Effect of Ca on the microstructure and magnetocaloric effects in the La1−xCaxFe11.5Si1.5 compounds

The effect of Ca on the microstructure and magnetocaloric effects has been investigated in the La_1−xCa_xFe_11.5Si_1.5 (x = 0, 0.1, 0.2 and 0.3) compounds. The introduction of Ca leads to the appearance of minor α-Fe and Ca-rich phases, which affects the actual compositions of the main phases for the Ca containing samples. With increasing the Ca concentration, the Curie temperature T_C increases from 183 to 208 K, and the maximum magnetic entropy changes |ΔS| at the respective T_C with a magnetic field change from 0 to 5 T are 21.3, 19.5, 16.9, and 11.2 J/kg K for x = 0, 0.1, 0.2, and 0.3, respectively. The nature of the magnetic transition changes from first-order to second-order with an increase in Ca concentration, which leads to a reduction of the hysteresis and a decrease of the magnetic entropy change. However, the relative cooling power for La_1−xCa_xFe_11.5Si_1.5 compounds remains comparable with or even larger than that of other magnetocaloric materials over a wide temperature range. The higher T_C and the smaller hysteresis in comparison with those of the parent compound suggest that the La_1−xCa_xFe_11.5Si_1.5 compounds could be suitable candidates for magnetic refrigerants in the corresponding temperature range.     

Structure, magnetic, and magnetocaloric properties of amorphous and crystalline La0.4Ca0.6MnO3+δ nanoparticles

We report a systematic study of the effects of size reduction on the magnetic and magnetocaloric properties of amorphous and crystalline La_0.4Ca_0.6MnO_3+δ nanoparticles. The materials were synthesized using a modified wet chemical Pechini route, starting with nitrate precursors to produce the perovskite structure. Phase purity, structure, size, and crystallinity were investigated using XRD and TEM. Thermal treatments resulted in nanocrystals with average diameters of 25 nm, 50 nm, and 130 nm, as well as amorphous particles ∼10 nm in diameter. Magnetic measurements revealed broad, second order ferromagnetic transitions in the nanocrystals. As particle size increased from 10 nm to 130 nm, the Curie temperature shifted from 40 K to 255 K. Magnetization, magnetic entropy change (ΔS_M), and refrigerant capacity (RC) also increased with size in the nanocrystalline samples. For a field change of 5 T, the 130 nm particles exhibit a magnetic entropy change of 2.8 J/kg K and a large refrigerant capacity of ∼240 J/kg at 250 K. Interestingly, the 10 nm amorphous particles undergo the sharpest magnetic transition, leading to a larger value of ΔS_M than in the 25 nm or 50 nm crystalline particles. These results reveal that size reduction has a significant impact on the magnetic and magnetocaloric properties of La_0.4Ca_0.6MnO_3+δ.     

Magnetocaloric effect in the Ce2Fe17−xMnx helical magnets

The Ce₂Fe_17−xMn_x (x = 0-2) compounds demonstrate a complex temperature dependence of the magnetocaloric effect MCE, which is inverse in a narrow temperature interval just below Néel temperature T_N and normal at higher or lower temperatures. The normal MCE exhibits two peaks in the vicinity of temperatures of ferromagnetic ordering Θ_T and T_N for compositions x = 0-0.35, 1.3-2 or one peak near T_N for antiferromagnets with x = 0.5-1. The maximal change of the peak entropy −S_M is about 3 J/kg K in a field of 5 T for the compounds with x = 0-0.5 at T ∼230 K close to T_N. The drastic decrease of the MCE, by half, in the Ce₂Fe_17−xMn_x system is traceable to a decrease of the spontaneous magnetization and the helical type of magnetic states in the compounds.     

The effect of the substitution of Cu for Ni on magnetoresistance and magnetocaloric properties of Ni50Mn34In16

The effect of the substitution of Cu for Ni on the magnetoresistance behaviour and the magnetocaloric properties of the Ni₅₀Mn₃₄In₁₆ alloy has been investigated. The (Ni-Cu)₅₀Mn₃₄In₁₆ alloys crystallize in the cubic L2₁ structure in austenite phase. While the M_s temperature is about 160 K for the Ni_47.5Cu_2.6Mn_34.0In_15.9 alloy, the martensitic transition is not observed for Ni_45.5Cu_4.6Mn_33.8In_16.1 alloy. To estimate the magnetic entropy change of the Ni_47.5Cu_2.6Mn_34.0In_15.9 alloy, the magnetization measurements as function of magnetic field are performed by continuous and noncontinuous heating methods. The Ni_47.5Cu_2.6Mn_34.0In_15.9 alloy shows the magnetostructural transition whose irreversible ratio is about 50% at 160 K. The magnetocaloric effect strongly depends on the sample history due to the occurrence of the irreversible magnetostructural transition. For the magnetic field change of 2 T, giant magnetoresistance value is about −68% that is rather big among the similar bulk alloys.     

Magnetic and magnetocaloric properties of partially disordered RFeAl (R = Gd,Tb) intermetallic

Magnetic and magnetocaloric properties of TbFeAl and GdFeAl were studied in a wide temperature region in magnetic field up to 10 T. They order magnetically below T_c = 196 K and 259 K for TbFeAl and GdFeAl, respectively. The temperature change of ΔT = 1.4 K and 1.6 K was observed in GdFeAl and TbFeAl respectively for a field change of 4 T. Interestingly wide temperature region of significant magnetocaloric effect was observed in both compounds which give rise to relative cooling power of 350 J kg⁻¹ for TbFeAl and 348 J kg⁻¹ for GdFeAl for 4 T field span.     

Structural, magnetic and magnetocaloric properties in Pr0.5M0.1Sr0.4MnO3 (M = Eu, Gd and Dy) polycristalline manganites

Structural, magnetic and magnetocaloric properties of Pr_0.5M_0.1Sr_0.4MnO₃ (M = Eu, Gd and Dy) powder samples have been investigated by X-ray diffraction (XRD) and magnetic measurements. Our samples have been synthesized using the solid state reaction method at high temperature. Rietveld refinements of the X-ray diffraction patterns show that all our samples are single phase and crystallize in the distorted orthorhombic system with Pbnm space group. Magnetization measurements versus temperature in a magnetic applied field of 50 mT show that all our samples exhibit a paramagnetic-ferromagnetic transition with decreasing temperature. The Curie temperature T_C is found to be 270 K, 258 K and 248 K for M = Eu, Gd and Dy, respectively. Arrott plots show that all our samples exhibit a second order magnetic phase transition. From the measured magnetization data of Pr_0.5M_0.1Sr_0.4MnO₃ (M = Eu, Gd and Dy) samples as a function of magnetic applied field, the associated magnetic entropy change |ΔS_M| and the relative cooling power RCP have been determined. In the vicinity of T_C, |ΔS_M| reached, in a magnetic applied field of 1 T, maximum values of 1.37 J/kg K, 1.23 J/kg K and 1.18 J/kg K for M = Eu, Gd and Dy, respectively.     

The Mn-doping effect on the charge ordering state of La1/3Sr2/3FeO3

The longitudinal ultrasonic velocity (V_l), as well as resistivity has been measured in single-phase polycrystalline La_1/3Sr_2/3Fe_1−xMn_xO₃ (x = 0, 0.025, 0.05) at a frequency of 10 MHz, from 20 K to 300 K. It is found that with increasing Mn-doping level, the resistivity increases and the charge ordering transition temperature T_CO shifts to lower temperature. For all samples, upon cooling down from 300 K, a substantial softening in V_l above T_CO and dramatic stiffening below T_CO are observed. This abnormal elastic softening above T_CO can be described well by the mean-field theory, which indicates that this feature is due to the electron-phonon coupling via the Jahn-Teller effect and this coupling is enhanced with the Mn doping. Below T_CO, another softening in V_l is observed for x = 0, and weakens with the increasing of Mn content. This character is attributed to the breathing-type distortion of Fe-O octahedron and suggests that the charge disproportionation (CD) transition is suppressed by the Mn substitution.     

Magnetic behaviour and experimental study of the magnetocaloric effect in the pseudobinary Laves phase Er1−xDyxCo2

We have investigated the crystal structure, the bulk magnetization characteristics and the magnetocaloric properties of Er_1−xDy_xCo₂ compounds. X-ray diffraction (XRD) analyses confirm that all these Laves phase type compounds crystallize in the cubic MgCu₂-type structure. First, the magnetization behaviour and the magnetic transition are analyzed in terms of Landau theory. Then, a direct correlation was pointed out between the character of the magnetic transition and the behaviour of the cell parameter versus x. Substitution of Dy to Er enhances markedly the Curie temperature T_C from 35 to 142 K, while ΔS the corresponding change of isothermal entropy decreases significantly. The refrigerant capacity of the Er_1−xDy_xCo₂ compounds is discussed and our experimental data are compared with the corresponding theoretical results reported in the literature [de Oliveira, von Ranke, J. Magn. Magn. Mater. 264 (2003) 55].     

Magnetism and magnetocaloric effect in the ternary equiatomic REFeAl (RE = Er and Ho) compounds

Magnetic properties and magnetocaloric effect (MCE) in ErFeAl and HoFeAl intermetallic compounds have been studied systematically. Both compounds undergo a second order magnetic phase transition from paramagnetic to ferromagnetic state together with a probable spin reorientation transition at low temperature. A considerable reversible magnetocaloric effect was observed around its own Curie temperatures T_C ∼55 K and 80 K for ErFeAl and HoFeAl, respectively. For a magnetic field change of 0–7 T, the maximum values of magnetic entropy change (−ΔS_M^max) are found to be 8.3 and 9.9 J/kg K for RE = Er and Ho, respectively. The corresponding values of refrigerant capacity (RC) are evaluated to be 384 and 647 J/kg.     

A-site-disorder-dependent magnetocaloric properties in the mono-valent-metal doped La0.7Ca0.3MnO3 manganites

The influence of mono-valence-metal (Li, Na, and K) doping effect on the structural, resistivity, magnetic and magnetocaloric properties of La_0.7Ca_0.3MnO₃ polycrystalline samples is studied for a fixed (5% at Ca site) dopant concentration. All the samples crystallize in orthorhombic structure and the lattice parameters increase continuously as the dopant atoms changes from Li to Na and then K. Paramagnetic-ferromagnetic phase transition at T_C and insulator-metal phase transition at T_p are observed for all studied samples. The transition temperature decreases as Ca atoms is replaced by Li, while the transition temperature shifts to higher values as Ca is substituted by Na or K. In addition, the maximum magnetic entropy change of the K-doped sample is much smaller than that of the free- and Na-doped samples. The results are discussed according to the change of A-site-disorder effect caused by the systematic variations of A-site average ionic radius 〈r_A〉 and A-site-cation mismatch σ².     

The studies of phase relation, microstructure, magnetic transition, magnetocaloric effect in (Gd1−xErx)5Si1.7Ge2.3 compounds

The phase relation, microstructure, Curie temperatures (T_C), magnetic transition, and magnetocaloric effect of (Gd_1−xEr_x)₅Si_1.7Ge_2.3 (x = 0, 0.05, 0.1, 0.15, and 0.2) compounds prepared by arc-melting and then annealing at 1523 K (3 h) using purity Gd (99.9 wt.%) are investigated. The results of XRD patterns and SEM show that the main phases in those samples are mono-clinic Gd₅Si₂Ge₂ type structure. With increase of Er content from x = 0 to 0.2, the values of magnetic transition temperatures (T_C) decrease linearly from 228.7 K to 135.3 K. But the (Gd_1−xEr_x)₅Si_1.7Ge_2.3 compounds display large magnetic entropy near their transition temperatures in a magnetic field of 0-2 T. The maximum magnetic entropy change in (Gd_1−xEr_x)₅Si_1.7Ge_2.3 compounds are 24.56, 14.56, 16.84, 14.20, and 13.22 J/kg K⁻¹ with x = 0, 0.05, 0.1, 0.15, and 0.2, respectively.     

The magnetic structures of RMgSn compounds (R = Ce, Pr, Nd, Tb)

The synthesis of the new compounds RMgSn (R = La-Nd, Sm, Gd-Tm, Lu and Y) has been recently reported. The compounds formed by La and Ce crystallise in the TiNiSi structure type (oP12, Pnma), while from Nd they adopt the CeScSi-type (tI12, I4/mmm); PrMgSn is dimorphic: its high-temperature form (HT) is TiNiSi-type while the low-temperature one (LT) is CeScSi-type.In this paper we now report the results of a neutron diffraction investigation which has been performed in order to refine the crystal as well as the magnetic structures for the RMgSn compounds with R = Ce, Pr, Nd and Tb. All these compounds see at low temperature the establishment of long range magnetic ordering with a predominantly antiferromagnetic interaction; only PrMgSn-HT orders ferromagnetically. These results agree with those from magnetic measurements recently reported.The magnetic structure of CeMgSn is of the amplitude-modulated type, the value of the magnetic propagation vector refined at 2 K is τ = [0, 0.1886(4), 0.3384(8)]. The PrMgSn-HT phase below T = 52 K adopts first a purely ferromagnetic structure, then at about T = 15 K a second magnetic coupling leads to a spin-canted magnetic structure. Both PrMgSn-LT and NdMgSn have the same antiferromagnetic commensurate magnetic structure. The TbMgSn compound below T_N = 35 K orders antiferromagnetically with an equal moment cycloidal structure; however a second magnetic transition at a temperature corresponding to T_N2 = 65 K is likely also present.     

Magnetic properties and magnetocaloric effect of FeCrNbYB metallic glasses with high glass-forming ability

The influences of Cr addition on the Curie temperature (T_C), glass-forming ability (GFA), and magnetocaloric effect were investigated in FeCrNbYB metallic glasses. It was found that the addition of Cr element slightly decreases the GFA and saturation magnetization, whereas effectively modulates T_C. By the method of copper mold casting, bulk metallic glasses (BMGs) with critical diameters up to 5 mm can be obtained in Fe_68−xCr_xNb₄Y₆B₂₂ (x = 2–6) alloys. The resulting metallic glasses exhibit T_C of 271–367 K and excellent magnetocaloric properties, including magnetic entropy change of 0.76–1.05 J/kg K, and refrigerant capacity of 83–93 J/kg under a low field change of 1.5 T. In addition, they exhibit a wide supercooled liquid region of 116–135 K. The successful synthesis of the FeCrNbYB BMGs with near room-temperature magnetocaloric properties is encouraging for the future development of Fe-based BMGs as a new magnetic refrigerant in magnetic cooling system.     

Properties of the GdTX (T = Mn, Fe, Ni, Pd, X = Al, In) and GdFe6Al6 intermetallics

The magnetic and magnetocaloric properties of GdTX (T = Mn, Fe, Ni, Pd, X = Al, In) and GdFe₆Al₆ ternary compounds for possible applications in magnetic refrigeration have been investigated. Magnetization measurements have been performed in the temperature range of 2-400 K and magnetic field range of 0-7 T. The magnetic entropy changes ΔS_m have been calculated indirectly from the magnetization measurements. The calculated values of entropy change ΔS_m for examined compounds amount −13.63 J/K kg, −13.05 J/K kg, −6.13 J/K kg, −3.72 J/K kg, −1.38 J/K kg and −0.94 J/K kg, respectively, for GdNiAl, GdPdAl, GdPdIn, GdFeAl, GdFe₆Al₆ and GdMnAl at 7 T.     

The magnetocaloric properties of GdScSi and GdScGe

The magnetocaloric properties of the ferromagnetic compounds GdScSi (T_C = 354(2) K) and GdScGe (T_C = 349(2) K) (tetragonal CeScSi-type structure refined from single crystal diffractometer data, space group I4/mmm) have been determined. The field dependence of the magnetic entropy ΔS_m change and of the refrigerant capacity RC have been obtained from magnetization measurements. At the T_C Curie temperature, ΔS_m for GdScSi and GdScGe is respectively equal to 2.5 and 3.3 J/kg_Gd K for a magnetic field change of 2 T; the values are comparable to those determined previously for Gd₇Pd₃ and Gd₄Bi₃ which exhibit also a T_C-temperature higher than room temperature. Both microstructural and the analysis of the magnetocaloric response are used to assess the single phase nature of the samples.     

Change of the equilibrium state of ferromagnetic MnBi by high magnetic fields

Differential thermal analysis was carried out for ferromagnetic material MnBi in the temperature range 300-773 K in magnetic fields up to 45 T to investigate the effect of high magnetic fields on its decomposition process and corresponding phase diagram. The decomposition temperature T_t (MnBi → Mn_1.08Bi + liquid Bi) increases from 632 K (at a zero field) to 714 K by applying a magnetic field of 45 T. Furthermore, the magnetocaloric effect of MnBi is observed in 11.5-45 T in the vicinity of 689 K, showing that a field-induced composition process occurs. The obtained results show that the equilibrium state of MnBi can be controlled by a high magnetic field.     

Influence of interstitial and substitutional atoms on the crystal structure of La(FeSi)13

The influence of interstitial hydrogen or carbon on the crystal structure of LaFe_11.5Si_1.5H_δ (δ = 0, 1.2, and 2) and LaFe_11.5Si_1.5C_δ (δ = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) has been investigated based on the Rietveld analyses of powder X-ray diffraction spectra. Effects of Ce substitution for La are also studied for comparison. The incorporation of interstitial atoms causes a lattice expansion of the compounds while leaves the structural symmetry unchanged. Accompanying the lattice expansion, Fe-Fe bond exhibits a concomitant variation. Four of the five Fe-Fe bonds show a tendency to expansion. The largest elongation occurs for the shortest inter-cluster bond, and the relative change is as large as ∼2.53% as δ increases from 0 to 2 for LaFe_11.5Si_1.5H_δ. In contrast, the longest Fe-Fe bond shrinks considerably (−0.97%). Similarly effects on Fe-Fe bonds are produced by the paramagnetic to ferromagnetic transition, though the bond variation is smaller. Increase in Ce content produces, fascinatingly, essentially the same effects as the decrease of interstitial content, though Ce occupies different crystallographic sites than those occupied by interstitial atoms. Influence of interstitial atoms on magnetic behaviors may be dominated by the change of the shortest Fe-Fe bond.     

TbRhSn and DyRhSn – Detailed magnetic and 119Sn Mössbauer spectroscopic studies

The results of magnetic studies and Mössbauer spectroscopic investigations are reported for the stannides TbRhSn and DyRhSn crystallizing in the hexagonal ZrNiAl-type structure. The polycrystalline samples of these ternary intermetallics were synthesized by arc melting from metallic precursors. Detailed ¹¹⁹Sn Mössbauer spectroscopic studies are used to investigate the hyperfine interactions and their temperature evolutions at places occupied by the diamagnetic tin nuclei. Magnetic properties of DyRhSn and TbRhSn were studied by AC/DC magnetometry in a wide temperature range. The results show that both compounds are magnetically ordered at low temperatures. DyRhSn is a non-collinear antiferromagnet with the Néel temperature T_N = 7.5 K, whereas TbRhSn undergoes a transition from a paramagnetic to an antiferromagnetic state at T_N = 20.2 K. An additional transition at T_SR = 10.3 K is detected for TbRhSn which corresponds to some changes in the magnetic moments ordering. The role of the magnetostriction effect in the evolution of the hyperfine parameters and its influence on the observed TbRhSn Mössbauer spectra is discussed. Triangular-like antiferromagnetic arrangements with rare-earth magnetic moments lying in the hexagonal plane are proposed for both compounds at very low temperatures.