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Magnetic refrigeration working based on the magnetocaloric effect can be the perfect replacement of the conventional gas compression-based refrigeration technology and reduces its harmful effects on the environment. The boundary between a first-order and a second-order phase transition would be where the perfect magnetocaloric material would be found. Therefore, establishing the sequence of phase transitions clearly is essential for the characterization of other phase change materials and for applied magnetocaloric research. A quantitative fingerprint of second-order thermomagnetic phase transitions is reported here in Si-substituted high content Mn-based inverse Heusler alloy systems, which are found to be crystallized in cubic structures. The second-order nature of the phase transition has been confirmed from the Arrott plot analysis and a correlation between magnetocaloric effect and local exponent is established. Using the Arrott plot, the critical exponents are evaluated employing different techniques such as modified Arrott plot, Kouvel–Fisher method, and critical isotherm. Their values are found to be in great agreement with each other and follow the mean-field model signifying the presence of long-range ordering in the materials. The high value of isothermal magnetic entropy change and the reversibility justifies the suitability of the reported materials in the practical application as magnetic refrigerants. [ABSTRACT FROM AUTHOR]

The advancement of magnetic materials with coupled magneto-structural phase transition (MST) to fulfill the ultimate objectives of practical solid-state cooling applications requires a better understanding of the hysteresis phenomenon linked across the phase transition region along with the large magnetocaloric parameters. For the present sample Mn0.65Ni0.65Fe0.70Si, the MST is associated with a sharp jump in magnetization along with a small thermal hysteresis of ∼13 K. A giant isothermal magnetic entropy change (| Δ S M max |) of ∼37.6 J kg−1 K−1 at 299 K and effective refrigerant capacity (R C effe ) of ∼214.3 J kg−1 under ΔH = 30 kOe is obtained with excellent compatibility between the martensite and austenite phases. The geometrical compatibility condition, i.e., very small (∼0.55%) deviation of the middle eigenvalue (λ 2) from unity justifies the observation of small hysteresis in the present material. The investigation of hysteresis behavior under different extents of the driving forces (temperature or magnetic field) reveals that both the driving forces trigger equally the phase transition and are responsible equivalently for the hysteresis phenomenon. The present study provides a pathway to understand the complexity of the hysteresis behavior, its impact on the reversibility of magnetocaloric effect, and its minimization by optimizing the geometrical compatibility condition between the austenite and martensite phases. [ABSTRACT FROM AUTHOR]

Lead-free ferroelectric materials are essential for environment-friendly solid-state cooling technology. In this respect, the electrocaloric effect of lead-free Ba0.8−xSr0.2SmxTiO3 (x = 0.01–0.05) ceramics has been investigated. The Rietveld refinement reveals that all the ceramics exhibit tetragonal symmetry and the tetragonality decreases for x > 0.03, which is the major reason behind the decrease in both isothermal entropy change and adiabatic temperature change. The degree of diffuseness parameter is enhanced by doping of Sm3+, which confirms the formation of polar nanoregion and the enhancement of the electrocaloric effect parameter by adding extra entropy. The relaxor behavior of the prepared samples is also confirmed by estimating the degree of deviation parameter (Δ T m = 77 K ) from the 1 ε r vs. T curve. So, the temperature stability of the prepared ceramics is improved in a broad temperature window. The highest isothermal entropy change (Δ S) , adiabatic temperature change (Δ T) , and electrocaloric strength ( Δ T Δ E ) are found to be 1.230 Jkg−1 K−1, 0.862 K, and 0.028 73 K cm/kV at 316 K, respectively, for Ba0.77Sr0.2Sm0.03TiO3. [ABSTRACT FROM AUTHOR]

Many physical properties behave differently in disorder systems from the order one. The effects of atomic disorder on the magnetic properties have been explored on Co2TiSi and Co2TiAl full Heusler alloys. Co2TiSi crystallizes in an ordered L21 crystal symmetry, and Co2TiAl mainly possesses a B2 structure. The B2 disorder affects not only the magnetic ordering but also the saturation magnetization, causing the deviation from the Slater-Pauling prediction for Co2TiAl. The critical exponents of the magnetic phase transition have been analyzed by employing different techniques like, the Arrott plot, Kouvel–Fisher plot, critical isotherm analysis, scaling magnetic behavior, and magnetocaloric analysis as well. The calculated critical exponents are close to the mean field theory which implies long-range ferromagnetic ordering in Co2TiSi. However, the exchange interaction in Co2TiAl deviates from the long-range mean field theory. This may be due to the magnetic contribution of the B2-disordered phase. In addition to the magnetic ordering, the magnetocaloric analysis provides the applicability of Co2TiSi alloy as a low-cost, hysteresis-free magnetic refrigerant with a large working temperature near room temperature. [ABSTRACT FROM AUTHOR]

The analysis of structural, electrical transport, critical magnetic behavior, and magnetocaloric properties of Co2CrAl Heusler alloys has been conducted here. Co2CrAl full Heusler alloy has been prepared by the conventional arc melting method in Ar atmosphere and post‐annealing in a vacuum‐sealed quartz tube. X‐ray diffraction (XRD) pattern reveals the crystallization of the alloy to the Fm3¯m space group with L21 crystal symmetry. The correspondence between the magnetic and electrical transport properties of the Co2CrAl alloy has been observed. This alloy exhibits paramagnetic to ferromagnetic transition (Curie temperature) just above room temperature at around 340 K. Metallic to semiconducting transition occurs near the magnetic Curie temperature. Critical magnetic behavior has been analyzed by employing the Arrott plot, Kouvel–Fisher methods, and critical isotherm analysis. These conclude that the exchange interaction in Co2CrAl alloy follows between mean‐field theory having spin–spin long‐range interaction and the 3D Heisenberg model. The magnetocaloric effect has been explored from the isothermal magnetization data by employing Maxwell's equations. The highest magnetic entropy change is found to be ≈0.99 J (kg K)−1 for a magnetic field change of 30 kOe. A large working temperature window of 57 K is the advantage of the present alloy. [ABSTRACT FROM AUTHOR]

Of late, d0 ferromagnetic oxide materials have been studied as potential material for spintronics application. We investigate the crystal structure, magnetic, optical, and dielectric properties of Ag-doped TiO2 compounds. XRD study reveals polycrystalline rutile form of TiO2 with tetragonal-type structure. SEM micrographs show spherical particles of size 2–3 µm. All the compounds exhibit ferromagnetism with Curie temperature beyond 400 K and with a typical coercivity value in the range 130–550 Oe. The bifurcation of ZFC and FC M–T curves points out magnetic irreversibility. All Ag-doped TiO2 compounds exhibited p-type conductivity as per the Hall effect study. Optical absorbance data indicate the widening of the band gap up to 6% of Ag doping followed by a decrease for higher doping and these materials exhibit transmittance in the range of 20–30%. The compounds exhibit zero dielectric loss and appreciable dielectric constant and their AC conductivity increases with Ag doping. [ABSTRACT FROM AUTHOR]

Heusler alloys have drawn considerable interest, due to their tunable properties, for technological applications. The literature review reveals that the critical exponents and magnetocaloric effect (MCE) of Co2TiAl0.75Si0.25 quaternary Heusler alloy need to be established. Hence, Co2TiAl0.75Si0.25 alloy is prepared by the arc melting method. Critical exponents of the alloy are obtained by analyzing magnetic isotherms with various techniques, such as Arrot plots, Kouvel–Fisher plots, critical isotherm analysis, and magnetocaloric analysis near the transition temperature. The paramagnetic to ferromagnetic transition temperature is found to be ≈235 ±2.5 K. The nature of the Arrot plots and the universal curve of the MCE confirm this as a second‐order phase transition. Despite the moderate magnetocaloric values, the large working temperature (≈75 K at 3 T) makes this alloy a potential candidate for a multistage magnetic refrigeration application. The critical exponents obtained from different techniques fall in between the theoretical prediction from the mean field theory, which indicates long‐range ferromagnetic exchange interaction, and the 3D Heisenberg model, implying short‐range interaction. The range of interaction (σ = 1.727) also confirms this. This complex magnetic interaction may arise from the competition between localized and itinerant electron present in the system. [ABSTRACT FROM AUTHOR]