Magnetic and transport properties of the emergent ferromagnetic superlattices<\/p>\n
LNO is a paramagnetic metal, and LFO is a G-type AFM insulator with intralayer antiparallel spin alignments, which shows one of the highest ordering temperatures of perovskite oxide (TN\u2009\u2248\u2009740\u2009K). Remarkably, when they were epitaxially grown along the (001) directions but absent for the (110) and (111) directions (Supplementary Fig.\u00a0S4<\/a>), the LNOn\/LFO1 SLs show strong signatures of ferromagnetism, which depends critically on the stacking thickness, n, of LNO. Figure\u00a02a<\/a> shows the hysteresis curves of the LNOn\/LFO1 SLs measured at 300\u2009K, i.e. the dependence of the magnetization (M) as a function of the magnetic field B, which is saturated under the applied field above 3.5\u2009kOe. All samples exhibited key characteristics of FM behavior, including hysteresis and remnant magnetization. At n\u2009=\u20095, the SL shows weak saturation ferromagnetism similar to LaFe0.5Ni0.5O3 thin films. Remarkably, as n decreases, the highest saturation magnetization increases significantly, reaching 252.8\u2009emu\/cm3 for n\u2009=\u20091, ~4.0\u2009\u03bcB\/p.l., comparable to the 4.96\u2009\u03bcB\/f.u. (f.u.\u2009=\u2009formula unit) for the FM DP La2NiMnO628<\/a>, and much larger than that of the LaFe0.5Ni0.5O3 thin films (Fig.\u00a02b<\/a>). Figure\u00a02c<\/a> shows the temperature dependence of magnetization (M\u2013T curve) for the typical LNOn\/LFO1 (n\u2009=\u20091, 3 and 5) SL samples in addition to the LaNi0.5Fe0.5O3 film. The Curie temperature, TC, could be determined based on the minimum value of the temperature coefficient TCM, defined as TCM\u2009=\u20091\/M(dM\/dT). Remarkably, all the SL samples show a TC much higher than room temperature, around TC\u2009=\u2009608, 589, and 419\u2009K, respectively, for n\u2009=\u20091, 3, and 5. It is also noteworthy that the LaNi0.5Fe0.5O3 film shows a TC\u2009=\u2009109\u2009K, much lower than those of the SL but higher than the bulk spin glassy temperature of 83\u2009K27<\/a>. The results strongly suggest that a unique FM order is accessed in the SLs structures with the strongest magnetism obtained at n\u2009=\u20091, an emergent La2NiFeO6 DP structure with B-site layered ordering configuration along (001) direction. As illustrated in Table\u00a01<\/a> its TC is close to the highest value (TC\u2009=\u2009625\u2009K) reported for Sr2CrReO6 among the DP compounds29<\/a>. The temperature-dependent resistivity, \u03c1, of the LNOn\/LFO1 (n\u2009=\u20091, 3 and 5) SL sample was also measured between 2 and 300\u2009K, as shown in Fig.\u00a02d<\/a>. All the SLs show a semiconductive behavior with small polaron conduction mechanism (n\u2009\u2264\u20094) (Supplementary Fig.\u00a0S5<\/a>), although the LaFe0.5Ni0.5O3 thin film is found to be highly insulating. This strong conduction difference, in addition to the strong difference in magnetism, may be related to the difference in the ordered arrangement of the Fe and Ni cation atoms over the B-sites, where the higher level of cations ordering prefers higher magnetization and higher conductivity19<\/a>. We thus obtained an emergent La2NiFeO6 DP with Curie temperature much higher than room temperature, which has been predicted to be a half-metallic ferromagnet26<\/a>.<\/p>\n Fig. 2: Magnetic and transport properties of the atomically engineered LNOn\/LFO1 SLs.<\/b> a<\/b> The hysteresis curves of the LNOn\/LFO1 SLs measured at 300\u2009K, for comparison the results of LaFe0.5Ni0.5O3 thin films, is also presented. b<\/b> The hysteresis curve of the LNO1\/LFO1 SL measured at 5\u2009K in comparison to the LaFe0.5Ni0.5O3 thin film. c<\/b> The temperature dependence of magnetization for the LNOn\/LFO1 (n\u2009=\u20091, 3 and 5) SLs and the LaNi0.5Fe0.5O3 film. d<\/b> The temperature-dependent resistivity, \u03c1, of the LNOn\/LFO1 (n\u2009=\u20091, 3, and 5) SLs, showing semiconductive behavior.<\/p>\n Table 1 The emerging ferromagnetic SLs and the typical double perovskites (DP) with ferromagnetism near and above room temperature<\/b>Magnetic domain microstructure characterized by magnetic force microscopy<\/p>\n The magnetic domain microstructure and its evolution with respect to the applied field for the LNO1\/LFO1 SL were also investigated by magnetic force microscopy (MFM). Generally, the magnetograms obtained by MFM provide information on the effective magnetization along the z direction of the local domains. Figure\u00a03<\/a> shows the magnetograms obtained at 2\u2009K in different out-of-plane magnetic fields corresponding to the critical points on the M\u2013H loop in Fig.\u00a02b<\/a>. Here the red or blue-colored regions represent net-magnetization oriented up (+Mz) or down (\u2212Mz), respectively, and the green color represents the zero Mz signal. As shown in Fig.\u00a03<\/a>, the as-grown LNO1\/LFO1 SL first demonstrates nearly zero-magnetization along the z direction after the zero-field-cooling (ZFC) process. By applying a positive magnetic field along the out-of-plane direction, discrete upward domains begin to appear and grow continuously (+900\u2009Oe), eventually reaching an upward single-domain state at the saturation field of +20\u2009kOe. By applying a negative magnetic field, the single-domain state turns to fragments of upward and downward domains at the coercive field of approximately \u2212250\u2009Oe, corresponding to the zero net magnetization in the M\u2013H loop. The typical size of the fragment domains after magnetization (measured at +250\u2009Oe) is about 300\u2013500\u2009nm (Supplementary Fig.\u00a0S6<\/a>). The magnetic domain evolution is repeatable during the field-sweeping process, as further evidenced by the downward single-domain state at \u221220\u2009kOe and then the fragment-domain state at +250\u2009Oe (the positive coercive field). The domain structures after ZFC and their evolution upon applying magnetic fields are consistent with the direct magnetization measurements. This further confirms that the strong magnetism is intrinsic to our SL samples, where the plausible magnetic contribution from the STO bulk substrate is negligible.<\/p>\n![\"figure](\"https:\/\/www.europesays.com\/uk\/wp-content\/uploads\/2025\/04\/41467_2025_58968_Fig2_HTML.png\")
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