Uniaxial magnetic anisotropies in Fe films on single crystal and virtual Ge(001) substrates studied with spin polarized inverse photoemission and MOKE

Fe films have been grown at room temperature on standard Ge(001) single crystals and virtual $\mathrm{Ge}∕{\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x∕\mathrm{Si}\left(001\right)$ substrates, and their magnetic properties extensively investigated in situ by spin polarized inverse photoemission and magneto-optical Kerr effect. Two different uniaxial anisotropies have been found to coexist. The first one, giving rise to a [110] easy axis, is associated to the $\mathrm{Fe}∕\mathrm{Ge}$ interface: it disappears at large thickness (more than 10 Fe layers), while dominates in very thin films (5 Fe layers) on virtual substrates. A second anisotropy, considerably smaller in strength, originates a [010] or [100] easy axis: it persists at large thickness (up to 60 Fe layers) and is essentially associated to bulk properties. However, this is not an intrinsic property, being related to the sample preparation conditions, i.e., substrate sputtering at oblique incidence for cleaning and Fe deposition at oblique incidence. The uniaxial easy axis is always perpendicular to the incidence plane, either of the Fe atoms flux or the ion beam, with a larger effect of deposition conditions with respect to sputtering. Our results give evidence of a strong correlation between morphology and magnetism in $\mathrm{Fe}∕\mathrm{Ge}∕{\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x∕\mathrm{Si}\left(001\right)$ and $\mathrm{Fe}∕\mathrm{Ge}\left(001\right)$ films, opening the way to the engineering of magnetic properties via the control of the preparation conditions.

Figure 1

(a) The reference frame for $\mathit{H}$, $\mathit{M}$, and $\mathit{P}$ supposed in plane. (b) The spin polarized inverse photoemission spectra at different angles $\alpha$ on 5 ML of $\mathrm{Fe}∕\mathrm{Ge}∕{\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x∕\mathrm{Si}\left(001\right)$, with $\mathit{H}$ applied along [100]; full and empty dots refer to different directions of the incident electron spin; for the case of 45° full dots correspond to minority-spin electrons.

Figure 2

MOKE loops on $\mathrm{Fe}∕\mathrm{Ge}\left(001\right)$ films of different thicknesses: (a) longitudinal loops, with $\mathit{H}$ applied along the [100] direction; (b) transverse loops, with $\mathit{H}$ applied along the [010] direction.

Figure 3

Experimentally determined orientation of ${\mathit{M}}_0$ after application of $\mathit{H}$ along [100] in Fe films with different thicknesses on (a) Ge(001) and (b) $\mathrm{Ge}∕{\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x∕\mathrm{Si}\left(001\right)$. The determination has been done by applying the SPIPE analysis as described in the text [cf. also Fig. 1b].

Figure 4

MOKE loops on 5 ML of $\mathrm{Fe}∕\mathrm{Ge}\left(001\right)$; the sputtering/MBE planes of incidence are: (a) $\left(010\right)∕\left(010\right)$; (b) $\left(100\right)∕\left(010\right)$; (c) $\left(100\right)∕\left(100\right)$. Filled (empty) dots refer to longitudinal (transverse) configuration. In the inset the measured (circles) and simulated (continuous lines) hysteresis loops from film $C$ are reported.

Figure 5

Spin polarized inverse photoemission spectra at different angles $\alpha$ on 5 ML of $\mathrm{Fe}∕\mathrm{Ge}\left(001\right)$, with $\mathit{H}$ applied along [100]. The sputtering/MBE planes of incidence are: (a) $\left(010\right)∕\left(010\right)$; (b) $\left(100\right)∕\left(100\right)$.

Figure 6

MOKE loops on $\mathrm{Fe}∕\mathrm{Ge}∕{\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x∕\mathrm{Si}\left(001\right)$ films of different thicknesses: (a) longitudinal loops; (b) transverse loops.

Figure 7

Optical microscope image showing the cross-hatch morphology typical of virtual substrates.

Figure 8

Measured (empty circles) and simulated (continuous line) hysteresis loops from 10 ML $\mathrm{Fe}∕\mathrm{Ge}∕{\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x∕\mathrm{Si}\left(001\right)$ film: (a) longitudinal loop; (b) transverse loop.

Figure 9

Measured (empty circles) and simulated (continuous line) longitudinal hysteresis loop on 5 ML $\mathrm{Fe}∕\mathrm{Ge}∕{\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x∕\mathrm{Si}\left(001\right)$ film with the external field $\mathit{H}$ applied along [100]. The direction of $\mathit{M}$ at different values of $\mathit{H}$ is indicated by arrows.