Critical Isotherm Technique Research Articles

Sm substitution effect on the critical behaviour of Laves phase Gd1−xSmxCo2 intermetallic nearby the ferromagnetic-paramagnetic phase transition

In this work, we present a comparative and detailed study of the critical behavior near ferromagnetic-paramagnetic phase transition in the intermetallic Laves phase compounds Gd1−xSmxCo2, x = 0.5, 0.6 and 0.7. The investigated samples exhibit a second-order phase transition at Curie temperature TC. The critical exponents β, γ, and δ have been estimated through diverse techniques: the Modified Arrott plot, the Kouvel-Fisher plot, the critical isotherm technique, and the magnetocaloric effect method. The critical exponents values that were determined have been validated using the universal scaling hypothesis, wherein the isotherm magnetization curves M(H) converge into two distinct universal branches below and above TC. The substitution of Gd by Sm changes the universality class in the (Gd,Sm)Co2 compounds. For the sample Gd0.5Sm0.5Co2, the critical exponents obtained are closely consistent with the predictions of the 3D-Heisenberg model featuring a magnetic system with a short-range exchange interaction. While, for both samples Gd0.4Sm0.6Co2 and Gd0.3Sm0.7Co2, the values deviate from known theoretical models and might belong to an unconventional universality class with an intermediate-range exchange interaction.

Magnetic properties, critical behavior and magnetocaloric effect in the nanocrystalline Pr2Fe16Al

We report the correlation between critical behavior and magnetocaloric effect around magnetic phase transition temperature, using magnetization isotherms of Pr2Fe16Al. We used four techniques to determine the critical exponents: the Modified Arrott plot, Kouvel-Fisher plot, critical isotherm technique, and magnetocaloric effect method. The critical exponents values for Pr2Fe16Al are similar to that found by the 3D-Heisenberg model. The validity and the reliability of our critical exponents were confirmed. The isotherm M(H) curves below and above the critical temperatures collapse into two independent universal branches with the found critical exponents. The exponents determined in this study are close to those calculated from the results of the renormalization group approach for a heuristic model of three-dimensional Heisenberg (d = 3, n = 3). A correlation between critical behavior and magnetocaloric effect is demonstrated.

Universality class change from Mean-Field to 3D-Heisenberg in magnetocaloric compounds [formula omitted][formula omitted

We report a study of universality class change from Mean-Field to 3D-Heisenberg in magnetocaloric compounds SmNi3-xFex. Three conventional techniques are used to identify the universal classes: the Modified Arrott plot, Kouvel-Fisher plot, and critical isotherm technique. The calculated critical exponents were confirmed and the isotherm M(H) curves in paramagnetic state and ferromagnetic state fall in two separated universal branches. The exponents found are similar to those determined from the renormalization group approach for a heuristic mean-field (d=3,n=3) and 3D-Heisenberg (d=3,n=3) model for SmNi3 and SmNi2.2Fe0.8, respectively.

Physical properties of double-doping Lanthanum manganite for bolometer applications

We have studied the structural, magnetic, critical behavior and electrical properties of La0.5Ag0.1Ca0.4MnO3 sample synthesized by traditional Solid State route. X-ray diffraction studies reveal that La0.5Ag0.1Ca0.4MnO3 crystallizes in the orthorhombic structure with Pnma space group. Vibrational study by means of infrared spectroscopy each give a part of the vibrational reality of the material explored. Studying the magnetic and electric properties of La0.5Ag0.1Ca0.4MnO3, it has been found the compound exhibits a second-order phase transition around the ferromagnetic-paramagnetic transition. The critical behavior of La0.5Ag0.1Ca0.4MnO3 compound is investigated based on the data of static magnetic measurements in the vicinity of its critical temperature TC. The estimated critical exponents derived from the magnetic data were estimated using modified Arrott plot (MAP) and critical isotherm technique (CI). The critical exponents find are comparable to the values theoretical predicted for the values corresponding to the 3D-Ising model and are confirmed by scaling equations of state. The investigated compound exhibits a semiconductor-metal transition at 380 K. For the important value of TCR at zero magnetic fields, we consider our sample is a good candidate infrared sensor for night vision bolometer applications.

Study of Critical Behavior and Magnetocaloric Effect in Nd1− x SrxMnO3 Compounds

In this paper, we have investigated the influence of substitution of Nd by Sr on the structural, magnetic, and magnetocaloric properties of Nd1 − xSrxMnO3 (x = 0.3 and x = 0.4). These compounds were prepared using the solid state method. The structural, morphological, and magnetic properties of our system were characterized by XRD, MEB techniques, and a magnetometer for magnetic characterization. The Rietveld refinement has revealed the coexistence of both Pnma orthorhombic and R-3c rhombohedral phases. The magnetic data indicate that the compounds exhibit a continuous paramagnetic (PM) to ferromagnetic (FM) phase transition. In addition, the critical behavior in both compounds associated with the magnetic phase transition has been studied by the magnetization isotherms. The critical exponents are deduced using various techniques such as the modified Arrott plot, Kouvel–Fisher plot, and critical isotherm technique. Finally, the results obtained of the magnetocaloric effect are in agreement with the reported values of previous researches. These results show that our samples can be used as a magnet refrigerant at room temperature.

Critical behavior and magnetocaloric effect in La0.67Sr0.33MnO3.01 compounds

The polycrystalline La0.67Sr0.33MnO3.01 sample was synthesized by standard solid state technique. Crystal structure of sample belongs to rhombohedral system with space group R3−c. The parameters are a = 0.55003(2) and c = 1.33753(4) nm for indexing in hexagonal system and a′ = 0.54689 nm and α = 60.38° for indexing in rhombohedral system. In order to obviously reveal the magnetic interaction we study the critical behavior by the modified Arrott plot, the Kouvel-Fisher method and the critical isotherm technique. Three main critical exponents, β, γ and δ, are derived from different three methods mentioned above. The three critical exponents obey the Widom relation and are able to collapse the different isothermal magnetization data into two branches below and above TC based on the single scaling equation M(H, ε) = εβf±(H/εβ+γ). The critical exponents, β and γ, are on the verge of the theoretical prediction of a mean-field model, implying the existence of long-range magnetic interaction with magnetic exchange distance decaying as J(r) ≈ r−4.57. The exponent n in the power relation ΔSm∝Hn dependence of temperature increases with temperature decreasing and approaches to 1 well below TC and increases with temperature increasing and approaches to 2 well above TC. Near TC the n = 0.669 is very close to 2/3. These data support that the magnetic interaction can assign to mean-field model. The RCP increases with applied magnetic field and is about 40 J/kg under 1.2 T applied field.

Griffiths-like phase, critical behavior near the paramagnetic-ferromagnetic phase transition and magnetic entropy change of nanocrystalline La0.75Ca0.25MnO3

In this work, we report the structural and magnetic properties of La0.75Ca0.25MnO3 nanoparticles synthesized by the sol-gel route. Rietvield refinement of X-ray powder diffraction confirms that our sample is single phase and crystallizes in orthorhombic system with Pnma space group. The facts that effective magnetic moment is large and the inverse susceptibility deviates from the Curie Weiss lawn indicate the presence of Griffiths-like cluster phase. The critical exponents have been estimated using different techniques such as modified Arrott plot, Kouvel-Fisher plot and critical isotherm technique. The critical exponents values of La0.75Ca0.25MnO3 are very close to those found out by the mean-field model, and this can be explained by the existence of a long-range interactions between spins in this system. These results were in good agreement with those obtained using the critical exponents of magnetic entropy change. The self-consistency and reliability of the critical exponent was verified by the Widom scaling law and the universal scaling hypothesis. Using the Harris criterion, we deduced that the disorder is relevant in our case. The maximum magnetic entropy change (ΔSM) calculated from the M-H measurements is 3.47 J/kg K under an external field change of 5 T. The ΔSM-T curves collapsed onto a single master curve regardless of the composition and the applied field, confirming the magnetic ordering is of second order nature. The obtained result was compared to ones calculated based on the Arrott plot and a good concordance is observed. Moreover, the spontaneous magnetization obtained from the entropy change is in excellent agreement with that deduced by classically extrapolation the Arrott curves. This result confirms the validity of the estimation of the spontaneous magnetization using the magnetic entropy change.

Influence of grain size and sintering temperature grain size on the critical behavior near the paramagnetic to ferromagnetic phase transition temperature in La0.67Sr0.33MnO3 nanoparticles

We have undertaken a systematic study of critical behavior in La0.67Sr0.33MnO3 nanoparticles, sintered at different temperatures (L6, L8, L10 and L12 sintered at 600 °C, 800 °C, 1000 °C, 1200 °C respectively), by magnetization measurements. The critical exponents are estimated by various techniques such as the Modified Arrott plot, Kouvel–Fisher plot and critical isotherm technique. Compared to standard models, the critical exponents are close to those expected by the Mean-field model (with β = 0.5 γ = 1, and δ = 3) for (L6, L8, and L10) samples and by the (3D) Heisenberg model (β = 0.365, γ = 1.336 and δ = 4.80) for L12 sample. We conclude that the reduction of grain size strongly influences the universality class.

Effect of sodium deficiency on the critical behavior near the paramagnetic to ferromagnetic phase transition temperature in La0.8Na0.2−x□xMnO3 oxides

The critical behavior associated with the magnetic phase transition has been investigated by in La0.8Na0.2−x□xMnO3 (0.00≤x≤0.15) (□ is the sodium deficiency). The critical exponents are estimated by various techniques such as the modified Arrott plot, Kouvel–Fisher plot and critical isotherm technique. Compared to standard models, the critical exponent values determined in our work are close to those expected by the tricritical model (β=0.25, γ=1, and δ=5) for x=0.00 and 0.05 samples and by mean field theory (β=0.5, γ=1, and δ=3) for x=0.10 and 0.15 samples. We conclude that the Na deficiency rate affect the critical exponents.

Enhanced magnetocaloric properties and critical behavior of (Fe0.72Cr0.28)3Al alloys for near room temperature cooling

Magnetic cooling is an environmentally friendly, energy efficient, thermal management technology relying on high performance magnetocaloric materials (MCM). Current research has focused on low cost, corrosion resistant, rare earth (RE) free MCMs. We report the structural and magnetocaloric properties of novel, low cost, RE free, iron based (Fe0.72Cr0.28)3Al alloys. The arc melted buttons and melt spun ribbons possessed the L21 crystal structure and B2 crystal structure, respectively. A notable enhancement of 33% in isothermal entropy change (−ΔSm) and 25% increase in relative cooling power (RCP) for the ribbons compared to the buttons can be attributed to higher structural disorder in the Fe–Cr and Fe–Al sub-lattices of the B2 structure. The critical behavior was investigated using modified Arrott plots, the Kouvel–Fisher plot and the critical isotherm technique; the critical exponents were found to correspond to the short-range order 3D Heisenberg model. The field and temperature dependent magnetization curves of (Fe0.72Cr0.28)3Al alloys revealed their soft magnetic nature with negligible hysteresis. Thus, these alloys possess promising performance attributes for near room temperature magnetic cooling applications.

Prediction of Magnetocaloric Effect by a Phenomenological Model and Critical Behavior for La0.78Dy0.02Ca0.2MnO3 Compound

The La0.78Dy0.02Ca0.2MnO3 (LDCMO) compound prepared via high-energy ball-milling (BM) presents a ferromagnetic-to-paramagnetic transition (FM-PM) and undergoes a second-order phase transition (SOFT). Based on a phenomenological model, magnetocaloric properties of the LDCMO compound have been studied. Thanks to this model, we can predict the values of the magnetic entropy change ΔS, the full width at half-maximum δ T FWHM, the relative cooling power (RCP), and the magnetic specific heat change ΔC p for our compound. The significant results under 2 T indicate that our compound could be considered as a candidate for use in magnetic refrigeration at low temperatures. In order to further understand the FM-PM transition, the associated critical behavior has been investigated by magnetization isotherms. The critical exponents estimated by the modified Arrott plot, the Kouvel–Fisher plot, and the critical isotherm technique are very close to those corresponding to the 3D-Ising standard model (β = 0.312 ± 0.07, γ = 1.28 ± 0.02, and δ = 4.80).Those results revealed a long-range ferromagnetic interaction between spins.

Anisotropic magnetic coupling with a two-dimensional characteristic in noncentrosymmetric Cr11Ge19.

In this work, we successfully synthesize the single crystal Cr11Ge19. The magnetism of the noncentrosymmetric Cr11Ge19 with itinerant ferromagnetic ground state is thoroughly investigated on the single crystal. Based on the variation measurements including the angular rotation, temperature, and magnetic field dependence of magnetization, we find that this material exhibits strong magnetic anisotropy along the c-axis. To clearly reveal the magnetic interactions, the critical behavior is studied using the modified Arrott plot, the Kouvel-Fisher method, and the critical isotherm technique. Combining these different methods, three main critical exponents (β, γ, and δ) are obtained. The critical exponent β is close to the theoretical prediction of a three-dimensional XY model with spin-dimensionality n = 2, indicating two-dimensional magnetic coupling. Meanwhile, the critical exponent γ suggests that the magnetic interaction is of long-range type with magnetic exchange distance decaying as J(r) ≈ r−4.61. We propose that the ferromagnetic ground state of Cr11Ge19 is formed by the polarized magnetic moments along the c-axis, while the long-range magnetic coupling is established within the ab plane.

Short-range ferromagnetic order in perovskite manganite La0.62Er0.05Ba0.33Mn0.95Fe0.05O3

The critical phenomena of a high quality La0.62Er0.05Ba0.33Mn0.95Fe0.05O3 sample are comprehensively studied using dc magnetometry in the vicinity of paramagnetic to ferromagnetic second order phase transition. The critical exponents are estimated through various techniques such as Modified Arrott plot, Kouvel–Fisher plot and critical isotherm technique. In comparison with the values given by standard models, the critical exponents β, γ and δ in our case are close to those expected from the three-dimensional Ising model (β = 0.322 ± 0.004, γ = 1.175 ± 0.002 and δ = 4.415 ± 0.02 at an average TC = 256.53±0.06 K). This proves the existence of a short-range ferromagnetic order in La0.62Er0.05Ba0.33Mn0.95Fe0.05O3. Thus, the scaling law γ + β = δβ is fulfilled. With these critical exponents, the isothermal magnetization data around TC fall into two branches of a universal function M(H, ε) = |ε|β f±(H/|ε|β+γ) where ε = (T−TC)/TC is the reduced temperature.

Critical behavior and magnetocaloric effect in Co50−xNixCr25Al25 (x = 0 and 5) full Heusler alloy system

This work reports the investigation of critical behavior of Co50−xNixCr25Al25 (x=0 and 5) and magneto caloric effect (MCE) of bulk Co2CrAl full Heusler alloy system. The alloy series of Co50−xNixCr25Al25 (x=0, 1, 2, 3, 4 and 5) have been prepared using arc melting technique. The magnetic properties of all the samples have been studied in the temperature range of 5–300K. The value of Curie temperature (TC) is found to decrease with increasing doping concentration of the Ni (substitution of Ni at Co site). The critical exponents behavior and scaling relation have been investigated using magnetic isotherms in Co50−xNixCr25Al25 (x=0 and 5) alloys. The critical exponents are estimated by various techniques such as, Modified Arrott plot, Kouvel–Fisher plot and critical isotherm technique. The value of critical exponents vicinity to the second order magnetic phase transition of Co50Cr25Al25 were found to be β=0.488 (7), γ=1.144 (16) and δ=3.336 (5) with TC=328.64 (5)K whereas for Co50Ni5Cr25Al25 the values are β=0.522 (13), γ=1.014 (6) and δ=3.043 (7) with TC=285.71 (11). The critical exponent values for both the samples are almost similar to the value as predicted by mean field theory. This has been best explained by long range mean field like ferromagnetic interaction in the entire Co50−xNixCr25Al25 (x=0, 5) highly spin polarized full Heusler alloy system. Under an external magnetic field maximum up to 5T, normal magneto caloric effect (MCE) have been observed with a maximum magnetic entropy change of 2.55J/kgK at 328.5K (near the ferromagnetic→paramagnetic second order magnetic phase transition in the austenite phase) in Co2CrAl full Heusler alloy system.

Size reduction effect on the critical behavior near the paramagnetic to ferromagnetic phase transition temperature in La0.9Sr0.1MnO3 nanoparticles

The critical behavior of La0.9Sr0.1MnO3 nanoparticles, annealed at different temperatures (H6, H8, H10 and H12 annealed at 600°C, 800°C, 1000°C, 1200°C, respectively), has been investigated by magnetization measurements. Indeed, the magnetic data indicate that the compound exhibits a continuous (second-order) paramagnetic (PM) to ferromagnetic (FM) phase transition. The critical exponents are estimated by various techniques such as the Modified Arrott plot, Kouvel–Fisher plot and critical isotherm technique. Compared to standard models, the critical exponent values determined in our work are close to those expected for the mean-field model (with β=0.5, γ=1, and δ=3) (H8, H10, and H12). Concerning the sample having a smaller crystallite size (H6), the obtained values of the critical exponents β and δ are similar to those predicted by the mean-field model. However, the value of γ shows a (3D) Heisenberg model-like. This behavior, which is quite new and surprising, shows that the reduction of grain size strongly influences the universality class. Moreover, the decrease of the critical exponents (β, γ, δ) with the increase of grain size has been explained by crossover phenomenon. This result and the other obtained values are explained taking into account the contribution of uncompensated spins at the surface, strain anisotropies, and noncollinear magnetic ordering.

Short-range ferromagnetic order in La0.67Sr0.16Ca0.17MnO3 perovskite manganite

The critical phenomena of a high quality La0.67Sr0.16Ca0.17MnO3 sample are comprehensively studied using dc magnetometry in the vicinity of paramagnetic to ferromagnetic second order phase transition. The critical exponents are estimated by various techniques such as the Modified Arrott plot, Kouvel–Fisher plot and critical isotherm technique. In comparison with the values given by standard models, the critical exponents β, γ and δ in our case are close to those expected from the three-dimensional Ising model (β=0.324±0.005, γ=1.176±0.03, and δ=4.724±0.03 at an average TC=333.18±0.2K). This proves the existence of a short-range ferromagnetic order in La0.67Sr0.16Ca0.17MnO3. Thus, the scaling law γ+β=δβ is fulfilled. With these critical exponents, the isothermal magnetization data around TC fall into two branches of a universal function M(H,ε)=|ε|βf±(H/|ε|β+γ) where ε=(T-TC)/TC is the reduced temperature.

Critical phenomena in Pr0.52Sr0.48MnO3 single crystal

Abstract The critical phenomena are comprehensively studied using dc magnetometry in the vicinity of paramagnetic to ferromagnetic second order phase transition for a high quality of Pr0.52Sr0.48MnO3 single crystal. The critical exponents obtained from the modified Arrott plot, Kouvel–Fisher plot and critical isotherm technique are found to be reasonably close, which do not correspond to any known universality class. The critical exponents are dependent on the direction of magnetic field. This direction dependent anisotropy is compatible with the 2D Heisenberg type character of the magnetic interaction as evident from the renormalization group theory approach.

Effect of calcium deficiency on the critical behavior near the paramagnetic to ferromagnetic phase transition temperature in La0.8Ca0.2MnO3 oxides

The critical behavior associated with the magnetic phase transition has been investigated by magnetization isotherms in La0.8Ca0.2MnO3 and La0.8Ca0.1□0.1MnO3 (□ is the calcium deficiency). The critical exponents are estimated by various techniques such as the Modified Arrott plot, Kouvel–Fisher plot and critical isotherm technique. The critical exponents values for La0.8Ca0.2MnO3 are very close to those found out by the 3D-Ising model (β=0.328, γ=1.180, and δ=4.826 at an average TC=181.676K). Moreover, the estimated critical exponents of La0.8Ca0.1□0.1MnO3 are consistent with the prediction of the 3D-Heisenberg model (β=0.357, γ=1.167, and δ=4.802 at an average TC=178.182K). We noted that the critical exponents γ are almost similar to the value of the mean-field theory which can be explained by the existence of a long-range dipole–dipole interaction. Following the Harris criterion, we deduced that the disorder in our case is relevant, which can be the cause of the change in the universality class.

Investigation of weak itinerant ferromagnetism and critical behavior of Y2Ni7

The weak itinerant ferromagnetic behavior of Y$_2$Ni$_7$ is investigated through magnetic, transport and calorimetric measurements. The low value of saturation moment, large Rhodes-Wohlfarth ratio, large value of the linear term in heat capacity and Fermi liquid like resistivity behavior with enhanced electron-electron scattering contribution establish firmly the weak itinerant ferromagetic nature of the sample. The critical exponents associated with the paramagnetic to ferromagnetic transition are also investigated from magnetization isotherms using modified Arrott plot, Kouvel-Fisher plot and critical isotherm technique. The more accurate Kouvel-Fisher plot provides the critical exponents to be $\beta$ = 0.306, $\gamma$ = 1.401 and $\delta$ = 5.578. These values are markedly different from the mean field values and correspond closely to the two dimensional Ising spin system with long range spin spin interaction.