NiO bilayers

Solid State Communications 220 (2015) 1–5

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Interface exchange coupling induced fourfold symmetry planar Hall effect in Fe3O4/NiO bilayers P. Li n, W.Y. Cui, H.L. Bai Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparation Technology, Institute of Advanced Materials Physics, Faculty of Science, Tianjin University, Tianjin 300072, China

art ic l e i nf o

a b s t r a c t

Article history: Received 9 May 2015 Received in revised form 27 June 2015 Accepted 30 June 2015 Available online 7 July 2015

An unexpected fourfold symmetry planar Hall effect was observed in Fe3O4/NiO bilayers. As the thickness of the antiferromagnetic layer exceeds 37 nm, the planar Hall effect of the bilayer further shifts to twofold symmetry, which is ascribed to the dying interfacial coupled effect with increasing antiferromagnetic NiO layer thickness. According to the fitting based on the Stoner–Wohlfarth model, it was notable that an extra cubic anisotropic field in the bilayer structure was obviously amplified by attenuating the thickness of the antiferromagnetic layer. First principle calculations reveal that the amplified cubic anisotropic field was ascribed to the synergistic effect from interfacial bonding structure and charge transfer. & 2015 Elsevier Ltd. All rights reserved.

Keywords: A. Bilayer C. Antiferromagnetism D. Interfacial coupling D. Planar Hall effect

1. Introduction The anisotropy stemming from the interface of ferromagnet (FM) and antiferromagnet (AFM) has been intensively explored recently in spintronic applications, for instance, spin valves, magnetic tunneling junctions, etc. Although the high data storage densities stray fields may destroy the FM set states, an AFM would be relatively insensitive to these stray fields and maintain the anisotropic magnetoresistance (AMR) in FM layer [1]. Especially, the exchange-induced anisotropy relevant to the exchange coupling at the FM/AFM interface attracts considerable attention recently for the tunable anisotropy in FM layer by the interfacial exchange couplings. For instance, in a Fe/CoO/MgO(001) system, the exchange coupling from the magneto-crystalline anisotropy of CoO AFM spin induces a strong uniaxial anisotropy in the Fe film with the easy axis along the CoOo 110 4 directions [2]. The planar Hall effect (PHE) has been proved to be an effective method to characterize the exchange coupling anisotropy in the FM/ AFM system [3]. Through the exchange coupling at the interface of FM/AFM bilayers, an unidirectional anisotropy (exchange bias) and a uniaxial magnetic anisotropy were usually introduced into the FM layer [4–7]. Recently, PHE has been used to detect the unidirectional magnetic anisotropy in Fe films, which serves to establish the direction of exchange bias [8]. In addition, since the magnetization direction is determined by the competition between the external

n

Corresponding author. E-mail address: [email protected] (P. Li).

http://dx.doi.org/10.1016/j.ssc.2015.06.025 0038-1098/& 2015 Elsevier Ltd. All rights reserved.

magnetic field and the magnetic anisotropy of the system, the PHE provides a powerful tool to study the in-plane magnetization rotation [9–13]. The typical example is the presence of strong cubic anisotropy in GaMnAs, resulting in four easy directions in the system [14–18]. Although giant magnetoresistance in the exchange bias based spin valves is fully understood, the effect of exchange coupling at the interface of FM/AFM on the anisotropic magnetotransport in FM layer is still lack of exploration, especially for PHE. In this work, we explored the in-plane magnetic anisotropy in Fe3O4/NiO/MgO(001) and Fe3O4/NiO/MgO(110) systems through AMR and PHE. An unexpected fourfold symmetric PHE observed in the bilayers with thinnest AFM layer suggests the extra cubic anisotropy was introduced into FM layer by the exchange coupling at the interface. By investigating the anisotropy as a function of the NiO thicknesses, we separated the surface anisotropy and volume magnetic anisotropy in the Fe3O4 layer induced by the NiO AFM order. The first principle calculations reveal that the cubic anisotropy can be modulated by the interfacial bond structure.

2. Experiments The epitaxial Fe3O4 and NiO films were fabricated by dc reactive magnetron sputtering from a pair of facing Fe targets. All of the Fe3O4 layers were maintained 24 nm while NiO layers were deposited with different thicknesses on the MgO(001) and MgO(110) substrates, respectively. During the sample depositing, electrodes of Fe3O4 layer were patterned with the five-terminal mask to obtain the electronic properties. Two sets of samples were

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P. Li et al. / Solid State Communications 220 (2015) 1–5

prepared: Fe3O4(24 nm)/NiO(t nm)/MgO(001), Fe3O4(24 nm)/NiO (t nm)/MgO(110). Both of Fe3O4 and NiO were deposited by reactive magnetron sputtering. Fe3O4 was deposited in a total pressure 0.3 Pa Arþ O2 mixed pressure with 0.4% of O2 partial pressure, and NiO film was deposited in a proportion of Ar:O2 ¼ 5:2. The epitaxial structure was described by x-ray diffraction (XRD) θ–2θ, φ scan and high-resolution transmission electron microscopy (HRTEM). The anisotropic transport properties were measured by a physical property measurement system (PPMS-9) equipped with a sample rotator. The AMR and planar Hall resistivity (PHR) were obtained simultaneously with a five-terminal mask covered on the top Fe3O4 layer. The electric properties were measured after cooling down the samples from room temperature with a magnetic field of 50 kOe. During the measurements, the magnetic field rotated in the film plane, the electric current was fixed along the [100] axis of the sample. The angle between the magnetic field and current direction was defined as α. For ab initio calculations, the Vienna ab initio simulation package (VASP) [19] was used with the generalized gradient approximation [20] and projector augmented wave potentials [21]. Full structural relaxation in shape and volume was performed until the forces become lower than 0.01 eV/Å for determining the most stable interfacial geometries and the total energy is converged to 1  10  5 eV. A 8  8  1 K-point mesh was used in our calculations, with the energy cutoff equal to 520 eV. In the calculation, the thickness of Fe3O4 was fixed to 9 monolayers (ML), the NiO thickness was 4, 5, 6 ML, respectively.

interval of 90o for Fe3O4/NiO/MgO(001), and two peaks with a interval of 180o for Fe3O4/NiO/MgO(110), suggesting the epitaxial structure in-plane. HRTEM was employed to confirm the epitaxial structure of these bilayers further. Fig. 2(a) shows the overall image of the NiO/Fe3O4/ MgO interface. The HRTEM image at the MgO/Fe3O4 and Fe3O4/NiO interfaces in Fig. 2(b) and (d) shows the epitaxial growth of the bilayer, confirming the results obtained by XRD measurements. The corresponding selected area electron diffraction (SAED) pattern shown in Fig. 2(c) further confirmed the epitaxial fcc structure. The AMR and PHR as a function of α between magnetic field and the electric current direction ([100] crystal axis) for (001) and (110) orientated Fe3O4/NiO bilayers under different temperatures are shown in Fig. 3(a)–(d) ((a) and (c): AMR; (b) and (d): PHE). The fourfold symmetric PHR was found in both (001) and (110)oriented Fe3O4 (24 nm)/NiO (37 nm) bilayers at 90 K, suggesting that the fourfold symmetric behavior is no accident. Furthermore, the fourfold symmetric characteristics are irrespective with the orientation of films. However, as a comparison, the fourfold symmetry disappeared in single Fe3O4 layer (24 nm) as seen in Fig. 3(f), indicating that the fourfold symmetric PHE in Fe3O4 layer is ascribed to the introduction of the AFM NiO layer. In order to separate the extra anisotropy in FM layer introduced by the AFM layer, we consider that the fourfold anisotropy of a FM/AFM 3 O4 bilayer is composed of two parts: the fourfold anisotropy H Fe cub

comes from the Fe3O4 itself and the extra fourfold anisotropy H NiO cub induced by the AFM layer. The effective fourfold anisotropy K cub in the Stoner–Wohlfarth model of the Fe3O4/NiO system should be 3 O4 þ K NiO K cub ¼ K Fe cub , as follows [14]: cub

3. Results and discussions The typical XRD θ–2θ patterns for Fe3O4 (24 nm)/NiO (t nm)/MgO (001), Fe3O4 (24 nm)/NiO (t nm)/MgO (110) are shown in Fig. 1(a) and (b), respectively. Only the diffraction peaks form Fe3O4(00l) and NiO (00l), Fe3O4(ll0) and NiO(ll0) can be observed in the corresponding oriented bilayer, indicating that all of the bilayers are single phase and have the fcc structure with a preferred c-axis orientation. Furthermore, the XRD φ scans performed at 2θ¼18.289o, α¼35.3o for (001)oriented bilayer and (110)-oriented bilayer are shown in Fig. 1(c) and (d), respectively. There are four symmetric diffraction peaks with a

2 3 O4 þK NiO E ¼ K uni cos 2 αM þ ½ðK Fe cub Þ=4 cos 2αM  MH cos ðαM  αH Þ; cub

ð1Þ where H is external magnetic field, K uni and K cub are uniaxial and cubic anisotropy fields, respectively. αM and αH are angles of magnetization and external magnetic field direction with respect to the current direction (along [100] axis). By using the relation RPHR ¼ ðk=tÞM 2 sin 2αM [14], the αM αH curves (not shown) were obtained from the PHR  α curves measured in Fig. 3(b) and (d). Since the direction of magnetization of the FM layer follows the position of magnetic energy minimum given by Eq. (1), fitting the αM  αH curve

Fig. 1. XRD θ–2θ patterns of epitaxial bilayers: (a) Fe3O4/NiO/MgO(001), (b) Fe3O4/NiO/MgO(110); XRD φ scans of epitaxial bilayers: (c) Fe3O4/NiO/MgO(001), and (d) Fe3O4/ NiO/MgO(110).

P. Li et al. / Solid State Communications 220 (2015) 1–5

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Fig. 2. (a) The overall cross-sectional TEM image of Fe3O4/NiO/MgO(001); (b) and (d) show the interfacial cross-sectional HRTEM image of epitaxial Fe3O4/MgO and Fe3O4/ NiO, respectively; (c) shows the corresponding selected area electron diffraction (SAED) pattern.

Table 1 The cubic and uniaxial anisotropic fields H cub and Huni extracted from the fitting by Eq. (1), both for (001) and (110)-oriented Fe3O4/NiO bilayers. Orientation tNiO (nm)

(001)

(110)

Fig. 3. (a) and (c) show the angular dependence of AMR and PHE of Fe3O4(24 nm)/ NiO(37 nm)/MgO(001) at different temperatures; (b) and (d) represent the angular dependence of AMR and PHE of Fe3O4(24 nm)/NiO(37 nm)/MgO(110) at different temperatures; (e) and (f) show the AMR and PHE of single Fe3O4 (001) layer (24 nm) at different temperatures (All of the data were measured at H¼ 50 kOe).

1  by αH ¼ αM  arcsin 2H ðH uni þ H cub cos 2αM Þ sin 2αM gives the paraNiO 3 O4 þ H NiO meters H uni and H Fe cub . To separate H cub from the total cubic cub 3 O4 was calculated with the same anisotropy field of Fe3O4 film, H Fe cub method from the PHR  α curve of the single Fe3O4 layer (Fig. 3(f)), as shown in Table 1. For both (001) and (110)-oriented bilayers, the extra

37 46 55 64 37 46 55 64

Anisotropy fields (Oe) H cub (Oe)

3 O4 H Fe cub (Oe)

H NiO cub (Oe)

Huni (Oe)

3 O4 H Fe uni (Oe)

H NiO uni (Oe)

152.24 116.43 84.56 43.56 68.3 42.93 25.16 13.24

37.1 35.64 36.25 34.67 11.07 12.83 12.03 11.56

115.14 80.79 48.31 8.89 57.23 30.10 13.13 1.68

27.85 46.67 68.26 84.57 17.4 22.74 46.66 89.54

15.2 14.54 15.32 15.42 1.26 1.70 1.45 1.78

12.65 32.13 52.94 69.15 16.14 21.04 45.21 87.76

cubic anisotropy H NiO cub was obviously amplified by the attenuating thickness of the AFM layer. It suggests that the fourfold symmetric behavior was attributed to the extra cubic anisotropy H NiO cub introduced in Fe3O4 by the NiO layer. The uniaxial anisotropy was enhanced by the thicker NiO layer, which is consistent with the symmetry of PHE shifted to twofold with the increase of NiO layer thickness. To investigate the origin of unaxial and cubic anisotropy, respectively, the angular dependences of AMR and PHE with different NiO thicknesses were studied. As exceeding 37 nm of the AFM layer, the PHE symmetry transformed from fourfold back to conventional twofold, as seen in Fig. 4(b) and (d). This phenomenon suggests that the magnetic interactions are

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P. Li et al. / Solid State Communications 220 (2015) 1–5

Fig. 4. (a) and (b) show the angular dependence of AMR and PHE of Fe3O4(24 nm)/NiO(t nm)/MgO(001) with different NiO thicknesses; (c) and (d) represent the angular dependence of AMR and PHE of Fe3O4(24 nm)/NiO(t nm)/MgO(110) with different NiO thicknesses; (e) and (f) show the anisotropy constant obtained from (a)–(d). (All of the data were measured at T ¼90 K, H¼ 50 kOe).

predominant at the interface, and the effect of AFM layer on the interfacial exchange coupling is tunable. The H cub and H uni are plotted in Fig. 4(e) and (f). Through maintaining the thickness of Fe3O4 unchanged, the cubic anisotropic field H cub was weakened within the bilayer with thicker AFM thickness, suggesting that the fourfold symmetry mainly origins from the cubic anisotropy. To confirm the speculation of the relationship between interfacial magnetic interaction and the PHE behavior in the FM layer, we investigated the magnetic anisotropy of the bilayer in details. Phenomenologically, magnetic anisotropy of a thin film usually consists of the volume anisotropy and the interface anisotropy [22]: surf K c;u ¼ K vol c;u þ2K c;u =d, d is the thickness of the thin film. Here we surf take H c;u ¼ 2K c;u =M and get H c;u ¼ H vol c;u þ 2H c;u =d. Separating the vol volume contribution H c;u and the interface contribution H surf c;u may provide a deeper insight into the origin of the uniaxial and cubic magnetic anisotropy of the bilayers [23]. The factor 2 results from the presence of the interfaces Fe3O4–NiO and the surface of the Fe3O4 layer. We use this relation to fit Fig. 4(e) and (f), the volume and surface contribution of H cub and H uni were separated from the fitting, respectively, as shown in Table 2. The derivative parameters indicated that for both Fe3O4/NiO/MgO(001) and Fe3O4/NiO/MgO (110) bilayers, the cubic anisotropy contribution has a clear interfacial origin, while the uniaxial anisotropy was dominant in volume contribution. Therefore, we can further deduce that the extra cubic anisotropy in Fe3O4 layer was induced by the interfacial exchange interaction between FM and AFM layer. We attribute the interfacial exchange interaction to the interfacial chemical bonding structure and charge transfer. Detailed first principle calculation of the electronic structure of the Fe3O4 films was performed in order to confirm this hypothesis. The bond length across the interface is dependent on the thickness of NiO AFM layer. As shown in Table 3, the Fe–O bond (2.158 Å) with the thinnest NiO (37 nm) is shorter than the Ni–O bond (2.219 Å), implying the stronger Fe–O interaction than Ni–O. While as the NiO layer becomes thicker (5 and 6 layers), the Fe–O bond becomes longer than the Ni–O bond, indicating that the Fe–O becomes weaker with the increasing NiO thickness. Furthermore, both Fe–O and Ni–O bonds were elongated by the increasing

Table 2 The coefficients obtained from Fig. 4(e) and (f). Orientation

(001) (110)

surf B ðHu ¼ H vol u þ 2H u =dÞ

surf C ðHc ¼ Hvol =dÞ c þ 2H c

H vol u (Oe)

H surf (Oe) u

Hvol (Oe) c

Hsurf (Oe) c

0.47191  0.00912

 33.6712  19.96442

0.01142  0.04498

3.42875 12.89772

Table 3 The calculated interfacial bond lengths of Ni–O and Fe–O in Fe3O4/NiO with different number of NiO layer by first principle calculations. Bond

Ni–O Fe–O

Bond length (Å) 4 layers

5 layers

6 layers

2.219 2.158

2.591 2.663

2.680 2.730

NiO thickness, indicating the dying interface coupling with increasing AFM layer thickness. For the purpose of investigating the bonding characteristics at the interfaces, the charge density difference due to adhesion is calculated, by subtracting the charge densities of Fe3O4 and NiO having the same atomic positions with the interface supercells from that of the interface, as shown in Fig. 5. The countermaps illustrate how the valence electrons are redistributed. At the interface of Fe3O4/NiO (4 layers), substantial and localized charge redistribution is established between the FeB–O and Ni–O, indicating strong ionic interaction. Especially, the charge transfer between Fe–O is more intensive than that between Ni–O, further confirmed the stronger interaction between Fe–O. The amplified fourfold symmetric orbit of Fe  dx2  y2 at the interface (light blue orbit around Fe atom) is the main source of the extra cubic anisotropy induced by NiO layer, while the twofold symmetric orbit of Fe  d3z2  r2 at the interface (yellow orbit around Fe atom) contributes to the uniaxial anisotropy mostly. However, there is less charge accumulation inside the NiO layer than the interface. It

P. Li et al. / Solid State Communications 220 (2015) 1–5

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Wohlfarth model fitting, extra cubic anisotropic field in the bilayer structure was obviously amplified by attenuating thickness of the AFM layer, which was ascribed to the synergistic effect from interfacial bonding structure and electron transformation via the first principle calculations. The AFM layer thickness modulation of magnetic anisotropy in FM layer by the interfacial interactions from the AFM layer further provides an opportunity for tuning magnetic interactions via tuning the interfacial bond structure and electrons transformation.

Acknowledgments This work was supported by the National Natural Science Foundation of China (11204207), Foundation of Doctoral Department of Education (20120032120074) and Natural Foundation of Tianjin City (12JCYBJC11100). References

Fig. 5. (Color online) Charge density difference map for the Fe3O4/NiO with (a) 4 layers, (b) 5 layers, (c) 6 layers NiO, respectively. (Light blue orbits around Fe atom denote the fourfold symmetric orbit of Fe  dx2  y2 ; Yellow orbits around Fe atom represent the twofold symmetric orbit of Fe  d3z2  r2 ).

reveals that the interface coupling is more obvious in the bilayer with thinner NiO AFM layer (Fig. 4(e) and (f)). In contrast, there are only minor charge redistributions for Fe3O4/NiO (5 layers), the Fe– O and Ni–O interactions become weaker, while it is amazing to find that there is apparently charge transformation inside the NiO. 4. Conclusions In summary, an unexpected fourfold symmetric PHE was observed in the bilayer Fe3O4/NiO with the thinnest NiO layer (37 nm), as exceed 37 nm of the AFM layer, the bilayer further shifted to twofold symmetry. It is because that the cubic anisotropy has a main contribution from the interface origin, whereas the uniaxial anisotropy has a main volume contribution. According to the Stoner–

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