ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e571–e573
Dynamic magnetic response of LaMn0.5Ga0.5O3 D. Rinaldia, R. Caciuffoa,*, J.J. Neumeierb, D. Fioranic, S.B. Oseroffd a
INFM, Dipartimento di Fisica ed Ingegneria dei Materiali, Universita" Politecnica delle Marche, Via Brecce Bianche, Ancona I-60131, Italy b Department of Physics, Montana State University, Bozeman, MT 59717, USA c Istituto di Chimica della Materia, CNR, P.O. Box 10, I-00016 Monterotondo Stazione, Rome, Italy d San Diego State University, San Diego, CA 92182, USA
Abstract The results of linear and non-linear magnetic AC susceptibility measurements are reported for polycrystalline LaMn0.5Ga0.5O3. The observation of an asymmetric peak in the linear AC response at 59 K, accompanied by the development of a sizeable second-harmonic component and of a double-peak third-harmonic signal, suggests the establishment of a long-range ordered magnetic ground state with unconventional dynamics. The results are compatible with a picture assuming a canted antiferromagnetic structure or the presence of ferromagnetic domains embedded in an antiferromagnetic matrix. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Ee; 75.50.Lk; 75.30.Cr; 75.20.g Keywords: Non-linear magnetic AC susceptibility; Cluster-glass; Mn perovskites
Early studies [1] and recent investigations of the LaMn1xGaxO3 system [2,3] have shown a magnetic behaviour in agreement with the phenomenological rules established by Goodenough and Kanamori for the sign of superexchange interactions in ionic crystals. For xo0:5; LaMn1xGaxO3 crystallizes in the O0 -orthorhombic structure (c=O2oaob). Ordering of the Mn eg orbitals and static Jahn–Teller (J–T) distortions stabilize anisotropic antiferromagnetic (AF) order. For x > 0:5 the system has the O (aoc=O2) or O (aEc=O2) orthorhombic structure. Isotropic ferromagnetic (F) interactions were predicted both in the former case (no J–T distortions) and in the latter (local J–T distortions fluctuating in three dimensions) [1]. For x ¼ 0:5; a spinglass freezing was suggested at 60 K [2]. On the other hand, neutron diffraction experiments on x ¼ 0:5 samples have been interpreted assuming a simple collinear F order below 75 K [3].
*Corresponding author. Tel.: +39-0712204423; fax: +39071-2204729. E-mail address:
[email protected] (R. Caciuffo).
Here we study the dynamics of the magnetic response in LaMn0.5Ga0.5O3, trough measurements of the linear and non-linear components of the AC magnetic susceptibility in a stoichiometric polycrystalline sample prepared following the procedure described in Ref. [4]. The sample was reacted in Ar atmosphere during all phases of the preparation process, whilst the samples studied in Ref. [3] were prepared in air, with a final treatment in Ar. Data were collected as a function of temperature using the mutual-inductance technique, on warming from 15 to 300 K after zero-field cooling of the sample. The leading field-dependent contributions to the susceptibility, wn ; were obtained from the nth harmonic (n ¼ 2; 3; 5; 7) of the voltage induced in the pickup coils by the time-varying sample magnetization. The primary coil was driven at a fundamental frequency f, while the reference input to the lock-in amplifier, set to a bandpass filter mode of operation, was at a frequency nf. A sharp peak is observed at T ¼ 59 K both in the real and in the imaginary component of the linear susceptibility (Fig. 1). The temperature variation of the maximum relaxation time, t, exhibits a critical slowing down behaviour, with a freezing temperature
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.01.004
ARTICLE IN PRESS D. Rinaldi et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e571–e573
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χ' (SI)
3
2
1
0 40
80
120 160 Temperature (K)
200
Fig. 1. Real (circles) and imaginary (crosses) part components of the linear AC magnetic susceptibility (f ¼ 5 Hz; HAC ¼ 80A=m). The inset shows the slowing down of the relaxation time 1/f.
Fig. 3. Real part of the n ¼ 5 harmonic susceptibility at f ¼ 1 kHz for HAC ¼ 100 A=m (circles) and 200 A/m (crosses). The inset shows the behaviour of the n ¼ 7 signal at the same frequency and field amplitudes.
0.0
0 -0.2 -0.4 106 χ'2 (SI)
107 χ'3 (SI)
-5
-10
-0.6 -0.8 -1.0
-15
-1.2
-20
40
80 120 Temperature (K)
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Fig. 2. Real part of the n ¼ 3 harmonic signal measured at 110 Hz (crosses) and 1 kHz (circles) with HAC ¼ 20 A=m: The inset shows the dependence from the field amplitude at f ¼ 40 Hz; HAC ¼ 100 A=m (circles), 200 A/m (crosses), 400 A/m (filled circles).
Tg ¼ 58:4 K and a critical exponent aE3:5; about half the value usually found for canonical spin glasses (inset Fig. 1). A frequency-dependent peak at the critical temperature is observed also in the real part of the non-linear susceptibility components. The amplitude of the peak in wn ðn ¼ 3; 5; 7Þ increases as the frequency decreases. This is shown for n ¼ 3 in Fig. 2. A divergent behaviour is 0 observed at T for w3 ; as predicted by the mean-field theory for spin-glass freezing. Fitting to the data recorded for T > Tg ; with a driving field of frequency f ¼ 40 Hz; gives a critical exponent g ¼ 2:3: However, 0 the shape of the w3 curve is strongly dependent from the amplitude of the magnetic field, and a double peak 0 structure appears in w3 for H > 200 A=m (inset Fig. 2).
-1.4 20
30
40
50 60 70 Temperature (K)
80
90
100
Fig. 4. Real part of the second harmonic susceptibility; f ¼ 1 kHz; HAC ¼ 100 A=m (circles) and 200 A/m (crosses).
Higher order odd components ðn ¼ 5; 7Þ are shown in Fig. 3. For n ¼ 5; a sharp positive peak centred at T grows with decreasing frequency of the driving field, whereas a negative peak develops at T for n ¼ 7 (inset Fig. 3). At constant frequency, the amplitudes of the n ¼ 5; 7 signal grows as the field amplitude decreases. For f > 1 kHz; the w07 curve shows a small positive signal at the low temperature side of the main peak. As shown in Fig. 4, a sharp peak at about 59 K is visible also in the second-harmonic signal. The height of the peak increases either when the frequency is decreased at constant field amplitude or when the amplitude is decreased at constant frequency. The presence of a second-harmonic response is an indication that spontaneous magnetization is established below T. Our results indicate that, despite some similarities with the spin-glass behaviour [5,6], a canonical spin-glass transition does not occur in LaMn0.5Ga0.5O3. The observed
ARTICLE IN PRESS D. Rinaldi et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e571–e573
features, the peaks asymmetry, and the signal measured for even-order harmonics at Tg can be interpreted as evidence for the coexistence of macroscopic regions with 3D orbital fluctuations and long-range F order, and AF regions in which the occupied eg orbitals are ordered into (0 0 1) planes, as proposed in Ref. [2]. Alternatively, they can indicate the existence of canted antiferromagnetic order, as suggested in Ref. [3] for lower x content.
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References [1] [2] [3] [4]
J.B. Goodenough, et al., Phys. Rev. 124 (1961) 373. J.-S. Zhou, et al., Phys. Rev. B 63 (2001) 184423. J. Blasco, et al., Phys. Rev. B 66 (2002) 174431. . J. Topfer, J.B. Goodenough, Eur. J. Solid State Inorg. Chem. 34 (1997) 467. [5] S. Chikazawa, et al., J. Phys. Soc. Japan 49 (1980) 1276. [6] S. Chikazawa, et al., J. Phys. Soc. Japan 50 (1981) 2884.
