Ferrimagnetism and Ferroelectricity in Cr-Substituted GaFeO3

Feb 1, 2018 - GaFeO3-type iron oxides are promising multiferroic materials due to the coexistence of a large spontaneous magnetization and polarizatio...
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Cite This: Chem. Mater. 2018, 30, 1436−1441

Ferrimagnetism and Ferroelectricity in Cr-Substituted GaFeO3 Epitaxial Films Tsukasa Katayama,* Shintaro Yasui, Takuya Osakabe, Yosuke Hamasaki, and Mitsuru Itoh Laboratory for Materials and Structures, Tokyo Institute of Technology, 4259-J2-19, Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan S Supporting Information *

ABSTRACT: GaFeO3-type iron oxides are promising multiferroic materials due to the coexistence of a large spontaneous magnetization and polarization near room temperature. However, magnetic substitution, which is a general method to control multiferroic properties, is difficult due to instability of the substituted GaFeO3. In this study, Ga0.5Cr0.5FeO3 epitaxial thin films are successfully fabricated through epitaxial stabilization. These films exhibit in-plane ferrimagnetism and out-of-plane ferroelectricity simultaneously. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements of the Ga0.5Cr0.5FeO3 film reveal that the oxidation states of the Fe and Cr ions are trivalent. In addition, some Fe ions are located at tetrahedral Ga1 sites. Compared to the GaFeO3 film, the Ga0.5Cr0.5FeO3 film shows a higher magnetic phase transition temperature (240 K), weaker saturation magnetization at 5 K, and a unique temperature dependence of the magnetization behavior. The effects of Cr substitution on the magnetic properties are strongly affected by the sites of the Fe3+ (3d5) and Cr3+ (3d3) ions. Furthermore, room-temperature ferroelectricity in the GaFeO3 and Ga0.5Cr0.5FeO3 films was demonstrated. Interestingly, the change in the ferroelectric parameters via Cr substitution is very small, which disagrees with the previously proposed polarization switching mechanism. Our findings are key to understanding the genuine polarization switching mechanism of the multiferroic GaFeO3 system.

1. INTRODUCTION Multiferroic materials simultaneously exhibiting electric and magnetic orders in a single phase have been eagerly studied due to their fascinating physics and future technological applications such as fast writing, power savings, and nondestructive data storage.1−4 However, such multiferroic materials rarely exhibit both spontaneous magnetization and polarization when the temperature is not low. GaFeO3-type iron oxides are promising multiferroic materials due to the coexistence of a large spontaneous magnetization and polarization near and above room temperature as well as their multiferroic properties such as magnetic-field-induced modulation of polarization.5−16 GaFeO3 has an orthorhombic structure with Pna21 (no. 33) and consists of one tetrahedral (Td) Ga1 site and three distorted octahedral (Oh) Ga2, Fe1, and Fe2 sites (Figure 1). It exhibits ferrimagnetism with a high magnetic transition temperature due to the superexchange interaction between the Fe ions at the Ga1 and Fe1 sites and those at the Ga2 and Fe2 sites. The ferrimagnetism can also be adjusted to be above room temperature by changing the Ga/Fe ratio.5−8 Furthermore, spin−orbit coupling due to the strong hybridization between the orbitals of Fe 3d5 at the Fe2 site and O 2p leads to a large coercive field (Hc) in the a-axis direction.9,10 Additionally, polarization switching is realized at room temperature in GaFeO3 films along the c-axis by applying a high electric field.11−13 © 2018 American Chemical Society

Figure 1. Crystal structure of GaFeO3, which consists of one tetrahedral Ga1 site and three octahedral Ga2, Fe1, and Fe2 sites. Spontaneous polarization and magnetization appear in the c- and a-axis directions, respectively.

In GaFeO3-type oxides, the magnetic moment of Fe plays a key role in not only the magnetic but also the multiferroic properties. Thus, substitution of 3d magnetic elements is expected to effectively control and expand the magnetic and dielectric properties in GaFeO3-type oxides. Indeed, magnetic element substitutions such as Cr, Mn, and Co for nonmagnetic Ga enhance TC of GaFeO3.17−21 However, the maximum substitution content x in Ga1−xMxFeO3 (M = Cr, Mn, and Co) Received: January 11, 2018 Revised: January 31, 2018 Published: February 1, 2018 1436

DOI: 10.1021/acs.chemmater.8b00144 Chem. Mater. 2018, 30, 1436−1441

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Chemistry of Materials has been limited to 0.1, and the substitution effect on ferroelectricity has not been investigated. To realize a large amount of substitution and ferroelectric measurements, an epitaxial thin-film growth technique is useful because interfacial strain due to a lattice mismatch between the film and the substrate enables stabilization of the metastable phase and control of the polarization orientation in out-of-plane direction. Herein, we report the roles of the chemical substituted Cr ions and the host Fe ions on the magnetic property in the GaFeO 3 -type structure by successfully fabricating Ga0.5Cr0.5FeO3 epitaxial thin films, which can be formed via epitaxial stabilization. Adaptation of high quality thin film allows ferroelectric measurements with little leakage current owing to a higher speed and smaller applied bias as compared to bulk measurements. The film has a c-axis orientation with three kinds of in-plane domains. X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements reveal that the oxidation states of the Fe and Cr ions are trivalent and some Fe ions are located at the Td Ga1 sites in the Ga0.5Cr0.5FeO3 film. These features differ from those of GaFeO3. The Ga0.5Cr0.5FeO3 film exhibits ferrimagnetism along the in-plane direction with a Curie temperature (TC) of 240 K. Through Cr substitution, the TC value increases due to increased amount of magnetic ions, but saturated magnetization at 5 K decreases due to increase of Fe at the Ga1 site compared to the GaFeO3 film. Additionally, the Ga0.5Cr0.5FeO3 film exhibits ferroelectricity along the c-axis at room temperature. Furthermore, magnetocapacitance effect is observed in the film.

Figure 2. (a) Out-of-plane 2θ−θ and (b) ϕ scan patterns for the Ga0.5Cr0.5FeO3 film.

the polarization of GaFeO3-type oxides appears along the caxis.5 To determine the lattice parameters, in-plane XRD measurements were also conducted. The Ga0.5Cr0.5FeO3 film has an orthorhombic structure with a = 5.06, b = 8.75, and c = 9.386 Å. The c-axis length is slightly smaller than that of the GaFeO3 film (9.398 Å) because Cr3+ has a smaller ionic radius (0.615 Å) than that of Ga3+ (0.62 Å). The in-plane orientation relationship between the film and substrate was evaluated using the XRD ϕ scans around the film{201} and STO{110} diffraction peaks (Figure 2b). The 201 diffraction peaks of the Ga0.5Cr0.5FeO3 film appear 6 times at every 60°, indicating that the film contains multiple in-plane domains. Each domain has a relation with the a-axis along the [112̅]STO, [12̅1]STO, or [2̅11]STO direction. This type of multiple in-plane domain is typically observed in GaFeO3type oxide films grown on STO(111) substrates (e.g., GaxFe2−xO3, AlFeO3, and InxFe2−xO322−24). Figure 3a shows the Fe L-edge XAS spectrum of the Ga0.5Cr0.5FeO3 film. Additionally, the XAS spectra of γ-Fe2O3 with both Oh and Td Fe3+ sites (with a 5:3 ratio), α-Fe2O3 with only Oh Fe3+ sites, and FeO with Fe2+ sites are shown as references.25−27 The spectrum of the Ga0.5Cr0.5FeO3 film has a similar peak shape and position as those of the reference Fe3+ oxides, indicating that the Fe ions in the film are in the trivalent

2. EXPERIMENTAL METHODS GaFeO3-type Ga0.5Cr0.5FeO3 and GaFeO3 thin films were fabricated on SrTiO3(111), (STO(111)), and 0.5 wt % Nb-doped STO(111) (Nb:STO(111)) substrates through pulsed laser deposition (PLD) using the fourth-harmonic wave of a Nd:YAG laser with a laser fluence of 2.4 J/cm2 per shot (5 Hz, wavelength = 266 nm). As PLD targets, Ga0.5Cr0.5FeO3 and GaFeO3 ceramic pellets were used. The pellets were prepared from mixed powders of α-Fe2O3(99.99%, Rare Metallic Co.), Cr2O3 (99.99%, Rare Metallic Co.), and Ga2O3(99.99%, Rare Metallic Co.) by a solid-state reaction (sintering at 1350 °C). The substrate temperature and oxygen partial pressure during deposition were 700 °C and 200 mTorr, respectively. Crystal structures of the films were determined using high-resolution X-ray diffraction (XRD) with Cu Kα1 radiation (Rigaku Smartlab). XAS and XMCD measurements were conducted at Beamline 16A at the Photon Factory, KEK. The XAS and XMCD spectra were measured at 35 K using the total electron-yield method. All the XAS and XMCD measurements were performed in an applied magnetic field (H) of 50 kOe, where H was parallel to the X-ray propagation direction. The inplane magnetization of the films was measured using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design Co. MPMS XL). The magnetization versus temperature (M−T) curves were obtained in H of 500 Oe. The ferroelectric properties of the films were investigated using a ferroelectric tester (Toyo Corporation FCE-1E). The dielectric properties were measured with a precision LCR meter (Agilent 4284A). In these measurements, a 100 μm-diameter Pt electrode and Nb:STO(111) substrate were used as the top and bottom electrodes, respectively.

3. RESULTS AND DISCUSSION 3.1. Structural Characterization. Figure 2a shows the out-of-plane 2θ−θ XRD pattern of the Ga0.5Cr0.5FeO3 film. The 00l diffraction peaks are observed without impurity peaks, indicating that the film has a c-axis orientation. This orientation is preferred for polarization switching measurements because

Figure 3. Fe L-edge (a) XAS and (b) XMCD (μ+ − μ−) spectra of the Ga0.5Cr0.5FeO3 film at 35 K in H of 50 kOe. XAS spectra of γ-Fe2O3, αFe2O3, and FeO25,26 and XMCD spectra of GaFeO3 and γ-Fe2O326,27 are included for comparison. Blue and green triangles indicate peaks due to the Fe3+ octahedral (Oh) and tetrahedral (Td) sites, respectively. 1437

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Figure 4b shows the Cr L3-edge XAS spectra taken with opposite circular polarizations, μ + and μ − , for the Ga0.5Cr0.5FeO3 film at 35 K in H of 50 kOe. The μ+ spectrum exhibits a lower intensity at 577.5 eV but higher intensity at 578.5 eV compared to the μ− spectrum. This indicates that similar to the Fe3+ ions, the total magnetic moment of the Cr3+ ions in the Ga0.5Cr0.5FeO3 film is positive.30 3.2. Magnetic Characterization. Figure 5a shows the M− H curves for the Ga0.5Cr0.5FeO3 and GaFeO3 films at 5 K. Both films show clear magnetic hysteresis loops, confirming the ferrimagnetism of the films. The coercive field (Hc) and saturated magnetization (Ms) of the Ga0.5Cr0.5FeO3 film are 4.3 kOe and 0.36 μB/f.u. at 5 K, respectively. The Ms value is lower than that of the GaFeO3 film (0.55 μB/f.u.) despite the magnetic Cr3+ substitution. In GaFeO3-type oxides, the magnetization is represented as MFe2 + MGa2 − MFe1 − MGa1, where MA (A is cation site) is the sublattice magnetization at each site.5,6 Thus, the decrease of Ms through Cr3+ substitution means an increase in MFe1 and MGa1 and/or a decrease in MFe2 and MGa2. Indeed, the XAS and XMCD measurements reveal that Cr3+ substitution increases the amount of the Fe ions at the Ga1 site, which consequently increases MGa1. On the other hand, MFe2 and MGa2 probably do not increase much through Cr3+ substitution. Although the XMCD measurements indicate that the amount of Cr3+ ions at the Fe2 and Ga2 sites is larger than that at the Fe1 and Ga1 sites, it is expected that the amount of the Fe ions at the Fe2 and Ga2 sites decreases through Cr3+ substitution. Notably, the magnetic moment of Cr3+ (3d3 high-spin) is weaker than that of Fe3+ (3d5 highspin). Therefore, one of the main causes for the decrease in Ms through Cr3+ substitution is the increase of the Fe3+ ions at Ga1 site. Figure 5b shows the field-cooling M−T curves for the Ga0.5Cr0.5FeO3 and GaFeO3 films in H of 500 Oe. When the temperature decreases, the magnetization of the GaFeO3 film rapidly increases at TC of 200 K but gradually increases below 200 K. On the other hand, the magnetization of the Ga0.5Cr0.5FeO3 film slowly increases at TC of 240 K, and the M−T curve has a peak at 60 K. The difference in the shape of the M−T curves indicates that Cr3+ substitution changes the type of ferrimagnetism. Ferrimagnetic materials show various types of M−T curves, which are derived from the difference in the temperature dependence of sublattice magnetization.6,31 Compared to GaFeO3, the Ga0.5Cr0.5FeO3 film has one additional magnetic element (Cr3+) and one additional Fe3+ site (Ga1 site). These factors play key roles in changing the M− T curve behavior. It is noteworthy that the TC value is enhanced through Cr substitution, which probably originates from the increase in the amount of magnetic ions. The temperature dependence of the magnetization was further investigated by performing M−H curve measurements as a function of temperature for the Ga0.5Cr0.5FeO3 film (Figure 5c). As the temperature increases, the Hc value rapidly decreases. Hc at 25 K (0.55 kOe) is about 8 times smaller than that at 5 K (4.3 kOe). At the peak temperature (60 K) of the M−T curve, the variation in M with H in a low magnetic field becomes larger, resulting in a larger M value at H of 500 Oe at 60 K compared to that at 25 K, as shown in Figure 5c. Consequently, the M−T curve of the Ga0.5Cr0.5FeO3 film has a peak. At 300 K, the magnetization becomes smaller, demonstrating that the XMCD spectra measured at 35 K have almost no distribution of room-temperature ferromagnetic and/or ferrimagnetic impurities such as Fe, γ-Fe2O3, and Fe3O4.

state. The L3-edge of the XAS spectrum of the Ga0.5Cr0.5FeO3 film has two peaks at 708.6 and 710 eV. The XAS peak intensity at 708.6 eV, which is due to Fe3+ at Oh sites,28 is higher than that of γ-Fe2O3 but lower than that of α-Fe2O3. Because the XAS spectrum is sensitive not only to the oxidation state but also the local bonding coordination, this result implies that the Fe ions in the Ga0.5Cr0.5FeO3 film are located both at the Oh and Td sites. Notably, the film has one Td site (Ga1 site) and three Oh sites (Ga2, Fe1, and Fe3 sites), as illustrated in Figure 1. To investigate the Fe coordination in more detail, we measured the Fe L-edge XMCD spectrum for the Ga0.5Cr0.5FeO3 film (Figure 3b). The spectrum was obtained by calculating the difference between two XAS spectra taken with opposite circular polarizations, μ+ and μ−, at 35 K in H of 50 kOe (Supporting Information, Figure 1). Figure 3b also includes the Fe L-edge XMCD spectra of GaFeO3 and γ-Fe2O3 as references for only Oh Fe3+ and a mixture of Oh and Td Fe3+ sites, respectively.26,27 The spectrum of the Ga0.5Cr0.5FeO3 film has two sharp negative peaks at 708.6 and 710.3 eV due to Fe3+ at the Oh sites and one sharp positive peak at 709.6 eV due to Fe3+ at the Td sites.26,27 Thus, the XAS and XMCD results indicate that the Fe ions in the Ga0.5Cr0.5FeO3 film are located at both Oh sites (Ga2, Fe1, and Fe2 sites) and Td sites (Ga1 sites). On the other hand, the XMCD spectrum of GaFeO3 does not have a sharp positive peak, showing that Fe ions in GaFeO3 are mostly located at the Oh sites (Ga2, Fe1, and Fe2 sites).27 Indeed, neutron diffraction analysis shows that the ratio of Fe ions at the Ga1:Ga2:Fe1:Fe2 sites in GaFeO 3 is 0.02:0.23:0.43:0.32.18 Therefore, Cr substitution into GaFeO3 affects the position of the Fe ions. Some Fe ions move to the Ga1 (Td) sites. It should be noted that the sum of the XMCD intensity at the Fe L3 edge for the Ga0.5Cr0.5FeO3 film is negative, confirming that the inner product between the total magnetic moment of Fe in the Ga0.5Cr0.5FeO3 film and H is positive. Figure 4a shows the Cr L-edge XAS spectrum of the Ga0.5Cr0.5FeO3 film together with those of Cr2O3 and CrO2 as

Figure 4. Cr L-edge (a) XAS spectrum and (b) spectra taken with opposite circular polarizations (μ+ and μ−) for the Ga0.5Cr0.5FeO3 film at 35 K in H of 50 kOe. XAS spectra of Cr2O3 and CrO229 are included for comparison.

references of the Oh Cr3+ and Cr4+ sites, respectively.29,30 The spectrum of the Ga0.5Cr0.5FeO3 film is nearly identical to that of Cr2O3, indicating that the Cr3+ ions are located at the Oh sites (Ga2, Fe1, and Fe2 sites) in the film. In general, Cr3+ ions in oxides tend to be in the Oh environment due to the 3d3 configuration. 1438

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Figure 5. (a) M−H curves at 5 K and (b) field-cooling M−T curves in H of 500 Oe for the Ga0.5Cr0.5FeO3 and GaFeO3 films. Panel b includes the coercive field of the Ga0.5Cr0.5FeO3 film as a function of T. (c) M−H curves for the Ga0.5Cr0.5FeO3 film at 5, 25, 60, 150, and 300 K.

Figure 6. (a−d) Polarization and current versus electric field curves for the GaFeO3, Ga7/8Cr1/8FeO3, and Ga0.5Cr0.5FeO3 films at 100 Hz. (e) Polarization curve measured using the PUND method for the Ga0.5Cr0.5FeO3 film. The measurements were conducted at 29 °C.

Notably, the Ga0.5Cr0.5FeO3 film shows a strong in-plane magnetic anisotropy (Supporting Information, Figure 2) because the magnetic easy axis (the a-axis for Pna21) is in the in-plane direction.5 The calculated effective magnetic anisotropy constants (Keff u ) of the Ga0.5Cr0.5FeO3 film are >7.2, 4.5, and 4.4 × 105 erg/cm3 at 5, 25, and 60 K, respectively. 3.3. Ferroelectric Characterization. Figures 6a−d show polarization and current versus electric field (P−E and I−E) curves for the GaFeO3 and Ga0.5Cr0.5FeO3 films at 29 °C with frequency of 100 Hz. The figures also include the P−E curves of the Ga7/8Cr1/8FeO3 film for comparison. All the films show clear P−E hysteresis loops. Additionally, the I−V curves have switching current peaks originating from polarization reversal at coercive electric field (Ec). These results strongly indicate the room temperature ferroelectricity of the films along c-axis direction. The P−E curve of the Ga0.5Cr0.5FeO3 film includes larger leakage current as compared to those of the other films. To confirm the ferroelectricity of the Ga0.5Cr0.5FeO3 film, positive-up-negative-down (PUND) measurements were also conducted (Figure 6e). The change of polarization at positive (negative) pulse is larger than that at up (down) pulse, showing the existence of a polarization reversal in the film. From the P− E and PUND measurements, we concluded that the Ga0.5Cr0.5FeO3 film shows out-of-plane ferroelectricity at room temperature.

The polarization switching mechanism of GaFeO3 was studied using theoretical calculations.11,32 Polarization switching is predicted to switch the cations at the Td Ga1 site with those at the Oh Ga2 sites.11,32 If the mechanism is applicable to the Ga0.5Cr0.5FeO3 film, the exchange of the Cr ions at the Oh Ga2 site for other atoms at the Td Ga1 site should occur during the polarization reversal. (Notably, Cr3+ ions prefer to be located at Ga2 site,17 which is consistent with our XMCD result.) However, this reversal is energetically unfavorable due to the 3d3 configuration of Cr3+. Thus, large change in polarization switching was expected. However, as shown in Figure 6d, change in the P−E curves via Cr substitution is very small. This indicates that the proposed polarization switching mechanism disagrees with our experimental result. In addition, another problem remains in the proposed mechanism; its predicted activation energy for the polarization switching (0.5− 0.6 eV/f.u.11,32) is so high that it does not tend to occur at room temperature. Notably, the activation energy is one order larger than those for conventional ferroelectric compounds (e.g., BaTiO3 (0.02 eV/f.u.) and PbTiO3 (0.03 eV/f.u.)33,34). Therefore, a new polarization switching mechanism, which agrees with the experimental P−E results and has lower activation energy, should be considered. 3.4. Magnetodielectric Characterization. Figure 7 shows the magnetic field (H) dependence of the magnetocapacitance (MC) and tan δ values for the Ga0.5Cr0.5FeO3 film at 50 K. MC 1439

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is calculated from the change of permittivity (MC = Δε′/ε′H=0). The MC value decreases with H, showing the negative MC effect. It is known that MC effect comes from magnetoelectric coupling and/or magnetoresistance (MR) properties.35,36 When magnetoelectric coupling is dominated in MC, the MC value tends to become proportional to M2.35 On the other hand, when negative (positive) MR behavior is dominated in MC, negative (positive) MC is observed accompanied by an increase (decrease) of tan δ.36 In the Ga0.5Cr0.5FeO3 film, the tan δ value keeps almost constant value against H. Furthermore, the MC value changes linearly with M2 at low M2 region (Supporting Information, Figure 3), although it is difficult to discuss the relation at high M2 region because of the diamagnetism of STO substrate. These results suggest the possible existence of magnetoelectric coupling effect in the Ga0.5Cr0.5FeO3 film as observed in GaFeO3.5

ACKNOWLEDGMENTS

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REFERENCES

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4. CONCLUSION We successfully fabricated a c-axis oriented Ga0.5Cr0.5FeO3 epitaxial thin film. XAS and XMCD measurements reveal that the Fe and Cr ions are in trivalent states and the Fe ions are partially located at the Td Ga1 sites unlike GaFeO3. Magnetic Cr3+ (3d3) substitution yields a Ga0.5Cr0.5FeO3 film with an inplane ferrimagnetism with TC of 240 K, which is higher than that of the GaFeO3 film. On the other hand, the Ms value decreases with Cr substitution at 5 K, which is derived from the increase of the Fe ions at Td Ga1 site. Furthermore, we found that the film exhibits ferroelectricity at room temperature along the c-axis direction. Our findings show that magnetic substitution is useful to modify and expand ferrimagnetism and ferroelectricity in multiferroic GaFeO3-type iron oxides. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00144. Details of XMCD data and magnetic properties of the films (PDF)

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This work was supported by a JSPS KAKENHI Grant-in-Aid for Young Scientist start-up (T.K., Grant 16H06794), Young Scientist (B) (S.Y., Grant15K18212), Scientific Research (A) (M.I., Grant 15H02292), and the MEXT Elements Strategy Initiative to Form Core Research Center, and Creation of Life Innovation, Materials for Interdisciplinary and International Researcher Development, Japan. We would like to thank Prof. Kenta Amemiya (KEK), Dr. Masaki Sakamaki (KEK), Dr. Hiroko Yokota, Dr. Hiroshi Naganuma, and Mr. Tomohiro Ichinose for supporting the synchrotron measurements (Proposal 2015G690).

Figure 7. Magnetic field dependence of magnetocapacitance and tan δ for the Ga0.5Cr0.5FeO3 film at 50 K and 5 kHz.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tsukasa Katayama: 0000-0003-2010-3966 Mitsuru Itoh: 0000-0001-6457-9152 Notes

The authors declare no competing financial interest. 1440

DOI: 10.1021/acs.chemmater.8b00144 Chem. Mater. 2018, 30, 1436−1441

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DOI: 10.1021/acs.chemmater.8b00144 Chem. Mater. 2018, 30, 1436−1441