3. Experimental Design and Setup
3.1 Sample assembly
For the experiments on the project two different probes were made, one contains fixed well-known resistances (probe 1) and the other contains the sample of FeRh (probe 2) where the exact value of the resistance remains unknown. Each of the probes were used for specific tasks and both are needed to complete all the steps of the project. The design of each of the probes is explained below.
3.1.1 Probe 1 design
This first probe was made in order to be able to improve the electrical measurements and to proper analyze the sources of noise having a well-known and fixed resistance.
22 | C H A P T E R 3 - E X P E R I M E N T A L D E S I G N A N D S E T U P
It was very important to learn about the sources of noise from the measurements before applying the probe containing the FeRh thin film so they could be easier identified later.
Figure 4 presents the first design of probe 1. The probe was made of a metal
container (Figure 4.A and 4.B) with four coaxial connectors 1 (Figure 4.A) to make the four-point measurements. Two of the inner connectors were first soldered to a 5.6 kΩ resistance (Figure 4.C) and connected to the two outer BNC connectors.
A) B) C)
Figure 4. Probe 1 first design with weld resistance. A) Metal container with the four
coaxial connectors. B) Metal container. C) Resistance weld to the inner coaxial connectors.
This first design was useful to improve the electrical measurements due to the need of a well-known fixed resistance to act as a reference for the experiment. It was practical to deepen the study of devices and programs involved including the Source Meter Unit (SMU) Keithley 2400 and LabVIEW, both explained later.
A) B) C)
Figure 5. Probe 1 second and final design. A) Metal container with the four coaxial
connectors. B) Metal container with some resistances. C) Resistance pinned the inner coaxial connectors.
23
Later, a second and improved design of the Probe 1 was made in order to find the nature of the noise obtained as well as to reduce it as much as possible.
Figure 5 presents the improved design mentioned above. In this design two crimp
connectors as shown in Figure 6.B were soldered instead of the single resistance. This assembly, enabled the switch to any resistance by simply attaching the other side of the crimp connector (Figure 6.A) in order to study the accuracy of the measurements and later, the noise tendency of them.
Figure 6. Crimp connectors used. A) Male crimp. B) Female crimp.
3.1.1.1
Resistances election
To study the noise tendency of the experiments a wide spectrum of resistances were needed. The spectrum started with the 5.6kΩ resistance, previously used, and its results were later used to compare the measurements with the new added resistances. Figure 7 shows all the resistances used for the spectrum and its values are 0.2kΩ, 0.56kΩ, 1.5kΩ, 3.6kΩ, 5.6kΩ and, finally, 24kΩ.
Figure 7. The six resistances used for the spectrum with its own values
As Figure 7 shows, all the resistances were previously customized with the crimp connectors shown in Figure 6.A and their values were pasted in one side to avoid possible mistakes on the connections.
3.1.2 Probe 2 design
This probe was made by Alex Eaton on his Master of Physics project to be able to measure the FeRh sample. The design is explained in detail on his interim report[5] but, for the sake of a deeper understanding, it is summarized below.
24 | C H A P T E R 3 - E X P E R I M E N T A L D E S I G N A N D S E T U P
3.1.2.1 Sample configuration
The sample was made by the PhD student William Griggs at The University of Manchester in 2018. It was fabricated via DC magnetron sputtering at a power of 100W. Made from an iron rhodium film of 100nm, the sample, was sputtered onto a MgO(001) substrate.
Figure 8. A) FeRh sample correctly placed in the holder with the four-point
measurement setup. B) Four-point configuration over the sample. [52]
Figure 8.A illustrates the FeRh sample placed in the sample holder explained in section 3.1.2.2. On the top of the holder, four pins are visible. They are used to
perform the four-point measurement technique over the sample when the holder is closed. Figure 8.B shows the technical configuration of the pins over the sample.
3.1.2.2
Sample holder design
The sample holder is illustrated by Figure 9 below. The image was created by Alex Eaton using SketchUp for manufacturing purposes:
25
This Polytetrafluoroethylene (PTFE) or Teflon sample holder is divided in two halves, the left-hand side (LHS) and the right-hand side (RHS). In the LHS, concentric cavities for housing the sample can be seen while in the RHS there is just one single cavity for housing a copper block containing the heaters. Both the innermost cavity of the LHS and the single cavity of the RHS have a slot for the wiring as an exit formed through the top and the bottom, respectively.
A composite aerogel is used which has an extremely low thermal conductivity [5,
50] of 3.2x10-4 Wm-1K-1 for insulation purposes to house the electrical probe pins.
3.1.2.3
Final assembly and full design
For the final design all the pieces were assembled together as shown in Figure 10
A and B:
A)
B)
Figure 10. Final design of probe 2. A) Schematic of the probe with each part [52].
26 | C H A P T E R 3 - E X P E R I M E N T A L D E S I G N A N D S E T U P
Figure 10.A shows how the four pins for the four-point measurements are
connected to the SMU Keithley 2400. The heaters installed have a power rating of 160W, for fast heating rates and they are connected from the power supply and control box to the cartridge heaters. The final results of the assembly in Figure
10.B can be compared to the schematics proposed in Figure 10.A.