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 ...

Electric control of multiferroic domains is demonstrated through polarized magnetic neutron diffraction. Cooling to the cycloidal multiferroic phase of Ni3V2O8 in an electric field E causes the incommensurate Bragg reflections to become neutron spin polarizing, the sense of neutron polarization reversing with E. Quantitative analysis indicates the E-treated sample has handedness that can be reversed by E. We further show close association between cycloidal and ferroelectric domains through E-driven spin and electric polarization hysteresis. We suggest that definite cycloidal handedness is achieved through magneto-elastically induced Dzyaloshinskii-Moriya interactions.PACS numbers: 75.25.+z, 75.60.-d, 75.80.+q, 77.80.-e Materials that are both ferroelectric and magnetic are classified as multiferroics. In some multiferroics where the ferroelectric and magnetic phases coexist, spin and charge are strongly coupled, leading to the possibility of controlling magnetic properties through an electric field E. Such a nonlinear magneto-electric response is of fundamental interest and holds the potential for applications that include sensing, spintronics, and microwave communication [1]. Recent studies have shown that an external E applied to multiferroics with non-collinear spin structures, such as TbMnO 3 and LiCu 2 O 2 , favors a particular handedness of the magnetic order [2,3]. Other studies have shown the E control of domain population related to equivalent magnetic propagation vectors [4]. Here we examine the suppression and promotion of cycloidal magnetic structures in Ni 3 V 2 O 8 (NVO) by an applied E. Our quantitative analysis of the polarized magnetic diffraction cross-section and hysteresis curve for this multiferroic material indicates that a clockwise cycloidal single crystal can be generated and is stabilized by magnetoelastically induced Dzyaloshinskii-Moriya interactions.NVO is an insulating magnet with spin-1 Ni 2+ ions arranged in a buckled kagomé-staircase geometry [5]. The spins occupy two distinct crystallographic sites denoted cross-tie and spine [See Fig. 1 (a)]. Competing nearest and next-nearest neighbor interactions along the spines yield a complex magnetic phase diagram [6]. Magnetic inversion symmetry breaking was inferred in the so-called low-temperature incommensurate (LTI) phase, where unpolarized neutron diffraction data indicate a magnetic cycloidal structure with spins in the a-b plane and pyrocurrent measurements find concomitant electric polarization along the b axis. A Landau mean field theory was previously devised to account for this multiferroic behavior [5,7,8]. The free-energy expansion isHere, a and b are constants, T is temperature, σ H and σ L are the magnetic order parameters in the high-T incommensurate and LTI phases, respectively, χ E is the electric susceptibility, and P is the electric polarization. The last term is the lowest order (trilinear) symmetryallowed multiferroic interaction, which in the LTI phase is given by V LT I = γ a γ σ H σ L P γ . Minimizing F with r...