IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 9, SEPTEMBER 2013 5043 How Extrinsic Is the Coercivity in NdFeB Bonded Magnets? E. A. Périgo , I. A. Cruz , B. Antunes , A. P. V. Braga , and F. J. G. Landgraf Laboratory for the Physics of Advanced Materials, University of Luxembourg, L-1511 Luxembourg, Luxembourg Institute for Technological Research, São Paulo 05508-901, Brazil Polytechnic School, University of São Paulo, São Paulo 05508-010, Brazil The influence of the densification step, performed by uniaxial pressing at room temperature, on the coercivity of NdFeB bonded magnets has been analyzed. The mass density does not play any role on of non-cured magnets prepared by the mentioned method. Due to the brittle and intergranular character of the Nd Fe B-based phase fracture, the nano-sized grains are not affected or the modification is negligible, which maintains unchanged. After curing, the intrinsic coercivity of the magnets is reduced as a function of the compaction pressure increase. Index Terms—Coercivity, magnetic materials, NdFeB, pressing. I. INTRODUCTION A LONG the last century, magnetic materials (MMs) have evolved from a laboratory subject to human society life. Nowadays, a large amount of equipment, such as high-perfor- mance motors and cooling systems, is constituted by MMs [1]. From the engineering viewpoint, such MMs are analyzed and classified based on their extrinsic properties. As an example, magnetic materials are considered soft when the intrinsic coer- civity is inferior to 1.2 mT [2] and possess low magnetic losses and high permeability; on the other hand, magnetic ma- terials are identified as hard when 1.2 mT and present high remanence and maximum energy product . All cited properties are achieved when the density of the bulk parts is as high as possible. The relation between and the characteristic parameters of the hysteresis loop can be linear, like versus in NdFeB and SmCo sintered magnets, or expo- nential for the permeability of soft magnetic composites [3], [4]. It has been reported that the of melt spun NdFeB bonded magnets can also be expressed as a function of by [5] (1) where is the intrinsic coercivity of the single mag- netic powder particle and is the theoretical density of the bonded magnet. However, the coercivity of NdFeB melt spun magnets (or even of any hard magnetic material) will hardly be expressed by (1) because if a magnet reaches its theoretical den- sity its coercivity will be null, which is in strong con- trast to experiment. In fact, (1) is a special case of a more general formula used for a powder containing ellipsoidal single-domain particles [6] (2) Manuscript received November 19, 2012; revised January 28, 2013; accepted March 12, 2013. Date of publication March 22, 2013; date of current version August 21, 2013. Corresponding author: E. A. Périgo (e-mail: eaperigo@ieee. org). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2013.2253788 According to Néel [7], and (returning to (1)). Such expression explains, from the qualitative viewpoint, the coercivity of soft magnetic composites prepared with distinct magnetic powders [8]. On the other hand, Kondorsky and Wohl- farth report alternative values of these parameters (Wohlfarth also indicates additional constants related to the interactions and geometrical arrangement of the particles [6]). As will be shown in this work, is independent of in melt spun NdFeB bonded magnets since the nano-sized grains responsible for the coercivity of the compound are not changed, or their modification is negligible. II. EXPERIMENT A commercially available MQP-B NdFeB melt-spun powder was used as the starting material. Isotropic bars and cylinders were prepared compacting the particles at distinct pressures (100 MPa 800 MPa) at room temperature . No additional polymer/additive, except the epoxy resin existent on the particles, was added before pressing. Two set of samples were analyzed: one before curing and other after curing, per- formed at 473 K during 3.6 ks in argon (99% purity—flow rate 3 Lmin ). All cured samples were prepared in the same batch. The density of the pieces before curing was obtained by using Archimedes’ principle with the average of 3 measurements. The only exception was the sample prepared at 100 MPa, charac- terized geometrically due to its low mechanical strength. The density was assumed to be the same after curing because the mass change was inferior to 0.5% and no geometrical variations were noticed. The magnetic properties of powders and cylin- drical magnets were obtained with a vibrating sample magne- tometer and a hysteresisgraph, respectively, at room tempera- ture. In both cases, the applied field to perform the magnetic measurements was 2.0 T, and no demagnetization corrections have been employed. In order to obtain the remanence for the powder in Tesla, it has been considered 7640 kg m [9]. III. RESULTS AND DISCUSSION Fig. 1 depicts the magnetic properties of the bulk samples, before and after curing, as a function of the compaction pres- sure. The remanence follows a linear relation with (for a brief comparison between and , the reader is referred to check 0018-9464 © 2013 IEEE