NONLINEAR SUSCEPTIBILITY AROUND THE HELIMAGNETIC-TO-COLLINEAR MAGNETIC TRANSITION IN Cr5S6

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NONLINEAR SUSCEPTIBILITY AROUND THE

HELIMAGNETIC-TO-COLLINEAR MAGNETIC

TRANSITION IN Cr5S6

S. Ohta, A. Kawamoto, S. Anzai, H. Sakamoto

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Suppl6ment au no 12, Tome 49, dkembre 1988

NONLINEAR SUSCEPTIBILITY AROUND THE HELIMAGNETIC-TO-COLLINEAR

MAGNETIC TRANSITION IN Cr5S6

S. Ohta (I), A. Kawamoto (2), S. -4nzai (2) and H. Sakamoto (2)

( I ) Hachinohe Institate of Technology, Hachinohe, 031 Japan (2) Keio University, Yokohama, 223 Japan

Abstract.

-

Linear ( x ~ ) and nonlinear (x2) susceptibilities around the transition temperature Tt mentioned in the tilte are investigated at various frequencies and thermal conditions. A v-shaped anomaly in ~ 7 , is observed around Tt,

although this magnetic order-order transition has a characteristic of 1st order one.

Upon heating process, the helical turn angle 6 and the phase angle

4

gradually decreases and increases, re- spectively, in the antiferromagnetic state (HS) of CrsSs [I]. Then, they coincide with those of the collinear ferrimagnetic state (CS) a t a transition temperature Tt. Very little is known on the critical behaviors of X o

and X 2 around a transition temperature betwenn mag-

netically ordered states. By using a usual Hartshorn- type induction technique, Xo and Xz are simultane- ously measured a t ac-field H = H' sin (2.rrft) with

H' = 40 Oe. For each f, the heating run measure- ments were carried out from 100 K to just above the ferrimagnetic Curie temperature, and then the cooling runs between vice versa.

As seen in figure 1, at all f a sharp decrease in Xo {c) and a sharp increase in Xo {h) are observed a t an inter- val of 5 K centered at about 160 K. Here, the abbrevia- tions h and c in { ) represent the heating (closed signs

used in Figs. 1-3) and cooling (open ones) runs, re- spectively. Only a sharp change in Xo{h and c, 80 Hz) is detected when each Xo was measured after holding several tens minutes under H at a fixed temperature. Such a temperature hysteresis is one of the character- istics of 1st order transitions1.

Figure 2 shows that dXo {c) /dt is positive at 162 K while it turns t o negative a t 161 K. Hence, the equilib- rium transition temperature Tt,

..

is assigned t o be 161.5 K in this sample.

At

161.5 K, Xo {h) vs. t curve exhibits a contribution from a long time relax- ation in addition t o a short time one as long as those observed in Xo {h) a t 158.2 and 166.9 K. Moreover, the fixed temperature measurements of X 2 shows a v-

shaped anomaly a t about Tt,

.,

although a t lower f it is smeared out with the background contribution being proportional t o the magnetization-T curves [2] (Fig. 3). It should be noticed that this type of anomaly has been predicted on a 2nd-order transition between spin-glass and paramagnetic states [3].

t , min TEMPERATURE, K

Fig. 2.

-

Time dependence of

xo

in arbitrary linear scales Fig. 1. - Temperature (T) dependence of xo at f : circles, at temperatures-

f = 20 Hz; squares, 30 Hz; inverted triangles, 60 Hz; tri- angles, 80 Hz. Common symbols are used in figures 1 and

3. Upper and lower data were taken at dT/dt = 0.5 K/min _{h he volume change estimated from the Clausius-CIapeyron} and middle one, at a fixed temperature. The dashed lines relation (0.0004 % ) is smaller than the experimental error in

represent the zero levels. the thermal expansion measurement [5].

JOURNAL DE PHYSIQUE

I

155 160 165 170

TEMPERATURE, K

Fig. 3.

-

Temperature dependence of ~ 2 . Upper and middle data were taken at dTfdt = 0.5 K/min and lower one, at a fixed temperature. Downward arrow indicates Tt,

.,.

We propose a tentative model for the transition ex- hibiting both characteristics of 1st and 2nd order tran- sitions. The HS-CS transition has been understood as

a 2nd order one through thermodynamical considera-

tion with no external magnetic field H e x [4, 51. Since

H,, lowers Tt [2], the 2nd order fea.ture is disturbed through the 3rd term in the Landau-like Gibbs free energy expression

Here, an order parameter q is related t o the change in magnetic symmetry concerning to B and

4,

and A,

B

and C are positive constants. Even if

He,

= 0, in CS the ferrimagnetic intra-domin magnetic field plays a role of H,, in

.

,

,

G

In ground state, HS has

no ferrimagnetic components. Therefore, the thermal fluctuation effects on the potential barrier due to the 3rd term would be responsible for the complex feature of this transition.

The present paper constitutes the first experimen- tal attempt to investigate the nonlin~ear susceptibility around a magnetic order-order transition temperature.

[I] Van Laar, B., Phys.

Rev.

156 (:1967) 654.

[Z] Dwight, K., Germann, R. W., Menyuk, N. and Wold, A., J. Appl. Phys. 33 (1962) 1341.

[3] Shirakawa, T. and Takayama, H., J. Phys.

C

19

(1986) L459.

[4] Dwight, K., Menyuk, N. and KaMas, J. A., Phys.

Rev.

B 2 (1970) 3630.