Tereshinac,f a Institute of Physics, Czech Academy of Sciences, 182 21 Prague, Czech Republic b Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, 119991
Trang 1Original article
A.V Andreeva,*, I.A Pelevinb,c, J Sebeka, E.A Tereshinaa, D.I Gorbunova,d, H Drulise,
I.S Tereshinac,f
a Institute of Physics, Czech Academy of Sciences, 182 21 Prague, Czech Republic
b 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
a r t i c l e i n f o
Article history:
Received 19 April 2016
Received in revised form
9 May 2016
Accepted 10 May 2016
Available online 13 May 2016
Keywords:
Rare-earth intermetallics
Hydrides
Magnetic anisotropy
Ferrimagnetism
High magnetic fields
Field-induced transition
a b s t r a c t
Crystal structure and magnetic properties were studied on a single crystal of HoFe6Al6H and compared with those of the parent HoFe6Al6compound with a tetragonal crystal structure of the ThMn12type Hydrogenation leads to a 1% volume expansion HoFe6Al6is a ferrimagnet with exact compensation of the Ho and Fe sublattices magnetizations at low temperatures Both the hydride and the parent com-pound 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 in-creases slightly (from 10 to 10.45mB) 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 0.45mB/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 molecularfield Hmolcreated on Ho ions
by Fe sublattice is 38 T in HoFe6Al6and 48 T in HoFe6Al6H The inter-sublattice exchange constant nHoFeis 3.8 T/mBand 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/)
1 Introduction
Intermetallic compounds of rare-earth metals R with 3d
tran-sition 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
inter-sublattice interactions of exchange and anisotropic origins are
strongly desirable for a deeper understanding of fundamental
problems of magnetism The inter-sublattice interaction is
especially interesting since it“pulls” the low temperature aniso-tropic properties of the 4f-sublattice towards elevated tempera-tures, which is the base for applications For this reason, the rare-earth intermetallics, especially those with a high content of a 3d-metal (in particular, with the RT5, R2T17, R2T14B and RT12 stoichi-ometries) 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 in-fluences the magnetic properties and can be in a good approxi-mation considered as an application of negative pressure It gives
an additional tool for a study of exchange and anisotropic in-teractions Since most of intermetallic compounds are very brittle and their elastic limit does not exceed strain of 103, whereas lat-tice expansion as a result of hydrogenation can reach several percent, absorption of hydrogen often leads to powderization of
* Corresponding author.
E-mail address: andreev@fzu.cz (A.V Andreev).
Peer review under responsibility of Vietnam National University, Hanoi.
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j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
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
Journal of Science: Advanced Materials and Devices 1 (2016) 152e157
Trang 2ingots 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
temper-atures but also on magnetic anisotropy andfield-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 HoFe6Al6belongs to the
RT12 intermetallics having the tetragonal crystal structure of the
ThMn12type Binary compounds exist only with T¼ Mn,
never-theless, the structure can be stabilized by a third element
Com-pounds 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
RFexM12 xquasibinaries can vary in a wide interval Compounds of
this group with a high Fe content, RFe11Ti, RFe10.5V1.5, RFe10Si2and
RFe10Mo2, are considered as promising materials for permanent
magnets[15e19] Compounds with M¼ Al, RFexAl12 x, are
char-acterized 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
TC¼ 315 K It displays a strong magnetic anisotropy of the
easy-plane type A large anisotropy is also observed in the easy easy-plane,
the [110] axis is the easy magnetization direction [20] The Fe
sublattice, as found from the magnetization study of LuFe6Al6with
non-magnetic Lu, has a magnetic moment MFe¼ 10mB[21] Since
the magnetic moment of single Ho3þion MHois also 10mB, the total
spontaneous magnetic moment Msis exactly zero at low
temper-atures due to antiparallel arrangement of the sublattices Due to a
faster temperature decrease of MHoas compared to MFe, Msgrows
in a wide temperature interval
In the present work, we study the influence of hydrogenation on
magnetism of HoFe6Al6by magnetization measurements on single
crystals
2 Experimental details
A single crystal of parent compound HoFe6Al6was grown by a
modified Czochralski method in a tri-arc furnace from
stoichio-metric mixture of pure elements (99.9% Ho, 99.98% Fe and 99.999%
Al) Back-scattered Laue patterns were used to confirm a
mono-crystalline 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,
a¼ 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 HoFe6Al6using a glass Sieverts-type apparatus
The hydrogenation was carried out using pure hydrogen gas under
pressures of up to 0.1 MPa obtained from TiH2hydrogen storage In
order to prevent the sample from powderization, hydrogenation
was performed rather slowly A short (1 h) thermal activation
process at 400C 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
650 C The HoFe6Al6 sample was annealed in the hydrogen at-mosphere 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 cali-brated reactor chamber We realized that at the conditions used hydrogen absorption is rather slow After 3 days the calculated hydrogen content y in HoFe6Al6Hywas only 0.56, whereas in other RFexM12xHycompounds it reached the value y¼ 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 Thefinal hydrogen con-centration close to y¼ 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 deter-mined to be about y¼ 1 (see below)
Thefield 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 magneticfields
3 Results and discussion Owing to the slow hydrogenation procedure, the single-crystalline 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 a¼ 868.3 pm
Trang 3and 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 RFe11TiHyoccupy
the 2b Wyskoff positions of the ThMn12structure 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 y¼ 3.1 obtained during the
hydrogenation procedure is unstable and after removing the
sam-ple from hydrogen atmosphere it decreased fast to a relatively
stable y¼ 1 value Hydrogen release from RFexM12 exHyhydrides
with M¼ 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
atmo-sphere both lattice parameters and magnetic properties
(sponta-neous 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 inFig 3 Whereas
HoFe6Al6exhibits no spontaneous magnetization along all the
di-rections because the sublattice magnetizations compensation point
is practically 2 K (a noticeable Ms becomes observable only at
20e30 K [20]), the hydride has Ms¼ 0.32mB/f.u and 0.45mB/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, M100
s =M110
s zcos45
,
reflects a good quality of the single crystal and its proper
orienta-tion in the magneticfield There is no projection of spontaneous
magnetic moment onto the [001] axis which is the hard magneti-zation 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 coercivefield 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 MFebecomes larger, the total Ms
should be along MFe and Ms(T) should increase with increasing temperature If MFebecomes smaller, the total Msshould 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 inFig 4 The magnetization in-creases with the increasing temperature up to 240 K It is seen also
in the temperature dependence of the magnetization infield of 1 T (Fig 5) No compensation point is observed It means that Msis along MFe in the whole temperature range of magnetic order including the ground state MFeincreases from 10mBin the parent compound to 10.45mBin the hydride TCof the hydride determined from M(T) in a lowfield (0.02 T) applied along the easy [110] axis is found to be 350 K compared to 315 K for HoFe6Al6 Thus, the hy-drogenation enhanced both the magnitude of the Fe magnetic moments and their exchange coupling Increase of TCfrom 315 K to
350 K, i.e, by 11%, is close to this (7e12%) in other isostructural compounds with the same H content, RFe11TiH (R¼ 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 magneticfield of 1 T as the compound approaches TC As seen inFig 4, pronounced anisotropy within the basal plane of the HoFeAlH single crystal vanishes above 240 K In
Fig 2 Crystal structure of HoFe Al H, view along the [001] axis.
Fig 3 Magnetization curves of HoFe 6 Al 6 and HoFe 6 Al 6 H single crystals measured along the principal axes at 2 K.
A.V Andreev et al / Journal of Science: Advanced Materials and Devices 1 (2016) 152e157 154
Trang 4HoFe6Al6it occurs at a lower temperature (200 K)[20] The T-metal
sublattice in ReT intermetallics can provide a noticeable
contri-bution to the magnetic anisotropy limited only by thefirst
anisot-ropy constant K1 Higher-order anisotropy constants which
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 Ho3þion is considered to
be in the molecularfield, 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 in-teractions being much weaker Using this approximation, the temperature dependence of the Ho magnetic moment is a function
of Hmolis
MHoðTÞ ¼ MHoð0ÞBJ
g
JJmBm0Hmol
kBT
where BJis the Brillouin function, gJ is the Lande factor, J is a quantum number of the total moment of Ho3þion and kBis the Boltzmann constant Since the molecularfield originates from the
Fe sublattice, it depends on its magnetic moment as
HmolðTÞ ¼ Hmolð0ÞMFeðTÞ
We approximated the MFe(T) dependence for HoFe6Al6H using data of the isostructural compound LuFe6Al6where Lu carries no ordered magnetic moment For LuFe6Al6, MFe¼ 10mB/f.u at 2 K and
TC¼ 325 K[21] We normalized these values to MFe¼ 10.45mB/f.u and TC ¼ 350 K for HoFe6Al6H The resulting temperature de-pendences of the Fe and Ho magnetic moments for HoFe6Al6H are given inFig 6 The bestfit 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
Fig 4 Magnetization curves of a HoFe 6 Al 6 H single crystal measured along the [100]
and [110] axes at elevated temperatures.
Fig 5 Temperature dependence of magnetization of HoFe 6 Al 6 and HoFe 6 Al 6 H single
crystals measured along the principal axes in a field of 1 T The low-field M(T) data
Fig 6 Temperature dependence of the spontaneous magnetic moment M s and sub-lattice moments M Fe and M Ho for HoFe 6 Al 6 H For comparison, the temperature dependence of M Ho in the parent compound HoFe 6 Al 6 is also shown The dashed lines fits of the M
Trang 5We obtained nHoFe¼ 4.6 T/mB A comparison withm0Hmol¼ 38 T
and nHoFe¼ 3.8 T/mBin the parent compound is indicative of a
substantial enhancement of the HoeFe exchange interactions in
HoFe6Al6as a result of hydrogenation The effective ReT interaction
parameter ARTdiscussed systematically in review[26]is equal to
8.8*1023J in the parent compound and 10.7*1023J in the hydride
for number of Fe atoms around the Ho ion ZRTas 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, seeFig 2) It
corresponds well to ARTcalculated in Ref.[26]as 12.4*1023J in
HoFe11Ti and 12.6*1023 J in HoFe10V2 (i.e., compounds
iso-structural with HoFe6Al6), because ZRTis larger for compounds with
higher Fe content, up to 20 in hypothetic RFe12
Both HoFe6Al6 and its hydride are strongly anisotropic
ferri-magnets 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 thefinal
forced ferromagnetic state.Fig 7demonstrates high-field
magne-tization 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
temper-atures (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 thefield interval above
available 58 T due to enhancement of the HoeFe exchange in-teractions 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 16mB/f.u in the highest availablefield applied along the both basal-plane axes This is lower than the forced collinear ferromagnetic alignment of the Ho and Fe magnetic moments, Mferro¼ MHoþ MFe¼ 20mB/f.u In the hydride, where Mferro¼ 20.45mB/f.u., the magnetization in 58 T is lower (10.5e12.5mB/f.u.) This also points to a stronger HoeFe exchange interaction Thefield 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 HoFe6Al6and again somewhat lower (13mB/f.u.) in HoFe6Al6H at the highest appliedfield 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 10.45mB/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 0.45mB/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 TCvalue in the hydride compared to the parent compound (350 K and 315 K, respectively) The HoeFe inter-sublattice interaction is also enhanced in the hydride as found from the temperature dependence of the Ho magnetic moment The molecularfield Hmolcreated on Ho ions by the Fe sublattice is
38 T (inter-sublattice exchange constant nHoFe ¼ 3.8 T/mB) in HoFe6Al6and 48 T in HoFe6Al6H (nHoFe¼ 4.6 T/mB) Measurements
in pulsed magneticfields 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 In-frastructures (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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