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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

          www.csr.ernet.in              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
                      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.
    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.




The magnetic moments         The rotation of the magnetic   The redistribution of
without the external field   moments within the twins.      the twin variants.
                 Live simulation of the FSMA effect




Rotation of magnetic moments:           FSMA effect: change in
[Magnetocrystalline anisotropy<<        shape [Magnetocrystalline
Zeeman energy]                          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
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).
     The FSMA mechanism




Magnetic field induced strain =1- c/a
               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.]
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
                      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 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.
                    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.
           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).
                     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                                            Ref: M. Kataoka, PRB, 63, 134435 (2001)
a clear jump at TM.

 •    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)
 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      Ref: www-phys.llnl.gov/Research/Metals_Alloys/Methods/AbInitio/LAPW/

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




  IPES spectrometer

  S. Banik, A. K. Shukla and S.R. Barman, RSI, 76, 066102 (2005).   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 (accepted).
                          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)                     Charge density in 110 plane      Spin density in 110 plane
TC=375K, TM=260K

Ni2MnGa: Four interpenetrating
                                 The Mn atom in Ni position (Mn1) is antiferrimagnetically
f.c.c. sublattice:               aligned to the original Mn (Mn2) and the total moment
Ni at (0.25,0.25,0.25) and      decreases. Reason for opposite alignment is direct Mn-Mn
(0.75, 0.75, 0.75)               interation. The nearest neighbours of Mn1 atoms are four Mn2
Mn at (0.5, 0.5, 0.5),          and four Ga atoms at a distance of 2.53Å.
Ga at (0,0,0).
Why Mn1 and Mn2 magnetic moments are different?

                                  Martensite         Austenite
                       Mn1        -2.21              -2.43
                       Mn2         2.91               3.2
                       Ni          0.27               0.32
                       Total      1.21               1.29




                        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)
            Origin of the structural transition (the martensitic phase)




                                     He II, UPS
            800                    martentic phase, 150K
                                   austentic phase, 300K
intensity




            600



            400



            200



              0                                                   Lowering of the electron states related to the
              39.0   39.5   40.0   40.5       41.0         41.5
                                                                  cubic to tetragonal structural transition: Jahn
                                                                  Teller effect (Fujii et al., JPSJ)
                     kinetic energy
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
Bungaro, Rabe, Dal Corso, PRB, 68, 134104, (2003)                occur at q resulting in a modulated
                                                                 pre-martensitic or martensitic phases.




                                                    Cross section of the Fermi surface (a) with the (001)
                                                    plane. The arrows are examples of nesting vectors
 (a) Minority spin Fermi surface of                 q0=0.34(1,1,0).
 cubic Ni2MnGa.
                        Highly nested FS of Mn2NiGa




010
       100

Minority spin hole type FS, Band 27,
NV: 0.4{100},NA= 0.17 a.u.2


              q1




                                          Majority spin FS, band 29;
Minority spin FS, Band 29;                NV: 0.44(100) & (010)
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
                                 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.
                  give you a flavour of this important
I hope I couldtotal energy calculations, magnetic moments, occupied
   Structure from
 VB are in .
material good agreement with experiment.
    Self energy effects in unoccupied DOS.
We will appreciate your suggestions and comments that
   Evidence of Ni cluster formation with Ni doping.
might lead to new collaborations….. of total energy;
  Origin of structural transition related to lowering
 redistribution of states near EF.
Thank you for your attention.
   Antiferrimagnetism in Mn2NiGa
   Highly nested Fermi surface
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.
Mn magnetic moment from XPS
           Exchange splitting:
           •Occurs when the system has unpaired electrons
           in valance band.
                            3d5 (6S)                  3s2

                                       Ground state
                                             hn
                 3d5 (6S)       3s (2S)         3d5 (6S)     3s (2S)


                        exchange
                              7S                            5S


           •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)
                  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                 EF                             EF
                                                                       •If screening is better:
          3d9                                               3d9        main peak, no satellite.
                             3d9 10
                              3d
     4s                 4s                             4s
                                                                       •If screening is poor:
                                                                       satellite arises.

                        Ground state                   Excited state
hn




                   2p                  2p                              2p
                                                              -1
           c -1                C
                                 -1                          C
                    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.
                  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.
              Crystal structure at room temperature

                                                                    M
A                                                                   a
u                                                                   r
s                                                                   t
t                                                                   e
e                                                                   n
n                                                                   s
i                                                                   i
t                                                                   t
e                                                                   e




              Mn                                                                 Ga
                              Ga
         Ga                                   Mn
                         Mn
                                         Ga                                      Mn               Ga
    Mn                                                              Ga
                    Ga
                                   Mn
                   Ni                                                     Ni1              Ni1
                                   Ni                                                 Ga
              Ni
                              Ni                                                 Mn
              Ga                                                          Ni1               Ni1
                              Mn
                                              Ga
         Mn
                         Ga                                         Mn            Ga              Mn
                                         Mn
    Ga
                   Mn
                                    Ga                                    Ni1              Ni1
                   Ni                                                             Mn
                                   Ni
              Ni                                                                 Ga
                              Ni                                           Ni1              Ni1
              Mn
                              Ga
                                              Mn                                 Mn               Ga
         Ga                                                         Ga
                         Mn
    Mn                                   Ga
                   Ga
                                   Mn              Pow r C l 1. 0
                                                      de el
                                                                                      Ga                  de el
                                                                                                       Pow r C l 1. 0




         Cubic                                                           Tetragonal
                         Martensitic phase at room temperature.
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.
                       DSC and ac-susceptibility of Ni2+xMn1−xGa
                                                                     x= 0                x= 0.24             x= 0.35
 x      Ms        Mf         As         Af
       (TM)
 0     205       189       216         234     DSC: [Rate 10
                                               C/min]
0.24   434       408       423         447
0.35   537       523       553        582




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


                                             PES      IPES
                       Characteristics of our PES workstation
Characteristics   PES station      Our aim..

Angle dependent   Yes
XPS
Angle resolved    No               Yes, using angle
PES                                resolved analyzer
Base pressure     6 x 10-11 mbar

LEED              Not available    Yes

Analyser energy   100 meV          1 meV
resolution in
UPS
Analyzer energy   0.8 eV           0.4 eV (by
resolution in                      monochromatic
XPS
                                   XPS)
Spatial           100 mm           <10 mm
resolution
Temperature of    150 K, RT        <15 K to RT
expt.                              (controlled)
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                   Stainless steel cathode                    Ceramic feedthrough




                              MgF 2 window       Tungsten anode         Teflon support

                                                                                          Pumping port

Operating principle                          Design                           S. Banik, A. K. Shukla and S.R. Barman, RSI, 76, 066102 (2005).
Surface composition from XPS for sputtered surface


      Ni 3p                    Ga 3d   •EDAX: Ni2.1Mn0.88Ga1.01

                  Mn 3p                •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)]




     Ion sputtering increases Ni content on the surface.
  Surface composition from XPS with annealing
                                   T (0C)    Surface Composition (20 A0)
                                    100           Ni2.47Mn0.44Ga1.09
                                    200           Ni2.42Mn0.5Ga1.09
                                    300           Ni2.25Mn0.71Ga1.03
                                    350           Ni2.14Mn0.76Ga1.1




With increasing annealing temperature Mn segregates to surface.
At about 390oC the Ni:Mn ratio is same as that of the bulk (2.3).
Valence band spectrum of Ni2MnGa in martensitic phase
DOS calculation using the actual modulated structure



                               Non-modulated




   7 layer modulated phase,      Modulated
   Pnnm space group, 56
   atoms/unit cell, a=4.215,
   b=29.302 and c=5.557 Å.
                     Comparison: photoemission and theory
                         Cu2MnAl




  D. Brown et al., PRB, 57, 1563 (1998)




Disagreement in Feature A.
Could overall agreement be
better if modulation is
considered?
                       Self energy effects in Ni2MnGa IPES
                                                The states near EF 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.

Inverse photoemission spectrum of               Deviation from DFT is quantified in terms
Ni2MnGa at room temperature in the FC
                                                of self-energy, where the real part gives the
phase, compared with the calculated
conduction band of Ni2MnGa FC phase             energy shift and the imaginary part gives
based on total, Mn, and Ni 3d PDOS. The         the broadening. Self energy effects in the
IPES spectrum of Ni2.24Mn0.75Ga1.02             unoccupied states have also been observed
(x=0.24) in the FT phase is also shown.
                                                in 3d transition metals like Cu.
    Banik et al Phys. Rev. B, 74, 085110 2006
Compare with IPES spectra of Nickel and Manganese metal
     Calculated Spin polarized energy bands of Ni2MnGa
        Majority spin                           Minority spin




* A parabolic majority spin band crosses EF 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 EF
and -0.7 eV and no EF 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.
Origin of the modulated phases in Ni2MnGa: Fermi surface nesting

                                                                                 Partial phonon dispersion
                                                                                 of Ni2MnGa in the fcc
                                                                                 Heusler structure, along
                                                                                 the G-K-X line in the
                                                                                 (110) direction. The
                                                                                 experimental data taken
                                                                                 at 250 K and 270 K.




                                                    Cross section of the minority-spin Fermi surface (a)
 (a) Fermi surface of cubic Ni2MnGa.                with the (001) plane. The arrows are examples of
 (b) The fcc BZ is shown as a reference.            nesting vectors q0=0.34(1,1,0).
Bungaro, Rabe, Dal Corso, PRB, 68, 134104, (2003)
Possibility of tuning the minority spin DOS near EF




                                x= 0.25




                                  x= 0
        Magnetoresistance and twin variant rearrangement




Ni2MnGa, 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, 202508 (2005)

				
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posted:10/8/2011
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