M-H 자기이력곡선 : SQUID, VSM

Transcription

1 자성특성측정방법 자기장측정 M-H 자기이력곡선 : SQUID, VSM 고주파특성 ( 투자율 )

2 (1) 자기장측정 자기센서기술연구동향 지구자기장 NVE InSb By Honeywell

3 휴대폰용 COMPASS 센서응용 SQUID Flux gate Magneto-Impedance Hall AMR 지구자기장 0.1 nt 1 nt 30 nt 0.1 nt 차세대 compass - 성능 - 가격 Micro-Size Low power 1 ft Low Cost =? 센서소자 + ASIC + Package & Test

4 COMPASS 센서의가격절감요소 센서소자 Red ocean Size 감소 재료비절감 단가절감 성능향상 ASIC 휴대폰연동 By Aichi Steel 가속도센서병행사용 단가절감 회로비용절감 AMP Gain A/DC 민감도향상 각도분해능향상 Offset 최적화

5 COMPASS 센서의가격절감요소 휴대폰과연동 자기장발생원 : 스피커, 진동자등 최적의위치선점 : ASIC Offset 기능최적화 0.10 V out 동작점이동 최적의위치 Magnetic Field (Oe) By Aichi Steel

6 E-compass 응용 수평 3 차원지자기 ( 방위 ) 센서 Size : 5mm X 5mm X 1.2mm

7 Hall 효과방법 홀전기장 E H v B 홀전압, V H R H IB t V J B ne E H ( R H H ( J ne v) w ( w : width 1 / ne, J I / ) wt ) Hall 효과 측정범위 : 수 Oe ~ 수십 KOe I B V V= R H B

8 (2) 자기물성측정 WHAT WE MEASURE: B OR M SI : B = μh = μ 0 (H+M) = μ 0 (1+χ)H (χ=m/h : susceptibility) cgs : B = H + 4πM B = Magnetic flux density, Magnetic induction susceptometer H = Magnetic field strength experimentally controllable (Current) M = (Volume) Magnetization magnetometer cgs SI Conversion B G T, Wb/m 2 1 G = 10-4 T H Oe A/m 1 Oe = 10 3 /4π A/m M emu/cm 3 A/m 1 emu/cm 3 = 10 3 A/m m (magnetic moment) emu A.m 2 1 emu =10-3 A.m 2 M-H vs B-H Loop

9 Magnetometer Sensitivity (emu) Dynamic range Applications χ ac 10-9 Wide Torque 10-3 ~ 10-8 (cap.) 10-9 ~ (piezo) narrow very narrow SQUID 10-8 ~10-11 narrow VSM AGM 10-6 (0.5 uemu ~ 1000 emu) 10-8 (1 nemu ~ 10 emu) Wide middle phase transitions superparamag. superconductors not quite abs. magnetization phase transitions magnetization anisotropy Superconductors all above absolute magnetization all above absolute magnetization all above absolute magnetization demanding instrumentation low T/high pressure Maintenance cost : low small signal for polycrystal difficult to calibrate Maintenance cost : middle Very high sensitivity Hybrid magnet Speed : slow Maintenance cost : high low sensitivity Temperature : 10 ~ 1073 K Maintenance cost : low Size : > 15mm (Pole cap) Mass : 10g middle sensitivity Temperature : 10 ~ 473 K Maintenance cost : low Size : 5x5x2 mm Mass : 0.2g

10 자화율측정 ) ( ) ( ) : ( M V m B m U U F 위치에너지힘 ) sample, of outside field : ( ) ( H μ B B H M V z H V z B M F o o

11 진동시료법 (Vibrating Sample Magnetometer : VSM) 전자기유도방법 d ( d t B dt 1 NA dt NAB) : 자속 ( maxwells ) 유도전압 ( Volt ) 면적 ( cm 2 ) 권선수 : A: N :

12 Introduction Magnetic measurement is a powerful method to characterize properties of materials. Among numbers of magnetic measurement equipment, Vibrating sample magneto-meter (VSM) is known as a very effective way to determine magnetization. VSM offers different measuring modes. By analyzing the results, many useful information of materials can be extracted.

13 Vibrating Sample Magnetometer (VSM) Vibrator Computer - Magnetic field (electro magnet, Power supply) - Detection part (pick-up coil Lock-in(m), Gaussmeter(H)) Electromagnet Power supply Electromagnet Lock-in & Gaussmeter Vibrator power - Vibrating part (loud speaker, feedback system, power supply) - Display & controller (Computer, Software) Pick-up coil

14 VSM Operation Principle If a sample of any material is placed in a uniform magnetic field, created between the poles of a electromagnet, a dipole moment will be induced. If the sample vibrates with sinusoidal motion a sinusoidal electrical signal can be induced in suitable placed pick-up coils. The signal has the same frequency of vibration and its amplitude will be proportional to the magnetic moment, amplitude, and relative position with respect to the pick-up coils system.

15 VSM Operation Principle Magnetization is often measured by induction method (for example in an extraction magnetometer): V: Induction voltage N: Number of coils : magnetic flux t: time A: area of coil BA V NA db dt B 0 ( H M ) 1 NA Vdt This method only measures B, not M The accuracy is not high. I N I N S S

16 VSM Operation Principle VSM: sample is vibrating with a standardized frequency ( ) during the measurement. H does not depend on t but M depends on t : sint db d dh dm V NA NA ( H M ) NA NA dt dt dt dt d 0 NA M sin t NA M cos t M cos t dt AC measurements always improve signal to noise ratio. By this method, the signal is directly proportional to magnetization.

17 VSM Sensitivity Vibration 4 coils Sample (t ) Oscillator (~ 80 Hz) Lock-in amplifier Reference Magnetization v( t) t kafm(h ) k : coil constant A : area f : vibrating frequency H-field Computer m(h) : magnetic moment The voltage V(t) across the VSM detection coils can be written as Magnetic moment: selection of the sample volume can be used to optimize signal; in general, larger is better; size may affect B uniformity & vibration load. Vibration amplitude: large amplitude increases sensitivity (if coils are large enough to capture full excursion with uniform sensitivity). Vibration frequency: higher freq. gives higher sensitivity, but other constraints limit max usable freq. (eddy currents in conducting samples, audio noise due to vibrator, interference from harmonics/subharmonics of power freq.). Detection coil sensitivity: coupling of detector is strong function of (inverse) separation between sample & coils; small separation makes sample mounting & shape issues more important.

18 VSM Sensitivity & Noise The main sources of noise to limit VSM sensitivity comes from background signals and the signal-to-noise ratio (SNR). A. Background signals These can include vibration of the detector coils due to mechanical coupling from the sample vibrator (need to insure vibration isolation here). Also stray signals can come from wire loops or drive wires leading to the vibrator (independent of B and present without a sample). Also pickup from other power sources (electrical & mechanical vibrations). B. Noise in VSM The main sources of noise include the usual culprits (Johnson, Shot, and 1/f noise). Johnson noise (thermal noise due to e fluctuations in R) is usually the most significant in VSM. It is given by V RMS = (4kTRf) 1 / 2 where k = Boltzmann s constant T = absolute T R = coil resistance f = freq. bandwidth of measure in Hz

19 M-H, B-H Loop (Hysteresis Loop) 1) 자기소거 (demagnetized state) 2) 초기곡선 (initial curve) 3) 이력곡선 (hysteresis loop) major loop minor loop 4) 잔류자화 ( Remanence magnetization, B r or M r ) 5) 보자력 ( Coercivity, H c ) 6) 포화자화 (Saturation, magnetization B s or M s ) Magnetic moment (EMU) Parallel to the field Ferromagnetic or Paramagnetic Perpendicular diamagnetic to the field Field (Oe)

20 연자성 : 작은보자력, 자화 탈자가쉬움 경자성 : 큰보자력, 자화 탈자가어려움 soft ferromagnetic material has both a small M r and H c. - Transformers - Magnetic Shield - Flux keeper for relays, printer, - motors, watches, and other magnetic system hard ferromagnetic material has a large M r and large H c. - Motors - Linear motors - Headphone - Balances - Microwave tubes, laser

21 (3) CNU Droplet 방법!! -50 L : emu at10,000 Oe (3 emu/cc) emu at 15 Oe - Drop of 40 pl : ~ emu -Signal intensity : 15 V - Noise level : 0.5 V Resolution : emu

22 Is it reasonable? Magnetometer Sensitivity (emu) Dynamic range VSM 10-6 Wide [1,5] AGM 10-8 Middle [2,5] SQUID 10-8 ~ Narrow [3,5] W.Ross et.al., Rev.Aci.Instrum., 51,612 (1980) 3. J. Diederichs et.al., Czechoslovak J. Phys., 46, 2803 (1996) 4. A. Bogach et.al., J. Electrical Engineering, 59, 11(2008) 5. C.D.Graham et.al., J.Mater.Sci.Technol., 16,97(2000) SQUID Dipole field 2 m 1 m H cos eˆ r sin eˆ r 4 r SQUID Our r SQUID m V H : / r r system PHR SQUID : r 3 PHR 10 cm (10 m) 10 m ( m) Measurable field : ~ ft (~10 15 T) emu 10 B 10 A m 2 Same moment resolution T T :