Magnetic Nanocomposites for PermanentMagnets

F. Tolea, M. Sofronie, A. Birsan, G. Schinteie, V. Kuncser, and M. Valeanu

Abstract The influence of different crystallization conditions on the microstructureand magnetic hardening of RE-Fe-B amorphous ribbons with different Fe concen-trations and Pr and Nd as rare earth elements was analyzed. The microalloyingeffects of Zr and Ti substitution on the evolution of crystallization process and themagnetic hardening was also discussed. The final aim of this experimental studywas to obtain performing exchange spring magnets with as much as lower contentof the expensive RE material.

1 Introduction

Exchange-spring magnets (or spring magnets) consist of interfaced hard and softmagnetic nano-phases that are coupled by exchange interactions. Essentially, springmagnets combine the high anisotropy typical to a hard magnetic phase (e.g.Nd2Fe14B, with high uniaxial magneto-crystalline-anisotropy, Ku) with the largemagnetization typical to a soft magnetic phase (e.g. α-Fe or Fe3B, with high mag-netization at saturation, Ms). The magnetic properties of RE-Fe-B/α-Fe (or Fe3B)nanocomposite magnets (where RE is a Rare Earth), are deeply related to theirmicrostructure. The optimal microstructure of magnetic nanocomposites consistsof uniformly distributed hard and soft magnetic nanophases, the nano-size dimen-sion of the two interconnected phases being a critical parameter for an enhancedmagnetic coupling [1–3]. Evidently, the cause of the magnetic strengthening (hard-ening) is the exchange interaction at the interface between the soft and hard phases.Since the exchange interaction is a short range interaction, the dimension of the softphase nano-clusters has to be less than twice the width of the domain wall in thehard phase (the hardening’s condition) [2, 4]. In principle, it is difficult to controlthe phase dimensions, but this becomes possible by starting from the amorphous

F. Tolea (B)National Institute of Materials Physics (INFM), 105 Atomistilor Street,077125 Magurele-Bucharest, Romaniae-mail: felicia@infim.ro

A. Aldea, V. Bârsan (eds.), Trends in Nanophysics, Engineering Materials,DOI 10.1007/978-3-642-12070-1_12, C© Springer-Verlag Berlin Heidelberg 2010

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state and applying suitable annealing treatments and/or different substitutions withalloying elements like Cu, Si, V, Nb, W, Al, Dy, etc. [5, 6].

Even if they are the most usual exchange spring magnets, the Nd-Fe-B+α-Fenanocomposites are not suitable to be used at low temperature. This is becausethe hard phase undergoes a spin reorientation at 135 K and develops a planar mag-netocrystalline anisotropy which destroys the exchange coupling. Pr-Fe-B + α-Fenanocomposites are available for low temperature purposes [4, 7].

This work reports on the effect of thermal treatments on the exchange couplingand microstructure of nanocomposites based on RE2Fe14B+ x%Fe (RE = Nd andPr and x less than 35). The influence of Zr and Ti substitution in Nd7Fe81B12 [8], onthe formation of the nanocomposite material for permanent magnets, is also consid-ered. The final aim of this experimental study was to obtain performing exchangespring magnets with as much as lower content of the expensive RE material.

2 Experimental Set-Up

Ingots with nominal compositions RE2Fe14B + x% Fe, where RE is Nd or Pr andx = 5, 10, 15, 20, 25, 30, 35 were prepared by arc melting. The amorphous rib-bons were obtained by the melt spinning technique in argon atmosphere. Differentannealing treatments were performed in vacuum, for 2 until 15 min, at temperaturesbetween 650◦C and 811◦C. We introduce the following notation of the samples: A1for Nd based sample with 5% additional Fe, A2 for Nd based sample with 10%additional Fe,. . .,A7 for Nd based sample with 35% additional Fe. Pr based samplewith similar additional Fe content as in the previous description were denoted byC1, C2,. . .,C7, respectively.

For structural information, room temperature X-ray diffractions (XRD) were per-formed by using a Seifert Diffractometer with Cu Kα radiation. The microstructureinformation has been completed by Transmission Electron Microscopy (TEM).

The structural influence of Ti and Zr substitution on Nd7Fe81B12, Nd7Fe81Ti2B10,Nd7Fe79Zr2B12 compositions was studied in situ with synchrotron radiation, byenergy-dispersive X-ray diffraction at high temperature (HASYLAB, F2.1 beam-line).

The Curie temperatures were acquired via the Faraday method, using a horizontalWeiss balance. The hysteresis loops were obtained by the extraction method at roomtemperature, in fields up to 1.8 T, applied along the length of the ribbons.

3 Results and Discussions

3.1 Nd2Fe14 B and Pr2Fe14 B + x% Fe

The crystallization behavior of the amorphous material imposes the subsequentmicrostructure of the nanocomposite. Figure 1 shows the diffraction patterns ofsample A3, as cast (or as quenched (AQ)) and after two different thermal treatments.

Magnetic Nanocomposites for Permanent Magnets 289

Fig. 1 The diffraction patterns of sample A3, in different annealing conditions

For the as cast ribbons, the XRD pattern confirms their amorphous state. Annealingtreatments at temperature higher than 600◦C promote the crystallization of bothhard – and soft – phases. A higher temperature treatment, even shorter, induces abetter refinement of the crystalline structure, maintaining the phase’s dimensions inrange of nanometers. After a short treatment at 786◦C, the mean grain sizes of a-Feand Nd2Fe14B phases, evaluated from X-ray broadening analysis using the Scherrerformula, are ∼ 5.5 and 11.0 nm, respectively.

The TEM analysis of sample A3 reveals an amorphous matrix for the as quenchedsample. One can notice that the thermal treatment (TT) promotes the crystallization,evidenced by the grain formation. At higher temperature the grains’ boundariesbecome more evident (see Fig. 2).

Thermo-magnetic measurements performed on A3 sample in the as quenchedstate revealed a magnetic order-disorder transition of about 150◦C, temperatureassumed to be the Curie temperature of the amorphous phase. The magnetizationof the sample is negligible above the mentioned temperature. The thermomagneticmeasurement is, in fact, equivalent to a thermal annealing during which crystalliza-tion is induced. Therefore, by heating the sample, as soon as the crystallizationbegins with the formation of a crystalline phase which presents magnetic orderabove the crystallization temperature, the magnetization starts to increase. That ishappened at ∼550◦C on the analyzed samples, where the formation of the hard(Nd2Fe14B) and the soft (α-Fe) magnetic phases is expected. It is worth to noticethat Nd2Fe14B enter the paramagnetic state at temperatures above 312◦C whereasthe soft phase, α-Fe in this case, enter the paramagnetic state above 740◦C. There-fore, the magnetization observed between 550◦C and 740◦C along the heating curve

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a b c

Fig. 2 Bright field transmission electron micrographs of the A3 (Nd10Fe85B5) alloy, in the follow-ing states: as quenched (a), annealed at 650◦C for 5 min (b) and respectively at 750◦C for 3 min(c) and the corresponding selected area electron diffractions shown in inset for (a) and (b)

has to be due to only the α-Fe phase. Further on, by cooling the already crystallizedsample, the magnetization reveals two augmentations which are attributed to themagnetic ordering of the two phases: the soft magnetic phase α-Fe, at about 750◦C,and the hard magnetic phase Nd2Fe14B, at about 312◦C (see Fig. 3).

For A3 TT 10 min/757◦C the heating curve indicates directly the two order-disorder transitions corresponding to the two magnetic phases. One should noticethat even this sample in the starting state is not yet totally crystallized. The magne-tization is continuously increasing before the Curie temperature of the hard phase,suggesting that the process of refining the crystalline structure and grain growthis going on. The thermo-magnetic cooling curve is almost identical with that forthe AQ sample (see also Fig. 3), suggesting the similar crystalline state of the twosamples after the thermo-magnetic heating up to 800◦C.

Magnetic measurements were performed by extraction method at room tempera-ture, in fields up to 1.8 T, applied along the length of the ribbons.

Fig. 3 Thermo-magnetic measurements performed in vacuum on A3 sample in as-quenched state(right side) and on A3 TT 10 min/757◦C (left side)

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Fig. 4 Histerezis loops for A1 A3 A5 and A7 samples with Nd as rare earth (left side) and for C1,C3, C4 and C5 samples with Pr as rare earth (right side)

The hysteresis loops for A1, A3, A5 and A7 samples annealed at 750◦C and725◦C, for either 3 or 5 min are presented in Fig. 4. The experimental data evidencethat by increasing the iron content, the saturation magnetization increases and thecoercive field decreases, suggesting the simultaneous augmentation of the relativeamount of the soft magnetic phase and the reduction of the magnetic hardness. Thesame tendency is observed for samples with Pr as rare earth (see also Fig. 4). Thisgeneral trend is also observed from the numerical parameters of all the analyzed Ndbased samples presented in Table 1.

As can be observed from data in Table 1, the annealing conditions are very crit-ical in regard to optimal size and crystalline structure of the involved phases, withdirect influence on the magnetic properties of the nanocomposites. The effect of thethermal treatment on the magnetic behavior of sample A3 (amorphous Nd10Fe85B5

Table 1 Characteristics of the A1–A7 samples, where: treatment= annealing conditions, remanentmagnetization = MR, coercive field = Hc, saturation magnetization = MS

Sample Treatment (time/temperature) MS(Gs∗cm3/g) MR(Gs∗cm3/g) Hc(kOe) MR/MS

A1 5 min/650◦C 113 77 8.7 0.683 min/750◦C 103 80 8.8 0.782 min/790◦C 116 86 7.6 0.74

A2 5 min/650◦C 120 69 4.4 0.573 min/750◦C 117 81 5.3 0.692 min/790◦C 120 88 5.3 0.74

A3 5 min/650◦C 123 54 2.9 0.443 min/750◦C 115 75 4.5 0.652 min/790◦C 118 73 3.3 0.61

A4 5 min/650◦C 131 68 3.2 0.553 min/750◦C 128 70 3.8 0.552 min/790◦C 122 69 3.4 0.56

A5 5 min/650◦C 139 63 3.1 0.455 min/750◦C 137 70 3.7 0.51

A6 5 min/650◦C 141 53 2.1 0.375 min/750◦C 138 71 2.8 0.52

A7 5 min/650◦C 144 52 1.1 0.365 min/750◦C 148 75 2.5 0.51

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Fig. 5 Histerezis loops for A3 sample in AQ state and annealed for 3 min at 750◦C, 2 min at 786◦Cand 15 min at 760◦C. The AQ state shows a negligible coercive field whereas the sample annealedfor 15 min at 760◦C presents a mixture of two independent ferromagnetic phases with differenthardness (constricted loop). The best coupling in sample A3 involves a thermal treatment of 3 minat 750◦C

in the AQ state) is evidenced also via Fig. 5. On a hand, the amorphous phase showsthe expected negligible coercive field. Oppositely, when the thermal treatment is toolong or the temperature is too high, the dimension of the soft magnetic phase exceedsthe critical dimensions for an optimal exchange coupling between the soft and thehard phase grains. Consequently, the two magnetic phases are only weakly coupledand a small kink appears in the demagnetization curve. Only optimal treatmentslead to suitable coupling with increased coercive field, remanent magnetization, andMR/MS ratio.

The best-achieved exchange couplings among the Nd based samples (see Table 1)have been obtained for samples A5 annealed at 750◦C for 5 min and A4 annealed5 min at 725◦C, for which the energy product, (BH)max, defined as the maximumproduct of the flux density, B, and the corresponding opposing field, H, is higherthan 90 KJ/m3.

3.2 Nd7Fe81B12 with Zr and Ti Substitutions

An interesting crystallization process was evidenced in some Nd based compo-sitions more rich in boron, for which the soft magnetic phase is expected to be

Magnetic Nanocomposites for Permanent Magnets 293

Fe3B [6, 9]. The influence of Zr and Ti substitution on the evolution of the crys-tallization process and magnetic hardening was studied, by starting from the par-ent compound Nd7Fe81B12. The temperature-induced crystallization processes onamorphous ribbons with nominal compositions Nd7Fe81B12, Nd7Fe81Ti2B10 andNd7Fe79Zr2B12 were studied in situ using synchrotron radiation in high-temperatureenergy-dispersive X-ray diffraction experiments performed at HASYLAB (F2.1beam line).

Figure 6 shows the diffraction patterns of the substitution-free sample in the tem-perature range from 300 – 850◦C. It is worth noticing that the Nd fluorescencelines at 36.84, 37.36, 42.27 and 43.32 keV are superposed to the diffraction spectra.The crystallization process starts at 600◦C with the precipitation of the Nd2Fe23B3metastable phase, which was detected as the predominant one even at the highesttemperature. The first diffraction peaks of the Nd2Fe14B hard phase and of the softFe3B/α-Fe phase become visible only above 700◦C. The reason for such a highdecomposition temperature is related to the sample composition, very closed to thestoichiometry of the Nd2Fe23B3 phase, as well as to a lack of composition gradientaround the metastable phase grains.

On the other hand, the high value of the decomposition temperature inducesa non-uniform microstructure, unsuitable for an exchange coupled magnet, as

Fig. 6 In situ high temperature energy dispersive XRD measurements performed on the rapidlysolidified Nd7Fe81B12 sample

294 F. Tolea et al.

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supported by additional thermo-magnetic measurements. The thermo-magneticmeasurements performed with a Weiss-type susceptometer on a sample with Tisubstitution, ex situ annealed at 650◦C for 5 min (Fig. 7) confirm the presence ofthe following phases: (i) Nd2Fe14B with a Curie temperature of 310◦C, (ii) themetastable Nd2Fe23B3 with a Curie temperature of 385◦C, (iii) the Fe3B with aCurie temperature of 518◦C and finally (iv) the α-Fe with 770◦C Curie temperature.However, after reaching approx. 800◦C, the Nd2Fe23B3 has fully decomposed intoFe3B/α-Fe and Nd2Fe14B, as proven by the cooling curve shown in Fig. 7.

Usual XRD measurements were performed on ex-situ thermally treated sampleswith Ti substitution (Nd7Fe81Ti2B10 initial composition). The crystallization startsat 600◦C, with the formation of the Nd2Fe23B3 which subsequently decomposesinto Fe3B and Nd2Fe14B at 700◦C, as illustrated in Fig. 8a and b, respectively.Surprisingly, in a similar sample with Zr substitution (Nd7Fe79Zr2B12 initial com-position), both Fe3B and Nd2Fe14B phases precipitate directly from the amorphousphase (Fig. 8c). The Fe3B phase decomposes into Fe2B and α-Fe, when the anneal-ing temperature is increasing.

The exchange coupling between the soft and hard magnetic phases in these sam-ples was checked by magnetic measurements. The room temperature hysterezisloops of the samples after optimal annealing treatments are represented in Fig. 9.The saturation magnetization decreases with increasing the annealing tempera-ture, due to decomposition of Nd2Fe23B3, as observed also from XRD pattern andthermo-magnetic measurements. The highest coercive field of Nd7Fe81B12 samplewas obtained after a thermal treatment at 775◦C for 5 min. However, the loop showsthe typical behavior of a hard/soft mixture without exchange coupling. The inter-granular exchange interaction is evidenced in Nd7Fe81Ti2B10 sample, annealed for2 min at 700◦C. Finally, the best exchange coupling is obtained for Nd7Fe79Zr2B12

Magnetic Nanocomposites for Permanent Magnets 295

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sample annealed at 725◦C for 2 min. This sample presents an energy product of87.2 kJ/m3.

4 Conclusions

It is known that the exchange coupling and the microstructure of magnetic nanocompositesare deeply related to the thermal treatment. Our magnetic measurements indicate that opti-mal magnetic hardening occurs for thermal treatments at higher temperatures and shorterannealing times applied to initially amorphous ribbons.

296 F. Tolea et al.

Suitable thermal treatments applied to amorphous compounds of compositionscorresponding to the combination RE2Fe14B+x%Fe (RE= Nd or Pr) with or with-out Ti or Zr substitutions can lead to performing exchange spring magnets of lowercosts – with reduced RE concentration. The best exchange coupling is obtained inthe Nd based series with x = 25% Fe addition, annealed for 5 min at 750◦C (onsample A5). This sample presents an energy product of 92 KJ/m3.

Corroborating detailed crystallographic, Mossbauer spectroscopy and magne-tometry studies, it can be proven that optimal magnetic properties are obtained fora microstructure in which hard and soft magnetic phases co-exist together with asmall amorphous phase [10, 11].

We also analyzed the 2% Ti substitution for B in Nd7Fe81B12 which leads to theundesirable formation of the metastable Nd2Fe23B3 phase with a high decomposi-tion temperature, requiring an inconveniently high thermal treatment temperature.Oppositely, 2% of Zr substituting Fe atoms changes the sequence of crystallizationprocess, promoting the direct formation of Nd2Fe14B/Fe3B nanocomposities withimproved magnetic properties [8].

Acknowledgements The authors wish to thank Ministry of Education and Research for financialsupport by national program PN2 72-186.

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