FERROMAGNETISM IN TbF3

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FERROMAGNETISM IN TbF3

L. Holmes, H. Guggenheim

To cite this version:

L. Holmes, H. Guggenheim. FERROMAGNETISM IN TbF3. Journal de Physique Colloques, 1971,

32 (C1), pp.C1-501-C1-502. �10.1051/jphyscol:19711166�. �jpa-00213987�

JOURNAL DE PHYSIQUE Colloque C1, supplément au n° 2-3, Tome 32, Février-Mars 1971, page C 1 - 501

FERROMAGNETISM IN TbF ³

by L. HOLMES and H. J. GUGGENHEIM

Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey

Résumé. — On signale des mesures d'aimantation sur des cristaux de TDF3 orthorhombique. On trouve des indi- cations de ferromagnétisme à T < T

= (3,95 ± 0,05) °K avec un moment de saturation suivant a de 8 #

/Tb

. On observe pour H || c une transition magnétique, induite par le champ. On interprète les résultats par le truchement d'un modèle à deux réseaux composants dans lequel les moments magnétiques de 9#B/Tb

sont fixés par le champ cristallin à des angles de ± (27 ± 2)° relativement à l'axe a dans le plan c-a.

Abstract. — Magnetization measurements are reported on crystals of orthorhombic TbF3. Evidence of ferroma- gnetism is found at T < Tc = (3.95 ± 0.05) °K, with a saturation moment along a of 8 /te/Tb

. A field-induced magnetic transitions is observed for H \\ c. The data are interpreted with a two-sublattice model in which magnetic moments of 9 /te/Tb

are fixed by the crystal field at angles ± (27 ± 2)° from the a axis in the c-a plane.

Magnetic ordering at low temperatures in aniso- tropic rare earth compounds is of interest when the combination of weak exchange interactions and high anisotropy (approaching the Ising limit) permits detailed experimental and theoretical study.

In the present investigation we have found that ortho- rhombic T b F

(space group T>\t — Pnma [1]) is ferromagnetic below 4 °K. In previous work on TbF

, the magnetic susceptibility has been measured on powder specimens above 4.2 °K [2] and optical absorp- tion and fluorescence spectra have been obtained [3].

The transparent, colorless crystals were grown from the melt using a modified Stockbarger technique [4].

Magnetization data were obtained in a vibrating- sample magnetometer at temperatures between 1.4°

and 300 °K in applied magnetic fields, H, of up to 14.5 kOe.

Magnetic susceptibilities, x, were evaluated from the slopes of magnetization curves along the three crys- tal axes. For the b axis, % is roughly independent of temperature below 40 °K and is < 8 % of x

or Xc at 4.2 °K (Fig. 1). The apparent reversal of anisotropy in the b-c plane at ~ 105 °K is presumably associa- ted with thermal population of excited crystal-field levels [3] of the T b

ions. The susceptibility, #

, of a powder sample has also been measured (Fig. 1), and our data are consistent with x

— (Xa + Xb + Xc)/3,

FIG. 1. — Inverse magnetic susceptibilities in TbF3. The filled-in points are for a powder specimen.

providing approximate demagnetizing corrections are applied. Above 30 °K, the data may be fitted to a Curie-Weiss law,

with

(as compared to 9.72 fi

for free T b

ions) and 0 = (0 ± 1) °K. These values may be compared to j"eff .= 9.47 /j.

and 9 = — 0.2 °K reported in a pre- vious work [2] on TbF

.

Magnetic ordering is achieved at pumped-helium temperatures. In low magnetic fields (H « 10 Oe), X

remains constant below T

= (3.95 + 0.05) °K and is determined by the demagnetizing factor of the sample, while in higher fields, the magnetization along the a axis saturates to M„ = (8.09 + 0.20) ^

at

1.4 °K (Fig. 2). A small discontinuity occurs in #

, and Xc decreases rapidly, at temperatures below T

(Fig. 1). We ascribe these results to ferromagnetic ordering in a highly anisotropic structure. Consistent with the magnetic data, a 1-type anomaly has been observed in specific heat measurements [5] on TbF

, with the peak occurring at T

= (3.913 + 0.002) °K.

A field-induced transition is observed in magnetic fields applied along the c axis at T < T

(Fig. 2).

The magnetization increases sharply at the «transi- tion » field, H

, where H

(1.4 °K) = (12.3 + 0.2) kOe.

The behavior has been followed in magnetic fields up to 60 kOe, and it has been found [6] that the tran- sition becomes increasingly step-like as the tempera- ture is lowered to 1.1 °K. At that temperature, the magnetization along the c axis [6] saturates abruptly to M

° = (3.84 ± 0.08) fi

at fields above

The magnetic behavior may be understood in terms of a two-sublattice model. The T b

ions occupy two magnetically inequivalent sites of point symmetry, C

, with principal magnetic axes related by a mirror reflection in the a-b plane. Making reference to an earlier interpretation [7] of metamagnetism in TbA10

, which belongs to the same space group, DH, we assume that the lowest crystal-field level of the T b

,

7

F

ground state is an « accidental» doublet (*) of the (!) The fact that TbFej orders magnetically below 4 °K shows that the tentative assignment of ? F

levels in Ref. [3] must be rearranged to put such a doublet at the bottom.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711166

C 1 - ⁵⁰²

L. HOLMES AND H. J. GUGGENHEIM

FIG. 2. - Magnetization curves at low temperatures in TbF3.

Sample was an irregular b platelet and demagnetizing correc- tions have not been applied.

form M j = +_ 6 . At each of the ~ b sites, the axis ~ + of quantization for the accidental doublet lies in the c-a plane at an angle @, from the a axis (Fig. 3). If

FIG. 3. - (A.) Symmetry-determined axes of quantization, z l and 22, at two Tb3+ sites in TbF3. (B.) Proposed two-sublat- tice model for ferromagnetic ordering in TbF3. Dashed line shows field-induced reversal of sublattice << 2

at H

Ht along

the

axis.

we neglect the small second-order terms which couple Mj = + 6 with higher-lying crystal-field levels, then the. magnetic moment per Tb3+ ion,

is strictly confined to the local axis of quantization.

For T < ^T,, the saturation magnetization is M: = I ^p I cos @,

along a, or M: = ] p ] sin cDo in high magnetic fields, H > H,, along c (Fig. 3). For T > T,, the paramagne- tic susceptibilities associated with the Mj = f 6 Ievel are given by, X, = Np2 cos2 cDo/k(T - 0,) and

xc ⁼ Np2 sin2 @,/k(T - 8,) ,

where B,, 8, are Curie-Weiss parameters. The measured magnetizations [6] at T 6 T, and the paramagnetic susceptibilities below 40 OK may be fitted t o these equations with cD, = (27 f 2j0, 8, = (4 + 1) OK and 8, = - (4 _+ 1) OK. An approximate correction t o zero demagnetizing factor has been applied to 8, and 8,. The probable magnetic structure in TbF, may be described in terms of linear combinations, F and C, of magnetic moment vectors for the Tb3+ ions, ana- logous to the base vectors which Bertaut has used [8]

in describing magnetic symmetries in the rare earth orthoferrites. The saturated state along the a axis may be described by the notation C , F, and along the c axis by F, C,, where z and x refer t o the c and a axes, respectively. The transition from C, F, to F, C , at H = H , may be interpreted as a

spin-flip

on one of the magnetic sublattices (Fig. 3), which occurs because the Tb3+ moments are confined to the axes of quantization in this highly anisotropic material.

In this case, one can show on a two-sublattice model that H, (0 OK) is proportional to the inter-sublattice molecular field.

The magnetic ordering in TbF, is unusual, but one may hope to understand much of the behavior with careful study. Experiments are planned on spherical specimens, which permit accurate corrections for demagnetizing fields, t o determine the dependence of M: and H, on temperature 151.

The authors are grateful to R. Hecker for technical assistance.

References ZALKIN (A.) and TEMPLETON (D. H.), J. Amer. Chem.

Soc., 1953, 75, 2453.

KERN (S.) and RACCAH (P. M.), J. Phys. Chem. Solids, 1965, 26, 1625.

KRUPKA (D. C.) and GUGGENHEIM (H. J.), J. Chem.

Phys., 1969, 51, 4006.

GUGGENHEIM (H. J.), J. Appl. Phys., 1963, 34, 2482.

HOLMES (L.), GUGGENHEIM (H. J.), MAITA (J. P.) and HECKER (R.) (to be published).

HOLMES (L.), GUGGENHEIM (H. J.) and HULL (G. W.), Solid State Commun. (in press).

HOLMES (L.), SHERWOOD (R.) and VAN UITERT (L. G.).

J . &l. Phys., 1968,' 39, 1373.

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New York, 1963), vol. 111, p. 150.