In-plane magnetic anisotropy and coercive field dependence upon thickness of CoFeB Lalminthang Kipgen, Himanshu Fulara, M. Raju, Sujeet Chaudhary n Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India article info Article history: Received 3 October 2011 Received in revised form 24 April 2012 Available online 22 May 2012 Keywords: Coercivity Magnetic anisotropy Amorphous/nanocrystalline material abstract The structural and magnetic properties of as-grown 5–50 nm thin ion-beam sputter deposited transition metal–metalloid Co 20 Fe 60 B 20 (CFB) films are reported in this communication. A broad peak observed at 2y 451 in the glancing angle X-ray diffraction pattern revealed the formation of very fine nano-sized grains embedded in majority amorphous CFB matrix. Although no magnetic field is applied during deposition, the longitudinal magneto-optic Kerr effect measurements performed at 300 K in these as-grown films clearly established the presence of in-plane uniaxial magnetic anisotropy (K u ). It is argued that this observed anisotropy is strain-induced. This is supported by the observed dependence of direction of K u on the angle between applied magnetic field and crystallographic orientation of the underlying Si(100) substrate, and increase in the coercivity with the increase of the film thickness. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Transition metal–metalloid (TM–M) alloy CoFeB is an amor- phous soft ferromagnetic material [1]. It gradually recrystallizes at annealing above a temperature which depends on its composi- tion [2,3]. The CoFeB is a widely used material in spintronic devices such as in magnetic tunnel junctions (MTJs), sensor applications, magnetic random access memories (MRAMs) and other novel devices [4–6]. In particular, CoFeB in conjunction with AlO x [7] and recently with MgO [8] has been shown to exhibit record high tunnel magnetoresistance (TMR) values at room temperature. In such devices, the in-plane magnetic anisotropy of the soft magnetic CoFeB films is a required property for their operation. Generally, the in-plane magnetic anisotropy is achieved by applying dc-magnetic field during the film deposition itself [9]. This anisotropy is due to short range chemical ordering dictated by the applied magnetic field. CoFeB can also be fabricated to exhibit uniaxial anisotropy in any arbitrary direction by post-deposition magnetic annealing [1], or by application of mechanical stress [10]. In this paper, we report the magnetization behavior of the as-deposited ion-beam sputtered CoFeB films synthesized at 300 K. Although, these films were not subjected to any of the above mentioned procedures, they however, exhibit an in-plane magnetic anisotropy in the as-grown state. Neel’s pair ordering anisotropy (POA) model [11] postulates a difference in the number of like and unlike nearest neighbor distances of the atoms for the in-plane and out-of-plane direc- tions as the origin of induced anisotropy in the former case. TM–M alloys have been widely studied and observations consistent with POA model have been made [12]. Garcia et al. [10] sputter deposited CoFeB on a glass substrate subjected to a compressive stress. The compression induced the magnetic anisotropy due to the negative magnetostriction. 2. Experimental Thin films of Co 20 Fe 60 B 20 of thickness ranging from 5 nm to 50 nm were grown on Si (100) substrates by ion-beam sputter deposition system (NORDIKO 3450) with a base pressure r1  10 6 Torr. The Argon (Ar) plasma was created by bleeding Ar in a differentially pumped rf-ion source, operated at 95 W. The Ar ion-beam was extracted via a dual grid assembly (grid voltages at  270 V and 500 V). This resulted into a pressure of 1.5  10 4 Torr in the chamber during the film deposition. The film deposition was carried out at room temperature using the 500 eV Ar ion-beam which sputtered a water cooled 6 00 diameter Co 20 Fe 60 B 20 target at 451. With a substrate to target distance of  26 cm, the deposition rate was estimated as 0.02 nm/sec by X-ray relectivity (XRR) measurement. The presence of clear Kiessig fringes in XRR profiles up to a wide range of incident angle indicates the thickness uniformity [13] in our case. It may be noted that no magnetic field is applied during the deposition. Prior to the deposition, the substrates were cleaned initially with acetone, followed by propanol-rinse in ultrasonic bath. Magnetic M–H characterization was performed at room temperature by a Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.05.012 n Corresponding author. Tel.: þ91 11 2659 1341. E-mail address: sujeetc@physics.iitd.ac.in (S. Chaudhary). Journal of Magnetism and Magnetic Materials 324 (2012) 3118–3121