Common use of Experimental Clause in Contracts

Experimental. The Seebeck coefficient was measured using a home-made sample holder built on a PPMS puck. It consists of two copper blocks separated by a thermal insulator plastic. The copper has a high thermal conductance so the blocks are at a uniform temperature while a temperature gradient is produced be- tween them. A small heater (maximum power of 5 W) is installed in the upper block. Its temperature is measured with a Pt-100 resistor and controlled with an external temperature controller. The temperature of the lower block is controlled by the set point of the PPMS, but the temperature was separately measured by a second Pt-100 resistor. The whole setup is covered with a stainless steal cup that isolates the sample holder to help stabilize the tem- perature gradient. The measurements were done in a relatively low vacuum of 10 mTorr. A schematic of the sample holder is given in Fig.6.1 The samples consisted of thin films, mostly on sapphire substrates, with an area of 10 × 10 mm2. CrO2 films were deposited by Chemical Vapor Deposition (CVD) on both isostructural TiO2(100) and sapphire (1000) sub- strates. CrO2 films deposit epitaxially on TiO2 in the form of rectangular grains aligned along c-axis but on sapphire the grains are aligned with six fold rotational symmetry coming from the hexagonal structure of the substrate, as detailed in Chapter 3. The Py thin films were deposited using dc sputter- ing in a UHV sputtering system, with a base pressure of 10−9 mbar, the Co films were deposited in Z-400 an RF sputtering system with base pressure of 10−6 mbar. Both Py and Co were deposited on sapphire substrates because of its better thermal conductivity. The Seebeck coefficient was recorded with reference to copper since Cu wires were connected at both ends of the film via pressed Indium. The po- tential difference was probed using a Nanovoltmeter (▇▇▇▇▇▇▇▇ 2018) in an open circuit geometry (J = 0). A dynamic technique was utilized to measure TEM as function of temperature in which the temperature difference between hot and cold point was always 5 K, while the temperature of the cold point was increased by 10 K in each step. In this way hot point and cold point interchanged in each step between the temperature range of 100 - 400 K [80]. To check the setup, TEP was measured for nonmagnetic Cu, Au, and Pt with reference to Cu. In principal, it should give a zero TEP on a Cu thin film, but we measured around 2.5 µV at temperature difference of 45 K with the hot terminal being at 300 K, which gives a TEP of the order of 0.05 µV/K. The small TEP measured with the Cu film validates the assumption that we can take Cu as a reference for the TEP measurements and it also the test of our home-built sample holder. The non-zero voltage appearing on Cu thin films can be attributed to two factors. Firstly, we used pressed Indium to contact the Cu wires with samples that can also contribute to the TEP. Secondly, the Cu wires may not be connected really with the temperature bath, which can still generate some temperature difference between the voltage pads. TEP measurements on Au and Pt unstructured thin films (100 nm and 10 nm thick, respectively) gave values of -0.4 µV/K and -5.8 µV/K reference to Cu. From the definition in Eq. 7.9 this means the voltage difference between the hot and the cold point is positive. Figure 6.1: A schematic of thermal transport sample holder built on a PPMS puck. Two copper blocks separated by a thermally insulator plastic, a heater is installed in the upper Cu block and two Pt-100 resistor are being used to measure the temperature difference between the Cu blocks. Whole set up is covered with a stainless steel cap that is not shown in this picture.