SAF Free Layer Biasing for High Resolution MR Read Heads
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Solution Overview
Problem
Magnetoresistive (MR) sensors in hard disk drives face challenges in achieving improved down-track linear resolution without reducing the read gap width, which would compromise signal-to-noise ratio due to reduced magnetic charge and increased thermal noise from domain perturbations.
Innovation Solution
A CPP MR read head design utilizing a synthetic-antiferromagnetic (SAF) free layer structure with exchange bias and hard magnetostatic bias, replacing traditional read shields to enhance resolution without narrowing the read gap, and employing a top exchange biasing structure to stabilize the SAF free layer magnetization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the read gap width is reduced to improve down-track linear resolution, then the spatial resolution is improved, but the signal-to-noise ratio deteriorates due to reduced magnetic charge and increased thermal noise
Solution Approach 1:
The patent changes the magnetic configuration parameters by introducing a SAF free layer with exchange bias coupling and longitudinal magnetization, replacing the traditional single-layer free layer. This parameter change allows the system to achieve both high resolution and high SNR by creating symmetric magnetic charge distribution that enhances signal while maintaining thermal stability.
Solution Approach 2:
The patent employs a composite magnetic structure consisting of two ferromagnetic layers (first and second magnetic layers) coupled through a nonmagnetic layer, forming a synthetic antiferromagnetic structure. This composite material approach enables the system to simultaneously achieve the properties needed for high resolution (narrow effective width) and high signal-to-noise ratio (symmetric magnetic charge distribution).
2Measurement precision
If the read gap width is reduced to improve down-track linear resolution, then the spatial resolution is improved, but thermal noise from domain perturbations increases
Solution Approach 1:
The patent introduces exchange bias coupling between the first magnetic layer and a pinned layer, and applies a longitudinal bias field to establish symmetric magnetization directions. This parameter change stabilizes the magnetic domains and reduces thermal fluctuations, thereby reducing thermal noise while maintaining high spatial resolution.
Solution Approach 2:
The patent applies preliminary stabilization measures by introducing exchange bias and longitudinal hard bias before the sensing operation. This preliminary anti-action prevents domain perturbations and thermal noise from degrading the signal, allowing the system to operate at narrow read gap widths without suffering from increased thermal noise.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design achieves high linear resolution and signal stability by symmetrically biasing the SAF free layer, reducing thermal noise and maintaining a reasonable signal-to-noise ratio, as demonstrated by simulated read-back signal waveforms showing peak-to-peak amplitudes comparable to wider bit lengths.
Implementation Method 1
a top exchange biasing structure exchange coupled to an upper layer of the SAF free layer to provide exchange biasing
Implementation Method 2
Hard bias (HB) magnets (5) (magnets formed of hard, i.e. high coercivity, magnetic material) are laterally disposed to either side of the sensor stack (6). These magnets, which stabilize the magnetization (arrow, 81) of the free layer (8) by a magnetostatic interaction with the free layer
Implementation Method 3
the resistance of the head varies in accord with the physical principles of the tunneling-magnetoresistive effect which is based on the tunneling of conduction electrons through a thin dielectric layer
Implementation Method 4
a synthetic antiferromagnetic (SAF) free layer structure comprising a first magnetic layer and a second magnetic layer with oppositely directed magnetizations
Data Source
AI summary
A CPP (Current Perpendicular to Plane) MR (Magnetoresistive) read head and its method of fabrication includes a patterned CPP MR sensor stack having a SAF (Synthetic Antiferromagnetic) free layer structure that is longitudinally biased by the combination of an exchange biasing layer formed over the sensor stack and hard biasing layers that are formed adjacent to the patterned sides of the stack. The combination provides the stack with high resolution reading capabilities without the necessity for a narrow read gap formed by closely spaced top and bottom shields. Sixteen embodiments are described that provide different versions of the exchange biasing layer, different positions of the hard biasing layers and different patternings of the CPP MR sensor stack.