Magnetic Properties of Sintered High Energy Sm-Co and Nd-FeB Magnets

Magnetic properties of permanent magnetic materials based on intermetallic compounds of Sm-Co and Nd-Fe-B are in direct dependence on the microstructure. In the first part of this paper, having in mind the importance of the regime of sintering and heat treatment to obtain the optimal magnetic structure, yet another approach in defining the most adequate technological parameters of the sintering process for applied heat treatment conditions was made. The goal of these investigations was to use the correlation that exists between sintering conditions (temperature and time) and intensity of the diffraction peak of the (111) plane of the SmCo5 phase to optimize. In the second part a brief overview of high energy magnetic materials based on Nd-Fe-B is presented with special emphasis to the current research and development of high remanent nanocomposite magnetic materials based on Nd-Fe-B alloys with a reduced Nd content. Part of experimental results gained during research of the sintering process of SmCo5 magnetic materials were realized and published earlier. The scientific meeting devoted to the 60 anniversary of Frankel’s theory of sintering was an opportunity to show once more the importance and role of sintering in optimization of the magnetic microstructure of sintered Sm Co5 magnetic materials.


Introduction
Permanent magnetic materials based on rare earth-transition metal compounds represent modern magnetic materials that play the key role as magnetic components, which are the source of constant and intensive magnetic flux in different devices, machines and equipment in almost every aspect of technology, especially where the request for miniaturization is present and powerful magnets are needed [1][2][3][4][5][6].Application of permanent magnetic materials based on rare earth-transition metal compounds in industry started with sintered SmCo 5 magnetic materials and continued with magnetic materials based on Nd-Fe-B alloys.Magnetic properties of these materials expressed in the energy product (BH) max as a basic quantity of the quality of permanent magnetic materials exceed the values of all previously produced hard magnetic materials [7].In a course of providing a high remanent induction a highly ordered crystal texture is needed.In order of providing the maximal values of coercive force, permanent magnetic materials of Sm-Co and Nd-Fe-B type have to be high density conglomerates with a small mean grain size, practically monocrystalline with parallel orientation of the axis of easy magnetization i.e. starting powders of materials have to have very small particles [1,2,3,8].
A direct dependence of the magnetic properties of these materials on microstructure gives the process of sintering special significance in the production of magnetic materials of this type by powder metallurgy techniques.
Sintering and heat treatment are the most important steps in the production of sintered SmCo 5 magnets.The aim of sintering is, on one side, achievement of a high texture density without opened connecting pores whereas, on the other hand, the growth of crystal grains should be avoided.Open pores cause intrinsic oxidation and, therefore, reduce the magnetic remanence [1,8,9,10].The growth of crystal grains directly reduces the coercitivity, making the nucleation of reversed domains easier [1].The heat treatment after sintering must be chosen to correspond to the chemical composition of the alloy, and its purpose is to obtain a microstructure that improves coercivity [2][3][4][5].
Because of the importance of the regime of sintering and heat treatment in obtaining the optimal magnetic structure, yet another approach in defining the most adequate technological parameters of sintering processes for given heat treatment conditions was made.In previous investigations [10][11][12], a direct relation between the magnetic properties and the intensity of the diffraction peak of the (111) plane of the SmCo 5 phase was observed, so that the best magnetic properties appear when this intensity is the highest.For this reason the investigations of the correlation between the sintering conditions (temperature and time) and the intensity of the diffraction peak of the (111) plane of the SmCo 5 phase were performed [10][11][12].
The purpose of this investigation was to establish the relation between the sintering conditions and amount of main hard magnetic phase SmCo 5 , expressed by the intensity of its diffraction peaks corresponding to the (111) plane.
The dependence of the content of the SmCo 5 phase on the regime of sintering and heat treatment was monitored by X-ray diffraction analysis of the polycrystalline specimens.According to obtained diffractograms for investigated regimes of sintering and heat treatment, using mathematical modeling the regression dependence for different heat treatment regimes was established [10][11][12].

Experimental procedure
A commercial SmCo 5 powder was used as a starting powder in this investigation.The chemical composition, given by the producer was: Sm = 34.7 ± 0.3 mass% and Co = 64.8± 0.3 mass%.The powder was milled in anhydrous toluene to an average particle size of 7.23 µm.Double side pressing using a pressure of 700 MPa, with simultaneous particle orientation in a magnetic field was performed.
Sintering was investigated in the temperature range from 1100 to 1180 o C in intervals of 20 o C, with sintering times of 30, 45 and 60 minutes for each investigated temperature.Sintering and heat treatment were performed consequently in the same furnace -an electro resisitive owen.The furnace had a maximal working temperature of 1500 o C, electronic programming and temperature control with a precision of ± 1 o C.
Two different heat treatment regimes were investigated, for all investigated sintering conditions.In the first case after each sintering regime, heat treatment was performed at 900 o C for 90 minutes, and in the second case the heat treatment was at 920 o C, also for 90 minutes.After heat treatment the specimens are rapidly cooled to room temperature.A vacuum of 150Pa as a protection atmosphere during the sintering and heat treatment was used.
The presence of the SmCo 5 phase was observed using powder X-ray diffraction analysis of the polycrystalline specimens.X-ray analysis was performed using a PHILIPS PW 1710 diffractometar and a copper anticathode (λ = 0.154178 nm).Mathematical-statistical methods were used on the experimental results of the X-ray analysis [13][14][15].Using regression analysis the influence of the sintering parameters (temperature and time), on the amount and, more exactly, the stability of the magnetic SmCo 5 phase was determined by establishing a correlation between the intensity of its most intensive diffraction peak, corresponding to the (111) plane [16], with the sintering parameters.

Results and discussion
The  The intensity of the most important diffraction peaks, corresponding to the (111) plane of the SmCo 5 phase, for all investigated sintering temperatures and times and for both applied heat treatment regimes are presented in Tab.I.
For the sintering temperature of 1100°C, data are given only for the sample sintered for the longest time of 60 minutes.Samples sintered at this temperature for shorter times exhibited poorer mechanical properties.Samples sintered at 1180°C for times longer than 30 minutes had a thicker oxide zone in the surface layer, and were not taken into consideration [9,12].
The increase of the intensity of the diffraction peak of the (111) plane of the SmCo 5 phase and of all the crystallographic planes corresponding to the SmCo 5 phase up to a sintering temperature of 1140 o C, under the investigated conditions, could be caused by better crystallized SmCo 5 particles.The decrease of the intensity of the diffraction peak of the (111) plane of the SmCo 5 phase with further increase of the sintering temperature above 1140 o C is caused by the appearance of less magnetic and non-magnetic phases, which reduce the content of the magnetic SmCo 5 phase.The presence of undesirable phases could not be identified by the Xray diffraction method because their content was smaller than the precision of this method.The undesired phases were observed by a metallographic investigation and confirmed by Micro X-ray Analysis using Energy Dispersion Spectra.

Tab. I.
The final conclusion about the reason for this trend in the dependence can only be given after completion of microstructural and X-ray investigations.
According to the results of X-ray analysis of the sintered and heat treated samples, the maximum of the intensity of the diffraction peak, which corresponds to the (111) plane of the SmCo 5 phase was observed in the investigated temperature and time interval.The parabolic dependence of the intensity of the diffraction peak on the sintering regime was noticed.To verify the observed dependence the simplest non-linear regression equations (Table II In Tab.II, k 1 , k 2 , k 3 , k 4 , k 5 and k 6 are parameters; t -the sintering temperature in o C; τthe sintering time in minutes.For both heat treatment regimes, the smallest average square errors are obtained when equation No. 3 is used.These functions were chosen as the most appropriate, and they can be presented by the following regression equations: (1) -for heat treatment at 900 o C and (2) -for heat treatment at 920 o C. I= -0.0174 t 2 -0.0387 τ 2 -0.0222 t τ + 40.7910 t + 28.9499 τ -23921 (1) I= -0.0175 t 2 -0.0220 τ 2 -0.0205 t τ + 40.8538 t + 25.3125 τ -23830 (2) A graphical interpretation of the dependence of the experimental results, obtained by measuring the intensity of the (111) plane peak of the SmCo 5 phase, on sintering temperature and time, as well as the values obtained using the regression equations (1 and 2) are shown in Fig. 2.  It is obvious that the obtained mathematical models, for different heat treatments, show the same type of complex dependence of the peak intensity of the sintering time and temperature.The intensity of the diffraction peak of the (111) plane of the SmCo 5 phase depends on the square of both the sintering time and temperature but also on tτ (mutual product of the sintering time and temperature).Considering that this dependence has a maximum, by calculating the values for the sintering time and temperature for which the given dependence has its maximum, the optimal sintering conditions should simultaneously be predicted.It was confirmed experimentally that the best results are obtained when sintering is performed under the stated conditions.In this way the obtained models confirmed their usability in optimization of the sintering process.
It can be noticed that the curve obtained for heat treatment performed at 920 o C has smaller values of the intensity of the diffraction peak of the (111) plane of the SmCo 5 phase than the curve obtained for heat treatment of 900°C.According to the similarity of the obtained dependencies it can be assumed that heat treatment at temperatures higher than 920°C would be even less appropriate and that these investigations should not be performed.Also, these dependencies indicate that if heat treatment is performed at temperatures within interval 900-920°C, the results will be better than on 920°C but poorer than on 900°C.
The hysteresis graph magnetizer with magnetic field strength in the range from 2100 to 2400 kA/m was used for measurements of the magnetic properties of investigated sintered SmCo 5 magnetic materials.The influence of sintering conditions and applied heat treatment regimes on magnetic properties of investigated SmCo 5 samples is illustrated with corresponding hysteresis loops presented on Fig. 5.
Fig. 5 shows hysteresis loops of SmCo 5 samples sintered under different sintering conditions heat treated at 900 o C for 90 min.
In spite of high temperature stability of magnetic properties of magnets of the Sm-Co type, nowadays magnets of the Nd-Fe-B type are more interesting owing to high validity of (BH) max and cheaper initial raw materials.Anisotropic, sintered Nd 2 Fe 14 B -type magnets [7] with energy products (BH) max exceeding 400 kJ/m 3 (50 MGOe) are practically the third generation of permanent magnetic materials based on rare earth-transition metal compounds.The basic requirements for good quality of the produced magnetic material of this type are a minimized oxygen content, maximal volume fraction of the hard magnetic 2:14:1 phase with a minimal content of non-ferromagnetic phases on grain boundaries, a small crystallite size with average grain size in the range from 2 to 6 µm and a maximum alignment of the easy axis of magnetization.Techniques of conventional powder metallurgy generally fulfill all presented requests and sintered Nd 2 Fe 14 B-type magnets are produced routinely since the middle of the 1980s [7].Current research in the field of magnetic materials based on Nd-Fe-B is directed in three main directions: increase of magnetic energy, increase of corrosion resistance and reduction of the amount of rare earth (Nd) in order to reduce the price of the final magnetic material while keeping high values of magnetic energy.
In the course of investigations of the microstructure of RQ Nd-Fe-B magnets the following factors were emphasized: the influence of the cooling rate on the magnetic properties, the influence of the Nd content on the value of the coercivity, as well as the influence of alloying additives on the increase of the Curie temperature and thermal stability of the final magnets of this type [7,17,18].
Permanent magnetic materials based on Nd-Fe-B alloys with reduced Nd content are the new type of nanocomposite permanent magnetic materials [7,17,18].The most common method for production of nanocomposite permanent magnets is the method of rapid quenching (melt-spining) [7,18].
Unlike monophase Nd 2 Fe 14 B magnetic alloys, these materials have a multiphase microstructure.In order to obtain an optimal magnetic microstructure which is the key to improvement of hard magnetic properties, heat treatment is needed.By heat treatment crystallization is aimed at the formation of the so called magnetic microstructure, in this case nanocomposite structure which provides optimal magnetic characteristics.Enhancement of remanence of investigated RQ Nd-Fe-B alloys with low Nd content after heat treatment, according to most authors are the result of "exchange coupling" between neighboring grains of soft Fe 3 B and hard Nd 2 Fe 14 B magnetic phases.Necessary conditions for exchange coupling, are that these phases should be crystalographically coherent and the mean grain size should be below 40 nm [19] which leads to remanence enhancement and consequently higher values of magnetic energy (BH) max .
Based on previous investigations [20][21][22][23] the influence of the Nd content on the magnetic properties is illustrated by hysteresis loops of three investigated Nd-Fe-B alloys with different Nd content presented on Fig. 6.Tab.III shows summarized results of magnetic measurements obtained by a SQUID magnetometer and calculated remanence ratios of investigated Nd-Fe-B alloys after heat treatment.A further increase of remanence and magnetic energy is obtained for a nonstoichiometric multiphase alloy with a lower amount of Nd by forming of a nanocomposite structure after application of an optimal heat treatment regime [7,22,23].

Tab. III Magnetic properties of investigated
According to the results of phase analysis of the Nd 4.3 Fe 76.2 B 19.5 alloy [20,23] a Fe 3 B soft magnetic phase, Nd 2 Fe 14 B hard magnetic phase and a whole set of Fe-Nd ferromagnetic phases were identified after annealing.Compared to measured remanence of investigated alloys with a dominant hard magnetic Nd 2 Fe 14 B phase Nd 14.5 Fe 78.5 B 7 (Br = 7.4 kG) and Nd 12.5 Fe 82.5 B 5 (Br = 8.6 kG) remanence of the Nd 4.3 Fe 76.2 B 19.5 alloy is increased (Br = 10.9 kG).The shape of the hysteresis loop of Nd 4.3 Fe 76.2 B 19.5 alloy with reduced Nd content compared to the hysteresis loops of Nd 14.5 Fe 78.5 B 7 and Nd 12.5 Fe 82.5 B 5 alloys (Fig. 6) also indicates enhancement of remanence.By correlation of the phase composition obtained after optimal heat treatment and measured magnetic properties [23], it might be concluded that enhancement of remanence and high value of maximal magnetic energy (BH) max = 10.7 MGOe is the consequence of the formation of nanocomposite Fe 3 B/Nd 2 Fe 14 B and that the grains of the Fe 3 B soft magnetic phase are coupled with neighboring grains of the Nd 2 Fe 14 B hard magnetic phase.
The calculated value of the remanence ratio Mr/Ms from the hysteresis loop of the Nd 4.3 Fe 76.2 B 19.5 alloy obtained by measurements on a SQUID magnetometer is 0.6 and it exceeds the theoretical limit of 0.5 predicted by the Stoner-Wohlfarth theory.This indicates that a nanocomposite structure is formed and that the magnetic properties of the investigated Nd 4.3 Fe 76.2 B 19.5 alloy are influenced by the effect of ferromagnetic exchange coupling between the grains of identified soft and hard magnetic phases.
The content of Nd in nanocomposite magnetic materials can be 15-50 at% lower than that in conventional melt-spun and sintered Nd-Fe-B magnets.Among the advantages of these magnets is low material cost due to reduction of the content of the expensive rare earth.Nanocomposite permanent Nd-Fe-B magnets are very attractive because of their suitability for production of bonded magnets.

Conclusion
The role of the sintering process in the production and improvement of magnetic properties of sintered SmCo 5 permanent magnetic materials is observed and analyzed to establish a correlation between the sintering conditions (temperature and time) and the amount of the main hard magnetic phase SmCo 5 , expressed by the intensity of its diffraction peaks which correspond to the (111) plane.
The new nanocomposite permanent magnetic material based on the Nd-Fe-B alloy with reduced Nd content is brieflly overviewed with a few experimental results showing the influence of the amount of Nd on the magnetic properties.The conditions and parameters which have dominant influence on the formation of a nanocomposite structure providing a stronger exchange coupling between the soft and hard magnetic phase and therefore higher remanence enhancement are also presented.

Садржај: Магнетна својства перманентних магнетних материјала на бази интерметалних једињења типа
representative diffractograms of samples of SmCo 5 sintered at 1140 o C, for 45 minutes are presented on Fig. 1.The applied heat treatment regimes were 90 minutes at 900 o C (graph a) and 90 minutes at 920 o C (graph b).
The temperature and time dependence of the most important diffraction peaks corresponding to the (111) plane of the SmCo 5 phase for investigated heat treatment regimes 900 o C/90 min and 920 o C/90 min.
The investigated regression equations for the dependence of the peak intensity of the (111) plane of the SmCo 5 phase on the sintering time and temperature