HAL Id: jpa-00214456
https://hal.archives-ouvertes.fr/jpa-00214456
Submitted on 1 Jan 1971
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
MAGNETIC RESONANCE IN FERROMAGNETIC CRYSTALS WITH M2CuX4.2H2O STRUCTURE
T. Klaassen, N. Poulis
To cite this version:
T. Klaassen, N. Poulis. MAGNETIC RESONANCE IN FERROMAGNETIC CRYSTALS WITH M2CuX4.2H2O STRUCTURE. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-1157-C1-1158.
�10.1051/jphyscol:19711414�. �jpa-00214456�
JOURNAL DE PHYSIQUE Colloque C I , suppliment au no 2-3, Tome 32, Fe'orier-Mars 1971, page C 1 - 11 57
MAGNETIC RESONANCE IN FERROMAGNETIC CRYSTALS WITH M ~ C U X ~ . 2HzO STRUCTURE
T. 0. KLAASSEN and N. J. POULIS
Kamerlingh Onnes Laboratorium, Leiden, The Netherlands
Rhume. - Nous avons entrepris une etude de l'arrangement magnetique des cristaux isomorphes MzCuX4.2 Hz0 (M = K , NH4, Rb, Cs ; X = Cl, Br). Pour tous ces cristaux la temp6rature de Curie se trouveentre 0,7 OK et 1,8 OK.
Dans l'etat'paramagnetique on a fait de la resonance quadrupolaire pure et de la R. M. N., des experiences qui nous four- nissaient les parametres des couplages quadrupolaires pour tous les noyaux. Dans l'etat ferromagnetique la variation des raies de resonance avec la tempkrature est mesuree pour tous ces noyaux, dont les lnteract~ons hyperfines ont Cte calculks par la sCparation des contributions quadrupolaires et magnetiques.
Abstract. - Magnetic ordering of the isomorphous tetragonal crystals M2CuX4.2 H20 (M = K, NH4, Rb, Cs and X = CI, Br) has been studied. The Curie temperature of these crystals varies between 0.7 OK and 1.8 OK. In the para- magnetic state p. q. r. and n. m. r. has been performed, yielding the parameters of the quadrupole coupling for the various nuclei. In the ferromagnetic state the temperature dependence of the resonance lines of all nuclei has been measured. By separating quadrupolar and Zeeman contributions, the hyperfine interactions for the nuclei have been evaluated.
The crystals are grown by evaporating an aqueous solution of the components. The tetragonal unit cell, shown in figure 1 for (NH4)2CuBr4.2 H 2 0 , consists
FIG. 1. -The unit-cell of ( N H ~ ) ? C U B ~ ~ . ~ H ~ O : the X I sites are situated along the short, the XII sites along the long X-X
diagonal.
of 2 formula units. Apart from magnetic dipole inter- actions, there are two, magnetically inequivalent X positions, XI and XI,, and only one M position. The ions on the XI position will have a strong, and those on the XI, positions a weaker magnetic interaction with the nearest Cu2+ ion.
In the paramagnetic state the pure quadrupole resonance frequencies of the nuclei with 1 > 4 are determined. By studying in a small external magnetic field the Zeeman perturbation on the large quadrupole interaction, the axes of the electric field gradient tensor and the asymmetry parameter ? are found.
The measurements in the ferromagnetic state are
performed in zero external field with a frequency modulated and frequency swept Pound-Wattkins type oscillator. The temperature dependence of the reso- nance line of all nuclei, including those of the Cu2+
and 02- ions, are measured. These results together with the PQR frequencies for the various nuclei yield, using the theory of Brown [I], the energy level dia- grams and the internal magnetic fields as a function of temperature.
In all crystals two sets of resonance lines for the X ions were observed. The XI site gives rise to a triplet, consisting of the three Am, = I transitions. The internal field at all these X, nuclei, extrapolated to T = O O K , is of the order of 15 kOe. A decrease of the internal field is observed with increasing atomic number of the M ion. The X I , position leads to 4 lines symmetrically placed around the PQR frequency (m, = + 4 -t m1 = + 3, a transitions and m, = _+ 3 -+ m, = T 3 , P transitions) and two reso- nances at very low frequencies (m, = + -$ -+ ml = - ) and m , = + 3- -, mI = - 3) [2]. For these XI, sites the internal field is calculated to be about 1 kOe, strongly dependent on the M ion.
These results on the XI, resonances show that the magnetic interaction can be treated as a small pertur- bation on the electric quadrupole interaction. The frequency interval Av, between the two i< cr )> reso- nances will then be proportional to the spontaneous magnetization M(7'). Both these cc resonance lines tend with increasing temperature to the finit PQR frequency.
Therefore the temperature dependence of the reso- nance frequency near the Curie point will be much smaller then in the case of a + 3 -+ - 3 transition.
This implies that, because of the absence of magnetic broadening of the resonance lines and the finit fre- quency at T = T,, the determination of the sponta- neous magnetization near T, is much easier and more accurately performed then with any other resonance lines. In figure 2 the temperature dependence of Av, in Cs2CuCI4.2 H 2 0 is plotted together with the frequency of the central line of the septet on a normalized scale. The magnetization derived from both
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711414
C 1 - 1158 T. 0. KLAASSEN AND N. J. POULIS
FIG. 2. -The temperature dependence of the spontaneous magnetization of CszCuC14.2 H z 0 as derived from Cl11 (0)
and Cs ( a) resonance.
resonances agrees perfectly at low temperatures so that near Tc surely the supposed relation
Av,(T) = const. M(T)
will be exact. From these results together with the exact values of Tc in the different salts the critical exponent for the magnetization near T, is calculated.
The value j3 = 0.325 k 0.005 is equal for all crystals within the experimental accuracy.
Apart from the above mentioned C1 and Br reso- nances all other resonances, including those of the C U ~ ~ , ~ ~ and 017 nuclei are determined. On the base of the measured transferred hyperfine fields, the Curie- temperatures and the crystal structures, calculations are in progress to determine the superexchange mecha- nisms in these crystals. As a first result we can already mention that apart from the mostly assumed Cu-X-0-Cu exchange path [3], also an important coupling via an alkali ion has to exist.
References
[ l ] BROWN (L.), Phys. Rev., 1955, 100, 1764.
[2] DAS (T.), Solid State Physics, 1958, Suppl. 1, 10.
[3] WIELINGA (R.), Le~den Thesis, 83.
