Magnetic properties for the Cu2MnSnSe4 and Cu2FeSnSe4 compounds

Magnetic susceptibility χ measurements in the range from 2 to 300 K were carried out on samples of the Cu₂FeSnSe₄ and Cu₂MnSnSe₄ compounds. It was found that Cu₂FeSnSe₄ was antiferromagnetic showing ideal Curie-Weiss behavior with a Néel temperature T_N of about 19 K and Curie-Weiss temperature θ=−200 K, while for Cu₂MnSnSe₄ the behavior was spin-glass with a freezing temperature T_f of about 22 K and Curie-Weiss temperature θ=−25 K. The spin-glass order parameter q(T), determined from the susceptibility data, was found to be in agreement with the prediction of conventional spin-glass theory.     

Raman study of BiFeO3 with different excitation wavelengths

Raman spectra of bismuth ferrite (BiFeO₃) over the frequency range of 100-1500 cm⁻¹ have been systematically investigated with different excitation wavelengths. The intensities of the two-phonon modes are enhanced obviously under the excitation of 532 nm wavelength. This is attributed to the resonant behavior when incident laser energy closes to the intrinsic bandgap of BiFeO₃. The Raman spectra of BiFeO₃ excited at 532 nm were measured over the temperature range from 77 to 678 K. Besides the abnormal changes of the peak position and the linewidth of the A₁ mode at 139 cm⁻¹, the prominent frequency shift, the line broadening and the decrease of the intensity for the two-phonon mode at 1250 cm⁻¹ were observed as the temperature increased to Néel temperature (T_N). All these results indicate the existence of strong spin-phonon coupling in BiFeO₃.     

Mössbauer study of CoFeRhO4

CoFeRhO₄ has been studied by Mössbauer spectroscopy and X-ray diffraction. The crystal is found to have a cubic spinel structure with the lattice constant a₀=8.451±0.005 Å. The iron ions are in ferric states. The temperature dependence of the magnetic hyperfine field is analyzed by the Néel theory of ferrimagnetism. The intersublattice superexchange interaction is antiferromagnetic and strong with a strength of J_AB=−12.39k_B while the intrasublattice superexchange interactions are weak with strengths of J_AA=−4.96k_B and J_BB=6.20k_B. As the temperature increases toward the Néel temperature T_N, a systematic line broadening effect in the Mössbauer spectrum is observed and interpreted to originate from different temperature dependences of the magnetic hyperfine fields at various iron sites.     

Spin-glass-like behaviour and magnetic order in Mn[Cr0.5Ga1.5]S4 spinels

Magnetization and susceptibility were investigated as a function of temperature and magnetic field in polycrystalline Mn[Cr_0.5Ga_1.5]S₄ spinel. The dc susceptibility measurements at 919 Oe showed a disordered ferrimagnetic behaviour with a Curie-Weiss temperature θ_CW=−55 K and an effective magnetic moment of 5.96 μ_B close to the spin-only value of 6.52 μ_B for Cr³⁺ and Mn²⁺ ions in the 3d³ and 3d⁵ configurations, respectively. The magnetization measured at 100 Oe revealed the multiple magnetic transitions with a sharp maximum at the Néel temperature T_N=3.9 K, a minimum at the Yafet-Kittel temperature T_YK=5 K, a broad maximum at the freezing temperature T_f=7.9 K, and an inflection point at the Curie temperature T_C=48 K indicating a transition to paramagnetic phase. A large splitting between the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations at a temperature smaller than T_C suggests the presence of spin-glass-like behaviour. This behaviour is considered in a framework of competing interactions between the antiferromagnetic ordering of the A(Mn) sublattice and the ferromagnetic ordering of the B(Cr) sublattice.     

Charge and lattice dynamics of ordered state in La1/2Ca1/2MnO3: infrared reflection spectroscopy study

We report an infrared reflection spectroscopy study of La_1/2Ca_1/2MnO₃ over a broad frequency range and temperature interval which covers the transitions from the high temperature paramagnetic to ferromagnetic and, upon further cooling, to antiferromagnetic phase. The structural phase transition, accompanied by a ferromagnetic ordering at T_C=234 K, leads to enrichment of the phonon spectrum. A charge ordered antiferromagnetic insulating ground state develops below the Néel transition temperature T_N=163 K. This is evidenced by the formation of charge density waves and opening of a gap with the magnitude of 2Δ₀=(320±15) cm⁻¹ in the excitation spectrum. Several of the infrared active phonons are found to exhibit anomalous frequency softening. The experimental data suggest coexistence of ferromagnetic and antiferromangetic phases at low temperatures.     

Effect of hydrostatic pressure on the Neel temperature and the electrical residual resistivity of α-manganese

The electrical resistivity of α-Mn was measured from 4.2 K to 290 K at pressures up to 8 kbar. The residual resistivity was found not to depend on pressure. The Néel temperature decreases linearly with pressure at a slope dT_N/dP = − 1.7 ± 0.2 K/kbar.     

Magnetoelastic properties of Nd6Fe13Cu intermetallic compound

Magnetoelastic properties of Nd₆Fe₁₃Cu intermetallic compound are reported. To study the magnetoelastic behaviour of this compound, the thermal expansion as well as the longitudinal (λ_l) and transverse (λ_t) magnetostriction were measured by using the strain gauge method in the selected temperature range of 80-500 K under applied magnetic fields up to 1.5 T. An anomaly and invar-type effects are observed in the linear thermal expansion and α(T) curves at the Néel temperature. The linear spontaneous magnetostriction decreases sharply by approaching the Néel temperature and also shows the short-range magnetic ordering effects when antiferromagnetic-paramagnetic transition occurs. In the low field region, the absolute values of the anisotropic magnetostriction are small and then start to increase with applied magnetic field. Each isofield curve of the anisotropic magnetostriction passes through a minimum and then approaches to zero with increasing temperature. This magnetostriction compensation arises from the difference in the magnetoelastic coupling constants of the sublattices in this compound.     

Crystal structure and magnetism of UFe3B2

The structure of the UFe₃B₂ compound has been refined down to R=0.022 and wR₂=0.052 from single crystal X-ray diffraction data. This uranium boride crystallizes in the CeCo₃B₂ type-structure (P6/mmm space group no. 191, Z=1, ρ=10.79 g/cm³), with lattice parameters at room temperature a=0.5052(1) nm, c=0.3002(1) nm and V=0.664(1) nm³. Magnetization measurements made between 2 K and 800 K suggested that UFe₃B₂ is an antiferromagnet with a rather high Néel temperature of T_N=268±5 K. No other magnetic transitions were observed down to the lowest studied temperature.     

Influence of temperature on critical fields in ZnCr2Se4

The complex ac dynamic magnetic susceptibility was used to study the influence of temperature on critical fields in polycrystalline ZnCr₂Se₄ spinel. An antiferromagnetic order with a Néel temperature T_N=20.7 K and a strong ferromagnetic exchange evidenced by a positive Curie-Weiss temperature θ_CW=55.1 K were established. An increasing static magnetic field shifts T_N to lower temperatures while a susceptibility peak at T_m in the paramagnetic region—to higher temperatures. The non-zero and negative values both of the second and third harmonics of susceptibility suggest only a parallel spin coupling in ferromagnetic clusters in the range between the Néel and Curie-Weiss temperatures. Below T_N the magnetic field dependence of susceptibility, χ_ac(H), shows two peaks at critical fields H_c1 and H_c2. The values of H_c1 decrease slightly with temperature while the values of H_c2 drop rapidly with temperature. The strong changes of H_c2 temperature induced are mainly responsible for a spin frustration of the re-entrant type in the spinel under study.     

Electrical and magnetic properties of TmCoIn5 and YbCoIn5 single crystals

Electrical and magnetic properties of TmCoIn₅ and YbCoIn₅ single crystals were investigated by means of electrical resistivity and magnetization measurements in the temperature range from 300 to 0.5 K under the magnetic field up to 5 T. TmCoIn₅ is an antiferromagnetic metal with a Néel temperature T_N=2.6 K. YbCoIn₅ shows non-magnetic behavior, reflecting of divalent Yb ion.     

High heat generation ability in AC magnetic field for nano-sized magnetic Y3Fe5O12 powder prepared by bead milling

Nano-sized magnetic Y₃Fe₅O₁₂ ferrite having a high heat generation ability in an AC magnetic field was prepared by bead milling. A commercial powder sample (non-milled sample) of ca. 2.9 μm in particle size did not show any temperature enhancement in the AC magnetic field. The heat generation ability in the AC magnetic field improved with a decrease in the average crystallite size for the bead-milled Y₃Fe₅O₁₂ ferrites. The highest heat ability in the AC magnetic field was for the fine Y₃Fe₅O₁₂ powder with a 15-nm crystallite size (the samples were milled for 4 h using 0.1 mm? beads). The heat generation ability of the excessively milled Y₃Fe₅O₁₂ samples decreased. The main reason for the high heat generation property of the milled samples was ascribed to an increase in the Néel relaxation of the superparamagnetic material. The heat generation ability was not influenced by the concentration of the ferrite powder. For the samples milled for 4 h using 0.1 mm? beads, the heat generation ability (W g⁻¹) was estimated using a 3.58×10⁻⁴ fH² frequency (f/kHz) and the magnetic field (H/kA m⁻¹), which is the highest reported value of superparamagnetic materials.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

Synthesis and physical characterization of superconductivity-magnetism crossover compound RuSr2EuCeCu2O10−δ

We carefully studied the nonsuperconducting sample of the magneto-superconducting RuSr₂(Eu_1−xCe_x)Cu₂O_10−δ series with composition RuSr₂EuCeCu₂O_10−δ. This compound seems to exhibit a complex magnetic state as revealed by host of techniques like resistivity, thermopower, magnetic susceptibility, and MR measurements. The studied compound exhibited ferromagnetic like M(H) loops at 5, 20, and 50 K, and semiconductor like electrical conduction down to 5 K, with −MR^7 T of up to 4% at low temperatures. The −MR^7 T decreases fast above 150 K and monotonically becomes close to zero above say 230 K. Below, 150 K −MR^7 T decreases to around 3% monotonically down to 75 K, with further increase to 4% at around 30 K and lastly having a slight decrease below this temperature. The thermopower S(T) behavior closely followed the −MR^7 T steps in terms of d(S/T)/dT slopes. Further, both MR^7 T steps and d(S/T)/dT slopes are found in close vicinity to various magnetic ordering temperatures (T_mag) of this compound.     

Antiferromagnetic resonance study of sodalite loaded with sodium by multi-frequency ESR

We have carried out electron spin resonance (ESR) measurements on powder samples of sodalite loaded with Na at several frequencies between 9.7 and 35 GHz and at temperatures between 1.5 and 60 K. The ESR absorption spectrum below a Néel temperature T_N turns into an asymmetric spectrum with a long tail at low fields from a symmetric one above T_N. The line shape of the spectra below T_N is analyzed by a powder pattern simulation of the antiferromagnetic resonance spectra with easy-plane anisotropy. The calculated line shape reproduces the experimental one considerably well by assuming a Gaussian distribution of the zero-field energy gap. We have evaluated a small anisotropy field of about 2×10⁻⁴ T by using the exchange coupling constant calculated from the Weiss and the Néel temperatures. This result indicates that the sodalite loaded with Na is quite an ideal Heisenberg antiferromagnet as expected from the s-electron character of Na clusters and the cubic arrangement of nano-spaces in the sodalite.     

Kondo behaviour of the solid solution (Ce1−xLax)PtGa

The solid solution (Ce_1−xLa_x)PtGa has been studied through X-ray diffraction, magnetization (σ(B)), magnetic susceptibility (χ(T)), electrical resistivity (ρ(T)), magnetoresistivity (MR) and heat capacity (C_P(T)) measurements. The Néel temperature (T_N=3.3 K) for CePtGa is lowered upon La substitution as observed from χ(T) and ρ(T) measurements. The Kondo temperature T_K as calculated from MR measurements is comparable to T_N and also decreases with La substitution. The volume dependence of T_K is in accordance with the compressible Kondo lattice model and a Doniach diagram of the results is presented. C_P(T) measurements are presented for CePtGa, Ce_0.2La_0.8PtGa and LaPtGa and the results are discussed in terms of the electronic and magnetic properties. Other features of interest are anomalies in ρ(T) and C_P(T) due to crystalline electric field effects and metamagnetism as observed in σ(B) studies for samples with 0≤x≤ 0.3.     

Magnetic ordering in the frustrated system FeSc2S4

The sample of FeSc₂S₄ was prepared by solid reaction method. The crystallographic structure and the magnetic properties of the fabricated compound were investigated by X-ray, and superconducting quantum interference device (SQUID) magnetometer and Mössbauer spectroscopy. The polycrystalline FeSc₂S₄ confirmed the normal cubic spinel structure (space group Fd3m). The lattice constants a₀ and anion parameter u are 10.519 Å and 0.255, respectively. The Mössbauer spectroscopy has been studied for the FeSc₂S₄ at various temperatures, ranging from 4.2 K to room temperature. The spectra consist of two doublets at 4.2 K while a single line at room temperature. It is noticeable that the Mössbauer spectra of two doublet patterns with large electric quadrupole splitting (ΔE_Q) remain over the Néel temperature. Those are interpreted as a result of large electric quadrupole interaction compared to magnetic dipole interaction. The magnetic susceptibility measurements were performed with a SQUID magnetometer for temperatures 2<T<320 K, in external fields up to 5 kOe. Magnetic behavior shows antiferromagnetic behavior and the magnetic superexchange interactions between the Fe ions are weakly antiferromagnetic. The paramagnetic susceptibilities follow Curie–Weiss (CW) law with CW temperature Θ_CW=−100 K, and frustration parameter f=−Θ_CW/T_N is of the order of 1000. We conclude that two sublattices are coupled antiferromagnetically, leading to strong frustration effects.     

Optical characterization of fourfold (Td)- and sixfold (Oh)-transition-metal species in MgAl2O4:Co by time-resolved spectroscopy

This work investigates the origin of novel visible photoluminescence (PL) bands observed in the spinel MgAl₂O₄:Co²⁺. Besides the well-known fourfold-coordinated Co²⁺(T_d) PL at 670 nm [N.V. Kuleshov, V.P. Mikhailov, V.G. Scherbitsky, P.V. Prokoshin and K.V. Yumashev, J. Lumin. 55 (1993) 265.], a rich structured PL band at 686 nm was also observed that we associate with uncontrolled impurities of sixfold coordinated Cr³⁺(O_h) by time-resolved spectroscopy and lifetime measurements and their variation with temperature. We also show that the lifetime of the Co²⁺(T_d) emission at 670 nm varies from τ=6.7 μs to 780 ns on passing from T=10 to 290 K. This unexpected behaviour for T_d systems is related to the excited-state crossover (⁴T₁↔²E), making the emission band to transform from a narrow-like emission from ²E at low temperature to a broad structureless band from ⁴T₁ at room temperature.     

High pressure behaviour of KHSO4 studied by electrical impedance spectroscopy

Measurements of the electrical conductivity were performed in KHSO₄ at pressures between 0.5 and 2.5 GPa and in the temperature range 120-350 °C by the use of the impedance spectroscopy. The temperatures of the α-β phase transition (T_Tr) and of the melting (T_m), determined from the Arrhenius plots ln(σT) vs. 1/T, increase with pressure up to 1.5 GPa having dT/dP∼+45 K/GPa. Above the pressure 1.5 GPa, the pressure dependencies of T_Tr and T_m are negative dT/dP∼−45 K/GPa. At pressures above 0.5 GPa, the reversible decomposition of KHSO₄ into K₃H(SO₄)₂+H₂SO₄ (and probably into K₅H₃(SO₄)₄+H₂SO₄) affects the electrical conductivity of KHSO₄, with the typical values of the protonic electrical conductivity, c. 10⁻¹ S/cm at 2.5 GPa.     

The effect of duration of diffusion on Ag diffusion coefficients in YBa2Cu3O7

The Ag diffusion in superconducting YBa₂Cu₃O₇ (YBaCuO) ceramic has been studied over the duration of the diffusion range 5-24 h in the temperature range 700-850 °C by the energy-dispersive X-ray fluorescence (EDXRF) technique. For the excitation of silver atoms, an annular Am-241 radioisotope source (50 mCi) emitting 59.543 keV photons was used. The temperature dependences of silver diffusion coefficients in grains (D₁) and over the grain boundaries in the range 700-850 °C (D₂) are described by the relations D₁=1.4×10⁻² exp[−(1.18±0.10)/kT] and D₂=3.1×10⁻⁴ exp[−(0.87±0.10)/kT].