The overall goal of this procedure is to prepare quality iron rhodium epi layers with a high degree of B two chemical ordering in order to gain control of the iron rhodium magneto structural properties. This is accomplished by first preparing clean single crystal substrates properly installing the iron rhodium target and a kneeling the samples overnight under vacuum. The second step is to deposit the iron rhodium layer while flowing liquid nitrogen through the Meisner trap.
The final step is to further anil the iron rhodium layers to improve the degree of B two ordering alpha prime phase. Ultimately, iron rhodium epi layers prepared following this procedure allow studies on magnetic and structural behaviors and can be inserted into more complex hetero structures in order to investigate the influence of structural changes on the magnetic phase transition. This method can help answer a key question in the field of non-magnet, such as the, the relationships between microstructure and magnetic properties of iron rium and related mathos.
The implications of this technique are to extend our understanding of the remarkable magneta structural phase transition in iron rhodium. This is because the causal links between the changes in electronic magnetic and crystallographic properties as the material undergoes its transition are not yet known. Visual demonstration of this method is important as there are several aspects of the putter growth that depart from standard puttering technique and all aspects are important to success.
Although this technique is excellent for preparing B two ordered iron rhodium epi layers are very high quality. It's also very useful for other ordered alloy materials such as L one zero, iron platinum, or iron palladium. To begin, rinse the magnesium oxide 0 0 1 substrates with isopropanol and mount them into substrate holders, then load the holders into a vacuum chamber.
Next mount an iron rhodium target into a magnetron gun and reassemble the gun for a sample with anatomic composition. A target composed of 47%iron and 53%rhodium is most suitable yielding the clearest magneto structural phase transition test that there is no short circuit between the magnetron and the surrounding shield. By using a multimeter to check for resistance between the target and the chamber, then close the vacuum chamber and pump it down.
Once the vacuum is better than one times 10 to the minus six tor heat the substrates to 600 degrees Celsius at a rate of 6.7 degrees Celsius per minute. Monitor the vacuum level carefully to ensure that the pressure does not rise above this level. When the substrate is at 600 degrees Celsius, hold it there overnight.
One hour before commencing layer growth begin to flow liquid nitrogen through the Meisner trap. After 20 minutes, the input of the Meisner trap is frozen. The vacuum should improve to better than four times 10 to the negative seventh tor.
Next, set the mass flow controller to 65 SCCM of working gas flow and open up the gas valve. Use a sputter gas of Argonne with 4%hydrogen to avoid sample oxidation during growth. Watch the pressure in the chamber After opening the valve, the pressure in the chamber should rise to the low milour range.
Then pre sputter the iron rhodium target for 1200 seconds at 30 watts. For deposition of the iron rhodium layer by DC magnetron sputtering adjust the set point of the mass flow controller to give a chamber pressure of four times 10 to the minus three to watch the pressure until it settles to a stable value. Additionally, ensure that the substrate temperature remains at 600 degrees Celsius and is stable.
Then apply power to the magnetron to yield an overall deposition rate of 0.4 angstroms per second. Open the shutter and deposit the iron rhodium on the heated substrate for a length of time suitable to give the desired thickness Under typical conditions. A 502nd deposition will yield a sample that is about 20 nanometers thick.
Then close the shutter, turn off the power to the magnetron and close the gas valve. Next, increase the sample temperature to 970 kelvins at a rate of 3.33 degrees Celsius per minute and allow the samples to an knee for one hour. Monitor the pressure and make sure it stays better than one times 10 to the minus six tor after one hour.
Shut off power to the heater and allow the samples to cool to room temperature in the setup shown here. This takes at least three hours. Once cooled deposit any capping layer that is required, the capping layer must be deposited at a temperature below 370 Kelvins to prevent inter diffusion of it into the iron rhodium layer.
When complete vent the chamber with dry nitrogen, open it and remove the samples. They will appear bright and shiny. In order to measure the sample's thickness first, mount the sample in the deflectometer and align it so that it makes a specular reflection of the x-ray beam into the detector with a detector angle of two theta equals one degree.
If available, chi should also be aligned. This aligns the sample surface to the refractometer. Then perform a low angle x-ray reflectivity scan.
Run a standard theta two theta scan with theta running from zero degrees until the noise floor of the instrument is reached. This typically occurs once theta is about six degrees or greater for a good quality sample. Then the epi layer thickness can be determined from the spacing of the thin film interference fringes, also known as kig fringes.
The fit to the data is based on a model of the layer with its thickness as an input parameter. A good fit indicates that the input parameter is correct. The inset shows the input to the model that gives rise to the simulation of the data that is shown as a fit.
Next, perform a high angle x-ray diffraction scan to determine the degree of chemical order with the sample still loaded in the deflectometer. Again, align the sample this time to the lattice planes of the substrate. Use the magnesium oxide 0 0 2 brag peak for alignment as the 0 0 2 brag peak is the out of plain direction for the type of substrate used.
Then run a theta two theta scan covering the range of theta from 12.5 degrees to 62.5 degrees. Find the substrate peaks from the scan for a magnesium oxide substrate. The peak should be located at two theta equals 42.9 degrees.
If a copper K sub alpha radiation source is used additionally, 0 0 1 and 0 0 2 iron rhodium peaks will also appear around two theta equals 29.88 degrees and 61.88 degrees respectively. Finally, perform a measurement of the temperature dependence of the sample resistivity to determine the transition temperature to accomplish this. First, make electrical contacts to the sample such that a standard four point measurement can be made using a DC method.
Make measurements for forward and reverse current directions and the resistances averaged order to null off any thermal electromotive force generated at elevated temperatures. Then place the sample on a temperature controlled hot stage positioned in a small turbo pumped high vacuum chamber to avoid oxidation. Measure the resistance as a function of temperature on both heating and cooling sweeps at a rate of two kelvins per minute so that any hysteresis in the first order magneto structural phase transition can be determined.
Shown here are transmission electron micrographs of an iron rhodium epi layer on a magnesium oxide substrate. The iron rhodium layer is estimated to be 30 nanometers thick with an additional four nanometer chromium layer capped with one nanometer of aluminum. The X-ray reflectometry data is shown here for a nominally 25 nanometer thick iron rhodium epi layer capped with a thin polycrystalline layer of aluminum.
The inset is a model of the scattering length density versus depth for the sample, which shows the thickness of the iron rhodium layer. Simulating the x-ray reflectivity that would arise from such a layer gives the solid line, which is seen to agree well with the experimental data confirming the correctness of the model layer, the pronounced sig fringes in the layer stack indicate that the interfaces at the top and bottom of the iron rhodium epi layer are smooth and well correlated. X-ray diffraction results from the same sample reveal three main peaks showing a strong 0 0 2 reflection of the magnesium oxide substrate and reflections of both 0 0 1 and 0 0 2 iron rhodium layer reflections.
It is possible to use this data to determine the chemical order parameter from the relative integrated intensities of the two iron rhodium peaks. When the structure is perfect, the integrated intens of the 0 0 1 peak should be 1.14 times the integrated density of the 0 0 2 peak.Shown. Here is an example of the temperature dependent resistivity of the iron rhodium layer.
The curve shows the anticipated thermal hysteresis between the heating and cooling curves of the material Once mastered. This technique can be done in 16 hour for the growth, three hour for measurement, and four hour for the resistivity if it's done properly. After watching this video, you should have an excellent understanding of how to prepare ordered alloy epi layers of materials like iron rhodium.
