The magnetic structure and magnetization dynamics of systems of plane frustrated Ising chain lattices are reviewed for three groups of compounds: , , and . The available experimental data are analyzed and compared in detail. It is shown that a high-temperature magnetic phase on a triangle lattice is normally and universally a partially disordered antiferromagnetic (PDA) structure. The diversity of low-temperature phases results from weak interactions that lift the degeneracy of a 2D antiferromagnetic Ising model on the triangle lattice. Mean-field models, Monte Carlo simulation results on the static magnetization curve, and results on slow magnetization dynamics obtained with Glauber's theory are discussed in detail.

Frustrated lattices of Ising chains

By using x-ray absorption spectroscopy at the $\text{Rh}\text{ }{L}_{2,3}$, $\text{Co}\text{ }{L}_{2,3}$, and $\text{Fe}\text{ }{L}_{2,3}$ edges, we find a valence state of ${\text{Co}}^{2+}/{\text{Rh}}^{4+}$ in ${\text{Ca}}_{3}{\text{CoRhO}}_{6}$ and of ${\text{Fe}}^{3+}/{\text{Rh}}^{3+}$ in ${\text{Ca}}_{3}{\text{FeRhO}}_{6}$. X-ray magnetic circular dichroism spectroscopy at the $\text{Co}\text{ }{L}_{2,3}$ edge of ${\text{Ca}}_{3}{\text{CoRhO}}_{6}$ reveals a giant orbital moment of about $1.7{\ensuremath{\mu}}_{B}$, which can be attributed to the occupation of the minority-spin ${d}_{0}{d}_{2}$ orbital state of the high-spin ${\text{Co}}^{2+}$ $(3{d}^{7})$ ions in trigonal prismatic coordination. This active role of the spin-orbit coupling explains the strong magnetocrystalline anisotropy and Ising-type magnetism of ${\text{Ca}}_{3}{\text{CoRhO}}_{6}$.

X-ray absorption and x-ray magnetic dichroism study on Ca3CoRhO6 and Ca3FeRhO6

For the spin-chain compound ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2}{\mathrm{O}}_{6}$, the magnetization curves as a function of the magnetic field are strongly out-of-equilibrium at low temperature, and they exhibit several steps whose origins are still a matter for debate. In the present paper we report on a detailed investigation of the temperature and time dependence of these features. First, it is found that some of the magnetization steps can disappear as the characteristic time of the measurement is increased. A comparison of the influence of temperature and time points to the existence of a thermally activated process that plays an important role in determining the form of the magnetization curves. Second, direct investigations of the magnetic response as a function of time show that this thermally activated process competes with a second relaxation mechanism of a very different nature, which becomes dominant at the lowest temperatures. These results shed new light on the peculiar magnetization process of this geometrically frustrated, Ising-like spin-chain compound.

Temperature and time dependence of the field-driven magnetization steps in Ca 3 Co 2 O 6 single crystals

In this paper, we report polarized optical microscopy and electrical transport studies of manganese oxides that reveal that the charge ordering transition in these compounds exhibits typical signatures of a martensitic transformation. We demonstrate that specific electronic properties of charge-ordered manganites stem from a combination of martensitic accommodation strain and effects of strong electron correlations. This intrinsic strain is strongly affected by the grain boundaries in ceramic samples. Consistently, our studies show a remarkable enhancement of low field magnetoresistance and the grain size effect on the resistivity in polycrystalline samples and suggest that the transport properties of this class of manganites are governed by the charge-disordered insulating phase stabilized at low temperature by virtue of martensitic accommodation strain. High sensitivity of this phase to strains and magnetic field leads to a variety of striking phenomena, such as unusually high magnetoresistance ${(10}^{10}%)$ in low magnetic fields.

Martensitic accommodation strain and the metal-insulator transition in manganites.

Reentrant charge ordering transition in the manganites as experimental evidence for a strain glass

Specific heat measurements were carried out on the closely related spin-chain oxides Ca3Co2O6 and Ca3CoRhO6. Both compounds consist of Ising magnetic chains that are arranged on a triangular lattice. The spin coupling along and between the chains is ferromagnetic and antiferromagnetic, respectively. Geometrical frustration is expected from the combination of these magnetic features. The present study reports that the specific heat data of these compounds exhibit similarities in the spin-freezing process at low T, whereas striking differences exist in the antiferromagnetic interchain ordering at higher T. These results are discussed in connection with previous magnetization and neutron diffraction data.

http://iopscience.iop.org/article/10.1088/0953-8984/15/33/307/pdf

Specific heat investigation of the magnetic ordering in two frustrated spin-chain oxides: Ca3Co2O6 and Ca3CoRhO6

Substitutions in the Mn sublattice of antiferromagnetic, charge, and orbitally ordered manganites was recently found to produce intriguing metamagnetic transitions consisting of a succession of sharp magnetization steps separated by plateaus. The compounds exhibiting such features can be divided into two categories, depending on whether they are sensitive to thermal cycling effects or not. One compound in each category has been considered in the present study. This article reports on the influence of two treatments: high-temperature annealing and grinding. It is shown that both of these treatments can dramatically affect the phenomenon of magnetization steps. The results provide us with information about the origin of these jumps in magnetization.

Staircase-like metamagnetic transitions in phase-separated manganites: Influence of thermal and mechanical treatments

Specific heat measurements were carried out on the closely related spin-chain oxides Ca3Co2O6 and Ca3CoRhO6. Both compounds consist of Ising magnetic chains that are arranged on a triangular lattice. The spin coupling along and between the chains are ferromagnetic and antiferromagnetic, respectively. Geometrical frustration is expected from the combination of these magnetic features. The present study reports that the specific heat data of these compounds exhibit similarities in the spin-freezing process at low T, whereas striking differences exist in the antiferromagnetic interchain ordering at higher T. These results are discussed in connection with previous magnetization and neutron diffraction data.

A complex structural, magnetic and electric transport investigation shows that the Cr doping on Mn sites in the A-type antiferromagnet Pr0.44Sr0.56MnO3 provokes a non-uniform magnetic state with coexisting FM and AFM regions. Irrespective of the ratio of magnetic phases, the samples exhibit a non-metallic behavior of resistivity and thermopower, pointing to the nanoscopic nature of the phase separation. A particularly large magnetoresistance encountered in a broad range of temperatures for samples with Cr doping of 4 - 6 % supports such idea.

C. Martin

Papers

Specific heat investigation of the magnetic ordering in two frustrated spin-chain oxides: Ca3Co2O6 and Ca3CoRhO6

Staircase-like metamagnetic transitions in phase-separated manganites: Influence of thermal and mechanical treatments

Specific heat investigation of the magnetic ordering in two frustrated spin-chain oxides: Ca3Co2O6 and Ca3CoRhO6

The magnetoresistive behavior of Cr-doped manganites Pr0.44Sr0.56MnO3

Ultra-sharp magnetization steps and anomalous field cooling effect in perovskite manganites