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Journal of Science: Advanced Materials and Devices 1 (2016) 152e157
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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
9 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
1. 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: [email protected] (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) do 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 103, 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 1 (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
RFexM12x 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, RFexAl12x, are characterized by competitive exchange and anisotropic interactions
within their homogeneity range, 4 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 Ho3þ 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 3 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 3 days the calculated
hydrogen content y in HoFe6Al6Hy was only 0.56, whereas in other
RFexM12xHy compounds it reached the value y ¼ 1 [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 1 (see below).
The field and temperature dependences of the magnetization in
fields up to 7 T were measured between 2 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 2 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.
3. 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
2. 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 1 MPa pressure and the system was heated to
153
Fig. 1. X-ray powder diffraction patterns of HoFe6Al6 and HoFe6Al6H.
154
A.V. Andreev et al. / Journal of Science: Advanced Materials and Devices 1 (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 RFexM12xHy
systems with a higher Fe content (RFe11TiHy) where y ¼ 1
[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 1 per the 1e12 formula unit, i.e., y ¼ 1
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 1 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 2 K for the
hydride and parent compounds are presented in Fig. 3. Whereas
HoFe6Al6 exhibits no spontaneous magnetization along all the directions because the sublattice magnetizations compensation point
is practically 2 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. 2. Crystal structure of HoFe6Al6H, view along the [001] axis.
Fig. 3. Magnetization curves of HoFe6Al6 and HoFe6Al6H single crystals measured
along the principal axes at 2 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 2 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. 4. 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 1 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 1 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 1 (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 Ho3þ ion is considered to
be in the molecular field, 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 ðTÞ ¼ MHo ð0ÞBJ
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 Ho3þ ion and kB is the
Boltzmann constant. Since the molecular field originates from the
Fe sublattice, it depends on its magnetic moment as
Hmol ðTÞ ¼ Hmol ð0Þ
Fig. 4. 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 2 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. 6. The best fit of MHo(T) using Eq. (1) was obtained for
m0Hmol ¼ 48 T (at 2 K). The inter-sublattice exchange constant,
nHoFe, can be calculated from the relation
m0 Hmol ¼ nHoFe MFe :
Fig. 5. Temperature dependence of magnetization of HoFe6Al6 and HoFe6Al6H single
crystals measured along the principal axes in a field of 1 T. The low-field M(T) data
along the [110] axis in the vicinity of TC are shown as well.
(2)
(3)
Fig. 6. 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 1 (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*1023 J in the parent compound and 10.7*1023 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*1023 J in
HoFe11Ti and 12.6*1023 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. 7 demonstrates high-field magnetization curves at 2 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
2 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 ¼ MHo þ MFe ¼ 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.
4. 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).
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Fig. 7. Magnetization curves of HoFe6Al6 and HoFe6Al6H single crystals measured
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