Modification of the secondary-electron spin polarization in Co/Cu(110) films via gaseous adsorbates

The effect of O and N adsorbates on the magnetic properties of ultrathin Co/Cu(110) films was investigated in situ by measuring the spin polarization of secondary electrons emitted from the surface using Mott polarimetry. The data have been fitted to a function to take into account the exponential attenuation of the signal through the layers above, yielding a room temperature paramagnetic-ferromagnetic transition thickness of $d_c=3.5\pm 1.0$ monolayers (ML). This evolution was compared to the equivalent evolution for cobalt films grown on O- and N-saturated surfaces. In the case of nitrogen, the onset of ferromagnetism occurs at a thickness of $4.9\pm 0.5$ ML and the saturation polarization is reduced to 65% of the value for the bare cobalt film, implying a dramatic quenching of the magnetic moment in the uppermost layer of the films. For films grown on the oxygen surface, a change in growth mode from three-dimensional to layer-by-layer growth dominates the behavior and this leads to a decrease in $d_c$ to $2.3\pm 0.3$ ML and a 38% increase in the saturation polarization value.

Figure 1

LEED images of the copper substrate and cobalt films for the different surfaces. The beam energy is shown in the bottom corner of each image. The crystallographic axes are presented in the legend below.

Figure 2

The evolution of Auger peak heights with cobalt thickness for cobalt grown on oxygen and nitrogen reconstructed copper surfaces, compiled over several growth runs. The AES signal is plotted relative to the bare copper peak height in each case. The gas peak remains roughly constant as the cobalt increases and copper decreases. The lines are guides for the eye.

Figure 3

A graph showing the thickness evolution of the M-H loop coercivity, measured using MOKE, for the bare cobalt and gas-covered films. The coercivity has been normalized with respect to the maximum value for each type of sample, as indicated in the legend. The lines are guides for the eye.

Figure 4

A graph showing the thickness evolution of the SESP for the different surfaces. The polarization has been normalized with respect to the saturation polarization for the uncovered surface, $P_{s,\text{Co}}$, in each case. The lines represent fits to the data as described in the text. Different style data points correspond to different experimental runs. In the case of Co, the solid square data set is compiled from separate time evolution experiments.