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Electronic properties of a ferromagnetic shape memory alloy: Ni-Mn-Ga Sudipta Roy Barman UGC-DAE Consortium for Scientific Research, Indore Talk at ‘Electronic.

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Presentation on theme: "Electronic properties of a ferromagnetic shape memory alloy: Ni-Mn-Ga Sudipta Roy Barman UGC-DAE Consortium for Scientific Research, Indore Talk at ‘Electronic."— Presentation transcript:

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

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 force microscopy image of Ni 2.23 Mn 0.8 Ga in the martensitic phase at room temperature clearly shows the twin bands (width 10 micron) and magnetic domains (width 2-3 microns) Magnetic domains and twin bands C. Biswas, S. Banik, A. K. Shukla, R. S. Dhaka, V. Ganesan, and S. R. Barman,, Surface Science, 600, 3749 (2006). Topography imageMFM image

6 Potential fields of applications Smart actuator materials

7 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). A real actuator made from FSMA by Adaptamat

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, (2006) ; Appl. Phys. Lett.. 86, (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, (2005); Phys. Rev. B 72, (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 Department of Science and Technology, Govt. of India through SERC project ( ) and Ramanna Research Grant. P. Chaddah and A. Gupta 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

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 L2 1 phase is obtained after annealing 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  Ferromagnetism due to RKKY indirect exchange interaction.  Heusler alloys are famous for localized large magnetic moments on Mn. Ni 2 MnGa 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).

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

14 Phase coexistence in Ni 2 MnGa (a) Hysteresis curve showing mole fraction of the cubic phase determined from Rietveld analysis of the XRD patterns. (b) Ac-susceptibity; Decrease at T M due to large magnetocrystalline anisotropy in martensitic phase. (c) Differential scanning calorimetry Nice agreement between structural, magnetic and thermal techniques. Small width of hysteresis K; highly thermoelastic (mobile interface, strain less).

15 Resistivity and magnetoresistance 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. Ref: M. Kataoka, PRB, 63, (2001) T/T c = 0.8 Metallic behaviour with a clear jump at T M. C. Biswas, R. Rawat, S.R. Barman, Appl. Phys. Lett., 86, (2005)

16 Ref: www-phys.llnl.gov/Research/Metals_Alloys/Methods/AbInitio/LAPW/ 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. WIEN code (P. Blaha, K. Schwartz, and J. Luitz, Tech. Universität, Wien, Austria, 1999)

17 Structure optimization for Ni 2 MnGa Experimental c/a= Previous theory: c/a= 1.2, 1, etc.

18 Total energy contours for structural optimization of Ni 2 MnGa  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, (2005)

19 Increase Nickel Ni 2 MnGa  Ni 2+x Mn 1-x Ga (Ni , Mn  )  Ni 3 Ga (x=1) Ni 2 MnGa  Ni-Mn-Ga Increase Manganese Ni 2 MnGa  Ni 2-y Mn 1+y Ga (Mn , Ni  )  NiMn 2 Ga or Mn 2 NiGa (y=1)

20 Structure optimization for Ni 2.25 Mn 0.75 Ga Good agreement between the experimental and theoretical lattice constants: Expt: a= Ǻ, c= Ǻ Theory: a= 5.38 Ǻ, c= 6.70 Ǻ) [within 1% for a and 2% for c].

21 Phase diagram of Ni 2+x Mn 1−x Ga C= cubic (austenite), T= tetragonal (martensite) x  T C and T M determined by DSC and ac-chi measurements.  T C increases with Ni content i.e. x.  T C = T M for x= 0.2, large magnetoelastic coupling and gaint magnetocaloric effect.  T C 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, (2006) P= paramagnetic, F= ferromagnetic

22 PC= paramagnetic cubic FC= ferromagnetic cubic FT= ferromagnetic tetragonal PT= paramagnetic tetragonal Total energies in meV/ atom FC PC 322 x= 0, Ni 2 MnGa FT 3.6 x= 0.25, Ni 2.25 Mn 0.75 Ga T M T C PC FT 253 PT Phase diagram vis-à-vis total energies k B T C ~ E tot (P) - E tot (F)  Decrease in T C for x= 0.25 k B T M ~ E tot (C) - E tot (T)  Increase in T M for x= 0.25

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

24 UPS VB of Ni 2 MnGa compared to VB calculated from DOS 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 E F. Chakrabarti, Biswas, Banik, Dhaka, Shukla, Barman, PRB, 72, (2005) Non-modulated Modulated Calculated DOS

25 Ni 2+x Mn 1−x Ga : 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 Ni 2+x Mn 1−x Ga Ni Mn Difference between expt. and theory: Mn related peak is shifted by 0.4 eV. Indicates existence of self energy effects. As x  : Ni peak intensity increases and Mn decreases. Small shift of Mn peak to higher energies.

27  Saturation magnetic moment of Ni 2 MnGa: MCP: 4  B Magnetization: 3.8  B FPLAPW: 4.13  B  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. Magnetic moments of Ni 2 MnGa 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 Mn 2 NiGa Increase Manganese : Ni 2 MnGa  Ni 2-y Mn 1+y Ga (Mn , Ni  )  NiMn 2 Ga or Mn 2 NiGa (y=1) Mn 2 NiGa: 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) T C =375K, T M =260K Ni 2 MnGa: 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). Charge density in 110 plane Spin 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.

29 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) MartensiteAustenite Mn Mn Ni Total Why Mn1 and Mn2 magnetic moments are different?

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

31 (a) Minority spin Fermi surface of cubic Ni 2 MnGa. Cross section of the Fermi surface (a) with the (001) plane. The arrows are examples of nesting vectors q 0 =0.34(1,1,0). Origin of the modulated phases in Ni 2 MnGa: Fermi surface nesting Bungaro, Rabe, Dal Corso, PRB, 68, , (2003) 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.

32 q1q Minority spin FS, Band 29; NV q 1 = 0.31{1,0,0};NA(q1)= 0.164a.u. 2 NV q 2 = 0.46(1,1,0); NA= 0.034a.u. 2 Majority spin FS, band 29; NV: 0.44(100) & (010) Highly nested FS of Mn 2 NiGa Minority spin hole type FS, Band 27, NV: 0.4{100},NA= 0.17 a.u. 2

33 Conclusions Phase diagram determined from T M and T C 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 Ni 2 MnGa. Ni 2 MnGa 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 E F. Antiferrimagnetism in Mn 2 NiGa Highly nested Fermi surface 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.

34 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 2p 3/2 and 2p 1/2 peak respectively. Band filling, U dc and 3d bandwidth are responsible for the binding energy shift of the main peak, satellite and decrease in satellite intensity. Ni 2p of Ni 2 MnGa shows an interesting satellite feature

35 Exchange splitting: Occurs when the system has unpaired electrons in valance band. 3d 5 ( 6 S)3s ( 2 S) 5S5S 3d 5 ( 6 S)3s ( 2 S) exchange 7S7S 3d 5 ( 6 S) Ground state h 3s 2 Exchange split peak is at 1167 eV (x=0, Austenite),  E ex = 4.3 eV  eV (x=0, Martensite),  E ex = 5.1 eV eV (x=0.1, Martensite),  E ex = 4.8 eV eV (x=0.2, Martensite).  E ex = 4.4 eV Mn moment decreasing with decrease in Mn content. From theory: 3.4  B (Fuji et al., JPSJ), 3.36  B (Ayuela et al.JOP:CM) Mn magnetic moment from XPS

36 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 3d 9 4s ground state of Ni. c EFEF 4s 3d 9 h 2p EFEF 4s 3d 9 2p C -1 3d 10 Ground state EFEF 4s 3d 9 2p C -1 Excited state Origin of satellite in Ni core level If screening is better: main peak, no satellite. If screening is poor: satellite arises.

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 Ni 2.23 Mn 0.8 Ga in the martensitic phase at room temperature.

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

41 Crystal structure at room temperature Martensitic phase at room temperature. AusteniteAustenite MartensiteMartensite Cubic Tetragonal

42 The spontaneous strain increases from 17.6% to 23% between x= 0.15 and 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. Lattice constant variation with x in Ni 2+x Mn 1-x Ga

43 DSC and ac-susceptibility of Ni 2+x Mn 1−x Ga Small width of hysteresis K for x=0; highly thermoelastic (mobile interface, strain less). Decrease of  at T M due to large magnetocrystalline anisotropy in martensitic phase. For x>0.2 TM>TC: change in  shape. Banik, Chakrabarti, Kumar, Mukhopadhyay, Awasthi, Ranjan, Schneider, Ahuja, and Barman, PRB, 74, (2006) DSC: [Rate 10 C/min] Susceptibility: [ 26 Oe field, Hz] x= 0x= 0.24x= 0.35 x M s (T M ) M f A s A f Albertini et al, JAP, , 2001

44 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.

45 PES IPES Photoemission (PES) and Inverse photoemission spectroscopy (IPES)

46 Characteristics of our PES workstation Characteristics PES stationOur aim.. Angle dependent XPS Yes Angle resolved PES NoYes, using angle resolved analyzer Base pressure 6 x mbar LEED Not availableYes Analyser energy resolution in UPS 100 meV1 meV Analyzer energy resolution in XPS 0.8 eV0.4 eV (by monochromatic XPS) Spatial resolution 100  m <10  m Temperature of expt. 150 K, RT<15 K to RT (controlled)

47 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 principleDesign S. Banik, A. K. Shukla and S.R. Barman, RSI, 76, (2005).

48 Surface composition from XPS for sputtered surface EDAX: Ni 2.1 Mn 0.88 Ga 1.01 Sputtering: 0.5 keV: Ni 2.6 Mn 0.4 Ga keV: Ni 2.45 Mn 0.4 Ga 1.1. 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)] Ni 3p Mn 3p Ga 3d Ion sputtering increases Ni content on the surface.

49 With increasing annealing temperature Mn segregates to surface. At about 390 o C the Ni:Mn ratio is same as that of the bulk (2.3). T ( 0 C)Surface Composition (20 A 0 ) 100Ni 2.47 Mn 0.44 Ga Ni 2.42 Mn 0.5 Ga Ni 2.25 Mn 0.71 Ga Ni 2.14 Mn 0.76 Ga 1.1 Surface composition from XPS with annealing

50 Valence band spectrum of Ni 2 MnGa in martensitic phase

51 DOS calculation using the actual modulated structure Non-modulated Modulated 7 layer modulated phase, Pnnm space group, 56 atoms/unit cell, a=4.215, b= and c=5.557 Å.

52 Comparison: photoemission and theory D. Brown et al., PRB, 57, 1563 (1998) Cu 2 MnAl Disagreement in Feature A. Could overall agreement be better if modulation is considered?

53 Self energy effects in Ni 2 MnGa IPES Inverse photoemission spectrum of Ni 2 MnGa at room temperature in the FC phase, compared with the calculated conduction band of Ni 2 MnGa FC phase based on total, Mn, and Ni 3d PDOS. The IPES spectrum of Ni 2.24 Mn 0.75 Ga 1.02 (x=0.24) in the FT phase is also shown. The states near E F are broader and the 1.9- eV peak is shifted toward higher energy by 0.4 eV w.r.t.calculated spectrum. These differences could be related to existence of correlation effects. DFT is a ground-state calculation and the electron-electron interaction is considered in an average way. Deviation from DFT is quantified in terms of self-energy, where the real part gives the energy shift and the imaginary part gives the broadening. Self energy effects in the unoccupied states have also been observed in 3d transition metals like Cu. Banik et al Phys. Rev. B, 74,

54 Compare with IPES spectra of Nickel and Manganese metal

55 Calculated Spin polarized energy bands of Ni 2 MnGa Minority spinMajority spin * A parabolic majority spin band crosses E F near M and R points. * Between -0.7 and -4 eV exhibit small dispersion and are related to Ni 3d-Mn 3d hybridized states. * In the ΓX, ΓM or ΓR direction, no majority spin bands are observed between E F and -0.7 eV and no E F crossing is observed. Half metallic character along certain directions ( ΓX, ΓM and ΓR ) of the Brillouin zone with a gap of about 0.7 eV * Future plan for experimental determination of band dispersion by ARPES.

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57 Partial phonon dispersion of Ni 2 MnGa in the fcc Heusler structure, along the  -K-X line in the (110) direction. The experimental data taken at 250 K and 270 K. (a) Fermi surface of cubic Ni2MnGa. (b) The fcc BZ is shown as a reference. Cross section of the minority-spin Fermi surface (a) with the (001) plane. The arrows are examples of nesting vectors q 0 =0.34(1,1,0). Origin of the modulated phases in Ni 2 MnGa: Fermi surface nesting Bungaro, Rabe, Dal Corso, PRB, 68, , (2003)

58 Possibility of tuning the minority spin DOS near E F x= 0 x= 0.25

59 Magnetoresistance and twin variant rearrangement  Ni 2 MnGa, in the martensitic phase exhibits a cusp like shape with two inflection points at 0.3 T and 1.3 T. This is due to the twinning and large magnetocrystalline anisotropy in the martensitic phase  At 150 K, x=0, x=0.1 and x=0.2 are at the martensitic phase. For x=0.1, the inflection points are observed at lower H. For x=0.2, MR is almost linear with a possible inflection point at 0.15 T. C. Biswas, R. Rawat, S.R. Barman, Appl. Phys. Lett., 86, (2005)


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