Takaya Mitsui
Magnetism Research Group, Synchrotron Radiation Research Center

The study of the surface and interface magnetism of 3 dtransition metals is of interest due to the essential role thatmagnetism plays in determining magnetic interactions and spintransport properties of nanomagnets and magnetic hetero
junctions. Over the past few decades, various techniques haveadvanced the research on surface and interface magnetism.
However, few experimental studies have investigated the depthdependent local magnetic structures of surfaces and interfaces at
the atomic layer level. This is due to the difficulties encounteredwhen performing depth-resolved studies at the uppermost surface
of a metal, such as with scanning tunneling microscopy, or due tothe signal arising from a relatively broad depth range, e.g., with
x-ray magnetic circular dichroism spectroscopy.
The surface magnetism of Fe(001) is a fascinating researchsubject for atomic-layer-resolved magnetic analysis. Theoreticalstudies predict a 30% enhancement of the magnetic moment MFeat the surface and an oscillatory behavior with increasing depth
in the individual layers, i.e., a magnetic Friedel oscillation [1]. Asa related phenomenon, Ohnishi et al. theoretically predicted thatthe hyperfine magnetic field Hint is reduced by 30% relative to thebulk value despite a significant increase in the surface MFe [2].
Recently, we determined the layer-by-layer Hint of theFe(001) surface by the in situ 57Fe probe layer method with a highbrilliance synchrotron Mössbauer source [3]. In this method, aresonant isotope probe layer is embedded in a thin film preparedwith a non-resonant isotope. The observed Hint at the nucleus
provides details on the local surface magnetism.
Fe(001) films were fabricated by alternatively evaporating
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Fe and 57Fe from 99.94% iron-56 and 95.93% 57Fe isotopic
sources onto pre-cleaned 10×10×0.5 mm3 MgO(001) substrates
under a vacuum pressure of approximately 10 8 Pa. A 0.8-ML
thick 57Fe probe layer (t = 0.1 nm) was embedded to the depth of
the Nth atomic layer below the surface where N = 1 to 4 and 7.
These samples are hereafter referred toNth probe layer

Figure 1b presents the Mössbauer spectra of the Nth probelayer samples (N = 1 to 4 and 7) recorded at 300 K. All samples
showed magnetically split Mössbauer patterns. The spectra of thefirst, second, and third probe layer samples exhibited complex
profiles composed of different magnetic components, i.e., smallHint (red lines, around 28 T), large Hint (blue lines, around 36 T),
and bulk-like Hint (green lines, around 32 T).

The experiments were performed at the BL11XU beamlineof SPring-polarized 14.4-rays
with a 15.4 neV bandwidth produced by a synchrotron Mössbauer-ray beam was vertically focused by an ellipticalFigure 1. (a) Experimental setup. (b) Mössbauer spectraof the Nth probe layer samples measured at 300 K. Blacksolid lines represent the fitted curves. Red, blue, andgreen lines represent the three different magneticcomponents. M(i) represents the magnetic component
assigned to the 57Fe atoms located in the ith layer below
the surface. Hex is the magnetic field (300 Oe).

flux was approximately 2.9×104 photons/s. This beam wasintroduced into the measurement chamber to perform grazing
incidence measurements (Fig. 1a). An external field of 300 Oe
was applied antiparallel to the beam direction to magnetize the-polarized incident beamm = ±1. The
Mössbauer absorption spectra were measured by collecting the-rays from the sample surface at an incident
angle of 0.1° with a reflectivity of approximately 80%. Eachspectrum was obtained within a few hours of sample preparation.
Such rapid measurements significantly reduced the residual gas
absorption and oxidation on the Fe(001) surfaces.

The ideal probe layer in the sample was surrounded byfinely distributed 57Fe atoms, which stemmed from the random
deposition and surface diffusion of iron atoms during the growth
process. Figure 1b (right) shows a conceptual diagram of theprocess. In this case, if the first, second, and third layers of theiron surface have a different Hint the spectra should exhibit acomplex profile with multiple components. Based on thesystematic behavior of the three components, the small Hint, large

42Hint, and bulk-like Hint represent the intrinsic hyperfine fields forthe first, second, and third layers from the surface, respectively.
In contrast, the spectra of the fourth and seventh probe layer
samples exhibited a single magnetic component with fourabsorption lines, even in the presence of finely distributed 57Fe
atoms. This is because the hyperfine fields of the neighboringlayers at these depths are bulk-like, and the overlapping
subspectra result in a simple absorption profile. The prominentsubspectrum with the largest percent area in the Nth probe layer
sample was assigned to the spectrum characterizing the 57Fe
atoms located in the Nth atomic layer from the surface.
The experimentally determined layer-by-layer Hint
exhibited a marked decrease at the surface and an oscillatory
decay toward the bulk value. Such a behavior was successfully
reproduced by theoretical calculations (Fig. 2). This result

provides the first experimental evidence for the magnetic Friedel
oscillations, which penetrate several layers below the Fe(001)surface. Theoretically, the oscillatory decay of Hint should bestrongly coupled with the Friedel oscillation of MFe, which iscaused by the surface electronic structure with a large spin
imbalance and d-band narrowing [1, 2, 3].
In summary, the surface magnetism of Fe(001) was studied
in an atomic layer-by-layer manner by using the in situ 57Fe probelayer method with a synchrotron Mössbauer source. The observed
Hint exhibited a marked decrease at the surface and an oscillatory
behavior with increasing depth in the individual upper four layers
below the surface. In the future, the in situ 57Fe probe layer
method with a synchrotron Mössbauer source should facilitate
additional studies on the surface and interface magnetism inadvanced magnetic and spintronic materials and devices.

The authors thank Prof. K. Mibu, Prof. Y. Yamada, Dr. H.
Naramoto, Dr. S. Entani, Dr. T. Inami, and Dr. Y. Katayama for
their helpful discussions.

1. C. S. Wang and A. J. Freeman, PRB 24 (1981)4364.
2. S. Ohnishi et al: PRB 28 (1983) 6741.
3. T. Mitsui et al: PRL 125 (2020) 236806.

Figure 2. The experimental and theoretical layer-by-layer
hyperfine magnetic fields.

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