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Magnetic and electronic properties of Hf2Fe hydrides
Journal of the Less-Common
MAGNETIC G. TEISSERONa,
Metals, 130 (1987)
AND ELECTRONIC P. VULLIEP
163 - 172
PROPERTIES
163
OF Hf,Fe
HYDRIDES*
and L. SCHLAPBACHb
%‘entre
d%tudes Nuclhires de Grenoble, Departement de Recherche Fondamentale/Service de Physique/MDIH, 85X-38041 Grenoble CLdex (France) et Uniuersitk Scientifique, Technologique et Mkdicale, Grenoble (France)
bLaboratorium fur Festkiirperphysik CH-8093 Zurich (Switzerland) (Received
Eidgenossische
Technische
Hochschule
Zurich,
May 28,1986)
The intermetallic compound Hf,Fe forms several hydride phases of different stability up to the composition Hf,FeH,. We have studied the hydrogen induced variation of the magnetic and electronic properties of Hf,Fe using static and dynamic susceptibility measurements and X-ray photoelectron spectroscopy (XPS). The photoelectron spectra revealed a very weak hydrogen induced band, a shift of the hafnium 4d and 4f core levels of 0.6 eV and hardly any shift of the iron core levels, in agreement with strong Hf- H bonding. The magnetic properties of Hf,FeH, changed from Pauli paramagnetism for x = 0 to Langevin pararnagnetism for ;Y= 1.5 to ferromagnetism for 3c> 2.5. This behaviour can be explained by the evolution of the iron 2p core levels upon charging with hydrogen. In Hf,Fe they are rather symmetric in contrast with elemental iron, indicating a weak partial density of iron states at the Fermi level EF owing to a lowering of the iron 3d conduction states below EF. In the hydride, the line asymmetry increases, in agreement with the reappearance of the magnetic moment. At low temperatures and weak magnetic fields we observed magnetic behaviour which characterizes spin glasses: temperature and field dependent irreversible effects in the static susceptibility, a cusp in the temperature dependence of the dynamic susceptibility and a very large typical relaxation time of the magnetization.
1. Introduction Hydrogen absorption intermetallic compounds.
considerably changes The lattice expansion,
*Paper presented at the International of Metal Hydrides V, Maubuisson, France, 0022-5088/87/$3.50
the physical properties the attractive potential
Symposium on the Properties May 25 - 30,1986. 0 Elsevier
Sequoia/Printed
of of
and Applications
in The Netherlands
164
the proton and the additional electron modify the electronic density of states and may induce important effects on the magnetic properties such as variation of the transition temperature in ferromagnetic samples, loss of the magnetism or, in contrast, setting up of new magnetic phases [ 11. Hf,Fe, which crystallizes in the cubic T&.Ni-type structure [2], is an interesting example for studying these types of effect. It was briefly pointed out [3] that this compound is Pauli paramagnetic and becomes ferromagnetic when charged with hydrogen. Recently, more detailed investigations [4], have given some complementary information: (i) the sample can be charged with up to five hydrogen atoms per formula unit (x = 5); (ii) the saturated magnetic moment reaches a maximum at about 3c = 3 ; (iii) in the hydrogen concentration range 1.5 < x < 5 there is evidence for a disordered magnetic phase at low temperatures. The purpose of this work was to study in more detail the appearance of the magnetic moment, to relate it to the changes in the electronic structure induced by hydrogen and to obtain more information on the disordered magnetic phase.
2. Sample preparation
and experimental
set-up
Ingots of Hf,Fe were prepared in an induction furnace under a highly purified argon stream. To obtain the hydride, the virgin compound was first heated in lop5 Torr vacuum at 1000 “C, then purified hydrogen was admitted at a pressure of 1 atm and finally the heating was switched off. The hydride formation was immediate. After about 30 min, the temperature of the sample was returned to room temperature and the hydride HfzFeH4.s was obtained. The hydrogen content was deduced by the volumetric method. Hydrides with intermediate concentrations were prepared by partial desorption of the fully hydrogenated compound. Susceptibility and magnetization measurements were performed on a superconducting quantum interference device (SQUID) apparatus (SHE Corporation). The temperature of the sample could be varied in the range 5 - 320 K. The accuracy of the temperature was lo-* K between 5 K and 70 K and 10-i K at higher temperatures. The precision of the applied magnetic field H was 10 Oe when H > 1 kOe and 1 Oe when H < 1 kOe using a high resolution device. The temperature and applied magnetic field variations were controlled by a computer. The valence band and the hafnium and iron core levels were analysed by photoelectron spectroscopy using Mg Ko radiation (1253.6 eV) in a VG Escalab spectrometer (1 X lo-lo mbar, gold 4f,,* at 84.0 eV with 1.2 eV full width at half maximum (FWHM)). The Kosa4 satellites were corrected numerically, but remaining ghost peaks from the hafnium 4f peaks appeared in the same energy range as the hydrogen induced band. Cylindrical shaped samples were fractured at 1 X lo-” mbar to obtain clean surfaces. The hydride was prepared in the high pressure cell of the spectrometer by the
165
exposure of a bulk sample to 20 bar H2 for 10 min at 300 K and subsequent cooling to 100 K where the high pressure was released. The hydride was analysed at 100 K. It desorbed hydrogen rather quickly so that the chamber pressure rose from 1 X 10-i’ to 6 X 1O-6 mbar. From p-T curves we estimated that the concentration of the sample was HfiFeH,,.s after hydrogenawhile taking the first spectra and Hf*FeH, at the end of the tion, Hf,FeH,, series, which was confirmed by X-ray analysis.
3. Experimental results 3.1. Magnetic results 3.1 .l. Static susceptibility
Before starting magnetization measurements, the following experimental procedure was carried out. The applied magnetic field H was established in the supraconductor magnet of the SQUID apparatus and the temperature of the measurement cell was stabilized to 6 K. The sample was introduced into the magnet coils; it was rapidly cooled from room temperature to 6 K and abruptly experienced the external magnetic field. The static magnetization was measured on samples with various hydrogen concentrations by increasing and then decreasing temperature under H = 1 kOe and H = 100 Oe. When H = 1 kOe, the inverse molar susceptibility reveals three typical different behaviours (see Fig. 1). As the hydrogen concentration is increased from 3c = 0 to x = 1.5, a change from Pauli paramagnetism to quasi-Langevin paramagnetism is observed. In the latter case, within the low temperature range (T < 80 K), it must be pointed out that the l/xm curve extrapolates to a negative temperature of about --lo K, which suggests the presence of antiferromagnetic interactions (see Fig. l(a)). When x is increased over two, a ferromagnetic behaviour is observed (see Fig. l(b)). The experimental high temperature limitation does not permit an accurate determination of the paramagnetic Curie temperature TP. However, the experimental data give evidence of a maximum value of T, in the close vicinity of x = 3. This behaviour is similar to that observed in the hydrogen concentration dependence of the saturated magnetic moment [ 41. For H = 100 Oe, the specific static susceptibility (see Fig. 2) reveals irreversible effects with increasing and decreasing temperature. These effects, which disappear for H 2 1 kOe, were earlier observed for x = 4.8 and H = 40 Oe and were attributed to a disordered magnetic phase. As similar effects are known for some iron containing oxides [ 51 and as surface segregation effects produce additional phases at the surface [6] which may contribute significantly to the magnetic properties of a powder sample [7] we made some additional measurements on samples with x = 0. The field-dependent magnetization at 6 K is unaffected by the grain size, an important factor in the oxidation process. Susceptibility measurements (300 K) as a function of the magnetic field (0 - 6 kOe) showed for fully desorbed Hf,Fe powder samples
166
800 -
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+
+
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t+
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', LOOJ L l
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t
*
++
+ ++ + ++++ +++++++ ++ + + + + + + + + +
x=0
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Fig. 1. Inverse molar static susceptibility vs. temperature for various hydrogen concentrationsx:(a)x~O,1.5;(b)~=2.6,3.0,4.0,4.8.TheappliedmagneticfieldH=1kOe. I”
”
I ”
“I’
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l ”
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TIK)
Fig. 2. Specific static susceptibility vs. increased then decreased temperature for various x. The applied magnetic field is 100 Oe.
167
a slightly enhanded, but field-independent, susceptibility (1.1 X 10m6 instead of 1.0 X 10V6 e.m.u. g-i) and a small ferromagnetic contribution (x H = 3 X 10e3 e.m.u. g-’ for H-t 0). Both contributions which are probably surface-related contributions are negligibly small compared with the effects shown in Fig. 2. When the temperature is slowly increased (the complete rise of the temperature requires several hours), the magnetization grows in several steps. Similar results have already been observed in other hydrogenated compounds [8]. This strongly suggests a time dependence in the growing of the magnetization, requiring a large characteristic rearrangement time of the magnetic carriers. 3.1.2. Dynamic susceptibility The dynamic susceptibility was measured as a function of the temperature for various values of the alternating magnetic field frequency. The amplitude of the magnetic field was 0.4 Oe and the hydrogen concentration x = 4.8 (see Fig. 3). A net maximum of the real part of the dynamic susceptibility occurs at the freezing temperature TF (approximately 61 K), characteristic of a spin glass [ 91. TF is very slightly frequencydependent (ATF = 4 K between 11 Hz and 11 kHz). The maximum value of the susceptibility cusp occurs when the temperature and size-dependent relaxation times of magnetic clusters are close to the period of the alternating field.
-03.
0
20
40
60
60
IOOTIK'
Fig. 3. Real part of the dynamic susceptibility vs. temperature for various frequencies : (a) f= 11 Hz; (b) f= 300 Hz; (c) f= 11 kHz. The hydrogen concentration x = 4.8.
3.1.3.
Critical behaviour
When the SQUID apparatus limitations permitted the investigation of the l/x,,, plot just above the Curie temperature, i.e. for x > 3.5 we analysed the data in terms of critical behaviour, using the generalized Curie law [lo] xT=C
T -i T-T,
’
1
168
This model is particularly advantageous as it can be applied in a temperature range from Tc to 4Tc, as was observed in a good Curie-Weiss ferromagnet, pure nickel [ll]. -A In T/A ln(xT) was plotted as a function of the temperature from the l/m, us. T data presented in Section 3.1.1 (see Fig. 4) and the ferromagnetic Curie temperature Tc and critical exponent 7 were deduced from the linear law (T - Tc)/y Tc (see Table 1).
O.LO-
+ x=L.8
+
+
+
0.30E i7 z
+ + + +,+ I
$0.20z I
P
,
,
O.lO-
I
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0
50
,
,
I
100
I
150 T (KI
200
250
300
Fig. 4. -A In T/A In&T) us. temperature for various X. The linear part corresponds to the (T - T,#yTc law. TABLE 1 Ferromagnetic Curie temperature Tc and critical exponent 7 for various hydrogen concentrations x (the paramagnetic Curie temperature T,, is estimated from l/&, us. T curves) X
Tc W
Y
Tp (K)
Tp - Tc F)
3.5 4.0 4.8
260 + 3 215”3 85 + 8
2.3 + 0.1 2.5 f 0.1 6.6 + 0.2
295 262 215
35 41 130
The great difference between the ferromagnetic Curie temperature Tc and the paramagnetic Curie temperature T,, and the large value obtained for the y exponent (y = 1.38 in the scale law model) reveals that the ferromagnetic behaviour of Hf,FeH, differs more and more from the Curie-Weiss law as x increases over three. These results suggest the infinite ferromagnetic cluster is replaced, when the concentration increases from x = 3 to x = 4.8, by finite ferromagnetic clusters which remain present well above the Curie temperature [12]. 3.1.4. Relaxation of the static magnetization Time-dependent magnetic measurements were performed on a fully hydrogenated (X = 4.8) sample, using the following procedure. The sample
169
was cooled from room temperature to the selected temperature in a 100 Oe static magnetic field and then the magnetic field was switched off. It was observed that the magnetization was reduced by 25% in about one hour. This long time relaxation is typical of a slow dynamic disordering. If plotted us. In t, the magnetization follows a linear dependence (see Fig. 5) which must be related to a large distribution of potential barrier heights [ 13 1.
+ 0.38-
+
+
0.36-
+
d, \
”
0.3Lc-
+
+
+ +
; . 0.32-
=
+ ++
++
++
0.300.28-
LOG(
++++
+++ *+++ +*++++ %,
%+*
t )
Fig. 5. Time relaxation of the magnetization us. logarithm of time after a 100 Oe applied field is switched off. Hydrogen concentration is 4.8. Temperature is 20 K.
3.2. X-ray photoelectron spectroscopy results The valence band spectrum of Hf,FeH,,, which was not present in the spectrum of Hf,Fe
20
16
12 BINDING
4
6 ENERGY
showed a weak emission and minor changes in the
0= EF
(eV)
Fig. 6. XPS spectra of the valence band of UHV fractured HfiFe (300 K) and of HfsFeHM4 (100 K). A hydrogen induced emission appears at 4 - 6 eV. The hafnium 4f core levels of the hydride are shifted by 0.6 eV to larger binding energy.
170
conduction band. The core levels hafnium 4f,,2. sj2(see Fig. 6) and hafnium 4dsn, sM of the hydride shifted by 0.6 eV to larger binding energies, whereas the iron 2p core levels showed hardly any shift (5 0.1 eV) indicating a strong H-Hf bond. The hafnium core level shift decreased within 10 min from 0.6 eV (Hf,FeH,,) to approximately 0.4 eV and remained constant for many hours (Hf,FeH,s). A small variation of the asymmetry of the iron 2p line was observed as a function of hydrogen concentration (see Fig. 7). The asymmetry, which is related to the partial density of iron states at EF, is much smaller in Hf,Fe and Hf,Fe hydrides than in elemental iron [ 141.
_.___Fe -
Hf2FeHs,
--
Hf2FeH,4
t
( eV
707
709
711 BINDING
ENERGY
705
(eV)
Fig. 7. Iron 2~33,~ core level spectra (XPS) of elemental iron, UHV fractured Hf,Fe (300 K), HfnFeHc4 and HfiFeHaa (100 K). The core level asymmetry, which is related to the partial density of iron states at EF, drops from iron to Hf*Fe and increases slightly in the hydride, in agreement with the disappearance and reappearance of the magnetic moment of iron.
4. Discussion The XPS spectra, measured on the intermetallic compound HfsFe reveal that the line of the iron 2p core level is by far less asymmetric than in pure iron, indicating a weak partial density of iron states at the Fermi level EF [ 141. Apparently, the iron 3d band shifts away from EF which explains the Pauli paramagnetic behaviour of the inter-metallic compound by the filling of the 3d band.
171
The variation of the magnetic properties when the hydrogen concentration is raised reveal the reappearance of a magnetic moment on iron at about x = 1.5 and then an increase of the ferromagnetic exchange interaction when x 5 2.0. Together with the reappearance of the magnetic moment, the asymmetry of the iron 2p line increases, caused by the moving back of the iron 3d band towards the position it had in pure iron. Moreover, the observed shift of the hafnium core level, owing to a charge transfer from hafnium to hydrogen, indicates a strong H-Hf bond. In contrast the H-Fe bonding is very weak. The formation of the Hf-H bond probably modifies the Hf-Fe hybridization and thus induces changes in the band structure which lead to the setting of the magnetic moment on iron. The stiffening of the ferromagnetism is not monotonic and reaches a maximum at about x = 3, observed both in the variation in the saturated magnetic moment [4] and in the variation in the paramagnetic and ferromagnetic Curie temperatures, which are related to the magnetic moment and to the exchange interactions. It has been shown that hydrogen induces a large lattice expansion particularly between x = 2 and x = 3 [4]. Recent neutron diffraction experiments on this hydride [15] reveal that the nearest neighbours Fe-Fe averaged distances do not follow the same expansion tendency, but show a minimum at about x = 3 (dFe_+ = 3.018. 2.84 A and 2.91 A for x = 0,2.85 and 4.5 respectively). This variation in the Fe-Fe distances with x is directly responsible for the ferromagnetic exchange interactions when x 2 2. X-ray and neutron diffraction data show that Hf,FeH, remains crystalline for all hydrogen concentrations. Iron atoms occupy only,one type of crystallographic site and are arranged in distinct tetrahedra. Within a tetrahedron, Fe-Fe distances differ by about 0.1 a owing to the statistical distribution of hydrogen [ 151. This leads to a local distribution of the direct ferromagnetic exchange. The nearest iron atoms of two adjacent tetrahedra are separated by about 5 a. This value is large enough to reduce strongly the probability of direct exchange interaction between tetrahedra. We suggest that the main magnetic coupling comes from the indirect Ruderman-KittelKasuya-Yosida (RKKY) interaction. Therefore we may summarize the magnetic phase of Hf*FeH, as constituted by the following: (i) total magnetic moments borne by each tetrahedron and resulting from distributed ferromagnetic intratetrahedral interactions and (ii) weak RKKY interactions between the randomly oriented total magnetic moments. This last interaction gives rise to the spin-glass behaviour we have observed. The previous discussion permits us to gain a better insight into the Mijssbauer results of ref. 4. The distribution in Fe-Fe distances explains the non-magnetic contribution in these spectra. This non-magnetic contribution increases from x = 2.8 to x = 4.8 in agreement with larger Fe-Fe mean distances and with the decrease of both the measured magnetic moment per iron atom and the Curie temperature. Timedependent magnetic measurements are in progress to more accurately determine the spin-glass behaviour.
172
Acknowledgment We are indebted to Dr. Tholence (CNRS Grenoble) for performing the dynamic susceptibility measurements and for discussing the corresponding data with us.
References 1 K. H. J. Buschow, P. C. P. Bouten and A. R. Miedema, Rep. Progr. Phys., 45 (1982) 937. 2 M. H. Miiller and H. W. Knott, Trans. Met. Sot. AZME, 227 (1963) 674. 3 K. H. J. Buschow and A. M. van Diepen, Solid State Commun., 31 (1979) 469; E. Tuscher, Monotsh. Chem., 110 (1979) 1275. 4 P. Vulliet, G. Teisseron, J. L. Oddou, C. Jeandey and A. Yaouanc, J. Less-Common Met., 104 (1984) 13. 5 T. Shigematsu, Y. Bandon and T. Takada, J. Phys. C, 40 (2) (1979) 153. 6 L. Schlapbach, A. Seiler, F. Stucki and H. C. Siegmann, J. Less-Common Met., 73 (1980) 145. 7 L. Schlapbach, J. Phys. F: Met. Phys., 10 (1980) 2477. 8 H. Fujii, F. Pourarian and W. E. Wallace, J. Less-Common Met., 88 (1982) 187. 9 J. L. Tholence, Physica B, 126 (1984) 157. 10 E. Carre, J. P. Renard and J. Souletie, J. Magn. Magn. Mater., 53 - 57 (1986) 697. 11 J. Souletie and J. L. Tholence, Solid State Commun., 48 (5) (1983) 407. 12 B. R. Coles, B. V. B. Sarkissian and R. H. Taylor, Philos. Mug. B, 37 (4) (1978) 489. 13 R. Gmari, J. J. Prejean and J. Souletie, J. Phys., 45 (1984) 1809. 14 J. C. W. Folmer and D. K. G. de Boer, Solid State Commun., 38 (1981) 1135. 15 D. Fruchard et al., J. Less-Common Met., 130 (1987) 89.
MAGNETIC G. TEISSERONa,
Metals, 130 (1987)
AND ELECTRONIC P. VULLIEP
163 - 172
PROPERTIES
163
OF Hf,Fe
HYDRIDES*
and L. SCHLAPBACHb
%‘entre
d%tudes Nuclhires de Grenoble, Departement de Recherche Fondamentale/Service de Physique/MDIH, 85X-38041 Grenoble CLdex (France) et Uniuersitk Scientifique, Technologique et Mkdicale, Grenoble (France)
bLaboratorium fur Festkiirperphysik CH-8093 Zurich (Switzerland) (Received
Eidgenossische
Technische
Hochschule
Zurich,
May 28,1986)
The intermetallic compound Hf,Fe forms several hydride phases of different stability up to the composition Hf,FeH,. We have studied the hydrogen induced variation of the magnetic and electronic properties of Hf,Fe using static and dynamic susceptibility measurements and X-ray photoelectron spectroscopy (XPS). The photoelectron spectra revealed a very weak hydrogen induced band, a shift of the hafnium 4d and 4f core levels of 0.6 eV and hardly any shift of the iron core levels, in agreement with strong Hf- H bonding. The magnetic properties of Hf,FeH, changed from Pauli paramagnetism for x = 0 to Langevin pararnagnetism for ;Y= 1.5 to ferromagnetism for 3c> 2.5. This behaviour can be explained by the evolution of the iron 2p core levels upon charging with hydrogen. In Hf,Fe they are rather symmetric in contrast with elemental iron, indicating a weak partial density of iron states at the Fermi level EF owing to a lowering of the iron 3d conduction states below EF. In the hydride, the line asymmetry increases, in agreement with the reappearance of the magnetic moment. At low temperatures and weak magnetic fields we observed magnetic behaviour which characterizes spin glasses: temperature and field dependent irreversible effects in the static susceptibility, a cusp in the temperature dependence of the dynamic susceptibility and a very large typical relaxation time of the magnetization.
1. Introduction Hydrogen absorption intermetallic compounds.
considerably changes The lattice expansion,
*Paper presented at the International of Metal Hydrides V, Maubuisson, France, 0022-5088/87/$3.50
the physical properties the attractive potential
Symposium on the Properties May 25 - 30,1986. 0 Elsevier
Sequoia/Printed
of of
and Applications
in The Netherlands
164
the proton and the additional electron modify the electronic density of states and may induce important effects on the magnetic properties such as variation of the transition temperature in ferromagnetic samples, loss of the magnetism or, in contrast, setting up of new magnetic phases [ 11. Hf,Fe, which crystallizes in the cubic T&.Ni-type structure [2], is an interesting example for studying these types of effect. It was briefly pointed out [3] that this compound is Pauli paramagnetic and becomes ferromagnetic when charged with hydrogen. Recently, more detailed investigations [4], have given some complementary information: (i) the sample can be charged with up to five hydrogen atoms per formula unit (x = 5); (ii) the saturated magnetic moment reaches a maximum at about 3c = 3 ; (iii) in the hydrogen concentration range 1.5 < x < 5 there is evidence for a disordered magnetic phase at low temperatures. The purpose of this work was to study in more detail the appearance of the magnetic moment, to relate it to the changes in the electronic structure induced by hydrogen and to obtain more information on the disordered magnetic phase.
2. Sample preparation
and experimental
set-up
Ingots of Hf,Fe were prepared in an induction furnace under a highly purified argon stream. To obtain the hydride, the virgin compound was first heated in lop5 Torr vacuum at 1000 “C, then purified hydrogen was admitted at a pressure of 1 atm and finally the heating was switched off. The hydride formation was immediate. After about 30 min, the temperature of the sample was returned to room temperature and the hydride HfzFeH4.s was obtained. The hydrogen content was deduced by the volumetric method. Hydrides with intermediate concentrations were prepared by partial desorption of the fully hydrogenated compound. Susceptibility and magnetization measurements were performed on a superconducting quantum interference device (SQUID) apparatus (SHE Corporation). The temperature of the sample could be varied in the range 5 - 320 K. The accuracy of the temperature was lo-* K between 5 K and 70 K and 10-i K at higher temperatures. The precision of the applied magnetic field H was 10 Oe when H > 1 kOe and 1 Oe when H < 1 kOe using a high resolution device. The temperature and applied magnetic field variations were controlled by a computer. The valence band and the hafnium and iron core levels were analysed by photoelectron spectroscopy using Mg Ko radiation (1253.6 eV) in a VG Escalab spectrometer (1 X lo-lo mbar, gold 4f,,* at 84.0 eV with 1.2 eV full width at half maximum (FWHM)). The Kosa4 satellites were corrected numerically, but remaining ghost peaks from the hafnium 4f peaks appeared in the same energy range as the hydrogen induced band. Cylindrical shaped samples were fractured at 1 X lo-” mbar to obtain clean surfaces. The hydride was prepared in the high pressure cell of the spectrometer by the
165
exposure of a bulk sample to 20 bar H2 for 10 min at 300 K and subsequent cooling to 100 K where the high pressure was released. The hydride was analysed at 100 K. It desorbed hydrogen rather quickly so that the chamber pressure rose from 1 X 10-i’ to 6 X 1O-6 mbar. From p-T curves we estimated that the concentration of the sample was HfiFeH,,.s after hydrogenawhile taking the first spectra and Hf*FeH, at the end of the tion, Hf,FeH,, series, which was confirmed by X-ray analysis.
3. Experimental results 3.1. Magnetic results 3.1 .l. Static susceptibility
Before starting magnetization measurements, the following experimental procedure was carried out. The applied magnetic field H was established in the supraconductor magnet of the SQUID apparatus and the temperature of the measurement cell was stabilized to 6 K. The sample was introduced into the magnet coils; it was rapidly cooled from room temperature to 6 K and abruptly experienced the external magnetic field. The static magnetization was measured on samples with various hydrogen concentrations by increasing and then decreasing temperature under H = 1 kOe and H = 100 Oe. When H = 1 kOe, the inverse molar susceptibility reveals three typical different behaviours (see Fig. 1). As the hydrogen concentration is increased from 3c = 0 to x = 1.5, a change from Pauli paramagnetism to quasi-Langevin paramagnetism is observed. In the latter case, within the low temperature range (T < 80 K), it must be pointed out that the l/xm curve extrapolates to a negative temperature of about --lo K, which suggests the presence of antiferromagnetic interactions (see Fig. l(a)). When x is increased over two, a ferromagnetic behaviour is observed (see Fig. l(b)). The experimental high temperature limitation does not permit an accurate determination of the paramagnetic Curie temperature TP. However, the experimental data give evidence of a maximum value of T, in the close vicinity of x = 3. This behaviour is similar to that observed in the hydrogen concentration dependence of the saturated magnetic moment [ 41. For H = 100 Oe, the specific static susceptibility (see Fig. 2) reveals irreversible effects with increasing and decreasing temperature. These effects, which disappear for H 2 1 kOe, were earlier observed for x = 4.8 and H = 40 Oe and were attributed to a disordered magnetic phase. As similar effects are known for some iron containing oxides [ 51 and as surface segregation effects produce additional phases at the surface [6] which may contribute significantly to the magnetic properties of a powder sample [7] we made some additional measurements on samples with x = 0. The field-dependent magnetization at 6 K is unaffected by the grain size, an important factor in the oxidation process. Susceptibility measurements (300 K) as a function of the magnetic field (0 - 6 kOe) showed for fully desorbed Hf,Fe powder samples
166
800 -
a
l
+
+
_
x=1.5++++ 2 600-
t+
2
t+ *+
', LOOJ L l
200-
t
t
*
++
+ ++ + ++++ +++++++ ++ + + + + + + + + +
x=0
oIIIIIII/I~IIIIIIIIIIIII~II~I 0 50
100
150
200
250
TIKI
I”“l”“l”“l”“l’il’I’~~~
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LO-
+
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+
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+'
?
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+
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+
+
+
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+
+
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150
+ + +;:
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+
+
+++
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+
200
+
'+X=2.6
_
z3:o
-
~
300
250
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Fig. 1. Inverse molar static susceptibility vs. temperature for various hydrogen concentrationsx:(a)x~O,1.5;(b)~=2.6,3.0,4.0,4.8.TheappliedmagneticfieldH=1kOe. I”
”
I ”
“I’
’ ”
I ”
”
I ”
”
l ”
”
1
’ ”
0.06-
-T O.OLP 2 s 9 0.02-
6_ o.oo-
TIK)
Fig. 2. Specific static susceptibility vs. increased then decreased temperature for various x. The applied magnetic field is 100 Oe.
167
a slightly enhanded, but field-independent, susceptibility (1.1 X 10m6 instead of 1.0 X 10V6 e.m.u. g-i) and a small ferromagnetic contribution (x H = 3 X 10e3 e.m.u. g-’ for H-t 0). Both contributions which are probably surface-related contributions are negligibly small compared with the effects shown in Fig. 2. When the temperature is slowly increased (the complete rise of the temperature requires several hours), the magnetization grows in several steps. Similar results have already been observed in other hydrogenated compounds [8]. This strongly suggests a time dependence in the growing of the magnetization, requiring a large characteristic rearrangement time of the magnetic carriers. 3.1.2. Dynamic susceptibility The dynamic susceptibility was measured as a function of the temperature for various values of the alternating magnetic field frequency. The amplitude of the magnetic field was 0.4 Oe and the hydrogen concentration x = 4.8 (see Fig. 3). A net maximum of the real part of the dynamic susceptibility occurs at the freezing temperature TF (approximately 61 K), characteristic of a spin glass [ 91. TF is very slightly frequencydependent (ATF = 4 K between 11 Hz and 11 kHz). The maximum value of the susceptibility cusp occurs when the temperature and size-dependent relaxation times of magnetic clusters are close to the period of the alternating field.
-03.
0
20
40
60
60
IOOTIK'
Fig. 3. Real part of the dynamic susceptibility vs. temperature for various frequencies : (a) f= 11 Hz; (b) f= 300 Hz; (c) f= 11 kHz. The hydrogen concentration x = 4.8.
3.1.3.
Critical behaviour
When the SQUID apparatus limitations permitted the investigation of the l/x,,, plot just above the Curie temperature, i.e. for x > 3.5 we analysed the data in terms of critical behaviour, using the generalized Curie law [lo] xT=C
T -i T-T,
’
1
168
This model is particularly advantageous as it can be applied in a temperature range from Tc to 4Tc, as was observed in a good Curie-Weiss ferromagnet, pure nickel [ll]. -A In T/A ln(xT) was plotted as a function of the temperature from the l/m, us. T data presented in Section 3.1.1 (see Fig. 4) and the ferromagnetic Curie temperature Tc and critical exponent 7 were deduced from the linear law (T - Tc)/y Tc (see Table 1).
O.LO-
+ x=L.8
+
+
+
0.30E i7 z
+ + + +,+ I
$0.20z I
P
,
,
O.lO-
I
o.oo-
0
50
,
,
I
100
I
150 T (KI
200
250
300
Fig. 4. -A In T/A In&T) us. temperature for various X. The linear part corresponds to the (T - T,#yTc law. TABLE 1 Ferromagnetic Curie temperature Tc and critical exponent 7 for various hydrogen concentrations x (the paramagnetic Curie temperature T,, is estimated from l/&, us. T curves) X
Tc W
Y
Tp (K)
Tp - Tc F)
3.5 4.0 4.8
260 + 3 215”3 85 + 8
2.3 + 0.1 2.5 f 0.1 6.6 + 0.2
295 262 215
35 41 130
The great difference between the ferromagnetic Curie temperature Tc and the paramagnetic Curie temperature T,, and the large value obtained for the y exponent (y = 1.38 in the scale law model) reveals that the ferromagnetic behaviour of Hf,FeH, differs more and more from the Curie-Weiss law as x increases over three. These results suggest the infinite ferromagnetic cluster is replaced, when the concentration increases from x = 3 to x = 4.8, by finite ferromagnetic clusters which remain present well above the Curie temperature [12]. 3.1.4. Relaxation of the static magnetization Time-dependent magnetic measurements were performed on a fully hydrogenated (X = 4.8) sample, using the following procedure. The sample
169
was cooled from room temperature to the selected temperature in a 100 Oe static magnetic field and then the magnetic field was switched off. It was observed that the magnetization was reduced by 25% in about one hour. This long time relaxation is typical of a slow dynamic disordering. If plotted us. In t, the magnetization follows a linear dependence (see Fig. 5) which must be related to a large distribution of potential barrier heights [ 13 1.
+ 0.38-
+
+
0.36-
+
d, \
”
0.3Lc-
+
+
+ +
; . 0.32-
=
+ ++
++
++
0.300.28-
LOG(
++++
+++ *+++ +*++++ %,
%+*
t )
Fig. 5. Time relaxation of the magnetization us. logarithm of time after a 100 Oe applied field is switched off. Hydrogen concentration is 4.8. Temperature is 20 K.
3.2. X-ray photoelectron spectroscopy results The valence band spectrum of Hf,FeH,,, which was not present in the spectrum of Hf,Fe
20
16
12 BINDING
4
6 ENERGY
showed a weak emission and minor changes in the
0= EF
(eV)
Fig. 6. XPS spectra of the valence band of UHV fractured HfiFe (300 K) and of HfsFeHM4 (100 K). A hydrogen induced emission appears at 4 - 6 eV. The hafnium 4f core levels of the hydride are shifted by 0.6 eV to larger binding energy.
170
conduction band. The core levels hafnium 4f,,2. sj2(see Fig. 6) and hafnium 4dsn, sM of the hydride shifted by 0.6 eV to larger binding energies, whereas the iron 2p core levels showed hardly any shift (5 0.1 eV) indicating a strong H-Hf bond. The hafnium core level shift decreased within 10 min from 0.6 eV (Hf,FeH,,) to approximately 0.4 eV and remained constant for many hours (Hf,FeH,s). A small variation of the asymmetry of the iron 2p line was observed as a function of hydrogen concentration (see Fig. 7). The asymmetry, which is related to the partial density of iron states at EF, is much smaller in Hf,Fe and Hf,Fe hydrides than in elemental iron [ 141.
_.___Fe -
Hf2FeHs,
--
Hf2FeH,4
t
( eV
707
709
711 BINDING
ENERGY
705
(eV)
Fig. 7. Iron 2~33,~ core level spectra (XPS) of elemental iron, UHV fractured Hf,Fe (300 K), HfnFeHc4 and HfiFeHaa (100 K). The core level asymmetry, which is related to the partial density of iron states at EF, drops from iron to Hf*Fe and increases slightly in the hydride, in agreement with the disappearance and reappearance of the magnetic moment of iron.
4. Discussion The XPS spectra, measured on the intermetallic compound HfsFe reveal that the line of the iron 2p core level is by far less asymmetric than in pure iron, indicating a weak partial density of iron states at the Fermi level EF [ 141. Apparently, the iron 3d band shifts away from EF which explains the Pauli paramagnetic behaviour of the inter-metallic compound by the filling of the 3d band.
171
The variation of the magnetic properties when the hydrogen concentration is raised reveal the reappearance of a magnetic moment on iron at about x = 1.5 and then an increase of the ferromagnetic exchange interaction when x 5 2.0. Together with the reappearance of the magnetic moment, the asymmetry of the iron 2p line increases, caused by the moving back of the iron 3d band towards the position it had in pure iron. Moreover, the observed shift of the hafnium core level, owing to a charge transfer from hafnium to hydrogen, indicates a strong H-Hf bond. In contrast the H-Fe bonding is very weak. The formation of the Hf-H bond probably modifies the Hf-Fe hybridization and thus induces changes in the band structure which lead to the setting of the magnetic moment on iron. The stiffening of the ferromagnetism is not monotonic and reaches a maximum at about x = 3, observed both in the variation in the saturated magnetic moment [4] and in the variation in the paramagnetic and ferromagnetic Curie temperatures, which are related to the magnetic moment and to the exchange interactions. It has been shown that hydrogen induces a large lattice expansion particularly between x = 2 and x = 3 [4]. Recent neutron diffraction experiments on this hydride [15] reveal that the nearest neighbours Fe-Fe averaged distances do not follow the same expansion tendency, but show a minimum at about x = 3 (dFe_+ = 3.018. 2.84 A and 2.91 A for x = 0,2.85 and 4.5 respectively). This variation in the Fe-Fe distances with x is directly responsible for the ferromagnetic exchange interactions when x 2 2. X-ray and neutron diffraction data show that Hf,FeH, remains crystalline for all hydrogen concentrations. Iron atoms occupy only,one type of crystallographic site and are arranged in distinct tetrahedra. Within a tetrahedron, Fe-Fe distances differ by about 0.1 a owing to the statistical distribution of hydrogen [ 151. This leads to a local distribution of the direct ferromagnetic exchange. The nearest iron atoms of two adjacent tetrahedra are separated by about 5 a. This value is large enough to reduce strongly the probability of direct exchange interaction between tetrahedra. We suggest that the main magnetic coupling comes from the indirect Ruderman-KittelKasuya-Yosida (RKKY) interaction. Therefore we may summarize the magnetic phase of Hf*FeH, as constituted by the following: (i) total magnetic moments borne by each tetrahedron and resulting from distributed ferromagnetic intratetrahedral interactions and (ii) weak RKKY interactions between the randomly oriented total magnetic moments. This last interaction gives rise to the spin-glass behaviour we have observed. The previous discussion permits us to gain a better insight into the Mijssbauer results of ref. 4. The distribution in Fe-Fe distances explains the non-magnetic contribution in these spectra. This non-magnetic contribution increases from x = 2.8 to x = 4.8 in agreement with larger Fe-Fe mean distances and with the decrease of both the measured magnetic moment per iron atom and the Curie temperature. Timedependent magnetic measurements are in progress to more accurately determine the spin-glass behaviour.
172
Acknowledgment We are indebted to Dr. Tholence (CNRS Grenoble) for performing the dynamic susceptibility measurements and for discussing the corresponding data with us.
References 1 K. H. J. Buschow, P. C. P. Bouten and A. R. Miedema, Rep. Progr. Phys., 45 (1982) 937. 2 M. H. Miiller and H. W. Knott, Trans. Met. Sot. AZME, 227 (1963) 674. 3 K. H. J. Buschow and A. M. van Diepen, Solid State Commun., 31 (1979) 469; E. Tuscher, Monotsh. Chem., 110 (1979) 1275. 4 P. Vulliet, G. Teisseron, J. L. Oddou, C. Jeandey and A. Yaouanc, J. Less-Common Met., 104 (1984) 13. 5 T. Shigematsu, Y. Bandon and T. Takada, J. Phys. C, 40 (2) (1979) 153. 6 L. Schlapbach, A. Seiler, F. Stucki and H. C. Siegmann, J. Less-Common Met., 73 (1980) 145. 7 L. Schlapbach, J. Phys. F: Met. Phys., 10 (1980) 2477. 8 H. Fujii, F. Pourarian and W. E. Wallace, J. Less-Common Met., 88 (1982) 187. 9 J. L. Tholence, Physica B, 126 (1984) 157. 10 E. Carre, J. P. Renard and J. Souletie, J. Magn. Magn. Mater., 53 - 57 (1986) 697. 11 J. Souletie and J. L. Tholence, Solid State Commun., 48 (5) (1983) 407. 12 B. R. Coles, B. V. B. Sarkissian and R. H. Taylor, Philos. Mug. B, 37 (4) (1978) 489. 13 R. Gmari, J. J. Prejean and J. Souletie, J. Phys., 45 (1984) 1809. 14 J. C. W. Folmer and D. K. G. de Boer, Solid State Commun., 38 (1981) 1135. 15 D. Fruchard et al., J. Less-Common Met., 130 (1987) 89.