Magnetization process of Hf0.8Ta0.2Fe2 in strong pulsed magnetic fields

~

Solid State Communications, Vol.49,No.12, pp. I113-1116, 1984. Printed in Great Britain.

MAGNETIZATION IN STRONG H. Nishihara, Institute

G. Kido*,

PROCESS

0038-1098/84 $3.00 + .00 Pergamon Press Ltd.

OF Hf0.sTa0.2Fe 2

PULSED MAGNETIC Y. Nishihara**,

FIELDS M. Itoh and H. Yasuoka

for Solid State Pbysics, University Roppongi 7, Tokyo 106

of Tokyo,

*The Research Institute for Iron, Steel and Other Metals, Tohoku University, Katahira, Sendai 980 **Electrotechnical (Received

Laboratory, 19 December

Sakura,

Ibaraki

305, Japan

1983 by J. Kanamori)

The magnetization process of an itinerant-electron magnet Hf~ 8Taa 9Fe9 has been measured in pulse@" m a ~ t i ~ fields up to 3@0 kOe at 4-33M K. The magnetic phase boundary in the high-field region has been determined and the temperature dependence of the high-field susceptibility in the ferromagnetic or s a t u r a t e d paramagnetic state is reported.

An intermetallic compound TaFegis a Pauli-paramagnet with a hexagonal Laves phase structure (]), while HfFe 2 is a ferromagnet with a Curie temperature of T = 6~0 K which can be crystalized into t~e same structure (2). An alloy of Hf_ 8Ta- 2Fe-is a peculiar magnetic m a ~ r i a ~ " wi~h the same hexagonal Laves phase structure which shows successive phase transitions with increasing temperature from a ferromagnetic state to an a n t i f e r r o m a g n e t i c one at T0=15@ K and the antiferromagnetic stat~ to a paramagnetic one at TN=338 K in zero field (3). Such a p o s s l b i l i t y of the phase transitions is quite difficult to interprete from the localized electron model; the e x c h a n g e - i n v e r s i o n model has been discussed to fail to explain the transition (5). However, it had been predicted to occur from self-consistent renormalization theory (SCR theory) for an itinerant electron system if the wave-vector dependent susceptibility without ~he electron-electron interaction X_ has two peaks at q=~ and q=Q, both I~X~ and I/X~ have strong temperature ~ependences ~ and some conditions like T c
is being expected to be the case (d). The magnetic phase diagram of Hf_ ^Ta- 9Fe 2 in the applied field less t h ~ ~ 7~'~Oe was determined by measuring the temperature dependence of the magnetization with a superconducting solenoid (5). This note reports a measurement of the m a g n e t i z a t i o n process in pulsed high magnetic fields up to 3@0 kOe. Powder samples of Hf~.sTa 0 _Fe2was prepared by melting p6W~er of stoichiometric amount with an arc furnace and annealing at 1000 C for a week. The magnetic fields were generated by discharging high voltages (5 kV-l@ kV) from a capacitor banks (16 mE-4 mF) to a multi-turned coil. The maximum field in repeated runs reaches 300 kOe when the coil is cooled down to the liquid nitrogen temperature and the half period of the field is approximately 16 or 9 ms, which is long enough to measure the magnetization of highly conductive powder of Hf0.8Ta 0 2Fe_. The induced voltages from a " pi~k-up coil for monitoring the field magnitude (H) and from a compensated pick-up coil assembly for measuring the time derivative of the magnetization (dM/dt) together with the integrated signal (M) are memorized in transient recorders and converted to a computer system. The data of M or dM/dt for a sample are obtained by sabstracting the blank signals. The detail of the a p p a r a t u s was reported (6, 7). An example of an observed time dependence of H, dM/dt and M for 1113

1114

Vol. 49, No. 12

MAGNETIZATION PROCESS OF Hf8.8Tao.2Fe 2

H

H -

~H T

T (o)

T

T

M

F, p

T" T (b)

T

T

8

H ( rns)~'~_

T

T

(c)

T

T

T

T* T (d)

T Fig.

M

M

2. An example of observed dependence of the field magnetization (M) , and dM/dt H f 0 . s T a 0 . 2 F e 2 at P2S K.

80

H (e)

H

M

H (f)

H

H

(h)

'

I

'

I

i

Hc

~D

H

I

285 K

H

M

H (g)

'

time (H), for

40

k

0

(a)

I00

200 H

300

(kOe)

80

Fig.

i. (a)-(d): Possible phase diagrams expected from the self-consistent renormalization theory for an itinerant electron system, where F, AF and P i n d i c a t e the ferro-, antiferroand paramagnetic phases, respectively, and F+AF the c o e x i s t e n t p h a s e for H = ~. (e)-(f) : M a g n e t i z a t i o n c u r v e s as a funcion of field passing t h r o u g h the c r i t i c a l fields shown in (a)-(d) (from ref. 4).

219 K Hk

7,, 40

,

O

(b)

L

H 80

Hip ~Ta_ ^Fe^ at 288 K is shown in Fig:~. ~ a m ~ l e s of m a g n e t i z a t i o n curves as a f u n c t i o n of H at three t e m p e r a t u r e s are given in F i g . 3 ( a ) , (b) and (c). A data at 77 K is e s s e n t i a l l y the same as that at 4.2 K in Fig.3(c) . The M-H curve in Fig.3(a) is q u i t e s i m i l a r to the t h e o r e t i c a l one in Fig.l (h) in that it is concave at low field r e g i o n and has a cusp at H= H although the divergence of d M / d H is ~ot o b s e r v e d and the field H~, where dM/dH takes a maximum value, is somewhat different from H . Each M-H curve at the low temperatures in Fig.3(b) and (c) has a hysteresis a r o u n d the c u s p field H. and suggests the t r a n s i t i o n is of the ~irst order in agreement with the l o w - f i e l d

I

I00

'

I

I

,

200

300

(kOe) '

I

'

f 40

4,2 K

0 (c)

Fig.

,

I

I00

,

I

200

,

300

H (kOe)

3. Examples of magnetization curves in Hfo gTa_I~./ ^Fe^~ .at three temperatures. "~'he cusp fzeJds and the fields with m a x i m u m slope are shown.

Vol. 49, No. ]2

d i f f e r e n t from H~ at which d M / d H takes a maximum value and both Hkand H~ together with Hcand H~are plotted in ~ig.4 as a function of ~emperature T. The phase '

1115

MAGNETIZATION PROCESS OF Hfz.8Ta0.2Fe 2

m e a s u r e m e n t (5). However, a sudden jump of th~ magnetization like Fig.l(g) observed in the low-field m e a s u r e m e n t is not seen in the figure and p r o b a b l y the change in the field {30 MOe/sec) is too fast to observe the sudden jump of M for the first order transition in the present case. The cusp field H k is also

I

i

I

i

i

i

-

Wahlfarth

ratio)

(5).

The maximum f i e l d

on the phase boundary, which c o r r e s p o n d s to the point where the first order and the second order transitions meet, is determined to be H*= i~@ kOe +7 % at the temperature of T*= 295 K+5 %.-Each high-field portion of the magnetization curves in Fig.3 is linear to the external field and we regard the slope to be the high-field s u s c e p t i b i l i t y (XHm) in m e t a l l i c system. The inverse 6f the high-field s u c e p t i b i l i t y d e t e r m i n e d at the field of 250 kOe is plotted as a function of temperature in Fig. 5. It is roughly constant at low t e m p e r a t u r e s and tends to zero at about 3~0 K. The high-field

if ! ++++

i

F /FI i 0

P

T(K)

I

I00

200 T

300

400

Fig.

(K)

Fig. ~. The cusp fields (triangles) and the fields with m a x i m u m slope (dots) in magnetization curves in Hf 0 ^Ta_ 2Fe_ are plotted as a fun6~io~" ~ o~ field. The phase boundary determined from the low-field measurement (ref. 5) are also shown with open circles. boundary determined in the low-field measurement (5) is also plotted in the same figure and it c o i n c i d e s with H~ or ;~', hence the fields H' and H' at which • K C . d~/dH take maxlmum values are consldered to be the phase boundary between the antiferromagnetic and the f e r r o m a g n e t i c or paramagnetic sates. The total phase diagram obtained quite looks like the theoretical prediction of Fig.l {d) although the theory is d e v e l o p p e d for weakly magnetic cases and the present material is not very weakly m a g n e t i c system as is seen from the m a g n e t i c moment of 1 up per iron at the f e r r o m a g n e t i c state and the ratio of the effective paramagnetic moment to the saturation momentof 2.2 (Rhodes-

5. The inverse of the h i g h - f i e l d susceptibility obtained at a field of 250 kOe versus temperature in Hf~.~TaH.2Fe 2-

suceptibility is a response of a system to a strong uniform m a g n e t i c field and considered to be something like XZ except an effect of the field strength. Hence T , where Xs diverges, is obtained from the h i g h - f i e l d s u s c e p t i b i l i t y to be 300+10 K. It is somewhat higher than the paramagnetic Curie temperature of 270 K (5), but still the c o n d i t i o n T o . ( = 1 5 0 K) < T < TN (=338 K) Is found experimentally to be satisfied. The absolute magnitude of XHF in t h ~ material at low temperatures Is 2.2x10 emu/g and is an order of maqnitude larger than XHF = 5x10 -6 emu/g for pure ion {P), suggesting a large d i f f e r e n c e of the density of states at the Fermi surface between up and down spin bands. Acknowledgement--We thank Professors T. Moriya, S. Ogawa, N. Miura and Drs. H. Hasegawa Y. Takahashi, Y. Yamaguchi and T. Tsuda for helpful discussions.

REFERENCES ].

2.

~. ~.

K. ~ai, T. Nakamichi and M. Yamamoto, J. Phys. Soc. Jpn. 29, ]09A (1970). T. Nakemichi, K. Mai, Y. Aoki, K. Ikeda and M. Yamamoto, J. Phys. Soc. Jpn. 99, 794 (1970). V. Nishihara and Y. Yamaguchi, J. Phys. Soc. Jpn. 51, 1333 (1982). T. Moriya and w. Usami, Solid State Commun. 23, 935 (1977).

5.

6.

Y. Nishihara and Y. Yamaguchi, J. Mag. and Mag. Mater. 31-34, 77 (]983) and J. Phys. Soc. Jpn. 52, 3630 (1983). G. Kido, N. Miura, K. Nakamura, H. Miyajima, K. Nakao and S. Chikazumi, High Field M a g n e t i s m (Proceedings of I n t e r n a - ~ n a l S y m p o s i u m on High Field Magnetism, ed. M. Date), P.309. N o r t h - H o l l a n d Publishing Co., A m s t e r d a m (]983).

]116 7. 8.

MAGNETIZATION PROCESS OF Hf~.8Ta~.2Fe 2 G. Rido, N. Miura, and K. Adachi, J. Mag. and Mag. Mater. 3]-34, 283 (1983). H. Pauthenet, High Field Magnetism

Vol. 49, No. 12

(Proceedings of International Symposium on High Field Magnetism, ed. M. Date), p.77. North-HoH'land Publishing Co., Amsterdam (1983).