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Pseudomorphic yttrium iron garnet thin films with low damping and inhomogeneous linewidth broadening

Brandon. M. Howe1,†,*, Satoru Emori2,†, Hyung-Min Jeon3, Trevor M. Oxholm2, John G. Jones1, Krishnamurthy Mahalingam1, Yan Zhuang3, Nian X. Sun2, Gail. J. Brown1 1. Materials and Manufacturing Directorate, Air Force Research Laboratory, WPAFB, OH 45433 2. Electrical and Computer Engineering, Northeastern University, Boston, MA 02139 3. Electrical Engineering, Wright State University, Dayton, OH 45435

We report on nanometer-thick yttrium iron garnet (YIG) films, grown on gadolinium gallium garnet substrates by pulsed laser deposition, exhibiting remarkably high crystallinity and atomically smooth surfaces. The pseudomorphic growth mechanism leads to large out-of-plane uniaxial anisotropy, narrow resonance linewidths, and minimal damping. Magnetic resonance measurements indicate that both the Gilbert damping parameter α and inhomogeneous linewidth broadening ΔHpp,0 are consistently low for films of various thicknesses. Even at film thickness ≈20 nm, we attain α ≈ 2×10-4 and ΔHpp,0 ≈ 1 Oe, which are among the lowest values ever reported.

†

These authors contributed equally to this work.

*email: [email protected]

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I. Introduction Yttrium iron garnet (Y3Fe5O12, YIG) exhibits the lowest damping parameter among known magnetic materials, and has been used extensively in various high-frequency electronic devices including microwave filters and oscillators. The exceptionally low damping also makes YIG suitable for magnonic and spintronic applications [1]–[4]. In particular, in a YIG film interfaced with a metal film, magnetization dynamics can be tuned electrically by a spin torque generated from the spin Hall effect in the metal [5]. By using YIG, whose damping parameter is one or two orders magnitude smaller than those of ferromagnetic metals (e.g., permalloy and CoFeB), the charge current density required to manipulate the magnetization may be substantially reduced. Since the spin torque arising from the spin Hall effect scales inversely with the thickness of the magnetic layer, the use of nanometer-thick YIG is desirable for efficient electrical control of magnetization. In recent years, there have been several reports of <200-nm thick YIG films grown using liquid phase epitaxy, sputtering, and pulsed laser deposition (PLD) [6]–[16], as summarized in Table 1. Significant advances have been made in understanding spin-current transmission across YIG-metal interfaces [13], [17] and in demonstrating potentials for spin-Hall-driven devices based on nanometer-thick YIG films [18]. For evaluating the performance of a magnetic material for high-frequency applications, the key parameter is the linewidth that represents the broadening of the ferromagnetic resonance (FMR) spectrum. This spectrum is often measured as a function of swept external magnetic field while a fixed microwave excitation of frequency f is applied using a microwave cavity, shorted waveguide, strip line, or coplanar waveguide. When lock-in detection is used to acquire the differential resonance spectrum, the most convenient definition of the linewidth is the peak-topeak linewidth ΔHpp, which is related to the Gilbert damping parameter α through

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H pp  H pp,0 

2 3

f,

(1)

where γ ≈ 2.8 MHz/Oe is the gyromagnetic ratio and ΔHpp,0 is the frequency-independent linewidth broadening due to sample inhomogeneity. It is clear from Eq. (1) that both α and ΔHpp,0 must be small to minimize the linewidth over a wide range of frequency. The damping parameters of many nanometer-thick YIG films reported recently (summarized in Table 1) are α ≈ 1×10-3, or about one to two orders of magnitude greater than micron-thick YIG films grown by liquid phase epitaxy. Chang et al. showed a very low damping parameter α ≈ 9×10-5, although the inhomogeneous broadening contribution to the linewidth is substantial at ΔHpp,0 ≈ 5 Oe [15]. d’Allivy Kelly et al. have reported on 20-nm thick YIG films with α ≈ 2×10-4 and ΔHpp,0 ≈ 1.4 Oe [10]. However, it remains a challenge to attain reliable growth of YIG thin films that simultaneously exhibit minimal damping and linewidth broadening. In this Letter, we report on nanometer-thick PLD-grown YIG films exhibiting consistently superior structural and magnetic properties. These films exhibit remarkably high crystalline quality, due to the pseudomorphic growth mechanism to well lattice-matched gadolinium gallium garnet (Gd3Ga5O12, GGG) substrates. This strained, pseudomorphic structure results in pronounced out-of-plane uniaxial anisotropy and, more importantly, exceptionally narrow linewidths with α ~10-4 and ΔHpp,0 ~ 1 Oe. We have consistently attained these structural and magnetic properties in more than 40 thin films grown in a span of several months. The ability to grow such high-quality YIG films is essential for conducting fundamental studies and device applications that leverage low-loss magnetization dynamics driven by spin current.

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II. Sample Growth and Structural Properties High-quality epitaxial YIG layers are grown on 5×5 mm2 GGG(111) single crystal substrates (MTI Corp) in a custom built ultra-high vacuum PLD system. Substrates are first cleaned by successive rinses in ultrasonic baths of acetone, isopropyl alcohol, deionized water, and isopropyl alcohol, and then blown dry with dry nitrogen before introducing into the loadlock chamber and evacuated to <10-8 Torr. The samples are then introduced into the growth chamber (base pressure <10-9 Torr) and heated to 825oC for one hour before pre-ablating the 1”diameter YIG target (Trans-Tech, Inc.) at 4 Hz, ≈2.7 J/cm2 (laser set to 750 mJ/pulse) in 100 mTorr O2 for 5 min (while keeping the sample protected with a shutter) using a Lambda Physik LPX 305i excimer laser (λ = 248 nm, 25ns/pulse). The target to substrate distance is 6 cm. Growth is performed at a substrate temperature of 825oC and O2 pressure of 100 mTorr (same parameters as pre-ablation) and deposition rate of ≈10 nm/min. No annealing was performed after the deposition. The sample was cooled at -10oC/min in 500 mTorr O2. Powder X-ray diffraction and subsequent Rietveld refinement analysis of the target showed it to be phase-pure and with no detectable deviations from the nominal stoichiometry or misplacement of the transition metal atoms. Results of typical microstructural and surface morphological measurements are summarized in Fig. 1. Fig. 1(a) is an atomic force micrograph showing very little surface features aside from atomic step edges appearing as inclined stripes. The overall RMS surface roughness of the film is <1.6 Å. This low roughness was confirmed by X-ray reflectivity, which was also used to measure the film thickness. A high-resolution X-ray diffractogram (coupled ω-2θ scan) about the film and substrate (444) peaks, is presented in Fig. 1(b). The only observable diffraction peaks, over the entire range investigated from 2θ = 10-110o, are those allowed by the

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film and substrate, i.e., the (444) and (888) family of peaks. Although not shown here, there were no peaks in the glancing angle scans using an incident angle of 0.9 o over the same range of 2θ =10-110o. Fig. 1(c) shows clear Laue oscillations around the YIG(444) peak, indicating the high quality and low roughness of the film. Fig. 1(d) is a high resolution reciprocal lattice map (HRRLM) about the asymmetric reflection (642) for both substrate and film peaks. Suggesting a slightly lower lattice constant than YIG, the GGG (642) diffracted intensity lies at higher scattering angle (Qz), while both peaks appear at the same Qx value. This behavior is indicative of strained, pseudomorphic layer growth in which the film retains the in-plane lattice constant of the substrate (12.383 Å). From this HR-RLM, no signs of film relaxation are evidenced, as there is no diffracted intensity from the film appearing towards lower Qx values. We find that the outof-plane lattice constant of YIG is 12.515 Å, compared to the bulk value of 12.376 Å, yielding an estimated unit cell volume of 1919 Å3, ≈1.3% larger than the unit cell volume of bulk stoichiometric YIG of 1896 Å3. This lattice expansion, also reported in an earlier study of PLDgrown YIG films [19], may be due to a small off-stoichiometry, e.g., oxygen deficiency. We characterized the film composition by X-ray photoemission spectroscopy: While no contaminants or dopants in the film were detected to within the experimental resolution of <1 at. %, we were unable to precisely quantify the ratios of Y, Fe, and O. The exact effects of stoichiometry on film structure and magnetic properties will be the subject of a future study. We confirmed the high-quality, pseudomorphic growth of YIG films with a transmission electron microscope (TEM). The samples were prepared by conventional ion milling with liquid nitrogen cooling, and imaged using an aberration (image) corrected Titan TEM operated at an acceleration voltage of 300 kV. Fig. 2 shows a representative TEM image of the YIG/GGG interface. The arrows in Fig. 2 denote the position of the YIG/GGG interface, indicating an

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abrupt interface. The film and substrate are in-plane lattice-matched, consistent with our HRRLM findings.

III. Magnetic Properties A. Static Properties We measured the static magnetic properties of YIG films by vibrating sample magnetometry. The saturation magnetization 4πMs was measured with field applied parallel to the film plane. We obtain 4πMs = 1600±100 G, averaged over 15 samples with thicknesses ranging from 18 to 190 nm, with no systematic dependence of 4πMs on the film thickness. The slightly smaller value of 4πMs, compared to the typical 4πMs ≈1750 G for bulk YIG, may be due to a deviation from the bulk stoichiometry of YIG as noted in Section II. The in-plane coercive fields of the films are <1 Oe as shown by representative hysteresis loops in Figs. 3(a),(c). Weak uniaxial anisotropy within the film plane, with saturation fields ≈1-6 Oe, is evidenced in in-plane hysteresis loops (Figs. 3(a),(c)). This small in-plane uniaxial anisotropy is unrelated to the crystalline orientation of the GGG(111) substrate, which should lead to three-fold anisotropy. Whereas anisotropy within the film plane is weak, significantly more pronounced anisotropy is revealed from out-of-plane magnetometry shown in Figs. 3(b),(d). The out-of-plane saturation field (equivalent to the effective demagnetizing field 4πMeff) is ≈2500 G, approximately 800 Oe larger than 4πMs, indicating the presence of strong uniaxial anisotropy perpendicular to the film plane. Similar out-of-plane uniaxial anisotropy has been reported in a few other studies of PLD-grown YIG films [20]–[22].

We argue that the pseudomorphic

structure simultaneously leads to narrow FMR linewidths and pronounced out-of-plane uniaxial anisotropy, two characteristics that have been previously reported to be correlated in [20]: The

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minimal defect density likely results in narrow linewidths (as shown below), while the strained growth lattice-matched to the substrate causes the out-of-plane lattice expansion (described in Section II) that may be the source of the large out-of-plane anisotropy.

B. Resonance Properties The X-band peak-to-peak resonance linewidth ΔHpp of YIG thin films was measured using an electron spin resonance system with a TE 102 microwave cavity, operated at frequency f = 9.56 GHz and power P = 1 mW. The bias magnetic field was applied parallel to the film plane. We consistently obtain ΔHpp,9.56GHz < 2 Oe for ≈200-nm thick films, and ΔHpp,9.56GHz ≈ 2-4 Oe for ≈20-30-nm thick films. Fig. 4 shows examples of FMR spectra, each fitted to the derivative of a modified Lorentzian [23] to quantify the peak-to-peak linewidth. These narrow X-band linewidths are observed in over 40 samples deposited over a span of 8 months, and are among the lowest ever reported so far (see Table 1). We note that the in-plane resonance field HFMR is in the range 2330-2400 Oe, which is lower than typically reported HFMR ≈2600-2700 Oe at f ≈ 9.5 GHz. By using the in-plane Kittel equation,

f   H FMR ( H FMR  4M eff ) ,

(2)

we estimate 4πMeff = 2460-2670 G with γ set at 2.8 MHz/Oe. This is in agreement with the 4πMeff estimated from out-of-plane magnetometry as described above. We quantified 4πMeff, γ, and α from a broadband FMR system based on the setup described in [24] with an input microwave power of 1 mW. FMR spectra under in-plane bias field were measured by sweeping the field at fixed microwave frequencies (e.g., Fig. 5(a)). FMR spectra with bias field applied perpendicular to the film plane were also measured, with H>5000 Oe to ensure that the sample was magnetized out-of-plane. Since the resolution of the field

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measurement in this range was limited at 1 Oe, out-of-plane FMR spectra were acquired by sweeping the microwave frequency at 1-MHz steps (e.g., Fig. 5(b)) for precise measurement of the linewidth. For the 23-nm thick YIG film shown in Fig. 5, by fitting of the out-of-plane FMR data to the appropriate Kittel equation,

f   ( H FMR  4M eff ) ,

(3)

we find γ = 2.83±0.004 MHz/Oe and 4πMeff = 2410±10 G. The 4πMeff from Fig. 5(d) is used to fit the in-plane data in Fig. 5(c) to Eq. (2), yielding γ = 2.83±0.01 MHz/Oe, confirming that the in-plane and out-of-plane FMR data are in quantitative agreement. The frequency dependence of ΔHpp yields α and ΔHpp,0 through Eq. (1). From the inplane case where ΔHpp is measured directly, we arrive at α = (1.8±0.3)×10-4 and ΔHpp,0 = 1.2±0.1 Oe. We similarly quantify α and ΔHpp,0 from the out-of-plane FMR data, as shown in Fig. 5(f). In the out-of-plane measurement geometry, the field linewidth ΔHpp and frequency linewidth Δfpp are related through [25] H pp 

f pp



.

(4)

From Eq. (1), we obtain from the out-of-plane data α = (2.1±0.4)×10-4 and ΔHpp,0 = 1.4±0.2 Oe. The damping α for both in-plane and out-of-plane configurations are identical within experimental uncertainty, and both of these values are comparable to α = 2.3×10-4 and ΔHpp,0 = 1.4 Oe reported in [10]. Two-magnon scattering, a defect-induced contribution to the resonance linewidth, is suppressed when the magnetization is oriented out of plane [26]. Since both α and ΔHpp,0 are nearly the same for the in-plane and out-of-plane measurement geometries, we might conclude that two-magnon scattering is negligible. However, it is possible that two-magnon scattering 8 1949-307X (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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contributes to the linewidth in the in-plane case, whereas different mechanisms may lead to a similar linewidth in the out-of-plane case. For example, the film may not be uniformly magnetized out-of-plane due to slight inhomogeneity of uniaxial anisotropy, thereby broadening the linewidth. Although the YIG thin films reported here exhibit high structural quality and narrow linewidths, we are unable to rule out two-magnon scattering conclusively from the outof-plane broadband FMR measurement. We carried out similar broadband FMR measurements on several ≈20-30-nm and ≈200nm thick YIG films and obtain α ≈2-4×10-4 and ΔHpp,0 ≈ 1-3 Oe. These findings are in agreement with the narrow linewidths measured with the X-band cavity setup (e.g., Fig. 4), and highlight the consistent high quality of our PLD-grown YIG thin films.

IV. Summary We have grown high-quality nanometer-thick YIG films by PLD. Structural characterization shows that these YIG films are pseudomorphic to the GGG substrate and have atomically smooth surfaces and interfaces. Static magnetometry reveals low coercivity <1 Oe and bulk-like saturation magnetization ≈1600 G. From magnetic resonance measurements, we find that both the damping parameter and inhomogenous linewidth broadening are among the lowest ever reported for nanometer-thick YIG. The ability to consistently grow highly crystalline, narrow-linewidth YIG films demonstrated here will enable advances in low-loss magnonic and spintronic applications.

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Acknowledgements This work was supported by the Air Force Research Laboratory through contract FA8650-14-C-5706 and in part by FA8650-14-C-5705.

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[10] O. d’Allivy Kelly, A. Anane, R. Bernard, J. Ben Youssef, C. Hahn, A. H. Molpeceres, C. Carrétéro, E. Jacquet, C. Deranlot, P. Bortolotti, R. Lebourgeois, J.-C. Mage, G. de Loubens, O. Klein, V. Cros, and A. Fert, “Inverse spin Hall effect in nanometer-thick yttrium iron garnet/Pt system,” Appl. Phys. Lett., vol. 103, no. 8, p. 082408, Aug. 2013. [11] T. Liu, H. Chang, V. Vlaminck, Y. Sun, M. Kabatek, A. Hoffmann, L. Deng, and M. Wu, “Ferromagnetic resonance of sputtered yttrium iron garnet nanometer films,” J. Appl. Phys., vol. 115, no. 17, p. 17A501, Jan. 2014. [12] P. Pirro, T. Brächer, A. V. Chumak, B. Lägel, C. Dubs, O. Surzhenko, P. Görnert, B. Leven, and B. Hillebrands, “Spin-wave excitation and propagation in microstructured waveguides of yttrium iron garnet/Pt bilayers,” Appl. Phys. Lett., vol. 104, no. 1, p. 012402, Jan. 2014. [13] H. L. Wang, C. H. Du, Y. Pu, R. Adur, P. C. Hammel, and F. Y. Yang, “Scaling of Spin Hall Angle in 3d, 4d, and 5d Metals from Y3,” Phys. Rev. Lett., vol. 112, no. 19, p. 197201, May 2014. [14] M. C. Onbasli, A. Kehlberger, D. H. Kim, G. Jakob, M. Kläui, A. V. Chumak, B. Hillebrands, and C. A. Ross, “Pulsed laser deposition of epitaxial yttrium iron garnet films with low Gilbert damping and bulk-like magnetization,” APL Mater., vol. 2, no. 10, p. 106102, Oct. 2014. [15] H. Chang, P. Li, W. Zhang, T. Liu, A. Hoffmann, L. Deng, and M. Wu, “NanometerThick Yttrium Iron Garnet Films With Extremely Low Damping,” IEEE Magn. Lett., vol. 5, pp. 1–4, 2014. [16] J. Lustikova, Y. Shiomi, Z. Qiu, T. Kikkawa, R. Iguchi, K. Uchida, and E. Saitoh, “Spin current generation from sputtered Y3Fe5O12 films,” J. Appl. Phys., vol. 116, no. 15, p. 153902, Oct. 2014. [17] C. Du, H. Wang, F. Yang, and P. C. Hammel, “Enhancement of Pure Spin Currents in Spin Pumping Y3Fe5O12/Cu/Metal Trilayers through Spin Conductance Matching,” Phys. Rev. Appl., vol. 1, no. 4, p. 044004, May 2014. [18] A. Hamadeh, O. d’Allivy Kelly, C. Hahn, H. Meley, R. Bernard, A. H. Molpeceres, V. V. Naletov, M. Viret, A. Anane, V. Cros, S. O. Demokritov, J. L. Prieto, M. Muñoz, G. de Loubens, and O. Klein, “Full Control of the Spin-Wave Damping in a Magnetic Insulator Using Spin-Orbit Torque,” Phys. Rev. Lett., vol. 113, no. 19, p. 197203, Nov. 2014. [19] Y. Krockenberger, H. Matsui, T. Hasegawa, M. Kawasaki, and Y. Tokura, “Solid phase epitaxy of ferrimagnetic Y3Fe5O12 garnet thin films,” Appl. Phys. Lett., vol. 93, no. 9, p. 092505, Sep. 2008. [20] P. C. Dorsey, S. E. Bushnell, R. G. Seed, and C. Vittoria, “Epitaxial yttrium iron garnet films grown by pulsed laser deposition,” J. Appl. Phys., vol. 74, no. 2, p. 1242, Jul. 1993. 11 1949-307X (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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[21] S. A. Manuilov, R. Fors, S. I. Khartsev, and A. M. Grishin, “Submicron Y3Fe5O12 Film Magnetostatic Wave Band Pass Filters,” J. Appl. Phys., vol. 105, no. 3, p. 033917, Feb. 2009. [22] M. B. Jungfleisch, A. V. Chumak, A. Kehlberger, V. Lauer, D. H. Kim, M. C. Onbasli, C. A. Ross, M. Kläui, and B. Hillebrands, “Thickness and power dependence of the spinpumping effect in Y3Fe5O12 /Pt heterostructures measured by the inverse spin Hall effect,” Phys. Rev. B, vol. 91, no. 13, p. 134407, Apr. 2015. [23] A. L. Stancik and E. B. Brauns, “A simple asymmetric lineshape for fitting infrared absorption spectra,” Vib. Spectrosc., vol. 47, no. 1, pp. 66–69, May 2008. [24] S. Beguhn, Z. Zhou, S. Rand, X. Yang, J. Lou, and N. X. Sun, “A new highly sensitive broadband ferromagnetic resonance measurement system with lock-in detection,” J. Appl. Phys., vol. 111, no. 7, p. 07A503, Feb. 2012. [25] S. S. Kalarickal, P. Krivosik, M. Wu, C. E. Patton, M. L. Schneider, P. Kabos, T. J. Silva, and J. P. Nibarger, “Ferromagnetic resonance linewidth in metallic thin films: Comparison of measurement methods,” J. Appl. Phys., vol. 99, no. 9, p. 093909, May 2006. [26] R. D. McMichael and P. Krivosik, “Classical Model of Extrinsic Ferromagnetic Resonance Linewidth in Ultrathin Films,” IEEE Trans. Magn., vol. 40, no. 1, pp. 2–11, Jan. 2004.

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Table 1. Nanometer-thick yttrium iron garnet films in recent literature Reference Deposition Thickness α (10-4) ΔHpp,0 Method (nm) (Oe) Heinrich et al. (2011) [6] PLD 9 7 5 Sun et al. (2012) [7] PLD 19 2.3 3 Weiler et al. (2013) [8] PLD 27 7 10 Hahn et al. (2013) [9] liquid phase 200 2.0 0.2* epitaxy d’Allivy Kelly et al. (2013) [10] PLD 20 2.3 1.4 Liu et al. (2014) [11] sputtering 26 9.9 2 Pirro et al. (2014) [12] liquid phase 100 2.8 0.8* epitaxy Wang et al. (2014) [13] off-axis 20 9.1 8 sputtering Onbasli et al. (2014) [14] PLD 79 2.2 0.8* Chang et al. (2014) [15] sputtering 22 0.86 6.8 Lustikova et al. (2014) [16] sputtering 83 7.0 1.7 this work PLD 23 1.8 1.2 * ΔHpp,0 converted from full-width-at-half-maximum linewidth.

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Figure 1. Summary of typical microstructural data from YIG/GGG(111) (here, a 200 nm thick YIG film): (a) atomic force microscopy image, (b) overall ω-2θ scan, (c) ω-2θ scan near the substrate/film (444) diffraction (exhibiting pronounced Laue oscillations), and (d) highresolution reciprocal lattice map about substrate and film (642).

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LMAG.2015.2449260, IEEE Magnetics Letters

Figure 2. A [0-11] conventional high-resolution transmission electron microscope image of the YIG/GGG interface.

Figure 3. (a,b) Hysteresis loops for a 190-nm thick YIG film with field applied in-plane (a) and out-of-plane (b). (c,d) Hysteresis loops for a 23-nm thick YIG film with field applied in-plane (c) and out-of-plane (d).

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LMAG.2015.2449260, IEEE Magnetics Letters

Figure 4. FMR spectra of (a) a 190-nm thick YIG film and (b-d) 23-nm thick YIG films, measured with a 9.56 GHz X-band ESR system.

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LMAG.2015.2449260, IEEE Magnetics Letters

Figure 5. (a,b) FMR spectra measured with a broadband FMR system: (a) field-sweep spectrum with in-plane bias field at fixed f = 15 GHz, and (b) frequency-sweep spectrum with fixed out-ofplane bias field H = 7710 Oe. (c,d) Resonance field HFMR versus microwave frequency for inplane bias field (c) and out-of-plane bias field (d). (e,f) Peak-to-peak linewidth ΔHpp versus microwave frequency f for in-plane bias field (e) and out-of-plane bias field (f). All data are measured from a 23-nm thick YIG film. Solid green symbols in (c) and (e) indicate values measured in a cavity operated at f = 9.56 GHz.

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