High-temperature ferromagnetic semiconductor with a field-tunable green fluorescent effect

Ferromagnetic semiconductors with luminescent effects provide a unique platform for studying magneto-electric-optical multifunctional devices. However, little is known about such materials with spin ordering well above room temperature. By using a unique high-pressure annealing method, a Cr and Fe disordered perovskite oxide SrCr0.5Fe0.5O2.875 (SCFO) with a simple cubic structure was prepared. Magnetic measurements demonstrated the ferromagnetic behavior with a spin ordering temperature as high as 600 K. In contrast to metallic SrCrO3 and SrFeO3, SCFO, with a moderate oxygen deficiency, is a direct bandgap semiconductor with an energy gap of 2.28 eV, which is within the visible light region. As a consequence, SCFO displays a green fluorescent effect arising from the d–p bonding and anti-bonding states. Moreover, the photoluminescence intensity can be tuned by a magnetic field. This work opens up a new avenue for research on room-temperature multifunctional materials with coupled magnetic, electrical, and optical performance. A magnetic semiconductor that retains its magnetic properties at high temperatures has been developed by researchers in China and Germany. Semiconductor materials do much of the processing in computers and cell phones whereas magnetic materials store and retrieve information. A magnetic semiconductor merges these two functions in a single material and offers unique functionality not seen in the other materials. However, the magnetic properties of most of the known magnetic semiconductors disappear at high temperatures, limiting their application. Youwen Long from the Beijing National Laboratory for Condensed Matter Physics and colleagues used high pressures and high temperatures to create a SrCrFeO compound. This perovskite (a compound with a similar crystal structure to CaTiO3) demonstrated ferromagnetic behavior up to 600 K. Ferromagnetic semiconductors are promising candidates for high-performance multifunctional spintronic devices due to the peculiar magneto-electric and magneto-optical properties. However, the low spin ordering temperature limits their applications. By using high pressure to stabilize the crystal structure and oxygen content, an oxygen-deficient perovskite SrCr0.5Fe0.5O2.875 was synthesized. This compound displays ferromagnetic behavior with a spin ordering temperature as high as 600 K. Benefiting from the semiconducting direct bandgap (~2.28 eV), a field-tunable green fluorescent effect is observed. This work opens up a new avenue for research on room-temperature multifunctional materials with coupled magnetic, electrical, and optical performance.


Introduction
Magnetic semiconductors display intriguing magnetoelectric and magneto-optical properties, providing promising candidates for high-performance multifunctional spintronic devices such as spin field-effect transistors and magnetic random access memory devices [1][2][3][4][5][6] . For practical applications, a high spin ordering temperature (T C ) above room temperature (RT) is desirable. However, despite tremendous efforts, most reported intrinsic ferromagnetic (FM) semiconductors without element substitution and doped diluted magnetic semiconductors have low critical temperatures [7][8][9][10][11][12] . Recently, a few insulating/semiconducting two-dimensional van der Waals crystals, such as Cr 2 Ge 2 Te 6 and CrI 3 , have been found to show ferromagnetism [13][14][15] , but the T C values are rather low. At present, the discovered FM compounds have rarely been known to show luminescence. Therefore, there is a pressing need to search luminescent ferromagnetic materials with spin ordering above RT for magnetoelectric-optical coupled applications.
The bandgaps of transition-metal perovskite oxides are usually developed with the formation of charge or spin density wave states. However, both SrCrO 3 and SrFeO 3 perovskites are metallic because of a small hole-type Fermi pocket around some high symmetry points of the Brillouin zone [16][17][18] . These two compounds may be converted into semiconductors if the small hole-type Fermi pocket is removed, i.e., by removing some electrons and its dominating bands around the Fermi level. Since the bands intercepting the Fermi level are dominated by the O-2p orbitals, we can remove some O atoms to tune the electron transport. Thus, by delicate control of O vacancies in SrCrO 3 and SrFeO 3 , the Fermi level can fall into the gap, leading to semiconducting/insulating behavior. On the other hand, the bandgap is tunable by transition metal doping, which can increase or decrease the local spin moment and therefore modify the magnetism. By decreasing the oxygen content and mixing Cr and Fe magnetic ions, an oxygen-deficient perovskite oxide SrCr 0.5 Fe 0.5 O 2.875 with a B-site disordered Cr/Fe distribution was designed and prepared under high-pressure and high-temperature conditions. Although the oxygenrich SrCrO 3 19-23 and SrFeO 3 24-27 are AFM metals with lower ordering temperatures (<140 K), the oxygendeficient SCFO was confirmed to be a high-temperature (T C~6 00 K) ferromagnetic semiconductor with a direct bandgap of 2.28 eV. Moreover, a green fluorescent effect, which is tunable by a magnetic field, was found to occur. These joint features make SCFO a rare material system to design multifunctional magneto-electric-optical coupling devices with promising applications at or even above RT.

Experimental section
Polycrystalline SCFO was prepared by a solid-state reaction under high-pressure and high-temperature conditions. The highly pure (>99.9 %) reactants SrO, Fe 2 O 3 , CrO 2 , and CrO 3 were mixed at a ratio of 8:2:3:1. The finely mixed reactants were treated at 6 GPa and 1173 K for 30 min on a cubic anvil-type high-pressure apparatus. The phase quality of SCFO was characterized by powder X-ray diffraction (XRD) using a Huber diffractometer with Cu Kα 1 radiation at RT. Rietveld refinement of the XRD data was performed using the FullProf program 28 . High-resolution selected area electron diffraction was performed along the [1 − 10] zone axis on an ARM200Fcold field emission transmission electron microscope. X-ray absorption spectroscopy at the Crand Fe-L 2,3 edges was performed at the BL11A beamline of the NSRRC synchrotron using total electron yield mode. The magnetic susceptibility and magnetization measurements were carried out using a superconducting quantum interference device magnetometer (Quantum Design, MPMS-VSM). The resistivity and specific heat were measured on a physical property measurement system (Quantum Design, PPMS-9T). We measured the visible and UV range frequency-dependent reflectivity from the surface at a near-normal angle of incidence on an Avaspec 2048 × 14 optical fiber spectrometer. The extinction coefficient was obtained through a Kramers-Kronig transformation of reflectivity at a near-normal angle of incidence in the range of 50-50,000 cm −1 on a Bruker 80v Fourier transform infrared spectrometer using an in situ evaporation technique. Steady-state photoluminescence measurements without a magnetic field were performed on an Edinburgh FLS 920 spectrometer by directing the excitation laser pulses of the pump fluence of~0.5 nJ cm -2 at 445 nm. For the magnetic fielddependent photoluminescence spectra, the magnetic field was provided by a superconducting magnet, and the excitation light source was changed to HeCd lasers with a 325 nm wavelength (Kimmon Koha). Figure 1 shows the powder XRD pattern and the refinement results of SCFO synthesized at 6 GPa and 1173 K. All the diffraction peaks can be well fitted on the basis of a simple cubic perovskite structure with space group Pm-3m. The goodness of fit parameter was within a satisfactory range, with R wp = 2.01%. Moreover, high-resolution electron diffraction was performed along the [1-10] direction. One cannot discern any diffraction spots with h + k + l = odd like (111) spot (see the inset of Fig. 1), further revealing the disordered distribution for Cr and Fe at the B site. Therefore, SrCr 0.5 Fe 0.5 O 2.875 possesses a crystal structure similar to that of undoped SrCrO 3 29 and SrFeO 3 30 . In comparison, the refined lattice constant of SCFO (3.9185 Å) is slightly larger than that of the oxygen-rich SrCrO 3 (3.818 Å) and SrFeO 3 (3.836 Å), which can be attributed to the oxygen deficiency in SCFO. To verify the valence states of Fe and Cr, elementsensitive soft X-ray absorption spectroscopy (XAS) was performed (see Supporting information Fig. S1) 31 Note that in addition to controlling the oxygen content, high pressure is also necessary to stabilize the cubic perovskite structure of SCFO, similar to the highpressure synthesis of isostructural SrFeO 3 and SrCrO 3 .  Figure 2a shows the temperature dependence of the magnetic susceptibility of SCFO measured under a magnetic field of H = 0.1 T using zero-field-cooling mode. Upon cooling to T C~6 00 K, the susceptibility experiences a sharp increase, suggesting an FM-like phase transition. Upon further cooling, the susceptibility smoothly increases, accompanied by an apparent upturn below approximately 80 K. If one looks at the field-dependent magnetization, as shown in Fig. 2b, remarkable magnetic hysteresis exists below T C . At 300 K, the coercive field is approximately 2000 Oe, which is almost temperature independent upon cooling to 2 K. The remnant magnetization measured at 300 K is approximately 0.01 μ B /f.u. and increases to 0.02 μ B /f.u. at 2 K. These features indicate the canted ferromagnetism of SCFO, as reported in the ferroelectric ferromagnets of BiFe 1-x Co x O 3 thin films and 1D SrFeO 2.5 chains 36,37 . In comparison, all of these FM compounds show similar magnitudes in remnant magnetization, but the coercive fields observed in SCFO and BiFe 1-x Co x O 3 thin films are much higher than that of the SrFeO 2.5 chain (130 Oe). In SCFO, magnetic Fe 3+ and Cr 4+ ions exist at a ratio of 4:3. Since the d-electron amount of Fe 3+ (3d 5 ) is considerably larger than that of Cr 4+ (3d 2 ), the Fe 3+ -O-Fe 3+ superexchange interactions are expected to dominate the magnetism. The canted spins of Fe 3+ ions are thus responsible for the hightemperature FM behavior of SCFO. In addition, the lower-temperature upturn observed in magnetic susceptibility below~80 K is probably indicative of some short-range FM correlations due to the Fe 3+ -O-Cr 4+ superexchange interactions.

Results and discussion
In comparison with the oxygen-rich perovskite oxides, i.e., the C-type AFM SrCrO 3 and spiral SrFeO 3 with lower Néel temperatures, Cr/Fe disordered SCFO with oxygen deficiency exhibits different magnetic features, with a high T C well above RT. Moreover, unlike the metallic electrical transport properties observed in SrCrO 3 and SrFeO 3 , the current SCFO displays semiconducting behavior, as evidenced by the sharply increased resistivity upon cooling (see Fig. S2). The resistivity data between 210 and 380 K follow a 3D Mott variable-range hopping model, as shown in the inset of Fig. S2 using the function ρ = ρ 0 exp (T 0 /T) 1/4 , where ρ 0 is the prefactor and T 0 represents the characteristic temperature 38 . As shown in Fig. S3, the specific heat of SCFO does not display any anomalies from 2 to 200 K. At lower temperatures (<12 K), the data can be fitted using the function C p = αT 3/2 + βT 3 , yielding α = 8.3 mJ/mol·K 5/2 and β = 0.16 mJ/mol·K 4 . The presence of the T 3/2 term is in agreement with the FM behavior. The absence of an electron contribution to specific heat (proportional to T) suggests the semiconducting or insulating nature with a considerable bandgap in SCFO.
To investigate the detailed energy bandgap of SCFO, reflectance spectra in the UV-visible region were obtained at RT, as shown in Fig. 3a. Although the reflectivity for a polycrystalline sample is usually not as strong in the wavelength range from 400 to 800 nm, a sharp peak is found to occur at 545.5 nm in SCFO. Moreover, other smaller peaks are also observed, which can be attributed to the formation of some bound exciton states 39,40 . We obtained the absorption coefficient α 0 according to the equation α 0 = 4πκ/λ, where the extinction coefficient κ was detected from 50 to 50,000 cm −1 . With the major optical transitions described, we used the Tauc and Davis-Mott models 41 based on the equation α 0 hν = A(hν − E g ) n , where A is a proportional constant, E g is the optical bandgap and n depends on the type of band transition of the material. For materials with a direct (indirect) bandgap, n = 1/2 (2) is assigned. A direct bandgap of Tauc's plots exhibiting (α 0 hν) 2 versus photon energy hν has been revealed for SCFO, as shown in Fig. 3b, yielding a bandgap of E g = 2.28 eV (λ = 545.2 nm), which is consistent with the strongest peak observed in the reflectance spectrum at 545.5 nm. The E g of SCFO is close to the bandgaps of LaFeO 3 (~2.34 eV) 42 and SrCrO 4 (~2.45 eV) 43 . Note that the wavelength of the bandgap energy lies in the visible light range, particularly in the green wavelength region. The direct-gap nature of SCFO makes it possible to generate light emission. Figure 4a shows the photoluminescence measurement result of SCFO. A broad green emission peak can be observed at approximately λ = 520 nm upon excitation by blue light at a wavelength of 445 nm. The peak energy is somewhat larger than E g (~545 nm), suggesting that the green light emission mainly emanates from an acrossbandgap (band-to-band) transition. There are some empty d-orbitals dominating the band between the d-p bonding and anti-bonding states. Therefore, except for the electronic transition between the d-p bonding and anti-bonding states, there are additional photoluminescence lines mediated by the empty and localized d-orbitals between the d-p bonding state and antibonding states, giving rise to photon energies larger than the bandgap. Moreover, the full width at half maximum completely covers the green light region (see the shaded part in Fig. 4a). This broadening effect may result from band-edge absorption and multiple exciton states, such as the charge trapping that exists in polycrystalline samples 44 . On the other hand, from the viewpoint of the electronic configuration, unlike the f-f transition emission spectra of rare earth ions 45 where f electrons are screened by outer 5s and 5p electrons so that the emission spectra peaks are sharp and more similar to atomic emission line spectra, d electronic transition emission spectra show peak broadening as a consequence of band broadening of the outer d electrons sensitive to the crystal field environment.
Since the transition-metal 3d electrons in SCFO take part in spin ordering, the related fluorescent effect can be tuned by an external magnetic field. Fig. 4b presents the photoluminescence spectra measured at zero field and 7.35 T using 325 nm excitation light. Apparently, applying a magnetic field can effectively change the luminescence intensity, suggesting the occurrence of magneto-optical coupling. Usually, fluorescent effects are observed in inorganic semiconductors with rare earth element activators or bound excitation states. The current SCFO provides a new photoluminescence system that is composed of 3d transition metals rather than expensive rare earth elements. The empty d-orbitals separate the dp bonding and anti-bonding states, which serve as forbitals from activators or exciton states. Moreover, the local spin moments of rare earth elements are paramagnetic at RT because of the weak exchange interactions between them, whereas the spin moments are ordered well above RT in SCFO, providing an excellent platform for research on the interaction between spin order and photoluminescence.
In conclusion, a magnetic semiconductor, SCFO, with a high T C (~600 K) was synthesized, and its green photoluminescence was studied. The magnetism and electron transport properties of SCFO are also presented in this report. Oxygen-vacancy modulation by the high-pressure and high-temperature synthesis method controls the crystal structure, electronic configurations, magnetic interactions, and bandgap energy. A specific direct bandgap structure induces green light emission, which provides an additional important degree of freedom for designing bulk materials with ferromagnetic and semiconducting properties. Our results suggest that SCFO is an available candidate for multifunctional magnetoelectric-optical coupling devices with potential applications above room temperature.