Influence of hydrogenation on magnetic interactions in intermetallic RNi (R = Gd, Tb, Dy) compounds

Article abstract of DOI:10.1016/j.jallcom.2010.10.140

The influence of hydrogenation on the magnetic properties of intermetallic compound RNi is reported. GdNiH_3.2, TbNiH_3.4, DyNiH _3.4 hydrides have been synthesized and their magnetic properties were investigated in the tempe

Full text of DOI:10.1016/j.jallcom.2010.10.140

Journal of Alloys and Compounds 509S (2011) S827–S829
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Journal of Alloys and Compounds
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Influence of hydrogenation on magnetic interactions in intermetallic RNi
(R = Gd, Tb, Dy) compounds
W. Iwasieczkoa, H. Drulisa, Yu. L. Yaropolovb,∗, S.A. Nikitinb, V.N. Verbetskyb
a Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wroclaw, Poland
b Lomonosov Moscow State University, GSP-3, Leninskije Gory, Moscow, 119992, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 July 2010
Received in revised form 11 October 2010
Accepted 26 October 2010
Available online 2 November 2010
The influence of hydrogenation on the magnetic properties of intermetallic compound RNi is reported.
GdNiH3.2, TbNiH3.4, DyNiH3.4 hydrides have been synthesized and their magnetic properties were inves-
tigated in the temperature range 1.8–300 K. It was established that hydrogen absorption in these
compounds makes the ferro- and antiferromagnetic interactions between magnetic moments much
weaker. The reasons of this phenomenon are discussed. The hydrogen presence at interstitial positions
also changes the symmetry of the crystal electric field at the rare earth ion sites making this symmetry
higher than those in initial intermetallics.
Keywords:
Rare earth compounds
Metal hydrides
© 2010 Elsevier B.V. All rights reserved.
Magnetic measurements
1. Introduction
effect at magnetic phase transition temperatures [6,7]. The field
dependence of the magnetization of some RNi compounds shows
The magnetic transition temperatures of many rare earth inter-
metallic compounds and alloys are very sensitive on hydrogen
presence in the crystal lattice. Hydrogenation can change the elec-
tronic structure of the compound as well. Good examples are the
rare earth R2Fe17, RFe11Ti compounds – with high content of iron
[1,2]. The apparent Curie transition temperature enhancement due
to the large increase of unit cell volume after hydrogenation is
observed. The enhancement of magnetic interaction can partly be
caused by the changes of the 3d electronic density of state (DOS) at
the Fermi level. In the hydrogen protonic (H+) model, the conduc-
tion band can be filled in by an additional number of the conduction
electrons coming from hydrogen atoms or might be depopulated
in an anionic (H−) hydrogen ion version.
Magnetic properties of the RNi compounds have already been
studied relatively in detail [3–7]. RNi compounds with R = Pr, Nd,
Sm, Gd, Tb, Dy, Ho, Er, Tm are ferromagnets with transition tem-
peratures TC in the region of the liquid nitrogen temperature. On
the other hand, the several compounds of series YNi, LaNi, CeNi,
YbNi and LuNi are Pauli paramagnets. This indicates that the nickel
atom is a nonmagnetic ion and that all magnetism is coming from
the rare earth atoms when RE is the magnetic element. Some of the
magnetically active compounds are believed to be important for
the magnetocaloric cooling or heating effects – for instance, GdNi,
DyNi, HoNi and ErNi which have shown a high magnetocaloric
an additional metamagnetic transition at temperatures lower than
TC [5–7]. The strong anisotropy of the magnetic properties is also
observed in these compounds.
The magnetic moments of the rare earth elements (except
Gd) are ordered non-collinearly, and apart from a ferromagnetic
component, there is also an antiferromagnetic component which
has a direction different to the ferromagnetic one [4,8,9]. This
type of magnetic structure appears as a result of competition
between exchange magnetic interactions and the magnetocrys-
talline anisotropy caused by the energy levels of the rare earth ions
split due to the crystal electric field interaction. The presence of
an antiferromagnetic component may cause the spin-reorientation
transitions in the external magnetic field oriented along some cho-
sen direction. This can be visible as characteristic steps on the
magnetization curve versus the magnetic field.
The magnetic properties of the RNi (R = Gd, Tb and Dy) hydride
phases have not been widely reported up to now. The prelim-
inary magnetic measurements of the RNi (R = Gd, Tb and Dy)
hydride phases have been reported in Ref. [10]. In [9] the temper-
ature dependence of magnetization of the CeNi compound and its
hydride phase were studied. The CeNiH2.9·hydride phase is a para-
magnetic material with paramagnetic Curie temperature T = −19 K.
The magnetic moment calculated per Ce atom ꢀeff = 2.46ꢀB is
very close to the magnetic moment of the free Ce3+ ion equal of
ꢀeff = 2.54ꢀB.
The aim of this report is to study the magnetic properties of the
RNi (R = Gd, Tb and Dy) hydride phases in the temperature range
1.8–300 K much wider than it was done in [10].
∗
Corresponding author. Tel.: +7 495 939 1413; fax: +7 495 932 8846.
E-mail address: yaropolov@inbox.ru (Yu.L. Yaropolov).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.10.140

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W. Iwasieczko et al. / Journal of Alloys and Compounds 509S (2011) S827–S829
Fig. 2. Temperature dependence of magnetization for TbNiH3.4 hydride and TbNi
compound. In the inset the field dependence of magnetization at 1.8 K is shown.
Symbol’s description as in Fig. 1.
Fig. 1. Temperature dependence of magnetization for GdNiH3.2 hydride and GdNi
compound for FC (open symbols) and ZFC (closed symbols) procedures. The inset
shows the field dependence of magnetization at 1.8 K.
and as reported in [10] it appears that the nickel atoms have a
non-magnetic configuration 3d10 in both the hydride phase and
in initial 1:1 compounds. The whole magnetism of the hydride
and respective alloys is determined by the rare earth ion magnetic
moments. However, it is interesting to point out that the ordering
temperatures of the hydrides are lower than those in corresponding
RNi compounds. These lower Curie temperature values reflect the
weakening of the ferromagnetic R–R exchange interactions after
RNi hydrogenation.
The temperature dependence of magnetization of GdNiH3.2
hydrides shown in Fig. 1 indicates that it behaves like a ferromag-
net similarly to its parent compound. However, the plots in the
insets of Fig. 1 show that the magnetization of GdNiH3.2 at 1.8 K
does not approach the saturation state even at the highest field
applied, whereas GdNi reaches the magnetic saturation already in
the field ꢀoH = 1.5 T. It is worth to stress that there is no the differ-
ences between the FC and ZFC magnetization in both gadolinium
compounds.
2. Experimental details
The rare earth nickel intermetallic compounds were prepared by arc-melting
stoichiometric amounts of the constituent elements. Nickel (purity of 99.99%) and
rare earth metals (99.9%) were used. For good homogeneity, the ingots were turned
and re-melted several times. The hydrogenation of intermetallic samples was car-
ried out using a Sieverts-type apparatus. The composition of the hydride samples
were determined by a volumetric method based on the comparison of the gas pres-
sure before and after hydrogen absorption. The hydrogen reacts with GdNi, TbNi,
and DyNi compound relatively quickly. The GdNiH3.2, TbNiH3.4, DyNiH3.4 hydrides
have been obtained at room temperature under hydrogen pressure lower than
0.1 MPa. The crystal structure and phase purity of the sample were analyzed by
the powder X-ray diffraction in [10]. This analysis showed that the GdNi crystal-
lizes in the orthorhombic CrB-type structure (Cmcm space group), DyNi crystallizes
in the orthorhombic FeB-structure (Pnma space group), whereas TbNi possess his
own structure type (P21/m space group). All the hydride samples crystallize in the
CrB-type structure. An increase of the crystal unit-cell volume of about 24–25% on
average was found for all hydrides under study.
The magnetization measurements in the temperature range of 1.8–300 K were
carried out in zero field (ZFC) and field cooling (FC) procedures in a field of
ꢀoH = 0.1 Tesla (T) with the SQUID Quantum Design equipment. The magnetiza-
tion isotherm measurements were performed in the field range 0–5 T at the lowest
temperature T = 1.8 K
The temperature dependence of magnetization for TbNiH3.2 and
TbNi alloy in Fig. 2 shows that there is an essential difference
between ZFC and FC measurements below 60 K. This is likely con-
nected with the presence of large magnetocrystalline anisotropy,
particularly in the TbNi sample. The magnetization isotherm in
TbNiH3.4 hydrides in Fig. 2 approaches saturation at a field of 5 T,
like its initial compound, but the magnetization value is of 50% less.
3. Results and discussion
The results of magnetization measurements as a function of
temperature for GdNiH3.2, TbNiH3.4, DyNiH3.4 hydrides, and their
initial GdNi, TbNi and DyNi compounds for FC and ZFC procedures
are presented in Figs. 1–3, respectively. In the insets, the respec-
tive magnetization measurements made at 1.8 K in the magnetic
fields up to ꢀoH = 5 T are shown. The Curie temperatures of the
ferromagnetic transition of parent RNi alloys and their hydride
were estimated from the ꢁ(T) curves taken for FC version of mea-
surements. The spontaneous magnetic moments were estimated
from magnetization ꢁ(H) isotherms at 1.8 K. The results of the
magnetic investigations are collected in Table 1. From this data,
Table 1
Magnetic data for GdNiH3.2, TbNiH3.4, DyNiH3.4 hydrides and they corresponding
RNi compounds.
Compound
TC (K)
Spontaneous
moment at 1.8 K
DyNi
DyNiH3.4
TbNi
TbNiH3.4
GdNi
GdNiH3.2
59
3.5
62
3.5
75
10
4.40
4.60
6.32
3.77
6.20
–
Fig. 3. Magnetization measurements as a function of temperature and magnetic
field for DyNiH3.4 hydride and DyNi alloy. In the inset the details of magnetization
for the lowest magnetic fields are displayed. Symbols description as in Fig. 1.

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S829
It is important to note that there is no evidence for the metam-
agnetic or spin reorientation transitions in the TbNiH3.4 hydride
phase, which were observed in TbNi alloy at about 0.5 T and 2.5–3 T
magnetic fields. It is obvious that the fields of ꢀoH > 3.0 T destroy the
subtle magnetic structure of Tb moments in TbNi alloy. The antifer-
omagnetic component of this structure, which exists at zero field,
disappears and a full collinear ferromagnetic ordering is estab-
lished.
ing this symmetry higher. Unfortunately, hydrogen ions occupy
interstitial sites rather statistically and the symmetry of electric
field potentials is somehow distributed more than those in the
initial compounds. In this case, the role of magnetocrystalline
anisotropy in the magnetic ordering processes in TbNi and DyNi
hydride samples is diminished or even canceled. This results in
the lack of a tendency for the metamagnetic transitions in the
hydride phases to take place.
The temperature dependence of the magnetization for DyNi and
DyNiH3.4 hydrides in Fig. 3 are similar to the dependencies in the
terbium compounds. However, from the dependence of magneti-
zation on the magnetic field shown in the inset of Fig. 3 appears
that metamagnetic or spin re-orientation transitions in DyNi alloy
takes place at very low magnetic fields contrary to those observed
in TbNi compound.
4. Upon analyzing the different magnetization characteristics for
GdNi in comparison to those in TbNi and DyNi, it is evidently
seen that the non-collinear magnetic structure in the two lat-
ter compounds arise from a competition between the exchange
interaction and the magnetocrystalline anisotropy. Gd3+ ion has
an angular orbital moment L = 0, therefore there should not be
any magnetocrystalline anisotropy and indeed GdNi appears as
a pure ferromagnetic compound. On the other hand, the field
dependence of magnetization for GdNiH3.2 at 1.8 K in Fig. 1
suggests that the hydride behaves rather as a ferrimagnetic
material.
4. Conclusions
From our investigations presented in this report, we have come
to the following conclusions:
References
1. Introduction of hydrogen atoms to the RNi compounds increases
the crystal unit cell volume and changes its structure type in
the case of TbNi and DyNi. However, this fact does not induce
the appearance of magnetic moment on the nickel ions in the
hydride phase as one can expect.
2. The hydrogen atoms seems to be electron donors (the vice-
versa situation should not be excluded). Nevertheless, the filling
or depopulation of the conduction band of RNi with electrons
diminishes the magnitude of electron density of state (DOS) at
the Fermi energy level. In such a way, according to the RKKY cou-
pling model, the exchange interaction between the rare earth
4f magnetic moments is reduced and the magnetic transition
temperatures are dramatically lowered in the hydride phases.
3. The hydrogen presence at interstitial positions changes also the
symmetry of crystal electric field at the rare earth ion sites mak-
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