Effect of second-order magnetic anisotropy on nonlinearity of conductance in CoFeB/MgO/CoFeB magnetic tunnel junction for magnetic sensor devices

Takahiro Ogasawara,Mikihiko Oogane,Muftah Al-Mahdawi,Yasuo Ando,Masakiyo Tsunoda

doi:10.1038/s41598-019-53439-0

Takahiro Ogasawara, Mikihiko Oogane + Show 3 more

Open Access

https://doi.org/10.1038/s41598-019-53439-0

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Abstract

We studied the effect of second-order magnetic anisotropy on the linear conductance output of magnetic tunnel junctions (MTJs) for magnetic-field-sensor applications. Experimentally, CoFeB/MgO/CoFeB-based MTJs were fabricated, and the nonlinearity, NL was evaluated for different thicknesses, t of the CoFeB free layer from the conductance. As increasing t from 1.5 to 2.0 nm, maximum NL, NLmax was found to decrease from 1.86 to 0.17% within the dynamic range, Hd = 1.0 kOe. For understanding the origin of such NL behavior, a theoretical model based on the Slonczewski model was constructed, wherein the NL was demonstrated to be dependent on both the normalized second-order magnetic anisotropy field of Hk2/|Hkeff| and the normalized dynamic range of Hd/|Hkeff|. Here, Hkeff, Hk2, are the effective and second-order magnetic anisotropy field of the free layer in MTJ. Remarkably, experimental NLmax plotted as a function of Hk2/|Hkeff| and Hd/|Hkeff|, which were measured from FMR technique coincided with the predictions of our model. Based on these experiment and calculation, we conclude that Hk2 is the origin of NL and strongly influences its magnitude. This finding gives us a guideline for understanding NL and pioneers a new prospective for linear-output MTJ sensors to control sensing properties by Hk2.

Highlights

Magnetic tunnel junctions (MTJs) using a MgO barrier layer have a large tunnel magnetoresistance (TMR) ratio at room temperature[1,2,3,4,5,6], and this has made them of interest for a number of spintronic applications such as read heads for hard disk drives and magneto-resistive random access memory
Hd is defined as the range of the magnetic field, H, where the sensing properties are evaluated within |H| < Hd; for example, Hd = 1.0 kOe means the sensing properties are evaluated within −1.0 kOe < H < 1.0 kOe
Regarding (1), since magnetic tunnel junctions (MTJs) sensors are composed of two ferromagnetic electrodes with orthogonal easy axes, the maximum Hd is determined by the smaller values of the effective anisotropy field, Hkeff, of the free layer or the switching field, Hsw, of the pinned layer

Summary

Introduction

Magnetic tunnel junctions (MTJs) using a MgO barrier layer have a large tunnel magnetoresistance (TMR) ratio at room temperature[1,2,3,4,5,6], and this has made them of interest for a number of spintronic applications such as read heads for hard disk drives and magneto-resistive random access memory. Note that the magnetic field is assumed to be applied along the easy axis of the pinned layer For this reason, utilization of perpendicular magnetic anisotropy (PMA) is especially useful due to the large Hsw of perpendicularly synthetic antiferromagnetic (p-SAF) coupled Co/Pt multilayers[13,14,15,16,17] and L10-ordered MnGa alloy[18]. According to the previous study, a conductance model taking account of only first-order magnetic anisotropy suggests that G is expressed by G = G0(1-P2H/Hk eff)[21,22], where G0 is the conductance at H = 0, P is the effective spin polarization and H is the magnetic field This equation means that G is perfectly proportional to H, resulting in NL to be 0 in theory. Based on these experimental and theoretical results, the origin and the controlling method of NL will be discussed

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