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Co-Ni-Ga alloys with room temperature
ferromagnetic martensite phase A.
Srinivasan and S. Sarma Department of Physics,
I.I.T. Guwahati, Guwahati 781
039 asrini_at_iitg.ernet.in http//www.iitg.ernet.i
n/physics
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Progress in the development of FSMAs

• Ni-Mn-Ga
• Giant MFIS gt 5 (60000?) in Ni48Mn31Ga21
• (1999) and 10 in Ni49Mn30Ga21 (2002).
• For 1 change in composition, TM ? 50 K.
• TM 160 K to 620 K and TC 320 K to 380 K in
• NixMnyGa1-x-y eg. Ni2MnGa, TM210K,
  TC376K.
• Average electron / atom (ie., e/a) ? 7.30

Binary systems Fe-Pt and Fe-Pd exhibit giant
MFIS ( 830 ?).
High cost of Pt, Pd, and the brittleness of
Ni-Mn-Ga alloys have created interest in other
Heusler alloys like Co-Ni-Ga(Al), Fe-Ni-Ga(Al)
alloys. Current research is focused on
development of FSMAs with TC gt Ms gt RT, more
ductility and lower material cost (large MFIS,
higher magnetisation).
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Why Co-Ni-Ga FSMAs?

• Some of the hypotheses .
• Similar to Ni-Mn-Ga in e/a ratio and saturation
  magnetisation.
• Presence of ? -phase precipitates in parent
  phase gives more ductility.
• Higher (gt RT) Ms and TC.
• Expect better magneto-mechanical properties due
  to Co and lack of Mn.
• Fewer studies on the system.

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Our Focus

• Obtain Co-Ni-Ga alloys with Ms and TC above RT
  (esp., TC gt Ms gt RT).
• Understand substitutional effects between
  constituent elements

Co70-xNixGa30 (20 x 26)
CoxNi25Ga75-x (43 x 50)

• Study the effect of varying homogenisation
  (annealing) temperature.
• Investigate the influence of cooling rate.
• Study the influence of ageing conditions.
• Studies restricted to bulk polycrystalline
  alloys prepared by casting route.
• (arc melting ? homogenization ? quenching)

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Structural Studies - effect of composition
Co70-xNixGa30 (20 x 26) Samples homogenized
at 1230 ºC and quenched in liquid nitrogen.
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Structural Studies effect of composition
CoxNi25Ga75-z (43 x 50) Sample homogenized
at 1230 ºC and quenched in liquid nitrogen.
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Structural studies effect of cooling rate
Co45Ni25Ga30 LN2 quenched, ice-water quenched and
slow cooled after homogenization at 1150 ºC
Slowly cooled sample showed the presence of A1
phase. Other samples were of single phase
structure. a b 2.72 Å and c 3.23 Å. Unit
cell volume 23.437 Å3.
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Structural studies effect of annealing
temperature
Co47Ni23Ga30 alloy annealed at 900, 1000, 1150
and 1230 ºC.
Samples annealed below 1150 C showed mixed phase
structure. For the ß-phase a b 2.71 ?, c
3.22 ? and unit cell volume was 23.65 ?3. For
the ?-phase a 3.59 ? and unit cell volume was
46.27 ?3.
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DSC studies effect of composition
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DSC studies effect of cooling rate and
annealing temperature
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DSC studies effect of thermal cycling and
ageing Co45Ni25Ga30 alloy
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Magnetic susceptibility effect of composition
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Magnetic susceptibility effect of cooling rate
annealing
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Summary and Conclusions
Co70-xNixGa30 series
Co70-xNixGa30 series
Possibility ot tailor Ms and TC by heat-treatment
(and higher e/a?) MFIS? Magnetic anisotropy?
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THANKS !