Micromechanical Instruments for Ferromagnetic Measurements

Transcription

1 Micromechanical Instruments for Ferromagnetic Measurements John Moreland NIST 325 Broadway, Boulder, CO, Phone: FAX: Presented at the THIC Meeting at the National Center for Atmospheric Research Boulder CO June 11-12, 2002

2 Colleagues NIST Boulder Michelle Chabot Albrecht Jander Markus Löhndorf Pavel Kabos Stephen Russek DongHoon Min NIST Gaithersburg Bob McMichael University of Nebraska Sy-Hwang Liou Dalhousie University Ted Hubbard

3 MEMS Integration of Electromagnetic Systems: Impact on the Next Generation of Magnetometers. Magnetometer Sub-Systems optical microwave mechanical magnetic MEMS Integration magnetism laboratory on a chip magnetic thin film MEMS torque resonator optic fiber Industrial Impact near term smart substrates new probe microscopes long term nanometer scale MRI spin quantum mechanics

4 Ferromagnetic resonance (FMR). H 0 M natural response Precession and relaxation of M in response to an applied field H. H rf driven response (FMR) Lorentzian absorption line typical of FMR showing microwave power absorption as a function of swept bias field.

5 Quick review of FMR parameters. = H H M s γ ω H = ω γ α = 0 0 2H M H M M s s s ω γ α χ H H 0

6 Instrumentation. DIFF AMP LASER DIODE IN REF LOCK-IN AMP PULSE OSC Z rms OUT FOUR QUADRANT PHOTODIODE A B SPLIT PHOTODIODE AM RF SWEEPER C D CANTILEVER MICROSTRIP RESONATOR RF AMP LASER SPOT Torsion mode (A+C)-(B+D) A+B+C+D CIRCULATOR RF DETECTOR TUNING SCOPE Deflection mode (A+B)-(C+D) A+B+C+D

7 TOP VIEW z END VIEW MICROSTRIP RESONATOR PERMANENT MAGNET H 1 CANTILEVER CANTILEVER COUPLING GAP MICROSTRIP LINE H 0 H T MICROSTRIP RESONATOR SUBSTRATE PERMANENT MAGNET

8 Cantilever characteristics Deflection K s = Ewt 3 /4l 3 Torsion 3 E( wt ) K s = 6(1 + m) l F noise Hz = 2K skbt Q f o Si Cantilevers T noise Hz = 2K s k Q f b o T K f o s = 0.2 N / m = 15 khz Q = 200 t = 3µm w = 50µm l = 450µm K f o s = 3 10 = 250 khz Q = N m/ rad F noise N / Hz T noise N m/ Hz

9 Apparatus for micromechanical detection of FMR postdoc dipole magnet mod. coils AFM co-ax / stripline transition rf power amp AFM laser beam-bounce head micrometer stage microstrip resonator cantilever

10 Micro torque magnetometer. m (10-10 A-m 2 ) nm Fe T =µ o MHV T H T applied field (mt) H o

11 FMR with a micro resonating torque magnetometer: magnetic moment modulation nm NiFe film T = µ MHV quasi static o T torque (10-14 N-m) H T H rf H o applied field (ka/m) M

12 FMR with a micro resonating torque magnetometer: spin angular momentum damping. cantilever torsion (arb.) T dynamic H rf = µ 0Ms 2 α (2H + M r s H ) 2 rf V applied field (ka/m) M H o

13 FMR with a micromechanical calorimeter sensor. µ 0M γ( Hr + M P= 2 α (2H + M ) ) H s s 2 rf r s V Cold H rf Hot H o

14 in situ monitoring during thin-film deposition. 0.5 nm/s 0.7 nm/s 0.7 nm/s Magnetic moment measured with a resonating torque microbalance versus film thickness measured with a quartz crystal microbalance.

15 in situ monitoring during thin-film deposition. Active damping feedback signal versus torque field for a 30 nm thick permalloy film.

16 Smart Substrates for Calibrated Magnetometry of Ultra Thin Magnetic Films standard film unknown film silicon magnetic thin films optic fiber MEMS torsional resonator Batch fabricate MEMS torsional oscillators are coated with standard magnetic film at the wafer level. M r t of the standard film is measured at the wafer level in the calibration lab. The wafer is diced up into coupons for MEMS test fixtures. A direct comparison of the unknown film and the standard film is made during the deposition and subsequent processing steps on the factory floor.

17 FMR absorption of photon energy and angular momentum. H rf M H o T = dn dt h E = hω J = h + σ Cold H rf H o Hot P = dn dt h ω P/T = ω

18 Combining cantilever torque and deflection (power) measurements of the same sample: P =γ ( H + 0 M ) T res s res torsion (arb.) P res /T res deflection (arb.) applied field (ka/m) -M s H o Deflection and torque measurements taken with the same experimental configuration. New way to measure M s or, alternatively, the ratio of γ s of different spin systems.

19 MEMS Magnetometer Projects at NIST-Boulder. Dielectric bimaterial cantilever Ultra thin cantilevers Triple torsional oscillator High Q torsional oscillators

20 Magnetic moment sensitivity comparisons (A m 2 ) conventional magnetometers NIST MEMS magnetometers (current status) NIST MEMS magnetometers (MEMS optimization) MRFM (predicted) T = 100 mk pole-tip of a writer spin-valve reader superparamagnetic particle (8 nm) 3 1 µ B single spin