MÖSSBAUER HFS STUDIES NEAR LOW-TEMPERATURE MAGNETIC CRITICAL POINTS IN FE (III) BIS (DITHIOCARBAMATES)

HAL Id: jpa-00216647

https://hal.archives-ouvertes.fr/jpa-00216647

Submitted on 1 Jan 1976

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.

MÖSSBAUER HFS STUDIES NEAR

LOW-TEMPERATURE MAGNETIC CRITICAL

POINTS IN FE (III) BIS (DITHIOCARBAMATES)

J. Grow, G. Robbins, H. Wickman

To cite this version:

JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, Dicembre 1976, page C6-633

MOSSBAUER HFS STUDIES NEAR LOW-TEMPERATURE MAGNETIC

CRITICAL POINTS IN FE (110 BIS (DITHIOCARBAMATES)

J. M. GROW (*), G. L. ROBBINS and H. H. WICKMAN

Department of Chemistry, Oregon State University, Corvallis, Oregon 97331, U. S. A.

R6sum6.

-

Les transitions magnetiques de plusieurs complexes dithiocarbonate de fer (111) dans des &tats intermediaires de spin ont Bte 6tudi6es par spectrometrie Mossbauer. Les temperatures de transitions magnetiques et une selection de spectres pour les etats ordonnes sont presentes ici. Les complexes peuvent &tre classes en deux categories suivant le signe du paramktre D du champ cristaliin. Les complexes ayant un D positif ont des spectres Mossbauer presentant A la fois des contributions magnetiques et non-magnetiques particulierement au voisinage de Tc. L'origine de ce comportement est decrite et on donne une estimation de la contribution superparamagnetique. Abstract.

-

Magnetic transitions in several intermediate-spin halobis (dithiocarbamate) iron 011) complexes have been studied by Mossbauer spectroscopy. Magnetic transition temperatures and selected spectra for the known ordered species are summarized here. The complexes may be placed in two classes according to the sign of their crystal field D parameter. Complexes with positive D parameter exclusively show both magnetic and non-magnetic spectral contributions to their Mossbauer hfs patterns, particularly near Tc. Origins of this behavior are summarized, and an estimate of superparamagnetic contributions to the spectra has been made in terms of microscopic magnetic anisotropies in the complexes.

1. Introduction.

-

A number of studies of 57Fe hyperfine structure have shown the coexistence of both magnetic and non-magnetic spectral contributions t o absorber Mossbauer patterns in iron salts in the region of their cooperative magnetic transition tempe- ratures [I-91. Typically, a two line quadrupole pattern is superposed upon a six-line magnetic hfs pattern. In most cases, the presence of the quadrupole doublet persists (with diminishing relative intensity to reduced temperatures, TITc, of about 0.9. These spectral features are referred to here as CPhfs. If experimental problems, such as temperature gradients, may be eliminated, there are several possible origins of this behavior. Simplest is a small spread in transition temperatures among the crystals in the absorber. Relaxation phenomena associated with critical point fluctuations are also possible. In addition, there is a suggested form of magnetic domain fluctuation, which has been termed critical superparamagnetism (CSP) [ l ] (and which may be compared with ordinary superparamagnetism occurring in small particles over extended temperature ranges [lo-131). In most cases, it is not possible to unambiguously separate all of these (or other) processes as sources of CPhfs. In the present work we report materials where CPhfs is ubiquitous, and where CSP may be identified as a likely source of this behavior.

The materials investigated are of certain halobis (dithiocarbamate) iron (111) complexes (denoted

bisdtcs). These complexes exhibit magnetic cooperative transitions only at low temperatures, T ;5 4 K. The materials are also of interest because they have a very simple electronic level structure in the orbital-singlet, spin-quartet state of the iron (111) which lends itself to ready interpretation of normal relaxation and magnetic hfs [14-171. For example, in earlier work, a successful analysis of spin relaxation in both the paramagnetic and ferromagnetic regime of a complex with negative axial crystal field term (see eq. (2), below) was possible [14]. In that case (which is the only reported example of an ordered complex with negative D) no evidence of superparamagnetic effects was found. By contrast, we have now investigated numerous magnetically ordered complexes which have positive axial parameters and in every case these materials show typical CPhfs. Because of the simple level structure of the Fe (111) ground state, we are able to suggest a contribution to the CPhfs in the form of CSP which depends on microscopic crystal field anisotropies in the complexes.

In any type of superparamagnetic behavior, the absence of mhfs below T, is attributed to rapid magnetic domain fluctuations which lead to a vanish- ing internal magnetic field at the nucleus. These fluctuations depend exponentially upon a product, KV, between anisotropy energy K and domain volume

V. The domain fluctuation rate QF is given by

(*) Present Address : Department of Physics, Portland State with 00 a frequency factor which is in the University, Portland, Oregon, U. S. A. range of 109-1011 HZ ; V is the effective domain

C6-634 J. M. GROW, G . L. ROBBINS AND H. H. WICKMAN volume, and K is the effective anisotropy barrier for

the domain flip [Ill. Representative values for K are 10'-lo6 erg . ~ m - ~ . Superparamagnetism is in all cases related to materials properties such as sample volume, microcrystallinity and surface effects. Direct interpretations of the anisotropy energy parameter K

in terms of microscopic properties of the magnetic ions is often difficult and this parameter generally is only of phenomenological significance. However, because of the simple level structure in bisdtcs, a direct calculation of K is possible. We show below that K is quite different for positive and negative axial crystal field interactions. This difference is suggested as an origin of the CPhfs in one case and its absence in the other.

2. Experimental.

-

The Mossbauer apparatus was of conventional design and employed a source of 5 7 C o / ~ d or 5 7 C ~ / R h . The samples were in all cases immersed in liquid helium and their temperatures were varied by pumping on the helium bath. Tempe- ratures were monitored by a calibrated germanium thermometer placed on the absorber holder. The absorber was sandwiched, without binder, between 0.13 mm thick beryllium discs. Temperature stability in all cases was

L-

0.0015 K and accuracy was $. 0.01 K. In this and other work, over 25 different bisdtcs have been studied by Mossbauer spectroscopy [14-17, 201. Table I lists those complexes and their transition

Magnetically ordered intermediate spin halobis (dithiocarbamate) iron (III) complexes

Axial crvstal field Darameter Apical

ligand Positive Negative

- - C1- diethyldtc, Tc = 2.48 K Br- a-MBr, TC = 3.47 K dimethyldtc, Tc = 1.9 K I- diethyldtc, Tc = 1.95 K pyrrolidyldtc, Tc = 2.18 K a-MI, To = 3.61 K /3-MI, Tc = 2.41 K

temperatures as determined by Mossbauer spectros- copy. In the positive D complexes, Mossbauer spectra show CPhfs. Representative data are given for a crystal polymorph (a-MI), figure 1, and a solvent adduct (P-MI), figure 2 of iodobis (morpholyldtc) iron. Spectra are also shown for a solvent adduct of bromobis (morpholyldtc) iron (a-MBr), figure 3. These are all well characterized crystalline solids. In the region near Tc, a sharp quadrupole doublet is superposed on the normal mhfs which shows a modest, sample-dependent broadening. At reduced tempzratures below about 0.85-0.9, the quadrupole doublet is absent and the mhfs spectra can be fit with

I

-0.8 0 .8

V E L O C I T Y ( C M / S )

Frc. 1.

-

Mossbauer spectra of a-MI, Fe(S2CNC.+Hs0)21. He is the internal magnetic field.

a stochastic relaxation calculation that yields a lower limit of 5 x 108 Hz for the ionic relaxation rate Q,.

Solid curves in figure 3 were obtained in that way. The relaxation calculation has been summarized in reference [9]. Fitting parameters for static hfs obtained at the lowest temperature available (1.35 K) are given in table 11. Other solid curves are static mhfs patterns which are simply illustrative of patterns expected in the fast relaxation limit for internal fields appropriate to the spectra.

The possibility that the non-magnetic hfs arises from a distribution of transition temperatures must be considered. In many cases, the spectra have a mhfs without the broadening that would result from a significant distribution of critical temperatures. A

M~SSBAUER HFS STUDIES NEAR LOW-TEMPERATURE MAGNETIC CRITICAL POINTS

_I

C6-635

I00 +i~,xi:$-{LF2z,

-:>y5

,.?*~Y<<(L~-~:G

:

.

.

.

.

T=4.20 U-MBr Tc = 3 . 4 7 R

/ . . .

:f:$.~.?*%Z;<i.

. -

-

.

.&.,I.%.~*~...*

. .

.

_.

_."

.

+,:.

*A. ,At'

..

.

.

"

. .

,C

. -<

A

:.

.

_.

_.

T z 3 . 4 5

..

..

Z

P4

z 0 H a cn _LT

+

.a (r + W w LT HE =I54 CM/S t

C FIG. 3. - Mossbauer spectra of a-MBr,

8 Fe(SzCNC4HsO)zBr. (5) CHzC12

.

at most

--

0.8 mm/s ; figure 2 is an example. It is HE =I95 _{often true that the narrower the range in wich CSP}

effects are seen, the narrower the linewidths near

- 0 . 8 o .8 T, ; figure 1 is an example. We have found that

V E L O C I T Y (CM/S! preparations leading to small crystallites yield spectra

with broader mhfs and an extended temperature

FIG. 2. - Mossbauer spectra of BMI, _{range for superparamagnetic effects. Similar spectra} Fe(S2CNC4HsO)zI. CHZC~Z

.

result from grinding the materials. All spectra shown

Crystallite orientation effects are present in this sample. The

rnhfs spectra are single crystal spectra with an unpolarized here correspond to carefully recrystallized samples ?-ray beam at angle ,g = 450 and g, = 550 with respect to the efg with crystallite dimensions in the range of 0.05-0.5 mm.

tensor systems. Hence, they might be expected to have maximal

TABLE 11.

-

Magnetic hyperJne structure parameters of Fe (III) bis (dithiocarbamate) complexes with positive axial crystal field parameters

Complex Mossbauer hyperfine structure parameters

(a) At sample temperatures of 1.35 K.

(0) Internal field orientation parameters with respect to efg system.

(C) Isomer shift (at 4.2 K) with respect to natural iron (at 300 K).

C6-636 J. M. GROW, G. L. ROBBINS A N D H. H. WICKMAN

domain volumes and minimal superparamagnetic properties. Indeed, in some samples, the linewidths would be consistent with a spread in transition tempe- ratures, CSP or possible critical fluctuations. In no case, however, was it possible to eliminate the CPhfs and, for this reason, we consider the electronic level structure as a possible explanation of the effect.

3. Discussion. -A dear presentation of factors which influence domain fluctuations and hfs near a magnetic transition has been given by Levinson et al. [I]. These may be considered in the general context of a magnetic ion with an isolated ground electronic state of multi- plicity 2 S

+

1. As indicated by the experimental data, a rapid relaxation over electronic levels in a spontaneously polarized - ion leads to a non-vanishing value for

<

S

>,,

=

S,. At a given instant, the direction of

3,

is the same for all ions in a reasonably sized domain. (We neglect complications such as pinning of spins near the domain surface). The relaxa- tion rate over the ionic levels is denoted 0,. Initially, we assume that Qc satisfies the relation Q, 9 Q,, where QL is the nuclear Larmor precession frequency in the internal field. In order for the mhfs to vanish, it is necessary that the domain direction S, fluctuate rapidly (and randomly with respect to a particular direc- tion) in comparison with QL. If this flip rate is denot- ed a,, then a criterion for the occurrence of super- paramagnetism is the inequality Q, Q QF

<

&.

Near the critical point, the exchange and hence anisotropy energy of the lattice vanishes [11, 181. Hence, the domain fluctuation rate may be quite rapid for temperatures close to Tc where KV

<

kT, so Q,

<

Q, is a relatively easy condition to satisfy. At most temperatures, it is also easy to satisfy the condition 0, Q Q,.

One might expect a range of fluctuation rates for Q,, depending upon the distribution of effective volume sizes in a particular specimen. However, it has been shown in several discussions that the rapid dependence of

a,

on volume can lead to an effective partitioning of the spectra into normal magnetic hfs and averaged hfs [I, 11, 18, 191. The distinction between spectral types corresponds to particles with volumes larger and smaller than a critical volume, Vc. Because of the exponential product KV,, a change in sample volume from Vc to 2 Vc can decrease Q, by several orders of magnitude, leading to a normal fast relaxation limit mhfs :

QF

<

QL

<

Q,. This argument may be extended as follows. If materials have a characteristic volume (or typical range of volumes), but the anisotropy energy K may vary due to different crystal field splittings in different samples, then it is possible to have a critical anisotropy constant Kc such that variations in this parameter by a factor of 2 or so will cause some materials to show superparamagnetic effects while others will not. We now show that such

a situation is consistent with the crystal field level structure in bisdtcs.

Crystal field interaction within the orbital-singlet, spin-quartet ground state of trivalent iron in bisdtcs are well described by the spin Hamiltonian

The zero-field splittings

1

2 D(l f ~ ( E / D ) ~ ) ' / ~

1

,

range from 3 to 40 cm-' in different homologues. In all cases, the magnetic transitions occur below approxi- mately 4.2 K and hence the magnetic behavior is influenced predominantly by the ground Kramers doublet of the iron ion. Since we are interested in behavior near Tc, where exchange fields are small, it is possible to use perturbation g-values for the ground doublet to estimate single ion magnetic anisotropies. These g-values depend upon the ratio E/D. Further, the anisotropy barrier will depend upon the direction of the exchange field with respect to the axes of eq. (2). The exchange interaction S,

.

J,,

.

S, is approxi- mated here by a molecular field term : H,, = gPHm. S. Crystal field splittings for all bisdtcs have not yet been determined. In the known cases, the ratio E/D is less than about 0.18 and we assume that 0

<

(E/D)

<

0.2 in the materials of Table I. In such cases, a straightforward analysis of the low temperature mhfs determines the sign of D for a particular complex. Determination of the direction of Hm with respect to the crystal field axes is less direct but it has been shown that in the negative D case of Fe(diethyldtc),Cl, H, is parallel to the z-axis of eq. (1). In positive D materials, the molecular field is normal to the z-axis [9, 171.

We now consider the difference in anisotropy energies between the case of negative and positive D. The anisotropy barrier is roughly estimated near Tc for an exchange field of 1-2 kOe and the E/D range noted above. For negative D, the barrier is in the range AE

-

2.5

+

0.5 x 10-l7 erg.molecule-l. However, for positive D, the magnetic anisotropy is much lower and the range for AE is 1

Ifl

1 x lo-'' erg. molecule- l . We convert the molecular anisotropy

energy to volume anisotropy energy by assuming a density of 2.00 and M. W. of 700. The result is K

--

lo5 erg.cmW3 for negative D and K 11 2 x lo4

MOSSBAUER HFS STUDIES NEAR LOW-TEMPERATURE MAGNETIC CRITICAL POINTS C6-637

the microscopic estimate for K (2 x lo4 e r g . ~ m - ~ ) and the phenomenological value (3

x

lo3 erg. cme3) is not unreasonable in view of the approximations that we have made. Within this range of values for K there is, of course, a rapid variation of QF

with domain volume, consistent with experiment. Because of the overall consistency of the argument, we conclude that domain linear dimensions in the range 100-1000

A

are reasonable estimates of the actual situation. This implies a rather high degree of microdomain structure and microcrystallinity in bisdtcs. This result is consistent with earlier obser- vations of large paramagnetic resonance linewidths in externally sound crystals and is also in agreement with single crystal Mossbauer experiments in the negative D case of Fe(diethyldtc),Cl, where intensities of mhfs lines were suggestive of microcrystallinity in the absorber crystal 1161. In conclusion, we note

that the present result, though suggestive, does not rule out the possibility of other origins of the CPhfs. In particular, a detailed relaxation calculation, including non-adiabatic, or non-white-noise approxi- mation, in the region of T, would be of interest. The foregoing discussion has also assumed that the magnetic units in the solids are monomeric. Recently, Petrouleas and Malliaris [20] have presented evidence for dimer formation in several iron bisdtcs. A discus-

sion of CPhfs and subcritical phenomena in dimeric and related systems will be given elsewhere (Petrouleas, Malliaris and Wickman, to be published).

Acknowledgement.

-

This work supported by the National Science Foundation. We thank W. E. Silver- thorn for the synthesis of materials used in these experiments and S. W. McCann for programming assistance.

References

[I] LEVINSON, L. M., LUBAN, M. and SHTRIKMAN, S., Phys.

Rev. 177 (1969) 864.

[2] WERTHEIM, G. K., GUWENHEIM, H. J., LEVENSTEIN, H. J.,

BUCHANAN, D. N. E. and SHER~OOD, R. C., Phys.

Rev. 173 (1968) 614.

[3] EIBSCHUTZ, M., SHTRIKMAN, S. and TREVES, D., Phys.

Rev. 156 (1967) 562.

141 BERTELSEN, V., KNUDSEN, J. M. and KROGH, H., Phys.

Stat. Sol. 22 (1967) 59.

[5] WEHNER, H. L., RITTER, G. and WEGENER, H. H. F.,

Phys. Lett. 46A (1974) 333.

'[6] YAMAMOTO, H., OKADA, T., WATANABE, H. and FUKASE, M., J. Phys. SOC. Japan 24 (1968) 275.

171 DEZSI, I., KESZTHELYI, L., KULGAWCZUCK, D., MOLNAR, B. and EISSA, N. A., Phys. Stat. Sol. 22 (1967) 617.

[8] VAN DER WOUDE, F. and DEKKER, A. J., P h y ~ . Stat. Sol. 13 (1966) 181.

[9] GROW, J. M. and WICKMAN, H. H., Amer. Inst. Phys. Proc. 24 (1974) 215.

[lo] N ~ E L , L., Ann. Geophys. 5 (1949) 99.

[ I l l JACOBS, I. S. and BEAN, C. P., in Magnetism, edited by

G. T. Rado and H. Suhl, Academic Press (1963),

Vol. 111, pp. 271-294.

[12] K~~NDIG, W., BOMMEL, H., CONSTABARIS, G . and LIND-

QUIST, R. H., Phys. Rev. 142 (1966) 327.

1131 AFANASEV, A. M., SUZDALEV, I. P., GEN, M. Ya., GOL-

DANSKII, V. I., KORNEEV, V. P. and MANYKIN, E. A.,

Sov. Phys. JETP 31 (1970) 65-69 [Zh. Eksp. Teor. Fig. 58 (1970) 1151.

[14] WICKMAN, H. H. and WAGNER, C. F., J. Chem. Phys. 51

(1969) 435.

1151 CHAPPS, G. E., MCCANN, S. W., WICKMAN, H. H. and SHERWOOD, R. C., J. Chem. Phys. 60 (1974) 990.

[16] WICKMAN, H. H., J. Chem. Phys. 56 (1972) 976.

1171 KOSTIKAS, A., PETRIDES, D., SIMOPOULOS, A. and PAS- TERNAK, M., Solid State Commun. 13 (1973) 1661.

[IS] SCHUDE, W. J., SHTRIKMAN, S. and TREVES, D., J. Appl. Phys. 36 (1968) 1010.

1191 BROWN, Jr., W. F. in Fluctuation Phenomena in Solids,

R. E. Burgess, Ed. (Academic Press, Inc., N. Y.)

1965, Chapter 2.

1201 PETROULEAS, V. and MALLIARIS, A., J. Phys. (to be publish-