Effect of annealing on Co${}_2$FeAl${}_{0.5}$Si${}_{0.5}$ thin films: A magneto-optical and x-ray absorption study

A series of Al and MgO-capped Co${}_2$FeAl${}_{0.5}$Si${}_{0.5}$ epitaxial thin films grown on MgO with various levels of L2${}_1$ ordering was obtained by in situ annealing. The films were studied by means of x-ray absorption spectroscopy, x-ray magnetic circular dichroism (XMCD), magneto-optical Kerr effect magnetometry, and Brillouin light scattering. We find the anisotropy constants decrease, while the spin wave stiffness increases as the samples are annealed to higher temperatures. The magnetization as determined by Brillouin light scattering reveals a maximum value at intermediate annealing temperatures. Surprisingly, the orbital-to-spin-moment ratio (as seen from XMCD) is essentially stable through the sample series and does not change upon annealing, despite the observed changes in anisotropy and exchange.

Figure 1

Geometry for the MOKE and QMOKE measurements in saturation (for the latter, each field direction $n$, ${\mu }_0{H}_n=50$ mT). The plane of incidence is along the $H_{180}$-$H_0$ direction, and the angle shown is $\alpha$ in the text.

Figure 2

X-ray absorption spectra (XAS) of Co${}_2$FeAl${}_{0.5}$Si${}_{0.5}$ annealed at 550 °C at (a) the Fe and (b) the Co $L_{2,3}$ edge averaged from the total electron yield (TEY) and transmission intensities (TM) measured at 300 K for the magnetization direction parallel (antiparallel) to the x-ray polarization, ($I^{+}+I^{-})/2$. (c,d) Corresponding XMCD spectra, $I^{+}-I^{-}$. The inset in (b) indicates the determination of the spectral weight of the satellite peak in the Co XAS spectra. Solid lines, TM spectra; dashed lines, TEY spectra. Blue represents Al-capped samples; orange, MgO-capped samples.

Figure 3

Spectral weight of the satellite peak after linear background subtraction vs. annealing temperature, indicating the increase in local atomic order with increasing annealing temperature. $\blacksquare$, Al-capped, TM; $•$, MgO-capped, TM; $\square$, Al-capped, TEY; $ˆ$, MgO-capped, TEY. Lines are guides for the eye.

Figure 4

Ratio of surface (TEY)- and bulk (TM)-related spin magnetic moment $m_{\mathrm{s}}(\mathrm{TEY})/m_{\mathrm{s}}(\mathrm{TM})$ for (a) Fe and (b) Co. Data for the Al-capped series are indicated by $\square$; data for the MgO-capped series, by $ˆ$. The ratio of orbital-to-spin magnetic moment $m_{\mathrm{l}}/m_{\mathrm{s}}$ for (c) Fe and (d) Co as a function of annealing temperature. Labels for (c) and (d) as in the captions to Fig. 3. Lines are guides for the eye.

Figure 5

Measured Kerr rotation loops at various in-plane orientations for the as-prepared sample. The vertical scale is the same for both graphs.

Figure 6

(a) Experimental data $ˆ$: Al-capped sample, $T_{\mathrm{anneal}}=600$ °C, in-plane orientation $\alpha =90$ °) and best fit (solid line). (b) Determined cubic ($K_C$; $•$) and uniaxial ($K_U$; $\blacksquare$) constants for the Al-capped series of samples, as a function of annealing temperature.

Figure 7

Polar plots of the coercive field measured for (a) as-prepared sample and samples annealed at (b) 480 °C, (c) 525 °C, (d) 550 °C, (e) 575 °C, and (f) 600 °C.

Figure 8

QMOKE contributions obtained at normal incidence as a function of the sample orientation for an annealing temperature of 600 °C. Lines are fit to Eq. (2).

Figure 9

Amplitude and offset of QMOKE components as a function of the annealing temperature (a) for the Al-capped sample series and (b) for the MgO-capped sample series. $\blacksquare$, $M_{\mathrm{L}}{M}_{\mathrm{T}}$ offset; $•$, $M_{\mathrm{L}}{M}_{\mathrm{T}}$ amplitude; $ˆ$, $M_{\mathrm{L}}^2-M_{\mathrm{T}}^2$ amplitude. Lines are guides for the eye.

Figure 10

Measured BLS spectra for the as-prepared sample (a) and for samples annealed in situ at (b) 480 °C, (c) 525 °C, (d) 550 °C, (e) 575 °C, and (f) 600 °C. All spectra measured under an applied field ${\mu }_{0}H=130$ mT and angle of incidence $\phi$ $=$ 45 °. Only the anti-Stokes part of the spectrum is shown.

Figure 11

(a) Determined saturation magnetization per formula unit (fu), (b) spin-wave stiffness $D$, and (c) exchange constant $A$ determined from fitting the spin-wave frequencies measured by BLS. Lines are guides for the eye.