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Realization of magnetostructural coupling and giant magnetocaloric effect in off-stoichiometric MnCoGe alloys
2nd International Conferences on Nanotechnology & Chemistry
April 06-07, 2022 | Webinar
Najam Ul Hassan
University of Education, Lahore, Pakistan
Scientific Tracks Abstracts : J Pharm Chem Chem Sci
Abstract:
Magnetostructural coupling, a coupled structural and magnetic transition exhibits variety of multifunctional effects such as Magnetocaloric effect, magnetoresistance, magnetic field induced strain and magnetic shape memory effect. These effects show applications in many research areas like magnetic refrigeration, sensors, actuators, energy-harvesting devices and magneto-mechanical devices. The magnetostructural coupling in off-stoichiometric MnCoGe alloys is realized through tuning the Mn/Co ratio, and it results in a significant entropy change (up to -30.8 J•K-1•kg-1) and wide Curie-temperature window (up to 96 K). Two scenarios have been studied, i.e., the Co-rich and Mn-rich compositions, respectively. In both scenarios, the orthorhombic-hexagonal structural transition temperature (Tt) can be shifted into the temperature range between the Curie temperatures of the orthorhombic and hexagonal phases, enabling the magnetostructural coupling between two phases. Although the magnetostructural coupling can be achieved in both scenarios, the Co-rich alloys show much higher entropy change and wider Curie-temperature window than the Mn-rich alloys. Based on the systematic structural and magnetic analyses, the composition-dependent phase diagrams for both scenarios have been constructed.Background:Magnetic refrigeration has drawn great attention from researchers because of its advantages over the conventional cooling technique, such as the compact size, low mechanical vibration, environmental friendliness and high efficiency. Materials exhibiting the large magnetic entropy changes (ΔSM) are potential candidates for magnetic refrigeration applications. Till now, the large ΔSM has been widely studied in materials experiencing the magnetic-field-induced first-order transition, e.g. La-Fe-Si, Gd-Si-Ge, Mn-Fe-P-As and Heusler-type NiMn-based magnetic shape memory alloys. Besides these materials, MMnX (M=Co or Ni, X=Si or Ge) alloys also attract considerable attention for its large and tenable magnetocaloric effect. Methods: The polycrystalline samples of Mn2-xCoxGe alloys with Co-rich compositions (x = 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.10, 1.11 and 1.12) and Mnrich compositions (x = 0.89, 0.88, 0.87, 0.86, 0.85, 0.84, 0.83, 0.82, 0.81 and 0.80) samples were prepared by arc melting the appropriate amount of raw materials with purities higher than 99.9% in an argon atmosphere using a water- cooled copper crucible. The ingots were remelted for several times to ensure the homogeneity. After that, the ingots were annealed at 1123 K for 5 days in vacuum quartz tubes followed by slow cooling to room temperature. The structural transitions were investigated by differential scanning calorimetry (DSC, Mettler Toledo) with a sweeping rate of 10 K/min during heating and cooling cycles. The crystal structures of powdered samples were identified by X-ray diffraction (XRD, Bruker, D8 Advance) using Cu-Kα radiations. Magnetic measurements were carried out on physical property measurement system (PPMS, Quantum Design, Dynacool. Results: We have managed to realize the magnetostructural coupling in off-stoichiometric MnCoGe alloys, leading to the significant magnetocaloric effect. This was achieved through tuning the Mn/Co ratio, i.e., the Mn-rich scenario and Co-rich scenario. In both approaches, the orthorhombic-hexagonal structural transition temperature (Tt) is lowered into the temperature range between the Curie temperatures of the orthorhombic and hexagonal phases, enabling the magnetostructural coupling between two phases. Wide CTWs of 96 and 34 K are obtained for Co-rich and Mn-rich alloys, respectively. Composition-dependent phase diagrams for both scenarios are also constructed. The large ΔSM (up to -30.8 J•K-1•kg-1) and wide CTW make the off-stoichiometric MnCoGe alloys very promising for magnetic cooling applications
Biography:
Magnetic refrigeration has drawn great attention from researchers because of its advantages over the conventional cooling technique, such as the compact size, low mechanical vibration, environmental friendliness and high efficiency. Materials exhibiting the large magnetic entropy changes (ΔSM) are potential candidates for magnetic refrigeration applications. Till now, the large ΔSM has been widely studied in materials experiencing the magnetic-field-induced first-order transition, e.g. La-Fe-Si, Gd- Si-Ge, Mn-Fe-P-As and Heusler-type NiMn-based magnetic shape memory alloys. Besides these materials, MMnX (M=Co or Ni, X=Si or Ge) alloys also attract considerable attention for its large and tenable magnetocaloric effect.
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