Room Temperature Planar Hall Transistor

Transcription

1 Room Temperature Planar Hall Transistor Bao Zhang 1, Kangkang Meng 1, Mei-Yin Yang 1, K. W. Edmonds 2, Hao Zhang 1, Kai-Ming Cai 1, Yu Sheng 1,3, Nan Zhang 1, Yang Ji 1, Jian-Hua Zhao 1, Kai-You Wang 1* 1 SKLSM, Institute of Semiconductors, CAS, P. O. Box 912, Beijing , People s Republic of China 2 School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom 3 Department of Physics, School of Sciences, University of Science & Technology Beijing, Beijing , China Controlling the spin transport in solids electrically is central to the application of spintronics for the development of information technology 1. Since the spin-transistor was proposed by Datta and Das in 1990s, due to the difficulties of spin injection and detection in semiconductors, until now there is still no efficient spin field effect transistor developed 2-4. Recently, the spin Hall transistor based on two-dimensional electron gas has been demonstrated, in which the spins were generated optically rather than electrically in the semiconductor channel 5. Here we report the tunability of the planar Hall resistance in ferromagnetic half metal Co 2 FeAl devices solely by piezo voltages from positive to negative and from negative to positive, which can be analogously used as the n-type and p-type field effect transistor, respectively. The magnetic NOT and NOR gates are demonstrated based on the Co 2 FeAl planar Hall transistors without external magnetic field at room temperature. Our demonstration can pave a way for the application of future spintronics, realizing both the information storage and processing using ferromagnetic materials. Apart from the proposals of spin based logic in semiconductors 6,7,approaches to control the domain mall motion, spin waves, spin Hall effect electrically using spin transfer torque, spin-orbit torque, spin Hall effect, magnetoelectrical coupling and * Corresponding author s kywang@semi.ac.cn

2 piezo voltages etc. have been proposed for spin logic based on ferromagnetic metals Among them, the piezo voltage is one of the most effective methods to control the magnetization, in which a deformation of the crystal structure of the magnetic materials, induces a change in the magnetocrystalline anisotropy which is directly related with the spin orbit interaction in the crystal The control of the charge transport in semiconductors by piezo voltages has also been demonstrated for high speed piezotronics, which has been proposed for post Complementary Metal Oxide Semiconductor (CMOS) technology 25. Here we propose and achieve planar Hall transistors controlled by piezo voltages which can be used for both the storage and logic devices. The active layer in our devices is the half metallic Co 2 FeAl 24,26. The magnetization of the Co 2 FeAl is controlled by the piezo voltages and the detection is provided by the planar Hall voltage across the Hall bar devices. The planar Hall voltage in magnetic materials is sensitive to the relative direction between the electrical current and magnetization vector in the plane 27. Here we demonstrate the planar Hall resistance (R H ) of the half metallic Co 2 FeAl devices can be tuned by piezo voltages from positive (negative) to negative (positive) effectively, which is associated with magnetization switching in the plane by 90. Without the semiconductor channel, the functionality of n-type field effect transistors (n-fet) and p-type field effect transistors (p-fet) can be realized in our planar Hall transistors using half magnetic metals by piezo control of the magnetization switching. Utilizing the planar Hall transistors, we demonstrate the NOT and NOR logic operations. The simple device structure allows us to build large scale building blocks for future logics. The half metallic Co 2 FeAl (CFA) thin film was grown on semi-insulating GaAs (001) by using molecular beam epitaxial (MBE) technique at The Hall bar devices along different in-plane major crystalline orientations were fabricated using standard photolithography (See Methods). After polishing the GaAs substrate down to 100 m, the devices were bonded to the piezoelectric ceramic transducer (PZT). The positive/negative voltage produces a uniaxial tensile/compressive strain perpendicular to the stacks (the direction of strain is marked in Fig.1a). In order to ensure the

3 deterministic switching of the Co 2 FeAl moment, all the devices were bonded ~2 from the [110] to [010] direction. The deformation of the Co 2 FeAl devices under the piezo voltage was measured using a strain gauge, and was found to be linearly changed with the applied piezo voltages, with tensile strain under positive piezo voltages and compressive strain under negative piezo voltages (See Supplementary Information Fig. S1). The magnetization vectors and the magnetic domains of the devices during magnetization reversal along in-plane orientations were measured by longitudinal magneto-optical Kerr microscopy (LMOKM). At different deformation states by the piezo voltages, the longitudinal and transverse resistances were measured simultaneously with a fixed electrical current injected through the channel. All measurements were performed at room temperature. The schematic diagrams of the two Co 2 FeAl Hall bars with the respect to the GaAs crystal orientation along [100] and [010] axes are shown in Fig. 1a. We first fully magnetized the Hall bar devices along the [110] direction with a rather large magnetic field of 500 Oe (which is much larger than the coercive field), then swept the external magnetic field to zero. The R H measured with a fixed current I = 50 μa along the channel was recorded with periodic piezo voltage pulses applied to both devices, as is shown in Fig. 1b. Strikingly, the R H of the [100] orientation device periodically switched from to 0.12 Ω with the voltage pulse changing from 0 to -30 V. In contrast, the R H of the [010] orientation device was periodic switched simultaneously from 0.12 to Ω with switching the voltage pulse from 0 to -30 V. It is worth noting, with negative current applied to the [100] or the [010] orientated device, same R H but opposite planar Hall voltage switching was observed in both devices during the piezo voltages changing from 0 to -30 V. The Hall resistance transitioned from negative low value to positive high value for sample [100], whereas the [010] orientated device changed from positive high to negative low under the same range of piezo voltage, which is analogous to the n-fet and p-fet in CMOS technology. The advantage of this Planar Hall transistor is that the R H (induced by planar Hall effect) changes sign during the tuning, whereas the conventional field effect transistors are switched on and off by accumulating and depleting the electrons

4 (holes) in the channel through electrical gating, while the resistance of the FET is always positive. However, in our piezo voltage control planar Hall devices, the R H sign change originates from the rotation of the magnetization vector respective to the electrical current under piezo voltages. Thus the two magnetic states of planar Hall transistor tuned by piezo voltages can not only be used for information storage, but also used as a building block for new functional logic devices. The change of the planar Hall resistance (ΔR H ) of [010] and [100] orientated devices as a function of the piezo voltage is shown in Fig. 1c. The ΔR H remains almost zero when switching the piezo voltages from 0 to -27 V for both devices. But a sharp jump was observed for both devices with switching the piezo voltages from zero to a further lower value, and then raised slightly before it reaches to a flat plateau. Opposite value of ΔR H (0.25 Ω and Ω) was observed for devices along [100] and [010] orientations when the piezo voltage is changed from zero to a value lower than -28 V. It is well established that the planar Hall resistance arises as a result of the non-equivalence of components of the resistance tensor which are perpendicular and parallel to the magnetization direction, leading to the appearance of off-diagonal resistance components. Thus the planar Hall resistance is strongly dependent on the relative direction between the electric current and the magnetization vector. The angular dependence of the R H was measured for devices along both samples at piezo voltage U P = 0, where a fixed 2000 Oe external magnetic field was rotated in the plane anticlockwise starting at [110] orientation, which is shown in Fig. 1d. The external magnetic field is much larger than the in-plane magnetic anisotropy fields, so thatthe magnetization vector of the Co 2 FeAl follows the external magnetic field direction. As shown in Fig. 1d, the angular dependence of the planar Hall resistance can be fitted well using the single domain model 29, R H = 1 (R 2 sheet R sheet ) sin [2( ± π/4) + γ], where R sheet is the sheet resistance with the current parallel to the magnetization, R sheet is the sheet resistance with the current perpendicular to the magnetization, θ + π/4 represents the angle between the electrical current and the magnetization vector for [010] orientated device while

5 θ π/4 represents the angle between the electrical current and magnetization vector for [100] orientated device, and γ is the deviation angle between the PZT strain/compress direction and [110] direction which is about 2. The periodic R H for both devices has the same magnitude and frequency, with π/2 phase shift. The maximum magnitude of the Hall resistance occurs when θ = π/4 + nπ/2, where n is an integer. The angular dependence of R H gave us the information that switching magnetization by 90 induces a change in R H of 0.27 Ω for both devices which coincides with the value shown in figure 1b on switching the piezo voltage from 0 to -30 V. Thus, the piezo voltage can fully switch the magnetization of the Co 2 FeAl devices by 90 in the plane. To have a more insight into the switching behavior of Co 2 FeAl planar Hall transistors, the magnetic properties Co 2 FeAl devices with various U P were investigated using LMOKM. The hysteresis loops and the corresponding magnetic domain structures were recorded with magnetic field applied in the plane. Fig. 2a shows the Kerr rotation angle during the magnetization reversal without the piezo voltage with magnetic field applied close to the in-plane major crystalline [110], [1 10] and [010] orientations, respectively. [110] orientation is the easy axis with the full magnetic moment at remanence. Hard-axis-like behavior is seen for the magnetic field in [010] direction with saturation occurring around 220 Oe (the black loop in Fig. 2a). However, the loop of the magnetic field applied along [1 10] shows a two-step switching with a continuous reversible rotation in between the two steps. The different switching behaviors for the [1 10] and [110] directions is not expected on the grounds of cubic crystal symmetry. The observed magnetic hysteresis loops along the major crystalline orientations are the consequence of the superposition of the uniaxial and the fourfold anisotropy, where the uniaxial easy axis is along [110] orientation and the cubic easy is along [110] and [1 10] orientations 30. The Kerr rotation angle during the magnetization reversal at ±30 V with magnetic field applied along in-plane major crystalline [110], [1 10] and [010] orientations was also investigated. To demonstrate the evolution of the magnetic anisotropy under piezo voltages, the magnetic hysteresis loops along [1 10] of

6 Co 2 FeAl under piezo voltages at zero and ±30 V are plotted in Fig. 2b. With U P = 30 V, although the two-step jumping was also observed during the magnetization reversal, the field range of the two sharp jumps increased dramatically by more than a factor of two compared to the original state namely U P = 0 V. The continuous reversible magnetic field range between the two step jumps increases with increasing the applied the piezo voltages (details are shown in Supplementary information), indicating the [1 10] becomes harder with increasing the tensile strain along [110] orientation. With the piezo voltage at U P = -30 V as shown in Fig. 2b, the magnetic hysteresis loop along [1 10] orientation has one step magnetization reversal, suggesting the magnetic easy axis has been switched by 90 from [110] to [1 10] in the plane under piezo voltages of -30 V. The magnetic anisotropy variation under piezo voltages is due to an extra uniaxial anisotropy introduced by the strain under the piezo voltages. The magnetic energy density of the system without deformation can be written as 31 : ε(θ) = 1 K 4 C sin 2 (2θ) + K U sin 2 (θ) HM s cos(θ α) (1), where θ is the angle between magnetization and easy axis [110] direction, α is the angle between the external magnetic field and [110] direction, K C is cubic anisotropy, K U is the uniaxial anisotropy, M S is the saturated magnetization, and the last term is the Zeeman energy 32. The magnetic anisotropy constants can be obtained by analyzing the magnetic hysteresis loops along the uniaxial hard orientation, where the two step jumping appears. The K C and K U were obtained to be 108M S and 41M S, respectively (details in Supplementary Information). Using the obtained magnetic anisotropy constants, the angular dependence of the magnetic energy for the state without deformation is plotted by the red line in Fig. 2c. The minimum values of the ε/m S are at [110] and [1 1 0] orientation when the piezo voltage is 0 V, so the [110] orientation is an easy axis. An additional stress-induced uniaxial magnetic anisotropy term K p sin 2 (θ) is added to the magnetic energy density equation (1) when U p 0, where K P has the same sign of K U at tensile strain (positive piezo voltages) and opposite sign at compressive strain (negative piezo voltages),

7 respectively. The angular dependence of the magnetic energy at U P = ±30 V obtained by analyzing the magnetic hysteresis loop using the modified magnetic energy density formula are also plotted in Fig. 2c. The obtained K P /M S is 25.7 Oe for U P = 30 V and Oe for U P = -30 V. The lowest energy state is along [110] orientation for piezo voltage at 0 and 30 V. However, because the piezo voltage at -30 V induced K P is larger than that of the K U and with the opposite sign, the lowest energy state is along the [1 10] orientation at U P = -30 V. The magnetization switching will happen if the gaining energy of magnetic domains is larger than the energy barrier of the two neighbor local minimums. The magnetic energy landscape in figure 2c confirms piezo voltages can switch the magnetic easy axis by 90. The piezo voltage control of the magnetization switching by 90 in the plane has also been confirmed using ferromagnetic resonance 24. The magnetic domain images of the Co 2 FeAl [100] orientated device without deformation were taken by LMOKM with magnetic field applied in [1 10] orientation, which are shown in Fig. 2d (a-e). At relatively large positive magnetic field of 80 Oe, the magnetic images are homogenously dark, because the magnetization vector of the device is fully aligned in [1 10] orientation. Decreasing the positive field to 12 Oe, part of the domain image turns from dark to grey as shown in Fig. 2d (b), indicating only part of the device was switched 90 to the easy axis of [110] orientation. Further decreasing external magnetic field to zero, the observed homogenously grey domain images shown in Fig. 2d (c) suggest the magnetization has been fully switched by 90 to [110] orientation due to the energy minimum is in [110] orientation, which is confirmed by the very small Kerr signal shown in Fig. 2a. Then with increasing negative magnetic field, part of the domain image of the device firstly turns into white from grey at around H = -33 Oe, which is shown in Fig. 2d (d). Then, the domain image of the whole device turns to white with increasing the negative magnetic field further to -80 Oe as shown in Fig. 2d (e), where the magnetization of the device is fully switched by the external magnetic field to [11 0] orientation. We then investigated the magnetic domain states controlled by piezo voltages at zero magnetic fields using the LMOKM configuration, which is shown in

8 Fig. 2d (f-j). On removing the external magnetic field after firstly magnetizing the device along [110] orientation with an external magnetic field of 100 Oe, the domain image is homogenously grey at zero piezo voltage as shown in Fig. 2d (f), indicating the magnetization vector stays in [110] orientation. When the piezo voltage of +30 V is applied, the color of the domain image does not change. However, when the piezo voltage of -30 V is applied as shown in Fig. 2d (h), the domain image of the device turns into dark, indicating the magnetization was fully switched by 90 to [1 10] orientation. After decreasing the piezo voltage back to 0, the domain image of the device returns back to grey in Fig. 2d (i). As shown in Fig. 2d (h-j), the homogeneous reversible switch from grey to dark with switching the piezo voltages between 0 and -30 V confirm that the magnetic states between [110] and [1 10] orientations are switchable by piezo voltages without external magnetic field. The two magnetic states in the planar Hall transistors tuned by piezo voltages cannot only be used to the information storage, but also can be used as logic devices. Firstly, a high Hall voltage state (ON, digital 1 ) can be defined as an output voltage of +5 μv or larger. A low Hall voltage state (OFF, digital 0 ) is defined as an output voltage of +2 μv or less. Based on the single planar Hall transistor shown in Fig. 3a, the piezo voltages can effectively switch the magnetization between the [110] and [1 10] magnetic states, which function as the NOT gate and produce the planar Hall voltage output as shown in Fig. 1b. When the piezo voltage is 0 V, the output Hall voltage (V o ) is 6.5μV, which is larger than the 5 μv so that output =1. For piezo voltage of -30 V, the V o is changed to -5.7 μv < 2 μv by switching the magnetization and the output = 0. The truth table in Fig. 3b represents the NOT gate operation Y = A. The p-type and n-type functionalities of the planar Hall transistors based on [100] and [010] orientation Co 2 FeAl devices can be realized. It is also worth noting that piezo voltages based on only [100] or [010] orientation devices can also fulfill the function of the p and n type transistor by applying opposite current. The NOR gate was built based on one p-type ([100] orientation) and one n-type ([010] orientation) planar Hall transistor, which is shown in Fig. 3c. The two devices are connected as shown in Fig.

9 3c, where the two piezo voltages (U P1 and U P2 ) control the magnetization of the [010] and [100] orientated devices separately. The magnetizations of two devices were preset to [110] orientation by external magnetic field. Then all the operations were executed without external magnetic fields with the fixed current of 50 μa. Inputting the [0,0] to the logic with both U P1 and U P2 equal to zero sets the magnetization of both devices along [110], so that the V o is 11.8 μv (the sum of the Hall voltages from these two devices). This is greater than the threshold voltage so that output = 1. On switching magnetization of both devices to [1 10] direction by piezo voltages, corresponding to the magnetic state [1,1], the V o is μv < 2 μv and output = 0. If only switching the magnetization of [100] or [010] device to [1 10] orientation, corresponding to the [1,0] or [0,1] magnetic states, the V o are -1.9 and 1.9 μv respectively. Thus the output is 0 since both V o are less than 2 μv (as shown in Fig. 3d). The non-zero value of Hall resistance at [1,0] and [0,1] states is because the magnitude of the Hall resistance between these two devices is different, which is either from the photolithography or the slightly different misalignment to [100] or [010] orientation of these two devices. If the two devices have very different planar Hall voltages, which can be realized by fabricating the electrical current channel along different in-plane orientations, very clear four states gates controlled by piezo voltages could be achieved through the above NOR logic design. The results are summarized in the truth table representing the NOR gate Y = A + B as shown in Fig. 3e. Using the similar methodology to integrating the piezoelectric layers, the application of a smaller input voltage switching the magnetization will be achieved by scaling down the devices to nanometer sizes 33. Thus the magnetic NOT and NOR gate functionalities can be realized by the logic gates designing. In summary, we have presented that the planar Hall transistor in half metal Co 2 FeAl devices can be controlled from positive (negative) to negative (positive) by piezo voltages without external magnetic field, which is associated with piezo voltages controlling the magnetization switching by 90 in the plane. The two magnetic states of the planar Hall transistor controlled by the piezo voltages can not

10 only be used as the information storage, but also can be used in logic devices. Our demonstration could pave a way for the application of future spintronics, realizing both the information storage and processing in only ferromagnetic metals. METHODS SUMMARY The Co 2 FeAl film was grown on a GaAs (001) substrate using molecular beam epitaxy (MBE) technology at 280. After deposition of 10 nm-thick CFA, the film was capped with an aluminum layer of 3 nm to avoid oxidation. The Hall bar devices along different in-plane major crystalline orientations were fabricated using standard photolithography and ion beam etching, where the device width is 20μm and the distance between the neighbor arms is 80μm. Ti/Au contacts were deposited by thermal evaporation. The GaAs substrate was polished down to 100 m, and then the heterostructure was bonded to the piezoelectric ceramic transducer (PZT). The magnetic hysteresis loops and the magnetic domain images were taken using the longitudinal magneto-optical Kerr microscopy Nano MOKE3. DC measurements were used to perform all the magnetotransport measurements, where the Hall voltages were detected using a Keithley nanovoltage meter 2182.

11 Figure 1 丨 The Planar Hall resistance of Co 2 FeAl device structure controlled by the Piezo voltages. a. The schematic diagrams for planar Hall effect controlled by piezo voltages measurements for both the [010] and [100] orientated devices. b. The periodic changes of the planar Hall resistances for both the [010] and [100] orientated devices with the periodic change of the piezo voltage pulses between 0V and -30 V without external magnetic field. c. The change of the planar Hall resistance dependent on the change of the piezo voltages from 0 to certain values for both the [010] and [100] orientated devices. d. The angular dependence of the planar Hall resistance for both the [010] and [100] orientated devices with a fixed magnetic field at 2000 Oe rotated in the plane, where the dots are the experimental results and the lines are the fitted results.

12 Figure 2 丨 The magnetic states of Co 2 FeAl device controlled by the piezo voltages. a. The magnetic hysteresis loops of Co 2 FeAl device measured by longitudinal magneto-optical Kerr system with magnetic field applied in the [110], [1 10] and [010] directions with piezo voltage at zero. b. The magnetic hysteresis loops measured using longitudinal magneto-optical Kerr system with magnetic field in [-110] orientation with piezo voltages at -30, 0 and 30 V. c. The angular dependence of the magnetic energy density at zero magnetic field for the Co 2 FeAl device with piezo voltages at -30 (red), 0(blue) and 30 V (green), where the minimum energy is switched from [110] to [1 10] orientation when the piezo voltage is -30 V. d. The magnetic domain images (a-e) of the Co 2 FeAl device during the magnetization reversal along [1 10] orientation without deformation. The magnetic domain images (f-j) of the Co 2 FeAl device were controlled by piezo voltages without external magnetic field.

13 Figure 3 丨 Programmable logic operation demonstrated by a NOT and a NOR gate. a. The schematic diagram of a piezo voltage controlled [100] orientated Co 2 FeAl device built for NOT gate. b. Truth table summary of the operation described in NOT gate. c. The schematic diagram of piezo voltages controlled [010] and [100] Co 2 FeAl devices built for NOR gate, where the piezo voltages U P1 and U P2 for the [010] and [100] devices, respectively. d. The output voltages of the NOR gates with varying the piezo voltages for both logic gates. e. Truth table summary of the operation described in NOR gate.

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