Since a GMR head utilizing a giant magnetoresistive effect (GMR effect) is widely known, a recording density of a magnetic recording becomes remarkably high. A GMR element includes a multilayer (spin bulb film), i.e., a ferromagnetic layer/a non-magnetic layer/a ferromagnetic layer. In the GMR element, an exchange bias is subjected to one ferromagnetic layer to fix a magnetization, a magnetization of another ferromagnetic layer is changed by an external magnetic field, and a change of relative angle between magnetization directions of two ferromagnetic layers is detected as a change of resistance value. Up to this time, a CIP-GMR element to detect a resistive change by applying a current onto a plane of the spin bulb film, and a CPP-GMR element to detect a resistive change by perpendicularly applying a current onto a plane of the spin bulb film are developed. The CIP-GMR element and the CPP-GMR element can acquire a magnetic resistive ratio (MR ratio) such as several percents, and can process a recording density such as 250 Gbit/inch2, respectively.
In order to cope with high density of magnetic recording, a TMR element utilizing a tunneling magnetoresistive effect (TMR effect) has been developed. The TMR element includes a multilayer (a ferromagnetic layer/an insulator/a ferromagnetic layer), and a tunnel current flows by applying a voltage between the ferromagnetic layers. In the TMR element, the tunnel current is changed by a relative angle between magnetization directions of two (top and bottom) ferromagnetic layers. Accordingly, a change of the relative angle by the external magnetic field is detected as a change of a tunnel resistance value.
The TMR element has the MR ratio larger than the GMR element, and a signal voltage is also larger. However, in the TMR element, not only a pure signal component but also a noise component by a shot noise becomes also larger. As a result, S/N ratio (signal to noise ratio) does not improve. The shot noise is due to fluctuation of a current generated by an electron irregularly passing through a tunnel wall, and increases in proportion to a square root of tunnel resistance. In order to acquire a necessary signal voltage by suppressing the shot noise, a tunnel isolation layer needs to be thin, i.e., the tunnel resistance needs to be lower.
As the recording density is higher, an element size needs to be smaller as the same size as a recoding bit. Accordingly, a junction resistance of a tunnel isolation layer needs to be small, i.e., the tunnel isolation layer needs to be thin. As to the recording density of 300 Gbit/inch2, the junction resistance should be smaller than 1 Ω·cm2. By converting this to a film thickness of a Mg—O (Magnesium oxidation film) tunnel isolation layer, a tunnel isolation layer having a thickness of two atomic layers should be formed. As the tunnel isolation layer is thinner, two (top and bottom) electrodes easily short-circuit, and the MR ratio falls. As a result, manufacture of this element is extremely difficult. By above-mentioned reason, a limit of the recording density to cope with the TMR element is estimated as 500 Gbit/inch2.
Broadly speaking, the above-mentioned elements utilize a magnetoresistive effect (MR effect). Recently, magnetic white noise has become a problem that is common to MR elements. This white noise occurs due to heat fluctuation of minute magnetization, different from an electric noise such as the shot noise. Accordingly, the white noise becomes a larger problem in proportion to the miniaturization of the MR element. At the recording density of 250˜500 Gbit/inch2, the white noise is more notable than the electric noise.
On the other hand, a spin wave oscillator is disclosed in “L. Berger “Emission of spin waves by a magnetic multilayer traversed by a current” Physical Review B 54, 9353 (1996) . . . non-patent reference 1”. The spin wave oscillator includes triple-layer (a first ferromagnetic layer/a non-magnetic layer/a second ferromagnetic layer). By perpendicularly applying a current onto a film plane of the spin wave oscillator, a movement of the magnetization is utilized. Concretely, by perpendicularly applying a direct current onto the film plane, an electron is spin-polarized while passing through the first ferromagnetic layer, and the spin-polarized current flows into the second ferromagnetic layer. By correlatively acting the spin of the electron upon a magnetization of the second ferromagnetic layer, a precession of the magnetization is induced in the second ferromagnetic layer. As a result, a micro wave occurs by the precession induced, and the spin wave oscillator oscillates with a frequency corresponding to the micro wave. As to the spin wave oscillator, a high-frequency oscillation voltage occurred at the spin wave oscillator is detected by the MR effect. This technique is disclosed in “S. I. Kiselev et al. “Microwave oscillations of a nanomagnet drives by a spin-polarized current” Nature 425, 380 (2003) . . . . Non-patent reference 2”.
The spin wave oscillator of a magnetic material using a spin-transfer effect is called a spin-transfer oscillator or a spin-torque oscillator (STO). By improved miniaturization, the spin-torque oscillator can be manufactured as a size equal to or smaller than “100 nm×100 nm”, and the micro wave can be locally generated. In the spin-torque oscillator, an amplitude and a frequency of the precession of a magnetization depend on a magnetic field acting upon the current and the magnetization. A magnetic sensor preparing the spin-torque oscillator having this feature is disclosed in “JP-A 2006-286855 (Kokai) . . . . Patent reference 1”.
However, the spin-torque oscillator is a non-linear oscillator, and a film thickness of the ferromagnetic layer is composed by a very thin film such as “nm” unit. Accordingly, a demagnetizing field strongly acts along a direction perpendicular to the plane having the magnetization with the precession. In an in-plane magnetization film, due to the effect of the demagnetizing field, a frequency shifts to a low frequency side when an oscillation amplitude of the magnetization increases. In this case, a fluctuation of the amplitude strongly correlates with a fluctuation of a phase, and a spectrum linewidth does not become narrow. As a result, if the spin-torque oscillator having a wide spectrum linewidth is used as a magnetic sensor, the S/N ratio is low, and sufficient characteristics are not acquired.
As mentioned-above, the spin-torque oscillator has a non-linearity. Concretely, the demagnetizing field which acts on the in-plane magnetization film (precession of magnetization is induced) is large, and the frequency decrease in proportion to increase of the amplitude. As a result, the spectrum linewidth cannot become narrow. Accordingly, a spin-torque oscillator having a narrow spectrum linewidth (stable oscillation frequency) is desired. Furthermore, a magnetic sensor having the spin-torque oscillator, and a magnetic recording system having the magnetic sensor, are desired.
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