Introduction of RKKY-pMTJ-Based Ultrafast Magnetic Sensor Architecture and Magnetic Multilayer Optimization

Abstract

A state-of-the-art tunnel magnetoresistance (TMR) sensor architecture, which is based on the perpendicularly magnetized magnetic tunnel junction (pMTJ), is introduced and engineered for ultrafast, high thermal stability, and linearity for magnetic field detection. Limitations in high-frequency environments, stemming from insufficient thermal stability and slow recovery times in conventional TMR sensors, are overcome by this approach. The standard MRAM structure is modified, and the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction is employed to give a strong, internal restoring torque to the storage layer magnetization. Sensor linearity is also ensured by this RKKY mechanism, and rapid relaxation to the initial spin state is observed when an external field is removed. The structural and magnetic properties of the multilayer stack are experimentally demonstrated. Robust synthetic antiferromagnetic (SAF) coupling is confirmed by using polar MOKE spectroscopy with an optimal Ru insertion layer thickness (0.6 nm), which is essential for high thermal stability. Subsequently, an ultrafast response of this TMR sensor architecture is probed by micromagnetic simulations. The storage layer magnetization rapidly recovers to the SAF state within an ultrashort time of 5.78 to 5.99 ns. This sub-6 ns recovery time scale suggests potential operation into the hundreds of MHz range.

Keywords: Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction; interlayer exchange coupling; magnetic multilayer optimization; magnetoresistance sensor; micromagnetic simulations; tunnel magnetoresistance; ultrafast switching.

Figure 1

RKKY-pMTJ-based MTJ sensor architecture and working principle: (a) conventional MRAM architecture, the modified MRAM architecture, and magnetic multilayer structure. (b) Initial spin configuration without external field, RKKY-pMTJ-based TMR sensor working principle with in-plane and out-of-plane applied magnetic fields.

Figure 2

(a) The magnetic multilayer structure of RKKY-pMTJ TMR sensor (thickness in nanometers). (b) The magnetic hysteresis loop of the SAF coupled structure with Ru = 0.6 nm. Red and blue arrows indicate the directions of the magnetizations of SL and RL, respectively. The red dotted lines are indicating the exchange field, H_ex. (c) The magnetic hysteresis loop of the synthetic ferromagnetically (SF) coupled structure with Ru = 1.6 nm. (d) Another SAF coupled structure with Ru = 2.3 nm.

Figure 3

Insertion layer thickness-dependent strength of RKKY interaction, Ru thickness-dependent H_ex measurements (Black filled circles) (inset). Conversion curve of H_ex to RKKY exchange energy (J_ex) (Red filled circles).

Figure 4

(a) Cross-sectional transmission electron microscopy (image) of RKKY-pMTJ-based ultrafast TMR sensor architecture with full magnetic multilayer stack, (b) polar MOKE measurement as a function of out-of-plane magnetic field (H_z). The full magnetic multilayer stack information and the magnetization configurations of all layers and each step in magnetic hysteresis loop are visualized. White and Black arrows indicate the directions of the magnetizations of RL1, RL2, and SL, respectively.

Figure 5

Micromagnetic simulations for ultrafast TMR sensors: (a) Schematic configuration of modeling geometry. J_RKKY between SL and RL1 is expressed. (b) Magnetic hysteresis as a function (Red filled circles) of H_z. (c) Top image: the profile of the out-of-plane magnetic field pulse as a function of simulation time (Black solid line). The amplitude of H_z is varied from 0.4 T to 0.15 T with a 0.05 T step. Bottom image: The magnetization profile of the modeling system as a function of simulation time(Black filled circles). The blue and red dashed lines mark the onset and termination of each magnetic field pulse, respectively.