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Guang Yang, Yongye Li, Xi Chen, Jingyan Zhang, Guanghua Yu, "Ultrasensitive Anomalous Hall Effect in Ta/CoFe/Oxide/Ta Multilayers", Advances in Condensed Matter Physics, vol. 2016, Article ID 9734610, 7 pages, 2016. https://doi.org/10.1155/2016/9734610
Ultrasensitive Anomalous Hall Effect in Ta/CoFe/Oxide/Ta Multilayers
Ultrahigh anomalous Hall sensitivity has been demonstrated in Ta/CoFe/Oxide/Ta multilayers. By changing oxides (MgO and HfO2) and annealing temperature, different annealing dependence of sensitivity was found in MgO-sample and HfO2-sample. For the MgO-sample, the anomalous Hall sensitivity reaches 18792 Ω/T in the as-deposited state and significantly reduces as annealing temperature increases. On the contrary, the sensitivity of the as-deposited HfO2-sample is only 765 Ω/T, while it remarkably increases with annealing temperature increasing, finally reaching 14741 Ω/T at 240°C. The opposite variation of anomalous sensitivity in two samples originates from the different change of magnetic anisotropy and anomalous Hall resistance during the annealing process. Our study provides a new perspective that both the choice of oxide material and the optimization of annealing treatment are important to the anomalous Hall sensitivity.
Magnetic sensors are playing an increasing important role in daily life and industrial production, with their wide applications ranging from read heads in the hard disk , to the speed and rotation angle detectors in the automotive industry , and even to the detection of DNA and proteins . The current design of magnetic sensors is based on the Hall effect in semiconductor materials or magnetoresistive effect including anisotropy magnetoresistance (AMR), giant magnetoresistance (GMR), and tunneling magnetoresistance (TMR) in magnetic materials. However, the sensors based on Hall effect and AMR effect always suffer a lower sensitivity. On the other hand, although high sensitivity can be obtained in GMR- and TMR-based sensors, the complex fabrication process with higher costs is also an obstacle. Recently, the anomalous Hall effect (AHE) of ferromagnets has attracted enormous attention owning to the abundant physics [4, 5] and potential applications [6, 7]. In 2007, Zhu and Cai  first demonstrated an anomalous Hall sensitivity as high as 1200 Ω/T in multilayers, which is better than the conventional semiconductor Hall sensitivity (about 1000 Ω/T). Subsequently, the strategy adapted to achieve a higher sensitivity was by using ultrathin ferromagnetic films/multilayers with enhanced spin-orbit scattering and tailored magnetic anisotropy that enables large anomalous Hall resistance and low saturation field [9–13]. In particular, Lu et al.  obtained a sensitivity of 12000 Ω/T in SiO2/FePt/SiO2 sandwich structure films with optimized FePt composition and thickness. Zhu et al.  demonstrated a sensitivity of 23760 Ω/T in MgO/CoFeB/Ta/MgO multilayers by tuning the thickness of CoFeB and adjacent Ta layer. More excitingly, a very recent study has reported the anomalous Hall sensitivity up to 106 Ω/T, which is two orders higher than the best of semiconductors .
Although the achieved ultrahigh sensitivity is remarkable, the compatibility between AHE materials and CMOS technology still needs further consideration. For example, heavy metals such as Pt are always used in AHE materials to enhance the spin-orbit scattering for a large anomalous Hall resistance, while it will cause a terrible shunting effect as well as increased costs. The CoFeB/MgO heterostructure seems a more promising material system, while the commonly used oxides in CMOS technology are high- materials such as SiO2 and HfO2. From the application point of view, it is better to introduce the same high- oxides into the AHE materials. Last but not least, AHE materials generally need additional annealing to exhibit a high sensitivity. Considering the postannealing is also essential to CMOS technology, it is necessary to further optimize the annealing process.
In this work, we demonstrate the ultrasensitive AHE in Ta/CoFe/Oxide/Ta multilayers. By changing oxides (MgO and HfO2) and annealing temperature (), opposite dependence of sensitivity was found in MgO-sample and HfO2-sample. For the MgO-sample, the anomalous Hall sensitivity reaches 18792 Ω/T in the as-deposited state and significantly reduces as increases. On the contrary, the sensitivity of the as-deposited HfO2-sample is only 765 Ω/T, while it remarkably increases with increasing, finally reaching 14741 Ω/T at 240°C. Based on the angular dependent ferromagnetic resonance (FMR) measurements and temperature dependent transport measurements, the different change of sensitivity in two samples comes from the different temperature dependence of the anomalous Hall resistance and the magnetic anisotropy. This study gives new insights that the choice of oxides and the optimization of are both important to obtain an ultrahigh anomalous Hall sensitivity.
All samples were deposited on Si substrates by magnetron sputtering at room temperature. The sample structure is Ta(0.8)/Co20Fe80(0.8)/Oxide(0.8)/Ta(1.0) (all in nm), where the oxide is MgO or HfO2. Thermal annealing was carried out in a vacuum furnace (better than Torr) for 15 min without external magnetic fields. Hall bars were patterned by optical lithography combined with Ar+ milling for transport measurements in a physical property measurement system. FMR measurements were performed in an electron spin resonance spectrometer (JEOL ESR FA-200) at X-band (9.0 GHz).
3. Results and Discussions
The anomalous Hall sensitivity is defined as [12, 14], where is the perpendicular saturation field and is the saturated anomalous Hall resistance that can be obtained via a linear extrapolation of at high field to zero field. The inset of Figure 1 exhibits the anomalous Hall loops of sample Ta(0.8)/Co20Fe80(0.8)/MgO(0.8)/Ta(1.0) (in nm) in the as-deposited and different annealed states, from which the corresponding value of is calculated. As a result, Figure 1 shows the sensitivity as a function of the annealing temperature . When is 25°C (as-deposited state), of MgO-sample has reached 18792 Ω/T. Nevertheless, the value of decreases significantly with the increase of . When reaches 140°C, the value of is 8145 Ω/T, decreasing 57% with respect to that in the as-deposited state. As further increases to 240°C, the value of is only 2572 Ω/T.
In contrast, Figure 2 shows as a function of for sample Ta(0.8)/Co20Fe80(0.8)/HfO2(0.8)/Ta(1.0) (in nm). Different from the MgO-sample, the value of in the as-deposited HfO2 sample is only 765 Ω/T. When increases to 180°C, the value of appears almost unchanged. However, as is above 200°C, the value of increases dramatically. When reaches 240°C, the value of is 14741 Ω/T, which is about 19 times larger than that in the as-deposited state. It is interesting to find that the variation trend of with respect to is opposite in the MgO-sample and HfO2-sample. To further illustrate the difference, four typical samples were chosen as below: as-deposited MgO-sample, 240°C annealed MgO-sample, as-deposited HfO2-sample, and 240°C annealed HfO2-sample.
As shown in Figure 3, the detailed -H curves of the above four samples are presented. In Figure 3(a), the curve of the as-deposited MgO-sample (black one) shows an obvious linear response without magnetic hysteresis. The saturated anomalous Hall resistance is 35.8 Ω and the perpendicular saturation field is 20 Oe. By annealing at 240°C, the linear shape of the curve began to degrade, with decreasing to 14.2 Ω and increasing to 150 Oe. Both the reduced and the increased are detrimental to the sensitivity, leading to a significant decrease of from 18792 Ω/T to 2572 Ω/T. Figure 3(b) shows the -H curves of the as-deposited and 240°C annealed HfO2-samples. The values of and for the as-deposited sample are 28.5 Ω and 1000 Oe. By annealing at 240°C, the value of reaches 36.4 Ω while the value of decreases to 30 Oe. Both the increased and the reduced are beneficial to an ultrahigh sensitivity, leading to a significant increase of from 765 Ω/T to 14741 Ω/T.
It is well known that the perpendicular saturation field is related to the magnetic anisotropy of the films. During the annealing process, the volume anisotropy as well as the interfacial anisotropy is likely to change [15, 16]. In order to characterize the evolution of magnetic anisotropy in the MgO- and HfO2-samples, out-of-plane angular dependent FMR measurements were performed. The typical FMR differential absorption spectrum is shown in the inset of Figure 4(a), where the resonance field and peak-to-peak linewidth are defined. Figure 4(a) presents the out-of-plane angular dependent for the as-deposited MgO-sample. Here, the angle is defined as the direction of applied magnetic field with respect to the film normal. The value of can be fitted by Kittel’s formula:where and . , , , and are the first-order, second-order uniaxial anisotropy constant, the saturation magnetization, and the equilibrium angle of the magnetization vector with respect to film normal, respectively. GHz is the frequency of AC magnetic fields in the machine. is the gyromagnetic ratio given as , where , , and are Landé factor, Bohr magneton, and Planck’s constant, respectively. As shown in Figure 4(a), the experimental value of as a function of can be well fitted, where above parameters can be obtained. Consequently, the fitting parameters , , , , the effective magnetic anisotropy constant , and the effective anisotropy filed calculated from Figures 4(a)–4(d) are listed in Table 1.
From Table 1, it is clearly seen that the variation trend of magnetic anisotropy is different in the MgO-sample and HfO2-sample. For the as-deposited MgO-sample, both values of the effective magnetic anisotropy constant and the second-order uniaxial anisotropy constant are positive, indicating the sample has perpendicular magnetic anisotropy (PMA) . For the sample with PMA, the perpendicular direction is the easy magnetization axis; thus the perpendicular saturation filed is small. It is also important to point out that since the calculated effective anisotropy field is very small (only about 94 Oe), the -H curve will not exhibit the obvious coercivity. For the 240°C annealed MgO-sample, the calculated values of and are −4.09 × 105 erg/cm3 and erg/cm3, respectively. Considering the value of is negative and < −(1/2), the annealed MgO-sample has in-plane magnetic anisotropy (IMA) . For the sample with IMA, the perpendicular direction is the difficult magnetization axis; thus the value of will be very large. On the other hand, for the as-deposited HfO2-sample, the value of is negative and < −(1/2), representing a typical IMA character. However, by annealing at 240°C, both the values of and change to positive, indicating the 240°C annealed HfO2 sample has PMA with a small . Therefore, the variation trend of magnetic anisotropy during annealing is opposite in the MgO-sample and HfO2-sample. For MgO-sample, the magnetic anisotropy changes from PMA to IMA, resulting in a significant increase of , while, for HfO2-sample, the magnetic anisotropy changes from IMA to PMA, leading to a remarkable decrease of .
For the ferromagnetic metal (FM)/Oxide heterostructures, the interfacial magnetic anisotropy plays a dominated role . In theory, first-principles calculation has been used to study the FM/Oxide interface, showing that the interfacial magnetic anisotropy is strongly affected by the hybridization between FM-3d and O-2p orbits [18, 19]. In addition, previous researches have reported that the orbital hybridization between FM and oxide is sensitive to the annealing process [20, 21]. By annealing, the activated oxygen atoms could migrate to the interface, producing a bonding between FM atoms and oxygen atoms. It is necessary to point out that the degree of bonding is important to the orbital hybridization, where an optimized bonding is beneficial to PMA, whereas the excessive and insufficient bonding will lead to a degradation of PMA . Here in our samples, the enthalpy of formation () for MgO is −601.6 kJ/mol, larger than that for HfO2 (−1144.7 kJ/mol). It means that the combination between Hf and O is more stable than that between Mg and O. Therefore, during the deposition and annealing process, MgO is more likely to deviate the stoichiometric ratio and transfer oxygen atoms to the adjacent CoFe layer, leading to the final difference of the FM-O bonding degree for the two samples. According to our recent work, the oxygen migration direction during annealing process may be inverse at different FM/Oxide interfaces . However, since the oxygen migration could also be affected by the film thickness and annealing temperature and so forth, the specific differences about oxygen migration in the two samples need further investigation.
In addition to , AHE sensitivity is also related to , whose value represents the magnitude of AHE. Previous work has reported that the annealing process will affect the intrinsic or extrinsic mechanisms, leading to a variation of AHE [24, 25]. To explain the change of in the MgO- and HfO2-sample as shown in Figure 3, contributions to the AHE by different mechanisms were analyzed. In general, , where is the saturated anomalous Hall resistivity, is the longitudinal resistivity, a represents the skew scattering contribution, and represents the side jump as well as the intrinsic contribution [26–30]. It is necessary to point out that the thickness change during annealing is eliminated; thus is equivalent to . The coefficients and can be obtained by plotting as a function of and linear fitting to the experimental data. Figure 5(a) shows the linear fitting for MgO-sample in the as-deposited and 240°C annealed states. The values of and are −0.029 and μΩ−1 cm−1 in the as-deposited state, respectively. By annealing at 240°C, the values of and change to 0.002 and μΩ−1 cm−1, respectively. Although the sign of alters from negative to positive, both the values of and decrease by one order of magnitude, finally weakening the AHE. For the HfO2-sample, the values of and are −0.015 and μΩ−1 cm−1 in the as-deposited state, respectively. By annealing at 240°C, both the values of and increase by one order of magnitude, reaching −0.437 and μΩ−1 cm−1, respectively. The competitive relation between and will affect not only the value but also the sign of . Considering the large enhancement of as well as the same positive sign between and , it suggests that the influence of on AHE is improved during annealing process for the HfO2-sample. Above analysis gives strong evidence that the variation trend of AHE is different during the annealing process in the MgO- and HfO2-sample. For the MgO-sample, both the intrinsic and extrinsic contributions to AHE are weakened by annealing, resulting in the significant decrease of as shown in Figure 3(a). In contrast, the side jump and the intrinsic contributions are remarkably enhanced, leading to the final increase of as shown in Figure 3(b).
In conclusion, the ultrasensitive AHE was demonstrated in Ta/CoFe/Oxide/Ta multilayers. For sample Ta/CoFe/MgO/Ta, AHE sensitivity is as high as 18792 Ω/T in the as-deposited state, while the value decreases significantly as the annealing temperature increases. For sample Ta/CoFe/HfO2/Ta, the value of sensitivity is small in the as-deposited state but increases to 14741 Ω/T by 240°C annealing. The opposite variation of AHE sensitivity in two samples originates from the different change of magnetic anisotropy and anomalous Hall resistance during the annealing process. This work gives new insights that both the choice of oxide material and the optimization of annealing treatment play an important role in the anomalous Hall sensitivity.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the National Basic Research Program of China (2015CB921502) and the Natural Science Foundation of China (51331002, 51371027, and 11504019).
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