Magnetic phase separation in double layer ruthenates Ca3(Ru(1-x)Ti(x))2O7

Magnetic Phase Separation in the Oxypnictide Sr2Cr1.85Mn1.15As2O2

Layered Sr₂M₃As₂O₂-type oxypnictides are composed of tetrahedral M₂Pn₂ and square planar MO₂ layers, the building blocks of iron-based and cuprate superconductors. To further expand our understanding of the chemical and magnetic properties of the Sr₂Cr_3–xMn_xAs₂O₂ solid solution, Sr₂Cr₂MnAs₂O₂ has been synthesized. The compound crystallizes in the I4/mmm tetragonal space group with a refined stoichiometry of Sr₂Cr_1.85Mn_1.15As₂O₂. The M(2) site within the M₂Pn₂ slab is occupied by 42.7% Cr and 57.3% Mn, and the magnetic moments order antiferromagnetically below T_N(M2) = 540 K with a C-type antiferromagnetic structure. The M(1) site within the MO₂ layers is fully occupied by Cr, and antiferromagnetic order is observed below T_N(M1) = 200 K. Along c, there are two possible interplanar arrangements: ferromagnetic with the (1/2, 1/2, 0) propagation vector and antiferromagnetic with the (1/2, 1/2, 1/2) propagation vector. Magnetic phase separation arises so that both propagation vectors are observed below 200 K. Such magnetic phase separation has not been previously observed in Sr₂M₃As₂O₂ phases (M = Cr, Mn) and shows that there are several competing magnetic structures present in these compounds.

Layered Sr₂M₃As₂O₂-type oxypnictides are composed of tetrahedral M₂Pn₂ and square planar MO₂ layers, the building blocks of iron-based and cuprate superconductors. Sr₂Cr_1.85Mn_1.15As₂O₂ is antiferromagnetic and semiconducting. Surprisingly magnetic phase segregation is observed in the oxypnictide Sr₂Cr_1.85Mn_1.15As₂O₂ because of competing magnetic exchange interactions along c.

Electronic and Magnetic Properties of Cation Ordered Sr2Mn2.23Cr0.77As2O2

Several different mechanisms of magnetoresistance (MR) have been observed in 1111 LnMnAsO_1-xF_x oxypnictides (Ln = lanthanide) as a result of magnetic coupling between the Mn and Ln. Such phases also exhibit interesting magnetic phase transitions upon cooling. Sr₂Mn₂CrAs₂O₂ has been synthesized to investigate if it is possible to observe MR and/or magnetic phase transitions as a result of magnetic coupling between the Mn and Cr. Sr₂Mn₂CrAs₂O₂ crystallizes in the tetragonal space group I4/mmm containing alternating MO₂^2- and M'₂As₂^2- layers, and neutron diffraction results demonstrate that the actual stoichiometry is Sr₂Mn_2.23Cr_0.77As₂O₂. Cation order is present between Mn and Cr, with Cr predominantly occupying the square planar MO₂^2- site. Below 410 K, the magnetic moments of the Mn/Cr ions in the M'₂As₂^2- sublattice exhibit G-type antiferromagnetic order. The Mn/Cr moments within the MO₂^2- layer order below 167 K with a K₂NiF₄-type antiferromagnetic structure that simultaneously induces a spin flip of the magnetic moments in the M'₂As₂^2- layers from a G-type to a C-type antiferromagnetic arrangement. The results demonstrate that the superexchange interactions are finely balanced in Sr₂Mn_2.23Cr_0.77As₂O₂. Sr₂Mn_2.23Cr_0.77As₂O₂ is semiconducting, and there is no evidence of MR.

In Situ Control of Diamagnetism by Electric Current in Ca_{3}(Ru_{1-x}Ti_{x})_{2}O_{7}

Nonequilibrium steady state conditions induced by a dc current can alter the physical properties of strongly correlated electron systems. In this regard, it was recently shown that dc current can trigger novel electronic states, such as current-induced diamagnetism, which cannot be realized in equilibrium conditions. However, reversible control of diamagnetism has not been achieved yet. Here, we demonstrate reversible in situ control between a Mott insulating state and a diamagnetic semimetal-like state by a dc current in the Ti-substituted bilayer ruthenate Ca_{3}(Ru_{1-x}Ti_{x})_{2}O_{7} (x=0.5%). By performing simultaneous magnetic and resistive measurements, we map out the temperature vs current-density phase diagram in the nonequilibrium steady state of this material. The present results open up the possibility of creating novel electronic states in a variety of strongly correlated electron systems under dc current.

Colossal Magnetoresistance in a Mott Insulator via Magnetic Field-Driven Insulator-Metal Transition

We present a new type of colossal magnetoresistance (CMR) arising from an anomalous collapse of the Mott insulating state via a modest magnetic field in a bilayer ruthenate, Ti-doped Ca_{3}Ru_{2}O_{7}. Such an insulator-metal transition is accompanied by changes in both lattice and magnetic structures. Our findings have important implications because a magnetic field usually stabilizes the insulating ground state in a Mott-Hubbard system, thus calling for a deeper theoretical study to reexamine the magnetic field tuning of Mott systems with magnetic and electronic instabilities and spin-lattice-charge coupling. This study further provides a model approach to search for CMR systems other than manganites, such as Mott insulators in the vicinity of the boundary between competing phases.