Spin-charge-lattice coupling near the metal-insulator transition inCa3Ru2O7

Abstract

We report x-ray scattering studies of the c-axis lattice parameter in Ca 3 Ru 2 O 7 as a function of temperature and magnetic field. These structural studies complement published transport and magnetization data, and therefore elucidate the spin-charge-lattice coupling near the metal-insulator transition. Strong anisotropy of the structural change for field applied along orthogonal in-plane directions is observed. Competition between a spin-polarized phase that does not couple to the lattice and an antiferromagnetic metallic phase that does gives rise to a rich behavior for B ʈ b.The interplay of spin, charge, and lattice degrees of freedom is believed to underlie the complicated physics exhibited by many correlated electron systems, such as the colossal magnetoresistant manganites. 1 An exciting corollary of this interplay is the tunability of ground state propertiese.g., inducing a metal-insulator transition via application of magnetic field or controlling magnetic properties via strainthat has the potential to turn these systems into technologically useful electronic materials. 2 Optimization of the useful properties, however, requires a thorough understanding of the spin-charge-lattice coupling in these materials.Bilayer Ca 3 Ru 2 O 7 provides a particularly rich example of spin-charge-lattice coupling in a correlated electron system. Metallic Ca 3 Ru 2 O 7 orders antiferromagnetically at T N = 56 K and exhibits a metal-insulator transition at T m−i =48 K, 3 although a quasi-two-dimensional metallic ground state has been reported for T Ͻ 30 K. 4 At T m−i , a collapse of the c-axis lattice parameter is observed, 5 with a concomitant in-plane expansion. 6 This structural change is surprising in that a smaller c-axis lattice parameter is expected to increase the orbital overlap, which would result in a stabilization of the metallic state. Neutron powder diffraction studies suggest, however, that a decrease in the Ru-O-Ca bond angle that decreases the interlayer electron transfer may explain this unexpected behavior. 6 Finally, a spin reorientation transition, which was indicated by magnetic susceptibility measurements, 3 occurs in the vicinity of T m−i . 4,7-9 Neutron powder diffraction 6 confirmed the proposed low-temperature magnetic structure 7 as consisting of ferromagnetic bilayers, antiferromagnetically coupled and with moments along the orthorhombic b axis, using the Bb2 1 m notation of space group 36, for which a Ͻ b. 10 The spin reorientation rotates the moments such that they lie along a for T Ͼ T m−i .The confluence of spin reorientation, structural change, and a metal-insulator transition clearly demonstrates the interplay of the spin, charge, and lattice degrees of freedom in Ca 3 Ru 2 O 7 near T m−i , which has been the focus of recent transport and magnetization measurements. 7,11 In this Brief Report, we further this investigation by reporting x-ray scattering studies of the c-axis lattice parameter in Ca 3 Ru 2 O 7 , with applied magnetic fields of up to 10 T. Combining this information with pre...

Transition between two metallic ferroelectric orders in multiferroic Ca$_3$Ru$_2$O$_7$, induced by magnetism-mediated orbital re-polarization

“…Interestingly, the transition is accompanied by a switching of two distinct FEOs of different order parameters. Our picture naturally account for the rich interplay among metallic FEO, AFM order, SOO and lattice structure observed previously [7,23,26,30,32,46]. Our LDA+U +V method should be generally applicable to the study of other strongly correlated multi-ferroic materials.…”

Field-induced orbital patterns in ferromagnetic layered ruthenates

“…In zero field, Ca 3 Ru 2 O 7 undergoes an antiferromagnetic (AFM) transition at T N =56 K while remaining metallic and then a Mott-like transition at T MI =48 K with a dramatic reduction (up to a factor of 20) in the conductivity for T<T MI 10,11,13,14,17 . This transition is accompanied by an abrupt shortening of the c axis lattice parameter below T MI 10,18 . We notice that in the AFM state, neutron scattering measurements indicate that the magnetic moments align ferromagnetically within the double layer, and antiferromagnetically between the double layers (Atype AFM structure) along the c axis 19,20 .…”

“…Indeed, in this regime the magnetization does not change significantly, thus suggesting that magnetic scattering is not the primary mechanism responsible for CMR 11,13,19,23 . These transitions are also accompanied by anisotropic structural changes and modifications of the coupling between the electrons and the lattice [13][14][15]18 . Notably, when the field is applied along the low temperature hard axis, there occurs a sharp structural change that ties the CMR to an increase of the c-axis lattice parameter 18 .…”

“…These transitions are also accompanied by anisotropic structural changes and modifications of the coupling between the electrons and the lattice [13][14][15]18 . Notably, when the field is applied along the low temperature hard axis, there occurs a sharp structural change that ties the CMR to an increase of the c-axis lattice parameter 18 . The joining togheter of CMR, structural changes and metal-insulator transition is a clear indication of the interplay between spin, charge and lattice degrees of freedom 21 .…”

Magnetic structure and orbital state ofCa3Ru2O7investigated by resonant x-ray diffraction

“…The Ca analogs of these two materials, Ca 2 RuO 4 and Ca 3 Ru 2 O 7 , have lower electrical conductivities and undergo first-order transitions between high-temperature metallic and low-temperature insulating phases at 357 and 48 K, respectively. The phase diagram of Ca 3 Ru 2 O 7 is particularly interesting as multiple phase transitions are induced by external magnetic fields, [3][4][5] uniaxial or hydrostatic pressure, [6][7][8] and doping. 9,10 Most of these transitions have been attributed to rearrangements of the magnetic and/or orbital ordering pattern.…”