Sputter-prepared (001) BiFeO3 thin films with ferromagnetic L10-FePt(001) electrode on glass substrates
© Chang et al.; licensee Springer. 2012
Received: 9 May 2012
Accepted: 14 July 2012
Published: 3 August 2012
Highly textured BiFeO3(001) films were formed on L10-FePt(001) bottom electrodes on glass substrates by sputtering at reduced temperature of 400°C. Good electric polarization 2Pr = 80 and 95 μC/cm2, comparable to that of the reported epitaxial films, and coercivity Ec = 415 and 435 kV/cm are achieved in the samples with 20-nm- and 30-nm-thick electrodes. The BiFeO3(001) films show different degrees of compressive strain. The relation between the variations of strain and 2Pr suggests that the enhancement of 2Pr resulted from the strain-induced rotation of spontaneous polarization. The presented results open possibilities for the applications based on electric-magnetic interactions.
BiFeO3 (BFO) with a rhombohedral perovskite structure has attracted considerable attention due to its multiferroic properties above room temperature (RT) including high ferroelectric (TC = 830°C) and G-type antiferromagnetic (AFM) (TN = 370°C) transition temperatures[1–4]. Different from the spiral spin structure in bulk, BFO thin film exhibits an antiparallel AFM structure along , allowing coupling to the spins of a ferromagnetic (FM) layer at the interface. The coupling permits the possibilities of various advanced spintronic and memory devices based on the electric-magnetic interactions[2–5].
Ferroelectric properties of BFO films highly depend on preferred orientation[6–11]. (111)-textured BFO shows the highest remanent polarization 2Pr of approximately 200 μC/cm2[6–10]. Nevertheless, BFO(001) (2Pr = 40 to 120 μC/cm2) shows more advantages for practical uses, such as lower electrical coercive field (Ec), better fatigue resistance, and higher piezoelectric coefficient[9–12]. For the BFO films prepared by either pulsed laser deposition (PLD) or sputtering, the preferred orientation can be well controlled by either using proper single crystal substrates or controlling the texture of the perovskite electrode underlayers[2, 5–13].
However, the high processing temperature (Tp > 600°C)[6–10] as well as the cost of using perovskite substrates is not favorable to industry. Although it has been reported that the use of the metal electrode Pt can reduce Tp to about 500°C, single crystal substrates are still necessary for texture control of both Pt and BFO. Considering that the electric-magnetic coupling is the fundamental mechanism to function the related spintronic devices, development of FM electrode that can induce a specific texture of BFO is thus one of the most effective ways to facilitate this coupling.
However, no related investigation has been reported prior to the presented study. In this letter, we demonstrate the induction of the BFO(001) preferred orientation for the sputter-prepared thin films by strongly textured ferromagnetic electrode of L10(001) FePt on glass substrates. Structural as well as ferroelectric properties are reported in detail.
Results and discussion
The induction of strong (001) texture of BFO films using the ferromagnetic FePt(001) bottom electrode with thicknesses of 20 and 30 nm on glass substrates by rf sputtering is reported. A degree of preferred orientation (LOF = 0.79) higher than that of the film prepared by PLD on SrTiO3(001) is achieved in the sample with 30-nm-thick electrode at a reduced temperature of 400°C. 2Pr values of 80 and 95 μC/cm2 are obtained in the films with 20-nm- and 30-nm-thck electrodes, respectively, much higher than that of the BFO(001) epitaxial films with Pt(001) bottom electrode grown on single crystal substrates. The BFO(001) films with 20-nm- and 30-nm-thick electrodes exhibit different compressive strains of 0.19% and 0.84%, respectively, and the relation between the increments of 2Pr and biaxial strain suggests that the strain-induced polarization rotation mechanism reported previously is responsible for the variation of 2Pr. The results of this study demonstrate the advantages of fabricating BFO(001) films using ferromagnetic bottom electrode on non-textured substrates and open wide possibilities for advanced applications based on electric-magnetic couplings.
This research was supported by the National Science Council of Taiwan under grant nos. NSC-98-2112-M-029-001-MY3 and NSC-100-2112-M-029-002-MY3 and Tunghai Green Energy Development and Management Institute (TGEI).
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