Spin valve element driving method and spin valve element

1. A spin valve element driving method for obtaining an electromagnetic signal from a spin valve element, the spin valve element including an intermediate layer, which is an insulating layer or an electrically conductive nonmagnetic layer, and a pair of ferromagnetic layers including a fixed layer and a free layer sandwiching the intermediate layer, the fixed layer having a higher coercivity than the free layer, and being magnetized in a direction substantially perpendicular to a film plane thereof,

the method comprising a driving step of passing current from one of the pair of ferromagnetic layers to the other through the intermediate layer, wherein

in the driving step, an amount X of the current and the value Y of a magnitude of an external magnetic field satisfy both

a first condition:

( A+BYD ) 2 −4 AD ( BY−CX )>0

and

a second condition:

−1<(( A+BYD )−(( A+BYD ) 2 −4 AD ( BY−CX )) 0.5 )/(2 AD )<(( A+BYD )+(( A+BYD ) 2 −4 AD ( BY−CX )) 0.5 )/(2 AD )<1

when A>0; and

(( A+BYD )−(( A+BYD ) 2 −4 AD ( BY−CX )) 0.5 )/(2 AD )<1

when A<0, where

X is positive when the current passes from the free layer toward the fixed layer,

A=αγHu and B=αγ, α being the Gilbert damping constant, γ being the gyromagnetic ratio, and Hu being the component of the magnetic field, which is the sum of a magnetocrystalline anisotropy field crystal anisotropy magnetic field and a demagnetizing field of the free layer, perpendicular to the free layer, and

C=μP/(Ms×V×e) and D=P 2 if the intermediate layer is an insulating layer, and C=(μP/(Ms×V×e))/(−16+3(P 1/2 +P −1/2 ) 3 ) and D=(P 1/2 +P −1/2 ) 3 /(−16+3(P 1/2 +P −1/2 ) 3 ) if the intermediate layer is an electrically conductive nonmagnetic layer, P being a spin polarization, μ being a Bohr magneton, Ms being a saturation magnetization of the free layer, V being a volume of the free layer, and e being an electronic charge.

2. The spin valve element driving method according to claim 1 , wherein the driving step includes driving the spin valve element without applying an external magnetic field, where the current X passing therethrough, satisfies both

a third condition:

A 2 +4 ADCX >0

and a fourth condition:

−1<( A− ( A 2 +4 ADCX ) 0.5 )/(2 AD )<( A+ ( A 2 +4 ADCX ) 0.5 )/(2 AD )<1,

when A>0, and

( A −( A 2 +4 ADCX ) 0.5 )/(2 AD )<1,

when A<0.

3. The spin valve element driving method according to claim 1 , further comprising passing a current through the spin valve element for a prescribed time prior to the driving step, the amount X of the current satisfying

1<(( A+BYD )+(( A+BYD ) 2 −4 AD ( BY−CX )) 0.5 )/(2 AD ),

where A>0.

4. The spin valve element driving method according to claim 1 , further comprising passing a current through the spin valve element for a prescribed time prior to the driving step, the amount X of the current satisfying

(( A+BYD )+(( A+BYD ) 2 −4 AD ( BY−CX )) 0.5 )/(2 AD )<−1,

where A<0.

5. A spin valve element, comprising:

an intermediate layer, which is an insulating layer or an electrically conductive nonmagnetic layer; and

a pair of ferromagnetic layers including a fixed layer and a free layer sandwiching the intermediate layer, the fixed layer having a higher coercivity than the free layer, such that an electromagnetic signal is obtained when a current passes from one of the ferromagnetic layers to the other through the intermediate layer during which

the fixed layer is magnetized in a direction substantially perpendicular to a film plane of the fixed layer, and

for the free layer, an amount X of the current and the value Y of a magnitude of an external magnetic field satisfy both

a first condition:

( A+BYD ) 2 −4 AD ( BY−CX )>0

and

a second condition:

−1<(( A+BYD )−(( A+BYD ) 2 −4 AD ( BY−CX )) 0.5 )/(2 AD )<(( A+BYD )+(( A+BYD ) 2 −4 AD ( BY−CX )) 0.5 )/(2 AD )<1

when A>0; and

(( A+BYD )−(( A+BYD ) 2 −4 AD ( BY−CX )) 0.5 )/(2 AD )<1

when A<0, where

X is positive when the current passes from the free layer toward the fixed layer,

A=αγHu and B=αγ, α being the Gilbert damping constant, γ being the gyromagnetic ratio, and Hu being the component of the magnetic field, which is the sum of a magnetocrystalline anisotropy field crystal anisotropy magnetic field and a demagnetizing field of the free layer, perpendicular to the free layer, and

C=μP/(Ms×V×e) and D=P 2 if the intermediate layer is an insulating layer, and C=(μP/(Ms×V×e))/(−16+3(P 1/2 +P −1/2 ) 3 ) and D=(P 1/2 +P −1/2 ) 3 /(−16+3(P 1/2 +P −1/2 ) 3 ) if the intermediate layer is an electrically conductive nonmagnetic layer, P being a spin polarization, μ being a Bohr magneton, Ms being a saturation magnetization of the free layer, V being a volume of the free layer, and e being an electronic charge.

6. The spin valve element according to claim 5 , wherein when the current passes from one to the other of the ferromagnetic layers,

an external magnetic field is not applied, and

in the free layer, the amount X of the current passing therethrough satisfies both

a third condition:

A 2 +4 ADCX> 0

and

a fourth condition:

−1<( A− ( A 2 +4 ADCX ) 0.5 )/(2 AD )<( A+ ( A 2 +4 ADCX ) 0.5 )/(2 AD )<1,

when A>0; and

( A− ( A 2 +4 ADCX ) 0.5 )/(2 AD )<1,

when A<0.

7. A spin valve element driving method for obtaining an electromagnetic signal from a spin valve element, the spin valve element including an intermediate layer, and a pair of ferromagnetic layers including a fixed layer and a free layer sandwiching the intermediate layer, the fixed layer having a higher coercivity than the free layer, and being magnetized in a direction substantially perpendicular to a film plane thereof, the method comprising a driving step of passing current from one of the pair of ferromagnetic layers to the other through the intermediate layer, without applying an external magnetic field to the free layer, the free layer exhibiting a saturation magnetization and anisotropy magnetic field for which in the driving step, the current passing in the spin valve element can take on a value satisfying a prescribed oscillation condition.

8. A spin valve element, comprising:

an intermediate layer; and

a pair of ferromagnetic layers including a fixed layer and a free layer sandwiching the intermediate layer, the fixed layer having a higher coercivity than the free layer, such that an electromagnetic signal is obtained when a current passes from one of the ferromagnetic layers to the other through the intermediate layer, the fixed layer, being magnetized in a direction substantially perpendicular to a film plane of the fixed layer, and the free layer exhibiting a saturation magnetization and anisotropy magnetic field for which the current passing in the spin valve element can take on a value satisfying a prescribed oscillation condition.

9. The spin valve element driving method of claim 7 , further comprising, in the driving step, adjusting the current for driving the spin valve element to satisfy the prescribed oscillation condition.

10. The spin valve element of claim 8 , further comprising current adjustment means, connected to at least one of the fixed layer and the free layer, for setting the current for driving the spin valve element to the prescribed oscillation condition.

11. The spin valve element of claim 8 , wherein the intermediate layer is an insulating layer or an electrically conductive nonmagnetic layer.