VMoalte. r7ia,l sN Roe.s e2ar, c2h,0 V0o4l. 7, No. 2, 355-357, 2004. Reentrant Spin Glass Behavior in Polycrystalline La Sr Mn Fe O © 2305054
0.7 0.3 1-X X 3
Reentrant Spin Glass Behavior in Polycrystalline La Sr Mn Fe O
0.7 0.3 1-X X 3
M.M. Xavier Jr.a*, F.A.O. Cabralb, J.H. de Araújob, T. Dumelowb, A.A. Coelhoc
aDepartamento de Física, Universidade do Estado do Rio Grande do Norte
59633-010 Mossoró - RN, Brazil
bDepartamento de Física Teórica e Experimental, Universidade Federal
do Rio Grande do Norte, 59072-970 Natal - RN, Brazil
cInstituto de Física Gleb Wataghin, Universidade Estadual de Campinas - SP, Brazil
Received: April 11, 2003; Revised: November 10, 2003
The magnetic and transport properties of the compound La Sr Mn Fe O  (0.1 ≤ x ≤ 0.4)
0.7 0.3 1-x x 3
have been studied by means of electrical resistivity, AC magnetic susceptibility, and DC magneti-
zation. At low concentrations (x ≤ 0.1), the system displays essentially para-to-ferromagnetic
transitions as the temperature is decreased, although a decrease in the magnetic moment has been
observed in previous studies at temperatures a little below T . This ferromagnetism is explained
C
by double exchange theory in terms of the formation of Mn+3/Mn+4 ions pairs in the system. At
concentrations in the range 0.1 < x ≤ 0.4 the system is more complex. Increased Fe doping not
only weakens the ferromagnetic (FM) order and augments the resistivity of the samples, but also
induces the appearance of a reentrant spin glass phase at low temperatures (T < 60 K). Irreversibil-
ity of the magnetization measured with zero field cooling and with field cooling has been ob-
served. In addition, the AC susceptibility peak position varies with frequency. All these effects are
characteristic of spin glass behavior. The results have been interpreted based in an increase of
frustration due to increasing competition between FM Mn+3/Mn+4 interactions and antiferromagnetic
interactions between ions at the boundaries of Fe clusters.
Keywords: spin glass, manganite, rare earths
1. Introduction
The discovery of important effects such as colossal tions that may influence ferromagnetic (FM) DE and
magnetoresistance and rich phase diagrams in manganites antiferromagnetic (AFM) superexchange diferently. In both
of the type LnAMnO3 (Ln = lanthanide: A = alkali metal) cases, interesting properties were observed. J. C. Nie et al.
6
enhanced interest in these materials within the scientific studied a series of (La1-xTbx)2/3Ca1/3MnO3 (0 ≤ x ≤ 0.4) ox-
community1-3. Partial substitution of the lanthanide (triva- ides and observed a transition from FM to spin-glass (SG)
lent) by the alkali metal (divalent), which results in a dou- order as <rA> decreases, and that, as <rA> decreases further,
ble exchange (DE) interaction4 between the mixed valence the ferromagnetism disappears and a direct transition from
Mn+3 and Mn+4 ions, not only introduces a net concentra- superparamagnetic to SG order occurs on cooling. R.
tion of charge carriers into the system, but also changes the Mathieu et al.7 observed a formation of a magnetic disorder
Mn-O-Mn bond angle and length, which directly affect the state as La is substituted by Nd in La0.96-yNdyK0.04MnO3 + d,
structural, magnetic and transport properties of these sys- reflecting the competition between FM DE and AFM
tems5. Recent studies have concentrated mainly on two superexchange interactions. Through time dependent zero
groups of these manganites: (1) compounds in which the field cooling (ZFC) magnetization measurements, they con-
Ln is partially substituted by another lanthanide; (2) com- firmed the presence of a reentrant spin glass (RSG) transi-
pounds in which the Mn is partially substituted by another tion (after a transition from paramagnetic to a FM phase),
transition metal. The latter substitution directly affects the with low field ageing properties in both RSG and FM phases,
DE interation. The former results in strong lattice distor- similar to those observed in archetypal spin glass materials.
*e-mail: milton@uern.br
Articles presented at the XV CBECIMAT, Natal - RN, November de 2002.
356 Xavier et al. Materials Research
J-Wen Feng et al.8 observed a nonlinearity of reciprocal
magnetization (1/M(T)) in the paramagnetic state, indicat-
ing clustering of magnetic moments in La Sr Mn Ni O
1-x x 0.8 0.2 3
(x = 0.2, 0.3). Jun Takeuchi et al9 studied the effect of Fe
doping on the Mn site in Nd Sr M Fe O  (x ≤ 0.3;
0.67 0.33 1-x x 3
M = Mn, Co) and argued that, for x = 0.1, this material shows
a spin-glass-like behavior instead of a long-range
ferromagnetic order. Results of recent works10 suggest the
formation of AFM clusters of Fe ions in the compound
La Sr Mn Fe O  (x ≤ 0.1). This was checked by meas-
0.7 0.3 1-x x 3
uring the Weiss constant (θ ) as a function of x, from a fit to
reciprocal susceptibility in the paramagnetic phase. It was
seen that θ decreased rapidly with increasing x, tending
towards a negative value. In this work, we present results of
measurements of DC electrical resistivity, AC magnetic sus- Figure 1. Temperature dependence of the magnetization (normal-
ceptibility, and DC magnetization on samples of ized to the maximum value) of the system La Sr Mn Fe O  for0.7 0.3 1-x x 3
La Sr Mn Fe O  with 0.1 ≤ x ≤ 0.4, through which we x = 0.1, 0.15 and 0.2. Inset: Temperature dependence of the mag-
0.7 0.3 1-x x 3
propose an explanation of how the increase of Fe concen- netization of the system La Sr Mn Fe O  for x = 0.3 and 0.4,0.7 0.3 1-x x 3
tration (and of AFM clusters) leads to an RSG behavior in using a 50 Oe field.
the system.
2. Experimental Method
Polycrystalline samples were prepared using the sol-gel
technique11 and examined by x-ray diffraction. The results
show a single phase perovskite structure.
All the samples were pressed in pellets with 5 ×  2 ×  1 mm³
to the measurements. Resistivity was measured by the usual
four point method. AC magnetic susceptibility and DC mag-
netization were measured using a superconducting quantum
interference device (SQUID).
3. Results and Discussion
Figure 1 shows curves of magnetization as a function of
temperature for x = 0.1, 0.15 and 0.2. AC magnetic
Figure 2. Temperature dependence of the magnetization of the
susceptiblity measurements yield similar curves. For x = 0.1, system La Sr Mn Fe O  for x = 0.2 and 0.3 measured for ZFC
the magnetization curve shows essentially FM behavior, as 0.7 0.3 1-x x 3and FC in a field of 1000 Oe.
previously seen for lower Fe concentrations10. For x = 0.15
and 0.2, the magnetization curves display, at temperatures
of about 160 K and 120 K respectively, a behavior similar AC magnetic susceptibility peak position was found change
to that of the paramagnetic/FM transition observed in the with frequency, it means the peak is shifting towards high
x = 0.1 sample at about 270 K. However, at lower tempera- temperature as frequency is increased such is expected for
tures (T < 60 K) the magnetization falls rapidly. The sharp an archetypal spin glass system, as shown in Fig. 3. These
drop in the magnetic moment indicates that the FM phase results lead us to believe that, below 60 K, the samples with
disappears at these temperatures. The magnetization curves x < 0.4 can be characterized by an RSG phase, with a mix-
for the x = 0.3 and x = 0.4 (inset Fig. 1) are very similar to ture of phases for 0.1 < x < 0.3.
each other and display the same behavior as those with The x = 0.15 sample displays metallic behavior in a the
x = 0.15 and 0.2 at low temperature (T < 60 K), suggesting narrow temperature at about 80 K, as shown in the resistiv-
that the magnetic phase in this temperature region is similar ity curve in Fig. 4. This behavior is associated with the mag-
for these four samples. netism of the material. It is seen that, for this value of x, the
Figure 2 shows ZFC and field cooled (FC) measurements compound shows ferromagnetism in a temperature band
for the x = 0.2 and 0.3 samples. Both exhibit a pronounced between 60 K and 160 K. As the metal-insulating transition
irreversibility characteristic of SG behavior. In addition, the occurs well below of the para-ferromagnetic transition (as
Vol. 7, No. 2, 2004 Reentrant Spin Glass Behavior in Polycrystalline La Sr Mn Fe O 357
0.7 0.3 1-X X 3
the competition between AF superexchange interactions
involving Fe ions and the FM double exchange interac-
tions between Mn ions. This leads to a large number of
frustrated couplings, so that at a certain x value FM order-
ing is completely destroyed, and SG behavior occurs. The
appearance of an SG transition at a temperature below that
of FM ordering characterizes the reentrant spin glass phase.
Sumary
In summary, we have studied the effect of Fe doping on
the magnetic and transport properties of the system
La0.7Sr0.3Mn1-xFexO3. An increase of the Fe concentration
up to 40% destroys the FM order and induces an RSG
behavior at low temperatures. These results have been ex-
Figure 3. Dependence of the AC susceptibility peak position on plained as due to an increase in the number of frustrated
frequency for polycrystalline La Sr Mn Fe O  measured in a
0.7 0.3 0.7 0.3 3 interactions, with the addition of Fe due to the competition
50 Oe field.
between the FM coupling between Mn+3/Mn+4 ions pairs
and the AF coupling between Fe ions.
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358 Xavier et al. Materials Research