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THE FIELD-INDUCED PHASE TRANSITIONS IN FeBr2 CRYSTAL

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THE FIELD-INDUCED PHASE TRANSITIONS IN

FeBr2 CRYSTAL

J. Kociński

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Suppl6ment a u no 12, Tome 49, dhcembre 1988

THE FIELD-INDUCED PHASE TRANSITIONS IN

FeBr2

CRYSTAL

Warsaw Technical University, Institute of Physics, Koszykowa 75, 00-662 Warszawa, Poland

Abstract.

-

The field-induced phase transitions in FeBr2-type crystal under hydrostatic pressure have been determined in terms of Landau's theory, with the paramagnetic phase described by a unitary or a non-unitary group. The sets of

low-temperature symmetry groups implied by these two descriptions partly overlap. Introduction

The field-induced phase transition in FeBr2 crystal from the paramagnetic phase (P) to the antiferromag- netic phase (AF) was experimentally detected by Vet- tier et al. [I]. Transitions to AF and ferrimagnetic (Fi) phases were discussed [2] and the corresponding phase diagrams were analysed [3]. A review of metamagnetic transitions was presented by Stryjewski and Giordano [4]. Applying Landau's theory, we will give a group- theoretical discussion of the low-temperature phases which can emerge after field-induced phase transitions, accounting for the influence of external pressure. S y m m e t r y g r o u p of t h e paramagnetic phase a n d o r d e r p a r a m e t e r s

The paramagnetic phase of a FeBr2 crystal in an external magnetic field parallel to the threefold axis is characterized by the symmetry group GoM=PJm' determined by the elements:

al, aa, as, 1, Rs, Rs, 1113, R17

numbered according to Kovalev [5, 61, where 8 is the time-reversal operation, and all a2, as are the basis vectors of the Bravais lattice. According to the Curie principle, this is the maximal common subgroup of the symmetry group of the unmagnetized crystal and the magnetic field, for a fixed mutual orientation of their symmetry elements. To distinguish between the AF and Fi phases, each consisting of two sublattices, two axial-vector order parameters are required. For the AF phase we define the order parameter by

where e, is the unit vector along the threefold axis and S& (x,)

<

0 is the mean spin *component at the point x, of the B-sublattice. The mean spin z-component in the A-sublattice is assumed t o be positive and antipar- allel to the external magnetic field. For the Fi phase we define the order parameter by

where Si {x,) and SG (x,+a) are the mean spins in A and B sublattices a t the points x, and xp+a, respec- tively. Notice that parameter equation (3) can also de- scribe the AF phase, however, parameter equation (2) cannot describe the Fi phase (cf. [4]). By definition, the symmetry of the AF and Fi phases is determined by the symmetry of parameters equations (2) and (3),

respectively.

Active representations a n d corepresentations The paramagnetic phase can be described in terms of a unitary or a non-unitary grpup and consequently, phase transitions can be induced by active representa- tions (reps) or corepresentations (coreps), respectively. These two methods are based on the isomorphism of the two groups: GOM in (1) and Go=P3ml, the latter obtained from the former by omitting the time-reversal operation. To obtain a representation of the magnetic group (1) of the P phase, the matrices of a rep of the group P3ml connected with these rotational elements which in (1) appear together with the time-reversal o p eration, have t o be multiplied by -1. For those groups, six real irreps, and two real and two physically irre- ducible type (a) coreps are connected with the vector k = b3/2 which must be chosen to obtain the observed AF phase (see Bradley and Cracknell 161, Kovalev and Gorbanyuk [7]). We have one-arm star and therefore d l the irreps and coreps are Landau-active, and it can be verified that they are Lifshitz-active. We assume that different low-temperature phases are induced by different reps or coreps. The two-dimensional irreps can be transformed to a real form with help of the matrix

which implies that the complex basis functions fulfil the equality cp2 = cpr. The order parameters equa- tions (2) and (3) are expanded in terms of the basis functions of an active irrep or corep,

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C8 1558 JOURNAL DE PHYSIQUE

with i, j = 1, 2, depending on the rep or corep dimen- sion. The symmetry groups of the low-temperature phases are determined on the basis of these two ex- pansions.

Low-temperature s y m m e t r y groups

An analysis of the spin structures connected with the order parameters equations (2) and (3) shows that the irreps 73 and 76 can induce the AF and Fi phases. The same holds for the coreps dl and d3 @ d 5 . The

remaining reps and coreps cannot induce those phases. The low-temperature symmetry groups connected with 73 and dl are GOM in (I), with the unit cell doubled compared to that of the P phwe. The low-temperature groups connected with 7 5 add d3 g, d5 are determined by the elements: all a 2 , 2a3, 1, R13, and all a2, 2a3,1, R13, OR7, 0R19, respectiv$ly. The symmetry groups

connected with 7 6 and d4 $i de are determined by the elements: all az, 2a3, 1, ( 8 1 3 ( a3)

,

and 81, a2, 2%

1, (R13

1

a3), OR?, Q {Rle

I

a s ) , respectively. The reps 72 and 74 induce still other symmetry groups. Thermodynamic potential density

It has the same form for reps and coreps, since for the two-dimensional reps we have the equality: 9 2 = 9;. The influence of hydrostatic pressure is accounted for by the invariants connected with the rep

rf

at k = 0. Expressed with accuracy to the sixth-degree the potential density is

where

where A; (T

-

Tc,i), i = 1, 2 with Tc,i denoting the transition temperature at zero external stress, Ci, Di,

E2, a , pi, y and 6; are temperature-independent, and where ev=&,,

+

zyy

+

E,, and a,= a,, +a,,

+

o,, are

the basis function of

I??

and the respective external stress. The term in the potential corlnected with the magnetization M in the external field does not influ- ence symmetry change, and has been omitted. The lowest-degree mixed invariants connected with the in- teraction of the order parameters with M, can be in- corporated into the second-degree invariants appearing in @I and @2, respectively. By applying the condi- tion a@/ae,= 0, we eliminate the strain tensor com- ponents and obtain an effective potential density a'. The second-degree invariants in @' have the effective coefficients

The form of A: explains the experimentally determined linear dependence of the critical end-point tempera ture on the hydrostatic pressure 11). According to equation (8), the bicritical end-point, is expected to behave in the same way.

[I] Vettier, C., Alberts, H. L. and Bloch, D., Phys.

Rev. Lett. 31 (1973) 1414.

(21 Kincaid, J. M. and Cohen, E. G. ID., Physics Lett.

A 50 (1974) 317.

[3] Stryjewski, E. and Giordano, N., Adv. Phys. 26 (1977) 487.

[4] Onyszkiewici, Z., Physica 103A (1980) 226. [5] Kovalev, 0. V., Irreducible Representations of the

Space Groups (Gordon and Bre,ach, New York, 1965).

[6] Bradley, C. J. and Cracknell, A. P., The Mathe- matical Theory of Symmetry in Solids. Represen- tation Theory for Point Groups anid Space Groups

(Clarendon Press, Oxford) 1972.

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