Nano-Structured Dilute Magnetic Semiconductors for E ﬃ cient Spintronics at Room Temperature

: In recent years, many e ﬀ orts have been made to develop advanced metal oxide semiconductor nanomaterials with exotic magnetic properties for modern applications w.r.t traditional analogues. Dilute magnetic semiconductor oxides (DMSOs) are promising candidates for superior control over the charge and spin degrees of freedom. DMSOs are transparent, wide band gap materials with induced ferromagnetism in doping, with a minor percentage of magnetic 3d cation to create a long-range antiferromagnetic order. Although signiﬁcant e ﬀ orts have been carried out to achieve DMSO with ferromagnetic properties above room temperature, it is a great challenge that still exists. However, TiO 2 , SnO 2 , ZnO and In 2 O 3 with wide band gaps of 3.2, 3.6, 3.2 and 2.92 eV, respectively, can host a broad range of dopants to generate various compositions. Interestingly, a reduction in the size of these binary oxides can induce ferromagnetism, even at room temperature, due to the grain boundary, presence of defects and oxygen vacancies. The present review provides a panorama of the structural analysis and magnetic properties of DMSOs based on binary metal oxides nanomaterials with various ferromagnetic or paramagnetic dopants, e.g., Co, V, Fe and Ni, which exhibit enhanced ferromagnetic behaviors at room temperature.


Introduction
Persistent efforts to achieve substantially smaller information storage devices open up many areas of interest for research, alongside certain challenges. In this regard, spintronics offers the utilization of electron spin and charge to enhance information density. The development of functional ferromagnetic semiconductors at room temperature, i.e., the combination of the functionality of semiconductors and ferromagnets, is the goal of spin-based electronics. In this way, interest in dilute magnetic semiconductors (DMSs), mainly dilute magnetic oxides (DMOs) or dilute magnetic semiconductor oxides (DMSOs), has been rapidly increasing due to their potential application in spintronics devices. Low-temperature ferromagnetism in (Ga,Mn)As, p-(Cd,Mn)Te and similar compositions, and calculations on doped, nitride and oxide semiconductors which might be able to explain the ferromagnetism via valence-band holes at lower temperatures, have been much sought-after areas in magnetism research [1]. At a low concentration of donors, the localized impurity band forms, Magnetite has a cubic inverse spinel crystal structure with space group Fd3 m and has revealed electric and magnetic behavior via electron exchange in a mixed valence Fe 2+ /Fe 3+ state, leading potential uses in catalysis, magnetic recording, ferro fluids, clinical diagnosis, etc. Additionally, monodispersed nanocrystals with suitable sizes/size distribution could add additional functionality to scientific and technological interests. Alloys based on iron/magnetite were unsuitable due to eddycurrent losses [17][18][19][20]. Magnetite shows the ferrimagnetic properties associated with congruent spins (e) Magnetite has a cubic inverse spinel crystal structure with space group Fd3m and has revealed electric and magnetic behavior via electron exchange in a mixed valence Fe 2+ /Fe 3+ state, leading potential uses in catalysis, magnetic recording, ferro fluids, clinical diagnosis, etc. Additionally, monodispersed nanocrystals with suitable sizes/size distribution could add additional functionality to scientific and technological interests. Alloys based on iron/magnetite were unsuitable due to eddy-current losses [17][18][19][20]. Magnetite shows the ferrimagnetic properties associated with congruent spins that are partially oriented in their field direction. However, the magnetic domains in magnetite-independent moment vectors display definite magnitudes and directions [21]. Importantly, the transformation of multi-domains to a single-domain state for ferromagnetic materials is possible via particle-size engineering [22][23][24].
However, binary oxides TiO 2 , SnO 2 , ZnO and In 2 O 3 are intrinsically diamagnetic, while the grain boundary, presence of defects and oxygen vacancies produced in the nano regime induce the unexplainable magnetism at room temperature (RT) [25,26]. These nanomaterials can be used in spintronics applications in place of magnetite.

Theory of Ferromagnetism in Oxides
The mechanism responsible for initiating room temperature ferromagnetism (RTFM) can be introduced in non-magnetic semiconductors (NMSs) through charge and spin of electrons as explained in Figure  A schematic representation of FM in Co:TiO2 is shown in Figure 3. Titanium 3d electrons travel around the material aligning the spin of cobalt atoms to point in the same direction [27]. Using various TM ions, dopants perturb the semiconductor oxides in two ways. In the first one, if dopants substitute at metal ion site will strongly perturb conduction band and therefore, reduce the mobility of electrons by dispersion of the electrons [28]. In another case, if dopants available at oxygen lattice, then it will perturb the valence band hence obtain higher mobility of electrons due to the conduction band free from dispersion. Furthermore, due to different ionic sizes and charges of dopants, distortion occur in the structure in several ways as shown in Figure 4 [29]. Hence, an appropriate selection of the dopants for metal oxide semiconductors is significant for acquiring better quality spintronics materials. A consideration of this paradox is vital to determine the type of electrons or charge carriers that mediate the ferromagnetism in a material to design efficient spintronic devices. A schematic representation of FM in Co:TiO 2 is shown in Figure 3. Titanium 3d electrons travel around the material aligning the spin of cobalt atoms to point in the same direction [27]. Using various TM ions, dopants perturb the semiconductor oxides in two ways. In the first one, if dopants substitute at metal ion site will strongly perturb conduction band and therefore, reduce the mobility of electrons by dispersion of the electrons [28]. In another case, if dopants available at oxygen lattice, then it will perturb the valence band hence obtain higher mobility of electrons due to the conduction band free from dispersion. Furthermore, due to different ionic sizes and charges of dopants, distortion occur in the structure in several ways as shown in Figure 4 [29]. Hence, an appropriate selection of the dopants for metal oxide semiconductors is significant for acquiring better quality spintronics materials. A consideration of this paradox is vital to determine the type of electrons or charge carriers that mediate the ferromagnetism in a material to design efficient spintronic devices. band free from dispersion. Furthermore, due to different ionic sizes and charges of dopants, distortion occur in the structure in several ways as shown in Figure 4 [29]. Hence, an appropriate selection of the dopants for metal oxide semiconductors is significant for acquiring better quality spintronics materials. A consideration of this paradox is vital to determine the type of electrons or charge carriers that mediate the ferromagnetism in a material to design efficient spintronic devices. Figure 3. A schematic representation of ferromagnetism in Co:TiO2. The orange, green and blue spheres correspond to titanium, oxygen and cobalt atoms, respectively [27]. Figure 3. A schematic representation of ferromagnetism in Co:TiO 2 . The orange, green and blue spheres correspond to titanium, oxygen and cobalt atoms, respectively [27]. It is well known that the magnetic properties in doped nanomaterials such as metal oxides, and superparamagnetic iron oxide, chalcogenide semiconductors [30,31] are dependent on nature and extent of doping along with synthesis characteristic. Incorporation of several transition metal dopant in host ranging from ZnO, SnO2, TiO2, etc. develop DMS material possessing different surface properties and size of materials etc. [32] from the various synthesis protocol and even saturated magnetic moment can be enhanced by annealing at a suitable temperature [33].

TiO2 and Ferromagnetism
TiO2 is the well-known and widely used semiconductor having both rutile and anatase structure ( Figure 1) with a wide bandgap that can be engineered easily on doping and also, various properties can be modified easily such as optical, electrical as well as magnetic properties. Choudhary et al. examined the effect of Mn on TiO2 nanoparticles formed by a sol-gel method of size range 6-11 nm displayed magnetic properties at room and low temperature. The mechanism of paramagnetism and antiferromagnetism is shown in Figure 5 [33]. Mn 2+ magnetic spins and exchange interaction of Mn 2+ -Mn 2+ are responsible for paramagnetic behavior whereas negative Curie-Weiss temperature showed antiferromagnetic interaction of Mn 2+ ions through lattice oxygen ( Figure 5) [34]. Sharma et al. reported Mn-doped TiO2 thin films on quartz by spray pyrolysis and investigated RTFM due to bound magnetic polarons [35]. A stable anatase phase, Ni-doped TiO2 nanocrystals synthesized by sol- It is well known that the magnetic properties in doped nanomaterials such as metal oxides, and superparamagnetic iron oxide, chalcogenide semiconductors [30,31] are dependent on nature and extent of doping along with synthesis characteristic. Incorporation of several transition metal dopant in host ranging from ZnO, SnO 2 , TiO 2 , etc. develop DMS material possessing different surface properties and size of materials etc. [32] from the various synthesis protocol and even saturated magnetic moment can be enhanced by annealing at a suitable temperature [33].

TiO 2 and Ferromagnetism
TiO 2 is the well-known and widely used semiconductor having both rutile and anatase structure ( Figure 1) with a wide bandgap that can be engineered easily on doping and also, various properties can be modified easily such as optical, electrical as well as magnetic properties. Choudhary et al. examined the effect of Mn on TiO 2 nanoparticles formed by a sol-gel method of size range 6-11 nm displayed magnetic properties at room and low temperature. The mechanism of paramagnetism and antiferromagnetism is shown in Figure 5 [33]. Mn 2+ magnetic spins and exchange interaction of Mn 2+ -Mn 2+ are responsible for paramagnetic behavior whereas negative Curie-Weiss temperature showed antiferromagnetic interaction of Mn 2+ ions through lattice oxygen ( Figure 5) [34]. Sharma et al. reported Mn-doped TiO 2 thin films on quartz by spray pyrolysis and investigated RTFM due to bound magnetic polarons [35]. A stable anatase phase, Ni-doped TiO 2 nanocrystals synthesized by sol-gel technique with different concentration of Ni, coercivity decreases on Ni addition and magnetic properties were observed because of oxygen vacancies and bound magnetic polarons (BMP) ( Figure 6). The defects confirmed by PL analysis and Langevin fitting used to determine the concentration of BMPs arising. The higher magnetic moment found in Ni-doped TiO 2 revealed to show applications in magneto-optics and spintronics [36]. Hou et al. synthesized Ni-doped anatase TiO 2 thin film by reactive magnetron sputtering on silica substrate. He investigated the effect of Ni-doped on RTFM which shows high correlation between oxygen vacancies [37].     Rodriguez et al. synthesized nanoparticles of Fe doped TiO 2 anatase phase by a microemulsion method and the X-ray absorption technique confirmed the presence of Fe 3+ at Ti 4+ site in the TiO 2 system. Using the Mossbauer and magnetic measurements, he observed that electronic defects also affect the ferromagnetic ordering along with oxygen defects and magnetic ions [38]. Chen et al. showed that oxygen vacancies increase the FM in the rutile structured Fe doped TiO 2 through DFT calculations. More profound defects due to oxygen vacancies or trapping of electrons by Fe atom origin of DMS [39]. Santara et al. synthesized Fe doped TiO 2 nanoribbons mixed-phase rutile and anatase by solvothermal method and observed strong RTFM because of oxygen vacancies and Fe content. RTFM reduction with Fe concentration due to superexchange interaction was mentioned in the report. In addition, it was found that a decrease in FM with increase in annealing temperature for 0.1% Fe doped TiO 2 due to migration of Ti 3+ towards the surface and oxidized to Ti 4+ which reduce the BMPs while highest magnetization (Ms) was observed on vacuum-annealing the same nanomaterials [40]. Fajariah et al. predicted FM in TiO 2 when various TM ions were introduced using DFT calculations [41]. Zero magnetic moments generated by Sc and Ni ions while maximum magnetism was observed in Fe and Mn. Rutile phased Fe doped TiO 2 was explored by Mallia et al. using hybrid-exchange DFT for DMS [42]. It was noted that oxygen stoichiometry and the presence of Fe in the TiO 2 lattice mainly affects ferromagnetism. Bapna et al. investigated the anatase form of Fe (4-8 at.%) doped TiO 2 thin film grown on silicon substrate by PLD technique for RTFM [43]. Ti 3+ defect state, oxygen vacancies and electronic structure are the main factors for RTFM. Ogale et al. explored the room temperature FM in Co-doped TiO 2 thin film [44]. Uniform distribution of Co and tiny Co cluster through the film cross-section driven the FM. Rutile structured Co:TiO 2 thin film fabricated by laser molecular beam epitaxy. Enormous generated oxygen vacancies and high spin Co 2+ ions cause charge imbalance and lattice distortion along with high charge carriers that favors high-temperature FM [45]. Song et al. investigated the effect of 6% Ga and Al as dopants on rutile TiO 2 thin film on Al 2 O 3 by PLD [46]. Magnetic polaron model explained the ferromagnetism behavior in Ga doped TiO 2 that higher the oxygen vacancies, more the Ti 3+ ions responsible for the ferromagnetism and antiferromagnetism coupling with high magnetic moment and T c . Zhu et al. grew defective anatase TiO 2 (001) facet on reduced graphene oxide (rGO) and investigated dilute RTFM behavior through the healing effect of rGO [47]. Two different types oxygen vacancies defects on surface and at subsurface was explored using first principle simulations where oxygen vacancies at subsurface attribute significant amount of FM. It also demonstrated that healing effect due to functional group of rGO removed oxygen defects at surface which cause better interface and interaction with oxygen vacancies at subsurface enhanced the same. Some of the recent reports are summarized in Table 1.

SnO 2 and Ferromagnetism
SnO 2 has various useful properties alike O-vacancies, pellucidity, excellent charge carrier, high thermal and chemical stability due to which it is used for spin electronic devices [48][49][50]. SnO 2 nanoparticles have various properties than their bulk oxide, such as the size of SnO 2 is reduced to nanoparticle range, then non-ferromagnetic oxide shows ferromagnetic character [26,51]. Chang et al. investigated that undoped tin oxide shows the ferromagnetic character at room temperature and also explained the importance of O-vacancy in ferromagnetic nature [52]. When the size of various DMS reduced to below 20 nm, then they show excellent ferromagnetic character as compared to when the size is more significant than 100 nm [53].
The magnetic and electronic characteristics of Mo doped with SnO 2 is studied with the help of gradient approximation from density functional theory. Spin functionality is induced on the DOS when the Sn atom is substituted by Mo atom in the crystal structure and this substitution is helpful to make easy the hybridization between the d-orbital of Mo and p-orbital of O atom. Due to this p-d hybridization, in d-state of splitting Mo atom spin that is responsible for antisymmetric DOS at the E F and this splitting in the high range can generate the magnetism at the site of Mo atom. These samples show excellent spin polarization value at the E F . Due to excellent spin polarization value, it can be used in devices that are based on electronic-spin for future respect [54].
The superconducting quantum interference device (SQUID) measurements 5K and 300K for 1% and 2% Fe doped SnO 2 nanoparticles synthesized via chemical co-precipitation method indicate negative effect of doping due to the substitution of dopant ions in the host lattice [55]. Importantly, electron trapped in oxygen vacancies due to Fe 3+ -Vo 2− -Fe 3+ cluster in the host lattice will compensate with two irons through F centers and leads to surprising FM ordering. Similarly, surfactant-less Co-doped SnO 2 nanocrystals prepared using hydrothermal process showed that introduction of Co (II) ions deforms the SnO 2 lattice resulting in reduction in unit cell volume with Co content without the Co-rich clusters. Linear magnetic response with maximum saturation magnetization (3.5 emu/g) of the flower-like morphology with a Co/SnO 2 mass ratio = 0.06 m/m was found. Ogale et al. predicted that Co-doped SnO 2 films synthesized by pulsed laser deposition technique grown on R plane sapphire show ferromagnetism at high-temperature. The 5% Co-doped film shows a high magnetic moment with a considerable value of 7 ± 5 µB per Co ion and Curie temperature (T c ) at 650 K [12]. 5% Co-doped SnO 2 was synthesized by Srinivas et al. using the tartaric gel method and investigated RTFM due to oxygen vacancies, vacancy clusters and surface diffusion of Co ions [56]. As the concentration of Co ions increases, the magnetic character also increases in Co-doped SnO 2 and as Mn ions doped with Co ions in SnO 2 lattice, the samples show superparamagnetic character instead of ferromagnetic character ( Figure 7) [57].
Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 24 al. investigated that undoped tin oxide shows the ferromagnetic character at room temperature and also explained the importance of O-vacancy in ferromagnetic nature [52]. When the size of various DMS reduced to below 20 nm, then they show excellent ferromagnetic character as compared to when the size is more significant than 100 nm [53]. The magnetic and electronic characteristics of Mo doped with SnO2 is studied with the help of gradient approximation from density functional theory. Spin functionality is induced on the DOS when the Sn atom is substituted by Mo atom in the crystal structure and this substitution is helpful to make easy the hybridization between the d-orbital of Mo and p-orbital of O atom. Due to this p-d hybridization, in d-state of splitting Mo atom spin that is responsible for antisymmetric DOS at the EF and this splitting in the high range can generate the magnetism at the site of Mo atom. These samples show excellent spin polarization value at the EF. Due to excellent spin polarization value, it can be used in devices that are based on electronic-spin for future respect [54].
The superconducting quantum interference device (SQUID) measurements 5K and 300K for 1% and 2% Fe doped SnO2 nanoparticles synthesized via chemical co-precipitation method indicate negative effect of doping due to the substitution of dopant ions in the host lattice [55]. Importantly, electron trapped in oxygen vacancies due to Fe 3+ -Vo 2− -Fe 3+ cluster in the host lattice will compensate with two irons through F centers and leads to surprising FM ordering. Similarly, surfactant-less Codoped SnO2 nanocrystals prepared using hydrothermal process showed that introduction of Cо (II) ions deforms the SnO2 lattice resulting in reduction in unit cell volume with Co content without the Co-rich clusters. Linear magnetic response with maximum saturation magnetization (3.5 emu/g) of the flower-like morphology with a Co/SnO2 mass ratio = 0.06 m/m was found. Ogale et al. predicted that Co-doped SnO2 films synthesized by pulsed laser deposition technique grown on R plane sapphire show ferromagnetism at high-temperature. The 5% Co-doped film shows a high magnetic moment with a considerable value of 7 ± 5 μB per Co ion and Curie temperature (Tc) at 650 K [12]. 5% Co-doped SnO2 was synthesized by Srinivas et al. using the tartaric gel method and investigated RTFM due to oxygen vacancies, vacancy clusters and surface diffusion of Co ions [56]. As the concentration of Co ions increases, the magnetic character also increases in Co-doped SnO2 and as  The value of lattice constants and the absorption gap are changed as the concentration of Mn ions increases in SnO2 films and also shows paramagnetic character, which is confirmed by magnetization measurements [58]. When SnO2 is doped with some 3d-transition metals such as Cr, Mn, Fe, Co and Ni ions, then it shows the ferromagnetic character at room temperature and these samples can be synthesized via pulsed-layer deposition method. Among these metal ions, Fe shows The value of lattice constants and the absorption gap are changed as the concentration of Mn ions increases in SnO 2 films and also shows paramagnetic character, which is confirmed by magnetization measurements [58]. When SnO 2 is doped with some 3d-transition metals such as Cr, Mn, Fe, Co and Ni ions, then it shows the ferromagnetic character at room temperature and these samples can be synthesized via pulsed-layer deposition method. Among these metal ions, Fe shows the highest magnetic moment and also found that the magnetic moment is independent with the concentration of doping ions [59]. Roy et al. investigated that when SnO 2 is doped with all 3d-transition metals one by one, then all 3d-transition metals show magnetic character whose moment values are from 0.8 µB to 5.95 µB except Sc, Ti and Ni transition metals (Figure 8). It was found that electronegativity of metal dopants show significant role for ferromagnetic behavior. Among the seven transition metals, Mn shows the highest magnetic moment and Cu shows the lowest magnetic moment value. Remaining all five transition metals shows more stable ferromagnetic states than anti-ferromagnetic states and also observed that Zn doped on SnO 2 have the highest stable ferromagnetic states among all transition metals which can be used as one of the suitable spintronics material [29]. Furthermore, Delgado et al. reported undoped and Zn-doped SnO 2 with flower-like morphology from acicular nanoparticles (NPs) synthesized via hydrothermal method. The morphology of the NPs can be tuned on Zn-doping from truncated rods to sharp needles due to surface free energy of the NPs' facets and growth direction. Higher ferromagnetic ordering was observed in Zn-doped samples due to Zn in Sn lattice positions (ZnSn) as compared with the undoped formulation. Further investigation using thermal annealing under reducing pressure leads to the an excess of V O , while, reduced magnetization suggests ZnSn defects controlled magnetism [60].  Similarly, Co 2+ -doped SnO2 shows an increase in magnetization with the Co 2+ ions and the average crystallite size estimated to be 10 ± 2 nm for the synthesized nanocrystals. In addition to this, Mn can also influence the magnetization of this composition, i.e., Mn 2+ ions transform the ferromagnetism to superparamagnetism. On the contrary, Sn0.83Mn0.12Co0.05O2 exhibits diamagnetism with Mn 2+ ions [57]. M-H (magnetic field-magnetization) hysteresis loop demonstrated the room temperature ferromagnetism for the mixed doped (Co, Zn) SnO2 composition. First-principles calculation estimates the increase of total magnetic moment with effective magnetic interactions between metal ions and oxygen atoms as suggested by experimental methods [61]. The doping of Cr 3+ at Sn 4+ lattice in SnO6 octahedra enhances the defects, i.e., oxygen vacancies to generate the RTFM via magnetic exchange interaction meditated F centers of oxygen vacancies and magnetic dopant impurities. However, excess Cr of beyond 2%, of solubility limits, decreases the magnetic moment significantly due to antiferromagnetic exchange interaction from interstitial Cr dopants governing over the BMP mechanism [62]. Interestingly, tetragonal shaped nanoparticles (10-40 nm) of Ni doped SnO2 showed room-temperature ferromagnetic properties with remarkable Ms (5×10 −4 emu/g) and the coercive field, Hc 83-96 Oe with variable Ni content [63]. Similarly, Co 2+ -doped SnO 2 shows an increase in magnetization with the Co 2+ ions and the average crystallite size estimated to be 10 ± 2 nm for the synthesized nanocrystals. In addition to this, Mn can also influence the magnetization of this composition, i.e., Mn 2+ ions transform the ferromagnetism to superparamagnetism. On the contrary, Sn 0.83 Mn 0.12 Co 0.05 O 2 exhibits diamagnetism with Mn 2+ ions [57]. M-H (magnetic field-magnetization) hysteresis loop demonstrated the room temperature ferromagnetism for the mixed doped (Co, Zn) SnO 2 composition. First-principles calculation estimates the increase of total magnetic moment with effective magnetic interactions between metal ions and oxygen atoms as suggested by experimental methods [61]. The doping of Cr 3+ at Sn 4+ lattice in SnO 6 octahedra enhances the defects, i.e., oxygen vacancies to generate the RTFM via magnetic exchange interaction meditated F centers of oxygen vacancies and magnetic dopant impurities. However, excess Cr of beyond 2%, of solubility limits, decreases the magnetic moment significantly due to antiferromagnetic exchange interaction from interstitial Cr dopants governing over the BMP mechanism [62]. Interestingly, tetragonal shaped nanoparticles (10-40 nm) of Ni doped SnO 2 showed room-temperature ferromagnetic properties with remarkable M s (5 × 10 −4 emu/g) and the coercive field, H c 83-96 Oe with variable Ni content [63]. SnO 2 doping with Ce exhibits room-temperature ferromagnetic behavior [64] which increases with Ce concentration due to exchange mechanism between 4f and 5d conduction electrons ( Figure 9). Notably, divergence between the ZFC and FC measurements concluded that both the Ce ions and V O attribute to RT ferromagnetism. Kumar et al. investigated the codoping with rare earth, Er and fluoride ion also induced strong RT ferromagnetism in SnO 2 as compared to Er doped SnO 2 [65]. Observed RTFM is due to high oxygen vacancies and shallow defects present in codoping nanoparticles. Conversely, rare-earth doped SnO 2 including Sm-SnO 2 showing ferromagnetism at RT can also exhibit utilization in magneto-optoelectronic devices [64][65][66]. Nomura et al. investigated the effects of codoping on SnO 2 synthesized by the sol-gel route and found that coping enhanced FM as compared to a single dopant [67]. Various dopants (Mn, Co, C, Cr, Mg, V, Fe, Ni, Zn, K) have been studied in detail to use SnO 2 for RTFM [68][69][70][71][72][73][74][75][76][77][78][79][80]. Table 2 briefly summarizes the literature on SnO 2 .  SnO2 doping with Ce exhibits room-temperature ferromagnetic behavior [64] which increases with Ce concentration due to exchange mechanism between 4f and 5d conduction electrons ( Figure  9). Notably, divergence between the ZFC and FC measurements concluded that both the Ce ions and VO attribute to RT ferromagnetism. Kumar et al. investigated the codoping with rare earth, Er and fluoride ion also induced strong RT ferromagnetism in SnO2 as compared to Er doped SnO2 [65]. Observed RTFM is due to high oxygen vacancies and shallow defects present in codoping nanoparticles. Conversely, rare-earth doped SnO2 including Sm-SnO2 showing ferromagnetism at RT can also exhibit utilization in magneto-optoelectronic devices [64][65][66]. Nomura et al. investigated the effects of codoping on SnO2 synthesized by the sol-gel route and found that coping enhanced FM as compared to a single dopant [67]. Various dopants (Mn, Co, C, Cr, Mg, V, Fe, Ni, Zn, K) have been studied in detail to use SnO2 for RTFM [68][69][70][71][72][73][74][75][76][77][78][79][80]. Table 2 briefly summarizes the literature on SnO2.

Doping in ZnO and Ferromagnetism
ZnO is n-type semiconductor with a wide band of 3.30 eV (exciton binding energy = 60 meV). The benefit of these materials having high stability for high-temperature uses and good photo-response and well-known doping and defect studies makes ZnO smart materials for spintronic application as compared to II-VI compounds [26,[82][83][84]. ZnO exhibits room-temperature ferromagnetism, while the intensity of the ferromagnetism enhanced in the presence of a magnetic field during the synthesis process along with surface morphology changes [84]. Furthermore, FM in ZnO doped with TM ions is primarily due to defect arrangements [85,86]. Additionally, incorporation of Zn 2+ ions in ZnO thin film enhanced RTFM synthesized on Si wafer by RF sputtering method leads to defects in the lattice, causing an alteration in electrical and magnetic properties [87]. This magnetic behavior is usually arbitrated due to defects on insertion of TM ions as dopants and hence, termed as defect-induced d 0 FM. While the exact mechanism is still controversial. X-ray centered microscopy and spectroscopy techniques can be helpful to find the intrinsic origin of d 0 FM in undoped ZnO, which would be attributed to the O2p orbitals arising from zinc vacancies (V Zn ). d 0 FM can be promoted by stabilize V Zn in ZnO, appropriate TM ions doping, maintaining crystalline structure [84]. Dietl et al. reported ferromagnetism in metal nitrides and oxides, p-type Mn-doped ZnO and GaN semiconductors by theoretical calculations using local spin density approximation [88,89]. In addition, it was experimentally revealed that the Curie temperature (T c ) and exchange interaction are efected by number of holes in the valence band above room temperature.
Sharma et al. have reported the synthesis of hexagonal phase of Er (1, 3, 5, 7%) doped ZnO cone-like nanostructures using the wet chemical route. Ferromagnetic hysteresis loops were observed on Er doped ZnO. Moreover, saturation magnetization (M S ), remanent magnetization (M R ) and the coercivity (H C ) increase with Er concentration (Figure 10) shows DMS properties [90].
Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 24 Sharma et al. have reported the synthesis of hexagonal phase of Er (1, 3, 5, 7%) doped ZnO conelike nanostructures using the wet chemical route. Ferromagnetic hysteresis loops were observed on Er doped ZnO. Moreover, saturation magnetization (MS), remanent magnetization (MR) and the coercivity (HC) increase with Er concentration (Figure 10) shows DMS properties [90]. Hexagonal wurtzite structure has been reported for Fe doped ZnO studied by high-temperature route and shows room temperature magnetization behavior on the addition of Fe ion into ZnO lattice as shown in Figure 11   Hexagonal wurtzite structure has been reported for Fe doped ZnO studied by high-temperature route and shows room temperature magnetization behavior on the addition of Fe ion into ZnO lattice as shown in Figure 11  Sharma et al. have reported the synthesis of hexagonal phase of Er (1, 3, 5, 7%) doped ZnO conelike nanostructures using the wet chemical route. Ferromagnetic hysteresis loops were observed on Er doped ZnO. Moreover, saturation magnetization (MS), remanent magnetization (MR) and the coercivity (HC) increase with Er concentration (Figure 10) shows DMS properties [90]. Hexagonal wurtzite structure has been reported for Fe doped ZnO studied by high-temperature route and shows room temperature magnetization behavior on the addition of Fe ion into ZnO lattice as shown in Figure 11     Fe doped zinc oxide promotes higher Ms values than undoped due to defects and exchange interaction Fe 3+ ions via BMP mechanism with conductive electrons of ZnO which leads to the spin polarization [92]. The M vs. H hysteresis loop at RT shows paramagnetic behavior in the Fe-doped ZnO samples ( Figure 10) and the signal increases Fe concentration [26]. Fe (1, 3, 5, 7%) doped ZnO was synthesized by a solid-state high-temperature method. With the increase in the concentration Fe, the inverted spinel ZnFe2O4 phase was obtained, which results in a gradual rise in the paramagnetic behavior in the ZFO samples [91]. It was observed that magnetization also depends upon synthetic procedure and for obtaining strong FM properties, low-temperature method is more suitable than a solid state synthetic method [66,69]. However, Torquato et al. reported DMO Co-doped ZnO with variable concentration achieved by combustion reaction claimed ferromagnetic behavior above room temperature. Co +2 doping increases Ms and also Tc, which showed that this materials can be beneficial for DMS [93]. Co-doped ZnO thin films were synthesized by dip-coating technique and showed strong ferromagnetism. Various defects, oxygen vacancies and zinc interstices increase with the increase in Co concentration, which supports suitable candidates for room-temperature ferromagnetism [94].
RT ferromagnetism is shown by Mn-doped ZnO nanoparticles films which was synthesized by sol-gel dip coating method at different withdrawal speed [95]. As the withdrawal speed of the coating increases, the bandgap of Mn-doped ZnO decreases from 3.74 eV to 2.76 eV and magnetic behavior increases with the thickness of the thin film. Strong magnetic ordering is disclosed to room temperature (RT) by the Er implanted and annealed ZnO nanoparticles. Due to intrinsic defects, ZnO shows ferromagnetic behavior, which is responsible for the mediator in the magnetic ordering in Er and annealed zinc oxide [96]. The common reason responsible for magnetic response in the oxides is due to the existence of oxygen vacancy. For the continuation of steady magnetic behavior, extensive effort and inclusion of dopants must be demanding conditions in these oxides [97]. Due to doping of Mn is doped in ZnO, the shape and extent of the electron spin resonance (ESR) spectra are changed. From these changes, it shows that a dilute magnetic semiconductor character is fabricated in ZnO. It has been reported that the concentration of doped Mn is 0.6%, then it shows a sharp resonance peak Fe doped zinc oxide promotes higher M s values than undoped due to defects and exchange interaction Fe 3+ ions via BMP mechanism with conductive electrons of ZnO which leads to the spin polarization [92]. The M vs. H hysteresis loop at RT shows paramagnetic behavior in the Fe-doped ZnO samples ( Figure 10) and the signal increases Fe concentration [26]. Fe (1, 3, 5, 7%) doped ZnO was synthesized by a solid-state high-temperature method. With the increase in the concentration Fe, the inverted spinel ZnFe 2 O 4 phase was obtained, which results in a gradual rise in the paramagnetic behavior in the ZFO samples [91]. It was observed that magnetization also depends upon synthetic procedure and for obtaining strong FM properties, low-temperature method is more suitable than a solid state synthetic method [66,69]. However, Torquato et al. reported DMO Co-doped ZnO with variable concentration achieved by combustion reaction claimed ferromagnetic behavior above room temperature. Co +2 doping increases M s and also T c, which showed that this materials can be beneficial for DMS [93]. Co-doped ZnO thin films were synthesized by dip-coating technique and showed strong ferromagnetism. Various defects, oxygen vacancies and zinc interstices increase with the increase in Co concentration, which supports suitable candidates for room-temperature ferromagnetism [94].
RT ferromagnetism is shown by Mn-doped ZnO nanoparticles films which was synthesized by sol-gel dip coating method at different withdrawal speed [95]. As the withdrawal speed of the coating increases, the bandgap of Mn-doped ZnO decreases from 3.74 eV to 2.76 eV and magnetic behavior increases with the thickness of the thin film. Strong magnetic ordering is disclosed to room temperature (RT) by the Er implanted and annealed ZnO nanoparticles. Due to intrinsic defects, ZnO shows ferromagnetic behavior, which is responsible for the mediator in the magnetic ordering in Er and annealed zinc oxide [96]. The common reason responsible for magnetic response in the oxides is due to the existence of oxygen vacancy. For the continuation of steady magnetic behavior, extensive effort and inclusion of dopants must be demanding conditions in these oxides [97]. Due to doping of Mn is doped in ZnO, the shape and extent of the electron spin resonance (ESR) spectra are changed. From these changes, it shows that a dilute magnetic semiconductor character is fabricated in ZnO. It has been reported that the concentration of doped Mn is 0.6%, then it shows a sharp resonance peak and if the concentration of doped Mn is greater than 0.8%, then there is a decrease in the intensity ( Figure 13) [98]. Hence, the appropriate level of dopants in metal oxide lattice help to find DMSOs.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 15 of 24 and if the concentration of doped Mn is greater than 0.8%, then there is a decrease in the intensity ( Figure 13) [98]. Hence, the appropriate level of dopants in metal oxide lattice help to find DMSOs. Zinc oxide films and copper-doped zinc oxide films was casted using screen-printing techniques a eco-friendly. As copper is doped on zinc oxide films, it increases the O-vacancies, which was confirmed by EPR signal obtained at g value 2.0018 [99]. Hence, these materials may be used in spin electronics and electro-optics devices. Table 3 provides a brief summary of the literature on ZnO.  Zinc oxide films and copper-doped zinc oxide films was casted using screen-printing techniques a eco-friendly. As copper is doped on zinc oxide films, it increases the O-vacancies, which was confirmed by EPR signal obtained at g value 2.0018 [99]. Hence, these materials may be used in spin electronics and electro-optics devices. Table 3 provides a brief summary of the literature on ZnO.

Doping in In 2 O 3 and Ferromagnetism
Several reports demonstrating ferromagnetism at RT for TM-ion doped-In 2 O 3 bulk/thin film were available in the literature [3,100,101]. In this way, ferromagnetism shown by Laser ablated transition-metal (TM)-doped In 2 O 3 thin films grafted on MgO and Al 2 O 3 substrates [102], milled In 2 O 3 powder [103], to name a few.
The RF-sputtered film of Fe, Cu codoped In 2 O 3 shows interesting local structural, optical, magnetic and transport properties [104]. Importantly, interexchange mechanism and overlapping of polarons via BMP explained the observed ferromagnetism. However, changing the synthesis protocol such as spin coating techniques and concentration of Fe highly affect the magnetic properties ( Figure 14) [105]. As the concentration of Fe increases FM decreases due to disappearance of nearest neighbor hopping (NNH) conduction, wheras sol gel method used for Fe doped In 2 O 3 followed by annealing at 300-600 • C in presence of H 2 showed similar magnetization behavior [106]. Strong interaction of localized electrons with Fe ions to polarize its spin cause magnetic behavior and ferromagnetism decreases as oxygen vacancies decreases on increasing Fe concentration.

Doping in In2O3 and Ferromagnetism
Several reports demonstrating ferromagnetism at RT for TM-ion doped-In2O3 bulk/thin film were available in the literature [3,100,101]. In this way, ferromagnetism shown by Laser ablated transition-metal (TM)-doped In2O3 thin films grafted on MgO and Al2O3 substrates [102], milled In2O3 powder [103], to name a few.
The RF-sputtered film of Fe, Cu codoped In2O3 shows interesting local structural, optical, magnetic and transport properties [104]. Importantly, interexchange mechanism and overlapping of polarons via BMP explained the observed ferromagnetism. However, changing the synthesis protocol such as spin coating techniques and concentration of Fe highly affect the magnetic properties ( Figure  14) [105]. As the concentration of Fe increases FM decreases due to disappearance of nearest neighbor hopping (NNH) conduction, wheras sol gel method used for Fe doped In2O3 followed by annealing at 300-600 °C in presence of H2 showed similar magnetization behavior [106]. Strong interaction of localized electrons with Fe ions to polarize its spin cause magnetic behavior and ferromagnetism decreases as oxygen vacancies decreases on increasing Fe concentration. Some other methods, such as 5% Fe-doped In2O3 films which were pulsed laser deposited under a partial pressure of 10 −3 , 10 −5 and 10 −7 torr, respectively [107] and polarized neutron scattering measurements reveal lower magnetized Fe-rich phase located at the interface than uniformly distributed phases. A similar study was reported by Garnet et al. showing ferromagnetic ordering with decreased saturation magnetization with concentration for Fe in In2O3 nanocrystalline films prepared by the sol-gel method [108]. Similarly, Fe doped In2O3 synthesized by solid-state reaction method and vacuum annealing with various levels of dopants [109]. Mn-doped In2O3 was synthesized by solid-state methods and that composition showed magnetic moment of 2.83 μB/Mn due to tetrahedrally or octahedrally coordinated Mn 3+ in the intermediate spin state [110]. A mesoporous In2O3 semiconductor implanted with Co ions shows a measurable ferromagnetic signature at RT [111]. Co-doped In2O3 synthesized by chemical solution route showed FM at RT and Some other methods, such as 5% Fe-doped In 2 O 3 films which were pulsed laser deposited under a partial pressure of 10 −3 , 10 −5 and 10 −7 torr, respectively [107] and polarized neutron scattering measurements reveal lower magnetized Fe-rich phase located at the interface than uniformly distributed phases. A similar study was reported by Garnet et al. showing ferromagnetic ordering with decreased saturation magnetization with concentration for Fe in In 2 O 3 nanocrystalline films prepared by the sol-gel method [108]. Similarly, Fe doped In 2 O 3 synthesized by solid-state reaction method and vacuum annealing with various levels of dopants [109]. Mn-doped In 2 O 3 was synthesized by solid-state methods and that composition showed magnetic moment of 2.83 µB/Mn due to tetrahedrally or octahedrally coordinated Mn 3+ in the intermediate spin state [110]. A mesoporous In 2 O 3 semiconductor implanted with Co ions shows a measurable ferromagnetic signature at RT [111]. Co-doped In 2 O 3 synthesized by chemical solution route showed FM at RT and LT [112]. It is observed that the Bohr magneton number remained consistent at a lower doping concentration of Co 2+ in the high spin state while it decreased on the addition of Co 3+ ions. On the contrary, Figure 15 shows the N-doped In 2 O 3 films, which exhibited room-temperature (RT) ferromagnetism and Mott variable range hopping (VRH) transport behavior [113]. Moreover, alkali metal-based doping such as Li-doped In 2 O 3 nanoparticles exhibits d 0 ferromagnetism at room temperature via FM coupling exerted by the LiIn-ONN-VIn-ONN-LiIn chains [114]. Table 4 provides a short summary on recent reports on In 2 O 3 .
LT [112]. It is observed that the Bohr magneton number remained consistent at a lower doping concentration of Co 2+ in the high spin state while it decreased on the addition of Co 3+ ions. On the contrary, Figure 15 shows the N-doped In2O3 films, which exhibited room-temperature (RT) ferromagnetism and Mott variable range hopping (VRH) transport behavior [113]. Moreover, alkali metal-based doping such as Li-doped In2O3 nanoparticles exhibits d 0 ferromagnetism at room temperature via FM coupling exerted by the LiIn-ONN-VIn-ONN-LiIn chains [114]. Table 4 provides a short summary on recent reports on In2O3. Figure 15. Magnetic hysteresis loops of N-doped In2O3 films with 2 and 5 at.% ratio of N recorded at 300 K (Reprinted with permission from American Chemical Society [113]).

Conclusions
To summarize, the review article has attempted to provide the insights regarding the structural analysis, magnetic properties of DMSO with various ferromagnetic or paramagnetic dopants into binary metal oxide nanomaterials which exhibit a ferromagnetic behavior at or above room temperature. Several factors govern the DMS in which oxygen vacancies, interstitial defects, particle size, especially in the nano range, synthesis methods are the probable reasons. It is believed the ferromagnetism arises due to the exchange interactions between unpaired electron spins emerging from oxygen vacancies at the surfaces of the nanomaterials. The oxygen vacancies present at the surface and subsurface, the presence of deeper, shallower and grain boundary barrier defects, interstitial metal defects, charge imbalance, formation of metallic clusters, electronic defects are responsible for FM even annealing temperature, with or without vacuum largely affecting it. Further, exchange interaction between localized electron spin moment with oxygen vacancy and annealing the NMs in the presence of magnetic field from North to South pole also enhances the FM. Morever, size of the nanomaterials reduced to below 20 nm, excellent FM was achieved at or above room temperature. The probable mechanism of DMS is still debatable and controversial field among the various researchers. Most of the host materials as the dopant concentration increase at concentration level below 10%, it found suitable for DMS. The origin of DMSO could be explained on the basis of the BMP theory. However, still more work has to done for clear understaning and governing of defects-induced ferromagnetism in non-magnetic bulk materials. Importantly, these materials can be easily integrated in spin-devices for long life-time of spin-alignment and additionaly, it is proposed that can also overcome the spin-scattering at the interfaces to avoid data loss. Off course, these nanomaterials are a potential materials for spintronics device due high spin polarization and high Tc without the doping of expensive inner-transition metal in general. There is much promising in this field as it may open a novel gateway for feasible efficient spintronics at room temperature in nanometric regime.
Author Contributions: A.G. and R.Z. did extensive research on literature. P.K. worked on conceptualization and validation. V.K. and A.K. worked on review and overall editing of manuscript. All authors have read and agreed to the published version of the manuscript.