"REENTRANT SPIN GLASS" MAGNETISM IN FeNiMn

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”REENTRANT SPIN GLASS” MAGNETISM IN

FeNiMn

B. Huck, J. Landes, R. Stasch, J. Hesse

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Supp16ment au no 12, Tome 49, dbcembre 1988

"REENTRANT SPIN GLASS" MAGNETISM IN FeNiMn

B. Huck, J. Landes, R. Stasch and J. Hesse

Institut fur Metallphysik und Nukleare Festkiirperphysik, Technische Universitat, 3300 Braunschweig, F.R.6'. A b s t r a c t . - On the alloy sysem (Fe0.65Ni0.35)1-, Mn, we observe the reentrant spin glass transition by AC susceptibility, DC magnetization and MGssbauer effect. At the first time two peaks in the low temperature region are observed by ACS in a cubic alloy. Their dependence on external fields is presented together with that on the Curie temperature Tc.

This is a preliminary report of our systema- tic investigation on the magnetic properties of the (Fe0.6~Ni0.35)~-, Mn, alloy series with competing ex- change interactions. Varying z from 0 to 0.245 we investigated 16 polycrystalline foil samples and found four magnetic phases a t low temperatures: imperfect ferromagnetism (0

5

x

<

0.03)

,

reentrant spin glass ' (0.03

<

x

<

0.12), pure spin glass (0.12

<

x

<

0.2) and imperfect antiferromagnetism (x

>

0.2) [I]. Here we confine ourselves t o the sample with x = 0.113 which is an example for the reentrant spin glass beha- viour

.

First this is seen in the de magnetization measu- rements (MM) (Fig. 1) which we performed using a Faraday balance. In the low temperature range we ob- serve a maximum of the spontaneous magnetization at the temperature Tm and additionaly the onset of irre- versibility, at zm revealed in two different branches of the magnetization in field cooling (fc) and zero field cooling (zfc) experiments.

Second we measured the AC susceptibility (ACS) as a function of low external magnetic fields which suppress the influence of the Bloch walls in the ferro- magnetic state. In this experiment we detected three peaks in the X'-signal (Fig. 2) at the temperatures Tc (Curie temperature), TR and

z.

The dependence of

Fig. 1. - Magnetization vs. temperature. The fc and zfc measurements are identically above 25 K. Below this temperature the two different branches (fc and zfc) can be interpreted as a strong irreversibility of t h e spin system. Note t h e measuring field of 100 mT in both cases while the fc field was 1.25 T. The arrow indicates the maximum of the magnetization.

T I K 1

-.

4

0 20 40 60 80 100 120 140 160 180 200 Fig. 2.

-

(a) AC-Susceptibility vs. temperature ( v = 137 H z , H0,ac=5.5 A/m). The measurement without exter- nal d c fidd show a ferromagnetic-like maximum just below

Tc and an anomalous break down below 60 K. Note the large influence of the small external dc field on the am- plitude of the X'-signal. (b) AC-Susceptibility vs. tempe- rature in external dc fields. At 2 mT there are two well distinguished peaks a t and Tc. With increasing ex- ternal field the low temperature peak is splitted into two resolved peaks indicating a different field dependence of

Ti

and TR.

C8 - 1142 JOURNAL DE PHYSIQUE Tc,

TR

and Ti on the external magnetic field is plot-

ted in figure 3. The simultaneous appearence of two characteristic temperatures (i.e. the ACS peaks a t TK and

n)

as predicted theoretically by [2] during one measurement is observed a t a first time for our knou-- ledge. We see this phenomenon for other values of x, too. Additionally these both temperatures were de- tected recently by another group [3] using our sample with x = 0.084. Our values TR (B) and

Z

(B) are not delivering the predicted power laws by [2]. But the correlation between T R , ~ and B as well as between Tc and B is in qualitative agreement with calculations of

PI.

As a third method we applied Mossbauer effect spec- troscopy (MES) in order to determine the mean hyper- fine field at the 5 7 ~ e nuclei. The results are plotted in figure 4. The MES measurements are performed wi- thout an external magnetic field.

In the mean hyperfine field we only detect the onset of the freezing of frustrated spins which manifest itself in a steep increase of the mean hyperfine field at a

Fig. 3. - Field dependence of the three characteristic tem- peratures Tc, TR and Ti. The extrapolations TR (B

-

>

0) and Tj(B

-

>

0 ) deliver different zero field values indica, ting an intrinsic anisotropy of the spin system.

Fig. 4. - The mean hyperfine field H 9s. temperature rnea- sured with Miissbauer effect spectroscopy without external fields. The kink temperature as well as the Curie tempe rature correspond very well with them from ACS measure- ments.

temperature which corresponds to TR and agree very

well with the picture given by [5]. Acknowledgments

We are grateful to Prof. Dr. Ch. Schwink for his continuous support. This work was aided by the Deutsche Forschungsgeme inschaft.

[I] Huck, B., Hesse, J., in preparation;

Huck, B., Thesis, T U Braunschweig (1988). [2] Gabay, M., Toulouse, G., Phys. Rev. Lett. 47

(1981) 201.

[3] Avirovic, M., Univ. Konstani, private communi- cation.

[4] Mitra, A., Ghatak, S. K., J. Magn. Magn. Mater.

51 (1985) 321.

[5] Saslow, W. M., Parker, G., Phys. Rev. Lett. 56