Photoinduced Electron Spin Resonance Phenomenon in α-Cr2O3 Nanospheres
The photoinduced phenomenon in α-Cr2O3 nanoscaled spherical particles was investigated in the temperature range of 150 up to 315 K. An X-band electron-spin resonance spectrometry was employed to probe the magnetic behavior in α-Cr2O3 under an IR illumination in the nanosecond regime. The photoinduced effect on both low and high field ESR signals appears above 280 K and is remarkably enhanced just below Néel temperature . Such a photoinduced ESR phenomenon disappears in a reproducible way in the paramagnetic insulating state which occurs above of crystalline α-Cr2O3. In the antiferromagnetic phase, that is, below , the shift of the low field absorption could be attributed to the interaction of the light with specific Cr3+ ions located in strongly distorted sites correlated to strong ligand-field effect.
Photoinduced phenomena including phase transition and surface photoactivation are becoming a hot topic in the light-matter interaction domain and are of a specific interest both from fundamental and technological viewpoints [1–11]. Such a multidisciplinary trend is unlocking new perspectives that allows manipulation of the materials’ properties by photons in a tunable way . Some studies have been reported in charge transfer complexes , halogen bridged metal complexes , and perovskite type oxides [4–6]. A singular specificity to these photoinduced phenomena is their electronic dynamic in the ultrafast temporal regime in various spectral ranges.
In the UV range, Takesada et al.  have observed a significant enhancement in the ferroelectric properties in quantum paraelectric oxides. As reported by Mochizuki et al. , when MgO and rutile TiO2 are irradiated with ultraviolet laser light ( nm) in vacuum, their photoluminescence spectra change notably in intensity and in spectral structure with increasing irradiation time. By irradiation with the same laser light at room temperature in O2 gas, their original photoluminescence reappears. It was concluded that such reversible photoinduced phenomena are results of light-induced oxygen surface desorption/adsorption. Unlike previously reported transient photoinduced effects in manganites, Smolyaninov et al.  have observed permanent reflectivity changes following local sample illumination with 488 nm light in charge-ordered Bi0.3Ca0.7MnO3. This was found to be correlated to charge-transfer transitions between oxygen 2p and Mn 3d states. These transitions induce strong perturbations into the periodic arrangement of the Mn3+ and Mn4+ ions in the charge-ordered state. It was proposed that this photoinduced disorder may lead to charge-order domain switching and causes a domain wall formation as the ones observed in doped BaTiO3 and LiNbO3 perovskite-type oxides which exhibit a pronounced photorefractive effect . The singular observation of Smolyaninov et al. in Bi0.3Ca0.7MnO3 indicates that photonic band-gap structures may be created using holographic recording in manganites. Recently, Mochizuki and Fujishiro  reported the first photoinduced photoluminescence in CeO2 in both its bulk and nanoscaled forms due to O photoreduction. Such a photoreduction is accompanied by both a valence number change of cerium ions (Ce4+ → Ce3+) and oxygen-defect formation, a process which could be applied to light control of the oxygen-storage and oxygen-release processes at metal-oxide surfaces.
In the IR spectral range, these photoinduced phenomena in simple and multioxides have a unique specificity to either induce, among others, photo-magnetic effects and/or to appear inultrafast phase transitions based systems, in particular in strongly correlated materials. Firstly, the photo-induced magnetism which is generally triggered by a spin crossover inter-conversion and known as light-induced excited spin state trapping is of an appealing interest for magneto-optical technological applications, like optical data storage and fast processing devices. As a typical example, specifically nanoscaled spinel ferrites Al0.2Ru0.8Fe2O4 systems by Kanki et al.  were found to exhibit a cluster-glass behavior near room temperature with the spin-freezing state under illumination. This latter glass spin-freezing was considered as originating from randomness in the local magnetic anisotropy. The additional ultrafast spectroscopic measurements using femtosecond laser excitation indicated that such a photoinduced magnetization was correlated to intervalence charge transfer between mixed valence ions which lead to the change of local magnetic anisotropy and responsibility to applied magnetic fields. The low saturation magnetization under 0.6 μB/unit cell and good conductivity accompanied with a high spin polarized electron level over 75% attribute to this spinel ferrites family an attractive spintronic technological applications. In this photoinduced solid state phenomenon, one should single out the case of the standard yet controversial oxide VO2 which exhibits a pure 1st order phase transition. Indeed, as reported recently by various authors, the photoinduced phase transition by femtosecond laser pulses in VO2 has allowed shedding light not only on the so long disputed nature of the transition itself, distinguishing hence the Mott–Hubbard-like transition from the electron trapping in homopolar bonds, but also on its dynamic and the duration of the phase transition itself which was found to be of about 180 fs [18, 19]. The additional complementary recent work of Lysenko et al.  indicated that, upon a laser excitation, an instantaneous response in the transient reflectivity and transmission was observed followed by a relatively longer relaxation process. This photoinduced 1st order phase transition characteristic of VO2 at about 68°C makes it an optical coating candidate of choice for ultrafast optical switching devices , field effect transistors, and electrooptical gates  as well as ultrafast tunable nanoplasmonics among others [23, 24] as well as optical limiting in the IR spectral region .
Another singular oxide is chromium (III) oxide, Cr2O3, which is considered in this communication, is of a major importance equivalent to VO2 , and is even superior in view of the magnetic aspect. Indeed, Cr2O3 was one of the first oxides which was demonstrated to exhibit specific magnetoelectric characteristics. These properties of Cr2O3 are of fundamental interest and have been investigated for their broad potential for technological applications. Following the theoretical calculations by Landau and Lifshitz that the magnetoelectric response is only allowed in media without time-reversal symmetry or inversion symmetry , Dzyaloshinskii predicted that Cr2O3 should be a magnetoelectric crystal . Based on its magnetic point group, the linear-induced magnetization by an external electric field [27, 28] and the inverse effect  were measured experimentally. Such a linear magnetoelectric effect can be decoupled into three contributions, namely, electronic (frozen-ion), ionic (lattice-mediated), and strain-mediated responses . Each term can be further subdivided into spin and orbital contributions. The early ab initio studies were focused on the spin-lattice  and spin-electronic  terms. First-principles methods have only recently been developed to calculate the full magnetoelectric response tensor, including both spin and orbital contributions . In the case of Cr2O3, the spin-lattice term is dominant . More precisely, Íñiguez has shown  that the lattice contribution can be written as a product of the Born charge, the force-constant inverse, and the dynamical magnetic charge, which is the magnetic analog of the dynamical Born charge. These magnetoelectric theoretical studies in Cr2O3 have been refined recently by Ye and Vanderbilt  using first-principles density functional methods to study the dynamical magnetic charges driven by the spin-orbital coupling mechanism.
The current quest for materials, which can be controlled by light illumination to enhance their electronic, magnetic, and structural properties [35–39], leads to the study of the light-induced effect in Cr2O3 polycrystalline thin films by Sasaki et al. . They reported the first photoinduced results demonstrating crystal-clearly a light induced enhanced electron spin resonance (ESR) response in 2D type thin films. In this work, complementary to Sasaki et al.’s on 2D films, we investigate the photoinduced ESR phenomena and their temperature evolution in nonagglomerated and quasimonodisperse α-Cr2O3 nanospheres using a pulsed Nd-YAG laser (064 nm) as an illumination exciting source in the temperature range of 150–315 K.
2. Experiments, Results, and Discussion
The synthesis of α-Cr2O3 nanospheres was performed with reagent grade chemicals. The general methodology is described in previous contributions [41, 42] and illustrated in Figure 1(a). An aqueous solution (MilliQ, 18.2 MΩ) of 1 mM of chromium potassium sulfate dodecahydrate (KCr(SO4)2·12H2O) mixed with polyvinylpyrrolidone (PVP) as a stabilizer in a glass bottle with an autoclavable screw cap was heated in a standard laboratory oven at a constant temperature of about 75°C. Subsequently, the green precipitates of Cr2O3·nH2O were formed onto quartz glass and silicon substrates. The precipitates onto substrates were heat-treated at about 500°C in air, based on the thermal analysis data, for about 1 h to obtain α-Cr2O3 spherically shaped particles. The crystalline structure of the spherical particles of α-Cr2O3 was investigated using an X-ray diffractometer (XRD, model Bruker AXS D8 Advance, Kα line of copper Å). Their structural characterization was investigated using Raman spectroscopy (Horiba Jobin Yvon, LabRAM HR UV/Vis/NIR) and a Perkin Elmer Spectrum ATR-FTIR with a diamond ATR accessory. Their morphology was investigated using a Leo-StereoScan 440 scanning electron microscope (SEM). The photoinduced phenomenon of the α-Cr2O3 nanospheres was investigated using an X-band ( GHz) electron-spin resonance (ESR) spectrometer under an external illumination of a 160 mW Nd:YAG laser operating at its fundamental regime that is with a wavelength nm and a pulse repetition frequency of ~30 Hz.
Figure 1(b) reports a typical SEM image of the synthesized chromium oxide particles. It demonstrates that isolated spherical chromium oxide particles can be produced with well-defined surface morphology, relatively narrow size distribution, and a prominent shape control. Figure 1(c) shows the corresponding particle size distribution histogram. The ordinate indicates the frequency, which is normalized as number of particles with /total number of particles. The distribution has a long tail toward larger size particles. The data which was fitted to a log-normal distribution gives a mean diameter of nm highlighting the nanoscale aspect of the chromium oxide spheres.
Figure 2 depicts a typical XRD pattern of a film of chromium oxide nanospheres. All observed Bragg peaks coincide with those given in the JCPDS card 74–0326 of the bulk α-Cr2O3. More precisely, these Bragg peaks are attributed to the rhombohedral structure (space group Rc) with unit cell parameters of about and (Å) of pure α-Cr2O3 green phase under a slight compressive stress. Hence, these nanoscaled α-Cr2O3 would not exhibit a size effect but likely surface ones if any.
Figure 3 reports a typical infrared vibrational spectrum at room temperature obtained using attenuated total reflection (ATR) geometry of the synthesized powder of α-Cr2O3 nanospheres. This result reveals two strongest bands centered at 539 and 603 cm−1. These values fall well within the range reported in the literature for samples having the adequate α-Cr2O3 stoichiometry. Both bands are associated with Cr–O stretching modes (symmetrical stretching and antisymmetrical stretching) in α-Cr2O3 and are due to various combinations of O2− and Cr3+ displacements in the lattice. The pair of sharp bands at 442 and 411 cm−1 corresponds to two specific O2− displacements in the lattice.
Figure 4 reports a typical room temperature Raman spectrum of the α-Cr2O3 nanospheres. As one could notice, two Eg and one modes of chromium (III) oxide are observed. As well established, bulk chromium (III) oxide has a corundum structure that belongs to the space group and consists of a hexagonal close packed array of oxide anions with 2/3 of the octahedral holes occupied by chromium. The site symmetry for the Cr atoms is C3, whereas the O atoms are on sites having C2 symmetry. The optical modes in the crystal are 2, 2, 3, , 5Eg, and vibrations with only two and five Eg vibrations which are Raman active. As mentioned above, in the present work related to isolated α-Cr2O3 nanospheres, it was found, in a reproducible manner on several similar samples that only the two Eg and one modes subsist. The observation of only this set of limited modes cannot be caused by size effects in the α-Cr2O3 nanospheres and/or to the breakdown of the 3D symmetry of surface site atoms. Compared to bulk α-Cr2O3 material, the shift of the most intense Raman peak, assigned to symmetry, from 533 to 543 cm−1, indicated a significant internal compressive stress which might be related to surface effects, as it was noted in the XRD patterns. As a preliminary conclusion and from the combined XRD, Raman, and IR-ATR results, one could preconclude on the phase purety of α-Cr2O3 nanospheres and that there are no substantial size effects but rather surface ones.
Figure 5 reports the electron spin resonance (ESR) profiles with and without IR laser illumination which are the major focus of this short contribution. The investigations were carried out at different temperatures ranging from 150 to 315 K. The solid and dash curves correspond to the ESR profiles in dark that is without laser illumination and under laser illumination, respectively. The observed ESR profiles retain a Lorentzian curvature centered at the high magnetic field resonance of 3440 G. As one can notice, there are two components in fact: a low and a high field component. This first observation of the ESR signature of the α-Cr2O3 nanospheres is in contrast to the pioneering work of Sasaki et al. and Yanagisawa et al. on 2D sputtered thin films where there was only one ESR resonance (the high field one). In addition, one can also distinguish that while the position of the high field ESR absorption does not vary with temperature up to nearly 280 K, the low field component shifts substantially towards high fields. More accurately, the higher is the temperature, the larger is the shift of the low field component. It adds to the high field component near the Neel temperature K. While shifting, the low field component becomes broader. Likewise, it is worth mentioning that this shift has not been observed by Sasaki et al. as well as by Yanagisawa et al. in their sputtered 2D thin films.
In terms of the photo-induction phenomenon, and as in the case of Sasaki et al. and Yanagisawa et al, there is no noticeable variation in the high field ESR absorption with the Nd-YAG illumination below 280 K. Above such a temperature up to K, the high field ESR absorption is enhanced indeed as in the case of Sasaki et al. and Yanagisawa et al. The enhancement of the ESR absorption at 3440 G is maximum just below and vanishes rapidly above such a critical temperature. Consequentially, this indicates crystal clearly that the photo-induction phenomenon is related to primarily magnetic fluctuations as the magnetic susceptibility presents a singularity at K. More precisely, one could conclude that the observed photoinduced enhancement both in our case (α-Cr2O3 nanospheres) as well as in the 2D thin films by Sasaki et al. and Yanagisawa et al. is directly related to the antiferromagnetic/paramagnetic phase transition of the α-Cr2O3 structure. It is worth noticing that the photoinduction effect starts as early as 220 K for the low field ESR absorption. Hence, the shift of the low field absorption in the antiferromagnetic state of α-Cr2O3 can be attributed to the interaction of the light with Cr3+ ions in the strongly distorted sites due to strong ligand-field effect. As mentioned before, while the ESR absorption enhancement is likely to be attributed to the magnetic fluctuations in the α-Cr2O3, the physical origin of this photoinduced effect could only be related to the Cr3+ ions. Cr2O3, is a well-known antiferromagnet with magnetic point group symmetry . Below the Néel temperature K, the four chromium spins in the unit cell are aligned along the threefold axis (-axis) of the crystal in a + − + − fashion, reducing the magnetic symmetry to . Because of the antiferromagnetic order, the crystal loses both space and time reversal symmetry while the product of the parity operations is still a symmetry operation. Cr2O3 exhibit the spontaneous nonreciprocal reflection of light that was predicted several decades ago  but first observed on α-Cr2O3 .
What is the physical origin of such an ESR absorption enhancement and the observed photo-induced effect in the investigated nanoscaled α-Cr2O3 spheres? This photoinduced ESR phenomenon could be caused by the peculiar optical properties of Cr3+ ions (3d3) in α-Cr2O3 that originate from the d-d electronic transitions between the splitted lower triply degenerated levels and the upper doubly degenerated eg levels of the fivefold degenerated 3d orbitals of the chromium ions in the centrosymmetric point group. The free ions Russell–Saunders coupling in symmetry are , , and . The ground state is , which splits into , , and . The excited state is and splits into , , , and . The optical transitions originate from the strong spin-allowed but parity-forbidden and transitions as well as from the weak spin- and parity-forbidden transitions , and . The bandwidths of such transitions are usually large due to the strong electronic coupling to vibrations of the local environment as well as the stoichiometry, size, and surface morphology of the samples. In addition, the observed ESR absorption enhancement and the photoinduced effect can be correlated to the experimental findings of Fiebig et al. [48, 49]. They have observed a strong second harmonic generation signal  originating from the transition of Cr3+ ions in α-Cr2O3 due to the two-photon excitation of levels below  and have been used to image the antiferromagnetic domains and determine magnetic symmetry. Likewise, it is noteworthy to highlight an analogous observed photoinduced effect by Terakado and Tanaka . They have reported the first studies on photoinduced changes in GeO2 glass system which has been investigated in terms of its optical and structural properties too. It was found that band-gap excitation gives rise to modifications in midgap optical absorption and photoluminescence spectra, producing electron-spin signals at and 2.008 which were related to photoinduced bond transformations from rutile-like to defective quartz-like structures and/or from small to large rings.
At last, one might suspect that the current observed photoinduced ESR phenomenon in the α-Cr2O3 nanospheres is driven by the Nd-YAG laser heating. The evidence against such a hypothesis is provided by the temperature dependence of ESR profiles themselves. As indicated in Figure 5, there is no difference among those profiles at low temperatures where the result is expected to be sensitive for this kind of heating. In fact, the ESR profile after the postmeasurement with the optical excitation shows almost identical with that obtained prior to the optical excitation. From experimental viewpoint, such a photoinduced aspect could be extended to engineer tunable bang gap engineering of core shell oxide/α-Cr2O3 nanostructures [51–54].
Photoinduced ESR effects and enhancement at high field in α-Cr2O3 nanospheres were investigated and observed. It was found that the light-induced ESR signal appears above 280 K in both low and high magnetic fields and is remarkably enhanced just below the vicinity of for the high field ESR absorption resonance. Consequentially, this latter was concluded to be correlated to magnetic fluctuations, specifically at the edge of the antiferromagnetic/paramagnetic phase transition where the magnetic susceptibility exhibits a singularity. Relatively to 2D continuous thin films, the photoinduced ESR absorption resonance of the nonagglomerated nanospheres presents an additional low field component which shifts towards high field values with the increasing of temperature. The present results open up a framework of more detailed future research on the light-induced effect in chromites as well as new possibilities of tunable photoinduced magnetooptical applications in α-Cr2O3 based nanoscaled devices. From fundamental viewpoint, this study could reopen the chapter of nonreciprocity. Indeed, the specificity of the magnetoelectric effect in α-Cr2O3 is characterized by the spontaneous nonreciprocal optical effects such as the gyrotropic birefringence and dichroism for which time and parity symmetry are broken.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was sponsored within the framework of the UNESCO UNISA Africa Chair in Nanosciences & Nanotechnology and the Nanosciences African Network (NANOAFNET) by the National Research Foundation of South Africa, the African Laser Centre (ALC), the University of South Arica (UNISA) in collaboration with iThemba LABS, and the Abdus Salam ICTP-Trieste. Likewise, the authors do acknowledge the contribution of the Institute for Micro- and Nanotechnologies, Ilmenau, Germany.
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