An insight into spin-chain magnetism through Moessbauer spectroscopic investigations in Eu-doped Ca3Co2O6 and Ca3CoRhO6

We report the results of 151Eu Moessbauer effect and magnetization measurements in the Eu-doped Ca3Co2O6 and Ca3CoRhO6, which are of great current interest in the fields of spin-chain magnetism and geometrical frustration. We find that there is a pronounced increase in the line-width of the Moessbauer spectra below a certain characteristic temperature which is well-above the one at which three-diensional ordering features set in. This unusual broadening of the spectra indicates the existence of a characteristic temperature in these 'exotic' magnetic systems, attributable to the onset of incipient one-dimensional magnetic order. This is inferred from an intriguing correlation of this characteristic temperature with the paramagnetic Curie temperature (a measure of intrachain coupling strength in these cases)

Magnetic structure and phase diagram in a spin-chain system: Ca3_3Co2_2O6_6

The low-temperature structure of the frustrated spin-chain compound Ca$_3$Co$_2$O$_6$ is determined by the ground state of the 2D Ising model on the triangular lattice. At high-temperatures it transforms to the honeycomb magnetic structure. It is shown that the crossover between the two magnetic structures at 12 K arises from the entropy accumulated in the disordered chains. This interpretation is in an agreement with the experimental data. General rules for for the phase diagram of frustrated Ising chain compounds are formulated.Comment: 4 pages, 2 figure

Magnetization reversal in mixed ferrite-chromite perovskites with non magnetic cation on the A-site

In this work, we have performed Monte Carlo simulations in a classical model for RFe$_{1-x}$Cr$_x$O$_3$ with R=Y and Lu, comparing the numerical simulations with experiments and mean field calculations. In the analyzed compounds, the antisymmetric exchange or Dzyaloshinskii-Moriya (DM) interaction induced a weak ferromagnetism due to a canting of the antiferromagnetically ordered spins. This model is able to reproduce the magnetization reversal (MR) observed experimentally in a field cooling process for intermediate $x$ values and the dependence with $x$ of the critical temperatures. We also analyzed the conditions for the existence of MR in terms of the strength of DM interactions between Fe$^{3+}$ and Cr$^{3+}$ ions with the x values variations.Comment: 8 pages, 7 figure

Magneto-elastic coupling and unconventional magnetic ordering in triangular multiferroic AgCrS2

The temperature evolution of the crystal and magnetic structures of ferroelectric sulfide AgCrS2 have been investigated by means of neutron scattering. AgCrS2 undergoes at TN = 41.6 K a first-order phase transition, from a paramagnetic rhombohedral R3m to an antiferromagnetic monoclinic structure with a polar Cm space group. In addition to being ferroelectric below TN, the low temperature phase of AgCrS2 exhibits an unconventional collinear magnetic structure that can be described as double ferromagnetic stripes coupled antiferromagnetically, with the magnetic moment of Cr+3 oriented along b within the anisotropic triangular plane. The magnetic couplings stabilizing this structure are discussed using inelastic neutron scattering results. Ferroelectricity below TN in AgCrS2 can possibly be explained in terms of atomic displacements at the magneto-elastic induced structural distortion. These results contrast with the behavior of the parent frustrated antiferromagnet and spin-driven ferroelectric AgCrO2

Stability of geometrically frustrated magnetic state of Ca3CoRhO6 to applications of positive and negative pressure

The influence of negative chemical pressure induced by gradual replacement of Ca by Sr as well as of external pressure (up to 10 kbar) on the magnetism of Ca3CoRhO6 has been investigated by magnetization studies. It is found that the solid solution, Ca(3-x)Sr(x)CoRhO6, exists at least till about x= 1.0 without any change in the crystal structure. Apart from insensitivity of the spin-chain feature to volume expansion, the characteristic features of geometrical frustration interestingly appear at the same temperatures for all compositions, in sharp contrast to the response to Y substitution for Ca (reported previously). Interestingly, huge frequency dependence of ac susceptibility known for the parent compound persists for all compositions. We do not find a change in the properties under external pressure. The stability of the magnetic anomalies of this compound to the volume change (about 4%) is puzzling

The enhancement of phase separation aspect in electron doped manganite Ca0.8Sm0.16Nd0.04MnO3

The complex lanthanide doping of electron manganites results in enhancement of various phase separation effects in physical properties of these compounds. Selecting Ca0.8Sm0.16Nd0.04MnO3 as a model case we show that the first order structural phase transition from paramagnetic semi-metallic phase into anti-ferromagnetic semi-metallic phase at TS ~ 158 +- 4 K is marked by an abrupt decrease in magnetization, a step like anomaly DL/L = 10-4 in thermal expansion and large latent heat DQ = 610 J/mol. In a certain temperature range below TS, the high field magnetization exhibits hysteretic metamagnetic behavior due to field-induced first order transformation. ac-susceptibility, magnetization and resistivity data suggest rather a non-uniform state in Ca0.8Sm0.16Nd0.04MnO3 at low temperatures. The metal - insulator transition occurs at TMI ~112 +- 3 K, accompanied by a step-like increase in magnetization. These features could be ascribed to "sponging" of electrons from neighboring anti-ferromagnetic matrix by clusters undergoing the ferromagnetic ordering.Comment: submitted to J.Phys. Cond. Matte

Negative chemical pressure effects induced by Y substitution for Ca on the exotic magnetic behavior of spin-chain compound, Ca3Co2O6

The magnetic behavior of a solid solution, Ca3-xYxCo2O6, based on an exotic spin-chain compound, Ca3Co2O6, crystallizing in K4CdCl6-derived rhombohedral structure is investigated. Among the compositions investigated (x= 0.0, 0.3, 0.5, 0.75 and 1.0), single phase formation persists up to x= 0.75, with the elongation of the c-axis. The present investigations reveal that the temperature at which the partially disordered antiferromagnetic structure sets in (which occurs at 24 K for the parent compound, x= 0.0) undergoes gradual reduction with the substitution of Y for Ca, attaining the value of about 2.2 K for the nominal x = 1.0. The trend observed in this characteristic temperature is opposite to that reported under external pressure, thereby establishing that Y substitution exerts negative chemical pressure. Anomalous steps observed in the isothermal magnetization at very low temperatures (around 2 K) for x= 0.0, which have been proposed to arise from quantum tunneling effects are found to vanish by a small substitution (x = 0.3) of Y for Ca. Systematics in ac and dc magnetic susceptibility behavior with Y substitution for Ca have also been probed. We believe that the present results involving the expansion of chain length without disrupting the magnetic chain may be useful to the overall understanding of the novel magnetism of the parent compound.Comment: Pramana - J. Phys. 65 (2005) 49

Inhomogeneous Magnetism in La-doped CaMnO3. (II) Mesoscopic Phase Separation due to Lattice-coupled FM Interactions

A detailed investigation of mesoscopic magnetic and crystallographic phase separation in Ca(1-x)La(x)MnO3, 0.00<=x<=0.20, is reported. Neutron powder diffraction and DC-magnetization techniques have been used to isolate the different roles played by electrons doped into the eg level as a function of their concentration x. The presence of multiple low-temperature magnetic and crystallographic phases within individual polycrystalline samples is argued to be an intrinsic feature of the system that follows from the shifting balance between competing FM and AFM interactions as a function of temperature. FM double-exchange interactions associated with doped eg electrons are favored over competing AFM interactions at higher temperatures, and couple more strongly with the lattice via orbital polarization. These FM interactions thereby play a privileged role, even at low eg electron concentrations, by virtue of structural modifications induced above the AFM transition temperatures.Comment: 8 pages, 7 figure

Geometrically frustrated magnetic behavior of Sr3NiRhO6 and Sr3NiPtO6

The results of ac and dc magnetic susceptibility isothermal magnetization and heat-capacity measurements as a function of temperature (T) are reported for Sr3NiRhO6 and Sr3NiPtO6 containing magnetic chains arranged in a triangular fashion in the basal plane and crystallizing in K4CdCl6-derived rhombohedral structure. The results establish that both the compounds are magnetically frustrated, however in different ways. In the case of the Rh compound, the susceptibility data reveal that there are two magnetic transitions, one in the range 10 -15 K and the other appearing as a smooth crossover near 45 K, with a large frequency dependence of ac susceptibility in the range 10 to 40 K; in addition, the features in C(T) are smeared out at these temperatures. The magnetic properties are comparable to those of previously known few compounds with partially disordered antiferromagnetic structure. On the other hand, for Sr3NiPtO6, there is no evidence for long-range magnetic ordering down to 1.8 K despite large value of paramagnetic Curie temperature.Comment: A slightly different version from that of PRB version (that is to appear in print). Phys, Rev, B, in pres