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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
What is a shape memory alloy? SMA effect involves structural transition called martensitic 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.
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. Source: www.fyslab.hut.fi/epm/heusler/
Live simulation of the FSMA effect Source: www.fyslab.hut.fi/epm/heusler/ 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.
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).
Smart actuator materials Potential fields of applications Source: www.adaptamat.com/technology/applications.php
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). Source: www.adaptamat.com/demos/
The FSMA mechanism Magnetic field induced strain =1- c/a Source: www.adaptamat.com/technology/mechanism.php
Overview of our collaborative work on studies 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-housePhys. 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 74, 224443 (2006); Phys. Rev. B in press, (2007)] • 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 in press, (2007)]
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 University, Banaras: D. Pandey, R. Ranjan S.N. Bose Centre for Basic Sciences: U. Kumar, P. Mukhopadhyay Sukhadia University, Udaipur: B. L. Ahuja Rajasthan university, Jaipur: B. K. Sharma Department of Science and Technology, Govt. of India through SERC project (2000-2005) and Ramanna Research Grant (2007-). P. Chaddah, Director and A. Gupta, Centre Director, UGC-DAE CSR.
Melt grown samples prepared 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.
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). TC= 375 K, TM= 210 K • Ferromagnetism due to RKKY indirect exchange interaction. • Heusler alloys are famous for localized large magnetic moments on Mn.
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 74, 224443 (2006).
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).
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. • MR behavior explained by s-d scattering; agrees with theory. • Magnetic spin disorder scattering increases with increasing x. • C. Biswas, R. Rawat, S.R. Barman, Appl. Phys. Lett., 86, 202508 (2005)
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.
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)
Structure optimization for Ni2MnGa Experimental c/a= 0.94. Previous theory: c/a= 1.2, 1, etc.
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. Compares 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)
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)
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].
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)
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
Experimental facilities for electronic structure studies IPES spectrometer fabricated in our laboratory uses GM type detector (inset) and Stoffel Johnson type electron gun. Details of fabrication published in:S. Banik, A. K. Shukla and S.R. Barman, RSI, 76, 066102 (2005); A .K. Shukla, S.Banik, and S.R. Barman, Curr. Sci. 90, 490, 2006. XPS/UPS spectrometer
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)
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.
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.
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, in press (2007).
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 interaction. 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).
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)
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., J. Phys. Soc. Jpn. 58, 3657 (1989).
Conclusions • 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