Remark: Evico magnetic‘s single-sheet magnet is especially designed for the investigation of electrical steel sheets in standard Epstein geometry (30 x 300 mm, thickness up to 0.5 mm), but it can

Remark: Evico magnetic‘s single-sheet magnet is especially designed for the investigation of electrical steel sheets in standard Epstein geometry (30 x 300 mm, thickness up to 0.5 mm), but it can as well be used for amorphous or nanocrystalline ribbons (< 30 mm width, length 300 mm). The magnet consists of two field coils with an opening for domain observation, a flux-closing yoke made of laminated electrical steel, and two pick-up coils for the inductive measurement of magnetization loops from quasistatic frequency up to the kHz range (program „Fluxlab“, see Sect. 3).

Tensile stress up to 100 MPa and compressive stress can be applied during domain observation and loop measurement. The magnet is placed directly on the (general) magnet holder. To change the position of observation, sample and magnet are moved together by hand.

1. Installation

Remark: Two pick-up coils with winding numbers 20 and 200 can optionally be chosen with the switch at the control box of the single-sheet magnet. The 20-turn coil is recommended for electrical steel samples in standard Epstein geometry, while the 200-turn coil should be chosen for amorphous and nanocrystalline ribbons of low cross sectional areas. Note that the winding number has to be selected on both, the control box of the magnet and the air-flux compensation box!. During switching, the measurement has to be stopped (AC run off, see Sect. 3).

2. Sample assembly

• Remove yoke screws, open clamp screws, pull out clamps, remove frame together with sample

• Insert new sample in coil slit

• Add yoke pieces on two sides

• Insert coils with sample and yoke pieces into frame

• Slip sample in clamp at load cell and tighten clamp screws

• Slip clamp on free side over sample and tighten screws

• Fix yoke screws

3. Magnetometry (FluxLab)

With the program FluxLab, the magnetization loops of sheets or ribbons in the single sheet magnet can be measured inductively

Note: If FluxLab is used in combination with KerrLab (e.g. for simultaneous domain observation), the magnetic field activation in KerrLab has to be disabled. To do so:

a) Open KerrLab

b) In the KerrLab menu bar, chose Preferences -> Magnetic field setup -> Device preferences. All control and measurement buttons have to be deactivated!

3.1 Turning-on and parameter setting

a) Switch on Kepco power supply

b) Start the FluxLab measurement program (icon „FluxLab“ on desktop). The main window MH-Loop will open

c) Select Preferences in the main window. The Magnetic field control and measurement setup window is opened. Define the following parameters:

• Pick-up coil: chose the winding number (20 or 200) that was selected on the control box of the magnet and on the air-flux compensation box. During switching at the two boxes, the measurement has to be stopped (AC run off)

• Air Flux Constant: This constant defines the air flux µ₀ H that is subtracted from the total pick-up signal B in order to obtain the pure sample polarization J (with B = µ₀ H + J)

If Standard is chosen, a predetermined value is used. Otherwise the user may set his own constant by selecting User defined and inserting a value. The constant is determined experimentally without a sample inserted in the magnet. In case of insufficient air flux compensation, a linear „magnetization“ curve with a certain slope will be obtained. By adapting the Air Flux Constant, the slope can be minimized. Although a change of the preset constant is not to be expected, it is recommended to prove the air flux compensation from time to time

Deactivate

• Integration Constant: This parameter determines the sensitivity of the measurement. It is recommended to use a high value (10^-1) for electrical steel sheets and small integration constants (10^-2) for amorphous ribbons. The smaller the integration constant, the higher the sensitivity, but there might also be an increase of the drift

• Field Offset: Default setting is zero. This parameter is only relevant if the Kepco power supply is operated in the voltage control mode, as recommended for the measurement of electrical steel at high frequency (see Sect. 3.4). The parameter should be in the range of ±10 V

• When all parameters are chosen, close the window by pressing Done

d) Type in the sample cross section in the window Crossection of the main MH-Loop window. The cross section is given in square-millimeters

3.2 Running the measurement

a) Select the proper parameters for the measurement in the MH-Loop main window. Like in KerrLab, the parameters can changed by typing the numbers or by rotating the wheel of the shuttle device after the corresponding field has been activated. The parameters can be changed in-situ while the measurement is running (note that Average has to be deactivated for changing the parameters)

• Signal type: sinusoidal or triangular waves for the drive field can be chosen

• Amplitude: select the amplitude in units A/m

• Frequency: The frequency can be freely chosen between 0.05 Hz and 1010 Hz, but the range of frequencies is divided in two modes: Normal (1 – 1010 Hz) and Ultra Low (0.05 – 2 Hz). Before typing-in the desired frequency, the corresponding mode must be selected by choosing Frequency Mode from the menu

• Gain: The gain determines the amplification of the integrated signal. Chose 1 by default. In case of noisy hysteresis curves, select higher values

b) Start the M (H)-Measurement by pressing AC run. The magnetic polarization µ₀ M [in units Tesla] as function of the magnetic field [in units A/m] is displayed in the main MH-Loop window. Parts of the loop can be magnified by using the LabView zoom tools (see Sect. 3.3)

c) Noisy curves can be improved by activating Average

d) The Symmetrize button in the main MH-Loop window shifts the magnetization loop along the field axis in case of a field offset in order to obtain a symmetric loop with respect to zero field

e) The following loop parameters, taken from the last or averaged loop, are listed below the loop: (i) maximum field amplitude in A/m, (ii) negative and positive coercivities in A/m, (iii) maximum magnetic polarization in Tesla, and (iv) loop area (loss) in J/m³

f) The raw data of the magnetization curve can be viewed by choosing Acquisition. Three graphs are displayed:

• Black: Drive voltage U _drive(t) of the field coil

• Red: Integrated pickup voltage ∫ U _ind(t)d t

• Blue: Induced voltage U _ind(t) in the pick-up coil

Note: The displayed voltages should not exceed ±10 V to prevent damage

g) Data storage:

• The magnetization loop can be stored on the hard disk by pressing the Save button. A file with the name specified in the field File name will be saved to the directory specified in the field Select data folder on disc to. To specify the data folder, press the button with the folder image to the left of the corresponding data folder field. A dialog will appear, in which the desired folder can be chosen

• The saved data file consists of two columns: (i) field in kA/m and (ii) magnetic polarization in Tesla. In addition to the data file, the main parameters of the

measurement are added to the info.txt file and stored in the same directory as the data file. Any comment, displayed in the field Comment, will also be stored in the info.txt file

h) Closing the Program: The program can be closed either by choosing the menu item Exit in the main window or by pressing the cross in the right upper corner. The measurement, if running, will stop automatically

3.3 Working with graphs in LabView

• By default, all graphs are auto-scaled. By clicking on the right mouse button, the graph scaling can be changed. Just check out AutoscaleX (or/and Y)

• After disabling autoscaling, the zoom tools below the graphs can be used

• After pressing the looking glass button, different zooming modes can be selected:

1. Zoom in the region

2. Autozoom (show all the curve points)

3. Zoom in the X scale

4. Zoom in the Y scale

3.4 Voltage-controlled experiment

For the measurement of electrical steel at frequencies higher than 100 Hz it is recommended to drive the Kepco power supply in the voltage-controlled mode

a) Chose the proper settings on the Kepco front panel

b) Consider that in the voltage-controlled mode the AC-amplitude, to be chosen in the main menu, is no longer valid. The actual amplitude now rather depends on the frequency. It has to be adjusted as mentioned in Sect. 3.1c

c) The ‘Offset Voltage’ (see Sect. 3.1c) is needed to eliminate a hardware offset of the KEPCO current source, due to which the curve becomes non-symmetric

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