BaTiO3

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Mar 17, 2010 - the mixed ferroelectric domain state at the BTO surface at E = −0.6 kV/cm induces a .... two well-defined magnetic states as indicated in Fig. 3(b) ...
Electric field controlled manipulation of the magnetization in Ni/BaTiO3 hybrid structures S. Gepr¨ags,1, ∗ A. Brandlmaier,1 M. Opel,1 R. Gross,1, 2 and S.T.B. Goennenwein1 1

Walther-Meißner-Institut, Bayerische Akademie

der Wissenschaften, D-85748 Garching, Germany 2

Physik-Department, Technische Universit¨at M¨ unchen, D-85748 Garching, Germany (Dated: March 11, 2010)

Abstract The manipulation of the ferromagnetic magnetization via electric fields is investigated in Ni/BaTiO3 hybrid structures. The application of an electric field to the ferroelectric BaTiO3 induces elastic strain. In the Ni/BaTiO3 hybrids, this strain is transferred into the ferromagnetic Ni layer, affecting its magnetization due to inverse magnetostriction. Two approaches to electrically alter the Ni magnetization are investigated. One approach exploits the strain-induced change of the nickel magnetic coercivity, and allows for both reversible and irreversible magnetization control. The other is based on irreversible ferroelectric domain effects in BaTiO3 , and yields two different electro-remanent magnetization states. PACS numbers: 75.85.+t 85.80.Jm 75.80.+q 75.70.Cn 75.30.Gw 75.60.Ej 75.60.JK 77.80.Dj

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Unconventional or novel functionalities can be expected if the magnetic and the dielectric degrees of freedom in a given material can be coupled. This magneto-electric interaction will be particularly strong in systems which simultaneously exhibit both ferromagnetism and ferroelectricity, i.e. in magnetoelectric multiferroics [1–3]. However, single-phase multiferroics with strong magnetoelectric coupling still are scarce. Attractive alternatives are hybrid or composite material systems, in which a ferromagnetic and a ferroelectric (or piezoelectric) compound are artificially assembled, e.g. in thin film hybrid structures [4, 5]. In these systems, the elastic coupling between the two ferroic phases enables a strong magnetoelectric coupling [6, 7]. More precisely, an electric field induces elastic strain in the ferroelectric via the inverse piezoelectric effect, or via ferroelectric domain reconfiguration. This strain is then transferred into the ferromagnetic film clamped onto the ferroelectric. In the film, inverse magnetostriction modifies the magnetic anisotropy, thus enabling an electro-mechanical magnetization control scheme [8, 9]. Several materials were used to realize such ferromagnetic/ferroelectric hybrid structures, e.g. La0.7 A0.3 MnO3 (A=Sr,Ca) on PMN-PT(001) [7], La0.67 Sr0.33 MnO3 on BaTiO3 [5], or Fe on BaTiO3 [10]. Even industrially produced multilayer capacitors with either Ni as electrodes [11] or Ni deposited on commercial actuators [9] were used to electrically change the magnetic properties. Upon using BaTiO3 (BTO) as the ferroelectric compound in such ferromagnetic/ferroelectric hybrids, the crystalline phase transitions of BTO and the corresponding large changes of the crystalline lattice parameters as a function of temperature can furthermore be exploited to tune the mechanical strain state [10, 12–16]. In this letter, we report on two different approaches to control the magnetization M in Ni thin films on BTO substrates via electric fields E at room temperature. On the one hand, M can be electrically reduced by 20% from its saturation value. This process is reversible, and driven by the mechanical strain transferred from the BTO substrate into the Ni thin film. On the other hand, irreversible ferroelectric domain effects in BTO as a function of E allow to switch M remanently. We compare the two mechanisms and extract the magneto-electric coupling coefficient α33 . A series of polycrystalline Ni thin films were deposited on 0.5 mm thick (001) oriented BTO single crystals by electron beam evaporation at a base pressure of 1×10−8 mbar. The Ni thin films were capped in-situ with a 10 nm thick Au layer to prevent oxidation. A Au bottom electrode was sputtered on the BTO backside so that an electric field E could be 2

applied across the BTO substrate. After deposition of the metallic layers, the Ni/BTO hybrid structure was heated to 450 K – well above the ferroelectric transition temperature of BTO (TC ≈ 393 K) – and slowly cooled down to 300 K at E = 4 kV/cm to pole the ferroelectric substrate. In order to investigate the manipulation of M , SQUID magnetometry measurements were performed as a function of electric E and magnetic H fields. In the following we will focus on measurements at 300 K, where BTO is ferroelectric with the polarization aligned along the c-axis of the tetragonal unit cell. This yields 6 possible polarization orientations, or three different ferroelastic domain types. If the polarization is pointing out-of-plane, the corresponding domains are called c-domains, while domains with in-plane polarization are denoted by a1 - and a2 -domains. Figure 1(a) shows the M (E) curves of a 100 nm thick Ni film, taken at a constant magnetic field H applied in the Ni film plane. Reversible changes of M up to 20% are observed as a function of E. Furthermore, the M (E) loops exhibit a butterfly like shape. This indirectly proves that the elastic strain transferred from the BTO substrate into the Ni thin film is the driving force here. We now discuss the results of Fig. 1(a) in more detail. Upon decreasing E from +4 kV/cm to −0.6 kV/cm, the BTO substrate crosses over from a ferroelectric single c-domain state into a mixed a- and c-domain state. The tetragonal unit cells of the emerging a-domains are oriented with the longer c-axis parallel to the BTO surface and therefore induce tensile strain in the Ni film. This causes a large uniaxial magnetic anisotropy due to inverse magnetostriction and thus changes the measured projection of M onto the direction of the magnetic field in those parts of the Ni thin film that are clamped to these a-domains. In contrast, the magnetic properties of Ni on top of c-domain regions stay unaffected. Therefore the mixed ferroelectric domain state at the BTO surface at E = −0.6 kV/cm induces a heterogeneous magnetic state in the Ni thin film. We note that the inverse linear piezoelectric effect of BTO leading to a maximum in-plane strain of ∆²k = d31 ∆E3 = 1.53 × 10−5 [17] can be neglected since the strain induced by a-domains ²a-domain =

cBTO −aBTO aBTO

= 0.0125 [18]

is almost three orders of magnitude larger. Hence the reduction of M when going from E = +4 kV/cm to E = −0.6 V/cm should be a direct measure of a-domains emerging in the BTO substrate. The BTO domain configuration for different applied E was determined using high resolution x-ray diffraction and no difference between E = +4 kV/cm and E = −4 kV/cm was observed. Therefore, in contrast to Xu et al. [19] there is no evidence for 3

non-switchable a-domains in our Ni/BTO hybrid structure. The magnitude of the electric field induced change of M is largest at µ0 H = 0 mT, reaching nearly 20%. This value is in good agreement with previous measurements on La0.67 Sr0.33 MnO3 /BTO epitaxial hybrid structures [5]. Moreover, at zero magnetic field, the M values measured at the beginning and at the end of the E sweep at +4 kV/cm, respectively, do not coincide. This is clear evidence for irreversible magnetic domain effects induced by E. By increasing H, the irreversible magnetic domain effects are suppressed, but a finite variation M (E) is still observed. In the inset of Fig. 1(a) the H dependence of the maximum magnetization change ∆M with E is shown. The magnetoelectric coupling decreases exponentially with H up to µ0 H = 400 mT (dashed line in the inset of Fig. 1(a)). At µ0 H > 400 mT, a weak variation of M as a function of E is still visible, although the magnetization of the Ni film is saturated (see Fig. 1(b)). These M (E) loops which are reminiscent of P (E) hysteresis loops, and might be due to a nonvanishing influence of the surface charges from ferroelectric c-domains on the saturation magnetization of the Ni thin film. Influences of strain on the saturation magnetization Ms can be excluded from symmetry considerations as strain should yield Ms (E) loops with even symmetry with respect to E = 0 kV/cm. The linear magnetoelectric coupling coefficient α33 can be directly measured by using the ac option of a SQUID magnetometer [20]. In this approach, an ac electric field with amplitude Eac = 0.4 kV/cm and a frequency of 10 Hz is superimposed on the dc electric field E, and the magnetic response is measured using a lock-in technique. Fig. 1(c) shows the linear magnetoelectric coupling constant α33,dc = µ0 ∆M/∆E and α33,ac = µ0 ∂M/∂E derived from the dc and ac measurements respectively. Both measurements were made at µ0 H=0 mT, after saturating M at µ0 H = 1000 mT. Overall, the two magnetoelectric coupling constants match very well. The slightly smaller value of α33,ac can be understood, considering the large time constant involved in the accumulation of a-domains in the BTO substrate. Figure 2(a),(b) show M (H) loops recorded for different E. A rectangular shape of the hysteresis loops is observed at E = +4 kV/cm, indicating that H is aligned in-plane along an easy direction of the Ni film. Upon decreasing E to −0.6 kV/cm, a-domains are formed, resulting in a ferromagnetic multi-domain state in the Ni thin film as discussed above. Therefore, the observed M (H) loop will be a superposition of the M versus H loops of the 4

different magnetic domain regions. As compared to M (H) at E = +4 kV/cm, the loop obtained at E = −0.6 kV/cm reveals not only a reduction of M below µ0 H = 200 mT, but also a change in the coercive field from µ0 Hc = 5.5 mT to 5.0 mT. Therefore, Hc can be manipulated by ≈ 10% via electric fields in this sample. This value is smaller than the one reported recently in Fe/BTO hybrid structures [10], but of course depends strongly on the amount of a-domains emerging in the respective samples. This E control of Hc can be utilized to switch M irreversibly. This is shown in Figs. 2(c) and (d). After preparing M in a well-defined state at µ0 H = −100 mT and E = +4 kV/cm, H is increased from −100 mT (A) to 5.2 mT (B1 ) at E = +4 kV/cm. At point (B1 ), M has not yet switched, and is therefore essentially oriented anti-parallel to H in a metastable state [9]. Reducing E from +4 kV/cm (B1 ) to −0.6 kV/cm (B2 ) at µ0 H = 5.2 mT (Fig. 2(d)) then results in an almost demagnetized Ni thin film with a multi-domain state and thus a low magnetization M = 52.7 kA/m. Upon increasing E again from −0.6 kV/cm (B2 ) to +4 kV/cm (B3 ), M aligns along the external field as evident from the large positive M = 290 kA/m. A subsequent scan of E from +4 kV/cm to −0.6 kV/cm and back (B3 →B4 →B5 ) only reduces M by nearly 20%, but does not switch the magnetization orientation again. Thus, Fig. 2(d) demonstrates that M can be switched irreversibly by sweeping the electric field. To restore the initial state, H must be decreased from µ0 H = 5.2 mT (B5 ) to −100 mT (C) at a constant E = +4 kV/cm (see Fig. 2(c)). Moreover, as can be seen in Fig. 2(d) the second electric field sweep (B3 →B4 →B5 ) yields a hysteretic M (E) loop with two different values of the remanent magnetization at E = 0 V/cm, depending on the sweep history. This is illustrated in more detail in Fig. 3(a). Two different M states at E = 0 kV/cm are achieved by sweeping E in a small range from E = +1 kV/cm to −0.2 kV/cm and back to +1 kV/cm. We attribute these two different states to irreversible ferroelectric domain wall effects in the BTO substrate. M can even be switched from one electro-remanent state at E = 0 kV/cm to another, and back to the initial state, by applying the electric field sequence +0.2 kV/cm→0 kV/cm→−0.2 kV/cm→0 kV/cm to the BTO substrate in zero magnetic field (see Fig. 3(b)). The two remanent magnetic states at 0 kV/cm differ by 0.7%. In other words, it is possible to change M electrically between two well-defined magnetic states as indicated in Fig. 3(b). We note that the time constant is of the order of seconds. Moreover, the dynamics of the switching strongly depend on the kinetics of the ferroelectric domain walls and can be influenced by poling the BTO substrate 5

as well as ramping or switching of the electric field [5]. In summary, we showed that the interaction between dielectric, mechanical and magnetic degrees of freedom allows to tune electrically the magnetization in Ni/BTO hybrid structures. M can be irreversibly switched using the electric field dependence of the magnetic coercivity. Alternatively, M also can be changed reversibly by more than 20% due to the combined action of electro-elastic strain and inverse magnetostriction. Furthermore, two different remanent magnetization states can be realized electrically. These electro-remanent magnetization states are due to irreversible domain wall effects in BTO. Financial support by the Deutsche Forschungsgemeinschaft via the priority program 1157 (project no. GR 1132/13), GO 944/3-1, and the German Excellence Initiative via NIM is gratefully acknowledged.



Electronic address: [email protected]

[1] M. Fiebig, J. Phys. D 38, R123 (2005). [2] W. Eerenstein, N. D. Mathur, and J. F. Scott, Nature 442, 759 (2006). [3] R. Ramesh, and N. A. Spaldin, Nature Mater. 6, 21 (2007). [4] C. W. Nan, Phys. Rev. B 50, 6082 (1994). [5] W. Eerenstein, M. Wiora, J. L. Prieto, J. F. Scott, and N. D. Mathur, Nat. Mater. 6, 348 (2007). [6] Y. Zhang, Z. Li, C. Deng, J. Ma, Y. Lin, and C.-W. Nan, Appl. Phys. Lett. 92, 152510 (2008). [7] C. Thiele, K. D¨orr, O. Bilani, J. R¨odel,and L. Schultz, Phys. Rev. B 75, 054408 (2007). [8] A. Brandlmaier, S. Gepr¨ags, M. Weiler, A. Boger, M. Opel, H. Huebl, C. Bihler, M. S. Brandt, B. Botters, D. Grundler, R. Gross, and S. T. B. Goennenwein, Phys. Rev. B 77, 104445 (2008). [9] M. Weiler, A. Brandlmaier, S. Gepr¨ags, M. Althammer, M. Opel, C. Bihler, H. Huebl, M. S. Brandt, R. Gross, and S. T. B. Goennenwein, New J. Phys. 11, 013021 (2009). [10] W. Sahoo, M. Polisetty, J. L. Duan, J. L. Sitaram, J. L. Jaswal, J. F. Tsymbal, and N. D. Binek, Phys. Rev. B 76, 092108 (2007). [11] C. Israel, S. Kar-Narayan, and N. D. Mathur, Appl. Phys. Lett. 93, 173501 (2008). [12] M. K. Lee, T. K. Nath, C. B. Eom, M. C. Smoak, and F. Tsui, Appl. Phys. Lett. 77, 3547 (2000).

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[13] D. Dale, A. Fleet, J. D. Brook, and Y. Suzuki, Appl. Phys. Lett. 82, 3725 (2003). [14] H. F. Tian, T. L. Qu, L. B. Luo, J. J. Yang, S. M. Guo, H. Y. Zhang, Y. G. Zhao, and J. Q. Li, Appl. Phys. Lett. 92, 063507 (2008). [15] C. A. F. Vaz, J. Hoffman, A.-B. Posadas, and C. H. Ahn, Appl. Phys. Lett. 94, 022504 (2009). [16] F. D. Czeschka, S. Gepr¨ags, M. Opel, S. T. B. Goennenwein, and R. Gross, Appl. Phys. Lett. 95, 062508 (2009). [17] E. W. Lee, Rep. Prog. Phys. 18, 184 (1955). [18] L. A. Shebanov, Phys. stat. sol. (a) 65, 321 (1981). [19] F. Xu, S. Trolier-McKinstry, W. Ren, B. Xu, Z.-L. Xie, and K. J. Hemker, J. Appl. Phys. 89, 1336 (2001). [20] P. Borisov, A. Hochstrat, V. V. Shvartsman,and W. Kleemann, Rev. Sci. Instrum. 78, 106105 (2007).

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Figures

FIG. 1: (a), (b) Magnetization M of a 100 nm thick Ni film on BTO plotted versus the electric field E applied across the BTO substrate at different applied magnetic fields H. M is normalized to the value Mpol obtained at E = +4 kV/cm. (c) Comparison of the magnetoelectric coupling coefficient α33 obtained from dc and ac SQUID measurements. For clarity, only one direction of the electric field sweep is displayed.

FIG. 2: (a), (b) In-plane magnetic hysteresis loops M (H) for different electric fields applied across the BTO substrate. (c) The magnetization orientation can be switched either via electric or via magnetic fields. (d) Evolution of the magnetization during the electric field sweep (+4 kV/cm (B1 )→ −0.6 kV/cm (B2 ) →+4 kV/cm (B3 )) at µ0 H = 5.2 mT. The second electric field sweep (+4 kV/cm (B3 ) →−0.6 kV/cm (B4 ) →+4 kV/cm (B5 )) is indicated by open symbols .

FIG. 3: (a) M (E) loops measured at µ0 H = 5 mT. The two different electro-remanent magnetic states at E = 0 kV/cm are indicated by the full circles. (b) By appropriately ramping the electric field, M can be switched back and forth between the two electro-remanent states, indicated by the full circles. The measurement was performed at µ0 H = 0 mT.

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(a)

1.00

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20 15 10 5 0

0.75 -5

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μ0H (mT)

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0mT 10mT 25mT 50mT 100mT 500mT 1000mT

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(c)

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μ0H = 5.2mT

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