Order by static disorder in the Ising chain magnet Ca3Co2-xMnxO6

Spin-state transition of Co ion (S2S5/2) in hole substituted 1D chain of Ca3Co2O6

We have discovered spin-state transition (S= 2 toS= 5/2) of Co ions due to Mg substitution in the Ca₃Co₂O₆apparent in the magnetic susceptibility, x-ray photoelectron spectroscopy (XPS), and first-principles study. We also examine the effect of Mg substitution on the magnetic and electronic structure of Ca₃Co₂O₆by first-principles calculations. It involves generalized gradient approximation with Coulomb interaction (U) in exchange-correlation energy functional. Our study shows a reasonable agreement between effective magnetic moment (μ_eff) determined from the Curie-Weiss fit with that from the XPS analysis and first-principles calculations study. We have attributed the decrease in positive intra-chain exchange interaction constant (J₁/k_B) to the antiferromagnetically coupled induced Co⁴⁺ions (S= 5/2) arising from the Mg²⁺ions substitution. The in-field metamagnetic transitions in the isothermalM(H) curves below the critical field (H_c) have been accurately mapped and successfully explained by the change in magnetic entropy (ΔS) calculations and Arrott plots. Electronic structure study reveals hole-type doping of Mg atom, and the Fermi level (E_F) shifts below. Density of state and band structure calculation indicates strong hybridization between partial states of Co-3d and O-2p orbitals for the Mg-doped compound due to which the band crossing at Fermi level is observed, and a hole-type Fermi surface is formed.

Rigorous determination of the ground-state phases and thermodynamics in an Ising-type multiferroic chain

To understand the ferroelectricity driven by collinear magnetism in a multiferroic spin-chain system, we have adopted an elastic diatomic Ising spin-chain model with axial next-nearest-neighbor interaction to describe its magnetoelectric properties. By employing magneto-phonon decoupling and the transfer-matrix method, the possible ground-state configurations and thermodynamic behaviors of the system have been determined exactly. The parameter relation for the appearance of electric polarization has been discussed from the perspective of the ground-state configuration. In the case of nearest-neighbor antiferromagnetic coupling, a novel series of zero-temperature transitions induced by magnetic field have been observed, from the ↑↑↓↓ spin configuration associated with ferroelectric order to the ↑↓↑ state with a peculiar 1/3 magnetization plateau, then to the ↑↑↑↓ state, and finally saturation in the ↑↑↑↑ state.

Disappearance of Ising nature in Ca3ZnMnO6 studied by high-field ESR

High-field electron spin resonance measurements of an antiferromagnet Ca3ZnMnO6 isostructure, with the Ising-chain multiferroic Ca3CoMnO6, have been carried out. Two distinct resonance modes were observed below TN = 25 K, which is well explained by conventional antiferromagnetic resonance theory with easy-plane anisotropy. The zero-field spin gap is derived to be about 166 GHz, originating from the easy-plane anisotropy and exchange interaction. Our result suggests that the Dzyaloshinsky-Moriya interaction, which may induce spin canting, is absent. Disappearance of Ising anisotropy in Ca3ZnMnO6 suggests that the Co(4+) ion, as well as the Co-Mn superexchange, plays an important role for the Ising nature in Ca3CoMnO6.

Modifications in the frustrated magnetism, oxidation state of Co and magnetoelectric coupling effects induced by a partial replacement of Ca by Gd in the spin-chain compound Ca3Co2O6

We have systematically investigated the influence of the gradual replacement of Ca by Gd on the magnetic and complex dielectric properties of the well-known geometrically frustrated spin-chain system Ca3Co2O6 (TN = 24 K with additional magnetic transitions below 12 K), by studying the series Ca3−xGdxCo2O6 (x ≤ 0.7), down to 1.8 K. Heat-capacity measurements establish that the reduction of TN with Gd substitution is much less compared to that by Y substitution. The magnetic moment data reveal that there are changes in the oxidation state of Co as well, unlike for Y substitution, beyond x = 0.2. Thus, despite being isovalent, both these substitutions interestingly differ in changing these magnetic properties in these oxides. We propose that the valence electrons of Y and those of R ions play different roles in deciding the magnetic characteristics of these mixed oxides. It is observed that a small amount (x = 0.3) of Gd substitution for Ca is enough to suppress glassy ac magnetic susceptibility behavior for the peak around 12 K. An additional low-temperature magnetic anomaly close to 5 K gets more prominent with increasing Gd concentration as revealed by heat-capacity data. Trends in temperature dependence of complex dielectric behavior were also tracked with varying composition and a frequency dependence is observed, not only for the transition in the region around 10 K (for some compositions), but also for the 5 K transition which is well resolved for a higher concentration of Gd. Thus, the Gd-substituted Ca3Co2O6 series is shown to reveal interesting magnetic and dielectric behaviors of this family of oxides.

Novel dielectric anomalies due to spin-chains above and below Néel temperature in Ca3Co2O6

We bring out novel dielectric behavior of a spin-chain compound, Ca₃Co₂O₆, undergoing Néel order at (T_N = ) 24 K. It is found that the virgin curve in the plot of isothermal dielectric constant (ε') versus magnetic-field lies outside the ‘butterfly-shaped’ envelope curve well below T_N (e.g., 2.6 K), with a signature of a partial arrest of the high-field magnetoelectric (ME) phase in zero-field after travelling through magnetic-field-induced magnetic transitions. This behavior is in contrast to that observed in the isothermal magnetization data. Thus, this work brings out a novel case for ‘phase-coexistence phenomenon’ due to ME coupling. Another strange finding is that there is a weak, but a broad, peak in ε' around 85–115 K well above T_N, attributable to incipient spin-chain magnetic ordering. This finding should inspire further work to study ME coupling on artificial assemblies of magnetic chains, also keeping in mind miniaturization required for possible applications.

Formation of magnetic microphases in Ca3Co2O6

We study a frustrated quantum Ising model relevant for Ca3Co2O6 that consists of a triangular lattice of weakly coupled ferromagnetic chains. According to our quantum Monte Carlo simulations, the chains become ferromagnetic and form a three-sublattice "up-up-down" structure for T≤T(CI). In contrast, long-wavelength spin-density-wave microphases are stabilized along the chains for T(CI)<T<T(c). Our mean field solutions reveal a quasicontinuous temperature dependence of the modulation wavelength, implying the existence of metastable states that explain the very slow dynamics observed in Ca3Co2O6. We also discuss implications of microphases for the related multiferroic compounds Ca3CoMnO6 and Lu2MnCoO6.