Thickness-driven polar spin reorientation transition in ultrathin Fe/Au(001) films

The magnetic properties of ultrathin Fe films grown on Au(001) were studied at room temperature as a function of iron thickness in the range of two to three Fe monolayers (ML). The magneto-optic Kerr effect (MOKE) indicated that a spin reorientation from an in-plane direction to the film normal direction takes place when the iron thickness is reduced from 2.3 to 2.0 ML. Values of the effective magnetic anisotropy constants were determined from MOKE and superconducting quantum interference device measurements. The flow analysis of the effective anisotropy constants in anisotropy space revealed that the transition occurs via an intermediate state of canted magnetization.

Figure 1

(Color online) Right column: LEED patterns collected for: (a) Au(001) buffer layer, (b) 2.0 ML of Fe, and (c) 3.0 ML of Fe. The energy of the incident electron beam is given for each pattern. Left column: [(d) and (e)] topographic STM images for $\sim 2.5\text{ML}$ of Fe grown on Au(001) with cross section taken along white line.

Figure 2

Polar and longitudinal MOKE loops measured in the range of the spin reorientation transition for successive iron thicknesses between 3.0 and 2.0 ML. The hard loops are presented with corresponding simulated curves.

Figure 3

Anisotropy field $H_a$ derived from the hard-axis MOKE loops as a function of the nominal Fe layer thickness.

Figure 4

SQUID magnetization loops for 3 ML of Fe measured at room temperature for in-plane (upper panel) and out-of-plane (lower panel) magnetic field configurations.

Figure 5

Anisotropy space spanned by the anisotropy constants $K_1{}^{eff}$ and $K_2{}^{eff}$ (after Refs. 3, 59). Areas marked by horizontal, vertical, canted, and criss-crossed lines correspond to phases with spontaneous in-plane, perpendicular, and canted magnetizations and to phases of coexistence, respectively. The insets show energy dependencies on the angle $\theta$ characteristic for the distinguished regions. The straight lines (with arrows) across the anisotropy space represent three possible scenarios of the spin reorientation transition for a linear thickness-driven evolution of anisotropy constants: (1) through the point with zero anisotropy, (2) through a canted phase, and (3) through the area of phase coexistence.

Figure 6

The values of $K_1{}^{eff}t$ and $K_2{}^{eff}t$ obtained from the fitted hard-axis MOKE loops and corresponding linear regressions (straight lines) as a function of the Fe layer thickness $t$.

Figure 7

Experimental values of $K_1{}^{eff}$ and $K_2{}^{eff}$ plotted in anisotropy space for Fe layers in the thickness range of 2.0–3.0 ML. The straight line through the experimental points corresponds to the thickness-driven evolution of the anisotropy constants and indicates that the SRT from the in-plane to out-of-plane easy direction occurs via the area of a canted phase. The insets show the character of the anisotropy energy for the respective areas of the anisotropy space.