SEEING Phase Transitions with. Magnetic Force Microscopy. Dept. of Physics, University of Texas at Austin. Alex de Lozanne

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1 SEEING Phase Transitions with Magnetic Force Microscopy Alex de Lozanne Dept. of Physics, University of Texas at Austin Defense: Mon Dec. 5, 10am, CURRENT GROUP: Dr.Weida Wu, Casey Israel, Tien-Ming Chuang, JeeHoon Kim, Junwei Huang, Changbae Hyun, Suenne Kim, Seosoong Kweon, Alfred Lee, Frank Ruzicka, Fred Castro. COLLABORATORS: Neil Mathur (Cambridge), Sang-Wook Cheong (Rutgers), Rick Greene (Maryland), Aminta Mendoza (Universidad del Quindío, Colombia), Brian Korgel (UT)

2 Scanning Probe Microscopy sensor amplifier sample probe scanning tube computer driving electronics probes: force, optical, thermal, capacitance, tunneling 3

3 Magnetic Force Microscopy (MFM) Ferromagnetic material on sharp tip attached to cantilever Assume dipole moment aligned with? Assume cantilever only responds to forces in? Magnetic force gradient causes effective change of k Drive cantilever at resonance, read out lever deflection, measure frequency shift as sample is scanned? m B sample

4 Fe Piezolevers: Deflection=resistance change oxide layer tip doped layer silicon gold pads ceramic chip 5

5 Electric/Magnetic Force detection Frequency Modulation Technique ω ω 0 = 1 2 k f Amp. Lift height FerroMagnetic Metal film (Co/Cr) Magnetic Force gradient MFM signal Electric Force gradient EFM signal 6

6 Colossal Magnetoresistive (CMR) Materials La 1-X D X MnO 3? Magnetoresistive effect Perovskite structure La or dopant O Mn H M Ferromagnetic Paramagnetic T? C T T C T IM Metallic Insulating T IM T 7 Slide: Tien-Ming Chuang

7 Magnetoresistance (MR): CMR 1994: La 0.67 Ca 0.33 MnO 3 films show MR ~ % at B = 6 T Named Colossal MR (CMR), related to metal-insulator transition at T C La 0.67 (Ca 0.33 Pb 0.67 ) 0.33 MnO 3 T C M. Salamon et al, RMP 73, 583 (2001) GMR MR = % MR = % 77 K B (T) S. Jin et al, Science 264, 413 (1994)

8 Manganite crystal structure BCC perovskite R 1-x A x MnO 3 R 3+ (rare earth) or A 2+ (alkali earth) Mn 3+ or Mn 4+ Free Mn: [Ar] 3d 5 4s 2 O 2-5 degenerate 3d orbitals split to triplet and doublet, Hund s coupled 1-x electron per Mn site to doublet Figure courtesy Tien-Ming Chuang 10

9 1951: Zener s Double exchange, simultaneous transfer of 2 electrons More likely if Mn core spins are aligned, t cos(?/2) B can align Mn core spins, qualitatively explains CMR MR = % B (T) Double Exchange Core spin (triplet) Free spin Mn 3+ O 2- Mn 4+? C. Zener, Phys. Rev. 82, 403 (1951) B 11

10 1995: Millis: Double Exchange misses the sharp drop of R at T C Proposed that electron in Mn ion doublet can couple to Jahn-Teller distortion of surrounding O 2- ions This tetragonal distortion splits doublet, lowering Coulombic energy for 1 orbital at the cost of lattice distortion energy Self-trapping Release of trapped electrons at T C causes sharp R drop Jahn-Teller Effect T C T C A. J. Millis et al, PRL 74, 5144 (1995) A. J. Millis et al, PRL 77, 175 (1996) 12

11 Nature 392, 147 (1998) COEXISTENCE OF COMPETING PHASES (ELECTRONIC INHOMOGENEITY) PERCOLATION 2-4 ev 13

12 La 0.65 Ca 0.35 Mn 0.8 O 3 thin film, 300 nm thick Temperature (K) 0 15 Resistivity (Ω-cm) Magnetization (emu/cm 3 ) without magnetic field with 5T magnetic field magnetization for H = 5 Oe magnetoresistance

13 Pg

14 Science 298, 805 (Oct 25, 2002) 17

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16 Cooling 6 µm 600Å La 0.33 Pr 0.34 Ca 0.33 MnO 3 /NdGaO 3 19

17 Warming (after field cooling) 600Å La 0.33 Pr 0.34 Ca 0.33 MnO 3 /NdGaO 3 20

18 New MFM Design Temperature control High vacuum operation External field in or perpendicular to the sample plane In-situ transport measurements during imaging Accurate in-situ tip-sample positioning + WINDOW 21

19 MFM head AFM cantilever with tip, coated with Fe Z screw for approach X-Y balls offset from center axis of rods -> provide x-y coarse motion Hole in tip plate allows for optical access Tip plate removable for access to sample Scan tube Thermal diode Z ball and screw Sample X-Y rod Ball Drive piezo Cantilever X-Y balls and rods Spring loaded clip 22

20 Phase Separation and Percolation in Single Crystal Manganites B = 0 12 K : Single crystal La 1/4 Pr 3/8 Ca 3/8 MnO 3 Doped in region of low T µm-scale phase separation 15 µm MFM images confirms isolated FM regions, consistent with TEM data Single crystal La 5/8-y Pr y Ca 3/8 MnO 3 PM 0 Hz 0.3 FM CO 23 M. Uehara et al, Nature 399, 560 (1999)

21 Phase Separation and Percolation in Single Crystal Manganites 12 K : T Single crystal La 1/4 Pr 3/8 Ca 3/8 MnO 3 Zero field imaging -> high field (saturation) imaging : phase discrimination (non-fm/fm) 15 µm B=0? 0 Hz 0.3 2? If field favors FM state, why no FM growth as field is increased? -> Some kind of energy barrier for phase growth at low temperature, frozen. 24

22 Phase Separation and Percolation in Single Crystal Manganites Scan with field: Phase imaging (dark=fm, bright=co) Movie: All images at 7 K, start from low T frozen state, (mostly CO, some FM), increase field to favor FM state, avalanche growth of conducting FM phase correlated with drop in R Movie goes here Phase separation evident, PS length scale up to many µm at percolation Twin boundary as phase boundary is evidence that strain energy barrier is the freezing influence 25

23 Variable temperature magnetic force microscopy of patterned magnetic films (La 0.6 Ca 0.4 MnO 3 ) with T C below room temperature Casey Israel Changbae Hyun, Alex de Lozanne Physics Department, University of Texas at Austin Bas van Aken, Neil Mathur Department of Materials Science, University of Cambridge 30

24 Tip-Sample Alignment Align at RT using coarse x-y manipulators and scan Fine tune tip-sample position during scanning Withdraw and cool down 50 µm 31

25 Additional structure associated with domain walls in LCMO? If FM and CO phases have similar energies: Phase balance very sensitive to field, strain, There is the possibility of inclusion of CO phase at DW since magnetic order is lowered Schematic picture of charge-order parameter (striped) and magnetization (grey) at a magnetic domain wall, in situations where the CO and FM phases are close in energy. ND Mathur and PB Littlewood, Solid State Commun. 119, 271 (2001) 32

26 La 0.6 Ca 0.4 MnO 3 40 nm film on NdGaO 3 T C = 228 K Hope to pin domain walls at constrictions in track, correlate transport data with MFM images R a La0.6Ca0.4MnO3 R=12 800(10) Ω 6DW thin track H H (mt) x 2 Easy axis H 8µm 50 µm 33 R (O) 78 K

27 MULTIFERROICS Nicola A. Spaldin and Manfred Fiebig, Science 309, 391 (2005) Physics Today, August 2005, pg

28 The immiscibility between La 5/8 Sr 3/8 MnO 3 and LuMnO 3 Park et al, PRL 92, (2004) Insulator metal Topograph MFM SMM Lee et al. APL 86, (2005) 44

29 (x)lsmo:(1-x)lmo prepared by Floating Zone Method Polarized Optical Microscopy 0.1 mm LuMnO 3 Hexagonal FerroElectric T c =900K Anti-FM T N =90K x=0.2 La 5/8 Sr 3/8 MnO 3 Orthorhombic. FerroMag: T c =370K 45

30 (x)lsmo:(1-x)lmo prepared by Floating Zone Method Polarized Optical Microscopy x=0.2 LuMnO 3 Hexagonal FerroElectric T c =900K Anti-FM T N =90K La 5/8 Sr 3/8 MnO 3 Orthorhombic. FerroMag: T c =370K 46