Electronic properties of a ferromagnetic shape memory

alloy: Ni-Mn-GaSudipta Roy Barman UGC-DAE Consortium for Scientific Research, Indore

Talk at ‘Electronic Structure of Emerging Materials: Theory and Experiment’ at Lonavala-Khandala, 8th 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.

www.csr.ernet.in

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.

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

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.

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)

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 image MFM image

Potential fields of applications

                                 

    

                              

    

                           

    

                                                

                      

Smart actuator materials

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

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

Department of Science and Technology, Govt. of India through SERC project (2000-2005) and Ramanna Research Grant.

P. Chaddah and A. Gupta

Phd students: S. Banik, C. Biswas, and A. K. ShuklaRRCAT, Indore: A. ChakrabartiUGC-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. MukhopadhyaySukhadia Univerisity, Udaipur: B. L. AhujaRajasthan univeristy, Jaipur: B. K. Sharma

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.

Ferromagnetism due to RKKY indirect exchange interaction.

Heusler alloys are famous for localized large magnetic moments on Mn.

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).

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.

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

• 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, 134435 (2001)

T/Tc= 0.8

Metallic behaviour with a clear jump at TM.

C. Biswas, R. Rawat, S.R. Barman, Appl. Phys. Lett., 86, 202508 (2005)

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)

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)

Increase Nickel

Ni2MnGa Ni2+xMn1-xGa (Ni,

Mn) Ni3Ga (x=1)

Ni2MnGa Ni-Mn-Ga

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 ǺTheory: a= 5.38 Ǻ, c= 6.70 Ǻ)[within 1% for a and 2% for c].

Phase diagram of Ni2+xMn1−xGa

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)

P= paramagnetic, F= ferromagnetic

PC= paramagnetic cubicFC= ferromagnetic cubicFT= ferromagnetic tetragonal PT= paramagnetic tetragonalTotal energies in meV/ atom

FC

PC

322

x= 0, Ni2MnGa

FT3.6

x= 0.25, Ni2.25Mn0.75Ga

TM<TCTM>TC

PC

FT

253PT

219

39

Phase diagram vis-à-vis total energies

kBTC ~ Etot(P) - Etot(F)

Decrease in TC for x= 0.25

kBTM ~ Etot(C) - Etot(T)

Increase in TM for x= 0.25

S. Banik, A. K. Shukla and S.R. Barman, RSI, 76, 066102 (2005).

IPES spectrometer

XPS/UPS spectrometer

Experimental facilities for electronic structure studies

UPS VB of Ni2MnGa 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 EF.

Chakrabarti, Biswas, Banik, Dhaka, Shukla, Barman, PRB, 72, 073103 (2005)

Non-modulated

Modulated

Calculated DOS

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

NiMn

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.

Saturation magnetic moment of Ni2MnGa: 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 Ni2MnGa

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)TC=375K, TM=260K

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).

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.

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)

Martensite Austenite

Mn1 -2.21 -2.43

Mn2 2.91 3.2

Ni 0.27 0.32

Total 1.21 1.29

Why Mn1 and Mn2 magnetic moments are different?

Origin of the structural transition (the martensitic phase)

800

600

400

200

0

41.541.040.540.039.539.0

martentic phase, 150K austentic phase, 300K

He II, UPS

Lowering of the electron states related to the cubic to tetragonal structural transition: Jahn Teller effect (Fujii et al., JPSJ)

kinetic energy

inte

nsit

y

(a) Minority spin Fermi surface of cubic Ni2MnGa.

Cross section of the Fermi surface (a) with the (001) plane. The arrows are examples of nesting vectors q0=0.34(1,1,0).

Origin of the modulated phases in Ni2MnGa: Fermi surface nesting

Bungaro, Rabe, Dal Corso, PRB, 68, 134104, (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.

q1

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

Majority spin FS, band 29; NV: 0.44(100) & (010)

Highly nested FS of Mn2NiGa

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

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

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.

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.

Ni 2p of Ni2MnGa shows an interesting satellite feature

Exchange splitting:

•Occurs when the system has unpaired electrons in valance band.

3d5 (6S) 3s (2S)

5S

3d5 (6S) 3s (2S)

exchange7S

3d5 (6S)

Ground stateh

3s2

•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 B (Fuji et al., JPSJ), 3.36 B (Ayuela et al.JOP:CM)

Mn magnetic moment from XPS

•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.

c

EF

4s

3d9

h

-12p

EF

4s3d9

2pC-1

3d10

Ground state

EF

4s3d9

2pC-1

Excited state

Origin of satellite in Ni core level

•If screening is better: main peak, no satellite.

•If screening is poor: satellite arises.

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.

                                                      

        

                                                      

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.

Crystal structure at room temperature

Martensitic phase at room temperature.

Austeni te

Martensi te

Mn

Ga

Ga

Mn

Mn

Mn

Ga

Ni

Ga

Ni

Ga

Mn

Ni

Mn

Ni

Mn

Ga

Ga

Ga

Mn

Ni

Mn

Ni

Mn

Ga

Ni

Ga

Ni

Ga

Mn

Mn

Mn

Ga

Ga

Mn

PowderCell 1.0

Ga

Mn

Ni1 Ni1

Ga Mn Ga

Ga

Ni1 Ni1

Mn Ga Mn

Ni1 Ni1

Ga

Ga Mn Ga

Ni1 Ni1

Mn

Ga

PowderCell 1.0

Cubic Tetragonal

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.

Lattice constant variation with x in Ni2+xMn1-xGa

DSC and ac-susceptibility of Ni2+xMn1−xGa

Small width of hysteresis 14-38 K for x=0; highly thermoelastic (mobile interface, strain less).

Decrease of at TM 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, 085110 (2006)

DSC: [Rate 10 C/min]

Susceptibility:[ 26 Oe field, 33.33 Hz]

x= 0 x= 0.24 x= 0.35 x Ms

(TM) Mf As Af

0 205 189 216 234

0.24 434 408 423 447

0.35 537 523 553 582

Albertini et al, JAP, 89 5614, 2001

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.

PES IPES

Photoemission (PES) and Inverse photoemission spectroscopy (IPES)

Characteristics of our PES workstation

Characteristics PES station Our aim..

Angle dependent XPS

Yes

Angle resolved PES

No Yes, using angle resolved analyzer

Base pressure 6 x 10-11 mbar

LEED Not available Yes

Analyser energy resolution in UPS

100 meV 1 meV

Analyzer energy resolution in XPS

0.8 eV 0.4 eV (by monochromatic XPS)

Spatial resolution

100 m <10 m

Temperature of expt.

150 K, RT <15 K to RT (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.

MgF2 window Tungsten anode Teflon support

Stainless steel cathode Ceramic feedthrough

Pumping port

Gas filled photon detector

Operating principle Design S. Banik, A. K. Shukla and S.R. Barman, RSI, 76, 066102 (2005).

Surface composition from XPS for sputtered surface

•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)]

Ni 3p

Mn 3p

Ga 3d

Ion sputtering increases Ni content on the surface.

With increasing annealing temperature Mn segregates to surface.

At about 390oC the Ni:Mn ratio is same as that of the bulk (2.3).

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

Surface composition from XPS with annealing

Valence band spectrum of Ni2MnGa in martensitic phase

DOS calculation using the actual modulated structure

Non-modulated

Modulated7 layer modulated phase, Pnnm space group, 56 atoms/unit cell, a=4.215, b=29.302 and c=5.557 Å.

Comparison: photoemission and theory

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

Cu2MnAl

Disagreement in Feature A.Could overall agreement be better if modulation is considered?

Self energy effects in Ni2MnGa IPES

Inverse photoemission spectrum of Ni2MnGa at room temperature in the FC phase, compared with the calculated conduction band of Ni2MnGa FC phase based on total, Mn, and Ni 3d PDOS. The IPES spectrum of Ni2.24Mn0.75Ga1.02 (x=0.24) in the FT phase is also shown.

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.

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, 085110 2006

Compare with IPES spectra of Nickel and Manganese metal

Calculated Spin polarized energy bands of Ni2MnGa Minority spin Majority 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.

Partial phonon dispersion of Ni2MnGa 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 q0=0.34(1,1,0).

Origin of the modulated phases in Ni2MnGa: Fermi surface nesting

Bungaro, Rabe, Dal Corso, PRB, 68, 134104, (2003)

Possibility of tuning the minority spin DOS near EF

x= 0

x= 0.25

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)