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Magnetic structure of rare earth compounds of the type RFe10V2
Journal of the Less-Common
Metals, 162 (1990)
MAGNETIC STRUCTURE TYPE RFe,OV,
285-295
285
OF RARE EARTH COMPOUNDS
OF THE
W. G. HAIJE and J. SPIJKERMAN* Netherlands Energy Research Foundation,
ECN, 1755 ZG Petten (The Netherlands)
F. R. DE BOER and K. BAKKER Natuurkundig Laboratorium, (The Netherlands)
Universiteit van Amsterdam,
Valckenierstraat 65, 1018 XE Amsterdam
K. H. J. BUSCHOW Philips Research Laboratories,
P. 0. Box 80 Ooo, 5600 JA Eindhoven
(The Netherlands)
(Received January 20,199O)
Summary We have studied the magnetic properties of several rare earth compounds of the type RFer,,V, (R= Nd, Tb, Dy, Ho and Er) by means of a.c. susceptibility measurements and neutron diffraction. When cooling to below room temperature most of these compounds undergo a magnetic phase transition from a collinear structure in which the moments are parallel to the c axis (300 K) to a collinear structure in which the moments deviate from the c axis (4.2 K).
1. Introduction Ternary iron-rich compounds derived from the tetragonal ThMn,, structure have been proposed as inexpensive alternatives for permanent magnet materials. These compounds have the general formula composition RFe,, _ *M,, where x = 1 for ME Ti and W and x = 2 for M = V, Cr, MO and Si [ 1,2]. Their Curie temperatures and magnetizations have values sufficiently high for technical applications, and there are several examples where a sufficiently high coercive force has been attained [3-51. From the physical side these compounds are just as interesting as the R*Fe,,B and R*Fe,,C compounds, since they give rise to numerous magnetic structures and phase transitions [6-lo]. The ThMn,, structure comprises three different 3d sites. Experimental data available for some of the compounds have revealed that there is a distinct site preference (8i site) for the M element in compounds with M = Ti, V and MO [8,11,
*Present address: University of Twente, 7500 AE Enschede, The Netherlands. 0022-5088/90/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
286
121. In all these series the iron sublattice anisotropy favours an easy magnetization direction parallel to the c axis. The evolution of the magnetic structures at temperatures well below T, can be described by the same formalism as used for the R2Ti4X compounds and involves a competition between the iron sublattice anisotropy and the rare earth sublattice anisotropy. In the present study we have focused our attention to several compounds of the RFe,,V, series. We have used a.c. susceptibility measurements to detect magnetic phase transitions and determined the corresponding magnetic structures by means of neutron diffraction.
2. Experimental details The compounds of the type RFe,,V, (R= Nd, Tb, Dy, Ho and Er) were prepared by arc melting starting materials of at least 99.9% purity. After arc melting, the samples were vacuum annealed at 900 “C for 2 weeks. After vacuum annealing, the samples were investigated by X-ray diffraction (XRD). All samples were found to be approximately single phase. The occurrence of magnetic phase transitions was investigated by means of a.c. susceptibility measurements. These measurements were made on powdered material in the temperature range 4.2-300 K. Cooling curves as well as heating curves were measured. Neutron diffraction measurements were made at temperatures between 300 and 4.2 K (configuration 30 min/sample/30 min; L = 2.5719 A). The diffraction patterns obtained were analysed by means of Rietveld’s profile refinement technique [ 131. The maximum absorption correction was about 5%, ,uR remaining below 0.8 even in the case of DyFe,,,V,. The powder of the latter compound was mixed with aluminium powder in order to reduce the absorption of neutrons by the dysprosium. For the scattering lengths we used the values 0.769 X lo-‘* cm for neodymium, 0.738 x lo-‘* cm for terbium, 1.69 X lo- ‘* cm for dysprosium, 0.808 x 10-l* cm for holmium, 0.954 x 10-l* cm for iron and -0.0382 x lo-l2 cm for vanadium. The corresponding magnetic form factors were taken from the papers published by Watson and Freeman [14], Blume et al. [15] and Steinsvoll et al. [16]. From results of magnetic measurements obtained earlier [6] it follows that all compounds are magnetically ordered at 4.2 and 300 K. For this reason, nuclear as well as magnetic reflections had to be considered in the diffraction patterns obtained at all temperatures applied. All neutron diffraction patterns obtained in the course of the present investigation were fitted with the constituent atoms placed in the following positions of the ThMn,, structure type (space group 14/mmm): R in 2a(OOO), iron and vanadium at 8i (x, 0, 0), iron in 8j (x, $, 0) and ironin 8f (& :, 4). Our choice of the 8i position for the vanadium atoms is based on the results of a previous neutron diffraction study in which we focused our attention of the preferred site occupation of the vanadium atoms in this class of compounds [l 11.The quality of the fit between the various experimental and the calculated diffractograms was assessed
287
by the expression Xz = c wi 1 yi(obs)-X(calc)12 V
where y,( obs) and y,( talc) are the observed and calculated intensity values of the ith measuring point, wi being its statistical weight. The quantity v represents the number of points minus the number of parameters. The parameters derived from the fitting procedure are given in Table 1. In the following section we have restricted ourselves to presenting only the data pertaining to the fits with the lowest values of x 2, R,,,, and Rmag.
3. Results and interpretation 3. I. NdFe,, V, The temperature dependence of the a.c. susceptibility when measured from room temperature to 4.2 K (cooling curve) shows a broad peak centred at T,= 120 K. This was taken as an indication that at this temperature a magnetic phase transition takes place. As may be seen from Fig. 1, the transition at T, takes place at a slightly lower temperature when the measurements are started at 4.2 K (heating curve). Neutron diffraction data were taken at a temperature below (4.2 K) and above (295 K) the phase transition temperature. Results of the refinement of the corresponding neutron diffraction patterns are given in the top part of Table 1. It follows from these data that the easy magnetization direction is parallel to the c direction at 295 K, but makes an angle with it at 4.2 K. It is reasonable to assume that the change in easy magnetization direction commences near T, where it is responsible for the enhancement of the a.c. susceptibility. The neodymium and iron moments remain collinear at all temperatures, the neodymium moments being parallel to the iron moments. 3.2. SmFe,,I/, The a.c. susceptibility does not indicate any change in the spin structure. No neutron diffraction data are available. 3.3. TbFe,, V, The temperature dependence of the a.c. susceptibility (heating curve) shows a shallow maximum at about 100 K and a further maximum at about 230 K. These features are less clear in the cooling curve (see Fig. 1). Earlier neutron studies made on this compound have shown that the easy magnetization direction is parallel to the c axis at 295 K, but makes an angle # = 50” with it at 4.2 K [lo]. In the course of the present investigation we have focused our attention to the intermediate temperature range. The neutron diffraction diagrams obtained at 120 and 170 K are shown in Fig. 2, and the data obtained from refining these diagrams are listed in the
288 TABLE 1 Parameters derived from fitting the neutron diffractograms obtained for various compounds of the type BFeIOV1” B Overall
Compound
T (K)
a=b(A)
c(A)
x (8i)
x (8j)
NdFe,,Vr
293
4.2
8.5553(3) 8.5515(4)
4.7737(2) 4.7633(2)
0.3634(4) 0.3654(5)
0.2722(2) 0.2714(3)
293 120(A)b 170 120(B)b 4.2
8.4856(4) 8.4814(3) 8.4827( 3) 8.4817(3) 8.4860(3)
4.7662(2) 4.7609(2) 4.7626(2) 4.7611(2) 4.7638(2)
0.3613(6) 0.3600(6) 0.3617(6) 0.3593(6) 0.3637( 11)
0.2755(4) 0.2750(3) 0.2750(3) 0.2755(3) 0.2746(6)
0.57(6) 0.69(5) 0.23(6) 0.49(6)
DyFer”%
293 160 4.2
8.4816(6) 8.4781(5) 8.4734(5)
4.7658(3) 4.7609(3) 4.7562(3)
0.3612(g) 0.3594(g) 0.3613(g)
0.2781(4) 0.2773(4) 0.2745( 5)
0.18(8) 0.09( 7) 0.37(8)
HoFe,,Vr
293 4.2
8.4661(4) 8.4565(4)
4.7608(2) 4.7507(2)
0.3627(6) 0.3620(g)
0.2765(4) 0.2761(4)
0.39(7)
TbFe,,V,
ErFe,,V,
4.2
8.451(4)
4.7539(2)
0.3607(g)
0.2831(5)
- 0.98(7)
YFereVr
4.2
8.483
4.764
0.3574(4)
0.2783(2)
-0.21(6)
BR
B,,
0.6(l) O.l( 1)
1.1(l) 0.1(2)
0.4( 1)
0.3(2)
1.9(2)
0.3(2)
aThe values of the temperature factors B are given in A’. The values of the total rare earth moments pa, the iron moments pFe are given in ,ue. The numbering Fe,, Fe, and Fes corresponds to the 8i, 8j and 8f sites respectively. The deviation of the easy magnetization direction from the c axis is given by the angle # (accurate within 2”). bA and B refer to the first and second run for TbFe,,V, (see main text).
middle part of Table 1. The magnetic structure is collinear at all temperatures, the terbium moments being coupled antiparallel to the iron moments. The easy magnetization direction is parallel to the c axis at room temperature, but deviates from it at lower temperatures. The temperature dependence of the tilt angle # is shown in Fig. 3. It may be derived from these data that the easy magnetization starts to deviate from the c direction at about 210 K, which is close to the high temperature maximum of the a.c. susceptibility. Since the a.c. susceptibility was found to give rise to a small temperature hysteresis (see Fig. 1) of the transition we measured a second neutron diffraction diagram at 120 K after having completed the data collection at 4.2 K and without allowing the sample to reach temperatures higher than 120 K. Results of the refinement procedure obtained from these data have been included in Table 1. It is seen that the tilt angle (4 = 22”) is slightly larger than the angle (# = 20”) obtained when cooling directly to 120 K, being in keeping with the hysteretic nature of the spin reorientation in TbFe,,V,. It should be noted, however, that this small difference in # is near the experimental error. 3.4. DyFe,, V, The a.c. susceptibility gives rise to a broad peak centred at 130 K in the heating curve and at 150 K in the cooling curve, as shown in Fig. 4. Neutron
289
2.6( 1) 2.0( 2)
1.7( 1) 1.7(2)
1.4(l) 1.7(2)
0 60
2.90
6.35
3.31
-5.6(l) -7.78(6) -7.37(5) -8.02(6) -8.77(7)
1.q 1) 1.60(6) 1.49(6) 1.55(6) 2.03(4)
1.8( 1) 2.04(5) 2.11(5) 1.99(5) 1.15(6)
1.9(l) 1.88(3) 2.01(3) 1.91(3) 1.97(4)
0 20 9 22 50
2.09 2.12 1.77 2.68 2.5
3.53 2.73 2.59 3.61 3.7
4.47 5.47 7.55 5.83 5.9
-7.0(l) -8.6(l) -9.8(l)
0.6( 1) 1.3( 1) 1.3( 1)
1.3(l) 1.7(l) 1.9(l)
1.5( 1) 1.8(l) 1.6( 1)
0 15 32
4.10 3.25 2.31
7.16 4.66 2.86
2.63 2.51 3.60
-5.0(l) - 10.03(8)
0.6( 1) 1.36(7)
1.2(l) 2.03(5)
1.2( 1) 1.83(4)
0 0
4.28 4.07
7.15 4.03
4.21 5.05
-10.4(l)
2.0( 1)
1.5(l)
1.9( 1)
90
5.4
7.7
10.7
0
1.9( 1)
1.5(2)
1.8( 1)
0
5.1
3.9
6.6
l.O( 1) 0.2(l)
0.8( 1) 0.2(l)
l.l( 1) 0.1(2)
1.4( 1) 3.1(2)
1.1(l)
0.8(l)
0.3(2)
1.2(l)
0.8(l)
0.3(2)
diffraction data obtained at 295, 160 and 4.2 K are listed in Table 1. It follows from these data that the easy magnetization direction is parallel to the c axis only at room temperature. It starts deviating from the c axis upon cooling. According to the neutron data this deviation occurs already at higher temperatures than the temperature dependence of the a.c. susceptibility suggests. 57Fe Mossbauer spectroscopy indicated the spin reorientation to occur already near 200 K [ 171. 3.5. HoFe,, V, The a.c. susceptibility of this compound is shown in the middle part of Fig. 4. There is no indication of any spin reorientation. From the neutron diffraction data, listed in the bottom part of Table 1, we derive that there is no change in the easy magnetization direction with temperature, the c direction (or a direction very close to it) being the preferred moment direction at 295 K as well as at 4.2 K. 3.6. ErFe,,I/, The temperature dependence of the a.c. susceptibility of ErFe,,V,, shown in the bottom part of Fig. 4, suggests a spin reorientation occurs at about 50 K. This is in agreement with the results of the magnetization measurements [ 181, 57Fe Miissbauer spectroscopy [ 191 and neutron diffraction [lo] reported earlier. At
290
E ‘5
4 m k
2000
1000 0
0
100
200
300
100 SmFe10U
TbFe,,Y
T(K)
-
Fig. 1. Temperature dependence of the a.c. susceptibility in several RFe,,,V2 (R=Nd, Sm, Tb) compounds. Measurements made with increasing temperature (heating curves) are represented by broken curves, measurements made with decreasing temperature (cooling curves) are given as full curves. The small differences in cooling and heating curves well below the phase transformation temperature in NdFe,,V, are probably due to experimental inaccuracies.
room temperature the easy magnetization direction is parallel to the c axis. At 4.2 K the easy magnetization direction is perpendicular to the c axis, or at least very close to the latter direction.
4. LXscussion
We will first discuss the magnetic moment values derived from the refinement of the neutron data. The rare earth moments found at 4.2 K are fairly close to the free ion values (gJpc,) and their relative orientation corresponds to an antiparallel
291
Calculated profile ............ Observed profile
TbFe,,,V, 293K
t
25 20
g
15
g
10
a 0 x .?
5
s F
=
0 25
120K r
0
60
40
20
80 20 (degrees)
F
Fig. 2. Neutron diffraction diagram of the compound TbFe,,V, at 293 K (top part) and 120 K (bottom part). The full lines represent calculated fits to the experimental data. The location of nuclear and magnetic reflections of the main phase are indicated by short vertical bars at the bottom of the diagrams. Absence of a full line indicates regions excluded because of impurity phases.
I
60 TbFe,oVp
0’
.
I
0
100
200
Temperature
Fig. 3. Temperature dependence
(K)
300
-
of the tilt angle in TbFe,,,V,.
coupling between the iron moments and the rare earth spin moment. These results are in satisfactory agreement with expectations. More difficult to interpret are the iron moments. There are three distinct iron sites in the ThMn12 structure. In the VQckoff notation these sites are indicated as
292
Fig. 4. Temperature dependence of the ac. susceptibility in several RFe,,V, (R=Dy, Ho, Er) compounds. Measurements made with increasing temperature (heating curves) are represented by broken curves, measurements made with decreasing temperature (cooling curves) are given as full curves. The small differences in cooling and heating curves well below the phase t~sformation temperature in DyFe,aV, and are probably due to experimental inaccuracies.
8i, 8j and 8f. The latter two sites are occupied exclusively by iron atoms while the 8i sites accommodate the iron atoms as well as the vanadium atoms in RFe,,V, [l 11.It was found that at the 8i site the iron atoms have the largest moments. This was confirmed in a number of s7Fe Mossbauer ~vestigations [7, 19, 20, 211. The 57Fe Mossbauer spectra of RFei0M2 are basically composed of three different subspectra corresponding to the three types of iron sites mentioned above. The assignment of these subspectra to the three sites is not free of ambiguity with respect to the sites 8j and 8f. However, the subspectrum attributed to the 8i site can clearly be identified owing to the fact that its relative intensity is reduced by 50% because of the presence of the M atoms at this site. Also, band structure calculations [22] have shown that the 8i has the largest moment in the various R(Fe,T),, compounds (M = Ti, V, W and MO).
293
When viewed in this light, the values of the iron moments at 4.2 K listed in Table 1 seem difficult to interpret. For comparative purposes we have included the neutron data obtained previously for TbFe,,V,, ErFe,,V, [l l] and YFe,,V2 [lo] in Table 1. The moment values of the 8i iron atoms exceed those of the 8j and 8f iron atoms only in the compounds in which R = Nd, Tb, Er or Y. However, the iron moments at the 8i sites are substantially lower than those at the 8j and 8f sites in the compounds where R = Ho and Dy. We have no explanation for this unsystematic variation of the iron moments observed in the neutron data. Attempts to refine the neutron data by restricting the value of the iron moments to those found in YFe,,V, resulted generally in larger values of the tilt angles 4. Owing to the substantially higher R values associated with these refinements we left the latter results out of consideration. The magnetocrystalline anisotropy in the RFe,,V, compounds consists of two separate contributions originating from the iron sublattice and the rare earth sublattice. The former contribution can be determined from high field measurements made on compounds RFe,,V, in which R represents gadolinium or a non-magnetic rare earth atom such as yttrium or lutetium. From high field measurements made on YFe,,V, and LuFe,,V, it was found that at 4.2 K pc,H, equals 4.5 T in YFe,,V, and 4 T in LuFe,,V,, the corresponding values of the magnetic saturation induction being 1, = 1.09 T and 1.27 T respectively [6]. From these data the first-order anisotropy constant is calculated to be equal to Kp/k= 24 K per formula unit for both YFe,,V, and LuFe,,V,, showing that the rare earth contribution to the anisotropy is fairly constant through the rare earth series. A considerably more complex behaviour is due to the rare earth contribution to the anisotropy. It is crystal field induced, the leading term being equal to
KY= -3/2a,(r2)A:(O'+5/3,(r4)A:(O~) Owing to the strong decrease with increasing temperature of the expectation values of the Stevens operators (0;) for n > 2 it is reasonable to assume that only the second-order term of KY contributes to the anisotropy at room temperature. Experimentally we showed that all compounds investigated have the same easy magnetization direction at room temperature. This easy magnetization direction is parallel to the c axis, corresponding to K, = Kp + KY > 0.It follows therefore that the rare earth sublattice contribution at room temperature is smaller than the iron sublattice contribution since for a fixed value of the second-order crystal field parameter A; (inherent in the crystal structure) K, remains positive irrespective of the value of the second-order Stevens factor a, of the various rare earth components. It is also known from magnetic measurements that samarium is the only rare earth having an anisotropy that adds constructively to the iron anisotropy at room temperature. From this it is derived that At < 0 since a,> 0 for samarium. Independent confirmation that A! is comparatively small in RFe,,V, was obtained from “‘Gd Mossbauer spectroscopy, showing that A! in RFe,,,V, is roughly only one-third of the value found in the series R2Fe14B, R2Fe14C, R,Co,,B [ 1,231. Owing to the small value of A! one expects that the rare earth sublattice contribution KY can become of a size comparable with that of Kp only at low
294
temperatures. However, then we are already in the temperature regime where higher order crystal field terms become spoilt. These will not only modify the value of KY but will also give rise to strongly temperate-dependent higher order anisotropy constants KF and KY. This is the reason why ErFe,,V, does not give rise to an easy magnetization direction parallel to the c axis, even at low temperatures, as would be expected on the basis of the term Kfl = - 3/2cr,(r2)A~(O~> with a,>O. Instead we found an easy magnetization direction perpendicular to the c axis, or at least very close to it [lo, 181. The compound HoFe,,VZ is an example where a prevailing second-order crystal field term would have led to an easy magnetization direction perpendicular to the c axis since here a,
References 1 K. H. J. Buschow, D. B. de Mooij, M. Brouha, I-I. H. A. Smit and R. C. Thiel, IEEE Trans. Mugn., 24‘(1988) 1611. 2 K. Ohashi, Y. Tawara and R. Osugi, J. Less-Common Met., I39 (1988) Ll. 3 L. Schultz and J. Wecker, J. Appl. Phys., 64 (1988) 5711. 4 J. Dig and M. Rosenberg, J. Mugs. ~agn. Mater. 5 M. Okada, K. Yamagishi and M. Homma, Mater. Trans.JfM, 30 (1989) 374. 6 F. R. de Boer, Huang Ying-Kai, D. B. de Mooij and K. H. J. Buschow, J. Less-Common Met., 135 (1987) 199.
295
7 Bo-Ping Hu, Hong-Shuo Li, J. P. Gavigan and J. M. D. Coey, J. Phys. Co&. Mater., I (1989) 755. 8 M. Solzi, L. Pareti, 0. Moze and W. I. F. David, J. Appl. Phys., 64 (1989) 5084. 9 E. B. Boltich, B. M. Ma, L. Y. Zhang, F. Pourarian, S. K. Mahk, S. G. Sankar and W. E. Wallace, J. Mugn. Magn. Mater., 78 (1989) 364. 10 R. B. HeImholdt, J. J. M. Vleggaar and K. H. J. Buschow, J. Less-Common Met., 144 (1988) 209. 11 R. B. Helmholdt, J. J. M. Vleggaar and K. H. J. Buschow, J. Less-Common Met., 138 (1988) Ll 1. 12 K. H. J. Buschow, J. Appl. Phys., 63 (1988) 3130. 13 H. M. Rietveld, J. Appl. Crysfullogr., 2 (1969) 65. 14 R. E. Watson and A. J. Freeman, Acta CrystuZZogr., I4 (1961) 27. 15 M. Blume, A. J. Freeman and R. E. Watson, J. Chem. Phys., 36 (1962) 1248. 16 0. Steinsvoll, G. Shirane, R. Nathans, M. Blume, H. A. Alperin and S. J. Pickart, Phys. Rev., 161 (1967) 499. 17 P. C. M. Gubbens, A. M. van der Kraan and K. H. J. Buschow, in C. Herget et al. (eds.), Proc. 5th ht. Symp. on Magn. Anisotropy, Bad Soden, 1987,1987, p. 117. 18 D. B. de Mooij and K. H, J. Buschow, J. Less-Common Met., 15.5 (1989) L9. 19 Th. Sinneman, K. Erdmann, M. Rosenberg and K. H. J. Buschow, Hyp. Inter., 50 (1989) 675. 20 Th. Sinneman, M. Rosenberg and K. H. J. Buschow, J. Less-Common Met., 146 (1989) 223. 21 C. J. M. Denissen, R. Coehoom and K. H. J. Buschow, J. Mugn. Magn. Mater., in the press. 22 R. Cdehoom, Phys. Rev. B, 39 (1989) 13-72. 23 M. W. Dirken, R. C. Thiel and K. H. J. Buschow, J. Less-Common Met., 146 (1989) L5. 24 K. H. J. Buschow, in E. P. WohIfarth and K. H. J. Buschow (eds.), Ferromagnetic Materials, Vol. 4, North-Holland, Amsterdam, 1988, p. 1.
Metals, 162 (1990)
MAGNETIC STRUCTURE TYPE RFe,OV,
285-295
285
OF RARE EARTH COMPOUNDS
OF THE
W. G. HAIJE and J. SPIJKERMAN* Netherlands Energy Research Foundation,
ECN, 1755 ZG Petten (The Netherlands)
F. R. DE BOER and K. BAKKER Natuurkundig Laboratorium, (The Netherlands)
Universiteit van Amsterdam,
Valckenierstraat 65, 1018 XE Amsterdam
K. H. J. BUSCHOW Philips Research Laboratories,
P. 0. Box 80 Ooo, 5600 JA Eindhoven
(The Netherlands)
(Received January 20,199O)
Summary We have studied the magnetic properties of several rare earth compounds of the type RFer,,V, (R= Nd, Tb, Dy, Ho and Er) by means of a.c. susceptibility measurements and neutron diffraction. When cooling to below room temperature most of these compounds undergo a magnetic phase transition from a collinear structure in which the moments are parallel to the c axis (300 K) to a collinear structure in which the moments deviate from the c axis (4.2 K).
1. Introduction Ternary iron-rich compounds derived from the tetragonal ThMn,, structure have been proposed as inexpensive alternatives for permanent magnet materials. These compounds have the general formula composition RFe,, _ *M,, where x = 1 for ME Ti and W and x = 2 for M = V, Cr, MO and Si [ 1,2]. Their Curie temperatures and magnetizations have values sufficiently high for technical applications, and there are several examples where a sufficiently high coercive force has been attained [3-51. From the physical side these compounds are just as interesting as the R*Fe,,B and R*Fe,,C compounds, since they give rise to numerous magnetic structures and phase transitions [6-lo]. The ThMn,, structure comprises three different 3d sites. Experimental data available for some of the compounds have revealed that there is a distinct site preference (8i site) for the M element in compounds with M = Ti, V and MO [8,11,
*Present address: University of Twente, 7500 AE Enschede, The Netherlands. 0022-5088/90/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
286
121. In all these series the iron sublattice anisotropy favours an easy magnetization direction parallel to the c axis. The evolution of the magnetic structures at temperatures well below T, can be described by the same formalism as used for the R2Ti4X compounds and involves a competition between the iron sublattice anisotropy and the rare earth sublattice anisotropy. In the present study we have focused our attention to several compounds of the RFe,,V, series. We have used a.c. susceptibility measurements to detect magnetic phase transitions and determined the corresponding magnetic structures by means of neutron diffraction.
2. Experimental details The compounds of the type RFe,,V, (R= Nd, Tb, Dy, Ho and Er) were prepared by arc melting starting materials of at least 99.9% purity. After arc melting, the samples were vacuum annealed at 900 “C for 2 weeks. After vacuum annealing, the samples were investigated by X-ray diffraction (XRD). All samples were found to be approximately single phase. The occurrence of magnetic phase transitions was investigated by means of a.c. susceptibility measurements. These measurements were made on powdered material in the temperature range 4.2-300 K. Cooling curves as well as heating curves were measured. Neutron diffraction measurements were made at temperatures between 300 and 4.2 K (configuration 30 min/sample/30 min; L = 2.5719 A). The diffraction patterns obtained were analysed by means of Rietveld’s profile refinement technique [ 131. The maximum absorption correction was about 5%, ,uR remaining below 0.8 even in the case of DyFe,,,V,. The powder of the latter compound was mixed with aluminium powder in order to reduce the absorption of neutrons by the dysprosium. For the scattering lengths we used the values 0.769 X lo-‘* cm for neodymium, 0.738 x lo-‘* cm for terbium, 1.69 X lo- ‘* cm for dysprosium, 0.808 x 10-l* cm for holmium, 0.954 x 10-l* cm for iron and -0.0382 x lo-l2 cm for vanadium. The corresponding magnetic form factors were taken from the papers published by Watson and Freeman [14], Blume et al. [15] and Steinsvoll et al. [16]. From results of magnetic measurements obtained earlier [6] it follows that all compounds are magnetically ordered at 4.2 and 300 K. For this reason, nuclear as well as magnetic reflections had to be considered in the diffraction patterns obtained at all temperatures applied. All neutron diffraction patterns obtained in the course of the present investigation were fitted with the constituent atoms placed in the following positions of the ThMn,, structure type (space group 14/mmm): R in 2a(OOO), iron and vanadium at 8i (x, 0, 0), iron in 8j (x, $, 0) and ironin 8f (& :, 4). Our choice of the 8i position for the vanadium atoms is based on the results of a previous neutron diffraction study in which we focused our attention of the preferred site occupation of the vanadium atoms in this class of compounds [l 11.The quality of the fit between the various experimental and the calculated diffractograms was assessed
287
by the expression Xz = c wi 1 yi(obs)-X(calc)12 V
where y,( obs) and y,( talc) are the observed and calculated intensity values of the ith measuring point, wi being its statistical weight. The quantity v represents the number of points minus the number of parameters. The parameters derived from the fitting procedure are given in Table 1. In the following section we have restricted ourselves to presenting only the data pertaining to the fits with the lowest values of x 2, R,,,, and Rmag.
3. Results and interpretation 3. I. NdFe,, V, The temperature dependence of the a.c. susceptibility when measured from room temperature to 4.2 K (cooling curve) shows a broad peak centred at T,= 120 K. This was taken as an indication that at this temperature a magnetic phase transition takes place. As may be seen from Fig. 1, the transition at T, takes place at a slightly lower temperature when the measurements are started at 4.2 K (heating curve). Neutron diffraction data were taken at a temperature below (4.2 K) and above (295 K) the phase transition temperature. Results of the refinement of the corresponding neutron diffraction patterns are given in the top part of Table 1. It follows from these data that the easy magnetization direction is parallel to the c direction at 295 K, but makes an angle with it at 4.2 K. It is reasonable to assume that the change in easy magnetization direction commences near T, where it is responsible for the enhancement of the a.c. susceptibility. The neodymium and iron moments remain collinear at all temperatures, the neodymium moments being parallel to the iron moments. 3.2. SmFe,,I/, The a.c. susceptibility does not indicate any change in the spin structure. No neutron diffraction data are available. 3.3. TbFe,, V, The temperature dependence of the a.c. susceptibility (heating curve) shows a shallow maximum at about 100 K and a further maximum at about 230 K. These features are less clear in the cooling curve (see Fig. 1). Earlier neutron studies made on this compound have shown that the easy magnetization direction is parallel to the c axis at 295 K, but makes an angle # = 50” with it at 4.2 K [lo]. In the course of the present investigation we have focused our attention to the intermediate temperature range. The neutron diffraction diagrams obtained at 120 and 170 K are shown in Fig. 2, and the data obtained from refining these diagrams are listed in the
288 TABLE 1 Parameters derived from fitting the neutron diffractograms obtained for various compounds of the type BFeIOV1” B Overall
Compound
T (K)
a=b(A)
c(A)
x (8i)
x (8j)
NdFe,,Vr
293
4.2
8.5553(3) 8.5515(4)
4.7737(2) 4.7633(2)
0.3634(4) 0.3654(5)
0.2722(2) 0.2714(3)
293 120(A)b 170 120(B)b 4.2
8.4856(4) 8.4814(3) 8.4827( 3) 8.4817(3) 8.4860(3)
4.7662(2) 4.7609(2) 4.7626(2) 4.7611(2) 4.7638(2)
0.3613(6) 0.3600(6) 0.3617(6) 0.3593(6) 0.3637( 11)
0.2755(4) 0.2750(3) 0.2750(3) 0.2755(3) 0.2746(6)
0.57(6) 0.69(5) 0.23(6) 0.49(6)
DyFer”%
293 160 4.2
8.4816(6) 8.4781(5) 8.4734(5)
4.7658(3) 4.7609(3) 4.7562(3)
0.3612(g) 0.3594(g) 0.3613(g)
0.2781(4) 0.2773(4) 0.2745( 5)
0.18(8) 0.09( 7) 0.37(8)
HoFe,,Vr
293 4.2
8.4661(4) 8.4565(4)
4.7608(2) 4.7507(2)
0.3627(6) 0.3620(g)
0.2765(4) 0.2761(4)
0.39(7)
TbFe,,V,
ErFe,,V,
4.2
8.451(4)
4.7539(2)
0.3607(g)
0.2831(5)
- 0.98(7)
YFereVr
4.2
8.483
4.764
0.3574(4)
0.2783(2)
-0.21(6)
BR
B,,
0.6(l) O.l( 1)
1.1(l) 0.1(2)
0.4( 1)
0.3(2)
1.9(2)
0.3(2)
aThe values of the temperature factors B are given in A’. The values of the total rare earth moments pa, the iron moments pFe are given in ,ue. The numbering Fe,, Fe, and Fes corresponds to the 8i, 8j and 8f sites respectively. The deviation of the easy magnetization direction from the c axis is given by the angle # (accurate within 2”). bA and B refer to the first and second run for TbFe,,V, (see main text).
middle part of Table 1. The magnetic structure is collinear at all temperatures, the terbium moments being coupled antiparallel to the iron moments. The easy magnetization direction is parallel to the c axis at room temperature, but deviates from it at lower temperatures. The temperature dependence of the tilt angle # is shown in Fig. 3. It may be derived from these data that the easy magnetization starts to deviate from the c direction at about 210 K, which is close to the high temperature maximum of the a.c. susceptibility. Since the a.c. susceptibility was found to give rise to a small temperature hysteresis (see Fig. 1) of the transition we measured a second neutron diffraction diagram at 120 K after having completed the data collection at 4.2 K and without allowing the sample to reach temperatures higher than 120 K. Results of the refinement procedure obtained from these data have been included in Table 1. It is seen that the tilt angle (4 = 22”) is slightly larger than the angle (# = 20”) obtained when cooling directly to 120 K, being in keeping with the hysteretic nature of the spin reorientation in TbFe,,V,. It should be noted, however, that this small difference in # is near the experimental error. 3.4. DyFe,, V, The a.c. susceptibility gives rise to a broad peak centred at 130 K in the heating curve and at 150 K in the cooling curve, as shown in Fig. 4. Neutron
289
2.6( 1) 2.0( 2)
1.7( 1) 1.7(2)
1.4(l) 1.7(2)
0 60
2.90
6.35
3.31
-5.6(l) -7.78(6) -7.37(5) -8.02(6) -8.77(7)
1.q 1) 1.60(6) 1.49(6) 1.55(6) 2.03(4)
1.8( 1) 2.04(5) 2.11(5) 1.99(5) 1.15(6)
1.9(l) 1.88(3) 2.01(3) 1.91(3) 1.97(4)
0 20 9 22 50
2.09 2.12 1.77 2.68 2.5
3.53 2.73 2.59 3.61 3.7
4.47 5.47 7.55 5.83 5.9
-7.0(l) -8.6(l) -9.8(l)
0.6( 1) 1.3( 1) 1.3( 1)
1.3(l) 1.7(l) 1.9(l)
1.5( 1) 1.8(l) 1.6( 1)
0 15 32
4.10 3.25 2.31
7.16 4.66 2.86
2.63 2.51 3.60
-5.0(l) - 10.03(8)
0.6( 1) 1.36(7)
1.2(l) 2.03(5)
1.2( 1) 1.83(4)
0 0
4.28 4.07
7.15 4.03
4.21 5.05
-10.4(l)
2.0( 1)
1.5(l)
1.9( 1)
90
5.4
7.7
10.7
0
1.9( 1)
1.5(2)
1.8( 1)
0
5.1
3.9
6.6
l.O( 1) 0.2(l)
0.8( 1) 0.2(l)
l.l( 1) 0.1(2)
1.4( 1) 3.1(2)
1.1(l)
0.8(l)
0.3(2)
1.2(l)
0.8(l)
0.3(2)
diffraction data obtained at 295, 160 and 4.2 K are listed in Table 1. It follows from these data that the easy magnetization direction is parallel to the c axis only at room temperature. It starts deviating from the c axis upon cooling. According to the neutron data this deviation occurs already at higher temperatures than the temperature dependence of the a.c. susceptibility suggests. 57Fe Mossbauer spectroscopy indicated the spin reorientation to occur already near 200 K [ 171. 3.5. HoFe,, V, The a.c. susceptibility of this compound is shown in the middle part of Fig. 4. There is no indication of any spin reorientation. From the neutron diffraction data, listed in the bottom part of Table 1, we derive that there is no change in the easy magnetization direction with temperature, the c direction (or a direction very close to it) being the preferred moment direction at 295 K as well as at 4.2 K. 3.6. ErFe,,I/, The temperature dependence of the a.c. susceptibility of ErFe,,V,, shown in the bottom part of Fig. 4, suggests a spin reorientation occurs at about 50 K. This is in agreement with the results of the magnetization measurements [ 181, 57Fe Miissbauer spectroscopy [ 191 and neutron diffraction [lo] reported earlier. At
290
E ‘5
4 m k
2000
1000 0
0
100
200
300
100 SmFe10U
TbFe,,Y
T(K)
-
Fig. 1. Temperature dependence of the a.c. susceptibility in several RFe,,,V2 (R=Nd, Sm, Tb) compounds. Measurements made with increasing temperature (heating curves) are represented by broken curves, measurements made with decreasing temperature (cooling curves) are given as full curves. The small differences in cooling and heating curves well below the phase transformation temperature in NdFe,,V, are probably due to experimental inaccuracies.
room temperature the easy magnetization direction is parallel to the c axis. At 4.2 K the easy magnetization direction is perpendicular to the c axis, or at least very close to the latter direction.
4. LXscussion
We will first discuss the magnetic moment values derived from the refinement of the neutron data. The rare earth moments found at 4.2 K are fairly close to the free ion values (gJpc,) and their relative orientation corresponds to an antiparallel
291
Calculated profile ............ Observed profile
TbFe,,,V, 293K
t
25 20
g
15
g
10
a 0 x .?
5
s F
=
0 25
120K r
0
60
40
20
80 20 (degrees)
F
Fig. 2. Neutron diffraction diagram of the compound TbFe,,V, at 293 K (top part) and 120 K (bottom part). The full lines represent calculated fits to the experimental data. The location of nuclear and magnetic reflections of the main phase are indicated by short vertical bars at the bottom of the diagrams. Absence of a full line indicates regions excluded because of impurity phases.
I
60 TbFe,oVp
0’
.
I
0
100
200
Temperature
Fig. 3. Temperature dependence
(K)
300
-
of the tilt angle in TbFe,,,V,.
coupling between the iron moments and the rare earth spin moment. These results are in satisfactory agreement with expectations. More difficult to interpret are the iron moments. There are three distinct iron sites in the ThMn12 structure. In the VQckoff notation these sites are indicated as
292
Fig. 4. Temperature dependence of the ac. susceptibility in several RFe,,V, (R=Dy, Ho, Er) compounds. Measurements made with increasing temperature (heating curves) are represented by broken curves, measurements made with decreasing temperature (cooling curves) are given as full curves. The small differences in cooling and heating curves well below the phase t~sformation temperature in DyFe,aV, and are probably due to experimental inaccuracies.
8i, 8j and 8f. The latter two sites are occupied exclusively by iron atoms while the 8i sites accommodate the iron atoms as well as the vanadium atoms in RFe,,V, [l 11.It was found that at the 8i site the iron atoms have the largest moments. This was confirmed in a number of s7Fe Mossbauer ~vestigations [7, 19, 20, 211. The 57Fe Mossbauer spectra of RFei0M2 are basically composed of three different subspectra corresponding to the three types of iron sites mentioned above. The assignment of these subspectra to the three sites is not free of ambiguity with respect to the sites 8j and 8f. However, the subspectrum attributed to the 8i site can clearly be identified owing to the fact that its relative intensity is reduced by 50% because of the presence of the M atoms at this site. Also, band structure calculations [22] have shown that the 8i has the largest moment in the various R(Fe,T),, compounds (M = Ti, V, W and MO).
293
When viewed in this light, the values of the iron moments at 4.2 K listed in Table 1 seem difficult to interpret. For comparative purposes we have included the neutron data obtained previously for TbFe,,V,, ErFe,,V, [l l] and YFe,,V2 [lo] in Table 1. The moment values of the 8i iron atoms exceed those of the 8j and 8f iron atoms only in the compounds in which R = Nd, Tb, Er or Y. However, the iron moments at the 8i sites are substantially lower than those at the 8j and 8f sites in the compounds where R = Ho and Dy. We have no explanation for this unsystematic variation of the iron moments observed in the neutron data. Attempts to refine the neutron data by restricting the value of the iron moments to those found in YFe,,V, resulted generally in larger values of the tilt angles 4. Owing to the substantially higher R values associated with these refinements we left the latter results out of consideration. The magnetocrystalline anisotropy in the RFe,,V, compounds consists of two separate contributions originating from the iron sublattice and the rare earth sublattice. The former contribution can be determined from high field measurements made on compounds RFe,,V, in which R represents gadolinium or a non-magnetic rare earth atom such as yttrium or lutetium. From high field measurements made on YFe,,V, and LuFe,,V, it was found that at 4.2 K pc,H, equals 4.5 T in YFe,,V, and 4 T in LuFe,,V,, the corresponding values of the magnetic saturation induction being 1, = 1.09 T and 1.27 T respectively [6]. From these data the first-order anisotropy constant is calculated to be equal to Kp/k= 24 K per formula unit for both YFe,,V, and LuFe,,V,, showing that the rare earth contribution to the anisotropy is fairly constant through the rare earth series. A considerably more complex behaviour is due to the rare earth contribution to the anisotropy. It is crystal field induced, the leading term being equal to
KY= -3/2a,(r2)A:(O'+5/3,(r4)A:(O~) Owing to the strong decrease with increasing temperature of the expectation values of the Stevens operators (0;) for n > 2 it is reasonable to assume that only the second-order term of KY contributes to the anisotropy at room temperature. Experimentally we showed that all compounds investigated have the same easy magnetization direction at room temperature. This easy magnetization direction is parallel to the c axis, corresponding to K, = Kp + KY > 0.It follows therefore that the rare earth sublattice contribution at room temperature is smaller than the iron sublattice contribution since for a fixed value of the second-order crystal field parameter A; (inherent in the crystal structure) K, remains positive irrespective of the value of the second-order Stevens factor a, of the various rare earth components. It is also known from magnetic measurements that samarium is the only rare earth having an anisotropy that adds constructively to the iron anisotropy at room temperature. From this it is derived that At < 0 since a,> 0 for samarium. Independent confirmation that A! is comparatively small in RFe,,V, was obtained from “‘Gd Mossbauer spectroscopy, showing that A! in RFe,,,V, is roughly only one-third of the value found in the series R2Fe14B, R2Fe14C, R,Co,,B [ 1,231. Owing to the small value of A! one expects that the rare earth sublattice contribution KY can become of a size comparable with that of Kp only at low
294
temperatures. However, then we are already in the temperature regime where higher order crystal field terms become spoilt. These will not only modify the value of KY but will also give rise to strongly temperate-dependent higher order anisotropy constants KF and KY. This is the reason why ErFe,,V, does not give rise to an easy magnetization direction parallel to the c axis, even at low temperatures, as would be expected on the basis of the term Kfl = - 3/2cr,(r2)A~(O~> with a,>O. Instead we found an easy magnetization direction perpendicular to the c axis, or at least very close to it [lo, 181. The compound HoFe,,VZ is an example where a prevailing second-order crystal field term would have led to an easy magnetization direction perpendicular to the c axis since here a,
References 1 K. H. J. Buschow, D. B. de Mooij, M. Brouha, I-I. H. A. Smit and R. C. Thiel, IEEE Trans. Mugn., 24‘(1988) 1611. 2 K. Ohashi, Y. Tawara and R. Osugi, J. Less-Common Met., I39 (1988) Ll. 3 L. Schultz and J. Wecker, J. Appl. Phys., 64 (1988) 5711. 4 J. Dig and M. Rosenberg, J. Mugs. ~agn. Mater. 5 M. Okada, K. Yamagishi and M. Homma, Mater. Trans.JfM, 30 (1989) 374. 6 F. R. de Boer, Huang Ying-Kai, D. B. de Mooij and K. H. J. Buschow, J. Less-Common Met., 135 (1987) 199.
295
7 Bo-Ping Hu, Hong-Shuo Li, J. P. Gavigan and J. M. D. Coey, J. Phys. Co&. Mater., I (1989) 755. 8 M. Solzi, L. Pareti, 0. Moze and W. I. F. David, J. Appl. Phys., 64 (1989) 5084. 9 E. B. Boltich, B. M. Ma, L. Y. Zhang, F. Pourarian, S. K. Mahk, S. G. Sankar and W. E. Wallace, J. Mugn. Magn. Mater., 78 (1989) 364. 10 R. B. HeImholdt, J. J. M. Vleggaar and K. H. J. Buschow, J. Less-Common Met., 144 (1988) 209. 11 R. B. Helmholdt, J. J. M. Vleggaar and K. H. J. Buschow, J. Less-Common Met., 138 (1988) Ll 1. 12 K. H. J. Buschow, J. Appl. Phys., 63 (1988) 3130. 13 H. M. Rietveld, J. Appl. Crysfullogr., 2 (1969) 65. 14 R. E. Watson and A. J. Freeman, Acta CrystuZZogr., I4 (1961) 27. 15 M. Blume, A. J. Freeman and R. E. Watson, J. Chem. Phys., 36 (1962) 1248. 16 0. Steinsvoll, G. Shirane, R. Nathans, M. Blume, H. A. Alperin and S. J. Pickart, Phys. Rev., 161 (1967) 499. 17 P. C. M. Gubbens, A. M. van der Kraan and K. H. J. Buschow, in C. Herget et al. (eds.), Proc. 5th ht. Symp. on Magn. Anisotropy, Bad Soden, 1987,1987, p. 117. 18 D. B. de Mooij and K. H, J. Buschow, J. Less-Common Met., 15.5 (1989) L9. 19 Th. Sinneman, K. Erdmann, M. Rosenberg and K. H. J. Buschow, Hyp. Inter., 50 (1989) 675. 20 Th. Sinneman, M. Rosenberg and K. H. J. Buschow, J. Less-Common Met., 146 (1989) 223. 21 C. J. M. Denissen, R. Coehoom and K. H. J. Buschow, J. Mugn. Magn. Mater., in the press. 22 R. Cdehoom, Phys. Rev. B, 39 (1989) 13-72. 23 M. W. Dirken, R. C. Thiel and K. H. J. Buschow, J. Less-Common Met., 146 (1989) L5. 24 K. H. J. Buschow, in E. P. WohIfarth and K. H. J. Buschow (eds.), Ferromagnetic Materials, Vol. 4, North-Holland, Amsterdam, 1988, p. 1.