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Unusual peculiarities of paramagnet to ferromagnet phase transition in La0.88MnO2.91
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 300 (2006) e159–e162 www.elsevier.com/locate/jmmm
Unusual peculiarities of paramagnet to ferromagnet phase transition in La0.88MnO2.91 V.A. Ryzhova,, A.V. Lazutaa, I.A. Kiseleva, V.P. Khavronina, P.L. Molkanova, I.O. Troaynchukb, S.V. Trukhanovb a Petersburg Nuclear Physics Institute RAS, Gatchina, St. Petersburg, 188300 Russia Institute of Physics of Solids and Semiconductors, National Academy of Sciences, ul. P. Brovki 17, 220072, Minsk, Belarus
b
Available online 18 November 2005
Abstract Study of the linear and nonlinear magnetic responses as well as electron spin resonance (ESR) is presented for insulating La0.88MnO2.91 with T C 216 K. According to the data on the second harmonic of magnetization (M2), this compound exhibits an unconventional paramagnet to ferromagnet (PF) phase transition. A critical behavior of the system is found to agree with a prediction for a 3D isotropic ferromagnet in the P-region above T*E247 K. Below T*, an anomalous phase develops whose properties differ radically from those of a magnet exhibiting an usual PF transition. This phase originates in a ferromagnetically ordered state and reveals a strong nonlinear behavior in the weak fields. The ESR data, which are obtained in the essentially higher field and frequency than those in the M2 measurements, agree with the conventional critical behavior above 225 K, indicating a strong suppression of the anomalous response by the field. These peculiarities are close to those found in the usually doped Nd1xBaxMnO3 (x ¼ 0.23, 0.25) manganites. It supports an assumption on universal character of this phenomenon, namely, formation of a heterogeneous magnetic state with the coexistence of the anomalous and traditional phases. Since this compound is near a border of a metal to insulator transition, the anomalous phase is assumed to exhibit a ferromagnetic metallic state whereas the traditional one is a ferromagnetic insulator. Formation of this nontrivial state is attributed to a strong coupling of the orbital, Jahn–Teller phonons and charge degrees of freedom with the magnetic ones. r 2005 Elsevier B.V. All rights reserved. PACS: 72.20.Ht; 72.80Ga; 75.40.Gb; 75.70.Pa Keywords: Magnetically ordered materials; Phase transition; Nonlinear response; Electron paramagnetic resonance
The interest in the study of the perovskite manganite oxides is due to their rich physical properties. The holedoped manganites can exhibit the colossal magnetoresistance effect (CMR) that is one of the central issues in their physics. They are traditionally doped by substituting a trivalent lanthanide to a divalent alkaline-earth ion. The doping usually causes a transition from an insulator (I) antiferromagnetic (AF) to I ferromagnetic (F) state and then to a F metallic phase. One of the important aspects of their behavior is a nature of a paramagnet (P) to F phase transition closely related to the CMR. This issue is still a matter of discussion [1]. The study of Nd1xBaxMnO3 Corresponding author. Tel.: +7 813 714 6842; fax: +7 813 713 2303.
E-mail address: [email protected] (V.A. Ryzhov). 0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.10.173
(x ¼ 0:23; 0:25) with the FI ground state showed that their critical behavior, corresponding to that of a 3D isotropic ferromagnet, proceeded above T* (ETC+20 K). Below T*, an anomalous behavior was observed. It was characterized by the appearance of a phase with strong nonlinear properties in the weak fields [2,3]. Recently, a new class of the manganites La0.88 MnOx (x ¼ 2:8222:95), where the doping is caused by variation in the oxygen content, has been synthesized. Their phase diagram, magnetic and transport properties were found to be similar to those of the traditionally doped manganites [4]. A question arises whether the PF transition in these compounds possesses the unusual peculiarities as in the NdBa system. To elucidate this issue, we investigated the linear and nonlinear magnetic susceptibilities as well as
ARTICLE IN PRESS V.A. Ryzhov et al. / Journal of Magnetism and Magnetic Materials 300 (2006) e159–e162
e160
electron spin resonance (ESR) of a high-quality singlephase powder La0.88MnO2.91 used in [4] in the critical Tregion. This compound exhibits the P–FI transition as well as CMR property and its crystal structure is close to that of the NdBa system. Fig. 1 shows w(T). As it is seen w00 =w0 o102 , therefore, 0 w Ew0 (w and w0 are AC and static susceptibilities, respectively). A value of TC that is needed to analyze w0 ðtÞ {t ¼ ðT T C Þ=T C } can be obtained from the data on the third harmonic of magnetization. The T C 216:5 K is determined as position of the |M3(T)| maximum at the largest T [2,3] (inset in Fig. 1). Note, the |M3(T)| exhibits a second maximum at T 211 K. A power law fit of w0 ðtÞ / tg (for 0:028ptp0:27) gives an unrealistic large value of g 1:75 compared with g 4=3 for a 3D isotropic ferromagnet. This result is associated with effect of the anomalous phase that begins to dominate in response to the weak fields at tot 0:14 (see below). Besides, as one can see from Fig. 1, below 220 K, the ratio w00 =w0 102 is much higher than a usual factor o=G104 (f 105 Hz was used and GX400 Oe is a relaxation rate of uniform magnetization) and increases fast down to TC in spite of G growth (see ESR data below). This result evidences also an unusual magnetic state of the sample in a range of the anomalous phase existence. To clarify the critical behavior above TC, the second harmonic of magnetization, M2(T), was investigated. The M2 measurements were performed in the parallel AC and steady magnetic fields H þ h sinð2pftÞ (f ¼ 15:7 MHz, hp35 Oe). The Re M2 and Im M2 were recorded as the functions of H that was scanned symmetrically relative to the point H ¼ 0 for detecting H-hysteresis of the signal. Fig. 3 displays the data for some characteristic temperatures that are related to the three (‘‘impurity’’, normal and 10 M3| (µV)
10 f=20 kHz
8
0.04
6 4 2
1
0 200 220 T (K)
240
0.02
4πχ''
4πχ'
180
f=95 kHz
0.1
0.00
0.01 200
250 T (K)
Fig. 1. The temperature dependencies of the linear susceptibility and amplitude of the third harmonic of magnetization (inset to panel). The lines are a guide for the eyes.
anomalous) T ranges with the different M2(H,T) dependencies. Similar ranges were found in the NdBa system [2,3]. In the ‘‘impurity’’ range (315–289 K), practically T–independent signal is observed. It exhibits weak Hhysteresis with a small M2 at H ¼ 0 that provides a clear evidence of a spontaneous magnetization M SP ðM 2 ð0Þ / M SP Þ of this phase. Indeed, M2 is a pseudovector and even function of h. Therefore, M2(H) is odd in H with M 2 ð0Þ ¼ 0 in the paramagnetic phase. The M 2 ð0Þa0 response in ‘‘impurity’’ range is associated with a small amount of a magnetically ordered impurity phase (probably with an AF phase possessing a weak ferromagnetism). In the normal range (289–247 K ¼ T*), the signal begins to increase with decreasing temperature, and we observe the typical response corresponding to a 3D isotropic ferromagnet in the weak fields (gmH5T C t5=3 ). Turning first to the Im M2, note that this component has the required sign structure (Im M 2 40 at Ho0) [2,3] which differs from the opposite structure of the impurity signal above 279.3 K (see Fig. 2b). The same quantitative analysis as in Refs. [2,3] shows that in this range, Im M2(H,t) dependence can be described in terms of the conventional critical behavior. A normal Re M2(H,t) dependence is Re M 2 ðH; tÞ / Htg 2 (g2 14=3) [2,3]. This behavior can be only observed below 279 K since the relatively large impurity signal masks the contribution of the normal phase above 279 K. In the remaining narrow T–interval, tdependence of the Re M2 agrees reasonably with the prediction. The anomalous range (T* ¼ 247 K–TC) is characterized by the appearance of the additional signal with an extraordinary Im M2(H) dependence (a hysteresis loop at jHjo100 Oe with the extreme in very weak field jHj10 Oe, Fig. 2b). The new signal increases sharply with decreasing T and exhibits maximal amplitude at T ðaÞ C 217 K that is in reasonable agreement with TC found from jM 3 ðTÞj data and should be considered as a temperature of F ordering the anomalous phase. The field position of anomalous signal extreme remains weakly T dependent down to T(a) C indicating the increasing volume of the anomalous phase. In the Re M2(H), the anomalous signal also accounts for formation of the extreme in a small jHj30 Oe and field hysteresis (Fig. 2a). A peculiarity of anomalous phase in our manganite is that it originates with the MSP because M 2 ð0Þa0. In the NdBa system, the F ordering both the anomalous and normal phases appeared at TC without H-hysteresis above it. In the La0.88MnO2.91, the signal from normal phase with the extreme in higher field appears in Im M2(H) below 223 K and exhibits the increasing of its value below T(a) C with maximum at other ordering temperature T(n) E211 K (Fig. 3b) that is in C accordance with position of second maximum in jM 3 ðTÞj. Note the H-position of this extreme and extreme in Re M2 is T dependent in accordance with second-order character of normal phase. Intriguing point is a decrease of Hhysteresis in Re M2 below 230 K. This evidences the increase in relative volume of normal phase, that is not
ARTICLE IN PRESS V.A. Ryzhov et al. / Journal of Magnetism and Magnetic Materials 300 (2006) e159–e162 Re M2 (H)
Im M2 (H) (10-4 emu/g)
(10-3 emu/g) 0.5
3
(10-3 emu/g) -3 (10 emu/g)
2
0.0
1
-0.5
2 (10-4 emu/g)
-2
10 -6
-6
-7 (10 emu/g)
2 1
Re M2 (10-7 emu/g)
0 0
-10
T=228.9 K
-2 -4
-2
T=243.1 K
-4 T=246.9 K
-1
0
T=259.7 K
-1 -1
(a)
T=285.3 K
H (Oe) -300
-150
0
150
2
-7 (10 emu/g) -7
(10 emu/g) 6
0
3
-2
0
-4
-8 (10 emu/g)
0
1 1
0
(a)
TC =217 K
-5
300
T=315.1 K
1
1
3
(b)
6
1
-3
0
-1 -6
T=168.9 K
-4
0 -3
0
TC(n)=211
-3 0
-6
TC(a)=217 K T=228.9 K
-1
T=243.1 K
0 Im M2 (10-8 emu/g)
(10-7 emu/g)
4
(10 emu/g) 4
-1
0 2
T=214.3 K
5
(10 emu/g) 4 -7 (10 emu/g)
-4 (10 emu/g)
0
0 -2
3
-4 (10 emu/g) 6
-1
(10-5 emu/g)
4
T=168.9 K
0 1
e161
T=246.9 K
3
T=259.7 K
-1 0
T=285.3 K -3
H (Oe)
-300
-150
0
T=315.1 K 150
300
Fig. 2. Two-phase components of the second harmonic of magnetization M2 as functions of the steady magnetic field H at some temperatures. Full and open symbols are used for curves recorded at direct and reverse H-scans, respectively.
1200
ΓC=23.94(42)Oe 1000
TC=215K γC=0.81(11) Λ=46.7(6)
800
γ =1.34(1)
ESR spectrum (a.u.)
Fit byΓ=ΓC[τ--γC+Λτγ]
T=219.7K 4
2
Γ (Oe)
0
T=240.3K 0 1 2 3 4 5 6 H (kOe)
600
400
200
200
250
300 T (K)
Fig. 3. Temperature dependencies of the spin relaxation rate G parameters found from the ESR spectra for zero field (ZF), slow cooling (full symbols) and ZF slow heating (open symbols) regimes of the T-treatments of the sample. Described in the text, fit of G(t) is presented by the solid line curve. Inset shows two ESR spectra registered in anomalous T-region.
hysteretic above TC, also. Thus, the Im M2(H) dependence at T* and just below T* reveals the coexistence of the normal (in H200 Oe) and anomalous (Ho50 Oe) re-
sponses (Fig. 2b). This observation suggests formation of a mixed two-phase magnetic state below T*. Note that the M2(T) data obtained at the cooling and heating do not reveal a noticeable T-hysteresis. To examine the critical behavior at the higher H and frequency, ESR study has been performed using a special X-range spectrometer (f 8:37 GHz) [5]. The spectra were obtained for 210–330 K region. The inset in Fig. 3 displays typical signals. Above 225 K, the spectra are well described by a Lorentzian line. A corresponding expression is given in Ref. [2]. We have obtained the g(T), A(T) and G(T) dependencies, where g is the g-factor, G is the relaxation rate of the uniform magnetization and Apw0 is the amplitude of the signal. The g is found to be weakly T dependent and close to an usual value for the manganites g 2:0. The A(t) and G(t) may be expected to follow the predictions for a 3D isotropic ferromagnet. In this case, A(t) has to obey a law AðtÞ / tg (g 4=3) in a weak-field regime as t4tH ¼ ðgmH=T C Þ3=5 2:3 102 (T4T H 221 K). The fit in the region 225 KpTp330 K yields a close value of g ¼ 1:38ð1Þ for our compound. This result differs radically from that obtained in the low-frequency measurements of the linear response (w0 (t)) to the weak AC field (see above). The G(t) is presented as [2,6] g GðtÞ ¼ GC ðtg C þ Lt Þ.
(1)
Here, GC is controlled by the dipolar forces, a single ion anisotropy and an exchange anisotropy (GC ðV An Þ2 =T C , where VAn is the characteristic scale of these interactions). The term with L describes uncritical contribution including a spin–lattice coupling. The fit with the fitting parameters GC, L and gC gives gC ¼ 0:8ð1Þ (Fig. 3) that is close to gC 1 for a 3D isotropic ferromagnet [6]. Using obtained
ARTICLE IN PRESS e162
V.A. Ryzhov et al. / Journal of Magnetism and Magnetic Materials 300 (2006) e159–e162
GC 24 Oe, we find V An 0:8 K that is the usual scale of the single ion and exchange anisotropy. These interactions account for the critical enhancement of G(t) since the dipolar3 energy is 4pðgmÞ2 =V 0 0:13 K, where V 0 ˚ is the volume of the magnetic unit cell. Thus, the ð3:9 AÞ A(t) and G(t) dependencies are in a reasonable agreement with the predictions. The anomalous phase is clearly observed in the M2 measurements, and is likely to modify the w0 (t); however, it does not affect the ESR spectra. This means that its contribution is strongly suppressed by the relatively strong field used in the ESR, and this phase occupies a small volume (an upper border does not presumably exceed 20%). Turning to the nature of the anomalous phase, note that our compound is near a border of an insulator to metal transition (x 2:93) [4]. Therefore, the anomalous behavior may be associated with the metallic-like regions. Fragmentation of the sample may be such that these regions do not form a percolative conductive cluster, and the system remains an insulator. At the same time, a
magnetic coupling of the regions through the critical normal phase can lead to their F-ordering. This work was supported from the joint Russian–Belorussian Foundation for Basic Research, Grants No. 04-0281051 RFBR-Bel2004_a, F04R-076.
References [1] M.B. Salamon, M. Jaime, Rev. Mod. Phys. 73 (2001) 583. [2] V.A. Ryzhov, A.V. Lazuta, I.D. Luzyanin, I.I. Larionov, V.P. Khavronin, Yu.P. Chernenkov, I.O. Troynchuk, D.D. Khalyavin, Zh. Eksp. Teor. Fiz. 121 (2002) 678. [3] V.A. Ryzhov, A.V. Lazuta, V.P. Khavronin, I.I. Larionov, I.O. Troynchuk, D.D. Khalyavin, Solid State Commun. 130 (2004) 803. [4] I.O. Troyanchuk, V.A. Khomchenko, M. Tovar, H. Szymczak, K. Barner, Phys. Rev. B 69 (2004) 054432. [5] V.A. Ryzhov, E.I. Zavatskii, V.A. Solov’ev, I.A. Kiselev, V.N. Fomichev, V.A. Bikineev, Zh. Tekh. Fiz. 65 (1995) 133. [6] S.V. Maleev, Sov. Sci. Rev. Sec. A 8 (1987) 1229.
Journal of Magnetism and Magnetic Materials 300 (2006) e159–e162 www.elsevier.com/locate/jmmm
Unusual peculiarities of paramagnet to ferromagnet phase transition in La0.88MnO2.91 V.A. Ryzhova,, A.V. Lazutaa, I.A. Kiseleva, V.P. Khavronina, P.L. Molkanova, I.O. Troaynchukb, S.V. Trukhanovb a Petersburg Nuclear Physics Institute RAS, Gatchina, St. Petersburg, 188300 Russia Institute of Physics of Solids and Semiconductors, National Academy of Sciences, ul. P. Brovki 17, 220072, Minsk, Belarus
b
Available online 18 November 2005
Abstract Study of the linear and nonlinear magnetic responses as well as electron spin resonance (ESR) is presented for insulating La0.88MnO2.91 with T C 216 K. According to the data on the second harmonic of magnetization (M2), this compound exhibits an unconventional paramagnet to ferromagnet (PF) phase transition. A critical behavior of the system is found to agree with a prediction for a 3D isotropic ferromagnet in the P-region above T*E247 K. Below T*, an anomalous phase develops whose properties differ radically from those of a magnet exhibiting an usual PF transition. This phase originates in a ferromagnetically ordered state and reveals a strong nonlinear behavior in the weak fields. The ESR data, which are obtained in the essentially higher field and frequency than those in the M2 measurements, agree with the conventional critical behavior above 225 K, indicating a strong suppression of the anomalous response by the field. These peculiarities are close to those found in the usually doped Nd1xBaxMnO3 (x ¼ 0.23, 0.25) manganites. It supports an assumption on universal character of this phenomenon, namely, formation of a heterogeneous magnetic state with the coexistence of the anomalous and traditional phases. Since this compound is near a border of a metal to insulator transition, the anomalous phase is assumed to exhibit a ferromagnetic metallic state whereas the traditional one is a ferromagnetic insulator. Formation of this nontrivial state is attributed to a strong coupling of the orbital, Jahn–Teller phonons and charge degrees of freedom with the magnetic ones. r 2005 Elsevier B.V. All rights reserved. PACS: 72.20.Ht; 72.80Ga; 75.40.Gb; 75.70.Pa Keywords: Magnetically ordered materials; Phase transition; Nonlinear response; Electron paramagnetic resonance
The interest in the study of the perovskite manganite oxides is due to their rich physical properties. The holedoped manganites can exhibit the colossal magnetoresistance effect (CMR) that is one of the central issues in their physics. They are traditionally doped by substituting a trivalent lanthanide to a divalent alkaline-earth ion. The doping usually causes a transition from an insulator (I) antiferromagnetic (AF) to I ferromagnetic (F) state and then to a F metallic phase. One of the important aspects of their behavior is a nature of a paramagnet (P) to F phase transition closely related to the CMR. This issue is still a matter of discussion [1]. The study of Nd1xBaxMnO3 Corresponding author. Tel.: +7 813 714 6842; fax: +7 813 713 2303.
E-mail address: [email protected] (V.A. Ryzhov). 0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.10.173
(x ¼ 0:23; 0:25) with the FI ground state showed that their critical behavior, corresponding to that of a 3D isotropic ferromagnet, proceeded above T* (ETC+20 K). Below T*, an anomalous behavior was observed. It was characterized by the appearance of a phase with strong nonlinear properties in the weak fields [2,3]. Recently, a new class of the manganites La0.88 MnOx (x ¼ 2:8222:95), where the doping is caused by variation in the oxygen content, has been synthesized. Their phase diagram, magnetic and transport properties were found to be similar to those of the traditionally doped manganites [4]. A question arises whether the PF transition in these compounds possesses the unusual peculiarities as in the NdBa system. To elucidate this issue, we investigated the linear and nonlinear magnetic susceptibilities as well as
ARTICLE IN PRESS V.A. Ryzhov et al. / Journal of Magnetism and Magnetic Materials 300 (2006) e159–e162
e160
electron spin resonance (ESR) of a high-quality singlephase powder La0.88MnO2.91 used in [4] in the critical Tregion. This compound exhibits the P–FI transition as well as CMR property and its crystal structure is close to that of the NdBa system. Fig. 1 shows w(T). As it is seen w00 =w0 o102 , therefore, 0 w Ew0 (w and w0 are AC and static susceptibilities, respectively). A value of TC that is needed to analyze w0 ðtÞ {t ¼ ðT T C Þ=T C } can be obtained from the data on the third harmonic of magnetization. The T C 216:5 K is determined as position of the |M3(T)| maximum at the largest T [2,3] (inset in Fig. 1). Note, the |M3(T)| exhibits a second maximum at T 211 K. A power law fit of w0 ðtÞ / tg (for 0:028ptp0:27) gives an unrealistic large value of g 1:75 compared with g 4=3 for a 3D isotropic ferromagnet. This result is associated with effect of the anomalous phase that begins to dominate in response to the weak fields at tot 0:14 (see below). Besides, as one can see from Fig. 1, below 220 K, the ratio w00 =w0 102 is much higher than a usual factor o=G104 (f 105 Hz was used and GX400 Oe is a relaxation rate of uniform magnetization) and increases fast down to TC in spite of G growth (see ESR data below). This result evidences also an unusual magnetic state of the sample in a range of the anomalous phase existence. To clarify the critical behavior above TC, the second harmonic of magnetization, M2(T), was investigated. The M2 measurements were performed in the parallel AC and steady magnetic fields H þ h sinð2pftÞ (f ¼ 15:7 MHz, hp35 Oe). The Re M2 and Im M2 were recorded as the functions of H that was scanned symmetrically relative to the point H ¼ 0 for detecting H-hysteresis of the signal. Fig. 3 displays the data for some characteristic temperatures that are related to the three (‘‘impurity’’, normal and 10 M3| (µV)
10 f=20 kHz
8
0.04
6 4 2
1
0 200 220 T (K)
240
0.02
4πχ''
4πχ'
180
f=95 kHz
0.1
0.00
0.01 200
250 T (K)
Fig. 1. The temperature dependencies of the linear susceptibility and amplitude of the third harmonic of magnetization (inset to panel). The lines are a guide for the eyes.
anomalous) T ranges with the different M2(H,T) dependencies. Similar ranges were found in the NdBa system [2,3]. In the ‘‘impurity’’ range (315–289 K), practically T–independent signal is observed. It exhibits weak Hhysteresis with a small M2 at H ¼ 0 that provides a clear evidence of a spontaneous magnetization M SP ðM 2 ð0Þ / M SP Þ of this phase. Indeed, M2 is a pseudovector and even function of h. Therefore, M2(H) is odd in H with M 2 ð0Þ ¼ 0 in the paramagnetic phase. The M 2 ð0Þa0 response in ‘‘impurity’’ range is associated with a small amount of a magnetically ordered impurity phase (probably with an AF phase possessing a weak ferromagnetism). In the normal range (289–247 K ¼ T*), the signal begins to increase with decreasing temperature, and we observe the typical response corresponding to a 3D isotropic ferromagnet in the weak fields (gmH5T C t5=3 ). Turning first to the Im M2, note that this component has the required sign structure (Im M 2 40 at Ho0) [2,3] which differs from the opposite structure of the impurity signal above 279.3 K (see Fig. 2b). The same quantitative analysis as in Refs. [2,3] shows that in this range, Im M2(H,t) dependence can be described in terms of the conventional critical behavior. A normal Re M2(H,t) dependence is Re M 2 ðH; tÞ / Htg 2 (g2 14=3) [2,3]. This behavior can be only observed below 279 K since the relatively large impurity signal masks the contribution of the normal phase above 279 K. In the remaining narrow T–interval, tdependence of the Re M2 agrees reasonably with the prediction. The anomalous range (T* ¼ 247 K–TC) is characterized by the appearance of the additional signal with an extraordinary Im M2(H) dependence (a hysteresis loop at jHjo100 Oe with the extreme in very weak field jHj10 Oe, Fig. 2b). The new signal increases sharply with decreasing T and exhibits maximal amplitude at T ðaÞ C 217 K that is in reasonable agreement with TC found from jM 3 ðTÞj data and should be considered as a temperature of F ordering the anomalous phase. The field position of anomalous signal extreme remains weakly T dependent down to T(a) C indicating the increasing volume of the anomalous phase. In the Re M2(H), the anomalous signal also accounts for formation of the extreme in a small jHj30 Oe and field hysteresis (Fig. 2a). A peculiarity of anomalous phase in our manganite is that it originates with the MSP because M 2 ð0Þa0. In the NdBa system, the F ordering both the anomalous and normal phases appeared at TC without H-hysteresis above it. In the La0.88MnO2.91, the signal from normal phase with the extreme in higher field appears in Im M2(H) below 223 K and exhibits the increasing of its value below T(a) C with maximum at other ordering temperature T(n) E211 K (Fig. 3b) that is in C accordance with position of second maximum in jM 3 ðTÞj. Note the H-position of this extreme and extreme in Re M2 is T dependent in accordance with second-order character of normal phase. Intriguing point is a decrease of Hhysteresis in Re M2 below 230 K. This evidences the increase in relative volume of normal phase, that is not
ARTICLE IN PRESS V.A. Ryzhov et al. / Journal of Magnetism and Magnetic Materials 300 (2006) e159–e162 Re M2 (H)
Im M2 (H) (10-4 emu/g)
(10-3 emu/g) 0.5
3
(10-3 emu/g) -3 (10 emu/g)
2
0.0
1
-0.5
2 (10-4 emu/g)
-2
10 -6
-6
-7 (10 emu/g)
2 1
Re M2 (10-7 emu/g)
0 0
-10
T=228.9 K
-2 -4
-2
T=243.1 K
-4 T=246.9 K
-1
0
T=259.7 K
-1 -1
(a)
T=285.3 K
H (Oe) -300
-150
0
150
2
-7 (10 emu/g) -7
(10 emu/g) 6
0
3
-2
0
-4
-8 (10 emu/g)
0
1 1
0
(a)
TC =217 K
-5
300
T=315.1 K
1
1
3
(b)
6
1
-3
0
-1 -6
T=168.9 K
-4
0 -3
0
TC(n)=211
-3 0
-6
TC(a)=217 K T=228.9 K
-1
T=243.1 K
0 Im M2 (10-8 emu/g)
(10-7 emu/g)
4
(10 emu/g) 4
-1
0 2
T=214.3 K
5
(10 emu/g) 4 -7 (10 emu/g)
-4 (10 emu/g)
0
0 -2
3
-4 (10 emu/g) 6
-1
(10-5 emu/g)
4
T=168.9 K
0 1
e161
T=246.9 K
3
T=259.7 K
-1 0
T=285.3 K -3
H (Oe)
-300
-150
0
T=315.1 K 150
300
Fig. 2. Two-phase components of the second harmonic of magnetization M2 as functions of the steady magnetic field H at some temperatures. Full and open symbols are used for curves recorded at direct and reverse H-scans, respectively.
1200
ΓC=23.94(42)Oe 1000
TC=215K γC=0.81(11) Λ=46.7(6)
800
γ =1.34(1)
ESR spectrum (a.u.)
Fit byΓ=ΓC[τ--γC+Λτγ]
T=219.7K 4
2
Γ (Oe)
0
T=240.3K 0 1 2 3 4 5 6 H (kOe)
600
400
200
200
250
300 T (K)
Fig. 3. Temperature dependencies of the spin relaxation rate G parameters found from the ESR spectra for zero field (ZF), slow cooling (full symbols) and ZF slow heating (open symbols) regimes of the T-treatments of the sample. Described in the text, fit of G(t) is presented by the solid line curve. Inset shows two ESR spectra registered in anomalous T-region.
hysteretic above TC, also. Thus, the Im M2(H) dependence at T* and just below T* reveals the coexistence of the normal (in H200 Oe) and anomalous (Ho50 Oe) re-
sponses (Fig. 2b). This observation suggests formation of a mixed two-phase magnetic state below T*. Note that the M2(T) data obtained at the cooling and heating do not reveal a noticeable T-hysteresis. To examine the critical behavior at the higher H and frequency, ESR study has been performed using a special X-range spectrometer (f 8:37 GHz) [5]. The spectra were obtained for 210–330 K region. The inset in Fig. 3 displays typical signals. Above 225 K, the spectra are well described by a Lorentzian line. A corresponding expression is given in Ref. [2]. We have obtained the g(T), A(T) and G(T) dependencies, where g is the g-factor, G is the relaxation rate of the uniform magnetization and Apw0 is the amplitude of the signal. The g is found to be weakly T dependent and close to an usual value for the manganites g 2:0. The A(t) and G(t) may be expected to follow the predictions for a 3D isotropic ferromagnet. In this case, A(t) has to obey a law AðtÞ / tg (g 4=3) in a weak-field regime as t4tH ¼ ðgmH=T C Þ3=5 2:3 102 (T4T H 221 K). The fit in the region 225 KpTp330 K yields a close value of g ¼ 1:38ð1Þ for our compound. This result differs radically from that obtained in the low-frequency measurements of the linear response (w0 (t)) to the weak AC field (see above). The G(t) is presented as [2,6] g GðtÞ ¼ GC ðtg C þ Lt Þ.
(1)
Here, GC is controlled by the dipolar forces, a single ion anisotropy and an exchange anisotropy (GC ðV An Þ2 =T C , where VAn is the characteristic scale of these interactions). The term with L describes uncritical contribution including a spin–lattice coupling. The fit with the fitting parameters GC, L and gC gives gC ¼ 0:8ð1Þ (Fig. 3) that is close to gC 1 for a 3D isotropic ferromagnet [6]. Using obtained
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GC 24 Oe, we find V An 0:8 K that is the usual scale of the single ion and exchange anisotropy. These interactions account for the critical enhancement of G(t) since the dipolar3 energy is 4pðgmÞ2 =V 0 0:13 K, where V 0 ˚ is the volume of the magnetic unit cell. Thus, the ð3:9 AÞ A(t) and G(t) dependencies are in a reasonable agreement with the predictions. The anomalous phase is clearly observed in the M2 measurements, and is likely to modify the w0 (t); however, it does not affect the ESR spectra. This means that its contribution is strongly suppressed by the relatively strong field used in the ESR, and this phase occupies a small volume (an upper border does not presumably exceed 20%). Turning to the nature of the anomalous phase, note that our compound is near a border of an insulator to metal transition (x 2:93) [4]. Therefore, the anomalous behavior may be associated with the metallic-like regions. Fragmentation of the sample may be such that these regions do not form a percolative conductive cluster, and the system remains an insulator. At the same time, a
magnetic coupling of the regions through the critical normal phase can lead to their F-ordering. This work was supported from the joint Russian–Belorussian Foundation for Basic Research, Grants No. 04-0281051 RFBR-Bel2004_a, F04R-076.
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