MAGNETIC AND CRYSTALLOGRAPHIC PROPERTIES OF MnRhAs

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MAGNETIC AND CRYSTALLOGRAPHIC

PROPERTIES OF MnRhAs

T. Kanomata, T. Suzuki, T. Kaneko, H. Yasui, S. Miura, Y. Nakagawa

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Supplement au no 12, Tome 49, decembre 1988

MAGNETIC A N D CRYSTALLOGRAPHIC PROPERTIES OF MnRhAs

T. Kanomata (I), T. Suzuki

('1,

T. Kaneko ('), H. Yasui ('), S. Miura (') and Y. Nakagawa (2)

(I) Department of Applied Physics, Faculty of Engineering, Tohoku Gakuin University, 13-1, Chuo-1 Tagajo City, Miyagi Pref. 985, Japan

(') Institute for Materials Research, Tohoku University, Sendai 980, Japan

Abstract. - The crystallographic and magnetic measurements of MnRhAs were carried out. The anomaly of the lattice parameter was found at the temperature Ts 2: 233 K , in which the discontinuity in d~;:,

/

dT was also observed. The

transition at T, is of the 1st-order with the lattice concentration of Aa/a = 0.03 x lo-'.

The intermetallic compound MnRhAs has the crystal structure of a FezP-type. Bacmann et al. [I]

proposed the magnetic structure of MnRhAs as shown in figure 1 on the basis of the results of neutron diffrac- tion: with increasing temperature, MnRhAs shows a first-order transition at Tt

=

160 K, from an AF state t o one in which F and AF components coexist. This state exists up to the Curie temperature Tc = 200 K [I]. Recent results of neutron diffraction [2] and spe- cific heat [3] experiments show that there exists a new AF state above Tc, the NQ1 temperature (T,) of which is 240.7 K.

MnRhAs b

t

TI

Fig. 2.

-

Temperature dependence of lattice parameters a, c and volume V for MnRhAs.

Fig. 1. - Magnetic structures of MnRhAs.

The main purpose of the present paper is to describe the magnetic and crystallographic measurements of MnRhAs.

The details of sample preparation are given in refer- ence [4]. Figure 2 shows the temperature dependence of the a and c axes and the unit cell volume of Mn- RhAs measured by X-ray diffraction technique. There appears an abrupt expansion of the lattice parame- ters at 160 K and a small anomaly at 207 K with in- creasing temperature. The temperature a t the abrupt expansion and the small anomaly of a and c axes cor- respond to Tt and T,, respectively. The magnitude of the relative change of the a and c axes at Tt are Aa/a = 0.02

x

lo-'

and Ac/c w 0.12

x

The value of A a l a is smaller than that given by Chenevier et aL [5]. The value of Ac/c is somewhat larger

than that reported in reference [5]. The thermal ex-

pansion coefficient, V-' (dV

/

dT)

,

is estimated to be

4.98

x

K-' below T, and 12.64

x

K-' above

Tc. The difference of the thermal expansion coefficient,

Aav, at Tc is -7.66

x

lo-'

K-l. We observed an abrupt contraction of the lattice parameter a at 233 K corresponding to Ts, but no anomaly appeared in the

c axis us. temperature curve. The magnitude of the relative change of a at T, is A a l a = 0.03

x

lo-'. It shoud be noted that the transitions at Tt and Ts are of the 1st-order and the transition a t Tc is of the 2nd- order. The entropy change ASt at Tt can be deduced from the Clausius-Clapeyron equations:

ASt = AV

/

(aTt

/

6'p),=-, ₍₁₎

where AV and (dTt

/

6 ' ~are the volume change ) ~ ~ ~

at the transition and the pressure derivative of the transition temperature, respectively. ASt is estimated

C8 - 170 JOURNAL DE PHYSIQUE

to be 0.5R by substituting the values of AV obtained in this study and (aTt

/

ap),=, [4] to the equation ( 1 ) .

This value is large compared with 0.12R obtained from specific heat experiment by Bartolome et al. [3]. The value of A S t determined in this paper is comparable to that of 0.5R measured at the transition from AF to F states for MnsGaC [6, 71. The pressure derivative of

T,

is calculated to be 0.12 K/kbar by using equation (1)

with the values of the volume change at

T,

and the entropy change at

T,

determined from the specific heat experiment [3].

The temperature dependence of the magnetic sus- ceptibility X, below Tt and above

T,

is shown in fig- ure 3. The

X,-T

curve above T, shows no singularity

Since the magnetic moments of Mn and Rh atoms in MnRhAs depend strongly on the magnetic state [ I ] ,

the magnetic properties should be discussed in terms of itinerant electron magnetism. Moriya and Usami

[8] and Isoda [9] obtained various types of magnetic phase diagrams on the basis of the SCR theory of spin fluctuation. However, the transitions from the AF to the AF+F state and then from the AF+F to the new AF state with decreasing temperature observed for MnRhAs does not occur in both theories mentioned above. A further investigation of the magnetic order-

order transition will be necessary experimentally and theoretically.

-

a t

T,.

However, the 1

/

,X

-

T curve exhibits a varia-

[I] Bacmann, M., Chenevier, B., Fruchart, D.,

tion in slope at the temperature 232.5 K, which is in

Laborde, 0 . and Soubeyroux, J. L., J. Magn. very good agreement with the temperature

Ts

= 233 K _{Magn. Mater. 54-57 (1986) 1541.}

of the anomaly in volume change. Thus, the crystal-

lographic and magnetic results in this paper is com- [2] Bartolome, J., Chenevier, B., Bacmann, M., patible with the result reported in neutron diffraction Fruchart, D., Rillo, C., Navarro, R., Regnault, study [2]. J. P., Erkelens, P., Mignod, J. R. and Fruchart,

R., NATO AS1 Ser. (1987).

1

,//,

, !& ;*

,

,

teur, J. P., Chaudouet, P. and Lundgren, L., L.,

Phys. Status Solzdi A 84 (1984) 199.

k..

..

[6] Garcia, J., Bartolome, J., Gonzalez, D., Navarro,

0.1

...*'

..

..'...

A

....

R. and Fruchart, D., J. Chem. T h e m o d y n . 15

(1983) 1059.

700 150 ZOO 250 300 [7] Kaneko, T., Kanomata, T. and Shirakawa, K., J. T I K I _{Phys. Soc J p n} 56 (1987) 4047.

[3] Bartolome, J., Garcia, J., Floria, L. M., Falo, F., Navarro, R., Fruchart, D., Bacmann, M. and De

Fig. 3. - Magnetic susceptibility vs. temperature curve for [81 M o ~ Y ~ , T. and Usami, K., Solid State Commun. MnRhAs. Inset in the figure is the inverse susceptibility vs. 23 (1977) 936.

temperature curve above Tc for MnRhAs. [9] Isoda, M., J. P h y ~ . SOC. Jpn 53 (1984) 3587.

0,5

-

MnRhAs Jongh, L. J., J. Magn. Magn. Mater. 54-57

(1986) 1547.

5/

5

4 Kanomata, T., Shirakawa, K . h i ,

I.

and

-

E"

0.3

Kaneko, T., J. Magn. Magn. Mater. 68 (1987) 286.

-