High field magnetization and spin reorientation in Sm2(Fe1−xAlx)17 single crystals

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AND CO~O~D5 Journal of Alloys and Compounds 222 (1995) 62-66

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High field magnetization and spin reorientation in Smz(Fel_xAlx)17 single crystals H. Kato ~'*, J. Shiomi a T. Koide a T. Iriyama b M. Yamada c, y . Nakagawa Institute for Materials Research, Tohoku University, Sendai 980-77, Japan b Central Laboratory, Asahi Chemical Industry Co., Ltd., Fuji 416, Japan c Department of Applied Physics, Tohoku Gakuin University, Tagajo 985, Japan Department of Electronics, Tohoku Institute of Technology, Sendai 982, Japan

Abstract High field magnetization measurements have been performed for single crystals of Sm/(Fe~_~AI,)17, with x=0.075, 0.093, 0.114, 0.145. The spontaneous magnetization at room temperature lies in the hexagonal c plane. Appreciable anisotropy within the c plane was observed at 4.2 K with the magnetization along the a axis being more than 3% smaller than that along the b axis, even in a field of 10 T. The c axis curve has been found to show a smaller spontaneous magnetization, indicating that the total magnetization vector is not within the c plane but in the plane spanned by the b and c axes. The angle between the magnetization direction and the b axis increases linearly with increasing x. The results are analyzed on the basis of crystalline electric field theory. Keywords: Spin reorientation; High field magnetization; Crystalline electric field

1. Introduction The magnetic properties of R2F17 (R, rare earth) containing interstitial nitrogen atoms have recently been studied extensively, because of the compound's drastic increase in Curie temperature Tc upon nitrogen absorption [1]. The increase in Tc is attributed to the increase in the average F e - F e distance resulting from the volume expansion, which greatly enhances the exchange interactions [2]. Another drastic change which occurs upon nitrogenation is the appearance of strong uniaxial anisotropy in the Sm2Fea7Nx system; the direction of easy magnetization changes from the c plane (Sm2Fe17) to the c axis (Sm2Fe17N3). Some theoretical studies [3] have dealt with this phenomenon in connection with the change in the crystalline electric field (CEF) potential resulting from nitrogen atoms located at the 9e site, which is quite close to the Sm site. It is not unlikely, however, that a simple expansion of the lattice gives rise to a change in the sign of the CEF coefficients. Experimentally, high field magnetization measurements on single-crystal samples are a powerful method * Present address: Department of Applied Physics, Faculty of Engineering, Tohoku University, Sendai 980-77, Japan. 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSD1 0925 - 8 3 8 8 ( 9 4 ) 0 4 9 1 8 - 1

to obtain quantitative information about the CEF interactions. It is not easy, however, to grow a single crystal of such an interstitially modified system; the nitrogenation of the single-crystal Sm2Fe17 is quite sluggish, owing primarily to the large volume expansion. However, the pseudo-binary systems R2(Fel_xM,)17 (M -= A1, Ga) also show the increase in Tc with increasing M concentration x for x <0.2 [4,5]. It is important to note that, in these substitutional systems, the unit cell volume V also expands significantly with increasing x. The rate of increase ATc/Tc as a function of the volume change AV/V is similar to that of the interstitial nitrogen system. It is also expected that such a substitutional system shows an appreciable change in the C E F parameters as a result of the volume expansion. To investigate this effect, we have grown single crystals of Sm2(Fel_xA1,)17 and systematically measured the high field magnetization.

2. Experimental Single-crystal samples of Smz(Fel _~Alx)17 were grown by the Bridgman method, using an A1203 crucible coated with BN, in an atmosphere of high pressure Ar gas to avoid the evaporation of Sm. Some amounts of A1 were

H. Kato et al. /Journal of Alloys and Compounds 222 (1995) 62--66

found to be transferred from the AI203 crucible to the sample, as a result of an appreciable reaction between A1203 and Sm. The final concentration of A1 was precisely determined using the inductively coupled plasma method. The single crystals with x = 0.075, 0.093, 0.114 and 0.145 were obtained, the lattice constants and Curie temperatures of which were confirmed to agree with those already published for polycrystalline samples [4,6]. To perform the magnetization measurements, each crystal was embedded into a cube of epoxy resin, so that the principal directions of the hexagonal cell, t.e. a ([100]), b ([120]) and c ([001]), become parallel to the three corresponding cube axes. High field magnetization measurements were performed using a vibrating sample method in fields of up to 15 T, generated by a high power water-cooled magnet.

63

15 T. The ratio of the magnetizations becomes nearly equal to 31/2/2 if extrapolated to zero field. Furthermore, the c axis curve exhibits a small spontaneous magnetization, indicating that the easy direction is not within the c plane. It can be concluded from these results that the total magnetization vector lies in the plane spanned by the b and c axes, with an angle a = 12° away from the b axis. By measuring the temperature dependence of the magnetization along the c axis, we found that this outof-plane deviation angle a vanishes above about 200 K. Such a spin reorientation is thought to originate from the competition between the Fe anisotropy energy and the CEF interaction at the Sm site; the CEF interaction contribution becomes more significant at low temperatures. Magnetization curves for the samples with x = 0.093, 0.115 and 0.145 at 4.2 K are shown in Fig. 2. The c axis curve in each sample shows a spontaneous mag-

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Fig l(a) shows magnetization curves of the sample with x=0.075 at 296 K, in fields applied along the three principal directions of the hexagonal lattice. Typical behavior of the easy-plane anisotropy is seen; the magnetization curve along the c axis starts from the origin and saturates at about 2 T. At 4.2 K, significant anisolropy within the c plane was observed, as shown in Fig. l(b); the magnetization along the a axis is 5% smaller than that along the b axis, even in a field of

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H. Kato et al. /Journal of Alloys and Compounds 222 (1995) 62-66

64

netization, which increases with increasing x. The field at which the c axis magnetization saturates, however, decrease with increasing x, indicating a rapid reduction in axial anisotropy, resulting from the A1 substitution for Fe. It also should be noted that the difference in the high field magnetization between the a and b axes remains quite large, even in the sample with x=0.145, although it decreases slightly with increasing x. In Fig. 3(a), the magnitude Ms of the spontaneous magnetization vector, deduced from the magnetization values extrapolated to zero field along the three directions, is plotted against the AI concentration x. This result has shown that Ms decreases almost linearly with x; the rate of this decrease is larger than that expected from the simple dilution of Fe by non-magnetic A1. We have deduced the magnetic moment per Fe atom (mve) by assuming that the Sm magnetic m o m e n t ( m s m ) is negligible, as shown in Fig. 2(b). A considerable reduction in m v e with increasingx is observed, in contrast to the fact that the concentration dependence of the magnetization in a-FeA1 alloys can be explained by the simple dilution picture. A possible origin of the decrease in mv~ is the preferential site occupation of AI, as

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already confirmed in the Nd2(Fel_xAlx)a7 system by neutron diffraction [7]. The direction of the total magnetization vector, i.e. the out-of-plane angle a, is deduced from the ratio of the extrapolated magnetization values, and is plotted against x in Fig. 3(c), showing a linear increase with increasing x. Recently, the interstitially modified system SmzFe17Co.5, the volume of which is similar to that of Sm2(Feo.gzsAlo.o75), has been reported to exhibit cometype anisotropy, according to the X-ray diffraction of aligned polycrystals [8]. Therefore, these results suggest that the easy direction is correlated with the unit cell volume in both substitutional and interstitial systems. However, Sm2Fe17Hs, which has a larger volume than the above systems, was confirmed to exhibit planar anisotropy [8]. This case can be regarded as an exception, since hydrogens in excess of three occupy the sites other than 9e, i.e. 18g sites. It is important to note that the calue of a extrapolated to x = 0 is zero, suggesting that the spontaneous magnetization of Sm2Fea7 is actually within the c plane. However, the anisotropy within the c plane is almost independent of the A1 content x, so the SmzFe~7 single crystal will exhibit considerable in-plane anisotropy. Thus, the easy direction of Sm2Fe17 is expected to be along the b axis, which is consistent with the results of M6ssbauer analysis by Hu et al. [9]. Direct evidence of these properties has been lacking so far, owing to the difficulty of obtaining good single crystals of Sm2Fe17.

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Although the simple dilution picture cannot be applied to the Fe(A1) sublattice, it is still useful to carry out the CEF analysis for the Sm sublattice to fit the observed magnetization curves. The method of calculation is essentially similar to that which we adopted for R2Fe14B [10] and SmzFelvN3 [11]. The primary assumption in this model is that the magnitude of the Fe-Fe exchange interaction is large enough compared with that of the F e - R interaction, so that the Fe sublattice can be regarded as rigid, being treated phenomenologically. We can estimate the moment and anisotropy of the Fe(A1) sublattice from the experimental data of the corresponding Y compounds. This assumption may not be justified for R2Fe17, the Tc value of which is as low as 300-400 K. At low temperatures, however, these are still thought to be good approximations. We first assume that the total free energy of the system is given by F ( H , 7 ) = - 2 k T in Z + 17

(1-x){KFe(T ) sin 2 0 - m F ¢ ( T ) . H }

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65

H. Kato et al. / Journal of Alloys and Compounds 222 (1995) 62--66

where H is the applied magnetic field and T the temperature. The term in braces in Eq. (1) expresses the contribution from the Fe(A1) sublattice, where the anisotropy energy KF~(T) and the magnitude of the magnetic moment mF~(T) per Fe atom are determined from the experimental data for YzFe17. It should be mentioned that the simple dilution picture is adopted here for the AI substitution. The: first term in Eq. (1) is the Sm contribution, where Z is the partition function defined by

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where A , m is the CEF coefficient and £jV,,m(j) is the non-normalized tesseral function for the jth 4f electron at the coordinates (xj, yj, zj), with the summation j over all the', 4f electrons. The Cartesian coordinates in this formulation were taken so that the z and x axes become parallel to the hexagonal c and b axes respectively. Since the effect of excited J multiplets on the magnetic properties of Sin-containing compounds is crucially important [10,11], first excited J = 7/2 and second excited J = 9 / 2 multiplets are taken into account, in addition to the ground J = 5/2 multiplet. In the actual calculation, we set mv~(0)=2.0~B and KFe(0)=--2.5 K, according to the magnetization data for YzFe~7 single crystals [12]. After performing some preliminary calculations, we have found that it is necessary to change the CEF parameters significantly for each A1 concentration x, in order to reproduce the variation of the magnetization curves with different x values, especially for those curves along the x axis. In Figs. 4(a) and 4(b), two examples of calculated magnetization curves for T= 0 K are shown, which correspond to the cases with x=0.075 and 0.10 respectively. The values of the second-order CEF coefficient used areA2 ° = 5 Kao 2 forx = 0.075 andA2 ° = - 50 Kao -2 for x=0.10, where ao is the Bohr radius. Other parameters are common to both cases: A4° = 16 Kao -4,

A6°=2.5 Kao -6, A 6 6 = 100 Kao -6, Aa3 = A 6 3 = 0 a n d the magnitude of molecular field Hm = 200 K. In these calculation examples, we needed to adopt quite a large value of A6 6 to fit the observed large inplane anisotropy. This value of A66 is at least one order of magnitude larger than that in Ho2Fe17 [13], as determined from the analysis of inelastic neutron scattering data• Although very little quantitative information about the non-axial CEF coefficients for R2Fe17 or R2C017 has been published, our recent investigation on Ndz(Fel _xAl,)17 single crystals revealed a similarly large in-plane anisotropy [14]• It must be emphasized that the sign of A2° is different for the two cases; in general, the negative A 2 tends to make the Sm moment align along the c axis, against the Fe anisotropy energy. In contrast to the case of Sm2Fe17N3, in which the CEF energy is much larger than the Fe anisotropy energy [10], the contribution from Kve is comparable with that from the A: ° terms here• In fact, the present calculations have shown that rove and msm are non-collinear, even at zero field; the angles between the two moments at H = 0 are 4•6° and 11•6°, respectively, for x=0.075 and x=0.10. It should be mentioned, however, that the Fe anisotropy is based on the simple dilution picture, which is not justified in the present case• Therefore, it is necessary to obtain accurate values of KFc and rove as

66

H. Kato et al. / Journal of Alloys and Compounds 222 (1995) 62~56

a function of the Al concentration x. This can be done by preparing single crystals of Yz(Fel_xAlx)17 with various x values, and measuring their magnetization curves. Such studies are now in progress, together with efforts to obtain good single crystals of S m 2 F e 1 7 . Acknowledgments We are grateful to M. Kudo, K. Sai and Y. Ishikawa of the High Field Laboratory at IMR for the operation of the high power water-cooled magnet. This work was partly supported by a Grand-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan. References [1] J.M.D. Coey and H. Sun, J. Magn. Magn. Mater., 87 (1990) L251.

[2] S.S. Jaswal, W.B. Yelon, G.C. Hadjipanayis, Y.Z. Wang and D.J. Sellmyer, Phys. Rev. Lett., 67 (1991) 644. [3] M. Yamaguchi and S. Asano, J. Phys. Soc. Jpn., 63 (1994) 1071. [4] D. McNeely and H. Oesterreicher, J. Less-Common Met., 44 (1976) 183. [5] P.C.M. Gubbens and A.M. van der Kraan, J. Less-Common Met., 159 (1990) 173. [6] X. Li, N. Tang, Z. Lu, T. Zhao, W.G. Lin, R. Zhao and F. Yang, J. AppL Phys., 73 (1993) 5890. [7] W.B. Yelon, H. Xie, G.J. Long, O.A. Pringle, F. Grandjean and K.H.J. Buschow, J. Appl. Phys., 73 (1993) 6029. [8] C.N. Christodoulou and T. Takeshita, J. Alloys Comp., 198 (1993) 1. [9] B. Hu, H. Li, H. Sun and J.M.D. Coey, J. Phys. Condens. Matter., 3 (1991) 3983. [10] M. Yamada, H. Kato, H. Yamamoto and Y. Nakagawa, Phys. Rev. B, 38 (1988) 620. [11] H. Kato, M. Yamada, G. Kido, Y. Nakagawa, T. Iriyama and K. Kobayashi, J. AppL Phys., 73 (1993) 6931. [12] M.T. Averbuch-Pouchot, R. Chevalier, J. Deportes, B. Kebe and R. Lemaire, J. Magn. Magn. Mater., 68 (1987) 190. [13] K. Clausen and B. Lebech, J. Phys. C, 15 (1982) 5095. [14] T. Koide, H. Kato, J. Shiomi, T. Iriyama and M. Yamada, J. Magn. Magn. Mater., 140-144 (1995) in press.