CO2‐Induced Strong Room‐Temperature Ferromagnetism in BiFeO3

With humongous demand for data storage and processing, two‐dimensional (2D) ferromagnetic‐based materials, is emerged as the next‐generation nanoelectronic devices due to their low power consumption and optimal memory and processing capabilities. BiFeO3 as a single‐phase multiferromagnetic material is expected to find potential applications in electromagnetic devices. Herein, 2D room‐temperature ferromagnetic BiFeO3 is obtained with the help of supercritical carbon dioxide (SC CO2). The rhombic phase of BiFeO3 is converted to cubic with the creation of Ov and Fe2+ defects over the SC CO2 treatment, leading to significant ferromagnetic enhancement. More importantly, it is found that SC CO2 can destroy the cycloidal spin structure of BiFeO3 leading to an increase in the Fe─O─Fe bond angle, which generates stronger superexchange interactions. Ultimately, the saturation magnetization strength of BiFeO3 is increased by nearly 23 times. A new strategy is provided for ferromagnetic induction in 2D materials, which is favorable for promoting their practical applications on device architectures in the future.


Introduction
Ferromagnetic materials (FM) can be used in many applications, such as memory and sensing probes or devices. [1]With the increasing demand for device size miniaturization, the research on two-dimensional (2D) ferromagnets has become urgent and important. [2]Perovskite, as important ferromagnetic material, is widely used in solar cells, data storage devices, and catalysis, etc. [3] BiFeO 3 is one of the most widely studied materials belonging to Perovskites.Besides, it is one of the DOI: 10.1002/aelm.202300626very few single-phase multiferroics that exhibit ferroelectricity and antiferromagnetism at room temperature, with a ferroelectric Curie temperature of T C = 1103 K and an antiferromagnetic Néel temperature of T N = 673 K. [4] Due to its high T C and T N ordered temperature, this material is expected to find potential applications in spintronics, [5] data storage and electromagnetic devices. [6]iFeO 3 is a twisted rhombic perovskite structure at room temperature, and neutron diffraction experiments showed it is a G-type antiferromagnet. [7]Usually, the G-type antiferromagnetic structure consists of a cubic structure stretched along the [111], and all ions are displaced along the [111] direction relative to the ideal centrosymmetric positions as well.The oxygen octahedra surrounding the transition metal cations are rotated around this axis, and the direction of the magnetic moments in the bulk rotates along a long-range cycloidal spin structure of 62-64 nm. [8]It was found that if this cycloidal spin structure is suppressed, the system will display weak ferromagnetism. [9]In 1982, the existence of spin pendulums was found by Sosnowska et al. by neutron diffraction. [8]Fe 3+ magnetic moments are ferromagnetically aligned in the (111) plane, all nearest neighbor Fe 3+ spin are antiparallel to each other, and then the spin cycloid makes the existence of very small deflection on the nearest neighbor Fe 3+ magnetic moments, which help form net magnetic moments and exhibit weak magnetization below 673 K (T N ) as well. [10]How to enhance the weak ferromagnetism of BiFeO 3 is significant, while it is challenging.Currently, the common approach used for enhancing magnetic properties is doping with transition or alkali metals, [11] adopting pulsed laser deposition (PLD), [12] and forming solid solutions with ABO 3 , e.g.BiFeO 3 -BaTiO 3 . [13]s there a novel route to obtain room-temperature ferromagnetism of BiFeO 3 ?Considering the nature of ferromagnetic ordering is time-reversal symmetry breaking, we need to obtain 2D BiFeO 3 .In our previous work, we have successfully prepared strong 2D room temperature ferromagnetic BaTiO 3 and SrTiO 3 using supercritical CO 2 (SC CO 2 ). [14]Herein, for the first time, we use SC CO 2 to help bulk BiFeO 3 exfoliate to 2D structure, more importantly in this process, the FeO 6 octahedral tilt and the Fe─O─Fe bond angles are changed and modulated, including the introduction of oxygen defects, all of which contribute to the greatly enhanced room-temperature ferromagnetism, which are nearly 23 times higher than the untreated BiFeO 3 in saturation magnetization strength.

Structure Characterization of BiFeO 3
The surface morphology, lattice structure, and dimensions of the BiFeO 3 samples treated at 12 MPa supercritical CO 2 (SC CO 2 )are shown in Figure 1.The successful exfoliation of layered BiFeO 3 nanosheets was samples treated at 12 MPa SC CO 2 is shown in Figure 1.The successful exfoliation of layered BiFeO 3 nanosheets was confirmed in low-resolution transmission electron microscopy.To obtain more detailed structural information, the area in the yellow rectangle in the figure was characterized using high-resolution transmission electron microscopy (HRTEM).Different exposed crystalline surfaces were obtained, the dspacings calculated from the HRTEM of Figure .1b are 0.1905 and 0.2702 nm, corresponding to (024) and (110) planes, respectively.a slight decrease compared to the 0.1981 and 0.2789 nm of the PDF card for BiFeO 3 can be observed, which indicates that BiFeO 3 is compressed during the SC CO 2 treatment.The thickness of BiFeO 3 nanosheets was characterized by atomic force microscopy (AFM), which are shown in Figure 1c,d.AFM results also confirmed the successful exfoliation of BiFeO 3 nanospheres.
The thickness distribution of the as-prepared sample obtained at 12 MPa is 2.4-4 nm, i.e., 6-10 layers, and they are accounted for 72.17% of the exfoliated amount.
The structure of BiFeO 3 nanosheets prepared by different SC CO 2 pressures was further characterized by X-ray diffraction (XRD) shown in Figure 2a.It indicates that the BiFeO 3 belongs to a distorted rhombic chalcogenide crystal structure with a spatial point group corresponding to R3c (PDF#86-1518).In addition, the enlarged detail of the (110), (006), and (024) plane (Figure 2b-d) shows that the characteristic diffraction peaks of BiFeO 3 move to the high-angle direction under different SC CO 2 pressure.
Information such as the internal crystal structure and molecular vibrations of the sample were characterized by Raman spectroscopy, as shown in Figure 2e.The experimentally obtained space group of BiFeO 3 is R3c, which corresponds to the space group theory yielding 13 (4A1 + 9E) Raman vibrational modes. [15]he low-frequency Raman vibrations of BiFeO 3 are associated with Bi─O bonding, while Fe─O plays a larger role in the higher frequency bands. [16]As the CO 2 pressure increases, the BiFeO 3 sample size decreases and the relative intensities of the A 1 -1 and A 1 -2 peaks become weaker and shift toward larger wave numbers. [17]When BiFeO 3 is treated with SC CO 2 , the OV content increases significantly, and oxygen defects are formed.The XPS spectra of Fe are shown in Figure 3b, the peaks at 711.6 eV (Fe 2p3/2) and 725 eV (Fe 2p1/2) belong to Fe 3+ while the peak at 710.3 eV (Fe 2p3/2) is consistent with Fe 2+ according to the literature. [19]As the CO 2 pressure increases, the increase in Fe 2+ content is due to the increase in O V content.The magnetic moments of Fe 2+ ions are arranged opposite to those of Fe 3+ , leading to a net magnetic moment and enhanced superexchange interaction. [20]From Figure 3a,b, it can be observed that the samples prepared by SC CO 2 at 12 MPa show the most defective structures.In addition, the electron paramagnetic resonance spectra (EPR) of the BiFeO 3 samples are shown in Figure 3c.The BiFeO 3 samples exhibit a weak signal at g = 2.005, which is enhanced after SC CO 2 treatment, demonstrating the increase in O V defects.

Characterization of Magnetic Behavior
Compared with the weak ferromagnetism of untreated BiFeO 3 Ms = 0.02127 emu g −1 , [21] the SC CO 2 treated BiFeO 3 exhibited significantly enhanced ferromagnetism with saturation magnetization (M S ) of 0.4943, 0.2338, and 0.2828 emu g −1 depending on different SC CO 2 pressure of 12, 14, and 16 MPa (Figure 4).The magnification of the hysteresis line near H = 0 is shown in Figure 4, which shows the coercivity field (Hc) of 100.00 Oe and the residual magnetization (Mr) of 0.0120 emu g −1 for 12 MPa, while with the increase of CO 2 pressure to 14 and 16 MPa, Hc increase to 175.11 and 175.26Oe (Table 1).

Mechanism Investigation on Effect of CO 2 Pressure
To further confirm the crystal structure changes of the samples after SC CO 2 treatment, the XRD diffraction data were refined by Rietveld structure refinement to test the changes of cell parameters and the refinement result parameters are listed in Table S2 (Supporting Information).Table S5 (Supporting Information) lists the changes of lattice constants at different pressures, apparently, SC CO 2 has an important effect on the crystal structure of BiFeO 3 , changing the cell parameters and crystal structure of BiFeO 3 , changing the cell parameters and ionic bond angles and bond lengths of the samples.In BiFeO 3 crystals, the exchange interaction between magnetic Fe atoms is very weak, and the antiferromagnetism in BiFeO 3 reaction is achieved by Fe atoms through nonmagnetic oxygen atoms. [22]In the superexchange interaction, the spins of two Fe atoms are always opposite, and the superexchange interaction becomes stronger when the Fe─O─Fe bond angle is close to 180°.In our experiment, it can be observed that the Fe─O─Fe bond angle is 161.8°after 12 MPa treatment, which is larger than the Fe─O─Fe bond angle of untreated BiFeO 3 (Figure 5b), so the SC CO 2 treated BiFeO 3 exhibits a stronger superexchange interaction, especially at 12 MPa.Usually, there are two types Fe─O bond lengths in the FeO 6 octahedra as shown in Figure 5d. [23]or the 12 MPa SC CO 2 treated BiFeO 3 , the Fe─O 1 bond length is shortened while the Fe─O 2 bond length is almost unchanged and the Fe─O─Fe bond angle is increased compared to the untreated sample (Table S4, Supporting Information).
Further another important parameter of the c/a lattice ratio is tested, which is a straining function for BiFeO 3 . [24]The decrease in c/a indicates a decrease in the rhombohedral distortion of the crystal cell. [25]the rhombohedral structure of BiFeO 3 is formed by the stretching of the cubic structure along the [111] direction, with hexagonal parameters a hex = 5.58 and c hex = 13.87Å. [26] The normalized lattice parameters are calculated from the following relation, a nor = a nor = a hex ∕ √ 2 and c nor = c nor = c hex ∕ √ 12 .
[27] According to the fine parameters shown in Table S5 (Supporting Information), at 12 MPa pressure, c nor /a nor −1, [28] the lattice parameters deviate from the bulk phase, the rhombic distortion and the cubic chalcogenide structure is approached.

Electronic Structure Analysis
To further investigate the origin of the magnetic properties of BiFeO 3 nanosheets, we performed first-principles densityfunctional theory (DFT) calculations.Specifically, we constructed BiFeO 3 models after different pressure treatments based on the experimental refinement results.The spatial distributions of spin-charge density and density of states (DOS) are shown in Figure 5e,f and Figures S6 and S7 (Supporting Information).After SC CO 2 treatment, the Fe─O─Fe bond angle increased and the magnetic moment was as high as 4.42 μB Co −1 (Figure 5e).In addition, the SC CO 2 treatment shows an asymmetric DOS signal indicating the emergence of ferromagnetism, and the asymmetric spin-up (majority) and spin-down (minority) occupancies shown near the Fermi energy level (E F ) indicate the presence of spin polarization, which suggests the emergence of an indisputable local magnetic moment.In addition, the distribution of the charge density is mainly concentrated near the iron and oxygen atoms, suggesting that the magnetic moment is mainly supplied by the p-orbitals of the iron atoms and the orbitals of the oxygen atoms.When the Fe─O─Fe bond angle is the smallest, the value of the magnetic moment calculated using the model shown in Figure S7b (Supporting Information) is ≈3.73 μB Co −1 .In this case, we have to take into account the structural alterations induced by the SC CO 2 , which have been shown in the characterization tests described above.This directly reveals the important role of SC CO 2 in improving the magnetic properties of BiFeO 3 .Figure S7c (Supporting Information) corresponds to the fact that a further increase in the Fe─O─Fe bond angle increases the spincharge density, and the magnetic moment becomes 3.96 μB Co −1 .The Fe─O─Fe bond angle is maximum after 12 MPa treatment, and the magnetic moment becomes 4.42 μB Co −1 (Figure 5f),  which indicates that the magnetic enhancement is closely related to the Fe─O─Fe bond angle.This calculation is in direct agreement with our experimental data.

Conclusion
In summary, room temperature ferromagnetism of BiFeO 3 has been successfully obtained with the help of SC CO 2 .2D nanosheets of BiFeO 3 were efficiently exfoliated using SC CO 2 , which changes the crystal structure of BiFeO 3 from the rhombic phase to the cubic phase.And it can be demonstrated that the critical influence of SC CO 2 on the BiFeO 3 crystal structure, and the defect of O v and Fe 2+ can be produced subsequently.More importantly, for the first time, we found out SC CO 2 can destroy the BiFeO 3 cycloidal spin structure, which leads to the increased Fe─O─Fe bond angle and as well as the enhanced super-exchange interaction, which can lead to a net magnetic moment to improve the ferromagnetism.
Hydrothermal Synthesis BiFeO 3 : According to the stoichiometric ratio of BiFeO 3 , the appropriate amount of Fe(NO 3 ) 3 •9H 2 O and Bi(NO 3 ) 3 •5H 2 O was mixed, and the appropriate amount of dilute nitric acid HNO 3 was added to the mixture above, then stir magnetically until complete dissolution.Further gradually adding an appropriate amount of 4 mol L −1 NaOH solution until the dark brown precipitate was completely generated.The precipitate was washed repeatedly with deionized water until the supernatant was neutral, and the hydrothermal precursor was obtained.The precursor was mixed with 2 mol L −1 NaOH solution, stirred at a constant speed for 30 min, and then poured into the reaction kettle and reacted at 220 °C for 28 h.After the reaction finished, the product was washed repeatedly with deionized water, and the obtained sample was dried at 100 °C for 10 h to obtain the powder sample.
Preparation of Ultrasonicated BiFeO 3 : 30 mg bulk BiFeO 3 was dispersed in 30 mL of ethanol/water (V ethanol :V water = 1:1) solution and subjected to ultrasonic treatment for 4 h.The resulting suspension was labeled as ultrasonicated BiFeO 3 .
CO 2 -Induced Phase Engineering: Ultrasonicated BiFeO 3 suspension was directly transferred into the supercritical CO 2 apparatus composed mainly of a stainless-steel autoclave with a heating jacket and a temperature controller.The autoclave was heated to designated temperature (100 °C), and then CO 2 was charged into the reactor to the desired pressures (12/14/16 MPa) and maintained for 4 h under continual stirring.After the CO 2 was slowly released, the supernatant was collected by centrifugation at 2000 r.p.m. for 15 min and the precipitates were collected by supernatant at 10 000 r.p.m. for 15 min.Finally, the precipitates were dried in the oven at 100 °C.
Characterization: Transmission electron microscope (TEM) images were recorded on a FEI Tecnai G2-F20 at an acceleration voltage of 200 kV.The thickness of nanosheets was measured by an atomic force microscope (Bruker Dimension Icon).X-ray diffraction (XRD) patterns were collected on a Bruker D8 Focus diffractometer (Bruker AXS, Germany) using Cu K radiation.Raman measurements were performed using LabRAM HR Evolution with a laser wavelength of 532 nm.X-ray photoelectron spectroscopy (XPS) was performed using the Thermo Scientific K-Alpha + system.The electron paramagnetic resonances (EPR) were obtained by Electron Paramagnetic Resonance Spectrometer (EMX-9.5/12).The magnetic measurement was carried out with a Physical Property Measurement System (quantum design, PPMS-9) and the magnetic hysteresis loop is observed in the range of −30 k Oe < H < 30 k Oe at room temperature.
Computational Details: The theoretical calculation based on density functional theory was completed by the VASP software package and the projector augmented wave (PAW) method was used to describe the ion-electron interaction.Furthermore, the Perdew-Burke-Ernzerhof (PBE) functional was used to describe the exchange-correlation energy of the simulation system and DFT-D3 was used to improve the calculation accuracy of dispersion force.The cutoff energy of plane wave basis was 500 eV, and the Monkhorst-Pack scheme was used to generate k-points with a density of 0.4 per Angstrom for Brillouin zone sampling.The self-consistent field (SCF) calculation was kept within the energy convergence criterion of 1.0 × 10 −5 eV.In addition, a correction based on the Hubbard U model5 was used for the d orbitals of Ti atoms to obtain more accurate correlation energy, and the U value was 3.4 eV.For the slab model, the vacuum layer with a thickness of 15 Å was established to avoid layer-to-layer interaction.
The XRD diffraction data were refined by Rietveld structure refinement through GSAS structure refinement software.

Figure 1 .
Figure 1.Characterization of BiFeO 3 treated with SC CO 2 at 12 MPa.a) Low-magnification TEM image.b) High-magnification TEM image.Inset: Diffraction spots obtained by Fast Fourier transform.c) The AFM images.Inset: The sample thickness diagram in (c).d) The thickness distribution diagram of samples.

Figure 3 .
Figure 3. Characterization of BiFeO 3 treated with different pressure SC CO 2 .a) XPS O 1s. b) XPS Fe 2p.The percentage in Figure 3a represents the content of O v .c) Cryo-electron paramagnetic resonance (EPR) spectra.

Figure 5 .
Figure 5.The Rietveld structure refinement and structure of BiFeO 3 treated with SC CO 2 .a-b) The Rietveld-refined XRD patterns and Lattice structure fitting diagram of BiFeO 3 under 12 MPa.c) Typical R3c rhombohedra of BiFeO 3 .d) The FeO 6 octahedral structure indicates Fe─O bond.TODs and spin-charge density distribution of BiFeO 3 treated with different pressure SC CO 2 .e) The TDOS and PDOS of 12 MPa.f) Spin charge density distribution of BiFeO 3 treated at 12 MPa.

Table 1 .
The Ms, Mr, and Hc values under different pressures of CO 2 .