JP3172143B2 - Spin-polarized scanning tunneling microscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a scanning tunneling microscope, and more particularly to a scanning tunneling microscope suitable for observing a magnetic structure of a magnetic material or a magnetic domain structure of a ferromagnetic material at a spatial resolution of several nm or less.

[0002]

2. Description of the Related Art A magnetic force microscope has been known as a magnetic domain observation means having the highest resolution for evaluating a fine magnetic domain structure on a magnetic material surface. Its resolution is at most about 10 nm. On the other hand, with the miniaturization of magnetic recording media and the like, further improvement in resolution of the hyperfine magnetic domain observation technique is desired.

A scanning tunneling microscope (Scanning)
Tunneling Microscopy) scans the probe relative to the sample on the surface of the sample by driving the sample or the probe, and the tunnel current is detected by the probe.
Utilizing that it is extremely sensitive to the distance between samples,
This is a method to evaluate the surface structure and physical properties at atomic resolution.
When a magnetic material sample or a probe is used, tunnel electrons are spin-polarized, so that if the spin state can be discriminated, magnetic information on the surface can be obtained with a scanning tunneling microscope at atomic level resolution.

[0004]

A method for obtaining spin information from a spin-dependent tunnel current change with a magnetic sample surface using a ferromagnetic probe as the spin-polarized scanning tunneling microscope described in the literature (see, for example, US Pat. R. Wissendan
ger, H .; -J. Guntherodt,
G. FIG. Guntherodt, R.A. J. Gamb
ino and R.S. Ruf, Physical
Review Letters, vol. 65, p
247 (1990). ). This method using a ferromagnetic probe is disclosed in Japanese Patent Publication No.
Compared to a spin-polarized scanning tunneling microscope using optical means as disclosed in JP-A-199757, there is a great advantage that the device configuration and principle are simple. However, as shown in the above-mentioned document, the obtained image is obtained by superimposing the spin information on the surface unevenness image, and it is impossible to extract only the spin image from the actual sample surface whose surface morphology and spin arrangement are unknown. There was a major drawback that you couldn't.
This defect is expected to occur similarly when an antiferromagnetic probe is used in addition to the ferromagnetic probe.

The present invention provides a method for taking out only a spin image by preventing superposition of a surface unevenness image and a spin image, which is a problem when a ferromagnetic or antiferromagnetic probe is used.

[0006]

SUMMARY OF THE INVENTION A spin-polarized scan according to the present invention.
Type tunnel microscope, samples are separated by 10 nm from O.1
Consists of a ferromagnetic or antiferromagnetic material located on the surface
A probe, and scanning the probe relative to the sample surface.
Drive mechanism and a bias voltage between the probe and the sample.
A bias voltage source for applying pressure between the probe and the sample.
With means for detecting tunnel current flowing through the spin
In the polarized scanning tunneling microscope, the probe material is:
Ferromagnetic composed of Fe, Co, Ni, Cr, Mn or their compounds
Body or antiferromagnetic material and the bias
The voltage consists of the probe and the sample at V1 and V2
When the spin polarization of the system is P1 and P2 respectively

(Equation 2) The value F expressed by is either F> 1.03 or F <0.97
For two bias voltages Vl and V2 that satisfy the conditions of
The tunnel current becomes constant (I ₁ ) at the bias voltage V ₁ .
To determine the distance between the probe and the sample
Then, without changing the distance between the probe and the sample,
Only the bias voltage is changed to V _2, and the tunnel current I ₂
Is an image signal.

In a tunnel microscope, a tunnel current I flowing between a probe and a sample is extremely sensitive to a distance d between the probe and the sample, and as the distance increases as approximated by I∞exp (−2 kd). Exponentially decreases. Therefore, by controlling the height (Z position) of the probe from the sample surface at the time of scanning so as to have a constant current and using the Z position as an image signal, an image (topographic image) representing surface irregularities can be obtained. . When the sample is a magnetic material and the probe is also a magnetic material, the conductance when the spins of the electrons contributing to the tunnel are parallel and antiparallel are given by the following equations, respectively, due to the difference in the tunnel conductance depending on the spin. Change.

[0008] _{I ↑↑ = I 0 (1 +} P), I ↑ ↓ = I 0 (1-P) where, P is spin polarization showing a spin detection sensitivity of the whole system, the tunnel probe and the sample If the spin polarization degrees of the contributing electrons are p ^T and p ^S , then P = p ^T × p ^S. I _O indicates the average conductance. Thus, the relative Z of the probe with respect to the sample is set to be constant.
If the position is controlled, if the spin states of the electrons contributing to the tunnel are parallel, the current will flow more easily, and the probe will move away. If it is antiparallel, the current will not flow easily, and the probe will move closer. The difference between the Z positions corresponds to the spin information. However, it is impossible for the actual sample surface to be flat at the atomic level in all observation regions except in a special case, and changes in Z reflecting the sample irregularities overlap. It will be. Moreover, the change due to the unevenness is larger than the change representing the spin information. For this reason, the image of the spin information could not be separated.
This was a conventional disadvantage.

On the other hand, in the present invention, probes having different degrees of spin polarization of the system at different bias voltages are used.
The conditions are as follows: When the spin polarization of the probe-sample system at bias voltages V ₁ and V ₂ is P ₁ and P ₂ , respectively,

(Equation 3) F satisfies either F> 1.03 or F <0.97. In this condition, first, when the tunnel current to control the probe relative height to be constant in the bias V _1, Z position is determined is taken into account spin information in topographic information. In that position, when changing the bias voltage to V _2, without changing the topo information, it is possible to change only the spin-dependent information, V ₂
The spin information can be extracted by using the current I2 at Here, it is a condition that the relationship between V ₁ and V ₂ satisfies the above equation. The optimum value of the bias voltage satisfying this condition varies depending on the electronic state of the sample and the probe. P
_{When 1} = P ₂ , F = 1, and the tunnel current I _{2 at} V ₂ does not change even if the spin state changes, so that a spin image cannot be obtained. Also, when F is too close to 1, the spin image cannot be obtained because it is buried in noise. F is 1
It is desirable to set the bias voltage so as to be as large as possible or as small as possible. F = 1.1
Above or at F = 0.9 or less, a spin image with sufficient contrast can be obtained. In particular, the condition where P ₁ × P ₂ is negative is preferable because a spin image with higher contrast can be obtained. Note that two bias voltage values V ₁ and V
_In both cases, the range of −5 V to +5 V is appropriate.

There are two methods for selecting V ₁ and V ₂ while scanning the relative position of the probe with respect to the sample surface.

In the first method, the same portion is scanned twice.
As shown in FIG. 1, the first scan (corresponding to time t ₁ to t ₂ ) is performed with the bias voltage V ₁ , and the Z position of the piezo that determines the probe-sample distance at which the constant current I ₁ is obtained is recorded. . (Corresponding to t ₃ from the time t ₂₎ 2 th scan is reproduced in accordance with the Z position recorded in the first to the scanning location, reading the current I ₂ (change the magnetization state) in the bias voltage V _2, An image signal.

The second method is to determine the Z position at V ₁ at each scanning point during scanning, and then to set the bias voltage to V
Reading I ₂ in place of _2, do this for each scanning point (pixel). As shown in FIG. 2, at each pixel, by adjusting the piezo height so that a constant current I ₁ is obtained by the voltage V ₁ at t ₂ from time t _1, then at time t ₂ the bias voltage V _The value of the current I ₂ is read in place of _{2 and} used as an image signal. This operation is performed for each pixel during scanning. In FIG. 2, the voltage is schematically changed by a rectangular wave. However, if the voltage changes between V ₁ and V ₂ , deformation such as a trigonometric wave may be performed. In each pixel, the above operation may be performed while scanning is stopped, or the above operation may be performed while scanning.

[0013] When the image signal a tunnel current I ₂ at V _2, the easiest way is a method of imaging an I ₂ itself. More if such desired to clarify the contrast, the ratio I ₂ / I ₁ or the difference between I ₁ (I ₂ -I ₁₎
For example, the result of addition, subtraction, multiplication and division of I ₂ can be used as an image signal.

The probe material applicable to the present invention includes F
A ferromagnetic material or an antiferromagnetic material composed of e, Co, Ni, Cr, Mn and a compound thereof becomes a probe material. In particular, Heusler alloys, CrO ₂ , Fe ₃ O ₄ , La _1-X SrX
A half metal magnetic material such as MnO ₃ is preferable because high sensitivity can be obtained. In addition to the above-described probe made of a single substance, a composite material such as a ferromagnetic layer / non-ferromagnetic layer / ferromagnetic layer containing the above-mentioned materials, or a granular film dispersed in a matrix can also be used. The advantage of the former is that the leakage magnetic field is small and damage to the magnetized state of the sample can be reduced, and the advantage of the latter is that the signal strength can be increased. Further, a laminated structure of a conductive ferromagnetic material and an insulating antiferromagnetic material disclosed in JP-A-6-94813 can also be used. According to the probe described above, according to the conventional method, the spin image overlaps the topographic image, so that only the spin image cannot be extracted. However, according to the present invention, only the spin image can be extracted.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The principle of operation of a scanning tunneling microscope according to the present invention will be described below in detail with reference to the drawings.

An example in which Co is used as a sample and Cr is used as a probe will be described. FIG. 3 schematically shows the electronic states of Co and Cr. As Co electronic state of seen, even at the Fermi energy E _F, also, is both a minority spin electrons becomes dominant even for high energy level than E _F, spin polarization are both negative It is. Meanwhile, Cr is a majority spin dominant over the magnetic moment direction of the atom located on the probe tip in E _F, minority spin is dominant in the state energy 2eV higher than E _F.

Now, when a sample bias voltage of +0.2 V is applied as V ₁ , electrons which contribute most to the tunnel are C 1
For o sample than E _F + 0.2eV energy of high electron, for Cr probe is an electron in the vicinity of E _F,
Therefore, the sign of the spin polarization P ₁ of the entire system represented by the product of the individual spin polarizations is negative. On the other hand, -2
When applying a sample bias voltage V ₂ and V, electrons EF of Co is also than E _F of Cr + 2 eV energy high level is most greatly contributes to the tunnel. P ₂ in this case takes a positive sign. Now, assuming that P ₁ and P ₂ are estimated to be smaller, −8% and P ₂ is + 3%, F is 0.80, which satisfies the condition.

As shown in FIG. 4, the magnetization direction of the sample surface is reversed at the sample surface left (region (a)), center (region (a)), and right (region (c)) as shown in FIG. Consider upward, downward, and upward states. By the way, the electronic states that greatly contribute to the tunnel when the bias is V ₁ are states having up and down spins in the regions (a) and (c), respectively (indicated by arrows circled in the figure). . Here, when the tip of the magnetic moment at the tip is brought close to the Cr probe, when the sample bias voltage V ₁ = + 0.2 V,
The electron spin (indicated by an arrow circled in the figure) that contributes to the tunnel in the Cr probe is an up spin.
Therefore, if the tip-sample distance is controlled so that a constant tunnel current flows, the tip-sample distance becomes farther in region (a) and in region (c) due to the difference in tunnel conductance depending on spin. Comes closer. Each probe-sample distance is represented by S ₁ = S ₀ + Δ
When S ₁ , S ₂ = S ₀ + ΔS ₂ , the difference ΔS between the tip and the sample due to the difference in magnetization is ΔS = ΔS ₁ −ΔS
₂ = (1 / 2κ) ln (1 + P ₁ / 1-P ₁ ).
When 2κ = 2A ⁻¹ is used as a general value, ΔS becomes −
It is about 0.08A.

Next, the bias voltage is set to V
₂ = + 2V. In the case of the region (a), the tunnel current is hard to flow because the probe is farther than the average, but the spin direction of the electrons contributing to the tunnel is also antiparallel in this bias. To become
The flowing current becomes smaller and smaller. On the other hand, in the region (c), the current is easy to flow because it is closer to the average,
Furthermore, since the spin direction is the same, it is easier to flow. When these current ratios are examined, I (region (A)) / I
(Area (c)) = exp (2κΔS) (1 + P ₂ ) /
(1−P ₂ ), and a contrast of about 0.8 is obtained. This change in tunnel current is fully detectable.

Next, an example of a probe applicable to the present invention and an example of a manufacturing method thereof will be described.

First, a ferromagnetic or antiferromagnetic material composed of a single substance such as Fe, Co, Ni, Cr, and Mn is searched from a wire-shaped material by chemical polishing, electrolytic polishing, or focused ion beam processing. Can be processed into needle shape. Further, it can be manufactured by pulling both ends of a wire whose thickness has been reduced.

For a ferromagnetic material or an antiferromagnetic material made of a compound containing Fe, Co, Ni, Cr, Mn, etc., in the case of a ferromagnetic material, the same chemical polishing or electrolytic polishing as described above, or a focused ion beam is used. Processing or the like can be used, and in the case of an antiferromagnetic material, electropolishing that does not break the crystal structure is preferable. CrO ₂ , Fe ₃ O ₄ , L
For non-ductile substances such as oxides such as a1-XSrXMnO ₃ , those having a decomposability are cleaved. For films grown on a substrate having a cracking property, a substrate with a film is used. , And by using the end on the membrane side, a probe can be obtained.

As a composite material containing Fe, Co, Ni, Cr, Mn and its compounds, a laminated film comprising a ferromagnetic layer / non-ferromagnetic layer / ferromagnetic layer containing the above-mentioned materials, or particles in a matrix A dispersed granular film or the like can be used. FIG. 5 shows an example using a laminated film.
FIG. 5A shows a laminated film composed of a ferromagnetic layer and a non-ferromagnetic layer, in which a part of the substrate is peeled off, and only the film part is thinned in parallel to the film surface direction by etching. The direction perpendicular to the plane is the probe axis direction. (C) uses the edge portion of the film. (D) shows a laminated film deposited on the tip of the probe. The above laminated film has a structure in which a ferromagnetic layer / a non-ferromagnetic layer is alternately repeated (at least, it is composed of a ferromagnetic layer / a non-ferromagnetic layer / a ferromagnetic layer).
Instead of this structure, a laminated film having a layer for fixing the magnetization direction, which includes a ferromagnetic layer / non-ferromagnetic layer / ferromagnetic layer / antiferromagnetic layer, may be used. In that case, (e), (f), and (g) correspond to (a), (c), and (d).

FIGS. 5 (c), 5 (c) and 5 (c) show examples in which a granular film in which particles are dispersed in a matrix is used.
(H) is a substrate obtained by removing the substrate and processing the granular film with a probe, (v) is a probe formed perpendicular to the film surface, and (v)
Is a method in which the substrate is divided by cleavage or the like while the substrate is attached, and a corner is used. (H) and (h) can be manufactured by etching after being finely processed with an ion beam or the like.

[0025]

As described above, according to the present invention, a spin image is taken out by using a ferromagnetic or antiferromagnetic probe, which is expected to have high sensitivity, separately from a topographic image showing irregularities on the sample surface. be able to.

[Brief description of the drawings]

FIG. 1 is a waveform diagram showing a change in a bias voltage during scanning.

FIG. 2 is a waveform chart showing another change in the bias voltage during scanning.

FIG. 3 is a schematic diagram showing electronic states of a Co sample and a Cr probe.

FIG. 4 is a diagram illustrating the principle of the present invention.

FIG. 5 is a sectional view showing an example of a probe according to the present invention.

────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Kuniyoshi Tanaka 1 Toshiba-cho, Komukai-shi, Kawasaki-shi, Kanagawa Prefecture Toshiba Research & Development Center Co., Ltd. No. 1 Toshiba Corporation R & D Center (56) References JP-A-6-268283 (JP, A) JP-A-1-320750 (JP, A) JP-A-7-333233 (JP, A) 9-196928 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 13/10-13/24 G12B 21/00-21/24 G01B 7/34 G01B 21/30 H01J 37 / 28 JICST file (JOIS)

Claims (1)

(57) [Claims] 1. A probe made of a ferromagnetic material or an antiferromagnetic material disposed on the surface of a sample at a distance of 10 nm from O.1 and a drive for scanning the probe relatively to the surface of the sample. Mechanism and a bias voltage between the probe and the sample.
The bias voltage source to be applied and the flow between the probe and the sample
Polarization detecting means for detecting a tunnel current
In a scanning tunneling microscope, the probe material is Fe,
Ferromagnetic materials made of Co, Ni, Cr, Mn or their compounds
Or when the spin polarization of the system consisting of the probe and the sample at bias voltages V1 and V2 is P1 and P2, respectively, In represented by the value F is, F> 1.03 or F <O.97 either satisfy two bias voltages Vl of, with respect to V2, first, the tunnel current is constant at a bias voltage V ₁ (I ₁₎ the probe and the distance between the sample and the control determined to be, then changing only the bias voltage without changing the probe and the sample distance is to V _2, the tunnel current I ₂ at that time
Is a spin-polarized scanning tunneling microscope.