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 18792Ω/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 765Ω/T, while it remarkably increases with annealing temperature increasing, finally reaching 14741Ω/T at 240C. 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.


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 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 [8] first demonstrated an anomalous Hall sensitivity as high as 1200 Ω/T in [CoFe/Pt]  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][10][11][12][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 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 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 2 Advances in Condensed Matter Physics 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 MgO-sample and HfO 2 -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 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.

Experiments
All samples were deposited on Si substrates by magnetron sputtering at room temperature.The sample structure is Ta(0.8)/Co20 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 3×10 −7 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).

Results and Discussions
The anomalous Hall sensitivity is defined as  =   / ≈  AH /  [12,14], where   is the perpendicular saturation field and  AH 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 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)/Co 20 Fe 80 (0.8)/HfO 2 (0.8)/Ta(1.0)(in nm).Different from the MgO-sample, the value of  in the as-deposited 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 as-deposited state.It is interesting to find that the variation trend of  with respect to   is opposite in the MgO-sample and HfO 2 -sample.To further illustrate the difference, four typical samples were chosen as below: as-deposited MgO-sample, 240 ∘ C annealed MgOsample, as-deposited 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 as-deposited MgO-sample (black one) shows an obvious linear response without magnetic hysteresis.The saturated 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, 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  res and peak-to-peak linewidth Δ pp are defined.Figure 4(a) presents the outof-plane angular dependent  res for the as-deposited MgOsample.Here, the angle   is defined as the direction of applied magnetic field with respect to the film normal.The value of  res can be fitted by Kittel's formula: where 1 ,  2 ,   , 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. = 9.0 GHz is the frequency of AC magnetic fields in the machine. is the gyromagnetic ratio given as  =   /7, where ,   , and 7 are Landé factor, Bohr magneton, and Planck's constant, respectively.As shown in Figure 4(a), the experimental value of  res as a function of   can be well fitted, where above parameters can be obtained.Consequently, the fitting parameters ,   ,  1 ,  2 , the effective magnetic anisotropy constant  eff =  1 −2 2  , and the effective anisotropy filed  eff = 2 eff /  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 HfO 2 -sample.For the as-deposited MgO-sample, both values of the effective magnetic anisotropy constant  eff and the second-order uniaxial anisotropy constant  2 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  eff 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  eff and  2 are −4.09× 10 5 erg/cm 3 and 1.02 × 10 5 erg/cm 3 , respectively.Considering the value of  eff is negative and  2 < −(1/2) eff , the annealed MgO-sample has in-plane 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 as-deposited HfO 2 -sample, the value of  eff is negative and  2 < −(1/2) eff , representing a typical IMA character.However, by annealing at 240 ∘ C, both the values of  eff and  2 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 MgO-sample and HfO 2sample.For MgO-sample, 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, 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 [22].Here in our samples, the enthalpy of formation (Δ  ) 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 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 [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  AH , 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  AH in the MgOand HfO 2 -sample as shown in Figure 3, contributions to the AHE by different mechanisms were analyzed.In general,  AH =   + 2  , where  AH 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][27][28][29][30].It is necessary to point out that the thickness change during annealing is eliminated; thus  AH is equivalent to  AH .The coefficients  and  can be obtained by plotting  AH /  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 2.12 × 10 −4 Ω −1 cm −1 in the as-deposited state, respectively.By annealing at 240 ∘ C, the values of  and  change to 0.002 and 8.74 × 10 −5 Ω −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 2.57 × 10 −4 Ω −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 3.89 × 10 −3 Ω −1 cm −1 , respectively.The competitive relation between  and  will affect not only the value but also the sign of  AH .Considering the large enhancement of || as well as the same positive sign between  and  AH , 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 MgO-sample, both the intrinsic and extrinsic contributions to AHE are weakened by annealing, resulting in the significant decrease of  AH as shown in Figure 3(a).In contrast, the side jump and the intrinsic contributions are remarkably enhanced, leading to the final increase of  AH as shown in Figure 3(b).

Figure 4 :
Figure 4: (a)-(b) Out-of-plane angular dependent resonance fields  res for sample Ta(0.8)/Co 20 Fe 80 (0.8)/MgO(0.8)/Ta(1.0)(in nm) in the as-deposited and 240 ∘ C annealed states.Hollow circles and solid lines represent experimental data and theoretical fitting of  res .Inset: typical FMR differential absorption spectra where the resonance field  res and peal-to-peak linewidth Δ pp are defined.(c)-(d) Out-of-plane angular dependent resonance fields  res for sample Ta(0.8)/Co 20 Fe 80 (0.8)/HfO 2 (0.8)/Ta(1.0)(in nm) in the as-deposited and 240 ∘ C annealed states.Hollow diamonds and solid lines represent experimental data and theoretical fitting of  res .