Increased room temperature ferromagnetism in Co-doped tetrahedral perovskite niobates

Dilute magnetic semiconductors (DMSs), such as (In, Mn)As and (Ga, Mn)As prototypes, are limited to III–V semiconductors with Curie temperatures (Tc) far from room temperature, thereby hindering their wide application. Here, one kind of DMS based on perovskite niobates is reported. BaMxNb(1−x)O3−δ (M = Fe, Co) powders are prepared by the composite-hydroxide-mediated method. The addition of M elements endows BaMxNb(1−x)O3−δ with local ferromagnetism. The tetragonal BaCoxNb(1−x)O3−δ nanocrystals can be obtained by Co doping, which shows strong saturation magnetization (Msat) of 2.22 emu g−1, a remnant magnetization (Mr) of 0.084 emu g−1 and a small coercive field (Hc) of 167.02 Oe at room temperature. The ab initio calculations indicate that Co doping could lead to a 64% local spin polarization at the Fermi level (EF) with net spin DOS of 0.89 electrons eV−1, this result shows the possibility of maintaining strong ferromagnetism at room temperature. In addition, the trade-off effect between the defect band absorption and ferromagnetic properties of BaMxNb(1−x)O3−δ is verified experimentally and theoretically.


Introduction
Since the discovery of ferromagnetism in Mn-doped InAs in 1992 [1], dilute magnetic semiconductors (DMSs) with doped transition metal elements have been increasingly studied due to their fascinating multifunctional spintronic properties [2][3][4]. However, the solubility of doped transition metal atoms in a matrix remains a technical challenge, limiting these materials to the preparation of epitaxial films with metastable phase structures. Furthermore, elevating the T c of DMSs to room temperature is a long-standing request for their industrial application, and the complex carrier doping strategy can increase the T c of traditional DMSs to 100-180 K [5,6]. However, reports show that developing DMSs with transition metal oxides, such as Zn 1−x Mn x O 2 and Ti 1−x Co x O2, can extend the critical temperature above 300 K while maintaining a relatively low M sat ferromagnetism value of approximately 10 −2 emu g −1 [7,8].
Efforts to develop the room temperature DMSs in the form of powders or bulk ceramics have been carried out on Mn-doped ZnO; however, the formation of Mn clusters under high processing temperature (T > 700°C) would lead to suppression or disappearance in ferromagnetism [7]. Hence, the exploration of methods with lower synthesis temperature is of great importance to the industry application of DMSs in the bulk device and, extend the room temperature DMSs to other oxides system.
Perovskite niobates, with the composition of ANbO 3 (A = Li, K, Na, Ag), are widely used in lead-free piezoelectric and nonlinear optical devices due to their excellent ferroelectricity and nonlinear optical properties [9][10][11][12][13][14]. Previous theoretical and experimental work has proven that local ferromagnetism can be obtained in transition-metal-doped perovskite niobates [15][16][17]. Specifically, as a prototype of an A-site atom with an oxidation state of +2, BaNbO 3 has shown great potential as a room temperature DMS, showing that the introduction of oxygen defects, modulation of the electric field and Co doping with transition metals can be achieved; however, its M sat is limited to approximately 10 −2 emu g −1 [18][19][20] M-doped BaNbO 3−δ nanocrystals were prepared by the composite-hydroxide-mediated method. In a typical synthesis, 3.8819 g of NaOH, 5.1181 g of KOH, 0.2660 g of Nb 2 O 5 and 0.4880 g of BaCl 2 · 2H 2 O were weighed and mixed, and then 0.2705 g of FeCl 3 · 6H 2 O, or 0.2379 g of CoCl 2 · 6H 2 O was added into, respectively. The mixture was stirred and put in a 40 ml Teflon beaker. The Teflon beaker was placed in a preheated furnace at 195°C for 24 h. Then the Teflon beaker was taken out for natural cooling to room temperature. The reaction mixture was washed and filtered by distilled water and alcohol alternately three times. The filtered M-doped BaNbO 3−δ powder was dried at 60°C for 4 h.

Sample characterization
XRD was performed on a PANalytical Empyrean instrument outfitted with a PIXcel 2D detector operating at 40 kV per 40 mA, using Cu-Kα radiation (λ = 1.5405 Å). A Quanta 200FEG field emission SEM with EDS attachment was used for SEM analysis. XPS data were collected by ESCALAB 250Xi photoelectron spectrometer, which is produced by ThermoFisher company; the gun source was Al-Kα radiation. The HRTEM and SAED experiment was using a Tecnai G2 F30 transmission electron microscope. A Lake Shore 7404 vibrating sample magnetometer, with an external magnetic field sweeping from −15 000 to +15 000 Oe, was employed to evaluate the ferromagnetism of M-doped BaNbO 3−δ nanocrystals.

Theoretical calculation
DFT calculation in this study was performed with the Cambridge Serial Total Energy Package (CASTEP). A 2 × 2 × 2 supercell crystal model of pure BaNbO 3 was established first. Then one of the Nb atoms in the crystal was replaced by M elements, accompanied by one O vacancy in the oxygen octahedral cage. Localized density approximation (LDA) was employed for geometry optimization and property calculations with CA-PZ exchange-correlation functional. The plane-wave cut-off energy was set to 400 eV, and the k-point sampling grid was 4 × 4 × 4.

Results
Here, we report DMSs based on M-doped BaNbO 3−δ , in which strong magnetism can be obtained at room temperature. The M-doped BaNbO 3−δ nanocrystals are prepared by the composite-hydroxide-royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 8: 210121 mediated (CHM) method. The as-synthesized pure BaNbO 3−δ has a cubic lattice with a = b = c = 4.135 Å (figure 1a,b) and belongs to the P-m3m space group. This lattice parameter is larger than the theoretical value of 4.080 Å. Additionally, this lattice expansion is beneficial for accommodating more dopants. It can be proven by the XRD results with M doping that no second phase can be observed, as shown in figure 1a. The addition of Fe leads to a slight blue shift of diffraction peaks, indicating a decrease of the lattice parameter. While a red shift of diffraction peaks can be observed with the addition of Co, indicating an increase in lattice parameter, as shown in figure 1b. It is in accordance with the ionic radius of Nb 4+ (68 pm), Fe 3+ (64.5 pm) and Co 2+ (74.5 pm). In addition, the emergence of a shoulder diffraction peak at 43.6°(figure 1b) with Co doping indicates a phase change from the cubic BaNbO 3 to tetragonal BaCo x Nb (1−x) O 3−δ . The change in phase structure of BaCo x Nb (1−x) O 3−δ powders indicates more Co atoms have been incorporated in the BaNbO 3 matrix, implying better ferromagnetic properties than BaFe x Nb (1−x) O 3−δ . Figure 1c,d shows the XPS results of BaM x Nb (1−x) O 3−δ nanocrystal, and the oxidation states of Fe and Co are +3 and +2, respectively, combining with the EDS mapping measurements (electronic supplementary material, figure S1), corroborating that the dopants are successfully doped in the corresponding compounds. Furthermore, the existence of oxygen defects in the as-synthesized samples by CHM methods is proven by the blueshift in the split binding energy of the O_1s states (electronic supplementary material, figure S2). The oxygen defects are another origin for the emergence of room temperature ferromagnetism in BaM x Nb (1−x) O 3−δ [18].
To obtain further insight into the doped crystal lattices, the high-resolution transmission electron microscopy (HRTEM) and selection area electron diffraction transmission electron microscopy (SAED TEM) are used to characterize the crystalline structure of as-grown BaM

Room temperature ferromagnetism characterization
To evaluate the ferromagnetism of BaM x Nb (1−x) O 3−δ nanocrystal, the field-dependent magnetization (M) was measured. Figure 3 shows the M versus applied magnetic field (H ) curve with field sweeping from −15 000 to +15 000 Oe. As shown in figure 3a, the addition of M increases the ferromagnetism of BaMxNb (

Mechanism of room temperature ferromagnetism
To understand the physical mechanism of strong ferromagnetism maintained at room temperature, we investigated the electronic structure of the prepared materials based on ab initio methods. The 2 × 2 × 2 supercell crystal models were built with M elements replacing one of the Nb atoms, accompanied by   ), which will contribute to maintaining strong ferromagnetism at room temperature. Figure 5a shows the predicted optical absorption coefficient (η) of BaM x Nb (1−x) O 3−δ based on DFT calculations. The absorption induced by the transition between occupied O_2p orbitals and defect  royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 8: 210121 bands can be observed in all of the models due to doping transition metal elements and the existence of O defects. Co doping shows a small contribution to the defect band absorption (hν < 3.5 eV), which is six times smaller than the major absorption between the O_2p and Nb_4d orbitals (hν > 3.5 eV). However, BaFe x Nb (1−x) O 3−δ has a strong defect band absorption with a maximum η higher than 10 5 , which is comparable to the absorption between the O_2p and Nb_4d orbitals. The large differences in the defect band absorption between Co and Fe doping are ascribed to the strong local spin splitting of Co in the BaNbO 3−δ matrix, while the Fe in the BaNbO 3−δ matrix shows weak local spin splitting with strong hybridization of the opposite spin states near E F . As shown in the schematic of figure 5b, only states that have the same spin momentum are allowed, and the transition from degenerate O_2p orbitals to split Co_3d orbitals is forbidden. The above theoretical predictions are proven by the UV-vis absorption measurement results (spectra captured in a range of 1.55-6.22 eV). Figure 5c shows the absorption spectra of the prepared nanocrystal of BaM