CPP-GMR Sensors with Damped Free Layers
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Solution Overview
Problem
Scissoring-type CPP-GMR sensors are susceptible to spin transfer torque (STT)-induced instability and magnetic instability, limiting the bias current density and sensitivity, especially due to the absence of a ferromagnetic pinned layer and strong magnetostatic interactions between free layers.
Innovation Solution
Incorporating damping layers made of Pt or Pd, or rare-earth metals like La, Ce, and Dy, in contact with the free layers to increase magnetic damping, which reduces STT effects and enhances the critical current density for stable operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the bias current density is increased to maximize signal and signal-to-noise ratio in CPP-GMR sensors, then the sensing performance is improved, but spin transfer torque induces continuous gyrations of magnetization resulting in substantial low-frequency magnetic noise and instability
Solution Approach 1:
A nonmagnetic spacer layer is introduced between the two ferromagnetic free layers to mediate their magnetic interaction. This spacer layer allows the layers to be positioned close together for strong magnetostatic coupling while preventing direct exchange coupling, enabling the scissoring mode operation with reduced spin transfer torque effects
Solution Approach 2:
The sensor uses a composite structure with two different ferromagnetic free layers having different magnetization directions (one in-plane, one perpendicular-to-plane). This composite approach allows the layers to respond differently to spin transfer torque, reducing overall magnetic instability while maintaining high signal-to-noise ratio
2Device complexity
If a ferromagnetic pinned layer is used in conventional GMR sensors to fix magnetization direction, then the sensor structure is simplified, but the pinned layer is susceptible to spin transfer torque and difficult to magnetically damp
Solution Approach 1:
The invention extracts and removes the ferromagnetic pinned layer from the sensor structure, replacing it with two free layers that are not exchange-coupled to an antiferromagnetic layer. This eliminates the pinned layer's susceptibility to spin transfer torque while maintaining the necessary magnetization reference through the antiparallel configuration of the free layers
Solution Approach 2:
The invention changes the magnetic parameters of the free layers by using different magnetization directions (in-plane vs. perpendicular-to-plane) and different thicknesses. This allows the layers to operate in a scissoring mode where their relative magnetization change detects the external field while being more resistant to spin transfer torque effects
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
The use of damping layers allows for higher bias current densities without magnetic instability, significantly increasing the critical current for current-induced noise and enhancing magnetoresistance, thereby improving sensor stability and performance.
Implementation Method 1
increase the magnetic damping of the ferromagnetic free layer, i.e., to increase the effective thermal coupling between the magnetization (spin-system) and that of its host lattice
Implementation Method 2
The spin-polarized bias or sense current flows perpendicularly through the ferromagnetic layers and produces a spin transfer torque (STT) on the local magnetization
Implementation Method 3
a sensor based on the giant magnetoresistance (GMR) effect that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack
Data Source
AI summary
A “scissoring-type” current-perpendicular-to-the-plane giant magnetoresistive (CPP-GMR) sensor has magnetically damped free layers. In one embodiment each of the two free layers is in contact with a damping layer that comprises Pt or Pd, or a lanthanoid (an element selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Th, Yb, and Lu). Each of the two free layers has one of its surfaces in contact with the sensor's electrically conducting nonmagnetic spacer layer and its other surface in contact with its associated damping layer. A nonmagnetic film may be located between each free layer and its associated damping layer. In another embodiment the damping element is present as a dopant or impurity in each of the two free layers. In another embodiment a nanolayer of the damping element is located within each of the two free layers.