Research Article  Open Access
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
Abstract
Ultrahigh anomalous Hall sensitivity has been demonstrated in Ta/CoFe/Oxide/Ta multilayers. By changing oxides (MgO and HfO_{2}) and annealing temperature, different annealing dependence of sensitivity was found in MgOsample and HfO_{2}sample. For the MgOsample, the anomalous Hall sensitivity reaches 18792 Ω/T in the asdeposited state and significantly reduces as annealing temperature increases. On the contrary, the sensitivity of the asdeposited HfO_{2}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.
1. Introduction
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 [1], to the speed and rotation angle detectors in the automotive industry [2], and even to the detection of DNA and proteins [3]. 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 TMRbased 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 [8] 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 spinorbit scattering and tailored magnetic anisotropy that enables large anomalous Hall resistance and low saturation field [9–13]. In particular, Lu et al. [11] obtained a sensitivity of 12000 Ω/T in SiO_{2}/FePt/SiO_{2} sandwich structure films with optimized FePt composition and thickness. Zhu et al. [12] 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 10^{6} Ω/T, which is two orders higher than the best of semiconductors [13].
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 spinorbit 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 SiO_{2} and HfO_{2}. 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 HfO_{2}) and annealing temperature (), opposite dependence of sensitivity was found in MgOsample and HfO_{2}sample. For the MgOsample, the anomalous Hall sensitivity reaches 18792 Ω/T in the asdeposited state and significantly reduces as increases. On the contrary, the sensitivity of the asdeposited HfO_{2}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.
2. Experiments
All samples were deposited on Si substrates by magnetron sputtering at room temperature. The sample structure is Ta(0.8)/Co_{20}Fe_{80}(0.8)/Oxide(0.8)/Ta(1.0) (all in nm), where the oxide is MgO or HfO_{2}. 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 FA200) at Xband (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)/Co_{20}Fe_{80}(0.8)/MgO(0.8)/Ta(1.0) (in nm) in the asdeposited 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 (asdeposited state), of MgOsample 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 asdeposited 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)/Co_{20}Fe_{80}(0.8)/HfO_{2}(0.8)/Ta(1.0) (in nm). Different from the MgOsample, the value of in the asdeposited HfO_{2} 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 asdeposited state. It is interesting to find that the variation trend of with respect to is opposite in the MgOsample and HfO_{2}sample. To further illustrate the difference, four typical samples were chosen as below: asdeposited MgOsample, 240°C annealed MgOsample, asdeposited HfO_{2}sample, and 240°C annealed HfO_{2}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 asdeposited MgOsample (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 asdeposited and 240°C annealed HfO_{2}samples. The values of and for the asdeposited 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.
(a)
(b)
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 HfO_{2}samples, outofplane 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 peaktopeak linewidth are defined. Figure 4(a) presents the outofplane angular dependent for the asdeposited MgOsample. 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 firstorder, secondorder 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.

(a)
(b)
(c)
(d)
From Table 1, it is clearly seen that the variation trend of magnetic anisotropy is different in the MgOsample and HfO_{2}sample. For the asdeposited MgOsample, both values of the effective magnetic anisotropy constant and the secondorder uniaxial anisotropy constant are positive, indicating the sample has perpendicular magnetic anisotropy (PMA) [17]. 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 MgOsample, the calculated values of and are −4.09 × 10^{5} erg/cm^{3} and erg/cm^{3}, respectively. Considering the value of is negative and < −(1/2), the annealed MgOsample has inplane magnetic anisotropy (IMA) [17]. 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 asdeposited HfO_{2}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 HfO_{2} sample has PMA with a small . Therefore, the variation trend of magnetic anisotropy during annealing is opposite in the MgOsample and HfO_{2}sample. For MgOsample, the magnetic anisotropy changes from PMA to IMA, resulting in a significant increase of , while, for HfO_{2}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 [16]. In theory, firstprinciples calculation has been used to study the FM/Oxide interface, showing that the interfacial magnetic anisotropy is strongly affected by the hybridization between FM3d and O2p 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 [22]. Here in our samples, the enthalpy of formation () for MgO is −601.6 kJ/mol, larger than that for HfO_{2} (−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 FMO 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 [23]. 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 HfO_{2}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 MgOsample in the asdeposited and 240°C annealed states. The values of and are −0.029 and μΩ^{−1} cm^{−1} in the asdeposited 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 HfO_{2}sample, the values of and are −0.015 and μΩ^{−1} cm^{−1} in the asdeposited 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 HfO_{2}sample. Above analysis gives strong evidence that the variation trend of AHE is different during the annealing process in the MgO and HfO_{2}sample. For the MgOsample, 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).
(a)
(b)
4. Conclusions
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 asdeposited state, while the value decreases significantly as the annealing temperature increases. For sample Ta/CoFe/HfO_{2}/Ta, the value of sensitivity is small in the asdeposited 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.
Competing Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work was supported by the National Basic Research Program of China (2015CB921502) and the Natural Science Foundation of China (51331002, 51371027, and 11504019).
References
 J. M. Daughton, “GMR and SDT sensor applications,” IEEE Transactions on Magnetics, vol. 36, no. 5, pp. 2773–2778, 2000. View at: Publisher Site  Google Scholar
 C. P. O. Treutler, “Magnetic sensors for automotive applications,” Sensors and Actuators, A: Physical, vol. 91, no. 12, pp. 2–6, 2001. View at: Publisher Site  Google Scholar
 S. G. Grancharov, H. Zeng, S. Sun et al., “Biofunctionalization of monodisperse magnetic nanoparticles and their use as biomolecular labels in a magnetic tunnel junction based sensor,” The Journal of Physical Chemistry B, vol. 109, no. 26, pp. 13030–13035, 2005. View at: Publisher Site  Google Scholar
 N. Nagaosa, J. Sinova, S. Onoda, A. H. MacDonald, and N. P. Ong, “Anomalous Hall effect,” Reviews of Modern Physics, vol. 82, no. 2, pp. 1539–1592, 2010. View at: Publisher Site  Google Scholar
 Y. Tian, L. Ye, and X. Jin, “Proper scaling of the anomalous hall effect,” Physical Review Letters, vol. 103, no. 8, Article ID 087206, 2009. View at: Publisher Site  Google Scholar
 J. Moritz, B. Rodmacq, S. Auffret, and B. Dieny, “Extraordinary Hall effect in thin magnetic films and its potential for sensors, memories and magnetic logic applications,” Journal of Physics D: Applied Physics, vol. 41, no. 13, Article ID 135001, 2008. View at: Publisher Site  Google Scholar
 A. Gerber, “Towards Hall effect spintronics,” Journal of Magnetism and Magnetic Materials, vol. 310, no. 2, pp. 2749–2751, 2007. View at: Publisher Site  Google Scholar
 Y. Zhu and J. W. Cai, “Ultrahigh sensitivity Hall effect in magnetic multilayers,” Applied Physics Letters, vol. 90, no. 1, Article ID 012104, 2007. View at: Publisher Site  Google Scholar
 S. L. Zhang, J. Teng, J. Y. Zhang et al., “Large enhancement of the anomalous Hall effect in Co/Pt multilayers sandwiched by MgO layers,” Applied Physics Letters, vol. 97, no. 22, Article ID 222504, 2010. View at: Publisher Site  Google Scholar
 J. Zhang, G. Yang, S. Wang et al., “Ultrahigh anomalous hall sensitivity in Co/Pt multilayers by interfacial modification,” Applied Physics Express, vol. 6, no. 10, Article ID 103007, 2013. View at: Publisher Site  Google Scholar
 Y. M. Lu, J. W. Cai, H. Y. Pan, and L. Sun, “Ultrasensitive anomalous Hall effect in SiO_{2}/FePt/SiO_{2} sandwich structure films,” Applied Physics Letters, vol. 100, no. 2, Article ID 022404, 2012. View at: Publisher Site  Google Scholar
 T. Zhu, P. Chen, Q. H. Zhang, R. C. Yu, and B. G. Liu, “Giant linear anomalous Hall effect in the perpendicular CoFeB thin films,” Applied Physics Letters, vol. 104, no. 20, Article ID 202404, 2014. View at: Publisher Site  Google Scholar
 G. Kopnov and A. Gerber, “MegaOhm extraordinary Hall effect in oxidized CoFeB,” Applied Physics Letters, vol. 109, no. 2, Article ID 022404, 2016. View at: Publisher Site  Google Scholar
 G. X. Miao and G. Xiao, “Giant Hall resistance in Ptbased ferromagnetic alloys,” Applied Physics Letters, vol. 85, no. 1, pp. 73–75, 2004. View at: Publisher Site  Google Scholar
 T. Maeda, T. Kai, A. Kikitsu, T. Nagase, and J.I. Akiyama, “Reduction of ordering temperature of an FePtordered alloy by addition of Cu,” Applied Physics Letters, vol. 80, no. 12, pp. 2147–2149, 2002. View at: Publisher Site  Google Scholar
 S. Ikeda, K. Miura, H. Yamamoto et al., “A perpendicularanisotropy CoFeB–MgO magnetic tunnel junction,” Nature Materials, vol. 9, no. 9, pp. 721–724, 2010. View at: Publisher Site  Google Scholar
 E. Y. Vedmedenko, H. P. Oepen, and J. Kirschner, “Microstructure of the spin reorientation transition in secondorder approximation of magnetic anisotropy,” Physical Review B, vol. 66, no. 21, Article ID 214401, 2002. View at: Google Scholar
 H. X. Yang, M. Chshiev, B. Dieny, J. H. Lee, A. Manchon, and K. H. Shin, “Firstprinciples investigation of the very large perpendicular magnetic anisotropy at FeMgO and CoMgO interfaces,” Physical Review B, vol. 84, no. 5, Article ID 054401, 2011. View at: Publisher Site  Google Scholar
 K. H. Khoo, G. Wu, M. H. Jhon et al., “Firstprinciples study of perpendicular magnetic anisotropy in CoFe/MgO and CoFe/Mg_{3}B_{2}O_{6} interfaces,” Physical Review B, vol. 87, no. 17, Article ID 174403, 2013. View at: Publisher Site  Google Scholar
 X. Chen, C. Feng, Z. Long Wu et al., “Interfacial oxygen migration and its effect on the magnetic anisotropy in Pt/Co/MgO/Pt films,” Applied Physics Letters, vol. 104, no. 5, Article ID 052413, 2014. View at: Publisher Site  Google Scholar
 G. Yang, J.Y. Zhang, S.G. Wang et al., “Magnetization reorientation induced by interfacial structures in ultrathin disordered FePt film sandwiched by SiO_{2} layers,” Applied Surface Science, vol. 353, pp. 489–493, 2015. View at: Publisher Site  Google Scholar
 A. Manchon, C. Ducruet, L. Lombard et al., “Analysis of oxygen induced anisotropy crossover in Pt/Co/MOx trilayers,” Journal of Applied Physics, vol. 104, no. 4, Article ID 043914, 2008. View at: Publisher Site  Google Scholar
 G. Yang, J.Y. Zhang, S.L. Jiang et al., “Effect of oxygen migration on magnetic anisotropy and damping constant in perpendicular Ta/CoFeB/Gd/MgO/Ta multilayers,” Applied Surface Science, 2016. View at: Publisher Site  Google Scholar
 S.L. Jiang, X. Chen, X.J. Li et al., “Anomalous Hall effect engineering via interface modification in Co/Pt multilayers,” Applied Physics Letters, vol. 107, no. 11, Article ID 112404, 2015. View at: Publisher Site  Google Scholar
 S.L. Jiang, G. Yang, J. Teng, Q.X. Guo, L.L. Li, and G.H. Yu, “Interfaceengineered spindependent transport in perpendicular Co/Pt multilayers,” Applied Surface Science, vol. 387, pp. 375–378, 2016. View at: Publisher Site  Google Scholar
 M.C. Chang and Q. Niu, “Berry phase, hyperorbits, and the Hofstadter spectrum: semiclassical dynamics in magnetic Bloch bands,” Physical Review B  Condensed Matter and Materials Physics, vol. 53, no. 11, pp. 7010–7023, 1996. View at: Publisher Site  Google Scholar
 D. Hou, Y. Li, D. Wei, D. Tian, L. Wu, and X. Jin, “The anomalous Hall effect in epitaxial facecenteredcubic cobalt films,” Journal of Physics Condensed Matter, vol. 24, no. 48, Article ID 482001, 2012. View at: Publisher Site  Google Scholar
 J. Smit, “The spontaneous hall effect in ferromagnetics II,” Physica, vol. 24, no. 1–5, pp. 39–51, 1958. View at: Publisher Site  Google Scholar
 L. Berger, “Sidejump mechanism for the hall effect of ferromagnets,” Physical Review B, vol. 2, no. 11, pp. 4559–4566, 1970. View at: Publisher Site  Google Scholar
 A. Crépieux and P. Bruno, “Theory of the anomalous hall effect from the Kubo formula and the Dirac equation,” Physical Review B, vol. 64, no. 1, Article ID 014416, 2001. View at: Google Scholar
Copyright
Copyright © 2016 Guang Yang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.