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Recommended and optimal sample dimensions.

SAMPLE: Recommended and optimal sample dimensions (area)

For condensed matter experiments it is important that samples be as big as possible, while taking account of the sample environment that is to be used. This is because in comparison to an x-ray source the flux of a neutron source is much lower. Couple this with the difficulty in focusing neutron beams, requires that samples be large to maximize the use of all available flux.

The average size of sample that is used is roughly 20mm x 20mm square this provides a decent count time and reasonable max Q range.

PolRef magnet only:

• Optimal: 100mm (long) x 40mm (wide)
• Average: 10 x 10mm to 20 x 20mm
• Minimum: 5mm (long) x 5mm (wide). Be warned that the count time will increase drastically and max Q reachable will decrease dramatically.

PolRef flow cryostat (magnet is always included):

• Maximum: 40mm x 28mm is the maximum size that can fit in the cryostat due to the sample mount and gives the best results.
• Average: 10 x 10 mm to 20 x 20mm
• Minimum: 5mm (long) x 5mm (wide). Be warned that the count time and max Q reachable will increase and decrease dramatically.

Free standing sample:

• Minimum: 5mm (long) x 5mm (wide). Be warned that the count time and max Q reachable will increase and decrease dramatically

The bigger, the better after that.

SAMPLE: Thickness

PolRef can measure film thicknesses up to about 4000Å (dQ/Q = 1%) and still resolve fringes. The lower limit on thin samples than can still obtain a measurable signal is about 10Å. This can be circumvented by the use of multilayers to boast the reflectivity.

The average sample thickness is usually between 0 -1500Å (dQ/Q = 2% to 4%) for condensed matter systems.

Before submitting a proposal or coming to ISIS, it is always worth reaching out to the local contact about the best sample dimensions for the experiment in question.

SAMPLE: Roughness, off-specular scattering and curvature

Roughness: Surface quality is paramount. PolRef can deal with roughness on the atomic scale of up to and around 200Å RMS (approximate maximum).

As a rule of thumb, if a sample is rough to the naked eye, then it is very unlikely a good specular reflection can be achieved. This will manifest itself often as a rainbow-coloured, oil slick-like reflection when the sample is held up to a light and tilted slightly. In extreme cases, it will look like scrunched up tin foil – this certainly will not work unless it can be flatted somehow.

Off-specular scattering: There are exceptions to this, for instance deliberately patterned media can give rise to in-plane Bragg peaks. Again, if you are not sure please contact the beamline scientist or assigned local contact.

In off-specular scattering mode, PolRef can resolve features on the order of 0.25 micrometres to approximately 40-50 micrometres, the best result being obtained for features on the order of about 2-4 microns in size. It is also important that the surface coverage is high, otherwise the count times become very large, especially if polarised neutrons are being used (on the order of a few days). Again, it is best to contact the instrument scientist or assigned local contact about this.

Curvature: Large area sample scans often suffer from this. Convex or concave samples act as focusing/defocusing mirrors; however, the focal point will not be at the detector (Well, so far anyway). In the past, samples have been checked for this using a profilometer (expensive) or simply by shining a laser pen (cheap) onto the surface and measuring the reflected spot size (at a suitable distance) as compared to a reflected spot from a piece of thick silicon wafer, or any other reflective surface that is almost atomically flat. Any sample that produces a spot that is a factor of 2 wider or more than the silicon wafer might not work too well.

An on-beamline solution to this is to make a smaller area sample out of the big area sample, often by cutting the sample up. Often, due to the growth, the central regions of samples are flatter than the outer regions, but this can be heartbreaking to do, so check beforehand.

The problem of curved samples due to the stress of the film can often be solved by simply growing on thicker substrates. A lot of polymer experiments are done on 10mm- or 5mm-thick silicon substrate blocks for this reason.

SAMPLE: Substrates and reference layers

Substrates: There are a few things to remember concerning substrates. Some materials are not as friendly as others as substrate materials.

Structural transitions with temperature can cause issues. The classic case is STO substrates, which are renowned for the structural transition they undergo around 120K. This drastically reduces the reflectivity of the sample below these temperatures. This is due to a massive roughening of the surface as the system moves through the transition. There are alternatives, like LSAT, that have similar lattice properties. However, if you need temperatures below 120K and can avoid using STO, we recommend you do so.

Sapphire substrates can also cause issues. However, if you are using D₂O in your experiment, the contrast between sapphire D₂O is very small. Aligning the system is going to be very hard and you will need extra time in your experimental plan.

Cleaning substrates can also be problematic. For instance, if you are using substrates for solid-liquid work, you also need to think about the substrate cleaning procedure and a suitable method to verify that the cleaning has worked. ISIS can provide various cleaning methods, from ozone cleaning to etching; however, you must contact your beamline scientist or assigned local contact.

Reference layers: There has been an increase in biological experiments using Au, Cr, Fe and Ni reference layers, to name a few, to provide extra contrast for biological or chemistry scattering experiments. Often, these reference layers are not sputtered by the groups that are using them and the quality can vary enormously.

It is wise to check this before coming to ISIS, however, we do have a lab-based X-ray machine for doing this off line. Please contact the instrument scientist or assigned local contact about using this machine beforehand to check your substrates and reference layers, before they are used on the beamline. This can save a lot of lost beamtime and experiments.

It has come to our attention that the magnetic reference samples often require some magnetic characterisation. We are at working on a MOKE system to do this off line. However, this is currently waiting for more funding.

Soft matter Experiments:

Liquid and biological experiments have standard sample sizes of 200mm (long) x 80mm (wide) due to the sample Teflon troughs provided. We can supply dimensions and designs if required. Some user groups build their own troughs.

Polymer samples:

Polymer films require large areas as well, but must be flat to the eye in-order to get good reflectivity.

Successful experiments have used polymer systems spin coated onto thick silicon blocks about 25 to 30mm in diameter or bigger. Again, the bigger the surface area, the better for neutron reflectivity measurements.

The thick nature of the films can result in stress on the substrate. In order to avoid curvature of the film, thick substrates need to be used 5 to 10mm thick. This is not always the case, though.

If two or more polymers are to be used, it is important that at least one polymer is deuterated. This is due to the fact that most polymer compounds have almost identical contrasts, making it very hard to distinguish between them with neutrons, unless deuterated material is used for at least one of them – deuterated polystyrene being a classic example. There is an ISIS Deuteration facility, which can be inquired about at the user office and through the ISIS proposal system.

Characterisation Measurements

It is highly recommended, but optional, that some basic characterisation measurements are made before coming to the beamline. It is understood that this may not be possible in the case of certain samples, systems and time constraints.

Condensed matter experiments: (minimum)

It is suggested, at a minimum, that a Cu Kα X-ray or equivalent structural X-ray be taken beforehand. (Both specular and off-specular curves time permitting).

Available at ISIS.

If the experiment is polarised, it is also recommended that the equivalent magnetometry also be performed. (MOKE/VSM/SQUID).

SQUID and VSM are available at ISIS.

With both of these, it is much easier to plan a successful CM experiment as you will have a rough idea of where to look and what fields to use. As count times can be on the order of days, this is invaluable in avoiding mistakes.

This data is also invaluable in fitting the neutron data as it allows you to tie down variables like the total moment of a sample and layer thickness and roughness. This makes the data analysis easier and more rigorous.

Other very helpful characterisation measurements:

Other characterisation techniques that are of great help in neutron experiment planning and data fitting are:

MFM/AFM: Can provide top surface roughness as well as magnetic domain size for estimating in-plane magnetic features. FFT of AFM/MFM images is also useful for validating off-specular data.

TEM: TEM images are brilliant for fitting data. If done well, you can literally see the Nb (SLD) variation in the images allowing you to set up a fitting model more accurately. They are also brilliant justification to referees for why a model has been set up.

Transport data – resistivity/TMR/GMR/etc.: Either as a function of field or temperature. This allows us to set fields and temperatures correctly if we are looking for transitions, such as superconducting phase change, and are again very useful for planning a successful experiment.

These techniques are but a few of the many available. It is worth reaching out to your local contact as some of these can be provided at ISIS.

If you have these measurements done beforehand, they are brilliant for papers/thesis write-ups/experimental reports/follow up proposals.

Soft matter experiments:

For soft matter experiments, there are a vast range of characterisation techniques available, some of which are listed below. It is understood that different groups have their own way of working and techniques. We strongly suggest characterising the property under study and optimising your experimental conditions prior to beamtime. Below is a list of surface characterisation techniques often employed in soft matter experiments to provide complimentary information to interfacial structure described by neutron reflectometry data. Some of these techniques are available at ISIS.

Atomic force microscopy: Widely used microscopy technique for examining structures at the solid/air or solid/liquid interfaces. (Available at ISIS)

Brewster angle microscopy/ imaging ellipsometry (BAM): Microscopy techniques that have high z-axis resolution and are generally employed to gain in-plane information on the structure of interfacial films. (Available at ISIS).

Surface infrared spectroscopy: Infrared techniques, such as Attenuated Total Reflectance (ATR) and external reflection (sometimes known as IRRAS), allow for the chemical composition of interfacial films to be examined and can be very effective in examining interactions at interfaces. (Available at ISIS).

Surface pressure measurements: Users working at the air/liquid interface are likely to measure surface pressure prior to or during NR measurements. It is highly desirable that users also conduct these measurements prior to beamtime. (Available at ISIS).

Surface plasmon resonance: A widely used technique to monitor interactions at the solid/liquid interface. Most universities will have access to this apparatus.

Transmission electron microscopy: An excellent imaging technique, although sample preparation for interfacial films can be challenging.

Quartz calorimetry: Surface calorimetry technique, provides similar information to surface plasmon resonance, although quartz calorimetry instruments are often orders of magnitude cheaper to obtain.

Please note that if you require help with sample preparation that you are advised to contact the ISIS Support Laboratories. This is a facility is available to assist users with sample preparation/optimisation measurements prior to ISIS beamtime.

If you have these measurements done beforehand, they are brilliant for papers/thesis write ups/experimental reports/follow up proposals, etc.

PolRef Data Reduction is handled by the Mantid data reduction software.

PolRef Data Fitting: At present, we are using three data reduction packages:

• GenX: Simultaneous X-ray and Neutron (NR/PNR/PA) fitting. Can fit Soft X-rays, Cu K alpha X-rays, NR, PNR and PA using a very efficient genetic algorithm for both magnetic and biological systems. We recommend starting here.
• Ref1D and BUMPS: (NR/PNR/PA) for when layer models simply won’t do and you need Bayesian fitting and analysis. (Warning can be difficult to get working.)
• RasCAL: Optimised for biological data sets. Now has bootstrapping and Bayesian data analysis and fitting routines. Requires Matlab.

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