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Susceptibility and N.M.R. study of the vitreous fluorides Pb 2MnFe1-xGaxF9 and Pb2FeMn 1-xZnxF9
C. Dupas, K. Le Dang, J.P. Renard, P. Veillet, J.P. Miranday, C. Jacoboni
To cite this version:
C. Dupas, K. Le Dang, J.P. Renard, P. Veillet, J.P. Miranday, et al.. Susceptibility and N.M.R. study of the vitreous fluorides Pb 2MnFe1-xGaxF9 and Pb2FeMn 1-xZnxF9. Journal de Physique, 1981, 42 (9), pp.1345-1350. �10.1051/jphys:019810042090134500�. �jpa-00209326�
Susceptibility and N.M.R. study of the vitreous fluorides
Pb2MnFe1-xGaxF9 and Pb2FeMn1-xZnxF9
C. Dupas, K. Le Dang, J. P. Renard, P. Veillet
Institut d’Electronique Fondamentale (*), Bâtiment 220, Université Paris-Sud, 91405 Orsay Cedex, France
J. P. Miranday (~) and C. Jacoboni
Laboratoire des Fluorures et Oxyfluorures Ioniques, Université du Maine (**), Route de Laval, 72017 Le Mans Cedex, France
(Reçu le 13 janvier 1981, révisé le 23 avril, accepté le 12 mai 1981 )
Résumé. 2014 Nous avons étudié l’effet de la dilution par des atomes diamagnétiques de Ga et Zn sur la température
de gel du verre de spin isolant amorphe Pb2MnFeF9, par des mesures de susceptibilité alternative et de R.M.N.
de 19F. Ces dernières ont également fourni des informations sur la structure microscopique de ces verres. Les résultats sont bien décrits par un modèle de chaines interconnectées d’octaèdres MF6 (M = Fe, Mn) liés par des sommets, avec Pb2+ en position interstitielle.
Abstract. 2014 The effect of dilution by the non magnetic atoms Ga and Zn, on the spin freezing temperature of the insulating spin-glass, amorphous compound Pb2MnFeF9 has been studied by ac susceptibility and 19F N.M.R.
techniques. Information was also obtained on the local structure of these glasses. The results are consistent with a
model of a glassy network of chains of corner sharing MF6 octahedra (M = Fe, Mn), with Pb2+ in interstitial sites.
Classification
Physics Abstracts
75.50K201376.60
1. Introduction. - In the past few years, many
experimental and theoretical studies have been devoted to disordered magnetic systems. In particular, it has
been shown that dilute metallic alloys such as Cu-Mn
or Au-Fe [1, 2] exhibit a new phase at low temperature : the spin-glass (SG) phase, characterized by a freezing
of the magnetic disorder [3]. The SG phase is related
to the random distribution of the magnetic atoms and
to the competition between the negative and the positive parts of the RKKY interaction [4]. The SG concepts’ were recently extended to magnetic insu-
lators [5] and it is now accepted that the randomly
diluted crystalline insulator EuxSr1 _xS for x 0.5
has a SG phase [6].
Another class of interesting disordered materials is that of the amorphous magnetic insulators with
predominantly antiferromagnetic interactions. These systems might exhibit a SG behaviour because of their
(*) Laboratoire associé au C.N.R.S.
(t) Deceased in 1980.
(**) Equipe de Recherche associée au C.N.R.S. n° 609.
topological disorder and frustration effects. Indeed the low temperature magnetic properties of the amorphous
manganese aluminosilicates are similar to those of the SG metallic alloys : a sharp cusp of the ac magnetic susceptibility [7, 8], no specihc heat anomaly, a rema-
nent magnetization below the freezing temperature
Tg [8, 9], and slowing down of the spin dynamics at Tg [10].
This paper contains a study of the magnetic pro-
perties of the vitreous fluorides of 3d transition metals.
It has previously been reported that the ac suscepti- bility of the fluoride glasses PbMnFeF7 and Pb2MnFeF9 exhibits a sharp cusp at the respective glass temperatures Tg = 11.77 and 5.35 K [11]. Fur-
thermore the field, temperature and time depen-
dences [12] of the remanent magnetization are similar
to those of the metallic spin-glasses. These materials with predominantly antiferromagnetic interactions
are certainly SG as are the amorphous Mn alumino-
silicates. One interesting property of these fluoride
glasses is their ability to be diluted by replacing Fe3 + by Ga3 + or Mn2+ by Zn2 +, thus enabling an expe-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019810042090134500
1346
rimental study of the dilution effect on the spin- freezing temperature of an insulating SG with short range interactions. Furthermore the 19F nuclear
magnetic resonance (N.M.R.) of the F atoms bonded
to non magnetic cations such as Ga3+ or Zn2+ can
easily be observed because of relatively long relaxation
times. Using this powerful microscopic technique,
information about the local structure of the glasses
and their electronic spin dynamics can be obtained.
The susceptibility and N.M.R. experiments were per- formed on several samples of « Pb2 MnFe 1 _ xGaxF9 »
and « Pb2FeMn 1 -,,Zn,,Fg », with x ranging from 0
to 1. The sample preparation has been described in a
previous, paper [ 13]. By analogy with crystalline mixed . fluorides of similar composition, it has been pro-
posed [13] that these glasses consist of disordered chains of corner sharing MF6 octahedra (M = Fe, Mn, Ga, Zn) with Pb2+ mainly in interstitial sites.
The present experimental results will be discussed on
the basis of this simple model.
2. Susceptibility measurements. - The ac suscep-
tibility was measured by means of a mutual inductance
bridge [14] operating between 10 and 1000 Hz. The
amplitude of the measuring field was about 5 Oe.
Temperatures between 1.2 and 4.2 K were obtained in
a pumped 4He cryostat. Between 1.2 and 0.32 K, a small 3 He glass cryostat was used. Temperature was
determined from the He vapour pressure and from calibrated germanium and carbon resistors.
The inverse molar susceptibility of the glasses
« Pb2MnFe1-xGaxF 9» with x = 0.2, 0.4, 0.6, 0.8 and 1, in the temperature range 0.32-4.2 K is shown
in figure 1. At 4.2 K, above the freezing temperature of these glasses, the molar susceptibility is almost
Fig. 1. - Inverse molar susceptibility xM-1, of the glasses Pb2MnFe1-xGaxF 9 with x = 0.2, 0.4, 0.6, 0.8 and 1 versus T below 4.2 K.
independent of x. This means that the reduction of the
number of magnetic ions by dilution is cancelled by a
similar reduction of the mean antiferromagnetic
interaction between the magnetic ions. The latter reduction is observed in the measured values of the Curie-Weiss temperature 10 1 estimated from the
susceptibility at 77 and 293 K (Table 1).
Table 1. - Curie-Weiss temperatures of’ the vitreous fluor ides Pb2MnFe1-xGaxF 9 and Pb2FeMn1-xZnxF 9
deduced ftom susceptibility measurements at 77 K and 300 K.
Below 4.2 K, a spin-freezing is clearly observed in all the studied samples. It is indicated by a net maximum
of x( T) for the less diluted samples with x = 0.2, 0.4
and by a net change of the slope of X-1(T) in samples
with x > 0.6.
The results for the glasses diluted by Zn are similar except that their Tg and 10 values are higher than
those of the Ga doped glasses for the same dilution.
The dependence of the freezing temperature on the measuring frequency v is small in the glasses with
x 0.6. The results for Pb2MnFeo.6Gao,4F9 are
shown in figure 2. The data can be fitted with a linear
Fig. 2. - Magnetic susceptibility per gram xm, of
Pb2MnFeo.6Gao.4F9
versus T for different values of the measuring frequency v. The corresponding freezing temperatures defined as the temperature of the susceptibility maximum are Tg = 2.75, 2.78, 2.80, 2.82 and 2.83 K for v = 20, 75, 200, 750 and 1 250 Hz respectively. Note
the increase of x with decreasing T due to small clusters of magnetic
ions. Their number increases strongly with dilution and x has no
maximum for x > 0.6.
relation between T; 1 and log v, which is expected on
the basis of independent clusters with a relaxation time s = ro exp(E/kT), although in the present case so and E have unphysical values : so = 9 x 10- 6’ s, E/k = 400 K. Note that :
(i) the quasi-independence of 7g on v is also observ- ed in the amorphous Mn aluminosilicates [9] and in
Cu-Mn SG [15] but not in some rare earth SG [16,17], (ii) it seems that a slight Tg(v) variation nevertheless appears in the most diluted samples which might be
related to the presence of finite clusters.
Finally the variation of Tg with the relative concen-
tration of magnetic cations C = (2 - x)/4 is shown
in figure 3. For the highest concentrations C > 0.35,
Tg is a linear function of C, and by extrapolation is
zero for C. - 0.3. For C 0.35, a crossover to a
different behaviour is observed. This concentration
dependence of Tg is different from that of the dilute
magnetic alloys such as CuMn where Tg is propor-
tional to the magnetic ion concentration [2].
In summary, the susceptibility measurements clearly
show two important features :
(i) For the same concentration; the magnetic properties are more affected by dilution with Ga than with Zn. This point will be discussed in section 3.
(ii) For strong Ga dilution (x > 0.6), the magnetic susceptibility does not exhibit any cusp but only
a change in the slope of its temperature dependence,
due to the paramagnetic contribution of many isolated magnetic ions or small clusters which do not freeze
even at very low temperatures. From figure 3, it
seems that the magnetic concentration of these glasses
is below the percolation threshold; their spin-glass
Fig. 3. - Freezing temperature Tg of vitreous Pb2 MnFe 1 -.,Ga.,F,
and Pb2FeMn, -.,Zn,,F, versus the concentration of magnetic
2 - x
cations C = 2 4 x . For the Ga-doped glasses, Tg(C) starts to
decrease linearly with C towards Co ~ 0.3 which could be the
percolation threshold. Note the change of slope of TiC) at lower
concentrations.
transition could be a progressive freezing of non interacting finite clusters.
3. 19F nuclear magnetic résonance. - The N.M.R.
of 19F nuclei was studied by the spin echo method in the paramagnetic and spin-glass phases of these compounds. The signal intensity was calibrated using Pb3Ga2F12 to measure the proportion of F atoms giving rise to the observed signal.
3 1 SIGNAL INTENSITY AND MICROSCOPIC STRUCTURE.
- At room temperature, i.e. in the paramagnetic region well above Tg, the 19F signal intensity increases linearly with x (Fig. 4). Only the F ions not bonded to a magnetic cation have a nuclear relaxation time long enough to allow the observation of a resonance signal.
Indeed, the nuclear spin-lattice relaxation rate Tï 1
is roughly equal to (yHloc)2 1:, where H10c is the local
fluctuating field and 1: the electronic correlation time.
For a magnetic bonded F, Hloc is the transferred
hyperfine field of the order of 50 kOe [18,19] whereas
for a non bonded one, H10c is a dipolar field not much greater than 1 kOe. The longest observed T1 is 1 ms showing that the relaxation time for the former should be shorter than 1 ps and thus cannot be observed.
The proportion of « non bonded » F, termed free F, is given by the empirical expressions
for the Ga3+ and Zn2+ substitutions respectively ; n(n’) is the average number of free F atoms produced by the substitution of one Ga (Zn) atom and po the
Fig. 4. - Proportion of free F in Pb2MnFe1-xGaxF9 and Pb2FeMn1-xZnxF9 as a function of Ga3+ and Zn2+ substitutions (0, Ga3+ ; X, Zn2+) from 19F nuclear intensity.
1348
residual amount of free F in the undoped sample.
From figure 4, n and n’ were determined to be 2.39 and 1.85, respectively.
It is clear that the n value depends on the structure of
the glasses. We assume here that Ga3+ and Zn2+
cations replace Fe3 + and Mn 21 cations respectively
i.e. n is the average number of unshared F bonded to a
trivalent cation. An example of the local structure around an impurity ion is given in figure 5 where the n value should be smaller than 4 because of the ramifi- cations and intersections between magnetic chains.
The average number of F ions belonging to a trivalent
cation is the sum of n unshared F and -1 (6 - n) shared
F giving 3 + n in total. The observed signal in the undoped sample shows that only 9(1 - po) F atoms
are bonded to transition metal ions M3+ and M2 + . Assuming also a sixfold coordination for M2+ ions
one obtains the following relation :
From the experimental po value of about 0.075 one
finds n + n’ = 4.65 in agreement with the observed n and n’ values, within the experimental uncertainties
of n, n’ and po. This result seems to confirm the above structure.
Fig. 5. - Local structure around a Ga3 + impurity. The free F ions
are bonded only to the latter.
It should be pointed out that the linear increase of the F signal with Ga concentration up to x = 1 means, in our model, that there are pratically no
nearest neighbour Ga-Ga pairs i.e. the M3+ and M2+
ions tend to alternate along the chains as expected for
local charge balance. This explains the strong decrease of the paramagnetic Curie temperature 0 with Ga concentration. For the Zn substitution 0 decreases rather smoothly with the concentration. This might
be due to the high value of the Fe3+-Fe3+ exchange
interaction as compared to that of the Mn2+ -Mn2+
interaction because of the higher covalency of the
former. Another factor that favours the exchange
interaction in Zn-doped sample is the slightly smaller
size of the Zn2 + ion in comparison with the Mn 21
one. The fact that n is higher than n’ is again ascribed
Fig. 6. - Nuclear spin-lattice relaxation time Tl at 77 K as a function of Ga3+ and Zn2+ substitution in Pb, MnFe, -.,Ga.F,
and Pb2FeMnl-,,Zn,,F, (0, Ga3+ ; X, Zn2+).
to the local charge balance : the average number of F- ions belonging to a cation increases with the
valency.
The nuclear spin-lattice relaxation time T 1 in the paramagnetic region decreases when the diamagnetic
substitution increases (Fig. 6). In the high-temperature region the electronic correlation time r is simply given by the reciprocal of the exchange frequency
weX. Since the dipolar field which depends only on the
distance F-Metal is assumed to be independent of doping, it is obvious that the decrease of (J)ex with the
magnetic concentration makes the nuclear relaxation process more efficient. Of course, the relation YF Hloc T 1 always holds since the smallest exchange
parameter J is a few degrees Kelvin in the sample
with x = 1, as deduced from the Curie-Weiss tem-
perature.
Fig. 7. -19F linewidth in an external field of 7.2 kOe as a function of reciprocal temperature for différent glassy compounds
Pb2MnFe1-xGaxF9 and Pb2Mn, -,,Zn,,Fg. (0, Ga3+ ; X, Zn2+) .
3.2 LINEWIDTH AND LOCAL MAGNETIZATION. - In
an amorphous system the linewidth Ah is proportional
to the induced magnetization under an applied magnetic field. The Ah values at 7.2 kOe were mea-
sured for different samples between 1.7 and 4.3 K
(Fig. 7). For the most diluted sample the temperature dependence follows the Curie law as expected from
the moderate field and the very low freezing point.
Below Tg it is interesting to find the linewidth extra-
polated to zero external field which will be rigorously proportional to the frozen spin. The linewidth due to the dipolar field at 0 K can be considered as nearly x independent and it is interesting to regroup the data for various compounds as a function of the reduced temperature TITG (Fig. 8). For T > Tg, Ah obviously
tends to zero with the applied field.
Fig. 8. - Zero-external-field 19F linewidth as a function of the reduced temperature T/Tg. The solid line represents the temperature dependence of the local frozen spin.
The fact that the linewidth increases abruptly at a temperature lower than Tg is explained by its local
character i.e. by the contribution from the frozen
spins nearest to a diamagnetic cation. We then define a local freezing temperature Tg ~ 0.65 7"g and tenta- tively compare this temperature dependence with
the Edwards-Anderson model [4], using the order parameter
There is a similarity between the temperature depen-
dence of Ah and fl mimicing precisely the local
frozen spin.
Fig. 9. - Reciprocal 19F linewidth in Pb2MnFe1-xGaxFq as a function of x at 4.3 K and in a dc field of 7.2 kOe.
3. 3 LINEWIDTH AND SUSCEPTIBILITY. - At 4.3 K > Tg, the reciprocal linewidth increases linearly
with x (Fig. 9) i.e. the product (x + xo) Ah is practi- cally independent of x, like the susceptibility. This
means that at this temperature the observed suscepti- bility arises chiefly from the unbalanced spins. The glassy magnetic structure may be compared to an anti- ferromagnetic system containing (x + xo) unbalanced spins 5/2 due to the presence of the diamagnetic cations, x Ga3+ and xo Pb2+, on the magnetic chains.
Under an applied field the average value of these
spins can be deduced from the relative linewidths at 4.3 and 0 K as follows,
Assuming all these spins pointing in the direction of the applied magnetic field, the corresponding molar susceptibility was evaluated to be about 0.4 emu in
qualitative agreement with the observed value of 0.3.
The smaller measured value probably arises because the spins are not exactly parallel to one another.
In conclusion, the N.M.R. measurements on the free F are quite consistent with the chain structure with ramifications and intersections. The susceptibility
at 4.3 K is related to the unbalanced spins due to the diamagnetic substitution. Below Tg, the behaviour
of the spins nearest to a diamagnetic cation reveals the presence of a local freezing temperature.
Acknowledgments. - The authors wish to thank Prof. R. De Pape for helpful and interesting discus-
sions.
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