News Excerpt:

Researchers have found a new alloy (Heusler alloys) that can act as an effective magnetic refrigerant that can be an alternative cooling agent for minimizing greenhouse gas emissions and meet the global demand for higher energy efficiency for tackling global warming.

About Magnetic Refrigeration:

• Magnetic refrigeration offers an energy-efficient and environment-friendly cooling technology as an alternative to the vapour-cycle refrigeration technology in use today.
• Magnetic cooling effect (MCE) is defined as the reversible temperature change of a magnetic material when it is subjected to an external applied magnetic field. 
• In the magnetic refrigeration cycle, a magnetic field is applied on the magnetic material under an adiabatic process (no exchange of heat with the surrounding). 
• Initially randomly oriented magnetic moments get aligned along the external magnetic field, resulting in the heating of the magnetic material. 
  • This heat is transferred from the material to the ambience. 
• When the magnetic field is removed during adiabatic demagnetization, the magnetic moments of the material become randomized, resulting in a decrease in temperature below the ambient temperature.
  • This process causes the material to absorb heat from the surrounding heat-transfer medium.

Fig 1. Schematic representation of magnetic refrigeration cycle from the internet

Properties required for new magnetic materials:

• The material must be capable of operating for millions of cycles without any fatigue and failure, 
• The material must have high thermal conductivity,
• The material should respond to an external magnetic field of about 2 T (Tesla) which can be generated by permanent magnets.

About Heusler alloys:

• A team at S.N. Bose National Centre for Basic Sciences, an autonomous institute of the department of science and Technology (DST) experimented with a certain type of alloys called all-transition metal based Heusler alloys (magnetic intermetallics with face-centred cubic crystal structure) in their search for material exhibiting giant reversible MCE (Magneto Caloric Effect).

Fig 2. Four FCC lattices of full-Heusler Ni(Co)-Mn-Ti Heusler alloy

• They chose Ni(Co)-Mn-Ti Heusler system because such systems often exhibit multifunctional properties with ultrahigh mechanical stability because of their intrinsic d-d hybridization.
• They found giant reversible MCE and magneto resistance (MR) in bulk Ni35Co15Mn34.5−xCuxTi15.5 (x = 1, 2, and 3) under an applied magnetic field of 5 T and 7 T.  
• Scientists have shown that Cu doping in the Mn site pulls the magnetic transition towards structural transitions and therefore the distance between them decreases. 
• This is because Cu strengthens the metallic character of the Ni–Ti bond and at the same time weakens the magnetic Mn–Mn exchange interaction.