1. Motivation Oscillatory magnetic anisotropy originating from
quantum well states in Fe films
Jia Li1 , Fikret Yildiz 2, XiaoDong Ma , Marek Przybylski2 , Yizheng Wu1
1. Department of Physics, Fudan University, Shanghai, China
2. Max-Planck-Institut für Mikrostrukturphysik, Halle, Germany
Motivation
Modern magnetoeletcronic devices are based on metallic films with thicknesses on the nanometer scale. The electrons in the ultrathin films with nanometer scale could be confined to form quantum well states (QWS), which could result in novel oscillatory behaviors of physical properties as a function of film thickness. For example, like the oscillation of the interlayer exchange coupling, magneto-optic effect, tunneling magnetic resistance, and magnetic anisotropy.
In this poster, we report our study of the step-induced in-plane magnetic anisotropy of Fe thin film grown on Ag(1,1,10) vicinal surface by Magneto-Optic Kerr Effect (MOKE). The strength and the easy axis direction of the uniaxial magnetic anisotropy exhibits a novel thickness-dependent oscillatory behavior with a periodicity of 5.7 monolayers(ML) with the temperature below 200K. Such anisotropy oscillation is attributed to the QWS of the d-electrons at the Fermi level of the Fe film, which is different from most of the other oscillatory properties due to the QWS in non-magnetic layers.
Experiment
Sample:
Ag(1,1,10) substrate was prepared by cycles of 1keV Ar+ sputtering at 450° and annealing at 600°.
Fe wedge films were grown epitaxially on Ag(1,1,10) vicinal surface at room temperature.
Anneal the sample at 150 °after deposition
Measurement: in situ Longitudinal MOKE
LEED pattern of Ag(1,1,10)
hysteresis loop for 60ML Fe/Ag(1,1,10) vicinal surface
we can get the double loop when perpendicular to the step which is quite similar to the previous results. We define the Hs as what is indicated in the right diagram.
The shape of double loop shown in the right diagram came from the competition between 4-fold anisotropy of bulk Fe and uniaxial step-induced anisotropy.
Longitudinal MOKE for //step and ⊥step
STM image of Ag(1,1,10) H Hs
Shift field as a function of Fe thickness measured for Fe/Ag(1,1,10)
Temperature dependent Hs~dFe for Au/Fe/Ag(1,1,10)
(a) RT and (b) 5 K
hysteresis loops for different Fe thicknesses measured at T = 5K
Inset shows the the 1/dFe dependence of the Hs measured at RT
The oscillation disappeared above 200K
Hs~T for Au/Fe/Ag(1,1,10) with dFe=(15ML,27ML,40ML)
Energy separation of QWS states
40ML Δ~60meV
27ML Δ~90meV
15ML Δ~160meV
Red line in (a) is a result of fitting as described in the text. Inset shows geometry of the MOKE experiment and a representative hysteresis loop.
Applied H:
(a)perpendicular to the steps
(b)parallel to the steps
mean free path of the 3d electrons which localized at the Fermi level should be large enough to form the QWS.
Splitting field Hs oscillate with the Fe thickness at 5K, indicating that step-induced uniaxial anisotropy oscillate with Fe thickness at 5K, which can be confirmed by the hysteresis loops measured by H //step and ⊥step.
This is the possible reason: the oscillation could only be observed at very low temperature
Conclusion:
A strong quantum oscillation of the uniaxial anisotropy is observed at Low Temperature with a period of 5.7 ± 0.2 ML.
For the Au-covered Fe films, the easy magnetization axis also oscillates between perpendicular and parallel to the steps with the same period.
Such a quantum oscillation is attributed to the QWS in a minority-spin d band at EF .
The lack of oscillation at RT may result from the thermal reduction of the Mean Free Path.
At RT, the 1/dFe behavior of the in-plane uniaxial anisotropy was observed.