Letter Cite This: Nano Lett. 2018, 18, 4273−4278
pubs.acs.org/NanoLett
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 M. Howe,# Harold Y. Hwang,†,‡ Elke Arenholz,⊥ and Yuri Suzuki†,‡ Downloaded via UNIV OF NEW ENGLAND on October 1, 2018 at 11:47:32 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
Geballe Laboratory for Advanced Materials, ‡Department of Applied Physics, and §Department of Physics, Stanford University, Stanford, California 94305, United States ∥ Department of Physics, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24060, United States ⊥ 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 S Supporting Information *
ABSTRACT: Pure spin currents, unaccompanied by dissipative charge flow, are essential for realizing energy-efficient nanomagnetic information and communications devices. Thin-film magnetic insulators have been identified as promising materials for spin-current technology because they are thought to exhibit lower damping compared with 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 ≠ 0 Tm3Fe5O1230 and Ce-doped YIG31 compared with YIG and LuIG. Here, we have developed a new epitaxial thin-film spinel ferrite, MgAl0.5Fe1.5O 4 (MAFO), that exhibits ultralow magnetic damping enabled by its L = 0 cation chemistry and high-quality 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 of ∼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 gfactor of close to 2.0 quantified 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 highquality 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 because 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 4274
DOI: 10.1021/acs.nanolett.8b01261 Nano Lett. 2018, 18, 4273−4278
Letter
Nano Letters Epitaxial MAFO exhibits robust room-temperature ordered magnetism along with low in-plane 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 the Supporting Information). We observe modest in-plane cubic anisotropy with ⟨110⟩ as the easy axes. For coherently strained MAFO, the saturation field along the ⟨100⟩ hard axes is still only ∼10 mT, and the inplane coercivity is
