Computer Controlled Thermostat for the Resistivity measurements of the La 1-xSrxMnO 3 thin films

Sistema susideda is termostato celės kuri yra patalpinama tarp elektromagneto polių. Celė susideda is Peltier elemento kuris gali kaitinti arba saldyti bandymų objektą. Pratekantis vanduo palaiko pastovią Peltier elemento kitos pusės temperatūrą. Grįžtamam rysiui naudojamas platininis temperatūros jutiklis prijungtas prie universalaus matuoklio „Tektronix DMM 4050“. Peltier elementas maitinamas is universalaus laboratorinio reguliuojamo srovės saltinio „TTI QL 564P“. Abu prietaisai sujungti su kompiuteriu per GPIB sąsają ir per GPIB-USB sąsajų konverterį. Naudojant LabView programavimo aplinką buvo sukurta programa naudojanti PID algoritmą bei valdanti prijungtus prietaisus. Programa užtikrina pasirinktos temperatūros stabilizavimą. Matavimų rezultatai parodė, kad norima temperatūra pasiekiama per kelias minutes, priklausomai nuo temperatūrų skirtumo. Pasiekus norimą temperatūrą, ji yra palaikoma su 0.02 °C paklaida. Toks tikslumas yra pakankamas manganitų sluoksnų varžos matavimams magnetiniame lauke.  Il. 8, bibl. 8 (anglų kalba; santraukos anglų ir lietuvių k.). DOI: http://dx.doi.org/10.5755/j01.eee.119.3.1363


Introduction
Manganites like La 1-x (Sr, Ca, Ba) x MnO 3 are of great interest for last few decades mainly since they exhibit a colossal magnetoresistance (CMR) effect.This has found many applications creating magnetic field sensors and magnetic memory [1].However, temperature plays even bigger role in the change of the resistance than magnetic field [2].Hence it is crucial to precisely stabilize the temperature during the measurements of the magnetoresistance (MR).Moreover it is necessary to adjust the temperature to the known values in order to study how it affects the resistivity [3], magnetoresistance and anisotropy [4].
There are many commercially available temperature controllers designed for the industrial or laboratory use of various functionality depending on the cost.Nevertheless in this work we have developed a temperature controlling system that satisfies the needs of setting and keeping the temperature in a small volume between the poles of an electromagnet.Moreover, the system consists mainly of the general purpose equipment which was and is used for the other purposes.In this way the costs were minimized while keeping the optimal performance.

Requirements
Magnetoresistance is defined as [5] MR= ρ H -ρ 0 ρ 0 ×100%, here ρ 0 is resistivity at the zero magnetic field and ρ H is resistivity at a given magnetic field H.The origin of the magnetoresistance lies on the quantum mechanical phenomenon known as double exchange which is inherent in manganites.The electron transport through the oxygen ions is greatly enhanced when the Mn +3 and Mn +4 ions are ordered ferromagnetically, hence the resistivity is lower.In contrast, resistivity is higher when there is no order (paramagnetic state).Obviously, magnetization M is the order parameter which affects resistivity.On the other hand, temperature is a disordering parameter which acts in an opposite way.In general, magnetoresistance can be described using a Brillouin function ℬ( , ) [6].
Since the resistance of the sample is both dependent on the magnetic flux density and temperature , it is possible to compare its sensitivity to these quantities.Fig. 1 shows magnetic flux density and temperature which affect resistance in the opposite way so, that eventually it is kept constant.The important thing which should be noticed is the slope of this graph.At low temperatures (0 ÷ 15 °C) it is equal 0.085 T/°C (Tesla per degree) but at high temperatures (40 ÷ 50 °C) it almost reaches 1 T/°C.While measuring the magnetoresistance, changes of the resistance can be wrongly interpreted as they appear due to the magnetic field.The standard deviation of the magnetic field during the measurements is about 5 mT.However in general 1% error is allowed meaning that the magnetic field deviation at 2.5 T might be equal 25 mT.Considering the slope of the graph in Fig. 1, requirements for the temperature stability are lowest at the low temperature -Δ = 0.06 ℃ to make measurements with precision of the magnetic field 5 mT and highest requirements are for the high temperatures, where Δ = 0.025 ℃ to make measurements with precision of the magnetic field 25 mT.

Measurement methodology
The most known algorithm for temperature stabilization is a PID controller.It was chosen to be used due to its simplicity and good performance [7].The PID controller algorithm is used to control the power source to adjust heating/cooling taking into account the feedback signal from the temperature sensor.A general purpose laboratory power supply "TTI QL564P" was used as a regulated current source and a multimeter "Tektronix DMM4050" was used as a temperature meter.A Pt 1000 temperature sensor was used as a probe since it is compatible with the multimeter and is not sensitive to the magnetic field.The drawing in Fig. 2 shows the components of the system and how they are interconnected.It is required that the temperature could be adjusted in a range of 0 ÷ 50 °C.It means that heating and cooling must be incorporated in the design.A simple way to do this is to use a Peltier element.A conventional Peltier element is a device which enables cooling one side and heating another when the current is passed.Reverse effect occurs when the current direction is reversed.A single Peltier device can create temperature difference of up to 70 °C.Keeping one side at a constant temperature, the other side can be heated above or cooled below the room temperature.An efficient way to keep the temperature of the one side constant is to use water flow.Air cooled system was also considered, but a heat sink and a fan would take too much space.Moreover, water cooling system is already present at the electromagnet; and is easy to connect to it.
A thermostat cell as shown in Fig. 3 was made.It consists of the brass block with a cavity (9) and water inlets (1, 2), a Peltier element (10), another brass plate on the other side (11) with the cavities for the temperature probe and the sample (4, 5) and the insulating materialstyrofoam on the top.All parts were glued together and fixed at the experiment site -between the poles of the electromagnet.Dimensions of the thermostat cell were chosen to fit in the gap between the poles, which is about 31mm.The cell can be inserted between the poles in different orientations, depending on which direction of the magnetic field is needed.The temperature probe and the sample are very close to each other to ensure that their temperatures are equal.

Automation using LabView
LabView is a graphical programming environment which offers straight through communication with the hardware devices and a user friendly graphical interface [8].The main function of the program is to ensure the temperature stabilization which consists of: communication with the power source and the multimeter, calculation of the current using the PID algorithm, applying different setting and options.Another purpose is visualization of the current temperature, so that user can decide when the temperature has stabilized.
The value called error ε which is the difference between the desired temperature and the current temperature is calculated in each cycle.It is a main variable in the PID algorithm.Moreover, time spent in each loop is calculated.These two variables and the P, I, D constants are used for the calculation of the current using a classical PID formula The program consists of several parts that run in the closed loop, as depicted in Fig. 4. First, the user supplied parameters and constants are collected.User is able to change some basic settings, as: the set point temperature, GPIB addresses and models of the power supply and the meter, probe type (Pt1000 or Pt100  The cycl than the therm enough to ave the system is circuit due to readings are a before they ar reached witho temperature se  In Fig. 8 the resistance dependence on temperature is shown.Resistance was measured in the range of temperatures of 0 ÷ 40 °C.Various preparation procedures were made between the measurements, such as annealing of the contacts at different temperatures, heating in different temperatures and humidity.Such procedures affect the resistivity of the sample, the resistance temperature coefficient and magnetoresistance.The change of the resistance of the order of less than 1% in the whole range of temperatures is observed.The temperature stabilization system for the magnetoresistance measurements of the La 1-x Sr x MnO 3 manganites is described in this paper.The thermostat cell with the Peltier heating/cooling element was manufactured specially to be placed between the poles of the electromagnet.The heat sink attached to the rear side of the Peltier element is cooled by the flowing tap water.Platinum film temperature probe was used for the temperature feedback signal.Universal multimeter "Tektronix DMM 4050" was used as a temperature meter and a regulated laboratory power supply "TTI QL 564P" was used to supply the current through the Peltier element.Both instruments were controlled by the computer software via the USB and GPIB interfaces.The software implementing a PID algorithm was written in the LabView graphical programming interface.The results show that the temperature of the sample can be changed in 2-3 minutes depending on the temperature step and is kept constant with precision of ±0.02 °C.Ill.8, bibl.8 (in English; abstracts in English and Lithuanian).Sistema susideda iš termostato kameros, kuri įtaisoma tarp elektromagneto polių.Kamera susideda iš Peltjė elemento, kuris gali kaitinti arba šaldyti bandymų objektą.Pratekantis vanduo palaiko pastovią Peltjė elemento kitos pusės temperatūrą.Grįžtamajam ryšiui naudojamas platininis temperatūros jutiklis prijungtas prie universalaus matuoklio "Tektronix DMM 4050".Peltjė elementas maitinamas iš universalaus laboratorinio reguliuojamo srovės šaltinio TTI QL 564P.Abu prietaisai sujungti su kompiuteriu per GPIB sąsają ir per GPIB-USB sąsajų konverterį.Naudojant LabView programavimo aplinką buvo sukurta programa, naudojanti PID algoritmą bei valdanti prijungtus prietaisus.Programa užtikrina pasirinktos temperatūros stabilizavimą.Matavimų rezultatai parodė, kad norima temperatūra pasiekiama per kelias minutes priklausomai nuo temperatūrų skirtumo.Ji palaikoma su ±0,02 °C paklaida.Toks tikslumas yra pakankamas manganitų sluoksnų varžai matuoti magnetiniame lauke.Il. 8, bibl.8 (anglų kalba; santraukos anglų ir lietuvių k.).

Fig. 1 .
Fig. 1.Graph showing magnetic field corresponding to the same change of resistance as is due to the change of temperature

Fig. 2 .
Fig. 2. Schematic diagram of the temperature stabilizing system

Fig. 8 .
Fig. 8. Resistance dependence on the temperature measured using the temperature stabilizing system.Several measurements of the same sample were made after different preparation procedures
), heating or cooling mode.More advanced settings are the P, I and D coefficients used in the PID algorithm, maximum current