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MAGNETIC PROPERTIES OF TmSe
J. Flouquet, F. Holtzberg, M. Papoular, O. Pena, R. Tournier, C. Vettier
To cite this version:
J. Flouquet, F. Holtzberg, M. Papoular, O. Pena, R. Tournier, et al.. MAGNETIC PROPERTIES OF TmSe. Journal de Physique Colloques, 1980, 41 (C5), pp.C5-255-C5-257. �10.1051/jphyscol:1980543�.
�jpa-00219978�
JOURNAL DE PHYSIQUE Colloque C5, supplément au n" 6, Tome 41, juin 1980, page C5-255
MAGNETIC PROPERTIES OF TmSe
J. Flouquet, F. Holtzberg , M. Papoular, 0. Pena, R.Tournier and C. Vettier
Centre des Reaherches sur les tres basses temperatures, C.N.E.S., B.P. 166 X, 38042 Grenoble Cedex, France.
Résumé.-Nous discutons les propriétés magnétiques de TmSe à partir des résultats récents obtenus sous pression par mesure d'aimantation et de diffraction neutroni- que. L'analyse est faite sur la base d'une compétition entre énergie d'échange et énergie de corrélation à un ion. Le diagramme de phase (H,T) est analysé suivant une analogie faite d'après le modèle de Blume-Emery-Griffiths développé pour les mélanges isotopiques d'hélium.
Abstract.- The magnetic properties of TmSe are discussed using recent results of neutron diffraction and magnetization under pressure. An analysis is made using a competition between exchange and a one ion correlation energy. The phase diagram
(H,T) is analyzed throught an analogy with the Blume-Emery-Griffiths thermodynamic model of isotopic helium mixtures.
Magnetization measurements performed under pressure p < 10 kbar have shown that the valence mixing remains constant under this range of pressure if its definition is given by the high temperature susceptibility law,x , /l/. Neutrons diffraction experi- ments / 2 / have shown :
(i) the persistence of the type I AF struc- ture up to 20 kbar, the highest applied
pressure. .„
(ii) the initial positive slope -y-pi MD.09 K/kbar of the ordering temperature TN with the pressure followed by a flat regime between 8 to 20 kbar,
(iii) the similar variation of the magnetic moment \i in the ordered phase.
The discussion of the magnetic proper- ties over a large range of pressure is quite a puzzle as i) the magnetic response of an intermediate valence compound is far to be understood for first principles and ii) the bandstructure must be known in order to take into account an eventual metal insula-
* Permanent address : Institut Laue Langevin, 38042, Grenoble, Prance.
+ Permanent address : IBM T.J. Watson Research Center, P.O. Box 218, Yorktown, Heights, N.Y. 10598, U.S.A.
tor transition /3/. A basic approach can be similar to that performed by Goodenough /4/ for the 3d metallic oxides where the point (ii) is extensively discussed.
We will present firstly an analysis of the experimental results based on the competition between a correlation energy called k T and the exchange energy among the Tm ions which are characterized by the nearest and next nearest neighbour exchange coupling Xiand \z. The correlation energy can be regarded as the analog of the Kondo energy for the abnormal cerium compounds/5/;
it reflects the low temperature properties due to the local exchange J coupling of the
localized moment of Tm with the itinerant electrons. The approximation is that k T is a one ion parameter. It arises for example in the low temperature properties such as the susceptibility as a translation in the temperature scale. The usual suscep- tibility law :
will be modified by :
x = T + T„ - e (2) It must be emphasized that the strength of TK at different temperatures can be modified by auxiliary couplings with the lattice.
This phenomena is wellknown for the Ce dilute alloys /6/.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980543
JOURNAL DE PHYSIQUE
Only the low temperature determination will be discussed. Accordingly TK is taken or not taken into account, the usual
expr6ssions of 8, TN,
x
(TN) as a function of h l r h 2 for a type I AF structure /7/ must be modified /14/.TABLE I
.-
Magnetization analysis Pa)TK=O 9 -2.2 +2.7 -0.8 8 kbar -5.5 +2.0 -2.8 0 0 -2.1 +2.7 -0.8 ao.1 b)T~fO 8 kbar -4.5 +7 -0.6 $6
X I and X2 is given in mole Oe/uem.
Table I represents the derivation of Xl,A2 and Xl/h2 takinq or not takinq into account the TK occurence. At p='0, the introduction of TK does not change significantly X
,
X 2whereas at p=8 kbar a strong variation is observed. For TK=O, under a 8 kbar pressure, the ratio AI/X2 is multiplied by 4 compared to the zero pressure value ; whereas with a TK'term. this ratio remains almost cons- tant and a stronq variation is observed in the TK estimation. We have chosen this last description assuming that the valence mixing remains constant under pressure /1/ and consequently the electronic structure
-
1characterized by the Fermi wavelength KF
,
since it can be assumed that the valence mixing gives the number of electronic carriers. A 1 and X2, -is related to J and KF by a RKKY like law :
r 1 and r being the first and second neighbour distances. The effect of the pressure is an increase of J ; K F r can be reqarded as constant. X1, h p and TK increase consequently with their J dependence but the ratio hl/X2 remains constant. The strong variation observed for TK is in excellent agreement with a tunnelling like dependence with the local exchange coupling J :
-
cte.
TK a cte exp
-
JThe neutron observation of the initial increase under pressure of TN followed by a flat maxima can be described bly our TK model.
Under pressure, Tx increase more ra9idlp than X 1, X L
.
The energy (the tendency of a non magnetic ground state) competes with the exchange energy. An optimum TN(p) appean for a critical value of J. It must be ampha-- sized that the true mechanism is more compli- cated than this description as i) a new insulating phase seems to occur at TN /6/at least for p < 6 kbar /8/ and ii) at high pressure p > 20 kbar the pure Tm3+ configu- ration must appear.
For dilute alloys of Ce such as E C e , the main effect of the pressure is the decrease of the magnitude
I E I
of the0
energy of the 4f level relative to the Fermi level /9/. When the cerium ions approaches under pressure a valence instability, in a Hartree Dock frame, the fictitious local exchange hamiltonian must be renormalized /lo/. An optimum in J(p) occurs for Eo equal to
-
6 , 6 being the half linewidth of the 4f virtual level due to the resonant mixing with the conduction electrons. The corres- ponding possibility of a TK optimum for TmSe is an open question. A supplementary striking point is the p range where the~ m ion can be regarded as an ordinary rare ~ + earth ion or an anomalous one like Ce in A1 ,Te or A1 g e /5/.
A similar analysis has been made for TmxSe samples. In the vicinity x .r 1 as x decreases, the main results are, the
decrease of the percent of Tm2+ contributing to the valence mixing, the increase of the magnitude of the antiferromagnetic coupling A l l the decrease of A 2 and finally the increase of the ratio X1/X2. We have
mentioned in reference /2/ that no evidence of the importance of the double exchange as proposed by Varma /11/ can be given by pressure measurements. We remark that the X 1 dependence with the percent of the ~ m ~ + ions is qualitative1y.h agreement with
Varma's prediction.
We would like now to point out that an /3/ Haen, P., Lapierre, F . , Mignot, J.M.
supplementary ferromagnetic coupling may Tournier, R. and Holtzberg, F., ~ h y s . Rev. Lett.
43
(1979) 304.explain the phase diagram (HIT) of TmSe
/4/ Goodenough, J.B., in Hog Solid State /12/. Whether or not this coupling is due Chemistry
2
(1971) 145.double exchange is an open question. Granted /5/ Benoit, A., Flouquet, J. and Riuault, with this coupling we can build up an M., J. Physique 40 (1979) C5-328.
analogy with the Blume-Emery-Griffiths /6/ FlouqUet, J., Phys. Rev. Lett.
27
(1971) 515.
(B.E.G.) model for isotopic helium mixtures /7/ Smart effective Field Theories of /13/. The real 3 ~ e
-
4 ~ e system exhibits Magnetism (Saunders, Philadelphia chemical potential A and thus the (LIT) 1966).phase diagram is similar to the (H,T) one. 18/ F1Ouquet, J * Haen, P. 1 Holtzberg,F. 1
Lapierre, F., Mignot, J.M., Ribault, B.E.G. showed numerically that the corres- M., and Tournier, R., TO be published ponding (A,T) would distort to display a in this conference.
triple point on a new line of 1st order /9/ See Coqblin, B., Maple, M.B., and Toulouse, G., Int. J. Magn.
1
(1971) transition points ending in a critical 333.point (line IV of the TmSe phase diagram /12/). The figure 6d of reference /13/
strikingly resembles the (HIT) diagram for TmSe. In order to develop in more detail our analogy, it will be stimulating to know the evolution of the phase diagram with the
~m'+ percent : experimental cases may be TrnSe,-xTex or TmxSe.
Conclusion.- We have developed a model b a s d on a magnetic delocalization parameter TK.
The choice of the pressure dependence of TK must be clarified in the future by transport and other dynamical properties measured under pressure.
References
/lo/
Toulouse, G. and Coqblin, B., Solid State Commun1
(1969) 853./11/ Varma, C.M., Solid State Commun
2
(1979) 537.
/12/ Ott, H.R., Andres, K. and Bucher, E.
A I P Conf. Proc.
24
(1974) 40./13/ Blune, PI., Emery, V.J. and Griffiths, R.B., Phys. Rev. (1971) 1071.
/14/ Pena, O., Thesis (1979) Grenoble unpublished.
/1/ Chouteau, G., Holtzberg, F., Pena, % 0.
Penney, T. and Tournier, R. J.
Physique
40
(1979) C5-361./2/ Vettier, C., Flouquet, J., Mignot, J.
M. and Holtzberg, F., to be published I C M 79.
