Magnetic anisotropy of textured CrO 2 thin films investigated by X-ray magnetic circular dichroism

Abstract

Highly a-axis-textured CrO 2 films have been deposited on Al 2 O 3 (0001) substrates by chemical vapor deposition. CrO 2 has been found to have highly a-axis (010)-oriented columnar growth on a Cr 2 O 3 (0001) initial layer. The sixfold surface symmetry of the Cr 2 O 3 initial layer leads to three equivalent in-plane orientations of the a-axis-oriented CrO 2 unit cell. We report Cr L 2,3 X-ray magnetic circular dichroism data along the surface normal and at 60 • off-normal sample orientation. For a 60 • sample alignment, a strong increase of the projected orbital moment could be observed for unoccupied majority t 2g states using moment analysis. Therefore, the c axis is identified as the intrinsic magnetic easy axis of CrO 2 . In addition, a small spin moment and a very strong magnetic dipole term T z have been found. PACS 75.30.Cr; 75.30.Gw; 78.20.Lj; 78.70.Dm; 78.20.Bk IntroductionThe theoretically predicted 100% spin polarization at the Fermi energy, ε F , of CrO 2 makes it a promising material for magnetoelectronic devices [1][2][3]. According to Jullière's model, the magnetoresistance (MR) of a ferromagnet/insulator/ferromagnet tunnel junction depends on the spin polarization of the electrode material used [4]. The MR increases with increasing spin polarization of the ferromagnetic electrode material. Also, injection of spins from ferromagnetic metals into semiconductors may be allowed only for highly polarized metallic electrodes [5]. Additionally, transition-metal oxides like CrO 2 , high-temperature superconductors and colossal magnetoresistance lanthanum-based manganates have attracted a lot of theoretical interest due to the complex interplay between orbital, structural and electronic degrees of freedom. This has revived research on half-metallic ferromagnetic transitionmetal oxides, especially CrO 2 , in the past few years. Unfortunately, CrO 2 is metastable at room temperature and atmospheric conditions and typically a thin Cr 2 O 3 layer covers the CrO 2 surface. Methods like photoemission (PES), ✉ Fax: +49-711/689-1912, E-mail: goering@mf.mpg.de X-ray absorption (XAS) or X-ray magnetic circular dichroism (XMCD) spectroscopy could not easily be used for examination of basic properties of CrO 2 , because of their intrinsic surface sensitivity. Therefore, only a few experimental results on single crystals or epitaxial-grown thin films have been published. Nevertheless, spectroscopic methods are able to give quantitative information about intrinsic magnetic properties of CrO 2 . Experiment and resultsCrO 2 crystallizes in a tetragonal rutile-type structure, where chromium atoms form a body-centered tetragonal unit cell. Cr sites are octahedrally surrounded by oxygen atoms. The apical axis of the oxygen octahedra is oriented along the [110] ([110]) direction for the edge (bodycentered) Cr ions. The lattice parameters are a = b = 4.421 Å and c = 2.916 Å [6,7]. CrO 2 is a metastable Cr-oxide phase at room temperature, orders ferromagnetically at T C = 393 K and degrades above 400 K at ambient atm...

Vanishing Fe 3d orbital moments in single-crystalline magnetite

“…This single crystal related effective electron escape length is much smaller compared to the λ e =5nm thin film result [33]. A very similar value of 0.7nm has been previously determined for another oxide system: CrO 2 [34].…”

“…The determined resonant absorption length scales are quite similar but also slightly reduced, compared to the absorption published by S. Gota et al [33]. This reduction, not significant for the conclusions given here, is related to the extended adaptation range, which we have used to reduce errors related to the oscillating region between 730-780eV [34]. Figure 4a shows the Fe L 2,3 edge region of the Henke table adapted nonmagnetic absorption lines for normal (0 • C) and 60 • C angle of incidence.…”

Direct correlation of Cr 3 d orbital polarization and O K -edge X-ray magnetic circular dichroism of epitaxial CrO 2 films

“…A Curie temperature of T c ≈ 385 K has been determined. Nearly no surface contamination of Cr 2 O 3 is present [14,15] which was also reported by other groups [16]. Further details of the sample growth and characterization have been discussed elsewhere [17].…”

“…Figure 1b shows the corresponding CrO 2 XMCD spectra for a fixed grazing incidence condition (φ = 60 • ) with a variation of the azimuthal Discussion. -As recently shown for a polycrystalline highly textured CrO 2 (100) sample [15] as well as for an epitaxially grown CrO 2 (100) thin film [14], two orbital contributions could be extracted from Cr L 2,3 XMCD spectra using the so-called moment analysis technique [22,23], originally used for the interpretation of pure 3d metal XMCD spectra [24][25][26]. Cr L 2,3 XMCD data has been previously fitted (fig.…”

Tuning the Néel temperature in an antiferromagnet: the case of NixCo1−xO microstructures

“…Estimated values are in good agreement with the deposited ratio for both samples (shown in Table 1). It should be noted that the probing depth is limited to the near surface region due to the short escape depth of photoelectrons (much more shorter than the x-ray penetration depth) 40,41 . Thus the composition of each sample is actually measured for the top few nm.…”