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Ultralow Damping in Nanometer-Thick Epitaxial Spinel Ferrite Thin Films Satoru Emori, Di Yi, Sam Crossley, Jacob J Wisser, Purnima P Balakrishnan, Behrouz Khodadadi, Padraic Shafer, Christoph Klewe, Alpha T. N'Diaye, Brittany T Urwin, krishnamurthy mahalingam, Brandon Howe, Harold Y. Hwang, Elke Arenholz, and Yuri Suzuki Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01261 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018
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Nano Letters
Ultralow Damping in Nanometer-Thick Epitaxial Spinel Ferrite Thin Films Satoru Emori,∗,†,‡ Di Yi,† Sam Crossley,† Jacob J. Wisser,†,¶ Purnima P. Balakrishnan,†,§ Behrouz Khodadadi,‡ Padraic Shafer,k Christoph Klewe,k Alpha T. N’Diaye,k Brittany T. Urwin,⊥ Krishnamurthy Mahalingam,⊥ Brandon M. Howe,⊥ Harold Y. Hwang,†,¶ Elke Arenholz,k and Yuri Suzuki†,¶ † Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305, United States ‡ Department of Physics, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24060, United States ¶ Department of Applied Physics, Stanford University, Stanford, California 94305, United States § Department of Physics, Stanford University, Stanford, California 94305, United States k Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433, United States E-mail:
[email protected] Abstract Pure spin currents, unaccompanied by dissipative charge flow, are essential for realizing energy-efficient nanomagnetic information and communications devices. Thinfilm magnetic insulators have been identified as promising materials for spin-current
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technology, since they are thought to exhibit lower damping compared to their metallic counterparts. However, insulating behavior is not a sufficient requirement for low damping, as evidenced by the very limited options for low-damping insulators. Here, we demonstrate a new class of nanometer-thick ultralow-damping insulating thin films, based on design criteria that minimize orbital angular momentum and structural disorder. Specifically, we show ultralow damping in 0.01) for L = 6 0 Tm3Fe5O12 30 and Ce-doped YIG 31 compared to YIG and LuIG. Here, we have developed a new epitaxial thin-film spinel ferrite, MgAl0.5Fe1.5O4 (MAFO), that exhibits ultralow magnetic damping enabled by its L = 0 cation chemistry and highquality coherent epitaxy. The nominal composition of MAFO indicates that its magnetism arises from Fe3+, while the Mg2+ and Al3+ cations have zero orbital angular momentum because of their full valence shells. In coherently strained MAFO thin films with thicknesses ≈10-20 nm, we achieve Gilbert damping parameters as low as α ≈ 0.0015. We verify the weak spin-orbit coupling in MAFO with a spectroscopic g-factor of close to 2.0 quantified
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by broadband ferromagnetic resonance (FMR), along with the confirmation of Fe3+ as virtually the sole source of magnetism by X-ray magnetic circular dichroism (XMCD). Our findings show that the deliberate choice of cation chemistry, combined with high-quality pseudomorphic growth, allows for ultralow damping in nanometer-thick insulating ferrite thin films. Films were synthesized on single-crystal substrates of MgAl2O4(001) by pulsed laser deposition from a polycrystalline target of stoichiometry MgAl0.5Fe1.5O4 (Toshima Manufacturing Co.). The details of the deposition process are described in the Supporting Information. We remark that MAFO films were deposited at a substrate temperature of 450◦C and that no post-annealing was performed at a higher temperature. From the standpoint of device fabrication, this relatively low thermal budget is a potentially attractive attribute of MAFO, as it is much lower than the typical deposition and annealing temperatures of ≈600-900◦C for epitaxial garnet films 6–16. The low deposition temperature may also be advantageous for reducing atomic intermixing between the film and substrate, which has been reported to be an issue for YIG thin films 32,33. Structural characterization reveals that epitaxial MAFO films of thicknesses .20 nm attain coherent epitaxy and high crystalline quality. In symmetric 2θ-ω X-ray diffraction scans of all MAFO films, only the substrate and film peaks corresponding to their (00l) planes appear. As shown in Fig. 1(a), for .20-nm thick films, we observe pronounced Laue oscillations, indicating smooth interfaces. For each of these .20-nm thick films, the full-widthat-half-maximum (FWHM) of the rocking curve about the (004) peak is ≈0.045-0.06◦, compared to the FWHM of ≈0.01-0.015◦ for the MgAl2O4 substrate, corroborating small mosaic spread. By contrast, much thicker films (e.g., 40 nm, as shown Fig. 1(a,b)) do not exhibit Laue oscillations (Fig. 1(a)), and the rocking curve FWHM increases to ≈0.2◦ (Fig. 1(b)). These findings suggest that thinner (.20-nm thick) MAFO films are of higher crystalline quality than their thicker counterparts. The structural difference between the thinner and thicker MAFO films is further shown in
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Figure 1: Structural properties of epitaxial MAFO. (a) Symmetric 2θ-ω X-ray diffraction scans for MAFO films with different thicknesses on MgAl2O4 (001) substrates. (b) Rocking curve scans about the (004) film peak for the same MAFO films shown in (a). (c,d) Reciprocal space maps of asymmetric scans about the (¯1¯15) substrate peak for (c) 18-nm and (d) 40-nm thick MAFO films. reciprocal space maps, as shown in Fig. 1(c,d). The in-plane lattice parameters of the 18-nm thick MAFO film and the MgAl2O4 substrate coincide (Fig. 1(c)), thus indicating that the film is coherently strained to the substrate. This coherent epitaxy is remarkable considering that the film undergoes an extremely large tetragonal distortion to accommodate the lattice mismatch of nearly 3% between MgAl2O4 (8.08 ˚ A) and MAFO (8.30 ˚ A, derived from the powder XRD data of the target), which is cubic in the absence of substrate-imposed strain 34. Based on the average out-of-plane lattice parameter c = 8.56 ± 0.015 of ˚ several .20-nm thick A films, derived from their (004) film peak position in the 2θ-ω scan, the tetragonal distortion, c/ a, is 1.059 ± 0.002. At a significantly larger film thickness, the film can relieve this large epitaxial strain by undergoing structural relaxation. The reciprocal space map of the 40-nm thick MAFO shows an in-plane (horizontal) offset between the centers of the broad film peak and the substrate peak, confirming that this thicker film is partially relaxed with respect to the substrate. Furthermore, from cross-sectional transmission electron microscopy (see Supporting Information), we find evidence for defects associated with strain relaxation in a 40-nm thick MAFO, whereas such defects are absent in .20-nm thick films. As we show below, the much higher crystalline quality of .20-nm thick MAFO films allows for soft
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Figure 2: In-plane static magnetic properties of epitaxial MAFO. (a,b) Magnetic hysteresis loops measured with a SQUID magnetometer, with field applied in the film plane along the [100] and [110] axes for (a) 18-nm and (b) 40-nm thick films. static magnetism and low damping. Epitaxial MAFO exhibits robust room-temperature ordered magnetism, along with low inplane coercive and saturation fields in coherently strained films. The saturation magnetization Ms at 300 K measured by SQUID magnetometry is 100 ± 7 kA/m, averaged over several films, with no significant dependence on film thickness. The Curie temperature determined from high-temperature vibrating sample magnetometry is ≈400 K (see Supporting Information). We observe modest in-plane cubic anisotropy with h110i as the easy axes. For coherentlystrained MAFO, the saturation field along the h100i hard axes is still only ∼10 mT, and the in-plane coercivity is
