Magnetic sensors are a key component of the disk drive. They determine how much and how fast the disk drive can "read." Because computer users are constantly demanding higher density disk drives (with more memory in the same physical space), manufacturers must constantly push to make smaller and smaller magnetic sensors. Unfortunately, practical size limits have been reached with present sensor technology. As sensors are made smaller, their performance degrades. The performance limits have to do with signal-to-noise ratio: smaller sensors give off smaller signals. At some point, the signals become too small to distinguish from the "noise" coming from the rest of the sensor environment.The team (pictured above) was working in some of the same areas of expertise as those in magnetic sensor research and development, and they became intrigued by the solutions that industry researchers were proposing for the magnetic sensor problem. They wondered what they might be able to do to help solve it.Industry researchers were investigating magnetoresistive and giant magnetoresistive (GMR) devices. Magnetoresistance--the change in a conducting material's resistivity when a magnetic field is applied to it--was also recognized by the Livermore team as a promising tool for sensing very small volumes of magnetic media. As they began thinking about the problem, they looked at the GMR devices already developed. They applied their expertise about thin, multilayer films, magnetics, and microfabrication technologies and emerged with a variation on the most promising GMR concept at the time, the "spin-valve" GMR sensor. They called their sensor the CPP-GMR sensor (CPP stands for current perpendicular to the plane).
Simply Different Structures
The CPP-GMR sensor is a microstructure made up of alternating ferromagnetic layers and nonmagnetic metal layers, or spacers. The layers are thin, generally less than 5 nanometers each, and each sensor may contain a total of 10 to 100 individual layers (depending upon material choices and applications). The layer thicknesses are selected to maximize the GMR effect. The total thickness of the sensor is often only approximately 100 nanometers thick, about one-thousandth the width of a human hair.
The width and length of the sensor are small enough to allow the individual ferromagnetic layers to form as single magnetic domains with a preferred magnetic orientation. These orientations are deliberately designed to be antiparallel to each other. Typically, the width of the sensor is 100 to 500 nanometers, and the length is 250 to 1,000 nanometers--a tiny shoe box shape.
In the absence of an external magnetic field, the sensor relaxes to its lowest energy state, with the alternating ferromagnetic layers aligning in an antiparallel configuration. Sensor resistivity is higher in this state because current that flows perpendicularly through the multilayer planes encounters greater electron scattering, which increases the resistivity. When a sufficiently large magnetic field is applied to the sensor, the ferromagnetic layers rotate into a parallel magnetization state. In this configuration, current flowing through the multilayer planes encounters reduced electron scattering and as a result, resistivity is lower. The two resistivity states can be used as the two states in a digital magnetic sensor.
The performance of the CPP-GMR sensor is a significant improvement over conventional GMR sensor design. Stearns' design has the GMR multilayers rotated 90 degrees so that the current flows perpendicular to the plane of the sensor. Because the signal from the CPP-GMR sensor is inversely proportional to its cross-sectional area, the signal actually increases as the sensor is scaled to higher and higher densities (and smaller sensor size). This scaling provides great manufacturing cost advantages for the future.
