Magnetic properties of HoFe6Al6H hydride A single crystal study 2016 Journal of Science Advanced Materials and Devices

Journal of Science: Advanced Materials and Devices (2016) 152e157 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original article Magnetic properties of HoFe6Al6H hydride: A single-crystal study a  A.V Andreev a, *, I.A Pelevin b, c, J Sebek , E.A Tereshina a, D.I Gorbunov a, d, H Drulis e, c, f I.S Tereshina a Institute of Physics, Czech Academy of Sciences, 182 21 Prague, Czech Republic Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, 119991 Moscow, Russia c International Laboratory of High Magnetic Fields and Low Temperatures, 53-421 Wroclaw, Poland d Dresden High Magnetic Field Laboratory (HLD-EMFL), Helmholtz-Zentrum Dresden-Rossendorf, D-01314 Dresden, Germany e Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wroclaw, Poland f Lomonosov Moscow State University, Faculty of Physics, 119991 Moscow, Russia b a r t i c l e i n f o a b s t r a c t Article history: Received 19 April 2016 Received in revised form May 2016 Accepted 10 May 2016 Available online 13 May 2016 Crystal structure and magnetic properties were studied on a single crystal of HoFe6Al6H and compared with those of the parent HoFe6Al6 compound with a tetragonal crystal structure of the ThMn12 type Hydrogenation leads to a 1% volume expansion HoFe6Al6 is a ferrimagnet with exact compensation of the Ho and Fe sublattices magnetizations at low temperatures Both the hydride and the parent compound display a high magnetic anisotropy of the easy-plane type, a noticeable anisotropy exists also within the easy plane with the [110] axis as the easy magnetization direction The hydrogenation increases slightly (from 10 to 10.45 mB) the magnetic moment of the Fe sublattice as a result of volume expansion It leads to a decompensation of the Fe and Ho sublattices and HoFe6Al6H has a spontaneous moment 0.45 mB/f.u The enhancement of the FeeFe intra-sublattice exchange interaction results in a higher Curie temperature (TC) value, 350 K in the hydride as compared to 315 K of HoFe6Al6 The HoeFe inter-sublattice interaction is also enhanced in the hydride The molecular field Hmol created on Ho ions by Fe sublattice is 38 T in HoFe6Al6 and 48 T in HoFe6Al6H The inter-sublattice exchange constant nHoFe is 3.8 T/mB and 4.6 T/mB, respectively High-field measurements confirm the enhancement of the HoeFe exchange interaction in the hydride found from the temperature dependence of magnetization © 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Rare-earth intermetallics Hydrides Magnetic anisotropy Ferrimagnetism High magnetic fields Field-induced transition Introduction Intermetallic compounds of rare-earth metals R with 3d transition metals T (first of all, with T ¼ Fe and Co) form a wide class of magnetic materials They are very interesting not only due to their high application potential (first of all permanent-magnet materials based on the R2Fe14B compounds) but also from the scientific viewpoint since they combine two principally different groups of electrons responsible for the magnetism: localized magnetism of the 4f sublattice is combined with (mainly) itinerant magnetism of the 3d sublattice Investigations of complicated intra- and intersublattice interactions of exchange and anisotropic origins are strongly desirable for a deeper understanding of fundamental problems of magnetism The inter-sublattice interaction is * Corresponding author E-mail address: andreev@fzu.cz (A.V Andreev) Peer review under responsibility of Vietnam National University, Hanoi especially interesting since it “pulls” the low temperature anisotropic properties of the 4f-sublattice towards elevated temperatures, which is the base for applications For this reason, the rareearth intermetallics, especially those with a high content of a 3dmetal (in particular, with the RT5, R2T17, R2T14B and RT12 stoichiometries) have been extensively studied for the last several decades (see for review [1e3]) Many fed intermetallics can form so-called interstitial solutions where atoms with small atomic radii (H, C, N) not substitute “main” atoms but rather reside in voids between them Up to a rather high content of such doping the lattice expands without qualitative changes of the crystal structure [4,5] This strongly influences the magnetic properties and can be in a good approximation considered as an application of negative pressure It gives an additional tool for a study of exchange and anisotropic interactions Since most of intermetallic compounds are very brittle and their elastic limit does not exceed strain of 10À3, whereas lattice expansion as a result of hydrogenation can reach several percent, absorption of hydrogen often leads to powderization of http://dx.doi.org/10.1016/j.jsamd.2016.05.001 2468-2179/© 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) A.V Andreev et al / Journal of Science: Advanced Materials and Devices (2016) 152e157 ingots This important feature is used in the hydrogen decrepitation desorption recombination (HDDR) process for preparation of powders of NdFeB magnets, however, it hinders preparation of single crystals which are needed for investigation of magnetic anisotropy, a very important property of the ReT compounds Nevertheless, hydrides of some compounds can be prepared under special conditions in the single-crystalline form Their study gives an important information of the hydrogen influence not only on the magnitude of magnetic moments and magnetic ordering temperatures but also on magnetic anisotropy and field-induced phase transitions in R2Fe14B [6e9], R2Fe17 [10,11] and RT12 [12,13] The present work addresses the problem of the influence of hydrogen on the magnetism of HoFe6Al6 HoFe6Al6 belongs to the RT12 intermetallics having the tetragonal crystal structure of the ThMn12 type Binary compounds exist only with T ¼ Mn, nevertheless, the structure can be stabilized by a third element Compounds R(T,M)12 (M is a stabilizing element) form a wide group of materials (see Ref [14] for review) The Fe content x in the RFexM12Àx quasibinaries can vary in a wide interval Compounds of this group with a high Fe content, RFe11Ti, RFe10.5V1.5, RFe10Si2 and RFe10Mo2, are considered as promising materials for permanent magnets [15e19] Compounds with M ¼ Al, RFexAl12Àx, are characterized by competitive exchange and anisotropic interactions within their homogeneity range, x 6, and therefore deserve special attention from the fundamental point of view HoFe6Al6 is a collinear ferrimagnet with Curie temperature TC ¼ 315 K It displays a strong magnetic anisotropy of the easyplane type A large anisotropy is also observed in the easy plane, the [110] axis is the easy magnetization direction [20] The Fe sublattice, as found from the magnetization study of LuFe6Al6 with non-magnetic Lu, has a magnetic moment MFe ¼ 10 mB [21] Since the magnetic moment of single Ho3ỵ ion MHo is also 10 mB, the total spontaneous magnetic moment Ms is exactly zero at low temperatures due to antiparallel arrangement of the sublattices Due to a faster temperature decrease of MHo as compared to MFe, Ms grows in a wide temperature interval In the present work, we study the influence of hydrogenation on magnetism of HoFe6Al6 by magnetization measurements on single crystals 650  C The HoFe6Al6 sample was annealed in the hydrogen atmosphere for h, cooled slowly (48 h) down to room temperature and then left at these conditions The amount of absorbed hydrogen was determined from the hydrogen pressure change in the calibrated reactor chamber We realized that at the conditions used hydrogen absorption is rather slow After days the calculated hydrogen content y in HoFe6Al6Hy was only 0.56, whereas in other RFexM12ÀxHy compounds it reached the value y ¼ [12,13,22e24] So, the temperature annealing program was repeated and the sample was left in a hydrogen atmosphere for 18 days During this time, sample slowly absorbed hydrogen The final hydrogen concentration close to y ¼ 3.1 was determined However, we found that a large portion of hydrogen was soon released by the crystals The concentration of hydrogen remaining in the samples was determined to be about y ¼ (see below) The field and temperature dependences of the magnetization in fields up to T were measured between and 350 K for fields applied along the principal crystallographic directions [100], [110] and [001] of 20e30 mg samples of hydride and parent compound using a MPMS magnetometer (Quantum Design) High-field magnetization curves were measured along the principal axes at K in pulsed magnetic fields up to 58 T (pulse duration 20 m) The magnetization was measured by the induction method using a coaxial pick-up coil system A detailed description of the set-up is given in Ref [25] Absolute magnetization values of were calibrated using data obtained by measuring the crystals in static magnetic fields Results and discussion Owing to the slow hydrogenation procedure, the singlecrystalline structure was preserved in hydride (confirmed by back-scattered Laue patterns) Powder X-ray analysis (Fig 1) showed a single-phase state with the tetragonal ThMn12-type structure (Fig 2) The obtained lattice parameters are a ¼ 868.3 pm Experimental details A single crystal of parent compound HoFe6Al6 was grown by a modified Czochralski method in a tri-arc furnace from stoichiometric mixture of pure elements (99.9% Ho, 99.98% Fe and 99.999% Al) Back-scattered Laue patterns were used to confirm a monocrystalline state and to orient the crystals to cut samples for the magnetization measurements Standard powder X-ray diffraction analysis performed on a part of the single crystal crushed into powder was used to check the crystal structure and to determine the lattice parameters The analysis confirmed a single-phase state with the tetragonal ThMn12-type structure The lattice parameters, a ¼ 864.2 pm and c ¼ 503.9 pm, are in a good agreement with literature [14] The hydride was prepared by direct reaction of gaseous hydrogen with small (20e30 mg) pieces of single crystal of the parent compound HoFe6Al6 using a glass Sieverts-type apparatus The hydrogenation was carried out using pure hydrogen gas under pressures of up to 0.1 MPa obtained from TiH2 hydrogen storage In order to prevent the sample from powderization, hydrogenation was performed rather slowly A short (1 h) thermal activation process at 400  C and at high vacuum was carried out to initiate the hydrogen absorption process and was completed by 1-h cooling to room temperature After that, reaction chamber was filled with hydrogen gas under MPa pressure and the system was heated to 153 Fig X-ray powder diffraction patterns of HoFe6Al6 and HoFe6Al6H 154 A.V Andreev et al / Journal of Science: Advanced Materials and Devices (2016) 152e157 and c ¼ 504.2 pm It means that the volume expansion during hydrogenation is only 1.0%, the same as for other RFexM12ÀxHy systems with a higher Fe content (RFe11TiHy) where y ¼ [12,13,22e24] It is known that hydrogen atoms in RFe11TiHy occupy the 2b Wyskoff positions of the ThMn12 structure type (Fig 2) The number of such positions is per the 1e12 formula unit, i.e., y ¼ corresponds to the full occupation of these sites by hydrogen atoms Such complete occupation is realized in all known RFexM12exHy systems, no other sites for hydrogen are reported We came to the conclusion that the hydrogen content y ¼ 3.1 obtained during the hydrogenation procedure is unstable and after removing the sample from hydrogen atmosphere it decreased fast to a relatively stable y ¼ value Hydrogen release from RFexM12exHy hydrides with M ¼ Al was confirmed by an unsuccessful attempt to prepare a hydride of HoFe5Al7 Similarly to HoFe6Al6Hy, the final y value reached 3.1 but after removing the sample from hydrogen atmosphere both lattice parameters and magnetic properties (spontaneous magnetic moment Ms, Cutie temperature TC and compensation temperature) were found to be exactly the same as in the parent compound, so in this case hydrogen left the alloy completely Based on the volume expansion, we finally came to the conclusion that magnetization measurements were performed for the composition HoFe6Al6H Magnetization isotherms at K for the hydride and parent compounds are presented in Fig Whereas HoFe6Al6 exhibits no spontaneous magnetization along all the directions because the sublattice magnetizations compensation point is practically K (a noticeable Ms becomes observable only at 20e30 K [20]), the hydride has Ms ¼ 0.32 mB/f.u and 0.45 mB/f.u along the [100] and [110] axes, respectively Taking into account the tetragonal symmetry of the crystal structure, the spontaneous  moment ratio along the [100] and [110] axes, Ms100 =Ms110 zcos45 , reflects a good quality of the single crystal and its proper orientation in the magnetic field There is no projection of spontaneous Fig Crystal structure of HoFe6Al6H, view along the [001] axis Fig Magnetization curves of HoFe6Al6 and HoFe6Al6H single crystals measured along the principal axes at K magnetic moment onto the [001] axis which is the hard magnetization direction Thus, the magnetic moments of HoFe6Al6H lie in the basal plane of the tetragonal lattice The easy-plane magnetic anisotropy with the easiest [110] axes is the same as in the parent compound The magnetization along the [100] and [110] axes demonstrates large hysteresis with the coercive field of T The appearance of spontaneous moment in the hydride shows that hydrogenation distorted the fine magnetization balance of the sublattices in the parent compound by changing the magnetic moment of the Fe sublattice, MFe If MFe becomes larger, the total Ms should be along MFe and Ms(T) should increase with increasing temperature If MFe becomes smaller, the total Ms should be along the Ho magnetic moment, MHo In this case Ms(T) should pass through a compensation point The temperature evolution of the magnetization curves is shown in Fig The magnetization increases with the increasing temperature up to 240 K It is seen also in the temperature dependence of the magnetization in field of T (Fig 5) No compensation point is observed It means that Ms is along MFe in the whole temperature range of magnetic order including the ground state MFe increases from 10 mB in the parent compound to 10.45 mB in the hydride TC of the hydride determined from M(T) in a low field (0.02 T) applied along the easy [110] axis is found to be 350 K compared to 315 K for HoFe6Al6 Thus, the hydrogenation enhanced both the magnitude of the Fe magnetic moments and their exchange coupling Increase of TC from 315 K to 350 K, i.e, by 11%, is close to this (7e12%) in other isostructural compounds with the same H content, RFe11TiH (R ¼ Sm, Er, Ho, Lu) [12,22,24] The M(T) curve along the hard [001] axis displays an anomaly at 330 K in the hydride and at 300 K in the parent compound (Fig 5) where the decreasing anisotropy field passes through the value close to that of an applied magnetic field of T as the compound approaches TC As seen in Fig 4, pronounced anisotropy within the basal plane of the HoFe6Al6H single crystal vanishes above 240 K In A.V Andreev et al / Journal of Science: Advanced Materials and Devices (2016) 152e157 155 describe, in particular, the in-plane anisotropy, originate only from the R sublattice Therefore, the increase of the temperature interval where the in-plane anisotropy is observed in the hydride indicates that not only the FeeFe exchange interaction, but also the HoeFe intersublattice interaction becomes stronger in the hydride This interaction is responsible for pulling R magnetism to the higher temperatures In order to determine the strength of the HoeFe inter-sublattice exchange interactions, the paramagnetic Ho3ỵ ion is considered to be in the molecular eld, Hmol, created by the Fe sublattice It is reasonable to assume that the main source of exchange in HoFe6Al6H is the FeeFe exchange; the HoeFe and HoeHo interactions being much weaker Using this approximation, the temperature dependence of the Ho magnetic moment is a function of Hmol is  MHo Tị ẳ MHo 0ịBJ  gJ J mB m0 Hmol ; kB T (1)  factor, J is a where BJ is the Brillouin function, gJ is the Lande quantum number of the total moment of Ho3ỵ ion and kB is the Boltzmann constant Since the molecular field originates from the Fe sublattice, it depends on its magnetic moment as Hmol Tị ẳ Hmol 0ị Fig Magnetization curves of a HoFe6Al6H single crystal measured along the [100] and [110] axes at elevated temperatures HoFe6Al6 it occurs at a lower temperature (200 K) [20] The T-metal sublattice in ReT intermetallics can provide a noticeable contribution to the magnetic anisotropy limited only by the first anisotropy constant K1 Higher-order anisotropy constants which MFe ðTÞ : MFe ð0Þ We approximated the MFe(T) dependence for HoFe6Al6H using data of the isostructural compound LuFe6Al6 where Lu carries no ordered magnetic moment For LuFe6Al6, MFe ¼ 10 mB/f.u at K and TC ¼ 325 K [21] We normalized these values to MFe ¼ 10.45 mB/f.u and TC ¼ 350 K for HoFe6Al6H The resulting temperature dependences of the Fe and Ho magnetic moments for HoFe6Al6H are given in Fig The best fit of MHo(T) using Eq (1) was obtained for m0Hmol ¼ 48 T (at K) The inter-sublattice exchange constant, nHoFe, can be calculated from the relation m0 Hmol ¼ nHoFe MFe : Fig Temperature dependence of magnetization of HoFe6Al6 and HoFe6Al6H single crystals measured along the principal axes in a field of T The low-field M(T) data along the [110] axis in the vicinity of TC are shown as well (2) (3) Fig Temperature dependence of the spontaneous magnetic moment Ms and sublattice moments MFe and MHo for HoFe6Al6H For comparison, the temperature dependence of MHo in the parent compound HoFe6Al6 is also shown The dashed lines are fits of the MHo(T) dependence according to Eq (1) 156 A.V Andreev et al / Journal of Science: Advanced Materials and Devices (2016) 152e157 We obtained nHoFe ¼ 4.6 T/mB A comparison with m0Hmol ¼ 38 T and nHoFe ¼ 3.8 T/mB in the parent compound is indicative of a substantial enhancement of the HoeFe exchange interactions in HoFe6Al6 as a result of hydrogenation The effective ReT interaction parameter ART discussed systematically in review [26] is equal to 8.8*10À23 J in the parent compound and 10.7*10À23 J in the hydride for number of Fe atoms around the Ho ion ZRT as 12 (8 in the 8f positions and 4, i.e, half, in 8j positions, shared by Fe and Al, whereas the 8i positions are occupied purely by Al, see Fig 2) It corresponds well to ART calculated in Ref [26] as 12.4*10À23 J in HoFe11Ti and 12.6*10À23 J in HoFe10V2 (i.e., compounds isostructural with HoFe6Al6), because ZRT is larger for compounds with higher Fe content, up to 20 in hypothetic RFe12 Both HoFe6Al6 and its hydride are strongly anisotropic ferrimagnets and, therefore, field-induced magnetic phase transitions can be expected as the magnetic moments rotate from the initial ferrimagnetic structure through a canted phase towards the final forced ferromagnetic state Fig demonstrates high-field magnetization curves at K In HoFe6Al6, field-induced transitions are observed along the basal plane directions: one along the easy [110] axis (at 44e51 T) and one along the [100] axis (49e55 T), the hard axis in the basal plane Both magnetization jumps display a wide hysteresis and thus the transitions are of the first order In HoFe6Al6H, the magnetization curve along the [100] axis exhibits a transition at 13e16 T This transition corresponds to a rotation of the total magnetic moment in a relatively low field It is not observed at low temperatures in the parent compound because of the zero total moment It becomes observable at elevated temperatures (in 12 T at 40 K) [20] This transition is seen in the hydride at K due to the non-zero Ms As regards to the high-field transition, it is absent in HoFe6Al6H because it moves to the field interval above available 58 T due to enhancement of the HoeFe exchange interactions A shift of the high-field transition to a higher field is evidently observed along the [110] axis The transition is seen there, however, only partly It is not completed at 58 T, for this reason the magnetization gain is lower than in the parent compound at the same transition In HoFe6Al6, the magnetization reaches 16 mB/f.u in the highest available field applied along the both basal-plane axes This is lower than the forced collinear ferromagnetic alignment of the Ho and Fe magnetic moments, Mferro ẳ MHo ỵ MFe ẳ 20 mB/f.u In the hydride, where Mferro ¼ 20.45 mB/f.u., the magnetization in 58 T is lower (10.5e12.5 mB/f.u.) This also points to a stronger HoeFe exchange interaction The field dependence of the magnetization along the hard [001] axis does not exhibit any magnetic transitions The magnetization grows gradually as the magnetic moments rotate towards the applied field and reaches a value of 14 mB/f.u in HoFe6Al6 and again somewhat lower (13 mB/f.u.) in HoFe6Al6H at the highest applied field Therefore, high-field measurements confirm the enhancement of the HoeFe exchange interaction in the hydride compared to the parent compound obtained from the temperature dependence of MHo Conclusion A magnetization study performed on single crystals of the intermetallic compound HoFe6Al6 and its hydride HoFe6Al6H showed that hydrogenation increases (from 10 to 10.45 mB/f.u.) the magnetic moment of the Fe sublattice as a result of a 1% volume expansion It leads to a decompensation of the antiparallel-coupled Fe and Ho sublattices and the hydride HoFe6Al6H has a spontaneous moment 0.45 mB/f.u compared to zero in the parent compound Similar to HoFe6Al6, HoFe6Al6H exhibits the easy-plane type of magnetic anisotropy, a noticeable anisotropy exists also within the easy plane with the [110] axis as the easy magnetization direction An enhancement of the FeeFe intra-sublattice exchange interaction results in a higher TC value in the hydride compared to the parent compound (350 K and 315 K, respectively) The HoeFe intersublattice interaction is also enhanced in the hydride as found from the temperature dependence of the Ho magnetic moment The molecular field Hmol created on Ho ions by the Fe sublattice is 38 T (inter-sublattice exchange constant nHoFe ¼ 3.8 T/mB) in HoFe6Al6 and 48 T in HoFe6Al6H (nHoFe ¼ 4.6 T/mB) Measurements in pulsed magnetic fields confirm the enhancement of the HoeFe exchange interaction in the hydride Acknowledgements The paper is dedicated to the memory of Professor Peter Brommer The work was partly performed in MLTL (http://mltl.eu/), which is supported within the program of Czech Research Infrastructures (Project No LM2011025) It was supported by Czech Science Foundation (grants P16-03593S) We acknowledge the support of the High Magnetic Field Laboratory Dresden (HLD) at HZDR, member of the European Magnetic Field Laboratory (EMFL) The work was also supported by the Russian Foundation for Basic Research (grant 15-02-08509) References Fig Magnetization curves of HoFe6Al6 and HoFe6Al6H single crystals measured along the principal axes at K in pulsed magnetic fields [1] J.J.M Franse, R Radwanski, Magnetic properties of binary rare-earth 3dtransition-metal intermetallic compounds, in: K.H.J Buschow (Ed.), Handbook of Magnetic Materials, vol 7, Elsevier, Amsterdam, 1993, pp 307e501 [2] A.V Andreev, Thermal expansion anomalies and spontaneous magnetostriction in ReT intermetallics (T¼Co and Fe), in: K.H.J Buschow (Ed.), Handbook of Magnetic Materials, vol 8, Elsevier, Amsterdam, 1995, pp 59e187 A.V Andreev et al / Journal of Science: Advanced Materials and 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Fig X-ray powder diffraction patterns of HoFe6Al6 and HoFe6Al6H 154 A. V Andreev et al / Journal of Science: Advanced Materials and Devices (2016) 152e157 and c ¼ 504.2 pm It means that the volume... projection of spontaneous Fig Crystal structure of HoFe6Al6H, view along the [001] axis Fig Magnetization curves of HoFe6Al6 and HoFe6Al6H single crystals measured along the principal axes at K magnetic. .. of the HoFe6Al6H single crystal vanishes above 240 K In A. V Andreev et al / Journal of Science: Advanced Materials and Devices (2016) 152e157 155 describe, in particular, the in-plane anisotropy,