Temperature dependence of the EPR spectra of Cu(AAB)2, a copper-aminoacid salt

Volume 108A, number 4

TEMPERATURE

PHYSICS LETTERS

DEPENDENCE

1 April 1985

O F T H E E P R S P E C T R A O F Cu(AAB)2 ,

A C O P P E R - A M I N O A C I D SALT Rafael CALVO and Manuel A. MESA lnstituto Fenezolano de lnvestigaciones Cient'~ficas, Centro de Ftsica, Apartado 1827, Caracas 1010A, Venezuela Received 8 November 1984

A strong temperature variation of the gyromagnetic factor of the EPR line of copper(II) bis(DL-a-amino-n-butyrato) has been observed in single crystal samples between 1.5 and 293 K. The data are interpreted in terms of the magnetic interactions within the crystal, which has a layered magnetic behavior.

In a previous paper [1], to be referred to as I, we reported EPR measurements at 293 K and 9.3 GHz in Cu(AAB)2, the copper salt of the aminoacid DLs-amino-n-butyric acid. The position and the linewidth of the EPR line were measured in three perpendicular planes of a single crystal sample. From the data on the position we obtained the molecular gyromagnetic factors and the orientation of the copper molecules within the unit cell of the crystal. The observed angular variation of the linewidth AH was attributed to a spin diffusive behavior of the spin dynamics, as is characteristic of two-dimensional magnetic systems [2], and it is supported by the crystal data [3,4] , t . Recently, a strong temperature (T) dependence of the gyromagnetic factor g of several Cu-aminoacid salts have been detected [5], and it was considered of interest to perform detailed measurements of the T dependence o f g in single crystals of Cu(AAB) 2. In this work the angular variations o f g and AH have been measured at X-band as a function of angle in three crystal planes, for fixed T at 1.5, 4, 77, and 293K, and as a function of T between 1.5 and 293 K, for fixed orientations of the applied magnetic field H along the crystal axes. The experimental details are similar to ,1 When we wrote I we were not aware of the article of Fawcett et al. [4] and t h e k results were not discussed there. We are very grateful to Professor Schugar for calling our attention to this paper.

0.375-9601/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

those given in refs. [1,6] ; the sample was larger, and the orientation uncertainty reduced. In fig. 1 we display the values of g2(0, ¢) measured in the three crystal planes at the different T. The angles 0 and ~ are defined in a coordinate system where R = a' =/~ × ~,p = b, and ~ = ~, and where

]~ = H~ IHI = (sin 0 cos ¢, sin 0 sin ¢, cos O). Fig. 2 displays the T variation ofg for H applied along the three crystal axes. The angular variation of AH was measured in the crystal planes at the different T. The change with T of AH(0, ~b) is very small and then the discussion on the origin of AH given in I still applies. The data in Figs. 1 and 2 show an unexpectedly large variation of g2(0, ¢) with T. The gyromagnetic factor increases with decreasing T when H is in the be plane of the copper layers, and it decreases with decreasing T when Itlla', the normal to those layers. The spin hamiltonian ~c = ~ s . gW" H ,

(1)

where/~ is the Bohr magneton, S the effective spin of Cu(II) (S = 1/2), and gT the gyromagnetic tensor, was used to fit the data at each T. The components of g2 were evaluated at 293, 77, 4, and 1.5 K with a least squares progra[n using the data in fig. 1 andg2(0, ~b) = h • gT "9T "h. The non-zero components, the eigenvalues and the eigenvectors o f ~ are given in table 1. 217

Volume 108A, number 4

PHYSICS LETTERS

1 April 1985

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measured at different temperatures in the three crystal planes of Cu(AAB)2 single crystals.

Fig. 2. Temperature variation of theg-factor measured between 1.5 and 293 K for H along the crystal axes. The insets display the value ofg as a function of inverse temperature.

The small differences between the results at 293 K given here with those reported in I are due to the better orientation of the sample. Using these values and the formulas given in I we calculated the direction (0M, @M) of the normal to one copper molecule in the unit cell of Cu(AAB)2, and the angle 2or between the normals to the two molecules. These angles, obtained assuming axial symmetry for the molecularg-tensors of Cu(II), are included in table 1 and should be compared to the values 0 M = 122.6 °, CM = 32"3°, and 2~t = 126.5 °, obtained from the crystallographic data of Fawcett et al. [4]. The agreement between the values

at 293 K is excellent, particularly considering that those calculated from the EPR data assume an axial molecular symmetry, which is not strictly true. However, the differences between these angles increase rapidly when T is lowered. The agreement at high T reflects the applicability of the electronic model discussed in I for the Cu(II) molecules. The values ofgll and g j_, calculated as in I at the different T, change with T but the changes are not as dramatic as those of (0M, @M)- Since no structural or magnetic phase transitions are observed and no changes of the lattice dimensions large enough to justify the variation o f g with T

Fig. 1. Angular variation of the squared gyromagnetic factor

218

Volume 108A, number 4

PHYSICS LETTERS

1 April 1985

Table 1 Components of the g2 tensors, its eigenvalues (g2)i and eigenvectorsai obtained at the different temperatures of the experiments. We de note with T = o. the values obtained by an extrapolation to high T of the low temperature data. The components xy and yz are zero due to the crystal symmetry. The molecular direction (OM,@M)'the angle 2~ between the two molecular directions, and the principal values of the molcular gyromagnetic tensor, gll and g±, are calculated as in I. 293K

(g2)xx (g2)y, (g2)zz

(g*)xz (g2) l al (42)2 d2 (g2)3 33 0M 0M 23 gll g±

77K

4.6301(4) 4.3792(4) 4.4685(4) -0.3385(5) 4.2013(5) (0.620, 0, 0.785) 4.8973(5) (0.785, 0, -0.620) 4.3792(4) (0, 1, 0) 123.6° 32.8° 126.4° 2.2528 2.0497

4K

4.6210(4) 4.3796(4) 4.4543(5) -0.3316(5)

4.4698(4) 4.4579(5) 4.5231(5) -0.3293(5)

4.1958(6) (0.615, 0, 0.789) 4.8795(6) (0.789, 0, -0.615) 4.3796(4) (0, 1,0) 123.1° 33.3° 125.2e 2.2502 2.0484

are expected, we attribute the observed temperature dependence to magnetic interactions between Cu(II) ions in Cu(AAB)2. The insets of fig. 2 show a close lIT temperature dependence o f g along the three crystal axes, at low T. These data were fitted to gd(T) = gd(0) + Cd/T, for d = a', b, and ~, and we obtained from least squares fits to the data in fig. 2: ga,(0) = 2.136,

gb(O) = 2.095,

Ca, = - 0 . 0 9 1 ,

Cb = 0.065,

gc(T) = 2.1175, Cc = 0.037.

To interpret the data in fig. 1 we introduce a gyromagnetic tensor gO for isolated Cu molecules. When these molecules are in the magnetic lattice they feel the applied field H, plus an internal field H i produced by the Cu neighbors. Then, eq. (1) is replaced by: ~f =/3S" go" ( / / + H i ) "

1.5K 4.3240(6) 4.5611(6) 4.5750(6) -0.3248(7)

4.1660(6) (0.735, 0, 0.678) 4.8269(6) (0.678, 0, -0.735) 4.4579(5) (0, 1, 0) 127.7° 44.4° 112.8° 2.2625 2.0411

T =** 4.5638(20) 4.3882(8) 4.4838(7) -0.3318(10)

4.1013(8) (0.825, 0, 0.565) 4.7977(8) (0.565, 0, -0.825) 4.5611(6) (0, 1, 0) 129.8° 55.2° 101.8° 2.2929 2.0252

H i = 4~rXT ° / / ,

(3)

and then, from eqs. ( 1 ) - ( 3 ) it is: gT = (1 + 47rXT) • g0"

(4)

The tensor g0 was calculated from the values of g2 andgd(T), in the T range where the lIT dependence is obeyed. Its components are included in table 1. Using eq. (4) we obtain -

= 8

(gT • X T ) s ,

(5)

where s means the symmetric part of the tensor in parentheses. The 1/Ttemperature dependence observed f o r g d ( T ) justify to assume that Z T = SIT. The components of S can be calculated considering that XT and S are symmetric tensors, using the data in figs. 1 and 2, and the values of the parameters C d with eq. (5). We obtained:

Sxx = --0.00321,

Syy = 0. 00131,

(2)

At high T the orientation of the spins is random and H i = 0, but H i 4= 0 at low T if there is a preferred spin orientation. We assume a tensorial (linear) dependence between H i and H,

Szz = 0.0024,

Sxz = 0.00002.

From symmetry arguments it is Sxy = Sy z = 0 in the P21/c group of Cu(AAB)2. The value obtained for Sxz is very small and then S is diagonal in the crystal 219

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PHYSICS LETTERS

coordinate system, within the experimental errors. The components of S in the plane of the crystallographic copper layers are positive, while the component along its normal is negative. These values for the components o r S have a symmetry that support a layered magnetic behavior, with some anisotropy within the layers, for Cu(AAB)2. The tensor S is not related to the orientation of the Cu(AAB)2 molecules in the crystal, but to the three-dimensional arrangement of the Cu(II) ions in the crystal. Since no phase transition is observed for Cu(AAB)2 above 1.5 K, a T dependence of the g-factor could be attributed to the paramagnetic bulk magnetization, or to short range magnetic order. The anisotropy of S, and the large values of its components rule out the first interpretation. We then hypothesize that when T is decreased below 50 K, short range order of the spins within the layers builds up. This short range order is enhanced by the low dimensional nature of the magnetic interactions. The internal field H i which produces the observed g-shifts is proportional to the short range order parameter and to the anisotropic magnetic interactions between one spin and its neighbors; its direction would be the same or opposite to the applied field depending i f / / i s applied in the plane of the layers or along its normal. This interpretation is similar to that given by Nagata and Tazuke [7] to explain theg-shifts observed in the Mn linear chain antiferromagnets TMMC and CsMnC13.2H20. These authors proposed that short range order within the linear chains is produced by the (isotropic) Heisenberg exchange interactions; the gshifts of these short range ordered spins are produced by the (anisotropic) dipolar interactions. In the case of Cu(II) compounds, the anisotropic exchange interac-

220

1April 1985

tions may be larger than the dipolar interactions, and they are the most important source of anisotropy. To evaluate the g-shifts using this hypothesis we would have to study the spin dynamics of Cu(AAB)2. In the case of the one-dimensional magnetic systems Nagata and Tazuke [7] used analytical results for the one-dimensional spin dynamics. These simple expressions are not valid in our system, and we can not at this moment perform a detailed calculation of thegshifts. More experiments, particularly low temperature magnetic susceptibility measurements are planned to provide more magnetic information. However, our most interesting results are the surprisingly large gshifts observed at temperatures up to two orders of magnitude above any magnetic transition. Also, we have given a phenomenological model to fit the experimental data. Similar results have been observed [5] in the copper salt of the aminoacid L-isoleucine, a system having a ferromagnetic transition at 0.117 K [8].

References [1] R. Calvo and M.A. Mesa, Phys. Rev. B28 (1983) 1244. [2] P.M. Riehards and M.B. Salamon, Phys. Rev. B9 (1974) 32. [3] A.J. Stosiek, J. Am. Chem. Soe. 67 (1945) 362. [4] T.G. Faweett, M. Ushay, J.P. Rose, R.A. Lalancette, J.A. Potenza and H.J. Sehugax, Inorg. Chem. 18 (1979) 327. [5] R. Cairo, J. Appl. Phys. 55 (1984) 2336. [6] R. Calvo, M.A. Mesa, G. Oliva, J. Zukerman-Schpector, O.R. Nascimento, M. Tovar and R. Arce, J. Chem. Phys., to be published. [7] K. Nagata and Y. Tazuke, J. Phys. Soe. Japan 32 (1972) 337. [8] P.R. Newman, J.L. Imes and J.A. Cowen, Phys. Rev. B13 (1976) 4093.