Experimental Setup for Magnetoresistance Analysis of Lanthanum Manganites Thin Films

Magnetoresistance (MR) is a property of the materials to change its electrical resistance when magnetic field is applied. Manganites like La1-x(Sr, Ca, Ba)xMnO3 exhibit a colossal magnetoresistance effect (CMR) at an external magnetic field. This property has found many applications in the creation of magnetic field sensors [1] or production of the magnetic memory [2]. However, the investigation of MR of the material involves a lot of measurements making the manual control of the equipment very complicated. The errors due to the human factor take place and must be considered. Also the thermal and magnetomechanic loads must be considered to ensure the accuracy of the experimental setup due to complexity of measurements in high magnetic fields [3]. The purpose of this work was to create a fully automated experimental setup for the MR measurements of the thin lanthanum manganites films. A measurement algorithm was developed and automated using LabVIEW software package. Resulting experimental data for La-Sr-Mn-O and La-Ca-Mn-O films was analyzed and conclusions formed.


Introduction
Magnetoresistance (MR) is a property of the materials to change its electrical resistance when magnetic field is applied.Manganites like La 1-x (Sr, Ca, Ba) x MnO 3 exhibit a colossal magnetoresistance effect (CMR) at an external magnetic field.This property has found many applications in the creation of magnetic field sensors [1] or production of the magnetic memory [2].However, the investigation of MR of the material involves a lot of measurements making the manual control of the equipment very complicated.The errors due to the human factor take place and must be considered.Also the thermal and magnetomechanic loads must be considered to ensure the accuracy of the experimental setup due to complexity of measurements in high magnetic fields [3].The purpose of this work was to create a fully automated experimental setup for the MR measurements of the thin lanthanum manganites films.A measurement algorithm was developed and automated using LabVIEW software package.Resulting experimental data for La-Sr-Mn-O and La-Ca-Mn-O films was analyzed and conclusions formed.

Magnetoresistance of the thin manganite films
Magnetoresistance is defined as where H  is resistivity of the material at magnetic field H, 0  is the resistivity of the material without magnetic field.At certain conditions anisotropy of the magnetoresistance appears in the manganite films.It can be described as the dependence of the magnetoresistance on the direction of the magnetic field with respect to the direction of the current and the surface plane of the film.There are three main reasons for the anisotropy in the ferromagnetic materials: (1) the existence of the preferential directions of magnetization (due to the crystalline structure and deformations), ( 2) resistivity dependence on the angle between the direction of the current and the magnetization vector which is inherent in ferromagnetic materials and (3) the shape anisotropy of the sample (demagnetization effect).The main reason for the anisotropy in the thin polycrystalline manganite films is due to the shape anisotropy [4].MR of the manganites depends on the magnetic field inside the sample.In turn, the magnetic field inside material depends on the external magnetic field and the demagnetizing field H d and can be calculated as: In this work, we developed equipment for investigation of magnetoresistance of thin polycrystalline manganite films having different chemical composition.Thickness of the films was 0.4 μm and dimensions of the samples in plane were 400x400 μm 2 .In this case, there will be a difference in the magnetoresistance when the magnetic field is applied in parallel and perpendicular to the surface directions.The main reason is magnetoresistance anisotropy, which can be quantitatively defined as     the user.It is needed to make sure that the magnetic field at the measurement site has stabilized.The sixth and the seventh algorithm steps are responsible for the magnetic field and the resistance measurements and checking whether precision requirements are fulfilled.Later the data is calculated and a MR graph versus magnetic field is plotted.The final algorithm step is the current check step.If the current is lower than the maximum selected by the user, the algorithm is restarted and the next higher current value is used.When the current is equal or exceeds the maximum selected by the user the algorithm comes to an end.

Automation using LabVIEW
LabVIEW is a graphical programming environment, which offers integration with the hardware devices and provides tools for advanced experimental data analysis and data visualization [7,8].Using the programming environment offered by the LabVIEW the measurement system has been successfully automated.In Fig. 4 the front panel design of the virtual instrumentation program is shown.

Fig. 4. Front panel design of the virtual instrumentation program
As it is shown in Fig. 4 the major part of the front panel was dedicated to the plot area where MR versus magnetic field is plotted.Also the front panel includes the waiting time switch, current steps selector and measurement indicators to show the measured values in real time.

Measurement results
The system was used for measurement of the magnetoresistance and the anisotropy of the manganite films.Fig. 5 presents the dependence of MR vs. magnetic flux density for two films of different chemical composition: La-Ca-Mn-O and La-Sr-Mn-O.The measurements were performed at two directions of the magnetic field.The current through electromagnet was changed from 0 to 50 A.This range was divided into four sub-ranges with different current steps.In the range 0-2 A the step was 0.25 A; in the range 2-5 A -0.5 A; for 5 -20 A the step was 1 A and in the range 20 -50 A the step was 5 A. The measurement results were written to the data file only when the fluctuations of the magnetic field and resistance were less than 0.01% during 200 ms.During measurements the temperature of thermostat was kept 20 0 C. Obtained results were used to calculate the MR of manganites films vs. magnetic flux density.One can see in Fig. 5 that for the La-Sr-Mn-O film the MR is much higher than for La-Ca-Mn-O.Also, the difference between MR when magnetic field is applied parallel and perpendicular to the surface of sample is higher too for La-Sr-Mn-O film.The small difference of MR for La-Ca-Mn-O films can be explained by weak magnetization of the films.It is known that films with chemical composition La-Ca-Mn-O have the temperature of maximal resistance lower than La-Sr-Mn-O.That is a reason for weak dependence of resistance vs. magnetic field.The resulting accuracy acquired in the experiments is sufficient for further calculations.Fig. 6 presents a MRA vs. magnetic field calculated according to the equation (2).Despite to small changes in magnetoresistance of La-Ca-Mn-O when magnetic field was applied in parallel to the surface of the film and perpendicular direction, the results obtained are very satisfying.

Fig. 6. MRA dependence on magnetic field
At low magnetic field the anisotropy of MR is highest and achieved 80%, whereas at magnetic field higher than 1 T the MRA decreases and at B = 2 T achieves 7.7% for La-Sr-Mn-O and 1.7 % for La-Ca-Mn-O films.

Conclusions
A measurement and automation equipment for measuring the manganite resistance dependence on magnetic field was created.Equipment consists of the electromagnet, the programmable power source, the magnetic field meter and the resistance measurement device.All devices are connected to PC by the GPIB cables and the GPIB-USB converter.The control program was created using the LabVIEW software package.The system was used for measurements of manganite films magnetoresistance dependence on magnetic field.The accuracy, which was achieved using this equipment, allows calculations of magnetoresistance anisotropy of the films despite to small changes of their resistance at low magnetic field.Also the measurement data allowed calculations of MR of manganites films vs. magnetic flux density to be performed.It was determined that for the La-Sr-Mn-O film the MR is much higher than for La-Ca-Mn-O.The difference between MR when magnetic field is applied parallel and perpendicular to the surface of sample is higher for La-Sr-Mn-O film.The small difference of MR for La-Ca-Mn-O films can be explained by weak magnetization of the films.Despite to small changes in magnetoresistance of La-Ca-Mn-O when magnetic field was applied in parallel to the surface of the film and perpendicular direction, the results obtained were very satisfying.

Fig. 1
Fig. 1.Schemat The therm the poles of th apply magnet perpendicular powered by th power supply measured usi magnetic field changing the to 50 A. A m measure resis devices was m cables and GP Requirement The m electromagnet measurement d vs. magnetic f Fig. depe Fig.

Fig. 5 .
Fig. 5. MR dependence on magnetic flux density for La-Sr-Mn-O and La-Ca-Mn-O films.Insert: Resistance of La-Ca-Mn-O film changing in low magnetic field.In both cases magnetic field is applied in two directions: parallel to plane of film (opened symbol) and perpendicularly (closed symbol)