JP6481191B2 - AC magnetic field measuring apparatus and AC magnetic field measuring method - Google Patents

Description

The present invention relates to an AC magnetic field measuring apparatus and an AC magnetic field measuring method for measuring an AC magnetic field (or AC magnetic field responsiveness) of a sample (a sample that generates an AC magnetic field by giving a signal) using a vibrating probe. About.
In particular, according to the present invention, even when a sample generates a high-frequency magnetic field having a frequency higher than several KHz (typically, a frequency of GHz or higher), the AC magnetic field (or AC magnetic field responsiveness) of the sample is concerned. The present invention relates to an AC magnetic field measurement apparatus and an AC magnetic field measurement method.

As shown in FIG. 3, the magnetic characteristics related to the applied magnetic field responsiveness of the sample 80 (magnetic head in FIG. 3) can be measured by a magnetic characteristic measuring device (magnetic force microscope) 8.
In the magnetic property measuring apparatus 8, a probe 83 made of a hard magnetic material (hereinafter also referred to as “hard magnetic probe”) 83 is formed on the lower surface of the tip of the cantilever 81 toward the sample 80. An exciter (piezo element) 82 is provided at the starting end of the cantilever 81. The exciter 82 mechanically vibrates the probe 83 at an angular frequency ω ₀ (frequency f ₀ (= ω ₀ / (2π)).
The mechanical vibration (angular frequency ω ₀ ) of the probe 83 is subjected to frequency modulation by an alternating magnetic field having an angular frequency ω _s generated from the sample 80.
That is, when the probe 83 receives a magnetic force from the AC magnetic field generated by the sample 80, the apparent spring constant of the cantilever 81 changes. As a result, frequency modulation occurs in the mechanical vibration of the probe 83 at the angular frequency ω ₀ .
In FIG. 3, the power source of the exciter 82 is indicated by AC ₁ , and the power source connected to the sample 80 is indicated by AC ₂ .

The vibration of the probe 83 (frequency modulated vibration) is detected by a vibration detector 84 including a laser light source 841 and a photodiode 842.
That is, the light from the laser light source 841 is reflected by the mirror formed on the top surface of the tip of the cantilever 81, and the reflected light is detected by the photodiode 842.
A signal from the photodiode 842 enters the frequency demodulator 85.
The demodulator 85 frequency-demodulates the input signal and sends the demodulated signal to the magnetic characteristic measurement circuit 86.
The magnetic characteristic measurement circuit 86 can measure the AC magnetic field (or AC magnetic field responsiveness) of the sample 80 by analyzing the demodulated signal received from the demodulator 85 (Patent Document 1).

WO2009 / 101991

In the alternating magnetic field measurement using the magnetic characteristic measuring apparatus of FIG. 3, the alternating magnetic field component in the direction (z direction) perpendicular to the sample surface generated from the sample 80 is expressed by the following equation. Here, the direction in which the probe 83 is displaced is a direction perpendicular to the sample surface.
_{H z (t) = H z0} ac cos (ω s t) = H z0 ac cos (2πf s t)
H _z0 ^ac : amplitude of the component of the alternating magnetic field perpendicular to the sample surface generated from the sample 80 ω _s : angular frequency of the alternating magnetic field generated from the sample 80 f _s : frequency of the alternating magnetic field generated from the sample 80 (f _s = ω _s / (2π))

  When a non-resonant AC magnetic field having a frequency different from the resonance frequency of the cantilever 81 is applied to the probe (hard magnetic probe) 83, a non-resonant AC magnetic force is applied to the probe 83, and as described above, Frequency modulation (FM) is induced in the mechanical vibration. Using this frequency modulation (FM), the alternating magnetic field generated from the sample 80 can be measured.

The probe 83 is a hard magnetic probe of a single magnetic pole type (a state of a magnetic probe in which the magnetic force received by the magnetic pole at the tip of the probe 83 is the main), and is perpendicular to the sample surface of the AC magnetic field generated by the sample 80. when the direction of the component is ^{_{H z0 ac cos (ω s t}} ), the AC magnetic force of the non-resonant is applied to the magnetic poles of the tip of the probe 83 (hard magnetic probe). At this time, the time change Δk (t) of the effective spring constant (apparent spring constant) of the cantilever 81 is given by the following equation.
Δk (t) = ∂F _z / ∂z = q _tip ^dc {∂H _z (t) / ∂z}
^{_{= Q tip dc {∂H z0 ac}} cos (ω s t) / ∂z}
q _tip ^dc : magnetic pole at the tip of the probe 83 (hard magnetic probe) H _z0 ^ac : amplitude of AC magnetic field component perpendicular to the sample surface generated from the sample 80 ω _s : angle of AC magnetic field generated from the sample 80 frequency

The probe 83 is a hard magnetic probe of a double magnetic pole type (a state of a magnetic probe in which the magnetic poles at both ends of the magnetization (magnetic moment) of the probe 83 receive a magnetic force), and the direction of magnetization of the probe is the sample. When the direction is perpendicular to the surface (z direction), a non-resonant AC magnetic force is applied to the magnetic moment of the probe 83 (hard magnetic probe). At this time, the time change Δk (t) of the effective spring constant of the cantilever 81 is given by the following equation.
Δk (t) = ∂F _z / ∂z = M _z ^dc {∂ ² H _z (t) / ∂z ² }
^{_{= M z dc {∂ 2 H}} z0 ac cos (ω s t) / ∂z 2}
M _z ^dc : The component perpendicular to the sample surface of the magnetic moment of the hard magnetic probe

When the time variation Δk (t) of the effective spring constant of the cantilever 81 is sufficiently small compared to the original spring constant k ₀ of the cantilever 81 (Δk (t) << k ₀ ), Δk (t) is Then, narrow band frequency modulation is induced in the mechanical vibration of the probe 83.
Here, the frequency demodulated signal, by detecting locking of a magnetic field and phase components the sample to produce (cos (ω _s t) component) in, in the vertical direction to the surface of the sample 80 an alternating magnetic field H _z0 ^ac cos (ω amplitude distribution of the gradient (∂H _z0 ^ac / ∂z) of _s t), or the distribution of the differential ^{_{(∂ 2 H z0 ac / ∂z}} 2) of the gradient can be measured.

In the present invention, when H _z0 ^ac is zero, the cos (ω _m t) component for lock-in detection disappears, so that zero detection of an alternating magnetic field is possible. In the present invention, the direction of H _z0 ^ac is inverted, the phase of the cos (ω _s t) is changed 180 °, has a characteristic that an upward-downward vertical magnetic field can also be detected simultaneously.

The spectrum of the narrowband frequency modulation induced by the time change Δk (t) of the effective spring constant includes a pair of frequency components in the vicinity of the mechanically excited resonance angular frequency ω ₀ (resonance frequency f ₀ ). It has a frequency component of the sideband spectrum (angular frequency (ω ₀ ± ω _s )).
Here, since the pair of sideband spectra of the angular frequency (ω ₀ ± ω _s ) moves away from the resonance angular frequency ω ₀ when ω _s increases, the resonance effect disappears. As a result, the effective spring constant of the cantilever 81 does not change from the original spring constant of the probe, and the probe 83 is difficult to displace.
Therefore, when ω _s increases, frequency modulation FM cannot be detected, and it may be difficult to measure an alternating magnetic field. Specifically, when the frequency exceeds several KHz, there arises a problem that it is difficult to detect an alternating magnetic field.

  Even if the sample generates a high-frequency magnetic field having a frequency higher than several KHz (typically a frequency of GHz or higher), the AC magnetic field (or AC magnetic field responsiveness) of the sample is increased. An object of the present invention is to provide an AC magnetic field measurement apparatus and an AC magnetic field measurement method that can perform measurement while maintaining the resolution.

The inventors of the present invention have heretofore been attributed to the above phenomenon that when the frequency of the alternating magnetic field increases, the effective spring constant of the probe device does not change from the original spring constant of the probe device and the probe is difficult to displace. In addition to the alternating magnetic field from the sample at the position of the probe, the inventor has obtained the knowledge that it can be eliminated by applying an alternating magnetic field having a slightly different frequency from the outside.
That is, in the present invention, the magnetic probe is changed from a hard magnetic probe in which the magnetization of the probe does not change with time in an alternating magnetic field to a probe made of a soft magnetic material in which the magnetization of the probe changes in time with an alternating magnetic field (hereinafter, “ In addition to the AC magnetic field from the sample, in addition to the AC magnetic field from the sample, the time-dependent magnetization of the probe and the AC magnetic field Take advantage of the magnetic interaction between. By using this magnetic interaction to convert the time variation of the effective spring constant of the probe device into a low frequency, a high frequency alternating magnetic field can be measured.

The gist of the AC magnetic field measurement apparatus of the present invention is (1) to (6).
(1)
An AC magnetic field measuring apparatus for measuring an AC magnetic field (or AC magnetic field response) of a sample that generates an AC magnetic field:
A probe device comprising a probe made of a magnetic material, the probe being magnetized by an alternating magnetic field generated from the sample (a beam member that performs spring vibration: basically according to Hooke's law);
An exciter for exciting the probe at an excitation frequency;
A sample driving source that drives the sample, generates the alternating magnetic field, and periodically reverses the magnetization of the probe according to the polarity of the alternating magnetic field (typically, a magnetic field that generates the probe magnetization by an electric current). Power source driven by
An AC magnetic field generator for generating a magnetic field having a frequency slightly different from the frequency of the AC magnetic field generated from the sample drive source (typically an AC magnetic field having at least one frequency component) at the position of the probe;
A vibration of the probe is detected, and a modulation signal generated with respect to the mechanical vibration by the exciter is generated from the detection signal (generated by changing an effective spring constant of the probe device by the modulation AC magnetic field). A signal extractor for extracting
An AC magnetic field response measuring device for measuring an AC magnetic field (or AC magnetic field response) of the sample based on the signal extracted by the signal extractor;
With
An AC magnetic field measuring apparatus characterized by that.
For example, a magnetic recording head is a sample that generates an alternating magnetic field and is formed with an exciting coil, which is measured by the present invention. The magnetic recording head is produced by a thin film process. In the present specification, the configuration and the like of the magnetic head produced by the thin film process are well known, and thus will not be described in detail.
In the present invention, the probe is typically a ferromagnetic material, but may be a paramagnetic material.
In the present invention, the AC magnetic field generator that applies a magnetic field having a frequency slightly different from that of the sample driving source to the probe has at least one frequency component.
(2)
An AC magnetic field measurement apparatus according to (1), wherein:
AC magnetic field generator
A coil that generates an alternating magnetic field;
A modulation power source for passing a current through the coil;
An AC magnetic field measuring apparatus characterized by that.
The AC magnetic field generator is typically composed of one coil and one current application power source.
When the intensity of the alternating magnetic field from the coil is small, the alternating magnetic field can be generated by a plurality of coils and the accompanying power supply for current application.
In the case where the AC magnetic field generator is constituted by one coil and one power supply for current application, the coil has two or more currents (for example, slightly different from the angular frequency (ω _s ) of the AC magnetic field generated from the sample). , Angular frequency ω _s −ω _m , ω _s + ω _{m, where} ω _m is a small value of angular frequency). As a result, an AC magnetic field having angular frequencies ω _s −ω _m and ω _s + ω _m is generated from the coil.
Further, the AC magnetic field generator can be constituted by, for example, a plurality of (typically two) coils and a plurality (typically the same number of coils) of current application power sources. When there are two coils and two current application power supplies (two pairs of coils and current application power supplies), an alternating magnetic field having an angular frequency ω _s −ω _m is generated from one coil and the other coil is used. An alternating magnetic field having an angular frequency ω _s + ω _m can be generated.
The coil can be provided at the tip (probe side or mirror side) of the probe device (cantilever).
(3)
The AC magnetic field measurement apparatus according to (1),
The signal extractor is
A probe vibration detector for detecting the vibration of the probe device;
A demodulation circuit that demodulates a signal corresponding to a magnetic signal applied to the vibration detection device from a signal relating to vibration detected by the vibration detection device;
With
An AC magnetic field measuring apparatus characterized by that.
(4)
The AC magnetic field measurement apparatus according to (1),
The probe device is
The probe;
A cantilever provided with a tip at the tip;
Consists of
The exciter is
AC power supply,
Cantilevers,
including,
An AC magnetic field measuring apparatus characterized by that.
(5)
The AC magnetic field measurement apparatus according to (1),
Furthermore, a scanning mechanism is provided,
The scanning mechanism moves the probe of the probe device three-dimensionally with respect to the sample;
An AC magnetic field measuring apparatus characterized by that.
(6)
The AC magnetic field measurement apparatus according to (5),
Furthermore, an image display device is provided,
The image display device measures an alternating magnetic field (or alternating magnetic field responsiveness) of the sample at each position on the surface of the sample of the probe scanned by the scanning mechanism, and displays the measurement result as an image. indicate,
An AC magnetic field measuring apparatus characterized by that.

The alternating magnetic field measurement method of the present invention is summarized as (7) to (11).
(7)
An AC magnetic field measurement method for measuring an AC magnetic field (or AC magnetic field responsiveness) of a sample that generates an AC magnetic field using a probe device including a probe made of a magnetic material:
An excitation step of exciting the probe at an excitation frequency with an exciter;
The sample is driven, the alternating magnetic field is generated, and the magnetization of the probe is periodically reversed according to the polarity of the alternating magnetic field, and the alternating magnetic field generated from the sample driving source at the position of the probe A mechanical modulation generating step of modulating the mechanical vibration of the probe by generating a magnetic field having a frequency slightly different from the frequency of (typically an alternating magnetic field having at least one frequency component);
A vibration of the probe is detected, and a modulation signal (a signal generated when an effective spring constant of the probe device is changed by the modulation AC magnetic field) is generated from the detection signal with respect to mechanical vibration. A signal extraction step to extract; and
A measuring step of measuring an alternating magnetic field (or alternating magnetic field responsiveness) of the sample based on the signal extracted by the signal extractor;
including,
AC magnetic field measurement method characterized by the above.
(8)
The AC magnetic field measurement method according to (7),
The signal extraction step includes a vibration detection step and a demodulation step,
In the vibration detection step, the vibration of the probe device is detected,
In the demodulation step, a signal corresponding to the magnetic signal is demodulated from the signal detected in the vibration detection step.
AC magnetic field measurement method.
(9)
The AC magnetic field measurement method according to (7),
In the excitation step, the piezoelectric element is driven by an AC power source to excite the probe device.
AC magnetic field measurement method characterized by the above.
(10)
The AC magnetic field measurement method according to (7),
Further comprising a scanning step,
In the scanning step, the probe is three-dimensionally moved with respect to the sample.
AC magnetic field measurement method characterized by the above.
(11)
The AC magnetic field measurement method according to (10),
Furthermore, an image display step is included,
In the image display step, the AC magnetic field (or AC magnetic field responsiveness) of the sample at each position on the surface of the sample of the probe scanned in the scanning step is measured, and the measurement result is imaged. indicate,
AC magnetic field measurement method characterized by the above.

Even when the frequency of the alternating magnetic field applied to the probe is high enough that the probe cannot cause mechanical vibration to be modulated (for example, GHz or higher), the high resolution is maintained while maintaining the high resolution. An alternating magnetic field (or alternating magnetic field response) can be measured.
In addition, upward and downward vertical magnetic fields can be detected simultaneously.

  In the present invention (magnetic characteristic measuring apparatus and magnetic characteristic measuring method), the operation when a soft magnetic probe is used as the probe will be described below. The AC magnetic field measurement method of the present invention can be implemented using the AC magnetic field measurement apparatus of the present invention.

In the present invention, an alternating magnetic field generated from a sample is applied to the probe. At the same time, an alternating magnetic field having a slightly different frequency (an alternating magnetic field for changing the magnetization of the probe over time) is applied to the position of the probe from an alternating magnetic field generator provided separately.
The sample is, for example, a magnetic recording head (write head) in which a coil is formed.
The AC magnetic field generated by the AC magnetic field generator must include at least one AC magnetic field component having different angular frequencies (basic angular frequency ω _s (fundamental frequency: f _s , ω _s = 2πf _s )) generated by the sample. There is.
For example, the spectrum may include one alternating magnetic field component different from the basic angular frequency ω _s .
In the case of amplitude modulation (AM modulation) or narrowband frequency modulation (FM modulation), the spectrum includes two alternating magnetic field components different from the basic angular frequency ω _s .

First, the case where the AC magnetic field generated by the AC magnetic field generator has one frequency component will be described below.
In this case, the basic angular frequency ω _s of the alternating magnetic field generated from the sample may be higher than several KHz, or may be higher than GHz.
The magnetization of the probe is modulated as follows.
An AC magnetic field having a fundamental angular frequency ω _s (in this embodiment, the same angular frequency as that applied to the sample during actual use) is generated in the sample.
On the other hand, an alternating magnetic field (modulated magnetic field in the present invention) of an angular frequency ω _s + ω _m or ω _s −ω _m (ω _m is a low angular frequency) slightly different from the angular frequency ω _s is applied to the probe from other than the sample. Apply. Here, considering that the magnetization of the soft magnetic probe is excited only by the AC magnetic field from the AC magnetic field generator, the probe is excited at the angular frequency ω _s + ω _m or ω _s −ω _m by the modulation magnetic field. The magnetization of the probe is modulated.
As a result, the probe is excited by the modulation magnetic field having the angular frequency ω _s −ω _m or ω _s + ω _m , and the magnetization is modulated.

By applying the modulated AC magnetic field as described above to the probe, an AC magnetic force having a low angular frequency (modulation frequency) ω _m is induced in the probe. The AC magnetic force at this low angular frequency ω _m is due to the magnetic interaction between the soft magnetic probe and the AC magnetic field.
As a result, frequency modulation occurs in the mechanical vibration of the probe, and by using this mechanical frequency modulation, the response of the AC magnetic field of the sample (magnetic head) or the AC magnetic field (high frequency magnetic field) generated by the sample Can be measured.

In the following, detection of the magnetic field of a sample that generates an alternating magnetic field with an angular frequency ω _s is performed using an alternating magnetic field generator, and is applied to a soft magnetic probe from the outside, which is slightly different from the angular frequency ω _s of this alternating magnetic field, _A case will be described in which an alternating magnetic field of _s + ω _m (ω _m is a low angular frequency) is generated and a soft magnetic probe is excited. Here, the direction in which the probe is displaced is a direction perpendicular to the sample surface, and this direction is the z direction. The z component of the alternating magnetic field generated from the sample is expressed below.
_{H z (t) = H sample} ac (t) = H z0 ac cos (ω s t) ( Equation 1)
H _z0 ^ac : Amplitude of component perpendicular to sample surface of AC magnetic field (angular frequency ω _s ) generated from sample ω _s : Angular frequency of AC magnetic field generated from sample In this case, basic angle of AC magnetic field generated from sample The frequency ω _s may be higher than several KHz and may be higher than GHz.
On the other hand, an angular frequency ω _s + ω _m is applied to the probe from the outside using an AC magnetic field generator. Here, the z component of the alternating magnetic field generated from the alternating magnetic field generator is expressed below.
H _z (t) = H _external ^ac (t)
= _{ΑH z0} ^ac cos ((ω _s + ω _m ) t) (Formula 2)
α: Ratio of the amplitude of the AC magnetic field component perpendicular to the sample surface generated by the AC magnetic field generator to the amplitude of the AC magnetic field component perpendicular to the sample surface generated from the sample ω _m : Very high compared with ω _s Low angular frequency (ω _m << ω _s )

  When measuring the AC magnetic field response of a sample, it is necessary to reduce the distance between the probe and the sample (referred to as “probe-sample distance”) in order to improve the spatial resolution. As the distance between the probe and the sample decreases, the magnetic behavior of the probe changes from a double-pole type (a state of a magnetic probe in which the magnetic poles at both ends of the magnetic moment of the probe's magnetization receive a magnetic force) to a simple magnetic field. It changes to a magnetic pole type (a state of a magnetic probe in which the magnetic force received by the magnetic pole at the tip of the probe is main).

First, consider the case where the magnetization of the soft magnetic probe is excited only by an alternating magnetic field from an alternating magnetic field generator.
Here, a case of a single magnetic pole type soft magnetic probe will be described.
Due to the AC magnetic field applied to the probe from the AC magnetic field generator, a magnetic pole of the following formula is generated at the tip of the probe.
ρ (t) = ρ _tip ^ac (t)
= Ρ ₀ ^ac cos ((ω _s + ω _m ) t) (Formula 3)

The effective spring constant is given by the following equation by the interaction between the alternating magnetic field generated from the sample and the magnetic pole of the probe, which changes with the alternating magnetic field from the alternating magnetic field generator provided separately.
Δk (t) = ∂F _z / ∂z
= Ρ (t) (∂H _z (t) / ∂z)
= Ρ _tip ^ac (t) (∂H _sample ^ac (t) / ∂z)
= Ρ ₀ ^ac cos ((ω _s + ω _m ) t)
^{_{(∂H z0 ac / ∂z) cos}} (ω s t)) ( Equation 4)

From the above equation, the effective spring constant component Δk _low that changes with time at the low angular frequency ω _m is given by the following equation.
Δk _low
= (1/2) ρ ₀ ^ac (∂H _z0 ^ac / ∂z) cos (ω _m t) (Formula 5)
The above equation is to use frequency modulation of the probe vibration by cos (ω _m t) component of .DELTA.k _low a (FM), a high frequency magnetic field ^{_{H z0 ac cos (ω s t}} ), a frequency conversion to a low angular frequency (down-conversion ) Means that it is possible to measure the amplitude (∂H _z0 ^ac / ∂z) of the high-frequency magnetic field.

Here, by detecting the amplitude of the cos (ω _m t) component related to the AC signal for probe excitation by the lock-in amplifier, the distribution of (∂H _z0 ^ac / ∂z) is used by the DC component. Compared to the case, an alternating magnetic field can be imaged near the sample surface with high sensitivity and improved spatial resolution. In the case of using a direct current component, it is difficult to measure an alternating magnetic field in the vicinity of the sample surface because a short distance force and a magnetic force due to the surface cannot be separated.
When H _z0 ^ac , which is a magnetic field component perpendicular to the sample surface, is zero, the cos (ω _m t) component due to lock-in detection disappears, so that zero detection of the magnetic field is possible. Note that when a DC component is used, zero detection of the magnetic field is impossible.
Further, when the direction of H _z0 ^ac is inverted, cos (ω _s t) component of the phase, as shown in the following equation, changes 180 °.
(1/2) ρ ₀ ^ac (−∂H _z0 ^ac / ∂z) cos (ω _m t)
= (1/2) ρ ₀ ^ac (∂H _z0 ^ac / ∂z) cos (ω _m t ± π) (Formula 6)
That is, in the present invention, the vertical magnetic field can be detected at the same time, upward and downward.

In the above example, to excite the probe, alternating magnetic field generates AC magnetic field generator provided separately, there in an alternating magnetic field with different one frequency component (ω _s + ω _m) and angular frequency omega _s It was.
Next, as a typical example in which a separately provided AC magnetic field generator for exciting the magnetization of the probe has two frequency components, an amplitude-modulated AC magnetic field will be described. At this time, the alternating magnetic field applied to the probe from the alternating magnetic field generator is given by the following equation.
H _z (t) = H _z0 ^ac cos (ω _s + ω _m ) t + H _z0 ^ac cos (ω _s −ω _m ) t
^{_{= (2H z0 ac cos (ω}} m t)) cos (ω s t) ( Equation 7)

In this case, the magnetization of the probe is excited by the alternating magnetic field of (Equation 7), and a magnetic pole given by the following equation is generated at the tip of the single magnetic pole type soft magnetic probe.
ρ (t) = ρ _tip ^ac (t)
= Ρ ₀ ^ac cos ((ω _s + ω _m ) t) + ρ ₀ ^ac cos ((ω _s −ω _m ) t)
^{_{= Ρ 0 ac cos (ω m}} t) cos (ω s t) ( Equation 8)

The effective spring constant varies with time by the interaction between the alternating magnetic field generated from the sample and the magnetic pole of the probe excited by the separately provided alternating magnetic field generator, and is given by the following equation.
Δk (t) = ∂F _z / ∂z
= Ρ (t) (∂H _z (t) / ∂z)
= Ρ _tip ^ac (t) (∂H _sample ^ac (t) / ∂z)
= (Ρ ₀ ^ac cos ((ω _s + ω _m ) t)
+ Ρ ₀ ^ac cos ((ω _s −ω _m ) t))
^{_{(∂H z0 ac / ∂z) cos}} (ω s t) ( Equation 9)

From the above equation, the effective spring constant component Δk _low of the low angular frequency is given by the following equation.
Δk _low = ρ ₀ ^ac (∂H _z0 ^ac / ∂z) cos (ω _m t) (Formula 10)
The use of an amplitude-modulated AC magnetic field is characterized in that the signal intensity of the cos (ω _m t) component is doubled compared to the case of the AC magnetic field of one frequency described above.

Next, let us consider a case where the magnetization of the soft magnetic probe is excited not only by the AC magnetic field from the AC magnetic field generator but also by the AC magnetic field from the sample.
Here, a case of a single magnetic pole type soft magnetic probe will be described.
Here, the direction in which the probe is displaced is defined as the direction perpendicular to the sample surface (z direction), and the component perpendicular to the sample surface of the AC magnetic field that excites the probe magnetization is expressed below.
H _z (t) = H _sample ^ac (t) + H _external ^ac (t)
= H _z0 ^ac cos (ω _s t) + _{αH z0} ^ac cos (ω _s + ω _m ) t (Formula 11)
H _z0 ^ac : Amplitude of component perpendicular to sample surface of AC magnetic field (angular frequency ω _s + ω _m ) generated from sample α: Amplitude of amplitude of AC magnetic field component perpendicular to sample surface generated from AC magnetic field generator Ratio to amplitude of AC magnetic field component perpendicular to sample surface generated from sample ω _s : Angular frequency of AC magnetic field generated from sample ω _m : Angular frequency very low compared to ω _s (ω _m << ω _s )
Due to the alternating magnetic field, the following magnetic pole is generated at the tip of the probe.
ρ (t) = ρ _tip ^ac (t)
^{_{= Ρ 0 ac cos (ω s}} + ω m) t + βρ 0 ac cos (ω s t) ( Equation 12)
ρ ₀ ^ac : Amplitude of the magnetic pole density (probe magnetic pole) at the tip of the probe due to the alternating magnetic field (angular frequency ω _s + ω _m ) generated by the AC magnetic field generator β: Generated by the alternating magnetic field (angular frequency ω _s ) from the sample Ratio of the amplitude of the probe magnetic pole to the amplitude of the probe magnetic pole generated by the AC magnetic field from the AC magnetic field generator

From (Equation 11) and (Equation 12), the time change of the effective spring constant is given by the following equation.
Δk (t) = ∂F _z / ∂z
= Ρ (t) (∂H _z (t) / ∂z)
= Ρ _tip ^ac (t) (∂H _sample ^ac (t) / ∂z)
= {Ρ ₀ ^ac cos (ω _s + ω _m ) t + βρ ₀ ^ac cos (ω _s t)}
^{_{{(∂H z0 ac / ∂z)}} cos (ω s t)
+ Α (∂H _z0 ^ac / ∂z) cos (ω _s + ω _m ) t} (Formula 13)
From (Equation 13), the effective spring constant component Δk _low of low angular frequency including direct current is given by the following equation.
Δk _low = {(α + β) / 2} ρ ₀ ^ac (∂H _z0 ^ac / ∂z)
+ {(1 + αβ) / 2} ρ ₀ ^ac (∂H _z0 ^ac / ∂z) cos (ω _m t)
(Formula 14)

(Equation 14), using the effective spring constant component .DELTA.k _low of cos (omega _m t) frequency modulation of the probe vibration by component (FM), a high frequency magnetic field ^{_{H z0 ac cos (ω s t}} ), a low angular frequency This means that the frequency of the high-frequency magnetic field (∂H _z0 ^ac / ∂z) can be measured by frequency conversion (down-conversion).
Here, by detecting the amplitude of the cos (ω _m t) component related to the AC signal for probe excitation by the lock-in amplifier, the distribution of (∂H _z0 ^ac / ∂z) is used by the DC component. Compared to the case, the alternating magnetic field can be imaged near the sample surface with high sensitivity and improved spatial resolution. In the case of using a direct current component, it is difficult to measure an alternating magnetic field in the vicinity of the sample surface because a short distance force and a magnetic force due to the surface cannot be separated.

Furthermore, in the present invention for detecting the cos (ω _m t) component, when H _z0 ^ac , which is a magnetic field component perpendicular to the sample surface, is zero, the cos (ω _m t) component disappears due to lock-in detection. Also, zero detection of the magnetic field is possible. Note that when a DC component is used, zero detection of the magnetic field is impossible.
When the direction of H _z0 ^ac is reversed, the phase of the cos (ω _m t) component changes by 180 ° as shown in the following equation.
ρ ₀ ^ac (−∂H _z0 ^ac / ∂z) {(1 + αβ) / 2} cos (ω _m t)
= Ρ ₀ ^ac (∂H _z0 ^ac / ∂z) {(1 + αβ) / 2} cos (ω _m t ± π) (Formula 15)
That is, the present invention has a feature that the direction of the vertical magnetic field (upward / downward) can be detected simultaneously.

In the above example, the alternating magnetic field that excites the probe magnetization is an alternating magnetic field having two frequency components, ω _s and (ω _s + ω _m ). Next, as a representative example in which an alternating magnetic field that excites probe magnetization has three frequency components, amplitude-modulated alternating current having angular frequency components of ω _s and (ω _s + ω _m ) and (ω _s −ω _m ). A case where a magnetic field is generated from an AC magnetic field generator will be described.

At this time, the alternating magnetic field for exciting the probe magnetization is given by the following equation.
H _z (t) = H _sample ^ac (t) + H _external ^ac (t)
= H _z0 ^ac cos (ω _s t) + _{αH z0} ^ac cos (ω _s + ω _m ) t
+ _{ΑH z0} ^ac cos (ω _s −ω _m ) t
^{_{= H z0 ac (1 + 2αcos}} (ω m t)) cos (ω s t) ( Equation 16)
A magnetic pole represented by the following formula is generated at the tip of the probe.
ρ (t) = ρ _tip ^ac (t)
= Ρ ₀ ^ac cos (ω _s + ω _m ) t + ρ ₀ ^ac cos (ω _s −ω _m ) t
^{_{+ Βρ 0 ac cos (ω s}} t) ( Equation 17)

From (Equation 16) and (Equation 17), the time change of the effective spring constant is given by the following equation.
Δk (t) = ∂F _z / ∂z
= Ρ (t) (∂H _z (t) / ∂z)
= Ρ _tip ^ac (t) (∂H _sample ^ac (t) / ∂z)
= {Ρ ₀ ^ac cos (ω _s + ω _m ) t + ρ ₀ ^ac cos (ω _s −ω _m ) t
^{_{+ Βρ 0 ac cos (ω s}} t)}
^{_{{(∂H z0 ac / ∂z)}} cos (ω s t)
+ Α (∂H _z0 ^ac / ∂z) cos (ω _s + ω _m ) t
+ Α (∂H _z0 ^ac / ∂z) cos (ω _s −ω _m ) t} (Formula 18)

From (Equation 18), the effective spring constant component Δk _low including DC is expressed by the following equation.
Δk _low = {(α + β)} ρ ₀ ^ac (∂H _z0 ^ac / ∂z)
+ {(1 + αβ)} ρ ₀ ^ac (∂H _z0 ^ac / ∂z) cos (ω _m t) (Formula 19)
From the above, it is possible to measure the amplitude (∂H _z0 ^ac / ∂z) of the high-frequency magnetic field using the frequency modulation (FM) of the probe vibration by the cos (ω _m t) component of Δk _low. I understand.
The use of an amplitude-modulated AC magnetic field is characterized in that the signal intensity of the cos (ω _m t) component is doubled compared to the case of the AC magnetic field of one frequency described above.

FIG. 1 is an explanatory view showing an embodiment of the AC magnetic field measurement apparatus of the present invention. FIG. 2 is a flowchart showing the AC magnetic field measurement method of the present invention. FIG. 3 is an explanatory diagram of the prior art (AC magnetic field measuring apparatus).

FIG. 1 is an explanatory diagram showing an AC magnetic field measuring apparatus according to the present invention. An AC magnetic field (or AC magnetic field responsiveness) of a sample that generates an AC magnetic field by applying an AC signal can be measured by a probe device. .
In FIG. 1, an AC magnetic field measuring apparatus 1 includes a probe device (cantilever) 11, an exciter 12, a sample driving source 131, an AC magnetic field generator 132, a signal extractor 14, and an AC magnetic field response measuring instrument 15. And a scanning mechanism 16 and an image display device 17.

In FIG. 1, the probe device 11 includes a cantilever 112 and a probe 111 made of a soft magnetic material provided at the tip (free end) of the cantilever 112. The probe 111 is made of a soft magnetic material, and the polarity of the magnetic pole at the tip of the probe 111 changes following the polarity of the magnetic field generated by the sample 2.
In this embodiment, the probe 111 is formed by coating a silicon probe with an FeCo alloy or the like having a small coercive force and a high saturation magnetic flux density.

The exciter 12 includes an AC power source 121 and a piezoelectric element (piezo element) 122, and can excite the probe 111 (mechanically vibrate). The angular frequency that the AC power supply 121 gives to the piezoelectric element 122 is ω ₀ . The exciter 12 excites the probe 111 at the natural frequency of the probe device 11 or an angular frequency ω ₀ close to the natural frequency (in this embodiment, the frequency f ₀ = about 300 kHz).
Note that the effective (apparent) spring constant of the probe device 11 changes as the probe 111 is affected by the AC magnetic field H ₀ . Thereby, frequency modulation occurs in the vibration of the probe 111.

The sample driving source 131 is a power source (current source or voltage source) that outputs an alternating current having an angular frequency ω _s . The sample driving source 131 drives the sample 2 to generate an AC magnetic field with an angular frequency ω _s from the sample 2 and periodically inverts the magnetization of the probe 111 according to the polarity of the AC magnetic field. That is, the sample driving source 131 causes an alternating current to flow through the coil 21 of the sample 2 and generates an alternating current having an angular frequency ω _s from the sample 2.
The AC magnetic field generator 132 includes a coil 1321 and a power source 1322. The power source 1322 passes a current through the coil 1321, and the coil 1321 generates an AC magnetic field. That is, the AC magnetic field generator 132 has a probe magnetization slightly different in frequency from the AC magnetic field H ₀ cos (ω _st t} (AC magnetic field generated from the sample 2) of the angular frequency ω _s at the position of the probe 11. A magnetic field for excitation (having one or more frequency components different from ω _s ) is applied to the position of the probe 111.
That is, the alternating magnetic field generator 132, look for an alternating magnetic field H ₀ cos of the angular frequency omega _s alternating magnetic field and the angular frequency is slightly different angular frequencies _{(ω s ± ω m) {} (ω s ± ω m) t} Give to needle 111.
In FIG. 1, the sample 2 is a magnetic generator provided with a coil 21, and the AC signal (ω _s ± ω _m ) given to the sample 2 is an AC current that flows through the coil 21. Sample 2 is typically a magnetic head for writing to a hard disk.

The signal extractor 14 includes a probe vibration detector 141 and a demodulation circuit 142.
The probe vibration detector 141 detects the vibration of the probe 111. The probe vibration detector 141 includes a laser 1411 and an optical sensor (photodiode) 1412.

The probe 111 oscillates at an angular frequency ω ₀ when no AC magnetic field is applied, but undergoes frequency modulation due to mechanical vibration when an AC magnetic field is applied. In the present invention, the alternating magnetic field is a modulated high-frequency magnetic field, and the probe device 11 behaves so that the spring constant changes at the modulation frequency ω _m . Therefore, the probe 111 vibrating at the angular frequency ω ₀ of the probe 111 is modulated at the modulation frequency ω _m .
The demodulation circuit 142 receives the detection signal of the probe vibration detector 141 and receives a modulation signal (vibration frequency periodically changing from ω ₀ to the modulation frequency) with respect to the mechanical vibration of the probe 111 included in the detection signal. The signal having the modulation angular frequency ω _m can be demodulated from the modulation signal.

The AC magnetic field response measuring device 15 measures the AC magnetic field (or AC magnetic field response) of the sample 2 based on the signal demodulated by the demodulation circuit 142. In other words, the AC magnetic field response measuring device 15 measures the AC magnetic field (or AC magnetic field responsiveness) of the sample 2 based on the degree of frequency modulation of the vibration of the probe 111 with respect to the signal of the modulation frequency ω _m .
The AC magnetic field response measuring device 15 can be composed of a lock-in amplifier 151, an amplitude measuring circuit 152, a phase measuring circuit 153, an in-phase component measuring circuit 154, and a quadrature component measuring circuit 155.
In the case of the above-described single magnetic pole type soft magnetic probe, which corresponds to the case where the distance between the probe and the sample is decreased in order to improve the spatial resolution, the amplitude measuring circuit 152 is obtained from the output of the lock-in amplifier 151. The amplitude of the periodically varying magnetic field ∂H _m / ∂z can be detected, and the phase measuring circuit 153 can detect the phase of the periodically varying magnetic field ∂H _zm / ∂z.

The scanning mechanism 16 can scan the probe 111 on the surface of the sample 2 by moving the probe device 11 relative to the sample 2. For example, the scanning mechanism 16 can be a three-dimensional moving stage (a stage that can move in the XYZ direction). A sample 2 is attached to the three-dimensional movement stage.
During scanning by the scanning mechanism 16, the distance between the probe 111 and the sample 2 can be detected in advance. In this embodiment, the function of the scanning mechanism 16 is configured to adjust the distance between the probe and the sample.

The image display device 17 can be composed of a computer and a display. The image display device 17 stores the measurement result of the alternating magnetic field (or alternating magnetic field responsiveness) on the surface of the sample 2 and the scanning position information of the scanning mechanism 16 (in this embodiment, xy coordinate information or xyz coordinate information) in a memory. You can remember it. Then, the image display device 17 displays the alternating magnetic field (or alternating magnetic field response: the strength of the magnetic field at each position) in a two-dimensional or three-dimensional manner corresponding to the scanning position information.
The image display device 17 measures the AC magnetic field (or AC magnetic field responsiveness) of the sample 2 at a number of positions on the surface of the sample 2 of the probe 111 scanned by the scanning mechanism 16, and displays the measurement result. Can be stored in memory. The measurement result can be displayed as an image.

FIG. 2 is a flowchart showing the AC magnetic field measurement method of the present invention.
The processes in steps S110 to S200 shown in FIG. 2 are performed in each component (probe device 11, exciter 12, sample driving source 131, AC magnetic field generator 132, signal extractor) in the above-described AC magnetic field measurement apparatus of the present invention. 14, AC magnetic field response measuring device 15, scanning mechanism 16 and image display device 17).

The scanning mechanism 16 starts scanning (coordinates are set to initial coordinates X = 0, Y = 0) (S110).
The sample driving source 131 drives the sample 2 with an AC signal having an angular frequency ω _s , and the sample 2 generates an AC magnetic field having an angular frequency ω _s and applies it to the probe 111. The AC magnetic field generator 132 generates an AC magnetic field having an angular frequency slightly different from the angular frequency ω _s (angular frequency difference: ω _m ) and applies the AC magnetic field to the probe 111 (S120).
The exciter 12 excites the probe 111 (mechanically vibrates), excites the probe 111 with an AC magnetic field having two or more frequency components, and is effective due to the magnetic interaction between the AC magnetic field and the probe magnetization. The spring constant is changed (S130).
The probe vibration detector 141 constituting a part of the signal extractor 14 detects the vibration of the probe 111 and generates a detection signal (S140).
The demodulation circuit 142 constituting a part of the signal extractor 14 demodulates a modulation signal (signal due to mechanical modulation) generated by mechanical vibration of the probe 111 included in the detection signal (S150).
The AC magnetic field response measuring device 15 measures the AC magnetic field (or AC magnetic field response) of the sample 2 based on the demodulated signal (S160). The AC magnetic field (or AC magnetic field responsiveness) of the sample 2 is measured from the degree of frequency modulation caused by the vibration of the probe 111.
The AC magnetic field response measuring instrument 15 stores the measurement result in a storage device (see reference numeral 171 in FIG. 1) (S170).
The AC magnetic field response measuring device 15 determines whether the scanning mechanism 16 has scanned all the coordinates (S180).
When the scanning mechanism 16 has not scanned all the coordinates, the process returns to S120, and the processes of S120 to S180 are performed for the new coordinates.
When the scanning mechanism 16 scans all coordinates, the measurement result is displayed as an image on the image display device 17 (S190).

DESCRIPTION OF SYMBOLS 1 AC magnetic field measuring device 2 Sample 11 Probe device 12 Exciter 13 Sample drive source 14 Signal extractor 15 AC magnetic field response measuring device 16 Scanning mechanism 17 Image display device 21 Coil 111 Probe 112 Cantilever 121 AC power supply 122 Piezoelectric element (piezo element)
131 Sample Drive Source 132 AC Magnetic Field Generator 141 Probe Vibration Detector 142 Demodulation Circuit 151 Lock-in Amplifier 152 Amplitude Measurement Circuit 153 Phase Measurement Circuit 154 In-phase Component Measurement Circuit 155 Vertical Component Measurement Circuit 171 Storage Device 1411 Laser 1412 Photosensor (Photo Sensor) diode)

Claims (11)

An AC magnetic field measuring apparatus for measuring an AC magnetic field or AC magnetic field response of a sample that generates an AC magnetic field:
Comprising a probe made of a magnetic material, the probe is magnetized by an alternating magnetic field generated from the sample, and the probe device is a beam member for spring vibration;
An exciter for exciting the probe at an excitation frequency;
A sample driving source that drives the sample, generates the alternating magnetic field, and periodically inverts the magnetization of the probe according to the polarity of the alternating magnetic field;
An alternating magnetic field generator for generating a magnetic field having a frequency different from the frequency of the alternating magnetic field generated from the sample drive source at the position of the probe to modulate the magnetization of the probe ;
Detecting the vibration of the probe, from the detection signal, a signal extractor which extracts a modulated signal generated with respect to mechanical vibration by the exciter;
An AC magnetic field response measuring device for measuring the AC magnetic field or AC magnetic field response of the sample based on the signal extracted by the signal extractor;
With,
An AC magnetic field measuring apparatus characterized by that. The AC magnetic field measurement apparatus according to claim 1, wherein:
AC magnetic field generator
A coil that generates an alternating magnetic field;
A modulation power source for passing a current through the coil;
With
An AC magnetic field measuring apparatus characterized by that. The AC magnetic field measurement apparatus according to claim 1, wherein:
The signal extractor comprises:
A probe vibration detector for detecting the vibration of the probe device;
A demodulation circuit that demodulates a signal corresponding to a magnetic signal applied to the vibration detection device from a signal relating to vibration detected by the vibration detection device;
With
An AC magnetic field measuring apparatus characterized by that. The AC magnetic field measurement apparatus according to claim 1, wherein:
The probe device is
The probe;
A cantilever provided with a tip at the tip;
Consists of
The exciter is
AC power supply,
Cantilevers,
including,
An AC magnetic field measuring apparatus characterized by that. The AC magnetic field measurement apparatus according to claim 1, wherein:
Furthermore, a scanning mechanism is provided,
The scanning mechanism moves the probe of the probe device three-dimensionally with respect to the sample;
An AC magnetic field measuring apparatus characterized by that. The AC magnetic field measurement apparatus according to claim 5, wherein:
Furthermore, an image display device is provided,
The image display device measures the alternating magnetic field response of the sample at each position on the surface of the sample scanned by the scanning mechanism, and displays the measurement result as an image.
An AC magnetic field measuring apparatus characterized by that. An AC magnetic field measurement method for measuring an AC magnetic field or AC magnetic field response of a sample that generates an AC magnetic field using a probe device including a probe made of a magnetic material:
An excitation step of exciting the probe at an excitation frequency with an exciter;
Driving the sample, generating the alternating magnetic field, periodically reversing the magnetization of the probe according to the polarity of the alternating magnetic field, and modulating the magnetization of the probe to the position of the probe A mechanical modulation generation step of modulating the mechanical vibration of the probe by generating a magnetic field having a frequency different from the frequency of the alternating magnetic field generated from the sample driving source;
A signal extraction step of detecting vibration of the probe and extracting a modulation signal generated with respect to mechanical vibration from the detection signal; and
A measurement step of measuring an alternating magnetic field or alternating magnetic field responsiveness of the sample based on the signal extracted by the signal extractor;
including,
AC magnetic field measurement method characterized by the above. The AC magnetic field measurement method according to claim 7, wherein:
The signal extraction step includes a vibration detection step and a demodulation step,
In the vibration detection step, the vibration of the probe device is detected,
In the demodulation step, a signal corresponding to the magnetic signal is demodulated from the signal detected in the vibration detection step.
AC magnetic field measurement method. The AC magnetic field measurement method according to claim 7, wherein:
In the excitation step, the piezoelectric element is driven by an AC power source to excite the probe device.
AC magnetic field measurement method characterized by the above. The AC magnetic field measurement method according to claim 7, wherein:
Further comprising a scanning step,
In the scanning step, the probe is three-dimensionally moved with respect to the sample.
AC magnetic field measurement method characterized by the above. The AC magnetic field measurement method according to claim 10, wherein:
Furthermore, an image display step is included,
In the image display step, the AC magnetic field or AC magnetic field responsiveness of the sample at each position on the surface of the sample scanned in the scanning step is measured, and the measurement result is displayed as an image.
AC magnetic field measurement method characterized by the above.