Ab initio investigations of magnetic properties of ultrathin transition ...

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4.1 3d-Monolayers on (001) oriented substrates 53 E [meV./3d-atom] Δ 500 400 300 200 100 0 −100 −200 −300 −400 −500 FM c(2x2)-AFM Rh(001) Rh(111) 1 ML 3d on Ag(001) Pd(001) Pd(001) W(001) Rh(001) W(001) Rh(001) V Cr Mn Fe Co Ni Figure 4.2: Total energy difference ΔE = EAF M − EFM per 3d atom between the c(2×2)- AFM and p(1×1)-FM phase for 3d monolayers on Ag(001)(circles), Pd(001) (diamonds) and W(001)(triangles). ΔE >0(< 0) means that the ferromagnetic (antiferromagnetic) configuration is the most stable one[3, 5, 7]. atoms within the monolayer and four substrate interface atoms. Considering that the 5d orbitals of W are more extended than the 3d ones of the overlayer and taking the additional interlayer relaxation into account, we can conclude that here the hybridization between the overlayer and the substrate is more important than the hybridization between the 3d atoms in the monolayer plane. Thus, the nature of the 3d-4d or -5d bond determines the physics. The strong 3d-5d hybridization effects also the magnetic moments, displayed in figure 4.3. Similar to fcc substrates the overall trend of the spin moments across the 3d series follows Hund’s first rule. This atomic-like behavior indicates that the magnetism is dominated by the local intra-atomic contribution. A comparison with unsupported monolayers (UML) using the experimental W lattice constant shows that the magnetic moment of the 3d overlayer is reduced due to the interaction with the substrate, which is a consequence of the 3d-5d hybridization. The magnetism of the overlayer also polarizes the substrate. For the FM configuration, the W atoms at the interface are antiferromagnetically coupled to the monolayer (apart from the case of Co) and carry a moment which is roughly proportional to that of the 3d TM. The induced polarization decreases rapidly with distance from the interface into the bulk and is already one order of magnitude lower at the second W layer. The sign of the magnetization oscillates from one W layer to the next, indicating a layered AFM (LAFM) susceptibility of W(001). For antiferromagnetic monolayers, the W moments in every second layer are suppressed due to symmetry.
54 4 Collinear magnetism of 3d-monolayers on Rh substrates Magnetic moment [μ Β ] 5.0 4.0 3.0 2.0 1.0 0.00 −0.2 −0.4 FM AFM 1 W(001) ML 3d/W(001) 3d UML W(I) V Cr Mn Fe Co Ni Figure 4.3: Local magnetic moments of 3d monolayers on W(001) calculated for the p(1×1) ferro- (solid symbols connected by solid line) and the c(2×2) antiferromagnetic configuration (open symbols connected by dashed line). For the FM case, the magnetic moment of interface W atoms is given by triangles[7]. 4.2 Results of 3d-Monolayers on Rh(001) Substrate In this section we will show results of ab initio calculations of 3d monolayers on the 4d TM substrate Rh(001). Rh has a large Stoner enhanced susceptibility as shown for Rh films on Fe [15] and FeRh is known to form ordered alloys in the cesium chloride (CsCltype) structure with subtle magnetic properties[16, 17, 18]. The lattice constant of Rh (ao =3.80 ˚A) is in between those of Cu and Ag and thus Rh serves as a potential substrate to grow artificial phases of 3d transition-metal films such as fcc-Fe stabilized under tensile strain or bcc-Co under compressive strain. The Rh(001) substrate provides favorable growth conditions for transition-metal films despite a large lattice mismatch of fcc Fe or Co and bcc Fe with Rh of about 6%, 8% and -7%, respectively. For example, no notable intermixing has been encountered at the interface of Fe/Rh(001) during growth of Fe films[19]. Epitaxial, pseudomorphic layer-by-layer growth of one and two layers of Co on Rh(001) was reported by Begley et al. [20] and several groups [19, 21, 22, 23] have been able to grow pseudomorphically even thicker films of face-centered tetragonal Fe on Rh(001). Hayashi et al.[22, 23] concluded on the basis of soft X-ray magnetic circular dichroism (XMCD) experiments measured at room temperature that a monolayer and a bilayer of Fe are not ferromagnetic and interpreted them as magnetically dead caused by the large strain exceeded in the interface of the thin film and the substrate. Hwang et al.[24] found experimentally a suppression of the ferromagnetic order of Fe overlayers on the Rh(001) surface, and he as well as Spisak and Hanfer[25] predicted a c(2 × 2) AFM order for 1 ML Fe on Rh(001) on the basis of DFT calculations.
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Mitglied der Helmholtz-Gemeinschaft
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Forschungszentrum Jülich GmbH Inst
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Abstract In this thesis, we investi
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”A human being is part of the who
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II Contents 4.2.1 Relaxations and m
Page 15 and 16: 2 INTRODUCTION (ao =3.80 ˚A) is in
Page 17 and 18: 4 INTRODUCTION understanding of our
Page 19 and 20: 6 INTRODUCTION
Page 21 and 22: 8 1 Density functional theory (DFT)
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Page 29 and 30: 16 2 The FLAPW method 1 0 0 1 Figur
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Page 33 and 34: 20 2 The FLAPW method metal systems
Page 35 and 36: 22 2 The FLAPW method Then one can
Page 37 and 38: 24 2 The FLAPW method Evac is the v
Page 39 and 40: 26 2 The FLAPW method of the operat
Page 41 and 42: 28 2 The FLAPW method a matrix expr
Page 43 and 44: 30 3 Magnetism of low dimensional s
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104 6 Co MCA from monolayers to ato
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116 rather subtle. To shed more lig
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118
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120 where, the relation � i H =
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122 Figure 6.8: The real (left) and
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124 E(q) = −M 2 ( 2J2 +2J6 +cos(
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126 For a 2D hexagonal lattice, the
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128 Bibliography [12] P. Ferriani,
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130 Bibliography [39] A. Dallmeyer,
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132 Bibliography [68] J. Perdew and
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134 Bibliography [101] L. Nordströ
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136 Bibliography [130] O. Le Bacq,
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138 Bibliography [160] P. M. Marcus
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140 Bibliography [190] R. Germar, W
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Acknowledgement I don’t know how
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Schriften des Forschungszentrums J
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