Page 1 of 1

Spin Valve differential conductance measurement

Posted: August 10th, 2011, 2:42 am
by debayanmitra
I am currently using the Keithley 6221/2182A to carry out high sensitivity differential and delta measurements of magnetoresistance of nanometric spin valves. However I have a big issue with the predefined delta and diff cond. modes of operation. Technically speaking, once the mode is initiated, the output current jumps from 0 to the desired value and then falls back to 0 as soon as the measurement is finished. Since the samples I use are extremely sensitive to brutal current changes, the longevity of samples is jeopardised by these modes.

I tried to program the delta mode manually using LabView but I cannot reach the same level of precision and execution time. It happens mostly because of two reasons:
1. I generate the waveform (square) in labview and send alternating polarised (positive or negative) current as a function of time through the GPIB
2. I request the 2182A to return the voltage reading for every current value (every trigger) through the series connection to the 6221 and then to the computer via GPIB. This process adds an unsurmountable delay of 150 ms between two consecutive readings

The present scenario affords two solutions :
1. If I can synchronise the trigger which sets the current in 6221 with a volatge measurement in 2182A that simply stores the data in the buffer (I tried it but I cannot synchronise, i.e. I cannot be sure later on, which voltage corresponds to which current)

2. Add another dc current source (like Keithley 2400) in series with 6221 to maintain the necessary dc current bias. But this generates a new problem of impedance mismatch and necessitates the design of a bias tee.

I would be glad to obtain any help whatsoever regarding these two possible solutions. Which one is more feasible ?

Re: Spin Valve differential conductance measurement

Posted: August 22nd, 2011, 7:23 am
by Mickael B
You should use built in triggering functionnalities of our models 6220 and 2182.
In remote mode, once a current level is settled, a pulse is outputted (action completed). This pulse can be used to trigger a voltage measurement on the nanovoltmeter (measurements stored in the buffer). This means that the nanovoltmeter must have the correct configuration (acquisition triggered with an external pulse)

Both instruments can be control with GPIB interface (set configurations, collect data). Instruments are also connected together with a TLink cable (Trigger Link) for triggering actions.

Pin 1 of model 6220 Tlink connector is defined as a trigger output , pin 2 as a trigger input .
Pin1 of model 2182 is defined as a voltmeter complete output, it means that at the end of each DCV measurement a pulse is outputted.
Pin 2 is an external trigger input.

Pin 1 of model 6220 Tlink connector must be connected to pin 2 of model 2182 Tlink connector (when the current is settled, a pulse is outputted on pin 1of the 6220 Tlink connector and triggers the nanovoltmeter.

This solution should limit communication time whatever the interface used.

Please kindly refer to the triggering sections of the reference manual and the user manual of both instruments.

http://www.keithley.com/products/dcac/c ... ocuments#6
http://www.keithley.com/products/dcac/s ... ocuments#6