Ferromagnetism in the unusual low-valence layered material LaSrNiRuO ₄

Abstract

Introduction. -Transition metal oxides encompass a lot of functional materials due to their varying chargespin-orbital states and the abundant electronic-magneticoptical properties to name a few [1][2][3][4][5][6][7][8]. For example, oxides with mixed 3d-4d-5d transition metals often display various and even unusual charge states [9-14] which arise from the much different chemical potentials of those transition metals. As a result, a special spin-orbital state associated with the unusual charge state could offer a remarkable property. In line with this expectation there appears a layered ferromagnetic (FM) insulator LaSrNiRuO 4 , which has an unusual low-valence Ni + /Ru 2+ state and a quite high Curie temperature of about 200 K [15].LaSrNiRuO 4 has a square planar NiO 2 -RuO 2 layer where the Ni and Ru atoms are checkerboard ordered, see fig. 1. This layered structure is due to a topochemical reduction and simultaneously a remarkably low valence Ni + -Ru 2+ state emerges [15]. Topochemical synthesis routes provide many opportunities to prepare novel extended oxide phases containing arrays of transition metal cations [15]. Here, layered LaSrNiRuO 4 is striking due to the ordered Ni-Ru arrangement in the infinite sheets, the unusual low-valence Ni + -Ru 2+ state, and the quite high

Contrasting Magnetism in Isovalent Layered LaSr3NiRuO4H4 and LaSrNiRuO4 due to Distinct Spin-Orbital States*

“…Therefore, we attempt to achieve it in a compromise way, by studying the hypothetic material LaSrNiZnO 4 with a substitution of the nonmagnetic Zn 2+ for the S=0 Ru 2+ . This choice is also justified by the consideration that the Zn 2+ ion (0.74 Å) has a very similar ionic size to the S=0 Ru 2+ (t 6 2g , probably 0.74 Å) which is unavailable but can be extrapolated from the ionic sizes of 0.565 Å/Ru 5+ (t [12]. Here again, we perform hybrid functional PBE0 calculations to verify this finding.…”

“…It is not surprising that this AF superexchange in LaSrNiZnO 4 due to the in-plane Ni + x 2 − y 2 orbital is 'stronger' than the AF superexchange in LaSr 3 NiRuO 4 H 4 with the out-of-plane Ni + 3z 2 − r 2 orbital, as the x 2 − y 2 orbital has a larger overlap with the in-plane ligands. It is now clear that the Ni + S=1/2 and Ru 2+ S=0 state can only maintain a very weak AF in LaSrNiRuO 4 as in LaSr 3 NiRuO 4 H 4 , and that this charge-spin state cannot at all yield the actual strong FM in LaSrNiRuO 4 .Our previous LSDA+U calculations have shown that LaSrNiRuO 4 is in the robust Ni + S=1/2 and Ru 2+ S=1 ground state[12]. Here again, we perform hybrid functional PBE0 calculations to verify this finding.…”

“…Relative total energies ∆E (meV/fu), total and local spin moments (µB) of LaSr3NiRuO4H4, LaSrNiRuO4, and hypothetic LaSrNiZnO4 (with Zn 2+ substitution for Ru 2+ ) in the FM or intra-layer AF state given by LSDA, LSDA+U, and PBE0 calculations. The LSDA+U results of LaSrNiRuO4 are adapted from Ref [12]…”

Possible High-TC Layered Ferromagnetic Insulator Sr2NiRuO4: An Ab Initio Study

“…The Ru 2+ (4d 6 ) shows an S = 1 state stabilized by a moderate Hund exchange along with a weak crystal field. 20 If the Ru 2+ ion were in the nonmagnetic S = 0 state as suggested in ref coupling would occur only in the diluted S = 1/2 Ni + sublattice, and it would thus be only weakly AFM due to a superexchange between the well-separated Ni + ions with the half-filled x 2 − y 2 orbital. Clearly, this would contradict the measured T C ∼ 200 K. 19 Starting from these considerations, here, we predict a surprisingly strong FM in the layered insulator Sr 2 NiRuO 4 .…”

“…18 Very recently, the Ni−Ruordered LaSrNiRuO 4 was synthesized and found to be an FM layered material with T C ∼ 200 K. 19 The FM order has been explained by considering the superexchange between the unusual low-valence Ni + and Ru 2+ cations. 20 The Ni + (3d 9 ) is in the S = 1/2 state and has an active x 2 − y 2 orbital under the influence of a square planar crystal field. The Ru 2+ (4d 6 ) shows an S = 1 state stabilized by a moderate Hund exchange along with a weak crystal field.…”