Technical Notes

Technical Note #1	obsolete
Technical Note #2	Instrument Calibration
Technical Note #3	Installation of DSP-1 Digital Signal Processing Option
Technical Note #4	Installation of High Voltage R/DR Upgrade
Technical Note #5	Year 2000 Compatibility of Model 108 and Model 109A
Technical Note #6	Drive Field Phase Adjustment on Model 109A
Technical Note #7	Amplifier Overheating
Technical Note #8	Instrument Adjustments and Maintenance Options
Technical Note #9	Theory of Operation of SHB Magnetic Measurement Systems

Unfortunately, there exist no thin-film flux standards that can be used to calibrate the vertical axis (total flux) of BH loop tracers like the SHB products. We have been working with NIST (National Institute of Standards) to make such standards available, but because of the varying wafer geometries used by our customers, as well as concerns about stability of standards, such standards are not yet available.

Our customers typically calibrate the vertical axis of their SHB instruments by setting the Hysteresis Looper readings to match reference samples they have calibrated on other instruments. We calibrate the instruments before shipment using an unofficial calibration standard provided to us by a knowledgeable customer.

Calibration really has two basic parts, Vertical (Bs), and Horizontal (Hc). Horizontal calibration is in principal easier, as one can purchase a gaussmeter (we recommend the Lakeshore Model 450), and place its probe in the center of the drive coils, and verify that the field produced agrees with the drive values we display.

Before beginning any calibration, make sure that the instrument internal adjustments have been done (under the Adjustments menu item – see Technical Note on Adjustments), and that the Earth’s Field compensation has been adjusted.

1)  Vertical Axis

Vertical (Bs) calibration is somewhat complex. Here we cannot use a simple instrument, as what we are measuring is the flux that the pickup coil captures from a particular sample. This is highly dependent on the pickup shape and number of turns, as well as the sample geometry. A calibration done for a 1” diameter sample in a given pickup will differ somewhat from that done for say a 4” diameter sample. Again, this is not an instrument limitation, but simply a result of the physics of the situation.

The flux captured by the pickup is to first order proportional to the magnetic thickness of the film being measured times the width of the sample within the pickup window. The distance the sample extends forward and backward (in the direction toward and away from the operator) is not critical, as long as it is least several times the dimension of the pickup windings in this direction, which is about ½ inch. Unfortunately the width of the sample within the pickup window is often poorly defined, as for example when dealing with round samples.

Here is an example of some calculations and calibration work done using a 3.2mm square piece of 4 mil Ni foil:

We were not able to fully saturate this material on the instrument (Model 109) used to measure the sample, but by extrapolating the loop we can estimate the value of Bs when saturation is reached as 210 nWb (good to perhaps 10%). To make the units consistent, we convert 210 nWb (nanoWebers) to 21 Mx (Maxwells).

What we want to calculate for the 4 mil thick, .32 cm wide Ni sample is the saturation induction, 4Pi Ms (in Gauss):

    4PiMs = [21 Mx] / [(.32 cm) ((.004 in) (2.54 cm/in))] = 6459 G

Therefore:

    Ms = 6459/(4Pi) = 514 emu/cm^3

The accepted value of saturation induction for Ni (per Bozorth) is 6084 G. These match to well within the approx. 10% certainty we have in the saturated Bs value. This shows that even with the 3.2mm square sample, which is "short" in the direction of the applied field, we are capturing essentially all of the flux with the pickup we used.

Therefore we can use the Bs measurements to calculate the product of the sample thickness in cm (t) and the saturation induction in Gauss (4PiMs):

Let Bs be the measured value for a given sample:

    4PiMs t = (Bs/10) / (.3 cm) = Bs / 3

Customers who care about accurate vertical axis measurements often do this calibration by starting with two similar samples, and then carefully measuring the Bs value of each sample. One sample is cut into small pieces which are then measured on a VSM (Vibrating Sample Magnetometer). Given the VSM measurement and a knowledge of the cross-sectional area of the film on the sample tested in the VSM, one can extrapolate to the Bs value to be expected for a larger cross-sectional area sample measure on the Looper. If the user is willing to assume good deposition uniformity, only one small piece need be measured in the VSM.

Note that for both vertical and horizontal calibrations is preferable to use a thick sample if possible in order to achieve a higher signal to noise ratio, and therefore a more accurate calibration.

There is only one vertical calibration constant (vertical gain) settable on the instrument. At the user’s choice the calibration can be done based on either the Bs or Br value of the material. There is no point in doing both – the second calibration will simply overwrite the first.

Note that the instrument has vertical attenuation modes to accommodate the measurement of very thick samples. These are called x10 and x100 modes, as opposed to the usual mode of operation, which is referred to as vertical x1 mode.

To perform a vertical calibration, the instrument should be set to the proper vertical sensitivity to display the desired reference loop, with a balance and a pattern memory store having been done before the sample is inserted, to assure a quality loop free of background noise.

The software must be at at least Engineer level before calibrating. The options for Cal Bs and Cal Br are identical, so only Cal Bs will be described here.

Select from the menu “Calibrate”, then “Cal Bs”. There will now be three options. The first, “Current Vertical Attenuation”, will only adjust the calibration constant for the vertical attenuation mode the instrument is now in. In other words, if the instrument is in vertical x10 mode (because we are measuring a thick sample), only the constant for x10 mode will be adjusted. Or if we are in the more normal vertical x1 mode, only that gain constant will be set. Separate constants are kept for each of x1, x10 and x100 modes, so in theory each can be calibrated to a different standard if desired. When this option is selected, the user will be asked for the value in nanoWebers that this sample should read, and the measurements and adjustments needed to make the sample measure at this value will be done.

As with all calibrations and adjustments, the results will be made permanent by being automatically saved to the “constant file” (c:\shbwin\constant.dat) on the hard disk, unless the option “No Constant automatic save” has been selected in the “File/Options” dialog box.

The second choice available under “Calibrate/Cal Bs” is “x1, x10 and x100…”. This option when selected, will calibrate the vertical range of the instrument for all three of the three vertical attenuation settings. Obviously this selection can only be used if a very thick calibration standard is being used, as we must be able to get a usable loop even with the x100 attenuator in place. If this is done, samples will then read the same on all three vertical attenuation ranges.

The third choice “Manual Adjust (delta %)”, is used when the Bs readings being obtained with the instrument are close to the desired value (within 10%), and the user wishes to simply adjust the vertical calibration up or down within the range of plus or minus 10%, without the need to perform a full calibration.

2)  Horizontal Axis

There are two ways to calibrate the horizontal axis of the instrument. The older (and less reliable) method involves having a standard sample with known Hc value. The instrument is set up to display this loop under the same conditions as were in place when the sample was originally measured, and the command “Calibrate Hc/User Value” is selected. The user will be asked for the desired Hc reading, and the instrument will be adjusted to make the sample read that Hc value. But the user must be aware that the measured value for Hc will be a function of sweep frequency (the lower the more accurate – we often use 1 or 2 Hz on the Easy axis), as well as the drive level selected (this affects the dV/dT of the drive waveform, much as does changing frequency). Note that “Calibrate/Cal Hc” also has a “Manual Adjust” option that allows for small percentage “tweaks”, just as for Bs. Calibration of the Transverse axis using the Hc method is only possible on older instruments that have Transverse pickups, using thick enough samples to be accurately measured using the Transverse pickups. The above process is simply repeated, using the Transverse pickup and Transverse field.

The second method of doing Horizontal calibration is more accurate, but require the use of an accurate DC gaussmeter (we recommend the Lakeshore Model 450 for both its AC and DC accuracy), as well as an axial (and ideally a transverse) probe. This method is selected from the menu as “Calibrate/Cal Field, and has two sub-options for “20 Oersteds” or “100 Oersteds” depending on the field range of the instrument being calibrated. An additional higher field choice is available on the high-field Model 110 instruments.

After selecting this option, the user will be taken through an automatic step-by-step process that involves measuring the DC field produced by the instrument using the gaussmeter, and having the instrument adjust its field to match the selected (20 Oe or 100 Oe) value. The process can be repeated several times if needed, and then “Canceled” when the calibration is adequately close.

This process should be repeated with the instrument set to Transverse field, using either a transverse reading probe on the gaussmeter, or an axial probe held in the transverse direction as close as possible to the center of the coils (not possible with solenoid drive coils).

Note that the Lakeshore 450 meter when switched from DC to Peak mode can be used to verify the field strength being produced by the instrument in normal AC operation, but its accuracy at 10 Hz is only 5%, and most other AC gaussmeters are worse.

3)  R Calibration

Resistance mode calibration of the instrument can be based either on an internal 0.02% standard resistor (“Calibrate/Cal R/Internal”), or a sample of known resistance can be put in place with the tool in R mode, and the command “Calibrate/Cal R/User Value” selected to allow the tool to be calibrated to read the proper value for the sample being measured.

Additionally, a manual adjustment feature is also available to allow percentage tweaks in the calibration constant.

4)  dR Calibration

Calibration of dR/R can also be accomplished using internal precision resistors, by selecting the command "Calibrate / Cal dR / Internal").

Further options allow calibration of dR/R with a sample of known dR/R in place ("Calibrate / Cal dR / User Value"). This command has three sub-choices. The first does not assume that a dR/R measurement has been made (but it does assume the tool has been properly put in first R mode and the probes lowered, and then in dR mode). It will make a dR measurement, and then ask the user what the proper reading is, and recalibrate the tool.

The second option assumes that a dR measurement is already present in the Results window, and will use that measurement, along with the desired value specified by the user to do the recalibration. It is called a single point calibration because it assumes the presence of only one dR value, and only asks for one from the user. It adjusts only the gain in dR mode, with an assumption that there is no offset.

The two-point calibration method is more advanced, and assumes that two dR measurements have already been done (typically one on a high-resistance and one on a low-resistance sample), and will ask the user for the correct values for both of these samples. Both a gain and offset constant are then calculated, and used for all future dR measurements. Manual adjust options exist for both the single- and dual-point calibrations.

The DSP-1 Digital Signal Processing option adds powerful signal processing capability to the Model 109A Hysteresis Loop Tracer. It is particularly effective in dealing with the low signal-to-noise ratios encountered when measuring very thin and/or small samples.

Installation of the DSP-1 is a simple process, basically involving installing the DSP-1 circuit board (labeled "Filter") in its pre-defined slot in the instrument chassis. Software support for the DSP-1 will be automatically enabled.

1. Turn off the power to the instrument.

2. Remove the four screws holding the front panel into the rack, and pull on the pair of handles to slide the instrument chassis fully out until it stops.

3. Identify the empty slot in the cardcage, just to the rear of the board labeled "Attenuator", and just in front of the board labeled "Sweep". If you look down at the motherboard next to the empty edge connector, it will be labeled "Filter".

4. Slide the DSP-1 circuit board through the guides of the card cage, and seat it firmly in the edge connector on the motherboard.

5. Turn on the instrument.

6. Operation of the instrument should be unchanged, unless the Digital Filtering algorithm is enabled by pressing the FILT button on the front panel, or using the test procedure command FilterOn. Pressing the FILT button again, or using the command FilterOff will turn off filtering, while FilterReset will reset the filtering process in order to speed up settling. Such resetting is done automatically when major changes to the instrument status are made, such as a sensitivity change.

7. From the main menu in the instrument software, select Adjustments/Trim Filter, to trim the hardware on the DSP-1. Then select Commands/Constants/Write File to make these new values permanent.

8. The menu choice Commands/Filter Algorithm is used to change the algorithm used by the DSP-1. The default choice (Exponential Averaging) is the only useful one at this time. Selecting Commands/Filter Time Constant brings up a dialog box that allows the time constant of the exponential averaging algorithm to be changed. A longer time constant will improve noise reduction, but at the cost of increased settling time for the displayed waveform. Any changes made here will only become permanent if Commands/Constants/Write File then done to save the new value.

9. Note that if both the filtering and pattern memory features are to be used (as is typically done), the filtering should be enabled before any patterns are stored. Please call or email if you have any problems or questions.

We are always interested in feedback on any problems you may be having with our products, or suggestions for changes that you would like us to make.

The upgrade to the new High Voltage R/DR option consists of the following:

1. R/DR Board, part number 109-060B
2. Upgrade of software to latest version (if necessary)
3. Plastic high-voltage shields with retaining rings for all pickup assemblies used on your instrument.

Please install one shield on each pickup, by unplugging the R/DR cable, slipping the shield in place over the R/DR probe assembly body, securing with a supplied retaining ring (each ring has two set screws), and then plugging the pickup R/DR cable back into its connector on the rear of the pickup. The shields come in three types 1) for 1" and 3" pickups, 2) for 6" pickups, and 3) for F88-type pickups. In addition, the shield provided for -34 and -64 magnetostriction pickups have an additional slot cut in them to accommodate the levers on the top of those pickups.

The shields are installed with the large side toward the front. If there is any problem with the fit of the shield let us know -- it may require some filing to fit older pickup assemblies.

The purpose of the shield is to prevent the user from making contact with the sample while the high voltage is on. The high voltage is on for one second or so after the probes are lowered in R mode. You can see when the high voltage is on, because the E (red light) is blinking during that time.

Also, you need to edit your constant file (C:\SHB\CONSTANT\CONSTANT.DAT). There is a constant called REV_R/DR. The value is now 1, and needs to be changed to 2, or the high voltage will not be enabled. This variable is used to keep track of the revision level of the R/DR PCB. To make this change, edit the above specified file. Search for the text string "REV_R/DR". The first occurrence will be a line beginning with a semicolon. This is a comment line. Search for the next occurrence. It should be a line that reads "%REV_R/DR D 1". Change it to read "%REV_R/DR D 2". Then save the file you are editing.

Now go to the "Adjustments" menu, and choose "Trim R/DR". When this completes do ALT-SAVE from front panel (press ALT, it will blink. While it is blinking press SAVE button). This saves the new trim value.

From the "Calibrate" menu, choose ""Cal R", then "Internal". When done, choose "Cal R/DR" also from the Calibrate menu. Do ALT-SAVE when done.

Operation of the R/DR measurements should be unchanged, other than the blinking of the E light when the probes are lowered, and the improved contact reliability.

Please contact us with any questions.

Issues relating to year 2000 compatibility of the Shb Instruments Model 108 and Model 109A Hysteresis loop tracers are discussed in detail below.

• The internal microprocessors on both the Model 108 and the Model 109A contain no date and/or time information.

• All automatic control of either instrument is handled by an external, independent personal computer, which in turn communicates with the instrument itself.

• The Shb-written control software that executes on the PC connected to the instruments makes no use of date and/or time information, other than to display it on the screen, write the date and time into the header information in data files written to the disk, and to display it at the top of a printout of measurement data.

• Any problems with Y2K compatibility will be determined by the hardware (BIOS) and operating system software on the controlling PC.

• Model 108s used a variety of older PC compatible computers, some of which may no longer be the original PCs that were shipped with the instrument. The operating system used on the PC controlling the Model 108s was MSDOS, versions ranging from 2.2 to 3.3. The Y2K compatibility of these machines and their operating system is unknown.

• Model 109As use Pentium-class PC compatibles. The early Model 109A instruments used a variety of brands, for which the Y2K compatibility is unknown. More recent Model 109A instruments have used Dell brand PCs, which are claimed by Dell to be Y2K compliant. The operating systems used on the Model 109A computers are one of Windows 95, Windows 98, or Windows NT 4.0. The Y2K compliance of these operating systems can be determined by contacting Microsoft.

• Determining Y2K compliance of the controlling computer (PC) and the operating system used on that PC are the responsibility of the user of the system.

• Even if the computer and/or operating system are not Y2K compliant, it is our expectation that the most serious consequence would be cosmetic, in that an incorrect date and/or time would be displayed and/or printed.

On a hysteresis loop tracer like the Shb Model 109A, when the rate of change of applied field increases, eddy currents induced in nearby conducting materials increase as well. These currents are 90 degrees out of phase with the drive field, and when detected by the pickup coil in the instrument, cause a phase error in the displayed loop. These phase errors can result in small changes in the displayed and measured Hc values. The two instrument settings that affect rate of change of drive field are drive field frequency, and drive field amplitude (horizontal sensitivity).

The Model 109A instrument has an adjustment (actually a constant value and a corresponding D-to-A converter) we call "drive field phase". This adjustment was added as a means of compensating for the increase of Hc values observed as frequency and drive level are increased.

When the DRIVE_PHASE_NORM constant is set to 0, there is no correction made for this phase shift. Maximum correction (usually more than is needed) is with this constant set to hex value FFF.

The amount of eddy current that flows, and therefore the amount of the phase shift, is proportional to the rate of change of the field. So the higher the drive frequency, and the higher the drive amplitude, the greater the phase shift that we need to correct for.

The method of adjusting this constant (described in detail below) is based upon making an Hch measurement at low frequency (0.5 Hz or 1 Hz) and low drive. Then the instrument is set for 10 Hz and high drive (10 or 20 Oe / division), and the phase constant is adjusted so the Hch value (which will have increased with no phase correction) is decreased back to roughly the same value it had at low drive and low frequency.

To start, make sure that you have done a Trim of the drivers board, and the earth’s field cancellation is properly adjusted in the Norm and Tran directions.

• Set a sample to the hard axis.

• Set frequency to 10 Hz, drive to 0.2 Oe/division ("low drive").

• Select from the menu Measurements/Horizontal/Hc. You will now get the Measurements dialog box.

• Click the Measure button. You will now have an Hc value for low drive on the screen.

• Change to 2 Oe/division ("high drive").

• Check the "Repeated measurements" checkbox in the Measurement dialog box.

• Click the Measure button to get automatically updated high drive Hc values on the second line of the screen.

• Select from the menu Commands/Low-level IO/Write DAC. You now get another dialog box.

• Click the arrow next to the "DAC Name" field, and select the name "NORM Field PHASE Correction" from the list. Record the hex value that the phase correction is currently set for (shown in red to the right of the slider). The decimal equivalent is in red to the left of the slider. Save this value (i.e. write it down) in case we need to go back to where we were. Use the mouse to change the slider (it will be sluggish). Clicking above and below the elevator box will change by decimal 100 steps. The arrows at the top and bottom are for finer steps.

• The goal here is to get the high drive Hc value (which is being constantly updated) about equal to the low drive value shown on the line above. No need for perfection here...just get it close. Now write down the new hex value that we want to use (red value to the right of the slider).

• Click the Cancel button in the "Write DAC" dialog box.

• Bring the Measurement dialog box back, by clicking the button on the task bar labeled "Measurements".

• Uncheck the "Repeated measurements" checkbox.

• Click the Cancel button to close the Measurements dialog box.

• Select from the menu File/Exit.

• Make a backup copy of the constant file C:\SHB109\CONSTANT\CONSTANT.DAT.

• Edit the constant file with a text editor (e.g. Notepad). Search for the second occurrence of #DRIVE_PHASE_NORM (the first is preceded by a semicolon, and is a comment). Note that there are 5 values for this constant. Each is preceded by a letter (T, S, M, L, H) indicating what size coil assembly it applies to (Tiny, Small, Medium, Large, Huge). Change the proper constant depending on the drive coils you were using (typically Small or Large) to the new hex value. You can be sure you have the right constant by the fact that the value before you change it should match the hex value next to the slider you wrote down earlier, before you changed the slider.

• After changing the value, save the constant file (as a text file).

• Restart the shb109 software, and check the Hc values at 1Hz, 10Hz and low and high drive. The increase in Hc at high drive and high frequency should be much smaller or gone.

• If you use both coil assemblies, repeat the process with a sample in the other coil assembly.

There has been a higher than expected rate of amplifier failures on the Model 109A instrument. We are now using a new, higher power transistor (IRF 150/9150), but that may not be sufficient to fully solve the problem.

Therefore we strongly recommend that the following steps be taken:

1. In the constant file, change the TEMP_HS constant from 75 to 60. This limits the amplifier maximum temperature to a lower, safer value.

2. The latest version of the software (2.61) has an option to allow the heatsink fan to run at full speed continuously. If the instrument is not operated in a noise sensitive environment this will aid in cooling. This version will also reduce the drive level automatically if the instrument is left unused at a high setting. Upgrades to the latest software version are available from us on request.

3. In order to minimize power dissipation in the amplifier, we control the voltages supplied to the amplifier, reducing them as the need for drive field is reduced. But we have found an error in those numbers in the constant file. It affects the 10 Oe/division horizontal scale with the large coil assembly. This change will not affect calibration in any way.

4. Search the constant file for the #PROG_DC constant. Then find the text "40 10 10 L" within that constant. Change that text to "30 10 10 L". On the line above change the "35 10 5 L" to "30 10 5 L". This change sets the voltage to +- 30 volts when at 10 Oe/div for 5 Hz and 10 Hz. Previously that voltage was +-40 volts. This change will reduce the heat sink temperatures by about 12 degrees C at the 10 Oe/division setting.

5. Make sure that your test procedures do not leave the instrument at high drive levels unnecessarily.

We apologize for the inconvenience, and are working on improvements to the design to eliminate this issue.

Adjustments

The adjustment of the Shb Instruments Magnetic Measurement Systems has been made much easier with the new Shbwin control software. Note that Adjustments typically account for and remove small internal offsets and errors in the instrument electronics, and are to be distinguished from Calibrations, which are used to make the instrument measurements agree with internal or external standards. All adjustments are contained under the Adjustments menu entry.

Most of the important adjustments can be invoked by selecting Adjustment / Trim Instrument / Auto. This will open a dialog box and step-by-step adjust each of the internal boards in the instrument. Any significant changes to these adjustments will be reported in the dialog box as Warnings, which are of significance only if unusually large. If an adjustment cannot be made at all, an Error will be reported, which is of more significance, and usually indicates a hardware failure. It is often useful to do this automatic adjustment twice.

Sometimes it is desirable to do only some of the internal adjustments, as for example when only one circuit board has been replaced. Individual board adjustments can be selected from the menu as Adjustments / Trim Instrument / Individual. There are six board adjustments, as well as an entry for Adjust Sweep Width, which can be used if the sweep does not properly fill the width of the screen. These seven adjustments are all done when the automatic option is selected.

One additional adjustment exists under Adjustments / Trim Instrument: Adjustments / Trim Instrument / Adjust Offset / Center. This adjustment is rarely done on its own, but will automatically be done following any Horizontal calibration of the instrument.

The next item on the Adjustments menu is Adjustments / Adjust Balance Constants / Norm Pickup. This adjustment is used to make sure that the instrument can properly tilt the trace to horizontal when needed, and should be done if a new, never before used pickup is installed. Routine changes between pickups should not require that this adjustment be redone, as once they are calculated, the adjustment values are saved in the constant file for each pickup. This adjustment will be automatically done after any vertical calibration operation.

Adjustments / Adjust Drive Field Phase is a specialized adjustment that corrects very low-level errors in Hc measurements. It requires the user to have a special Dysprosium paramagnetic sample, which is available from Shb Instruments. It can be done separately for each frequency range, as it is a slow adjustment, and is typically done only for frequency ranges of interest to the customer.

Adjustments / Adjust Earth’s Field / Adjust is an automated process that leads the user step-by step through the adjustment of the compensations for the Earths DC magnetic field. This must be done for both the Normal and Transverse axis, and should be checked on approximately a monthly basis. If the Normal axis Earth’s field is misadjusted, the hysteresis loop will be offset horizontally. If the Transverse Earth’s field is misadjusted, it will often be seen as an inappropriate asymmetry in a Hard axis loop.

Adjustments / Adjust Tilt / Region Control… is an advanced adjustment that allows the user temporary control over what part of the BH loop is assumed to be saturated and is therefore used to tilt the loop to the horizontal position. Much more sophisticated control of the tilt regions is available on a recipe by recipe basis using the Autotest automated measurement facility.

Maintenance

The Maint menu has as its first entry Internal Voltage Diagnostics. When selected, this will display a dialog box with various internal DC voltages being repeatedly measured. Any voltages that are out of range will be displayed in red, and may indicate a hardware failure. On the Model 110 instrument, an additional display indicating the remaining life of the resonating capacitors will also be displayed. In Factory mode an advanced button is available, which will cause measurements of various AC sine waves in the instrument to be measured, and have the waveforms optionally displayed. This is usually used only by the factory when doing remote diagnosis of an instrument.

Maint / Skew and Up / Down Switch Test is used to enable testing of the mechanical switches that are used in the pickup assemblies to detect the position of the skew latch and whether or not the R/dR probes are lowered.

Maint / Repeatability Tests enables an option for doing static repeatability measurements of a sample. The sample must be in place with a proper loop displayed, and the user can then select the number of measurements to be done, as well as the time interval between measurements. The results are automatically written into an Excel spreadsheet, as well as being graphed.

Typical measurements on hysteresis loop tracers include Saturation Magnetization (Bs), Remanence Magnetization (Br), Coercivity (Hc), Dispersion (statistical distribution of angles of magnetic domains), and Skew (misalignment between magnetic and physical axis).

Additionally, the instrument includes a set of probes which can be lowered onto the surface of the material, to measure its sheet resistance, as well as the change in resistance with applied magnetic field (magnetoresistance, or deltaR (dR)). The later is of particular importance to the manufacturers of disk drive heads, as the latest technology heads rely on the magnetoresistive (MR) or giant magnetostrictive (GMR) effects.