Making an electric-field-controlled giant magnetoresistance device using multiferroic material
We make giant magnetoresistance (GMR) magnetic multilayer devices (NiFe/Cu/NiFe) on top of BiFeO3 films that are simultaneously ferroelectric and antiferromagnetic. We observe an exchange-bias pinning on the bottom magnetic layer of the multilayer due to interaction with BiFeO3, which can be altered by an electric field applied to the BiFeO3 and is partially reversible by reversing the field. We are working to use control over the exchange bias to achieve electric-field-driven manipulation of the magnetization direction of the bottom NiFe magnetic layer.
Chen Wang and Dan Ralph
BiFeO3 is the only known room-temperature multiferroic material showing both ferroelectric and antiferromagnetic order. Because the two order parameters are intrinsically coupled to each other, it has been shown that an applied electric field that switches the electric polarization of a BiFeO3 domain will switch the antiferromagnetic order parameter as well . It has also been observed that BiFeO3 can produce an exchange bias pinning field on an adjacent magnetic film (CoFe) at the interface, and this exchange bias field can be switched along with the antiferromagnetic order parameter . This presents a possibility that we can make a magnetic storage device controlled entirely by an applied electric field, with negligible power consumption.
We grow BiFeO3 films of 50-100 nm on top of SrTiO3 , DyScO3, or TbScO3 substrates by pulsed laser deposition or molecular beam epitaxy, and then transfer the substrates into a sputtering chamber after a brief sonication cleaning with acetone/IPA solvent. We sputter a multilayer stack (from bottom to top) of Py (2.5 nm) / Cu (4-8 nm) / Py (2.5 nm) / Pt (2.5 nm) [where Py is permalloy = Ni81Fe19] onto the BiFeO3 surface in a 200 Oe magnetic field, and then etch the metallic layers into either micron-scale (50 µm × 2 µm) or nano-scale (80 nm × 5 µm) devices by ion milling. The former pattern is done by photolithography and the latter is done by electron-beam lithography. We put copper contacts on both ends of the stripe-shaped devices for current-in-plane electrical measurements. Most device stripes are designed to be tilted 45° away from the exchange bias direction but other angles were also attempted. We also deposit Cu electrodes 1.5-2 µm away and along both sidelines of the device stripe using photolithography and ion beam deposition. This pair of electrodes is used to apply an electric field across the area of BiFeO3 that provides exchange bias pinning under the magnetic stripes (Fig. 1).
We choose permalloy (Py) as our magnetic material because its magnetization direction can be switched easily relative to other magnets. We observe exchange bias pinning between Py and BiFeO3 that usually exceeds the coercive field of the Py film on BiFeO3 and can sometimes be as large as 90 Oe for the 2.5 nm Py film. Our resistance measurements on the devices exhibit the current-in-plane giant magnetoresistance effect, and the exchange bias is demonstrated by a shift of the magnetic hysteresis loop corresponding to the bottom Py layer pinned by BiFeO3(Fig. 2).
An applied electric field can switch the net in-plane polarization of the ferroelectric BiFeO3 by 90°, and the exchange bias direction is expected to be switched by 90o as well (as shown in Fig. 1) so that its projection along the device stripe is reversed. Since the coercive field of the magnetic layer is smaller than the magnitude of exchange bias (as shown in Fig. 2), such a switching of exchange bias can in principle drive switching of the pinned layer magnetization direction near zero applied magnetic field. We have applied voltages on BiFeO3 up to 140-200 V in 20 ms pulses on the 4-6 µm electrode spacing, producing electric fields larger than typical electric polarization coercivity of the BiFeO3 on these substrates. However, thus far we have not observed full reversal of the exchange bias. Instead in some samples we find that the exchange bias is altered by an applied electric field in a partially reversible fashion. Electric field in one direction can reduce the exchange bias, while a reversed electric field can produce a partial restoration (Fig. 3), but after repeated electric poling the exchange bias eventually decreases and saturates to a small value.
Piezo-force microscopy studies of our devices show that the domain structures of the BiFeO3 between the device and one of the side electrodes change after the electric field has been applied, in the sense that the direction of the domain walls has rotated by 90° (Fig. 4). This is consistent with the domain structure changes in previous studies and with the expected 90° switch of the net in-plane polarization. However, we also note that after back-and-forth poling the domain size becomes much larger and more spatially-varied than in its pristine state, indicating some irreversibility of the complex multi-domain ferroelectric ordering. The origin of exchange bias between BiFeO3 and a ferromagnetic metal is also still an open question, but some inverse correlation between the ferroelectric domain size and exchange bias has been established in the case of CoFe . In future work we will use nano-scale devices to study the exchange pinning within a single domain of BiFeO3 so as to greatly reduce the complexity associated with the changing domain structures.
Figure 1: Microscope image of the device geometry.
Figure 2: Resistance of the device versus magnetic field applied along the stripe.
Figure 3: The GMR hysteresis loop for the pinned Py layer between a sequence of positive and negative applied electric field. Note that the center of the loop (exchange bias) responds to the polarity of the electric field.
Figure 4: Comparison of the ferroelectric domain structure before (left) and after (right) electric field has been repeated applied.
- T. Zhao et al. Electrical control of antiferromagnetic domains in multiferroic BiFeO3 films at room temperature, Nat. Mater. 5, 823 (2006).
- Y.-H. Chu et al. Electric-field control of local ferromagnetism using a magnetoelectric multiferroic, Nat. Mater. 7, 478 (2008).
- L. W. Martin et al. Nanoscale Control of Exchange Bias with BiFeO3 Thin Films, Nano Lett. 8, 2058 (2008).
Last updated: 28-Jul-2011