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Strain-induced magnetoresistance and magnetic anisotropy properties of Co/Cu multilayers C. Rizal, P. Gyawali, I. Kshattry, and R. K. Pokharel Citation: J. Appl. Phys. 111, 07C107 (2012); doi: 10.1063/1.3671788 View online: http://dx.doi.org/10.1063/1.3671788 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v111/i7 Published by the American Institute of Physics.

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JOURNAL OF APPLIED PHYSICS 111, 07C107 (2012)

Strain-induced magnetoresistance and magnetic anisotropy properties of Co/Cu multilayers C. Rizal,1,a) P. Gyawali,2 I. Kshattry,1 and R. K. Pokharel3 1

Department of Electrical and Computer Engineering, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada 2 Laboratory for Nanospintronics and Nanoelectronics, Catholic University of America, 620 Michigan Ave., Washington, DC 20064, USA 3 E-JUST Center, Department of Electrical and Communication Engineering, Kyushu University, Fukuoka 819-0395, Japan

(Presented 2 November 2011; received 18 September 2011; accepted 20 October 2011; published online 15 February 2012) [Co (tCo) nm/Cu 1.5 nm]50 multilayers were grown onto 15-nm Cu=polyimide buffer layers. The relationship between stress, r, and strain, e, for the [Co 1.0 nm/Cu 1.5 nm]50 multilayers has been presented. The effects of induced strain on the magnetoresistance (MR) and magnetic anisotropy have been examined. The [Co 1.0 nm/Cu 1.5 nm]50 multilayer exhibited a maximum MR ratio of 3.4% at a Co layer thickness of 1.0 nm, b of 0.1, and a strain of 1.5%. The multilayers exhibited a remarkable magnetic anisotropy with the easy axis of magnetization always lying in a plane perC 2012 American Institute of Physics. pendicular to the direction of the induced strain. V [doi:10.1063/1.3671788]

I. INTRODUCTION

The giant magnetoresistance (GMR) and magnetic anisotropy properties exhibited by nano-scale magnetic multilayers continue to attract great attention in terms of fundamental research and technological applications.1,2 The GMR effect in the Co/Cu multilayers is believed to arise because of the interaction of 3d electrons of Co and 4s electrons of Cu, whereas the anisotropic magnetoresistance (AMR) effect is believed to arise by spin-orbit interaction because of the change in the relative angle between the direction of spin and that of current.3,4 The Co/Cu multilayers seldom exhibit magnetic anisotropy properties; however, it is possible to induce them via external means, such as: (i) the application of high magnetic fields,5 (ii) magnetic annealing,6 and (iii) introducing strain.7 Several reviews on inducing magnetic anisotropy using (i) and (ii) are available in the literature.5,6,8–10 However, the questions of how and to what extent strain has an effect on the physical properties of Co/Cu multilayers are yet to be understood. The GMR and magnetic anisotropy properties of the Co/ Cu multilayers are found to be strongly influenced by the interfacial roughness between the Co and Cu layers.11–13 The quality of the interfacial states affecting magnetoresistance (MR) and magnetic anisotropy is further dependent on the types of growth mechanisms employed.14 The primary physical mechanisms involved in the growth of the Co/Cu multilayers are normal15,16 and oblique incidence evaporation,17 both at high vacuum, whereas chemical methods involved potentiostatic and galvanostatic electrodeposition in aqueous electrolytes.18,19 However, it is still uncertain as to what a)

Author to whom correspondence should be addressed. Electronic mail: [email protected].

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extent the growth mechanisms have an effect on the physical characteristics of the multilayers of Co/Cu. Pulsed-current electrodeposition is one of the preferred electrochemical methods in which multilayers and alloys can be grown via reducing the reaction of metal ions from electrolytes. This is also a room temperature technology, which offers precision in growth on an atomic scale. It is possible to manipulate MR and magnetic anisotropy properties by controlling the film composition (i.e., pulse amplitude) and thicknesses (i.e., pulse widths) of the Co and Cu layers on the atomic scale.8,10,20 In this work, we report experimental results on the relationship between stress, r, and strain, e, for the [Co 1.0 nm/ Cu 1.5 nm]50 multilayer deposited onto the polyimide substrate, and examine the effect of strain on the MR and magnetic properties. We also examine the extent to which MR effects vary with the direction of the applied magnetic field and the current in the film plane as the strain is changed. II. EXPERIMENT

An experimental setup of the pulse generator circuit and electrochemical cell used for growing the Co/Cu multilayers is demonstrated in Ref. 21. The electrolyte was composed of CoSO47 H2O, CuSO42 H2O, Na3C6H5O7, and NaCl. The details of the chemical compositions are available in Refs. 22–24. The solution was prepared using double-distilled water and the pH was adjusted in the range of 3.0 to 5.0. Each of the substrates were 15-nm-thick Cu layers with surface areas of 1.69 cm2 that were e-beam evaporated onto 125-lm-thick polyimide. In the plating bath, the Co concentration was changed while keeping the concentration of Cu constant; whereas the ratio between the Co and Cu was varied. The compositions of Co and Cu in the deposited film were determined using energy-dispersive x-ray analysis.

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The anisotropy constant, Ku , and the degree of anisotropy, b, were calculated from the magnetization curves measured using a vibrating sample magnetometer. Ku was calculated using a Stoner-Wolf (S-W) model, assuming that the magnetization process proceeds through a coherent rotation of magnetization as in Co/Ag multilayers, as given in Ref. 25, and b was obtained from the measured saturation magnetizations when the magnetic field is applied perpendicular and parallel to e. The easy axis of magnetization is found along the perpendicular direction (in-plane) of the applied force or strain.22 It is useful to recall here the expression for the MR ratio, which is given by ½ðRH R0 Þ=R0 100 (%), where RH is the resistance at the applied magnetic field, and R0 is the resistance at the zero applied field. The AMR ratio is defined by ½ðRk R? Þ=ðRk þ R? Þ 100 (%), where Rk is the saturated longitudinal ðH k IÞ MR, and R? is the saturated transverse (H \ I) MR. Measurements on the MR ratio were performed at room temperature by varying the relative direction of the field, H, and the current, I, along the easy and hard axes. Details of these measurements are given in Ref. 22.

III. RESULTS AND DISCUSSION

Figure 1 shows a stress, r, versus strain, e, profile26,27 for the Co/Cu multilayers deposited onto the polyimide substrate. For easy observation, different strain limits and regions are labeled in the diagram. In the elastic region, r is proportional to e.27 At this limit, the slope of the tangent gives rise to Young’s modulus, E. In the experiment, the e was changed from the elastic region to a plastic region of up to 1.5% as this is the allowable strain that could be introduced into the multilayer without breaking up its crystalline structure.26 It is the region where the microstructural changes take place and, thus, the rearrangement of the magnetic spins in the Co layers.3 This process is irreversible.26 Figure 2(a) shows a schematic of the direction of the applied stress and magnetic field. Figure 2(b) is a twodimensional view of a Co/Cu multilayer film. It shows the direction of the induced strain, of the induced easy axis of magnetization, and of the applied magnetic field. The easy

FIG. 1. A diagram of stress, r, vs strain, e. The slope of the tangent gives rise to Young’s modulus, E, and rYS is the yield strength. The elastic and plastic regions and allowable strain limits are labeled in the diagram.

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FIG. 2. (Color online) A schematic of inducing strain in the sample. It shows: (a) the position of the multilayer films, the direction of the applied force, and the position of the electromagnets; and (b) the directions of the applied force, magnetic field, and the induced magnetic easy axis.

axis of magnetization is found to lie in a plane perpendicular to the direction of the strain.22 Figure 3 illustrates the MR ratio of anisotropic [Co 1.0 nm/Cu 1.5 nm]50 multilayers against the degree of anisotropy, b, when the magnetic field is applied parallel and perpendicular to e. The MR ratio increases with b and peaks at around b ¼ 0:1 followed by a decrease, which is consistent with the Co layer thickness dependence of the MR ratio.22 The maximum MR ratio of 3.2% observed at b of 0.1 corresponds to e ¼ 1:5 (%) in Fig. 1. The increasing and decreasing behaviors of the MR ratio with b when H k e and H?e are different, and the gap between these two curves widens as b increases. The MR result suggests that MR with H k e is always larger than when H?e. The inset shows a difference between the MR ratios measured when H k e:a: (easy axis) and H?e:a: (easy axis) as b is increased from 0 to 0.35. The DMR(%) increases from 0.54 to 0.85 owing to the increase in b. The position of the MR ratio peak at b ¼ 0:1 in Fig. 3 corresponds to the Co layer thickness of 1.0 nm in Ref. 22. The DMR ratio fits linearly with the degree of anisotropy with the coefficient of determination r2 ¼ 0:95. Figure 4 shows a relationship between the magnetic anisotropy constant, Ku , and e. The inset shows magnetization curves measured when the magnetic field is applied parallel

FIG. 3. (Color online) The MR ratio vs the degree of anisotropy, b, a ratio of mrk (magnetization measured when the applied magnetic field is perpendicular to the strain, i.e., parallel to the easy axis) and mr? (magnetization measured when the applied field is parallel to the strain, i.e., perpendicular to the easy axis). The inset shows the percentage change in MR vs the degree of anisotropy, b, being superimposed with a 95% best linear fit.


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between the layers of Co and Cu.27,28 This assumption holds well with our earlier results of MR verses Ku .22 Similar results were also obtained for Co/Ag multilayers.25,29 IV. CONCLUSIONS

The magnitude of the MR ratio is found to be proportional to the induced strain, showing more enhancement for the field perpendicular to the strain than in the parallel case. These findings corroborate the results we found earlier for the Co/Ag multilayers. These multilayers, with their remarkable magnetic properties, have been considered potentially useful for many technological applications including magnetic sensors in the automobile and biomedical sectors. ACKNOWLEDGMENTS FIG. 4. (Color online) A relationship between the magnetic anisotropy constant, Ku , and the induced strain, e. The inset shows magnetization curves when the strain is changed in the range of 1.5%.

and perpendicular to the direction of current. It shows the extent to which the Ku changes with e when e is changed from 0 to 1.5%. The Ku sharply increases with e up to 1, followed by a slow increase. Figure 5 shows the effect of e on the MR ratio for (a) H k e and H? I, and (b) H?e and H k I. The result suggests that the MR ratio decreases with increasing e and the difference in the MR ratio increases with e. The MR ratio of the [Co 1.0 nm/Cu 1.5 nm]50 multilayers differs as e is changed for H? Ie:a: and H k Ie:a: . The MR ratio decreases with the increase in e, and the MR ratio with H? Ie:a: is always larger than when H k Ie:a: , i.e., the MR ratio with H k e is more significant than when H?e, and the gap between the two widens as e is increased. The decrease in the MR ratio is in good agreement with the change in strain, as it changes the interfacial states of the layers between the Co and Cu layers, independent of the direction of current and the applied field. The difference in the MR ratio with the field parallel and perpendicular to strain is considered to be caused by the modification of the magnetic states owing to induced strain at the interface

FIG. 5. (Color online) A correlation between the MR ratio and strain, e: (a) (closed circles), H? Ie:a: ; and (b) * (open circles), H k Ie:a: . The directions of magnetic field, current flow, and strain are indicated with the corresponding arrows.

The authors are grateful for the partial financial support provided by a Natural Science and Engineering Research Council (NSERC) Scholarship, Government of Canada, for this work. 1

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