Proximity effects across oxide-interfaces of superconductor-insulator-ferromagnet hybrid heterostructure

Abstract

A case study of electron tunneling or charge-transfer-driven orbital ordering in superconductor (SC)-ferromagnet (FM) interfaces has been conducted in heteroepitaxial YBa₂Cu₃O₇(YBCO)/La_0.67Sr_0.33MnO₃(LSMO) multilayers interleaved with and without an insulating SrTiO₃(STO) layer between YBCO and LSMO. X-ray magnetic circular dichroism experiments revealed anti-parallel alignment of Mn magnetic moments and induced Cu magnetic moments in a YBCO/LSMO multilayer. As compared to an isolated LSMO layer, the YBCO/LSMO multilayer displayed a (50%) weaker Mn magnetic signal, which is related to the usual proximity effect. It was a surprise that a similar proximity effect was also observed in a YBCO/STO/LSMO multilayer, however, the Mn signal was reduced by 20%. This reduced magnetic moment of Mn was further verified by depth sensitive polarized neutron reflectivity. Electron energy loss spectroscopy experiment showed the evidence of Ti magnetic polarization at the interfaces of the YBCO/STO/LSMO multilayer. This crossover magnetization is due to a transfer of interface electrons that migrate from Ti^(4+)-δ to Mn at the STO/LSMO interface and to Cu²⁺ at the STO/YBCO interface, with hybridization via O 2p orbitals. So charge-transfer driven orbital ordering is the mechanism responsible for the observed proximity effect and Mn-Cu anti-parallel coupling in YBCO/STO/LSMO. This work provides an effective pathway in understanding the aspect of long range proximity effect and consequent orbital degeneracy parameter in magnetic coupling.

Figure 1

XRD and SQUID measurements of specimens S1 and S2. (a) XRD data from S1 and S2 grown on (001) STO substrates. (b) Field cooled curves as a function of temperature for the S1 and S2 MLs showing their ferromagnetic transitions. (c) The bottom inset shows the zero field cooled curves for both MLs. The typical kink in the curves indicate the superconducting transition temperatures T_SC = 50 K and T_SC = 60 K for S1 and S2, respectively. (d) The inset on the top right hand corner shows the hysteresis loop measurement at 100 K for the S2 ML.

Figure 2

STEM measurements of specimen S2. (a) Bright-field STEM cross-sectional image of the specimen S2, showing trilayer repetitions of the stack on the STO substrate. (b) HAADF STEM (Z-contrast) image of the YBCO layer on the STO substrate. (c) LSMO, STO and YBCO trilayer where a thin STO layer (about 5 nm) is sandwiched between LSMO and YBCO.

Figure 3

EELS measurements of specimen S2. (a) EELS spectrum in the energy-loss range of 400 eV–900 eV showing the Ti, O, Mn, Ba and La related energy loss edges of the specimen S2. (b–d) EELS spectra from LSMO, STO and YBCO. (e) Z-contrast image. (f) Plot of EELS elemental profiles within the sample. (g–n) Elemental EELS maps of Ti, La, Sr, Mn, Y, Ba, Cu and O.

Figure 4

EELS L edge spectra of specimen S2. (a) EELS spectra in the energy-loss range of 450 eV–475 eV showing the Ti-edge of the specimen S2. Background subtraction and D-scan related energy shift correction were applied to all spectra. The plots have been normalized to L₂ e_g peak height and have been vertically shifted for clarity. (b) Comparison of the Ti L-edge EELS spectra from STO5 and STO substrate without a vertical shift. (c) Energy difference between the e_g and t_2g peaks of the Ti L₃ (red squares) and Ti L₂ (black circles) absorption edges of the subsequent layers STO1–STO5 plotted as a function of the STO layer numbers (n=1,2...5) in the ML stack. The lines are a guide to the eye and the error bars are the experimental energy resolution of EELS. The horizontal lines are the ΔE L₃ (red line) and ΔE L₂ (black line) experimental values of the STO substrate of S2.

Figure 5

Cu L edge XAS and XMCD spectra of specimens S1 and S2. FY XAS with two different photon helicities (ρ⁺ and ρ⁻) and the corresponding XMCD signals of the Cu L_3,2 edges from the S1 (a,b) and S2 (e,f) MLs at 100 K measured at 1 kOe and from the S1 (c,d) and S2 (g,h) MLs at 10 K measured at remanence after field cooling at 1 kOe. The XMCD signals have been multiplied by a factor of 10². The XMCD signal at 100 K for the S2 ML is ambiguous. The horizontal arrows (black and blue) indicate the Cu magnetic moments direction with respect to the H_a direction. The vertical arrows indicate the positions of positive and negative XMCD signals of the L_3,2 edges. Schematic pictures of the experimental configuration for (i) S1 and (j) S2 are also shown in the adjacent right panels.

Figure 6

Mn L edge XAS, XMCD spectra and sum rules for s_z of specimens S1 and S2. TEY XAS signals of the Mn L_3,2 edges with two different photon helicities (ρ⁺ and ρ⁻) from the (a) S1 and (d) S2 MLs measured at 100 K and 1 kOe (saturation). XAS (summation) and XMCD (difference) signals corresponding to the XAS signals for S1 (b,c) and S2 (e,f) after baseline correction that takes into account of the linear increase of the background. The p, q and r values are indicated inside. The red lines are the integrated area of each signal. The arrows (black and orange) indicate the Mn magnetic moments direction with respect to the H_a direction.

Figure 7

Mn L edge XAS and XMCD spectra of specimens S1 and S2. TEY XAS with two different photon helicities (ρ⁺ and ρ⁻) and the corresponding XMCD signals of the Mn L_3,2 edges measured at remanence after field cooling at 1 kOe from the S1 (a–d) and S2 (e–h) MLs. The XAS measurements (a,e) at 100 K and (c,g) at 10 K are shown in two separate panels while the XMCD data are plotted together in the middle panels (b,f) for S1 and S2. Temperature dependence of the L₃ edge XMCD signals are shown for S1 (d) and for S2 (h). The arrows (black and orange) indicate the Mn magnetic moments direction with respect to the H_a direction. The maximum dichroic signal is ≈62% at high T and decreases significantly to ≈50% as T crosses below T_SC for S1. The maximum dichroic signal is ≈22% at high T and decreases nominally to ≈20% as T crosses below T_SC for S2. The percentages are calculated with respect to the XMCD signal at 10 K from a reference sample of LSMO layer on STO without a YBCO layer.

Figure 8

XMCD at the Mn L₃ edge of specimens S1 and S2. The temperature dependence of XMCD peak heights of the Mn L₃ edge from S1 (black square) and S2 (red circle). The dotted line indicates T_SC as obtained from the magnetization data. The error bars are typically the symbol sizes. The data has been compared with a reference Mn signal (violet triangle) from a LSMO layer not interfaced with YBCO. Inset shows the percentage difference in XMCD signal with temperature, normalized to the signal from a LSMO reference sample, indicating a 30% loss of magnetization in S1 with respect to S2 at 10 K.

Figure 9

PNR measurements of specimen S3. Specular neutron reflectivity patterns (solid symbols) along with their best fits (open symbols) as a function of Q_z for the NSF [R₋₋ (black) and R₊₊ (red)] channels measured at the saturation field H_a = +1.0 kOe and 10 K for the trilayer sample S3 using model 1, model 2 and model 3. Inset shows a schematic of the magnetic field measurement and neutron scattering geometry. The nuclear (ρ_n) and magnetic (ρ_m) SLDs versus the thickness of the trilayer are shown alongside. Also shown are the spin asymmetry data and the corresponding fits using the three different models, model 1, model 2 and model 3.

Figure 10

SE and DE coupling mechanisms. Schematic diagram of (a) Superexchange interaction between Mn 3d-O 2p-Ti 3d (Mn³⁺-Ti³⁺) and (b) Double-exchange coupling between Ti 3d-O 2p-Cu 3d (Ti³⁺-Cu²⁺) orbitals in the hybridization process. In one case 3d(3z² − r²) orbital is occupied (LSMO), whereas in the other case the 3d(3z² − r²) orbital with higher energy and 3d(x² − y²) orbital with lower energy are occupied.