TY - GEN
T1 - Wide-dynamic-range magnetic sensor based on magnetic tunnel junctions using perpendicularly magnetized synthetic antiferromagnetic reference layer
AU - Nakano, T.
AU - Oogane, M.
AU - Furuichi, T.
AU - Ando, Y.
N1 - Funding Information:
Magnetic tunnel junctions (MTJs) with an MgO barrier, which exhibit a giant tunnel magnetoresistance (TMR) effect [1]-[4], have been intensively studied for application to various magnetic sensors in the automotive industry. It is estimated that the magnetic sensors should have a dynamic range wider than ±2 kOe in certain cases. A linear TMR curve to an external magnetic field has been demonstrated in MgO-based MTJs by utilizing perpendicular magnetic anisotropy (PMA) [5],[6]. In our previous study [6], we reported linear TMR curves in CoFeB/MgO/CoFeB-MTJs with an in-plane magnetized free layer and a perpendicularly magnetized [Co/ Pd]-based reference layer. Thanks to the high PMA energy and large coercivity value the Co/Pd multilayer exhibited, we achieved a relatively wide dynamic range up to ±0.6 kOe in the MTJs. To widen the dynamic range further, it seems effective to modify the [Co/Pd]-based reference layer into a perpendicularly magnetized synthetic antiferromagnetic (p-SAF) structure. The strength of the interlayer exchange coupling in a p-SAF structure is expressed by the exchange field Hex = Jex / Mst (in the case where Ku > Jex / t) [7],[8], where Jex is an exchange coupling energy and Ms, t, and Ku are respectively the saturation magnetization, thickness, and perpendicular magnetic anisotropy energy of the ferromagnetic layers. Previously, Yakushiji et al. demonstrated a quite strong interlayer exchange coupling with Hex = 5.5 kOe in MTJs with a [Co/Pt]-based p-SAF structure [9]. If we integrate a p-SAF structure as a reference layer into an MTJ for magnetic sensor applications, the value of Hex should correspond to the dynamic range of the MTJ in the case that the value is smaller than the anisotropy field Hk value of the free layer. Hence, we attempted to achieve a wide dynamic range by developing a [Co/Pd]-based p-SAF reference layer [10]. In this study, we developed CoFeB/MgO/CoFeB-MTJs using a p-SAF reference layer for magnetic sensor applications. The MTJs showed dynamic ranges more than ±2.5 kOe, which are wider than those in CoFeB/MgO/CoFeB-MTJs reported so far. We also evaluated sensor performance metrics of the MTJs, i.e., sensitivity and nonlinearity. Film depositions were carried out on thermally oxidized Si (001) substrates by using a dc/RF magnetron sputtering system. The stacked structures of the MTJs were substrate/Ta (5)/Ru (10)/Pd (0.2)/ [Co (0.2)/Pd (0.4)]9/Co (0.2)/Ru (0.4)/Co (0.2)/[Pd (0.4)/Co (0.2)]7/Ta (0.3)/ Co40Fe40B20 (0.8)/MgO (2)/Co40Fe40B20 (tCoFeB = 1.8, 2.4, and 3)/Ta (5)/ Ru (8) (in nm). The films of the MTJs were microfabricated into circular junctions 80 μm in diameter by a photolithography process. All the samples were annealed at 300°C for 1 h in a vacuum. We measured TMR properties by a dc four-probe method and magnetic properties with a vibrating sample magnetometer under out-of-plane magnetic fields. Fig. 1 shows a magnetization curve of the [Co/Pd]-based p-SAF reference layer. There exists a plateau near zero magnetic fields where the two magnetizations in the p-SAF reference layer are antiferromagnetically aligned and completely compensated by each other. The Hex value of the p-SAF reference layer was determined to be 2.7 kOe, where one of the magnetizations is sharply reversed. Note that we defined Hex as the magnitude of a magnetic field at the center of a hysteresis originating from the p-SAF reference layer. This Hex value satisfies our requirement for the reference layer with a dynamic range more than ±2 kOe. Next, we investigated minor TMR loops without the flips of the magnetizations in the p-SAF reference layer. Fig. 2 shows normalized minor TMR loops of the MTJs. All the MTJs exhibited linear TMR curves, thus confirming we achieved a dynamic range more than ±2.5 kOe. The observed maximum TMR ratio decreased as the values of tCoFeB increased. This mainly reflects the magnitudes of the free layers’ Hk, which monotonically increase as the values of tCoFeB increase because the demagnetization energy becomes dominant compared with the PMA energy at the MgO/CoFeB interface: 6, 12, and 13.5 kOe for tCoFeB = 1.8, 2.4, and 3 nm respectively. A small Hk value leads to a large change in the relative angle between the magnetizations of the free and reference layers, resulting in a high TMR ratio. Finally, we evaluated and analyzed the performance metrics of the MTJ sensors. The sensitivities and the nonlinearities were found to depend significantly on the Hk values of the free layers. We explained the dependences by a simple model based on the Stoner-Wohlfarth and Sloncze-wski models, which gives us a guideline to design the sensor performance metrics. These results demonstrated that MTJs with a p-SAF reference layer are promising candidates for wide-dynamic-range magnetic sensors. This work was supported in part by the Center for Spintronics Research Network, in part by the Center for Innovative Integrated Electronic System, and in part by JSPS KAKENHI Grant Number JP24226001 and JP16J01812.
Publisher Copyright:
© 2017 IEEE.
PY - 2017/8/10
Y1 - 2017/8/10
N2 - Magnetic tunnel junctions (MTJs) with an MgO barrier, which exhibit a giant tunnel magnetoresistance (TMR) effect [1]-[4], have been intensively studied for application to various magnetic sensors in the automotive industry.
AB - Magnetic tunnel junctions (MTJs) with an MgO barrier, which exhibit a giant tunnel magnetoresistance (TMR) effect [1]-[4], have been intensively studied for application to various magnetic sensors in the automotive industry.
UR - http://www.scopus.com/inward/record.url?scp=85034662694&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85034662694&partnerID=8YFLogxK
U2 - 10.1109/INTMAG.2017.8007558
DO - 10.1109/INTMAG.2017.8007558
M3 - Conference contribution
AN - SCOPUS:85034662694
T3 - 2017 IEEE International Magnetics Conference, INTERMAG 2017
BT - 2017 IEEE International Magnetics Conference, INTERMAG 2017
PB - Institute of Electrical and Electronics Engineers Inc.
T2 - 2017 IEEE International Magnetics Conference, INTERMAG 2017
Y2 - 24 April 2017 through 28 April 2017
ER -