Nd-Fe-B films

Nd₂Fe₁₄B has excellent intrinsic properties such as a very high saturation polarization and high magnetocrystalline anisotropy. While this material is already used in high performance bulk magnets, thin films are interesting for future applications in microelectromechanical systems such as motors and actuators. Since high remanence along the application direction is required, a texture of the easy magnetization axis is needed. For many applications a perpendicular magnetic anisotropy is suitable, which is achieved by an alignment of the Nd₂Fe₁₄B c-axis perpendicular to the substrate. By suitable choice of buffer and substrate and variations of deposition parameters such as temperature and composition, it is possible to control phase formation, texture and grain size, and thus to optimize coercivity and magnetic texture.
Nd-Fe-B films were prepared by pulsed laser deposition on Ta, Cr or combined Cr/Ta buffer layers using  Al₂O₃ (0001), MgO (100) or amorphous SiN substrates. Comparing Cr and Ta buffers, films on Ta grow more perfect as they grow epitaxially, smoother and with a well-defined layer architecture, while films on Cr are only c-axis textured, showing an increased roughness and exhibiting interdiffusion. From this point of view Ta is a more appropriate buffer. On all heated substrates, Nd-Fe-B films possess an out-of-plane c-axis texture with a high remanence to saturation magnetization ratio of around 0.95 and high coercivities between 1.3 T and 2.0 T. The magnetic properties of films on Al₂O₃(0001) are visible in the hysteresis loop of a Nd-Fe-B film with an almost perfect perpendicular magnetic anisotropy and the coercivity of 2 T (fig. 1). The isolated rectangular Nd₂Fe₁₄B grains in these films are ideal to study the domain pattern by MFM. Films on amorphous SiN substrates are grown with a fiber texture. Films on MgO(100) have a single in-plane orientation of the NdFeB crystallites, growing epitaxially on the substrate. It allows us to investigate in-plane anisotropy analysis of these films through the spin reorientation (fig. 2). 
In order to reach these values, an optimization of the buffer and protection layers was necessary as oxidation can easily destroy the magnetic properties.

Fig. 1: Hysteresis curve of a Nd-Fe-B film on Al₂O₃(0001)

Fig. 2: Spin reorientation measurement of a Nd-Fe-B film on MgO(100)

Key publications

A. R. Kwon, S. Fähler, V. Neu, L. Schultz
Effect of composition on phase formation, microstructure and magnetic properties of Nd-Fe-B thin films
J. Mag. Mag. Mat. 302 (2006) 252

V. Neu, S. Melcher, U. Hannemann, S. Fähler, L. Schultz
Growth, microstructure and magnetic properties of highly textured and highly coercive Nd-Fe-B films
Phys. Rev. B 70 (2004) 144418

U. Hannemann, S. Fähler, V. Neu, B. Holzapfel, L. Schultz
Intrinsic and extrinsic properties of epitaxial Nd₂Fe₁₄B films
Appl. Phys. Lett. 82(21) (2003) 3710

All our publications on Nd-Fe-B films

A. R. Kwon, V. Neu, L. Schultz, S. Fähler
Morphology and hard magnetic properties of epitaxial Nd-Fe-B films grown on Mo and Cr/Ta buffers
J. Appl. Phys. 101 (2007) 09K506

A. R. Kwon, S. Fähler, V. Neu, L. Schultz
Effect of composition on phase formation, microstructure and magnetic properties of Nd-Fe-B thin films
J. Mag. Mag. Mat. 302 (2006) 252

S. Oswald, S. Fähler, S. Baunack
XPS and AES investigations of hard magnetic Nd-Fe-B films
Appl. Surf. Sci. 252(1) (2005) 218

S. Oswald, I. Mazilu, S. Fähler, W. Gruner, R. Hermann, W. Löser, O. Filip, H. Wendrock, L. Schultz, B. Schmidt, W. Rodewald
SIMS measurements of oxygen content in the Nd₂Fe₁₄B phase
J. Mag. Mag. Mat. 290-291, 2 (2005) 1240

A. R. Kwon, U. Hannemann, V. Neu, S. Fähler, L. Schultz
Microstructure and Magnetic Properties of highly textured Nd-Fe-B Films grown on Ta (100)
J. Mag. Mag. Mat. 290-291, 2 (2005) 1247

V. Neu, F. Großmann, S. Melcher, S. Fähler, L. Schultz
A local magnetization study of epitaxial Nd-Fe-B films by magnetic force microscopy
J. Mag. Mag. Mat. 290-291, 2 (2005) 1263

V. Neu, S. Melcher, U. Hannemann, S. Fähler, L. Schultz
Growth, microstructure and magnetic properties of highly textured and highly coercive Nd-Fe-B films
Phys. Rev. B 70 (2004) 144418

S. Fähler, U. Hannemann, M. Weisheit, V. Neu, S. Melcher, S. Leinert, S. C. Wimbush, A. Singh, A. Kwon, B. Holzapfel, L. Schultz
Pulsed laser deposition of hard magnetic films
Appl. Phys. A 79 (2004) 1529

U. Hannemann, S. Melcher, S. Fähler
Highly textured Nd-Fe-B films grown on amorphous substrates
J. Mag. Mag. Mat. 272-276(1) (2004) E859

S. Fähler, V. Neu, M. Weisheit, U. Hannemann, S. Leinert, A. Singh, A. Kwon, S. Melcher, B. Holzapfel, L. Schultz
High performance thin film magnets
Proc. 18th Int. Workshop on High Performance Magnets and their Applications Vol. 2 (2004), 566

U. Hannemann, S. Fähler, V. Neu, B. Holzapfel, L. Schultz
Intrinsic and extrinsic properties of epitaxial Nd₂Fe₁₄B films
Appl. Phys. Lett. 82(21) (2003) 3710

U. Hannemann, S. Melcher, S. Fähler, V. Neu, B. Holzapfel, L. Schultz
Microstructure and coercivity mechanism of highly textured Nd-Fe-B films
IEEE Trans. Mag. 39(5) (2003) 2726

S. Fähler, U. Hannemann, S. Oswald, V. Neu, B. Holzapfel, L. Schultz
Corrosion of highly coercive, highly textured Nd-Fe-B films
IEEE Trans. Mag. 39 (5) (2003) 2950

S. Fähler, U. Hannemann, V. Neu, V. Hoffmann, B. Holzapfel, L. Schultz
Highly Coercive Textured NdFeB Films prepared by Pulsed Laser Deposition
17th Int. Workshop on Rare-Earth Magnets and their Applications; ed. G. C. Hadjipanayis, M. J. Bonder (Eds.), Rinton Pres, Princeton (2002) 449

U. Hannemann, V. Neu, S. Fähler, B. Holzapfel, L. Schultz
Hard Magnetic NdFeB and SmCo Films: Influence of the Cr Buffer Microstructure on the Magnetic Properties
IEEE Trans. Mag. 38 (5) (2002) 2949

U. Hannemann, S. Fähler, V. Neu, B. Holzapfel, L. Schultz
Magnetic and Microstructural Properties of Hard Magnetic NdFeB Films Prepared on a Ta Buffer by Pulsed Laser Deposition
IEEE Trans. Mag. 38 (5) (2002) 2805

V. Neu, U. Hannemann, S. Fähler, B. Holzapfel, L. Schultz
Effect of Rare Earth content on microstructure and magnetic properties of SmCo and NdFeB thin films
J. Appl. Phys. 91(10) (2002) 8180

U. Hannemann, S. Fähler, S. Oswald, B. Holzapfel, L. Schultz
Effect of Cr and Ta buffers on hard magnetic Nd₂Fe₁₄B films
J. Mag. Mag. Mat. 242-245 (2002) 1294