HALL EFFECT AND MAGNETORESISTANCE MEASUREMENTS ON PERMALLOY Py THIN FILMS AND Py/Cu/Py MULTILAYERS

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1 Journal of Optoelectronics and Advanced Materials, Vol. 4, No. 1, March 2002, p HALL EFFECT AND MAGNETORESISTANCE MEASUREMENTS ON PERMALLOY Py THIN FILMS AND Py/Cu/Py MULTILAYERS M. Volmer, J. Neamtu a, I. Inta Transilvania University, Physics Department, Eroilor 29, 2200 Brasov, Romania a Research and Development Institute for Electrical Engineering, Splaiul Unirii 313, Bucharest, Romania We have performed both Hall effect and magnetoresistance measurements on Permalloy (Py), t Py = 10 nm thin films of and Py(t Py )/Cu(t Cu )/Py(t Py ) multilayers deposited on thermally oxidized Si substrates, where t Py = 4 and 10 nm, t Cu = 4 and 8 nm. The measurements were carried out at room temperature in a setup with four contacts forming a square of 3 to 5 mm each side. This setup allows us to perform both Hall effect and magnetoresistance measurements. These structures exhibit the so-called anisotropic magnetoresistance (AMR) effect which was measured with the magnetisation vector rotating in plane (IP) of the film. The Hall effect measurements were performed varying the angle between the magnetic field direction and the normal to the film plane, θ, from 0 to 90 degrees. The measured voltage presents a hysteresis loop at low magnetic field even for θ = 0 when it been expected to have only Hall effect. Considering the IP-AMR effect we have calculated the real values of the Hall voltages. The results suggest that the magnetisation is expected to rotate coherently. We discuss some possible applications for characterization of the reversal behaviour of thin magnetic films. (Received January 2, 2001; accepted March 3, 2002) Keywords: Hall effect, Magnetoresistance, Thin films, Multilayers 1. Introduction Currently there is a great interest in the transport and magnetic properties of metallic multilayer systems since the giant magnetoresistance (GMR) effect has been discovered. Usually, the magnetic properties of the thin films are investigated using a vibrating sample magnetometer (VSM) or other methods like Kerr magnetometry. In this study we characterised Ni 80 Fe 20 (Py) thin films and Py/Cu/Py multilayers by Hall voltage hysteresis loop measurements. Because the easy axis of the magnetisation lies in the film plane and because these structures exhibit the so-called anisotropic magnetoresistance effect (AMR) [1], we expected that the Hall voltage would be distorted at low magnetic fields. In this paper we show the importance of the sample orientation regarding the applied magnetic field and how can we obtain useful information about the switching fields from these measurements. Also the field dependences of the Hall voltages for different angular orientations are presented. 2. Experimental The samples were prepared by vacuum deposition at a base pressure of 10-9 Torr on thermally oxidized Si substrates. Two types of samples were obtained: thin films and Py(t Py )/Cu(t Cu )/Py(t Py ) multilayers with t Py = 4 and 10 nm and t Cu = 4 and 8 nm. The samples were cut in circular shape with in diameter. The measurements were made at room temperature in a setup with

2 80 M. Volmer, J. Neamtu, I. Inta four contacts forming a square about 3 to 5 mm each side. The current, I, was kept constant. This setup gives the possibility to perform both Hall effect and magnetoresistance measurements. In Fig. 1(a), we present the schematic setup and the four-resistor arrangement model, Fig. 1(b), to account for the electric behavior of contacts configuration. R l and R t are related with longitudinal and transversal resistivities, respectively. H is the applied magnetic field which makes the angle θ with respect to the surface normal n. We denote with H p, the in plane component of H and with H n the component of H normal to the sample plane. Resistance behavior is anisotropic with respect to the field direction, the MR being positive when the magnetic field, H p, is parallel to the current (longitudinal effect) and negative when the magnetic field, H p, is perpendicular to the current (transversal effect). This is known as in - plane anisotropic magnetoresistance (IP-AMR). Fig. 1. The four-lead setup for magnetoresistance and Hall effect measurements (a) and the four-resistor arrangement model (b). The current-voltage field configuration of Fig. 1 enhances the AMR effect [2] and allow us to make Hall effect measurements. The AMR effect can be minimized if we rotate the sample with an angle α = 45 or 90 ; in this case H P makes an angle of 45 both with R t and R l. Magnetisation curves and hysteresis loops were measured by means of a VSM at room temperature. For surface characterisation we used an atomic force microscope (AFM). 3. Results and discussion The AFM surface topography of a Py(5 nm) layer deposited under the mentioned conditions shows a rough surface with a rms-roughness of 1.1 nm. The maximum height of the surface roughness was about 10.7 nm. This is a characteristic for evaporated thin Py layers [3,4]. During the first stage of Py deposition, isolated Py islands are formed on the surface. At the percolation stage the available space between Py layers is filled and a continuous Py layer is formed. It is found that the percolation occurs at Py thickness of about 2 nm when the samples are deposited onto SiO 2 substrates [4]. This suggests that the Py(4 nm)/cu(t Cu )/Py(4 nm), with t Cu = 4 and 8 nm, samples present distortions of the multilayer structure and intermixing of Py and Cu layers [3, 4]. Fig. 2(a), shows the field dependences of Hall resistivities, ρ H, for Py(4 nm)/cu(4 nm)/py(4 nm) and Py(10 nm)/cu(4 nm)/py(10 nm). The hysteresis loop for a Py(4 nm)/cu(4 nm)/py(4 nm) is presented in Fig. 2(b) [3]. From this plots results a degradation of the magnetic properties and a smaller value for Hall resistivity for the sample with thin Py layer (4 nm). This shows a decreases of the sample magnetisation because the layers are not well defined due to intermixing between Py and Cu layers. When the Py thickness increses to 10 nm the layers becomes more well defined and the magnetisation increases and the coercive field decreases from about 200 Oe, Fig. 2(b), to about 60 Oe [3]. The MR ratio found for Py(4 nm)/cu(4 nm)/py(4 nm) isn t greater than 8 %. These facts suggest that the predominant conduction mechanism here is diffusive scattering at interfaces and grain boundaries that

3 Hall effect and magnetoresistance measurements on permalloy Py thin films 81 alter the MR and Hall effects. For this reason we will focus our discussion on two samples: and Py(10 nm)/cu(4 nm)/py(10 nm). 0.4 (a) 0015 (b) /Py (4 nm)/cu (4 nm)/py (4 nm) 0010 ρ H (µωm) 0.3 Py (10 nm)/cu( 4 nm) Py (4 nm)/cu(4 nm)/py(4 nm) Moment (e.m.u.) H Film H = Oe C Fig. 2. Field dependences of Hall resistivities for Py thicknesess 4 nm and 10 nm (a) and in plane hysteresis loop (b) for Py(4 nm)/cu(4 nm)/py(4 nm) sample at room temperature Hall effect measurements on Fig. 3 shows field dependences of the output voltage U for a Py(10 nm) thin film. The experimental setup is shown in Fig. 1(a). The current, I, was kept constant in 2.51 ma. The setup coresponds to maximum AMR effect (H P is parallel to R l and perpendicular to R P ) [2]. This means α = 0. θ was varied from 0, that means field normal to the sample plane, to 90 i.e. field in the sample plane U θ=5 o θ=15 o θ=90 ο - AMR effect 3.5 Fig. 3. Field dependences of the output voltage, U, for different θ orientations of the /Py sample. Fig. 3 shows the influence of the AMR efect on the output voltage even for θ = 0 when we expect to have only Hall effect. Because of the high values of the demagnetising fields the film magnetisation tends to stay in the film plane for low magnetic fields and follows the orientation of the H P. This means that the output voltage, U, can be expresed as: U = + U AMR (1) where is the Hall voltage and U AMR is a voltage that corresponds to the IP-AMR effect and contains in addition a term U 0 which is the offset voltage that is a constant. We observe a hysteretic behaviour at low magnetic field even for θ = 0. For θ > 0 the IP-AMR effect can be considerable and alter the shape of the output voltage U versus applied field as we can see from Fig. 3. To calculate the Hall voltage we must extract from U the U AMR term. This task is easy to be done for θ > 0. For θ = 0 we made the assumption that it is virtually impossible to

4 82 M. Volmer, J. Neamtu, I. Inta achieve the conditions H n = H and H P = 0. Every small misalignment of the sample and field distortions can produce an in plane field component H P. This is responsible for the U AMR (H = 0) voltage that appears when the magnetic field goes to 0 from Oe or from 8000 Oe. It is interesting to note that U AMR ( ) and U AMR ( ) takes the same values in Fig. 1 for different orientations θ of the sample. In Fig. 4, we present the results of these calculations carried out for θ = 0, 5 and (a) (b) 3.9 U =697 Oe (c) (d) - - θ=5 o =200 Oe =108.3 Oe Fig. 4. Field dependences of the output voltage, U, at θ = 0 (a) and the Hall voltages for θ = 0 (b), 5 (c) and 10 (d) measured for /Py sample; the arrows are guides for the eyes. Fig. 4(a) shows the output voltage for θ = 0 as was measured. It is visible the hysteresis loop. The shape remains the same if we modify de orientation with a θ up to ± 2. Figs. 4(b)-4(d) show the Hall voltages obtained from our calculations for θ = 0, 5 and 10. The field dependences of Hall voltages are the same for these orientations. The assymetry between positive and negative field regions is probably due to some other in-plane magnetoresistive effects like the assymetry of the IP-AMR, as we can see from Fig. 3. This asymetry appears because it is verry dificult to have the same values for the resistors R l and R t even if the contacts are placed on the film surface with an accuracy of mm. The sharp anomalies of the Hall voltages that appear in Fig. 4 can be connected with the reversal processes that take place in the film plane which produce a jump of the IP-AMR ratio. With we denote the critical values of the field normal to the film plane for which the magnetization reversal takes place in the film plane for different values of θ. For θ = 15 one obtain = Oe. From Fig. 4 we can make some assumptions regarding the reversal proceses that take place in the film. For θ = 0 we see in Fig. 4 (b) a relatively large transition region. This suggests a reversal process mainly due to domain wall motion. As θ increases the width of the transition regions decreases as we can see from Figs. 4(c)-4(d). For θ > 10 the reversal will only take place by coherent rotation. We have calculated the corresponding values of H for which the reversal take place Hall effect measurements on /Cu (4 nm) Fig. 5 shows the field dependences of the output voltage U for a Py(10 nm)/cu(4 nm)/ Py(10 nm) multilayer. The current, I, was 10 ma, α = 0 and θ was varied from 0 to 90.

5 Hall effect and magnetoresistance measurements on permalloy Py thin films /Cu (4 nm) -3.1 U θ=30 o -3.5 θ=60 o -3.6 θ=90 o AMR effect Fig. 5. Field dependences of the output voltage, U, for different orientations θ of the /Py/Cu/Py sample /Cu (4 nm) (a) /Cu (4 nm) (b) -3.1 U =250 Oe Fig. 6. Field dependences of the output voltage, U, at θ = 10 (a) and the Hall voltage for θ = 10 (b); the arrows are guides for the eyes. Fig. 6 shows the field dependences of the output voltage,u (a), and the Hall voltage when θ = 10 (b). The transition is not so pronounced as in the case of Py (10 nm) layer as we can see from Fig. 4(d) and Fig. 6(b). Also we found, for θ = 10, an increases of the critical value from 108 Oe to 250 Oe. This increases of is due to a positive coupling between feromagnetic layers that appears if the non-flatness of the layers is taken into account [4, 5]. Using the above mentioned procedure we can determine the values of for different θ. In Fig. 7, we plot the angular dependences of coercive force for both samples: (Oe) /Cu (4 nm) θ (degree) Fig. 7. The angular dependences of coercive field for Py(10 nm) and Py(10 nm)/ Cu(4 nm)/py(10 nm).

6 84 M. Volmer, J. Neamtu, I. Inta To minimiase the IP-AMR effect we rotate, the sample in his plane with an angle α = 45, like in Fig. 1(a). We presents, in Fig. 8, the results of measurements for θ = 0 and 10 in order to compare them with the case corresponding to α = 0. U /Cu (4 nm) -2.2 (a) /Cu (4 nm) (b) U /Cu (4 nm) -2.2 (c) /Cu (4 nm) (d) =400 Oe Fig. 8. Field dependences of the output voltage, U, and the Hall voltage for θ = 0 (a) and (b) and θ = 10 (c) and (d); the arrows are guides for the eyes. As we can observe the field dependence of the output voltage U presents hysteresis loops both for θ = 0 and θ = 10. The IP-AMR effect was minimized and the Hall voltages resulting from our calculations is very symmetric regarding the field polarity and do not exhibit anomalies described previously for Py thin film, when α = Conclusions We performed Hall effect and magnetoresistance measurements on thin films of Py (10 nm) and Py(t Py )/Cu(t Cu )/Py(t Py ) multilayers. The orientation of the sample in magnetic field plays a crucial role in achieving useful information about the coercive field and the reversal mechanism. This kind of measurements can be a very useful method for a rapid and cheap characterisation of magnetic thin films. References [1] Th. G. S. M. Rijks, R. Coehoorn, M. J. M. de Jong, W. J. M. de Jonge, Phys. Rev. B 51, 283 (1995). [2] C. Prados, D. Garcia, F. Lesmes, J. J. Freijo, A. Hernando, Appl. Phys. Lett. 67 (5), 718 (1995). [3] J. Neamtu, M. Volmer, A. Coraci, Thin Solid Films , 218 (1999). [4] T. Lucinski, G. Reiss, N. Mattern, L. van Loyen, J. Magn. Magn. Mater. 189, 39 (1198). [5] J. C. S. Kools, Th. G. S. M. Rijks, A. E. M. De Veirman, R. Coehoorn, IEEE Trans. Magn. 31 (6), 3918 (1995).