Magnetic imaging at the nanoscale

Transcription

1 Magnetic imaging at the nanoscale M. Ghidini Department of Materials Science, University of Cambridge, UK Department of Physics, University of Parma, Italy

2 Magnetic imaging at the nanoscale Comparison of magnetic imaging techniques Scanning Probe Microscopies for Magnetic Materials Magnetic microscopy studies of multiferroics heterostructures

3 Magnetic imaging at the nanoscale Benchtop Facility Highly Specialised Imaging Quant Probe depth (nm) Resolution (nm) Contrast Bitter Parallel N bulk 100 Stray field N MOKE Parallel Y (300) M Y Time Res. Synch. Rad. Techniques Electron Microsc. Scanning Probes (SPM) XMCD-PEEM Parallel N 5 50 MTXM N TEM SPLEEM SEMPA Lorentz Holography MFM MExFM SP STM Magn. field sensor scanning Parallel Par.(Offline) Parallel Scanning Scanning Y Y N Y N bulk Stray field N N Y < < 500 < 1 nm < 1 nm (5) bulk < 1000 nm M M B B Exchange F. SP emission Exchange F. SP current B Y Y Y N N N N N N N

4 Progress in magnetic imaging Domain patterns in FeSi Magnetization of individual Co atoms on Pt (111) mm F. Bitter, Phys. Rev., 38 (1931) 1903 H. J. Williams, R. M. Bozorth, and W. Shockley, Phys. Rev., 75 (1949) 155 F. Meier, L. Zhou, J. Wiebe, and R. Wiesendanger, Science 320, (2008), 32 Bitter powder method Spin Polarised Scanning Tunneling Microscopy (SP STM)

5 Bitter powder method Principle : the fine magnetic particles of a suspension are held in place by the magnitude of the stray fields from the domain walls. Resolution : limited by particle size and utilized microscope (optical, SEM, STM...) SEM image Vortex lattice (const ~ 1mm) in BSSCO crystal decorated by Fe clusters Z. Yao, S. Yoon, H. Dai, S. Fan, C.M. Lieber, Nature, 371 (1994) 777 STM image of decorated magnetic bits P. Rice and J. Moreland, Rev.Sci.Instrum., 62 (1991) 844

6 MOKE microscopy Principle : magneto-optical rotation of the plane of polarization depends on local M direction (circular magnetic birifrangence) Resolution : ~ 0.3 mm (F. Schmidt and A. Hubert. J. Magn. Magn. Mat. 61, (1986)) Polar effects with M perpendicular Longitudinal par (M in plane) Longitudinal perp (M in plane) Transverse (M in plane)

7 XMCD-PEEM Principle : X-ray absorption depends on orientation of local M with respect to beam direction Resolution : ~ 50 nm (typ.)

8 XMCD PEEM at Diamond Light Source ~200 m p m I I D(%) C M qˆ p m I I IO6 Nanoscience Beamline, S. Dhesi and F. Maccherozzi

9 Lorentz microscopy Principle : TEM with Lorentz force deflection Resolution: 2-20 nm F qv B Fresnel mode: domain wall contrast Foucault mode: domains contrast

10 Electron holography Principle : interference of a reference wave passing through vacuum with one passing scattered by the sample. A digital reconstruction allows both the amplitude and phase of the exit wave-function to be determined directly. Reconstructed Phase images reveal electrostatic and magnetic fields in the sample. Resolution: ~ 5 nm

11 SPLEEM Principle : contrast due to exchange interaction between low energy polarized beam of electrons and the electrons in the sample. Resolution: ~ 10 nm.

12 SEMPA (spin-uhv SEM) Principle : contrast from polarization analysis of secondary electrons Resolution : typically nm, but 5 nm demonstrated T.Kohashi and K. Koike, Jpn. J. Appl. Phys. 40, L1264 (2001)

13 Scanning Probe Microscopies for Magnetic Materials

14 Scanning Force Microscopy Scanning Probes principle deflection sensor The first Atomic Force Microscope (AFM) feedback sample force sensor tip approach vibration damping Data acquisition G. Binnig, Ch. Gerber and C.F. Quate, Phys. Rev. Lett. 56, 930 (1986)

15 MFM Principle : mapping stray field gradient via the dipolar forces exerted on a force sensor with a magnetic tip Resolution: nm

16 Force Measurement: static vs. dynamic f A f 1 f 2 f 3 z DC = F/k z AC FQ 2 m 0 z DC Q

17 Dynamic measurement: point mass modeling in the harmonic approximation Change in resonance curve can be detected by: Lock-in (A or ) (as in Tapping mode) FM detection (PLL, f) Albrecht, Grutter, Horne and Rugar J. Appl. Phys. 69, 668 (1991)

18 Nanopositioning Actuators for Scanning and Control PZT-5H d 31 = nm/v Vertical movement : elongation of a tube Horizontal scanning obtained by the bending of the tube x 2 2 2d31l U Dh x

19 Displacement sensors

20 Magnetic Force Microscopy Upper limit in ideal case 1 nn Typically forces are smaller 1-10 pn Co tip If force and tip are aligned along z: z mag Fmag ; k eff k eff k l F nonmag z F mag F=q tip H sample = N~1 nn Ni sample

21 Amplitude and phase variation in the presence of a magnetic forces gradient Close to resonance f z F z m z 2 z 2 H z Amplitude Phase 21

22 Non-magnetic vs. magnetic tip-sample forces Non-magnetic forces are short range and can be made negligible with respect to magnetic (or electrostatic) forces by scanning at higher tip- sample distances. Line by line Lift Mode

23 Magnetic tip-sample forces: theory of magnetic imaging by MFM Energy with no perturbation leads to dipolar forces only E m0m sam H tip Isolated stripe domain ( Charge contrast ) Reversible perturbations (H tip << H c ~ bm sam lead to a supplementary magnetic force which is always actractive 1 M samh 2 ( Susceptibility contrast ) 2 2 E bm sam sin m0 ( tip Dipolar Force gradient J.J. Saenz, N. Garcia and J.C. Slonczewski, Appl. Phys Lett. 53, (1988) 1449

24 Tip-sample interactions and contrast Charge contrast : no perturbation so M tip and H sam do not change with tipsample position, and so does contrast Susceptibiliy contrast : reversible perturbation of sample magnetization by the tip magnetic field (actractive force which is a function of tip/sample distance only) Hysteretic or irreversible modification: M tip and/or H sam are changed irreversibly during scan. Contrast depends also on the history of tip-sample position.

25 Charge contrast d D.Rugar et al. J. Appl. Phys, 68,1169, 1990 t H( x,z ) 4M 4M r t n ( 1) n r t n ( 1) z ( z nd ) n 2 x nd 2 ( x nd ) ẑ 2 z z 2 xˆ

26 Hard disks

27 Comparison of different observations of magnetic vortices Susceptibiliy contrast Charge contrast

28 Nominal coverage (atomic layers, AL) Flux closures in Fe (110) dots T <370K, Q r >6AL Q r T >400K, >6AL (for Mo) (for Mo) T =700K, Q [2AL,6AL] r t=6al (for Mo) Q~3.5AL 2mm >6AL Compact 3D dots [-110] (110) 50 0 nm Not explored 4AL [001] 3AL 5mm 2AL 1AL Flat islands t~1nm (for Fe/Mo) Compact 3D dots t>30nm 1m m ~750K? 300K 500K 700K 900K Deposition temperature, T (K) S O. Fruchart et al., J. Phys.: Condens. Matter 19, , Topical Review (2007)

29 Irreversible tip-induced sample modification

30 Magnetic Exchange Force Microscopy (MExFM) U.Kaiser, A. Schwarz, and R. Wiesendanger, Nature 446, (2007), 522.

31 Scanning Tunneling Microscopy (STM) Spin-polarised STM (SP STM) G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel Phys. Rev. Lett. 50, (1983) R. Wiesendanger, Rev. Mod. Phys., 81 (2009) 1495

32 Magnetization of individual atoms SP STM Atomic scale domain walls F. Meier, L. Zhou, J. Wiebe, and R. Wiesendanger, Science 320, (2008), 32 2D antiferromagnetism on the atomic scale Pratzer, M., H. J. Elmers, M. Bode, O. Pietzsch, A. Kubetzka, and R. Wiesendanger,, Phys. Rev. Lett. 87, (2001) Magnetic Vortex : in plane and out of plane M Wachowiak, A., J. Wiebe, M. Bode, O. Pietzsch, M. Morgenstern, and R. Wiesendanger,, Science 298, (2002) Heinze, S., M. Bode, A. Kubetzka, O. Pietzsch, X. Nie, S. Blügel, and R. Wiesendanger, Science 288, (2000),

33 Magnetic field sensor scanning microscopes Principle : SPM techniques where the micro-probe is a magnetic field sensor (Hall m-probes, GMR or MTJ, SQUID) Resolution: traditionally poor but improving Scanning m Hall probe Scanning Magnetoresistance Scanning SQUID

34 Magnetic imaging of multiferroics heterostructures

35 Multiferroic materials Single phase Composite systems W. Eerenstein, N.D. Mathur, J.F. Scott, Nature, 442, (2006) 759

36 Routes for non-volatile electrical control of magnetism in magnetoelectric heterostructures Exchange coupling Strain-mediated N. D. Mathur, Nature, 591, (2008) 454

37 Magnetic field assisted electrically driven M reversal Ni(100 nm)/ BTO S. Geprägs, A. Brandlmaier, M. Opel, R. Gross, and S. T. B. Goennenwein, Appl. Phys. Lett. 96, (2010)

38 Magnetic field assisted electrically driven M reversal V. Skumryev, V. Laukhin, I. Fina, X. Martı, F. Sa nchez, M. Gospodinov, and J. Fontcuberta Phys. Rev. Lett., 106 (2011)

39 Macroscopic magnetoelectric coupling in MLCs s m -1 Nature Materials 7 (2008) 93 Appl Phys Lett 93 (2008)

40 MLC characterization

41 Phase ( ) Phase ( ) Y Axis Title Volatile electrically driven magnetization reversal Apply & remove m 0 H x = 200 mt 3 mm Apply +100 V -2-2 Set 0 V X Axis Title Position (mm)

42 Phase ( ) Phase ( ) MFM-signal asymmetry due to tip-field reversal 1.8 mm Position (mm) 4 peak-to-peak 2 for sample tape

43 Phase ( ) Phase ( ) Non-volatile electrically driven repeatable magnetization reversal with no applied magnetic field A&R +200 V A&R -200 V A&R +200 V Position (mm) mm

44 Mechanism for non-volatile electrically driven repeatable magnetization reversal H f from surrounding domains H k from strain H k z y x M H f H f H f H k H k E e x e y 0 0 H H k k_hi 0 H k_lo t t t

45 Mechanism for non-volatile electrically driven repeatable magnetization reversal H f from surrounding domains H k from strain H k z y x M H f H f H f H k H k E e x e y 0 0 H H k k_hi 0 H k_lo t t t

46 3D imaging of Ni/BTO by XMCD PEEM NiBTO2: Electric and magnetic virgin state as beam angle varies hn hn = K = K hn hn Stripe domains modulate large in-plane domains = K = K FOV = 20 mm

47 Phase (deg) Variation of magnetic microstructure with T hn NiBTO2: Electric and magnetic virgin state C H 297 K 127 K H H cf. T K 297 K 20 µm FOV 297 K -0.5

48 Phase (deg) Height (nm) MFM of Electrically virgin state (unpoled substrate) AFM NiBTO2: Electric virgin state Cu(4 nm)/ Ni(100 nm)/ BTO (0.5 mm) Electrically virgin state (unpoled substrate) H app = 0 H app = 500 Oe V 0 MFM H app = 0 H app = 500 Oe 2-2 Remanent state after magnetizing in 1 T out-of-plane

49 Varying electric field with positive voltages NiBTO2 Electric virgin state 4 mm 0 V 90 V 110 V 150 V 210 V 250 V 300 V 350 V 400 V 0 V 400 V 0 V Electrical remanence Electrical remanence

50 Varying electric field with variable-sign voltages NiBTO2-50 V V V V 0 V 100 V Stripes disappear with negative voltages

51 Height (nm) Phase (deg) Poled NiBTO2: non-volatile control of perpendicular magnetic anisotropy Second example 0 V -100 V 0 V 100 V Stripes are rotated when they reappear

52 Unpoled NiBTO2: volatile control of perpendicular magnetic anisotropy -200 V 0 V 0 V 0 V 0 V 200 V 0 V Stripes disappear fast and reappear slowly

53 Ni/PMN PT : MFM imaging in applied electric fields d c -150 VVV 0100 V00 VV -100 e B 220 m(memu) agdecf b 200 f 180 A V(Volts) a g 100

54 Conclusions Imaging techniques have progressed tremendously and while they continue to have a key role in nanomagnetism their impact is growing also in multiferroics research. MFM and XMCD-PEEM are complementary techniques with the the suitable resolution and field of views to study the local details of magnetoelectric coupling in multiferroic heterostructures