Effects of Ru and Ag cap layers on microstructure and magnetic properties of FePt ultrathin films
© Liu et al.; licensee Springer. 2015
Received: 3 February 2015
Accepted: 16 March 2015
Published: 2 April 2015
The effects of Ru and Ag cap layers on the microstructure and magnetic properties of the FePt ultrathin films have been investigated. The results indicate that i) The Ag cap layer segregates from the FePt/Ag bilayer, lowers the FePt ordering temperature, promotes the FePt thin films to form island structure, and enhances the coercivity; ii) The Ru cap layer increases the FePt ordering temperature, helps to maintain smooth continuous structure film, and restrains the FePt (001) orientation and perpendicular magnetic anisotropy (PMA). The effects become more pronounced for the 3-nm-thick FePt thin films. The effects can be mainly attributed to the different melting point and thermal expansion stress between the cap layer and FePt thin films.
The L10 phase FePt thin film is proposed to be a potential candidate for the next generation high-density perpendicular magnetic recording media for its high magnetocrystalline anisotropy (MCA) . Generally, the FePt thin films deposited at room temperature are face-centered cubic (FCC) phase and show soft-magnetic behavior , a high annealing temperature above 550°C is needed for the ordering of face-centered tetragonal (FCT) L10 phase FePt . Several methods have been used for reducing the ordering temperature and enhancing the perpendicular magnetic anisotropy (PMA) of the L10 phase FePt thin films, for example, i) introduction of an underlayer, ii) introduction of a cap layer, and iii) addition of a third element to form a ternary alloy compound. For the underlayer method, different underlayers have been investigated, such as Ag [4,5], Pt , Hf , Al , and Ti [9,10]. For the cap layer method, the Ag , Au , etc. have been introduced. The third additive elements, such as Au , Ag , Zn , Sn , and Cu  have been added to the FePt thin films.
A suitable cap layer is essential in magnetic recording media because it can affect the morphology, microstructure, and magnetic property of the magnetic films greatly. For example, Zhao et al.  found that the crystallographic ordering of the L10 FCT FePt phase was significantly promoted when a Ag layer was deposited on top of the film; Yuan et al.  found that a Au cap layer was beneficial for FePt layer forming isolated grains and larger coercivity; Chen et al.  found that the exchange couple and magnetic reversal process can be controlled by the Cu cap layer diffusion in Cu/FePt/Pt/CrW multilayers. In this study, the effect of Ru and Ag cap layer on the microstructure and magnetic properties of ultrathin FePt films was investigated by using X-ray diffraction (XRD), scanning electron microscope (SEM), vibrate sample magnetometer (VSM), and anomalous Hall effect (AHE). We found that the Ag cap layer segregates from the FePt/Ag bilayer after annealing, which lowers the FePt ordering temperature, promotes the FePt thin films to form island structure, and enhances the coercivity, whereas the Ru cap layer shows a completely inverse behavior. The effects can be mainly attributed to the different melting point and thermal expansion stress between the cap layer and the FePt thin films.
The FePt (t nm, t = 3, 10) single layers and FePt (t nm, t = 3, 10)/X (5 nm, X = Ru, Ag) bilayers were deposited on the thermally oxided Si (100) substrates by Fe and Pt magnetron co-sputtering using a Kurt Lesker CMS-18 sputtering system (Kurt J. Lesker Company, Jefferson Hills, USA) at room temperature. The chamber pressure of the sputtering system was less than 1 × 10−7 Torr, and the deposition gas was 5 mTorr high purity Ar gas. After the deposition, the samples were annealed in vacuum for an hour with a base pressure less than 1.2 × 10−5 Torr. The structure of the samples was characterized by Rigaku D/Max-2000 XRD (Rigaku, Tokyo, Japan) with Cu Kα radiation. The ordering parameters of FePt phase were calculated by following equation: S 2 = (I 001/I 002)meas/(I 001/I 002)calc [18,19], where I 001 and I 002 are the integrated intensity of (001) superlattice and (002) fundamental diffractions, respectively. The composition of the FePt layer is Fe59Pt41, confirmed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) method. The morphology of the samples was obtained by Hitachi S4800 field emission SEM (Hitachi, Tokyo, Japan). The magnetic property was characterized by TOEI VSM-5S VSM (Toei Industry Co., Ltd., Tokyo, Japan) at room temperature. The perpendicular magnetic hysteresis loops of FePt (3 nm)/X (5 nm) ultrathin films were characterized by a home-built AHE measurement system, because of the limited accuracy of the VSM for the FePt ultrathin films.
According to the above results, we can find that different cap layers show significant different effects on the FePt film, including the structure, ordering temperature, orientation, surface morphology, and magnetic properties. The different morphology in the annealed bilayers can be attributed to the surface energy and melting point of the cap layers. Surface energy and melting point are important factors that influence various surface phenomena including faceting, roughening, crystal growth, catalytic behavior, and surface segregation during annealing . Such as the FePt/Ag bilayer, at 500°C, the Ag cap layer segregates from the FePt/Ag bilayer and forms Ag(111) and Ag(200) islands, while the FePt layer maintains continuous smooth surface, which can be attributed to the low melting point (961°C) of the metal Ag and the high melting point of FePt (>1,519°C) . For the FePt/Ru bilayer, because of a high melting point and surface energy of Ru, the films keep being continuous after 600°C annealing.
On the contrary, the Ru cap layer shows a completely inverse behavior compared to the Ag cap layer. This can be attributed to the higher melting point (2,250°C) than the FePt film, and the negative thermal stress −0.54 GPa on FePt thin films listed in the Table 1. Based on the discussion above, the high melting point of Ru helps the Ru cap layer maintain a smooth continuous film until the ordered FePt particles are formed. And the negative thermal stress of the cap layer generates a total in-plane compression stress on the FePt film, which restrains the FePt ordering process. Lastly, we note that the effects of the cap layer on magnetism of 3-nm-thick FePt films are much more pronounced than the 10-nm-thick films under the same annealing temperatures.
In summary, the melting point and thermal expansion stress originating from different CTE plays an important role on the FePt/X (5 nm, X = Ag, Ru) bilayer annealing process. The Ag cap layer segregates from the FePt/Ag bilayer, lowers the FePt ordering temperature, promotes the FePt thin films to form an island structure, and enhances the coercivity. The Ru cap layer increasing the FePt ordering temperature helps to maintain smooth continuous surface, restrains the FePt (001) orientation and the perpendicular magnetic anisotropy. The effects become much more pronounced for the 3-nm-thick FePt thin films. The effects can be mainly attributed to the low melting point and positive thermal expansion stress originating from the Ag cap layer and the high melting point and negative thermal expansion stress originating from the Ru cap layer.
This work was supported by the National Natural Science Foundation of China (NSFC Nos. 61272076, 61102002, 51171086) and the Fundamental Research Funds for the Central Universities (No. lzujbky-2013-31).
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