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Nanomagnetometry W. Wernsdorfer, E. Bonet Orozco and B. Barbara Lab - PowerPoint PPT Presentation

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Nanomagnetometry W. Wernsdorfer, E. Bonet Orozco and B. Barbara Lab L. Nel - CNRS, Grenoble, France A. Benoit CRTBT - CNRS , Grenoble, France D. Mailly L2M, Bagneux, Paris, France and a lot of collaborators Different techniques Torque

  1. Nanomagnetometry W. Wernsdorfer, E. Bonet Orozco and B. Barbara Lab L. Néel - CNRS, Grenoble, France A. Benoit CRTBT - CNRS , Grenoble, France D. Mailly L2M, Bagneux, Paris, France and a lot of collaborators

  2. Different techniques • Torque balance [Morrish 1956] • "Rotation method" [Knowles 1978] • Vibrating sample magnetometer 10 7 µ B [Richter 1989] • Lorentz microscopy 10 7 µ B [Salling 1991] • MFM 10 7 µ B [Chang 1993, Ledermann 1994] • Hall sensor 10 6 µ B [Kent 1994] • Micro - SQUID 10 4 µ B [Wernsdorfer 1995] • Transport measurements 10 4 µ B [Giordano 1995] ...

  3. ✁ ✁ � 2D Hall probes H I V Principle: Deviation of electrons induced by a magnetic field (Lorentz force) Semi-conductor heterostructure : GaAs - GaAlAs (à 4K) electron density : n = 3 10 11 cm -2 mobility : 800 000 cm 2 V -1 s -1 Hall resistant : 2000 /T resistance : 20 at 4K and 2000 at 300K

  4. 2D Hall bridge I H sample A.D. Kent, D.D. Awschalom et al., JAP, 76, 6656 (1994) sensitivity of 10 6 µ B A.K. Geim et al. APL, 71 (16), (1997) Luise Theil Hansen, <theil@meyer.fys.ku.dk> I sensitivity of 10 4 µ B V

  5. Electric transport measurements Magnetoresistance K. Hong, N. Giordano, JMMM, 151, 396 (1995) depinning of a domain wall in an isolated Ni wires F. Coppinger et al., PRL 75, 3513 (1995) Single domain switching of small ErAs clusters investigated using telegraph noise spectroscopy Giant magnetoresistance V. Gros, A. Fert et al. Co/Cu/Co structures Spin-dependent tunneling with Coulomb blockade L.F. Schelp, A. Fert et al., PRB, 56. R5747 (1997) Co/Al2O3/Co tunnel junctions with cobalt clusters in the Al2O3 layer

  6. Superconducting Quantum Interference Device (SQUID) Different types of Josephson junctions : Theoretical limit : 1 µ B !!! • point junctions • tunnel junctions with a coupling factor of 4*10 7 µ B / Φ o • micro bridge junctions

  7. Roadmap of the micro-SQUID technique Quantum limit of a SQUID 1 S (µ B ) 100 0 3 nm 10 4 information storage 10 6 10 8 Years 10 10 1993 1995 1997 1999 2001 2003

  8. Studied nanostructures Ho submicron molecular individual wires nanoparticles clusters particles clusters spins 20 10 10 10 6 10 3 10 8 10 5 10 4 10 2 10 1 S = 10

  9. ✁ � Micro-SQUID magnetometry stray field particle B ≈ 1 µm Josephson junctions • fabricated by electron beam lithography (D. Mailly, LPM, Paris) 10 -4 Φ Φ o Φ Φ • sensitivity : 10 2 – 10 3 µ B , i.e. (2 nm) 3 of Co 10 -18 – 10 -17 emu

  10. ✁ ✁ � � SQUID details • fabricated by electron beam lithography D. Mailly, L2M - CNRS, Bagneux • dimension : 1 - 2 µm • material : Nb • temperature : < 7K • direct coupling with the SQUID 10 -4 Φ o • sensitivity : 10 4 µ B i.e. (6nm) 3 of Co 10 -16 emu conventional SQUID : 10 -7 emu

  11. 2 4 conne ct e d m ic ro -SQUID s e t -up SQUIDs Curre nt s ource s 0 .0 3 - 6 K x 1 0 A/ 1 0 V y 1 0 A/ 1 0 V z 1 0 A/ 1 0 V 1 ...2 4 Scann e r 1 µ A - 3 mA I PLD Macint os h t 2 PCI cart e s SQUID Ele ct roniq ue

  12. Naïve theory I 1 n � 0 I 2

  13. Critical current measurements I D 15 U(mV) 10 C ? 300 ns 5 B 0 I c A -5 E Ic min Ic -10 F -15 -150 -100 -50 0 50 100 150 I(µA) t I c(µA) 120 110 100 90 80 70 60 50 Φ ο Φ Φ Φ ο ο ο 40 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 µ µ µο µ ο Η(µΤ) ο ο Η(µΤ) Η(µΤ) Η(µΤ)

  14. � � ✁ I c statistics 700 • Magnetization measurement : average of N count 600 measurements of I c precision increases with 500 400 • limitation of the cycling frequency of I c measurement : length of the current ramp 100 µs 300 cooling of SQUID 1 µs 200 100 • sensitivity : 10000 measurements per second : 0 0 0.5 1 1.5 2 Ex. : our sensitivity : 10 4 µ B Ic -180µA cluster of Co of 5 nm in diameter Histogram of 60000 I c measurements

  15. � ✁ ✁ ✁ ✁ ✂ Feedback mode I c I c0 stable zone measure I c continuously if I c > I c0 , apply positive external flux if I c < I c0 , apply negative external flux external flux compensates sample's fux

  16. � ✁ ✁ � ✁ � � � Jump detection: “cold mode” 120 0.3 110 0.2 100 Flux( ) Φ/Φο Φ/Φο Φ/Φο Φ/Φο 0.1 90 I c (µA) + – 80 0 Β Β Β Β Α Α Α Α 70 -0.1 60 50 -0.2 40 -0.3 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 -150 -100 -50 0 50 100 150 µ 0 H Η(µΤ) Η(µΤ) Η(µΤ) Η(µΤ) µ µ µ µ ο ο ο ο SQUID polarized below the critical current magnetization jump SQUID transition the SQUID heats only after the magnetization jump

  17. ✂ ✁ ✁ ✁ Blind mode H Hsat ² t t ( i) -H sat H t est M + 1 t -1 T 0.04 K < T < 30 K measure 0.04 K t apply a test field, we may (or may not) have reversal measure after the fact with a second field field out of plane, high T, microwaves...

  18. Ex: “large” particles Co particle: 70 nm x 50 nm x 25nm Ni wires: (40-100) nm x (4-5) µm 90¡ 1 120¡ 60¡ 0.3 h sw 30¡ 0.2 0.5 Flux( ) Φ/Φο Φ/Φο Φ/Φο Φ/Φο S < 1 0.1 0¡ 0 -0.1 S = 1.4 -30¡ -150¡ -0.2 S = 2.4 -0.3 -60¡ -120¡ -150 -100 -50 0 50 100 150 -90¡ Η(µΤ) Η(µΤ) Η(µΤ) Η(µΤ) µ µ µ µ ο ο ο ο

  19. Smaller systems FeS particle: length 200 nm, diameter 20 nm Co nanoparticles: diameter 20 nm 90¡ 90¡ 120¡ 60¡ 250 120¡ 60¡ 0.4 200 0.3 30¡ 150 30¡ 0.2 100 (µΤ) (µΤ) (µΤ) (µΤ) 50 (Τ) (Τ) 0 (Τ) (Τ) 0.1 σω σω σω σω σω σω σω σω 0¡ Η Η Η Η Η Η Η Η 0¡ ο ο ο ο µ ο ο ο ο µ µ µ µ µ µ µ 210¡ 330¡ 210¡ 330¡ 240¡ 300¡ 240¡ 300¡ 270¡ 270¡

  20. Coupling between nanoparticles 0.165 0.16 µ o H y (T) 0.155 90¡ 0.3 0.15 120¡ 60¡ dH/dt 0.2 µ 0 H(T) 30¡ 0.145 0.1 0.14 -0.125 -0.12 -0.115 -0.11 0 0¡ µ o H x (T) 210¡ 330¡ 240¡ 300¡ 270¡

  21. 3 nm cobalt cluster DPM - Villeurbanne: LASER vaporization and inert gas condensation source Low Energy Cluster Beam Deposition regime HRTEL along a [110] direction blue: 1289-atoms truncated octahedron grey: added atomes, total of 1388 atomes fcc - structure, faceting Ideal case: truncated octagedron with 1289 or 2406 atoms for diameters of 3.1 or 3.8 nm

  22. Low energy cluster beam deposition clusters Nb embedded clusters

  23. � ✁ � Micro-SQUID magnetometry embedded clusters embedded 3 nm clusters 1 µm Josephson junctions SQUID is fabricated by electron beam lithography D. Mailly, LPN-CNRS 10 2 - 10 3 µ B i.e. (2 nm) 3 of Co sensitivity : 10 -18 - 10 -17 emu i.e. clusters in Nb - matrix M. Jamet, V. Dupuis, A. Perez, DPM-CNRS, Lyon Acknowledgment: B. Pannetier, F. Balestro, J.-P. Nozières

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