P19S

23AG037

BL-9B

Spin-polarized electronic states of FeCo thin filmon Rh(001) substrate
K. Kunitomoa, K. Sumidab, K. Miyamotob, C. Zhanga,
Y. Sakurabac, and T. OkudabaGraduate School of Advanced Science and Engineering Hiroshima University, 1-3-1 Kagamiyama,
Higashi-Hiroshima 739-8526, Japanb
Hiroshima Synchrotron Radiation Center (HSRC), Hiroshima University 2-313 Kagamiyama,
Higashi-Hiroshima 739-0046, Japanc
National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

Keywords: FeCo alloys, spin polarization, perpendicular magnetic anisotropy (PMA)

The emergence of perpendicular magnetic anisotropy (PMA) in magnetic thin films is essential from theviewpoint of practical applications, e.g., increasing the recording density of storage devices. Many magnetic
thin films exhibiting PMA, such as Co/Pt, Co/Pd, FePt, TbFeCo, and GdFeCo, have been extensivelyinvestigated. Among them, “rare-earth and noble-metal elements free” FeCo alloy films are considered
promising candidates exhibiting strong PMA. Burkert et al. predicted that a magnetic anisotropy energy may
increase in FeCo alloys when the tetragonal distortion is applied [1]. Remarkably, around a c/a ratio of 1.201.25, the magnetic anisotropy energy exceeds 700-800 μeV/atom, which is one or two orders of magnitudelarger than that of pure Fe or Co. The emergence of strong PMA was experimentally confirmed in thetetragonally distorted FeCo ultra-thin-films grown on Rh(001) (c/a = 1.24) with a thickness of 13-17
monolayers (ML) [2].
Generally, it is known that the band structure near the Fermi energy (EF) plays a very important role inachieving PMA [3,4]. The theoretical calculations suggest that the microscopic origin of PMA in thetetragonally distorted FeCo alloy is attributed to the minority-spin states composed of dxy and dx2-y2 orbitals
near EF, hybridized by spin-orbit interactions [1]. However, the spin- and orbital-dependent electronic statesof the tetragonally distorted FeCo alloy films are poorly investigated experimentally.
In this study, we have fabricated FeCo thin films on Rh(001) substrate by molecular beam epitaxy andinvestigated the spin-polarized electronic states by in-situ spin- and angle-resolved photoemission
spectroscopy (spin-ARPES) at BL-9B of Hiroshima Synchrotron Radiation Center [5]. The quality of thesample surface was checked by low energy electron diffraction and Auger electron spectroscopy. Thethickness of the film was 13.6 ML. We also carried out the first-principles calculations for the tetragonally
distorted FeCo alloy (c/a =1.24) with/without considering the spin-orbit coupling using WIEN2k program[6]. The exchange-correlation was treated using the generalized gradient approximation.
To determine the high symmetry points along kz direction [Fig. 1(a)], we first performed the photonenergy-dependent measurement. Figure 1(b) shows the ARPES image acquired at various incident photon
energies from 45 to 70 eV with s-polarization. We can recognize the mostly non-dispersive bands around EB= 1 eV with kz = 3.7 Å-1 and just below EF with kz = 4.4 Å-1. The observed features are reproduced by thecalculations (not shown here), and we determined that kz = 3.6 Å-1 (hν = 45 eV) corresponds to the Γ point.
Figure 1(c) shows the ARPES image along the Γ-X line recorded at 45 eV with p-polarization. A very steep
band crossing EF around the X point is recognized. In addition, weakly dispersive bands are found in between
EF to 1.0 eV around the Γ point.
To experimentally verify the spin-polarization, we next performed the spin-resolved measurements. The
sample was magnetized along (001) direction at room temperature prior to the measurements. Figures 2(a)and 2(b) display the out-of-plane spin-resolved energy distribution curves (EDCs) and spin-polarization
recorded at the Γ and X points, respectively. Here, red and blue represent the majority- and minority-spincomponents. For both momenta, we can clearly see the out-of-plane spin-polarization. At the Γ point, wehave observed a double peak structure in the minority-spin channel around EB = 0.2 and 0.6 eV, although thebroad feature can be seen in the wide energy range in the majority-spin channel [Fig. 2(a)]. On the otherhand, at the X point, a prominent majority-spin peak exists at EF [Fig. 2(b)]. These findings tell us that theweakly dispersive bands around the Γ point and the steep band around the X point mainly comprise themajority-spin and majority-spin characters, respectively. Based on the theoretical investigations [2], themajority-spin states composed of dxy and dx2-y2 orbitals play an important role in the emergence of the PMAof the tetragonally distorted FeCo films. We finally compare the spin-ARPES results with the first-principles
calculations to scrutinize the orbital characters. Figure 2(c) shows the calculated band dispersion along theΓ-X line in the minority-spin channel. The radii of the orange (black) circles are proportional to the net
contribution of dxy (dx2-y2) orbital. By a comparison of Figs. 2(a), 2(b) and 2(c), it signifies that the observed
minority-spin peak located just below EF at the Γ point (black arrow) and at 1.1 eV at the X point (orangearrow) are mainly attributed to the dx2-y2 and dxy orbitals, respectively.
In summary, we have investigated the spin-polarized electronic states of the FeCo/Rh(001) films by insitu spin-ARPES and first-principles calculations. The out-of-plane spin-polarization indicating theemergence of PMA was clearly observed at room temperature with zero-field. We conclude that the minorityspin states near EF are mainly composed of dx2-y2 and dxy orbitals. Our findings provide the fundamental
properties of the FeCo thin films and pave the way for the PMA-based applications, such as the photoelectron
spin polarimeter, enabling the detection of the out-of-plane spin components.

FIGURE1 (a) Brillouin Zone of bct FeCo. (b) ARPES image of FeCo/Rh(001) film acquired from 45 to 75 eV with p-polarizedlight. (c) ARPES image along the Γ-X line acquired at 45 eV with p-polarized light.

FIGURE2 (a) Out-of-plane spin-EDCs and spin-polarization at the Γ point. (b) Same as (a) but at the X point. (c) Calculatedband dispersion along the Γ-X line in the minority-spin channel. Black and orange represent the dx2-y2 and dxy orbitals,respectively.

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