Second-order Magnetic Phase Transition Research Articles

Analysis based on scaling relations of critical behaviour at PM-FM phase transition and universal curve of magnetocaloric effect in selected Ag-doped manganites.

The critical behaviour of Pr0.5Sr0.5−xAgxMnO3 (0 ≤ x ≤ 0.2) samples around the paramagnetic–ferromagnetic phase transition is studied based on isothermal magnetization measurements. The assessments based on Banerjee's criteria reveal the samples undergoing a second-order magnetic phase transition. Various techniques such as modified Arrott plot, Kouvel–Fisher method, and critical isotherm analysis were used to determine the values of the ferromagnetic transition temperature TC, as well as the critical exponents of β, γ and δ. The values of critical exponents, derived from the magnetization data using the Kouvel–Fisher method, are found to be (β = 0.43 ± 0.002, 0.363 ± 0.068 and 0.328 ± 0.012), (γ = 1.296 ± 0.007, 1.33 ± 0.0054 and 1.236 ± 0.012) for x = 0.0, 0.1 and 0.2, respectively. This implies that the Pr0.5Sr0.5−xAgxMnO3 with 0 ≤ x ≤ 0.2 systems does not belong to a single universality class and indicates that the presence of magnetic disorder in the system must be taken into account to fully describe the microscopic interaction of these manganites. With these values, magnetic-field dependences of magnetization at temperatures around TC can be well described following a single equation of state for our samples. From magnetic entropy change (ΔSM), it was possible to evaluate the critical exponents of the magnetic phase transitions. Their values are in good agreement with those obtained from the critical exponents using a modified Arrott plot (MAP). We used the scaling hypotheses to scale the magnetic entropy change and heat capacity changes to a universal curve respectively for Pr0.5Sr0.5−xAgxMnO3 samples.

Structural, magnetic and magnetocaloric properties of HoNi2 and ErNi2 compounds ordered at low temperatures

In this work we report on the structural, magnetic and magnetocaloric properties of polycrystalline HoNi2 and ErNi2 Laves-phase compounds. Powder X-ray diffraction studies at room temperature show that the compounds crystallize in the cubic C15 structure (MgCu2 type). The magnetic measurements reveal that the second-order magnetic phase transition from ferromagnetic to paramagnetic state appears at TC equal 13.5 K for HoNi2 and 6.5 K for ErNi2. Using direct measurements over a wide field range (up to 11 T), along with indirect methods, we have found low and high field regularities of the magnetocaloric effect characterized by the adiabatic temperature change, ΔTad, and the isothermal magnetic entropy change, ΔSmag. The magnetocaloric properties near the phase transition are discussed in the framework of the Landau theory for the second-order phase transitions.

Structural, magnetic and electrical properties of a new double-perovskite LaNaMnMoO6 material.

Structural, magnetic, magnetocaloric, electrical and magnetoresistance properties of an LaNaMnMoO6 powder sample have been investigated by X-ray diffraction (XRD), magnetic and electrical measurements. Our sample has been synthesized using the ceramic method. Rietveld refinements of the XRD patterns show that our sample is single phase and it crystallizes in the orthorhombic structure with Pnma space group. Magnetization versus temperature in a magnetic applied field of 0.05 T shows that our sample exhibits a paramagnetic–ferromagnetic transition with decreasing temperature. The Curie temperature TC is found to be 320 K. Arrott plots show that all our double-perovskite oxides exhibit a second-order magnetic phase transition. From the measured magnetization data of an LaNaMnMoO6 sample as a function of the magnetic applied field, the associated magnetic entropy change |−ΔSM| and the relative cooling power (RCP) have been determined. In the vicinity of TC, |−ΔSM| reached, in a magnetic applied field of 8 T, a maximum value of ∼4 J kg−1 K−1. Our sample undergoes a large magnetocaloric effect at near-room temperature. Resistivity measurements reveal the presence of an insulating-metal transition at Tρ = 180 K. A magnetoresistance of 30% has been observed at room temperature for 6 T, significantly larger than that reported for the A2FeMoO6 (A = Sr, Ba) double-perovskite system.

A-site deficiency effects on the critical behavior of La0.6Ca0.15·0.05Ba0.2MnO3

The aim of the present work is to study the critical behavior of calcium deficient La0.6Ca0.15·0.05Ba0.2MnO3 (LCBMO), synthetized by the conventional solid-state reaction method, around the paramagnetic (PM)-ferromagnetic (FM) phase transition. X-ray diffraction revealed that these manganites crystallized in the orthorhombic structure with Pbnm space group. Then, the magnetic properties of this compound are discussed in detail, building on the magnetization and the susceptibility. The temperature dependence of magnetic susceptibility at higher temperature confirms the presence of the Griffiths phase above the Curie temperature which proves the existence of ferromagnetic clusters in the paramagnetic domain. Experimental results revealed that our sample exhibit a second-order magnetic phase transition. The estimated critical exponents derived from the magnetic data were estimated using various techniques such as modified Arrott plot, Kouvel-Fisher method, and critical magnetization isotherms M(TC, H). The obtained values are very close to those representative of the mean-field model (β = 0.547, γ = 1.23, and δ = 3.092 at an average TC = 201.74 K).

Direct measurement of the magnetocaloric effect (ΔTad) of Mn5−x (Fe,Co)xGe3

Magnetic field–induced adiabatic temperature change, ΔTad, measures the temperature gradient that enables a magnetic refrigerant to transfer heat from reservoirs. The ΔTad of Mn5Ge3 was evaluated by directly recording the temperature of a Mn5Ge3 sample while subjecting it to an external field. The peak ΔTad of 0.89 ± 0.006 at 0.8 T for Mn5Ge3 at its Curie temperature was consistent with the value previously obtained from specific heat capacity measurements. In addition, the peak values of ΔTad for Mn4.75Co0.25Ge3 and Mn4.75Fe0.25Ge3 were measured to be 0.77 ± 0.009 and 0.93 ± 0.011, respectively, at 0.8 T near their respective Curie temperatures. ΔTad measurements under cyclic application of a 0.8 T field showed that the alloys, whose magnetocaloric effect stems from the second-order magnetic phase transition, exhibited no hysteresis in ΔTad and maintained their peak ΔTad within 1.3% under cyclic field. The ΔTad of sandwiched Mn4.75Co0.25Ge3 and Mn4.75Fe0.25Ge3 plates was also measured to demonstrate that a combination of the alloys can be used to form a graded regenerator for the active magnetic regenerator cycle.

Potassium doping induced changes of magnetic and magnetocaloric properties of La0.78Cd0.22−xKxMnO3 (x = 0.00, 0.10, 0.15 and 0.20) manganites

In this paper, we have studied the effect of K substitution on the structural, magnetic and magnetocaloric (MCE) properties of the perovskite manganites La0.78Cd0.22−xKxMnO3 (x=0.00, 0.10, 0.15 and 0.20) which were prepared by standard solid-state reaction method. The phase purity of prepared samples were verified by analyzing the X-ray diffraction patterns and shows that all compositions crystallize in the rhombohedral structure with the R3‾c space group. The zero field cooled (ZFC) and field cooled (FC) magnetization measurements reveal that all the samples undergo a paramagnetic-ferromagnetic (PM-FM) phase transition and the transition temperature is found to increase from 202K for x=0.00 to 326K for x=0.20. Magnetic entropy change (ΔSM) was also studied which reaches a maximum values of 3.6, 4.86, 5.66 and 5.98J/kgK at 5T for x=0.00, 0.10, 0.15 and 0.20 composition, respectively. However, the relative cooling power (RCP) decreases with increasing K content from 274 (x=0.00) to 216 (x=0.20) J/kg for 5T due to the decrease of full width at half maximum of ΔSM curves with increasing K dopant. All the studied samples exhibit a second-order magnetic phase transition at TC. The field dependence of the magnetic entropy change curves show a power law dependence, ΔSMmax∞Hn, around the TC. The obtained n values are 0.89, 0.82, 0.86 and 0.75 for x=0.00, 0.10, 0.15 and 0.20 respectively, confirms not only the invalidity of the mean field model. Thus, the calculated ‘n’ values corroborate the absence of long range magnetic order and suggest the existence of magnetic inhomogeneities in all the samples reported here.

Direct Measurement of the Magnetocaloric Effect inLa(Fe,Si,Co)13Compounds in Pulsed Magnetic Fields

To bring magnetic refrigeration to market, superior magnetocaloric materials are still needed. In particular, it is important to tune alloy composition to promote abrupt, second-order magnetic phase transitions, for suitable cooling performance in a magnetic field that can vary as rapidly as 1000 T/s in a real device. Studying candidate quaternary alloys in very high pulsed magnetic fields, the authors find that magnetoelastic coupling determines the order of the transition, and that both candidates offer a large magnetocaloric response, which promises good performance under realistic conditions.

Improvement of magnetocaloric properties over a large temperature range in 0.5La0.7Ca0.2Sr0.1MnO3/0.5La0.7Ca0.15Sr0.15MnO3 composite

In the present research work, we investigate the magnetocaloric properties of a composite system by experiments and numerical calculations. The ability to adjust the magnetic entropy change in a large temperature range revealed to be possible by the mixture of both phases La0.7Ca0.2Sr0.1MnO3 and La0.7Ca0.15Sr0.15MnO3. Our composite 0.5La0.7Ca0.2Sr0.1MnO3/0.5La0.7Ca0.15Sr0.15MnO3 was synthesized using the solid–state reaction. Crystallographic structure and magnetocaloric properties were investigated by X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). Rietveld refinements of the X-ray diffraction patterns show that both samples are single phase and we have noticed a structural transition from orthorhombic structure with Pnma space group for La0.7Ca0.2Sr0.1MnO3 to rhombohedral structure with R-3C space group for La0.7Ca0.15Sr0.15MnO3. A second-order magnetic phase transition from ferromagnetic to paramagnetic state is observed and confirmed in terms of Landau theory. Therefore, two successive magnetic transitions in La0.7Ca0.2Sr0.1MnO3 and La0.7Ca0.15Sr0.15MnO3 resulting a nearly constant “table-like” magnetic-entropy change. Both peaks are partially overlapped with each other and induced an enhancement of the RCP in the composite when comparing with that of individual components. These results make that the proposed composite has substantial advantages for magnetic refrigeration.

Effect of Small Fe Content on the Structure, Magnetic and Magnetocaloric Properties of SmNi3−x Fe x (x = 0; 0.3 and 0.8) Intermetallic Compounds

The SmNi3−x Fe x (x = 0; 0.3 and 0.8) were synthesized by arc melting and annealed at 1073 K for 1 week. The Rietveld refinement showed that these compounds crystallize in the PuNi3-type structure ( $R\bar 3m$ -Space group) and the substitution of nickel by iron leads to an increase of the unit cell volume. The Curie temperature (T C ) of the SmNi3−x Fe x (x = 0; 0.3 and 0.8) samples increases from 60 K for SmNi3 to 230 K for SmNi2.2Fe0.8, the enhancement of T C is ruled by electronic and magnetovolumic effects. All the samples exhibit a second-order magnetic phase transition at Curie temperature T C . The magnetic entropy change ΔS M was estimated from isothermal magnetization curves and it decreases with the increase of Fe content from 2.3 J/kg.K (x = 0) to 0.6 J/kg.K (x = 0.8) at 50 kOe. The critical properties of SmNi2.7Fe0.3 (as an example) intermetallic compound at the ferromagnetic-paramagnetic (FM-PM) transition have been analyzed. The estimated critical exponents derived from the magnetic data using various methods such as modified Arrott plot, Kouvel-Fisher method and critical magnetization isotherms M(T C , H). The critical exponent values for the material are in a close agreement with those predicted for the mean-field theory. The reason for this behavior may be long-range interactions between spins in this system.

Effect of Sodium Doping on Magnetic and Magnetocaloric Properties of La0.65Sr0.35MnO3 Manganites

A series of Na-doped manganites La0.65Sr0.35−x Na x MnO3 (x = 0.20, 0.25, 0.30, and 0.35) were synthesized via sol-gel method. The structural, magnetic, and magnetocaloric properties of the samples were investigated by XRD, TEM, Raman spectroscopy, and magnetization measurement. Rietveld refinements of XRD patterns shown that samples were single phase with a space group R-3c. The variation of magnetization versus temperature in a magnetic applied field demonstrated a phase transition from ferromagnetic to paramagnetic. The value of T C decreased from 345 to 330 K when x increases from 0.20 to 0.35. Arrott plots shown that all samples exhibit a second-order magnetic phase transition. The maximum absolute values of the magnetic entropy change were found to be 0.91, 0.93, 1.12, and 1.10 J/kg for x = 0.2, 0.25, 0.3, and 0.35, respectively. Moreover, relative cooling power has been discussed.

Retraction Note: Magnetic properties and magnetocaloric effect of LaMnO3 nanoparticles prepared by using the sol-gel method

We synthesized LaMnO3 nanoparticles (NPs) with an average particle size of ~25 nm by using a sol-gel method with polyvinyl alcohol (PVA). X-ray diffraction studies revealed that the samples exhibited a rhombohedral single phase. The Curie temperature (T C) of the ferromagneticparamagnetic (FM-PM) phase transition increased from 218 K for PVA = 0 ml to 287 K for PVA = 5 ml. Concurrently, the magnetic properties of the sample prepared with PVA were improved remarkably. This was ascribed to the presence of excess oxygen which increased the Mn4+/Mn3+ ratio and the FM interactions in the NPs. Particularly, we found that the samples underwent a second-order magnetic phase transition at the T C. At H = 10 kOe, although their maximum magnetic-entropy change (ΔS m ) was small, 0.2 ~ 0.3 J/kg K, the linewidths of the |ΔS m (T)| curves were remarkably large, 50 ~ 70 K, leading to a quite large relative cooling power of ~17 J/kg.

Investigation of the magnetic, electronic and magnetocaloric properties of La0.7(Ca,Sr)0.3Mn1-xGdxO3 manganites

La0.7(Ca,Sr)0.3Mn1−xGdxO3 (with x=0; x=2% and x=6%) manganites samples have been studied by several techniques including X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), 55Mn and 139La nuclear magnetic resonance (NMR) spectroscopy and magnetometric measurements. The X-ray powder diffraction shows that all the prepared samples are single phase. La0.7Ca0.15Sr0.15MnO3 manganite is crystallized in the rhombohedral structure, whereas a structural transition towards orthorhombic system is observed for x≥2%. Magnetization as a function of temperature shows that all samples exhibit a ferromagnetic (FM)-paramagnetic (PM) phase transition. Gadolinium doping in Manganese site was found to induce a strong initial decrease of the Curie temperature TC. The single double-exchange-narowed 55Mn NMR line, though broadened by Gd, demonstrates a metallic state in all samples. The current manganites exhibit a second-order magnetic phase transition at TC, which is also confirmed by the criterion of Banerjee. In addition, the normalized entropy change curves of all the compounds collapse onto a universal master curve, a further signature of a second-order transition. The Gd-doped compounds undergo a large magnetocaloric effect, with an enhanced width TFWHM of the entropy change peak as compared to the Gd-free materials, and have consequently potential applications in magnetic refrigeration.

Entropy and magnetocaloric effect in ferrimagnets RCo2

The equations of state for the magnetic and elastic subsystems of a ferrimagnet are obtained in terms of the exchange–striction model. In the formulas derived for magnetic entropy, it is represented as the sum of the contributions of two magnetic sublattices of the ferrimagnet. One of the main characteristics of the magnetocaloric effect, viz., isothermal variation ΔS iso of the entropy in a magnetic field, is calculated for compounds RCo2 (R = Er, Ho, Dy) that experience a first-order magnetic phase transition and TbCo2 that experiences a second-order magnetic phase transition. It is shown that the calculated values of ΔS iso for these compounds are in satisfactory quantitative agreement with experimental results. The change in the entropy of a ferrimagnet in a strong magnetic field upon a transition from the ferrimagnetic to ferromagnetic ordering is calculated. The peculiarities in the magnetic field dependences of the magnetic entropy of a two-sublattice ferrimagnet are analyzed.

Magnetocaloric effect in Laves-phase rare-earth compounds with the second-order magnetic phase transition: Estimation of the high-field properties

High-field magnetocaloric effect of the rare-earth Laves-phase compounds, which show the second-order magnetic phase transition at low temperatures, has been studied theoretically and experimentally. Both direct and indirect methods are used to characterize the high-field properties of the adiabatic temperature change, ΔTad, and the magnetic entropy change, ΔSmag, for TbNi2 and DyNi2 compounds, which order ferromagnetically below 37 and 22 K, respectively. Experimental data are compared with theoretical results obtained in the frame of the microscopic model which takes into account the Zeeman exchange interaction and the crystal electric field anisotropy. The high-field magnetocaloric properties near the phase transition are also discussed in the framework of the Landau theory for the second-order phase transitions.

Effects of rare earth ions substitution on the magnetocaloric and critical behavior of La0.6A0.2Sr0.2MnO3 (A=Pr, Nd, Ce) manganite

We have studied the influence of Ce, Pr and Nd elements substitution on the magnetic and magnetocaloric properties of La0.6A0.2Sr0.2MnO3 (A = Pr, Nd, Ce) polycrystalline manganites. X-ray diffraction patterns show that the samples crystallize in the rhombohedral structure with R-3C space group. The Pr and Nd doped samples are single phase, while Ce doped sample has a CeO2 secondary phase. The temperature dependent magnetization exhibits a sharp paramagnetic (PM)–ferromagnetic (FM) transition at 333, 252 and 243 K for Ce, Pr and Nd doped samples, respectively. The results show that the saturation magnetization of the pure and Nd doped samples are higher than Pr doped sample, while the Ce doped sample exhibits lowest magnetization. The magnetic entropy change (ΔSM) was calculated by applying the Maxwell equation and the estimated values in H = 25 kOe were found to be 3.20, 2.63, 3.34 and 3.50 J/kgK for pure, Pr, Nd and Ce doped samples, respectively. Also, improved relative cooling power was observed for the Pr and Nd doped samples. By using three different methods including analyzing magnetization data using Banerjee's criterion, normalizing entropy change versus reduced temperature and using ΔSM(T) data which is related to the local exponent (nL), it is found that all samples display a second-order magnetic phase transition. The critical exponents of the second-order magnetic phase transition for the samples has been calculated, using scaling law and Widom scaling relation and their reliability has also been confirmed with the scaling hypothesis.

Structural characterization and magnetic field dependence of the magnetocaloric properties in Pr0.8Na0.05K0.15MnO3 ceramic

Pr0.8Na0.05K0.15MnO3 sample was prepared by solid-state reaction method. Refinement of the X-ray diffraction patterns shows that the sample is single phase and crystallizes in the orthorhombic structure with Pbnm space group. This result was confirmed by using the electron diffraction pattern for the sample using the [001] orientation. Analyzing temperature and field dependences of magnetization around the ferromagnetic-paramagnetic transition reveals the sample undergoing the second-order magnetic phase transition. The isothermal entropy changes ΔS as a function of temperature in different magnetic fields were determined. A phenomenological model is used to calculate magnetic and magnetocaloric properties of Pr0.8Na0.05K0.15MnO3 compound. The maximum values of ΔSmax and the relative cooling power (RCP) increase with increasing applied magnetic field. Landau theory and phenomenological universal curves of entropy change confirm the second order phase transition.

Magnetic and magnetocaloric properties of La0.6Ca0.4−xCexMnO3

La0.6Ca0.4−xCexMnO3 (x=0, 0.03, 0.06, and 0.09) compounds are fabricated by a solid-state reaction, and their structural, magnetic, and magnetocaloric properties are investigated. The Curie temperature at which a ferromagnetic–paramagnetic transition occurs decreases from 260 to 221K as x increases from 0 to 0.09. The saturation magnetization also decreases with the increase of x. The experimental results for the magnetization with respect to the temperature and magnetic field are analyzed using the Banerjee criterion, revealing that all the samples undergo the second-order magnetic phase transition. The maximum magnetic entropy change measured at a magnetic-field span of 50kOe, which occurs near the Curie temperature, slightly increases from 6.31 to 7.62J/kg K as x increases from 0 to 0.09.

Thermo-Magneto-Electric Generator Arrays for Active Heat Recovery System

Continued emphasis on development of thermal cooling systems is being placed that can cycle low grade heat. Examples include solar powered unmanned aerial vehicles (UAVs) and data storage servers. The power efficiency of solar module degrades at elevated temperature, thereby, necessitating the need for heat extraction system. Similarly, data centres in wireless computing system are facing increasing efficiency challenges due to high power consumption associated with managing the waste heat. We provide breakthrough in addressing these problems by developing thermo-magneto-electric generator (TMEG) arrays, composed of soft magnet and piezoelectric polyvinylidene difluoride (PVDF) cantilever. TMEG can serve dual role of extracting the waste heat and converting it into useable electricity. Near room temperature second-order magnetic phase transition in soft magnetic material, gadolinium, was employed to obtain mechanical vibrations on the PVDF cantilever under small thermal gradient. TMEGs were shown to achieve high vibration frequency at small temperature gradients, thereby, demonstrating effective heat transfer.

Rare-earth high-entropy alloys with giant magnetocaloric effect

In this paper, we report the development of rare-earth high-entropy alloys (RE-HEA) with multiple principle elements randomly distributed on a single hexagonal close-packed (HCP) lattice. Our work demonstrated that it is the entropy, rather than other atomic factors such as enthalpy, atomic size and electronegativity, that dictates phase formation in the current rare-earth alloy system. The high configuration entropy stabilized the crystalline structure from phase transformation during cooling, whereas a second-order magnetic phase transition occurred at its Neel temperature. The quinary RE-HEA exhibited a small magnetic hysteresis and the largest refrigerant capacity (about 627 J kg−1 at the 5 T magnetic field) reported to date, along with respectable mechanical properties. Our analysis indicates that the strong chemical disorder resulted from the high configuration entropy makes magnetic ordering in the HEA difficult, thus giving rise to a sluggish magnetic phase transition and enhanced magnetocaloric effect. Our findings evidenced that RE-HEAs have great potential to be used as magnetic refrigerants and the alloy-design concept of HEAs can be employed to develop novel high-performance magnetocaloric materials.

Microstructure, magnetic and magnetocaloric properties of Fe2–x Mn x P0.4Si0.6 alloys

The present work is devoted to investigating the microstructure, magnetism and magnetocaloric effects of Si- and Mn-rich FeMn(P,Si) alloys. The Mn-substituted alloys with Fe2– x Mn x P0.4Si0.6 ( x = 1.25, 1.30, 1.35, 1.40, 1.45 and 1.50) were prepared by high-energy ball milling and solid-state reaction. Experimental results show that the alloys crystallized into a majority Fe2P-type hexagonal structure, coexisting with minor amounts of (Mn,Fe)3Si and (Mn,Fe)5Si3 phases. The Curie temperature decreased linearly from 321 to 266 K with increasing Mn content from 1.25 to 1.50 in Fe2– x Mn x P0.4Si0.6 alloys. The first-order magnetic phase transition became weakened and the second-order magnetic phase transition became dominated with increasing Mn content. Fe0.75Mn1.25P0.4Si0.6 alloy presents a maximum isothermal magnetic-entropy changes of 7.2 J (kg K)–1 in a magnetic field change of 0–1.5 T. The direct measurement shows that Fe0.7Mn1.3P0.4Si0.6 and Fe0.65Mn1.35P0.4Si0.6 alloys exhibit a maximum adiabatic temperature change of 1.8 K in a magnetic field change of 0–1.48 T. The thermal hysteresis for all alloys is less than 4 K. These experimental results reveal that Fe2– x Mn x P0.4Si0.6 alloys could be a candidate material for magnetic refrigeration.