First-order Magnetic Transition Research Articles

Second-order magnetic critical points at finite magnetic fields: Revisiting Arrott plots

The so-called Arrott plot, which consists in plotting $H/M$ against ${M}^{2}$, with $H$ the applied magnetic field and $M$ the magnetization, is used to extract valuable information in second-order magnetic phase transitions. Besides, it is widely accepted that a negative slope in the Arrott plot is indicative of a first-order magnetic transition. This is known as the Banerjee criterion. In consequence, the zero-field transition temperature ${T}^{*}$ is reported as the characteristic first-order transition temperature. By carefully analyzing the mean-field Landau model used for studying first-order magnetic transitions, we show in this work that ${T}^{*}$ corresponds in fact to a triple point where three first-order lines meet. More importantly, this analysis reveals the existence of two symmetrical second-order critical points at finite magnetic field $({T}_{c},\ifmmode\pm\else\textpm\fi{}{H}_{c})$. We then show that a modified Arrott plot can be used to obtain information about these second-order critical points. To support this idea we analyze experimental data on ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_{3}$ and discuss an estimate for the location of the triple point and the second-order critical points.

Latent heat of the first-order magnetic transition ofMnFeSi0.33P0.66

The latent heat of a magnetoelastic phase transition is used as a measure of the magnetocaloric effect since it is directly proportional to the entropy change. Taking ${\mathrm{MnFeSi}}_{0.33}{\mathrm{P}}_{0.66}$ as a model magnetocaloric material, density functional theory calculations in addition to the phonon calculations based on the density functional perturbation theory were performed in order to calculate the latent heat of the magnetoelastic phase transition. The Curie temperature (${T}_{C}$) was determined by taking into account the quasiharmonic approximation and the configurational entropy. The material exhibits a first-order magnetic transition accompanied by a large latent-heat (19.97 kJ/kg) near-room-temperature operation.

Temperature dependences of the hyperfine parameters of Fe2+in FeTiO3as determined by57Fe-Mössbauer spectroscopy

The temperature variations of the ferrous Mossbauer parameters for a synthetic ilmenite (FeTiO 3 ) have been determined and interpreted over a very wide temperature range (5–900 K). The Debye model of the lattice vibrations was used in interpreting the temperature dependence of the center shift, yielding a characteristic Mossbauer temperature of 350 ± 20 K and a zero-Kelvin intrinsic isomer shift of 1.30 ± 0.01 mm/s. The temperature dependence of the ferrous Mossbauer quadrupole splitting was interpreted using crystal field theory. A most adequate description of the experimental Δ E Q ( T ) curve was obtained assuming an energy shift of at the most ca. 500 ± 50 cm −1 for the highest orbital T 2g level relative to the lowest level within this T 2g triplet. The temperature dependence of the hyperfine field was interpreted within the molecular field theory of magnetism assuming the magnetic exchange energy being a function of interatomic spacing, indicating a first-order magnetic transition at the magnetic-paramagnetic transition temperature of 59.0 ± 0.5 K. This detailed presentation of Mossbauer parameters as a function of temperature can serve as a basis for easily detecting ilmenite ore at, for example, the lunar surface and for monitoring by means of Mossbauer spectroscopy the reduction process of the mined mineral, for the purpose of supplying a future Moon base site with oxygen and water.

Effect of disorder on the dimer transition of the honeycomb-lattice compoundLi2RuO3

We report the dependence of magnetic properties on the crystalline disorder in Li$_{2}$RuO$_3$ with Ru honeycomb lattice. This oxide exhibits unconventional Ru-dimer transition below $T_\text{d} \sim$ 540 K. We demonstrate that the cell parameters, related to the coherence of the dimer formation, are strongly dependent on the synthesis procedure. We show that the magnetic behavior at the dimer transition is closely related to the lattice parameters. In particular, we revealed that samples with well-ordered dimers exhibit a first-order magnetic transition with the onset exceeding 550~K, higher than that reported previously. We discuss possible dimer configurations leading to this magneto-lattice coupling.

Anomalous magnetism of superconducting Mg-doped InN film

We report on the Meissner effect of Mg-doped InN film with superconducting transition onset temperature Tc,onset of 5 K. Mg-doped InN is magnetically ordered and exhibits a simultaneous first-order magnetic and electric transition near 50 K. Its behavior is similar to that of iron-based superconductors. A strong correlation is proposed to exist between structural distortion and superconductivity when Mg is doped into InN. The suppression of magnetic ordering close to Tc by doping is further demonstrated by anisotropic magnetoresistance and M-H measurements. The findings suggest that the superconducting mechanism in the system may not be conventional BCS.

Unconventional thermal effects across first-order magnetic transition in the Ta-dopedHfFe2intermetallic

Novel thermal effects across the first order antiferromagnetic (AFM) - ferromagnetic (FM) transition in an intermetallic alloy are reported. They show instances of warming when heat is extracted from the sample, and cooling when heat is provided to the sample across AFM-FM transition in Ta doped $HfFe_2$, thus providing indisputable evidence of metastable supercooled AFM and superheated FM states, respectively. Such thermal effects in a magnetic solid prepared from commercially available materials has been reported for the first time. The transition proceeds in multiple steps which is interpreted in the framework of quenched disorder broadening of AFM-FM transition and classical nucleation theory. Measurements in the presence of magnetic field conform to above frame work.

A pathway to optimize the properties of magnetocaloric Mn2-xFexP1-yGey for magnetic refrigeration

Magnetocaloric materials can be useful in magnetic refrigeration applications, but to be practical the magneto-refrigerant needs to have a very large magnetocaloric effect (MCE) near room temperature for modest applied fields (<2 T) with small hysteresis and magnetostriction, and should have a complete magnetic transition, and environmentally friendly. One system that may fulfill these requirements is Mn2-xFexP1-yGey, where a combined first-order structural and magnetic transition occurs between the high temperature paramagnetic and low temperature ferromagnetic phase. We have used neutron diffraction, differential scanning calorimetry, and magnetization measurements to study the effects of Mn and Ge location in the structure on the ordered magnetic moment, MCE, and hysteresis for a series of compositions of the system near optimal doping. The diffraction results indicate that the Mn ions located on the 3f site enhance the desirable properties, while those located on the 3 g sites are detrimental. The phase fraction that transforms, hysteresis of the transition, and entropy change can be affected greatly by both the compositional homogeneity and the particle size, and an annealing procedure has been developed that substantially improves the performance of all three properties of the material. We also establish a correlation between applied magnetic field to complete the transition and the temperature range of coexistence of the PM and FM phase. On the basis of these results we have identified a pathway to understand the nature and to optimize the MCE properties of this system for magnetic refrigeration applications.

Magnetic and Magnetocaloric Properties in Ferrimagnetic Mn2−xCoxSb ($x=0.15$ and 0.20) Alloys

Due to the energy-efficiency and environment-friendliness, magnetic refrigeration based on the magnetocaloric effect (MCE) has extensively been investigated during the past years, which is considered as a promising alternative to conventional gas-compression refrigeration [1-3]. In this context, the materials undergoing magnetic transition, especially the first-order magnetic transition (FOMT), have attracted the increasing attentions due to sharp change of magnetization ΔM observed near phase transition, which are generally associated with the large magnetocaloric effect (MCE) [1-3]. In this work, we reported on the structure and transitional behaviors in the Mn 2x Co x Sb (x=0.15, 0.20) alloys prepared by arc-melting method followed by annealing at 800 °C for 8 days. These alloys show an FOMT between AFM and FRI state. From the standard and loop process method, respectively, the isothermal magnetizations were measured and the magnetocaloric properties were studied for these alloys.

Magnetic refrigeration: an eco-friendly technology for the refrigeration at room temperature

Magnetic refrigeration is an emerging, environment-friendly technology based on a magnetic solid that acts as a refrigerant by magneto-caloric effect (MCE). In the case of ferromagnetic materials MCE is a warming as the magnetic moments of the atom are aligned by the application of a magnetic field, and the corresponding cooling upon removal of the magnetic field. There are two types of magnetic phase changes that may occur at the Curie point: first order magnetic transition (FOMT) and second order magnetic transition (SOMT). The reference cycle for magnetic refrigeration is AMR (Active Magnetic Regenerative cycle) where the magnetic material matrix works both as a refrigerating medium and as a heat regenerating medium, while the fluid flowing in the porous matrix works as a heat transfer medium. Regeneration can be accomplished by blowing a heat transfer fluid in a reciprocating fashion through the regenerator made of magnetocaloric material that is alternately magnetized and demagnetized. In this paper, attention is directed towards the near room-temperature range. We compare the energetic performance of a commercial R134a refrigeration plant to that of a magnetic refrigerator working with an AMR cycle. Attention is devoted to the evaluation of the environmental impact in terms of a greenhouse effect. The comparison is performed in term of TEWI index (Total Equivalent Warming Impact) that takes into account both direct and indirect contributions to global warming. In this paper the AMR cycle works with different magnetic refrigerants: pure gadolinium, second order phase magnetic transition (Pr0.45Sr0.35MnO3) and first order phase magnetic transition alloys (Gd5Si2Ge2, LaFe11.384Mn0.356Si1.26H1.52, LaFe1105Co0.94Si110 and MnFeP0.45As0.55). The comparison, carried out by means of a mathematical model, clearly shows that GdSi2Ge2 and LaFe11.384Mn0.356Si1.26H1.52 has a TEWI index always lower than that of a vapor compression plant. Furthermore, the TEWI of the AMR cycle working with FOMT materials is always better than that of SOMT materials. Gd5Si2Ge2 is the best FOMT material.

In-situ neutron investigation of hydrogen absorption kinetics in La(FexSi1−x)13 magnetocaloric alloys for room-temperature refrigeration application

Promising magnetocaloric material La(Fe,Si)13 with a first-order magnetic transition has been widely investigated. The observed instability of hydrogen in the material is detrimental for its industrial upscale and a better control of the hydrogen absorption/desorption is necessary to optimize its application potential. In this article, the hydrogen absorption kinetics is studied through an in-situ neutron diffraction experiment. The results allow us to have an inside look at the structure “breathing” to accommodate the interstitial atoms and compare the effect of hydrides with carbohydrides.

Second order magnetic phase transition and scaling analysis in iron doped manganite La0.7Ca0.3Mn1−xFexO3 compounds

We investigated magnetic properties of La0.7Ca0.3Mn1−xFexO3 (x=0.09 and 0.11) compounds in terms of isothermal magnetization analysis and scaling behavior with various critical exponents. From the Landau theory of magnetic phase transition, we found that the paramagnetic to ferromagnetic phase transition in La0.7Ca0.3Mn1−xFexO3 (x=0.09 and 0.11) compounds is the type of second order magnetic transition (SOMT), which contrary to the first order magnetic transition (FOMT) for low Fe-doped compounds (x<0.09) in previous reports. When we investigate the critical behavior of the compounds near T=Tc by the modified Arrott plot, Kouvel–Fisher plots, and critical isothermal analysis, the estimated critical exponents β, γ, and δ are in between the theoretically predicted values for three-dimensional Heisenberg and mean-field interaction models. It is noteworthy that the scaling relations are obeyed in terms of renormalization magnetization m=ε−βM(H,ε) and renormalized field h=|ε|β+γH. Temperature-dependent effective exponents βeff and γeff correspond to the ones of disordered ferromagnets. It is shown that the magnetic state of the compounds is not fully described by the conventional localized-spin interaction model because the ferromagnetic interaction has itinerant character by increasing Fe-doping concentration.

Critical behavior near the ferromagnetic–paramagnetic phase transition in Sm0.55−xPrxSr0.45MnO3 compounds (0.3 ≤ x ≤ 0.4)

We present a detailed study of the static magnetization M (H, T) in the vicinity of the ferromagnetic–paramagnetic (FM–PM) phase–transition temperature (TC) in polycrystalline Sm0.55−xPrxSr0.45MnO3 (0.3 ≤ x ≤ 0.4) manganites. The critical exponents β, γ, and δ are determined by using various techniques such as modified Arrott plot, Kouvel–Fisher method and critical isotherm analysis, and are found to be β = 0.255 ± 0.021 and γ = 0.957 ± 0.014 with TC = 226 K for Sm0.25Pr0.3Sr0.45MnO3 and β = 0.324 ± 0.024 and γ = 1.212 ± 0.027 with TC = 254 K for Sm0.15Pr0.4Sr0.45MnO3. A careful analysis of the magnetic data recorded in Sm0.55−xPrxSr0.45MnO3 demonstrates the existence of a tri-critical point that separates discontinuous first-order magnetic transition (FOMT) from continuous second-order magnetic transition (SOMT).

1:13 phase formation mechanism and first-order magnetic transition strengthening characteristics in (La,Ce)Fe13–x Si x alloys

The effects of the introduction of Ce to La1−xCexFe11.5Si1.5 alloys on 1:13 phase formation mechanism, the first-order magnetic phase transition strengthening characteristics, and magnetocaloric property were studied, respectively. The results show that the formation mechanisms of 1:13 and LaFeSi phases in La1−xCexFe11.5Si1.5 alloys are the same as those of Ce2Fe17 and CeFe2 phases in Ce–Fe binary system, respectively. The substitution of Ce in 1:13 phase which is limited can make the first-order magnetic phase transition characteristics strengthen, which can make thermal and magnetic hysteresis increase, the temperature interval of temperature-induced phase transition decrease, and the critical magnetic field of field-induced magnetic phase transition (HC) increase, respectively. Owing to the lattice shrink of 1:13 phase with the increase in Ce content, the Curie temperatures (TC) show a linear decrease. The maximum change in magnetic entropy gradually increases due to the decrease in temperature interval of temperature-induced phase transition, but the relative cooling capacities are all about 80 J·kg−1 at magnetic field of 2 T.

First-order magnetic and magnetostructural transitions in the magnetocaloric compound MnNi0.73Fe0.27Ge

Magnetic and magnetostructural transitions of MnNi0.73Fe0.27Ge have been studied by calorimetry and by magnetic measurements. The first-order MST from paramagnetic to ferromagnetic martensite shows a giant magnetic entropy change of −27J/kgK for a magnetic-field change of 50kOe around 233K. A weak first-order magnetic transition is observed in the martensite phase over a large low-temperature range, with clear thermal hysteresis and increased saturation field. The origin is discussed on the basis of the competitiom between ferromagnetic and antiferromagnetic coupling with magneto-elastic coupling.

Magnetic properties and magnetocaloric effect of Hf–Ta–Fe–(CO) alloys

The as-cast intermetallic compounds Hf0.8Ta0.2Fe1.98−xCox (x=0, 0.02 and 0.04) show a temperature or magnetic field induced first-order magnetic transition (FOMT) between aferromagnetic (F) and anantiferromagnetic (AF) state. The maximum magnetic entropy change (|ΔS|) is about 4.8J/kgK when x=0 in a magnetic field change of 5T. A small amount of Co substitution for Fe does not significantly reduce the maximum |ΔS| but broadens the working temperature range.

Magnetism and magnetocaloric effect study of CaFe0.7Co0.3O3

The CaFe0.7Co0.3O3 single crystal was grown for the first time by a two-step method and its magnetism and magnetocaloric effect were investigated. This compound experiences a second-order paramagnetism-to-ferromagnetism transition in a wide temperature window between 200 and 150 K due to the presence of multiple ferromagnetic interactions. Since the spin entropy is gradually released above the ferromagnetic Curie temperature (∼177 K), no sharp λ-type anomaly is observed in specific heat. On the basis of magnetization measurements, however, a considerable entropy change is found in this perovskite oxide. More interesting, this compound exhibits a broadening working temperature, and a significant refrigerant capacity (∼355 J kg−1 at 6 T) which is comparable with those found in some giant magnetocaloric alloys with first-order magnetic transitions. The present study therefore provides an example on how to enhance the refrigerant capacity by extending the working temperature of magnetocaloric material.

Effect of boron substitution on the ferromagnetic transition of MnFe0.95P2/3Si1/3

The changes in structural, magnetic and calorimetric properties induced by B substitution of P or Si in MnFe0.95P2/3Si1/3 are reported. The evolution of the structure and in particular of the ratio in cell parameters c/a is specific to the B substitution. The disappearance of the first-order magnetic transition (FOMT) with B substitution is shown to be systematic and much more pronounced than observed when varying the Mn/Fe and P/Si ratios or using other non-metallic elements. The correlation between the evolutions of the discontinuities Δ(c/a) and ΔStr suggests that the weakening of the FOMT results from a modification of the magneto-elastic coupling. The evolution seems in line with theoretical studies on Fe2P1-xXx materials and points towards a combined effect of size differences and valence changes at the non-metallic site.

Observation of the second-order magnetic and reentrant spin-glass transitions in LiNi0.5Mn0.5O2

The structure and magnetic properties of LiNi0.5Mn0.5O2 were studied by synchrotron X-ray diffraction, dc and ac susceptibilities. The material showed a continuous magnetic transition from paramagnetism into ferrimagnetism, followed by a spin glass with the decrease of temperature. Using a criterion given by Banerjee to distinguish first-order magnetic transition from second-order ones, it is shown that the ferrimagnetic transition at 108 K belongs to the second-order type. The frequency dependence of peak intensity and the shift in ac susceptibility at 14 K suggest a reentrant spin glass transition in LiNi0.5Mn0.5O2.

Structure, magnetic properties and giant magnetocaloric effect of Tb4Gd1Si2.035Ge1.935Mn0.03 alloy

Tb4Gd1Si2.035Ge1.935Mn0.03 alloy was prepared by arc melting followed by annealing at 1193 K for 168 h. Structural characterizations reveal that monoclinic (Tb, Gd)5Si2Ge2-type phase, secondary phase with orthorhombic 5-4 type structure and hexagonal 5-3 type structure coexist in the alloy. The paramagnetic Curie temperature (θp) is 120 K, indicating that the dominant exchange interaction is ferromagnetic or ferrimagnetic. That the thermal hysteresis of 13 K between heating and cooling and the negative slopes of Arrott plots derived from M–H curves between 116 K and 170 K confirm a typical first-order magnetic transition from ferromagnetism to paramagnetism occurs. The maximum magnetic entropy changes of the Tb4Gd1Si2.05Ge1.95Mn0.03 alloy for magnetic field changes of 0–1 T, 0–2 T, 0–3 T, 0–4 T and 0–5 T are about 3.3, 8.6, 14.0, 18.9 and 22.4 J/kg K, respectively. And the effective refrigeration capacity (RCeff) value is 231 J/kg with a subtracted magnetic hysteresis loss of 30 J/kg for a magnetic field change from 0 to 5 T. Large −ΔSM and RCeff suggest that Tb4Gd1Si2.035Ge1.935Mn0.03 alloy is an attractive potential magnetocaloric material working in the vicinity of 143 K.

Thermodynamic modeling of magnetic hysteresis in AMRR cycles

Current models of Active Magnetic Regenerative Refrigeration (AMRR) cycles are not able to capture the effect of magnetic hysteresis and are therefore strictly limited to second order magnetic transition (SOMT) materials. The discovery of the giant magnetocaloric effect (GMCE) in first order magnetic transition (FOMT) materials has generated substantial interest. FOMTs yield large adiabatic temperature changes but also exhibit significant magnetic hysteresis. This work quantifies the effects of magnetic hysteresis. Thermodynamically, hysteresis is treated as a source of entropy generation that is proportional to the area swept by the hysteresis loop experienced locally by the material during one refrigeration cycle. The 1-D numerical model presented by Engelbrecht (2008) is modified to include magnetic hysteresis. Hysteresis losses are shown to be directly proportional to regenerator volume. Therefore, at large refrigeration capacity to volume ratios, AMRR beds using layered FOMT materials significantly outperform the same cycle using layered SOMT refrigerants.