Syntropic spin alignment at the interface between ferromagnetic and superconducting nitrides

ABSTRACT The magnetic correlations at the superconductor/ferromagnet (S/F) interfaces play a crucial role in realizing dissipation-less spin-based logic and memory technologies, such as triplet-supercurrent spin-valves and ‘π’ Josephson junctions. Here we report the observation of an induced large magnetic moment at high-quality nitride S/F interfaces. Using polarized neutron reflectometry and DC SQUID measurements, we quantitatively determined the magnetization profile of the S/F bilayer and confirmed that the induced magnetic moment in the adjacent superconductor only exists below TC. Interestingly, the direction of the induced moment in the superconductors was unexpectedly parallel to that in the ferromagnet, which contrasts with earlier findings in S/F heterostructures based on metals or oxides. First-principles calculations verified that the unusual interfacial spin texture observed in our study was caused by the Heisenberg direct exchange coupling with constant J∼4.28 meV through d-orbital overlapping and severe charge transfer across the interfaces. Our work establishes an incisive experimental probe for understanding the magnetic proximity behavior at S/F interfaces and provides a prototype epitaxial ‘building block’ for superconducting spintronics.


Introduction
The interface between a superconductor (S) and a ferromagnet (F) is a topic of ongoing research in condensed matter physics.The interaction between superconductivity and ferromagnetism leads to fascinating phenomena, including (inverse) magnetic proximity effects [1][2][3][4][5], spin-triplet superconductivity [6,7], and the emergence of Majorana fermions [8].Of particular interest is the magnetic proximity effect, which directly reflects the exchange interaction between the spins of electrons across S/F interfaces, resulting in the suppression of magnetic order or the appearance of unconventional superconductivity [9,10].When a magnetic material is proximity to a superconductor, the magnetic order parameter can penetrate the superconductors over a short distance (usually a few nanometers) [11].This causes spatial variation of the superconducting (SC) order parameters near the interface and disrupts the Cooper pairs, significantly impacting the macroscopic physical properties of materials on both sides.In addition to the fundamental understanding based on S/F interfaces, extensive research has been conducted on the coupling between superconducting condensate states and magnetic-exchange spin coupling.This research has opened up a new field of study known as superconducting spintronics, which aims to develop dissipation-less spin-based devices, including memories [12], supercurrent spin-valves [13][14][15], and "π" Josephson junctions [16].Therefore, it is particularly relevant to experimentally verify the creation of a spin-polarized superconducting state with parallel or anti-parallel spin alignment at proximity-engineered S/F interfaces, as this is a crucial step in advancing this research direction.
So far, the genuine mechanism behind the magnetic proximity effect at S/F interfaces is still debated.Earlier research on S/F heterostructures composed of metal alloys reported an oscillation of the superconducting transition temperature as a function of the ferromagnetic layer thickness, demonstrating the possible unconventional propagation of superconducting pair waves in the system due to the strong exchange field [17][18][19][20][21].With the development of advanced thin-film synthesis techniques, increasing attention has been focused on high-quality epitaxial S/F interfaces between a high-temperature superconductor (YBa2Cu3O7) and a fully spin-polarized half-metal ferromagnet (La1-xCaxMnO3).[22][23][24][25].Evidence from both X-ray magnetic circular dichroism (XMCD) [26] and polarized neutron reflectometry (PNR) [27][28][29][30] confirm a reduction of the magnetic moment at S/F heterointerfaces and the antiparallel spin alignment between Cu and Mn ions in each component.The magnetic proximity effects in YBa2Cu3O7/La1-xCaxMnO3 interfaces are rather complex and depend on many impact factors, such as the electronic state of magnetic layers [26], the thickness of SC layer [29], nonhomogeneous domain structures [23][24], etc.Although the extensive studies had been conducted in both metals and oxides, the opposite magnetic coupling through the S/F interfaces makes it difficult to control the spin collinearity by external magnetic fields.Furthermore, the experimental preliminary confirmation of the existence of the triplet component in S/F structures or the formation of cryptoferromagnetic state in SC had seldomly be explored.The active and reversible control of triplet supercurrents in spin-valve structure can be highly advantageous for developing superconducting spintronics in future.
In this work, we investigated transition metal nitride bilayers composed of the superconducting VN and the ferromagnetic Fe3N using conventional electrical and magnetic transport measurements as well as PNR technique.Our findings indicate that the proximity of the SC layers to Fe3N can significantly reduce the SC order parameters and result in unconventional magnetic behaviors.We also confirmed that an induced large magnetic moment extends a few nanometers into the SC layers, with its sign unexpectedly parallel to the magnetic moment in the Fe3N layer, which contradicts earlier reports and theoretical predictions.Instead of superexchange coupling at oxide interfaces, the Heisenberg direct exchange coupling at nitride interfaces is further verified by first-principles calculations.This study provides a detailed microscopic picture that offers intriguing insights into the magnetic correlations at S/F interfaces.

Results
The Fe3N/VN bilayer, as well as Fe3N and VN single layers, were grown on (00l)-oriented Al2O3 substrates by pulsed laser deposition (PLD) assisted by RF nitrogen plasma (for experimental details, see Methods) [31][32][33][34].Further details on the structural characterizations of the Fe3N and VN single layers can be found in our previous works [34,35].Figure 1a shows an XRD θ-2θ scan of the Fe3N/VN bilayer, which exhibits narrow diffraction peaks with high-order Laue thickness fringes, indicating the epitaxial growth of a high-quality bilayer.The VN layer has a (111) orientation grown on Al2O3 substrates, while the Fe3N layer, grown on (111)-oriented VN, has a (100) orientation.
Notably, the structural orientation of Fe3N in the bilayer is differs from that of direct growth of Fe3N single films on Al2O3 [35].The insets of Figure 1a illustrate the atomic arrangements of the related nitrides and substrates.The epitaxial relationship between two nitride layers is enabled by their compatible crystallographic symmetry and relatively small misfit strain.XRR measurements were used to examine the well-defined interfaces and chemical compositions within the bilayer.The rootmean-square roughness of the Fe3N/VN interface, averaged over the coherence of the X-ray beam projected on the sample's surface (≈ tens of millimeter 2 ), was found to be 5.2 ± 0.5 Å. Figure 1b shows the chemical depth profile obtained from fitting a model to XRR experimental data.The X-ray scattering length density (SLD) profile revealed that the electron density at the surface of Fe3N is significantly lower compared to the interior part of the Fe3N layer.Additionally, the SLDs within the VN layer was nonuniform, with the SLD of the VN interfacial layer, having a thickness of 4.6 nm, greater by ~5% compared to that of the rest of the VN layer.
We performed STEM measurements on the identical Fe3N/VN bilayer.Figure 1c indicates the high crystallinity of both Fe3N and VN layers, and the bilayer appears to be free of noticeable defects.The Fe3N surface layer has a relatively lower HAADF brightness compared to the rest of the layer, consistent with XRR fitting results.Further investigation of the chemical composition was carried out using elemental-specific electron energy loss spectroscopy (EELS), as shown in Figure 1d and Supplementary Figures S1 and S2.We discovered that the Fe3N surface layer contains a significant amount of oxygen, indicating the oxidization of Fe ions.High-magnification STEM imaging was taken at the surface region of the Fe3N/VN bilayer (Supplementary Figure S3), revealing that the top surface has a spinel (Fe3O4) structure with clearly visible octahedral and tetrahedral iron atomic columns.Therefore, the surface of the Fe3N/VN bilayer comprises a mixture of Fe3N and Fe3O4 single-crystalline films.The oxidization of the Fe3N layer is unavoidable phenomenon when exposed to ambient conditions, which is commonly observed in other iron nitride compounds [36].Notably, the Fe3O4 surface layer does not affect the quality of the buried interfaces.
The atomic-resolved interfaces of Fe3N/VN and VN/Al2O3 are shown in Figures 1e and 1f, respectively.Both interfaces are clearly abrupt, with well-aligned atoms and no significant intermixing inside the VN layers.Additionally, XAS measurements were performed on the Fe3N/VN bilayer at room temperature with a small incident angle (large portion of irradiated area).The resulting spectra were taken at both resonant N Kand Fe L-edges (Supplementary Figure S4), indicating that the Fe3N layers have sufficient nitrogen content, and the valence state of iron ions remains +3.In contrast to the severe charge transfer observed between Fe and Mn ions at the YBa2Cu3O7/La1-xCaxMnO3 interfaces [24], no significant oxidization state changes were observed for both Fe and V ions at the Fe3N/VN interfaces.
Figure 2a presents the temperature-dependent resistivity of a VN single film and a Fe3N/VN bilayer at zero magnetic field.Prior to the sharp superconducting transition, all samples exhibit metallic phases.The normal state Hall resistivity measured at 10 K confirms that the charge carriers are electrons, and the carrier densities of the VN single film and Fe3N/VN bilayer fall within the range of (19.1 -26.3) × 10 22 cm -3 (Table I).The superconducting transition temperature (TC) of the VN single film is ~ 7.78 K and is suppressed by ~ 1.55 K after capping a ferromagnetic Fe3N layer.This effect is typically attributed to the presence of spin-polarized quasiparticles at the interface, which can enhance the scattering of the Cooper pairs responsible for superconductivity.The temperature-dependent resistivity of the two samples were recorded under different magnetic fields, both parallel and perpendicular to the sample's surface plane, up to 9 T (Supplementary Figure S5).
Both samples exhibit conventional suppressed superconductivity with increasing field.The VN is a typical type-II superconductor with upper critical field (Hc2) [37].Figures 2b and 2c show the in-plane and out-of-plane magnetic field-dependent resistivity of a Fe3N/VN bilayer, with temperature fixed at values from 3 to 8.5 K, respectively.The same transport measurements were conducted repeatedly on a VN single film (Figures 2d and 2e).The Hc2 is determined at fields when resistivity reduces to the half value of its normal state.In Figure 2f, we plotted the temperature dependence of the Hc2 for two samples when H // ab and H // c.Notably, both samples exhibit a smaller Hc2 (H // c) compared to Hc2 (H // ab).These data can be well-fitted by a single-band Werthamer-Herlfand-Hohenberg (WHH) equation (dashed lines) [38].The fitting parameters for the two samples are summarized in Table I.The mean free path of the VN layer increases by ~ 30% after proximity to a Fe3N layer.Simultaneously, the estimated coherence length increases from ~405 nm (VN) to ~422 nm (Fe3N/VN).
Earlier studies have shown that the ferromagnetism in a F layer can be influenced by superconductivity when the thickness of the F layer is much smaller than the coherence length of the SC layer [2,3].To text this hypothesis, we measured the field-dependent magnetization (M) of a Fe3N single layer and a Fe3N/VN bilayer (Figures 3a and 3b and Supplementary Figure S6).We recorded both in-plane and out-of-plane M at fixed temperatures across TC.The magnetic easy-axis is along the in-plane direction, consistent with the magnetic behavior of 2D thin films [35].The total M of both samples was normalized to the thickness of the Fe3N layer.At the high field regions, M of both samples is saturated.Interestingly, we observed that the Msat of a Fe3N/VN bilayer is lower than that of a Fe3N single layer below TC, and these values returned to the same level as that of a Fe3N single layer when the temperature is above TC.We believe that the exchange interactions between the superconducting condensate and the magnetic order parameter reduce the energy of the bilayer system, possibly leading to a cryptoferromagnetic state [39].When the temperature is below TC, the VN becomes superconducting, and some of the electrons located in the Fe3N layers may condensate into Cooper pair, reducing the absolute value of the total magnetization in the Fe3N/VN bilayer, as they do not contribute to the total magnetization.
In additional to the change in the total magnetization of a Fe3N/VN bilayer, the magnetic state of the superconducting VN layer can be correspondingly modified due to the magnetic proximity effect [1][2][3][4][5].The magnetization measurements on the Fe3N/VN bilayer confirm the presence of antimagnetic signals when the VN layer enters the SC state (Figure 3c).The TC obtained from the M-T curves is consistent with the electrical transport measurements (inset of Figure 3c).From the M-H measurements, we observe abnormal behavior in the hysteresis loops of the Fe3N/VN bilayer in the low field region when the temperature is lowered below TC.When H // ab, the magnetic moment initially increases with applied fields and then returns to a constant value of Msat beyond a critical field (HSC).This behavior suggests the presence of an induced magnetic moment in the VN layer for temperatures below TC.Simultaneously, the out-of-plane magnetic moment of the Fe3N/VN bilayer jumps to incredible values at the small fields and exhibits a greatly increased coercive fields (HC) when the VN layer is in the superconducting state.Figures 3d and 3e show HSC and HC as a function of the temperature, respectively.Clearly, HSC and HC in the SC state are orders of magnitudes larger than those in the normal state and reduce progressively as the temperature increases.When the temperature is above TC, the anomaly in the magnetization along both in-plane and out-of-plane direction disappears.
To quantitatively determine the interfacial magnetization profile, we perform systematic PNR measurements on a Fe3N/VN bilayer, as shown in Figure 4a.The specular reflectivity (R) of the bilayer was measured as a function of the wave vector transfer (q), with R + and R -representing the reflectivities for neutrons with spins parallel or anti-parallel to the applied magnetic field, respectively.The PNR measurements were performed under a magnetic field of 1 T at 3.5 K, when the Fe3N/VN bilayer was in the SC state.Figure 4b shows the PNR data with the reflectivity normalized to the asymptotic value of the Fresnel reflectivity RF (= 16π 2 /q 4 ).The large q-dependent splitting between the two reflectivities reflects the large net magnetization of the Fe3N/VN bilayer.
To fit the PNR data, we used the chemical depth profile obtained from XRR fitting to strictly constrain a model for PNR fitting.We obtained the nuclear and magnetic SLD profiles corresponding to the chemical and magnetization distribution as a function of film thickness, as shown in Figures 4e   and 4f.The best fits demonstrate that the Fe3N layer has a magnetization of 1630.9 ± 7.1 kA/m and the Fe3O4 surface layer has a much lower magnetization of 339.5 ± 6.3 kA/m.The magnetization of Fe3N interior layer is significantly larger than the magnetization measured by SQUID, which underestimated the value by assuming a uniformed distribution of magnetization across the entire Fe3N layer.Additionally, we observed that the VN interfacial layer in close proximity to Fe3N exhibits a significant net moment of 115.6 ± 2.5 kA/m ,which aligns parallel to the Fe3N magnetization.The spin texture schematic at the Fe3N/VN interfaces is depicted in Figure 4a.
Confidence in this interpretation of syntropic spin alignment at the interface is further reinforced by fitting the spin asymmetry (SA) curve derived from experimental data (Figure 4c).To fit the PNR-SA data, we considered three different scenarios: (1) parallel spins (positive M); (2) no spins (zero M); and (3) antiparallel spins (negative M) of the VN interfacial layer with respect to the spins of Fe3N layer (Figure 4c and Supplementary Figure S7).The best fit to the PNR data, with a lowest χ 2 metric ~ 1.1, demonstrates that the ferromagnetic coupling across the interface is an intrinsic nature.
Under the same magnetic field of 1 T, we conducted PNR measurements on the Fe3N/VN bilayer at 15 K (Figure 4d).The induced magnetic moment in the VN interfacial layer decreases to zero (Figure 4f), which is consistent with SQUID results.
To confirm the observation of the induced moment in VN interfacial layer, we performed further control measurements on the Fe3N/VN bilayer at a fixed temperature of 6.5 K with varying magnetic fields between 2 kOe and 1 T (Supplementary Figure S8).The measuring temperature was carefully chosen at the boundary of the phase transition between the SC state and normal state.Under a magnetic field of 2 kOe, the Fe3N/VN bilayer remained in the SC state.The VN interfacial layer exhibits a net magnetic moment of 96.5 ± 3.5 kA/m (Figure 4g).As the magnetic field was increased to 1 T, the magnetization of the Fe3N layer increased only by ~ 5% because the Curie temperature of ferromagnetic Fe3N is far beyond room temperature.However, no induced moment in the VN interfacial layer was observed.This was because the SC state of the bilayer was destroyed, and it enters the normal state at high magnetic fields.Therefore, our experimental results provide clear evidence that a parallel magnetic moment of in the VN layer close to the interface only exists once the bilayer enters the SC state.At this moment, we could not argue the formation of magnetic domains below TC due to the presence of magnetic vortices creating electrodynamic forces [23,40].
The off-specular PNR experiments did not observe the clean Bragg peak diffraction below TC.At least, our results clearly rule out the existence of a triplet supercurrent at the interface that previously had been proposed to explain the observed magnetism at S/F interfaces [41][42][43], otherwise the induced moment of VN interfacial layer should have an opposite sign with respect to that of Fe3N.Furthermore, earlier work demonstrates the magnetic exchange coupling in the YBa2Cu3O7/La1-xCaxMnO3 heterostructures strongly depends on the thickness of superconducting layer [23,26,44].We measured the transport properties of a VN single film and a Fe3N/VN bilayer with a VN layer thickness of ~ 6 nm (Figure 5a).The 6-nm-thick VN film exhibits a superconducting behavior with TC ~ 4.5 K.The superconducting order parameters show an increased anisotropy compared to those of a thick VN single layer (Supplementary Figure S9).Moreover, the Fe3N/VN bilayer maintains the ferromagnetic metallic phase with clear anisotropic MR when temperature is lowered down to 300 mK (Figure 5b and Supplementary Figure S10).These results suggest the strong suppression of superconducting ordering by a ferromagnetic capping layer.We note that the induced ferromagnetism and superconductivity cannot coexist in the interfacial VN layers.
Previous experimental evidence of long-range penetrated ferromagnetism into conventional superconducting oxides reveals an antiparallel magnetic coupling with respect to the ferromagnetic layers.[23][24][25][26][27][28][29][30].In those cases, the superexchange coupling between 3d transitional metal elements via oxygen 2p orbitals results in an antiferromagnetic spin alignment.However, our findings demonstrate that spins can be aligned parallel to each other across S/F interfaces (Figure 5c).We performed the first-principles calculations on Fe3N/VN interfaces based on density functional theory (DFT).Both FM and AFM magnetic configurations are initially texted (Supplementary Figure S11).
We calculated Heisenberg exchange coupling constant (J) between Fe and V atoms near the interface is 4.28 meV.The positive sign indicates the ferromagnetic coupling across the interfaces.Besides, we find that the electron clouds (marked in yellow) around the interfaces is denser than those in the interior parts of VN and Fe3N layers (Figure 5d).The calculation results indicate the electron transfer from Fe ions to V ions across the interface due to the charge density difference.We further calculated the contour density for the spin up (red) and spin down (yellow) components in Figure 5e.We found that the spin polarization exhibits a large positive value for the constructed heterointerface.The density of states (DOS) for spin-up channels are significantly larger than those of spin-down channels at the Fermi level, implying that the heterointerfaces exhibit the ferromagnetic coupling.
The ferromagnetically spin alignment between Fe and V ions is in sharp contrast to the earlier works on the spin coupling at oxide interfaces.We propose a qualitative physical explanation for the observed magnetic behaviors at the Fe3N/VN interfaces.When the temperature falls below TC, magnetic vortices form in the superconducting VN layer, but they do not influence the in-plane magnetization of the bilayer (Figure 3f).In the Fe3N/VN heterostructures with strong exchange fields, the number of spin-up and spin-down electrons is imbalanced, leading to a magnetic "leakage" from the Fe3N layer to into the VN layer and electron polarization in the VN layer persists over a short length scale.In this scenario, the direction of the induced magnetic moment in the VN interfacial layer aligns parallel to that in the Fe3N layer.This finding is supported by numerical solutions to the Bogoliubov-de Gennes equations [2,3], which are consistent with our SQUID and PNR results.However, when the magnetic field switches to the out-of-plane direction (Figure 3g), a different situation arises.At small fields, the Fe3N/VN bilayer remains in SC state, and the magnetic flux only penetrates through the vortices, resulting in ultra-large effective magnetic fields.This leads to the observation of incredibly large magnetic moments in the Fe3N/VN bilayer at small magnetic fields.Additionally, the strong competition between ferromagnetic and superconducting order parameters leads to enhanced coercive fields when the VN in the SC state.When the magnetic field exceeds Hc2 or the temperature increases above TC, the Fe3N/VN bilayer enters the normal state, the impact from the superconducting VN layer vanishes, greatly weakening the strength of interfacial magnetic coupling.The Msat and HC of the Fe3N/VN bilayer return to the values that are nearly identical to those of a Fe3N single layer.
Finally, we performed repeated measurements on a high-quality Fe3N/TiN bilayer and compared the results to those obtained from a TiN single layer (Supplementary Figures S12-S14).Similar effects, such as the large anisotropy of Hc2 and the suppression of TC, were observed in the Fe3N/TiN bilayer.Before TiN enter the SC state, we observe the coexistence of magnetoresistance and the opening of a superconducting pseudogap, suggesting the presence of strong competing orders at the critical transition temperature.Due to the significant suppression of TC to ~ 1.28 K, systematic magnetometry and PNR measurements would be very challenge.Testing whether the induced moment in TiN remains parallel to that of ferromagnetic Fe3N will be examined in future studies.

Discussions and conclusions
In summary, our work presents new findings regarding the induced magnetic moment in a superconducting VN layer in proximity to a strong ferromagnetic Fe3N layer.Our results differ from previous studies, as we have discovered that the spin orientation in VN is ferromagnetically coupled to the magnetization of the adjacent Fe3N layer, revealing a unique form of magnetic coupling across the S/F interface.Furthermore, we propose the possibility of cryptoferromagnetic states in the S/F bilayers, where superconductivity can persist despite the presence of a ferromagnetic background.
Our research sheds light on the potential for exploring complex competing orders in the large family of transition metal nitrides, such as ZrN and NbN [45,46], which are known to host superconducting transitions.Such materials have not been thoroughly investigated in the past, and our study provides a strategy for future exploration.Additionally, we recommend prioritizing the testing of binary or perovskite-type nitrides containing 5d elements [47,48].These materials not only have competing phases, but also exhibit a strong spin-orbital (SO) interaction.This interaction has an energy scale comparable to the superconducting gap, which may have a significant impact on the penetration length of the triplet component into the superconductor, thereby dominating the long-range proximity effect in S/F structures.

Sample synthesis
The VN and Fe3N thin films were fabricated on (001)-oriented single-crystalline Al2O3 substrates by pulsed laser deposition (PLD).Nearly stoichiometric binary nitride targets were synthesized using a high-pressure reaction route at the high-pressure Laboratory of South University of Science and Technology (SUSTech).The VN films were deposited at a substrate temperature of 750 o C, while the Fe3N films were deposited at a lower substrate temperature of 300 o C to maintain their high crystallinity.
During deposition, the density of laser fluence was maintained at ~ 1 J/cm 2 , and the base pressure was kept at around 10 -8 Torr.A radio frequency (RF) plasma source with tunable input power (100-400 W) and partial pressure of N2 gas (10 -3 -10 -6 Torr) was applied to generate highly active nitrogen atoms.
The generated nitrogen plasma helped to compensate the nitrogen vacancies in the films, enabling the as-grown films to maintain their intrinsic characteristics.The thickness of VN (Fe3N) films was controlled by counting laser pulses.The TiN films were fabricated using a home-made RF magnetron sputtering system at the Ningbo Institute of Materials Technology and Engineering, CAS.The base pressure of the sputtering chamber is below 3×10 -8 Torr.The TiN films were deposited at 800 o C under a pure nitrogen pressure of 0.02-0.03Torr.The RF generator power was maintained at 100 W during the deposition.The thickness of the TiN films was controlled by counting sputtering time.
Subsequently, the TiN films were capped with a Fe3N films using PLD.

Structural and electronic characterizations
X-ray reflectivity (XRR) measurements were conducted to confirm the structural integrity, layer thickness, interface/surface roughness, and densities of all layers, from which we could obtain the chemical profiles of Fe3N/VN and TiN/VN bilayers.The θ-2θ scans were performed on a highresolution four-circle X-ray diffractometer (Panalytical MRD X'Pert 3) with Cu Kα radiation.The synchrotron based XRD measurements were performed at the beamline 1W1A of the Beijing Synchrotron Radiation Facility (BSRF).The wavelength of synchrotron X-ray is 1.24 Å.The microstructures of a Fe3N/VN bilayer were examined using JEM ARM 200CF electron microscopy at the Institute of Physics, Chinese Academy of Sciences.The samples were prepared using Ga + ion milling after the mechanical thinning.Both HAADF and ABF imaging were performed in the scanning mode.Elemental-specific EELS mapping were performed at the O K-, N K-, V L-, Fe L-edges from the interested regions.X-ray absorption spectroscopy (XAS) measurements were performed on a Fe3N/VN bilayer using the total-electron yield (TEY) method.All structural characterizations were performed at room-temperature.

Transport and magnetic characterizations
Transport properties of VN and TiN single films and Fe3N/VN and Fe3N/TiN bilayers were measured using standard van der Pauw geometry.The temperature and magnetic field dependent resistivity measurements were conducted using a 9T-PPMS.The ac current was kept at a minimum requirement of 1 μA to avoid the Joule heating.The magnetic properties of a Fe3N single film and a Fe3N/VN bilayer were characterized by a SQUID equipped with high-temperature unit.Both in-plane and outof-plane magnetization were obtained at the variable temperatures and fields.For the antimagnetic measurements of a Fe3N/VN bilayer, the sample was zero-field cooled and measured during sample warming up process at a field of 200 Oe.

PNR measurements
PNR measurements were carried out at MR beamline of Chinese Spallation Neutron Source (CSNS), Dongguan, Guangdong Province.The size of a Fe3N/VN bilayer is 10 × 10 × 0.5 mm 3 .We performed PNR measurements at fixed temperatures of 3.5 and 15 K under an in-plane magnetic field of 1 T.These temperatures were chosen because the VN films stay in the superconducting (3.5 K) and normal state (15 K) during the PNR measurements.Additionally, we conducted a control measurement by fixing the temperature at 6.5 K. Similarly, the VN films can be switched between superconducting and normal state under magnetic fields of 2 kOe and 1 T, respectively.The specular neutron reflectivity (R) of a Fe3N/VN bilayer were recorded as a function of the wave vector transfer q (= 4πsinα/λ), where α is the incident angle of neutron beam and λ is the wavelength of neutrons.Both R + and R -were recorded when neutrons with spins parallel or antiparallel to the applied fields (corresponding to the spin-up and spin-down neutrons), respectively.We fitted PNR data to a model (including the layer thickness and chemical roughness) that was obtained by XRR fitting using the formalism of Parratt.In these cases, the nuclear scattering length densities (nSLD) for the Fe3N and VN were fixed to their bulk values of ~ 8.9 × 10 -6 Å -2 and ~ 5 × 10 -6 Å -2 , respectively.

First-principles calculations
The first-principles calculations within the framework of density-functional theory (DFT) based on Vienna ab initio Simulations Package (VASP) [49].Projected augmented wave (PAW) and generalized gradient approximation (GGA) methods are adopted to treat the valence electrons-ion and exchangecorrection effects, respectively [50].In order to correctly describe the 3d electrons, the GGA+U method is employed with an effect Hubbard U value UFe = 1.5 eV for Fe element and UV = 3 eV for V element [51].Otherwise, a 500 eV cutoff energy for plane-wave expansion and 6 × 6 × 1 k-point meshes are set for determining the most stable structure.Meantime, the Fe3N/VN heterointerface is relaxed until the Hellmann-Feynman force acting on each atom become smaller than 1×10 -3 eV/Å.Finally, the Heisenberg exchange coupling constant (J) between interfacial Fe and V elements is solved via comparing the energy between ferromagnetic (FM) and antiferromagnetic (AFM) configurations (as shown in a 1×√3×1 supercell is constructed, Supporting Information Fig. S11).I.

contributions:
The nitride samples were grown by Q.J.; TEM lamellas were fabricated with FIB milling and TEM experiments were performed by Q.H.Z. and L.G.; PNR measurements were conducted by H.B. under the guidance of T.Z.XAS measurements were performed by Q.J., S.R.C., H.T.H., T.C., and J.O.W.; Q.J., and S.R.C., worked on the structural and magnetic measurements.Y.L.G. and H.X.Y. performed the first-principles calculations based on density functional theory.Y.T.Z. and Z.G.C. performed the ultra-low temperature transport measurements.C.W. participated the discussions and K.J.J. provided important suggestions during the manuscript preparation.E.J.G. initiated the research and supervised the work.Q.J. and E.J.G. wrote the manuscript with inputs from all authors.Competing interests: The authors declare that they have no competing financial interests.

Figures and figure captions
Figures and figure captions

Figure 1 .
Figure 1.Atomically sharp interface between ferromagnetic Fe3N and superconducting VN.(a) XRR and θ-2θ scan of an Fe3N/VN bilayer grown on an Al2O3 substrate.The open symbols represent the experimental data, and the red line represents the best fit.The insets show the plane-view of crystal structures for all layers and substrates.(b) X-ray scattering length density (SLD) depth profile across the Fe3N/VN heterointerfaces.(c) High-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) image, and (d) combined electron-energy loss spectroscopy (EELS) spectrum image collected from a Fe3N/VN bilayer.The colors in the EELS mapping indicate the distribution of elements.Representative highresolution TEM image at (e) Fe3N/VN and (f) VN/Al2O3 interfaces are also shown.The colored insets in (e) and (f) show the integrated intensities of N Kand O K-edges, respectively.The nitrogen content in Fe3N is comparably lower than that in VN.

Figure 2 .
Figure 2. Suppression of superconducting transition temperature (TC) by a strong ferromagnet Fe3N.(a) Temperature dependent resistivity of a VN single layer and a Fe3N/VN bilayer at zero magnetic field.The TC of the VN films was suppressed by ~1.55 K. (b) and (c) Magnetic-field dependent resistivity of the Fe3N/VN bilayer at various temperatures when the magnetic field was applied parallel to the ab-plane and the c-axis, respectively.Similar measurements were performed on the VN single layer, as shown in (d) and (e).(f) Temperature dependence of the upper critical field (HC2) for the VN single layer and Fe3N/VN bilayer.The square and circle symbols represent HC2 when the magnetic field was applied parallel to the abplane and the c-axis, respectively.The dashed lines are fitting curves based on the Werthamer-Helfand-Hohenberg (WHH) model.The details of fitting parameters are summarized in Table

Figure 3 .
Figure 3. Strong magnetic coupling at the interfaces between Fe3N and VN.(a) and (b) M-H hysteresis loops measured from the Fe3N single layer and Fe3N/VN bilayer, respectively, with magnetic fields applied parallel to the ab-plane and c-axis.The temperature for M-H loops was varied across the superconducting transition.(c) M-T curves of a Fe3N/VN bilayer.The measurements were carried out during the sample warm-up after zero-field cooling (ZFC) and field cooling (FC) at 1 kOe.The inset of (c) shows the M-T curve of a Fe3N/VN bilayer when an out-of-plane magnetic field of 200 Oe was applied.Temperature dependence of (d) the critical field (HSC) and (e) coercive field (HC) under in-plane and out-of-plane magnetic fields.(f)and (g) Illustrations of the spin alignment and magnetic vortices within Fe3N and VN layers when magnetic fields are parallel or perpendicular to the interfaces, respectively.The illustrations of the vortices are guide for eyes.The orientations of vortices are perpendicular to the applied fields in reality.

Figure 4 .
Figure 4. Magnetic depth profiling across Fe3N/VN heterostructures.(a) Schematic of PNR set-up.PNR measurements were performed at fixed temperatures under external in-plane magnetic fields.The inset shows a high-resolution HAADF-STEM image at the interface region of a Fe3N/VN bilayer, with illustrated spin textures.(b) Measured (open symbols) and fitted (solid lines) reflectivity curves for spin up (R + ) and spin down (R -) polarized neutrons as a function of wave vector (q).The reflectivities were normalized to the Fresnel reflectivity (RF).PNR spin-asymmetry (SA) ratio SA = (R + + R -)/( R + -R -) obtained from the experimental and fitted reflectivities when VN in (c) the superconducting (SC) state (T = 3.5 K) and (d) the normal state (T = 15 K) under an external magnetic field of 1 T. The error bars represent one standard deviation.Fits in (c) with zero and negative magnetization in the VN interfacial layer were shown in dashed lines, demonstrating large deviations from experimental data.(e) Nuclear scattering length density (nSLD) and (f) magnetic scattering length density (mSLD) profiles measured for a Fe3N/VN bilayer at 3.5 and 15 K were presented as a function of the distance from substrate.The scale on the right-hand side shows the absolute magnetization (M).(g) PNR mSLD profiles of a Fe3N/VN bilayer at T = 6.5 K with magnetic fields of 2 kOe and 1 T. The magnetization measured inside VN layer in (f) and (g) is marked with arrows.

Figure 5 .
Figure 5. First-principles calculations on the electronic and spin states across Fe3N/VN interfaces.(a) Temperature dependent resistivity of a 6-nm-thick VN single layer and a Fe3N/VN(6nm) bilayer at zero magnetic field.The bilayer maintains metallic phase down to 300 mK, suggesting the ferromagnetic Fe3N completely suppressed the superconductivity.Inset shows the temperature dependence of HC2 for the 6nm-thick VN single layer.The square and circle symbols represent HC2 when the magnetic field was applied parallel to the ab-plane and the c-axis, respectively.The dashed lines are fitting curves based on the WHH model.(b) Fielddependent magnetoresistance (MR) of a Fe3N/VN(6nm) bilayer at 300 mK.Both in-plane and out-of-plane MR are recorded.(c) The side-view of interface region in a Fe3N/VN bilayer.The blue arrows indicate the spin orientation of ions.(d) Charge density differential distributions for FM state at the interface when the charge density is equal to ± 5×10 -3 e/Å 3 .The colored illustrations indicate the isosurfaces, corresponding to the charge accumulation (yellow) and depletion (green) in the space.(e) Contour density corresponding to the FM state for spin up (red) and spin down (yellow) states when the spin density is equal to ± 3×10 -3 μB/Å 3 .

Table I . Summary of the parameters for the VN and TiN superconducting single layers, and Fe3N/VN and Fe3N/TiN bilayers
, including layer thickness (t), superconducting transition temperature (T C ), upper critical field (H C2 ), carrier density (n), Fermi vector (κF), coherence length (LC), and mean free path (λ).The electronic parameters, such as resistivities, carrier densities, and mobilities, of single layer and bilayers were extracted from transport measurements conducted in the normal state (at 10 K).Estimated errors are listed in parentheses.