We have investigated a wide variety of soft magnetic layers as sense layers for magnetic-field sensors. We find that in thin-film form, some of these soft materials can have susceptibilities (?) approaching those of the corresponding bulk material. In general, the highest ? values occur in trilayer structures with a non-magnetic film separating two soft magnetic films. The alloy Ni77Fe14Cu5Mo4 of the mu-metal family is the softest thin-film material we have found, and we can achieve hard-axis ? values above 105 in trilayer structures. The hard axis is preferred for magnetic sensors due to its near-linear response. The major impediment we have found to using these very soft layers in low-field sensors is that the X value decreases by almost two orders of magnitude when the soft structure is incorporated in a standard spin valve or tunnel junction. The problem appears to be stiffening of the soft layer by the stray field from ripple in the pinned layer. A partial solution is found in the use of a synthetic antiferromagnetic as the pinned film. The antiferromagnetic alignment appears to have a canceling effect on the stray field.
Citation: Thin Solid Films
Pub Type: Journals
magnetic sensors pinning, synthetic antiferro-magnet