Spatiotemporal magnetic imaging at the nanometer and picosecond scales

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1 AFOSR Nanoelectronics Review, Oct. 24, 2016 Spatiotemporal magnetic imaging at the nanometer and picosecond scales Gregory D. Fuchs School of Applied & Engineering Physics, Cornell University T M V TRANE : time-resolved anomalous Nernst effect

2 Outline 1. The basics of TRANE and TRLSSE microscopy: what it can sense and how we do it Bartell et al. Nat. Commun Guo et al., Phys. Rev. Appl Phase-sensitive FMR imaging: Imaging dynamic modes and consequences for the spin Hall effect Guo et al., Phys. Rev. B Time-resolve spin Seebeck effect: spatiotemporal imaging of ultrathin YIG/Pt films Bartell et al., in prep (2016) 4. How to reach nanoscale spatiotemporal nanoscale magnetic microscopy: progress and perspectives

3 Principles of magneto-thermal microscopy V x, y, t β m y x, y, t Z T x, y, t + ηδρ T, x, y, t J(x, y, t) M z y x Local moment Local heat gradient Local thermal resistivity change current density T determines the spatial and temporal Magnetic metals: Anomalous resolutions Nernst Effect V T is local: ~700 nm E ANE = Nμ o M T V M T is transient: <30 ps (>20 GHz) Magnetic insulators/heavy metal (or TI): Longitudinal spin Seebeck effect + inverse spin Hall effect E ISHE = S s ξ SH ρ Pt μ o M T Sensitivity: ~0.1 / Hz Bartell et al. Nat. Commun. 6, 8460 (2015) Guo et al., Phys. Rev. Appl (2015)

4 S Time-Resolved Anomalous Nernst Effect (TRANE) 3 ps Reflectivity mv Ti:Sapphire laser RF current source circulator chopper I RF NA 0.9 y z NM FM Sapphire x 5 μm Magnetic signal H app =0 μv m y Lock-In H app mixing ref mixer 5 μm

5 FMR phase measurement V total = η ANE T m y (t) + I RF t R heat Magnetic signal (m y ) RF current V total 0 With a modulation field: m y / H H app

6 FMR phase measurement V total V total = η ANE T m y (t) + I RF t R heat Magnetic signal (m y ) RF current μv Current source off Guo et al., Phys. Rev. Applied 4, (2015) 0 H app V total Current source on With a modulation field: m y / H Precession angle: 1.5 H app V mod Current source on V

7 FMR phase measurement V total V total = η ANE T m y (t) + I RF t R heat Magnetic signal (m y ) RF current Guo et al., Phys. Rev. Applied 4, (2015) Line shape is determined by FMR phase. m y 0 With a modulation field: m y / H H app φ FMR = 0 m y =m y0 0 m y χ (Re) H app χ (Im) φ FMR = 90 m y =0 0 H res H app V total ~χ cos φ FMR + χ sin φ FMR

8 Spin Hall physics and measurement techniques A very short and incomplete list: Dyakonov and Perel, Phys. Lett. 35A, 459 (1971) Hirsch, PRL 83, 1834 (1999). Kato et al.,science 306, 1910 (2004) Tulapurkar et al., Nature 438, 339 (2005) Valenzuela and Tinkham, Nature 442, 176 (2006) Sankey et al., PRL 96, (2006) Saitoh et al., APL 88, (2006) h RF Kimura et al., PRL 98, (2007) Ando et al., PRL 101, (2008) Mosendz et al., PRL 104, (2010) Pi et al., APL 97, (2010) Liu et al., PRL 106, (2011) Azevedo et al., PRB 83, (2011) Miron et al., Nature 476, 189 (2011) Liu et al., arxiv: (2011) Liu et al., Science 336, 555 (2012) Kim et al., Nat Mater 12, 240 (2013) Garello et al., Nature Nanotechnol. 8, 587 (2013) Emori et al., Nature Mater. 12, 611 (2013) -J c m J s Ryu et al., Nature Nanotechnol. 8, 527 (2013) Pai et al., APL 104, (2014) Hayashi et al., PRB 89, (2014) Fan et al., Nat Commun 5, 3042 (2014) Mellnik et al., Nature 511, 449 (2014) Liu et al., Nat Phys 10, 561 (2014) Zhang et al., Nat Phys 11, 496 (2015)

9 How do we usually measure the Spin Hall effect? Spin-Torque Ferromagnetic Resonance (All Electrical) Spin Torque (spin current) χ (Re) STFMR voltage Oersted Torque (charge current) 0 0 H app χ (Im) H H app res Assumes: Uniform RF Drive Uniform precession phase ξ SHE J s J c Liu et al., Science 2012

10 Imaging spin torque vectors: Pt/Hf/FeCoB FMR Phase FMR Amplitude Systematic error in allelectrical measurement using STFMR Guo et al., PRB 93, (2016)

11 True separation of J c and J s : Magnetic Insulators Brillion Light scatter measurements of FMR Magneto optical Kerr effect measurements Spin Hall magneto resistance (SMR) Inductive measurements of auto-oscillations MOKE SMR BL S Inductive Pickup M.Montazari et al. Nat. Commun. 6, 8958 (2015) M. Jungfleisch et. al. PRL. 116, (2016) J. Sklenar et al. Phys. Rev. B 92, (2015) M. Collet et al. Nat. Commun. 7, (2016)

12 Ultrafast spin Seebeck effect in magnetic insulators J s M 6 nm 20 nm The longitudinal spin Seebeck effect is fast: < 100 ps Heating Pt electrons Pt phonons YIG phonons YIG magnons V Time-resolved LSSE voltage With Dan Ralph (Cornell) and Fenyuan Wang (OSU) Bartell et al., in prep (2016)

13 High-resolution and sensitive magnetic microscopy Reflectance Spatial resolution: 0.8 µm (focused light) Sensitivity to in-plane magnetic angle: 0.3 / Hz at 0.6 mw of optical power (9.4 mj/cm 2 fluence) LSSE: saturated field The most sensitive measurement of ultra-thin YIG films (<20 nm) using any technique. LSSE: remnant field With Dan Ralph (Cornell) and Fenyuan Wang (OSU) Bartell et al., in prep (2016)

14 Dynamic magnetic microscopy with TRLSSE: FMR Driving Torques Oersted magnetic field spin Hall effect torque H Detect FMR via TRLSSE microscopy Phase-sensitive: if we rotate the drive by 180, the resonance curve inverts. (torque vector imaging) Stroboscopic FMR m y =m y0 Bartell et al., in prep (2016)

15 FMR imaging with LSSE microscopy Resonance amplitude Resonance field Resonance phase Resonance linewidth Bartell et al., in prep (2016)

16 Bringing TRANE to the nanoscale Finite element thermal model Resolution = thermal spot size Same behavior at 50 nm Focused laser Plasmon antenna 5 µm Bartell et al. Nat. Commun. 6, 8460 (2015)

17 Development of spatiotemporal nanoscale magnetic microscopy SPP Not intrinsically diffraction limited Near-field microscopy Our original vision Today Sapphire Pulsed Laser Plasmon Antenna Chemically etched Au wire V J. M. Bartell*, D. H. Ngai*, Z. Leng, and G. D. Fuchs, Nat. Commun. 6, 8460 (2015)

18 Surface plasmon polaritons to create nanoscale heat Surface Plasmon Polariton (SPP) - Coupling between charge oscillations and electric field at a metal-dielectric boundary Field enhancement at tip + imaging effect confined heating SPP Pulsed laser Fabrication via electrochemical etching

19 Integration with optical set-up Laser on Laser focused on gold tip Sample scanning Sample mounted in microwave box 18 GHz contacts

20 AFM topology of magnetic devices False color Next step: current density and magnetic imaging

21 TRANE microscopy summary & acknowledgement Time-resolved measurements of stimulus-response using Heat Time resolved (<30 ps), Spatial resolution ( 0.7 μm) Phase sensitive, local magnetic measurement Simultaneously measure time-resolved charge currents Observe spatial variations in FMR phase, but no spatial variations in RF current phase in micron-scale, uniform samples Observe large corrections to SHE efficiency in relation to all-electrical FMR measurements Most sensitive probe of YIG thin films known Simultaneous nanoscale spatial and picosecond temporal resolution in the works! Jason Bartell Darryl Ngai Feng Guo J. M. Bartell*, D. H. Ngai*, Z. Leng, and G. D. Fuchs, "Towards a table-top microscope for nanoscale magnetic imaging using picosecond thermal gradients." Nat. Commun. 6, 8460 (2015). Feng Guo, J. M. Bartell, D. H. Ngai, and G. D. Fuchs, "Phase-sensitive imaging of ferromagnetic resonance using ultrafast heat pulses." Phys. Rev. Appl. 4, (2015). Feng Guo, J. M. Bartell, and G. D. Fuchs "Ferromagnetic resonance phase imaging in spin Hall multilayers." Phys. Rev. B 93, (2016). J. M. Bartell*, C. L. Jermain*, S. Aradhya, H. Wang, R. A. Burhman, F. Yang, D. C. Ralph, and G. D. Fuchs, Ultrafast spin Seebeck imaging of magnetism in thin YIG thin films. in preparation (2016).

22 Frequency (GHz) Confirmation we measure unperturbed dynamics Ni 81 Fe 19 (permalloy) 16.4 GHz Field (Oe) 4πM s = 840 emu/cm 3, N y = 0.015, N z = α = ± Nernst Coefficient: 3.7 ± V / (T K) A. Slachter, F.L. Bakker & B.J. van Wees Phys. Rev. B 84, (2011)

23 Imaging Current and FMR V x, y, t βn m y x, y, t Z T x, y, t + ηδρ T, x, y, t, J(x, y, t) No microwave current 5.7 GHz DC Level shift! 5.7 GHz DC shift rejected

24 Phase sensitive detection of FMR & RF current φ AWG = 260 φ AWG = 5 At H app =H res m y χ (Im) 1. Phase of RF driving field is constant m y (t)=0 2. FMR amplitude and phase is not constant due to variations in: Oe field Demag field Magnetic m y (t)=m y0 inhomogeneities? 0 0 m y H res H app χ (Re) H app

25 M pulsed (au) Preliminary: spin-torque switching of YIG Switching phase diagram M M Evidence for spin-transfer torque induced switching, but: Not simple thermal activation Questions about strain and interface anisotropy Likely domain-wall mediated reversal H switch H ext 90 o 6 ma 5 ma magnetic easy axis Raw switching measurement Field (Oe)

26 T M V