Sm(Co,Cu)5/FeCo anisotropic nanocomposite film
The nanocomposite magnets that are composed of exchange coupled hard and soft magnetic phases have been a subject of recent studies for their potential to achieve the maximum energy product (BH)max that is higher than those achieved in the existing sintered magnets [1,2]. Although various types of nanocomposite magnets were reported in the Nd-Fe-B system, their coercivity (Hc), remanence (Br), and (BH)max are lower than those for commercial sintered magnets of the same system due to their isotropic feature; thus, nanocomposite magnets are considered as economical medium performance materials for bonded magnet applications [3]. However, crystallographically textured anisotropic nanocomposites still have a great technological potential to achieve a higher (BH)max. Since many complicated metallurgical processes must be involved in producing anisotropic bulk nanocomposites, textured multilayer thin films will serve as a convenient model system for studying the fundamental properties of exchange-spring magnets [4,5]. The calculation by Skomski and Coey [6] predicted that the upper limit of (BH)max for anisotropic Sm2Fe17N3/Fe65Co35 multilayer with a volume fraction of only 9% of the hard phase was 137MGOe. Thus, an experimental demonstration of high (BH)max achievable by an anisotropic multilayer exchange spring magnet will boost the research toward new high performance permanent magnetic materials. In this work, we selected SmCo5 with a huge Ku (Ku>108 erg/cc ) as a hard phase and Fe as a soft phase. The previously reported textured multilayers were fabricated by depositing the Sm-Co and Fe or Co layers onto a heated substrate or by directly depositing the layers at room temperature [4,7] and the reported (BH)max was only in the range of 10 to 20GMOe. In this paper, we report the good textured Sm(Co,Cu)5/Fe multilayer films fabricated by sputtering and subsequent annealing. After the post-annealing, the multilayer structure was kept with a good in-plane c-axis texture. A high (BH)max of 32GMOe was obtained, which is larger than that achieved in the SmCo5 and Sm(Co,Fe,Cu,Zr)7 type commercial sintered magnets.

The multilayer films were prepared by sequentially depositing Sm-Co, Cu, and Fe layers on a 100 nm thick Cr underlayer that was sputter deposited on a thermally oxidized Si wafer. The reason for adding the Cu interlayers is that it is immiscible with Fe, but can dissolve in the Sm-Co layer. The Sm-Co layers with a nominal composition of SmCo6 were deposited by co-sputtering Sm and Co targets. After depositing the multilayer films, a 50 nm thick Cr capping layer was deposited to protect the oxidation of the films. The structure of the Sm-Co layer was amorphous in the as-deposited condition, so the as-deposited films were Cr(50 nm)/[a-(Sm-Co) (9 nm)/Cu (x nm)/Fe (5 nm)/Cu (x nm)]6/Cr (100 nm)/a-SiO2, where a- stands for amorphous. Then, the films were heat treated at temperatures ranging from 450oC to 525oC for 30min. The magnetic properties were measured using a superconducting quantum interference device (SQUID) magnetometer with a maximum magnetic field of 5.5 T. A preliminary examination of the film structure was made by x-ray diffraction (XRD) and the detailed microstructure was investigated by transmission electron microscopy (TEM).


Fig 1 (a) Cross sectional TEM image of [Sm(Co,Cu)5/FeCo]5 multilayer thin film and (b) in-plane and out-of-plane magnetization curves. The in-plane coercivity is 7.24 KOe and (BH)max=32 MGOe which is larger than the theoretical limit for SmCo5 single phase magnet (29 MGOe). (c) GIF elemental mapping of Sm, Co, Fe, Cu and the intensity profile obtained from the GIF mapping. Cu interlayer initially sandwitched between a-SmCo6 and Fe diffuse into the SmCo5 phase forming Sm(Co,Cu)5 and the excess Co in the a-SmCo6 phase diffuse in the Fe layer.

As shown in Fig. 1, we have successfully fabricated the exchange coupled [Sm(Co,Cu)5/Fe]6 multilayer films that approach the ideal exchange-spring magnets with aligned hard and soft layers by post-annealing the Cr(50 nm)/[a-(Sm-Co) (9 nm)/Cu (x nm)/Fe (5 nm)/Cu (x nm)]6/Cr (100 nm)/a-SiO2 multilayers. The addition of a Cu interlayer between the a-Sm-Co and Fe layers with an appropriate thickness improved the coercivity and (BH)max, especially at the low annealing temperatures. This is related to the reduced crystallization temperature of the amorphous Sm-Co phase as well as the substitution of Cu for Co to form the Sm(Co,Cu)5 phase. The largest (BH)max was 32 MGOe, which is larger than the theoretical limit for a SmCo5 single phase magnet, 28.8 MGOe. This value is also larger than those of the commercial SmCo5 and Sm(Co,Fe,Cu,Zr)7 type sintered magnets. By replacing Fe with Fe65Co35 or adjusting the thickness of hard and soft layers, it will be possible to enhance the (BH)max further. This work has demonstrated that there is a potential to develop high performance exchange spring magnets by fabricating strongly exchnge coupled anisotropic soft and hard phase nanocomposites.

Reference
  1. R. Skomski and J. M. D. Coey, Phys. Rev. B 48, 15812 (1993).
  2. D. J. Sellmyer, Nature 420, 374 (2002).
  3. V. Archambault, and D. Pere, Mat. Res. Soc. Symp. Pro., 577, 153 (1999)
  4. E. E. Fullerton, J. S. Jiang, C. H. Sowers, J. E. Pearson, and S. D. Bader, Appl. Phys. Lett. 72, 380 (1998); E. E. Fullerton, J. S. Jiang, M. Grimsditch, C. H. Sowers, and S. D. Bader, Phys. Rev. B 58, 12193 (1998).
  5. R. R?hlsberger, H. Thomas, K. Schlage, E. Burkel, O. Leupold, and R. R?ffer, Phys. Rev. Lett. 89, 237201 (2002); V. K. Vlasko-Vlasov, U. Welp, J. S. Jiang, D. J. Miller, G. W. Crabtree, and S. D. Bader, Phys. Rev. Lett. 86, 4386 (2001).
  6. R. Skomski and J. M. D. Coey, Phys. Rev. B 48, 15812 (1993).
  7. I. A. Al-Omari and D. J. Sellmyer, Phys. Rev. B 52, 3441 (1995).
Related Publications

Sm(Co,Cu)5/Fe exchange spring multilayer films with high energy product
J. Zhang, Y. K. Takahashi, R. Gopalan, and K. Hono, Appl. Phys. Lett. 86, 122509 - 122601 (2005) .
Copyright (2004) American Institute of Physics

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