Magnetic encoder apparatus
To provide a magnetic encoder having high resolution and a high output, and a magnetoresistive sensor for an encoder. A fine sensor having a reduced demagnetization field is produced by: forming a free layer consists of a laminated soft magnetic film in which two soft magnetic layers are coupled in anti-parallel, and setting the width of a sensor unit to 2 μm to 4 μm.
1. A magnetic encoder, comprising:
a magnetic medium on which an area with the width of λa magnetized in a first direction and an area with the width of λb magnetized in a second direction are alternately disposed; and
a magnetoresistive sensor formed by arranging a plurality of sensor units at intervals in a width direction of the area, each of the sensor units being formed of a magnetoresistive film patterned into a strip-like shape, the magnetoresistive film having a structure in which a soft magnetic free layer, a non-magnetic intermediate layer and a ferromagnetic pinned layer are stacked, a magnetization direction of the soft magnetic free layer changing in response to an external magnetic field,
wherein the soft magnetic free layer of the sensor units has a structure in which a first soft magnetic film, an anti-parallel coupling film and a second soft magnetic film are stacked in sequential order from a side of the non-magnetic intermediate layer; and magnetizations of the first soft magnetic film and the second soft magnetic film are magnetically coupled to each other in anti-parallel directions, and
the magnetic encoder outputs an electric signal having a period of λ=(λa+λb)/2.
2. The magnetic encoder according to claim 1 , wherein the magnetoresistive film has an antiferromagnetic film which applies an exchange coupling magnetic field substantially pinning magnetization of the ferromagnetic pinned layer.
3. The magnetic encoder according to claim 1 , wherein the following equations are satisfied:
M1>M2;
and
1(nm· T )<M1−M2<3(nm· T )
where M 1 denotes a product of saturation magnetization and a thickness of the first soft magnetic film, and M 2 denotes a product of saturation magnetization and a thickness of the second soft magnetic film.
4. The magnetic encoder according to claim 1 , wherein the magnetoresistive film has the width of 4 μm or less and the length of 50 μm or longer.
5. The magnetic encoder according to claim 1 , wherein the magnetoresistive sensor has a first sensor unit; a second sensor unit disposed apart from the first sensor unit in a width direction by (2n-1) λ, where n denotes a positive integer; a third sensor unit disposed apart from the second sensor unit in the width direction by (2n-1) λ/2; and a fourth sensor unit disposed apart from the third sensor unit in the width direction by (2n-1) λ;
and the first, second, third and fourth sensor units form a bridge circuit.
6. The magnetic encoder according to claim 5 , wherein
a power terminal, the first sensor unit and the second sensor unit which are electrically coupled in serial, a middle connected point, the third sensor unit and the fourth sensor unit which are electrically connected in serial, and a ground part are electrically coupled in sequential order; and
a middle point voltage of the middle connected point is detected.
7. The magnetic encoder according to claim 1 , wherein the λ is 10 μm or less.