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Magnetization, specific heat, thermal diffusivity, and magnetocaloric effect were studied in Pr0.7Sr0.3-xCaxMnO3 system (x = 0; 0.1; 0.2; 0.3) in the temperature range of 80–320 K and in a magnetic field of 18 kOe. A correlation was found between the coefficients under study and microscopic characteristics (〈rA〉 – average ionic radius and σ2 – cationic disorder). The decrease in σ2 and 〈rA〉, caused by the substitution of Sr atoms by Ca atoms, causes a weakening of the exchange interactions and a decrease in ТС. The observed minima in the temperature dependence of thermal diffusivity η(T) near ТС are associated with the appearance of an additional channel of phonon relaxation in fluctuations of the magnetic order parameter. A relationship was found between the MCE value and the change in thermal diffusivity in a magnetic field near ТС. This relationship can be applied as another MCE evaluating indirect method. It is shown that MCE value estimation from thermal diffusivity data is consistent with the results of caloric measurements.

We report on the structural, mechanical, and magnetocaloric properties of annealed melt-extracted Gd 53 Al 24 Co 20 Zr 3 amorphous microwires of ∼70 μm diameter. During heat treatment small islands of nanocrystallities are generated and isolated in the amorphous region for the wires. The size of the nanocrystallities ranges from 5 nm to 10 nm. The observed lattice distortion from the nanocrystallities causes changes in the magnetic properties of the wires. The annealing temperature of 100 °C has the largest strength (1845 MPa) as compared to wires annealed at other temperatures. This is likely to trigger nanophase transformation in the amorphous region and these nanocrystals have been preserved through the increase of annealing temperature. The formulation of the nanocrystalline islands is also verified by the selected-area electron diffraction (SAED). The microwires exhibit a large and reversible magnetocaloric effect (MCE), with the maximum isothermal magnetic entropy change (−Δ S M ) and refrigerant capacity ( RC ) values of 9.5 J/kg K and 689 J/kg respectively for the microwire annealed at 100 °C. This RC is about 35%, 67%, and 91% larger than those of bulk Gd 53 Al 24 Co 20 Zr 3 (∼509 J/kg), Gd (∼410 J/kg), and Gd 5 Si 2 Ge 1.9 Fe 0.1 (∼360 J/kg) regardless of their ordering temperatures. These results demonstrate the ability to tune the mechanical and magnetic properties of the microwires by thermal annealing.

The magnetic properties of La 0.7- x Pr x Ca 0.3 MnO 3 ( x = 0.0, 0.2, 0.3, 0.4, and 0.5) compounds fabricated by a solid-state reaction are studied by using Arrott plots and universal curves. The Arrott plots near the Curie temperature show negative slopes over the entire magnetic field range up to 50 kOe, indicating a first-order magnetic phase transition (according to Banerjee's criterion) for the x = 0.0, 0.2, 0.3, and 0.4 samples. For the x = 0.5 sample, however, the plots show positive slopes at low magnetic fields below 10 kOe, indicating a second-order transition. These results are further checked by universal curves of the normalized entropy change versus reduced temperature. The universal curves show a divergence among the curves with different magnetic fields for a first-order magnetic transition, but for a second-order transition, they collapse onto the same curve. The Curie temperature, around which the maximum magnetic entropy change occurs, decreases continuously from 260 to 110 K as x increases from 0.0 to 0.5. In the same x range, the maximum entropy change measured at a magnetic field span of 50 kOe decreases from 10.70 to 7.14 J/kg K, but the relative cooling power increases from 278 to 380 J/kg under the same magnetic field span.

The specific heat (CP), thermal diffusivity (η), thermal conductivity (κ), magnetization, resistivity (ρ), and magnetocaloric effect (MCE) were studied in fast-quenched ribbon samples of the Ni50Mn37-xAlxSn13 alloy (x = 2, 4). Anomalies are observed in the form of a jump for the temperature-dependent η(T) and κ(T) near the magnetic and magnetostructural phase transitions, which are associated with critical scattering of heat carriers by distortions of the crystal structure, fluctuations of the order parameter, or structural/magnetic inhomogeneities. The transition to the martensitic phase is found to be accompanied by a sharp increase in the resistivity, suggesting an increase in the electron relaxation rate due to structural distortions inherent in martensite. The value of the magnetoresistive effect at 1.8 T near the martensitic transition reaches ∼40%. The temperature hysteresis was investigated in terms of ρ(T) near TC, which is explained based on the coexistence of the magnetic two-phase state of FM + AFM and the structural magnetic heterogeneity of the samples with dominant austenitic phase. For a sample with x = 2, the electron and phonon components of thermal conductivity were estimated (κph » κe).

We have studied the magnetic and magnetocaloric (MC) properties of Ni50−xPrxMn37Sn13 (x = 0, 1, 3, and 5) alloy ingots fabricated by arc-melting. Experimental results revealed a coexistence of the conventional and inverse MC effects in the samples with x = 0, 1, and 3. There is a gradual decrease of the magnetization and magnetic-entropy change (ΔSm) when Pr-doping content increases. However, the Curie temperature of the austenitic phase ( T C A ) around room temperature was not changed very much. Particularly, doping a small amount of Pr with x = 1 enhances remarkably the linewidth of the peaks in its ΔSm(T) curves. This increases the relative cooling power (RCP) of Ni50−xPrxMn37Sn13 alloys when they are used as coolants in refrigeration devices. For example, under an applied field of H = 50 kOe, the RCP increases from 150 for x = 0 to ∼200 J/kg for x = 1. Higher Pr contents reduce quickly the MC effect because of the formation of nonmagnetic secondary phases.

We prepared single-phase rhombohedral samples La0.7Sr0.3Mn1−xCoxO3 (x = 0, 0.06, 0.08 and 0.1) by conventional solid state reaction, and then studied in detail their magnetic and critical properties. Investigations into temperature and magnetic-field dependences of magnetization indicated a gradual decrease of the ferromagnetic–paramagnetic transition temperature (TC) from 360 to 281 K with increasing Co-doping concentration up to 10%. Having used Banerjee’s criteria, we found all the samples undergoing a second-order phase transition. The values of the critical exponent β = 0.387–0.487 determined from modified Arrott plots are located in between those expected for the 3D Heisenberg and mean-field models while the values of γ = 1.109–1.166 are close to those of the 3D Ising and mean-field models. These features reveal an existence of ferromagnetic short-range order and magnetic inhomogeneity in La0.7Sr0.3Mn1−xCoxO3. Particularly, with increasing Co concentration, the critical exponent values trend to shift towards those of the mean-field theory. Such the results prove the Co doping favors establishing ferromagnetic long-range order.

The magnetic phase transition behavior were investigated in detail in Ni 50 − y Gd y Mn 37 Sn 13 ( y = 1 and 3) alloys prepared by arc-melting method. The martensite phase was found to be strongly suppressed by a small amount of Gd doping. Based on isothermal magnetization curves around Curie temperature of the austenite ( T C A ) phase, critical behavior in the austenite phases of both alloys were determined carefully by the Kouvel–Fisher method. The critical exponents were found to be β = 0.473 ± 0.020 and γ = 1.141 ± 0.017 with T C A = 299.0 ± 0.2 K for y = 1, and β = 0.469 ± 0.068 and γ = 1.214 ± 0.042 with T C A = 302.9 ± 0.7 K for y = 3, respectively. The values of the critical exponents for the ferromagnetic phase transition in the A phase of two alloys can be basically ascribed in the mean-field model (with β = 0.5, γ = 1) with slightly deviation, revealing a long-range order of ferromagnetic interactions. Such critical behavior can be attributed to the magnetic inhomogeneities originated from the atomic disorder introduced by Gd doping.

We have determined the values of critical exponents of two polycrystalline samples (Nd1−xYx)0.7Sr0.3MnO3 (x = 0 and 0.07) from the magnetization data versus temperature and magnetic field, M(H, T), to learn about their magnetic and magnetocaloric (MC) properties. The results reveal the samples exhibiting the crossover of first-order and second-order phase transitions, where the exponent values β = 0.271 and γ = 0.922 for x = 0, and β = 0.234–0.236 and γ = 1.044–1.063 for x = 0.07 determined by using modified Arrott plots and static-scaling hypothesis are close to those expected for the tricritical mean-field theory (β = 0.25 and γ = 1.0). Particularly, the TC of x = 0 and 0.07 can be any value in the temperature ranges of 240–255 K and 170–278 K, respectively, depending on the magnitude of applied magnetic field and determination techniques. Around the TC, studying the MC effect of the samples has revealed a large magnetic-entropy change (ΔSm) up to ∼8 J/kg K for the applied field interval ΔH = 50 kOe, corresponding to refrigerant capacity values of 200–245 J/kg. These phenomena are related to the crossover nature and the persisting of FM/anti-FM interactions even above the TC, as further confirmed by electron-spin-resonance data, Curie–Weiss law-based analyses, and an exponential parameter characteristic of magnetic order n = dLn|ΔSm|/dLnH.

The nature of ferromagnetic phase transition and its critical properties in La0.7Ca0.3−xSrxMnO3 (x = 0, 0.05, 0.1, 0.2 and 0.25) single crystals have been studied systematically. Based on magnetic measurements and critical behavior analysis using Banerjee criterion and Kouvel–Fisher method, we demonstrate the existence of a tri-critical point, with critical exponents (β = 0.26 ± 0.01, γ = 1.06 ± 0.02) at x ∼ 0.1, that separates “discontinuous” first-order magnetic transition (FOMT) for x 0.1 compositions. Above the tricritical point, the system (e.g. x = 0.2) shows a SOMT with the critical exponents (β = 0.36 ± 0.01, γ = 1.22 ± 0.01) belonging to the Heisenberg universality class (β = 0.365 ± 0.003, γ = 1.336 ± 0.004). This suggests that the magnetic interaction in these manganites is of short-range type. Our systematic studies show that chemical (internal) pressure induced by substituting larger Sr ions for smaller Ca ions, cooperative Jahn–Teller distortions, antiferromagnetic coupling, and formation of ferromagnetic clusters all have significant impact on the nature of the ferromagnetic phase transition, the conduction mechanism and colossal magnetoresistance (CMR) in the doped manganites.

The extremely soft magnetic behavior in the nanocrystalline Fe 81 B 11 Nb 7 Cu 1 alloy annealed at 450 and 550°C for 1 h, respectively, in a vacuum were obtained, and examined by means of the magnetoimpedance (MI) effect and the incremental permeability. Because the MI effect can be obtained only in ultra-soft magnetic materials, the improvement of magnetic softness by proper thermal treatment was carefully monitored by the MI effect for all annealed samples. The changes of the incremental permeability as a function of an external field were also measured to verify the magnetic softness along with the MI measurement. Our result show that the maximum value of magnetoimpedance ratio increases drastically in the sample annealed at 450 and 550°C indicating that the sample is ultra-softened by nanocrystallization.

The nanocrystalline Fe 84 Zr 7 B 6 Cu 1 Al 2 alloy was annealed at 450 and 550°C for 1 h to achieve the ultra-soft magnetic properties such as large magnetoimpedance ratio (MIR), the incremental permeability ratio (PR), nearly zero coercivity, zero magnetostriction, etc. The MIR and PR of the sample were measured from 100 kHz to 10 MHz in a cryogenic chamber where the temperature was varied from 10 to 300 K. The increment of MIR value is proportional to increasing temperature. The maximum PR values measured above 1 MHz remain almost temperature independent. However, the maximum PR values measured below 1 MHz show drastic increment at above 150 K due to thermal activation of magnetic domains.