In this tutorial we will build a nickel pillar and compute the hysteresis loop. Start the builder, java –jar JaB.jar. Click on the Grid button. Generate the initial grid with n=m=15 and l=20 and dx=dy=dz=10e-09. We chose n and m equal to 15 so that the cylinder can be centered on the grid. This really isn’t important, it’s really for asthetics.
Now, click on the Regions button. On the RegionsChooser widget click the AddRegion button. The MaterialsChooser widget will appear. From the MaterialsChooser widget select nickel, Ni, and then click OK. From the MaterialsChooser widget click on the cell for the Ni row and the Sample column and select cylinder from the combobox.
The RegionChooser widget supplies hints for the information needed in the additional columns.
Fill in the rest of the appropriate columns of the Ni row. We will put the middle of the cylinder at (8,8) starting at the bottom edge (z=0) and with a height of 20 (all the way to the top layer) and a radius of 7. Be sure to tab past the last cell or click on the last cell. Java has a problem that the value entered in the cell is not always updated until the cell is exited.
When you click the Done button the following should appear…
This is the pillar looking from the top. You can step through the layers using LayerUp and LayerDown buttons, however each layer looks the same for the pillar. Save the result. When you save the result the program will request the exchange coupling constants for Ni-Ni. Choose the defaults. Then the program will request the initial spin and easy axes directions, choose random for both. The chooser widget also specifies a cell constraint. Cells in a given region can be fixed, meaning their dimensions cannot change, or variable, meaning the dx,dy and dz values can change. This is currently used in computing the strain contribution to another program. For the micromagnetics program this is currently not used, so use the default. The program will inform you that the cml file has been saved.
Now, submit the job. Start the scheduler via, java –jar JaSCH.jar and then click on the Refresh button to see nodes that are available. Select one of the nodes and click on the Schedule button. Click on the GetSampleFile button and find and click on the cml file you saved the pillar to, mine was nipillar.cml. Follow tutorial2 until you get to the JaMES Configuration widget. For a hysteresis loop we need to apply an external magnetic field. Also, we will change the basename to something appropriate, such as nipillar.
Click on the OK button and the Acqusition widget appears. We set some of the usual values for a micromagnetics simulation as given below. Note that we set Output on Termination to true. This way the final magnetic configuation at the end of each field step in the hysteresis loop will be saved to disk. Click on the OK button. In the Output Properties widget that appears next you must set the Magnetization? field to true, the others are option. Click on the OK button. The AppliedField widget appears. Note, as the header says, the fields are in milliTesla.
In this example we will compute a virgin loop from zero field to the saturating field of 300.0 mT in 10 steps and then the outer loop from 300.0 mT to –300.0 mT in 30 field steps and then back again from –300.0 mT to 300.0 mT in 10 field steps. Click on the Add Field button twice and then enter the information as follows…
Click on the Done button. The LLG Parameters widget appears. Use the default values and select OK. In the Schedule widget click the Submit button. The job should now be running. Click on the Status button in the JaSCH widget and bring up the Status widget.
The widget shows the job we just submitted. The first column shows the node upon which the job is running, the second column shows the name of the job (with the node that submitted the job), the third column shows how many iterations of the current field step have been completed and the last column the number of stages that have been completed. Let’s look at the stages first. We configured the job to have 10 field steps in the virgin loop, 30 field steps in the upper loop and 30 field steps in the lower loop. That’s a total of 70 field steps, so there are 70 stages. We haven’t completed any yet, so were are on stage 0. So far we have completed 1 out of 2000. We will let the job run at each stage until the maxumim torque on any spin is less than the convergence or until we have reached 2000 iterations. Periodically check the job’s progress by clicking on the Refresh button in the Status widget. When all of the stages are done the background will turn red. The other buttons do the following: Stop kills the selected job, Suspend temporarily suspends execution of the selected job, Resume resumes execution of a suspend (selected) job, Clear permanently clears the entry of the job status from the status widget, Write forces the job to write an intermediate cml file to the remote node and Done exits the status widget. The Write button is useful for saving a result from a job that is taking too long to complete. If you get tired of waiting on a job you can write the output and then stop the job.
When the runs are complete and the background is red, then you can transfer the files and view the hysteresis loop. Start JaSEE via java –jar JaSEE. Click on the OpenFiles button and select all of the files you transferred. Then click on OK. You should see a list of those files in the JaSEE widget.
Click on the files you wish to plot. Note that you can use the standard windows techniques of holding down the shift or control keys while selecting field to easily select more than one filename. In this example we’ll skip the “virgin” loop and start selecting from the 0010 stage. Click on the 2D plot button and select the correct x-value and y-value from the 2DplotProperties widget. Since the applied field was along the x-axis I choose hx as the x-variable and mx as the y-variable. NOTE JaSEE has CHANGED since this tutorial was written. I am in the process of writing a new tutorial. However, the new version should be fairly obvious on usage.
Finally, click on the Done button. A progress bar appears showing the progress in parsing the files and then the hysteresis plot should appear.
This one looks a little funny because of the relatively large field step chosen, i.e. only 30 stages over 600.0 mT. Try running the input file with a larger number of steps.