High Field Linear Magnetoresistance Sensors with Perpendicular Anisotropy L 10-FePt Reference Layer

High field linear magnetoresistance is an important feature for magnetic sensors applied in magnetic levitating train and high field positioning measurements. Here, we investigate linear magnetoresistance in Pt/FePt/ZnO/Fe/Pt multilayer magnetic sensor, where FePt and Fe ferromagnetic layers exhibit out-of-plane and in-plane magnetic anisotropy, respectively. Perpendicular anisotropy L10-FePt reference layer with large coercivity and high squareness ratio was obtained by in situ substrate heating. Linear magnetoresistance is observed in this sensor in a large range between +5 kOe and −5 kOe with the current parallel to the film plane. This L10-FePt based sensor is significant for the expansion of linear range and the simplification of preparation for future high field magnetic sensors.


Introduction
Research on spintronics is fast developing due to its widely practical application [1][2][3][4].Magnetic tunnel junctions and magnetic spin valves as the important contents in spintronics are popularly used as magnetic sensors, such as magnetic read heads for hard-disk drive, position, and speed detectors in low magnetic field measurement [5,6].In general, the majority of the linear magnetic sensors are based on Hall effect or anisotropic magnetoresistance (AMR) with some shortcomings of poor thermal stability and limited operation frequency range [7].Recently, the linear giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR) sensors, in which a linear response can be achieved by applying a bias field [8], using a shape anisotropy of the free layer [9], or introducing a pinned layer [10], were proposed as promising candidates due to their high field sensitivity and good thermal stability [11][12][13].Particularly, in some applications such as magnetic levitating train and high field positioning measurements, the high field linear magnetoresistance (MR) sensors are investigated due to high sensing field and fine repeatability [14,15].An approach of making a high field linear magnetic field response is to use a sandwich structure with a reference layer having high perpendicular magnetic anisotropy and an in-plane anisotropy free layer [16][17][18][19].However some problems such as narrow linear range and complicated design are still unresolved.
In this work, we design and fabricate L1 0 -FePt/ZnO/Fe multilayer films to shed light on this problem by choosing perpendicular anisotropy L1 0 -FePt with a large coercive force (  ) and a high squareness ratio as the reference layer.The MR curve is nearly linear when the applied field is in the range ±5 kOe measured at 300 K, which is improved greatly compared to previously reported results [18].Meanwhile, this design simplifies effectively the structure of linear magnetic sensors through applying L1 0 -FePt with a large   as reference layer.

Experimental
Multilayer films with a structure of Pt(4)/FePt(15)/ZnO(5)/ Fe(5)/Pt(3) (thicknesses in nm) were prepared on single crystal MgO (001) substrates by magnetron sputtering system as shown schematically in Figure 1(a).A high field linear MR sensor is designed using a perpendicular anisotropy L1 0 -FePt reference layer and in-plane anisotropy Fe free layer separated by a ZnO barrier layer in the magnetic multilayers as shown in Figure 1  are perpendicular to each other.With the increase of external magnetic field ( ex ), the free layer's magnetization rotates until becoming perpendicular to the film plane.And a reversible linear MR response in a large range of magnetic field was expected.
The pressure of high-purity argon was 2.0 Pa during sputtering.The Pt and FePt layers were deposited at an in situ temperature of 450 ∘ C followed by annealing for 2 h at 450 ∘ C. Then the ZnO, Fe, and Pt layers were successively deposited at the room temperature after the deposition of FePt layer.The FePt film was manufactured by cosputtering an Fe target and a Pt target, whose deposition rates were calibrated to achieve Fe 55 Pt 45 .The base vacuum was under 8.0 × 10 −5 Pa.The crystalline structures and microstructure of multilayer films were characterized by means of X-ray diffraction (XRD) and transmission electron microscope (TEM).The morphologies of the samples were observed by scanning electron microscopy (SEM).The magnetic properties of films were determined using a superconduction quantum inference device (SQUID) and a vibrating sample magnetometer (VSM).The magnetic field dependence of MR was measured using a four-point method with current-in-plane (CIP) and applied field perpendicular to plane using a Keithley 2400 source meter and Keithley 2182 nanovolt meter in a physical property measurement system (PPMS, Quantum Design Inc.).

Results and Discussion
XRD pattern of the thin film with Pt/L1 0 -FePt bilayers is shown in Figure 1(c).The peaks of (001) and (002) were observed for FePt film, indicating formation of the L1 0 ordered phase with easy axis perpendicular to the substrate surface.Thus, the transformation of FePt film from facecentered cubic (fcc) phase to face-centered tetragonal (fct) phase was fulfilled by in situ substrate heating.The SEM image shows that the deposited Pt/L1 0 -FePt bilayer surface is continuous and smooth (see Figure 1(d)).The lattice misfit between FePt (001) and MgO (001) is 8.8% [20]; the FePt film received tensile stress along the in-plane direction, which causes the film tend to form granular film [21].Whereas the mismatch value is 1.8% between Pt (001) and MgO (001) [22], the Pt buffer layer deposited between MgO substrate and FePt film effectively reduces the stress, causing the forming of a continuous film.
In order to reflect the structure information of the multilayers more accurately, we carried out the cross section TEM image and high-resolution TEM (HRTEM) image of the Pt(4)/FePt(15)/ZnO(5)/Fe(5)/Pt(3) multilayer sensor.The result present that the films are continuous as shown in Figure 2(a).The structure of the multilayer films is consistent with the schematic structure in Figure 1(a).From the HRTEM image shown in Figure 2(b), the lattice fringe spacing is measured to be 0.387 nm, corresponding to the d FePt (001) (0.384 nm) of L1 0 -FePt phase [23].Also, the FePt (001) and FePt (002) can be found obviously from its fast Fourier transform (FFT) image in Figure 2(c), suggesting that L1 0 ordered phase of FePt was obtained in our films.
Figure 3(a) shows the magnetic hysteresis loops of Pt(4)/ FePt(15) films deposited in situ substrate heating at 450 ∘ C. The coercivity (  ) of out-of-plane and in-plane films is 7 kOe and 4 kOe, respectively.The remanence (  ) of outof-plane and in-plane direction is 610 emu/cm 3 and 176 emu/ cm 3 , respectively.The squareness ratios (  /  ) of in-plane and out-of-plane are about 0.5 and 0.9, respectively.These data indicate the preferred alignment of the easy magnetization axis along the out-of-plane direction, which agrees with the result of Figure 1(c).It should be mentioned that the saturation magnetization (  ) of out-of-plane is larger than that of in-plane as shown in Figure 3(a).This may be due to the fact that   of in-plane is not completely saturated under the maximum applied field of 3 Tesla, which has been reported in other publications about L1 0 -FePt films [24,25].The ZnO barrier layer, Fe free layer, and Pt protective layer were deposited on the Pt/FePt film at room temperature to prepare the L1 0 -FePt based sensor.The in-plane and out-ofplane magnetic hysteresis loops of Pt(4)/FePt(15)/ZnO(5)/ Fe(5)/Pt(3) multilayers are shown in Figure 3(b).There hardly exists coupling between top Fe free layer and bottom FePt reference layer for arising apparent kinks in it.The magnetization of multilayers reversal process (represented by arrows) can be understood precisely in out-of-plane loop when external magnetic field is applied perpendicular to film plane.The free layer is aligned perpendicular to reference layer when external magnetic field hits zero (point O).When magnetic field reached −7 kOe (or +7 kOe) (point A or B), the bottom FePt would begin to reverse from the up (down) to down (up) perpendicular to the films.
A typical MR curve of a nonpatterned sample is shown in Figure 4, showing regular current-in-plane (CIP) MR effect with an applied magnetic field out of the film plane.The MR values are found to be 0.5% and 1.2% measured at room temperature and 10 K, respectively.A double-peak feature at the coercive fields can be observed in the curves, which is in agreement with the magnetic hysteresis loop in Figure 3. Here, the MR is defined as MR = [R(H) − R(max)]/R(max) × 100%, where () and (max) are the resistance at external field  and the maximum resistance, respectively.This value is the typical magnitude of MR in L1 0 -FePt based multilayers [14,26,27], because that ZnO cannot be perfectly grown on L1 0 -FePt layer as shown Figure 3(a).Besides, we believe that the MR value of the film may be higher due to the fact that the MR curve is not saturated at 20 kOe, and the MR ratio might obtain a higher value if a higher magnetic field was applied to our sample.
The expected high field linear characteristic and nonhysteretic variation of the MR with highly reversibility is obtained when the magnetic field varies between +5 kOe and −5 kOe at room temperature as shown in Figure 4(c).The sensitivity is 0.2%/T in the MTJ composed of Pt(4)/FePt(15)/ ZnO(5)/Fe(5)/Pt(3) multilayers.The magnetic configuration and the resistance evolve with the field as shown in Figure 4(c).When the applied field is +5 kOe, the magnetization direction of the L1 0 -FePt layer points out-of-plane and the one of the Fe layer is not completely parallel to the magnetization of L1 0 -FePt layer (see point A in Figure 4(c)) due to the fact that the magnetic loop and MR are not saturated at 5 kOe shown in Figures 3(b) and 4(a).At this point (point M), the device has a smaller resistance.The value of resistance increases linearly with decreasing magnetic field until the field is reduced to zero at point O, where the magnetization of the Fe layer is rotated to perpendicular to the one of L1 0 -FePt layer.With the magnetic field increases in the reverse direction, the resistance continues to increase until the magnetic field reaches −5 kOe at point N.It is important to note that the magnetization of L1 0 -FePt layer is always invariant throughout the process, which plays a key role in the MR linear variation with the magnetic field.

Conclusion
In summary, linear MR in Pt/FePt/ZnO/Fe/Pt multilayer magnetic sensor was investigated.In this design, perpendicular anisotropy L1 0 -FePt reference layer and in-plane anisotropy Fe free layer separated by ZnO barrier layer are perpendicular to each other.The L1 0 -FePt film with large coercivity and high squareness ratio was obtained by in situ substrate heating.The MR values of 0.5% and 1.2% were obtained at room temperature and 10 K, respectively.Linear MR response is observed in this sensor in a large range between +5 kOe and −5 kOe at room temperature.This L1 0 -FePt based linear MR is significant for high field linear magnetic sensors.

2 Advances
(b).Here the free and reference layers

Figure 4 :
Figure 4: The MR of the multilayer films measured at 300 K (a) and 10 K (b); (c) the linear magnetic field response of the multilayer films when the external field varies between +5 kOe and −5 kOe at 300 K.The insets in (c) show the three repeated measures of MR curve of the same sample.