Nanopillars with E-field accessible multi-state (N ≥ 4) magnetization having giant magnetization changes in self-assembled BiFeO3-CoFe2O4/Pb(Mg1/3Nb2/3)-38at%PbTiO3 heterostructures

We have deposited self-assembled BiFeO3-CoFe2O4 (BFO-CFO) thin films on (100)-oriented SrRuO3-buffered Pb(Mg1/3Nb2/3)0.62Ti0.38O3 (PMN-38PT) single crystal substrates. These heterostructures were used for the study of real-time changes in the magnetization with applied DC electric field (EDC). With increasing EDC, a giant magnetization change was observed along the out-of-plane (easy) axis. The induced magnetization changes of the CFO nanopillars in the BFO/CFO layer were about ΔM/MrDC = 93% at EDC = −3 kv/cm. A giant converse magnetoelectric (CME) coefficient of 1.3 × 10−7 s/m was estimated from the data. By changing EDC, we found multiple(N ≥ 4) unique possible values of a stable magnetization with memory on the removal of the field.

phase ME materials 23 . Furthermore, lateral strain control is limited beyond a critical thickness (∼10 0nm), above which the strain may fully relax 23 : (1-3) heterostructures thus can have significantly reduced thickness effects, resulting in thicker films with higher ME coupling. Wang et al. 15 showed that by controlling the size, shape and volume fraction ratio of the CFO nanopillar phase, the magnetic properties could be tailored. The nanopillar morphology was shown to provide a contribution to the shape anisotropy 15 that can constrain the rotation of the magnetization direction. This may offer an approach to a new multi-state magnetization dependent on electric field history.
Because the STO substrate lacks a piezoelectric effect, the piezoelectric response of nanopillar heterostructures of BFO-CFO is completely determined by the BFO matrix phase of the epitaxial layer. In addition, BFO-CFO heterostructures suffer from notable leakage current 21,24 , as the E-field must be applied to the BFO-CFO layer to induce piezoelectric shape changes 18,20,21 . This limits the BFO-CFO/STO heterostructures from realizing the potential of its full ME coefficients 21 . Recently, a CFO single phase layer was deposited on piezoelectric Pb(Mg 1/3 Nb 2/3 ) 0.62 Ti 0.38 O 3 (PMN-38PT) substrates by Wang et al. 16,25,26 . Because of the high piezoelectric coefficient of PMN-38PT, a large magnetization change was observed in the CFO film under application of a large E-field. Furthermore, the E-field was applied to the PMN-PT substrate rather than the CFO layer, resulting in reduced leakage currents, and enhanced magnetization changes in the CFO layer.
A vertically integrated nanopillar BFO-CFO heterostructure has also recently been epitaxially deposited on SrRuO 3 buffered Pb(Mg 1/3 Nb 2/3 ) 0.70 Ti 0.30 O 3 (SRO/PMN-30PT) substrates 27 . This vertically integrated (1-3) heterostructure allows for a large magnetic anisotropy, which enables E-field tunable magnetic switching 28 . As a substrate, PMN-38PT has a small lattice mismatch with both CFO and BFO 17,29 . Unlike BFO-CFO/STO heterostructures, the E-field induced strain of BFO-CFO/PMN-PT heterostructures is mainly provided by domain reorientation in the PMN-PT substrate 28,30,31 . The combination of the large d 33 value of PMN-38PT 28 and the unique constraint of the vertically integrated two-phase structure results in a large ME coefficient. Recently, a giant ME coupling has been reported for BFO-CFO/PMN-30PT heterostructures by our group 28 . However, the number of magnetization states under different E-fields was not studied. Here, we report a self-assembled two-phase vertically integrated BFO-CFO/SrRuO 3 /PMN-38PT heterostructure by pulsed laser deposition (PLD). This BFO-CFO heterostructure possesses large magnetization changes in the CFO nanopillars by application of a DC electrical bias (E DC ) to the substrate. A giant ME coefficient has been obtained. It was also found feasible to access multiple ( ≥ N 4) stable magnetization states with memory.  Magnetization measurements were then performed in response to an E DC applied to the substrate. The electric field was applied along the out-of-plane direction. Figure 2(a,b) show the M-H loops under various E DC along the OP and IP directions, respectively. From these data, it can be clearly seen that the easy axis of the CFO nanopillars lies along the OP direction. This is a reflection of the shape anisotropy of the nanopillar structure, which has a much larger thickness than width. As shown in the insert of Fig. 2(a,b), the remnant magnetization (M r ) increases along OP with increasing E DC . Furthermore, M r decreases with increasing E DC along IP, although the change is small. This is due to the combination of the anisotropy of the magnetostriction coefficient of CFO (λ CFO ) 16 and the piezoelectric coefficient of PMN-xPT. The E-field induced strain in BFO-CFO/PMN-PT heterostructure is mainly due to the domain reorientation in the PMN-PT substrate, unlike that in BFO-CFO/STO heterostructures 28,30,31 . The BFO matrix has an important effect in imparting a large shape anisotropy to the CFO nanopillars. With increase of the aspect ratio of the CFO nanopillars, the shape anisotropy is significantly enhanced 15 , altering the easy axis from IP to OP directions. From the right hand axis of Fig. 2(c), it can be seen that the PMN-38PT substrate undergoes a compressive stress along the OP direction under E DC , resulting in the BFO-CFO nanocomposite layer experiencing a tensile stress along IP. Since λ CFO < 0, under a tensile IP stress, the easy axis of the CFO will rotate towards the OP direction 34 . As a consequence, the nanopillars will experience an increase in M r with increasing E DC , and vice versa a lower M r along the IP.

Results and Discussion
The remnant-to-saturation (M M / r s ) magnetization ratio in response to E DC (−7 kV/cm < E DC < 7 kV/cm) applied to the PMN-38PT substrate is shown in Fig. 2(c). Data are given for E DC applied along the OP and IP directions. As can be seen in Fig. 2 Fig. 2(d)), which were slightly greater than zero. The highest value of ΔM/M DC (∼0.90) was found near E DC = −3 kV/cm, which corresponded to the electric coercive field under negative polarity. This evidences that BFO-CFO/SRO/PMN-38PT heterostructures have their largest E DC induced ME coupling when the polarization reverses. The maximum value of ΔM/M rDC (∼90%) is notably higher than the largest value (66%) previously reported for a single CoFeB layer on PMN-30PT 1 .   Fig. 2(c). The magnetization direction with positive or negative E DC did not switch, but increased in value following a trajectory corresponding to the M-H loop (see Fig. 2(a)). After removing E DC , two stable magnetization states were accessible depending on the E DC direction.
These data in Fig. 3(a) reveal a strong coupling between the PMN-PT substrate and the BFO-CFO nanocomposite layer. The magnetic domains in the nanocomposite layer may rotate under < E 3kV/cm DC , resulting in good E DC tunable properties. The converse magnetoelectric coefficient (α) was then calculated from the data in Fig. 3(a), using the equation: , where µ 0 is the permeability of free space, and α is in units of s/m. The value of α was estimated to be 1.3 × 10 −7 s/m, again taken under = H 0 DC in a previously magnetized state. This magnetization coupling coefficient is much higher than that previously reported (about 10 −10 s/m) for BiFeO 3 -CoFe 2 O 4 /SrRuO 3 /SrTiO 3 heterostructures 21 , and is close to values reported by Eerenstein et al. 35 for LSMO/PMN-PT (2 × 10 −7 s/m). However, this prior investigation 35 found such high values only over a limited temperature range. Our results show a large magnetoelectric coupling tunable under E DC , which could be used over a wide range of temperatures below 375 K.
Two different magnetization values were found to be stable upon removing E DC , whose values were dependent on E DC histories (see Fig. 3(a)). To better illustrate these two states, the value of the M M / r s ratio is shown as a function of E DC in Fig. 3(b). We note that these data were taken under 14. The trends were also consistent with the data in Fig. 2(d), where it can be seen upon removing E DC that the E-induced strain relaxed, but its value under E DC = 0 was different between positive and negative bias sweeps. The highest M M / r s ratio value  Fig. 4. These states were for a previously magnetized condition, beginning from M r0 . As also shown in Fig. 4 . We note that our investigations were done in a vertically integrated two phase ME layer on PMN-PT for < E kV cm 3 / . This vertically integrated heterostructure with multistate ( ≥ N 4) values was tunable by E DC are more relevant to integrated memories and logic than layer-by-layer ones. They have a high number of magnetic nanopillars; offer multiple stable magnetization states, which are accessible by E DC ; and consume little power on changing states (i.e., passive). These multi-state heterostructures thus have the potential for neuromorphic-like applications.
There are several approaches to further improve the E-field induced coupling and to increase the number of stable magnetization states. One is the composition of the PMN-xPT substrate and its proximity to the morphotropic phase boundary (MPB). Our substrates were in the T-phase field with x = 38. However, compositions closer to the MPB (x = 35) have monoclinic (M), tetragonal (T), rhombohedral (R) and orthorhombic (O) phases that are close in energy 37 . In this case, application of E DC results in induced phase transformations, where the induced phase remains metastable on removal of E DC ; for example, the R → O phase transformation in PMN-32PT 38 . The availability of these metastable phases near the MPB offers the possibility of additional multistate magnetization values with E DC for BFO-CFO/PMN-PT heterostructures. Second is the composition of the two phase BFO-CFO target, which was selected at 65at%BFO/35at%CFO in this study. By changing the composition of the BFO-CFO target, the aspect ratio of the CFO nanopillars could be modified, which would result in a change in the shape anisotropy (K shape ). A previous report 15 has shown with increasing aspect ratio of the CFO nanopillars (from 3:1 to 5:1) that the values of M r and H C along OP were significantly increased. Increasing K shape could also help stabilize additional non-volatile multi-state magnetization values.

Conclusion
In summary, self-assembled nanopillar BFO-CFO two-phase layers have been deposited on SRO buffered PMN-xPT (100) substrates. Epitaxial growth of the vertical two-phase layers was shown by XRD, and a dense nanopillar surface was observed in AFM/MFM images. Large magnetization changes under applied E DC were found along the easy magnetization axis, where the M M / r s ratio exhibited a butterfly loop with E DC . The value of ΔM/M rDC was calculated, and the maximum was found to be ∼90%. The converse magnetoelectric coupling coefficient was calculated to be 1.3×10 −7 s/m. Real time changes in the magnetization with E DC were measured, and multiple stable magnetization states ( ≥ N 4) were found on the removal of field.

Methods
A 65%BFO-35%CFO composition ratio was chosen for the substrates. All thin films were deposited by PLD. PMN-38PT (100) single crystal substrates were grown by the Shanghai Institute of Ceramics Chinese Academy Sciences. Prior to the deposition, the substrates were cleaned with acetone and alcohol via ultrasonication. First, a 10 nm SRO layer was deposited on the PMN-38PT at 700 °C, 1.5 cm J/ 2 energy density and 150 mTorr O 2 atmosphere. After annealing under 700 °C and 150 mTorr O 2 atmosphere for 30 min, a 200 nm BFO-CFO heterostructure was deposited at 650 °C, 1.2 cm J/ 2 energy density and 90 mTorr O 2 atmosphere. The sample was then annealed at 700 °C and 100 Torr O 2 . Crystal structures were determined by X-ray diffraction (Philips X'Pert system) scans. Magnetic hysteresis curves were recorded using a vibrating sample magnetometer (VSM, Lakeshore 7300 series). Atomic force microscopy (AFM) and magnetic force microscopy (MFM) images were obtained (Dimension 3100, Vecco), which were used to study the film surface quality and magnetic domain structures.