Field Dependence Of Magnetocaloric Effect Research Articles

Magnetocaloric properties of a ribbon sample of Heusler alloy Ni-=SUB=-45-=/SUB=-Co-=SUB=-5-=/SUB=-Mn-=SUB=-31-=/SUB=-Al-=SUB=-19-=/SUB=-: experimental and theoretical studies

The results of experimental and theoretical studies of magnetocaloric properties of a ribbon sample of alloy Ni45Co5Mn31Al19 in the range of T=80-350 K in magnetic fields of up to 8 T are given. This alloy demonstrates a first-order magnetostructural phase transition (MSPT) in the temperature range of 270 K, as well as a second-order transition --- at the Curie temperature of 294 K. The magnetocaloric effect (MCE) was studied both by the direct method of magnetic field modulation in cyclic fields up to 8 T and by the classical extractive method. Field dependence of MCE have a different nature for first- and second-order phase transitions. A reverse MCE near MSPT is irreversible, i.e. the final sample temperature is 0.75 K below the initial one. Theoretical studies of magnetic properties and MCE of the studied sample were performed by ab initio calculations and Monte Carlo modeling. Theoretical temperature dependences of MCE are characterized by a similar interval of effect manifestation in the region of martensitic transformation and a narrower interval in the region of austenite Curie temperature as compared to the experiments, which is conditioned by the presence of a heterogeneous mixed state of austenite in the experimental sample. On the whole, theoretical data qualitatively and quantitatively reproduce the experimental dependences. Keywords: magnetocaloric effect, cyclic fields, Heusler alloy, Monte Carlo method.

The role of Ni in modifying the order of the phase transition of La(Fe,Ni,Si)13

In this work, we study the effect of Ni substitution on the magnetocaloric properties of La(Fe,Si)13 compounds. Sample quality has been optimized by a combination of induction melting and suction casting techniques, which allowed to shorten the annealing time by an order of magnitude and expand the existing range of LaFe11.6-xNixSi1.4 phase with cubic NaZn13-type structure up to x = 0.4. According to our Density Functional Theory calculations, Ni addition does not significantly alter the crystal structure (finding that Ni atoms occupy preferably 96i positions) as well as the total magnetic moment; both predictions are in agreement with experimental results. With increasing of Ni concentration, the transition temperature increases and the order of the phase transition changes from first to second-order type. In addition, we show that the magnetic field dependence of magnetocaloric effect enables a clear analysis of the order of phase transition even for compositions near the critical point, surpassing the accuracy of conventionally used techniques for determining the order of magnetic phase transitions (e.g. Banerjee's criterion).

Linear dependence of magnetocaloric effect on magnetic field in Mn0.6Fe0.4NiSi0.5Ge0.5 and Ni50Mn34Co2Sn14 with first-order magnetostructural transformation

The study on the field dependence of magnetocaloric effect (MCE) is considered to be of fundamental and practical importance, since it not only guides us in understanding and optimizing the MCE, but also helps us estimate the MCE for higher magnetic field which is not available in some laboratories. The magnetic field (0H) dependence of magnetic entropy change (△SM) has been studied extensively in many materials with second-order magnetic transition. However, the field dependence of MCE for first-order magnetic transition (FOMT) materials has not been sufficiently studied due to their complexity and diversity. In the present work, polycrystalline Mn0.6Fe0.4NiSi0.5Ge0.5, Ni50Mn34Co2Sn14, and LaFe11.7Si1.3 compounds with FOMT are prepared, and the magnetic and magnetocaloric properties are investigated systematically. In order to avoid a spurious △SM, the M-0H curves are measured in a loop process. The M-0H curves are corrected by taking into account the demagnetization effect, i.e. Hint=Hext-NdM. It is found that the -△SM follows a linear relationship -△SM=-△S0 +0H with the variation of magnetic field in Mn0.6Fe0.4NiSi0.5Ge0.5 compound when 0H 1 T. In addition, it is also noted that the △SM is approximately proportional to the square of 0H at low field. The origin of this linear relationship between △SM and 0H at high field and the deviation at low field are discussed by numerically analyzing the Maxwell relation. In addition to the △SM peak value, it is found that other △SM values at different temperatures also follow the linear relation at high field by performing the same numerical analysis. Moreover, it is found that the fitted △SM curve matches the experimental data very well. This result indicates that the linear relationship between △SM and 0H could be utilized to predict the △SM for higher magnetic field change when the field is lower than the saturation field. The applicability of this linear relationship is also verified in other systems with first-order magnetostructural transformation, such as Ni50Mn34Co2Sn14. However, it fails to describe the field dependence of △SM in LaFe11.7Si1.3, which exhibits a strong field dependence of transition temperature. Consequently, our study reveals that a linear dependence of △SM on 0H could occur in magnetostructural transition materials, which show the field independence of transition temperature.

Structural characterization, magnetic, magnetocaloric properties and critical behavior in lacunar La0.5Eu0.2Ba0.2□0.1MnO3 nanoparticles

The lacunar La0.5Eu0.2Ba0.2□0.1MnO3 nanoparticles where □ is a barium deficiency has been prepared using the sol-gel method. The M-T data suggests that the sample undergoes a paramagnetic (PM) to ferromagnetic (FM) transition upon cooling. Deviation of the temperature dependence of inverse susceptibility curves from the Curie Weiss law and less than unity susceptibility exponents indicating the existence of a Griffiths singularity. The magnetocaloric effect and the critical behavior of the sample are systematically studied using bulk magnetization M (H, T) in the vicinity of the Curie temperature. The critical exponents have been obtained using different methods such as modified Arrott plot, critical isotherm analysis and field dependence of magnetocaloric effect. Though the self-consistency and reliability of obtained critical exponents of β = 0.581 ± 0.022, γ = 0.165 ± 0.008 and δ = 3.12 ± 0.03 are verified by the Widom scaling law and scaling equations, the critical parameters do not obey magnetocaloric effect scaling laws, indicating that the nanometer size effect play a significant role on the magnetic phase transition for La0.5Eu0.2Ba0.2□0.1MnO3 nanocrystalline.

Normal and inverse magnetocaloric effect and short-range ferromagnetic interaction in (Pr,Sm)0.5Sr0.5MnO3 phase separated manganite

The dependence of critical phenomenon on disorder induced by R/A-site cational size mismatch in polycrystalline manganite Pr0.5-xSmxSr0.5MnO3 (x = 0.25) has been investigated by means of bulk magnetization M (H, T) in the vicinity of the ferromagnetic-paramagnetic phase-transition. The critical exponents characterizing this second order phase transition, have been obtained using different methods such as modified Arrott plot, Kouvel-Fisher plot, critical isotherm analysis and field dependence of magnetocaloric effect. Moreover, the self-consistency and reliability of obtained critical exponents are verified by the Widom scaling law and scaling equations. We demonstrate that at x = 0.25 doping level, the β value match well with those theoretically predicted for the 3D-Heisenberg model, while the γ value is close to that expected for the 3D-Ising model. This suggests that the presence of R/A-site disorder in the system must be taken into account to fully describe the magnetism of phase separated manganites. Hence, the short-range ferromagnetic originates from phase inhomogeneity and evolves with addition of disorder depending on Sm content. Moreover, this sample also shows a coexistence of normal and inverse magnetocaloric effect.

Magnetocaloric effect and critical behavior in Mn2-imidazole-[Nb(CN)8] molecular magnetic sponge

A comprehensive study of magnetocaloric effect (MCE) and critical behavior in the {Mn2(imH)2(H2O)4[Nb(CN)8]·4H2O}n molecular magnet is reported. The compound is an example of a magnetic sponge, where structural changes provoked by dehydration process lead to the increase of Tc critical temperature from 25K for the as-synthesized sample (1) up to 60K for the anhydrous one (2). MCE and critical behavior were investigated by magnetization measurements. The maximum value of magnetic entropy change ΔS, determined by the magnetization measurements for 1 is 6.70Jmol−1K−1 (8.95Jkg−1K−1) at µ0ΔH=5T, while for 2 it is equal to 4.02Jmol−1K−1 (7.73Jkg−1K−1) at the same magnetic field change. The field dependence of MCE at Tc for 1 and 2 was consistent with critical exponents, which allowed to classify both phases to 3D Heisenberg universality class. The Tc-2/3 dependence of the maximum entropy change has been tested using data of 1 and 2 together with MCE data previously reported for other members of the ferrimagnetic Mn2-l-[Nb(CN)8] (L=imidazole, pyridazine and pyrazole) series. Experimental MCE results have been compared with the spin contribution to the magnetic entropy change estimated using a molecular field approximation.

Magnetocaloric Effect in a Mn2‐Pyridazine‐[Nb(CN)8] Molecular Magnetic Sponge

AbstractA comprehensive study of the magnetocaloric effect (MCE) in the Mn2‐[Nb(CN)8] molecular magnet with a two‐step magnetic sponge behavior is reported. The structural transformations provoked by dehydration bring about an increase in the magnetic ordering temperature (Tc) from 43 K through to 68 K and up to 98 K. All three phases are soft isotropic ferrimagnets. The change in magnetic entropy in the partially and fully dehydrated phases is smaller than that in the “as synthesized” form by a factor of 0.6. The field dependence of MCE at Tc is consistent with the values of the critical exponents derived from the magnetization data.

Anomalous magnetic field dependence of magnetocaloric effect at low temperature in Pr0.52Sr0.48MnO3 single crystal

We report the magnetocaloric effect (MCE) in a Pr0.52Sr0.48MnO3 single crystal. A peak in the temperature dependence of magnetic entropy change (ΔSM) with a fairly large negative value (≈3.8 J/kg K) is observed at 275 K close to Curie temperature. MCE is spread over a wide temperature range resulting in a considerable refrigerant capacity (≈293 J/kg). At low temperature the sign of ΔSM changes, below which anomalous field dependence of ΔSM is observed indicating the coexistence of ferromagnetic and antiferromagnetic interactions. Interplay between the interactions is strongly influenced by direction and magnitude of applied magnetic field in the ordered state.