Nd-Fe-B Films With Perpendicular Magnetic Anisotropy and Extremely Large Room Temperature Coercivity

Abstract

Nd-Fe-B thin films have potential applications in high sensitive magnetic sensors and high density magnetic recording media due to its large magneto-crystalline anisotropy and high saturation magnetization [1–3]. They are also being investigated for energy harvesting. For all those applications, controlling the magnetic anisotropy to prepare films with perpendicular magnetic anisotropy are important [4]. In this digest, we present a novel process to prepare Nd-Fe-B thin films with perpendicular magnetic anisotropy. Substitution of various elements such as Tb, Dy and Sm were carried out. Of particular interest is that a 1 at% addition of Tb, Dy, or Sm can strongly enhance the perpendicular magnetic anisotropy of the Nd-Fe-B films. Films with additions of Tb show coercivity larger than 30 kOe. A clear correlation between perpendicular magnetic anisotropy of annealed state and the domain formation in the as-deposited state were noticed [5, 6]. In this experiment, Nd-Fe-B films were deposited by a facing targets sputtering system. The films were deposited onto thermally oxidized silicon wafer. The substrate temperature was ranging from 100°C to 400°C. It was confirmed by X-ray diffractometry (XRD) that all the deposited films are amorphous. The films were then subjected to a flash annealing process. Crystallized, magnetically hard films were obtained by annealing the samples at 650°C for 5 minutes [7, 8]. All the as-deposited amorphous films showed soft magnetic properties with coercivities less than 100 Oe in both perpendicular and in-plane direction. Fig. 1 shows magnetic force microscopy (MFM) images of as-deposited films without Tb addition (a) and with 1 at% Tb addition (b). We could not find any particular domain configuration in films deposited without Tb addition. However, well defined stripe domains with period of around 110 nm were present in the as-deposited films with Tb addition. The stripes were found to be aligned in the same direction as the stray magnetic field direction of facing targets sputtering system. Well aligned stripe domains are originating from the induced in-plane anisotropy field and perpendicular magnetic field as indicated by micromagnetic simulations. Further investigation of the perpendicular magnetic anisotropy from the saturation magnetization and domain period showed that the films have perpendicular magnetic anisotropy of around $5.0 \times 10 ^{4}\mathrm {J}/ \mathrm {m}^{3}$. Films became magnetically hard and crystallized after annealing at 650°C. Fig. 2 shows hysteresis loops of films deposited without Tb addition (a) and with Tb addition (b). According to Fig. 2(a). films deposited without Tb addition show almost identical perpendicular and in-plane hysteresis loops. However, as shown in Fig. 2(b), films deposited with addition of Tb show excellent perpendicular magnetic anisotropy with large perpendicular remanence but very low in-plane remanence. It should be noted that the films have very large coercivity that a maximum applied field of 24 kOe could not even reverse the magnetization. Films show almost zero coercivity at in-plane direction further confirms the excellent perpendicular magnetic anisotropy. XRD results shows c-axis normal to the film plane crystallographic properties for films with Tb addition, which further confirmed the perpendicular magnetic properties in those films. Our results show clear magnetic correlations between as-deposited amorphous films and annealed crystallized films. Our research provides a simple way to evaluate crystallographic and magnetic properties of Nd-Fe-B films even without annealing the films. Addition of Tb, Dy, and Sm were found can enhance the perpendicular magnetic anisotropy in the Nd-Fe-B films.