FEL & Utilization Section
FEL & Utilization Section

Research on magnetic and superconducting materials

1. Magnetic materials:

The research on magnetic materials is currently focused on magnetocaloric effect and its possible usage in:

(i) Refrigeration in and around room temperature and

(ii)Gas liquefaction.

(i) Magnetic refrigeration is an energy-efficient and environmentally sound technology alternative to the vapour-cycle refrigerators and air conditioners. It offers considerable saving of operating cost by eliminating the most inefficient part of the existing refrigerators– the compressor. It uses a solid refrigerant and a common heat transfer fluid (e.g. water, air or helium gas) with no ozone-depleting and global-warming effects. The technology right now is at a nascent stage, and its development will largely depend on discovering materials with a large magnetocaloric effect at or close to room temperature. The ongoing research activity is presently focused on two classes of materials namely NiMnX (X= In, Sn, Al etc.) based ternary Heusler alloys and FeRh based binary alloys.

Large magnetocaloric effect with significant refrigerant capacity has been observed around 240 K in a Ni50Mn34In16 alloy, and the working temperature has been pushed further to 275 K by partial chemical substitution of Cr and Cu in this Ni50Mn34In16 alloy. Further research has revealed that the same first order magneto-structural phase transition, which is responsible for the magnetocaloric effect in these NiMnIn based alloys, also gives rise to a large magnetoresistance and large magnetic field induced strain. These results highlight the multifunctional nature of this alloy system. The same multifunctional properties associated with the first order magneto-structural phase transition have been found in the equi-atomic FeRh alloy. A very large and reproducible magnetocaloric effect with giant magnetic refrigeration capacity has been observed FeRh alloys in and around the room temperature.

The present research on magnetocaloric materials, which could be useful for room temperature refrigeration, is being carried out on earth abundant materials, such as Mn-Co-Ge alloys. A large magnetocaloric effect has been observed near room temperature in an off-stoichiometric composition of this system. The work is aimed at finding a suitable alternative to costly rare-earth materials which are difficult to purify. The alloys being pursued do not consist of elements such as arsenic, which are toxic in nature

At the root of such multifunctional properties is a disorder influenced first order phase transition, which in turn is a manifestation of an interesting interplay between electronic and lattice degree of freedom observed in various classes of magnetic materials. This generality has been highlighted with the help of an interesting model intermetallic compound CeFe2. Further it has been shown that under certain circumstances in the presence of an applied magnetic field, this first order magneto-structural phase transition often gets kinetically arrested, and thus gives rise to a highly non-equilibrium glass-like magnetic state. This magnetic-glass state is distinctly different from ‘spin-glass’ state, and has also been observed in NiMnIn, FeRh and Gd5Ge4, apart from the CeFe2 based alloys

(ii) Liquid hydrogen, with its high volumetric density, is a useful medium for storing and transporting hydrogen efficiently and economically. In conventional liquefiers, the figure of merit currently is approximately 35%. In order to obtain higher efficiency (>50%), magnetic refrigeration based on magnetocaloric effect is a promising cooling method. The ongoing research in this direction is focused on finding materials with large magnetocaloric effect in the temperature regime 20 to 70K. Several new magnetocaloric materials with significant potential in this direction- DyCu2, DyPt2, MnSi, NdRu2 and GdCu6 have been identified.

2. Superconducting materials:

The research in superconducting materials is currently focused on:

(i) Superconductors alternate to Nb based materials for high-current high magnetic field applications (i.e. high-field superconducting magnets)

(ii) Superconductors for cryogenic radiation detector applications

(i) Superconductors alternate to Nb based materials for high-current high magnetic field applications:

The commercially available superconducting magnets are currently based on NbTi alloys (for fields < 7 T) and Nb3Sn (for fields 7 T < H < 15– 20 T). The increasing demand for high magnetic fields motivates research on newer superconducting materials with superior current carrying capability. However, the Nb-based superconductors are not compatible with neutron irradiation environment (as in a fusion based reactor) as the latter gives rise to long term radioactivity in these materials. Moreover, with the increasing usage of superconductivity in different fields in the modern world, the use Nb everywhere may lead to a scarcity of this element in future. Hence there is a need for looking into alternate superconducting alloys. The FEL & Utilization Section is strongly interested in the study of refractory metal alloys involving Zr, Ti, V, Mo and Re as alternate to the Nb based superconductors. Of particular current interest are the Ti-V alloys because of their excellent mechanical properties and other interesting physical properties. Experimental results indicate that magnetic spin fluctuations and superconductivity co-exist in the Ti-V alloys, where the spin fluctuations suppress the superconducting transition temperature by nearly 50% as compared to the theoretically predicted values. Addition of rare earth elements in the Ti-V alloys reduces the grain size in this system and also introduces additional flux line pinning centres. As a result, the alloys become capable of carrying large currents in higher magnetic fields, where the critical current density (Jc) is enhanced up to 20 times in magnetic fields up to 6 T. Addition of Gd is found to introduce ferromagnetism in the Ti-V alloys near room temperature. The superconductivity in the system is not only sustained in the presence of ferromagnetism, in fact, the superconducting transition temperature (Tc) is found to increase marginally along with the enhancement of Jc in the Gd-containing Ti-V alloys.

While both the Ti-V and Mo-Re alloy systems are low temperature superconductors, they are found to exhibit high field paramagnetic Meissner effect and in fact, this effect can be observed up to 7 T fields and beyond in the T-V alloys after Y-addition. The Mo-Re alloys are found to exhibit higher Jc in lower fields as compared to the Ti-V alloys, and experimental evidence indicates the presence of a surface mixed-state state or ‘Kulik vortex-state’ in the bulk Mo-Re alloys and also the occurrence of a vortex–liquid to vortex–glass transition.

The critical current density of the V-Zr alloys is very high and comparable to the commercial Nb based superconductors. This is found to be due to the presence of very large number of grain boundaries generated during eutectic reaction. The V-Zr alloys form with five different metallurgical phases. The β-V phase becomes superconducting below 5.2 K, whereas the α and β-Zr phases remain normal down to 2 K and lead to the formation of a normal channel of heat conduction in parallel to the superconducting channel even when bulk of the sample remains superconducting. This configuration makes these alloys more efficient in removing the heat than what is expected theoretically for a bulk superconductor, which is an important information for technological applications.

(ii) Superconductors for cryogenic radiation detector applications:

A superconducting film, electro-thermally biased at the superconducting-to-normal phase transition can be utilized as an extremely sensitive thermometer. These devices are known as Transition Edge Sensors (TES). Such sensors detect radiations ranging from γ and X-rays to the sub-millimeter and millimeter waves, which makes TES an indispensable tool for studies in basic sciences, enforcement of nuclear non-proliferation, nuclear forensics, non-destructive analysis of spent nuclear fuel and astronomy. The use of superconductors at sub-Kelvin temperatures helps in attaining the highest possible sensitivity as the superconducting gap is substantially lower than the band gap of the conventional detector materials. Moreover, the operation at very low temperatures reduces the noise thereby boosting the signal to noise ratio of the detectors.

As a part of the activity on the TES detectors, studies on the use of disordered Mo films for microwave and THz radiation has been taken up. The superconducting transition temperature of the disordered Mo films (nano-grain morphology, residual resistivity ratio ~1) is much higher than bulk Mo. The Tc (3-4 K) and microstructure of the films are being tuned with the Ar pressure maintained during deposition. Temperature dependent terahertz spectroscopy of these superconducting Mo films enable the experimental determination of the temperature dependence of penetration depth, superfluid density and superfluid stiffness. The disordered films show promise for use as active materials in microwave kinetic inductance bolometers for THz and IR radiation detection.

Figure 7: Temperature dependence of (a-b) penetration depth (λ) of Mo thin films of two different thicknesses, fitted with the dirty-limit BCS model, (c) Superfluid density (n) along with films and the two-fluid model fits and (d) Superfluid stiffness (J).

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