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Keywords:

  • L10-FePt;
  • crystallography;
  • perpendicular magnetic anisotropy;
  • high anisotropy constant

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental details
  5. Results and discussion
  6. Conclusion
  7. Acknowledgements
  8. References

The effect of seed layers on the magnetic and structural properties of FePt thin films was investigated. For the case of Cr(200) seed layer, no ordered L10-FePt phase in (001) orientation was observed. The insertion of a thin MgO barrier between the Cr and FePt layers improved the perpendicular magnetization orientation of L10-FePt by limiting diffusion of Cr into the FePt magnetic layer. Ordered L10-FePt thin films can be developed on Pd[100] at high substrate temperature with the lattice mismatch of below 1% and therefore low roughness compared to MgO or Cr seed layers. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental details
  5. Results and discussion
  6. Conclusion
  7. Acknowledgements
  8. References

Spin valve and magnetic tunnel junction (MTJ) devices based on layers with perpendicular magnetic anisotropy (PMA) have attracted great research interest recently 1, 2. Compared to in-plane anisotropy devices, these devices are suitable for spintronics applications such as read head sensors because the device size can be reduced without compromising thermal stability 3, 4.

Potential PMA materials, such as CoPt and FePt alloys, have been intensively investigated due to their large magnetocrystalline anisotropy (Ku) and high saturation magnetization (Ms) in their ordered L10 tetragonal phase 5–7 which enables potential scaling down to 5 nm. However, achieving ordered face-centered-tetragonal (fct) CoPt and FePt phase requires a high substrate temperature (Ts) during deposition or post annealing process 5–7. Several studies to tailor the growth of FePt for magnetic recording media using seed layers have been reported 5–7. The main role of seed layers in recording media applications is to induce a preferred textured growth. Moreover, the attention of the recording media related FePt studies is focused on the formation of a segregated grain structure which is not desired in spintronics applications.

Here, we focus on the tailoring of L10-FePt growth for spintronics applications. The effect of different seed layers on the crystallographic structure and magnetic properties of L10-FePt films will be discussed.

Experimental details

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental details
  5. Results and discussion
  6. Conclusion
  7. Acknowledgements
  8. References

All the films studied in this Letter were prepared using ultrahigh vacuum (UHV) magnetron sputtering at base pressures below 5 × 10–9 Torr. The thin film samples were characterized at room temperature using an alternating-gradient magnetometer (AGM) system for out-of-plane MH measurement, atomic force microscopy (AFM) for roughness measurements, X-ray diffraction (XRD) to investigate the crystallography of the thin films, and XPS to characterize the interlayer diffusion.

Results and discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental details
  5. Results and discussion
  6. Conclusion
  7. Acknowledgements
  8. References

To investigate the effect of different seed layers on magnetic and structural prop-erties of ordered L10-FePt, thin films with structure of SiO2 (substrate)/Cr (50 nm, Ts)/Fe55Pt45 (15 nm, 500 °C)/Pd (5 nm) were first prepared. Figure 1 shows the XRD patterns of the FePt/Cr thin films for Ts ranging from 300 °C to 400 °C, with steps of 50 °C. As shown in Fig. 1, Cr has (200) texture only at Ts higher than 350 °C. Moreover, the face-centered-cubic (fcc) FePt(200) peak appeared when Ts is 350 °C. However, the ordered L10-FePt in fct phase does not appear, even at the maximum temperature used in our experiments. It is known that the ordered L10-FePt can grow epitaxially along base-centered-cubic (bcc) (200) 8. The absence of FePt(001) peaks and perpendicular magnetization curves (from the hysteresis loop, not shown here) suggest that Cr could have diffused into the FePt magnetic layer. XPS data reveal that Cr diffusion into FePt increases by 40% when Cr Ts increases from 300 °C to 400 °C.

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Figure 1. XRD patterns for thin films of Cr/FePt deposited on SiO2 substrate. The Cr seed layer is deposited at different Ts.

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Additional samples were prepared by inserting a thin MgO layer between Cr and FePt layers with the stack of SiO2 (substrate)/Cr (50 nm, 350 °C)/MgO (t)/Fe55Pt45(15 nm, 500 °C)/Pd (5 nm) to verify the effect of Cr diffusion. MgO was chosen as it can act as a diffusion barrier and has a (200) texture with only 3.3% lattice mismatch. Moreover, it can provide a hetero-epitaxial growth condition for ordered L10-FePt. Figure 2 displays the XRD pat-terns of the FePt/MgO/Cr thin films with fixed Ts of 350 °C for the Cr seed layer and various thicknesses (t) for the MgO layer. As shown in Fig. 2, a Cr(002) peak was present in all samples. When MgO is thicker than 1.5 nm, a clear L10-FePt can be observed. However, the fcc FePt(111) was observed for thinner MgO barriers. By increasing the MgO layer thickness, the peak intensity of fcc-FePt(111) decreases and L10-FePt(001) increases, as shown in Fig. 2. It suggests that thicker MgO layers could act as a barrier of Cr diffusion and enhance L10-FePt growth.

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Figure 2. XRD patterns for thin films with different MgO thicknesses. The Cr layer was deposited at fixed Ts of 350 °C.

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Figure 3 shows the MH loops for films with different thicknesses of MgO layer. The hysteresis curves reveal a larger remnant moment and coercivity for layers deposited on thicker MgO barrier layer, indicating that the MgO barrier layer improves the PMA of the FePt thin films.

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Figure 3. Hysteresis curves of FePt deposited on Cr (50 nm)/MgO seed layers. MgO thicknesses were varied.

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Although the diffusion of Cr into L10-FePt could be avoided and eliminated by inserting a thin MgO layer, it is only of academic interest. This is because MgO will increase the device resistance, making it unsuitable for spintronics applications. Therefore, attention was paid towards metallic seed layer materials such as Pd which can provide good conductivity. Here, MgO(100) substrate was used with the stack of Pd (40 nm, Ts)/Fe55Pt45 (15 nm, 500 °C)/ Pd (5 nm) grown on it. Pd and FePt have a low lattice mismatch of below 1% which makes Pd promising compared to the large lattice mismatch between FePt(001) and MgO(200) of 9.07%, and FePt(001) and Cr(200) of 5.8% 9, 10. Moreover, under optimized conditions, Pd can grow with a (200) texture, suitable for L10-FePt. Figure 4 shows the XRD patterns of the FePt/Pd thin films for various Ts of the Pd seed layer. The XRD scan for the Pd films deposited at room temperature shows no L10-FePt phase because the low energy phase of Pd is fcc (111) 11. However, at higher Ts, Pd starts to grow with a (200) texture and the fcc-FePt(200) orientation decreases. That leads to an increase of fct-FePt(002) orientation. Thus, the L10-FePt(001) peak intensity starts to improve which is also confirmed by the full width at half maximum (FWHM) of the rocking curve (Δθ50) of Pd(200) and FePt(001) versus the Pd deposition temperature, as shown in the inset of Fig. 4.

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Figure 4. XRD patterns for thin films with different Ts of Pd seed layer. Inset: Δθ50 of Pd(200) and FePt(001).

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The results indicate that the sample with Pd seed layer deposited at 300 °C produces a stronger and narrower Pd(200) peak intensity with a smaller Δθ50 at the mentioned peak position. Moreover, the FePt(001) texture

shows strong peak intensity and small Δθ50 at around this deposition temperature confirming well-ordered L10-FePt growth. The long range ordering parameter (S) is calculated according to the ratio of the integrated intensity of fct-FePt(001) to that of fct-FePt(002) with increasing Ts of the Pd layers 12. The S value varies from 0.67 to 0.78 at different Pd deposition temperatures with a maximum value at 300 °C. Finally, the effect of different Ts of Pd on the magnetic properties of the FePt layer was investigated. The out-of-plane MH loops are shown in Fig. 5. The sample deposited at room temperature shows in-plane magnetization which also confirms the absence of L10-FePt and the presence of FePt(111) orientation in the XRD scan, shown in Fig. 4. The out-of-plane magnetization is achieved at Ts higher than 250 °C while a decrease of remnant magnetization (MR) was observed beyond 450 °C. This decrease of MR at high temperature may arise from deterioration in the easy axis direction or from a decrease of anisotropy. However, as the FePt(001) peak was strong with a FWHM of less than 2°, easy axis dispersion can be ruled out. The XPS results, not shown here, confirm the diffusion effect, clearly. It was observed that the Pd diffusion into the FePt magnetic layer increases with increasing Ts. It is well understood that the Pd diffusion into the FePt magnetic layer makes an alloy form of (Fex Pt1–x)y Pd1–y which changes the magnetic property of the FePt layer. The XPS data show minimum Pd diffusion at Ts of 300 °C, increasing to the maximum diffusion rate at 500 °C. Therefore, the degradation of anisotropy due to increasing Pd quantity in the FePt magnetic layer could be concluded. It is also interesting to note that the hysteresis loops show two reversals; a faster one after nucleation and a slower one near/after the coercive point. Such an observation has been reported in CoCrPt:SiO2 media with a significantly large exchange coupling 15. It has been reported that the faster reversal around nucleation field happens due to the reversal of magnetization in certain regions, which results in reduced magnetostatic energy. The large tail in the hysteresis loop beyond coercivity is due to the field needed to overcome the magnetostatic and anisotropy energy. While a high exchange coupling is a problem in conventional recording media, this is a desired property in spintronics and patterned media applications as the patterned magnetic dots have to behave as a single magnetic domain.

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Figure 5. MH loops for thin films with different Pd seed layers (Ts indicated). Inset figure magnifies the original graphs.

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Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental details
  5. Results and discussion
  6. Conclusion
  7. Acknowledgements
  8. References

The effect of Cr, MgO/Cr and Pd seed layers on the magnetic and structural properties of ordered L10-FePt at different Ts was studied. Ordered L10-FePt appeared when a thin MgO was used as barrier layer to prevent the diffusion between Cr and FePt layers. Pd seed layer was found to promote good L10-FePt growth with small surface roughness at temperatures between 300 °C and 400 °C. The growth of Pd at 300 °C was found to provide a minimum roughness value of 4.6 Å and a larger ordering parameter of 0.78, indicating its potential as a seed layer for L10-FePt.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental details
  5. Results and discussion
  6. Conclusion
  7. Acknowledgements
  8. References

Taiebeh Tahmasebi would like to express her gratitude for the support from the A*STAR-SINGA scholarship.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental details
  5. Results and discussion
  6. Conclusion
  7. Acknowledgements
  8. References