The magnetic entropy change in the double perovskite La2NiMnO6 with strong spin–phonon coupling

The magnetocaloric effect and the critical behavior of La₂NiMnO₆ are investigated by measurement of the magnetization around $T_C$. The magnetic entropy change $\left|\Delta S\right|$ of La₂NiMnO₆ for a field change of 0–45 kOe near the Curie temperature is about 5% of the theoretical expectation. The critical behavior of the La₂NiMnO₆ deviates from the mean field theory. These abnormal phenomena are understood in the context of the strong spin–phonon coupling in La₂NiMnO₆. It is suggested that a method of modulating this coupling would enhance the magnetic entropy change greatly, which makes the La₂NiMnO₆ a promising candidate for room-temperature magnetic refrigeration.     

Heat capacity of the spinel superconductors CuRh2S4 and CuRh2Se4

We report low-temperature heat capacity measurements on CuRh₂S₄ and CuRh₂Se₄. These compounds undergo a superconducting transition at $T_c=4.72$ and 3.38 K, respectively, with a fully opened superconducting gap. With increasing magnetic field $H$, the Sommerfeld coefficient $\gamma \left(H\right)$ shows a crossover from linear to sublinear $H$ dependence that is consistent with the predictions of recent theoretical calculations. Compared to CuRh₂Se₄, CuRh₂S₄ exhibits a smaller region of the linear-$H$ dependence in $\gamma \left(H\right)$ and pronounced positive curvature in the temperature dependence of the upper critical field.     

Unusual superconducting behavior in HfV2Ga4

Bulk superconductivity in HfV₂Ga₄ with critical temperature close to 4.1 K was determined via magnetic susceptibility, electrical resistivity and specific heat measurements. Both the upper and lower critical field dependence with reduced temperature ($T/T_c$) exhibit non-conventional behavior. The electronic component of specific heat shows a double-jump, the first close to $T_c$ and the other close to $0.75T_c$. We speculate about the nature of the douple jump observed in specific heat considering two plausable scenarios: bulk inhomogeneities and the existence of a second gap.     

YbNi2: A heavy fermion ferromagnet

Measurements of the magnetization and specific heat of YbNi₂ binary alloy are reported. The DC magnetic susceptibility displays a ferromagnetic behavior with a Curie temperature T_C=10.5 K, one of the highest found in Yb compounds. Moreover, the temperature dependence of the specific heat exhibits a lambda anomaly with a peak of 5.12 J/mol K at 9.4 K. The analysis also shows an additional magnetic contribution around 32 K stemming from the crystalline electric field of a quartet at ${\Delta }_1=72\mathrm{K}$ and a doublet at ${\Delta }_2=126\mathrm{K}$, according to the splitting of the Yb³⁺ ion in cubic symmetry. From the magnetic contribution to the specific heat, a relatively high Kondo temperature $T_K=27\mathrm{K}$ is estimated. Below the magnetic transition, the specific heat shows a huge value of the electronic coefficient ${\gamma }_{\mathit{LT}}=573\mathrm{mJ}/\mathrm{mol}\mathrm{K}$, which is a signature of a heavy fermion behavior. Therefore, this alloy is a fine example of enhanced ferromagnetism and heavy fermion behavior among Yb compounds.     

Table-like magnetocaloric effect and enhanced refrigerant capacity in crystalline Gd55Co35Mn10 alloy melt spun ribbons

Gd₅₅Co₃₅Mn₁₀ ribbons were prepared by melt-spinning and subsequent crystallization treatment. Crystallization resulted in the precipitation of the Gd₃Co-type and Gd₁₂Co₇-type phases in the amorphous matrix. Under a magnetic field change of 0–5 T, a table-like magnetocaloric effect, with a maximum magnetic entropy change ${(?\mathrm{\Delta }{S}_{\mathrm{M}})}^{\max }$ of $5.46\text{J}{\text{kg}}^{?1}{\text{K}}^{?1}$ in the temperature range of 137–180 K and enhanced refrigerant capacity (RC) of $536.4\text{J}{\text{kg}}^{?1}$, was achieved in Gd₅₅Co₃₅Mn₁₀ ribbons crystallized at 600 K for 30 min. The table-like ${(?\mathrm{\Delta }{S}_{\mathrm{M}})}^{\max }$ feature and enhanced RC values make Gd₅₅Co₃₅Mn₁₀ crystallized ribbons promising for Ericsson-cycle magnetic refrigeration in the temperature range from 137 to 180 K.     

Magnetic properties of terbium dihydrides

Magnetization measurements of terbium dihydride samples with different hydrogen concentration, TbH_1.92, TbH_1.99, TbH_2.03 and TbH_2.09, in the temperature range from 1.8 to 300 K and at an applied field of 1 kOe are reported. Two first-order antiferromagnetic transitions have been registered in the narrow temperature range between 15–18 K for all hydride compositions. Isothermal magnetization ($\sigma {\left(H\right)}_{T=\mathrm{const}}$) measurements at several temperatures in the vicinity of magnetic transitions have been carried out and used for the magnetic-entropy change, $\Delta S_{\sigma }$, calculations. The magnetic-entropy changes have negative values, and in an applied magnetic field of 5 T they reach 2.5 J/kg K, which is only about 26% of the $\Delta S_{\sigma }\left(H\right)$ value expected from thermal measurements. A much larger magnetic field must be applied for terbium hydrides to be used as refrigerant materials for low-temperature needs.     

Spin-glass behavior in the antiperovskite manganese nitride Mn3CuN codoped with Ge and Si

Antiperovskite manganese nitrides ${\mathrm{Mn}}_3\left({\mathrm{Cu}}_{0.6}{\mathrm{Si}}_x{\mathrm{Ge}}_{0.4?x}\right)N$ ($x=0.05$, 0.1, 0.15) were prepared and their negative thermal expansion, magnetic and specific heat properties were investigated. A frozen state with a freezing temperature was found at ～207 K in ${\mathrm{Mn}}_3\left({\mathrm{Cu}}_{0.6}{\mathrm{Si}}_{0.15}{\mathrm{Ge}}_{0.25}\right)N$. This indicates that ${\mathrm{Mn}}_3\left({\mathrm{Cu}}_{0.6}{\mathrm{Si}}_{0.15}{\mathrm{Ge}}_{0.25}\right)N$ exhibits a spin glass state at low temperatures. We discussed the cause of spin glass behavior and correlated this spin glass behavior with broadening of the negative thermal expansion operation-temperature window of the manganese nitrides ${\mathrm{Mn}}_3\left({\mathrm{Cu}}_{0.6}{\mathrm{Si}}_{0.15}{\mathrm{Ge}}_{0.25}\right)N$.     

Analysis of relationship between magnetic property and crystal structure of La1−xSrxCrO3 (x=0.13, 0.15)

Dependence of dc magnetization, $M$, of La_1?xSr_xCrO₃ ($x=0.13$, 0.15) polycrystalline specimens on external magnetic field, $H$, was measured at various temperatures. At higher temperatures above 274 K and 265 K for the specimens with $x=0.13$ and 0.15, respectively, almost linear relationships were observed, indicating paramagnetic property. At the lower temperatures, hystereses, indicating canted antiferromagnetic property, were observed in $M\text{–}H$ curves. The temperatures where the magnetic phase transition was observed showed agreement with those where base line shift was observed in differential scanning calorimetry (DSC) measurement. In the $M\text{–}H$ curves of La_1?xSr_xCrO₃ ($x=0.13$, 0.15), saturation magnetization, $M_s$, was not observed at external magnetic fields as high as 70 kOe. At 226 K and 189 K for the specimens with $x=0.13$ and 0.15, respectively, structural phase transition from orthorhombic- to rhombohedral-distorted perovskite was observed in DSC curves. At the temperature, abrupt decreases of residual magnetization, $M_r$, and variation of temperature dependence of coercive force, $H_c$, were detected. We regard that the abrupt decrease of $M_r$ and the variation of temperature dependence of $H_c$ at the structural phase transition temperature can be ascribed to discontinuous variation of Cr–O–Cr angles.     

Exchange integrals and magnetic short range order in the system CdCr2−xGaxSe4(0⩽x⩽0.06)

High-temperature series expansions are derived for the magnetic susceptibility and two-spin correlation functions for a Heisenberg ferromagnetic model on the B-spinel lattice. The calculations are developed in the framework of the random phase approximation and are given for both nearest and next-nearest neighbour exchange integrals $J_1$ and $J_2$, respectively. Our results are given up to order 6 in $\beta =(k_{\mathrm{B}}T{)}^{-1}$and are used to study the paramagnetic region of the ferromagnetic spinel CdCr_2−xGa_xSe₄. The critical temperature $T_{\mathrm{c}}$ and the critical exponents $\gamma$ and $\nu$ associated with the magnetic susceptibility $\chi (T)$ and the correlation length $\xi (T)$, respectively are deduced by applying the Padé approximate methods. The results as a function of the dilution x obtained by the present approach are found to be in agreement with the experimental ones.     

Persistent luminescence and synchrotron radiation study of the Ca2MgSi2O7:Eu2+,R3+ materials

The tetragonal ${\mathrm{Ca}}_2{\mathrm{MgSi}}_2{\mathrm{O}}_7$:${\mathrm{Eu}}^{2+},{\mathrm{R}}^{3+}$ persistent luminescence materials were prepared by a solid state reaction. The UV excited and persistent luminescence was observed in the green region centred at 535 nm. Both luminescence phenomena are due to the same ${\mathrm{Eu}}^{2+}$ ion occupying the single ${\mathrm{Ca}}^{2+}$ site in the host lattice. The ${\mathrm{R}}^{3+}$ codoping usually reduced the persistent luminescence of ${\mathrm{Ca}}_2{\mathrm{MgSi}}_2{\mathrm{O}}_7$:${\mathrm{Eu}}^{2+}$, which differs from the ${\mathrm{M}}_2{\mathrm{MgSi}}_2{\mathrm{O}}_7$:${\mathrm{Eu}}^{2+}$ $(\mathrm{M}=\mathrm{Sr},\mathrm{Ba})$ and ${\mathrm{MAl}}_2{\mathrm{O}}_4$:${\mathrm{Eu}}^{2+}$ $(\mathrm{M}=\mathrm{Ca},\mathrm{Sr})$ materials. Only the ${\mathrm{Tb}}^{3+}$ ion enhanced slightly the persistent luminescence. With the aid of synchrotron radiation, the band gap energy of ${\mathrm{Ca}}_2{\mathrm{MgSi}}_2{\mathrm{O}}_7$:${\mathrm{Eu}}^{2+}$ was found to be about 7 eV that is very similar to those of the ${\mathrm{M}}_2{\mathrm{MgSi}}_2{\mathrm{O}}_7$:${\mathrm{Eu}}^{2+}$ $(\mathrm{M}=\mathrm{Sr},\mathrm{Ba})$ materials. Thermoluminescence results suggested that the ${\mathrm{R}}^{3+}$ ions might act as electron traps, but only the TL peaks created by ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Sm}}^{3+}$ can be found in the temperature range accessible. Lattice defects (e.g. oxygen vacancies) are also important, since the same main thermoluminescence peak was observed at about $100{}^{\circ }\mathrm{C}$ with and without ${\mathrm{R}}^{3+}$ codoping.     

Heavy-fermion superconductivity in Ce2PdIn8

The compound Ce₂PdIn₈ is a recently discovered novel member of the series Ce_nTIn_3n+2, where T=d-electron transition metal, and $n=1$ or 2. So far, only the phases with T=Co, Rh and Ir have been intensively studied for their unconventional superconducting behavior at low temperatures. By means of magnetic susceptibility, electrical resistivity and heat capacity measurements we provide evidence that Ce₂PdIn₈ also has a superconducting ground state with a strong heavy-fermion character. The clean-limit superconductivity sets in at $T_c=0.7K$ at ambient pressure, likely at a verge of a quantum phase transition that manifests itself in a form of distinct non-Fermi liquid features in the bulk normal state characteristics.     

Complex structures adapted to magnetic flows

Let $M$ be a compact real-analytic manifold, equipped with a real-analytic Riemannian metric $g$, and let $\beta$ be a closed real-analytic 2-form on $M$, interpreted as a magnetic field. Consider the Hamiltonian flow on $T^{1}M$ that describes a charged particle moving in the magnetic field $\beta$. Following an idea of T. Thiemann, we construct a complex structure on a tube inside $T^{1}M$ by pushing forward the vertical polarization by the Hamiltonian flow “evaluated at time $i$”. This complex structure fits together with $\omega -{\pi }^1\beta$ to give a Kähler structure on a tube inside $T^{1}M$. When $\beta =0$, our magnetic complex structure is the adapted complex structure of Lempert–Szőke and Guillemin–Stenzel.We describe the magnetic complex structure in terms of its $(1,0)$-tangent bundle, at the level of holomorphic functions, and via a construction using the embeddings of Whitney–Bruhat and Grauert. We describe an antiholomorphic intertwiner between this complex structure and the complex structure induced by $-\beta$, and we give two formulas for local Kähler potentials, which depend on a local choice of vector potential $1$-form for $\beta$. Finally, we compute the magnetic complex structure explicitly for constant magnetic fields on ${\mathbb{R}}^2$ and $S^2$.     

Magnetic properties and the metal–insulator transition in GdXMn1−XS solid solutions

The structural, magnetic, electrical, and thermoelectric properties of Gd_XMn_1?XS ($0.01\le X\le 0.3$) solid solutions synthesized for the first time on the basis of $\alpha$-MnS have been studied experimentally in the temperature range 77–1000 K in magnetic fields up to 10 kOe. The synthesized samples are antiferromagnetic semiconductors with an NaCl-type cubic lattice typical of $\alpha$-MnS. Both concentration ($X_c=0.3$) and temperature ($T_c=450K$) metal–insulator phase transitions have been observed. The concentration metal–insulator transition is accompanied by a decrease in resistivity and thermopower by twelve and two orders of magnitude, respectively, with the change in conductivity from p-type to n-type.