JP2015049134A - Ac magnetic field measurement instrument and ac magnetic field measurement method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow measurement of an AC magnetic field generated from a sample by an instrument which measures the AC magnetic field by using an oscillating probe, even in the case where the sample generates the AC magnetic field having such a high frequency that an effective spring constant of the probe is not varied.SOLUTION: A probe 111 excited at an excitation frequency ωis magnetized by an AC magnetic field generated from a sample 2. A current modulated by a prescribed frequency is made to flow to a coil 21 of the sample 2, and a modulated AC magnetic field obtained by modulating a fundamental AC magnetic field (angular frequency ω) by an angular frequency ωis given to the probe 111. An effective spring constant of the probe 111 is varied by magnetic interaction of probe magnetization and the modulated AC magnetic field. Oscillation of the probe 111 is detected by a signal extractor 14, and a modulation signal generated in excited oscillation (a signal generated by variation in effective spring constant of the probe due to the modulated AC magnetic field) is extracted from the detection signal. An AC magnetic field response measurement instrument 15 measures an AC magnetic field response of the probe 111 on the basis of the extracted signal.

Description

The present invention relates to an AC magnetic field measuring apparatus that measures an AC magnetic field (or AC magnetic field responsiveness) of a sample (a sample that generates an AC magnetic field by applying an AC signal) using a vibrating probe, and an AC magnetic field measurement Regarding the method.
In particular, the present invention can measure the alternating magnetic field (or alternating magnetic field responsiveness) of the sample even when the sample generates a high-frequency magnetic field (frequency of GHz or higher) whose frequency is higher than several KHz. The present invention relates to an alternating magnetic field measuring apparatus and an alternating magnetic field measuring method.

As shown in FIG. 4, the magnetic characteristics related to the applied magnetic field response of the sample 80 (magnetic head in FIG. 4) can be measured by a magnetic characteristic measuring device (magnetic force microscope) 8.
In the magnetic characteristic measuring device 8, a probe 83 made of a hard magnetic material (hereinafter also referred to as “hard magnetic probe”) 83 formed on the lower surface of the tip of the cantilever 81 is provided at the start end of the cantilever 81. The exciter (piezo element) 82 is mechanically vibrated at an angular frequency ω ₀ (frequency f ₀ (= ω ₀ / (2π)).
The mechanical vibration of the probe 83 at the angular frequency ω ₀ 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. 4, 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. 4, 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 alternating 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 alternating magnetic force is applied to the probe 83. 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 alternating 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 z direction is perpendicular to the surface, a non-resonant alternating 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 probe vibration. Here, by detecting locking cos (ω _s t) component is a phase component with respect to the sample field frequency demodulated signal in the slope of the amplitude of the AC in the vertical direction to the sample surface magnetic field ^{_{H z0 ac cos (ω s t}} ) The distribution of (∂H _z0 ^ac / ∂z) or the derivative of the gradient (∂ ² H _z0 ^ac / ∂z ² ) 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. The present invention has been obtained based on the knowledge that the inconvenience can be eliminated by generating an alternating magnetic field modulated from a sample, exciting the probe magnetization, and utilizing the magnetic interaction between the probe magnetization and the alternating magnetic field. It came to an eggplant.
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, “ Instead of “soft magnetic probe”, a modulated AC magnetic field is applied to the probe, thereby utilizing the magnetic interaction between the time-varying magnetization of the probe and the AC magnetic field from the sample. And the high frequency alternating current magnetic field can be measured by converting the time change of the effective spring constant of the probe device into a low frequency.

The gist of the AC magnetic field measuring apparatus of the present invention is (1) to (5).
(1)
An AC magnetic field measuring apparatus for measuring an AC magnetic field (or AC magnetic field responsiveness) of a sample that generates an AC magnetic field, on which an exciting coil is formed,
A probe device comprising a probe made of a magnetic material, wherein the probe is magnetized by an alternating magnetic field generated from the sample (typically a beam member in accordance with Hooke's law);
An exciter for exciting the probe at an excitation frequency;
A sample driving source for causing an electric current to flow through the coil to generate an alternating magnetic field from the sample, and periodically reversing the magnetization of the probe according to the polarity of the alternating magnetic field by the alternating magnetic field;
The probe vibration is detected, and a signal relating to the mechanical modulation generated in the excitation vibration (the effective (apparent) spring constant of the probe device is changed by the modulation AC magnetic field) from the detection signal. A signal extractor for extracting a signal generated by
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
The drive source has a modulation function for outputting a current obtained by modulating an alternating current having a fundamental frequency at a predetermined frequency;
By passing the modulated current through the coil formed in the sample that generates the alternating magnetic field, the alternating magnetic field generated from the sample is generated,
Reversing the magnetization of the probe according to the polarity of the alternating magnetic field by the modulated alternating magnetic field, and changing the effective spring constant by the modulation component of the modulated alternating magnetic field,
An AC magnetic field measuring apparatus characterized by that.
For example, a magnetic head (magnetic recording head) is a sample that generates an alternating magnetic field and has an exciting coil formed in 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.
The modulation frequency of the modulation alternating magnetic field is set sufficiently smaller than the frequency of the magnetic field generated from the sample.
In the present invention, the magnetization modulation of the probe may be either AM modulation or FM modulation.
(2)
The AC magnetic field measurement apparatus according to (1),
The signal extractor comprises:
A vibration detection device for detecting 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.
(3)
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.
(4)
The AC magnetic field measurement apparatus according to (1),
Furthermore, a scanning mechanism is provided,
The scanning mechanism moves the probe three-dimensionally with respect to the sample;
An AC magnetic field measuring apparatus characterized by that.
The scanning mechanism can learn the surface shape by causing the probe to scan on the surface of the sample.
(5)
The AC magnetic field measurement apparatus according to (4),
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 AC magnetic field measurement method of the present invention is summarized as (6) to (10).
(6)
This is an AC magnetic field measurement method for measuring the AC magnetic field (or AC magnetic field responsiveness) of a sample that generates an AC magnetic field on which an exciting coil is formed, using a probe device having a probe made of a magnetic material. And
An excitation step of exciting the probe at an excitation frequency by an exciter;
A sample coil driving step for generating an alternating magnetic field from the sample by passing an electric current through the coil;
A mechanical modulation generating step for magnetizing the probe by an alternating magnetic field generated from the sample, changing an effective (apparent) spring constant of the probe device, and generating mechanical modulation in the excitation vibration;
A signal extracting step of detecting vibration of the probe and extracting a signal relating to mechanical modulation generated in the excitation vibration from the detection signal; and
A measurement step of measuring an alternating magnetic field (or alternating magnetic field response) of the sample based on the signal extracted by the signal extractor;
Including
In the sample coil driving step, a current obtained by modulating an alternating current having a fundamental frequency with a predetermined frequency is output,
By passing the modulated current through the coil, a modulated alternating magnetic field is generated from the sample,
The modulated alternating magnetic field reverses the magnetization of the probe according to the polarity of the alternating magnetic field, and the magnetic interaction between the probe magnetization and the alternating magnetic field effectively (apparently) the probe device. Change the spring constant,
AC magnetic field measurement method characterized by the above.
(7)
The AC magnetic field measurement method according to (6),
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.
(8)
The AC magnetic field measurement method according to (6),
In the signal extraction step, the piezoelectric element is excited by an AC power source.
AC magnetic field measurement method characterized by the above.
(9)
The AC magnetic field measurement method according to (6),
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.
(10)
The alternating magnetic field measurement method according to (9),
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, the modulated alternating magnetic field applied to the probe is generated by the sample.
The sample is a sample that generates an alternating magnetic field in which an exciting coil is formed.
The AC magnetic field generated by the sample needs to include at least one AC magnetic field component having a different angular frequency in addition to an AC magnetic field having a fundamental angular frequency ω _s (fundamental frequencies: f _s , ω _s = 2πf _s ).
For example, there may be a total of two frequency components including one AC magnetic field component different from the fundamental angular frequency ω _s in the spectrum.
In addition, in amplitude modulation (AM modulation) and narrowband frequency modulation (FM modulation), there may be a case where a total of three frequency components including two alternating magnetic field components different from the basic angular frequency ω _s appear in the spectrum.

First, the case where a total of two frequency components appear in the spectrum will be described below.
In this case, the basic angular frequency ω _s of the alternating magnetic field may be higher than several KHz.
The magnetization of the probe is modulated as follows.
An AC magnetic field having a fundamental angular frequency (in this embodiment, the same angular frequency given to the specimen during actual use) ω _s and an angular frequency slightly different from the fundamental angular frequency ω _s (ω _s −ω _m ) Or (ω _s + ω _m ) (ω _m is a low angular frequency) and a combined alternating magnetic field (modulated magnetic field in the present invention) is applied.
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 alternating magnetic field as described above, an alternating magnetic force having a low angular frequency (modulation frequency) ω _m is induced in the probe. This alternating magnetic force at the low angular frequency ω _m is due to the magnetic interaction between the soft magnetic probe and the alternating magnetic field.
As a result, frequency modulation occurs in the mechanical vibration of the probe. Using this 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 is measured. Is possible.

A magnetic field obtained by adding an alternating magnetic field (modulated magnetic field of angular frequency (ω _s + ω _m )) slightly different from the basic angular frequency ω _{s to} an alternating magnetic field (high frequency magnetic field) of the basic angular frequency ω _s. Is generated perpendicular to the sample surface, and the AC magnetic field (or AC magnetic field responsiveness) of the sample is changed to a single magnetic pole type (a magnetic probe mode in which the magnetic force received by the magnetic pole at the tip of the probe is the main). The case of measuring with a probe will be described.
The alternating magnetic field (modulated magnetic field) H _z (t) perpendicular to the sample surface is expressed as follows.
H _z (t) = H _sample ^ac (t)
= H _z0 ^ac cos (ω _s t) + _{αH z0} ^ac cos (ω _s + ω _m ) t (Formula 1)
H _z0 ^ac : Amplitude of component perpendicular to sample surface among AC magnetic field (basic angular frequency ω _s ) generated from sample α: Modulation degree of AC magnetic field from sample due to angular frequency ω _s + ω _m component ω _s : Sample Frequency of alternating magnetic field generated from ω _m : Modulation angular frequency

By this alternating magnetic field H _z (t), the soft magnetic probe is excited by the alternating magnetic field generated from the sample.
When measuring the AC magnetic field (or AC magnetic field responsiveness) of the sample, it is necessary to reduce the distance between the probe and the sample (distance between the probe and the sample) in order to improve the spatial resolution. As the probe-to-sample distance decreases, the magnetic behavior of the probe changes from the double-pole type (the 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 Change to magnetic pole type.
At this time, the density of the magnetic pole generated at the tip of the probe changes with time as shown in the following equation in response to the alternating magnetic field.
ρ (t) = ρ _tip ^ac (t)
= Ρ ₀ ^ac cos (ω _s t) + βρ ₀ ^ac cos (ω _s + ω _m ) t (Formula 2)
ρ ₀ ^ac : Amplitude of the magnetic pole density at the tip of the probe (probe magnetic pole) due to the angular frequency ω _s component β: Modulation degree of the probe magnetic pole due to the angular frequency ω _s ± ω _m component

The degree of modulation of the magnetic pole of the probe is generally 0 <= β <= α <1. When the intensity of the alternating magnetic field ( _{αH z0} ^ac cos (ω _s + ω _m ) t: modulation magnetic field added to the alternating magnetic field (high frequency magnetic field) generated from the sample) is low, the modulation degree of the probe magnetic pole is Usually smaller than the modulation degree of the alternating magnetic field.
From (Equation 1) and (Equation 2), 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 t) + βρ ₀ ^ac cos (ω _s + ω _m ) t}
^{_{{(∂H z0 ac / ∂z)}} cos (ω s t)
+ Α (∂H _z0 ^ac / ∂z) cos (ω _s + ω _m ) t} (Formula 3)
From (Equation 3), the effective spring constant component Δk _low including DC is given by the following equation.
Δk _low = {1/2 + (α + β) / 2} ρ ₀ ^ac (∂H _z0 ^ac / ∂z)
+ {(Α + β) / 2} ρ ₀ ^ac (∂H _z0 ^ac / ∂z) cos (ω _m t) (Formula 4)

(Equation 4) 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, 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.

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.

In the above example, the AC magnetic field applied to the probe is an AC magnetic field having two frequency components ω _s and (ω _s + ω _m ).
Next, as a representative example of an alternating magnetic field having three frequency components, an amplitude-modulated alternating magnetic field having angular frequency components of ω _s and (ω _s + ω _m ) and (ω _s −ω _m ) is used as the modulating magnetic field. The case will be described.

At this time, the alternating magnetic field applied to the probe is given by the following equation.
H _z (t) = H _sample ^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 5)
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 t) + βρ ₀ ^ac cos (ω _s + ω _m ) t
+ Βρ ₀ ^ac cos (ω _s −ω _m ) t (Formula 6)

From (Equation 5) and (Equation 6), 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 t) + βρ ₀ ^ac cos (ω _s + ω _m ) t
+ Βρ ₀ ^ac cos (ω _s −ω _m ) 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 7)

From (Equation 7), the effective spring constant component Δk _low including DC is expressed by the following equation.
Δk _low = {1/2 + (α + β)} ρ ₀ ^ac (∂H _z0 ^ac / ∂z)
+ (Α + β) ρ ₀ ^ac (∂H _z0 ^ac / ∂z) cos (ω _m t) (Formula 8)
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 having the two frequencies 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. 3A is a schematic diagram of the magnetic recording head, FIG. 3B is a perpendicular magnetic field image (Lock-in X image) acquired immediately above the main magnetic pole for perpendicular magnetic recording, and FIG. It is the cross-sectional profile. FIG. 4 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 drive source 13, a signal extractor 14, an AC magnetic field response measuring instrument 15, a scanning mechanism 16, and an image. And a display device 17.

The probe device 11 is a cantilever in FIG. 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 the present embodiment, the probe 111 is formed by covering a silicon probe with FeCo.

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 drive source 13 includes a first drive source (current source or voltage source) 131 that outputs an alternating current having an angular frequency ω _s and a second drive source (current source or voltage source) that outputs a signal having a low angular frequency ω _m. 132 and a combiner 133 that combines the output from the first drive source 131 and the output from the second drive source 132.
AC signal of the angular frequency omega _s applied to the sample 2 (current), the angular frequency omega is modulated at a sufficiently low modulation angular frequency omega _m than _s, the AC signal i of the modulated angular frequency (ω _s ± ω _m) Is given to sample 2.
When the AC signal i modulated with the modulation angular frequency ω _m is supplied, the sample 2 has an AC magnetic field H ₀ cos {() in which the AC magnetic field H ₀ cos (ω _st t} is modulated with the modulation angular frequency ω _m. ω _s ± ω _m ) t}.
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. However, when the AC magnetic field is applied, the angular frequency periodically varies due to the magnetic interaction between the probe magnetization and the AC magnetic field. Subject to changing frequency modulation. 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. The lock-in amplifier 151 receives a reference signal from the second drive source 132 for synchronization.
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). 171 can be stored. 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 processing in steps S110 to S200 shown in FIG. 2 is performed by each component (probe device 11, exciter 12, drive source 13, signal extractor 14, AC magnetic field response measuring device in the AC magnetic field measuring device of the present invention described above. 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 drive source 13 modulates an AC signal having an angular frequency ωs with a modulation angular frequency ω _m lower than this frequency, and drives the sample 2 with this modulation signal (AC current i). The sample 2 generates an alternating magnetic field (modulated alternating magnetic field) having a modulation frequency corresponding to the modulated signal (alternating current i) (S120).
The exciter 12 excites the probe 111 (mechanically vibrates), and excites the magnetization of the probe 111 with a modulated alternating magnetic field. The magnetization of the probe excited by the alternating magnetic field changes the effective spring constant of the probe by magnetic interaction with the alternating magnetic field, thereby generating frequency modulation in the probe vibration. (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 that constitutes a part of the signal extractor 14 demodulates a modulation signal (signal by frequency 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 response of the sample 2 based on the demodulated signal (S160). The AC magnetic field responsiveness of the sample 2 is measured from the degree of frequency modulation that occurs in 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).

《Experimental example》
Below, the experiment example in this embodiment is shown.
(A)
The conditions of this experimental example are as follows.
Sample 2 is a magnetic head.
The probe 111 was configured by forming a 30 nm FeCo film on a Si probe.
The frequency of the alternating magnetic field is 700 MHz.
The modulation method of the alternating magnetic field is amplitude modulation (AM), the modulation frequency is 70 Hz, and the modulation degree is 90%.

(B)
In the experiment, the probe 111 was excited with the above-described amplitude-modulated magnetic field, and an alternating magnetic force of 70 Hz was generated on the probe 111.
A mechanical modulation signal (frequency modulation signal (FM)) of the probe 111 generated by this alternating magnetic force was measured in the air at room temperature.

(C)
FIG. 3A shows a schematic diagram of a magnetic recording head (sample 2) in which an alternating magnetic field was measured. The magnetic flux Φ of the AC magnetic field generated from the magnetic recording head flows from the main magnetic pole 22 surrounded by the coil 21 to which an AC signal is applied toward the return yoke 23. Here, in the gap Gap portion between the main magnetic pole and the return yoke, the magnetic field component perpendicular to the head surface is zero and the magnetic field component parallel to the head surface is maximum.
FIG. 3B shows a vertical magnetic field image (Lock-in X image) acquired immediately above the main magnetic pole 22 and the return yoke 23 of the magnetic recording head (sample 2), and FIG. 3C shows the cross-sectional profile thereof.
As shown in FIG. 3B, the contrast of the magnetic field image is reversed in the down-track direction from the main magnetic pole 22 toward the return yoke 23 with the gap indicated by the solid line interposed therebetween.
It is clearly observed that the magnetic flux Φ from the main magnetic pole PM flows to the return yoke RY side across the gap.
As shown in FIG. 3C, the signal value of the Lock-inX image is positive on the main pole side, zero in the gap gap region, and negative on the return yoke 23 side. The magnetic flux flow indicated by is clearly measured.

DESCRIPTION OF SYMBOLS 1 AC magnetic field measuring device 2 Sample 11 Probe device 12 Exciter 13 Drive source 14 Signal extractor 15 AC magnetic field response measuring device 16 Scanning mechanism 17 Image display device 21 Coil 22 Main magnetic pole 23 Return yoke 111 Probe 112 Cantilever 121 AC power supply 122 Piezoelectric element
131 First Drive Source 132 Second Drive Source 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 Optical Sensor ( Photodiode)
gp21211 gap between main pole and return yoke Φ magnetic flux

Claims (10)

An AC magnetic field measuring apparatus for measuring an AC magnetic field (or AC magnetic field responsiveness) of a sample that generates an AC magnetic field, on which an exciting coil is formed,
A probe device comprising a probe made of a magnetic material, wherein the probe is magnetized by an alternating magnetic field generated from the sample;
An exciter for exciting the probe at an excitation frequency;
A sample driving source for causing an electric current to flow through the coil to generate an alternating magnetic field from the sample, and periodically reversing the magnetization of the probe according to the polarity of the alternating magnetic field by the alternating magnetic field;
The probe vibration is detected, and a signal relating to the mechanical modulation generated in the excitation vibration from the detection signal (a 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
The drive source has a modulation function for outputting a current obtained by modulating an alternating current having a fundamental frequency at a predetermined frequency;
By passing the modulated current through the coil formed in the sample that generates the alternating magnetic field, the alternating magnetic field generated from the sample is generated,
Reversing the magnetization of the probe according to the polarity of the alternating magnetic field by the modulated alternating magnetic field, and changing the effective spring constant by magnetic interaction between the probe magnetization and the alternating magnetic field,
An AC magnetic field measuring apparatus characterized by that. The AC magnetic field measurement apparatus according to claim 1,
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,
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,
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 4,
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. This is an AC magnetic field measurement method for measuring the AC magnetic field (or AC magnetic field responsiveness) of a sample that generates an AC magnetic field on which an exciting coil is formed, using a probe device having a probe made of a magnetic material. And
An excitation step of exciting the probe at an excitation frequency by an exciter;
A sample coil driving step for generating an alternating magnetic field from the sample by passing an electric current through the coil;
A mechanical modulation generating step of periodically reversing the magnetization of the probe by an alternating magnetic field generated from the sample, changing an effective spring constant of the probe device, and generating mechanical modulation in the excitation vibration;
A signal extracting step of detecting vibration of the probe and extracting a signal relating to mechanical modulation generated in the excitation vibration from the detection signal; and
A measurement step of measuring an alternating magnetic field (or alternating magnetic field response) of the sample based on the signal extracted by the signal extractor;
Including
In the sample coil driving step, a current obtained by modulating an alternating current having a fundamental frequency with a predetermined frequency is output,
By passing the modulated current through the coil, a modulated alternating magnetic field is generated from the sample,
In the mechanical modulation generation step, the effective spring constant is changed by magnetic interaction between the probe magnetization and the alternating magnetic field, and the mechanical modulation is generated.
AC magnetic field measurement method characterized by the above. The AC magnetic field measurement method according to claim 6,
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 6,
In the signal extraction step, the piezoelectric element is excited by an AC power source.
AC magnetic field measurement method characterized by the above. The AC magnetic field response measuring method according to claim 6,
Further comprising a scanning step,
In the scanning step, the probe is three-dimensionally moved with respect to the sample.
AC magnetic field response measuring method characterized by the above. The AC magnetic field measurement method according to claim 9,
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.