Composition dependence of magnetocaloric effect in Pr0.6Ca0.4Mn(1-x)Cr(x)O3 (x = 0.02-0.08)

We report the effect of varying Cr content on magnetic and magnetocaloric properties of Pr0.6Ca0.4Mn(1-x)Cr(x)O3 samples (x = 0, 0.02, 0.04, 0.06 and 0.08). While the parent compound (x = 0) is a charge ordered and antiferromagnetic insulator, Cr doped compounds are ferromagnetic metals with nearly same Curie temperature (T(c) approximately 140 K). We find unusual field-induced meta-magnetic transition above T(c) in x = 0.02 and 0.04 which is absent in x = 0.06 and 0.08. It is suggested that the paramagnetic phase in these compounds is inhomogeneous with coexistence of nano-size ferromagnetic clusters and short range charge ordered clusters. Field induced growth of ferromagnetic nano-clusters and destruction of short-range charge ordering leads to the observed metamagnetic transition, which results in large magnetic entropy change of -deltaS(M) = 5.043, 6, 5.509 and 4.375 J/kg K under deltaH = 5 T, for x = 0.02, 0.04, 0.06 and 0.08, respectively. In addition, large relative cooling power (RCP) found in these materials (327.384, 286.36, 272.22 and 279.936 J/kg) makes it interesting for practical applications. Our study suggests that creation of ferromagnetic nano-clusters in the paramagnetic phase by Mn-site doping in charge ordered compounds provides an alternative approach to achieve high AS(M) and RCP values.     

Room-temperature Magnetocaloric Effect in (Co_(0.35)Mn_(0.65))_2P Compound

The (Co0.35 Mn0.65)2P compound,prepared by a mechanical alloying plus solid sintering process,exhibits a second-order transition from a ferromagnetic state to a paramagnetic one at Curie temperature of about 320 K with no clear thermal hysteresis.A magnetic-entropy change (△SM) value above 1.6J·kg-1·K-1 for a 5 T field change is obtained in the whole temperature range of 292.5-352.5 K and the maximum value of the △SM is 2.3 J·kg-1·K-1 at 337.5 K.The study on the magnetocaloric effect of the (Co0.35 Mn0.65)2P compound may be helpful for exploring good candidates for room-temperature magnetic refrigeration.     

Magnetic transitions and magnetocaloric effects in Fe0.75Mn1.35As

In Fe_0.75Mn_1.35As compound, a metamagnetic transition from an antiferromagnetic phase to a ferrimagnetic phase can be induced above its phase transition temperature T _s = 165 K by an external magnetic field, which leads to large magnetocaloric effects around T _s. The sign of the magnetic entropy change ΔS _M in the Fe_0.75Mn_1.35As compound is negative, not as expected as an inverse magnetocaloric effect, and the maximum value of ΔS _M is 4.2 J/kg K at 167.5 K for a magnetic field change of 5 T. Although it induces an irreversible lattice expansion, the cycling of a magnetic field does not induce an irreversible change in the magnetic transitions and magnetocaloric behaviors. The antiferromagnetism-related metamagnetic transitions with a large magnetic entropy change may provide with an opportunity in searching novel materials for magnetic refrigeration.     

Magnetic and magnetocaloric properties of nanocrystalline Pr(1-x)A(x)Mn(1-y)Co(y)O3 (A = Ca, Sr) (x = 0.3; y = 0.5) manganites

Structural, magnetic and magnetocaloric properties of sol-gel prepared, nanocrystalline oxides Pr(1-x)A(x)Mn(1-y)Co(y)O3 (A = Ca, Sr) (x = 0.3; y = 0.5) (cubic, space group Fm3m) have been studied. From the X-ray data, the crystallite size of Pro.7Ca0.3Mn0.5Co0,503 and Pr0.7Sr0.3Mn0.5Co0.5O3 samples is found to be approximately 24 nm and approximately15 nm respectively. High resolution transmission electron microscopy image shows average particle size of approximately 34 nm and approximately 20 nm. Magnetization measurements indicate a Curie temperature of approximately 153 K and approximately172 K in applied magnetic field of 100 Oe for Pr0.7Ca0.3Mn0.5Co0.5O3 and Pr0.7Sr0.3Mn0.5Co0.O3 compounds. The magnetization versus applied magnetic field curves obtained at temperatures below 150 K show significant hysteresis and magnetization is not saturated even in a field of 7 T. The magnetocaloric effect is calculated from M versus H data obtained at various temperatures. Magnetic entropy change shows a maximum near T(c) for both the samples and is of the order approximately 2.5 J/kg/K.     

Large magnetocaloric effect and magnetoresistance in ErNi single crystal

The magnetic properties,magnetocaloric effect and magnetoresistance in ErNi single crystal have been investigated in detail.With decreasing temperature,ErNi single crystal undergoes two successive mag-netic transitions:a paramagnetic to ferromagnetic transition at Tc=11 K and a spin-reorientation transition at TSR=5 K.Meanwhile,a sharp field-induced metamagnetic transition is observed below the Tc along the a axis.ErNi single crystal possesses a giant magnetocaloric effect around Tc.The maximum magnetic entropy change is-36.1 J(kg K)-1 along the a axis under the field change of 0-50 kOe.In par-ticular,the rotating magnetocaloric effect in ErNi single crystal reaches its maximum under a relatively low field,and the maximum rotating entropy change with a value of 9.3 J(kg K)-1 is obtained by rotat-ing the applied field from the[011]to[100]directions under 13 kOe.These results suggest that ErNi could be a promising candidate for magnetic refrigeration working at liquid-helium temperature region.Moreover,a complicated transport behavior is uncovered in ErNi single crystal,which is attributed to the complex magnetic states and magnetic polaronic effect.Both positive and negative magnetoresistance are observed.A considerable large magnetoresistance with the value of-34.5％is acquired at 8 K under 50 kOe when the field is along the[100]direction.     

(Nd0.75Na0.25)1-x(Nd0.5Ca0.5)xMnO3中电荷有序稳定性关系的超声研究

系统研究了（Nd0.75Na0.25）1-x（Nd0.5Ca0.5）xMnO3（x=0、0．25、0．5、0．75、1）单相多晶样品在低温下的电磁输运性质和超声特性．电阻和磁化率的测量表明所有样品均发生了电荷有序相变．随着钠掺杂量的增加，电荷有序相变温度（Tco）向低温移动同时低温端磁化强度增大，并且电荷有序态趋向于不稳定和短程化．超声纵波声速从室温开始随着温度的降低逐渐减小，在Tco之后声速急剧硬化．这种超声异常表明体系中存在着强烈的电-声子相互作用，该电-声子耦合来源于Mn^3＋的Jahn-Teller效应．对纵波模量软化部分的拟合显示，随着钠的掺入，反映Jahn-Teller效应大小的Jahn-Teller耦合能EJT变小．分析认为电荷失配效应是导致电荷有序被抑制和Jahn-Teller耦合能EJT变小的主要因素．     

Effect of B-doping on the structural, magnetotransport and magnetocaloric properties of La0.67Ca0.33MnO3 compounds

In this study, the effect of Mn site substitution of B on the structural, electrical and magnetocaloric properties of manganites was investigated. Polycrystalline manganites with the chemical composition La_0.67Ca_0.33Mn_1−xB_xO₃ (x = 0, 0.1, 0.2 and 0.3) were prepared by the standard solid-state process. It was found that the magnetisation, the Curie temperature and the maximum value of the magnetic entropy change |ΔS_m| decrease with increasing concentration of B.     

Effect of Fe Substitution on the Structural,Magnetic and Magnetocaloric Properties of Pr0.6La0.1Mg0.3Mn1−x Fe x O3 (0≤x≤0.3) Perovskite Manganites Prepared by Sol Gel Method

Studies of the structural, magnetic, and magnetocaloric properties of polycrystalline Pr_0.6La_0.1Mg_0.3Mn_1?x Fe _x O₃ (0≤x≤0.3) perovskite manganites were carried out. The compounds were synthesized using the sol–gel method. X-Ray diffraction (XRD) analysis using Rietveld refinement shows that all our compounds crystallize in the orthorhombic structure with the Pnma space group. The surface morphology and elemental analysis of both samples were carried out by scanning electron microscopy (SEM) and the energy dispersive X-ray technique (EDX), respectively. Magnetization measurements versus temperature under magnetic applied field of 0.05 T showed that all our investigated samples display a ferromagnetic-paramagnetic transition. The substitution of Mn by Fe leads to an increase of the Curie temperature T _C from 64 K for x=0 to 380 K for x=0.3. The Arrott plots show that the phase transition is of second order. A large magnetic entropy change |ΔS _M | deduced from isothermal magnetization curves has been recorded in the parent compound Pr_0.6La_0.1Mg_0.3MnO₃ reaching a maximum of 0.79 J/kg K under a magnetic applied field of 2 T close to T _C . Our results on magnetocaloric properties suggest that the Pr_0.6La_0.1Mg_0.3MnO₃ compound is attractive as a possible refrigerant for low temperature magnetic refrigeration.     

Ba0.999Ce0.001Ti0.9Sn0.1O3–La0.75Sr0.25MnO3 Ferroelectric–Ferromagnetic Material

The semiconducting ferroelectric Ba_0.999Ce_0.001Ti_0.9Sn_0.1O₃ and ferromagnetic manganite La_0.75Sr_0.25MnO₃ were prepared by solid-state reactions. The resistivity of Ba_0.999Ce_0.001Ti_0.9Sn_0.1O₃ increases by three orders of magnitude between 350 and 450 K. The Curie temperature of La_0.75Sr_0.25MnO₃ is 370 K, which is 20 K higher than the temperature of the metal–semiconductor transition, accompanied by an increase in resistivity by a factor of 2.5. By firing a mixture of these perovskite oxides, a material was obtained which consisted of La_0.75Sr_0.25MnO₃- and Ba_0.999Ce_0.001Ti_0.9Sn_0.1O₃-based solid solutions. At room temperature, this material possesses both ferroelectric and ferromagnetic properties. At 100 K, its magnetoresistance in a magnetic field of 0.95 T is 18%.     

Pb(Mn1/3Sb2/3)0.05Zr0.47Ti0.48O3压电陶瓷的温度稳定性研究

探讨了硬性添加物MnO2、软性添加物Nb2O5和两性添加物Cr2O3对锑锰锆钛酸铅Pb(Mn1/3Sb2/3)0.05Zr0.47Ti0.48O3(简称PMSZT5)压电陶瓷的相组成及温度稳定性的影响.研究结果发现:各掺杂组成在900℃的煅烧温度下,都可以得到钙钛矿结构.随着各掺杂离子的增大,四方相含量减少,准同型相界向三方相移动.综合考虑离子掺杂对PMSZT5压电陶瓷的机电性能及温度稳定性的研究结果表明:锰过量较其它铌和铬掺杂的温度稳定性更好,机电性能最佳的PMSZT5+0.1wt%MnO2的组成,ε33T/ε0=1560,d33=350pC/N,Kp=0.63,25~80℃的fr、K31和d31平均温度系数分别为72×10-6/℃、0.027%/℃和0.100%/℃.     

Microwave assisted hydrothermal synthesis and magnetocaloric properties of La0.67Sr0.33MnO3 manganite

We report microwave assisted hydrothermal synthesis and magnetocaloric properties of La0.67Sr0.33MnO3 manganite. The synthesized La0.67Sr0.33MnO3 nanoparticles was characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and magnetization measurements. The XRD results indicated that La0.67Sr0.33MnO3 nanoparticles have polycrystalline nature with monoclinic structure. FE-SEM results suggested that La0.67Sr0.33MnO3 nanoparticles are assembled into rod like morphology. Magnetization measurements show that La0.67Sr0.33MnO3 nanoparticles exhibit transition temperature (Tc) above room temperature. The maximum magnetic entropy change (deltaS(M))max was found to be 0.52 J/kg K near Tc approximately 325 K at applied magnetc field of 20 kOe. This compound may considered as potential material for magnetic refrigeration near room temperature.     

Co0.525Fe0.475MnP化合物的室温磁热效应和磁电阻效应

采用多步骤固态烧结方法合成了具有单一Co₂P相的Co_0.525Fe_0.475MnP化合物,其反铁磁有序温度在室温附近。在升温过程中,这种化合物经历两个连续的磁转变：在285 K发生反铁磁到铁磁的一级相变,在375 K发生由铁磁到顺磁的二级相变。在0~5 T的外磁场中,两个相变点温度对应的最大磁熵变分别为1.1 J/(kg·K)(303 K)和-2.0 J/(kg·K)(383 K)。外磁场为零时,随着温度的降低电阻率曲线在铁磁到反铁磁转变温度附近出现极小值,是铁磁有序与反铁磁有序的竞争所致。在35 K再次出现的电阻率极小值,可归因于由Fe替代Co引起的自旋无序所导致的金属-绝缘体转变。在5 T磁场中磁电阻率的最大值对应温度为200 K时的-2.5%,在反铁磁温度以上磁电阻率迅速减小。这表明,这种化合物的磁电阻效应源于外磁场对反铁磁有序的影响。     

Effect of crystal grain dimension on the magnetic properties and magnetocaloric effects in DyCuAl compound

The influence of crystal grain dimension (approximately 38 nm and approximately 90 nm) on magnetic properties and magnetocaloric effects has been investigated for DyCuAl compound. The reduction of crystal grain dimension leads to the decrease of the Curie temperature (from 27 K to 24 K) and the evident enhancement of magnetic anisotropy. The peak values of magnetic entropy change under a field change of 0-5 T are 14.9 J kg(-1) K(-1) and 20.4 J kg(-1) K(-1) for DyCuAl compounds (approximately 38 nm and approximately 90 nm), respectively. A large refrigerant capacity is also produced in DyCuAl compounds (423 J kg(-1) and 379 J kg(-1) for the same field change, respectively). Large magnetic entropy changes and refrigerant capacities jointly make them attractive candidates for low-temperature magnetic refrigerant.     

Mn_2Sb_(0.95)Bi_(0.05)化合物的磁性和电输运性能

通过首先熔炼Sb0.95Bi0.05固溶体,然后再把该固溶体和Mn一起熔炼的多步骤熔炼法,成功地制备了Mn2Sb0.95Bi0.05化合物。样品的热磁曲线测量表明在亚铁磁的Mn2Sb基体中,通过Bi对Sb的替代可以在90K附近诱发亚铁磁到反铁磁转变。与这个一级变磁转变相对应,随着温度的降低,该化合物的电阻曲线在90K附近呈现异常的向上跳跃。电阻异常变化的机理可以被归结为反铁磁态下的超级布里渊区带隙的形成。     

(La0.6Dy0.1)Sr0.3MnO3的磁热效应研究

对庞磁电阻材料(La0.6Dy0．1)Sr0.3MnO3的磁热效应进行了研究．通过不同温度下的等温磁化(M-H)曲线的测量和计算，发现伴随铁磁-顺磁(PM—FM)相变出现大的磁热效应，额外的磁性交换作用将导致额外的磁熵变化．结果表明，(La0.6Dy0．1)Sr0.3MnO3可以作为室温下使用的磁制冷工质候选材料．     

Structural,magnetic and magnetocaloric properties of La0.65Sr0.35V0.1Mn0.9O3 perovskite

A new complex magnetic material La_0.65Sr_0.35V_0.1Mn_0.9O₃, suitable for the magnetic refrigeration, has been investigated. X-ray diffraction result showed that this compound had rhombohedral structure. The substitution of manganese with V leads to a decrease in the Curie temperature, T_C from 378 K to 353 K. Using Arrott plots; it was found that the phase transition for this compound is of the second-order. The magnetocaloric study exposed a quite large value of the magnetic entropy change ～ 1.56 J/kg K and the relative cooling power value of 67 J/kg at magnetic field variation of 1 T.     

Crystal structure,energy gap and photoluminescence investigation of Mn2+/Cr3+-doped ZnS nanostructures by precipitation method

Mn added ZnS (Zn_0.97Mn_0.03S) and Mn–Cr-doped ZnS (Zn_0.95Mn_0.03Cr_0.02S) nanostructures were synthesized by co-precipitation process. XRD pattern confirmed the cubic phase with highest intensity along (111) orientation. The shrinkage of crystallite size from 36 Å (Zn_0.97Mn_0.03S) to 26 Å (Zn_0.95Mn_0.03Cr_0.02S) and the influence of Cr/Mn on microstructural, optical and photoluminescence properties in ZnS were investigated. The substitution of Cr in Zn_0.97Mn_0.03S lattice not only diminished the crystallite size and also produced more defect-associated luminescent activation centres. The elevated micro-strain from 9.71?×?10^–3 (Zn_0.97Mn_0.03S) to 13.11?×?10^–3 (Zn_0.95Mn_0.03Cr_0.02S) by Cr substitution is due to the decrease of size and the higher micro-strain at Cr?=?2% is owing to the drop off of activation energy which is originated from higher electro-negativity of Cr ions than Zn²⁺ ions. The enhanced lattice parameters by Cr doping may be due to the coexistence of both Cr³⁺ ions and Cr²⁺ ions where the existence of Cr²⁺ ions is higher than Cr³⁺ ions and substitute Zn²⁺ basic ions with the ionic radius of 0.74 Å in the Zn–Mn–S host lattice. The presence of Zn²⁺, Mn²⁺ and Cr³⁺ ions in Zn–Mn–Cr–S lattice was confirmed by XPS spectra. SEM/TEM micrographs explored the microstructure and confirmed the sized reduction by Cr doping. The elevation in band gap from 3.50 eV (Zn_0.97Mn_0.03S) to 3.63 eV (Zn_0.95Mn_0.03Cr_0.02S, ?E_g?~?0.13 eV) by Cr addition was explained by Burstein–Moss effect and reduced crystallite size. The tuning of band gap and crystallite size of basic ZnS nanostructure by Mn/Cr substitution encourages these materials for modern electronic applications. FTIR spectra established the occurrence of Mn/Cr in Zn–S lattice by their characteristic bondings. The elevated yellowish-orange emission at 594 nm in Mn/Cr substituted ZnS is due to the exchange communication among the s–p electron states of Cr³⁺, Mn²⁺ and Zn²⁺ ions in Zn–S lattice. The inclusion of Mn /Cr provides an efficient control over modification of various emissions which suggests their applications in organic LED materials.

     

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 second-order magnetic phase transformation temperature, near 200 K in all ribbons. Under the field change of 7 T, a maximum entropy change (ΔS_m^max) of 6 J kg⁻¹ K⁻¹ was achieved in Gd₆₅Cu₁₀Co₂₀Mn₅ ribbon and slightly dependent on the compositions. With varied composition, a full width at half of ΔS_m^max was greatly widened to be over than 180 K for Gd₆₅Cu₁₅Co₁₀Mn₁₀ ribbon. The refrigeration capacity was also greatly enhanced to 1000 J kg⁻¹ for Gd₆₅Cu₂₀Co₁₀Mn₅ ribbon, which was even excellent compared with other Gd-based amorphous systems.     

Magnetic and Magnetocaloric Properties of La0.67Pb0.33?x Ag x MnO3 Compounds

In this paper, we have studied the magnetic and magnetocaloric properties of the perovskite manganites La_0.67Pb_0.33?x Ag _x MnO₃ (0??x??0.15), which show a sharp paramagnetic-ferromagnetic phase transition over a wide temperature range (T=250?C401?K). The Curie temperature has been analyzed by two methods: using the numerical derivative dM/dT and the thermodynamic model. The experimental results indicate that T _C decreases from 368 to 288?K with increasing Ag substitution independently of the method used to obtain?T _C. Upon 10?KOe applied magnetic field, the large magnetic-entropy change (|??S _M|) reaches values of 2.75, 3, 3.25 and 3.5?J/kg?K for x=0, 0.05, 0.10 and 0.15 compositions, respectively, which are comparable to that of?Gd. The relative cooling power (RCP) increases with increasing Ag content from 68.75 (x=0) to 156.27?J/kg (x=0.15) for ??H=10?KOe. Through these results, La_0.67Pb_0.33?x Ag _x MnO₃ materials are strongly suggested for the use of active refrigerants for magnetic refrigeration technology near room temperature.     

LSCM/3YSZ离子-电子混合导体的研究

采用固相法制备了LSCM(La0.75Sr0.25Cr0.5Mn0.5O3)粉体,并以不同质量比与3YSZ((Y2O3)0.03(ZrO2)0.97)混合制成了LSCM/3YSZ离子-电子混合导体(MIEC).采用X射线衍射、扫描电镜、热膨胀分析等手段对该混合导体的各项物理特性进行了研究,发现随着LSCM含量的逐渐增多,混合导体中Zr与Sr、Cr等元素的反应有少量杂相生成;LSCM相也逐步形成相连的空间网络结构;LSCM含量在0～40%范围的混合导体其热膨胀系数在0～1350℃测试温度范围内有一个明显的拐点.使用Wagnet直流极化法分离了混合导体的离子电导率和电子电导率.当LSCM质量百分比达到40%时,离子电导率以及总电导率开始大幅度地增大;当LSCM含量为50%测试温度800℃时样品的总电导率和离子电导率分别为0.24和0.08S/cm.