MOKE 1 Manual

David Lorang Summer 2004

Optics and Instrument Calibration

  1. Try to eliminate any sources of background intensity (turn off lights, shield photodiode, etc.)
  2. Make sure the PEM modulation is turned off
  3. Position the sample such that the laser reflects from the Germanium mirror and passes through the center of the optics. Make sure the beam is focused down so that all of the intensity enters the photodiode
  4. Set the polarizer to the desired angle, and set the analyzer to the angle of maximum extinction. Note that the photodiode output may be negative at extinction, so find the largest negative value.
  5. Now turn the PEM on
  6. If the optics were previously well calibrated, the PEM should be moved out of calibration by about 1-2 degrees
  7. Adjust the frequency (using 3 knobs across the top) and phase (using two knobs in bottom center) of the 2w lock-in to maximize the output signal intensity. You should set the frequency to maximum intensity, then the phase, then repeat a few times until it seems that no improvement can be made.
  8. Now, adjust the PEM angle to minimize the output signal of the 2w amplifier. It should be possible to get the signal very close to zero while set to 200 nV range. Be careful not to bump anything (such as the cables) during this process, because it will cause a temporary jump and confuse the lock-in.
  9. Finally, set the analyzer angle to 45 degrees. One of the 45 degree orientations will produce a positive 2w signal, and the other will produce a negative signal. You want to maximize the 2w signal at 45 degrees, but there is often a large plateau where little change in signal can be seen, so try to set the angle at the center of the plateau.
  10. Finally, move the beam to the sample of interest, and re-align the tilt stage so that the light passes through the center of the optics again. Move the coarse adjustment knobs so that the entire sample can be seen within the useful range of the motors. In the Y-direction, this range is 0 to 12,500. In the X-direction, 1000-5000 is preferred, but values outside this range may or may not cause problems.

How to perform MOKE

  1. Use step moter-test beam2.vi to determine the centerline of the sample. The first three columns in the output files are absolute X, absolute Y, DC intensity. If the home box is checked, both motors will return to home before taking the first point. The positioning controls work exactly like step moter2.vi, described below.
  2. Set up frontpanel.vi and step moter2.vi with the desired parameters. frontpanel.vi contains information about the magnet setup, and step moter2.vi determines the motion of the motors.
  3. The parameters in step moter2.vi work as follows:
  4. Xstart, Ystart: The coordinates of the start of the first scan line
  5. Xend, Yend: The coordinates of the start of the last scan line
  6. Xendline, Yendline: The coordinates of the end of the first scan line
  7. # of scanlines: The number of scanlines. For simple linear data, this is the number of points.
  8. # of Pts – 1st / Last L: The number of points in the first and last scanlines. For linear data, set both of these to 1. For triangular or trapezoidal data, the 1st line must have more than 1 point, but the last line may have 1 or more.

If the vi is running but not yet started, the path of data collection will be graphed. While running, the current point is highlighted in red.

  1. To begin moke, ensure that everything is ready, and run frontpanel.vi. If all of the settings are as desired, press start. step moter2.vi should come to the front. Click start here as well.
  2. The data files generated by frontpanel.vi are numbered corresponding to the point number of that location.

Other Useful Programs

c:\backmeup\ZaberLabview6Demo\testpanel.vi

Manually sends commands to Zaber motors. For more information, see Zaber Linear Actuator Manual

moke1polarity.vi and moke1ramp.vi

These two programs can be used to manually set the magnet direction and voltage (these are not affected by the a and b fit parameters in frontpanelglobals.vi). This is useful for calibration of the magnet. Remember to ramp to zero before switching the polarity. Also beware of changing the voltage in large increments, especially when already at high voltage, as this can cause the magnet power supply to cut off.

With the exception of testpanel.vi, all relevant programs are located in c:\backmeup\mokeVersion4.

Matlab Data Columns:

1: Field Strength

2: DC

3: DC uncertainty

4: 1w

5: 1w uncertainty

6: 1w phase

7: phase uncertainty

8: 2w

9: 2w uncertainty

10: 1w/DC

11: 1w/DC uncertainty

12: 2w/DC

13: 2w/DC uncertainty

14: 1w*cos/DC

15: 1w*cos/DC uncertainty

Fits1 Columns:

1: Negative Field Slope

2: Negative Field Intercept

3: Uncertainty in 2

4: Ascending Zero (remanence)

5: Uncertainty in 4

6: Positive Field Slope

7: Positive Field Intercept

8: Uncertainty in 7

9: Descending Zero (remanence)

10: Uncertainty in 9

1-10 are for 1w/DC

11-20: 2w/DC

21-30: 1w*cos/DC

Fits2 Columns:

1: 1w/DC Saturation difference

2: Uncertainty in 1

3: 1w/DC Remanence difference

4: Uncertainty in 3

5-8: 2w/DC

9-12: 1w*cos/DC

Fits3 Columns:

1: 1w/DC Descending Zero (Coercivity)

2: 1w/DC Ascending Zero (Coercivity)

3-4: 2w/DC

5-6: 1w*cos/DC

Segments 1 (52 per loop):

3000 to 1000 by 500 steps

800 to 200 by 200 steps

150 to 0 by 50 steps

0 to –150 by 50 steps

-200 to –800 by 200 steps

…

Segments 2 (56 per loop):

6000 to 2000 by 800 steps

1200 to 300 by 300 steps

225 to 0 by 75 steps

0 to –225 by 75 steps

-300 to –1200 by 300 steps

…