Synthesis and crystallographic studies of garnet-type AgCa2Mn2V3O12 and NaPb2Mn2V3O12

High-purity powder specimens of AgCa2Mn2V3O12 and NaPb2Mn2V3O12 have been successfully synthesized by solid-state chemical reaction. The Rietveld refinements from X-ray powder diffraction data verified that these compounds have the garnet-type structure (space group Ia-3d, No. 230) with the lattice constant of a = 12.596(2) Å for AgCa2Mn2V3O12 and a = 12.876(2) Å for NaPb2Mn2V3O12. Calculation of the bond valence sum supported that Mn is divalent and V is pentavalent in these garnets. Estimation of the quadratic elongation and the bond angle variance showed that the distortions of the MnO6 octahedra and the VO4 tetrahedra are significantly suppressed. Our new results of AgCa2Mn2V3O12 and NaPb2Mn2V3O12 are compared to those of AgCa2M2V3O12 and NaPb2M2V3O12 (M = Mg, Co, Ni, Zn)

Van Vleck paramagnetism of the thulium garnet Tm3Al5O12

The magnetic susceptibility of the garnet-type single crystal Tm3Al5O12 exhibits the typical Van Vleck temperature independent paramagnetism below ≈8 K. The temperature dependence of the susceptibility over the range 2.0–300 K has been analyzed on the assumption that the cubic crystal-electric-field dominates the energy level on 3H6 (J=6) ground multiplet for Tm3+ ion having 12-electrons in 4f shell. The ground state of the 3H6is nonmagnetic with Γ2 singlet, avoiding the Kramers doublet. The energy separation between Γ2 and the first excited state Γ(2)5 triplet is evaluated to be 68.0 K. The whole energy interval Δ between Γ2 and the highest state Γ1 in 3H6 is estimated to be 339.5 K

Ferromagnetism and the metal-insulator transition in the thiospinel Cu(Ir1-xCrx)2S4

A thiospinel CuIr2S4 exhibits a temperature-induced metal-insulator (M-I) transition at 230 K with a simultaneous spin-dimerization and charge-ordering transition although a three-dimensional system. On the other hand, CuCr2S4 has the same spinel structure without any structural transformations. CuCr2S4 remains metallic and is ferromagnetic with the Curie temperature TC≃377 K. In order to see the effect of substituting Cr for Ir on the M-I transition, we have carried out a systematic experimental study of electrical and magnetic properties of Cu(Ir1-xCrx)2S4 system. The M-I transition temperature decreases steeply with increasing Cr-composition x and this transition is not detected above x≃0.05. The value of TC decreases with decreasing x from 1.0, then TC disappears below x≃0.20. The ferromagnetic state suggests the non-collinear spin alignment. In the intermediate composition range over x=0.08 to 0.20, the B-site undergoes a local crystal distortion around 180 K, where the energy level t2g splits into lower symmetry. Then the low-spin state within the t2g subspace is realized for Cr3+ ion with s=1/2. The magnetic state of Cr3+ ion indicates a crossover from high temperature s=3/2 to low temperature s=1/2 state around 180 K

Magnetic susceptibility of vanadium garnets NaPb2Co2V3O12 and NaPb2Ni2V3O12

Vanadium garnets NaPb2Co2V3O12 and NaPb2Ni2V3O12 have been successfully synthesized. The X-ray diffraction experiments indicate that these compounds have the garnet structure of cubic symmetry of space group Ia3?d(Oh10) with the lattice constant of 12.742 Å (NaPb2Co2V3O12) and 12.666 Å (NaPb2Ni2V3O12), respectively. The magnetic susceptibility of NaPb2Ni2V3O12 shows the Curie Weiss paramagnetic behavior between 4.2 and 350 K. The effective magnetic moment μeff of NaPb2Ni2V3O12 is 3.14 μB due to Ni2+ ion at A-site and the Weiss constant is -3.67 K (antiferromagnetic sign). For NaPb2Co2V3O12, the simple Curie Weiss law cannot be applicable. The ground state is the spin doublet E2(t26e) and the first excited state is spin quartet T4(t25e), according to Tanabe-Sugano energy diagram on the basis of octahedral crystalline symmetry. This excited spin quartet state just a bit higher than ground state influences strongly the complex temperature dependence of magnetic susceptibility for NaPb2Co2V3O12

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 TN = 6.39 K for AgCa2Co2V3O12 and 7.21 K for AgCa2Ni2V3O12, respectively. The magnetic susceptibilities exhibit broad maximum, and these TN correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χmax). The magnetic entropy changes from zero to 20 K per mol Co2+ and Ni2+ ions are 5.31JK-1 mol-Co2+-ion-1 and 6.85JK-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 350K with the effective magnetic moment μef f= 3.23μBNi2+-ion-1 and the Weiss constant&thetas; = -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 20K which is in agreement with the result of the heat capacity

Ferrimagnetic order in the mixed garnet (Y1-xGdx)3Fe5O12

Ferrimagnetic order in the mixed rare-earth iron garnet (Y1-xGdx)3Fe5O12 system has been reinvestigated to cover the temperature range 5.0 to 700 K. The magnetization versus temperature exhibits a systematic variation with changing concentration x. The ferric-ion exchange coupling is strong enough to determine the Curie temperature ≈559 K for all the values x. The compensation temperature at which the magnetization crosses zero (shows the minimum) demonstrates the applicability of the three-sublattice model. The magnetic moment at 5.0 K indicates reasonable agreement with the relation nB = |21x-5.0|μB for the (Y1-xGdx)3Fe5O12 system. The compensation temperature decreases with decreasing x from 1.0 and reaches zero near x = 0.24. An enlargement of coercive force in the hysteresis loop for the low-field M-H curve is clearly seen in near the compensation temperature, indicating that a single domain is formed and the rotation of this single domain occurs without building up a multidomain structur

A new ferromagnetic thiospinel CuCrZrS4 with re-entrant spin-glass behaviour

A new thiospinel CuCrZrS4 has been successfully synthesized by a solid-state chemical reaction. This CuCrZrS4 exhibits ferromagnetic properties below the Curie temperature at Tc=60±2 K. The appearance of irreversible effect between field-cooled and zero-field-cooled magnetization is prominent below around 5 K in a magnetic field of less than 150 Oe. The ac susceptibility χAC shows a rapid decrease below about 10 K. This low magnetic-field behaviour indicates the existence of a re-entrant spin-glass phase below about 10 K. The dc magnetic susceptibility above 100 K shows Curie-Weiss behaviour with an effective magnetic moment of 3.61 μB, which is a little less than the spin-only value of 3.87 μB for the Cr3+ ion. The asymptotic Curie temperature &thetas;p is approximately 65 K, which is a little higher than Tc. The valence state is confirmed to be Cu+Cr3+Zr4+S42- on the basis of magnetic properties. The electrical resistivity ρ shows a semiconducting temperature dependence over the temperature range from 4.2 to 280 K with an activation energy of 6.84×10-3 eV in the higher temperature range from 50 to 283

Ferromagnetic-phase transition in the spinel-type CuCr2Te4

Ferromagnetic-phase transition in spinel-type CuCr2Te4 has been clearly observed. CuCr2Te4 is a telluride-spinel with the lattice constant a=11.134 a*, which has been synthesized successfully. The heat capacity exhibits a sharp peak due to the ferromagnetic-phase transition with the Curie temperature TC=326 K. This value of TC corresponds exactly to that of the negative peak of dM/dT in low field of 1.0Oe. The magnetic susceptibility shows the Curie-Weiss behavior between 380 and 650K with the effective magnetic moment μeff=4.14 μB/Cr-ion and the Weiss constant &thetas;=+357 K. The low temperature magnetization indicates the spin-wave excitations, where the existence of first term of Bloch T3/2 law and the next T5/2 term are verified experimentally. This spin-wave excitation is detected up to approximately 250K which is a fairly high temperature