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Electronic properties of a ferromagnetic shape memory alloy: Ni-Mn-Ga

Talk at ‘Electronic Structure of Emerging Materials: Theory and Experiment’ at Lonavala-Khandala, 8 th February, 2007. Electronic properties of a ferromagnetic shape memory alloy: Ni-Mn-Ga. Sudipta Roy Barman. UGC-DAE Consortium for Scientific Research , Indore.

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Electronic properties of a ferromagnetic shape memory alloy: Ni-Mn-Ga

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  1. Talk at ‘Electronic Structure of Emerging Materials: Theory and Experiment’ at Lonavala-Khandala, 8th February, 2007 Electronic properties of a ferromagnetic shape memory alloy: Ni-Mn-Ga Sudipta Roy Barman UGC-DAE Consortium for Scientific Research, Indore Part of university system fully funded by UGC. Besides in-house research, we provide advanced research facilities to University researchers. Emphasis on Researchers in different academic institutions to work together. www.csr.ernet.in Max Planck partner group project

  2. What is a shape memory alloy? SMA effect involves structural transition called martensitic (after F. Martens) transformations which are diffusionless. It is a first order transformation and occurs by nucleation and growth of a lower symmetry (tetragonal/orthorhombic) martensitic phase from the parent higher symmetry (cubic austenitic) phase.

  3. The magnetic moments without the external field The rotation of the magnetic moments within the twins. The redistribution of the twin variants. Ni-Mn-Ga is ferromagnetic, and exhibits magnetic SMA SMA: Transformation from the martensite to austenite phase by temperature or stress. FSMA: Entirely within the martensite phase, actuation by magnetic field, faster than conventional stress or temperature induced SMA. 10% Magnetic Field Induced Strain in Ni50Mn30Ga20 reported.

  4. Live simulation of the FSMA effect Rotation of magnetic moments: [Magnetocrystalline anisotropy<< Zeeman energy] FSMA effect: change in shape [Magnetocrystalline anisotropy>> Zeeman energy] 10% Magnetic Field Induced Strain in Ni50Mn30Ga20 reported. Highest in any system till date.

  5. Magnetic domains and twin bands Topography image MFM image Magnetic force microscopy image of Ni2.23Mn0.8Ga in the martensitic phase at room temperature clearly shows the twin bands (width 10 micron) and magnetic domains (width 2-3 microns) C. Biswas, S. Banik, A. K. Shukla, R. S. Dhaka, V. Ganesan, and S. R. Barman, , Surface Science, 600, 3749 (2006).

  6. Smart actuator materials Potential fields of applications

  7. A real actuator made from FSMA by Adaptamat This demo is animated, but it shows the motion of the axis. The actuator can be driven faster/slower (average 70mm/s) and in bigger/smaller steps (accuracy <1μm).

  8. The FSMA mechanism Magnetic field induced strain =1- c/a

  9. Overview of our collaborative work on study of fundamental properties of Ni-Mn-Ga • Polycrystalline ingot preparation in Arc furnace, EDAX [In house] • Thermal, transport and magnetic studies: Differential Scanning calorimetry, Ac susceptibility; magnetization; resistivity; magnetoresistance; AFM, MFM [Collaboration: SNBCBS,Kolkata; Suhkadia University, Udaipur; TIFR, Mumbai; RRCAT, Indore & In-housePhys. Rev. B, 74, 085110 (2006) ; Appl. Phys. Lett. . 86, 202508 (2005); Surface Science, 600, 3749 (2006).] • Structural studies: X-ray diffraction [Collaboration: Banaras Hindu University, Banaras  Phys. Rev. B (2006, in press); Phys. Rev. B (2007, in press)] • Electronic structure: Photoemission spectroscopy (UPS and XPS); Inverse photoemission spectroscopy; theory (FPLAPW) [Collaboration: In-house and CAT, Indore Phys. Rev. B, 72, 073103 (2005); Phys. Rev. B 72, 184410 (2005); Applied Surface Science, 252, 3380 (2006)] • Compton scattering[Collaboration: Rajasthan University, Jaipur; Sukhadia university, Udaipur, Spring-8, Japan Phys. Rev. B (2007), accepted.]

  10. Acknowledgments to the collaborators and funding agencies Phd students: S. Banik, C. Biswas, and A. K. Shukla RRCAT, Indore: A. Chakrabarti UGC-DAE CSR, Indore: R. Rawat, A. M. Awasthi, N. P. Lalla, D. M. Phase, A. Banerjee, V. Sathe, V. Ganesan. Banaras Hindu Univeristy, Banaras: D. Pandey, R. Ranjan S.N. Bose Centre for Basic Sciences: U. Kumar, P. Mukhopadhyay Sukhadia Univerisity, Udaipur: B. L. Ahuja Rajasthan univeristy, Jaipur: B. K. Sharma Department of Science and Technology, Govt. of India through SERC project (2000-2005) and Ramanna Research Grant. P. Chaddah and A. Gupta

  11. Samples grown in house Polycrystalline ingots of Ni-Mn-Ga alloys were prepared by melting in Arc furnace. Appropriate quantities of Ni, Mn, and Ga of 99.99% purity melted under Argon atmosphere. 0.5 to 1% maximum loss of weight, possibility of difference in intended and actual composition. The L21 phase is obtained afterannealing at 1100K in sealed quartz ampules. Annealing time for each sample is more than a week: to ensure homogenization. The ingots were quenched in ice water.

  12. Ni2MnGa is a Heusler alloy • L21 structure: Four interpenetrating f.c.c. sublattices with : • Ni at (1/4,1/4,1/4 ) and (3/4,3/4,3/4) • Mn at (1/2,1/2,1/2), • Ga at (0,0,0). • Ferromagnetism due to RKKY indirect exchange interaction. • Heusler alloys are famous for localized large magnetic moments on Mn.

  13. Temperature dependent XRD: evidence of modulation Austenite Martensite structure more complicated than tetragonal! 7 layer (7M) modulation in 110 direction. Ranjan, Banik, Kumar, Mukhopadhyay, Barman, Pandey, PRB (2006).

  14. Phase coexistence in Ni2MnGa (a) Hysteresis curve showing mole fraction of the cubic phase determined from Rietveld analysis of the XRD patterns. (b) Ac-susceptibity; Decrease at TM due to large magnetocrystalline anisotropy in martensitic phase. (c) Differential scanning calorimetry Nice agreement between structural, magnetic and thermal techniques. Small width of hysteresis 14-38 K; highly thermoelastic (mobile interface, strain less).

  15. Resistivity and magnetoresistance T/Tc= 0.8 Metallic behaviour with a clear jump at TM. Ref: M. Kataoka, PRB, 63, 134435 (2001) • Highest known magnetoresistance at room temperature for shape memory alloys. For x=0.35, MR is around 7.3% at 8T. • Experimental MR behavior agrees with the theoretical calculation. • Magnetic spin disorder scattering increases with increasing x. • C. Biswas, R. Rawat, S.R. Barman, Appl. Phys. Lett., 86, 202508 (2005)

  16. Total energy calculations using Full potential linearized augmented plane wave (FPLAPW) method • Total energy includes the electron kinetic energy and electron-electron, electron-nuclear and nuclear-nuclear potentials. • Ab-initio i.e. no requirement of input parameters. • FPLAPW solves the equations of density functional theory by variational expansion approach by approximating solutions as a finite linear combination of basis functions. What distinguishes the LAPW method from others is the choice of basis. Ref: www-phys.llnl.gov/Research/Metals_Alloys/Methods/AbInitio/LAPW/ WIEN code (P. Blaha, K. Schwartz, and J. Luitz, Tech. Universität, Wien, Austria, 1999)

  17. Structure optimization for Ni2MnGa Experimental c/a= 0.94. Previous theory: c/a= 1.2, 1, etc.

  18. Total energy contours for structural optimization of Ni2MnGa • For ferromagnetic martensitic phase, a= 5.88 Ǻ and c= 5.70 Ǻ, with c/a=0.97. Comapres well with expt. c/a=0.94. • Good agreement with experimental lattice constants: a= 5.88Ǻ, c= 5.56 Ǻ within 2.5%. • Tetragonal phase more stable than the cubic phase by 3.6 meV/atom. Barman, Banik, Chakrabarti, Phys Rev B, 72, 184410 (2005)

  19. Ni2MnGa Ni-Mn-Ga Increase Nickel Ni2MnGa Ni2+xMn1-xGa (Ni, Mn)  Ni3Ga (x=1) Increase Manganese Ni2MnGa  Ni2-yMn1+yGa (Mn, Ni)  NiMn2Ga or Mn2NiGa (y=1)

  20. Structure optimization for Ni2.25Mn0.75Ga Good agreement between the experimental and theoretical lattice constants: Expt: a= 5.439 Ǻ , c= 6.563 Ǻ Theory: a= 5.38 Ǻ, c= 6.70 Ǻ) [within 1% for a and 2% for c].

  21. Phase diagram of Ni2+xMn1−xGa P= paramagnetic, F= ferromagnetic C= cubic (austenite), T= tetragonal (martensite) x • TC and TM determined by DSC and ac-chi measurements. • TC increases with Ni content i.e. x. • TC = TM for x= 0.2, large magnetoelastic coupling and gaint magnetocaloric effect. • TC < TM for x> 0.2, emergence of the new paramagnetic tetragonal phase, confirmed by high temperature XRD. Banik, Chakrabarti, Kumar, Mukhopadhyay, Awasthi, Ranjan, Schneider, Ahuja, and Barman, PRB, 74, 085110 (2006)

  22. Phase diagram vis-à-vis total energies x= 0.25, Ni2.25Mn0.75Ga x= 0, Ni2MnGa TM>TC TM<TC PC PC= paramagnetic cubic FC= ferromagnetic cubic FT= ferromagnetic tetragonal PT= paramagnetic tetragonal Total energies in meV/ atom PC 39 PT 322 253 219 kBTC ~ Etot(P) - Etot(F)  Decrease in TC for x= 0.25 FC 3.6 FT FT kBTM ~ Etot(C) - Etot(T)  Increase in TM for x= 0.25

  23. Experimental facilities for electronic structure studies IPES spectrometer XPS/UPS spectrometer • S. Banik, A. K. Shukla and S.R. Barman, RSI, 76, 066102 (2005).

  24. UPS VB of Ni2MnGa compared to VB calculated from DOS Calculated DOS Non-modulated Modulated • Good agreement between expt. and theory ; VB dominated by Ni 3d–Mn 3d hybridized states. • Ni 3d states with peak at –1.75 eV. Mn 3d states exhibit two peaks at –1.3 eV and –3.1 eV. • VB for non-modulated structure in better agreement with expt. So, influence of modulation diminishes at the surface. • Mn 3d dominated peak above EF. Chakrabarti, Biswas, Banik, Dhaka, Shukla, Barman, PRB, 72, 073103 (2005)

  25. Ni2+xMn1−xGa :effect of excess Nickel Ni clustering, formation of Ni1 3d – Ni2 3d hybridized states at expense of Ni 3d– Mn 3d hybridized states.

  26. Unoccupied states of Ni2+xMn1−xGa Difference between expt. and theory: Mn related peak is shifted by 0.4 eV. Indicates existence of self energy effects. Mn Ni As x : Ni peak intensity increases and Mn decreases. Small shift of Mn peak to higher energies.

  27. Magnetic moments of Ni2MnGa • Saturation magnetic moment of Ni2MnGa: • MCP: 4 mB • Magnetization: 3.8 mB • FPLAPW: 4.13 mB • Large magnetic moments on Mn, clear from spin polarized DOS. • Ni moment 10% of Mn, both aligned in same direction. • Decrease in saturation magnetization with increasing x. B. L. Ahuja, B. K. Sharma, S. Mathur, N. L. Heda, M. Itou, A. Andrejczuk, Y. Sakurai, A. Chakrabarti, S. Banik, A. M. Awasthi and S. R. Barman, Phys. Rev. B (accepted).

  28. Magnetic moments of Mn2NiGa Increase Manganese : Ni2MnGa  Ni2-yMn1+yGa (Mn, Ni)  NiMn2Ga or Mn2NiGa (y=1) Mn2NiGa: Ni : (0.25,0.25,0.25) Mn1: (0.75, 0.75, 0.75) Mn2: (0.5, 0.5, 0.5) Ga : (0,0,0) TC=375K, TM=260K Spin density in 110 plane Charge density in 110 plane The Mn atom in Ni position (Mn1) is antiferrimagnetically aligned to the original Mn (Mn2) and the total moment decreases. Reason for opposite alignment is direct Mn-Mn interation. The nearest neighbours of Mn1 atoms are four Mn2 and four Ga atoms at a distance of 2.53Å. • Ni2MnGa: Four interpenetrating f.c.c. sublattice: • Ni at (0.25,0.25,0.25) and (0.75, 0.75, 0.75) • Mn at (0.5, 0.5, 0.5), • Ga at (0,0,0).

  29. Why Mn1 and Mn2 magnetic moments are different? Strong hybridization between the down spin 3d states of Ni and Mn2 (n.n. 2.55Å) compared to Weaker hybridization between the up spin M=Ni and Mn1 3d states (2.73 Å)

  30. Origin of the structural transition (the martensitic phase) intensity Lowering of the electron states related to the cubic to tetragonal structural transition: Jahn Teller effect (Fujii et al., JPSJ) kinetic energy

  31. Origin of the modulated phases in Ni2MnGa: Fermi surface nesting If the Fermi surface (FS) has flat parallel portions i.e. if it is nested with nesting vector (vector joining the parallel portions of the FS), a pronounced phonon softening can occur at q resulting in a modulated pre-martensitic or martensitic phases. Bungaro, Rabe, Dal Corso, PRB, 68, 134104, (2003) Cross section of the Fermi surface (a) with the (001) plane. The arrows are examples of nesting vectors q0=0.34(1,1,0). (a) Minority spin Fermi surface of cubic Ni2MnGa.

  32. q1 Highly nested FS of Mn2NiGa 010 100 Minority spin hole type FS, Band 27, NV: 0.4{100},NA= 0.17 a.u.2 Majority spin FS, band 29; NV: 0.44(100) & (010) Minority spin FS, Band 29; NV q1= 0.31{1,0,0};NA(q1)= 0.164a.u.2 NV q2= 0.46(1,1,0); NA= 0.034a.u.2

  33. Conclusions I hope I could give you a flavour of this important material . We will appreciate your suggestions and comments that might lead to new collaborations….. Thank you for your attention. • Phase diagram determined from TM and TC variation as function of Ni excess (x). For x> 0.2, martensitic transition occurs in paramagnetic phase. • Phase co-existence shown, existence of a 7 layer modulated structure at low temperature for Ni2MnGa. • Ni2MnGa shows large negative magnetoresistance (7%) at room temperature due to s-d spin scattering. • Structure from total energy calculations, magnetic moments, occupied VB are in good agreement with experiment. • Self energy effects in unoccupied DOS. • Evidence of Ni cluster formation with Ni doping. • Origin of structural transition related to lowering of total energy; redistribution of states near EF. • Antiferrimagnetism in Mn2NiGa • Highly nested Fermi surface

  34. Ni 2p of Ni2MnGa shows an interesting satellite feature Satellite feature at 6.8 eV and 5.9 eV below Ni 2p3/2 and 2p1/2 peak respectively. Satellite feature in Ni metal at 6 eV and 4.6 eV below Ni 2p3/2 and 2p1/2 peak respectively. Band filling, Udc and 3d bandwidth are responsible for the binding energy shift of the main peak, satellite and decrease in satellite intensity.

  35. Mn magnetic moment from XPS 3d5 (6S) 3s2 Ground state hn 3d5 (6S) 3s (2S) 3d5 (6S) 3s (2S) exchange 5S 7S • Exchange splitting: • Occurs when the system has unpaired electrons in valance band. • Exchange split peak is at • 1167 eV (x=0, Austenite), Eex = 4.3 eV  • 1166.2 eV (x=0, Martensite), Eex = 5.1 eV • 1166.5 eV (x=0.1, Martensite), Eex = 4.8 eV • 1166.9 eV (x=0.2, Martensite).Eex = 4.4 eV • Mn moment decreasing with decrease in Mn content. • From theory: 3.4 mB (Fuji et al., JPSJ), 3.36 mB (Ayuela et al.JOP:CM)

  36. EF EF 3d9 3d9 4s 4s 2p 2p C-1 C-1 Origin of satellite in Ni core level • The partially filled d states are treated as non-degenerate state interacting with s conduction states through s-d hybridization and with d states of other atoms through d-d transfer interaction giving rise to narrow d-band. • This initial mixing gives 3d94s ground state of Ni. EF • If screening is better: main peak, no satellite. • If screening is poor: satellite arises. 3d9 3d10 4s Excited state Ground state hn 2p c -1

  37. Microscopic twin structure with field Ref: Pan et. al. JAP. 87, 4702 (2000) Magnetic domains and twin bands clearly observed. MR explained by twin variant rearrangement with field. Magnetic force microscopy image of Ni2.23Mn0.8Ga in the martensitic phase at room temperature.

  38. A basic actuator structure A basic actuator consists of a coil and a MSM element. Actuator An actuator produced by AdaptaMat which controls pressure in a pneumatic valve. When magnetic field is applied, the MSM element elongates in the direction perpendicular to the magnetic field.

  39. Tetragonal Cubic Crystal structure at room temperature Martensi te Austeni te Martensitic phase at room temperature.

  40. Lattice constant variation with x in Ni2+xMn1-xGa The spontaneous strain increases from 17.6% to 23% between x= 0.15 and 0.35. Linear variation of lattice constants in alloys can be explained by Vegard’s law, This is expected because both Ni and Mn are 3d elements with similar electronic configuration and small size difference.

  41. DSC and ac-susceptibility of Ni2+xMn1−xGa x= 0 x= 0.24 x= 0.35 DSC: [Rate 10 C/min] Susceptibility: [ 26 Oe field, 33.33 Hz] Albertini et al, JAP, 89 5614, 2001 Small width of hysteresis 14-38 K for x=0; highly thermoelastic (mobile interface, strain less). Decrease of c at TM due to large magnetocrystalline anisotropy in martensitic phase. For x>0.2 TM>TC: change in c shape. Banik, Chakrabarti, Kumar, Mukhopadhyay, Awasthi, Ranjan, Schneider, Ahuja, and Barman, PRB, 74, 085110 (2006)

  42. Structure and magnetization of x= 0.35 Magnetization versus field M-H hysteresis loop at 293 K, the region close to H=0 is shown in the inset.

  43. Photoemission (PES) and Inverse photoemission spectroscopy (IPES) PES IPES

  44. Characteristics of our PES workstation

  45. The Inverse Photoemission Spectrometer work station • Photon detector and electron gun fabricated, interfaced with Labview • Two level Mu metal (Ni77Fe15CoMo) chamber. • Sample heating up to 950°C. • Indigenous design and assembly of the entire system involving purchase of more than 100 different items from 25 companies. Gas filled photon detector Operating principle Design • S. Banik, A. K. Shukla and S.R. Barman, RSI, 76, 066102 (2005).

  46. Surface composition from XPS for sputtered surface Ni 3p • EDAX: Ni2.1Mn0.88Ga1.01 • Sputtering: • 0.5 keV: Ni2.6Mn0.4Ga0.99. • 3.0 keV: Ni2.45Mn0.4Ga1.1. • Sputtering yield of Ni is less than Mn and Ga [For 0.5 keV Ar ions, Ni (1.3 atoms/ion) and Mn(1.9 atoms/ion)] Ga 3d Mn 3p Ion sputtering increases Ni content on the surface.

  47. Surface composition from XPS with annealing • With increasing annealing temperature Mn segregates to surface. • At about 390oC the Ni:Mn ratio is same as that of the bulk (2.3).

  48. Valence band spectrum of Ni2MnGa in martensitic phase

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