Magnetic properties of Fe/Mn/Fe/Cu 3 Au(001)

Figure 4.12: Two different average vertical interlayer distances related to fcc-like (full circles) and bcc-like Fe films (open circles), respectively. adapted from [2]

4.4 Magnetic Properties of Fe/Mn/Fe/Cu

Au(001)

Coercivity enhancement phenomenon

Fe films grown on Mn/Fe/Cu₃Au(001) reveal different magnetic properties from those grown on Mn/Cu₃Au(001) at room temperature. In Fig. 4.13 the coercivity of magnetic hysteresis loop of 6.3 ML Fe/4.5 ML Mn/2.5 ML Fe/Cu₃Au(001) in perpendicular direction is greatly enhanced by the 2.5 ML fcc-like Fe buffer layer as compared with 6.3 ML Fe/4.5 ML Mn/Cu₃Au(001). The increase of H_𝐶 is intu-itively simple to understand. For an AFM (Mn) layer with small anisotropy, when the FM (Fe) layer rotates, it drags the AFM (Mn) layer spins irreversibly, hence increasing the FM (Fe) layer coercivity. In Fig. 4.14 the coercivity versus tempera-ture indicates two important results. One is the coercivity in perpendicular direction enhanced by 2.5 ML fcc-Fe buffer layer with perpendicular magnetization. The pos-sible explanation is that the perpendicular component of Mn layer spin configuration somehow ehanced by 2.5 ML fcc Fe buffer layer with perpendicular magnetization during growth. Thus the exchange coupling in perpendicular direction between Fe overlayer and Mn layer is enhanced, and the coercivity of the Fe overlayer is en-hanced. The other is that the exchange coupling of 6.3 ML Fe/4.5 ML Mn/2.5 ML Fe in perpendicular direction is larger than 6.3 ML/6.5 ML Mn. Generally

speak-4.4. Magnetic Properties of Fe/Mn/Fe/Cu3Au(001) 38

Figure 4.13: Hysteresis loops of 6.3 ML Fe/4.5 ML Mn/2.5 ML Fe/Cu₃Au(001) and 6.3 ML Fe/4.5 ML Mn/Cu₃Au(001). At 196 K coercivity is extraordinary enhanced caused by wetting layer effect.

ing, Mn layer with larger thickness results in larger exchange coupling between Fe overlyer and Mn layer. However the influence on excange coupling in perpendicular direction by fcc-like Fe buffer layer with perpendicular magnetization apparently remarkable.

4.4. Magnetic Properties of Fe/Mn/Fe/Cu3Au(001) 39

Figure 4.14: Coercivity with respect to temperature. All the films were grown at 300 K.

Observation of thickness dependent SRT

The spin reorientation transition (SRT) phenomenon is an interesting subject in magnetism which describes the switch of magnetic easy axis. There are seveal different mechanism to induce SRT phenomenon. The thickness and temperature dependent spin reorientation transition of Fe/Mn/Fe/Cu₃Au(001) will be described.

In Fig. 4.15 hysteresis loops of n ML Fe/4 ML Mn/2.5 ML Fe/Cu3Au(001) was mea-sured at 195 K. When the thickness of Fe overlayer less than 7 ML, the coercivity gradually increased with thickness due to more ferromagnetic moments. Then the magnetic easy axis was switched from perpendicular direction to in-plane direction.

M𝑟 versus thickness curves (Fig. 4.16) also supports the same result. The reason why the magnetic easy axis aligns in different directions at different thickness is discussed in the following. When the thickness of top Fe layer is less than about 7 ML , the symmetry breaking on the surface induces perpendicular magnetization.

However, when the thickness of Fe overlayer is larger than about 7 ML, the vol-ume shape anisotropy which arises from the dipolar interaction forces the magnetic

4.4. Magnetic Properties of Fe/Mn/Fe/Cu3Au(001) 40

Figure 4.15: Hysteresis loops of various coverage Fe grown on 4 ML Mn/2.5 ML Fe/Cu₃Au(001). The onset thickness for SRT is about 7 ML.

Figure 4.16: M_𝑟 versus thickness curves of n ML Fe/4 ML Mn/2.5 ML Fe/Cu₃Au(001). The transition region is 7 ML to 10 ML.

4.4. Magnetic Properties of Fe/Mn/Fe/Cu3Au(001) 41 easy axis aligned in in-plane direction. In Fig. 4.17 hysteresis loop of 8 ML Fe/5 ML Mn/Cu₃Au(001) was measured only in in-plane direction at 195 K. When 2, 2.5, and 3 ML fcc-like Fe buffer layer was added individually between Mn film and Cu₃Au(001), the magnetic easy axis was switched from in-plane direction to perpen-dicular direction. The coercivity of those system with fcc-like Fe buffer layer seems almost the same.

Figure 4.17: Hysteresis loops of 8 ML Fe/5 ML Mn/n ML Fe/Cu₃Au(001). The magnetic easy axes switch from perpendicular direction to in-plane direction by fcc-like Fe buffer layers.

Observation of temperature dependent SRT

In Fig. 4.18 hysteresis loops of 10 ML Fe/6 ML Mn/2.5 ML Fe/Cu₃Au(001) were measured at different temperature. At lowest temperature (195 K), the magnetic easy axis lay in perpendicular direction. With rising temperature, the magnetic easy axis started to switching from perpendicular to in-plane direction which meant the exchange coupling in perpendicular direction decreses. The key factor of tempera-ture dependent SRT is the lattice vibration caused by rising temperatempera-ture. At low coverage region, symmetry breaking which dominates the perpendicular magnetiza-tion is easily disturbed by lattice vibramagnetiza-tion.

4.4. Magnetic Properties of Fe/Mn/Fe/Cu3Au(001) 42

Figure 4.18: Hysteresis loops of 10 ML Fe/6 ML Mn/2.5 ML Fe/Cu₃Au(001) mea-sured at different temperature.

4.4. Magnetic Properties of Fe/Mn/Fe/Cu3Au(001) 43 Overview of SRT

Figure 4.19: Hysteresis loops of m ML Fe/6 ML Mn/2.5 ML Fe/Cu₃Au(001) mea-sured at different temperature.

4.4. Magnetic Properties of Fe/Mn/Fe/Cu3Au(001) 44

Figure 4.20: Hysteresis loops of m ML Fe/4 ML Mn/2.5 ML Fe/Cu₃Au(001) mea-sured at different temperature and 8 ML Fe/5 ML Mn/n ML Fe/Cu₃Au(001) mea-sured at 195 K.

4.4. Magnetic Properties of Fe/Mn/Fe/Cu3Au(001) 45

Figure 4.21: Hysteresis loops of m ML Fe/8 ML Mn/2.5 ML Fe/Cu₃Au(001) mea-sured at different temperature and overview of SRT with a magnetic phase diagram.

Chapter 5 Discussion

Mn/fcc-like Fe/Cu₃Au(001) and Mn/Cu₃Au(001)

From an intuitive aspect thin films grown on different substrate should appear differ-ent lattice orders but in our case Mn/fcc-like Fe/Cu₃Au(001) and Mn/Cu₃Au(001) present almost the same vertical interlayer distance. The thickness of the Fe buffer layer is 2.5 ML which means the vertical interlayer distance almost the same as the substrate Cu₃Au(001) (fcc, d_⊥= 1.89 ˚𝐴) (Fig. 5.1(a)). If the growth process is assumed to be epitaxial growth, although the interfaces are different, Mn films grown on fcc-like Fe/Cu₃Au(001) and Cu₃Au(001) should appear the similar lattice order. For Mn grown on Cu₃Au(001), Mn films with low coverage reveal a d⊥ al-most the same as that of the substrate Cu₃Au(001) (d_⊥= 1.89 ˚𝐴) and the d_⊥ is reduced to about 1.77 ˚𝐴 at a higher thickness. Since the LEED patterns indicates the coherence growth of Mn on Cu₃Au(001), the Mn films are concluded to perform a structural transition from a fcc to a fct structure and the critical thickness is about 12 ML (Fig. 5.1(b)). For Mn grown on fcc-like Fe/Cu₃Au(001), Mn films with low coverage reveal a d⊥about 1.91 ˚𝐴 and the d⊥ is reduced to about 1.83 ˚𝐴 at a higher thickness. The Mn films perform a structural transition from a fcc to a fct structure and the critical thickness is about 14 ML (Fig. 5.1(c)).

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