bruno [85]. For LT grown Fe films deposited on flat Pt(111) surface have much rougher films surface than RT grown films then the easy axis is in the out-of-plane direction.
The 1 ML Fe/Pt(111) sample has been grown at substrate temperature of 193 K and slowly warms up at different annealing temperature for 15 min. To measure the polar and longitudinal Kerr hysteresis loops at each annealing temperature. Only polar Kerr signal can be observed during the annealing process. Fig. 4-15 shows the evolution of polar and longitudinal Kerr hysteresis loops versus sample temperature for 1 ML Fe/Pt(111) grown at 193 K. The Kerr intensity and coercivity in out-of-plane gradually decrease to zero
from 193 K to 300 K. The result can confirm that a different surface roughness may modify the surface magnetocrystalline and magnetostatic anisotropy.
Figure 4-15: The evolution of polar and longitudinal Kerr hysteresis loops versus sample temperature (TS) for 1 ML Fe/Pt(111) grown at 193 K. (b)Temperature dependence of Kerr intensity and coercivity of 1 ML Fe/Pt(111) at 193 K.