Proximity‐Induced Novel Ferromagnetism Accompanied with Resolute Metallicity in NdNiO3 Heterostructure

Abstract Employing X‐ray magnetic circular dichroism (XMCD), angle‐resolved photoemission spectroscopy (ARPES), and momentum‐resolved density fluctuation (MRDF) theory, the magnetic and electronic properties of ultrathin NdNiO3 (NNO) film in proximity to ferromagnetic (FM) La0.67Sr0.33MnO3 (LSMO) layer are investigated. The experimental data shows the direct magnetic coupling between the nickelate film and the manganite layer which causes an unusual ferromagnetic (FM) phase in NNO. Moreover, it is shown the metal–insulator transition in the NNO layer, identified by an abrupt suppression of ARPES spectral weight near the Fermi level (E F), is absent. This observation suggests that the insulating AFM ground state is quenched in proximity to the FM layer. Combining the experimental data (XMCD and AREPS) with the momentum‐resolved density fluctuation calculation (MRDF) reveals a direct link between the MIT and the magnetic orders in NNO systems. This work demonstrates that the proximity layer order can be broadly used to modify physical properties and enrich the phase diagram of RENiO3 (RE = rare‐earth element).

FIG. 1. RHEED patterns acquired at the end of the growth (panels d-f) and reflected spot intensity oscillations of the studied films (panels a-c). Yellow arrows in panels d-f point toward the 2x1 reconstruction observed in the RHEED pattern: this is a signature of the orthorhombic unit cell of NNO and linear photon polarizations. Spectra were acquired over a temperature range from 20 to 200 K. The energy and momentum resolutions were set to about 20 meV and ≈ 0.009/0.019Å −1 (parallel/perpendicular to the analyzer slit), respectively. The binding energy scale was calibrated with a polycrystalline copper reference sample in direct electrical and thermal contact with the film. The base pressure of the UHV chamber during the measurements was below 5×10 −11 mbar. Despite the ultra-thin limit of the top NNO layer, it is worth pointing out that the electron inelastic mean free path of electrons in the energy range used in this study is in the order of fewÅ [6], therefore much smaller than the NNO thickness. However, both 5 u.c. NNO layers show very similar FS topology with thick NNO film, formed by cuboids centered at A points and a sphere centered at the Γ point, showing 3D character of the band structure. Indeed, the band structures near EF for all 5 u.c. NNO layers grown on LSMO or just NGO are practically identical to the band structure of thick NNO film [1]. Based on these factors, we are very confident that the measured spectral weight in our ARPES data represents the whole NNO layer, not just the surface.

DETAILED TEMPERATURE DEPENDENT NNO/LSMO QUASIPARTICLE EVOLUTION
The nature of the electronic structure and its temperature evolution were depicted through the low-energy band dispersions acquired at 8 different temperatures for the 5 u.c. NNO/LSMO/NGO. Here we discuss only EDCs and MDc presented in Figure 3. The NNO in proximity to FM layer showed only the soft reduction of the spectral weight of quasiparticle bands down to 25 K. However, this soft reduction of the spectral weight can be a sing of emerging magnetism and/or electron -phonon correlation.
In the AFM state, our model reproduces the "Mott-Hubbard" gap when the system is in AFM state (See Figure 7), which is consistent with the insulator phase in this material. By contrast, for any value of the FM ordering parameter, a fully insulating ground state does not emerge while the quasiparticle peak splits into upper and lower magnetic bands (U/LMBs). Therefore, the NNO-FM at low T can be less metallic, which can account for the spectral weight reduction from the region near the Fermi level seen by ARPES. In addition, the reduced spectral weight is observed in layered (2D) LSMO system in the metallic state, due to electron-phonon interaction [8] XMCD MEASUREMENTS Freshly grown NdNiO 3 based heterostructures where transferred ex-situ for XMCD measurements in the EPFL/PSI X-Treme beamline [2]. The end-station is equipped with a split-pair of superconducting coils to apply up to 7T along the x-ray beam. The variable temperature insert consists of a pumped He-4 cryostat, allowing it to reach 2-3K at the sample. Figure 4 shows typical XMCD spectra at Ni and Mn L3,2 edges for the NNO/LSMO bilayer. The intense peak at ca. 849eV corresponds to La M4 edge. The measurements have been performed in total electron yield mode with the resolution around 0.1eV.
Absorption spectra at fixed magnetic field values were acquired alternating light with right-handed and left-handed circular polarisation (C + and C − respectively). XAS spectra (top panels) are then calculated simply XAS = C + +C − , while XM CD = C + − C − . In figure 1d of the main text the XMCD around the L3 edge was measured in short scans while slowly ramping the temperature. All measurements were performed after zero field cooling.
The magnetisation curves shown in figure 1c were measured by detecting the total electron yield as a function of applied field at two energies: at the maximum XMCD contrast and at the pre-edge where no XMCD contrast is found. The measurement is repeated for both x-ray helicity and both sweep field directions. The shift of the magnetisation curve in the applied field axis is caused by the flux trapped in the superconducting coils. We have characterized both NNO and LSMO single films grown on NGO (110) by XAS and XMCD. The measurements are shown in the figures below. Figure 5 displays XAS and XMCD of Mn in the bilayer versus single LSMO layer. The probed magnetism in LSMO is improved by the capping with the nickelate as already observed in similar nickelate/manganite heterostructures [9]. Other than that, both systems are very similar in the shape of the XAS and XMCD. Figure 6 shows the XAS and XMCD measured at the Ni edge for NNO/LSMO compared to NNO single layer. An XMCD signal is observed in NNO single layer when magnetic field is applied. This magnetization is however not remanent, as shown in figure 2c. The existence of a magnetic signal with applied field which turns out to be nonremanent indicates there could be some paramagnetic moments in Ni. Those paramagnetic moments could come from the top surface or interface with LSMO. This figure exemplifies well how different the magnetic behavior of single layer and bilayer are.
The interaction effect has two parts. First, due to nesting and proximity effects, magnetic order parameter(s) develops in the low-energy spectrum, which is treated within Hartree-Fock formalism. Again due to magnetic fluctuations, the momentum-dependent self-energy correction gives band renormalization and spectral weight distribution which is computed within the momentum-resolved density fluctuation (MRDF) theorem. [3] For the multiband structure, the interacting Hamiltonian consists of intra-orbital (U ), inter-orbital (V ) and Hund's coupling (J H ) interactions, but neglects the weak pair-hopping term. For simplicity, we consider all interaction parameters to be onsite and orbital independent. So we obtain, where i and j are orbital indices. Taking n i = n i↑ + n i↓ , and fixing the spin orientation axis along the z−direction, we have S iz = n i↑ − n i↓ . Now replacing the number operator with n iσ = k c † i,kσ c † i,kσ , arrive at For the antiferromagnetic (AFM) state, we know that the nesting vector is Q ∼ (0.5, 0.5, 0.5)2π, which arises from the nesting between the d x 2 −y 2 and d z 2 orbitals dominating the electron-pocket at the Γ-point, and the hole-pockets at the R-point. This result is consistent with the susceptibility calculation and with the previous ARPES experiment [1]. Based on these results, we introduce the AFM and FM order parameters as Employing the mean-field theory, we obtain the interaction Hamiltonians for the two cases as, where the spin index σ = −σ = ±. The corresponding quasiparticle gaps are defined as ∆ AFM = (V − J H )O AFM , and ∆ FM = (U + 2V )O FM , assuming the gap remains the same for both orbitals. For the AFM state we choose a Nambu' spinor as Ψ AFM k = (c 1,k↑ , c 2,k↑ , c 3,k↑ , c 1,k+Q↑ , c 2,k+Q↑ , c 3,k+Q↑ ) † , and one for the FM state is Ψ FM k = (c 1,k↑ , c 2,k↑ , c 3,k↑ , c 1,k+Q↑ , c 2,k+Q↑ , c 3,k+Q↑ ) † . In these basis, the corresponding total Hamiltonians read: and for the FM, we have Since the third orbital (hybridized d xy orbital) does not cross the Fermi level in this system, it remains unaffected by the AFM FS nesting. For the same reason, we also do not consider the FM order for this orbital. An insulating gap occurs only in the AFM state for a critical value of ∆ AFM 0.9 eV, giving a quasiparticle gap in the band structure ∼50-60 meV, as also seen experimentally. In contrast, for any value of the FM gap, a fully insulating ground state does not occur in this case. When the dynamical correlations due to spin-, charge-, and orbital density fluctuations are included, as discussed below, both AFM and FM bands split further by the corresponding self-energy correction. Such a four-band structure (for each orbital) is evident in the calculated spectrum. In the magnetic state, the magnetically split bands are referred to upper and lower magnetic bands (U/LMBs). The self-energy split bands are referred to here as upper and lower-Hubbard bands (U/LHBs). In the AFM state, our model reproduces the coexistence of the AFM and 'Mott-Hubbard' gap, in consistent with the two metal-insulator transitions in this material. In the FM state, the 'Mott-Hubbard' gap localizes the quasiparticle states by reducing their spectral weight from the low-energy region. Therefore, in the FM state, although a fully insulating quasiparticle gap does not open the insulating-like behavior arises due to spectral weight distributions.

ADDITIONAL CORRELATIONS TO THE MEAN-FIELD GROUND STATE
In an earlier study, we have demonstrated that the self-energy correction due to various density-density fluctuations leads to substantial renormalization of both quasiparticle bands and associated their spectral weights as a function of both energy and momentum in the paramagnetic state [1] Here we extend the momentum-resolved density fluctuation (MRDF) calculations [3][4][5] to various magnetic ground states discussed above.
The single-particle Green's function is defined asG 0 (k, iω n ) = iω n1 −H tot −1 , where iω n is the Matsubara frequency for the fermions, and H tot is the total magnetic Hamiltonian defined in Eqs. (10, 11). The explicit form of G is then obtained as Here q and ω p are the bosonic excitation momentum and frequency, respectively. φ ν k,m is the eigenstate for the ν th tight-binding band (E ν k ), projected onto the m th orbital. The non-interacting density fluctuation susceptibility is χ st 0,mn (q, ω p ) = − 1 Ω BZ β k,n G mn (k, iω n )G st (k + q, iω n + ω p ), where β = 1/k B T , and k B is the Boltzmann constant and T is temperature. Ω BZ is the electronic phase space volume. f ν k and n p are the fermion and boson occupation numbers, respectively. After performing the Matsubara summation over the fermionic frequency ω n and taking analytical continuation to the real frequency as ω n → ω + iδ, we get χ st 0,mn (q, ω p ) = − 1 Ω BZ k,ν,ν φ ν † k+q,s φ ν k+q,t φ ν k,n φ ν † k,m