LT 21 Proceedings of the 21st International Conference on Low Temperature Physics Prague, August 8-14, 1996

Part $4 - LT Properties of Solids 1: Magnetism (experiment)

Susceptibi l i ty measurement s on Y b l G as a test o f a refined theoretical model

J. Kol~i~ek, I. Veltrusk~, Z. ~im~a

Institute of Physics ASCR Cukrovarnickh 10, CZ-162 00 Praha 6, Czech Republic

1. INTRODUCTION

Owing to the complexity of its magnetic structure, ytterbium iron garnet (YblG) possesses a very rich magnetic phase diagram at low temperatures and in moderate magnetic fields. While the phase diagram is theoretically well understood [11 and the model experimentally confirmed [2] for magnetic field applied along [100] and [111] axis, for magnetic field applied in the [110] direction some controversy persists.

Experimental data [2-3] clearly indicated a first order phase transition in both the low temperature and low magnetic field region, whereas theory based on the

free-energy expansion in the neighbourhood of the tetracritical point predicted only phase transition of the second order 141. Our previous calculations 15l revealed the existence of the first order transition for temperatures above the compensation point. On the other hand, our far infrared measurements that indicated a phase transition below this point [3] remained unexplained.

To resolve this long lasting controversy the ac magnetic susceptibility measurements were made and the refined theoretical model was used to calculate the susceptibility. Both methods confirmed the existence of the low-temperature low-field phase transition.

o:

-m .Q 4d eL o u =1 m

Io w

m u m

,Q

eL o o M

u

a ) o o o o o H : 0 . 2 T

o o

o

o o o

o o

o 0 . 4 ~

~ - 0 . 6

ib 2o 3b 4o

b) / H = 0.2T

40 ~.o ~o 4o Temperature [K]

c) ..... " J - " ,,mr:: . . . . .

..D -" . / .:,,,-* 7 ..... . - ..~-- . . " 6 / ' , , . - J /

../" ../..."..-"'T : 5 K ,m . - " / /

, . .e . . . j . . . ' " " ) e - 8 . . ' u /

...... m . " J ' " .IR:s

�9 ......... te ........ �9

0 .0

o.o o~t o~2

o~s t~o

4 T : 7 K

M a g n e t i c f i e l d [ T ]

Figure 1. AC susceptibility of YbIG with dc magnetic field applied along [110] axis. Measured temperature and magnetic field dependence a) and c) are qualitatively explained by the theoretical calculations b) and d).

Czechos lovak Journal of Physics, Vol. 46 (1996), Suppl. $4 2159

2. SUSCEPTIBILITY MEASUREMENTS AND C O M P A R I S O N W I T H T H E O R Y

AC magnetic susceptibility was measured at frequency lkHz and drive amplitude 3 0 e by a sensitive SQUID magnetometer Quantum Design MPMS-5S in the temperature region 2-40K and applied dc magnetic fields up to 5T. The temperature dependence of the magnetic susceptibility for several applied fields is displayed on Fig.In. Two sharp drops on the susceptibility curves may be observed: the one at temperature of about 7K is ahnost independent on the applied magnetic field; the position of the other monotonously decreases with increasing magnetic field. These features are nmnifestations of the magnetic plmse transition predicted by the tlleoretical calculations (Fig.lb). The existence of the low temperature phase transition (below the compensation point 8K) is documented by measurements of the dependence of the susceptibility on magnetic field for several selected temperatures, which are displayed in Fig. lc. Here the phase transition results in the increase of the susceptibility with the increase of magnetic field. For increasing temperature the magnetic filed at which the transition takes place decreases, but for temperatures above the compensation point it increases very quickly. As can be seen from Fig.ld, such a behaviour is expected also theoretically, only the temperature of compensation point deduced from the theoretical calculation is slightly lower than it follows from the experiment. Also, the theoretical magnetic field needed to induce this low temperature phase transition is found considerably lower than it is experimentally observed.

A comparison of the theoretical and experimental phase diagram is made in Fig.2. Taking into account that all parameters for numerical calculations were taken from independent optical measurements [6] and no fitting was made, we consider the presented qualitative agreement as a justification of the applicability of our theoretical model.

3. CONCLUSIONS

The ac susceptibility measurements were made for magnetic fields along the [ 110] axis and the results were compared with the numerical calculations based on a two level model of Yb ions in the molecular field approximation. Good qualitative agreement confirms validity of the model, further improvement can be achieved by a slight variation of the input parameters. The existence of low-temperature low-magnetic field phase transition is confirmed. In our previous far

infrared measurements [3] this transition was observed at considerably higher fields. This discrepancy may be attributed to the fact that samples for the .two measurements were not identical and the position of the phase transition may be very sensitive with respect to the quality of the crystal.

4-

E o u

�9 e

lU

E e

0 , 0 S 1 0 t ' S 2 '0 2 'S ~ ' 0 ~ 'S , iO

Temperature [K]

Figure 2. Phase diagram of YblG for the applied magnetic field along [110] axis. The theory (lines) is compared with the results obtained from the susceptibility measurements (points).

REFERENCES

[11 R. Alben, Phys. Rev. B2 (1970) 2767 [2] J.L. Feron, G. Fillion, G. Hug, A. Berton,

J. Chaussy, Sol. St. Comm. 10 (1972) 641 131 J. Kolfi~ek, Z. Sim~a, R. TesaL Meas. Sci. Technol.

4 (1993) 1085 [4] J. Lots, phys. stat. sol.O) 84 (1977) 457 [5] I. Veltruslq~, J. Kolh~ek, Z. Sim~a, 9 th Czech and

Slovak Conference on Magnetism, Ko~ice, August 1995

[61 W.P. Wolf, M. Ball, M.T. Hutchings, M.J.M. Leask, A.F.G. Wyatt, J. Phys. Soc. Japan Suppl 17 (1962) 443

2160 Czech. J. Phys. 46 (1996). Suppl. $4