Post by Professor Lake Shore on Sept 7, 2017 14:44:19 GMT -5
Adding a sensor curve without a serial number isn’t working properly when using the CRVHDR command. Why is this?
The instrument ignores spaces and commas when parsing the CRVHDR command. Therefore, you need to enter a term in the serial number field even if the sensor doesn’t have a serial number. An example command that will work reliably for a sensor without a serial number would be CRVHDR 21,SENSOR21,NONE,4,325,1 where NONE is used as the serial number. See Section 6 of the instrument manual for more details on this command.
Post by Professor Lake Shore on Aug 29, 2017 9:09:54 GMT -5
Even researchers who are experienced in electronic device measurement sometimes do not get the results they expect. Lake Shore Cryotronics has published a new whitepaper, “5 Electronic Measurement Pitfalls You Learned About in School But Probably Forgot.”
This resource addresses several common, yet often overlooked, pitfalls to avoid, helping researchers prevent problems such as:
Post by Professor Lake Shore on Mar 29, 2017 14:28:16 GMT -5
Short of upgrading to a “dry” cryogenic probe station, what are some things that I can do to reduce ongoing LHe consumption in my station?
Here are steps you can take to help minimize LHe consumption in a “wet” cryogenic probe station:
Ensure that you have good vacuum in the probe station. In short, poor vacuum equates to a higher cooling load. Proper vacuum is important because it reduces heat transfer to the cold stage via the residual gas in the chamber as well as the thermal load due to the solidification of the gases on the cold surfaces. To operate efficiently, the probe station should be able to achieve a vacuum of <10-3 Torr at room temperature using the vacuum system and with the gauge on the chamber. Chapter 6 in the user manual provides diagnostic procedures for verifying that the vacuum pumping system and probe station vacuum chamber are functioning properly.
Ensure that you have good vacuum in the transfer line. If you see that you are requiring more and more LHe to cool the probe station (as shown in the cryogen consumption spec listed in the user’s manual), the problem may be a loss of vacuum in the jacket of the transfer line resulting in a “soft” transfer line. A tell-tale sign is frosting of the transfer line during operation. Soft vacuum can be caused by leaks that have developed in the jacket and O-ring seal as well as the buildup of outgassing in the absorbent charcoal filters within the line. The solution is to attach the line to a turbo vacuum pump and evacuate the line at room temperature, typically over the course of a day*. This will release what is stored in the charcoal and “recharge” the vacuum space. (See Chapter 6 in the user manual, which provides more details on troubleshooting for vacuum integrity.)
Ensure proper maintenance of storage Dewars. Typical storage Dewars boil-off roughly 1 to 2 L of liquid helium per day; excessive boil-off rates point to faulty or poorly maintained storage Dewars where loss of vacuum in the Dewar jacket is often to blame. Contact your helium supplier or Dewar manufacturer for maintenance procedures.
Regulate the pressure on the Dewar during operation. The correct amount of pressure creates efficient cryogen flow (see Chapter 3 in the user manual for recommended gas pressure rates). Excessive pressure in the Dewar can result in higher transfer rates in which more hot gas flows into the storage Dewar and increases boil-off. Additionally, with higher transfer rates, the enthalpy of the cold helium is not fully utilized in the heat exchange mechanisms of the probe station before being vented to atmosphere – effectively wasting cooling power.
Limit the flow rate during cooldown and at higher temperatures. At the expense of cryogen consumption, higher cryogen flow rates are conventionally used for faster cooldown and increased measurement throughput. When cooldown time is not a significant factor, helium can be conserved by slowing the cooldown with a reduction in the foot valve opening – typically 2 to 3 turns. Additionally, when you’re experimenting at higher temperatures, you don’t need as much cooling capacity. In this situation, adjust the flow as needed to conserve helium usage. (Chapter 4 in the user manual details this procedure. Pay particular attention to the recommended heater output power.)
* A compact turbo pump is required to perform all of the evacuation procedures listed above. It is also recommended that you contact Lake Shore Cryotronics for assistance before evacuating lines.
Post by Professor Lake Shore on Jan 31, 2017 18:04:25 GMT -5
I have a sample with unknown properties. What sort of VSM measurement should I start with?
Typically, a good starting point is to measure first the magnetic hysteresis (moment vs. field) for the full range of field at room temperature. Then the experiment can be optimized to focus on an important part of hysteresis or to focus on certain properties.
Post by Professor Lake Shore on Jan 31, 2017 18:01:45 GMT -5
What is the best way to attach the sample to the sample holder used in a VSM?
One should judge the application to determine the best way to attach a sample. In the case of measurements performed at room temperature, a sample can often be held in the sample holder with a screw nut – you put a sample in, tighten the nut, and it stays there. However, if you have a thin film to be attached to either a side- or bottom-mount sample holder, the sample can be secured to the sample substrate with some type of adhesive, such as glue or even vacuum grease. But vacuum grease may not work for a heavier sample. For heavier thin film samples, Super Glue can be used. The glue, when it dries, is very easy to peel off from a holder, allowing the sample to be removed easily from the holder at end of an experiment. Also, Super Glue does not have magnetic moment. However, when high or low temperatures are involved, it’s a little bit more complicated. For high-temperature measurement applications, it’s best to use a ceramic cement. For low-temperature applications, GE varnish (available from Lake Shore as VGE-7031 varnish) is the preferred adhesive because it has no signal at any temperatures and it’s easy to dissolve with acetone when cleaning the sample and holder after an experiment.
Post by Professor Lake Shore on Jan 31, 2017 17:59:41 GMT -5
Is cooling water always needed for VSM operation?
Yes, because both the electromagnet power supply and the electromagnet itself need cooling to dissipate the heat during operation. The water flow values are very important. You cannot do even a limited field range type of measurement without cooling.
Post by Professor Lake Shore on Jan 31, 2017 17:58:29 GMT -5
What is the difference between continuous and point by point modes of data acquisition?
When using a VSM in Point by Point Mode, the measurement software sets the field for the desired point, allows the system to settle, then measures the moment under the constant field value. In contrast, when operating in Continuous Mode, the software sweeps the field and measures the moment as the field continuously changes. In this acquisition mode, VSM sensitivity is fixed. A continuous measurement is a very fast mode of acquisition but less precise than point by point acquisition.
Post by Professor Lake Shore on Jan 31, 2017 17:57:02 GMT -5
How does the Moment AutoRange function work?
During an experiment, it adjusts to the best range of sensitivity automatically. Without Moment AutoRange turned on, the VSM will use the moment range specified with the experiment. With AutoRange enabled, the system picks the range which is best to measure the sample, automatically switching ranges as needed during an experiment. For experiments where you need to measure on a broad range of sensitivity, enable Moment AutoRange. In Point by Point Acquisition Mode, operation is different from Moment AutoRange enabled while operating in Continuous Mode. When operating a VSM in Continuous Mode, a modified version of Moment AutoRange is used at the beginning of a Continuous Mode experiment, and the applied field is ramped to the maximum field selected. While the sample is at the maximum field, an appropriate moment range is determined. That moment range is used for the rest of the experiment. The experiment then continues normally. The AutoRange mode of operation is particularly useful in experiments that are used in profiles where moment magnitudes are likely to change. However, please note: Do not use Moment AutoRange with Continuous Mode in experiments where the initial excursion to high applied field will interfere with the experimental data (such as when performing an initial magnetization curve measurement).
Post by Professor Lake Shore on Jan 31, 2017 17:55:14 GMT -5
Are there any limits in sample size and weight for a VSM?
In general, the sample should fit in a sample holder, because the detection geometry of the detection coils is optimized for measuring a sample no larger than that of the holder. If the sample is too large or too small for the holder, the detector loses measurement information because it is unable to “see” the edges of a larger sample and because it is not close enough to a smaller sample. The weight is also important. If the sample exceeds a certain weight, the head drive will be unable to move the sample effectively as it vibrates.
Post by Professor Lake Shore on Jan 31, 2017 17:54:05 GMT -5
Can I have more data points in a certain portion of my measurement?
Yes. This is done to concentrate, for example, on hysteresis curve slope changes (i.e. switchings). One can program such an experiment as a custom ramp using the Ramp Generator function of the Lake Shore IDEAS VSM software. This is done by clicking the Custom option on the Field, Temperature, and Angle experiment setups. When you click the Custom option, the Ramp Generator will appear, presenting the ramp that was previously set up on the experiment setup form. This allows you to construct a basic skeleton of an experiment and then use the Ramp Generator to edit the ramp to fit the needs of your experiment. For example, you can set up a hysteresis experiment that goes from 10 kOe to -1 kOe with smaller increments of 5 Oe to determine a more precise coercivity. For more on how to use the Ramp Generator, see the help file provided with the IDEAS VSM software.
Post by Professor Lake Shore on Jan 31, 2017 17:51:19 GMT -5
Is saddling (centering) the sample important for a magnetometer, especially with a VSM?
Yes, because the position of the sample will directly affect the detection efficiency and, in turn, the accuracy of the moment values. If you don’t saddle it by positioning the sample at the center of the detection, the moment value will be incorrect. Your measurement will be only qualitative not quantitative – that is, you see the shape, but you cannot rely on the numbers of the measurement. For more guidance on this, see Section 4.3.3. in the 7400-S Series VSM user manual, which shows moment vs. position on all three (X-Y-Z) axes. It explains how much moment is affected by a sample being off-center.
Post by Professor Lake Shore on Jan 31, 2017 17:47:46 GMT -5
What field should I apply during a temperature scan type of experiment?
The field applied during a temperature-dependent experiment depends on the material properties you’re trying to study. For example, at higher temperatures when we’re looking for Curie point in an experiment, we usually apply a saturating field and then observe sample behavior as the temperature is increased. In the case of a low-temperature experiment, one should choose between cooling a sample with field applied or in zero field. Typically, the field applied during a low-temperature experiment is well below saturation since we try to observe the sample’s behavior when thermal agitation increases with the increase in temperature. An example of this is an experiment where measurements are used to determine blocking temperature (TB) for a sample. (For further reading, see this 2014 Journal of Nanomaterials article examining into the effect of magnesium (Mg) ion substitution on microstructure and magnetic properties of nickel cadmium (NiCd) ferrite nanoparticles; for this, zero-field-cooled/field-cooled (ZFC/FC) measurements were used to determine blocking temperature for the ferrite nanoparticle samples.)
Post by Professor Lake Shore on Jan 31, 2017 17:45:15 GMT -5
Are there any health concerns associated with working with the magnetic field generated by the electromagnet of a VSM?
It is normal to be concerned because the fields are relatively high. But with a VSM, only a DC field is generated, which will not induce currents in the human body. (In contrast, variable AC fields – for example, the fields generated by RF inductors – will induce currents in the tissue of the body and are dangerous.) But it should be noted that anyone with a heart pacemaker or other medical implants will be affected by any magnetic field and they should not go near an electromagnet. Fringe fields of any type are hazardous to these types of medical devices.
Post by Professor Lake Shore on Jan 31, 2017 17:41:55 GMT -5
When setting excitation current in my Hall system, I want the current high enough that the desired voltage is large compared to the unwanted signals but not so high that it adversely affects my measurement and damages my sample. How can this be done?
This is a multi-step process. First, use the toolbox resistance measurement to find a current the produces a voltage across the contact between 0.1 volt and 0.01 volt. This is a range that we find useful, but you may have to move out of this range depending on the material and the contacts. We typically start at a milliamp of current and move up or down from there to find a starting voltage. For the sake of discussion, assume the current is 10 microamps. Then go to measurement section and select only ohmic check. Set the max. current to value selected in the toolbox (10 microamp in this case) and the min. current to 1/10th of the max. current (1 microamp in this case). Set the number of points 5 and run the ohmic check. If the correlation is 0.9999 or higher, you are good to go. If the correlation coefficient is less than 0.9999, try decreasing the max. current and min. current by a factor of 3 and repeat the ohmic check. The purpose of this is to eliminate self-heating, which will distort the linear ohmic check. This is particularly important if the AC field option is used. Self-heating during an AC field measurement can inject noise into the measurement. Sometimes, decreasing the current used in the AC field measurement can increase the signal to noise ratio of the measurement.
Post by Professor Lake Shore on Jan 31, 2017 17:39:16 GMT -5
What is the field uniformity across the sample stage in the vertical probe stations?
Field uniformity (or homogeneity) for the CPX-VF and CRX-VF stations is 0.5% over a 10 mm diameter and 1% over 25 mm diameter. For those interested in seeing field uniformity plots, Lake Shore can provide BZ contour plots of percent deviation of the field on or 1 mm above the grounded sample holder in a CPX-VF or CRX-VF station. To obtain these plots, contact Sales at email@example.com.