富山大学水素同位体科学研究センター研究報告34：37－44，2014.

ノート
TiFe0.9Co0.1 水素化物の磁気特性
赤丸悟士 1、高英之 2、原正憲 1、西村克彦 2、松山政夫 11）富山大学水素同位体科学研究センター、930-8555 富山市五福 3190
2）富山大学工学部、930-8555 富山市五福 3190

Magnetic Properties of Hydrogenated TiFe0.9Co0.1
Satoshi Akamaru1, Hideyuki Taka2, Masanori Hara1, Katsuhiko Nishimura2,and Masao Matsuyama1
1) Hydrogen Isotope Research Center, University of Toyama, Gofuku 3190,Toyama 930-8555, Japan
2) Faculty of Engineering, University of Toyama, Gofuku 3190, Toyama
930-8555, Japan(Received: December 12, 2014, Accepted: July 10, 2015 )Abstract
The thermodynamic properties of the TiFe0.9Co0.1–H system and its magnetic properties withvarious hydrogen contents were examined. The pressure-composition isotherm curves showed aplateau region, and the change in the enthalpy with monohydride formation was similar to that ofTiFe. From the conventional magnetic properties examinations, TiFe0.9Co0.1 was found to show nomagnetic transition down to 10 K. The magnetic susceptibility of TiFe0.9Co0.1 increased withhydrogen uptake, but no ferromagnetic behavior was observed up to the hydrogen content of
TiFe0.9Co0.1H0.6 at room temperature. The change in magnetic susceptibility with hydrogen uptake
could be explained qualitatively by the band structure calculation.

1. Introduction
TiFe is a candidate for hydrogen storage material consisting of abundantly existing,37

赤丸悟士・高英之・原正憲・西村克彦・松山政夫

inexpensive elements [1]. This intermetallic compound is also expected to be amagnetic material because of its iron-rich composition, and to be a functional material
applied to hydrogen sensing. However, no magnetic order has been found to occur inTiFe down to 4.2 K [2]. It was reported that TiFe0.74Co0.26 with the partial substitution ofCo for Fe showed ferromagnetic behavior at the temperature of 36 K [2,3]. A problem
with this substitution is that Co is an expensive element, and the use of Co results in acost increase.
The appearance of the magnetic transition of TiFe0.74Co0.26 was caused by theaddition of an electron to the intermetallic compound; the substitution of Co for Fe
introduces one more electron per atom without changing the electronic band structure.
Hydrogen also contributes to the addition of electrons [4]. A synergistic effect onmagnetic properties can, therefore, be expected with only modest substitution of Coatoms. Although TiFe1−yCoy absorbs hydrogen under high hydrogen gas pressure [5,6],it quickly releases hydrogen under normal pressure. Therefore, the magnetic properties
of hydrogenated TiFe1−yCoy must be examined in-situ under high hydrogen pressure.
The aim of this paper is to report the results of preliminary examination onhydrogen-induced large magnetic moments in TiFe1-yCoy compounds. The magneticsusceptibilityofTiFe0.9Co0.1

underhighhydrogenpressureand

thepressure-composition isotherm curves were measured. The relationship betweenhydrogenation and magnetic properties were also discussed.

2. Experimental
Ti, Fe and Co grains, the initial materials used for preparing TiFe0.9Co0.1, werepurchased from Kojyundo Kagaku Co. Ltd. All these materials had 99.9% purity.

38

TiFe0.9Co0.1水素化物の磁気特性

Stoichiometric amounts of the grains were weighed out, and melted together in an arc
furnace under an argon atmosphere. To obtain a homogeneous sample, the ingot wasturned upside down and melted again, and this process was repeated 5 times. After thearc melting, the surface of the ingot was grinded to remove any impurities on thesurface. The TiFe0.9Co0.1 ingot thus obtained was mechanically crushed into powder.
X-ray diffraction (XRD) measurement for the prepared powder was performed using anX’part system (Panalytical). In the measurements, crystallized Si powder was added tothe sample powder for diffraction angle calibration.
Pressure-composition isotherm curve measurements were performed using anautomated measurement system (Suzuki Syokan Co. Ltd.). Approximately 1 g of
TiFe0.9Co0.1 powder was placed in a stainless steel vessel, which was attached to thesystem. The powdered sample was activated in the following procedure. First, thesample was heated up to 673 K for 2 hours in a vacuum given by an oil-sealed rotary
pump. Then, hydrogen gas was introduced stepwise up to 4 MPa (40 atm) at room
temperature, and the hydrogen gas was evacuated at 473 K. The hydrogenation process
was repeated 5 times to stabilize hydrogen absorption characteristics. The amount ofabsorbed hydrogen was estimated as follows. In closed system, the hydrogen pressure
decreased when the sample contacted to hydrogen gas. The consumption of hydrogengas by contacting the sample was regarded as the amount of hydrogen in the sample,
which was calculated by applying the pressure difference, temperature, and a volume inthe closed system to ideal gas law.
The magnetization measurement of TiFe0.9Co0.1 was performed by SQUID
magnetometer MPMS-7 (Quantum design) at temperatures from 10 to 300 K under themagnetic field up to 7 T. The alternating-current magnetic susceptibility of39

赤丸悟士・高英之・原正憲・西村克彦・松山政夫

TiFe0.9Co0.1Hx was measured by using a system specially prepared for the measurementsof magnetic susceptibility under high hydrogen pressure. The detail of the system aredescribed elsewhere [4,7].

3. Results and Discussion
Figure 1 shows the XRD pattern of the prepared powder sample without hydrogen.
The diffraction peaks from crystallized Si powder are labeled by “Si” in this figure. The
diffraction angles of the observed peaks almost exactly corresponded to those of TiFe,
which has a CsCl type crystal structure (JCPDS1200

peaks were detected in this measurement,
indicating that Co atoms formed a complete
solid solution with Fe and the crystal structure
remained unchanged after substitution of Co for200020SiSi(211)400Si600

Si

reported elsewhere (0.2975 nm). No other800(110)which was also the same as that of TiFe

Intensity, I / cpsof TiFe0.9Co0.1 was calculated to be 0.2978 nm,

1000(200)

database no. 01-083-1653). The lattice constant

30405060
70
Diffraction angle, 2 / deg

80Figure 1. X-ray powder diffraction
pattern of TiFe0.9Co0.1. The label“Si” means the diffraction peaks
from crystallized Si powder.

Fe [6].
The temperature dependence of the magnetization for TiFe0.9Co0.1 was measured
under the external magnetic field of 0.1 T. The result is shown in Figure 2(a).
Magnetization decreased steeply with increase in temperature up to about 30 K, and its
temperature dependence was far weaker above 30 K. It seemed that the magnetization
was in inverse proportion to temperature, but it did not obey exactly the Curie-Weisslaw. No magnetic phase transition appeared in the measurement. The magnetic field40TiFe0.9Co0.1水素化物の磁気特性

30.3-1(a) TiFe Co
0.90.1

under 1 kOeMagnetization, M / emu gMagnetization, M / emu g

-1

0.35

0.25
0.2
0.15
0.1
0.05
0050

100150200Temperature, T / K

250300(b) TiFe Co
0.90.1

2.5

T = 10 K

21.51T = 300 K

0.50010203040506070

Magnetic Field, H / kOe

Figure 2. Magnetic properties of TiFe0.9Co0.1 measured by using SQUIDmagnetometer.
dependence of magnetization at 10 K and 300 K is shown in Figure 2(b). Themagnetization at 300 K increased linearly with the magnetic field. The magnetization at
10 K showed the saturation behavior, but did not reach the saturated value up to 7 T. Inaddition, no metamagnetic transition was observed, suggesting that TiFe0.9Co0.1 showedparamagnetic behavior at 10 K. From these results, we confirmed that TiFe0.9Co0.1 has a
non-magnetic ground state [3].
Figure 3 shows the pressure-composition isotherms for TiFe0.9Co0.1–H system attemperature showed a plateau region, whichshifted upwards in pressure and shrunk inwidth with the increase in temperature. Thesingle  phase (monohydride phase) appearedat[H]/[TiFe0.9Co0.1]

>

0.55.

Thepressure-composition isotherm curves at 303
K and 313 K showed slight swelling at[H]/[TiFe0.9Co0.1]≃0.60

and0.55,

Equilibrium pressure, P / Padifferent temperatures. The isotherm at each610303 K
313 K
323 K

51000.1

0.2 0.3 0.4 0.5 0.6[H]/[TiFe Co ], x / 0.90.7

0.1

Figure 3. Pressure-composition
isothermcurvesfortheTiFe0.9Co0.1–H system.

41

赤丸悟士・高英之・原正憲・西村克彦・松山政夫

respectively (indicated by allows in figure 3). The swelling probably reflected theappearance of a dihydride phase as reported for TiFe [1]. The changes in standard
enthalpy (H°-) and entropy (S°-) for monohydride formation were estimated by
the van’t Hoff plot using the plateau pressures at [H]/[TiFe0.9Co0.1] = 0.2; the H°-and S°- were -25 kJ/molH2 and -95 J/K·molH2, respectively. The obtained value forH°- was in a range of scattering of reported values for TiFe (from -22 − -28kJ/molH2) [1,5,6,8].
Figure 4 shows the magnetic susceptibility of hydrogenated TiFe0.9Co0.1 at room
temperature. The output voltage on the y-axis in the figure is theoretically proportional
to the magnetic susceptibility, but the conversion of this voltage to magnetic
susceptibility is difficult because of the unknown proportional coefficient and thebackground signals from the stainless steel vessel. The magnetic susceptibility increased
linearly with hydrogen content, but the hydride (TiFe0.9Co0.1H0.6) showed only twotimes as large magnetic susceptibility as TiFe0.9Co0.1. It was suggested that TiFe0.9Co0.1
monohydride showed paramagnetic behavior

moments appeared by hydrogenation. If theband structure of TiFe0.9Co0.1 could be thesame as that of TiFe, the enhancement ofmagnetic susceptibility by hydrogenation
could be explained by the electronic band
structure of TiFe calculated earlier [9,10]. The
results of electronic band structure calculationof TiFe indicated that the Fermi energy was42

Output voltage, V / mVat room temperature, and no large magnetic

0.7
0.6
0.5
0.4
0.3

00.2
0.40.6[H]/[TiFe Co ], x / 0.90.1

Figure4.
Alternating-current
magnetic
susceptibilityofhydrogenated TiFe0.9Co0.1. The
dashed line indicates the result of
linear fitting by the least-squares
method.

TiFe0.9Co0.1水素化物の磁気特性

positioned near the trough. The density of states rose sharply to reach the peak at anenergy level slightly higher than the Fermi energy. Hydrogen provided the doping
electrons and lifted the Fermi energy. As a result, the density of states at the Fermi
energy increased and the magnetic susceptibility also increased by hydrogenation. To
investigate magnetic properties in the ground state, it is necessary to examine themagnetic properties at low temperature under high hydrogen gas pressure.

4. Conclusions
The magnetic susceptibility of hydrogenated TiFe0.9Co0.1 was investigated. The
hydrogen absorption properties were similar to those of TiFe. TiFe0.9Co0.1 had aparamagnetic ground state. The magnetic susceptibility of hydrogenated TiFe0.9Co0.1
increased with hydrogen uptake but no large magnetic moments appeared at roomtemperature by hydrogen uptake. The change in the susceptibility with hydrogen uptake
was qualitatively explained by the electronic band structure of TiFe. Consequently, large
magnetic moments could not be induced by hydrogenation up to [H]/[TiFe0.9Co0.1] ≃0.60 for TiFe0.9Co0.1 at room temperature.

Acknowledgement
This study was supported by Grant-in-Aid for KAKENHI (23560830) from JSPS,
Japan.

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赤丸悟士・高英之・原正憲・西村克彦・松山政夫[3] G. Hilscher, G. Wiesinger, R. Hempelmann, Journal of Physics F: Metal Physics 11(1981) 2161-2177.
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