Double Exchange Effect Research Articles

Effect of A-site rare-earth ions on structure and magnetic properties of novel (Ln0.2Gd0.2La0.2Nd0.2Sm0.2)MnO3 (Ln = Eu, Ho, Yb) high-entropy perovskite ceramics

Single-phase high-entropy perovskite ceramics (Ln0.2Gd0.2La0.2Nd0.2Sm0.2)MnO3 (Ln-GLNSMO HEPCs) have been synthesized using solid-state reactions. The effect of A-site rare-earth ions (Ln = Eu, Ho, Yb) on the structure and magnetic properties was systematically investigated. The results show that all samples are orthorhombic phases (with a space group of Pbnm) and exhibit a strong crystallization trend sintered at 1300 °C for 16 h. All the HEPCs are composed of irregular spherical particles with smooth surfaces and show obvious hysteresis effects at T = 5 K. The changing Mn–O bond distance, Mn–O–Mn bond angle and the increase of Mn4+ ion concentration enhance the double exchange effect of Mn3+-O2--Mn4+, leading to different magnetic interactions. The residual magnetization Mr (coercivity Hc) of the samples is significantly enhanced, which is increasing from 0.13 emu/g (16.34 Oe) to 3.17 emu/g (97.07 Oe) due to the increase of configuration entropy, lattice distortion and the double exchange effect (DE). In addition, there is a strong relationship between the magnetic properties and the average radius of the A-site ions, in which Ho-GLNSMO HEPCs have a high Ms magnetism. This work is a valuable reference for the magnetic regulation of high-entropy ceramics based on rare-earth and entropy-stabilized perovskite types.

Changing the Ca2+ ion ratio to modulate the temperature coefficient of resistivity (TCR) of La1-xCaxMnO3 films prepared by the sol-gel spin coating method

Perovskite manganese oxides are highly desirable for infrared acquisition owing to their characteristics of ferromagnetic metal-paramagnetic insulators. In this work, the sol-gel spin-coating technique was employed to prepare La1-xCaxMnO3 (LCMO) films with varying Ca2+ content (x = 0.1, 0.2, 0.3, 0.4, 0.5) on the (l00) LaAlO3 (LAO) substrates. In comparison to previous studies, the pre-sintering temperature, final sintering temperature, and final sintering period were modified. The crystal structure, surface topography, elemental valence states, and electrical transport performance of the LCMO films were characterized and analyzed as well. The results demonstrated that the diffraction peaks of the above films were aligned with those of the LAO single-crystal substrate. The change in Ca2+ content caused an increase in the Mn–O bond length, the amount of Mn4+, and the Mn–O–Mn angle in LCMO films, promoting the double exchange effect. These modifications had a considerable impact on the resistivity of LCMO films, increasing their TCR value. The maximum TCR of 22.92% K−1 was reached at x = 0.3 and peak TCR temperature (Tk) of 241.93 K. Therefore, the film has great application potential for infrared bolometers.

Effects of phase transition driven by cation vacancies on magnetism and electricity in LaxMnO3 thin films

The attractive physical properties of perovskite manganese make them fundamental building blocks for oxide spintronic devices. However, defects and lattice distortions can lead to extrinsic properties, especially cation vacancies, severely compromising device performance. In particularly, many intriguing properties, such as the emergence of magnetism and two-dimensional high-mobility electron gases, are closely related to cation vacancies. In this study, we deposited a series of LaxMnO3 epitaxial thin films with varying contents of La3+ vacancies on SrTiO3 (100) substrates, with the presence of La3+ vacancies verified by using X-ray diffraction, X-ray photoemission spectroscopy, X-ray absorption spectroscopy, and energy dispersive spectroscopy spectrum. The reduction in La3+ vacancies led to stronger ferromagnetism with an easy axis oriented along the (110) direction in LaMnO3 thin films, along with the appearance of a monoclinic phase, indicating that the ferromagnetism possibly originated from the three-dimensional d3z2−r2/dx2−y2-alternated orbital order rather than the Mn3+/Mn4+ double exchange effect. This result implies that the consistent regulation of cation vacancies by pulsed laser deposition can modulate both the structure and physical properties of manganate films, thereby enabling control over device properties.

Double exchange, itinerant ferromagnetism, and topological Hall effect in moiré heterobilayer

Double exchange, itinerant ferromagnetism, and topological Hall effect in moiré heterobilayer

Effect of Co doping of (Ga, In) and 2N preferred orientation on the magnetic and optical properties of ZnO

Current research on the Zn1−xMxO1−yNy (M = Ga, In) system mainly focuses on the p-type property, where Ga–N or In–N has different preferred orientations for bonding. Studies on the magnetic, bandgap, and optical properties of ZnO co doped with N are also lacking. The generalized gradient approximation (GGA+U) plane wave ultrasoft pseudopotential method based on density functional theory is used to systematically investigate the effects of the magnetic, band gap, and optical properties of the system for addressing the aforementioned problems. Results show that the formation energy of the Zn1−xMxO1−yNy (M = Ga, In) system is negative, all doping is easy, and all doped systems are stable and magnetic. The magnetism comes from the double exchange effect of electron spin polarization. Research has found that the band gap in Ga–N or In–N bonding along the c-axis direction is narrower than that along the a-axis direction. In the low-energy region, the redshift of the absorption spectrum is more significant, and the absorption or reflection coefficient and the carrier polarization and activity are stronger. In particular, In–N forms bonds along the c-axis direction and is co doped with another N in ZnO, which results in the narrowest band gap and the most significant redshift in the absorption spectrum. The absorption of sunlight can also be achieved in the ultraviolet visible near-infrared region. The absorption or reflection coefficient and the carrier polarization and activity are relatively strongest. This work has certain reference value for the design and preparation of new solar cell light absorbing materials based on ZnO.

Improvements of magnetic performances for carbonyl iron soft magnetic composites (CI-SMCs) induced by Mn-based phosphating coating

A novel soft magnetic composites coated with Mn-based phosphate was successfully prepared in this work. Microstructural analysis showed that the hybrid layers coated on the surface of carbonyl iron powder were composed of Mn0.43Fe2.57O4, FeMnO3, and Fe7(P2O7)4. The hysteresis loss of carbonyl iron soft magnetic composites was significantly improved due to the grain growth caused by the introduction of Mn. Meanwhile, the double-exchange effect between Mn4+ and Mn3+ promoted the transformation from antiferromagnetic to ferromagnetic for FeMnO3 and Mn3+ oxides so as to increased the saturation magnetization and permeability. Compared with the carbonyl iron soft magnetic composites coated with phosphate, the permeability increased from 49 to 87 and the core loss decreased from 501.9 to 361.3 kW/m3 for the composites coated with Mn-based phosphate. Notably, the magnetic performance (principally permeability) of carbonyl iron soft magnetic composites could be optimized by employing the Mn-based phosphate coating.

Impact of La3+ doping on temperature coefficient of resistivity and peak temperature of La x Ca0.89-xSr0.11MnO3 films prepared by sol-gel spin coating method

Herein, refined LaxCa0.89-xSr0.11MnO3 (LCSMO, x = 0.65, 0.68, 0.71 and 0.74) films were prepared through the sol-gel spin-coating. The influence of La3+ content on the structural properties of LCSMO films was investigated by X-ray diffraction and Atomic force microscope, demonstrating that LCSMO films can grow well on SrTiO3 (00l) substrate. Besides, X-ray photoemission spectroscopy verified the double exchange (DE) effect was weakened with La3+ dopant. The La3+ doping and interconnected grains boundaries (GBs) led to the weakening DE effect and GBs scattering, respectively. Due to superior GBs connectivity, the resistivity of LCSMO films was less than 7.1 × 10−4 Ω·cm at low temperature of 100 K. Importantly, it is an effective control method to keep the temperature (Tk) corresponding to temperature coefficient of resistivity (TCR) at room temperature with Sr2+ content as constant in LCSMO films. At x = 0.71, the peak TCR value was found to be 8.84%/K and corresponding Tk was 283.15 K. These results are beneficial for advanced application of uncooling infrared bolometer.

An investigation on strain and magnetic properties of perovskite manganite superlattice films

The mixed-valence perovskite manganites attracted much attention because of their interesting electro-magnetic properties, and strain modulation on magnetic properties of perovskite manganites is worth exploring. In this paper, [La[Formula: see text]Ca[Formula: see text]MnO3/La[Formula: see text]Ca[Formula: see text]MnO3][Formula: see text] superlattice films with different La[Formula: see text]Ca[Formula: see text]MnO3 layer thicknesses are prepared by pulsed laser deposition (PLD). The crystal structures (lattice constants) of samples are measured by X-ray diffraction (XRD). The strain of the films is determined according to their crystal structures. The magnetic hysteresis loops (M-H loops) and magnetization versus temperature (M-T) curves of these superlattice films are measured by the physical property measurement system (PPMS). From the M-H loops, the coercive field (H[Formula: see text] of the samples can be measured. From the M-T curves, the Curie temperature (T[Formula: see text] of the samples can be obtained. All samples show a ferromagnetic to paramagnetic transition at TC, and the [La[Formula: see text]Ca[Formula: see text]MnO3/La[Formula: see text]Ca[Formula: see text]MnO3][Formula: see text] sample with the La[Formula: see text]Ca[Formula: see text]MnO3 layer thicknesses of 72[Formula: see text]Å has an antiferromagnetic Néel transition at TN. According to the strain state and magnetic phases, the magnetic properties are comprehensively analyzed. It is found that with the increase of ferromagnetic La[Formula: see text]Ca[Formula: see text]MnO3 layer thickness, the ferromagnetic phase is increased, and the double exchange effect can also be enhanced, resulting in the increase of TC. When the thickness of La[Formula: see text]Ca[Formula: see text]MnO3 reaches 96[Formula: see text]Å, the uneven strain distribution in the superlattice can induce the reduction of ferromagnetic phase compared with antiferromagnetic phase (even antiferromagnetic phase starts to appear), and the double exchange effect can be weakened, and finally leading to the decrease of TC. In addition, with increasing the La[Formula: see text]Ca[Formula: see text]MnO3 layer thickness, HC gradually decreases. The change of HC is related to strain states and magnetic phase interactions in the samples. The analysis of the strain and magnetism can contribute to the understanding of perovskite manganite superlattice films.

Novel coexistence of resistive switching and ferromagnetism modulation in Ta/La2/3Sr1/3MnO3/Nb:SrTiO3/Ag device

The Ta/La2/3Sr1/3MnO3/Nb:SrTiO3/Ag device has novel coexistence of resistive switching and ferromagnetism modulation. The inactive Ta electrode can effectively balance the Schottky barrier height on both sides of La2/3Sr1/3MnO3, making the current–voltage (I–V) curves of the device exhibit symmetry. The conduction mechanism of the device is conductive filaments of oxygen vacancies at low resistance state (LRS), and Schottky barrier at high resistance state (HRS). The migration of oxygen ions affects not only the resistance but also the ferromagnetism of the device. The suppression of double exchange effect by oxygen vacancies results in higher saturation magnetization (Ms) at HRS than at LRS. Our first-principles calculations are consistent with the experimental results. The stable coexistence of resistive switching and ferromagnetism modulation may provide a possible way to realize multi-state storage.

The optimal tuning of electronic structure, magnetic, and optical properties of (Fe, V + VO/VSn) co-doped SnO2 via first-principles calculations.

Dilute magnetic semiconductors (DMSs) with both charge and spin degrees of freedom have emerged as promising candidates in the spintronic industry. However, the Curie temperature below room temperature and uncertainty about the origin of ferromagnetism hinder the application of DMSs. To address these issues, we explored a better SnO2-based co-doped method (Fe, V + VSn) using ab initio calculations. The calculation results show that the Sn13FeVO32 (Fe, V + VSn) has a high Curie temperature (716K), good ferromagnetic properties, stronger covalency of bonds, and better optical transparency in the visible light range. In addition, the holes or electrons generated by the complexes in the (Fe, V + VO/VSn) co-doped system cause a spin-polarized double exchange effect in the Fe-3d, V-3d, and O-2p orbitals, which leads to magnetism of the co-doped systems. The static dielectric constant ɛ1(0) of the system increases after doping. Among them, Sn14FeVO31 (Fe, V + VO) has the largest ɛ1(0), indicating that Sn14FeVO31 has the strongest polarization ability and better photocatalytic properties. In Sn14FeVO31, the imaginary part of the dielectric function and the absorption spectrum all have new peaks in the low-energy region, which are caused by the jump of electrons from the guide band of the spin-polarized impurity energy level. This paper proposes a new method for preparing dilute magnetic semiconductors in spin electronic devices with high room temperature ferromagnetic properties and excellent optical properties through the (Fe, V + VO/VSn) co-doped SnO2.

Critical field analysis and magnetic properties of perovskite manganese oxide La0.8Ca0.2Mn1-xNixO3(x = 0.0,0.1,0.2)

Critical field analysis and the magnetocaloric effect of the polycrystalline perovskite manganese oxide samples, La0.8Ca0.2Mn1-xNixO3 (x = 0.0,0.1,0.2), which were prepared using a traditional solid phase reaction method, were explored. The data obtained from X-ray diffraction (XRD) measurements of the polycrystalline samples show good single-phase properties. The replacement of a given Mn3+ ion fraction by Ni2+ions decreased the crystal parameters and unit cell volume, Curie temperature (Tc), and the magnetic and the magnetic entropy change. These property changes originate from the partial substitution of Mn3+ ions by Ni2+, which changes the Mn3+/Mn4+ ratio and consequently weakens the double exchange (DE) effect. At an applied magnetic field of 7 T, the maximum difference in magnetic entropy of the undoped sample is 6.15 J/kg•K, and the refrigeration efficiency is 422.94 J/kg. The critical parameters of the parent material and the doped sample fit well to the Tricritical model. The La0.8Ca0.2Mn1-xNixO3 samples with x = 0.0 exhibit crossover of the phase transitions, which can be first or second order. The observed phase transitions of samples with x = 0.1 and 0.2 are exclusively second order. Around the TC of La0.8Ca0.2Mn1-xNixO3 (x = 0.0, 0.1 and 0.2) compounds, the correlation of the magnetic field with the maximum change in magnetic entropy follows a power law.

Room-temperature magnetoresistive and magnetocaloric effect in La1−xBaxMnO3 compounds: Role of Griffiths phase with ferromagnetic metal cluster above Curie temperature

The evolution of the Griffiths phase (GP) with a ferromagnetic metal (FMM) cluster above the Curie temperature (TC) and its effect on the magnetic properties, electrical transport, magnetoresistance (MR), and magnetocaloric effect (MCE) is studied comprehensively, using bulk compounds of La1−xBaxMnO3 (0.15 ≤ x ≤ 0.25) with different lattice distortions but with the same structural symmetry and space group. These La1−xBaxMnO3 samples show ferromagnetic transition at TC increasing from 229 K for x = 0.15–300 K for x = 0.25, in addition to the presence of GP with FMM clusters in the paramagnetic (PM) region, which have been confirmed by the combination of magnetization (susceptibility) measurements, the GP theory, and electron paramagnetic resonance technology. With increasing the Ba2+ ion doping, GP temperature (TG) and TC of La1−xBaxMnO3 are increased, and the GP regime is strengthened. The GP ratio in the PM region reached 27.7% for the sample with x = 0.20. The resistivity decreases and the FMM phase increases with increasing x from 0.15 to 0.25, which can be explained by the decrease in the bandgap (Eg) and the enhancement of the double-exchange effect. Remarkably, large room-temperature MR (∼44.7%) can be observed in the sample with x = 0.25 under 60 kOe, which is related to the presence of the GP regime. Furthermore, the MCE is also affected by the GP regime, and it is deduced that the magnetic transition is of second order. The value of magnetic entropy change (|ΔSM|) reaches 3.04 J/kg K near room temperature for the sample with x = 0.25 under 50 kOe. This value is associated with a relative cooling power (RCP) of 248.1 J/kg. For the sample with x = 0.15, the value of RCP reaches 307.6 J/kg under 50 kOe. The discovery of the MR and MCE near room temperature is of great significance from the practical application of perovskite manganites in magnetic sensors and magnetic refrigerants.

Bivalent Sr2+ doping to improve room-temperature TCR of La0.8-xSrxAg0.2MnO3 polycrystalline ceramics

Rare earth manganites have attractive wide attentions due to their promising applications in infrared bolometers and magnetic information storage. Their temperature coefficient of resistivity (TCR) need further improved so as to achieve its applications at room temperature. In this work, La0.8-xSrxAg0.2MnO3 (LSAMO, x = 0.00, 0.10, 0.15, 0.175, and 0.25) ceramics are fabricated by sol-gel method. Their structure and morphology as well as the optical, electrical and magnetic properties are studied. The structural distortion of Sr doped specimens is identified by the shift of the X-ray diffraction peaks toward the low angles and the red-shift of their Fourier transform infrared spectra. Meanwhile, the Mn4+ ion concentration increases, indicating that the double exchange effect enhances in the ceramics. Furthermore, the TCR values are obviously influenced by the doped Sr content. It enhances from 2.5% K−1 at 211.5 K for La0.8Ag0.2MnO3 to 16.9% K−1 at 299.8 K for La0.625Sr0.175Ag0.2MnO3. The origin of this improvement is attributed to the transformation of rare earth manganites from ferromagnetic metal to paramagnetic insulator and the distortion of MnO6 octahedra. These results uncover the effect of Sr2+ doping on the room-temperature TCR values of ceramics, making it suitable for advanced uncooling device applications.

Magnetic blue shift of Mott gaps enhanced by double exchange

A substantial energy gap of charge excitations induced by strong correlations is the characteristic feature of Mott insulators. We study how the Mott gap is affected by long-range antiferromagnetic order. Our key finding is that the Mott gap is increased by the magnetic ordering: a magnetic blue-shift (MBS) occurs. Thus, the effect is proportional to the exchange coupling in the leading order in the Hubbard model. In systems with additional localized spins the double-exchange mechanism induces an additional contribution to the MBS which is proportional to the hopping in the leading order. The coupling between spin and charge degrees of freedom bears the potential to enable spin-to-charge conversion in Mott systems on extreme time scales determined by hopping and exchange only, since a spin-orbit mediated transfer of angular momentum is not involved in the process. In view of spintronic and magnonic applications, it is highly promising to observe that several entire classes of compounds show exchange and double-exchange effects. Exemplarily, we show that the magnetic contribution to the band-gap blue-shift observed in the optical conductivity of $\alpha$-MnTe is correctly interpreted as the MBS of a Mott gap.

Synergistic effect of dielectric resonance and plasma oscillation on negative permittivity behavior in La1-xSrxMnO3 single-phase ceramic

Ceramic-based composites with negative permittivity property can be realized by dielectric resonance or plasma oscillation, but it is still a challenge to integrate two kinds of negative permittivity into a single-phase material. In this study, the La1-xSrxMnO3 (LSMO) single-phase ceramic (x = 0.3, 0.4, 0.5, 0.6 and 0.7) was successfully fabricated by a sol-gel auto-combustion and pressureless sintering method. Just by adjusting the content of Sr2+, the Lorentz-like and Drude-like negative permittivity were simultaneously observed in LSMO ceramic under the synergistic effect of dielectric resonance and plasma oscillation. The amplitude of dielectric resonance was lessened because of the increase of damping factor, and the resonance frequency gradually moved to a lower frequency region with the increase of Sr2+. When the content of Sr2+ reached 0.7, the Drude-like negative permittivity played an overwhelming role due to an enhanced double-exchange effect in La1-xSrxMnO3. Meanwhile, the tanδ of LSMO was as low as 0.085, which was scarcely observed in previous investigations. The generation and regulation mechanism of negative permittivity was further clarified. This work opened up a way to adjust negative permittivity behavior in single-phase ceramic material which has great value in high-power microwave filters and wireless ring resonators.

A-site Na-doping to enhance room-temperature TCR of La1-xNaxMnO3 polycrystalline ceramics

During the exploration on the temperature coefficient of resistance (TCR) of A-site cation-doped perovskites, divalent cation-doped material systems have received increasing attention; however, the TCR performance of monovalent cation-doped perovskites has been less studied. In order to investigate the effect of monovalent Na cation doping on the TCR of La1-xNaxMnO3 (LNMO, x = 0.1, 0.2, 0.3, 0.4, and 0.5) and obtain LNMO materials possessing high TCR values at room temperature, the LNMO perovskite ceramics were synthesized by the Pechini sol–gel method. The effect of sodium cation doping on their structural and electrical properties was investigated by various characterization methods. An increase in Na content affected the grain size and Mn4+ ion concentration in the LNMO ceramics. Furthermore, increasing quantity of Na significantly increased the TCR from 5.89 % K−1 at x = 0.1 to 8.56 % K−1 at x = 0.4. For a Na-doping level x = 0.4, the peak TCR temperature (Tk) at room temperature reached 299.42 K. Among the feasible reasons that could improve the above electrical parameters are the weakened grain boundary scattering, the increase in A-site vacancy, the enhanced double-exchange effects, and the generation of Jahn–Teller distortions. According to the results of this study, the obtained LNMO ceramics seem to be promising for the application in uncooled infrared bolometer technology.

Impact of K doping on room-temperature temperature coefficient of resistivity of La0.7(Ag0.3-xKx)MnO3 (0.160 ≤ x ≤ 0.180) polycrystalline ceramics

Herein, La0.7(Ag0.3-xKx)MnO3 (LAKMO, x = 0.160, 0.165, 0.170, 0.175 and 0.180) polycrystalline ceramics are prepared by the sol-gel method and the influence of K content on the structure of LAKMO ceramics is investigated using X-ray powder diffraction (XRD), field-emission scanning electron microscope (FESEM), Fourier transform infrared (FTIR) and Raman spectroscopy, demonstrating that variations in K content cause significant structural changes. For instance, K doping led to the distortion of MnO6 octahedron and grain growth. Moreover, the energy dispersive spectrometer (EDS), elemental mapping and X-ray photoemission spectroscopy (XPS) confirmed the uniform distribution of constituent elements on the surface. The increase in K-doping ratio resulted in the distortion of MnO6 octahedron, weakening grain boundaries scattering and enhancing the double exchange effect. Consequently, the electrical transport properties of LAKMO ceramics were improved with the increase of K doping, especially the decrease in resistivity and increase in temperature coefficient of resistivity (TCR) values. At x = 0.175, the peak TCR value was found to be 11.15% K−1 at room temperature (Tk = 292.22 K). These results indicated that the K-doped LAKMO ceramics are promising candidates for uncooled infrared bolometers.

Achieving 20% photovoltaic efficiency by manganese doped methylammonium lead halide perovskites

We report a transition metal such as manganese doped methylammonium lead halide perovskite (MA(Pb:Mn)I3) solar cell with an power conversion efficiency (PCE) over 20%. The rational design and fabrication of MA(Pb:Mn)I3 lead to the enhancements of all the photovoltaic parameters. To incorporate Mn can effectively eliminate the trap-assist and bi-molecular recombination. The photo-absorption ability at shorter wavelengths (i.e., less than 500 nm) and charge carrier lifetime can be elaborated. More importantly, the existence of the Mn2+-I−-Mn3+ motif contributes for the double exchange effect, giving rise to the charge/spin transport. By a combination of linearly and circularly polarized photo-excitations, we have explicitly determined the role of intrinsic spin–orbit coupling (SOC) in MA(Pb:Mn)I3. More dark states are expected to be available for the photocurrent generation. This study may pave the way for deep understandings of transition metals doped hybrid perovskites for highly efficient solar cell applications.

Mechanism for enhanced ferroelectricity in multi-doped BiFeO3 thin films

The thin films of BFO, binary-doped Bi0.96Sr0.04Fe0.98Co0.02O3, and multi-doped Bi0.96Sr0.04Fe0.94Mn0.04Co0.02O3 are fabricated using the sol–gel method. To study the mechanism for the enhanced intrinsic ferroelectricity in multi-doped BFO, this work uses related aspects including the electrostatic potential energy, the Fermi level, and the Schottky interface barrier. Using multi-doped BFO can reduce the height of the electrostatic barrier and change the Fermi level so that the Schottky barrier height is increased to reduce the leakage current, thereby improving the intrinsic ferroelectricity. Also, using multivalent Mn with a double-exchange effect improves the ferromagnetism. These improved intrinsic ferroelectric and ferromagnetic properties make BFO applicable in various practical devices and fields.

Enhanced Ferromagnetism of CrI3 Bilayer by Self-Intercalation*Supported by the National Natural Science Foundation of China (Grant Nos. 11974068 and 11874097), the China Postdoctoral Science Foundation (Grant Nos. BX20190052 and 2020M670739), and the Fundamental Research Funds for the Central Universities of China (Grant Nos. DUT20LAB110 and DUT19LK12). The authors acknowledge the computer resources provided by the Supercomputing Center of Dalian University

Two-dimensional (2D) ferromagnets with high Curie temperature have long been the pursuit for electronic and spintronic applications. CrI3 is a rising star of intrinsic 2D ferromagnets, however, it suffers from weak exchange coupling. Here we propose a general strategy of self-intercalation to achieve enhanced ferromagnetism in bilayer CrI3. We show that filling either Cr or I atoms into the van der Waals gap of stacked and twisted CrI3 bilayers can induce the double exchange effect and significantly strengthen the interlayer ferromagnetic coupling. According to our first-principles calculations, the intercalated native atoms act as covalent bridge between two CrI3 layers and lead to discrepant oxidation states for the Cr atoms. These theoretical results offer a facile route to achieve high-Curie-temperature 2D magnets for device implementation.