A1258690454OTYSq

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 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
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/
4
Live simulation of the FSMA effect
Source www.fyslab.hut.fi/epm/heusler/
Rotation of magnetic moments Magnetocrystalline
anisotropyltlt Zeeman energy
FSMA effect change in shape Magnetocrystalline
anisotropygtgt 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
Source www.adaptamat.com/technology/applications.
php
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 lt1µm).
Source www.adaptamat.com/demos/
8
The FSMA mechanism
Magnetic field induced strain 1- c/a
Source www.adaptamat.com/technology/mechanism.php
9
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-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 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)

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 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.
11
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 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
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.

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 74, 224443 (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 x0.35,
  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)
16
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.
17
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/Meth
ods/AbInitio/LAPW/
WIEN code (P. Blaha, K. Schwartz, and J. Luitz,
Tech. Universität, Wien, Austria, 1999)
18
Structure optimization for Ni2MnGa
Experimental c/a 0.94. Previous theory c/a
1.2, 1, etc.
19
Total energy contours for structural optimization
of Ni2MnGa

• For ferromagnetic martensitic phase, a 5.88 ?
  and c 5.70 ?, with c/a0.97. Compares well with
  expt. c/a0.94.
• Good agreement with experimental lattice
  constants a 5.88?, c 5.56 ? 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)
20
Ni2MnGa ? Ni-Mn-Ga
Increase Nickel Ni2MnGa ?
Ni2xMn1-xGa (Ni?, Mn?) ? Ni3Ga (x1)
Increase Manganese Ni2MnGa ?
Ni2-yMn1yGa (Mn?, Ni?) ? NiMn2Ga or Mn2NiGa
(y1)
21
Structure optimization for Ni2.25Mn0.75Ga
Good agreement between the experimental and
theoretical lattice constants Expt a 5.439 ? ,
c 6.563 ? Theory a 5.38 ?, c 6.70 ?) within
1 for a and 2 for c.
22
Phase diagram of Ni2xMn1-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 lt TM for xgt 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)
23
Phase diagram vis-à-vis total energies
x 0.25, Ni2.25Mn0.75Ga
x 0, Ni2MnGa
TMgtTC
TMltTC
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
24
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 inS. 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
25
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 3dMn 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)
26
Ni2xMn1-xGa effect of excess Nickel
Ni clustering, formation of Ni1 3d Ni2 3d
hybridized states at expense of Ni 3d Mn 3d
hybridized states.
27
Unoccupied states of Ni2xMn1-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.
28
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).
29
Magnetic moments of Mn2NiGa
Increase Manganese Ni2MnGa ? Ni2-yMn1yGa (Mn?,
Ni?) ? NiMn2Ga or Mn2NiGa (y1)
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) TC375K, TM260K
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).

30
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 MNi
and Mn1 3d states (2.73 Å)
31
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).
32
Conclusions

• Phase diagram determined from TM and TC
  variation as function of Ni excess (x). For xgt
  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