Magnetocaloric Effect in Perovskite Manganites

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Department of Physics
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The Perovskite Manganites

• Perovskite metal oxide has the general formula
  ABO3.
• A ? rare earth
• B ? 3d transition metal
• O ? oxygen.
• Perovskite manganites
• B site occupied by Manganese.
• AMnO3

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Structure

• Ideal cubic perovskite manganites structure.
• MnO6 octahedra
• Position
• A at vertices
• Mn at body center
• oxygen ions at the faces of unit cell.

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Tolerance factor

• Defined as,
• Where rA, rMn and rO are average radius of A-site
  cation, Mn ion and Oxygen ion respectively.
• Ideal cubical structure, f 1
• Deformation of octahedra from cubical shape to
• 1. rhombohedral, when 0.96 lt f lt 1
• 2. orthorhombic, when f lt 0.96
• etc.

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Jahn-Teller distortion

• Any non-linear molecule with degenerate
  electronic states will be unstable and will under
  go a distortion which form the system of lower
  symmetry and lower energy there by removing the
  degeneracy.

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The Magnetocaloric Effect

• A magneto-thermodynamic phenomenon.
• Defined as heating or cooling of a magnetocaloric
  material when it magnetized or demagnetized
  respectively.
• Discovered in iron by Emil Warburg in 1881.
• Debye(1926) and Giauque(1927) explained its
  origin independently and proposed that it could
  be used to reach low temperature by adiabatic
  demagnetization process.

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Entropy

• Randomness of system.
• Total entropy of system,
• S SM (magnetic entropy) ST (thermal entropy)
• Adiabatic Condition dQ 0
• S CONSTANT

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Adiabatic Magnetization (H?)

• H increases ? SM decreases
• To compensate it ST increases.
• ?
• ? Temperature of the material increases.
• Why?
• Magnetic field does work (force to align spins)
  to higher order state. dQ0, by 1st law of TD
  internal energy increases ? Temp. increases.
• As ST (due to atomic vibration) increases to
  compensate.

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Adiabatic Demagnetization (H?)

• H decreases ? SM increases
• To compensate it ST decreases.
• ?
• Electron spin return to its original position. As
  ST decreases, vibration of lattice decreases.
• ? Temperature of the material decreases.

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Suitable MCE materials

• Two parameters of MCE are
• 1. Isothermal Magnetic entropy change,
• 2. Adiabatic temperature change,
• C (T,H) is the Specific Heat of the material.

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• Materials should have
• 1. large
• 2. small C
• at the temperature of interest.
• is large for a ferromagnet near its
  Curie temperature( TC ).
• So, one needs a ferromagnetic material with
  TC around
• Room Temperature.

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• Relative Cooling Power (RCP), measures the amount
  of heat transfer among the cold and hot
  reservoirs in a refrigeration cycle.
• ???? ???????? full-width at half-maximum
  ?? 2 - ?? 1

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• Gd with TC around 290K is a potential MCE
  material for RT refrigeration.
• But the operation process is very complex.
• Also Gd is costly!
• Perovskite Manganites with different type of
  doping show large MCE at moderate magnetic fields.

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Sample Preparation

• These kind of perovskite transition metal oxides
  are prepared by many way, like as solid state
  reaction ,sol-gel method etc.
• Here only solid state reaction is discussed.

Solid state reaction
Weighting pre-cursors in stoichiometric proportion
Dry mixing
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grinding
Drying , sieving, pelleting and pre-calcination
High Temperature calcination
Grinding and Final product
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Measurement

• Structural Analysis ? XRD, Reitveld Analysis.
• Magnetic measurement
• Field cooled cooling (FCC) magnetization
• measurement w.r.t temperature(T)
• in constant magnetic field.
• Zero field cooling(ZFC).
• Field cooled worming(FCW)
• Magnetization (M) varies with varying
• magnetic field in constant temp. (T)etc.

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• Change of magnetization with temperature.

• Arrot plot (H/M vs ?? 2 curves)

• Magnetic phase transition
• Positive slopes ? second order
  (Pr0.6Sr0.2K0.2MnO3)
• Negative slopes ? first order (La3/8
  Nd1/4Ca3/8MnO3)

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Applications

1. Magnetic household refrigeration appliances.
2. Magnetic cooling and air conditioning in
   buildings, houses and cars.
3. Superconducting technology.
4. Refrigeration in medicine.
5. Cooling in food industry and storage.
6. Cooling of electronic equipments. Etc.

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Motivation

• CFCs, HCFCs, HFCs ? harmful to the environment.
• Magnetocaloric solid ? environmentally friendly
  coolants.
• CO2-compressors need pressures more than 100 bar.
• Magnetic cooling devices need only 1-2 bar.
• Gas compression cycles Carnot-efficiency about 40
  .
• Magnetic coolers reached about 60.
• Compressors are noisy.
• Magnetic cooling devices are silent.

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• THANK YOU