Structure and magnetism of the Pr2Fe14-xCoxB system

Journal of Magnetism and Magnetic Materials 62 (1986) 29-35 North-Holland, Amsterdam

29

STRUCIZIRE AND MAGNETISM OF THE Pr2Fet4_ xCOxB SYSTEM *

A.T. PEDZIWIATR *, S.Y. JIANG and W.E. WALLACE Magnetics Technology Center and MEMS Department, Carnegie-Mellon University, Pittsburgh, PA 15213, USA Received 4 May 1986; in revised form 30 June 1986

Isostructural, tetragonal materials were synthesized for the entire composition range (x = 0 to 14). They were studied by X-ray and magnetometry methods. Magnetization measurements confirm a ferromagnetic arrangement of 3d and Pr magnetic moments and reveal a sharp increase of the Curie temperature with the increase of Co concentration. Saturation magnetization at 295 and 77 K reaches a slight maximum around x = 1 and gradually decreases for higher Co content. Anisotropy fields initially decrease and afterwards increase very quickly as more Co is introduced to the system. A spin reorientation occurs in alloys with x > 9.5 at temperature higher than 660 K. It marks a transition between uniaxial anisotropy and planar anisotropy. Spin reorientation temperature becomes lower for materials with higher Co content. A discussion of the Curie temperature change is conducted in terms of preferential substitution of Co into 16k 2 sites. The weakening of the exchange interaction between Pr and 3d sublattices, as well as the decreasing Pr-sublattice auisotropy with rising temperature, are considered to be responsible for anisotropy field changes. The interplay between crystal field and exchange field interactions is emphasized. A comparison with the Nd2Fe14_xCoxB system is included.

1. Introduction

Research activities of many magnetics laboratories have been focused recently on studies of R2Fel4B intermetallic compounds (R = rare earth) due to the discovery of a new type of permanent magnets based on Nd2Fel4B [1-7]. The improvement of magnetic properties essential to permanent magnets was achieved in 2 : 14 compounds by a small addition of cobalt [8-14]. This sparked an interest in fundamental studies of R2FeI4_xCO~,B systems with the purpose of gaining a better understanding of the role of transition metal sublattice, magnetic interactions between sublattices and the interplay between crystal field and exchange interactions in these systems. So far, the effect of Co substitution was studied in R2FeI4B systems for R = Nd, Y, Gd [8-14]. RCo14B compounds were also investigated [8,12]. * The work was supported by a contract with the Army

Research Office. * On leave from Institute of Physics, Jagiellonian University, Cracow, Poland.

Generally, the cobalt substitution for iron in the above systems results in a monotonic decrease of lattice parameters and thus in changes of interatomic distances which are crucial for exchange interactions. The Curie temperature, which reflects the overall exchange, increases nearly 50 ° per one substituted Fe atom by Co (in the region o f low Co concentration). The saturation magnetization of the R2Fel4_xCoxB compounds increases slightly to a maximum around x --- 2.0, and subsequently decrease for larger x values. The occurrence of such maxima in ternary systems proves the similarity between ternary and binary systems, as it resembles the behavior of Co-Fe alloys [15]. Anisotropy field decreases in the case of Nd2Fel4_xCOx B as cobalt replaces ion. The same trend is observed for spin reorientation temperatures, which indicates the transition between conical and uniaxial anisotropy. In the crystal structure of the Nd2Fel4B-type, space group P42/mnm [3,4], there are six ineqnivalent crystallographic Fe sites, namely: 4e, 4c, 8jl , 8j2, 16k t and 16k 2. As was shown by M/Sssbauer spectroscopy for Nd2(Fe, CO)laB [11], the Co

0304-8853/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

30

A.T. Pedziwiatr et aL / Structure of the Pr: Fet 4- xCo~B system

atom show a preference for occupying 16k 2 sites, while Fe atoms occupy preferentially the 8j 2 sites. Cobalt atoms favor sites with a relatively high rare earth atom coordination, while iron atoms occupy sites with a low number of rare earth nearest neighbors. The Pr2Fe14B compound can equally well be used as a basis for permanent magnets manufacturing as is Nd2Fe14B. It is then of practical interest to study how the cobalt substitution can alter the magnetic properties of PrEFe14B. In addition, because of the differences in the low temperature magnetic anisotropy between Nd2Fel4B and Pr2Fe14B (Nd-compound undergoes spin reorientation at = 135 K while the Pr-compound does not), it is an interesting opportunity to compare the influence of transition metal sublattice on the anisotropy of both systems. In this paper we present our crystallographic and magnetic studies on Pr2Fe14_xCoxB system, as a part of our research program on REFeI4B phases.

the steep part of the curve to M 2 = 0. The magnetic measurements were made on a PAR vibrating sample magnetometer with external fields up to 20 kOe. The magnetization isotherms were analyzed according to the approach to saturation law. The anisotropy fields were measured at room and liquid nitrogen temperature in aligned powders set in wax.

3. Results and discussion The X-ray diffraction measurements showed that for the entire composition range (x = 0-14) the materials were isostrnctural. They crystallize in a tetragonal Nd 2Fe14B-type crystal structure. The lattice parameters decrease monotonically with the increasing cobalt concentration, as shown in fig. 1. Cobalt atoms, being smaller in size than iron atoms, accommodate themselves in the crystal structure in such a way that they provide a better space packing. It should be noted that the decrease of the c parameter is non-linear and faster

2. Experimental i

The materials studied were Pr2Fe14_xCo~B with x ranging from 0 to 14 (twelve compositions). The samples were prepared from 99.9% (or better) purity starting materials by means of induction melting in purified argon atmosphere. As-cast ingots were wrapped in Ta-foil, sealed into evacuated quartz tubes and annealed at 900 ° C for two weeks. This turned out to be an adequate procedure for obtaining single-phase materials, as proved by Xray and TMA analyses. X-ray diffraction analysis was performed on powdered samples with the use of Rigaku diffractometer and Cr-Ka radiation. Optical metaUographical microscopy was also employed for initial check of the quality of the samples. Thermomagnetic analysis was performed by measuring magnetization vs. temperature curves at low external magnetic field ( = 300 Oe) in the temperature range 295-1100 K, with the use of the Faraday-type magnetic balance. The Curie temperatures were determined from these measurements by plotting M2(T) and extrapolating

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31

A.T. Pedziwiatr et aL / Structure of the Pr2 Fe ~,l _ xCox B system

than that of parameter a ( c / a ratio changes from 1.389 for x - - 0 to 1.376 for x--14). The same trend was observed for the Nd2Fel4_xCoxB system [8,9], which may indicate some from of preferential (instead of random) occupation of 3d-metal crystal sites, similarly as shown in ref. [11] for Nd-based systems. Changes in interatomic distances in the Pr2Fei4_xCoxB system result in alterations of magnetic interactions involving different 3d sites. These are the most important to consider because the 3d-3d exchange interactions are the strongest, and they mainly determine the value of the Curie temperature. The nearest neighbor distances between 3d atoms cover a large range of values in the Nd2Fex4B-type crystal structure. For interatomic distances between iron atoms, dFe_Fe, smaller than -- 2.45 ,~,, the magnetic interactions between iron atoms are negative while for iron atoms located at greater distances these are positive. In Pr2Fel4B the interactions between Fe(jl ) and Foe(k2) atoms, located at dj_k~ distance of 2.401 A [16] are strongly negative. The interactions between iron atoms located in Jl and J2 sites at djx_a2= 2.637 ]k and k 1 sites located a t djt_kl = 2.589 A are strongly positive. The contribution of negative exchange interactions determines the low ordering temperature for R2Fel4B compounds.

For the Pr2Fex4_xCoxB systems, as in Nd 2Fe14_xCOxB, the Curie temperature, T~, is sharply increased by Co substitution (fig. 2, table 1). At low (x _<6) cobalt concentration the increase is -- 50 K per one Fe atom substituted by C o . For higher Co content the change of Tc is much smaller (--14 K per atom). This could indicate that for low Co concentration the cobalt atoms replace iron atoms in crystallographic sites involved in negative exchange interactions, namely, 16k 2 and 8j2. Fe atoms locate themselves preferentially in 8j2 sites which are involved in positive exchange interactions. When more cobalt is introduced to the system (x _>6) and the preferred sites are already occupied, cobalt atoms occupy the remaining sites which have less influence on. T~. The above speculation is supported by conclusions of ref. [11], proving that Co shows indeed a preference for 16k 2 site and avoids 8j2 sites in Nd2(Fe, Co)14B system. Also, a preferential site occupation was proved by neutron diffraction for the Nd2Fe17_xCo~ systems [17]. The above described preferential occupation of crystallographic 3d sites leads to a gradual decrease of negative exchange interactions within the 3d sublattice and consequently to an increase of Tc when Co replaces Fe in Pr2Fe14_~CoxB system, as experimentally observed. It is to be noticed that a very

Table 1 The magnetic properties of the Pr2Fe14_xCoxB system. All H A values determined at 77 and 295 K for alloys with x > 12 are very large. They can be treated only as approximate values since they are too large to be measured accurately with our present technique

H^(kOe)

rsR

Composition

T~

Ms(~b/f.u. )

x

(K)

295K

77 K

295K

77K

(K)

0 1 3 5 6 7 8 9.5 10 11 12 14

556 631 747 831 863 893 920 950 958 967 979 986

29.8 30.3 29.6 28.8 27.8 27.3 26.2 25.2 24.5 23.6 22.7 20.9

34.2 34.5 33.0 31.6 30.8 29.9 28.9 27.6 272. 26.2 25.1 23.2

81 79 76 72 70 68 68 66 67 72 84 137

190 182 173 167 166 168 170 180 187 200 232 348

836 760 684 660 664

T~ and TsR error: + 3 K. Ms error: +0.2/t B.

32

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Fig. 2. The dependence of the Curie temperature (Tc) and the spin reorientation temperature (TsR) on the Co concentration in the PrEFez4_xCoxB system. The T~ values for amorphous PrEFe9.5_xCox B alloys [18] are plotted by the dashed line.

similar increase of T~ was also reported for amorphous Pr16(Fez00_yCOy)76B s alloys [18]. It is indicated in fig. 2 for comparison with the crystalline material. The composition dependencies of saturation magnetization, M s, at 295 K and 77 K are plotted in fig. 3 and shown in table 1. As Co substitutes for Fe, the M s increases slightly to a maximum around x = 1 and then decreases monotonically for larger x values. This behavior is almost the

36!

o f the P r 2 Fe~ 4 - xCo~ B system

same as that observed in the case of the Nd2(Fe, Co)14B system and can be explained in the same way [9], in terms of a rigid band model and boron acting as donor of electrons. Pr 2(Fe, Cob4 B can be treated as corresponding to an intermediate situation between the Fel_xCo x binary system (which shows a characteristic maximum in saturation magnetization for 30 at% Co) and the Fez_xCOxB ternary system (showing a monotonic decrease of saturation magnetization). The magnetic properties of Fel_xCO~ alloys were recently discussed in ref. [15]. The magnetic structure of Pr2Fez4B was studied by neutron diffraction [19] revealing a ferromagnetic coupling of P r - F e magnetic moments and their dependence on crystallographic sites. The average Pr moment at 77 K was found to be 2.7# B. By assuming that the above value is constant throughout the entire series of Pr2Fe14_xCoxB and a collinear ferromagnetic coupling takes place, one can calculate a change of mean magnetic moment of the transition metal at 77 K for different compositions in the series. As shown in fig. 4, the mean 3d moment decreases monotonically (for x > 1) as more cobalt is introduced to the system. Cobalt has a lower magnetic moment than iron, so when iron is replaced by cobalt the net 3d magnetization is gradually decreased. The decreasing magnetization of the 3d sublattice has an important consequence for magnetocrystalline anisotropy in the Pr2Fe14_xCo~B sys-

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A.T. Pedziwiatr et al. / Structure of the Pr2Fel4 _ xCoxB system

tem. The dependence of the anisotropy field, H A, on cobalt content is shown in fig. 5 and table 1. At both temperatures (295 and 77 K) there is an initial decrease of H A observed. As more cobalt is introduced to the system, H A reaches a flat minimum and finally increases rapidly to relatively high values. As it is generally understood, the bulk anisotropy of R2Fe14B compounds arises from two contributions - the 4f sublattice and 3d sublattice anisotropies. The 3d sublattice favors an easy direction of magnetization parallel to the c-axis (as in Y2Fex4B and La2Fe14B ). The 4f anisotropy dominates over the 3d anisotropy in all cases where the orbital moment of the R atom is nonzero, but the easy direction of magnetization along the c-axis develops only in those compounds in which the R component has a negative Stevens coefficient. This coefficient is related to the shape of the electric charge distribution in the rare earth atom. In the case of R2Cot4B compounds, the 4f sublattice anisotropy shows qualitatively the same 380

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33

behavior as in R2Fe14B, but the contribution of the 3d anisotropy is entirely different [12], namely, 3d anisotropy gives rise to a planar bulk anisotropy, perpendicular to the c-axis (as in Y2COl4B and La2Co14B). The initial decrease of H A observed in Pr2Fe14_xCoxB for samples with low Co concentration can be ascribed to the lowering exchange field as the 3d net magnetization becomes smaller, because of the increasing amount of cobalt. The increase of H A observed for higher Co concentrations is not well understood. It may be due to the change in crystal field parameters brought on by a change in the electric charge of the 3d element. It seems obvious, however, that the strong anisotropy observed for compound with high Co concentration originates from single ion Pr anisotropy, which entirely dominates and overrules the 3d anisotropy. The observed values of H A are smaller at 295 than at 77 K. This is because the Pr anisotropy is decreasing with rising temperature [21]. The same reasoning can explain the fact that the minimum of the H A vs. composition curve is shifted towards lower Co concentrations at lower temperature (see fig. 5). Simply, at 77 K the Pr anisotropy is large enough (as compared with its anisotropy at 295 K) to dominate the 3d sublattice anisotropy even for compounds with high Fe content. The effect of competing anisotropies of 3d and 4f sublattices is manifested by the occurrence of a spin reorientation temperature, TSR. As plotted in fig. 2, TSR is observed only for compositions higher than 9.5, that is, in a composition region where the exchange interaction (Pr-3d) acting on Pr ions is decreased due to high Co concentration. For a given composition (x>9.5), the decreased exchange interaction increases the relative importance of the crystal field interaction and for certain temperature the spin reorientation takes place. The TsR in Pr2Fet4_xCOxB marks a transition between uniaxial and basal bulk anisotropy [20]. the value of TsR decreases when cobalt content is increasing because the more cobalt in the alloy, the net transition metal anisotropy has more planar tendency and can manifest itself over a larger temperature region. One may try to construct an alternative ex-

34

A.T. Pedziwiatr et al. / Structure o f the P r 2 Fe 14 - xCox B system

planation of the occurrence of TsR and basal anisotropy in alloys with x > 9.5. It could go as follows: it is known that iron anisotropy is uniaxial and cobalt anisotropy seeks the plane; therefore the change from uniaxial to basal anisotropy for alloys with high Co concentration may be attributed solely to the competition between Fe and Co inside the 3d sublattice, with no 4f sublattice involvement. It could be treated as an intra-transition metal sublattice effect. Simply, anisotropy of cobalt overcomes that of iron when Co concentration is much higher than the concentration of iron. Additional support for such speculation could come from the fact that TSR appears at relatively high temperatures where the 3d sublattice presumably dominates in magnitude over

the 4f sublattice. Such an explanation, however, is not justified because no TSR was observed either in the Y2Fe~4_xCoxB system (Y has no magnetic moment, no anisotropy) or in the Gd2Fe14_xCOxB system (Gd has a magnetic moment but no anisotropy) [8]. The magnetization curves in the vicinity of the spin reorientation temperature measured at different external fields have a "cusp-like" shape. A set of such curves is presented in fig. 6, as an example for an arbitrarily chosen composition x = 12. It is evident that a relatively small external magnetic field can wipe out the effect of spin reorientation. The shape of these curves is terms of changing anisotropy constants K~ and K 2 is described theoretically elsewhere [20].

4. Conclusions

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The Pr2Fe14_xCOxB alloys crystallize in P42// mnm-type structure in the composition range x = 0 to 14. The substitution of Fe atoms by Co results in decreasing lattice parameters. The Curie temperature increases by -~- 50 K per atom of cobalt for x ~< 6. It is postulated that for low Co concentration, cobalt atoms locate themselves preferentially in 16k 2 sites, as in the case of the Nd2Fe14_xCOxB system. The composition dependence of saturation magnetization at 295 and 77 K shows a decrease monotonically with Co insertion after reaching an initial slight maximum around x - - 1 . Ferromagnetic coupling of Pr and 3d moments is confirmed. Though in reality the values of magnetic moments of 3d metals in the Nd2Fe14B-type crystal structure depend on the crystal site, a mean value of 3d moment was calculated for all compositions by assuming a constant Pr moment derived from neutron diffraction measurements on Pr2Fe14B. The substitution of cobalt in place of iron results in a decrease of the 3d sublattices magnetization and, as a consequence, in weakening the exchange interaction between 3d and Pr sublattices. As the exchange interaction is lowered, the Zeeman splitting - and hence the high temperature anisotropy of the rare earth ion - is decreased. When it decreases to a level where it is comparable to the transition metal

A.T. Pedziwiatr et al. / Structure of the P ~ Fe: 4 - xCox B system

sublattice anisotropy, a spin reorientation occurs. It is observed for alloys with x >_ 9.5. Anisotropy fields reach very high values for materials with high Co concentration, especially at low temperatures (see table 1), due to the fact that 3d sublattice anisotropy is entirely overruled by the 4f sublattice anisotropy in Co-rich compounds. The Pr single ion crystal-field-induced anisotropy is the origin of high anisotropy fields observed in Co-rich compounds. Some striking similarities are evident between Pr2Fei4_xCoxB and the Nd-based analog. The change of lattice parameters, probable mode of preferential occupation, the saturation magnetization, the Curie temperature have almost the same composition dependence in both systems. However, the anisotropy field dependence on composition is different for the two systems; the spin reorientation phenomena are of different nature [22]. As was recently established [22] in the Nd 2Fei4_xCo~B system for high cobalt concentration there are two spin re,orientations - one (cone to axis) at temperatures lower than 100 K and a second (axis to plane) at temperatures higher than 546 K. In Pr-based compounds with x > 9.5, only the high temperature spin reorientation (axis to plane) was detected. The above-mentioned features clearly indicate that although both systems are exchange-dominanted ferromagnetic systems, they show significant differences in crystal field interactions. Discussion of differences between Prand Nd-based compounds is included in refs. [21] and [22].

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[2] J.J. Croat, F.J. Herbst, R.W. Lee and F.E. Pinkerton, Appl. Phys. Lett. 44 (1984) 148. [3] J.F. Herbst, J.J. Croat, F.E. Pinkerton and W.B. Yelon, Phys. Rev. B29 (1984) 4176. [4] D. Givord, H.S. Li and J.M. Morean, Solid State C o m mun. 50 (1984) 497. [5] S. Sinnema, R.J. Radwanski, J.J.M. Franse, D.B. de Mooij and K.H.J. Buschow, J. Magn. Magn. Mat. 44 (1984) 333. [6] M.Q. Huang, E. Oswald, E.B. Boltich, S. Hirosawa, W.E. Wallace and E. Schwab, Physica 130B (1985) 319. [7] E. Burzo, E. Oswald, M.Q. Huang, E. Boltich and W.E. Wallace, J. AppL Phys 57 (1985) 4109. [8] M.Q. Huang, E.B. Boltich, W.E. Wallace and E. Oswald, J. Magn. Magn. Mat. 60 (1986) 270. [9] Y. Matsuura, S. Hirosawa, H. Yamamoto, S. Fujimura and M. Sagawa, Appl. Phys. Lett. 46 (1985) 308. [10] Ying-chang Yang, Wen-Wang Ho, Hai-ging Chen, Jin Wang and Jian Lan, J. Appl. Phys. 57 (1985) 4118. [11] H.M. van Noort and K.H.J. Buschow, J. Less-Common Metals 113 (1985) L9. [12] K.H.J. Buschow, D.B. de Mooij, S. Sinnema, R.J. Radwanski and J.J.M. Franse, J. Magn. Magn. Mat. 51 (1985) 211. [13] E. Burzo, L. Stanciu and W.E. Wallace, J. Less-Common Metals 111 (1985) 83. [14] Z. Maocai, M. Deqing, Y. Xiuling and L. Shigiang, in: Proc. of the 8th Intern. Workshop on Rare Earth Magnets and Their Applications, Dayton, Ohio, 6-8 May 1985, ed. K.J. Strnat, University of Dayton, p. 541. [15] A.R. Victora and I.M. Falicov, Phys. Rev. B 30 (1984) 259. [16] A.T. Pedziwiatr, E. Burzo and W.E. Wallace, to be published. [17] J.F. Herhst, J.J. Croat, R.W. Lee agd W.B. Yelon, J. Appl. Phys 53 (1982) 250. [18] R.A. Overfeld and J.J. Becker, Appl. Phys. Lett. 44 (1984) 925. [19] J.F. Herbst and W.B. Yelon, J. Appl. Phys. 57 (1985) 2343. [20] E.B. Boltich, A.T. Pedziwiatr and W.E. Wallace, to be published. [21] E.B. Boltich and W.E. Wallace, Solid State Commun. 55 (1985). 529. [22] A.T. Pedziwiatr and W.E. Wallace, to be published.