JP2000314697A - Spin-polarized scanning tunneling microscope - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To maintain stable tunneling of spin-polarized electrons from a tip of a probe to a sample in a spin-polarized scanning tunneling microscope. A spin-polarized scanning tunneling microscope that irradiates a probe with laser light to excite spin-polarized electrons includes a rotation mechanism that can rotate the probe in parallel to a sample surface. Prepare.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spin-polarized scanning tunneling microscope, and more particularly, to a spin-polarized scanning scanning microscope characterized by a mechanism for irradiating a laser beam on a probe surface which is always in a better state. It relates to a tunnel microscope.

[0002]

2. Description of the Related Art In recent years, the recording density of a recording medium such as a hard disk has risen remarkably. However, if the recording density further increases in the future and the evaluation method is applied to a finer area, it is expected that the above-mentioned method will have limitations. As a method for evaluating magnetic information in such a fine area, a spin-polarized scanning tunneling microscope has begun to be developed. In principle, this spin-polarized scanning tunneling microscope has atomic resolution. I have.

[0003] For example, a spin-polarized scanning tunneling microscope described in Japanese Patent Application Laid-Open No. 62-139240,
The GaAs probe is irradiated with circularly polarized light perpendicularly to generate spin-polarized electrons, and a tunnel current is detected by injecting the spin-polarized electrons into the sample by a tunnel effect. The detection method scans the sample surface while switching the circularly polarized light to the left and right, detects the average value of the tunnel current corresponding to the magnetic domain of the sample as an unevenness signal on the sample surface, and detects the average value of the sample surface unevenness information and the magnetic domain information. The tunnel current is obtained at the same time.

However, in such a detection method, when sudden noise is mixed into the tunnel current due to the vibration of the apparatus or the like, the average value of the tunnel current fluctuates, and accurate unevenness information cannot be obtained. When the sample surface is scanned on the basis of inaccurate irregularity information, the distance between the GaAs probe and the sample surface cannot be kept constant, and the noise of the detected magnetic information is affected. is there.

The present inventor has proposed a scanning method for solving such a problem (if necessary, see Japanese Patent Application Laid-Open No.
A conventional spin-polarized scanning tunneling microscope used for such a scanning method will be described with reference to FIG. FIG. 5 is a conceptual configuration diagram of a conventional spin-polarized scanning tunneling microscope. An excitation light source 41 for irradiating a GaAs probe 50 with a laser beam 44 is driven by a laser driving circuit 43. An AlGaAs semiconductor laser 42 that outputs near-infrared laser light having a wavelength of 830 nm, a Pockels cell that rotates the polarization plane of the laser light by 0 ° or 90 ° according to the value of a binary drive signal supplied from a Pockels cell drive circuit 49. 45, λ that converts laser light 44 into circularly polarized light
/ 4 plate 46, shutter 47, lens 4 for focusing circularly polarized light
8.

The GaAs probe 50 is connected to the probe holder 5.
When the GaAs probe 50 is irradiated with the circularly polarized laser light 44 supported and fixed by 1, electrons in the valence band of the GaAs probe 50 are excited to the conduction band. 3/4 has a spin in the same direction as the propagation direction of the circularly polarized light, and the remaining about 1/4 of the electrons have a spin in the opposite direction.

GaAs in which the spin-polarized electrons are generated
When the probe 50 is moved closer to the sample 52, the spin-polarized electrons flow into the sample 52 due to the tunnel effect.
At this time, the flowing current changes depending on whether the magnetization direction of the magnetic domain of the sample 52 is the same as or opposite to the spin direction of the spin-polarized electrons. Therefore, magnetic information is detected by detecting the tunnel current.

The sample 52 in this case is, for example, a magnetic material having an easy axis in the sample plane, and the sample 52 is fixed on a piezo element 53 fixed on a coarse movement stage 54. The coarse movement stage 54 is driven by the coarse movement controller 59 to move in the X, Y, and Z directions orthogonal to each other, and the piezo element 53 is moved by the piezo controller 58 in the X, Y, and Z directions orthogonal to each other. The sample is displaced minutely with an accuracy of 1 °.
2 is displaced relatively to the GaAs probe 50.

The coarse movement controller 59 and the piezo controller 58 are under the control of a control unit 56. The tunnel current flowing through the GaAs probe 50 is amplified by the preamplifier 55 and supplied to the control unit 56. The control unit 56 scans the GaAs probe 50 on the sample 52 in accordance with a command from the computer 57, and further controls the excitation light source 41 or the Pockels cell drive circuit 49.

As described above, in the spin-polarized scanning tunneling microscope, it is essential that the spin-polarized electrons tunnel stably. The spin-polarized electrons that tunnel from the GaAs probe 50 to the sample 52 are: GaAs
The condition of the surface of the sample 52 made of the probe 50 and the magnetic material depends on the quality of the surface.

A GaAs probe 50 is used as a probe.
When the surface state is pinning by the surface state (p
inning)
As a method to reduce pinning due to various surface states,
A method called sulfur termination is used. this
The sulfur terminating process means that the GaAs probe 50 is set to (NH_Four) _Two
By dipping in S and then performing heat treatment, G
Terminate unbonded dangling bonds on aAs surface
This sulfur-termination treatment is relatively
Stable tunneling is realized.

[0012]

However, even when the above-mentioned sulfur-termination treatment is performed, the surface of the GaAs probe is irradiated with laser light at the time of measurement, so that the measurement time is reduced due to the ablation effect. Becomes longer, the terminated As may be scattered and stable tunneling may not be maintained.

Although the cause of such a tunneling defect has not been elucidated, it has been reported that the Alvarade causes deterioration even in an ultra-vacuum, and that tunneling is good for several hours from the start of measurement. It is considered that the failure involves laser irradiation for a long time, and the laser irradiation deteriorates the surface state of the GaAs probe.

Therefore, an object of the present invention is to maintain stable tunneling of spin-polarized electrons from the tip of a probe to a sample.

[0015]

FIG. 1 is an explanatory view of the principle configuration of the present invention. Referring to FIG. 1, means for solving the problems in the present invention will be described. See FIG. 1. (1) The present invention relates to a spin-polarized scanning tunneling microscope that irradiates a probe 2 with a laser beam 1 to excite spin-polarized electrons. It is characterized by having a rotating mechanism 5 that can rotate.

As described above, when the tunneling current is reduced by providing the rotation mechanism 5 on the probe 2, the laser beam 1 is always irradiated on a new surface having a good surface condition by rotating the probe 2. Therefore, stable tunneling of spin-polarized electrons can be maintained.

(2) Further, according to the present invention, in a spin-polarized scanning tunneling microscope for irradiating a probe 2 with a laser beam 1 to excite spin-polarized electrons, the laser beam 1 is applied to an optical path of the laser beam 1. An optical path scanning mechanism 6 for scanning is provided.

As described above, the laser beam 1
By providing an optical path scanning mechanism 6 for scanning
Different positions in the same plane of the probe 2 can be irradiated with the laser light 1, whereby when the tunnel current is reduced, the laser light 1 is scanned and the laser light 1 is moved to a position where the surface condition is good. By irradiation, stable tunneling of spin-polarized electrons can be maintained.

(3) Further, according to the present invention, in the above (1) or (2), a signal dependent on spin-polarized electrons is lock-in detected, and the rotation mechanism 5 or the optical path is adjusted so that the detected signal is always maximized. A means for controlling at least one of the scanning mechanisms 6 is provided.

As described above, by detecting the signal dependent on the spin-polarized electrons, that is, the tunnel current, by lock-in detection, only the change in the tunnel current can be extracted, whereby the probe 2 and the sample surface can be extracted. The magnetic information can be accurately detected without being affected by the change in the tunnel current due to the change in the distance from the tunnel.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Here, an embodiment of the present invention will be described with reference to FIGS. FIG. 2 is a conceptual configuration diagram of a spin-polarized scanning tunneling microscope according to an embodiment of the present invention. An excitation light source 11 for irradiating a GaAs probe 18 with laser light 14 includes a laser power supply 13. Semiconductor laser 12, which is driven by the laser and outputs near-infrared laser light having a wavelength of 831 nm.
PEM (Photo Elast) that generates left and right circularly polarized light in accordance with a 50 kHz signal supplied from the EM power supply 17.
ic Modulator) 15, optical path scanning mechanism 1
6 is comprised. The Glan Thompson prism, λ
Optical elements such as a / 4 plate or a focusing lens are not shown.

The GaAs probe 18 is supported and fixed by a probe rotating mechanism 19 composed of a piezo element via a probe holder (not shown), and irradiates the GaAs probe 18 with circularly polarized laser light 14. Then, the GaAs probe 1
Eight electrons in the valence band are excited into the conduction band, and about ／ of the excited electrons have a spin in the same direction as the propagation direction of circularly polarized light, and the remaining about １ ／ of electrons Has a spin in the opposite direction.

FIG. 3A is a schematic view showing the shape of the GaAs probe 18, having a (100) plane as a main surface and an impurity concentration of, for example, about 10 ¹⁷ cm ^-3. Using a p-type GaAs substrate of (0
11) The substrate is cleaved to reveal a plane to form a rod-shaped chip, and then cleaved obliquely again so that a (1-10) plane appears. By rotating the GaAs probe 18, the tip is obtained. The (100) plane, the (011) plane, or the (1-10) plane is irradiated with the laser beam 14. In the present specification, for convenience of preparation of the specification, the surface index usually represented by “1 bar” is represented by “−”.
1 ".

FIG. 3B is a view showing a state in which the GaAs probe 18 is held by the probe holder 27, and the probe holder 2 is shown in FIG.
The GaAs probe 18 is inserted into the concave portion 28 provided in 7 and fixed by screws 29 to hold the probe.

Referring again to FIG. 2, for example, a sample 20 made of a magnetic material is fixed on a piezo element 21 fixed on a coarse movement stage 22, and the coarse movement stage 22 is coarsely moved by a command from a control unit 23. The piezo element 21 is driven by a motion controller (not shown) to move in the X, Y, and Z directions orthogonal to each other. , In the Z direction, for example, with a small displacement of 0.1 °.
By these operations, the sample 20 is displaced relative to the GaAs probe 18.

The tunnel current flowing through the GaAs probe 18 is amplified by the preamplifier 25, and is amplified by the control unit 23.
And, it is supplied to the lock-in amplifier 26. Also, PE
The M power supply 17 is operated by a command from the control unit 23.
For example, driven at a frequency of 50 kHz,
The output of the M power supply 17 is used as a reference of the lock-in amplifier 26.

As described above, by detecting the tunnel current lock-in by the lock-in amplifier 26, only the change in the tunnel current can be extracted, whereby the GaAs probe 18 and the sample surface of the sample 20 can be extracted. Magnetic information can be accurately detected without being affected by a change in tunnel current due to a change in distance.

4 (a) and 4 (b) FIG. 4 (a) is a conceptual configuration diagram when the optical path scanning mechanism 16 is viewed from the traveling direction of the laser light 14, and FIG. 4 (b) 3 is a conceptual configuration diagram when the optical path scanning mechanism 16 is viewed along the traveling direction of the laser light 14. As shown in the figure, a pair of piezo elements 32, 3 in which a circular mirror 30 is arranged at a position of a vertex of an equilateral triangle with respect to a fixed point 31
The laser beam 14 is scanned by the inclination of the mirror 30 by expanding and contracting the mirror 3. Although the laser light 14 is shown in FIG. 2 as transmitting through the optical path scanning mechanism 16, the laser light 14 is actually reflected in a different direction by the mirror 30. The position 18 is also arranged at the position where the reflected laser light 14 irradiates.

Based on such a configuration, the operation of the optical path scanning mechanism 16 and the probe rotating mechanism 19, which are features of the present invention, will be described. First, as described above, GaAs
A predetermined position on a predetermined crystal plane of the probe 18 is irradiated with the circularly polarized laser light 14 to measure a tunnel current. When the signal of the magnetization state of the sample 20 is weakened, the lock-in detected signal is also measured. The laser beam 14 is scanned in the same crystal plane by applying a predetermined voltage to the piezo elements 32 and 33 constituting the optical path scanning mechanism 16 in accordance with the strength of the lock-in detection signal and inclining the mirror 30. I do. As described above, the scanning is stopped at the position where the detection signal obtained while scanning in the same crystal plane becomes large, and the normal measurement is restarted again.

Next, when the signal detected as lock-in again becomes weaker during the measurement, the laser beam 14 is again scanned by the optical path scanning mechanism 16. However, if the signal detected as lock-in does not increase, the scanning is stopped. Stop,
This time, by rotating the probe rotation mechanism 19, the surface irradiated with the laser beam 14 is set to a new crystal surface having a favorable crystal state different from that before.

In this case, the probe rotation mechanism 19 is
This is a rotation mechanism using a piezo element that is generally used, and simulates rotation of the GaAs probe 18 by combining displacements in the X, Y, and Z directions by the piezo element.

As described above, according to the present invention, when the magnetic information on the surface of the sample 20 such as a magnetic material is detected using the spin-polarized scanning tunneling microscope, the probe rotating mechanism 19 or the optical path scanning mechanism 16 is used. Is used to irradiate the laser beam 14 to the crystal plane having a good surface state where a large lock-in detection signal can be obtained. Measurement becomes possible.

Although the embodiments of the present invention have been described above, the present invention is not limited to the configurations and conditions described in the embodiments, and various changes can be made. For example,
In the description of the above embodiment, the probe rotation mechanism 19
Although a piezo element capable of controlling a small displacement is used as the piezo element, the invention is not limited to the piezo element, and a stepping motor that is easily available may be used.

In the above embodiment, the piezo elements 32 and 3 are used as the optical path scanning mechanism 16.
The mirror 30 driven by the mirror 3 is used.
However, a prism driven by a pair of piezo elements may be used similarly to the mirror 30.

Further, in the above description of the embodiment, the PEM 15 is used as an optical element for rotating the plane of polarization of the laser light 14, but the present invention is not limited to the PEM, and is different from the conventional example shown in FIG. Similarly, a Pockels cell may be used.

In the above embodiment, the GaAs probe 18 is used as a probe.
It is not limited to As, but is a direct transition type such as InP.
It is sufficient to use a III-V compound semiconductor, and in that case, it is necessary to change the oscillation wavelength of the semiconductor laser 12 according to the forbidden band width of the III-V compound semiconductor to be used.

In the description of the above embodiment, when the detection signal decreases, the optical path scanning mechanism 16 is operated first, and then the probe rotating mechanism 19 is operated. The rotation mechanism 19 may be operated, and then the optical path scanning mechanism 16 may be operated.

Further, in the description of the above embodiment, both the optical path scanning mechanism 16 and the probe rotating mechanism 19 are provided in the spin-polarized scanning tunneling microscope, but in some cases, only one of them is provided. Only, and in particular, only the probe rotating mechanism may be provided.

[0039]

According to the present invention, since a probe rotating mechanism or an optical path scanning mechanism is provided, a new crystal plane having a good surface condition can always be irradiated with a laser beam, thereby providing a spin polarization. Since stable tunneling of electrons can be maintained, stable measurement of magnetic information can be performed, and this greatly contributes to improvement of the recording density of the magnetic recording medium.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a basic configuration of the present invention.

FIG. 2 is a conceptual configuration diagram of a spin-polarized scanning tunneling microscope according to an embodiment of the present invention.

FIG. 3 is an explanatory diagram of a GaAs probe used in the embodiment of the present invention.

FIG. 4 is an explanatory diagram of an optical path scanning mechanism used in the embodiment of the present invention.

FIG. 5 is a conceptual configuration diagram of a conventional spin-polarized scanning tunneling microscope.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 laser light 2 probe 3 sample 4 sample surface 5 rotation mechanism 6 optical path scanning mechanism 11 excitation light source 12 semiconductor laser 13 laser power supply 14 laser light 15 PEM 16 optical path scanning mechanism 17 PEM power supply 18 GaAs probe 19 probe rotation mechanism 20 sample Reference Signs List 21 Piezo element 22 Coarse movement stage 23 Control unit 24 Computer 25 Preamplifier 26 Lock-in amplifier 27 Probe holder 28 Concave part 29 Screw 30 Mirror 31 Fixed point 32 Piezo element 33 Piezo element 41 Excitation light source 42 Semiconductor laser 43 Laser drive circuit 44 Laser light 45 Pockels cell 46 λ / 4 plate 47 Shutter 48 Lens 49 Pockels cell drive circuit 50 GaAs probe 51 Probe holder 52 Sample 53 Piezo element 54 Coarse stage 55 Preamplifier 56 Control unit 57 Computer 58 Piezo controller 59 Coarse controller

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2F063 AA50 CA40 DA01 DA05 DB06 DD02 EA16 EA20 EB23 EB27 FA07 LA30 ZA10

Claims (3)

[Claims] 1. A spin-polarized scanning tunneling microscope for irradiating a probe with laser light to excite spin-polarized electrons, comprising a rotation mechanism capable of rotating the probe parallel to a sample surface. A spin-polarized scanning tunneling microscope characterized by the following. 2. A spin-polarized scanning tunneling microscope for irradiating a probe with laser light to excite spin-polarized electrons, comprising: an optical path scanning mechanism for scanning the optical path of the laser light with the laser light. Features a spin-polarized scanning tunneling microscope. 3. A means for lock-in detecting a signal dependent on the spin-polarized electron and controlling at least one of the rotation mechanism and the optical path scanning mechanism so that the detection signal is always maximized. Claim 1.
Or a spin-polarized scanning tunneling microscope according to 2.