Tailoring Curie temperature and magnetic anisotropy in ultrathin Pt/Co/Pt films

The dependence of perpendicular magnetization and Curie temperature (Tc) of Pt/Co/Pt thin films on the thicknesses of Pt seed (Pts) and presence of Ta buffer layer has been investigated in this work. Pt and Co thicknesses were varied between 2 to 8 nm and 0.35 to 1.31 nm (across the spin reorientation transition thickness) respectively and the Tc was measured using SQUID magnetometer. We have observed a systematic dependence of Tc on the thickness of Pts. For 8nm thickness of Pts the Co layer of 0.35nm showed ferromagnetism with perpendicular anisotropy at room temperature. As the thickness of the Pts was decreased to 2nm, the Tc went down below 250K. XRD data indicated polycrystalline growth of Pts on SiO2. On the contrary Ta buffer layer promoted the growth of Pt(111). As a consequence Ta(5nm)/Pt(3nm)/Co(0.35nm)/Pt(2nm) had much higher Tc (above 300K) with perpendicular anisotropy when compared to the same stack without the Ta layer. Thus we could tune the ferromagnetic Tc and anisotropy by varying the ...


I. INTRODUCTION
Ultra thin films of ferromagnetic metals such as Co, Fe and their alloys in contact with heavy metals like Au, Pt, Pd and Ta are interesting systems to study due to the rich physics of spin orbit interactions responsible for perpendicular magnetic anisotropy (PMA), 1 enhanced magnetization 2 and appearance of anti-symmetric exchange interactions responsible for chiral magnetic structures. 3any of these materials also have potential applications in high density magnetic recording 1 and is widely investigated for current induced spin orbit torques. 4,5Innovations in thin film growth techniques like electron beam evaporation and sputtering has helped us build these thermodynamically metastable structures 6 and thereby understand the origins of PMA. 7,8But, the evolution of PMA in sputter grown films is still under investigation.It is quite difficult to get controlled growth of ultra thin films by sputtering primarily due to the high energy of the sputtered species and high rates of deposition.As a result there are issues of growth texture, interface damages, grain boundaries and porosity in these films at nanoscale.Even then, most of the present works on PMA systems including domain wall motion experiments, 9 generation of new magnetic phases like skyrmions 10 etc. are demonstrated on sputter grown films.
Even though there are extensive studies on magnetic anisotropy [11][12][13] and temperature dependence of magnetization 14 in multilayers of Pt/Co, there are only few similar studies on simple trilayer stacks of Pt/Co/Pt.In this work we have investigated the role of Pt s thickness and its crystallinity on the ferromagnetism of sputter grown ultra thin Co layer.The role of Ta buffer layer in promoting the crystallinity of Pt s layer and its effect on PMA has also been investigated.

II. EXPERIMENTAL
Thermally oxidized silicon (100) substrates were sequentially cleaned by ultra-sonication (3 minutes each) in acetone, IPA and DI water.Pt/Co/Pt and Ta/Pt/Co/Pt films were deposited using DC magnetron sputtering at an argon pressure of 5x10 −3 mbar in a high vacuum chamber maintained at a base vacuum less than 3x10 −7 mbar.For the samples with and without Ta buffer layer, T c dependence was studied as a function of Pt s and Co thicknesses.A 2nm thick Pt capping layer was grown on the samples to prevent the oxidation of Co. Polar magneto optic Kerr effect (PMOKE) measurements were performed at room temperature on all the samples to confirm the PMA.Magnetization measurements were carried out using SQUID magnetometer from Quantum Design.Measurements were done to find the out of plane component of magnetic moment and its temperature dependence down to 10K for determination of transition temperature. 15The magnetization was estimated by considering the volume of the cobalt layer (area of the film × thickness of Co).All the measurements were done in the presence of a small out of plane magnetic field of 5Oe to keep the magnetization of the sample from going to remnant field direction of the super conducting magnet.The structural aspects of the Pt s films were studied by XRD (Rigaku) in grazing incidence mode.

A. T c dependence on Co thickness
Figure 1(a) shows the room temperature out of plane hysteresis of samples with Pt s thickness of 3nm in which the Co layer thickness is varied from 0.35 nm to1.31nm.The samples with Co thickness less than 0.87 nm exhibit perfect squareness with 100% remanence clearly indicating PMA.This is due to the fact that at low Co thicknesses the interfacial anisotropy causes the magnetization to be out of plane whereas dipolar energy dominates at higher thicknesses, making the easy axis of magnetization in-plane, thus exhibiting the spin re-orientation transition.
The temperature dependence of magnetization down to 10K of SiO 2 /Pt/Co(0.35-0.87nm)/Pt samples are shown in Fig 1(b) indicating a decrease in T c with decrease in thickness of Co film.As the thickness of Co was reduced to 0.35nm (which was the minimum Co thickness to show ferromagnetic signal) the T c reduced to 216K.Also we see the systematic increase in magnetization with lowering of temperature for all samples below 0.87nm.For Co thicknesses above 0.87nm the effective PMA of the sample decreases with decrease in temperature causing the magnetization to tilt away from the out of plane direction and appear as an in-plane component.

B. T c dependence on the Pt thickness
Figure 2(a) shows the magnetization as a function of temperature of sample with 0.35nm of Co films where the Pt s thickness was varied from 2 to 8nm.The hysteresis curves of all the films taken below their T c shows perfect square loop (data not presented) which confirms the presence of PMA in all the samples.The data in the inset of Fig 2(a) shows the variation of T c of the films with Pt s thickness.At 2nm thickness of Pt s we get a T c of 208K which rises to 305K at 8nm of Pt s .The dependence of T c may be related to growth of Pt s on SiO 2 surface.To study this Pt films with thicknesses ranging from 1-8nm were grown on SiO 2 .Grazing incidence XRD which was done to increase the scattering cross section from different planes of the films reveal that all the films from 1 to 8nm exhibited polycrystalline growth.This is evident from the appearance of peaks corresponding to all the major planes of fcc Pt as shown in Fig 2(b).As the thickness was increased from 1nm to 8nm we observe that there was a systematic increase in all the peaks of polycrystalline Pt.The broadening of the XRD peaks can result from small size of crystallites, mosaicity and textured growth of the films.AFM imaging done on all the Pt films indicated low roughness (less than 0.3nm).The SiO 2 surface also had same surface roughness.Also no distinguishable surface morphologies were observed.As suggested by S. Wei et.al 16 the Pt films might have adopted Volmer Weber growth mode on SiO 2 .This would explain the low T c of Pt/Co/Pt samples with lower Pt s thicknesses.The coverage of the Pt s at low thickness (1-3nm) may not be 100% and hence the Co layer grown on top will have lesser Pt neighbors.As a result, Co atoms will be in direct contact with SiO 2 surface which do not promote PMA.Further an increase in the thickness of Pt s leads to a better surface coverage due to coalescence of Pt grains thereby giving a better template for Co growth promoting PMA.Pt being a fcc crystal grows predominantly in (111) orientation due to its lower surface energy compared to other planes. 16Pt(111) is reported to promote PMA in ultrathin Co films. 17But in our case Pt films are observed to be polycrystalline even at higher thicknesses (up to 8nm).In this case the evolution of T c with increase in Pt thickness may be mainly attributed to better surface coverage and formation of Pt(111) surface.

C. Effect of Ta buffer layer
In order to understand the effect of crystallinity of Pt s on the T c of Pt/Co/Pt stack, Pt s films were grown on a buffer layer of Ta.Fig. 3(a) shows the grazing incidence XRD of Ta5nm/Pt(1-8nm) samples.As compared to the SiO 2 /Pt samples, where all Pt fcc peaks emerged as the thickness of Pt was increased, Ta buffer layer promoted the formation of only Pt (111) and Pt (220) planes.As we increase the thickness from 1nm to 8nm, we can see that Pt (220) peak at 67.6 o becomes more prominent and the (111) peak at 39 o diminishes.This implies that the Pt s plane parallel to the surface of the film is (111) [which is at 35 o to the (220) plane].To find any in-plane orientations of the crystallites, a pole figure analysis was done for major peaks, Pt(111) and Pt(220) for the Ta5/Pt8nm sample.This confirms the fact that Pt films grown on Ta are textured polycrystalline.These results in correlation with the previous report 18,19 confirms that Pt layers deposited on Ta buffer grows with a fcc (111) preferred orientation in two dimensional Frank-van der Merwe mode.Fig. 3(b) shows the room temperature PMOKE measurements on Pt(1-5nm)/Co0.35/Pt2 sample grown on 5nm Ta buffer layer.In contrast to the samples without Ta layer (with T c below room temperature) we observe square hysteresis loop for samples with even 2nm Pt s .This correlates well with the appearance of 67.6 o peak for samples with Pt s thicknesses of and above 2nm.This indicates that Ta buffer layer has promoted PMA by improving the textured growth of Pt(111) 17 planes parallel to film plane.Even the sample with 1nm Pt s layer tends to saturate near 400 Oe of applied field [inset of Fig. 3(b)].The role of Ta on T c is also confirmed by the fact that even a 0.26nm thick Co layer displays PMA below 216K [Fig.3(c)].
Another important observation is that, at a temperature of 10K, for all thicknesses of Pt the saturation magnetization was observed to be higher [Fig.2(a)] than the bulk Co (1400emu/cc).This went upto 2225±84 emu/cc when a Ta buffer layer was used.This can be attributed to the induced moment of Pt layer due to the polarization by the Co proximity. 2The dependence of enhanced magnetization on the orientation of Pt layer needs further investigation.

IV. CONCLUSION
The dependence of T c and PMA on the thickness of Co and seed Pt layers as well as the presence of Ta buffer layer has been investigated in this work.The challenge here was to grow room temperature ferromagnetic structures of cobalt (thickness of Co<0.6nm) with PMA and square hysteresis loop.It was observed that the T c increased with increase in thickness of Co upto 0.87nm beyond which easy axis of magnetization tilted to in-plane direction.This was the result of net increase in dipolar energy as compared to the interfacial anisotropy.It was also noted that the increase in the thickness of Pt lead to increase in T c which was due to better Pt coverage over SiO 2 and the formation of Pt(111) crystallites.Further attempts to improve the growth of Pt(111) with the introduction of a 5nm Ta buffer layer resulted in enhanced T c even at a very low Co thickness of 0.35nm.All the above experiments show that we can tailor Pt/Co/Pt systems to obtain required Tc, magnetization and PMA, which are crucial for developing perpendicular magnetic data storage devices with high thermal stability.

FIG. 1
FIG. 1. a) Room temperature polar MOKE data for samples Pt 3nm/Co xnm /Pt 2nm, where the Co thickness is varied from 0.35nm to 1.31nm across the spin re-orientation transition.b) Magnetization variation with temperature for same samples, with Co thicknesses from 0.35 to 0.87nm.The error bar for the measurement is also indicated.

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FIG. 2. a) Magnetization versus temperature for SiO 2 /Pt(2-8nm)/Co 0.35nm/Pt2nm samples.b) Grazing incidence X-ray diffraction data for various Pt thicknesses.The polycrystalline nature of the Pt films on SiO 2 is evident from this.
FIG. 3. a) The grazing incidence XRD of various thicknesses of Pt films grown on 5nm of Ta buffer layer.The gradual increase in intensity (with Pt thickness) of 67 • peak corresponding to (220) plane indicates the textured growth of Pt on Ta. b) The room temperature polar MOKE data for samples Ta5/Pt xnm/Co 0.35 nm /Pt 2nm, where the Pt seed thickness is varied from 1nm to 5nm.The appearance of the square hysteresis for sample with Pt s 2nm coincides with the appearance of (220) peak in grazing incidence XRD.c) The comparison of magnetization versus temperature data showing the effect of Ta buffer layer.Ta buffer layer significantly improves the T c of the Pt/Co/Pt trilayers.