Controlling domain configuration of the sensing layer for magnetic tunneling junctions by using exchange bias

Sina Ranjbar,Muftah Al-Mahdawi,Yasuo Ando,Mikihiko Oogane

doi:10.1063/1.5130486

Abstract

The control of magnetic domain formation and fluctuations in the sensing layer is important to progress for low noise in magnetic tunnel junction sensors. We studied the effect of exchange bias on the domain structure in micro-patterned Permalloy (Py: Ni80Fe20) sensing layer. We deposited single Py films, and Pt48Mn52/Py films, where the latter showed exchange bias. By controlling the thickness of Py, Pt48Mn52 (15nm)/Py (t=235 nm) showed a small coercivity and exchange bias of 7 Oe. After micro-fabrication into circular pillars 80 µm in diameter, we measured the domain structure by Magneto Optical Kerr Effect (MOKE) microscopy. MOKE images showed that single Py pillars have a simple closure domain, where the domain wall at the center moved with the applied field. The exchange-biased Py pillars exhibited a more complicated structure, but with fixed domains at the center region due to the exchange bias overcoming the magnetostatic energy. The uniform rotation of magnetization at the center of the sample is promising for decreasing the domain hopping magnetic noise.

Highlights

There has been an immense research into the exchange bias (EB) phenomenon at the interface between an antiferromagnet (AFM) and a ferromagnet (FM)
EB have been mostly used for the pinned layers of spin valve structures1 of giant-magnetoresistive (GMR) and magnetic tunnel junctions (MTJs) devices for applications, such as in read heads of hard disk drives, magnetic random access memory (MRAM) cells, and field sensor
One of the main origins of magnetic noise is linked the fluctuating domain walls,10,11 which are at the tunnel junction area in the center of MTJ

Summary

Introduction

There has been an immense research into the exchange bias (EB) phenomenon at the interface between an antiferromagnet (AFM) and a ferromagnet (FM). Magnetic tunnel junctions are becoming important for magnetic field sensing applications, and recently, ultra-sensitive MTJ sensors have been demonstrated for bio-magnetic field detection from the heart and brain activity.. One of the main origins of magnetic noise is linked the fluctuating domain walls, which are at the tunnel junction area in the center of MTJ device. Controlling the domain configuration is important for controlling the magnetic noise in MTJ sensors. The common method for domain control and noise reduction was by using a bias field orthogonal to the sensing field.. The common method for domain control and noise reduction was by using a bias field orthogonal to the sensing field.3,12 This approach is not scalable for large MTJ sensor arrays. A similar function can be achieved by a small EB field from an AFM layer coupled to the sensing free layer. The effect of EB on domain formation in micrometer-sized Py pillars have been shown to exhibit single domains at the pillar’s center, which are uniformly rotating in response to applied field.

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Conclusion