Discovery of ferromagnetic-half-metal-to-insulator transition in K2Cr8O16

Two Magnetic Orderings and a Spin-Flop Transition in Mixed Valence Compound Mn3O(SeO3)3

A mixed-valence manganese selenite, Mn₃O(SeO₃)₃, was successfully synthesized using a conventional hydrothermal method. The three-dimensional framework of this compound is composed of an MnO₆ octahedra and an SeO₃ trigonal pyramid. The magnetic topological arrangement of manganese ions shows a three-dimensional framework formed by the intersection of octa-kagomé spin sublattices and staircase-kagomé spin sublattices. Susceptibility, magnetization and heat capacity measurements confirm that Mn₃O(SeO₃)₃ exhibits two successive long-range antiferromagnetic orderings with T_N1~4.5 K and T_N2~45 K and a field-induced spin-flop transition at a critical field of 4.5 T at low temperature.

Orbital Effects in Solids: Basics, Recent Progress, and Opportunities

The properties of transition metal compounds are largely determined by nontrivial interplay of different degrees of freedom: charge, spin, lattice, and also orbital ones. Especially rich and interesting effects occur in systems with orbital degeneracy. For example, they result in the famous Jahn-Teller effect, leading to a plethora of consequences for static and dynamic properties, including nontrivial quantum effects. In the present review, we discuss the main phenomena in the physics of such systems, paying central attention to the novel manifestations of those. After shortly summarizing the basic phenomena and their descriptions, we concentrate on several specific directions in this field. One of them is the reduction of effective dimensionality in many systems with orbital degrees of freedom due to the directional character of orbitals, with the concomitant appearance of some instabilities that lead in particular to the formation of dimers, trimers, and similar clusters in a material. The properties of such cluster systems, which are largely determined by their orbital structure, are discussed in detail, and many specific examples of those in different materials are presented. Another big field that has acquired special significance relatively recently is the role of the relativistic spin-orbit interaction. The mutual influence of this interaction and the more traditional Jahn-Teller physics is treated in detail in the second part of the review. In discussing all of these questions, special attention is paid to novel quantum effects.

Ferromagnetic Half-Metal Cyanamides Cr(NCN)2 Predicted from First Principles Investigation

The stability, physical properties, and electronic structures of Cr(NCN)₂ were studied using density functional theory with explicit electronic correlation (GGA+U). The calculated results indicate that Cr(NCN)₂ is a ferromagnetic and half-metal, both thermodynamically and elastically stable. A comparative study on the electronic structures of Cr(NCN)₂ and CrO₂ shows that the Cr atoms in both compounds are in one crystallographically equivalent site, with an ideal 4+ valence state. In CrO₂, the Cr atoms at the corner and center sites have different magnetic moments and orbital occupancies, moreover, there is a large difference between the intra- (12.1 meV) and inter-chain (31.2 meV) magnetic couplings, which is significantly weakened by C atoms in Cr(NCN)₂.

Magnetic phase diagram of K2Cr8O16 clarified by high-pressure muon spin spectroscopy

The K₂Cr₈O₁₆ compound belongs to a series of quasi-1D compounds with intriguing magnetic properties that are stabilized through a high-pressure synthesis technique. In this study, a muon spin rotation, relaxation and resonance (μ⁺SR) technique is used to investigate the pressure dependent magnetic properties up to 25 kbar. μ⁺SR allows for measurements in true zero applied field and hereby access the true intrinsic material properties. As a result, a refined temperature/pressure phase diagram is presented revealing a novel low temperature/high pressure (p_C1 = 21 kbar) transition from a ferromagnetic insulating to a high-pressure antiferromagnetic insulator. Finally, the current study also indicates the possible presence of a quantum critical point at p_C2 ~ 33 kbar where the magnetic order in K₂Cr₈O₁₆ is expected to be fully suppressed even at T = 0 K.

Magnetostructural phase transition assisted by temperature in Ag-αMnO2: a density functional theory study

A crystalline material formed by parallel chains of silver atoms inside one-dimensional tunnels of hollandite manganese dioxide, Ag-αMnO2, is investigated through first-principles total energy calculations. Two different magnetic phases have been identified; one structure containing linear Ag chains with an antiferromagnetic ordering in the direction perpendicular to the MnO2 tunnels for T = 0 K (I4/m) and another configuration with zigzag Ag chains in a non-magnetic regime for higher temperatures (P21/c). According to phonon dispersions, both structures are stable. On the other hand, the structure with linear Ag chains in the non-magnetic state is unstable. A critical temperature of Tc≃ 125 K for the magnetostructural phase transition between the two stable structures I4/m and P21/c is predicted.

Bi3Cr2.91O11: a ferromagnetic insulator from Cr(4+)/Cr(5+) mixing

The search for materials with ferromagnetic and semiconducting/insulating properties has intensified recently because of their potential use in spintronics. However, the number of materials is rather limited because of conflicting requirements needed for the appearance of ferromagnetic and insulating properties. Here we show that Bi3Cr2.91O11 belongs to the scarce family of ferromagnetic insulators. Bi3Cr2.91O11 was synthesized at high pressure of 6 GPa and high temperature of 1570 K. Its crystal structure and properties were studied using single crystals. It crystallizes in the KSbO3-type structure with space group Pn3 and the lattice parameter a = 9.2181(2) Å. Bi3Cr2.91O11 has almost a 1:1 mixture of Cr(4+) and Cr(5+) ions distributed in one octahedral crystallographic site. Bi3Cr2.91O11 is a rare example of oxides having chromium ions in unusual oxidation states. The presence of Cr(4+) and Cr(5+) results in ferromagnetic properties with ferromagnetic Curie temperature TC = 220 K.

Double exchange ferromagnetism in the Peierls insulator state

We study the effects of opening the band gap on the double exchange ferromagnetism. Applying the density-matrix renormalization group method and an analytical expansion from the dimer limit to the one-dimensional double exchange model, we demonstrate for a relevant region of the exchange coupling that, in the weak dimerization regime, the Peierls gap opens in the fully spin-polarized conduction band without affecting its ferromagnetism, whereas in the strong dimerization regime, the ferromagnetism is destroyed, and the Mott gap opens instead, leading the system to the antiferromagnetic quasi-long-range order. An insulator version of double exchange ferromagnetism is thus established.

Peierls mechanism of the metal-insulator transition in ferromagnetic hollandite K2Cr8O16

Synchrotron x-ray diffraction experiment shows that the metal-insulator transition occurring in a ferromagnetic state of a hollandite K(2)Cr(8)O(16) is accompanied by a structural distortion from the tetragonal I4/m to monoclinic P112(1)/a phase with a √2×√2×1 supercell. Detailed electronic structure calculations demonstrate that the metal-insulator transition is caused by a Peierls instability in the quasi-one-dimensional column structure made of four coupled Cr-O chains running in the c direction, leading to the formation of tetramers of Cr ions below the transition temperature. This provides a rare example of the Peierls transition of fully spin-polarized electron systems.

Dimerization and charge order in hollandite K₂V₈O₁₆

The metal-insulator transition occurring in hollandite K₂V₈O₁₆ has been studied by means of neutron and x-ray diffraction as well as by thermodynamic and electron-spin resonance measurements. The complete analysis of the crystal structure in the distorted phase allows us to identify dimerization as the main distortion element in insulating K₂V₈O₁₆. At low-temperature, half of the V chains are dimerized perfectly explaining the suppression of magnetic susceptibility due to the formation of spin singlets. The dimerization is accompanied by the segregation of charges into chains.

Charge ordering induced ferromagnetic insulator: K2Cr8O16

Usually metallicity accompanies ferromagnetism. K2Cr8O16 is one of the less common examples of magnetic materials, exhibiting ferromagnetism in the insulating state. Analyzing the electronic and magnetic properties within first principles electronic structure calculations, we find that the doped electrons due to K induce a charge-ordered and insulating ground state and interestingly also introduce a ferromagnetic coupling between the Cr ions. The primary considerations driving the charge ordering are found to be electrostatic ones with the charge being localized on two Cr atoms that minimize the electrostatic energy. The structural distortion that accompanies the ordering gives rise to a rare example of a charge-order driven ferromagnetic insulator.