Multiferroic and magnetodielectric properties of the Al_1−xGa_xFeO₃ family of oxides

Abstract

AlFeO₃, GaFeO₃ and Al_0.5Ga_0.5FeO₃, all crystallizing in a non-centrosymmetric space group, are multiferroic and also exhibit magnetodielectric properties, with Al_0.5Ga_0.5FeO₃ showing an unusually large magnetocapacitance at 300 K.

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Multiferroic oxides are not common since it is difficult to satisfy the criteria required for magnetism and ferroelectricity in the same material.^1–3 However, there are a few materials that show multiferroic properties due to mechanisms different from those in conventional ferroelectrics, such as BaTiO₃. All multiferroic materials are not necessarily magnetoelectric because the latter property arises from the interaction between the magnetic and the electric order parameters.^1–3 It has been recently reported that AlFeO₃, which is derived from Fe₂O₃ by the substitution of one Fe³⁺ by Al³⁺, shows ferrimagnetic as well as ferroelectric properties, unlike the parent binary oxides, Al₂O₃ and Fe₂O₃.^4–6AlFeO₃ has an orthorhombic structure with the chiral, non-centrosymmetric Pna2₁ space group. GaFeO₃ has a structure similar to that of AlFeO₃ with a non-centrosymmetric space group.⁷GaFeO₃ is also ferrimagnetic and ferroelectric.^6–10 In both AlFeO₃ and GaFeO₃, there are four different cation sites Fe1, Fe2, A1 and A2 (A = Al, Ga), of which Fe1, Fe2 and A2 have an octahedral oxygen environment. The cation in the A1 site is in the tetrahedral oxygen environment. The octahedral environment of Fe1, Fe2 and A2 are slightly distorted, while the A1O₄ tetrahedron is almost regular. The cause for this local deformation of the lattice is attributed to the difference in the octahedral radii of Fe³⁺ and A³⁺ ions and the disorder in the occupation of the octahedral sites. Low temperature magnetic ordering in these oxides is due to the cation–oxygen–cation superexchange antiferromagnetic interaction. Magnetodielectric properties of multiferroic AlFeO₃ and Al_0.5Ga_0.5FeO₃ are not established. In this communication, we present the multiferroic and magnetodielectric properties of Al_1−xGa_xFeO₃.† More importantly, we have investigated the properties of Al_0.5Ga_0.5FeO₃ since the nature of the A-cations as well as cation disorder would have a significant effect on dielectric and related properties.† We find that Al_0.5Ga_0.5FeO₃ is not only multiferroic, but also shows remarkably large magnetocapacitance at 300 K, exceeding the performance of AlFeO₃ and GaFeO₃.

In Fig. 1, we show the X-ray diffraction patterns of AlFeO₃, GaFeO₃ and Al_0.5Ga_0.5FeO₃ along with profile fits and difference patterns to show how they possess the same structure.†AlFeO₃ is ferrimagnetic with a T_N of 250 K and shows magnetic hysteresis at low temperature (Fig. 2a). GaFeO₃ exhibits a ferrimagnetic T_N of 210 K, while Al_0.5Ga_0.5FeO₃ shows a magnetic behaviour similar to AlFeO₃ and GaFeO₃ with a T_N of 220 K (Fig. 2b). We see divergence between the field-cooled (FC) and zero-field-cooled (ZFC) magnetization data just as in AlFeO₃ or GaFeO₃. We observe magnetic hysteresis at low temperatures (see inset of Fig. 2b). The value of saturation magnetization, remnant magnetization, and coercive field in the case of Al_0.5Ga_0.5FeO₃ are 10.0 emu g⁻¹, 19.1 emu g⁻¹ and 10.3 kOe respectively, not very different from the values found in AlFeO₃ and GaFeO₃.

Fig. 1 XRD patterns of (a) GaFeO₃ and (b) Al_0.5Ga_0.5FeO₃ along with profile fits, difference patterns and Bragg positions. Inset in (a) shows the XRD pattern of AlFeO₃.

Fig. 2 Temperature dependent magnetization of (a) AlFeO₃ and (b) Al_0.5Ga_0.5FeO₃ under field cooled (FC) and zero field cooled (ZFC) conditions. Magnetic hysteresis at 5 K is shown in the inset.

We show the temperature variation of the dielectric properties of GaFeO₃ and Al_0.5Ga_0.5FeO₃ in Fig. 3(a) and (b), respectively. We see dielectric dispersion below T_N and a significant increase in the dielectric constant above T_N, behaviour similar to that found in AlFeO₃. The dielectric behaviour shown in Fig. 3 is not unlike that of relaxor ferroelectrics. Al_0.5Ga_0.5FeO₃ shows ferroelectric hysteresis at relatively low temperatures (<300 K) just as with AlFeO₃ and GaFeO₃, with the hysteresis loops reflecting the leaky nature of these materials. In Fig. 4, we show hysteresis loops in the case of GaFeO₃ and Al_0.5Ga_0.5FeO₃. The maximum (saturation) polarization (P_m) and remnant polarization (P_R) are very much higher in Al_0.5Ga_0.5FeO₃ than in GaFeO₃ or AlFeO₃, the values being P_m = 1.1 μC cm⁻², P_R = 0.5 μC cm⁻² and coercive field (E_c) = 3.8 kV cm⁻¹ at 200 K for an applied voltage of 500 V compared to P_m = 0.05(0.07) μC cm⁻², P_R = 0.02(0.04) μC cm⁻² and E_c = 1.22(6.4) kV cm⁻¹ found in AlFeO₃ (GaFeO₃). Clearly, Al_0.5Ga_0.5FeO₃ is a multiferroic with better ferroelectric properties.

Fig. 3 The temperature variation of dielectric properties of (a) GaFeO₃ and (b) Al_0.5Ga_0.5FeO₃. The effect of 2 T is also shown for both (a) and (b).

Fig. 4 The ferroelectric hysteresis loops of (a) GaFeO₃ and (b) Al_0.5Ga_0.5FeO₃ at 200 K (1 kHz) showing leaky behaviour.

A marked increase in the dielectric constant around T_N signifies interaction between electric and magnetic order parameters. Application of a magnetic field has a marked effect on the dielectric properties of GaFeO₃ and Al_0.5Ga_0.5FeO₃, but the effect is considerably smaller in AlFeO₃. We show the effect of magnetic field of 2 T on the properties of GaFeO₃ and Al_0.5Ga_0.5FeO₃ in Fig. 3. Magnetocapacitance in AlFeO₃ reaches a value of around 20% at 300 K at 0.5 kHz and 2 T and does not vary significantly with frequency. The effect of magnetic field is however large in the case of GaFeO₃ and Al_0.5Ga_0.5FeO₃. Both these oxides show large magnetocapacitance and a significant variation of the magnetocapacitance with frequency, as shown in Fig. 5(a). Maximum magnetocapacitance is observed in these two oxides around 40 kHz and at this frequency, Al_0.5Ga_0.5FeO₃ shows 60% magnetocapacitance at 2 T and 35% at 1 T. Such a large magnetocapacitance is indeed noteworthy. In Fig. 5(b), we show the variation of % magnetocapacitance with magnetic field for all three oxides. The marked variation of magnetocapacitance in GaFeO₃ and Al_0.5Ga_0.5FeO₃, is noteworthy, the value increasing with the magnetic field. We have examined the magnetoresistance properties of the three oxides. All the three oxides are insulators and show negligible magnetoresistance around 300 K, being 0.5% or less in the case of AlFeO₃ and GaFeO₃ at 2 T and 300 K. It is around 2.5% in the case of Al_0.5Ga_0.5FeO₃. It therefore, appears that the origin of magnetodielectric effect observed by us is not entirely due to the magnetoresistance.

Fig. 5 (a) The variation of % magnetocapacitance with frequency for GaFeO₃ and Al_0.5Ga_0.5FeO₃ at a magnetic field of 2 T. In the inset, the variation of % magnetocapacitance with frequency at 1 T is shown, (b) the variation of % magnetocapacitance with magnetic field for AlFeO₃ (0.5 kHz, 300 K), GaFeO₃ (40 kHz, 300 K) and Al_0.5Ga_0.5FeO₃ (40 kHz, 300 K).

In conclusion, it is significant that simple iron oxides of the type AFeO₃ (A = Al, Ga) are both multiferroic and magnetodielectric. The large magnetodielectric effect exhibited by GaFeO₃ and Al_0.5Ga_0.5FeO₃, specially the latter, demonstrates the importance of the A-site cations. Displacements of the A cations and the disorder associated with their occupancies markedly affect the dielectric and magnetodielectric properties in a significant manner.⁶

Notes and references

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Footnote

† AlFeO₃ and GaFeO₃ were prepared by the co-precipitation method and sintered at 1350 °C. Al_0.5Ga_0.5FeO₃ was prepared by heating the appropriate mixture of Al₂O₃, Ga₂O₃ and Fe₂O₃ at 1400 °C for 5 h. Al_0.5Ga_0.5FeO₃ has a similar structure as AlFeO₃ and GaFeO₃. (a = 5.0292(1) Å, b = 8.6435(1) Å, c = 9.3155(1) Å). The lattice parameters of AlFeO₃ (GaFeO₃) are a = 4.9813(1) Å (5.0822(1) Å), b = 8.5516(2) Å (8.7447(1) Å), c = 9.2429(3) Å (9.3917(1) Å).

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