Apparatus for measuring exchange force

In an apparatus for measuring an exchange force between a specimen and a probe, the specimen and probe are faced to each other with a distance within a close proximity or RKKY-type exchange interaction region from a distance at which conduction electron clouds begin to be overlapped with each other to a distance at which localized electron clouds are not substantially overlapped with each other. In order to prevent the probe from being attracted to the specimen by a force between the specimen and the force, a piezoelectric element is provided on a cantilever and a control signal supplied to the piezoelectric element is produced in accordance with a displacement of the cantilever to control a spring constant of the cantilever. The exchange force between the specimen and the probe is calculated from the control signal supplied to the piezoelectric element.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for measuring an exchange 
force between a surface of a specimen and a probe which is faced to the 
specimen surface by a very small distance. 
2. Description of the Related Art 
Heretofore, in many known methods of analyzing solid specimens using an 
electron beam, the intensity (the number of electrons) and the kinetic 
energy are adopted as a measure for analysis. Another measure for the 
investigation is electron spin. There have been proposed several methods 
of evaluating a microscopic surface magnetism of a solid substance on the 
basis of the electron spin. For instance, there have been proposed several 
methods of determining directions of magnetic moments of respective atoms 
with atomic resolution as illustrated in FIG. 1. 
In accordance with recent progress in electronics, a recording density on 
magnetic recording medium has become higher year after year. FIG. 2 is a 
chart representing a variation of the recording density in accordance with 
progress in the magnetic recording medium and various methods of 
evaluating the surface magnetism. The horizontal axis denotes a time in 
the Gregorian year, the left-hand vertical axis a linear recording density 
(cycle/cm), and the right-hand vertical axis represents a resolution of 
methods of evaluating the surface magnetism in .mu.m and nm. Magnetic 
recording began in 1900 having a wavelength of 1 mm and have become more 
and more dense. The linear recording density has been improved in audio 
magnetic tape, .beta. magnetic tape and VHS magnetic tape. In a recent 
evaporation tape, a length of one bit is 0.3-0.4 .mu.m. In a modern hard 
disc, a length of one bit has shortened to 0.16-0.19 .mu.m. By the 
electron holography, magnetic bits of 0.085 .mu.m were observed on Co-Cr 
media. The resolution in evaluation methods of surface magnetism has been 
also improved. The resolution of the Bitter technique has been improved 
from 1 .mu.m to 0.7 .mu.m, and the resolution of the Kerr effect method 
has improved from 1 .mu.m to 0.5 .mu.m. The resolution of the 
spin-polarized scanning electron microscopy (SP-SEM) has improved from 
100-200 .mu.m in 1984 to 20 nm in 1994. The magnetic force microscopy 
(MFM) had a resolution of 100 nm in 1987 and had a resolution of 10 nm in 
1988. The electron holography had a resolution of 10 nm in 1991 and the 
Lorentz microscopy has a resolution of 10 nm now and will have a 
resolution of 0.7 nm in a near future. 
As explained above, the resolution of surface magnetic evaluation has 
become higher and higher. However, a higher resolution is required on in 
either basic studies of material properties or engineering, for instance 
magnetic recording. Hence, it has been earnestly required to develop an 
evaluation method which can evaluate magnetic properties of a solid 
surface with an atomic resolution. The inventors of the present 
application have proposed a spin-polarized scanning tunneling microscopy 
(SP-STM). 
FIG. 3 is a schematic view illustrating an experimental apparatus for 
proving the utility of SP-STM. In an actual SP-STM, a specimen is made of 
a magnetic material and a probe is made of gallium arsenide (GaAs). 
However, in the experimental apparatus, a specimen was made of GaAs and a 
probe was made of nickel (Ni). This does not cause any problem as long as 
the principle of the SP-STM is investigated. A single-mode laser diode 1 
was used as a linearly polarized light source of about 830 nm in 
wavelength and about 30 mW in maximum output power. Linearly polarized 
laser beam was made incident upon a Pockels cell 3 by means of an lens 2. 
To the Pockels cell 3, was applied a high voltage from an oscillator 4 via 
a high voltage amplifier 5. Then, an excited circularly polarized laser 
beam was modulated into right-hand circularly polarization and left-hand 
circularly polarization at a modulation frequency of about 400 Hz. In this 
manner, the spin-polarization of excited electrons was changed. The 
modulated laser beam was made incident upon a specimen 11 as exciting 
light by means of reflection mirror 6-8, .lambda./4 plate 9 and lens 10. A 
probe 12 made of a crystal wire of Ni was biased by a DC voltage source 13 
was brought into a close proximity of the surface of specimen 11 under the 
control of a Piezoelectric element 14 such that a tunneling current could 
flow from the specimen to the probe. The generated tunneling current was 
detected by a control unit 15, and an output signal of the control unit 
was supplied to a monitor 16 together with an output signal from the 
oscillator 4. In this manner, the tunneling current depending upon the 
spin-polarization of the surface of specimen 11 was detected. 
In the above explained SP-STM, the tunneling current produced by the 
radiation excitation is detected, and thus could not be applied to 
electrically insulating magnetic materials. The inventors have proposed a 
possibility of an atomic force microscopy (AMF) which could detect the 
exchange force between a sample and a probe. Such an atomic force 
microscopy could be applied to insulating objects. 
In the known atomic force microscopy, the measurement is performed within a 
non-contact region in which the tip of the probe is separated from the 
specimen surface by a relatively large distance or within a direct contact 
region in which the tip of probe is brought into contact with the specimen 
surface. In the measurement within the non-contact region, magnetic forces 
produced between magnetic dipoles are measured. However, these forces are 
of a long-range force, and thus it is impossible to realize an atomic 
resolution. In the measurement within the direct contact region, although 
it would be possible to evaluate the surface structure with an atomic 
resolution, it is impossible to measure the exchange force between the 
specimen and the probe in an accurate manner, because the probe tip is 
brought into contact with the specimen surface and is influenced by 
magnetic properties of the specimen surface. Therefore, it is impossible 
to evaluate inherent magnetism of the specimen surface in an accurate 
manner. 
In order to overcome the above mentioned drawback, the inventors have 
proposed, in a co-pending patent application, a method of measuring an 
exchange force between a probe and an electrically conductive or 
electrically insulating specimen with an atomic resolution. 
In this method, in order to measure an exchange force between a specimen 
and a probe each of which contains localized electrons and at least one of 
which contains conduction electrons, the specimen and probe are faced to 
each other by a distance within a close proximity region from a distance 
at which conduction electron clouds (wave function) begin to be overlapped 
with each other to a distance at which localized electron clouds (wave 
function) are not substantially overlapped with each other, and an 
exchange force between said two substances is measured. The above close 
proximity region is called RKKY type exchange interaction region. 
FIG. 4 is a graph showing variations of force and energy between the 
specimen and the probe in accordance with a distance therebetween. It 
should be noted that the force may be derived by differentiating the 
energy. The RKKY type exchange interaction region is between the contact 
region in which a direct exchange interaction is taken place and the 
non-contact region in which an interaction between magnetic dipoles is 
carried out. In the known atomic force microscope, the direct exchange 
interaction region or non-contact region is used. In these regions, the 
force between the specimen and the probe could not be measured with an 
atomic resolution. It should be noted that in FIG. 4, boundaries between 
the direct exchange interaction region, RKKY-type exchange interaction 
region and magnetic dipole interaction region are denoted by broken lines, 
but in practice, these boundaries could not be determined clearly. 
When a specimen and a probe are faced to each other by a distance within 
the RKKY-type exchange interaction region, an exchange force between the 
specimen and the probe is of an order of 10.sup.-10 N. Presently available 
atomic force microscope has a measuring limit of an order of about 
10.sup.-12 -10.sup.-13 N. Therefore, the exchange force of an order of 
10.sup.-10 N could be measured. 
However, if an exchange force within the RKKY-type exchange interaction 
region is measured using a cantilever of the known atomic force microscope 
in which the non-contact region is utilized, the probe is brought into 
contact with the specimen, because a distance between the specimen and the 
probe could not be controlled precisely. Since a spring constant of the 
cantilever is very small, when the probe is brought into a close proximity 
of the specimen, a resilient force of the cantilever might be against a 
force between the specimen and the probe and the cantilever is attracted 
to the specimen. When a spring constant of the cantilever is increased, a 
sensitivity of the cantilever might be decreased largely and the exchange 
force of an order of 10.sup.-10 N between the specimen and the probe could 
not be measured precisely. 
SUMMARY OF THE INVENTION 
The present invention has for its object to provide a novel and useful 
apparatus for measuring precisely with an atomic resolution an exchange 
force between a specimen and a probe which are faced to each other by a 
very small distance within the RKKY-type exchange interaction region. 
According to the invention, an apparatus for measuring an exchange force 
between a specimen and a probe each of which contains localized electrons 
and at least one of which contains conduction electrons, comprises: 
a means for holding said specimen; 
a resilient member for supporting said probe such that said specimen and 
probe are faced to each other by a distance within a close proximity 
region from a distance at which conduction electron clouds begin to be 
overlapped with each other to a distance at which localized electron 
clouds are not substantially overlapped with each other; 
a displacement measuring means for measuring a displacement of said 
resilient member due to a force between the specimen and the probe; 
a controlling means for controlling a resiliency of said resilient member 
against the force between the specimen and the probe such that the probe 
is prevented from being brought into contact with the specimen; and 
an exchange force detecting means for detecting an exchange force between 
the specimen and the probe in accordance with said displacement of the 
resilient member. 
According to the invention, it is preferable that said resilient member is 
formed by a resilient cantilever having one end secured to a stationary 
member, and said displacement measuring means includes a first 
piezoelectric element secured to the cantilever, an oscillator for 
supplying a driving signal having a given frequency and a given amplitude 
to said first piezoelectric element, and an opto-electric position 
detecting device for detecting a displacement of the cantilever. 
In a preferable embodiment of the apparatus according to the invention, 
said controlling means includes a spring constant adjusting means for 
adjusting a spring constant of the resilient member in accordance with the 
displacement of the resilient member supplied from the displacement 
measuring means. 
Alternatively, the controlling means may be constructed by a means for 
preventing the probe from being brought into contact with the specimen by 
means of an electromagnetic force or an electrostatic force. 
In a preferable embodiment of the apparatus according to the invention, 
said displacement measuring means includes a first piezoelectric element 
secured to the cantilever, an oscillator for generating a driving signal 
having a given frequency and a given amplitude, and an optical position 
detecting device for detecting a displacement of the cantilever in an 
opto-electric manner, said spring constant adjusting means comprises a 
second piezoelectric element and a control circuit for supplying a control 
signal to said second piezoelectric element such that the cantilever 
vibrates at the given frequency with a given amplitude, and said exchange 
force detecting means includes a calculation circuit for processing said 
control signal supplied from said control circuit to derive the exchange 
force between the specimen and the probe.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
At first, we consider a simple model, in which two thin films made of iron 
which is a 3d transition metal are brought together in close proximity as 
shown in FIG. 5. One of the thin iron films may be a specimen and the 
other may be a probe. In actual measurement, the specimen may be 
considered as a thin film, but the probe is a very sharp tip and could not 
be considered as a thin film. However, in a microscopic view point, the 
probe may be also considered as a thin film. It is also assumed that each 
of the two thin films has a structure constructed by three atom layers as 
illustrated in FIG. 6, (001) surfaces of the thin films are faced to each 
other by a distance d, and a lattice constant of thin films is a (2.83 
.ANG.). 
When an origin of coordinates is set at a middle point between the two 
films, a position of atoms of a first layer x.sub.1 of the first thin film 
is expressed by x.sub.1 (0, 0, d/2), a position of atoms of a second layer 
x.sub.2 is expressed by x.sub.2 (a/2, a/2, d/2+a/2), and a position of 
atoms of a third layer x.sub.3 is expressed by x.sub.3 (0, 0, d/2+a). 
Similarly, in the second thin film, a position of atoms of a first layer 
x.sub.1 ' is expressed by x.sub.1 '(a/2, a/2, -d/2), a position of atoms 
of a second layer x.sub.2 is expressed by x.sub.2 '(0, 0, -d/2-a/2), and a 
position of atoms of a third layer x.sub.3 is expressed by x.sub.3 '(a/2, 
a/2, -d/2-a). The surface relaxation is not considered. So, lattices is 
assumed to be rigid. 
Since the exchange force between the two thin films can be derived as a 
difference between a force obtained under a condition that directions of 
magnetic moments of these thin films are in parallel with each other and a 
second force obtained under a condition that directions of magnetic 
moments of the thin films are in anti-parallel with each other. Therefore, 
a dependency of these first and second forces upon a distance between the 
two thin films has been investigated. It has been derived by the first 
principle calculation using the local-spin approximation to the 
density-functional theory. Upon calculation, the full potential linear 
argumented plane wave (LAPW) method was employed. The inventors have 
reported in, for instance Japanese Journal of Applied Physics, Vol. 33 
(1994), pp. 2692-2695, Materials Science and Engineering B31 (1995), pp. 
69-76, and Physical Review B56(1995), pp. 3218-3321, calculation results 
of forces applied to respective atoms under such a condition that 
directions of magnetic moments are parallel with each other. In order to 
measure an actual exchange force, it is necessary to derive a difference 
between a force measured under such a condition that directions of 
magnetic moments are in parallel with each other and a force measured 
under such a condition that directions of magnetic moments are in 
anti-parallel with each other. 
According to the invention, not only force applied to the respective thin 
films under the parallel condition of magnetic moments, but also force 
applied to the respective thin films under the condition of the 
anti-parallel condition of magnetic moments are calculated in an extremely 
precise manner, and calculation results shown in FIG. 7 could be obtained. 
In FIG. 7, the horizontal axis denotes a distance d normalized by the 
lattice constant a (d/a) and the vertical axis shows a force F (10.sup.-9 
N). A curve F.sub.P represents the force in the parallel condition and a 
curve F.sub.AP shows the force in the anti-parallel condition. 
The force between the thin films contain forces other than the exchange 
force, and therefore in order to derive only the exchange force, it is 
necessary to cancel out the forces other than the exchange force by 
deriving a difference between them. In FIG. 7, the calculated exchange 
force is shown by a curve F.sub.ex =F.sub.AP -F.sub.P. As can be seen from 
the curve F.sub.ex, the exchange force has a large dependency upon the 
distance d between the two thin films. Within a region of d/a.ltoreq.1.7, 
the exchange force appears. Particularly, within a region of d/a &lt;1.0, a 
large exchange force is recognized. Within a region of 
1.0.ltoreq.d/a.ltoreq.1.7, the existence of the exchange force is 
recognized. However, in a region of d/a&gt;2.0, no exchange force could 
appear. 
Then, a dependency of the magnetic moment of the thin film under the 
parallel and the anti-parallel conditions has been investigated and a 
result shown in FIG. 8 was obtained. In FIG. 8, the horizontal axis 
denotes a distance a between the thin films normalized by the lattice 
constant a (d/a), and the vertical axis represents the magnetic moments 
m(.mu.B). Curves X.sub.1 (P) and x.sub.1 (AP), x.sub.2 and x.sub.3 show 
magnetic moments of atoms in the layers x.sub.1, x.sub.2 and x.sub.3, 
respectively. The curve X.sub.1 (P) shows a change in the magnetic moment 
under the parallel condition and the curve x.sub.1 (AP) represents a 
change in the magnetic moment under the anti-parallel condition. The 
magnetic moments of atoms in the second layer x.sub.2 are substantially 
identical with that of the bulk. When the normalized distance d/a between 
the two thin films is smaller than 1.0, the magnetic moment of the first 
layer x.sub.1 is greatly deceased. This means that atoms in this first 
layer x.sub.1 are subjected to the direct exchange interaction. Within the 
region in which the normalized distance d/a is smaller than 1.0, spins in 
the first thin film are directly exchange interacted. Therefore, in the 
present invention, this region is called a direct exchange interaction 
region. As explained above with reference to FIG. 7, in the direct 
exchange interaction region of the normalized distance d/a smaller than 
1.0, it is possible to attain a large exchange force. However, within this 
direct exchange interaction region, the magnetic moment changes largely, 
and therefore the magnetic structure of a specimen surface might be 
affected by the probe and the magnetic property of the specimen surface 
could not be evaluated accurately. 
According to the invention, it is extremely preferable to measure the 
exchange force within the region of 1.0.ltoreq.d/a.ltoreq.1.7. In the 
direct exchange interaction region of d/a&lt;1.0, the localized electron 
clouds (wave functions) of, for instance 3d orbitals are overlapped with 
each other as shown in FIG. 9A, and in the region of 
1.0.ltoreq.d/a.ltoreq.1.7, although the localized electron clouds are 
separated from each other as depicted in FIG. 9B, the conduction electron 
clouds (wave functions) of 4s and 3p orbitals are overlapped with each 
other. Therefore, according to the invention, the exchange force is 
measured by separating a specimen surface and a probe from each other by a 
distance within a region from a distance at which the conduction electron 
clouds of 4s and 3p orbitals begin to be overlapped with each other to a 
distance at which the localized electron clouds of 3d orbital are not 
substantially overlapped with each other. In the present specification, 
such a region is called a RKKY-type exchange interaction region. The 
measurement of the exchange force according to the invention is applied 
not only to the above mentioned 3d transition metal, but also to molecules 
revealing magnetism, 4f rare earth metals and compounds and magnetic 
semiconductors. It should be noted that the present invention may be 
equally applied to two substances each of which includes localized spins 
and at least one of which contains conduction electrons. 
As explained above with reference to FIG. 7, the magnitude of the exchange 
force F.sub.ex measured in the RKKY-type exchange interaction region, i.e. 
1.0.ltoreq.d/a.ltoreq.1.7 is smaller than that measured in the direct 
exchange interaction region, but is still of order of 10.sup.-10 N. The 
exchange force having such a magnitude can be measured, because the 
conventional atomic force microscope has a resolution of about 10.sup.-12 
to 10.sup.-13 N. Furthermore, this RKKY-type exchange force changes in a 
sinusoidal manner, and thus the exchange force can be measured accurately 
using such a characteristic. 
Now several embodiments of the apparatus for measuring the exchange force 
between a specimen and a probe according to the invention will be 
explained. 
FIG. 10 is a schematic view showing a first embodiment of the apparatus for 
measuring the above mentioned RKKY-type exchange force according to the 
invention. A specimen 21 whose magnetic properties are to be evaluated is 
placed on a stage 22 which can be moved in a three-dimensional manner. 
Above the specimen stage 22 is arranged a resilient cantilever 23 whose 
one end is secured to a stationary member by means of a first 
piezoelectric element 25. The resilient cantilever 23 is formed by a 
resilient strip made of silicon, silicon nitride, stainless steel, 
phosphor bronze and so on. A probe 24 is secured on a lower surface of the 
resilient cantilever 23 near its distal end. It is preferable that the 
probe 24 has a sharp tip. According to the invention, there is no special 
limitation for a combination of materials of the specimen 21 and probe 24, 
the probe may be made of any suitable material from a view of workability, 
except for a condition due to a property of a magnetic specimen. 
Therefore, according to the invention, the cantilever 23 and probe 24 may 
be formed as a single integral body. 
As explained above, the cantilever 23 is secured to the stationary member 
by means of the first piezoelectric element 25, which is connected to an 
oscillator 26 which generates a driving signal having a frequency of 
several hundreds KHz to vibrate the cantilever 23 at such a frequency. On 
the upper surface of the cantilever 23 is provided a reflection member 27, 
and a laser light beam emitted by a laser light source 28 is made incident 
upon the reflection member from an inclined direction. The laser beam 
reflected by the reflection member 27 is received by a position sensor 29. 
The position sensor 29 comprises an array of a plurality of light 
receiving elements and a position upon which the laser beam is made 
incident can be detected. In this manner, a position of the probe 24 in a 
direction Z perpendicular to the surface of the specimen 21 can be 
detected in a very precise manner. 
The specimen stage 22, cantilever 23, laser light source 28 and position 
sensor 29 are all installed within a vacuum chamber 30 to which a vacuum 
pump (not shown) is connected. In this manner, a space within the chamber 
30 can be maintained at ultra-high vacuum condition, and thus the accurate 
measurement of the exchange force can be achieved without being influenced 
by dusts deposited on the specimen 21. If the apparatus is placed in an 
extremely clean space, it is not necessary to use the vacuum chamber. 
In order to prevent the probe 24 from being brought into contact with the 
specimen 21 against a resilient force of the cantilever 23 by means of a 
force between the specimen 21 and the probe 24, in the present embodiment, 
there is provided a spring constant controlling means. That is to say, a 
second piezoelectric element 31 is provided on the cantilever 23. This 
second piezoelectric element 31 is connected to a control circuit 32. 
Outside the vacuum chamber 30, there are arranged, in addition to the above 
mentioned oscillator 26 and control circuit 32, a driving circuit 33 for 
driving the specimen stage 22, a displacement measuring circuit 34 
connected to said position sensor 29, a calculating circuit 35 for 
calculating output signals supplied from the control circuit 32 to derive 
an exchange force applied to the probe 24, and a processing circuit 36 for 
processing an output signal supplied from the calculating circuit 35 to 
evaluate magnetic properties of the specimen 21 on the basis of the 
measured exchange force applied to the probe 24. 
After placing the specimen 21 on the stage 22 and exhausting the vacuum 
chamber 30, the specimen stage 22 is driven by the driving circuit 33 such 
that the tip of probe 24 is faced to a given portion of the specimen. In 
this case, a distance between the specimen 21 and the tip of probe 24 is 
set to a value within the above mentioned RKKY-type exchange interaction 
region. Then, the piezoelectric element 25 is driven by the oscillator 26 
such that the resilient lever 23 and thus the probe 24 are vibrated in the 
direction Z at a given frequency. The frequency is preferably set to a 
resonant frequency of the cantilever 23. Due to this vibration, the 
position of the laser beam impinging upon the position sensor 29 is 
changed in a periodic manner. When the probe 24 is placed remote from the 
specimen 21 and any force is not induced between the specimen 21 and the 
probe 24, the vibration of the resilient lever 23 is not affected at all 
and the lever is vibrated at given frequency and amplitude. However, when 
the probe 24 is brought closer to the specimen 21 by a distance within 
said RKKY-type exchange interaction region, a force is induced between the 
specimen 21 and the probe 24 and the vibration of the lever 23 is 
influenced by this force. Then, the frequency and amplitude of the 
vibration of the resilient cantilever 23 are changed. In the present 
embodiment, a displacement signal generated by the displacement measuring 
circuit 34 is supplied to the control circuit 32. The control circuit 32 
produces a control signal and the thus produced control signal is supplied 
to the second piezoelectric element 31. The control signal is formed such 
that the cantilever 23 vibrates at said given frequency and amplitude 
irrespective of the exchange force between the specimen 21 and the probe 
24. In other words, in the present embodiment, to the second piezoelectric 
element 31 is supplied such a control signal that a spring constant of the 
resilient cantilever 23 is increased in accordance with a decrease in a 
distance between the specimen 21 and the probe 24. 
As explained above, the cantilever 23 is controlled by the second 
piezoelectric element 31 such that the cantilever 23 vibrates at said 
given frequency and amplitude, and thus the control signal supplied from 
the control circuit to the second piezoelectric element 31 represents the 
force induced between the specimen 21 and the probe 24. Therefore, the 
control signal is also supplied to the calculating circuit 35 to derive 
the force between the specimen 21 and the probe 24. The thus calculated 
force is once stored in the calculating circuit 35. As stated above, 
forces between the specimen 21 and the probe 24 are measured under such 
conditions that directions of magnetic moments are in parallel with each 
other and directions of magnetic moments are in anti-parallel with each 
other, and the exchange force is measured as a difference between both two 
forces. These parallel mode and anti-parallel mode may be attained by 
providing an electromagnetic coil around the probe 24 and flowing a 
current into a first direction to magnetize the probe in a first 
direction. After measuring one of the first and second forces, a current 
is flowed through the coil in a second direction opposite to said first 
direction to magnetize the probe in a second direction opposite to the 
first direction, and the other of said first and second forces is 
measured. During the measurement of the forces, no current flows through 
the coil, and therefore the measurement of force is not affected at all. 
Then, a difference between the first and second forces calculated and 
stored in the calculating circuit 35 is calculated to derive the exchange 
force between the specimen 21 and the probe 24. Finally, the calculated 
exchange force is supplied to the processing circuit 36 and magnetic 
properties of the specimen 21 are evaluated on the basis of the exchange 
force. 
FIG. 11 is a schematic view showing a major portion of a second embodiment 
of the exchange force measuring apparatus according to the invention. In 
the first embodiment explained above, the spring constant of the resilient 
cantilever 23 is controlled by the second piezoelectric element 31 
provided on the cantilever in such a manner that the probe 24 is prevented 
from being brought into contact with the specimen 21 by the force between 
the specimen and the probe. In the present embodiment, the attraction of 
the cantilever is prevented by means of a magnetic force. To this end, a 
resilient cantilever 42 is vibrated by means of a piezoelectric element 41 
and a magnetic member 43 is secured to a distal end of the cantilever 42. 
Above the magnetic member 43 is arranged a magnetic coil 44 which is 
connected to a control circuit 46. The control circuit 46 is connected to 
a displacement measuring circuit 45. 
In the present embodiment, by controlling a current supplied to the 
magnetic coil 44 by the control circuit 46 in accordance with a 
displacement signal supplied from the displacement measuring circuit 45 to 
produce a magnetic force applied to the magnetic member 43 such that the 
magnetic member is moved upward and a probe 48 secured to the cantilever 
42 is not attracted to a specimen 47 by a force between the specimen and 
the probe. 
In this manner, by controlling the current supplied to the magnetic coil 44 
such that the cantilever 42 vibrates at the given frequency and amplitude. 
Then, a control signal produced by the control circuit 46 represents the 
exchange force between the specimen 47 and the probe 48. In the present 
embodiment, the magnetic coil 44 is arranged above the cantilever 42, but 
it may be provided under the cantilever. 
FIG. 12 is a schematic view showing a major portion of a third embodiment 
of the exchange force measuring apparatus according to the invention. In 
the present embodiment, a resilient cantilever 52 is vibrated at given 
frequency and amplitude by a piezoelectric element 51 and an electrode 53 
is arranged above the cantilever. A variable DC voltage source 54 is 
connected across the cantilever 52 and the electrode 53. A probe 56 is 
secured to a distal end of the cantilever 56 to be faced with a specimen 
55. By adjusting a DC voltage applied by the variable DC voltage source 54 
across the cantilever 52 and the electrode 53 in accordance with a control 
signal which is produced on the basis of the displacement of the 
cantilever 52. Then, the cantilever 52 is subjected to an electrostatic 
force produced between the cantilever 52 and the electrode 53 and the 
probe 56 is prevented from being attracted to the specimen 55. By 
adjusting the control signal such that the cantilever 52 is vibrated at 
the given frequency and amplitude irrespective to the force between the 
specimen 55 and the probe 56, it is possible to measure the exchange force 
between the specimen and the probe by processing the control signal. 
The present invention is not limited to the embodiments explained above, 
but many alternations and modifications may be conceived by a person 
skilled in the art within the scope of the invention. In the above 
embodiments, the mechanical force, magnetic force and electrostatic force 
are used to prevent the probe provided on the cantilever from being 
attracted to the specimen. However, according to the invention, any other 
means may be utilized to avoid the attraction of the probe to the specimen 
by the exchange force between the specimen and the probe. 
As explained above, in the apparatus for measuring the exchange force 
according to the invention, the controlling means is provided for 
controlling the resiliency of the resilient cantilever against the force 
between the specimen and the probe such that the probe is prevented from 
being attracted to the specimen, and therefore the exchange force between 
the specimen and the probe can be measured accurately with an atomic 
resolution in regardless of compositions of the specimen and probe and the 
magnetic property of the specimen can be evaluated accurately on the basis 
of the measured exchange force.