Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-12-19
2004-02-17
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C360S313000, C360S324000, C428S065100, C428S690000
Reexamination Certificate
active
06692847
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic sensor having a GMR magnetic laminated film, more particularly, a magnetic sensor with an improved thermal stability.
2. Description of the Related Art
In a magneto resistive element (hereinafter referred to as an MR element) used in a magnetic sensor in a magnetic encoder, a NiFe alloy film or NiCo alloy film utilizing an anisotropic magneto resistive effect is used. Such materials have a magnet resistance ratio due to the anisotropic magneto resistive effect on the order of 2.5%. An output (signal voltage) of the magnetic encoder using the MR element ranges from 40 to 50 mV. Increasing a magnetic gap of the magnetic encoder used in a machine tool reduces the output. In order to obtain a sufficient output of the magnetic sensor even if the increased magnetic gap, the magneto resistance ratio (&Dgr;R/R) is required to become larger.
Recently, there has been contemplated that a magnetic laminated film of [Ni—Fe—Co/Cu] or [Ni—Fe/Cu], each of which is a giant magneto resistive element (GMR element) having a large magneto resistance ratio, is used as the magnetic sensor. For example, the Japanese Patent No. 2812042 discloses a magnetic sensor of a metal superlattice film having a NiCoFe layer and a nonmagnetic metal thin layer laminated with each other. According to the disclosure, in this magnetic sensor, directions of a current and an applied magnetic field are parallel with each other, and thus the magneto resistance ratio can be increased to 15 to 20%. Such a GMR element typically has the magneto resistance ratio two to four times larger than that of the MR element made of a Ni—Fe alloy or Ni—Co alloy film using the anisotropic magneto resistive effect. Since the output signal of the magnetic sensor can be increased when the magneto resistance ratio is increased, using the GMR element enables the output two or more times higher than that of the MR element (80 to 100 mV, or higher) to be obtained. With the higher output, mounting can be accomplished with a widened magnetic gap when the magnetic encoder is assembled. With the widened magnetic gap, ease of assembly is enhanced, and therefore the production yield is enhanced. Besides, through the application of the GMR element with the higher output, a fill bridge circuit that currently involves the MR element can be replaced with a half bridge circuit that involves the GMR element, thereby miniaturizing the magnetic sensor.
However, in a manufacturing process of the magnetic sensor for the magnetic encoder, the magnetic sensor may be heated in the step of soldering wires, the step of forming a protective film, or the like after the magnetic laminated film (GMR element) is fabricated. In the soldering step, for example, heat used for applying solder to a terminal section of the magnetic sensor or soldering a flexible wiring board to the terminal section may be conducted into the magnetic sensor. Thus, the magnetic laminated film having already been fabricated may be heated.
The conventional magnetic laminated film (GMR element) of [Ni—Fe—Co/Cu] or [Ni—Fe/Cu] has an insufficient thermal resistance compared to the MR element, and if the magnetic sensor is heated in the steps after the magnetic laminated film is formed, the magneto resistance ratio of the magnetic laminated film is decreased. It is considered that lead-free solder (having a composition free of Pb) will be used as the solder applied to a terminal of the magnetic sensor in the future. Compared to an eutectic point of lead solder that is 183 degrees centigrade in the case of Sn—Pb solder, the eutectic point of the lead-free solder is higher, specifically, 221 degrees centigrade in the case of Sn—Ag eutectic solder. As for the conventional magnetic laminated film of [Ni—Fe—Co/Cu] or [Ni—Fe/Cu], &Dgr;R/R is decreased by about 10% when it is heated at the temperature of 230 degrees centigrade, so that the lead-free solder is difficult to use.
The GMR magnetic laminated film is used in the magnetic encoder. The magnetic encoder is used for a machine tool, precision machine, and optical instrument. Particularly in the precision machine and optical instrument, miniaturization of the magnetic sensor is required. For the miniaturization, a glass substrate is reduced in thickness compared to conventional one. In addition, an aluminum oxide (alumina) film or silicon oxide film is formed on a part of the protective film by sputtering. When the glass substrate is used and the protective film is formed by sputtering, the temperature of the substrate is raised to 180 to 190 degrees centigrade. If the glass substrate is reduced in thickness for the sake of miniaturization, it is difficult for heat to escape from the substrate to the sputtering apparatus, so that the temperature of the substrate is considered to be raised to 200 degrees centigrade or more. In the case of the magnetic laminated film of [Ni—Fe—Co/Cu] or [Ni—Fe/Cu], if it is heated to a temperature of 200 degrees centigrade or more, the magneto resistance ratio thereof is decreased. For example, if it is heated at 250 degrees centigrade for one hour, the magneto resistance ratio (&Dgr;R/R) is decreased by as much as 20%. If such a magnetic laminated film undergoes the manufacturing process of the magnetic encoder, the magnetic encoder with a high output can be hardly provided.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a magneto resistive sensor having a magnetic laminated film with a large magneto resistance ratio and improved thermal resistance and a magnetic encoder using the same.
The magneto resistive sensor according to the invention includes a nonmagnetic substrate, an underlayer deposited on the substrate, and a magnetic laminated film formed on the underlayer, in which the magnetic laminated film has a plurality of magnetic thin layers and a plurality of nonmagnetic thin layers alternately laminated. The magnetic thin layer is 5 to 30 angstrom thick, and the nonmagnetic thin layer is 5 to 30 angstrom thick. The magnetic thin layer has a composition represented by the formula: [(Ni
x
Co
1-x
)
y
Fe
1-y
]
z
B
1-z
, where 0.50<x≦1.00, 0.70≦y<1.00, 0.90≦z<1.00. Preferably, in the formula of composition, x follows the inequality: 0.60<x<1.00.
The formula of composition of the magnetic thin layer is represented in terms of atomic contents ratio. The atomic ratio of Ni content to the sum of Ni and Co contents is denoted by x. The atomic ratio of the sum of Ni and Co contents to the sum of Ni, Co, and Fe contents is denoted by y, in other words, the atomic ratio of Fe content to the sum of Ni, Co, and Fe contents is denoted by 1−y. Similarly, the atomic ratio of the sum of Ni, Co and Fe contents to the sum of Ni, Co, Fe, B contents is denoted by z, in other words, the atomic ratio of B content to the whole contents is denoted by 1−z.
According to the invention, the magnetic thin layer contains B, as well as Ni, Co, and Fe that are ferromagnetic metallic elements. B is added in order to improve the thermal resistance, that is, thermal stability of the magneto resistive sensor of the invention, and should satisfy the relationship: 1−z>0. If the content of B is higher than 10 atomic % (at. %), the content of the ferromagnetic metallic elements is insufficient so that the magnetization of the magnetic thin layer is reduced, and therefore, the magneto resistance ratio is decreased. Then, the content of B should be equal to or less than 10 at. %, that is, the relationship: 1−z≦0.1 (0.9≦z) should be satisfied.
Among the ferromagnetic metallic elements, Fe is an element with a large ferromagnetic spin. Therefore, the magnetic thin layer has a large spontaneous magnetization by containing Fe. In an artificial lattice GMR multi-layered film, the magnetic thin layers with a nonmagnetic thin layer interposed therebetween have spins alternately orien
Harata Hitoshi
Itabashi Hiromitsu
Mima Hiroyuki
Shirasaki Fumio
Hitachi Metals Ltd.
Jones Deborah
Koppikar Vivek
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