Quest for magnons in ultrathin nickel films

High-momentum spin waves (magnons) in ultrathin films of cobalt and iron have been explored thoroughly using inelastic scattering of low-energy electrons. The search for magnons in ultrathin nickel films failed, however, although high-energy magnons do exist in bulk nickel. The failure might be due to the weak coupling of nickel magnons to scattering electrons. In order to increase the coupling we deposited layers of cobalt onto Ni films and successfully studied the magnons of such films. The acoustic modes show the same dispersion as pure Co films, which is consistent with the nearly identical stiffness of bulk magnons in nickel and cobalt. Standing magnons in Ni films covered with Co are strongly damped at the Ni/Cu(100) interface when their total wave vector exceeds about $3\mathrm{n}{\mathrm{m}}^{-1}$.

Figure 1

Classical precession amplitudes of the two lowest eigenmodes of a six-layer film (a) with homogenous exchange coupling shown as red bars (see text for details).

Figure 2

Illustration of the investigated layer systems.

Figure 3

Illustration of the scattering geometry.

Figure 4

LEED pattern at 167 eV of a $\mathrm{N}{\mathrm{i}}_3\mathrm{C}{\mathrm{o}}_3$ film (total of six layers).

Figure 5

Elastic intensity as function of the wave-vector transfer for an eight-layer Co film $\mathrm{C}{\mathrm{o}}_8$ and a $\mathrm{N}{\mathrm{i}}_4\mathrm{C}{\mathrm{o}}_4$ film is shown as red open squares and blue solid circles, respectively. The blue dashed line is a simulation assuming that the diffuse scattering is caused by a random array of steps with a mean terrace width $w=45$ nm.

Figure 6

Magnon spectra for $\mathrm{C}{\mathrm{o}}_6$ and $\mathrm{N}{\mathrm{i}}_3\mathrm{C}{\mathrm{o}}_3$ films deposited on Cu(100). The spectra are corrected for the Bose occupation number. Data accumulation time is 6 s/channel. The $\mathrm{C}{\mathrm{o}}_6$ spectrum is obtained with less resolution to boost the intensity of the intrinsically broader standing mode.

Figure 7

Magnon spectra of a $\mathrm{N}{\mathrm{i}}_4\mathrm{C}{\mathrm{o}}_2$ film corrected for the Bose occupation number at 300 K and 90 K. The acoustic mode is at the same energy at both temperatures.

Figure 8

Dispersion data of the acoustic mode of four different films, $\mathrm{N}{\mathrm{i}}_3\mathrm{C}{\mathrm{o}}_3,\mathrm{N}{\mathrm{i}}_4\mathrm{C}{\mathrm{o}}_2,\mathrm{C}{\mathrm{o}}_6$, and $\mathrm{C}{\mathrm{o}}_3$, are shown as red squares, magenta diamonds, olive up triangles, and blue down triangles. The dashed line is a common parabola fit to $\mathrm{N}{\mathrm{i}}_3\mathrm{C}{\mathrm{o}}_3,\mathrm{N}{\mathrm{i}}_4\mathrm{C}{\mathrm{o}}_2$, and $\mathrm{C}{\mathrm{o}}_6$ according to Eq. (2), the solid line the same fit to the three-layer Co film.

Figure 9

Sample spectra of $\mathrm{C}{\mathrm{o}}_8,\mathrm{N}{\mathrm{i}}_3\mathrm{C}{\mathrm{o}}_8$, and $\mathrm{N}{\mathrm{i}}_5\mathrm{C}{\mathrm{o}}_8$ films for a wave vector of $q_{\parallel }=1.8\mathrm{n}{\mathrm{m}}^{-1}$. The acoustic mode has the same energy in all three films (dash-dotted line) are shown in (a), (b), and (c), respectively. All spectra show a standing mode. In films with a nickel underlay (b), (c) the energy is downshifted (dashed line).

Figure 10

(a) Dispersion data for the acoustic and standing modes of $\mathrm{C}{\mathrm{o}}_6$ (red solid circles and squares, respectively) and $\mathrm{N}{\mathrm{i}}_3\mathrm{C}{\mathrm{o}}_6$ (open blue circles and squares, respectively). (b) Data for the acoustic and standing modes of $\mathrm{C}{\mathrm{o}}_8$ (red solid circles and squares, respectively) and $\mathrm{N}{\mathrm{i}}_3\mathrm{C}{\mathrm{o}}_8$ (open blue circles and squares, respectively). The lines are calculated in a Heisenberg model (see text for discussion).

Figure 11

Comparison of spectra at $q_{\parallel }=1.8\mathrm{n}{\mathrm{m}}^{-1}$ for the $\mathrm{N}{\mathrm{i}}_3\mathrm{C}{\mathrm{o}}_8$ and $\mathrm{N}{\mathrm{i}}_3\mathrm{C}{\mathrm{o}}_6$ films. The standing wave for the latter film is significantly more damped.