Layering influence on Nd agglomeration and hard magnetic properties of Mo/NdFeB/Mo thick films

Abstract

The hard magnetic characteristics of the Nd-Fe-B films deteriorate and the c-axis alignment of anisotropic Nd2Fe14B hard magnetic grains tends to be deviated from the out-of-plane orientation when thickness of the films increases over a critical value. At the same time, a Nd agglomeration effect was found in the Nd-Fe-B layer volume, which is accentuated by the increase in the treatment temperature and is stronger when the Nd content is higher. In order to prevent these drawbacks we have divided the Nd-Fe-B thick films in a number of n NdFeB(x nm)/Mo(5 nm) bilayers using molybdenum (Mo) film as interlayer. The variable thickness, x, of the Nd-Fe-B magnetic layers along with the annealing conditions (temperature and time) influences both the microstructure and the hard magnetic characteristics of Nd-Fe-B thick films. Single-layer and multilayer Nd-Fe-B films of various total thickness ranging between 540 nm and 1620 nm, have been deposited by vacuum r.f. sputtering on a (001) silicon (Si) substrate heated at about 470 °C. The resulted multilayer Nd-Fe-B thick films with a total thickness of about 1620 nm (three times higher than 540 nm thin film), stratified in 9 sequences - NdFeB(180 nm)/Mo(5 nm) bilayers and annealed for 40 min at low temperature (550 °C) exhibit coercive field and remanence of the same order of magnitude with the single-layer anisotropic Nd-Fe-B thin films having the thickness of about 540 nm. At the same time, these multilayer Nd-Fe-B thick films preserve the out-of-plane anisotropic character and present a 26% increased maximum energy product as compared to the single-layer Mo/NdFeB(540)/Mo thin films.