Second-order Magnetic Transition Research Articles

Magnetic properties and magneto-caloric effect in pseudo-binary intermetallic (Pr,Dy)2Fe17

Abstract Magnetic cooling could be a potential different energy solution substituting conventional vapour compression refrigeration in the future. For the largest cooling effects of most potential refrigerants, we need to exploit the different degrees of freedom such as magnetism and crystal structure. In order to realize this type of study, we have synthesized five pseudo-binary of bulk and nanostructured Pr 2− x Dy x Fe 17 intermetallics alloys, where praseodymium has been partially substituted by dysprosium for obtaining the maximum magneto-caloric response of the alloys around room temperature. The influence of Dy substitution for Pr on the structure and magnetic properties of Pr 2− x Dy x Fe 17 compounds with x = 0–0.5 was investigated. The analysis of the X-ray powder diffraction patterns show that the samples crystallize in the rhombohedral Th 2 Zn 17 -type crystal structure (space group R 3 ¯ m ). The unit cell volume decreases slightly with increasing dysprosium content while the c / a ratio remains nearly constant. The Curie temperature increases from 285 K at x = 0 to 314 K at x = 0.5.This variation is due to electronic effect. The temperature dependence of the magnetization, the magnetic entropy change Δ S M , as well as the relative cooling power around the second-order magnetic transition and the Arrott plots for the alloys are reported.

Large reversible magnetocaloric effects in ErFeSi compound under low magnetic field change around liquid hydrogen temperature

Magnetic properties and magnetocaloric effects (MCEs) of ternary intermetallic ErFeSi compound have been investigated in detail. It is found that ErFeSi exhibits a second-order magnetic transition from ferromagnetic to paramagnetic states at the Curie temperature TC = 22 K, which is quite close to the liquid hydrogen temperature (20.3 K). A thermomagnetic irreversibility between zero-field-cooling and field-cooling curves is observed below TC in low magnetic field, and it is attributed to the narrow domain wall pinning effect. For a magnetic field change of 5 T, the maximum values of magnetic entropy change (−ΔSM) and adiabatic temperature change (ΔTad) are 23.1 J/kg K and 5.7 K, respectively. Particularly, the values of −ΔSM and refrigerant capacity reach as high as 14.2 J/kg K and 130 J/kg under a magnetic field change of 2 T, respectively. The large MCE without hysteresis loss for relatively low magnetic field change suggests that ErFeSi compound could be a promising material for magnetic refrigeration of hydrogen liquefaction.

Mechanical and magnetocaloric properties of Gd-based amorphous microwires fabricated by melt-extraction

We report a systematic study of a new class of melt-extracted Gd53Al24Co20Zr3 amorphous microwires in terms of fabrication, structural characterization and evaluation of mechanical and magnetic properties. The tensile properties of the wires are characterized by a precision video gauge method and analyzed using the Weibull and lognormal methods. The three-parameter Weibull model and lognormal model based on the median rank value show consistent results and prove to be superior to the two-parameter Weibull model for the studied microwire with a smaller variation. The statistical mean tensile strength and fracture strain are calculated to be ∼1200MPa and ∼2.0%, respectively, which are comparable with those of other metallic glasses. Remarkably, the microwires exhibit a large and reversible magnetocaloric effect (MCE), with the isothermal magnetic entropy change (−ΔSm) and refrigerant capacity (RC) reaching the large values of 5.32Jkg−1K−1 and 467Jkg−1 for a field change of 3T. Albeit with a low Curie temperature, these values are superior to those reported for pure Gd and other Gd-based bulk or ribbon-shaped glasses. The mean field model-based and Langevin function analyses reveal that the second-order magnetic transition behavior of the studied wire originates from the local anisotropy associated with the fine size of the wire and derogation and fluctuation of the exchange integral. These results demonstrate the adaptability and overall excellence of the newly developed Gd-based microwires, making them multifunctional elements for MCE-based cooling applications, especially for liquid nitrogen liquefaction.

The magnetic and magnetocaloric properties of NdFe12−xMox compounds

The crystal structure and magnetic properties of NdFe12−xMox (x = 2.25, 2.5, 2.75, 3.0) compounds were synthesized and investigated. These compounds crystallized in ThMn12-type single phase, and the Curie temperature decreased with increasing Mo content. NdFe9.5Mo2.5 has a Curie temperature around room temperature, and a reversible magnetic entropy change was observed ∼290 K due to the second-order magnetic transition. The maximum value of −ΔSM is 2.38 J kg-1 K-1 at 290 K for a magnetic field change of 50 kOe. The working temperature span is more than 150 K and the relative cooling power reaches 357 J/kg, which makes the compound a promising candidate for magnetic refrigerator vicinal room temperature.

Large reversible magnetocaloric effect in TmTiO3 single crystal

The magnetic properties and magnetocaloric effect (MCE) of TmTiO3 single crystal were studied by magnetization and heat capacity measurements. A large reversible MCE was observed at the Curie temperature TC of 65 K, which is related to a second-order magnetic transition from paramagnetic to ferrimagnetic state. The values of maximum magnetic entropy change can reach 7.2 and 13.5 J kg−1 K−1 for a field change of 2 and 5 T, respectively, with no obvious hysteresis loss in the vicinity of the Curie temperature. The corresponding maximum adiabatic temperature changes are found to be 3.5 and 6.8 K. The magnetic transition and the origin of large MCE in TmTiO3 single crystal are discussed.

Large magnetocaloric properties in single-crystal dysprosium titanate

The magnetocaloric effect (MCE) of DyTiO3 single crystal was studied by magnetization and heat capacity measurements. A large reversible MCE has been observed at the Curie temperature TC of 65K, which is related to a second-order magnetic transition from paramagnetic to ferrimagnetic state. The values of maximum magnetic entropy change can reach 9.64 and 15.88Jkg−1K−1 for a field change of 2 and 5T, respectively, with no obvious hysteresis loss in the vicinity of the Curie temperature. The magnetic transition and the origin of large MCE in DyTiO3 single crystal are discussed.

Magnetocaloric effect in Er-Al-Co bulk metallic glasses

A series of Er-Al-Co bulk metallic glasses (BMGs) have been prepared by the copper mold casting method. The glass forming ability and magnetocaloric effect (MCE) for these alloys have been investigated. The second-order magnetic transition from para-magnetic to ferromagnetic states takes place at about 9 K. These BMGs exhibit excellent MCE because of their large effective magneton number; Er56Al24Co20 BMG has a maximum entropy change and refrigeration capacity of 16.06 J kg−1 K−1 and 546 J kg−1, respectively, under the field of 50 kOe (10 kOe=795.775 kA/m) indicating that these BMGs are potential candidate magnetic materials for hydrogen liquefaction.

First- and second-order magnetic and structural transitions in BaFe2(1−x)Co2xAs2

We present here high-resolution magnetization measurements on high-quality BaFe${}_{2(1\ensuremath{-}x)}$Co${}_{2x}$As${}_{2}$, $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.046$ as-grown single crystals. The results confirm the existence of a magnetic tricritical point in the $(x,T)$ plane at ${x}_{\text{tr}}^{m}\ensuremath{\approx}0.022$ and reveal the emergence of the heat-capacity anomaly associated with the onset of the structural transition at ${x}^{s}\ensuremath{\approx}0.0064$. We show that the extrapolated ${T}_{c}$ onset doping could be close to the magnetic tricritical point x${}_{\text{tr}}^{m}$.

From first-order magneto-elastic to magneto-structural transition in (Mn,Fe)1.95P0.50Si0.50 compounds

We report on structural, magnetic, and magnetocaloric properties of MnxFe1.95−xP0.50Si0.50 (x ≥ 1.10) compounds. With increasing the Mn:Fe ratio, a first-order magneto-elastic transition gradually changes into a first-order magneto-structural transition via a second-order magnetic transition. The study also shows that thermal hysteresis can be tuned by varying the Mn:Fe ratio. Small thermal hysteresis (less than 1 K) can be obtained while maintaining a giant magnetocaloric effect. This achievement paves the way for real refrigeration applications using magnetic refrigerants.

Origin of the magnetic anomaly and tunneling effect of europium on the ferromagnetic ordering inEu8−xSrxGa16Ge30(x=0,4)type-I clathrates

Systematic dc magnetization studies using the Banerjee criterion, Kouvel-Fisher, and magnetocaloric effect methods provide physical insights into the origin of the magnetic anomaly and the tunneling effect of europium on the ferromagnetic ordering in Eu${}_{8}$Ga${}_{16}$Ge${}_{30}$ type-I clathrates. We show that Eu${}_{8}$Ga${}_{16}$Ge${}_{30}$ undergoes a second-order magnetic transition (SOMT) at ${T}_{C}$ \ensuremath{\sim} 35 K, resulting from the magnetic interaction between the Eu${}^{2+}$ ions at the Eu2 sites, followed by a secondary magnetic transition at ${T}_{L}$ \ensuremath{\sim} 10 K (indicated as a magnetic anomaly in previous studies), as a result of the magnetic interaction between the Eu${}^{2+}$ ions at the Eu1 and Eu2 sites. The critical exponent \ensuremath{\beta} = 0.388 is close to that predicted from the three-dimensional Heisenberg model (\ensuremath{\beta} = 0.365), while the critical exponent \ensuremath{\gamma} = 0.956 is close to that predicted from the mean-field model (\ensuremath{\gamma} = 1). The substitution of Sr${}^{2+}$ for Eu${}^{2+}$ retains the SOMT but largely reduces the transition temperatures (${T}_{C}$ \ensuremath{\sim} 15 K and ${T}_{L}$ \ensuremath{\sim} 5 K), with the critical exponents \ensuremath{\beta} = 0.521 and \ensuremath{\gamma} = 0.917 close to those predicted from the mean-field model (\ensuremath{\beta} = 0.5 and \ensuremath{\gamma} = 1). These results point to the important fact that the tunneling of Eu${}^{2+}$ between the four equivalent sites in the tetrakaidecahedral cage tends to prevent the occurrence of a long-range ferromagnetic ordering in the type-I clathrate materials.

Large roomtemperature magnetocaloric effect with negligible magnetic hysteresis losses in Mn1−xVxCoGe alloys

The magnetic and magnetocaloric properties have been investigated in a series of Mn1−xVxCoGe (x=0.01, 0.02, 0.03, and 0.05) alloys. The substitution of V for Mn reduces the structural transformation temperature of MnCoGe alloy effectively and results in a second-order magnetic transition in Mn0.95V0.05CoGe alloys. Large room temperature magnetocaloric effect and almost zero magnetic hysteresis losses are simultaneously achieved in the alloys with x=0.01, 0.02, and 0.03. The reasons for the negligible magnetic hysteresis losses and the potential application for the roomtemperature magnetic refrigeration are discussed.

Effects of the Mn/Co ratio on the magnetic transition and magnetocaloric properties of Mn1+xCo1−xGe alloys

We have investigated the magnetic transition and magnetocaloric effects of Mn1+xCo1−xGe alloys by tuning the ratio of Mn/Co. With increasing Mn content, a series of first-order magnetostructural transitions from ferromagnetic to paramagnetic states with large changes of magnetization are observed at room temperature. Further increasing the content of Mn (x = 0.11) gives rise to a single second-order magnetic transition. Interestingly, large low-field magnetic entropy changes with almost zero magnetic hysteresis are observed in these alloys. The effects of Mn/Co ratio on magnetic transition and magnetocaloric effects are discussed in this paper.

Magnetic properties and large magnetocaloric effect in Gd–Ni amorphous ribbons for magnetic refrigeration applications in intermediate temperature range

Amorphous Gd 68− x Ni 32+ x ( x = −3, 0, 3) ribbons were prepared by melt-spinning method. The crystallization onset temperatures T x1 for Gd 68− x Ni 32+ x amorphous ribbons with x = −3, 0, and 3 are 561, 568, and 562 K, respectively. All the samples undergo the second-order magnetic transition at temperatures between ∼122 ( x = −3 and 3) and 124 K ( x = 0). The Curie temperature T C does not change with the composition significantly. The maximum isothermal magnetic entropy changes (−Δ S M) max of Gd 71Ni 29, Gd 68Ni 32, and Gd 65Ni 35 amorphous ribbons for a magnetic field change of 0–5 T were 9.0, 8.0, and 6.9 J kg −1 K −1, respectively. Large values of the refrigerant capacity (RC) were obtained in these ribbons. For example, Gd 71Ni 29 amorphous ribbon has a maximum RC value of 724 J kg −1. Large magnetic entropy change and RC values together with high stability enable the Gd 71Ni 29 amorphous alloy a competitive candidate among the magnetic refrigeration materials working at temperatures near 120 K.

Magnetic properties and magneto-caloric effect in pseudo-binary intermetallic (Ce,R) 2Fe 17 compounds (R = Y, Pr and Dy)

We have synthesized three pseudo-binary (Ce,R) 2Fe 17 intermetallic alloys, where cerium has been partially substituted by R = Y, Pr and Dy, with the aim of tuning the Curie temperature (between 253 and 273 K) for obtaining the maximum magneto-caloric response of the alloys just below room temperature. The analysis of the x-ray powder diffraction patterns show that the three samples crystallize in the rhombohedral Th 2Zn 17-type crystal structure (space group R 3 ¯ m ). We report the temperature dependence of the isothermal magnetic entropy change, |Δ S M |( T), the magnetic field dependence of its maximum value, |Δ S M ma x|( H), and the relative cooling power around the second-order magnetic transition for magnetic field changes μ 0Δ H max = 5 T. The collapse of the normalized Δ S M ( θ)/Δ S M max vs. temperature (where θ is a rescaled temperature) into a single master curve allows the extrapolation of |Δ S M |( T) curves for higher magnetic field changes, and/or the estimation of the temperature dependence of the magnetic entropy for other 2:17 pseudo-binary compounds. The lower values of |Δ S M ma x| compared with those of the R 2Fe 17 compounds with R = Pr or Nd are explained in terms of the decrease of the saturation magnetization. The magnetic field dependence of the |Δ S M ma x| indicates that only Pr 1.5Ce 0.5Fe 17 alloy roughly follows a mean-field behaviour.

Magnetic properties of barium uranate Ba 2U 2O 7

Unique magnetic properties of a ternary uranate Ba 2U 2O 7 are reported. Magnetic susceptibility measurements reveal that this compound undergoes a magnetic transition at 19 K. Below this temperature, magnetic hysteresis was observed. The results of the low-temperature specific heat measurements below 30 K support the existence of the second-order magnetic transition at 19 K. Ba 2U 2O 7 undergoes a canted antiferromagnetic ordering at this temperature. The magnetic anomaly which sets in at 58 K may be due to the onset of one-dimensional magnetic correlations associated with the linear chains formed by U ions. The analysis of the experimental magnetic susceptibility data in the paramagnetic temperature region gives the effective magnetic moment μ eff=0.73 μ B, the Weiss constant θ=−10 K, and the temperature-independent paramagnetic susceptibility χ TIP=0.14×10 −3 emu/mole. The magnetic susceptibility results and the optical absorption spectrum were analyzed on the basis of an octahedral crystal field model. The energy levels of Ba 2U 2O 7 and the crystal field parameters were determined.

Large reversible magnetocaloric effect in Er 2In compound

The magnetic properties and magnetocaloric effects (MCEs) of the Er 2In compound have been investigated. It is found that Er 2In undergoes a second-order magnetic transition from ferromagnetic (FM) to paramagnetic (PM) state at the Curie temperature T C = 46 K and no other transition is observed. These observations are in consistent with previously published works. For a magnetic field change from 0 to 5 T, the maximum magnetic entropy change (Δ S M) and refrigerant capacity (RC) are −16 J/kg K and 490 J/kg, respectively, which are comparable to or larger than those of some magnetic refrigerant materials with first-order magnetic transition around the similar magnetic ordering temperature. Large reversible MCE and high RC value indicate that the Er 2In compound is a suitable candidate of magnetic refrigerants in low temperature range.

Reversible room-temperature magnetocaloric effect in Mn5PB2

Magnetic properties and magnetocaloric effect of the Mn5PB2 compound have been investigated. The maximum values of magnetic entropy change −ΔSM at 302 K are 4.93 and 2.64 J kg−1 K−1 for magnetic field changes of 5 and 2 T, respectively, which are closely related with a second-order magnetic transition from the ferromagnetic to the paramagnetic state. The reversible magnetocaloric effect with relatively large magnetic entropy change makes the Mn5PB2 compound (free of rare earths) and Mn5PB2-based materials attractive candidates for room-temperature magnetic refrigeration.

Excellent Magnetocaloric Effect in Er60Al18Co22 Bulk Metallic Glass

Excellent magnetocaloric effect with a maximum entropy change and refrigeration capacity of 17.6 J·kg−1·K−1 and 546 J·kg−1, respectively, has been discovered in the Er60Al18Co22 bulk metallic glass under the field of 50 kOe in the temperature range of helium liquefaction. This MCE results from the second-order magnetic transition from the paramagnetic to the ferromagnetic state. Our analysis based on mean-field theory suggests that the excellent MCE is attributed to the strong exchange of magnetic moment in the glassy structure.

The influence of Al substitution on the phase transitions and magnetocaloric effect in Ni 43Mn 46Sn 11− xAl x alloys

The effects of Al substitution on the phase transitions and magnetocaloric effect of Ni 43Mn 46Sn 11− x Al x ( x=0–2) ferromagnetic shape memory alloys were investigated by X-ray diffraction and magnetization measurements. With the increase of Al content, the cell volume decreases due to the smaller radius of Al, and the martensitic transformation temperature increases rapidly, while the Curie temperature of austenitic phase shows a small increase. A large positive and a negative magnetic entropy change were observed near the first-order martensitic transition and the second-order magnetic transition, respectively. The magnetic entropy changes, hysteresis behavior, and refrigerant capacity near the two transitions are compared.

Improvement of refrigerant capacity of La 0.7Ca 0.3MnO 3 material with a few percent Co doping

The influence of first and second order magnetic phase transitions on the magnetocaloric effect (MCE) and refrigerant capacity or relative cooling power (RCP) of La 0.7Ca 0.3MnO 3 and La 0.7Ca 0.3Mn 0.95Co 0.05O 3 materials has been investigated. Large low-field-induced magnetic entropy changes are observed in La 0.7Ca 0.3MnO 3 and La 0.7Ca 0.3Mn 0.95Co 0.05O 3 materials. The La 0.7Ca 0.3MnO 3 material experiences a large entropy change with a first-order magnetic phase transition at the Curie temperature, T C . On the other hand, La 0.7Ca 0.3Mn 0.95Co 0.05O 3 displays a smaller entropy change with a second order phase transition. While a first-order magnetic transition material induces a larger MCE (7.528 J/kg K at 5 T) at T C , this is limited to a narrow temperature range, resulting in a relatively small RCP (218 J/kg), while the Co-doped second-order magnetic transition material induces a smaller MCE (7.14 J/kg K for 5 T), but it is spread over a broader temperature range, resulting in a larger RCP (308 J/kg). The maximum magnetoresistance (MR, defined as ρ(0)/ ρ( H)–1) under a field of 5 T is about 206% and 333% for La 0.7Ca 0.3MnO 3 and La 0.7Ca 0.3Mn 0.95Co 0.05O 3, respectively. The refrigeration capacity (RCP) is enhanced in La 0.7Ca 0.3Mn 0.95Co 0.05O 3 (by about 41%) due to small changes from Co doping. The magnetocaloric features of these materials at lower magnetic fields (MCE=3.163 for La 0.7Ca 0.3Mn 0.95Co 0.05O 3 and 4.63 J/kg K for La 0.7Ca 0.3MnO 3 at 1 T), and the high RCP and MR can provide some ideas for exploring novel magnetic refrigerants that can operate with permanent magnets rather than superconducting ones as the magnetic field source.