JP4174228B2 - Frequency drift tracking method and magnetic resonance force microscope having the tracking function - Google Patents

Description

【００２４】
以下では、最も簡単かつ有効な２個所の周波数位置で測定した雑音密度強度からω_n のドリフト量を見積もる方法を説明する。
【００２５】
〔数１〕で記述された雑音密度強度：Ｓ（ω）がＳ_max ／２になる周波数位置は、回路のＱ値を定義する位置でもあり、中心周波数ω_n から±ω_n ／２Ｑだけ離れた周波数で与えられる。これを
【００２６】
【数２】

【００２７】
と定義する（図１のグレー丸で表される２点）。
【００２８】
ある時刻：ｔ＝ｔ₀ で測定した周波数位置：ω_lower （ｔ₀ ）とω_higher（ｔ₀ ）における雑音密度強度をそれぞれＳ（ω_lower （ｔ₀ ），ｔ₀ ）とＳ（ω_higher（ｔ₀ ），ｔ₀ ）とすれば、ドリフトが存在しない場合、両強度比
【００２９】
【数３】

【００３０】
はＲ（ｔ₀ ）＝１である。
【００３１】
一方、時刻ｔ＝ｔ₁ でドリフトが発生した場合（図１の点線で表されるスペクトル）、そのドリフト量：ｄωを
【００３２】
【数４】

【００３３】
として、〔数１〕は、
【００３４】
【数５】

【００３５】
と書き表せる。ドリフトした雑音密度強度：Ｓ（ω，ｔ₁ ）を、先に定義したω_lower （ｔ₀ ）とω_higher（ｔ₀ ）の周波数位置で測定しその比をとると
【００３６】
【数６】

【００３７】
となるが（図１の黒丸で表される２点）、この量は、先に測定した共振／共鳴周波数：ω（ｔ₀ ）からのドリフト量を反映した情報を含む。
【００３８】
特にαが小さい場合、αに関するテイラー展開１項目で〔数６〕をよい精度で近似でき、その結果〔数４〕のドリフト量は、
【００３９】
【数７】

【００４０】
で与えられる。比：Ｒを評価するので、雑音密度の強度変化の影響は理論上排除することができる。
【００４１】
Ｑ値に関し大きな変化がないと仮定すれば、〔数７〕によって、先に測定したω_n （ｔ_i ）とＱ値から評価される周波数２個所
【００４２】
【数８】

【００４３】
で雑音密度を測定し、その比：Ｒ（ｔ_i+1 ）を議論することでドリフト量が評価でき、逐次最新の共振／共鳴周波数を知ることができる。
【００４４】
以上の説明では、測定ポイントを２個所としたが、雑音密度の強度に大きなドリフト・揺らぎがなければ、１個所の測定でも原理上可能である。また、複数個所での測定、及び共振／共鳴周波数とＱ値から定義される周波数位置ではなく、雑音密度スペクトルの任意のポイント、例えば裾野に位置するポイントで強度を測定してもよい。
【００４５】
さらに、測定周波数バンド幅を狭めたり、雑音密度スペクトルの裾野を測定したりすれば、より高いＱ値を持つ振動子に対してもドリフトモニターが可能である。取得した雑音密度強度、又は見積もられた周波数ドリフトの不確定性が大きい場合には、過去に測定したそれらの複数個のデータを重み付け平均して現時点の雑音密度強度、又は周波数ドリフトを評価することも可能である。
【００４６】
次に、本発明に係る雑音密度測定による周波数ドリフト追跡装置について説明する。図２は本発明に係る雑音密度測定による周波数ドリフト追跡装置の実施の形態を示す図、図３は本発明に係る雑音密度測定による周波数ドリフト追跡装置の他の実施の形態を示す図、図４は今回実施するために用いた共振／共鳴回路の詳細構成例を示す図である。図中、１は共振／共鳴回路、２は交流信号増幅器、３はバンドパスフィルタ、４は位相検波用参照信号、５は交流信号強度測定器、６は読み取り用レコーダ、７はロックインアンプ、１１はカンチレバー、１２は光ファイバ干渉計、１３はフォトダイオード、１４は電流増幅器を示す。
【００４７】
図２において、バンドパスフィルタ３又は位相検波器は、位相検波に必要な安定した位相検波用参照信号４が供給され、特定の周波数成分のみを抽出するものであり、それは、信号が出現する共振／共鳴周波数位置から適度の周波数分だけ離れ、つまり、信号が出現しても雑音密度強度に変化が生じない周波数位置である。交流信号増幅器２は、共振／共鳴回路１が発生する雑音を増幅し、交流信号強度測定器５は、バンドパスフィルタ３又は位相検波器の抽出した特定の周波数成分の強度を測定し、その雑音密度測定による周波数ドリフトを評価／追跡するのが読み取り用レコーダー６、又はコンピュータである。
【００４８】
このような構成の雑音密度測定による周波数ドリフト追跡装置により、共振／共鳴回路１が発生する雑音は、交流信号増幅器２によって増幅され、狭域のバンドパスフィルタ３又は位相検波器を通して特定の周波数成分のみを抽出し、交流信号強度測定器５によって測定して、読み取り用レコーダー６、又はコンピュータに格納することにより、周波数ドリフトが評価／追跡される。したがって、検出すべき目的の信号が共振／共鳴周波数位置（黒く鋭いピーク）に現れても、信号出現の影響がない周波数位置で雑音強度が測定される。
【００４９】
バンドパスフィルタ３又は位相検波器として、図３に示すようにより現実的にロックインアンプ７を用い、外部から位相検波に必要な安定した位相検波用参照信号４を供給する構成がある。ロックインアンプ７の中には雑音密度測定モードが既に用意されているものもあり、ロックインアンプ７の時定数と周波数フィルタスロープから決定されるバンド幅によって位相検波用参照信号４の周波数を中心に雑音密度強度が測定される。
【００５０】
具体的な実施例として、原子間力顕微鏡に使用されているカンチレバーの自由振動状態の振動雑音密度を測定し、カンチレバーの共振周波数ドリフトをモニターした。カンチレバーは非常に高いＱ値を持つ振動子である。自由振動状態で、カンチレバーの振幅は、１０^-9ｍ以下であり、振動を誘起する外部信号を与えない限りその共振周波数を直接見積もることは難しい。しかし、以下に示すように雑音密度を精度よく測定することで、共振現象のパラメータを取得／評価することができる。
【００５１】
ドリフトモニターのための測定周波数位置は、雑音密度強度がＳ_max ／２となる２個所で行った。つまり、先に説明した方法を実行した。また、過去に得られた周波数ドリフト量をデータベースとして保存しておき、ｔ＝ｔ₁ 時のドリフト量を過去２点の周波数ドリフトを含めた次の〔数９〕で定義される重み付け平均で近似した。
【００５２】
【数９】

【００５３】
上記共振／共鳴回路１の具体的な構成を、実施例に沿って示したのが図４である。図４において、カンチレバー１１の振動振幅強度は、レーザ光を用いた光てこ法、又は光ファイバーを用いた光ファイバー干渉計１２によって検出用フォトダイオード１３に導かれ、誘起された光干渉強度は、電流増幅器１４を通して増幅され検出される。
【００５４】
実際の測定においては、まず始めにカンチレバー１１の共振状態を記述するパラメータを取得するために雑音密度スペクトルを測定しておく。図５は本発明の検証に使用したカンチレバー１１の雑音密度スペクトルを示す図である。共振周波数：約１０ｋＨｚ、Ｑ値：約９５０が得られた他、√Ｓ_max ＝０．７５Å√Ｈｚの雑音が存在することを確認した。
【００５５】
図６は本発明に係る方法を用いてカンチレバーに対するドリフト量：黒丸の経時変化と、測定開始時に取得した共振周波数（固定）位置で測定した雑音密度：白丸の経時変化をプロットした図である。測定開始時の共振周波数は、１００分後にはドリフトの影響によりほぼ完全に共振領域外になってしまっていることが判る。
【００５６】
図７は同じカンチレバーに対し本発明に係る方法によって見積もられたドリフト量を測定開始時の共振周波数に加えた周波数（可変）位置（追跡モード）で雑音密度を測定した結果を示す図である。４２０分＝７時間後の共振周波数ドリフト量：黒丸は、２５０Ｈｚ、測定開始時と比べ２．５％にもなるが、モニターしている共振／共鳴周波数で測定した雑音密度：白丸は、測定開始時に測定された雑音密度強度と比べ、７時間後でも標準偏差：約５％で一定となった。図８は図７に示した同じ雑音密度データをヒストグラム表示した図である。ガウス型分布をしていることから、雑音密度測定における精度を上げることでドリフト量をより精度よく見積もることが可能であることを示唆している。
【００５７】
図９は共振／共鳴周波数のドリフト量の経時変化に対する理論式は存在しないので、多項式フィットを行い、フィッティング曲線からのズレをヒストグラム表示した図である。標準偏差：１．５Ｈｚを持つことが判った。
【００５８】
次に、周波数ドリフト追跡機能を備えた磁気共鳴力顕微鏡などの装置について説明する。図１０は本発明に係る周波数ドリフト追跡機能を備えたＭＲＦＭ装置の実施の形態を示す図、図１１は本発明に係る周波数ドリフト追跡機能を備えたＭＲＦＭ装置の他の実施の形態を示す図である。
【００５９】
試料に対してカンチレバーの先端に装着した磁気チップと外部磁石により静磁場を与え、高周波コイルにより高周波磁場を照射して磁気共鳴力強度を観察する磁気共鳴力顕微鏡において、図１０に示すように、コンピュータ２１は、上記雑音密度測定による周波数ドリフト追跡法を用いて、カンチレバーの共振周波数のドリフトをモニター、評価する。そして、取得した共振周波数情報は、各周波数のもつ信号を出力する信号発信器２２〜２４に送る。この信号発信器２２〜２４は、非調和変調法の場合、高周波ＡＭ変調周波数、磁場変調（ＨＭ）周波数、及び位相検波器の参照信号周波数の３種の周波数を発生させる信号発生器が該当する。位相同期用信号発生器２５は、全ての周波数情報が正しく送られた後に、ある時刻にて位相を一致させるための基準信号（例えばトリガーバルス）を各信号発生器２２〜２４に送る。各信号発生器は内部基準クロックを同期化させておくと、長時間に渡って位相のドリフトを最小限にする事ができるので、より一層の安定性が得られる。
【００６０】
図１１に示す実施形態では、図１０に示す実施形態の場合と同様に、コンピュータ３１によりモニターしているカンチレバーの共振周波数を基に決定された高周波（ＡＭ）変調周波数：ｆ_AM、磁場変調（ＨＭ）周波数：ｆ_HMを、該当する信号発生器３２、３３に設定する。そして、これら２つの信号出力を分岐させ、周波数ミキサー３５によってｆ_AM＋ｆ_HMとｆ_AM−ｆ_HMを合成し、フィルター３６を通して位相検波に必要な周波数成分を位相検波器３９に送る。位相同期用信号発生器３４は、図１０に示す実施形態の場合と同様に、各信号発生器３２、３３に、それらの位相をある時刻で同期させるための信号（例えばトリガーパルス）を送る。本実施形態においても、各信号発生器３２、３３は、内部基準クロックを同期化させておくと、長時間に渡って位相のドリフトを最小限にする事ができるので、より一層の安定性が得られる。必要とあらばアンプ４０を挿入し、位相検波に十分な参照信号強度を得るようにすればよい。
【００６１】
上記実施形態では、ＡＭ変調用周波数と磁場変調用周波数を使って、位相検波に必要な参照信号を周波数ミキサー３５とフィルター３６を使って合成した例を示したが、他の組み合わせ、例えば、ＡＭ変諷周周波数と位相検波周波数を使って、磁場変調用周波数を周波数ミキサー３５とフィルター３６を使って合成する事も可能であり、磁湯変調用周波数と位相検波周波数を使って、ＡＭ変調用周波数を周波数ミキサー３５とフィルター３６を使って合成する事も可能である。
【００６２】
以下に、実施形態で紹介した装置構成で取得した室温における検証実験結果を示す。ＭＲＦＭ信号測定モードは、非調和変調法であり、試料（ＤＰＰＨ）を載せたステージを空間的に１次元走査し、信号を観測した。測定開始時のカンチレバーは、共振周波数：約１１．８７６ｋＨｚ、Ｑ値：約１０００であった。
【００６３】
図１２は装置を校正した後に、モニターを開始したカンチレバー共振周波数の経時変化（図１２白丸：○）を示す図ある。校正が終了した時点からモニターしているカンチレバー共振周波数は７時間後には１００Ｈｚものドリフトを示しており、Ｑ値１０００を持つ共振状態下での測定環境は周波数追跡が無い限り容易に壊されてしまう。一方、共振周波数のドリフトを雑音密度測定による周波数ドリフト追跡法に基づきモニターした結果が、図１２黒丸：●で示してある。この結果では、標準備差１．３９を示し、共振周波数：約１１．８７６ｋＨｚに対し±１×１０^-4の精度で共振周波数を追跡できている事を示している。図１２内矢印は、以下に示す測定を開始した時点を示したものである。
【００６４】
図１３は測定開始位置（Ｚ＝２０μｍ）で最新のカンチレバー共振周波数に更新し、測定終了位置（Ｚ＝５２μｍ）に到達した後、ステージを測定開始位置に戻しカンチレバー共振周波数を更新し走査を繰り返すと言う方法で取得したスペクトルを示す図である。繰り返し回数は２０回、全測定時間は３０分である。測定中に位相の同期化を行っていないため、積算後の結果は、ノイズと共に消失してしまっている。
【００６５】
図１４は位相の同期化を行った結果を示す図である。また、繰り返し回数は５０回、全測定時間は１時間を越えるが、ＭＲＦＭ信号自身は減少することなく、ノイズ成分のみ減少させることができた。
【００６６】
本発明の方法により、共振周波数を取得するために測定を中断すると言う不必要な時間／工程を排除し、いつでも共振周波数を１０^-4と言う高い精度でモニターする事ができ、測定にかかる全時間を短縮する事ができた。また、意図的にＱ値を下げ信号検出感度を損ねる代わりに、微弱信号を常に高いＱ値を持つ最新の共振状態下で連続的に測定できる環境を与える事ができた。ＭＲＦＭ測定結果からＭＲＩ画像を得るために適した非調和変調法において、必要とされる３種の周波数信号に対し同期パルスによる位相同期化を行うことで、スペクトルを取得することが出来た。ドリフトが発生する環境下にて長時間の積算を行える測定環境が実現され、極微なＭＲＦＭ信号を効率良く、且つ、安定して測定できた。
【００６７】
【発明の効果】
以上の説明から明らかなように、本発明によれば、共振／共鳴状態下で信号を測定する装置の周波数ドリフト追跡法であって、共振／共鳴周波数位置近傍で雑音密度の信号を測定し、信号が出現しても雑音密度強度に変化が生じない周波数位置で雑音密度強度を随時モニターし、測定開始前の雑音密度スペクトルを基に共振／共鳴周波数のドリフト量を見積もって信号出現周波数にフィードバックをかけることにより、共振／共鳴周波数のドリフトを追跡するので、信号が出現する共振／共鳴周波数位置から適度な周波数分だけ離れた、つまり、信号が出現しても雑音密度強度に変化が生じない周波数位置で雑音密度強度を随時モニターし、測定開始前に雑音密度スペクトルを基に共振／共鳴周波数のドリフト量を見積もって信号出現周波数にフィードバックをかけることができる。
【００６８】
また、試料に対してカンチレバーの先端に装着した磁気チップと外部磁石により静磁場を与え、高周波コイルにより高周波磁場を照射して磁気共鳴力強度を観察する磁気共鳴力顕微鏡において、前記カンチレバーの共振周波数位置近傍で雑音密度の信号を測定し、信号が出現しても雑音密度強度に変化が生じない周波数位置で雑音密度強度を随時モニターし、測定開始前の雑音密度スペクトルを基に共振周波数のドリフト量を見積もって信号出現周波数にフィードバックをかけることにより、共振周波数のドリフトを追跡して共振周波数を基に決定される信号発生手段の周波数を更新し、さらに位相同期信号発生手段を備え、該位相同期信号発生手段により前記信号発生手段の位相を同期させるので、各測定場所で信号検出を始める前に最新の共振周波数に更新すると同時に、３種の周波数を同期させ、測定開始から終了までカンチレバー共振周波数を更新しながら位相情報を一定に保つことができる。
【００６９】
本発明によれば、測定を中断し共振／共鳴周波数を取得するために必要な時間／工程を排除して、いつでも共振／共鳴周波数をモニターすることができ、測定にかかる全時間を短縮することができる。また、意図的にＱ値を下げ信号検出感度を損ねる代わりに、微弱信号をより高いＱ値を持つ共振／共鳴状態下で測定できる環境を与えることができる。
【００７０】
さらに、ＭＲＦＭ測定結果からＭＲＩ画像を得るために適した非調和変調法において、必要とされる３種の周波数信号に対し同期パルスによる位相同期化を行うことで、スペクトルを取得することができ、ドリフトが発生する環境下にて長時間の積算を行える測定環境が実現され、極微なＭＲＦＭ信号を効率よく且つ安定して測定することができる。
【図面の簡単な説明】
【図１】 共振／共鳴回路の雑音スペクトルとドリフト量を見積もる方法を説明するための図である。
【図２】 本発明に係る雑音密度測定による周波数ドリフト追跡装置の実施の形態を示す図である。
【図３】 本発明に係る雑音密度測定による周波数ドリフト追跡装置の他の実施の形態を示す図である。
【図４】 Ｒ；ＷＬ実施するために用いた共振／共鳴回路の詳細構成例を示す図である。
【図５】 本発明の検証に使用したカンチレバー１１の雑音密度スペクトルを示す図である。
【図６】 本発明に係る方法を用いてカンチレバーに対するドリフト量：黒丸の経時変化と、測定開始時に取得した共振周波数（固定）位置で測定した雑音密度：白丸の経時変化をプロットした図である。
【図７】 同じカンチレバーに対し本発明に係る方法によって見積もられたドリフト量を測定開始時の共振周波数に加えた周波数（可変）位置（追跡モード）で雑音密度を測定した結果を示す図である。
【図８】 図７に示した同じ雑音密度データをヒストグラム表示した図である。
【図９】 共振／共鳴周波数のドリフト量の経時変化に対する理論式は存在しないので、多項式フィットを行い、フィッティング曲線からのズレをヒストグラム表示した図である。
【図１０】 本発明に係る周波数ドリフト追跡機能を備えたＭＲＦＭ装置の実施の形態を示す図である。
【図１１】 本発明に係る周波数ドリフト追跡機能を備えたＭＲＦＭ装置の他の実施の形態を示す図である。
【図１２】 装置を校正した後に、モニターを開始したカンチレバー共振周波数の経時変化（図１２白丸：○）を示す図である。
【図１３】 測定開始位置（Ｚ＝２０μｍ）で最新のカンチレバー共振周波数に更新し、測定終了位置（Ｚ＝５２μｍ）に到達した後、ステージを測定開始位置に戻しカンチレバー共振周波数を更新し走査を繰り返すと言う方法で取得したスペクトルを示す図である。
【図１４】 位相の同期化を行った結果を示す図である。
【図１５】 ＭＲＦＭ装置の簡略化した検出部の構成を示す図である。
【符号の説明】
１…共振／共鳴回路、２…交流信号増幅器、３…バンドパスフィルタ、４…位相検波用参照信号、５…交流信号強度測定器、６…読み取り用レコーダ、７…ロックインアンプ、１１…カンチレバー、１２…光ファイバ干渉計、１３…フォトダイオード、１４…電流増幅器[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a frequency drift tracking method of an apparatus for measuring a signal under a resonance / resonance state and a magnetic resonance force microscope having the tracking function.
[0002]
[Prior art]
In the measurement method that relies on the resonance / resonance phenomenon, the weak signal appearing at the resonance / resonance frequency position at that time is amplified by a Q value. Therefore, in a device that measures signals under the resonance / resonance state of an electrical / mechanical circuit having a very high Q value, if drift occurs due to the influence of ambient temperature changes, measurement with a high Q value can be performed. Disappear.
[0003]
As an apparatus for measuring a signal under a resonance / resonance state, for example, there is a magnetic resonance force microscope (MRFM). MRFM combines the technology of the conventional image processing device using magnetic resonance: MRI (Magnetic Resonance Imaging) and the atomic force microscope capable of observing the atomic image of the sample surface: AFM (Atomic Force Microscopy). This MRI apparatus is expected to have a spatial resolution at the atomic level. It is a developing device currently under development by several groups, and the current spatial resolution is tens of microns (see, for example, Stripe et. Al. Physical Review Letters, 87 277602-1 (2001)). is there. The purpose of use, which also serves as the final goal, is quantitative analysis mainly for imaging and analyzing the three-dimensional structure of a single gene, protein, biomolecule and other very small samples.
[0004]
FIG. 15 is a diagram showing the configuration of a simplified detection unit of the MRFM apparatus. 41 is an optical fiber, 42 is a magnetic chip, 43 is a cantilever, 44 is a sample stage, 45 is a high-frequency coil, and 46 is a sample. Known technical documents relating to MRFM include, for example, US Pat. No. 5,266,896, Japanese Patent Publication No. 7-69280, Japanese Society of Applied Magnetics Vol. 22, No. 1, p. 19, (1998), etc., and its configuration is composed of AFM components and elements necessary for magnetic resonance as shown in FIG.
[0005]
The components of the AFM are a laser that passes through the optical fiber 41, a cantilever 43 with a magnetic chip 42 attached to the tip, and a sample stage 44. On the other hand, elements necessary for magnetic resonance are a radio frequency (RF) coil 45 and an abbreviation. It is an external static magnetic field. The magnetic field gradient essential for MRI is created by a magnetic field with extremely poor spatial uniformity generated by the magnetic tip 42 of a high permeability magnetic material (including a permanent magnet) attached to the tip of the cantilever 43. The MRFM apparatus composed of these elements operates as follows.
[0006]
The magnetic resonance phenomenon in the MRFM is a resonance determined by a unique relationship between a static magnetic field defined by the sum of a magnetic field applied from the outside and a magnetic field generated by the magnetic chip 42 and a frequency of a high-frequency magnetic field irradiated by the RF coil 45. Occurs when the condition is met. When the resonance condition is not satisfied, the cantilever 43 feels the magnetic force given by the product of the magnetization of the sample 46 polarized by the static magnetic field and the magnetic field gradient generated by the magnetic chip 42, and the magnetic field and the magnetic field gradient are changed. Deflection from an equilibrium position defined when not present. When the resonance condition occurs, the magnetic force is weakened by the decreased polarization magnetization, and the cantilever 43 returns to the position direction in the equilibrium state. The change in force generated at this time is called magnetic resonance force.
[0007]
The measurement amount in the MRFM is the amplitude displacement amount of the cantilever 43. In the AFM, the displacement amount is measured by using an optical interference method or an optical lever method which are established techniques. By scanning the relative position between the magnetic chip 42 and the sample 46, the magnetic resonance force intensity distribution at each location can be obtained. This magnetic resonance force intensity distribution is subjected to computer processing while taking into account the known magnetic field distribution and magnetic field gradient distribution, thereby reproducing the real space image.
[0008]
[Problems to be solved by the invention]
Conventionally, in a device that detects a signal by a method that relies on a resonance / resonance phenomenon, it is necessary to know the resonance / resonance frequency of the device before measurement. However, many measurement means do not acquire the subsequent resonance / resonance frequency or optimize the device to the resonance / resonance frequency each time. The current situation is. Changes in resonance / resonance frequency that may occur during measurement: Since drift is not taken into account, if drift always occurs in the resonance / resonance frequency, the signal strength that is finally obtained decreases.
[0009]
On the other hand, it is possible to excite the resonance / resonance circuit and acquire the frequency, but in this case, it is also necessary to suspend the measurement. However, there has been no simple method for monitoring and acquiring the resonance / resonance frequency while continuing the measurement for signal detection.
[0010]
In order to make it less sensitive to the resonance / resonance frequency drift described above, the Q value, which is one of the parameters describing the resonance / resonance phenomenon, was intentionally lowered to create a wide resonance / resonance state. . In this case, it becomes less sensitive to resonance / resonance frequency drift, but the benefit of using the resonance / resonance state, that is, an environment in which a so-called weak signal can be amplified by a Q value, is lost.
[0011]
The principle of the signal detection method in the MRFM measurement is based on the weak magnetic resonance force (<10 using the mechanical resonance phenomenon of the cantilever. ^-15 N) is amplified and the cantilever resonance frequency component is detected by the phase detector. Since the resonance state of the cantilever has an extremely high Q value, a high mechanical amplification effect can be obtained, while the environment has a little It is difficult to accurately maintain the resonance state for a long time due to the drift of the cantilever frequency caused by a change or the like. For this purpose, the MRFM signal detector is kept at a low temperature to minimize the effect of temperature changes, and a damping circuit that intentionally reduces the Q value of the cantilever is introduced to avoid a decrease in the strength of the MRFM signal due to drift. It was. However, in the former case, it is necessary to construct a low temperature environment, and there remains a problem in operability. In the latter case, the MRFM measurement method that relies on mechanical resonance is not a preferable choice.
[0012]
The IBM group performed phase detection at the cantilever frequency by applying magnetic field modulation at half the cantilever frequency while using a method in which the cantilever was vibrated by the excitation signal input and the mechanical resonance frequency was measured with a frequency counter. Rugar et al., Nature, 360,563 (`92)). According to this measurement method, it is possible to obtain a spectrum corresponding to the second derivative of dispersion: χ ′ and absorption: χ ″ describing the magnetic resonance phenomenon. However, it is necessary to temporarily stop the measurement in order to obtain the cantilever frequency. In addition, a vibrator having a high Q value: the cantilever must be excited during measurement, which may cause physical defects / destruction.
[0013]
In addition, in order to obtain an MRI image of the specimen which is the final purpose of MRFM, an intensity spectrum in real space is required, but there is a constant part that cannot be determined even if the spectrum corresponding to the second derivative is integrated twice. appear. Therefore, this measurement mode is not suitable for obtaining an MRI image. That is, it is necessary to acquire a spectrum corresponding to at least one differentiation and use an intensity spectrum obtained by performing integration once for image processing. However, in the MRFM measurement, when various modulations are performed at the cantilever frequency, vibrations with an amplitude several hundred times that of the MRFM signal are excited, which is not the point of measurement.
[0014]
To avoid this phenomenon, a group at the University of Washington proposed an anharmonic modulation method (Bruland et al., Rev. Sci. Instrum. 66, 2853 (`95)). In this method, a high frequency amplitude modulation (AM) frequency: f _AM And magnetic field modulation (HM) frequency: f _HM Cantilever resonance frequency: f _C And the absolute value of the difference between them, including harmonics, | f _AM -F _HM | Or the absolute value of the sum: _AM + F _HM | _C To be equal to Phase detection with frequency f _C By performing the above, it is possible to obtain a spectrum having almost the same content as the single differentiation. In this measurement method, a desired spectrum can be obtained, but it is necessary to handle a total of three types of modulation signal frequencies.
[0015]
In order to efficiently generate the MRFM signal and detect the signal under stable conditions, the cantilever frequency is monitored at any time, the magnetic field modulation frequency or the high frequency amplitude modulation frequency is updated to the corresponding frequency, and the signal is For detection, it is necessary to update the reference signal frequency to the phase detector to the resonance frequency.
[0016]
Since the measured amount of MRFM is described by dispersion: χ ′ and absorption: χ ″ as in the magnetic resonance phenomenon, it is given as a coefficient of Cos (φ) term and Sin (φ) term for the reference signal to the phase detector, respectively. Both measured quantities are extremely sensitive to the phase difference between the signal and the reference signal: φ If the phase difference changes randomly with the frequency update, the integration / requirement required to detect the MRFM signal with high sensitivity. In the averaging process, the average value of Cos (φ) and Sin (φ) becomes zero, and the measurement noise is reduced by integration / average, and at the same time, the signal itself disappears.
[0017]
In other words, the anharmonic modulation method proposed by the University of Washington, which is suitable for acquiring the intensity spectrum of real space, synchronizes the phase while sequentially updating the various modulation frequencies and the reference signal frequency to the phase detector. There was no specific measuring device diagram that could withstand the integration / average of acquired data over time.
[0018]
[Means for Solving the Problems]
The present invention eliminates the time / steps required to interrupt the measurement and acquire the resonance / resonance frequency, so that the resonance / resonance frequency can be monitored at any time and the total time taken for measurement can be reduced. It is. Further, instead of intentionally lowering the Q value and impairing the signal detection sensitivity, an environment is provided in which a weak signal can be measured in a resonance / resonance state having a higher Q value.
[0019]
To this end, the present invention is a frequency drift tracking method for a device that measures a signal under resonance / resonance conditions, and measures a noise density signal near the resonance / resonance frequency position, The noise density intensity is monitored as needed at a frequency position where the noise density intensity does not change even if a signal appears, and the resonance / resonance frequency drift is estimated based on the noise density spectrum before the measurement is started and fed back to the signal appearance frequency. By applying Resonance / resonance frequency drift is tracked, and at least at two points on both sides of the resonance / resonance frequency, a noise density signal is measured at a frequency position where the noise density intensity is one half of the maximum value. It is characterized by.
[0020]
In addition, the present invention makes it possible to monitor the cantilever frequency with high accuracy successively in the MRFM in a room temperature environment where the influence of drift is always large. In addition, the signal is updated to the latest resonance frequency before starting signal detection at each measurement location. At the same time, the phases of the three frequencies are synchronized with the synchronization pulse, and the phase information is updated while the cantilever resonance frequency is updated from the start to the end of measurement. Can be kept constant.
[0021]
Therefore, the present invention provides a magnetic resonance force microscope in which a static magnetic field is applied to a sample by a magnetic chip attached to the tip of a cantilever and an external magnet, and a high frequency magnetic field is irradiated by a high frequency coil to observe the magnetic resonance force intensity. Measure the noise density signal near the resonance frequency position of the cantilever, The noise density intensity is monitored as needed at a frequency position where the noise density intensity does not change even if a signal appears, and the drift amount of the resonance frequency is estimated based on the noise density spectrum before the measurement is started, and the signal appearance frequency is fed back. By The frequency of the signal generation means determined based on the resonance frequency is updated by tracking the drift of the resonance frequency, and further includes a phase synchronization signal generation means, and the phase synchronization signal generation means The phase is synchronized.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram for explaining a method of estimating a noise spectrum and a drift amount of a resonance / resonance circuit. In general, the noise spectrum of a resonance / resonance circuit is derived from the dissipative fluctuation theorem and is described in the function form of [Equation 1] having a peak at the resonance / resonance frequency of the circuit.
[0023]
[Expression 1]

[0024]
In the following, ω from the noise density intensity measured at the two frequency positions, which is the simplest and most effective. _n A method for estimating the amount of drift will be described.
[0025]
Noise density intensity described in [Equation 1]: S (ω) is S _max The frequency position that becomes / 2 is also the position that defines the Q value of the circuit, and the center frequency ω _n To ± ω _n Is given at a frequency 2Q apart. this
[0026]
[Expression 2]

[0027]
(2 points represented by gray circles in FIG. 1).
[0028]
A certain time: t = t ₀ Frequency position measured at: ω _lower (T ₀ ) And ω _higher (T ₀ ) Is the noise density intensity at S (ω _lower (T ₀ ), T ₀ ) And S (ω _higher (T ₀ ), T ₀ ), If there is no drift,
[0029]
[Equation 3]

[0030]
Is R (t ₀ ) = 1.
[0031]
On the other hand, time t = t ₁ When a drift occurs in (a spectrum represented by a dotted line in FIG. 1), the drift amount: dω
[0032]
[Expression 4]

[0033]
[Equation 1] is
[0034]
[Equation 5]

[0035]
Can be written. Drifted noise density intensity: S (ω, t ₁ ) _lower (T ₀ ) And ω _higher (T ₀ ) At the frequency position and take the ratio
[0036]
[Formula 6]

[0037]
(2 points represented by black circles in FIG. 1), this amount is the previously measured resonance / resonance frequency: ω (t ₀ ) Includes information reflecting the amount of drift from
[0038]
Especially when α is small, [Expression 6] can be approximated with good accuracy by one Taylor expansion related to α. As a result, the drift amount of [Expression 4] is
[0039]
[Expression 7]

[0040]
Given in. Since the ratio: R is evaluated, the influence of noise density intensity change can be theoretically excluded.
[0041]
Assuming that there is no significant change in the Q value, the previously measured ω _n (T _i ) And two frequencies evaluated from the Q value
[0042]
[Equation 8]

[0043]
The noise density is measured by the ratio: R (t _{i + 1} ) Can be evaluated, and the latest resonance / resonance frequency can be known sequentially.
[0044]
In the above description, two measurement points are used. However, if there is no large drift / fluctuation in the intensity of noise density, it is possible in principle to perform measurement at one place. Further, the intensity may be measured at an arbitrary point of the noise density spectrum, for example, a point located at the bottom, instead of the measurement at a plurality of positions and the frequency position defined from the resonance / resonance frequency and the Q value.
[0045]
Further, if the measurement frequency bandwidth is narrowed or the base of the noise density spectrum is measured, drift monitoring can be performed even for a vibrator having a higher Q value. If the uncertainty of the acquired noise density intensity or the estimated frequency drift is large, the current noise density intensity or frequency drift is evaluated by weighted averaging of a plurality of data measured in the past. It is also possible.
[0046]
Next, a frequency drift tracking device based on noise density measurement according to the present invention will be described. FIG. 2 is a diagram showing an embodiment of a frequency drift tracking device by noise density measurement according to the present invention, FIG. 3 is a diagram showing another embodiment of the frequency drift tracking device by noise density measurement according to the present invention, and FIG. These are figures which show the detailed structural example of the resonance / resonance circuit used in order to implement this time. In the figure, 1 is a resonance / resonance circuit, 2 is an AC signal amplifier, 3 is a bandpass filter, 4 is a reference signal for phase detection, 5 is an AC signal strength measuring device, 6 is a recorder for reading, 7 is a lock-in amplifier, Reference numeral 11 denotes a cantilever, 12 denotes an optical fiber interferometer, 13 denotes a photodiode, and 14 denotes a current amplifier.
[0047]
In FIG. 2, a bandpass filter 3 or a phase detector is supplied with a stable phase detection reference signal 4 necessary for phase detection and extracts only a specific frequency component, which is a resonance in which a signal appears. / A frequency position that is separated from the resonance frequency position by an appropriate frequency, that is, a noise density intensity does not change even if a signal appears. The AC signal amplifier 2 amplifies the noise generated by the resonance / resonance circuit 1, and the AC signal strength measuring device 5 measures the strength of a specific frequency component extracted by the bandpass filter 3 or the phase detector, and the noise. The reading recorder 6 or the computer evaluates / tracks the frequency drift due to the density measurement.
[0048]
The noise generated by the resonance / resonance circuit 1 is amplified by the AC signal amplifier 2 by the frequency drift tracking device based on the noise density measurement having such a configuration, and a specific frequency component is passed through the narrow bandpass filter 3 or the phase detector. The frequency drift is evaluated / tracked by extracting only the signal and measuring it with the AC signal strength measuring device 5 and storing it in the reading recorder 6 or a computer. Therefore, even if the target signal to be detected appears at the resonance / resonance frequency position (black sharp peak), the noise intensity is measured at the frequency position where there is no influence of the signal appearance.
[0049]
As a bandpass filter 3 or a phase detector, there is a configuration in which a lock-in amplifier 7 is realistically used as shown in FIG. 3 and a stable phase detection reference signal 4 necessary for phase detection is supplied from the outside. Some lock-in amplifiers 7 already have a noise density measurement mode, and the frequency of the reference signal 4 for phase detection is centered on the bandwidth determined from the time constant of the lock-in amplifier 7 and the frequency filter slope. The noise density intensity is measured.
[0050]
As a specific example, the vibration noise density of a free vibration state of a cantilever used in an atomic force microscope was measured, and the resonance frequency drift of the cantilever was monitored. The cantilever is a vibrator having a very high Q value. In the free vibration state, the amplitude of the cantilever is 10 ^-9 It is difficult to estimate the resonance frequency directly unless an external signal that induces vibration is given. However, it is possible to acquire / evaluate the parameters of the resonance phenomenon by accurately measuring the noise density as described below.
[0051]
The measurement frequency position for the drift monitor has a noise density strength of S _max The test was performed at two locations where the ratio was / 2. That is, the method described above was executed. Also, the frequency drift amount obtained in the past is stored as a database, and t = t ₁ The amount of drift at the time was approximated by the weighted average defined by the following [Equation 9] including the frequency drift of the past two points.
[0052]
[Equation 9]

[0053]
FIG. 4 shows a specific configuration of the resonance / resonance circuit 1 according to the embodiment. In FIG. 4, the vibration amplitude intensity of the cantilever 11 is guided to the detection photodiode 13 by an optical lever method using a laser beam or an optical fiber interferometer 12 using an optical fiber, and the induced optical interference intensity is expressed as a current amplifier. 14 is amplified and detected.
[0054]
In actual measurement, first, a noise density spectrum is measured in order to obtain parameters describing the resonance state of the cantilever 11. FIG. 5 is a diagram showing a noise density spectrum of the cantilever 11 used for the verification of the present invention. Resonance frequency: about 10 kHz, Q value: about 950, √S _max It was confirmed that there was a noise of = 0.75Å√Hz.
[0055]
FIG. 6 is a graph in which the drift amount with respect to the cantilever using the method according to the present invention: the change with time of the black circle and the noise density measured at the resonance frequency (fixed) position obtained at the start of measurement: the change with time of the white circle are plotted. It can be seen that the resonance frequency at the start of measurement is almost completely outside the resonance region after 100 minutes due to the effect of drift.
[0056]
FIG. 7 is a diagram showing the result of measuring the noise density at a frequency (variable) position (tracking mode) obtained by adding the drift amount estimated by the method according to the present invention to the resonance frequency at the start of measurement for the same cantilever. . 420 minutes = resonance frequency drift after 7 hours: black circle is 250 Hz, 2.5% compared to the start of measurement, but noise density measured at the resonance / resonance frequency being monitored: white circle is measurement start Compared to the noise density intensity measured occasionally, the standard deviation was constant at about 5% even after 7 hours. FIG. 8 is a histogram display of the same noise density data shown in FIG. The Gaussian distribution suggests that the drift amount can be estimated more accurately by increasing the accuracy in noise density measurement.
[0057]
FIG. 9 is a diagram in which a deviation from the fitting curve is displayed as a histogram by performing polynomial fitting since there is no theoretical formula for the change over time in the drift amount of the resonance / resonance frequency. Standard deviation: found to have 1.5 Hz.
[0058]
Next, an apparatus such as a magnetic resonance force microscope having a frequency drift tracking function will be described. FIG. 10 is a diagram showing an embodiment of an MRFM apparatus having a frequency drift tracking function according to the present invention, and FIG. 11 is a diagram showing another embodiment of the MRFM apparatus having a frequency drift tracking function according to the present invention. is there.
[0059]
As shown in FIG. 10, in a magnetic resonance force microscope in which a static magnetic field is applied to a sample by a magnetic chip attached to the tip of a cantilever and an external magnet, and a high frequency magnetic field is irradiated by a high frequency coil to observe the magnetic resonance force intensity. The computer 21 monitors and evaluates the drift of the resonant frequency of the cantilever using the frequency drift tracking method based on the noise density measurement. And the acquired resonant frequency information is sent to the signal transmitters 22-24 which output the signal which each frequency has. In the case of the anharmonic modulation method, the signal transmitters 22 to 24 correspond to signal generators that generate three types of frequencies: a high frequency AM modulation frequency, a magnetic field modulation (HM) frequency, and a reference signal frequency of a phase detector. . After all the frequency information has been correctly sent, the phase synchronization signal generator 25 sends a reference signal (for example, trigger pulse) for matching the phases to each of the signal generators 22 to 24 at a certain time. If each signal generator is synchronized with the internal reference clock, the phase drift can be minimized over a long period of time, so that further stability can be obtained.
[0060]
In the embodiment shown in FIG. 11, as in the embodiment shown in FIG. 10, the high frequency (AM) modulation frequency determined based on the resonance frequency of the cantilever monitored by the computer 31: f _AM Magnetic field modulation (HM) frequency: f _HM Are set in the corresponding signal generators 32 and 33. Then, these two signal outputs are branched, and the frequency mixer 35 _AM + F _HM And f _AM -F _HM And a frequency component necessary for phase detection is sent to the phase detector 39 through the filter 36. As in the case of the embodiment shown in FIG. 10, the phase synchronization signal generator 34 sends signals (for example, trigger pulses) for synchronizing their phases at a certain time to the signal generators 32 and 33. Also in this embodiment, the signal generators 32 and 33 can minimize the phase drift over a long period of time if the internal reference clock is synchronized, so that the stability is further improved. can get. If necessary, an amplifier 40 may be inserted so as to obtain a reference signal intensity sufficient for phase detection.
[0061]
In the above embodiment, an example in which a reference signal necessary for phase detection is synthesized using the frequency mixer 35 and the filter 36 using the AM modulation frequency and the magnetic field modulation frequency has been described. However, other combinations, for example, AM It is also possible to synthesize the magnetic field modulation frequency using the frequency mixer 35 and the filter 36 using the variable frequency and the phase detection frequency. For the AM modulation using the magnetic water modulation frequency and the phase detection frequency. It is also possible to synthesize the frequencies using the frequency mixer 35 and the filter 36.
[0062]
The verification experiment results at room temperature obtained with the apparatus configuration introduced in the embodiment will be shown below. The MRFM signal measurement mode is an anharmonic modulation method, and a stage on which a sample (DPPH) is placed is spatially one-dimensionally scanned and a signal is observed. The cantilever at the start of measurement had a resonance frequency of about 11.7676 kHz and a Q value of about 1000.
[0063]
FIG. 12 is a diagram showing the change with time in the cantilever resonance frequency (FIG. 12, white circles) when monitoring was started after the apparatus was calibrated. The cantilever resonance frequency monitored from the end of calibration shows a drift of 100 Hz after 7 hours, and the measurement environment under a resonance state having a Q value of 1000 can be easily destroyed unless frequency tracking is performed. . On the other hand, the result of monitoring the resonance frequency drift based on the frequency drift tracking method by noise density measurement is shown by black circles in FIG. This result shows a standard deviation of 1.39 and a resonance frequency of about 11.766 kHz with respect to ± 1 × 10. ^-Four It shows that the resonance frequency can be tracked with the accuracy of. The arrows in FIG. 12 indicate the time points when the following measurements are started.
[0064]
FIG. 13 updates the latest cantilever resonance frequency at the measurement start position (Z = 20 μm), and after reaching the measurement end position (Z = 52 μm), returns the stage to the measurement start position, updates the cantilever resonance frequency, and repeats scanning. It is a figure which shows the spectrum acquired by the method of saying. The number of repetitions is 20, and the total measurement time is 30 minutes. Since the phase is not synchronized during the measurement, the result after integration disappears with noise.
[0065]
FIG. 14 is a diagram illustrating a result of phase synchronization. In addition, the number of repetitions was 50 times, and the total measurement time exceeded 1 hour, but only the noise component could be reduced without decreasing the MRFM signal itself.
[0066]
The method of the present invention eliminates the unnecessary time / step of interrupting the measurement to obtain the resonant frequency, and always sets the resonant frequency to 10 ^-Four It was possible to monitor with high accuracy, and the total time required for measurement could be shortened. Further, instead of deliberately lowering the Q value and deteriorating the signal detection sensitivity, it was possible to provide an environment in which a weak signal can be continuously measured under the latest resonance state having a constantly high Q value. In the anharmonic modulation method suitable for obtaining the MRI image from the MRFM measurement result, the spectrum was able to be obtained by performing phase synchronization by the synchronization pulse on the three kinds of required frequency signals. A measurement environment that can perform integration for a long time in an environment where drift occurs is realized, and a very small MRFM signal can be measured efficiently and stably.
[0067]
【The invention's effect】
As is apparent from the above description, according to the present invention, there is provided a frequency drift tracking method for an apparatus for measuring a signal under a resonance / resonance state. The noise density intensity is monitored as needed at a frequency position where the noise density intensity does not change even if a signal appears, and the resonance / resonance frequency drift is estimated based on the noise density spectrum before the measurement is started and fed back to the signal appearance frequency. By applying Since the resonance / resonance frequency drift is tracked, the noise density is separated from the resonance / resonance frequency position where the signal appears by an appropriate frequency, that is, at a frequency position where the noise density intensity does not change even if the signal appears. The intensity can be monitored from time to time, and the drift amount of the resonance / resonance frequency can be estimated based on the noise density spectrum before the measurement is started, and the signal appearance frequency can be fed back.
[0068]
In addition, in the magnetic resonance force microscope in which a static magnetic field is applied to the sample by a magnetic chip attached to the tip of the cantilever and an external magnet and a high frequency magnetic field is irradiated by a high frequency coil to observe the magnetic resonance force intensity, the resonance frequency of the cantilever Measure the noise density signal near the location, The noise density intensity is monitored as needed at a frequency position where the noise density intensity does not change even if a signal appears, and the drift amount of the resonance frequency is estimated based on the noise density spectrum before the measurement is started, and the signal appearance frequency is fed back. By The frequency of the signal generation means determined based on the resonance frequency is updated by tracking the drift of the resonance frequency, and further provided with a phase synchronization signal generation means, and the phase of the signal generation means is synchronized by the phase synchronization signal generation means So, it is possible to keep the phase information constant while updating the cantilever resonance frequency from the start to the end of the measurement by synchronizing the three kinds of frequencies and simultaneously updating to the latest resonance frequency before starting signal detection at each measurement location. .
[0069]
According to the present invention, it is possible to monitor the resonance / resonance frequency at any time by interrupting the measurement and eliminating the time / step required to acquire the resonance / resonance frequency, and to reduce the total time required for the measurement. Can do. Further, instead of intentionally lowering the Q value and impairing the signal detection sensitivity, it is possible to provide an environment in which a weak signal can be measured in a resonance / resonance state having a higher Q value.
[0070]
Furthermore, in the anharmonic modulation method suitable for obtaining the MRI image from the MRFM measurement result, the spectrum can be acquired by performing phase synchronization by the synchronization pulse for the three types of frequency signals required, A measurement environment in which integration can be performed for a long time in an environment where drift occurs is realized, and a very small MRFM signal can be measured efficiently and stably.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining a method of estimating a noise spectrum and a drift amount of a resonance / resonance circuit.
FIG. 2 is a diagram showing an embodiment of a frequency drift tracking device based on noise density measurement according to the present invention.
FIG. 3 is a diagram showing another embodiment of a frequency drift tracking device using noise density measurement according to the present invention.
FIG. 4 is a diagram showing a detailed configuration example of a resonance / resonance circuit used for carrying out R; WL;
FIG. 5 is a diagram showing a noise density spectrum of the cantilever 11 used for verification of the present invention.
6 is a graph plotting drift with respect to a cantilever using a method according to the present invention: time variation of a black circle and noise density measured at a resonance frequency (fixed) position acquired at the start of measurement: time variation of a white circle. .
FIG. 7 is a diagram showing a result of measuring noise density at a frequency (variable) position (tracking mode) obtained by adding the drift amount estimated by the method according to the present invention to the resonance frequency at the start of measurement for the same cantilever. is there.
FIG. 8 is a histogram display of the same noise density data shown in FIG.
FIG. 9 is a diagram in which there is no theoretical formula for the change over time in the drift amount of the resonance / resonance frequency, so that polynomial fitting is performed and the deviation from the fitting curve is displayed in a histogram.
FIG. 10 is a diagram showing an embodiment of an MRFM apparatus having a frequency drift tracking function according to the present invention.
FIG. 11 is a diagram showing another embodiment of the MRFM apparatus having a frequency drift tracking function according to the present invention.
FIG. 12 is a diagram showing a change with time in the cantilever resonance frequency (FIG. 12 white circle: ◯) when monitoring was started after the apparatus was calibrated.
13 is updated to the latest cantilever resonance frequency at the measurement start position (Z = 20 μm), and after reaching the measurement end position (Z = 52 μm), the stage is returned to the measurement start position and the cantilever resonance frequency is updated to perform scanning. It is a figure which shows the spectrum acquired by the method of repeating.
FIG. 14 is a diagram illustrating a result of phase synchronization.
FIG. 15 is a diagram showing a simplified configuration of a detection unit of the MRFM apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Resonance / resonance circuit, 2 ... AC signal amplifier, 3 ... Band pass filter, 4 ... Reference signal for phase detection, 5 ... AC signal strength measuring device, 6 ... Reading recorder, 7 ... Lock-in amplifier, 11 ... Cantilever , 12 ... optical fiber interferometer, 13 ... photodiode, 14 ... current amplifier

Claims (5)

A frequency drift tracking method for a device that measures a signal in a resonance / resonance state, where a noise density signal is measured in the vicinity of the resonance / resonance frequency position and the noise density intensity does not change even if the signal appears. To monitor the drift of the resonance / resonance frequency by monitoring the noise density intensity at the position as needed, estimating the drift amount of the resonance / resonance frequency based on the noise density spectrum before the start of measurement, and applying feedback to the signal appearance frequency. Frequency drift tracking method characterized by   2. The frequency drift tracking method according to claim 1, wherein a signal of noise density is measured at least at two points on both sides of the resonance / resonance frequency.   The frequency drift tracking method according to claim 1, wherein a noise density signal is measured at a frequency position where the noise density intensity is a half of the maximum value. In a magnetic resonance force microscope in which a static magnetic field is applied to a sample by a magnetic chip attached to the tip of a cantilever and an external magnet, and a high frequency magnetic field is irradiated by a high frequency coil to observe the magnetic resonance force intensity, in the vicinity of the resonance frequency position of the cantilever Measure the noise density signal at any time, monitor the noise density intensity at any frequency position where the noise density intensity does not change even if the signal appears, and determine the resonance frequency drift amount based on the noise density spectrum before the measurement starts. Magnetic resonance with a frequency drift tracking function characterized in that the frequency of the signal generation means determined based on the resonance frequency is updated by tracking the drift of the resonance frequency by estimating and applying feedback to the signal appearance frequency. Force microscope.   5. The magnetic resonance force microscope with a frequency drift tracking function according to claim 4, further comprising phase synchronization signal generation means, wherein the phase of the signal generation means is synchronized by the phase synchronization signal generation means.

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