Tunable Room Temp. Skyrmions in Ir/Fe/Co/Pt Multilayers

Transcription

1 Tunable Room Temp. Skyrmions in Ir/Fe/Co/Pt Multilayers Anjan Soumyanarayanan Panagopoulos Group, NTU, Singapore Data Storage Institute, A*STAR, Singapore A.S. et al., arxiv: (2016)

2 Collaborators M. Raju Anthony Tan Alex Petrovic Bhartendu Satywali NTU, Singapore Films, Transport MFM Transport FMR Christos Panagopoulos DSI, Singapore Anibal Gonzalez Pin Ho Lisen Huang Shikun He Michael Tran Franck Ernult Micromagnetics Nanostructures FMR IHPC, Singapore DFT Calculations Amy Khoo Chee Kwan Gan LBL, USA X-Ray Microscopy Mi-Young Im Technion, Israel Low Temp MFM Alon Yagil Avior Almoalem Ophir Auslaender SERC PHAROS FUND NATIONAL RESEARCH FOUNDATION Data Storage Institute NTU Panagopoulos Group Singapore Ministry of Education 2

3 DMI & Skyrmions: Bulk Interface Magnetic Skyrmions Competition b/w Exchange & DMI B20 crystals: Bulk DMI Skyrmions o Predicted: Bogdanov, (1992) o Observed: Pfleiderer, Tokura, ( ) Interfacial DMI FM / SOC interface generates DMI o Predicted: Fert (1990) o Observed: Bode/Wiesendanger (2007) Int. DMI can also stabilise skyrmions Pfleiderer Group, Munich (2011) Z.X. Yu, Nature 11 Fert, Nat Nano 13 3

4 Interfacial DMI Skyrmions Interfacial DMI Neel skyrmions A. Fert, Nat Nano 13 First 4 K Pd/Fe/Ir (and Fe/Ir) w/ MBE SP-STM Imaging Pd/Fe/Ir, 8 K (SP-STM) Hamburg, Science 13 Multilayer RT Co/Pt based sputtered films XMCD-based techniques (STXM, MTXM, PEEM) RT Skyrmions (STXM) [Ir/Co/Pt] x10 [Ta/Co/Pt] x15 CNRS, PSI, BESSY Nat Nano 16 MIT, LBL, BESSY Nat Mat 16 4

5 Skyrmion Manipulation 4K: [Pd/Fe/Ir] RT: Ta/CFB/TaO x Nucleation & Deletion Hamburg, Science 13 Argonne, Science 15 RT: [Ir/Co/Pt] x10 RT: MgO/Co/Pt Confinement CNRS, PSI, BESSY Nat Nano 16 SPINTEC, Nat Nano 16 RT: [Ta/Co/Pt] x15 Dynamics Road to Skyrmion Memory? MIT, LBL, BESSY, Nat Mat 16 5

6 The Path To Functional Skyrmions Vary Magnetic Interactions (e.g. D, K) Tune Skyrmion Properties Size: Towards 10 RT Stability: Isolated vs Lattice Density Sampaio, Nat Nano 16 Tabletop Detection & Manipulation Electrical Detection Imaging in Device Configuration von Bergmann, Science 15 6

7 Outline Multilayer Stack w/ Tunable Interactions Ir/Fe(x)/Co(y)/Pt Stack & DFT Calculations Skyrmion Imaging & Electrical Detection MTXM & MFM Imaging Topological Hall Effect Tuning Skyrmion Properties Fe/Co Composition Mag. Interactions Tuning Sk. Stability, Size & Density Thermodynamic Stability & Confinement Exploring Skyrmion Phase Diagram (H,T) Confinement Effects 7

8 DMI: Interfaces Trilayers FM/SOC Interface DMI Skyrmions Single interfaces low T C A. Fert, Nat Nano 13 Co-based Trilayers tot Co/Pt: Strong DMI (d Co/Pt +2 mev) Co/Ta, Co/Ir: Weak DMI ( d tot < 0.5 mev) Asymmetric trilayer (e.g. Ir/Co/Pt) tot d Ir/Co/Pt 2 mev Moreau-Luchaire, Nat Nano 16 How to enhance DMI? 8

9 4 Layer Stack for Tunable DMI Use 2 Large DMI Interfaces? Co/Pt: Large, Pos. DMI tot (d Co/Pt +2 mev, H.X. Yang, PRL 15) Fe/Ir: Large, Neg. DMI tot -2 mev, B. Dupe, Nat Comms 15, 16) (d Fe/Ir Combine into 4-Layer Stack w/ larger DMI? Ir / Fe / Co / Pt Multilayer Stack arxiv:

10 Ir/Fe/Co/Pt: DMI Calculations Ir/Fe/Co/Pt: Is DMI enhanced w.r.t. Ir/Co/Pt? DFT Calculation of DMI Stack: Ir[3] / Fe[a] / Co[b] / Pt[3] d tot = E CW E CCW /m H.X. Yang, PRL 15 d tot for varying Fe[a]/Co[b] H.X. Yang, PRL 16 Comparison w/ Fe[0]/Co[3] (Ir/Co/Pt) arxiv:

11 Ir/Fe/Co/Pt: DMI Enhancement Fe[1]/Co[2] vs. Fe[0]/Co[3] DMI can be substantially enhanced Enhancement wrt. Ir/Co[3]/Pt persists even for ~ 2x FM thickness Similar reported results for Fe[1]/Co[2] Fe[a]/Co[b] Tuning D, K D dome shape K monotonic decrease H.X. Yang, arxiv: ( 16) arxiv: H.X. Yang, arxiv: ( 16) 11

12 Ir/Fe/Co/Pt: Film Properties Sputtered Film Deposition Chiron System (Bestec GmbH, P ~ 10-8 torr) Sub-monolayer precision Fe (x): 0-6 Å, Co (y): 4-6 Å X-ray: Si 3 N 4 membranes, 20x repeats 20 x Structural Properties (111) texture w/ sub-nm roughness XRR fringes sharp interfaces Magnetic Properties Sheared Hysteresis Loops Zero Field: Labyrinthine Domains arxiv:

13 Ir/Fe/Co/Pt: Magnetic Microscopy MTXM (full field) XMCD contrast M z (r) XM1 (ALS, Berkeley), 25 nm resolution RT, OP field: ±250 mt Films on Si 3 N 4 membranes MFM (scanning probe) Deflection M z r stray field Park Systems, 30 nm ULM tip RT, in air, OP field: ±200 mt Films on SiO 2 substrates P. Fischer, Z. Phys B ( 98) Shimadzu Inc. 13

14 Setup Fe(3)/Co(6): Co Edge Co Edge: 778 ev +250 mt, down sweep Features Co: -100 mt Co: +100 mt Scale Bar: 500 nm +H: Round-ish, < 100 nm 0: Stripe-like -H: Round-ish, < 100 nm Co: 0 mt high low 14

15 Setup Fe(3)/Co(6): Fe Edge Fe Edge: 708 ev +250 mt, down sweep Features Fe: -100 mt Fe: +100 mt Scale Bar: 500 nm +H: Round-ish, < 100 nm 0: Stripe-like -H: Round-ish, < 100 nm Fe: 0 mt high low 15

16 Setup Fe(3)/Co(6): MFM MFM +250 mt, down sweep Features Fe: -100 mt Fe: +100 mt Scale Bar: 500 nm +H: Round-ish, < 100 nm 0: Stripe-like -H: Round-ish, < 100 nm Fe: 0 mt high low 16

17 Fe(3)/Co(6): Comparisons Domain Periodicity Fourier Analysis of Magnetic Contrast MTXM (Fe/Co) agree to ~10 nm MFM & MTXM agree to ~ 15% (SiO 2 vs. Si 3 N 4 substrates) Bubble Width 2D isotropic Gaussian fit Raw widths (no deconvolution) overestimates bubble size nm, reduces with increasing H MTXM (Fe/Co) agree to < 5% MTXM & MFM agree to < 10% 17

18 Ir/Fe/Co/Pt: Bubbles Skyrmions? Ir/Fe/Co/Pt: Bubble Properties Size: d meas ~ nm (as measured) Size: d meas reduces with H Density: increases w/ H (< H S ) D est /A est ~ 15-20% (later ) Skyrmions? Similar to Ir/Co/Pt and Ta/Co/Pt results Neel skyrmions from interfacial DMI Ongoing: Image in-plane spin texture with Lorentz TEM arxiv: Woo, Nat Mat 16 Moreau-Luchaire, Nat Nano 16 18

19 Magneto-transport on Ir/Fe/Co/Pt Transport Expts J exc 10 4 A/m 2, sub-nv resolution Field Offset removal for M H & R(H) ρ xx : constant to 0.05% I exc V xy ρ xy : fit using AHE form ρ fit xy H = R 0 H + M H A ρ xx (H) + B ρ2 xx H Residual Hall Signal ρ TH xy H = ρ xy H ρ fit xy H V xx arxiv:

20 Residual Hall Signal Topological Hall Effect ρ TH xy H = ρ xy H ρ fit xy H Consistent with skyrmion phase Topological Hall effect Berry Phase accumulated by electrons traversing skyrmions Max ρ TH xy (~ 30 nω cm) similar to B20 Films S.X. Huang, PRL 12 N.A. Porter, PRB 14 arxiv:

21 Quantifying Magnetic Interactions Measure Anisotropy, K (M(H)) Estimate DMI, D & Exchange, A ZF domain periodicity, MFM vs. μ-magnetics simulations C. Moreau-Luchaire, Nat Nano ( 16) S. Woo, Nat Mater ( 16) μ-magnetics: mumax 3, 2 μm size, 4 nm mesh, vary D & A 2D χ 2 fit to get D est and A est Fit Results A est 12 pj/m (across Fe/Co) D est mj/m 2 MFM, 0 T Simulation, 0 T high low 21

22 DMI Estimation Tuning DMI w/ Fe(x)/Co(y) A est 12 pj/m (across Fe/Co) Ir/Co(6)/Pt: D est ~ 1.6 mj/m 2 consistent w/ CNRS results Ir/Fe(4)/Co(4)/Pt: D est ~2. 3 mj/m 2 enhanced by ~ 50% Dome-shaped D est does not track K eff Tunable Parameters Continuous Tuning of D est and K How does this affect skyrmions? 22

23 Fe(x)/Co(y): Skyrmion Properties Fe(2)/Co(6) D est ~1.7 mj/m 2, K eff ~0.25 MJ/m 3 Large skyrmions, isolated config. Fe(4)/Co(6) D est ~1.9 mj/m 2, K eff ~0.05 MJ/m 3 Small skyrmions, disordered lattice Skyrmion Lattice Stability D c = 4 A K eff /π S. Rohart, PRB ( 13) FFT 23

24 Fe(x)/Co(y): Skyrmion Density D/D c (K eff ) skyrmion density Isolated: n Sk ~ 5 10 /μm 2 Lattice: n Sk ~ /μm 2 n Sk can be varied by 10x! 24

25 Fe(x)/Co(y): Skyrmion Size D > D c d Sk reduces as D increases Fe(2)/Co(6): nm (D~1.7 mj/m 2 ) Fe(4)/Co(6): nm (D~1.9 mj/m 2 ) Fe(4)/Co(4): nm (D~2. 3 mj/m 2 ) d Sk can be tuned by ~ 2x d Sk includes MFM tip size (~ 30 nm) real skyrmion size may be smaller! 25

26 Skyrmions in Nanodots Nano-dot Fabrication EBL (10 nm) and IB Etching Dot Sizes: μm H = H S 0.5 μm Nanodots Scale Bar: 0.1 μm 26

27 Fe(x)/Co(y): Confinement Effects Fe(x)/Co(y): Sk. Density Fe(4)/Co(6): n Sk (H) indep. of dot size Fe(2)/Co(6): n Sk (H) higher for 0.5 μm Consistent w/ CNRS, SPINTEC results Enhanced skyrmion nucleation in geometrically confined structures Confine skyrmions at zero field? H = nm 400 nm 300 nm 27

28 Conclusions Multilayer Stack w/ Tunable Interactions Ir/Fe(x)/Co(y)/Pt Multilayer Stack Skyrmion Imaging & Electrical Detection MTXM & MFM Imaging of Skyrmions Topological Hall Effect Detection Tuning Skyrmion Properties Enhanced D (~ 2.3 mj/m 2 ) & Varying K Sk. Stability (Isolated / Lattice), Density (10x), and Size ( nm) Thermodynamic Stability & Confinement Skyrmion Phase Boundary, Anisotropy Effects Zero Field Skyrmions: Nucleation & Confinement 28