Magnetism and structural chemistry of ternary silicides: (RE,Th, U)Pt2Si2 (RE = rare earth)

Ternary silicides (RE, U, Th)Pt₂Si₂ have been prepared from the elements. All the compounds (RE= Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and U, Th) were found to be isotypic and crystallize with the primitive tetragonal CePt₂Si₂-type structure closely related to the CaBe₂Ge₂-type. The magnetic properties of these alloys were studied in the temperature range 1.5 K < T < 1100 K and in fields up to 1.3 T revealing a typical Van Vleck paramagnetism of free RE³⁺-ions for temperatures T > 200 K. A nonmagnetic ground state is reflected from the magnetic susceptibility data of CePt₂Si₂, which are interpreted in terms of interconfiguration fluctuations (ICF). The magnetic results of SmPt₂Si₂ (μ_eff = 0.7 BM) compare well with the ideal Van Vleck behavior of Sm₃₊ ions with a $\text{J =}\text{5}\text{2}$ ground state and a low-lying excited first level $\text{J =}\text{7}\text{2}$. At temperatures below 40 K antiferromagnetic ordering is found for (Gd, Tb, U)Pt₂Si₂; whereas in case of (Dy, Ho, Er, Tm)Pt₂Si₂ the onset of ferromagnetism is indicated below 4 K. None of the samples exhibited a superconducting transition above 1.8 K.     

Magnetic properties and structural data of ternary silicides: (RE,Th,U) Os2Si2 (RE= rare earth)

New ternary silicides (RE,Th,U) Os₂Si₂ have been synthesized from the elements. All the compounds (RE= Y,La,Ce,Pr,Nd,Sm,Gd,Tb,Dy,Ho,Er,Tm,Yb, Lu) were found to be isotypic and to crystallize with the ordered BaAl₄-type of structure (ThCr₂Si₂-type). Magnetic properties of these alloys — studied in the temperature range 1.5<T<1100 K — reveal a typical Van Vleck paramagnetism of free RE³⁺ ions at temperatures higher than 300 K. The observed effective paramagnetic moment of CeOs₂Si₂, μ(eff)=0.98 BM, is compatible with a rather low concentration (15%) of Ce³⁺. The effective moment of SmOs₂Si₂, μ(eff)=0.47 BM, is in reasonable agreement with a Hund's rule $\text{J=}\text{5}\text{2}$ ground level for free Sm³⁺. For temperatures above 25 K, the magnetic susceptibility as a function of temperature corresponds to the Van Vleck behavior for free Sm³⁺ (closely spaced multiplet, $\text{J=}\text{5}\text{2}$,$\text{J=}\text{7}\text{2}$. Magnetic ordering temperatures of REOs₂Si₂ silicides are generally below 42 K. (Pr,Nd,Ho,Er,Tm) Os₂Si₂ exhibit ferromagnetic ordering whereas (Sm,Gd, Tb,Dy) Os₂Si₂ show antiferromagnetic behavior. Above 1.8 K none of the samples was found to be superconducting.     

Magnetic properties of the rare-earth diborodicarbides (RB2C2)

The magnetic susceptibility of RB₂C₂ has been measured in the temperature range of 3–300 K. Curie-Weiss fits to the susceptibilities led to effective moments in agreement with those expected for R³⁺ ions. The RB₂C₂ (R = Ce, Nd, Sm, Gd, Tb, Er, and Tm) compounds are antiferromagnetic. Metamagnetic transitions at low fields were observed for CeB₂C₂ and TbB₂C₂. The compounds, DyB₂C₂ and HoB₂C₂, are ferromagnets with complex magnetic structures. Praseodymium borocarbide becomes a Van Vleck paramagnet at low temperature. The magnetic ordering temperatures of these compounds are discussed in terms of their crystal structure and the RKKY theory.     

Itinerant and local magnetism,superconductivity and mixed valency phenomena in RM2Si2, (R = rare earth,M = Rh,Ru)°

X-Ray, magnetization and Mossbauer (¹⁵¹Eu, ¹⁵⁵Gd, ¹⁶¹Dy and dilute ⁵⁷Fe) studies of RM₂Si₂ reveal that when R is a magnetic ion the compounds order antiferromagnetically. For M = Rh a second antiferromagnetic phase transition is observed, corresponding to Rh itinerant electron magnetic ordering. In EuRh₂Si₂ the Eu ion is predominantly divalent with a mixed valent component. In EuRu₂Si₂ the Eu is predominantly trivalent. LaRu₂Si₂ and LuRu₂Si₂ display enhanced electron paramagnetism and become superconducting at 3.5 K and 2.4 K respectively. LaRh₂Si₂, YRh₂Si₂ and LuRh₂Si₂ display an itinerant electron magnetic phase transition, T_M (LaRh₂Si₂) = 7 K, and at lower temperatures a superconducting phase transition, T_c(LaRh₂Si₂) = 3.8 K. There is evidence that in the superconducting phase the itinerant magnetic order survives.     

Magnetic properties of CaNi2P2 and the corresponding lanthanoid nickel phosphides with ThCr2Si2 type structure

CaNi₂P₂ is Pauli paramagnetic with slightly increasing susceptibility at lower temperatures. The same temperature dependence is observed for the susceptibility of LaNi₂P₂. The absolute values of this metallic conductor, however, are shifted to the diamagnetic region. At temperatures above 100 K the compounds LnNi₂P₂ with Ln  Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm show normal Curie-Weiss behavior with magnetic moments slightly lower than those of the free Ln³⁺ ions. The paramagnetic Curie temperatures of these compounds vary between −5 and +12 K. The well established intermediate valence compound EuNi₂P₂ also shows a liner reciprocal susceptibility vs temperature dependence, however, the corresponding paramagnetic Curie temperature is much lower: Θ = −124 K. Ferromagnetic ordering is inferred from the positive paramagnetic Curie temperatures for PrNi₂P₂ (Θ = 12 K) and NdNi₂P₂ (Θ = 10 K). GdNi₂P₂ is antiferromagnetic with a Néel temperature of T_N = 10.5 K. SmNi₂P₂ shows Van Vleck paramagnetism. CeNi₂P₂ and YbNi₂P₂ exhibit temperature dependent paramagnetism, which may be rationalized with a mixed valence of cerium and ytterbium. Down to 1.8 K no superconducting transitions could be observed for CaNi₂P₂, LaNi₂P₂ or CeNi₂P₂. The magnetic properties of these compounds are discussed in accordance with a simple band structure model derived from Zintl's concept.     

Magnetic properties of RCoSi2 compounds (R=rare earth)

New ternary silicides of composition RCoSi₂ (R=rare earth and Y) have been prepared and found to crystallize in the orthorhombic CeNiSi₂-type structure. Their magnetic properties have been studied by means of susceptibility measurements between 2 and 250 K. The Ce and Y compounds show essentially temperature independent Pauli paramagnetism. The compounds with R=Nd, Sm, Gd, Tb, Dy, Ho, Er and Tm show antiferromagnetic ordering below 20 K. The effective rare earh moments in the paramagnetic state agree well with the free ion values, and, for the heavy rare earths, the Néel temperatures vary with the De Gennes factor. There is no indication for a magnetic contribution from the Co sublattice.     

Magnetic properties of RE2Au compounds (RE = Pr,Nd, Gd,Tb, Dy,Ho, Er,Tm and Y)

Magnetic properties of nine RE₂Au compounds have been studied in fields of up to 19 kOe in the temperature range 4.2K–300K. It has been found that all compounds are paramagnetic at room temperature except Gd₂Au. The compounds with Pr, Nd, Ho, Er and Tm exhibit Curie-Weiss behaviour with paramagnetic moments in close agreement with those expected for the free RE³⁺ ion. The moment of gold was found to be zero. The compounds with Pr, Nd, Tb, Dy, Er and Tm are antiferromagnetic at low temperatures. It appears that Ho₂Au is ferromagnetically ordered below 4.5 K. No evidence for magnetic ordering was found for Y₂Au. The compound with Tb exhibits metamagnetic behaviour.     

Magnetic ordering in NdRh2Si2 and ErRH2Si2

Neutron diffraction and magnetization study of polycrystalline NdRh₂Si₂ and ErRh₂Si₂ was performed in the temperature range from 4.2 to 293 K. Both compounds are of ThCr₂Si₂ type crystal structure and exhibit antiferromagnetic ordering below T_N = 53 K and T_N = 12.8 K respectively. The magnetic structure wave vector is τ = [0, 0, 1].     

Directional frustration of magnetic moments in (U0.50Dy0.50)Ni2B2C

Polycrystalline (U_0.50Dy_0.50)Ni₂B₂C solid solution was prepared and found by X-ray diffraction to crystallize in BCT LuNi₂B₂C-type structure (space group I4/mmm) of the end compounds UNi₂B₂C and DyNi₂B₂C. AC susceptibility and magnetization show paramagnetic behavior down to 6.5 K, with the values θ=−5(5) K and μ_eff=7.7(1) μ_B, compatible with those of the end compounds, and indicate possible cooperative phenomena at lower temperatures. The observed paramagnetism, at variance with antiferromagnetic ordering in (Pr_0.50Dy_0.50)Ni₂B₂C, is attributed to a directional frustration of the magnetic moments on the (U,Dy) site.     

Structural and magnetic phase transitions of kagome-like compounds REBaCo4O7 (RE=Dy,Ho, Er,Tm, Yb,Lu)

In a temperature range 5–300 K the specific heat C(T) on a new mixed valence cobalt oxides REBaCo₄O₇ (RE=Dy, Ho, Er, Tm, Yb, Lu) was investigated. The first-order structural phase transitions from hexagonal P6₃mc to orthorhombic Cmc₂1 phase was indicated by a peak-like anomaly in C(T) curves at T_S～160, 178, 224, and 280 K for RE=Lu, Yb, Tm, and Er correspondingly. The magnetic phase transitions was indicated as the changes of slope on the C(T) curves were found at corresponding temperatures: T_N～50, 74, 98, and 98 K for RE=Lu, Yb, Tm, and Er, correspondingly.     

The effects of elements doping on transport and thermoelectric properties of Sr3Ti2O7

The Ruddlesden–Popper (RP) phase compounds (Sr_0.95R_0.05)₃Ti₂O₇ (R=Er, Y, Dy, Gd, Eu, Sm, Nd and La) were prepared, and their transport and thermoelectric properties were investigated. The results indicate that high-T electrical resistivity ρ (300 K<T<1000 K) increases monotonically with temperature and basically has a relation ρ∝T^M, with M varying from 0.91 to 1.92 at temperatures T>~650 K, suggesting acoustic phonon scattering is dominant. At low temperatures (5 K<T<300 K), ρ for (Sr_0.95R_0.05)₃Ti₂O₇ (R=Nd and La) decreases monotonously with decreasing temperature, whereas ρ for (Sr_0.95R_0.05)₃Ti₂O₇ (R=Er, Y, Dy, Gd, Eu and Sm) decreases first, and then increases instead as T decreases to a critical temperature T_c. Moreover, electrical conductivity σ∝T^1/2 holds at lower temperatures, indicating that the electron–electron interaction caused by the presence of disorder dominates the transport process at the low temperatures. Besides, experiments show that at T<~400 K the lattice thermal conductivity of the doped compounds basically decreases with increase of the atomic mass of dopants. Generally, the figure of merit (ZT) at 1000 K increases first, and then decreases with the increase of the dopants' ionic radius, and the largest ZT is achieved in (Sr_0.95Gd_0.05)₃Ti₂O₇ mainly owing to its lower lattice thermal conductivity.     

Magnetic properties of R3Co29Si4B10 (R=La, Ce, Pr, Nd, Sm, Gd and Dy) borides

The crystal structure and magnetic properties of quaternary rare-earth intermetallic borides R₃Co₂₉Si₄B₁₀ with R=La, Ce, Pr, Nd, Sm, Gd and Dy have been studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in a tetragonal crystal structure with the space group P4/nmm. Compounds with R=La, Ce, Pr, Nd and Sm are ferromagnets, while ferrimagnetic behavior is observed for R=Gd and Dy. The Curie temperatures vary between 149 K and 210 K. The Curie temperatures in R₃Co₂₉Si₄B₁₀ (R=Ce, Pr, Nd, Sm, Gd, Dy) compounds are roughly proportional to the de Gennes factors.     

Magnetic and structural studies of the alloy system REIn0.5Ag0.5

The structural and magnetic properties of the alloy system REIn_0.5Ag_0.5 [RE = Gd, Tb, Dy, Ho, Er, Tm and Yb] are reported. All these alloys (except that of Yb) crystallize in a cubic CsCl type structure at room temperature. Low temperature X-ray diffraction data does not reveal any structural phase transformation down to 8 K. On the basis of magnetic susceptibility data at a different temperature (3–300 K) and applied magnetic field (2 × 10⁵ to 8 × 10⁶ A m^-1, it has been concluded that GdIn_0.5Ag_0.5 is ferromagnetic (T_c = 118 K), TbIn_0.5Ag_0.5 and DyIn_0.5Ag_0.5 are meta magnetic (T_N = 66 and 30 K, respectively) and alloys involving Ho, Er, Tm and Yb are ferrimagnetic with Néel temperatures (T_N) equal to 24, 22, 21 and 20 K, respectively. The evaluated effective magneton number (p) is found to be slightly larger compared to theoretical values for tripositive ions of Gd, Tb and Dy and a bit smaller for Ho, Er, Tm and Yb. The results have been qualitatively explained using appropriate theories.     

Investigation of TmFe4Al8 and DyFe4Al8 by means of 169Tm and 161Dy Mössbauer spectroscopy

We have performed ¹⁶⁹Tm and ¹⁶¹Dy Mössbauer spectroscopy on TmFe₄Al₈ and DyFe₄Al₈. From the temperature dependence of the electric quadrupole splitting of the ¹⁶⁹Tm spectra of TmFe₄Al₈ we have determined the second order crystal field potential V⁰₂ = (100 ± 10) K and the exchange field term g_Jμ_BH_M = (1 ± 1) K. The temperature dependence of the hyperfine field of the ¹⁶¹Dy spectrum of DyFe₄Al₈ gives g_Jμ_BH_M = (15 ± 3) K. With these exchange fields magnetic transition temperatures of the rare earth sublattices were found, which are consistent with experiment. The relaxation behaviour of the Tm sublattice below T_N = 187 K is discussed.     

Magnetic anomalies in single crystalline ErPd2Si2

Considering certain interesting features in the previously reported ¹⁶⁶Er Mössbauer effect, and neutron diffraction data on the polycrystalline form of ErPd₂Si₂ crystallizing in the ThCr₂Si₂-type tetragonal structure, we have carried out magnetic measurements (1.8–300 K) on the single crystalline form of this compound. We observe significant anisotropy in the absolute values of magnetization (indicating that the easy axis is c-axis) as well as in features due to magnetic ordering in the plot of magnetic susceptibility χ versus temperature T at low temperatures. The χ(T) data reveal that there is a pseudo-low-dimensional magnetic order setting in at 4.8 K, with a three-dimensional antiferromagnetic order setting in at a lower temperature (3.8 K). A new finding in the χ(T) data is that, for H∥〈1 1 0〉 but not for H∥〈0 0 1〉, there is a broad shoulder in the range 8–20 K, indicative of the existence of magnetic correlations above 5 K as well, which could be related to the previously reported slow-relaxation-dominated Mössbauer spectra. Interestingly, the temperature coefficient of electrical resistivity is found to be isotropic; no feature due to magnetic ordering could be detected in the electrical resistivity data at low temperatures, which is attributed to magnetic Brillioun-zone boundary gap effects. The results reveal the complex nature of magnetism of this compound.     

Oscillations in magnetocaloric effect and magnetic properties of RE2Al3Si2 (for RE=Dy,Ho and Er) and REAlSi (for RE=Ce and Pr)

We present the magnetic and thermal properties of a series of compounds RE₂Al₃Si₂ for RE=Dy, Ho, Er, and REAlSi for RE=Pr, Ce. The 2–3–2 family crystallizes with themonoclinic Y₂Al₃Si₂-type structure while the 1–1–1 family crystallizes in the body-centered tetragonal α-ThSi₂-type structure. The measurements were done on single crystals, grown using high-temperature flux technique and molten Al as a solvent . Susceptibility and heat capacity data were taken from 1.8 to 200 K, from the heat capacity data, the isothermal magnetic entropy change was obtained. Our results indicate signal oscillations in magnetocaloric properties for those compounds, in particular, Dy₂Al₃Si₂ that shows an anomaly that can be associated with a spin reorientation. Similar results are known for some Dy discilicides and dialluminades.     

NMR studies of magnetic superconductor RuSr2RECu2O8 (RE=Gd, Eu and Y)

We have carried out ^99/101Ru and ^63/65Cu nuclear magnetic resonance experiments in order to investigate magnetic and electronic properties of the magnetic superconductor RuSr₂RECu₂O₈ (RE=Gd, Eu and Y). The two kinds of ^99/101Ru signals were observed in the magnetically ordered state for each system, suggesting a charge segregation of Ru⁵⁺ (S=3/2) and Ru⁴⁺ (S=1) ions in the RuO₂ layers. The internal field at the Cu sites is revealed to be of the order of kilo Oe, indicating weak magnetic interactions between the CuO₂ and RuO₂ planes. The temperature dependence of nuclear spin-lattice relaxation time T₁ of ⁶³Cu in RE=Y shows a ‘spin gap’ like behavior, suggesting the system is under-doped.     

Low field magnetoresistance and conduction noise in layered manganite La1.4Ca1.6Mn2O7

Temperature dependence of conduction noise and low field magnetoresistance of layered manganite La_1.4Ca_1.6Mn₂O₇ (DLCMO) are reported and compared with the infinite layered manganite La_0.7Ca_0.3MnO₃ (LCMO). The double layered manganite was prepared using standard solid state reaction method and had a metal-insulator transition temperature (T_M-I) of 155 K. The temperature dependence of susceptibility showed evolution of ferromagnetic ordering at 168 K. The observed voltage noise spectral density (S_V) shows 1/f^α type of behaviour at all temperatures from 77 K to 300 K. In the ferromagnetic region (T<168 K), S_V/V² shows two peaks at 164 K and 114 K. The observed two peaks in normalised conduction noise of DLCMO is attributed to the excess noise generated due to setting up of short range 2D-ferromagnetic ordering and long range 3D-ferromagnetic ordering at two different temperatures T_C2 and T_C1. In temperature range between T_C1 and T_C2, the magnetoresistance (MR) showed a gradual increase with the magnetic field. The observed MR has been explained in the framework of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions].     

Thermoluminescence studies of LiBa2B5O10:RE (RE=Dy, Tb and Tm)

LiBa₂B₅O₁₀:RE³⁺ (RE=Dy, Tb and Tm) was synthesized by the method of high-temperature solid-state reaction and the thermoluminescence (TL) properties of the samples under the irradiation of the γ-ray were studied. The result showed that Dy³⁺ ion was the most efficient activator. When the concentration of Dy³⁺ was 2 mol%, LiBa₂B₅O₁₀:Dy³⁺ exhibited a maximum TL output. The kinetic parameter of LiBa₂B₅O₁₀:0.02Dy was estimated by the peak shape method, for which the average activation energy was 0.757 eV and the frequency factor was 1.50×10⁷ s⁻¹. By the three-dimensional (3D) TL spectrum, the TL of the sample was contributed to the characteristic f-f transition of Dy³⁺. The dose-response of LiBa₂B₅O₁₀:0.02Dy to γ-ray was linear in the range from 1 to 1000 mGy. In addition, the decay of the TL intensity of LiBa₂B₅O₁₀:0.02Dy was also investigated.     

Magnetic interactions in RECu2Si2 compounds (RE = Tb-Tm)

The RKKY theory and the molecular field model including CEF effects (by Noakes and Shenoy) are applied for the explanation of magnetic interactions in the RECu₂Si₂ family (RE = Tb-Tm). The negative sign of the obtained exchange integral J_sf may point to the presence of interband mixing. The predicted magnetic anisotropy and enhancement of ordering temperatures T_N-5 above the de Gennes values are consistent with experimental ones only on the basis of the B⁰₂ model with parameter A⁰₂ as for CdCu₂Si₂. They are not consistent if one uses the full CEF Hamiltonian with 5 A^m₁ parameters as for TmCu₂Si₂. A short discussion about the role of magnetic interactions other than the simple RKKY exchange is also given.