INFLUENCE OF HYDROGENATION ON MAGNETIC CHARACTERISTICS OF Lu 2 (Fe,M) 17 (M = Fe, Cr, Ni and Si) COMPOUNDS

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1 INFLUENCE OF HYDROGENATION ON MAGNETIC CHARACTERISTICS OF (Fe,M) 17 (M = Fe, Cr, Ni and Si) COMPOUNDS E.A. Tereshina 1,2, A.V. Andreev 1, I.S. Tereshina 3, H. Drulis 4 1 Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, Prague, 18221, Czech Republic 2 Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, Prague, 12116, Czech Republic 3 Baikov Institute of Metallurgy and Material Science RAS, Leninskii pr. 49, Moscow, , Russian Federation 4 Institute of Low Temperature and Structure Research, Okolna 2, Wroclaw, , Poland e.tereshina@seznam.cz Abstract Influence of substitution (chromium, nickel and silicon) and interstitial (hydrogen) atoms on magnetic properties of Fe 17 compound was investigated. Study was performed on single-crystalline samples grown by the Czochralski method. Hydrogenation regimes which allowed us to prepare hydrides without destruction of the single-crystalline structure have been developed. Magnetic ordering temperatures of the compounds under study were found to be strongly influenced by hydrogenation, while the saturation magnetization and the magnetic anisotropy field were changed weakly. No changes of the type of magnetic anisotropy from the easy-plane to the easy-axis were observed in compounds studied. 1. INTRODUCTION Investigation of rare-earth (R) intermetallic compounds with high 3d metal (T) content is important from both scientific and application viewpoints. In this field, a systematic study has begun in the early 1960s and in 1966 led to a discovery of the high uniaxial magnetic anisotropy in YCo 5 [HOFFER G., STRNAT K., 1966]. This fundamental result has found an immediate application in the first generation of the rare-earth permanent magnets based on SmCo 5. Related compounds with even higher content of T metal, with stoichiometry have also attracted much attention as potential hard magnetic materials, and the second generation of the rare-earth permanent magnets based on Sm 2 Co 17 appeared in the middle 70s [BUSCHOW K.H.J., 1988; STRNAT K., 1988]. Besides high values of magnetization, Curie temperature and magnetic anisotropy required for permanent magnet production, its cost has a lot to do with that, thus, another excellent materials were found as the third generation where the expensive Co was replaced by cheap Fe (the best are Nd-Fe-based alloys [BUSCHOW K.H.J., 1997; HERBST J.F., 1991]), and investigation of the other Fe-containing compounds (including R 2 Fe 17 ) is still in progress. From the scientific point of view, a coexistence of localized 4f electrons with the itinerant 3d electrons makes such intermetallics to be very interesting objects (see review papers [FRANSE J.J.M., RADWANSKI R., 1993; LI H.S., COEY J.M.D., 1991]). Compounds of rare earths with the 3d transition metals can be adequately considered as two-sublattices magnets (study of alloys with nonmagnetic Y or Lu provides an analysis of the 3d-metal sublattice). Competition of the exchange intraand inter-sublattice interactions in these materials together with the crystal-field interaction frequently leads to an appearance of various spontaneous and field-induced magnetic phase transitions. By means of different substitutions or by an introduction of light interstitial atoms (H, N and C) into the crystal lattice of the rare-earth intermetallics, great changes of magnetic characteristics of the compounds studied were observed. Thus, in previous works [SHEN B.G. et al., 1993, SHEN B.G. et al., 1995], the effect of various non-magnetic substitutions (such as Ga, Al and Si) for Fe in R 2 Fe 17 compounds was shown to play an important role in determining the easy magnetization direction (EMD). An increase 1

2 of Curie temperature (T C ) by 150 K/at.N upon the introduction of the nitrogen atoms into the crystal lattice of Sm 2 Fe 17 compound and a simultaneous appearance of the strong uniaxial anisotropy were found, which resulted in a development of the high-energy permanent magnets based on this material [STRNAT K., 1988; COEY J.M.D., 1992]. Thus, R 2 Fe 17 Z x (where Z = H, N and C) compounds became the objects of intensive study, in order to reveal the regularities of the fundamental characteristics changes induced by the introduction of the light atoms into the crystal lattice of these compounds. Purpose of the present work was to investigate the combined effect of both interstitial atoms (hydrogen) and substitution atoms (chromium, nickel and silicon) on magnetic characteristics of Fe 17 with non-magnetic Lu in order to reveal the behavior of the 3d-sublattice of R 2 Fe 17 compounds. 2. EXPERIMENTAL DETAILS The starting materials Lu (99.9%), Fe, Ni, Cr (99.99%), Si (99.999%) were weighted in the desired atomic ratio and melted together several times in a tri-arc furnace under an argon atmosphere. In order to ensure a perfect homogeneity, alloy buttons were turned several times and then kept in a melting state for about 1 hour. Single crystals were grown by means of modified Czochralski method in a tetra-arc furnace using a tungsten wire as a seed under 10 mm/h pulling speed. Powder X-ray diffraction experiments used for the initial phase analysis were performed on a Siemens diffractometer in Bragg Brentano geometry with monochrome Co K α radiation. The diffraction patterns were refined by means of Rietveld analysis using the Fullprof/Winplotr software [RODRIGUEZ-CARVAJAL J., 1993]. The back Laue patterns were used to check the mono-crystalline state and to orient crystals for cutting the samples. Hydrogenation of Fe 16 M (M = Fe, Cr, Ni and Si) single crystals was performed using a vacuum glass apparatus by means of the direct absorption of hydrogen by the compound after a short thermal activation procedure at 500 C. At this temperature high purity hydrogen obtained by thermal decomposition of titanium hydride was admitted at a given pressure to obtain a stable compound without powdering of the sample. To achieve good homogenization, the product was slowly cooled down (about 20 C/h) to room temperature. Depending on the desirable hydrogen content, temperature and time of hydrogenation process were varied. For (Fe,M) 17 (M = Fe, Si, Cr, Ni) the following hydrogen concentrations (H at. per formula unit) were obtained: ~0.8 and ~1.5. Hydrogen concentration was determined by the volumetric method. Temperature and field dependencies of magnetization of Fe 16 M (M = Fe, Cr, Ni and Si) single crystals and their hydrides were measured along the principal axes in magnetic fields up to 5 T at K using PPMS9 and SQUID magnetometers (Quantum Design, USA). Magnetization curves presented below were corrected for the demagnetizing field. 3. RESULTS AND DISCUSSION For all Fe 16 M (M = Fe, Cr, Ni and Si) compounds, the X-ray powder diffraction analysis confirmed the hexagonal crystal structure of the Th 2 Ni 17 (space group P6 3 /mmc) type [FLORIO J.V. et al, 1956] characteristic for R 2 Fe 17 compounds with heavy rare earths (crystallographic parameters of Fe 17 -based intermetallics and their hydrides are shown in Table 1). It is known that in Fe 17, Fe atoms occupy the Wyckoff positions 4f, 6g, 12j, and 12k. In the ideal Th 2 Ni 17 structure, Lu atoms in 2b-positions form pure Lu chains along the c-axis and Lu atoms located at the positions 2d alternate with Fe atoms in 4f-positions which create dumbbell-like pairs along the c-axis. The equilibrium composition of the R 2 Fe 17 compounds is shifted from the 2 17 stoichiometry to approximately 2 19 by substitution of part of the R atoms with Fe dumbbells in 4e-positions [GIVORD D. et al., 1972]. Presence of dumbbells - Fe atoms in 4f positions (with critical distances between them) along the c- axis of hexagonal Th 2 Ni 17 -type of crystal structure results in extremely delicate balance of ferro- (F) and antiferro- (AF) magnetic interactions. Fe 17 compound exhibits two types of magnetic ordering: an incommensurate helical magnetic structure with the collinear arrangement of Fe moments in the basal plane below Neel temperature T N = 274 K with the transition into a ferromagnetic phase below Curie temperature T C = 130 K (see Fig.1). As it was revealed earlier [ANDREEV A.V. et al., 2003], the real 2

3 sample composition and the local atomic ordering play a crucial role in the magnetic state of Fe 17 compounds for some compositions, a total suppression of the helimagnetic state was observed with a considerable increase of Curie temperature. Moreover, different substitutions in both, rare-earth and Fe sublattices, application of hydrostatic pressure [KAMARÁD J. et al., 2007; PROKHNENKO O. et al., 2003] and introduction of light atoms (i.e. hydrogen) are found to affect this subtle balance of positive (F) and negative (AF) exchange interactions in Fe-sublattice and lead to a change of the type of magnetic ordering. Table 1. Crystallographic and magnetic characteristics of Fe 17 -based intermetallics and their hydrides. M s is the spontaneous magnetic moment per formula unit, H A is the anisotropy field. Compound a, Å c, Å V, Å 3 V/V*, % T C, K M s, µ B /f.u. at 5 K H A, T at 5 K Fe ** Fe 17 H Fe 16 Ni Fe 16 NiH Fe 16 NiH Fe 16 Cr Fe 16 CrH Fe 16 CrH Fe 16 Si Fe 16 SiH Fe 16 SiH *relative change of volume in comparison to Fe 17 compound is shown for substituted compounds, and relative change of volume in comparison to its host compound is shown for hydrides. **for Fe 17 compound, T N is shown. Fe 17 has the easy-plane type of magnetic anisotropy (MA) with the easy magnetization directions (EMD) lying in the basal plane and hard magnetization direction (HMD) along the c-axis. Hydrogenation of Fe 17 compound (introduction of one hydrogen atom per formula unit) is found to cause an increase of the unit cell volume for 0.42 % without the crystal structure type change (see Table 1 for crystallographic data). According to Ref. [TERESHINA I.S. et al., 2001], hydrogen atoms occupy 6h positions in the hexagonal Th 2 Ni 17 -type of crystal structure. For Fe 17 H, a suppression of the high temperature antiferromagnetism and an initiation of the ferromagnetic states in the whole range of magnetic ordering with T C = 354 K (see Fig. 1) were found, however, no change of the type of magnetic anisotropy was observed upon hydrogenation. There are few mechanisms responsible for the change of Curie temperature: the effect of the lattice expansion (volume effect) which causes the 3d-band narrowing and thus, an increase the local Fe-atoms moments (see Table 1); and the effect of charge transfer from the interstitial atom to Fe-d-orbitals (chemical effect). In order to understand which mechanism is working in this particular case, we refer to the article [TERESHINA E.A. et al., 2007] where a reentrance of the antiferromagnetism was shown in Fe 17 H compound under an influence of hydrostatic pressure. In Fe 17 H, a change of the ordering temperature occured with the average rate of 190 K/GPa upon hydrogenation. On the other hand, under external hydrostatic pressure, T C was decreased with the rate of 50 K/GPa, i.e. considerably slower than it was increased upon hydrogenation. A reentrance of antiferromagnetism was observed above 0.6 GPa. Thus, the change of Curie temperature upon hydrogenation in Fe 17 H is conditioned not only by the volume effect but also by the electronic structure change - charge transfer from the hydrogen atoms to 3dorbitals of Fe. 3

4 M (µ B /f.u.) H // a-axis H = 0.01 T Fe 17 H x x = 0 x = T (K) Fig. 1. Temperature dependence of magnetization of Fe 17 H x (x = 0, 1) compounds measured along the easy a-axis in magnetic field of 0.01 T M (µ B /f.u.) Fe 16 M M = Cr M = Ni M = Si M = Fe T = 5 K H i (T) Fig. 2. Magnetization curves along the principal axes of Fe 16 M single crystals at 5 K. Magnetic properties of substituted Fe 16 M (M = Ni, Cr, Si) compounds were investigated previously [KAMARÁD J. et al., 2006] (see Fig 2). Substitution of one atom of Ni, Cr and Si for Fe 4

5 was found to stabilize ferromagnetism in all Fe 17 -based intermetallics under study. Drastic increase of magnetic ordering temperatures for more than 100 K per one substitution atom was observed along with a simultaneous decrease of the unit cell volume and with a decrease of the spontaneous magnetic moment of Fe 16 M (M = Ni, Cr and Si). Moreover, if one compares V/V (see Table 1) for Ni-, Crand Si-substituted compounds, V/V is more than 3 times larger for a case of Si due to its smaller atomic radius compared to Fe. Si is known to occupy primarily 6g and 12k positions [ANDREEV A.V. et al., 2004] in Th 2 Ni 17 - type of crystal structure and thus, 4f Fe-positions responsible for the negative exchange interactions in Fe 17 are Si free. Thereby, a decrease of spontaneous magnetic moment upon Si-substitution for Fe points to a hybridization of Fe- and Si-electron states. Apparently, a charge transfer from the valence band of Si to the 3d band of Fe takes place, locally reducing the magnetic moments on Fe atoms. This effect can disturb a subtle balance of negative and positive exchange interactions and therefore, destroy the non-collinear AF arrangement in the parent Fe 17 compound leading to onset of ferromagnetism in Fe 16 Si. In case of Ni- and Cr-substitution, values of the average magnetic on Fe atoms remain almost unchanged: m Fe = 1.99, 2.01 and 1.98 µ B /at.fe for Fe 17, Fe 16 Ni and Fe 16 Cr, respectively [KAMARÁD J. et al., 2006]. However in contrast to Si, atoms of Ni and Cr can probably occupy even the "dumbbell" 4f positions or dilute the ferromagnetic intra-plane bonds by lowered Ni-moments or by the anti-parallel to Fe orientation of Cr-moments. Thus, it can be concluded that both, intra- and inter-plane exchange interactions are strongly influenced by the substitution resulting in a change of the type of magnetic ordering. 30 M s, µ B /f.u Fe 16 M H x M = Cr, x = 0.9 M = Cr, x = 1.6 M = Ni, x = 0.8 M = Ni, x = 1.7 M = Si, x = 0.8 M = Si, x = T, K Fig. 3. Temperature dependencies of spontaneous magnetic moments of Fe 16 M (M = Ni, Cr, Si) compounds with different hydrogen content. As shown in Ref. [KAMARÁD J. et al., 2006], magnetocrystalline anisotropy of Fe 17 is only slightly affected by the substitution that does not change the crystal symmetry and the crystal electric field (CEF) interactions. The anisotropy field H A is almost identical in the Fe 17, Fe 16 Ni and Fe 16 Si intermetallics ( H A = 3.6 ± 0.25 T) (see Table 1 and Fig 2). Only for a case of Cr substitution, a considerable decrease of the anisotropy is observed, but it correlates well with a significant decrease of the saturation magnetization of Fe 16 Cr compound. 5

6 Simultaneous effect of substitution (Cr, Ni and Si) and interstitial (H) atoms on the temperature of magnetic ordering and type of magnetocrystalline anisotropy was studied on Fe 16 MH x single crystals with different hydrogen content. Hydrogenation didn t change the crystal structure of the samples under study (see Table 1 for crystallographic data). The lattice expansion upon hydrogenation was found to be anisotropic: it occurred rather in the basal plane of the investigated compounds. Due to the lack of possibility to control the hydrogen content during heating the samples, the Curie temperatures were extrapolated from the temperature dependencies of the spontaneous moment of Fe 16 MH x compounds. The Curie temperature was found to rise monotonously with hydrogen content increase. The values of the saturation magnetization of the compounds studied remained unchanged within the limits of experimental error (see Fig. 4) M, µ B /f.u Fe 16 CrH x x = 0, a-axis x = 0, c-axis x = 0.90, a-axis x = 0.90, c-axis x = 1.60, a-axis x = 1.60, c-axis 5 T = 5 K H i, T Fig. 4. Magnetization curves along the principal axes of Fe 17-x Cr x H y single crystals at 5 K. In order to make a conclusion which effect (volume or chemical) gives the main contribution to the change of Curie temperature, data on the influence of the applied hydrostatic pressure on T C are required. As it was mentioned before, since we have obtained hydrides of Fe 16 MH x without a destruction of the single-crystalline state, investigation of the influence of hydrogen on magnetocrystalline anisotropy of the compounds was possible to carry out. A slight decrease of the anisotropy field (see Fig. 2) upon hydrogenation was observed which agrees well with data on influence of hydrogen on magnetocrystalline anisotropy of the initial Fe 17 compound presented in Ref. [TERESHINA I.S. et al., 2001]. SUMMARY Drastic increase of Curie temperatures in Fe 17 compounds upon substitution (chromium, nickel and silicon atoms) and subsequent interstitial (hydrogen atoms) modifications was observed. It was shown that physical mechanisms responsible for the Curie temperature change have a quite complicated nature and depend on the unit cell volume, local magnetic moments on Fe atoms, interatomic distances between Fe atoms and the charge transfer from the interstitial and substitution atoms to the 3d-band of Fe. Moreover, in case of substitution, a local position of the substitution atom plays an important role. 6

7 Since the investigation was carried out on single crystalline samples, reliable data on the type of magnetic anisotropy and values of anisotropy fields in Fe 17 based compounds were obtained. AKNOWLEDGEMENTS The work is a part of the research project AVOZ and has been supported by grants GACR 202/06/0185 and GAUK / LITERATURE REFERENCES ANDREEV A.V., RAFAJA D., KAMARAD J., ARNOLD Z., HOMMA Y., SHIOKAWA Y., 2003, J. Alloys and Comp., v.361, p. 48. ANDREEV A.V., RAFAJA D., KAMARÁD J., ARNOLD Z., HOMMA Y., SHIOKAWA Y., 2004, Alloys and Comp., v. 383, p.40. ANDREEV A.V., RAFAJA D., KAMARÁD J., ARNOLD Z., HOMMA Y., SHIOKAWA Y., 2004, Physica B, v. 348, p BUSCHOW K.H.J., 1988, In: Handbook of Ferromagnetic Materials, v. 4, Amsterdam, North- Holland, p. 1. BUSCHOW K.H.J., 1997, In: Handbook of Magnetic Materials, v. 10, Amsterdam, North-Holland, p COEY J.M.D., 1992, Proc. Of the 6th Int. Conf. on Ferrites (ICF6), Kyoto, Japan, p FLORIO J.V., BAENZIGER N.C., RUNDLE R.E., 1956, Acta Cryst. v. 9, p FRANSE J.J.M., RADWANSKI R., 1993, In: Handbook of Magnetic Materials, Ed. K.H.J. Buschow, Elsevier, Amsterdam, v. 7, p GIVORD D., LEMAIRE R., MOREAU J.M., ROUDAUT E., 1972, J. Less-Common Met. v. 29, p HERBST J.F., 1991, Rev. Mod. Phys., v. 63, p HOFFER G., STRNAT K., 1966, IEEE Trans. on Magnetism, v. 2, 487. KAMARÁD J., ANDREEV A.V., MACHÁTOVÁ Z., ARNOLD Z., 2006, J. Alloys Comp., v , p KAMARÁD J., PROKHNENKO O., PROKEŠ K., ARNOLD Z., ANDREEV A.V., 2007, J. Magn. Magn. Materials, v. 310, p LI H.S., COEY J.M.D., 1991, In: Magnetic Materials, Ed. K.H.J. Buschow, Elsevier, Amsterdam, v. 6, p. 1. PROKHNENKO O., RITTER C., MEDVEDEVA I., ARNOLD Z., KAMARÁD J., KUCHIN A., 2003, J. Magn. Magn. Materials, v , p RODRIGUEZ-CARVAJAL J., 1993, Physica B, v. 192, p SHEN B.G., KONG L.S., WANG F.W., CAO L., 1993, Appl. Phys. Lett., v. 63, p SHEN B.G., CHENG Z.H., LIANG B., ZHANG J.X., GONG H.Y., WANG F.W., YAN Q.W., ZHAN W.S., 1995, Appl. Phys. Lett., v. 67, p STRNAT K., 1988, In: Handbook of Ferromagnetic Materials, vol. 4, ed. E.P. Wohlfarth and K.H.J. Buschow, Amsterdam: Elsevier, p. 131 TERESHINA I.S., NIKITIN S.A., STEPIEN-DAMM J., GULAY L.D., PANKRATOV N.Y., SALAMOVA A.A., VERBETSKY V.N., SUSKI W., 2001, J. Alloys Comp. v. 329, p. 31. TERESHINA I.S., NIKITIN S.A., LOUCHEV D.O., TERESHINA E.A., ANDREEV A.V., DRULIS H., 2006, J. Magn. Magn. Mater., v. 300, p. e497 e499. TERESHINA E.A., YOSHIDA H., ANDREEV A.V., TERESHINA I.S., KOYAMA K., KANOMATA T., 2006, High Pressure Research, v. 26, p