Tuning Martensitic Phase Transition by Non-Magnetic Atom Vacancy in MnCoGe Alloys and Related Giant Magnetocaloric Effect

The effects of non-magnetic atom vacancy on structural, martensitic phase transitions and the corresponding magnetocMoric effect in MnCoGel-x alloys are investigated using x-ray diffraction and magnetic measurements. The introduction of non-magnetic atom vacancy leads to the decrease of the martensitic transition temperature and realizes a temperature window where magnetic and martensitic phase transitions can be tuned together. Moreover, the giant magnetocaloric effect accompanied with the coupled magnetic-structural transition is ob- tained. It is observed that the peak values of magnetic entropy change of MnCoGeo.97 are about -13.9, -35.1 and -47.4J.kg-1K-1 for △H = 2, 5, 7T, respectively.

Metamagnetic transition and reversible magnetocaloric effect in antiferromagnetic DyNiGa compound

Rare-earth(R)-based materials with large reversible magnetocaloric effect(MCE)are attracting much attention as the promising candidates for low temperature magnetic refrigeration.In the present work,the magnetic properties and MCE of DyNiGa compound with TiNiSi-type orthorhombic structure are studied systematically.The DyNiGa undergoes a magnetic transition from antiferromagnetic(AFM)to paramagnetic state with Néel temperature TN=17 K.Meanwhile,it does not show thermal and magnetic hysteresis,revealing the perfect thermal and magnetic reversibility.Moreover,the AFM state can be induced into a ferromagnetic state by a relatively low field,and thus leading to a large reversible MCE,e.g.,a maximum magnetic entropy change(-ΔSM)of 10 J/kg·K is obtained at 18 K under a magnetic field change of 5 T.Consequently,the large MCE without thermal or magnetic hysteresis makes the DyNiGa a competitive candidate for magnetic refrigeration of hydrogen liquefaction.

Magnetocaloric properties and Griffiths phase of ferrimagnetic cobaltite CaBaCo_(4)O_(7)

We present a study on the magnetocaloric properties of a CaBaCo_(4)O_(7) polycrystalline cobaltite along with research on the nature of magnetic phase transition.The magnetization as a function of temperature identifies the ferrimagnetic to paramagnetic transition at a Curie temperature of 60 K.Moreover,a Griffiths-like phase is confirmed in a temperature range above T_(C).The compound undergoes a crossover from the first to second-order ferrimagnetic transformation,as evidenced by the Arrott plots,scaling of the universal entropy curve,and field-dependent magnetic entropy change.The maximum of entropy change is 3 J/kg⋅K for ΔH=7 T at T_(C),and a broadening of the entropy peak with increasing magnetic field indicates a field-induced transition above T_(C).The analysis of the magnetic entropy change using the Landau theory reveals the second-order phase transition and indicates that the magnetocaloric properties of CaBaCo_(4)O_(7) are dominated by the magnetoelastic coupling and electron interaction.The corresponding values of refrigerant capacity and relative cooling power are estimated to be 33 J/kg and 42 J/kg,respectively.

Structure, magnetism and magnetocaloric effects in Er_(5)Si_(3)B_(x)(x=0.3,0.6) compounds

We investigate the structure, magnetic properties, magnetic phase transitions and magnetocaloric effects(MCEs) of Er5Si3Bx(x=0.3,0.6) compounds. The Er5Si3Bx(x = 0.3, 0.6) compounds crystalize in a Mn5Si3type hexagonal structure(space group: P63/cm) and exhibit a successive complicated magnetic phase transition. The extensive magnetic phase transitions contribute to the broad temperature range of MCEs exhibiting in Er_(5)Si_(3)B_(x)(x=0.3,0.6) compounds, with maximum magnetic entropy change(-ΔSM_(max)) and refrigeration capacity of 10.2 J·kg^(-1)·K^(-1), 356.3 J/kg and 11.5 J·kg^(-1)·K^(-1),393.3 J/kg under varying magnetic fields 0–5 T, respectively. Remarkably, the δTFWHMvalues(the temperature range corresponding to 1/2×|-ΔSM_(max)|) of Er5Si3Bx(x=0.3,0.6) compounds were up to 41.8 K and 39.6 K, respectively. Thus, the present work provides a potential magnetic refrigeration material with a broad temperature range MCEs for applications in cryogenic magnetic refrigerators.

Phase structure and magnetocaloric effect of (Tb_(1–x)Dy_x)Co_2 alloys

Phase structure and magnetocaloric effect of （Tb1-xDyx）Co2 alloys with x=0, 0.2, 0.4, 0.6, 0.8, and 1.0 were investigated using X-ray diffraction analysis, differential thermal analysis, and magnetization measurement. The samples were single phase with cubic MgCu2- type structure; with the increase of Dy content, Tc decreased from 240 K （TbCo2） to 130 K （DyCo2）, and the maximum magnetic entropy change ｜ △SM,max｜ increased from 3.133 to 8.176 J/kg-K under low magnetic field of 0-2 T. The Arrott plot and the change of ｜△SM,max｜ showed that magnetic phase transition from second order to first order occured with the increase of Dy content between x=-0.6 and 0.8.

Review of magnetic properties and magnetocaloric effect in the intermetallic compounds of rare earth with low boiling point metals

The magnetocaloric effect （MCE） in many rare earth （RE） based intermetallic compounds has been extensively in- vestigated during the last two decades, not only due to their potential applications for magnetic refrigeration but also for better understanding of the fundamental problems of the materials. This paper reviews our recent progress on studying the magnetic properties and MCE in some binary or ternary intermetallic compounds of RE with low boiling point metal（s） （Zn, Mg, and Cd）. Some of them exhibit promising MCE properties, which make them attractive for low temperature magnetic refrigeration. Characteristics of the magnetic transition, origin of large MCE, as well as the potential application of these compounds are thoroughly discussed. Additionally, a brief review of the magnetic and magnetocaloric properties in the quaternary rare earth nickel boroncarbides RENi2B2C superconductors is also presented.

Magnetic properties and magnetocaloric effects in NaZn_(13)-type La(Fe,Al)(13)-based compounds

In this article, our recent progress concerning the effects of atomic substitution, magnetic field, and temperature on the magnetic and magnetocaloric properties of the LaFe13-xAlx compounds are reviewed. With an increase of the aluminum content, the compounds exhibit successively an antiferromagnetic （AFM） state, a ferromagnetic （FM） state, and a mictomagnetic state. Furthermore, the AFM coupling of LaFe13 -xAlx can be converted to an FM one by substituting Si for A1, Co for Fe, and magnetic rare-earth R for La, or introducing interstitial C or H atoms. However, low doping levels lead to FM clusters embedded in an AFM matrix, and the resultant compounds can undergo, under appropriate applied fields, first an AFM-FM and then an FM-AFM phase transition while heated, with significant magnetic relaxation in the vicinity of the transition temperature. The Curie temperature of LaFe13-xAlx can be shifted to room temperature by choosing appropriate contents of Co, C, or H, and a strong magnetocaloric effect can be obtained around the transition temperature. For example, for the LaFel 1.5All.5Co.2Hl.o compound, the maximal entropy change reaches 13.8 J.kg-1.K-1 for a field change of 0-5 T, occurring around room temperature. It is 42% higher than that of Gd, and therefore, this compound is a promising room-temperature magnetic refrigerant.

Magnetocaloric effect of (Gd_(1-x)Nd_x)Co_2 alloys in low magnetic field

The phases and magnetocaloric effect in the alloys （Gd1-xNdx）Co2 with x = 0, 0.1, 0.2, 0.3, and 0.4 were investigated by X-ray diffraction analysis and magnetization measurement. The samples are single phase with a cubic MgCu2-type structure. The To decreases obviously with increasing Nd content from 404 K of the alloy with x = 0 to 272 K of the alloy with x = 0.4; forx = 0.3, the To is 296 K, which is near room temperature. In the samples （Gd1-xNdx）Co2 with x = 0.0, 0.1, 0.2, 0.3, and 0.4, the maximum magnetic entropy change is 1.471, 1.228, 1.280, 1.381 and 1.610 J·kg^-1·K^-1, respectively, in the applied field range of 0-2.0 T. The results of Arrott plots confirmed that the transition type were second order magnetic transition forx = 0, 0.3, and 0.4.

Magnetocaloric effect study of SrFe_(0.8)Co_(0.2)O_3 single crystal prepared under high pressure

A high-quality SrFe0.8Co0.2O3 single crystal is prepared by combining floating-zone and high-pressure treatment methods. Its Magnetocaloric effect is investigated by magnetic measurements. A paramagnetism-to-ferromagnetism tran- sition is found at about 270 K and this transition is a second-order one in nature as confirmed by Arrott plots. The saturated moment obtained at 2 K and 7 T is 3.63 μB/f.u. The maximal value of magnetic entropy change measured at 5 T is about 4.0 J·kg-1 ·K-1. The full wide at half maximum for a magnetic entropy change peak observed in SrFe0.8Co0.2O3 is considerably large. As a consequence, the relative cooling power value of SrFe0.8Co0.2O3 obtained at 5 T is 331 J/kg, which is greatly higher than those observed in other perovskite oxides. The present work therefore provides a promising candidate for magnetic refrigeration near room temperature.

Influence of Tb substitution on low-field magnetocaloric effect in Gd_5Si_(1.72)Ge_(2.28) alloy

The lattice parameters, magnetic phase transition, Curie temperature and magnetocaloric properties for (Gd1-xTbx)5Si1.72- Ge2.28 alloys with x = 0, 0.15, 0.20 and 0.25 were investigated by X-ray powder diffractometry and magnetization measurements. The results show that suitable partial substitution of Tb in Gd5Si1.72Ge2.28 compound remains the first-order magnetic-crystallographic transition and enhances the magnetic entropy change, although Tb substitution decreases the Curie temperature (TC) of the compounds. The magnetic entropy change of (Gd1-xTbx)5Si1.72Ge2.28 alloys retains a large value in the low magnetic field of 1.0 T. The maximum magnetic entropy change for (Gd0.80Tb0.20)5Si1.72Ge2.28 alloy in the magnetic field from 0 to 1.0 T reaches 8.7 J/(kg·K), which is nearly 4 times as large as that of (Gd0.3Dy0.7)5Si4 compound (|-Smax| = 2.24 J/(kg·K), T_C = 198 K).

High-temperature phase transition behavior and magnetocaloric effect in a sub-rapidly solidified La–Fe–Si plate produced by centrifugal casting

A sub-rapidly solidified LaFe11.6Si1.4 plate was fabricated directly from liquid by centrifugal casting method. The phase constitution, microstructure and magnetocaloric effect were investigated using backscatter scanning electron microscopy, X-ray diffraction, differential scanning calorimetry and phys- ical property measurement system. When the plate was annealed at 1373 K, rl phase was formed by a solid-state peritectoid reaction. A first-order magnetic phase transition occurred in the vicinity of 188 K, and the effective refrigeration capacities reached 203.5J/kg and 209.7J/kg in plates annealed for I h and 3 h, respectively, under a magnetic field change of 3T. It is suggested that centrifugal casting may become a new approach to prepare high-performance La-Fe-Si magnetocaloric plates for prac- tical applications, which could largely accelerate the formation of rl phase during high-temperature heat-treatment process due to refined and homogeneous honeycombed microstructure.

Martensitic transformation,magnetocaloric effect and phase transition strain in Ni50Mn36-xGexSn14 Heusler alloys

In present work,the effect of Ge substitution for Mn on crystal structure and martensitic transformation was carefully investigated in magnetic shape memory Ni_(50)Mn _(36-x)Ge_(x)Sn_(14)(x=0,1) alloys.From X-ray diffraction(XRD) patterns,it can be found that each sample possesses cubic austenitic structure(L21) at room temperature and the main peak(220) shifts to higher degree with Ge doping,indicating that the cell volume of austenitic phase shrinks.With Ge content increasing,martensitic transformation(MT),temperature shifts to higher temperature region and the difference of magnetization between martensitic and austenitic phases(AM) also increases.In addition,the magnetocaloric effect(MCE) and phase transition strain(ΔL/L) were investigated in Ni50Mn35-GeSn14alloy.The maximal magnetic entropy change(ΔSm) associated with martensitic transition is 3.9 J·kg^(-1)-K^(-1)with applied magnetic field change of 5 T and the maximal ΔL/L reaches 0.18% in this alloy.

Quantum Phase Transition and Magnetocaloric Effect in a Tetrameric Chain with Three-Spin Interaction

We investigate the quantum phase transition(QPT) and magnetocaloric effect(MCE) of a tetrameric chain with three-spin interaction using Green's function theory. The magnetization and gap analysis reveals a variety of quantum phases tuned by magnetic field and three-spin interaction, which can open up an energy gap, giving rise to the occurrence of zero magnetization plateau. However, strong three-spin couplings causing strong frustration will destroy the intermediate 1/2 plateau with emergence of a new gapless phase between two cusps. It favors achieving an enhanced MCE at the critical fields, where the minima of isoentropes as well as the valley-peak structure of Gru¨neisen ratio, signaling the accumulation of entropy, lead to cooling via adiabatic(de)magnetization processes. It is also found that the temperature dependence of specific heat combined with Gru¨neisen ratio can testify various quantum phases explicitly.

Magnetic transitions and magnetocaloric effect in MnAs_(0.9)P_(0.1)

The compound MnAs0.9P0.1 exhibits a multistep magnetic order-order transition from a helimagnetic γ-phase with Hα-type magnetic order to a ferromagnetic β-phase at 80 K and then to a helimagnetic α-phase at 203 K. The γ-β transition exhibits the characteristics of a first-order transition with a thermal hysteresis as large as 6 K, while the β-α transition is of second order with a thermal hysteresis smaller than 2 K and without magnetic hysteresis. With these two successive helimagnetism-related transitions, magnetic-entropy changes of -2.1 J/（kg·K） at 203 K for a field change from 0 to 5 T and 0.1 J/（kg·K） at 83 K for a field change from 0 to 1 T are obtained. Investigation of the magnetocaloric effect associated with a transition from Hα-type magnetic order to FM order may open a new route to explore candidates for magnetic refrigeration.

Large reversible cryogenic magnetocaloric effect in rare earth iron carbides of composition RE_(2)FeC_(4)(RE=Ho,Er,and Tm)

At cryogenic temperatures,the investigations of magnetic phase transition and magnetocaloric effect in RE_(2)FeC_(4)(RE=Ho,Er,and Tm) compounds were performed.Ho_(2)FeC_(4)and Er_(2)FeC_(4)compounds undergo two magnetic phase transitions with the temperature decreasing:from paramagnetic(PM) to ferromagnetic(FM) transition at their respective Curie temperature(Tc) and from FM to antiferromagnetic(AFM) or ferrimagnetic(FIM) transition below 2 K.Tm_(2)FeC_(4)compound exhibits only a second-order PM to FM phase transition at TC=K.Large reversible MCE without hysteresis loss is observed in RE_(2)FeC_(4)(RE=Ho,Er,and Tm) compounds.Particularly,the maximum value of magnetic entropy change(-ASM)is 21.62 J/(kg K) under the magnetic field change(Δ_(μ0)H) of 0-5 T for Er_(2)FeC_(4).The Er_(2)FeC_(4)compound presenting excellent magnetocaloric performance makes it a competitive cryogenic magnetic refrigeration material.

Sn掺杂增强Mn-Fe-P-Si巨磁热化合物的力学性能和一级磁相变特性

利用机械合金化和固相烧结法制备了Mn_(1.25)Fe_(0.65)P_(0.50)Si_(0.50)Sn_(x)(x=0,0.04,0.10,0.20)系列化合物。利用X射线衍射仪、振动样品磁强计、台式电子显微镜、电子万能试验机分析了Sn加入后对化合物结构、磁性、磁热及力学性能的影响。结果表明,所有样品的均表现为P-62m六方相结构。随着Sn含量增加,化合物的一级磁相变特性得到显著增强,磁熵变呈增大趋势。在磁场变化为0~2 T时,在x=0,0.04,0.10和0.20时最大磁熵变(-ΔS_(M))分别为6.9,8.0,11.5和17.1 J/(kg·K)。同时,由于Sn的填充效应,化合物力学性能显著提高,最大抗压强度提高了近50%。

相变调控、磁热效应和反常热膨胀

相变作为广泛存在于自然界中的一种现象很早就受到了广泛的关注,并且已经被应用于相变制冷、相变存储、相变储能和负热膨胀等领域中.基于磁热、电热和机械热效应不断发展起来的固态制冷技术具有环保、高效、低噪声和易小型化等优点,被视为替代汽压缩制冷的新型制冷技术.其中,磁热效应是研究历史最悠久的一种.然而,单磁场驱动磁热效应的诸多不足限制了其固态制冷应用,如热效应幅度不够高、滞后损耗大、制冷温跨窄等,因此多场调控和多卡效应应运而生.本文主要介绍笔者团队近期开展的多场调控磁热效应、以及磁热材料的反常热膨胀行为的研究.

Itinerant-electron metamagnetic transition and giant magnetic entropy change in La_(0.8)Ce_(0.2)Fe_(11.4)Si_(1.6) compound

The crystal structure, itinerant-electron metamagnetic transition (IEMT) and magnetocaloric effect (MCE) in the iron-based rare-earth intermetallic compound La0.8Ce0.2Fe11.4Si1.6 have been investi- gated. The powder X-ray diffraction revealed that the ingot of La0.8Ce0.2Fe11.4Si1.6 annealed at 1373 K in vacuum for only 5 days could be crystallized in the cubic NaZn13-type structure. The La0.8Ce0.2Fe11.4Si1.6 compound exhibited giant values of the isothermal entropy change ?SM around the Curie temperature TC (about 186 K). And the maximum value ΔS M max is about 78.29 J/(kg·K) under a field change of 0—3 T, which can be calculated by the magnetization iso- therms around TC. Such a large MCE is attributed to the sharp change of magnetization and susceptibility around TC and the first-order magnetic transition of field-induced IEMT above TC.

Magnetic properties and magnetocaloric effects of Gd65（Cu,Co,Mn）35 amorphous ribbons

The magnetic phase transition and magnetocaloric effects of Gd65(Cu,Co,Mn)35 amorphous ribbons were investigated. The increased Mn substitution for Co or Mn substitution for Cu led into a higher secondorder magnetic phase transformation temperature, near 200 K in all ribbons. Under the field change of 7T, a maximum entropy change(Sm max) of 6 J kg^-1K^-1 was achieved in Gd(65)Cu10Co20Mn5 ribbon and slightly dependent on the compositions. With varied composition, a full width at half of Sm maxwas greatly widened to be over than 180 K for Gd65Cu15Co10Mn(10) ribbon. The refrigeration capacity was also greatly enhanced to 1000J kg^-1 for Gd65Cu20Co10Mn5 ribbon, which was even excellent compared with other Gd-based amorphous systems.

LaFe_(11.17-x)Co_(0.78)Si_(1.05)B_x合金磁热效应的研究

用工业纯原料研究了LaFe11.17-xCo0.78Si1.05Bx(x=0、0.1、0.2、0.3、0.4、0.5)合金在室温附近的磁热效应。B元素作为间隙原子进入晶胞,随着B含量的增加,磁相变点从x=0时的277K升高到x=0.3时的289K。同时,磁熵变和直接测量的绝热温变也随着B含量的增加而增大了,在低磁场具有较大的磁热效应。测量表明合金的热滞和磁滞均很小。