JPH05302965A - Scanning surface magnetic microscope - Google Patents

Abstract

PURPOSE:To observe a magnetic-domain structure on the same face with high accuracy by a method wherein a ferromagnetic probe which has been spin-polarized is installed at the tip of a cantilever, the probe is scanned along the surface of a sample and magnetic information is obtained by a change in an electric current value between the probe and the sample. CONSTITUTION:A tunneling bias 11 is applied across a probe 1 and a sample 3; an electric current value at this time is detected by using a current detection circuit 12. Then the polarization rate of a spin 24 for an upper-part magnetic film is equal to that of a spin 25 for a lower-part magnetic film, an electric current 27 across the probe 1 and the sample 3 is definite even when the direction of a spin 26 for the sample 3 is changed alternately. When a coil 13 for excitation is installed around the probe 1 and the coil 13 is excited in terms of a direct current by means of a power supply 14 for excitation use, the difference in the polarization rate between the spins 24 and 25 is caused. When the probe 1 provided with the polarization rate caused. When the probe 1 provided with the polarization electric current 27 which is large as compared with a region in an opposite direction can be detected in a region provided with a spin in the same direction as that of the probe 1. The magnetic information on the sample 3 can be separated and measured on the basis of a change in an electric current between the probe 1 and the sample 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microscope apparatus suitable for measuring the morphology and magnetic information of the surface of a magnetic sample.

[0002]

2. Description of the Related Art A conventional scanning tunneling microscope uses a tunnel current and a field emission current obtained when a voltage is applied between a probe and a sample and the probe and the sample are brought close to each other. This is an apparatus for examining the surface morphology of a conductor sample. On the other hand, the atomic force microscope is a device for investigating the surface state by using the atomic force generated when the probe approaches the conductor or insulator sample. Atomic force microscopy is described in U.S. Patent No. 8021.
No. 23 (Japanese Patent Laid-Open No. 62-130302), and an example of its realization is Journal. of. Physics Magazine,
1987, 61, 4723-4729. On the other hand, the magnetic force microscope is a device that uses a magnetic substance as a probe and uses the magnetic force generated between the magnetic probe and the magnetic field leaked to the surface of the magnetic sample to examine the magnetization state of the sample. For the conventional method of acquiring magnetic information of a sample in a scanning magnetic force microscope using a magnetic force obtained by bringing a magnetic probe and a sample close to each other, see Journal of Vacuum Science Technology A6 (19).
1988) p. 279 to 282, or Applied Physics Letters, Vol. 50 (1987) p. 1455 to 1457.

The operation modes of these atomic force microscopes and magnetic force microscopes are roughly classified into two types. In the first method, the tip of the cantilever is scanned on the surface of the sample in the region where atomic force acts (tens of nm or less from the surface), and the DC component of the change in force is detected to detect the morphology and magnetic field of the sample surface. Measure the force distribution. The second method vibrates the cantilever in the vicinity of the resonance frequency, detects the change in the resonance frequency of the cantilever due to the force acting on the probe, and measures the force gradient. In this case, the probe operates while vibrating vertically with respect to the sample surface with a small amplitude in a region tens to hundreds of nm away from the sample surface.

For the means for detecting the tunnel current between the magnetic sample and the magnetic probe to detect the magnetic domain structure on the sample surface, see Journal. of. vacuum. Science. Technology Magazine, B9 (2), 1991, pages 519-52.
Discussed on page 4. In this case, a probe (for example, chromium oxide) that is spin-polarized in the direction perpendicular to the sample surface is used, and the tunnel current flowing between the probe and the sample is parallel or antiparallel to the spin direction on the magnetic sample surface. This is a detection method that utilizes different things.

The above-mentioned magnetic information detection system has some drawbacks. The method of detecting the DC component of the force in the magnetic force microscope has a drawback that it is difficult to separate it from the surface structure of the sample because the magnetic force acting between the probe and the sample is small. The method of detecting the gradient of the magnetic force enables high-sensitivity detection, but it is difficult to improve the resolution because it measures in a region away from the sample surface. Further, the method of detecting the tunnel current is a method suitable for measuring the magnetic domain structure of a sample having a flat surface such as a single crystal sample, but in the case of detecting magnetic information of a general magnetic sample, It has the drawback that accurate information cannot be obtained due to the mixture of surface structure and magnetic information, and the detection sensitivity is low.

[0006]

In the above conventional technique, the distribution of the leakage magnetic field on the sample surface can be measured by controlling the position of the probe so that the magnetic force or magnetic force gradient acting on the probe is kept constant. However, since magnetic and morphological information are mixed and measured, it is difficult to predict true magnetic information. On the other hand, in the method of detecting the tunnel current between the spin-polarized probe and the sample, the magnetic domain structure on the sample surface can be measured by controlling the position of the probe so as to keep the tunnel current constant. There has been a problem that irregularities on the sample surface are mixed and detected.

The object of the present invention is to observe the surface morphology of a sample,
It is to provide a scanning surface magnetic microscope that uses atomic force and tunnel current, which enables highly accurate observation and measurement of magnetic domain structures on the same plane.

[0008]

To achieve the above object, in the present invention, a means for providing a spin-polarized ferromagnetic probe at the tip of a cantilever, a means for detecting a direct current displacement of the cantilever, and a probe. Means for detecting a current flowing between the needle and the sample, means for changing the magnetization intensity of the ferromagnetic needle at a selected specific frequency, and means for detecting a current between the sample and the probe having the specific frequency component , A means for scanning the probe along the sample surface was provided, and the structure of the sample was simultaneously measured by the direct current displacement of the cantilever and the magnetic information was simultaneously measured by the change in the current value between the probe and the sample.

[0009]

[Function] When the ferromagnetic probe provided at the tip of the cantilever is brought close to the sample surface up to the region where the repulsive interatomic force acts,
The atomic force causes the cantilever to bend. The displacement due to the bending of the cantilever is measured by a position detecting means or the like using laser light. The direct current displacement of the cantilever is measured and the z-axis of the sample or the probe (perpendicular to the sample surface is measured so that the force acting on the probe becomes constant, that is, the direct current displacement becomes constant. The surface structure of the sample is measured by controlling the position of (axis).

At the same time, the tunnel current flowing between the ferromagnetic probe and the sample surface is measured. The tunnel current I is
It changes depending on the spin polarization ratio P of the ferromagnetic probe. That is, when the directions of spin polarization of the sample and the probe are parallel, I =
As I ₀ (1 + P), in the case of antiparallel I = I ₀ (1-
It is detected as a tunnel current of P). Here, I ₀ is a current when there is no spin polarization.

The z axis of the sample or the probe (axis perpendicular to the sample surface) so as to keep the direct current displacement of the cantilever constant.
It is possible to separate the morphological information from the magnetic domain structure on the sample surface and measure them by associating them with each other by measuring the tunnel current at the same time as controlling the position. When the ferromagnetic probe approaches the sample surface, the magnetic field of the sample surface may change the magnetization of the probe. Since the change in the tunnel current is small, S /
The N ratio (ratio of signal and noise) may be small. In this case, the problem can be solved by providing an exciting coil or the like on a part of the ferromagnetic probe, exciting the probe at a specific frequency, and measuring the maximum current of the frequency component.

The ferromagnetic probe is most preferably a single domain structure made of a single crystal, but a thin film formed by a physical vapor deposition method such as a sputtering method or a vacuum vapor deposition method may be used. The direction of spin polarization of the probe may be perpendicular or parallel to the sample surface, and can be arbitrarily selected depending on the sample. Further, it is necessary that the probe and the surface of the sample are made of a conductive material. As the cantilever displacement detection method,
It can be realized by adopting the optical lever method, the optical interference method, the electrostatic capacity method, the optical critical angle method, the tunnel current detection method, and the like.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram for explaining the structure of a basic device of the present invention. When the cantilever 2 having the probe 1 at its tip approaches the surface of the sample 3 to a region where repulsive atomic force acts, the force received by the probe 1 causes the cantilever 2 to bend. The optical lever type detector has a function of detecting the displacement of the cantilever 2 due to this bending. The optical lever type detector includes a laser source 4, a position sensor 5, and a position detection circuit 6. The output signal of the position detection circuit 6 is input to the servo control circuit 7, and the Z-axis piezoelectric element of the XYZ scanner 9 controls the displacement of the cantilever 2 to be constant, that is, the distance between the probe 1 and the sample 3 to be constant. To do. Further, the sample 3 is XY scanned by the XY scanning circuit 8 and the control signal of the Z-axis piezoelectric element with respect to the probe position in the XY directions is displayed on the display device, so that the surface structure (AFM image) of the sample can be observed. ..

On the other hand, as the probe 1, a ferromagnetic needle spin-polarized in a specific direction, for example, a direction perpendicular to the sample surface is used to detect a tunnel current flowing between the probe 1 and the sample 3. The tunnel current is the tunnel bias 11 between the probe 1 and the sample 3.
Is applied and detected by the current detector 12. By displaying the output signal of the current detector 12 with respect to the probe position in the XY directions on the display device in synchronization with the control signal of the Z-axis piezoelectric element, magnetic information such as magnetic domain structure is separated from the surface structure of the sample. Can be detected. This is based on the fact that the tunnel current I flowing between the probe 1 and the sample 3 differs depending on the spin direction and spin polarization P. That is, when the spin directions of the probe 1 and the sample 3 are the same, I = I ₀ (1 + P)
If the current given by is in the opposite direction, I = I
₀ (1-P) is detected. Here, I ₀ is the current when P = 0. Excitation coil 13 around the cantilever 2
By providing an excitation power source for exciting this, the spin direction and the polarization rate at the tip of the probe 1 can be controlled.

An example of detailed operation will be described with reference to FIG.
A magnetic film 22 made of a ferromagnetic material such as permalloy (Ni—Fe alloy) on a probe substrate 23 such as Si or SiO _2.
To form a probe 1 in which a non-magnetic film 21 such as copper is laminated. This laminated film is preferably an epitaxially grown film. The magnetic film 22 may be made of a magnetic material containing at least one element of Fe, Ni, and Co in addition to permalloy, and the nonmagnetic film 21 may be made of C in addition to Cu.
A film containing at least one element of r, Pt, Au, Ag, Al, and C may be used. Further, the probe 1 can also be configured as a magnetic probe having the same effect by alternately stacking non-magnetic materials such as Mn and Al.

The spin P ₁ 24 of the upper magnetic film of the probe 1
The spin P ₂ 25 of the lower magnetic film is generally antiparallel to each other, but it goes without saying that the same effect can be obtained even in the same direction. The spin P ₁ 24 of the upper magnetic film and the spin P ₂ 25 of the lower magnetic film have substantially the same polarizability.

Now, the case where the spins of the probe 1 are formed perpendicularly to the surface of the sample 3 and the spins 26 of the sample are formed alternately perpendicular to the surface of the sample will be described. A tunnel bias 11 is applied between the probe 1 and the sample 3, and the current value at this time is measured by the current detection circuit 1.
Detect with 2. As shown in FIG. 2A, when the spin P ₁ 24 of the upper magnetic film and the spin P ₂ 25 of the lower magnetic film have the same polarizability (P ₁ = P ₂ ), the orientation of the spin 26 of the sample changes alternately. Even so, the current 27 between the probe 1 and the sample 3 is constant. When the exciting coil 13 is provided around the probe 1 and the exciting power source 14 excites the exciting coil 13 in a direct current,
As shown in FIG. 2B, the spin P ₁ 24 of the upper magnetic film is
And the polarizability of the spin P ₂ 25 of the lower magnetic film can be made different (P ₁ ≤P ₂ ). In addition, by exciting in the opposite direction (P ₁ ≧
It goes without saying that P ₂ ) can be realized. (P ₂ -P ₁₎
When the probe 1 having the polarization ratio given by is scanned on the surface of the sample 3, in a region having spins in the same direction as the probe 1, the region in the opposite direction is examined and a large value of the current 27 is detected. it can.

Such a probe 1 is installed at the tip of the cantilever 2 shown in FIG. 1 so that the atomic force between the probe 1 and the sample 3 is constant, that is, the distance between the probe 1 and the sample 3 is kept constant. The Z-axis piezoelectric element of the XYZ scanner 9 is controlled so as to keep it constant. From the control signal of the Z-axis piezoelectric element, the surface morphology of the sample can be measured by separating the magnetic information of the sample from the current change between the probe 1 and the sample 3.

Although the case where the spin directions of the probe 1 and the sample 3 are perpendicular to the sample surface has been described here, it is needless to say that the same effect can be obtained in the case of the spin parallel to the sample surface. In addition, in order to detect the displacement of the cantilever 2, in addition to the optical lever method, a non-contact, large area displacement detection method,
That is, it can be realized by adopting an optical interference method, an electrostatic capacity method, an optical critical angle method, a tunnel current detection method, or the like.

The ferromagnetic film forming the probe 1 may be a single-layer ferromagnetic film other than the laminated film as shown in FIG.

FIG. 3 is an explanatory diagram of another application example of the present invention. The exciting coil 13 is AC-excited by the oscillator 15 at a specific frequency in the range of several tens to several megaHz, and the spin polarization rate of the probe 1 at the tip of the cantilever 2 is changed. At this time, the current between the probe 1 and the sample 3 also changes in an alternating current.
The excitation frequency of the oscillator 15 is introduced into the lock-in amplifier 16 as the reference signal 17, and the peak value of the tunnel current of the same frequency component as this is detected. Others operate and measure in the same manner as the above-mentioned embodiment. With this method, the detected current signal /
The noise ratio can be improved.

[0022]

According to the present invention, it is possible to separate and measure a sample surface structure and a magnetic domain structure of a sample, which are not possible with the prior art, and to perform a correlation analysis between the structure of the sample and the magnetization state.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic device configuration according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a specific example of a measuring method for realizing the present invention.

FIG. 3 is a block diagram showing a device configuration for improving a signal / noise ratio according to an embodiment of the present invention.

[Explanation of symbols]

1 ... Probe, 2 ... Cantilever, 3 ... Sample, 4 ... Laser source, 5 ... Position sensor, 6 ... Position detector, 7 ... Servo control circuit, 8 ... XY scanning circuit, 9 ... XYZ scanner, 10
... Display device, 11 ... Tunnel bias, 12 ... Current detector, 13 ... Excitation coil, 14 ... Excitation power supply, 15 ... Oscillator, 16 ... Lock-in amplifier, 17 ... Reference signal, 21
... non-magnetic film, 22 ... magnetic film, 23 ... probe substrate, 24,
34 ... Spin P ₁ of upper magnetic film, 25, 35 ... Spin P ₂ of lower magnetic film, 26 ... Spin of sample, 27 ... Current.

Claims (4)

[Claims] 1. A flexible cantilever having a pointed needle, b) means for detecting a direct current displacement of the cantilever, and c) a probe for maintaining a constant direct current displacement of the cantilever. Means for controlling the position of d), d) means for detecting a current flowing between the tip and the sample, e) means for forming the tip with a spin-polarized ferromagnetic needle, and f) the ferromagnetism. Means for changing the spin polarization of the needle; g) means for detecting a current between the sample of the specific frequency component and the probe; h) means for scanning the tip needle along the sample surface; i. ) A scanning surface magnetic microscope comprising: a displacement of the cantilever and a means for displaying the current between the sample and the needle at each scanning position. 2. A scanning surface magnetic microscope according to claim 1, further comprising means for changing the spin polarization rate of the ferromagnetic needle at a selected specific frequency. 3. A scanning surface magnetic microscope according to claim 1, wherein the ferromagnetic needle is made of a thin film. 4. The scanning surface magnetic microscope according to claim 1, wherein the ferromagnetic needle is composed of a thin film in which a ferromagnetic material and a non-magnetic material are alternately laminated.