Spin Stripe Order in a Square Planar Trilayer Nickelate

Abstract

Trilayer nickelates, which exhibit a high degree of orbital polarization combined with an electron count (d 8.67 ) corresponding to overdoped cuprates, have been identified as a promising candidate platform for achieving high-Tc superconductivity. One such material, La4Ni3O8, undergoes a semiconductor-insulator transition at ~105 K, which was recently shown to arise from the formation of charge stripes. However, an outstanding issue has been the origin of an anomaly in the magnetic susceptibility at the transition and whether it signifies formation of spin stripes akin to single layer nickelates. Here we report single crystal neutron diffraction measurements (both polarized and unpolarized) that establish that the ground state is indeed magnetic. The ordering is modeled as antiferromagnetic spin stripes that are commensurate with the charge stripes, the magnetic ordering occurring in individual trilayers that are essentially uncorrelated along the crystallographic c-axis. Comparison of the charge and spin stripe order parameters reveals that, in contrast to single-layer nickelates such as La2-xSrxNiO4 as well as related quasi-2D oxides including manganites, cobaltates, and cuprates, these orders uniquely appear simultaneously, thus demonstrating a stronger coupling between spin and charge than in these related low-dimensional correlated oxides. Main text:There has been intense interest in stripe phases due to the interplay of charge, spin and lattice degrees of freedom as well as their relevance to high-temperature superconductivity in cuprates [1][2][3][4][5][6][7][8][9]. Uncovering cuprate-like superconductivity in oxides containing transition metals other than copper remains a daunting challenge [10], and in this regard R4Ni3O8 (R=La, Pr, or Nd) compounds have emerged as potential candidates [11][12][13]. These layered materials possess structures that resemble the n=3 Ruddlesden-Popper phase (Rn+1NinO3n+1) [14], but they differ in that all apical oxygens are absent, resulting in trilayers of NiO2 planes in which all Ni ions possess square-planar coordination of oxygen anions. The electron count (3d 8.67 ) coincides with the over-doped regime of cuprates [12,15]. Recent work indicates that these nickelates possess a low-spin state of Ni, large orbital polarization of the eg states with predominantly 2 − 2 orbital character near the Fermi energy, and significant O 2p-Ni 3d hybridization, all of which are considered to be important ingredients for superconductivity in the high-Tc cuprates [12]. Thus R4Ni3O8 compounds (particularly Pr4Ni3O8 which is metallic in its ground state [12]) are more similar to the superconducting cuprates than previously studied nickelates with octahedral coordination, such as La2-xSrxNiO4 (LSNO) [16][17][18] and LaNiO3-based heterostructures [19].Unlike metallic Pr4Ni3O8, La4Ni3O8 undergoes a semiconductor-insulator transition upon cooling through ∼105 K [11,13,[20][21][22][23][24][25][26][27][28], and we have recently shown that the insulating state is characterized by the formation ...

Charge-stripe order, antiferromagnetism, and spin dynamics in the cuprate-analog nickelate La4Ni3O8

“…µSR is a magnetic resonance technique [8], conceptually similar to NMR, that uses muon spins implanted into the sample as microscopic probes of their static and dynamic magnetic environment. We find that the 105-K transition is abrupt (discontinuous to within experimental resolution), and that the commensurate AFM configuration is consistent with both magnetic and charge stripe order as found in x-ray [5] and neutron [6] diffraction studies, respectively. Observed spectra of muon spin precession frequencies resemble histograms of calculated Ni dipolar fields at candidate muon sites for specific AFM stripe configurations, with reduced Ni moments 0.4-0.5µ B .…”

“…Observed spectra of muon spin precession frequencies resemble histograms of calculated Ni dipolar fields at candidate muon sites for specific AFM stripe configurations, with reduced Ni moments 0.4-0.5µ B . We present µSR evidence that the magnetic transition at T N is quasi-2D in nature, in which the trilayers are ordered but fluctuate independently down to ∼25 K. Below this temperature threedimensional (3D) static magnetism is observed, with disorder along the normal to the trilayer planes as found by neutron diffraction [6].…”

“…2 and listed in Table I. It is customary [5,6,16,23] to describe magnetic and stripe structure in La 4 Ni 3 O 8 using the setting F 4/mmm, in which basal-plane unit vectors are rotated 45 • to define a face-centered square lattice of Ni ions. The lattice constants in these directions are then √ 2 times the Ni-Ni nearest neighbor distance a in the basal plane.…”

“…The crystal structure of this compound involves square planar NiO 2 trilayers, isoelectronic to the Cu 2+ O 2 layers of the cuprates if the Ni valence were 1+. Further interest in comparing these systems arises from an antiferromagnetic (AFM) phase transition in La 4 Ni 3 O 8 at a Néel temperature T N = 105 K into a state with commensurate spin and charge stripes [4][5][6], similar in some respects to the AFM transition occurring * Present address: Lawrence Livermore National Laboratory, Livermore, California 94550, USA.…”

“…Very recently Zhang, Pajerowski et al [6] reported results of neutron Bragg diffraction experiments on La 4 Ni 3 O 8 . Their measurements confirm the magnetic ground state, with stripe AFM order commensurate with the charge stripes in individual trilayers but little or no correlation between trilayers.…”