Field induced transition in the heavy fermion compound CeCu2

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PhysieaC 185-189 (1991) 2641-2642 North-HoHand

FIELD INDUCED TRANSITION IN THE HEAVY FERMION COMPOUND CeCu 2

ashigeru TAKAYANAGI, blzuru UMEHARA, bKazuhiko SATOH, and byoshichika ONUKI a

.

.

.

.

aPhyslcs Department, Sapporo Campus, Hokkaldo-Kyoulku Unlversity, Sapporo, 002, Japan. ~Institute of Material Science, University of Tsukuba, Ibaraki 305, Japan.

We have measured the specific heat of CeCu9 under magnetic fields up to 6 T in the temperature range between O.7-16 K. When the magnetic field applied to the b-axis, it gives almost no effect on the specific heat peak up to 6 T. Above 2 T applied along the c-axis, however, a new specific heat peak appears in addition to the peak at 3.5 K. The new field-induced peak shifts to high temperature as growing up the intensity with increasing magnetic fields. We also found a change of the specific heat, which is corresponding to the metamagnetic transition at 2 T observed in the measurement of the magnetization along the a-axis.

i. INTRODUCTION

performed on

Cerium intermetallic compound is particularly

interesting with respect to the competition

small single crystal sample by an

adiabatic method in magnetic fields from 0 to 6 T applied along the a-, b- and c-axes.

between Kondo effect and magnetic interactions. Therefore,

a lot of works has been done on the

3. RESULTS AND DISCUSSION

Ce-compounds. The intermetallic compound CeCu 2 is

an

ordered heavy fermion system

with

the

temperature

at

shifted

to

K. 1'2 The temperature dependence of the

field, 2

At 2 T,

reaches

almost

antiferromagnetic TN=3.5

When the magnetic field is applied to the a-

transition

magnetic susceptibility had been measured on

a

single crystal of the CeCu2 .3 The susceptibili-

axis

below 2 T,

above

2

the depressing main

lo~er temperatures with

K°

constant

value of

4

This critical field H =

and

J/K.mol 2

T

is

C

tyshowsa strong uniaxial anisotropy. of

the Ce moment is pointing

The easy along

the

correspond[n~

surements. 1'3

a-axis

The

hump

the

netic field.

indicates metamagnetism with 2

anisotropy

is

considered

to be

T.

due

to

orthorhombic crystalline electric field. 4 These

nously

Ising-like system but the magnetic structure is not simple°

that in

Single

method. solid

transition

2 T,

the

mea-

Schottky-type

increasing

mag-

For H > 2 T, the maximum tempera-

tures of the C/T vs. T curves increases monoto-

magnetic

tungsten

Above

develops gradually with

properties indicate that CeCu 2 belongs to an

tive

2. EXPERIMENTALS

t(~ th~ metamagnetic

field found by the previous magnetization

crystalline a-axis. The magnetization along the

a

is

increasing

the main peak disappears

i

axis

peak

with

increasing

phase

magnetic

diagram is in

field.

agreement

The with

determined by the electrical resistivity 4 the magnetic field. We estimate the effecBohr

magneton as i.8 ~B for

the

a-axis

crystal

of CeCu 2 was grown

from

a

from the C/T)max vs. T relation of the Schett~v

crucible

by the Czochralski

pulling

specific heat. The magnetic moment is in agree3 ment with 1.6 ~B obtained by the magnetization

The single crystal rod was purified by state electro-transport method

at

a

pressure of ixlO-9 Torr. CeCu2erystallizes into

measurement along the a-axis at 8 T° In the case of the magnetic field along

the orthorhombic structure with a=4o44 A,b=6o97

the b-axis,

A and c=7.56 A. Specific heat measurements were

dependence up to

0921-4534/91I$03,50 © 1991 - Elsevier Science Publishers B.V.

All fights reserved.

the specific heat shows no field 6 T.

However, for

& Taka~nagi et aL

2642

/ Field induced transition in the heavy fermion compound CeCu 2 along the c-axis, the main specific heat peak

CeCu2

8

a

H/lc-~xis

Z-

7

moves gradually toward lower temperatures with increasing field, as shown in fig. I. Above 2 T, a new sharp peak appears and is shifted

6

5

I

toward higher temperatures with increasing

o"

4.

magnetic field.

3 2

#

s

,

;

/

H=O.O (TI ~

- - ~ - °o°°~°°''°°°'"

We

Y" ."

/ /

.

can

not find any evidence

of

the

new

~.~--- 1.5(TI

.,"..-" /, °

into a single sharp peak. The resulted magnetic phase diagram is determined as shown in fxg. 2.

,

2

At 6 T, the two peaks merged

magnetic

transition

in

the

measurement

of

•

:.-,./

k.,..---...

3.0IT)

magnetization

curve below T N.

So

the

field-

induced new phase transition should be a second

E

:2.

order one.

--3

.'/ ." ,;.~•

C.D

•

i

,/

~

In

4.0(T]

•

conclusion,

4.5(T1

tion

which

above

~.....~

5.0(T)

'~..,/.

6.0(T)

#

/

/

is induced by the

H=2.0

CeCu 2.

We

T applied along note

that two

magnetic the

/

for

cusp-like

small anomalies on the specific heat are observed

mental

[K]

field

c-axis

apparent

at 2.6 and 3.2 K at zero-field.

Temperaiure

specific

heat peak accompanied with the new phase transi-

//"..i :.

we have found the

result

indicates that

magnetically

ordered

the

magnetic

complex

The the

experi-

antiferro-

heavy fermion CeCu 2 structure

below

has T N-

Detailed results will he reported elsewhere. Fig. i. Temperature dependence of the specific heats of CeCu 2 in various magnetic fields applied parallel to the c-axis.

ACKNOWLEDEMENTS This work was supported by Grant-In-Aid

~

01 6

the

50

REFERENCES

-=

for

Scientific and Culture in Japan and by the

Supply

Center for low temperature

liquids

at

Hokkaido University.

CeCu2

H//c -a x i s

4.0 6Z o 3.0

~ 2.o

1:

I.

E. Gratz, E.Bauer, B. Barbara, S. Zermirli, F. Steglich, C. Do Bredl and W. Lieke, Jo Phys. F, 15 (1985) 1975.

2.

S. Takayanagi, l.Umehara, Ko Satoh,

Y. Onuki and

I.

N. Wada, J. Magn. Magn. Mat.

90 & 91 (1990) 479.

Temperature

3.

Y. Onuki, Y. Nakai, 1. Omi, T. Yamazaki and T. Komatsubara, J. Magno Magn. Mat. 76& 77 (19~7) 119.

4.

K.Satoh, I. Hmehara, A. [-ukuda, Y. Kurosawa, Y. On ~Pi end k Maez~wa, J° Magn° Magn. Mat. 90 891 (199[)) 141

[KI

Fig. 2. Magnetic phase diagram for CeCu 2 in the magnetic field along th~ c-axis.