Magnetoelectronics with magnetoelectrics

Magnetoelectric fi lms are proposed as key components for spintronic applications. The net magnetic moment created by an electric fi eld in a magnetoelectric thin fi lm infl uences the magnetization state of a neighbouring ferromagnetic layer through exchange coupling. Pure electrical control of magnetic confi gurations of giant magnetoresistance spin valves and tunnelling magnetoresistance elements is therefore achievable. Estimates based on documented magnetoelectric tensor values show that exchange fi elds reaching 100 mT can be obtained. We propose a mechanism alternative to current-induced magnetization switching, providing access to a wide range of device impedance values and opening the possibility of simple logic functions. The emerging fi eld of spintronics is already successful for applications in magnetic readheads and sensors. They involve giant magnetoresistance (GMR) [1-3] and tunnel magnetoresistance (TMR) effects [4], which provide a change of resistance related to a modifi cation of the magnetic confi guration between two neighbouring ferromagnetic fi lms. Evolution beyond passive magnetoelectronic components is envisioned in the next generation of spintronics devices, which should combine memory and logical functions and promises to set new standards in future information technology [5]. There has been growing interest in studying a direct method for magnetization reversal involving spin transfer from a spin-polarized current injected into the device. This effect has been theoretically predicted by Slonczewski [6] and Berger [7], and has been experimentally confi rmed by several groups [8-12].Experiments and theory agree on the necessity of applying signifi cant current densities (larger than 1011 A m 2) for switching the orientation of the magnetic nanoparticle. Scaling down of the device area makes spin-transfer an attractive alternative to stray magnetic fi eld techniques for samples of size below 100 nm × 100 nm. However, the technical diffi culties involved in making reliably such small structures, the necessity to apply large currents and avoid heating of the samples, and the intrinsic low sample resistance (of the order of a few ohms) are limiting the practical use for GMR devices. Applications in TMR devices are hindered by the large current density enforced in a very thin insulator, and the few reports on TMR systems are not conclusive [13, 14]. Copyright © 2005 IOP Publishing Ltd. Used by permission. 39 INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. PHYS.: CONDENS. MATTER 17 (2005) L39–L44 DOI:10.1088/0953-8984/17/2/ 40 LETTER TO THE EDITOR We propose device architectures combining a GMR or TMR device with a magnetoelectric (ME) fi lm, where the electric fi eld is used as alternative means for controlling the magnetic confi guration of spintronics devices. ME properties were originally predicted by Curie, concurrently to the discovery of ferromagnetism [15]. In an ME material, an electric fi eld E induces a magnetic moment M = α E, where α is the tensor of ME susceptibility. Landau and Lifschitz clarifi ed the necessary crystalline symmetry conditions [16], which are the absence of temporal and spatial inversion, but invariance with respect to the combination of both operations. Breaking time and spatial inversion symmetry can be realized by magnetic order and electric polarization, respectively. The prototypical ME example is the antiferromagnetic Cr2O3 [17, 18], where electric polarization and magnetization are fi eld induced. Evidence of ME properties has also been found for several other materials: GaFeO3, Ni3B7O13I, LiMnPO4 or Y3Fe5O12 [19]. Recently, major attention has been paid to the ME effect in magnetic ferroelectrics where polarization spontaneously occurs [20]. Among these, the multiferroic systems exhibit a coexistence of spontaneous ferromagnetic and ferroelectric order, and the coupling between magnetization and polarization gives rise to a pronounced ME response [21]. It is needless to say that a variety of spectacular applications, in particular in the fi eld of non-volatile data storage, can be expected if ultimately reversal of one order parameter is achieved by applying the conjugate fi eld of its coexisting counterpart. Recently, there have been promising demonstrations of cross coupling involving polarization reversal by magnetic fi elds in TbMn2O5 and electric fi eld stimulated ferromagnetic order in the hexagonal HoMnO3 [22, 23]. Epitaxially grown fi lms of BiFeO3 provide an attractive alternative to bulk multiferroic phases due to enhanced polarization and magnetization caused by constraints of heteroepitaxial growth [24]. In this letter we focus on spintronic applications which take explicit advantage of the antiferromagnetic properties of ME material. Antiferromagnetic order is only slightly affected by moderate external magnetic fi elds, but provides control of the magnetic confi guration of spintronic devices through strong exchange coupling to adjacent ferromagnetic fi lms. Electric fi elds tune the coupling via the ME effect and provide a novel degree of freedom, allowing the implementation of simple logical functions. In the simplest microscopic description of the prototypical ME compound Cr2O3 [25], Cr3+ ions located on different sublattices are shifted by the fi eld to non-equivalent positions and experience distinct strength of the crystal fi eld. This changes the g-tensor and the single ion anisotropy in a non-equivalent way, and modifi es the exchange integrals between the ions. The magnetic moment created by an electric fi eld remains small. For Cr2O3 with typical applied electric fi elds of the order of 10 5 V m 1 on a millimetre-sized sample, the magnetic moment is only of the order of 10-5 μB per Cr atom. However, if we apply electric fi elds reaching dielectric breakdown values of thin fi lms (109 V m-1), the extrapolation of the linear M(E) behaviour predicts a magnetic moment of several percent of μB per atom. If an ME layer is adjacent to a ferromagnetic fi lm, exchange coupling allows a shift μ0He of the ferromagnet hysteresis, with magnitudes becoming relevant for applications as spin valves. Estimates by Hochstrat et al [26] indicate that an exchange fi eld of 0.2 mT is expected for an applied electric fi eld of 105 V m-1. Linear extrapolation predicts values of the order of 100 mT if electric fi elds approaching dielectric breakdown values are applied. Experimental data providing indications of ME infl uence on magnetoresistance properties have been reported in CrO2/Cr2O3/CrO2 junctions, where the Cr2O3 fi lm is naturally growing between two CrO2 crystallites. These results were originally presented in the framework of current-induced switching models, questioning however the small current density occurring in these devices [27]. Figure 3 shows that the conductance versus voltage of these junctions exhibits hysteretic and asymmetry behaviour at temperatures higher LETTER TO THE EDITOR 41 Figure 1. Conductance versus bias voltage for a CrO2/Cr2O3/CrO2 junction. Asymmetry and hysteresis of the curve are absent at temperatures typically below 20 K, corresponding to the negligible magnetoelectric properties of Cr2O3. Figure 2. Schematics of the magnetoresistance curve of a TMR device involving an ME fi lm as a tunnel barrier. Half-hysteresis curves are shown, after saturation at positive fi eld values. The arrows denote the magnetization directions, with the bottom layer FM1 being harder (or pinned) than the top one FM2. The dashed curve is the expected TMR behaviour. The change of voltage polarity changes the direction of the net magnetization of the ME layer, adding an exchange bias magnetic fi eld to the resistance curve. The two colours indicate shifting of half-hysteresis curves towards positive or negative fi elds, depending on the polarity of the applied voltage. At zero magnetic fi eld, the change of voltage polarity changes the resistance value of the device (dots). than typically 50 K, corresponding to the occurrence of signifi cant ME longitudinal tensor components in Cr2O3 [18]. Applying an external magnetic fi eld destroys the effect, indicating the magnetic nature of the phenomenon, and suggesting that the magnetic confi gurations modifi ed by the applied voltage are zero-fi eld-cooled metastable states. The lack 42 LETTER TO THE EDITOR Figure 3. Schematics of the magnetoresistance curve of a GMR device involving an ME fi lm as a pinning layer. The bias voltage ΔU measures the resistance of the device in a current-in-plane geometry. Half-hysteresis curves are shown, after saturation at positive fi eld values. The change of polarity of the ME layer voltage V changes the direction of the net magnetization of the pinning fi eld. The pinned layer FM1 switches fi rst at large positive fi eld (red), or second at large negative fi eld (blue). The low fi eld magnetic confi guration is therefore either antiparallel (red) or parallel (blue), controlled by the ME voltage V. of control of the growth of the Cr2O3 fi lms, and the presence of impurities governing the magnetoresistance properties, makes the interpretation of the data more complicated [28]. We have however strong indications that the applied voltage modifi es the local magnetic confi guration, in a temperature range corresponding to the existence of signifi cant ME properties of Cr2O3. We propose to take advantage of an antiferromagnetic ME thin fi lm as a dielectric tunnel junction between two ferromagnetic metallic layers. A tunnel barrier is the ideal system for sustaining very high electric fi elds, reaching 1 V nm-1, corresponding to the bulk dielectric breakdown values previously mentioned. We expect therefore a signifi cant net magnetization to occur in an ME barrier. The basic principle of operation of the device involves taking advantage of the exchange fi eld between the magnetized ME layer and the two adjacent ferromagnetic fi lms. This creates a shift of the magnetization curves of both ferromagnetic layers proportional to the magnetization in the ME layer, or the applied voltage in the device. Following fi gure 1, we consider a TMR device made of a soft magnetic layer FM2 and a hard (or pinned by a bottom layer, not shown in the fi gure) bottom layer FM1. An exchange fi eld value of the order of the saturation fi eld of the soft magnetic layer, i.e. of several millitesla, will provide control of the magnetization direction of the soft layer, allowing the resistance state of the device to be set by the electric fi eld in the ME fi lm. The half-hysteresis magnetoresistance curves of fi gure 1 illustrate how a change of bias voltage allows a switching between the two resistance values at zero applied magnetic fi eld. This effect is similar to the current-induced magnetization switching, with the advantages of larger devices and larger resistance values, more suited for applications. LETTER TO THE EDITOR 43 Other advantages worth mentioning are that large currents are not needed, and no small lateral size scaling is necessary. The other proposed device is based on spin valves used in GMR systems (fi gure 3). We consider a maximum exchange fi eld μ0He due to coupling between the ME layer and the bottom ferromagnetic layer. A voltage difference V controls the ME layer magnetization, tuning the exchange coupling with the bottom pinned layer. Voltage control of the ME layer will therefore allow switching of the bottom layer at fi eld values between μ0He-μ0Hi and μ0He-μ0Hi for the half-hysteresis shown in fi gure 3 (after positive saturation fi eld), where μ0Hi is the small intrinsic switching fi eld of a free bottom layer. The exchange fi eld μ0He must have a magnitude corresponding to typical pinning fi eld values of spin valve devices, i.e. several tens of millitesla. The intrinsic longitudinal ME properties necessitate a perpendicular spin valve confi guration, which has been shown to be a possible alternative to standard spin valves [29]. A voltage ΔU is used to characterize the magnetoresistance of the device in a current-in-plane standard geometry (a current-perpendicular-toplane geometry requires a voltage ΔU applied between top and bottom ferromagnetic layers, and would correspond to a modifi ed drawing geometry in fi gure 3). Inverting the voltage V provides a change of the pinning direction of the bottom layer. The drawing of fi gure 3 illustrates how the voltage V controls which layer will fi rst switch its magnetization after applying a positive saturating fi eld. The parallel or antiparallel magnetic confi guration at low fi eld depends on whether the pinned layer has already switched direction or not. For small applied magnetic fi elds, controlling the magnetization state of the free layer, the magnetic confi guration of the device can be therefore modifi ed by changing the polarity of V, providing the desired electric control of the system. This device provides additional fl exibility over the TMR-type device of fi gure 2, as the ME control voltage is distinct from the sample bias voltage. One simple application is in terms of a logic device, which is a very attractive use of magnetic storage elements, with speed, information retention, and fl exibility advantages [30, 31]. The polarity of the ME voltage provides one logical input and the direction of an external fi eld is the other logical input, resulting in a high or low value of the resistance as logical output. For example, we assign a logical `0’ to the positive ME voltage (blue curve of fi gure 3), and logical value `1’ to the negative ME voltage (red curve of fi gure 3). A positive applied fi eld is identifi ed to a logical `1’ input, and a negative applied fi eld to a logical `0’. If both inputs are `0’ or `1’ the resistance value is high, assigned to logical output `0’. The two other logical input confi gurations result in a low resistance output, assigned to output `1’. This example corresponds to an exclusive OR (XOR) operation. The polarity of the saturating fi eld provides selection of the logic functionality (the example becomes an NXOR operation if the device was previously saturated at negative fi eld values), opening the possibility of high-speed reprogrammable logic functionality [32]. A further extension of this idea involves combining currentinduced switching due to a voltage ΔU in a perpendicular geometry with the ME control due to V. Simple logics using direct electric inputs only can be achieved, avoiding completely the necessity of stray magnetic fi eld inputs. In conclusion, spin-dependent transport devices involving resistance states controlled by applied voltages only have been presented. By taking advantage of the magnetoelectric effect occurring in an insulating antiferromagnetic fi lm, the electric-induced magnetization provides control of the magnetic pinning in exchange biased GMR or TMR structures. Our estimates show that signifi cant pinning fi elds should result from exchange coupling between magnetoelectric and ferromagnetic layers, providing a control of the magnetization state of the ferromagnet by the applied voltage through the ME fi lm. In TMR devices, the ME fi lm is used as a tunnel barrier and it controls the magnetization of the soft magnetic layer. ME tunnel barriers are attractive because of the intrinsic large electric fi eld occurring in these structures. The infl uence of this fi eld on the ions’ positions 44 LETTER TO THE EDITOR and possible related magnetic properties should be kept in mind when studying currentinduced-switching properties. In GMR devices, we propose to use the ME thin fi lm as a tunable pinning bottom layer. These devices offer new versatility for making logic and memory devices based on the change of resistance due to the change of magnetic confi gurations. Our proposal requires well-controlled thin crystalline structures, where the required property involves applying a large electric fi eld, which is in signifi cant contrast to previous devices asking for large current density capabilities. This research was supported by the MRSEC Program of the NSF (DMR-0213808), the Offi ce of Naval Research (N00140210610), the Keck Foundation, and the Nebraska Research Initiative.