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Effect of cation substitution on the magnetic and magnetotransport properties of epitaxial Fe3−xVxO4 films
Applied Surface Science 332 (2015) 70–75
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of cation substitution on the magnetic and magnetotransport properties of epitaxial Fe3−x Vx O4 films Chao Jin, Jie Liu, Dongxing Zheng, Min Tang, Peng Li, Haili Bai ∗ Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Institute of Advanced Materials Physics, Faculty of Science, Tianjin University, No. 92 Weijin Road, Nankai District, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 12 October 2014 Received in revised form 30 December 2014 Accepted 19 January 2015 Available online 28 January 2015 Keywords: Fe3−x Vx O4 films Magnetoresistance Reactive cosputtering Spin canting
a b s t r a c t The effect of cation on the structure, magnetic and magnetotransport properties of epitaxial Fe3−x Vx O4 (0 ≤ x ≤ 0.6) films fabricated by reactive cosputtering was investigated systematically. Four kinds of cations (Fe2+ , Fe3+ , V2+ and V3+ ) exist in the Fe3−x Vx O4 films. The Fe3−x Vx O4 films reveal semiconducting property and increased resistivity with increasing V content. The systematic change of the decreased saturation magnetization and enhanced exchange bias is closely related to the spin canting and antiferromagnetic coupling, which is caused by the V substitution on B sites. The presents of V2+ (3d3 ) enlarge the anisotropy, and further increase the coercivity. With the combined effects of the larger anisotropy, spin canting and enhanced antiferromagnetic coupling caused by V substitution, the Fe3−x Vx O4 films exhibit enhanced four-fold symmetric anisotropic magnetoresistance. © 2015 Published by Elsevier B.V.
PACS: 68.55.–a 73.43.Qt 81.15.Cd 75.25.−j
1. Introduction Spinel-type multiple oxides are commonly used in the spintronic devices to generate highly spin-polarized electrons, either for spin injection into semiconductors or for magnetoresistive effects [1–4]. Spinel oxides have the general AB2 O4 structure, in which the A and B cations are distributed in one-eighth of the tetrahedral (A) sites and half of the octahedral (B) sites, respectively. The cation occupancy leads to two different ordered cation distribution schemes, i.e. the normal and inverse spinels. One of the most worldwide used spinel oxides with the inverse spinel structure is Fe3 O4 , which is a half-metallic material predicted by the band structure calculation [7]. Moreover, the magnetic and transport properties of Fe3 O4 can be tailored by substituting Fe cations on the A or B sublattice with one or two isovalent magnetic or nonmagnetic transition metal ions, such as Zn [4], Mn [5], Ni [6,7], Co [8] and Cu [9]. Recently, spinel vanadates have attracted extensive attentions for their novel phenomena: dielectric and ferroelectric properties
∗ Corresponding author. Tel.: +86 22 27406991; fax: +86 22 27403425. E-mail address: [email protected] (H. Bai). http://dx.doi.org/10.1016/j.apsusc.2015.01.137 0169-4332/© 2015 Published by Elsevier B.V.
in spinel MnV2 O4 [10], FeV2 O4 [11–13]. FeV2 O4 is a normal spinel oxide which is appropriate to study the effects of spin and orbital degrees of freedom, because both Fe3+ (3d5 ) on A-sites and V3+ (3d2 ) on B-sites are magnetic and have orbital degrees of freedom. Moreover, FeV2 O4 is a multiferroic in particular, containing both ferroelectric and ferrimagnetic moments at low temperatures [11–13]. It is suggested that the magnetic structure with canted spins, which has not been clearly understood, may be responsible for the appearance of ferroelectricity in FeV2 O4 [12]. Thus, it is urgent to investigate the magnetic structure of spinels vanadates. In this respect, the vanadium-substituted magnetite (Fe3–x Vx O4 ) is a good choice used to investigate the cation effect (V) on the spin structure, magnetic and transport properties from ferrimagnetic Fe3 O4 to multiferroic FeV2 O4 , which would be helpful for understanding the mechanism of ferroelectricity in spinels vanadates. Previously, only the polycrystalline Fe3–x Vx O4 films were studied [14,15]. In this work, we fabricated Fe3–x Vx O4 epitaxial films on MgO (0 0 1) substrates by reactive cosputtering and investigated the structure, magnetic and spin-dependent transport properties. The Fe3–x Vx O4 films exhibit decreased saturation magnetization (MS ) while enhanced coercivity (HC ) and exchange bias (HEB ) with
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Fig. 2. (a) XRD ϕ scans of the Fe3–x Vx O4 (1 1 1) plane, (b) tilting scans of Fe3–x Vx O4 (1 1 1) plane. Fig. 1. XRD –2 scans of the Fe3–x Vx O4 films with various V contents.
increasing V content. The increased anisotropy and antiferromagnetic coupling caused by V2+ cations lead the enhanced four-fold symmetric anisotropic magnetoresistance (AMR). 2. Experimental details Epitaxial Fe3−x Vx O4 (0 ≤ x ≤ 0.6) films on (0 0 1)-oriented MgO substrates were fabricated by using reactive cosputtering from pure Fe (5 N) and V (5 N) targets in a gas mixture of Ar (5 N) and O2 (5 N) with flow rate of 100 and 2.0 sccm, respectively. The substrate temperature was kept at 420 ◦ C during the film deposition. After that, the films were annealed in situ at 420 ◦ C for 1.5 h, and then cooled down to room temperature (RT) at the rate of 1 K/min. The film thickness was ∼80 nm, determined by a Dektak 6 M surface profiler. The microstructure was characterized by X-ray diffraction (XRD, including –2 and ϕ scans). The Fe and V atomic fractions were measured by an energy-dispersive X-ray spectroscopy (EDX). The chemical valence was characterized by X-ray photoelectron spectroscopy (XPS). The magnetic properties were measured by using a Quantum Design magnetic property measurement system (SQUIDVSM). The transport properties were measured using a standard four-probe configuration by a Quantum Design physics property measurement system (PPMS-9). 3. Results and discussion 3.1. Microstructure The XRD –2 scan patterns of the Fe3−x Vx O4 films are shown in Fig. 1. Only the diffraction peaks from MgO (0 0 l) and Fe3−x Vx O4 (0 0 l) can be observed, demonstrating that all of the Fe3−x Vx O4 films are single phase with a preferred (0 0 l) orientation. The (0 0 4) reflection from the MgO substrates and the (0 0 8) reflection from
the Fe3−x Vx O4 films show double peaks due to the separation between the Cu-K˛1 and K˛2 doublets in the high 2 range. Moreover, compared with the –2 pattern of MgO (0 0 1), a peak located at ∼95.0◦ appears in that of Fe3−x Vx O4 /MgO. In order to confirm the epitaxial growth of the preferred oriented films, an XRD ϕ-scan was performed at 2 = 37.8◦ , ˛ = 35.3◦ , where no peak from MgO substrate appears and only peaks from Fe3−x Vx O4 (l l l) can be detected. It is clear from Fig. 2(a) that fourfold symmetric peaks for Fe3−x Vx O4 (l l l) with identical intervals appear. In addition, for further clarifying the epitaxial structure, the tilting 2 scans were carried out by rotating the sample to a certain angle, and the results were shown in Fig. 2(b). In the (l l l) lattice plane (˛ = 35.3◦ , ˇ = 45.0◦ ), only the overlapped (1 1 1) and (2 2 2) diffraction peaks from the Fe3−x Vx O4 are visible, indicating that this lattice plane is the Fe3−x Vx O4 (1 1 1) plane. Consequently, from the XRD –2, ϕ, and tilting scans, it can be concluded that the epitaxial relationship of the present films is Fe3−x Vx O4 (4 0 0)[0 0 1] MgO(2 0 0)[0 0 1] . Fig. 3(a) exhibits the XPS data on Fe 2p3/2 and 2p1/2 core levels of Fe3–x Vx O4 with x = 0.2. Due to the spin–orbit coupling, the Fe2p core levels split into 2p3/2 and 2p1/2 components, situated at 711 and 724 eV, respectively. The broadening of the Fe 2p3/2 and 2p1/2 peaks indicates the presence of Fe2+ . Fig. 3(b) shows the XPS data on V 2p3/2 and 2p1/2 core levels. Each experimental peak was well fitted by two lines, with the peaks at the binding energy around 516.5 and 514.8 eV, corresponding to V3+ and V2+ , respectively [14]. Here, the peak at 520 eV is due to the O 1s satellite peak, which indicates the hybridization between V 3d and O 2p levels [16]. It is clearly seen that the V3+ ions are dominant over the V2+ in Fe3–x Vx O4 . The results indicate that V3+ is mainly substituted for Fe3+ in Fe3 O4 , with some ˚ is almost V2+ substituted for Fe2+ . As the ionic radius of V3+ (0.780 A) ˚ no significant change for the the same as that of Fe3+ (0.785 A), lattice constant was observed. Hence, the (0 0 8) diffraction peaks of Fe3–x Vx O4 in the XRD –2 scans keep unchanged, as shown in Fig. 1. Moreover, Fig. 3 also presents the XPS data of Fe3–x Vx O4 with
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Fig. 3. Fe 2p (a) and V 2p (b) XPS of the Fe3–x Vx O4 films with different V contents.
different V contents. No significant different can be observed in the V 2p3/2 peaks of Fe3–x Vx O4 with different x, indicating the ratios of V2+ /V3+ are almost the same for various x, which are important for the discussions on magnetic properties. 3.2. Magnetic properties Fig. 4(a) shows the magnetization curves of the Fe3–x Vx O4 films with various V contents. All the films exhibit room temperature ferrimagnetism. The MS as a function x is shown in Fig. 4(b). It is clearly shown that the MS decreases with increasing V content, from 400
Fig. 4. (a) Room-temperature magnetization curves of the epitaxial Fe3–x Vx O4 films. (b) Saturation magnetization of the Fe3–x Vx O4 with various V contents.
(x = 0) to 220 (x = 0.6) emu/cm3 . XPS results display that V is mainly +3 (3d2 ) in Fe3–x Vx O4 film. Note that the spin magnetic moment of V3+ is 2 B while the substituted Fe3+ ion is 5 B , and hence the decreased MS as a function of x indicates that the V3+ is mainly substituted for the Fe3+ at octahedral B sites, which is consistent with the Mössbauer results [15]. Moreover, considering the inverse spinel structure of Fe3–x Vx O4 , in which the tetragonal A sites are occupied by the Fe3+ ion and octahedral B sites are occupied by Fe3+ , Fe2+ , V2+ , and V3+ (with the V2+ /V3+ ratio around 1/10), the magnetic moment of one formula unit can be estimated as MS = (4 − 2.8x)B according to Néel’s theory, indicating a linearly decreased MS with increasing x (the redline in Fig. 4(b)). By comparing the theoretical and experimental data, one can observe that the measured MS is smaller than the theoretical value. This smaller magnetization between the film and bulk values is generally considered to be caused by the antiphase boundaries (APBs) [17], surface spin disorder [18], cation vacancy on B sites or interstitial cations Fe3+ on A sites [19]. On the other hand, the MS did not linearly decrease with the increase of V content. Here, the essence of this characteristic is that the V spins tend to be canted to have a Yafet–Kittel-type triangular spin configuration [20]. As V on B sites induce some short range of spin canting, resulting in the non-180◦ antiparallel alignment, and thus a further reduction in the saturation magnetization. The previous works have demonstrated the spin canting existed in vanadium spinel oxides [21–24]. Furthermore, earlier investigations on the magnetic properties of vanadium spinels indicated strong antiferromagnetic (AF) B–B interactions between V cations existed in the system [25]. The spin canting or AF coupling acts as the role of APBs in Fe3–x Vx O4 films. It was reported that the exchange bias (EB) can be induced by domain wall pinning due to APBs [26]. Hence, in order to directly observe the trend of AF coupling strength in Fe3–x Vx O4 with V content, the exchange bias fields (HEB = (HC+ − HC− )/2) of the Fe3–x Vx O4 films were investigated through 50 kOe field cooling to 5 K. As shown in Fig. 5(a), the exchange bias increases with the increase V content, from −40 Oe at x = 0.1 to −75 Oe at x = 0.4, directly demonstrating the enhancement of AF coupling strength caused by spin canting and APBs (shown in Fig. 5(b)). As the density of APBs in epitaxial spin ferrite films is film thickness dependent, the increasing HEB here is mainly related to the spin canting. On the other hand, it is found that the coercivity of Fe3–x Vx O4 films also increases with increasing x, which is attributed to the enhanced anisotropy by
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Fig. 5. (a) Low-temperature (5 K) magnetization curves of the epitaxial Fe3–x Vx O4 films, the samples were cooled to 5 K with a magnetic field of 50 kOe. (b) Schematic diagram of the spin chain model with spin canting and APBs.
V2+ (3d3 ) substitution of the B sites, similar to the case of CoFe2 O4 with Co2+ (3d7 ) ions [15]. The zero-field-cooling (ZFC) and field-cooling (FC) curves at external magnetic field of 100 Oe are shown in Fig. 6. With the relatively lower V content (x = 0.1), a maximum in the ZFC curve corresponding to the spin freezing appears at a temperature of ∼220 K. The spin freezing temperature gets lower with increasing
Fig. 7. (a) Temperature-dependent resistivity of the epitaxial Fe3–x Vx O4 films, (b) MR–H curves for the epitaxial Fe3–x Vx O4 films.
V content. The lower spin-freezing temperatures reflect the weakened superexchange interaction between Fe3+ ions in A and B sites, JAB , which is caused by the V substitution for Fe3+ . Moreover, attributed to the strong AF coupling between antiphase domains and nonlinear alignment of spin moments near APBs in the epitaxial Fe3 O4 films, the temperature dependent ZFC and FC magnetizations always show significant branch [27]. Here, the divergence of the ZFC and FC branches gets stronger with the increase of V content, which originates from the increase of AF coupling strength in Fe3–x Vx O4 films with increasing V content. The characteristic is consistent with the increasing HEB shown in Fig. 5(a). Besides, one can also observer that all the FC and ZFC curves exhibit a rapid decrease with the temperatures below 20 K, which is the characteristic of the pure MgO substrate. 3.3. Transport properties The temperature dependence of resistivity for the Fe3–x Vx O4 films is shown in Fig. 7(a). All the films show clear semiconducting behavior with a longitudinal resistivity increases from 104 (x = 0) to 105 (x = 0.6) cm. Compared to the epitaxial Fe3 O4 films, the increased resistivity with the increasing V content is probably ascribed to the destroy of small polaron conduction due to the decrease of Fe2+ /Fe3+ pairs, spin canting or enhanced AF coupling in the thin film. The magnetoresistance (MR) of the Fe3–x Vx O4 films was measured by applying the field parallel to the film plane, and is defined as MR =
Fig. 6. ZFC and FC curves of the Fe3–x Vx O4 films measured at 100 Oe for (a) x = 0.1, (b) x = 0.2 and (c) x = 0.4.
[R(H) − R(0)] × 100%, R(0)
(1)
where R(H) is the resistance under applied field and R(0) magnetic the resistance at H = 0. The decrease of MR with the increase of
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Fig. 8. In-plane angular dependent anisotropic magnetoresistance for the epitaxial Fe3–x Vx O4 films, (a) x = 0, (b) x = 0.1, (c) x = 0.2, and (d) x = 0.4.
V content in Fig. 7(b) can also be correlated to the enhanced AF coupling Fe3–x Vx O4 films. Fig. 8 displays the in-plane variation of anisotropic magnetoresistance (AMR) for the epitaxial Fe3–x Vx O4 films measured under a magnetic field of 50 kOe. The AMR was measured by the standard four-probe configuration using PPMS–9 equipped with a sample rotator. Here, AMR ratio is defined as AMR =
( − max ) , max
(2)
where max is the maximum resistivity while the magnetic field rotates in the film plane. It can be seen that all the curves are consisted of the two-fold term and four-fold ones. Our earlier results indicate that the two-fold traditional AMR is decided by the scattering far away from the APBs and the four-fold symmetric AMR is based on magnetocrystalline anisotropy and APBs [28–31]. As the alignment of the spins near AF coupling dominates the magnetotransport properties in epitaxial Fe3 O4 films [32], the magnetocrystalline anisotropy field superimposed onto the external magnetic field modifies the alignment of the spins near APBs, leading to the oscillation in scattering possibility of the electrons, and further results in the four-fold symmetric AMR. Here, the AMR of the (1 0 0)-oriented Fe3–x Vx O4 epitaxial films were be fitted by AMR = P1 + P2 cos(2ϕ + ϕ0 ) + P4 cos(4ϕ + ϕ0 )
Fig. 9. Temperature dependence of the fitting parameters P2 and P4 obtained by using Eq. (3) for AMR with (a) x = 0, (b) x = 0.1, and (c) x = 0.2, the insets give the values of P4 /P2 , respectively.
4. Conclusion We have investigated the magnetic and magnetotransport properties of epitaxial Fe3–x Vx O4 films. The Fe3–x Vx O4 films exhibited semiconducting property and increased resistivity with increasing V content. The V substitution Fe cations on B sites induced spin canting and enhanced AF coupling, which further leads decreased saturation magnetization and enhanced exchange bias. The presents of V2+ enhanced the anisotropy of Fe3–x Vx O4 . The enhancement of the four-fold symmetric AMR was associated with the larger anisotropy, spin canting and enhanced AF coupling. Our results make a deep understand of the magnetic structure of the spinels vanadates.
(3)
where P2 and P4 represent the amplitudes of two-fold and fourfold symmetric AMR, respectively. ϕ is the angle between current and magnetic field, and ϕ0 is caused by the misalignment of sample rotator at the beginning of the measurements. The fitted parame ters of P2 and P4 are shown in Fig. 9. An increase of P4 /P2 with the increase of V content x was found, as shown in the inset of Fig. 9. The enhanced four-fold symmetric contribution is mainly attributed to the increased anisotropy by V2+ (3d3 ) substitution of the B sites, which has been verified by the increased coercivity in Fig. 5(a). Moreover, as the four-fold symmetric AMR is found to be correlated to the magnetocrystalline anisotropy modulated anisotropic scattering near the APBs, which is associated with the AF coupling strength. The enhancement of four-fold AMR should also be ascribed to the spin canting and strengthened AF coupling with V substitution, which has been verified by the increased HEB in Fig. 5(a).
Acknowledgements This work was supported by National Science Foundation of China (11304221) and Natural Science Foundation of Tianjin City (13JCZDJC32800). References [1] J.S. Moodera, L.R. Kinder, T.M. Wong, R. Meservey, Phys. Rev. Lett. 74 (1995) 3273. [2] Y. Kajiwara, K. Harii, S. Takahashi, J. Ohe, K. Uchida, M. Mizuguchi, H. Umezawa, H. Kawai, K. Ando, K. Takanashi, S. Maekawa, E. Saitoh, Nature (London) 464 (2010) 262. [3] G. Hu, Y. Suzuki, Phys. Rev. Lett. 89 (2002) 276601. [4] C. Jin, P. Li, W.B. Mi, H.L. Bai, J. Appl. Phys. 115 (2014) 213908. [5] M. Ishikawa, H. Tanaka, T. Kawai, Appl. Phys. Lett. 86 (2005) 222504. [6] C. Jin, Q. Zhang, W.B. Mi, E.Y. Jiang, H.L. Bai, J. Phys. D: Appl. Phys. 43 (2010) 385001. [7] C.Y. Chou, P.C. Kuo, Y.D. Yao, A.C. Sun, S.C. Chen, I.J. Chang, J.W. Chen, IEEE Trans. Magn. 41 (2005) 906.
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[19] J.P. Shepherd, J.W. Koenitzer, R. Aragon, J. Spalek, J.M. Honig, Phys. Rev. B 43 (1991) 8461. [20] Y. Yafet, C. Kittel, Phys. Rev. 87 (1952) 290. [21] D.B. Rogers, R.J. Arnott, A. Wold, J.B. Goodenough, J. Phys. Chem. Solids 24 (1963) 347. [22] A. Wold, D. Rogers, N. Menyuk, R.J. Arnott, J. Appl. Phys. 33 (1962) 1208. [23] S.H. Baek, N.J. Curro, K.Y. Choi, A.P. Reyes, P.L. Kuhns, H.D. Zhou, C.R. Wiebe, Phys. Rev. B 80 (2009) 140406. [24] J.S. Kang, J. Hwang, D.H. Kim, E. Lee, W.C. Kim, C.S. Kim, S. Kwon, S. Lee, J.Y. Kim, T. Ueno, M. Sawada, Phys. Rev. B 85 (2012) 165136. [25] S. Di Matteo, G. Jackeli, N.B. Perkins, Phys. Rev. B 72 (2005) 020408. [26] S.K. Aror, R.G.S. Sofin, A. Nolan, I.V. Shvets, J. Magn. Magn. Mater. 286 (2005) 463. [27] P. Li, E.Y. Jiang, H.L. Bai, J. Phys. D: Appl. Phys. 43 (2010) 265002. [28] C. Jin, P. Li, W.B. Mi, H.L. Bai, EPL 100 (2012) 27006. [29] P. Li, B.L. Guo, H.L. Bai, EPL 94 (2011) 57007. [30] P. Li, E.Y. Jiang, H.L. Bai, J. Appl. Phys. 108 (2010) 093921. [31] P. Li, E.Y. Jiang, H.L. Bai, Appl. Phys. Lett. 96 (2010) 092502. [32] W. Eerenstein, T.T.M. Palstra, S.S. Saxena, T. Hibma, Phys. Rev. Lett. 88 (2002) 247204.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of cation substitution on the magnetic and magnetotransport properties of epitaxial Fe3−x Vx O4 films Chao Jin, Jie Liu, Dongxing Zheng, Min Tang, Peng Li, Haili Bai ∗ Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Institute of Advanced Materials Physics, Faculty of Science, Tianjin University, No. 92 Weijin Road, Nankai District, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 12 October 2014 Received in revised form 30 December 2014 Accepted 19 January 2015 Available online 28 January 2015 Keywords: Fe3−x Vx O4 films Magnetoresistance Reactive cosputtering Spin canting
a b s t r a c t The effect of cation on the structure, magnetic and magnetotransport properties of epitaxial Fe3−x Vx O4 (0 ≤ x ≤ 0.6) films fabricated by reactive cosputtering was investigated systematically. Four kinds of cations (Fe2+ , Fe3+ , V2+ and V3+ ) exist in the Fe3−x Vx O4 films. The Fe3−x Vx O4 films reveal semiconducting property and increased resistivity with increasing V content. The systematic change of the decreased saturation magnetization and enhanced exchange bias is closely related to the spin canting and antiferromagnetic coupling, which is caused by the V substitution on B sites. The presents of V2+ (3d3 ) enlarge the anisotropy, and further increase the coercivity. With the combined effects of the larger anisotropy, spin canting and enhanced antiferromagnetic coupling caused by V substitution, the Fe3−x Vx O4 films exhibit enhanced four-fold symmetric anisotropic magnetoresistance. © 2015 Published by Elsevier B.V.
PACS: 68.55.–a 73.43.Qt 81.15.Cd 75.25.−j
1. Introduction Spinel-type multiple oxides are commonly used in the spintronic devices to generate highly spin-polarized electrons, either for spin injection into semiconductors or for magnetoresistive effects [1–4]. Spinel oxides have the general AB2 O4 structure, in which the A and B cations are distributed in one-eighth of the tetrahedral (A) sites and half of the octahedral (B) sites, respectively. The cation occupancy leads to two different ordered cation distribution schemes, i.e. the normal and inverse spinels. One of the most worldwide used spinel oxides with the inverse spinel structure is Fe3 O4 , which is a half-metallic material predicted by the band structure calculation [7]. Moreover, the magnetic and transport properties of Fe3 O4 can be tailored by substituting Fe cations on the A or B sublattice with one or two isovalent magnetic or nonmagnetic transition metal ions, such as Zn [4], Mn [5], Ni [6,7], Co [8] and Cu [9]. Recently, spinel vanadates have attracted extensive attentions for their novel phenomena: dielectric and ferroelectric properties
∗ Corresponding author. Tel.: +86 22 27406991; fax: +86 22 27403425. E-mail address: [email protected] (H. Bai). http://dx.doi.org/10.1016/j.apsusc.2015.01.137 0169-4332/© 2015 Published by Elsevier B.V.
in spinel MnV2 O4 [10], FeV2 O4 [11–13]. FeV2 O4 is a normal spinel oxide which is appropriate to study the effects of spin and orbital degrees of freedom, because both Fe3+ (3d5 ) on A-sites and V3+ (3d2 ) on B-sites are magnetic and have orbital degrees of freedom. Moreover, FeV2 O4 is a multiferroic in particular, containing both ferroelectric and ferrimagnetic moments at low temperatures [11–13]. It is suggested that the magnetic structure with canted spins, which has not been clearly understood, may be responsible for the appearance of ferroelectricity in FeV2 O4 [12]. Thus, it is urgent to investigate the magnetic structure of spinels vanadates. In this respect, the vanadium-substituted magnetite (Fe3–x Vx O4 ) is a good choice used to investigate the cation effect (V) on the spin structure, magnetic and transport properties from ferrimagnetic Fe3 O4 to multiferroic FeV2 O4 , which would be helpful for understanding the mechanism of ferroelectricity in spinels vanadates. Previously, only the polycrystalline Fe3–x Vx O4 films were studied [14,15]. In this work, we fabricated Fe3–x Vx O4 epitaxial films on MgO (0 0 1) substrates by reactive cosputtering and investigated the structure, magnetic and spin-dependent transport properties. The Fe3–x Vx O4 films exhibit decreased saturation magnetization (MS ) while enhanced coercivity (HC ) and exchange bias (HEB ) with
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Fig. 2. (a) XRD ϕ scans of the Fe3–x Vx O4 (1 1 1) plane, (b) tilting scans of Fe3–x Vx O4 (1 1 1) plane. Fig. 1. XRD –2 scans of the Fe3–x Vx O4 films with various V contents.
increasing V content. The increased anisotropy and antiferromagnetic coupling caused by V2+ cations lead the enhanced four-fold symmetric anisotropic magnetoresistance (AMR). 2. Experimental details Epitaxial Fe3−x Vx O4 (0 ≤ x ≤ 0.6) films on (0 0 1)-oriented MgO substrates were fabricated by using reactive cosputtering from pure Fe (5 N) and V (5 N) targets in a gas mixture of Ar (5 N) and O2 (5 N) with flow rate of 100 and 2.0 sccm, respectively. The substrate temperature was kept at 420 ◦ C during the film deposition. After that, the films were annealed in situ at 420 ◦ C for 1.5 h, and then cooled down to room temperature (RT) at the rate of 1 K/min. The film thickness was ∼80 nm, determined by a Dektak 6 M surface profiler. The microstructure was characterized by X-ray diffraction (XRD, including –2 and ϕ scans). The Fe and V atomic fractions were measured by an energy-dispersive X-ray spectroscopy (EDX). The chemical valence was characterized by X-ray photoelectron spectroscopy (XPS). The magnetic properties were measured by using a Quantum Design magnetic property measurement system (SQUIDVSM). The transport properties were measured using a standard four-probe configuration by a Quantum Design physics property measurement system (PPMS-9). 3. Results and discussion 3.1. Microstructure The XRD –2 scan patterns of the Fe3−x Vx O4 films are shown in Fig. 1. Only the diffraction peaks from MgO (0 0 l) and Fe3−x Vx O4 (0 0 l) can be observed, demonstrating that all of the Fe3−x Vx O4 films are single phase with a preferred (0 0 l) orientation. The (0 0 4) reflection from the MgO substrates and the (0 0 8) reflection from
the Fe3−x Vx O4 films show double peaks due to the separation between the Cu-K˛1 and K˛2 doublets in the high 2 range. Moreover, compared with the –2 pattern of MgO (0 0 1), a peak located at ∼95.0◦ appears in that of Fe3−x Vx O4 /MgO. In order to confirm the epitaxial growth of the preferred oriented films, an XRD ϕ-scan was performed at 2 = 37.8◦ , ˛ = 35.3◦ , where no peak from MgO substrate appears and only peaks from Fe3−x Vx O4 (l l l) can be detected. It is clear from Fig. 2(a) that fourfold symmetric peaks for Fe3−x Vx O4 (l l l) with identical intervals appear. In addition, for further clarifying the epitaxial structure, the tilting 2 scans were carried out by rotating the sample to a certain angle, and the results were shown in Fig. 2(b). In the (l l l) lattice plane (˛ = 35.3◦ , ˇ = 45.0◦ ), only the overlapped (1 1 1) and (2 2 2) diffraction peaks from the Fe3−x Vx O4 are visible, indicating that this lattice plane is the Fe3−x Vx O4 (1 1 1) plane. Consequently, from the XRD –2, ϕ, and tilting scans, it can be concluded that the epitaxial relationship of the present films is Fe3−x Vx O4 (4 0 0)[0 0 1] MgO(2 0 0)[0 0 1] . Fig. 3(a) exhibits the XPS data on Fe 2p3/2 and 2p1/2 core levels of Fe3–x Vx O4 with x = 0.2. Due to the spin–orbit coupling, the Fe2p core levels split into 2p3/2 and 2p1/2 components, situated at 711 and 724 eV, respectively. The broadening of the Fe 2p3/2 and 2p1/2 peaks indicates the presence of Fe2+ . Fig. 3(b) shows the XPS data on V 2p3/2 and 2p1/2 core levels. Each experimental peak was well fitted by two lines, with the peaks at the binding energy around 516.5 and 514.8 eV, corresponding to V3+ and V2+ , respectively [14]. Here, the peak at 520 eV is due to the O 1s satellite peak, which indicates the hybridization between V 3d and O 2p levels [16]. It is clearly seen that the V3+ ions are dominant over the V2+ in Fe3–x Vx O4 . The results indicate that V3+ is mainly substituted for Fe3+ in Fe3 O4 , with some ˚ is almost V2+ substituted for Fe2+ . As the ionic radius of V3+ (0.780 A) ˚ no significant change for the the same as that of Fe3+ (0.785 A), lattice constant was observed. Hence, the (0 0 8) diffraction peaks of Fe3–x Vx O4 in the XRD –2 scans keep unchanged, as shown in Fig. 1. Moreover, Fig. 3 also presents the XPS data of Fe3–x Vx O4 with
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Fig. 3. Fe 2p (a) and V 2p (b) XPS of the Fe3–x Vx O4 films with different V contents.
different V contents. No significant different can be observed in the V 2p3/2 peaks of Fe3–x Vx O4 with different x, indicating the ratios of V2+ /V3+ are almost the same for various x, which are important for the discussions on magnetic properties. 3.2. Magnetic properties Fig. 4(a) shows the magnetization curves of the Fe3–x Vx O4 films with various V contents. All the films exhibit room temperature ferrimagnetism. The MS as a function x is shown in Fig. 4(b). It is clearly shown that the MS decreases with increasing V content, from 400
Fig. 4. (a) Room-temperature magnetization curves of the epitaxial Fe3–x Vx O4 films. (b) Saturation magnetization of the Fe3–x Vx O4 with various V contents.
(x = 0) to 220 (x = 0.6) emu/cm3 . XPS results display that V is mainly +3 (3d2 ) in Fe3–x Vx O4 film. Note that the spin magnetic moment of V3+ is 2 B while the substituted Fe3+ ion is 5 B , and hence the decreased MS as a function of x indicates that the V3+ is mainly substituted for the Fe3+ at octahedral B sites, which is consistent with the Mössbauer results [15]. Moreover, considering the inverse spinel structure of Fe3–x Vx O4 , in which the tetragonal A sites are occupied by the Fe3+ ion and octahedral B sites are occupied by Fe3+ , Fe2+ , V2+ , and V3+ (with the V2+ /V3+ ratio around 1/10), the magnetic moment of one formula unit can be estimated as MS = (4 − 2.8x)B according to Néel’s theory, indicating a linearly decreased MS with increasing x (the redline in Fig. 4(b)). By comparing the theoretical and experimental data, one can observe that the measured MS is smaller than the theoretical value. This smaller magnetization between the film and bulk values is generally considered to be caused by the antiphase boundaries (APBs) [17], surface spin disorder [18], cation vacancy on B sites or interstitial cations Fe3+ on A sites [19]. On the other hand, the MS did not linearly decrease with the increase of V content. Here, the essence of this characteristic is that the V spins tend to be canted to have a Yafet–Kittel-type triangular spin configuration [20]. As V on B sites induce some short range of spin canting, resulting in the non-180◦ antiparallel alignment, and thus a further reduction in the saturation magnetization. The previous works have demonstrated the spin canting existed in vanadium spinel oxides [21–24]. Furthermore, earlier investigations on the magnetic properties of vanadium spinels indicated strong antiferromagnetic (AF) B–B interactions between V cations existed in the system [25]. The spin canting or AF coupling acts as the role of APBs in Fe3–x Vx O4 films. It was reported that the exchange bias (EB) can be induced by domain wall pinning due to APBs [26]. Hence, in order to directly observe the trend of AF coupling strength in Fe3–x Vx O4 with V content, the exchange bias fields (HEB = (HC+ − HC− )/2) of the Fe3–x Vx O4 films were investigated through 50 kOe field cooling to 5 K. As shown in Fig. 5(a), the exchange bias increases with the increase V content, from −40 Oe at x = 0.1 to −75 Oe at x = 0.4, directly demonstrating the enhancement of AF coupling strength caused by spin canting and APBs (shown in Fig. 5(b)). As the density of APBs in epitaxial spin ferrite films is film thickness dependent, the increasing HEB here is mainly related to the spin canting. On the other hand, it is found that the coercivity of Fe3–x Vx O4 films also increases with increasing x, which is attributed to the enhanced anisotropy by
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Fig. 5. (a) Low-temperature (5 K) magnetization curves of the epitaxial Fe3–x Vx O4 films, the samples were cooled to 5 K with a magnetic field of 50 kOe. (b) Schematic diagram of the spin chain model with spin canting and APBs.
V2+ (3d3 ) substitution of the B sites, similar to the case of CoFe2 O4 with Co2+ (3d7 ) ions [15]. The zero-field-cooling (ZFC) and field-cooling (FC) curves at external magnetic field of 100 Oe are shown in Fig. 6. With the relatively lower V content (x = 0.1), a maximum in the ZFC curve corresponding to the spin freezing appears at a temperature of ∼220 K. The spin freezing temperature gets lower with increasing
Fig. 7. (a) Temperature-dependent resistivity of the epitaxial Fe3–x Vx O4 films, (b) MR–H curves for the epitaxial Fe3–x Vx O4 films.
V content. The lower spin-freezing temperatures reflect the weakened superexchange interaction between Fe3+ ions in A and B sites, JAB , which is caused by the V substitution for Fe3+ . Moreover, attributed to the strong AF coupling between antiphase domains and nonlinear alignment of spin moments near APBs in the epitaxial Fe3 O4 films, the temperature dependent ZFC and FC magnetizations always show significant branch [27]. Here, the divergence of the ZFC and FC branches gets stronger with the increase of V content, which originates from the increase of AF coupling strength in Fe3–x Vx O4 films with increasing V content. The characteristic is consistent with the increasing HEB shown in Fig. 5(a). Besides, one can also observer that all the FC and ZFC curves exhibit a rapid decrease with the temperatures below 20 K, which is the characteristic of the pure MgO substrate. 3.3. Transport properties The temperature dependence of resistivity for the Fe3–x Vx O4 films is shown in Fig. 7(a). All the films show clear semiconducting behavior with a longitudinal resistivity increases from 104 (x = 0) to 105 (x = 0.6) cm. Compared to the epitaxial Fe3 O4 films, the increased resistivity with the increasing V content is probably ascribed to the destroy of small polaron conduction due to the decrease of Fe2+ /Fe3+ pairs, spin canting or enhanced AF coupling in the thin film. The magnetoresistance (MR) of the Fe3–x Vx O4 films was measured by applying the field parallel to the film plane, and is defined as MR =
Fig. 6. ZFC and FC curves of the Fe3–x Vx O4 films measured at 100 Oe for (a) x = 0.1, (b) x = 0.2 and (c) x = 0.4.
[R(H) − R(0)] × 100%, R(0)
(1)
where R(H) is the resistance under applied field and R(0) magnetic the resistance at H = 0. The decrease of MR with the increase of
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Fig. 8. In-plane angular dependent anisotropic magnetoresistance for the epitaxial Fe3–x Vx O4 films, (a) x = 0, (b) x = 0.1, (c) x = 0.2, and (d) x = 0.4.
V content in Fig. 7(b) can also be correlated to the enhanced AF coupling Fe3–x Vx O4 films. Fig. 8 displays the in-plane variation of anisotropic magnetoresistance (AMR) for the epitaxial Fe3–x Vx O4 films measured under a magnetic field of 50 kOe. The AMR was measured by the standard four-probe configuration using PPMS–9 equipped with a sample rotator. Here, AMR ratio is defined as AMR =
( − max ) , max
(2)
where max is the maximum resistivity while the magnetic field rotates in the film plane. It can be seen that all the curves are consisted of the two-fold term and four-fold ones. Our earlier results indicate that the two-fold traditional AMR is decided by the scattering far away from the APBs and the four-fold symmetric AMR is based on magnetocrystalline anisotropy and APBs [28–31]. As the alignment of the spins near AF coupling dominates the magnetotransport properties in epitaxial Fe3 O4 films [32], the magnetocrystalline anisotropy field superimposed onto the external magnetic field modifies the alignment of the spins near APBs, leading to the oscillation in scattering possibility of the electrons, and further results in the four-fold symmetric AMR. Here, the AMR of the (1 0 0)-oriented Fe3–x Vx O4 epitaxial films were be fitted by AMR = P1 + P2 cos(2ϕ + ϕ0 ) + P4 cos(4ϕ + ϕ0 )
Fig. 9. Temperature dependence of the fitting parameters P2 and P4 obtained by using Eq. (3) for AMR with (a) x = 0, (b) x = 0.1, and (c) x = 0.2, the insets give the values of P4 /P2 , respectively.
4. Conclusion We have investigated the magnetic and magnetotransport properties of epitaxial Fe3–x Vx O4 films. The Fe3–x Vx O4 films exhibited semiconducting property and increased resistivity with increasing V content. The V substitution Fe cations on B sites induced spin canting and enhanced AF coupling, which further leads decreased saturation magnetization and enhanced exchange bias. The presents of V2+ enhanced the anisotropy of Fe3–x Vx O4 . The enhancement of the four-fold symmetric AMR was associated with the larger anisotropy, spin canting and enhanced AF coupling. Our results make a deep understand of the magnetic structure of the spinels vanadates.
(3)
where P2 and P4 represent the amplitudes of two-fold and fourfold symmetric AMR, respectively. ϕ is the angle between current and magnetic field, and ϕ0 is caused by the misalignment of sample rotator at the beginning of the measurements. The fitted parame ters of P2 and P4 are shown in Fig. 9. An increase of P4 /P2 with the increase of V content x was found, as shown in the inset of Fig. 9. The enhanced four-fold symmetric contribution is mainly attributed to the increased anisotropy by V2+ (3d3 ) substitution of the B sites, which has been verified by the increased coercivity in Fig. 5(a). Moreover, as the four-fold symmetric AMR is found to be correlated to the magnetocrystalline anisotropy modulated anisotropic scattering near the APBs, which is associated with the AF coupling strength. The enhancement of four-fold AMR should also be ascribed to the spin canting and strengthened AF coupling with V substitution, which has been verified by the increased HEB in Fig. 5(a).
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