- Email: [email protected]
Electron spin resonance study of Y1−xCaxMnO3
ARTICLE IN PRESS
Physica B 398 (2007) 464–467 www.elsevier.com/locate/physb
Electron spin resonance study of Y1xCaxMnO3 M.T. Causa, E. Winkler, D. Tobı´ a, M. Tovar Centro Ato´mico Bariloche, Comisio´n Nacional de Energı´a Ato´mica, Av. Ezequiel Bustillo 9500, San Carlos de Bariloche, RN, Argentina
Abstract ESR spectra were measured in the series Y1xCaxMnO3 (YCa) (0pxp1) obtaining the dependences with x and T. Anomalies detected in the ESR parameters (resonance field, line width and line intensity) allowed us to determine magnetic and structural transitions. We have constructed a phase diagram T vs. x in order to contrast ESR results with those obtained by susceptibility and X-ray diffraction. r 2007 Elsevier B.V. All rights reserved. PACS: 75.30.m; 76.30.v; 76.50.+g Keywords: ESR; Manganites
Y1xCaxMnO3 (YCa) is a family of compounds that crystallize in a perovskite structure with the Pnma orthorhombic symmetry [1–3] for all x values, from x ¼ 0 (YMnO3) to x ¼ 1 (CaMnO3). X-ray diffraction performed at different T shows that, in the range xo0.93, transitions between two different orthorhombic phase takes place at T ¼ TOO0 (x). At TOO0 the lattice parameter ratio p ¼ (b/ O2)/c changes from p41, obtained at high T (O-phase), to po1 where the phase is more distorted (O0 -phase). For x40.93 [4] the O-phase is present in all the experimental range, T410 K. The O-phase is usually associated with high Curie–Weiss (CW) temperatures (Y) and low resistivity [5,6]. Changes in electric and magnetic properties observed at ToTOO0 were explained with charge localization and orbital ordering involving double-exchange (DE) interactions and lattice distortions [7]. In the extreme compounds of the series, Mn is single valence: Mn3+ (3d4t32ge1g) for x ¼ 0 and Mn4+ (3d3t32g) for x ¼ 1. Both compounds are G-type antiferromagnet (AFM). There is an important difference between the YCa series and the much-studied LaCa or LaSr series: in YCa, due to the small value of the Y3+ ionic radius, a ferromagnetic (FM) phase is never found. Magnetization measurements performed at ToTm (the magnetic ordering Corresponding author. Tel.: +54 2944 445207; fax: +54 2944 445299.
E-mail address: [email protected] (M.T. Causa). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.04.061
temperature) were compatible with AFM and ferrimagnetic (FIM) states in Y-rich samples [2,3]. In Ca-rich samples [5,8], instead, M vs. H behaviors at different T and Monte-Carlo calculations allowed to describe the system in terms of localized polarons embedded in an AFM matrix. The polarons are identified with the residual magnetization M0 and the AFM background with the differential susceptibility wdiff measured at relatively high fields where M(H, T) ¼ M0(T)+wdiff(T)H. In the paramagnetic (PM) phase, T4Tm, w(T) shows different behaviors for different x [9]. For 0oxo0.2 and 0.8oxo1, localized Mn3+ (S ¼ 2) and Mn4+ (S ¼ 3/2) coexist in the lattice, and w(T) is FIM-like. For intermediate concentrations (0.2oxo0.8), the experiments were explained considering that only t2g electrons remain localized while the eg electrons are itinerant. In this case w(T) is CW-like with positive Y. The main interaction is then FM, coming from DE mechanisms which are mediated by itinerant eg electrons. Notice that in spite of the presence of DE in the PM-phase, the FM long-range ordering is not reached, as mentioned above. In this paper, we analyze the different magnetic and structural behaviors found in the YCa series by means of electron spin resonance (ESR) experiments. The experiments were performed using a Bruker spectrometer operating at E9.5 GHz between 100 and 500 K. The ESR spectrum consists of a single line with Lorentzian-type
ARTICLE IN PRESS M.T. Causa et al. / Physica B 398 (2007) 464–467
shape and is observed only in the doping range xX0.2. In ESR experiments, three parameters may be determined, the resonance field (Hr), the line width (DH), and the line intensity (IESR).
1. Resonance field In the PM-phase, Hr is independent of T except in the neighborhood of Tm where internal fields, proportional to M, become important. In our samples, two, strongly interacting, magnetic species coexist. In this case [10], for T4Tm, the g-factor is given by g¼
xg4 C 4 þ ð1 xÞg3 C 3 . xC 4 þ ð1 xÞC 3
(1)
Here, gi and Ci are the corresponding g-factors and Curie constants for localized Mn3+ and Mn4+ spins. Then, Eq. (1) should be valid for the x ranges (0, 0.2) and (0.8, 1). For 0.2oxo0.8, where localized Mn4+ coexist with itinerant eg electrons, Eq. (1) must be replaced [11] by g¼
ge we ðTÞ þ g4 w4 ðTÞ , we ðTÞ þ w4 ðTÞ
(2)
where ge ¼ 2.0036 is the free-electron g and we, w4 are the itinerant and localized contributions to the total susceptibilityU In Fig. 1, we show the experimental g(x). Solid line corresponds to g(x) calculated from Eq. (1) where, as the ESR of YMnO3 was not visible, we assume the value g(Mn3+) ¼ 1.97, measured in LaMnO3 [12]. Deviations from Eq. (1) are observed at xo0.8 (see Fig. 1). In this range, g should be given by Eq. (2). In reasonable agreement, we observed an approximately constant g but some larger than g4 ¼ 1.99 (dotted line) derived from Eq. (2) for we5w4 [9].
465
2. Line width The line width, DH, is related to relaxation mechanisms and for strongly interacting spins a T-dependence is expected in the PM-phase [13] C DH 1 f1 þ AðT T m Þa g, (3) DHðTÞ ¼ ½TwðTÞ where A6¼0 only for TETm and the parameter DHN is the high T-value for DH(T) when w(T)EC/T. To DHN contribute anisotropic and exchange interactions that, respectively, enlarges and narrows the lines [12]. As was shown in Refs. [2,9] when x decreases, Y (proportional to the exchange interactions) diminishes and the distortions (associated with anisotropic interactions) increase. In Fig. 2, we have schematized the behavior expected from Eq. (3) for three different x values. This diagram was constructed from experimental susceptibilities, exchange interactions (obtained from Y), and the experimental DHN of CaMnO3. We conclude that, in our experiments, the ESR lines would be hardly observable for xo0.2. On the other hand, for TETm the second factor of Eq. (3) must be considered. The parameter a depends on the lattice symmetry and is usually ao0 giving a divergent DH(T-Tm). The exceptions are SC and BCC cubic magnetic lattices where, with a40, line narrowing is expected [14]. In Fig. 3, we show the T-dependence of the line width for some representative x values. For x ¼ 1 (see inset in Fig. 3a), the experiment is in agreement with Eq. (3) and the line width decreases at Tm in accordance with a SC and homogenous magnetic lattice. This behavior changes dramatically with small substitutions of Mn4+ by Mn3+ and DH(T-Tm) shows a divergent behavior (see Fig. 3a). In Fig. 3b, we show the behavior for x ¼ 0.5 and 0.75, where structural O–O0 transitions were reported [4,7]. The temperatures, where DH(T) deviates from a constant value,
20 2.02
Linewidth (kGauss)
g-factor
15 2.00
1.98
YMnO3 TN
10
Y0.9 Ca0.1 MnO3 5 CaMnO3
1.96 0.2
0.4
0.6 Ca concentration, x
0.8
1.0
Fig. 1. g vs. x: circles, experimental values; solid line, Eq. (1); dashed line, g ¼ 1.99, measured in CaMnO3.
0 0
100
200
300 T (K)
400
500
Fig. 2. Expected behavior for DH(T) vs. T (see text).
600
ARTICLE IN PRESS M.T. Causa et al. / Physica B 398 (2007) 464–467
466 4
a x = 1 .0 0.5 T (K) 0.0
2
(arb. units)
0.10
N
ESR
T 1.0
10
x = 1 .0 0
I
3
0
200
400
600
IESR (emu / mol)
ΔH (kGauss)
1.5
TN
0 0
100
200
300 T (K)
400
500
0.05 x = 0.90 x = 0.95
1 Y1-xCaxMnO3
x = 0.95
0.00 100
0
150
200
250
300
b 3
χdc
2
x = 0.2 x = 0.3 x = 0.33 x = 0.35
0
100
0 0
100
200
300
200
300 T (K)
400
0
χ
0.02
400
100
200
300
400
500
600
T (K)
(emu/mol)
2000
0.00
500
x = 0.75
0.01
ESR
x = 0.75
4000
IESR
x = 0.45
100
x = 0.5
0.02
I
1
ΔH (Gauss)
6000
emu / mol
x = 0.50
IESR (arb. units)
ΔH (kGauss)
x = 0.90
0.00
500
100
0
T (K)
0
200 300 T (K)
400
100
200
Fig. 3. DH vs. T for the indicated x values.
500
500 O -PM
300
3. Line intensity
ESR not observed
400
200
The intensity (IESR), or area under the absorption curve, was obtained by means of numerical integration. If all the magnetic species contribute to the resonance, IESR(T) should be proportional to w(T) and this usually occurs in the PM-phase. Below Tm, large anisotropy and exchange fields are present, the spins equilibrium state changes, and the PM resonance modes cannot be excited. The AFM resonance in polycrystalline samples is hard to observe and Tm is usually determined by the disappearance of the ESR spectrum [12]. This is the case for x in the range (0.9–1.0) shown in Fig. 4a, where IESR(T) is compared with w(T). Note that IESR(T)ow(T) below 150 K indicating an
400
Fig. 4. IESR vs. T for the indicated x values.
T (K)
are consistent with TETOO0 in both cases. This behavior resembles that found in the LaMnO3+d series [12] where similar anomalies in DH(T) coincide with the Jahn–Teller transition. A complete analysis of the DH(T) behavior for concentrations in the neighborhoods of x ¼ 0.75 and 0.50 is in progress. Finally, lines from Y-rich samples (see inset in Fig. 3b) are very large and approximately constant in our experimental T-range. Notice that, in spite of the different T-dependences, the lines enlarge when x decreases, supporting our assumptions (see Fig. 2).
300 T (K)
O´-PM
100 O´- Magnetic state 0 0.0 AFM / FIM
0.2
0.4
0.6 x
0.8
1.0 AFM + Pol.
Fig. 5. Phase diagram T vs. x. Circles, triangles and squares are results from ESR, susceptibility and structural experiments, respectively. Lines are eyes guide.
increasing fraction of spins becomes non-resonant maybe due to the localization of the, already mentioned, magnetic polarons. Finally, in Fig. 4b, we show IESR(T) for doping
ARTICLE IN PRESS M.T. Causa et al. / Physica B 398 (2007) 464–467
concentrations where structural O–O0 transitions are expected. In these cases, besides the maximum at TETm, a second peak appears at TETOO0 4Tm. This peak marks the transition between two magnetic regimes: FM-like in the O-phase and AFM-like in the O0 -phase [7]. We resume all the experimental facts in the phase diagram T vs. x shown in Fig. 5. Here, we show also Tm and TOO0 derived from magnetic and structural studies. As broad O–O0 transitions are observed in X-ray diffraction studies [3], we have taken TOO0 as a characteristic T where discontinuities in the cell-volume derivative (dV/dT) take place. Regarding the magnetic phases, the magnetic ordering type reached at intermediate concentrations is not known yet and detailed studies at low temperatures are in progress. In summary, we have studied the series Y1xCaxMnO3 that have recently attracted the interest of the researchers due to its relation with multiferroic materials. We have carried out our investigation through measurements of ESR parameters as function of x and T. ESR is a powerful technique that has allowed us to progress in the description of the YCa manganites providing some details that are hidden in ordinary magnetic and structural experiments.
467
Acknowledgments Argentinean institutions ANPCyT (PICT 20770-53 and 25317) and CONICET (PIP 5250 and DT Ph.D.-fellowship) have partially supported this work. E.W. and M.T. are CONICET researchers. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
D. Vega, et al., J. Solid State Chem. 156 (2001) 458. O. Agu¨ero, et al., Physica B 320 (2002) 47. D. Tobı´ a, Graduate Thesis, U. Buenos Aires, November 2005. D. Vega, et al., Physica B 320 (2002) 37. H. Aliaga, et al., J. Phys.: Condens. Matter 15 (2003) 249. H. Aliaga, et al., J. Magn. Magn. Mater. 226–230 (2001) 791. M.T. Causa, et al., J. Magn. Magn. Mater. 272–276 (2004) 81. H. Aliaga, et al., Physica B 320 (2002) 75. D. Tobı´ a, et al., Physica B 384 (2006) 41. D.L. Huber, Phys. Rev. B 12 (1975) 31. M. Tovar, et al., Phys. Rev. B 66 (2002) 024409. D.L. Huber, et al., Phys. Rev. B 60 (1999) 12155. M.T. Causa, et al., Phys. Rev. B 58 (1998) 3233. M.S. Seehra, D.L. Huber, in: C.D. Graham Jr., H. Lander, J.J. Rhyne (Eds.), AIP Conference Proceedings No. 24 on Magnetism and Magnetic Materials, New York, 1975, p. 241.
Physica B 398 (2007) 464–467 www.elsevier.com/locate/physb
Electron spin resonance study of Y1xCaxMnO3 M.T. Causa, E. Winkler, D. Tobı´ a, M. Tovar Centro Ato´mico Bariloche, Comisio´n Nacional de Energı´a Ato´mica, Av. Ezequiel Bustillo 9500, San Carlos de Bariloche, RN, Argentina
Abstract ESR spectra were measured in the series Y1xCaxMnO3 (YCa) (0pxp1) obtaining the dependences with x and T. Anomalies detected in the ESR parameters (resonance field, line width and line intensity) allowed us to determine magnetic and structural transitions. We have constructed a phase diagram T vs. x in order to contrast ESR results with those obtained by susceptibility and X-ray diffraction. r 2007 Elsevier B.V. All rights reserved. PACS: 75.30.m; 76.30.v; 76.50.+g Keywords: ESR; Manganites
Y1xCaxMnO3 (YCa) is a family of compounds that crystallize in a perovskite structure with the Pnma orthorhombic symmetry [1–3] for all x values, from x ¼ 0 (YMnO3) to x ¼ 1 (CaMnO3). X-ray diffraction performed at different T shows that, in the range xo0.93, transitions between two different orthorhombic phase takes place at T ¼ TOO0 (x). At TOO0 the lattice parameter ratio p ¼ (b/ O2)/c changes from p41, obtained at high T (O-phase), to po1 where the phase is more distorted (O0 -phase). For x40.93 [4] the O-phase is present in all the experimental range, T410 K. The O-phase is usually associated with high Curie–Weiss (CW) temperatures (Y) and low resistivity [5,6]. Changes in electric and magnetic properties observed at ToTOO0 were explained with charge localization and orbital ordering involving double-exchange (DE) interactions and lattice distortions [7]. In the extreme compounds of the series, Mn is single valence: Mn3+ (3d4t32ge1g) for x ¼ 0 and Mn4+ (3d3t32g) for x ¼ 1. Both compounds are G-type antiferromagnet (AFM). There is an important difference between the YCa series and the much-studied LaCa or LaSr series: in YCa, due to the small value of the Y3+ ionic radius, a ferromagnetic (FM) phase is never found. Magnetization measurements performed at ToTm (the magnetic ordering Corresponding author. Tel.: +54 2944 445207; fax: +54 2944 445299.
E-mail address: [email protected] (M.T. Causa). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.04.061
temperature) were compatible with AFM and ferrimagnetic (FIM) states in Y-rich samples [2,3]. In Ca-rich samples [5,8], instead, M vs. H behaviors at different T and Monte-Carlo calculations allowed to describe the system in terms of localized polarons embedded in an AFM matrix. The polarons are identified with the residual magnetization M0 and the AFM background with the differential susceptibility wdiff measured at relatively high fields where M(H, T) ¼ M0(T)+wdiff(T)H. In the paramagnetic (PM) phase, T4Tm, w(T) shows different behaviors for different x [9]. For 0oxo0.2 and 0.8oxo1, localized Mn3+ (S ¼ 2) and Mn4+ (S ¼ 3/2) coexist in the lattice, and w(T) is FIM-like. For intermediate concentrations (0.2oxo0.8), the experiments were explained considering that only t2g electrons remain localized while the eg electrons are itinerant. In this case w(T) is CW-like with positive Y. The main interaction is then FM, coming from DE mechanisms which are mediated by itinerant eg electrons. Notice that in spite of the presence of DE in the PM-phase, the FM long-range ordering is not reached, as mentioned above. In this paper, we analyze the different magnetic and structural behaviors found in the YCa series by means of electron spin resonance (ESR) experiments. The experiments were performed using a Bruker spectrometer operating at E9.5 GHz between 100 and 500 K. The ESR spectrum consists of a single line with Lorentzian-type
ARTICLE IN PRESS M.T. Causa et al. / Physica B 398 (2007) 464–467
shape and is observed only in the doping range xX0.2. In ESR experiments, three parameters may be determined, the resonance field (Hr), the line width (DH), and the line intensity (IESR).
1. Resonance field In the PM-phase, Hr is independent of T except in the neighborhood of Tm where internal fields, proportional to M, become important. In our samples, two, strongly interacting, magnetic species coexist. In this case [10], for T4Tm, the g-factor is given by g¼
xg4 C 4 þ ð1 xÞg3 C 3 . xC 4 þ ð1 xÞC 3
(1)
Here, gi and Ci are the corresponding g-factors and Curie constants for localized Mn3+ and Mn4+ spins. Then, Eq. (1) should be valid for the x ranges (0, 0.2) and (0.8, 1). For 0.2oxo0.8, where localized Mn4+ coexist with itinerant eg electrons, Eq. (1) must be replaced [11] by g¼
ge we ðTÞ þ g4 w4 ðTÞ , we ðTÞ þ w4 ðTÞ
(2)
where ge ¼ 2.0036 is the free-electron g and we, w4 are the itinerant and localized contributions to the total susceptibilityU In Fig. 1, we show the experimental g(x). Solid line corresponds to g(x) calculated from Eq. (1) where, as the ESR of YMnO3 was not visible, we assume the value g(Mn3+) ¼ 1.97, measured in LaMnO3 [12]. Deviations from Eq. (1) are observed at xo0.8 (see Fig. 1). In this range, g should be given by Eq. (2). In reasonable agreement, we observed an approximately constant g but some larger than g4 ¼ 1.99 (dotted line) derived from Eq. (2) for we5w4 [9].
465
2. Line width The line width, DH, is related to relaxation mechanisms and for strongly interacting spins a T-dependence is expected in the PM-phase [13] C DH 1 f1 þ AðT T m Þa g, (3) DHðTÞ ¼ ½TwðTÞ where A6¼0 only for TETm and the parameter DHN is the high T-value for DH(T) when w(T)EC/T. To DHN contribute anisotropic and exchange interactions that, respectively, enlarges and narrows the lines [12]. As was shown in Refs. [2,9] when x decreases, Y (proportional to the exchange interactions) diminishes and the distortions (associated with anisotropic interactions) increase. In Fig. 2, we have schematized the behavior expected from Eq. (3) for three different x values. This diagram was constructed from experimental susceptibilities, exchange interactions (obtained from Y), and the experimental DHN of CaMnO3. We conclude that, in our experiments, the ESR lines would be hardly observable for xo0.2. On the other hand, for TETm the second factor of Eq. (3) must be considered. The parameter a depends on the lattice symmetry and is usually ao0 giving a divergent DH(T-Tm). The exceptions are SC and BCC cubic magnetic lattices where, with a40, line narrowing is expected [14]. In Fig. 3, we show the T-dependence of the line width for some representative x values. For x ¼ 1 (see inset in Fig. 3a), the experiment is in agreement with Eq. (3) and the line width decreases at Tm in accordance with a SC and homogenous magnetic lattice. This behavior changes dramatically with small substitutions of Mn4+ by Mn3+ and DH(T-Tm) shows a divergent behavior (see Fig. 3a). In Fig. 3b, we show the behavior for x ¼ 0.5 and 0.75, where structural O–O0 transitions were reported [4,7]. The temperatures, where DH(T) deviates from a constant value,
20 2.02
Linewidth (kGauss)
g-factor
15 2.00
1.98
YMnO3 TN
10
Y0.9 Ca0.1 MnO3 5 CaMnO3
1.96 0.2
0.4
0.6 Ca concentration, x
0.8
1.0
Fig. 1. g vs. x: circles, experimental values; solid line, Eq. (1); dashed line, g ¼ 1.99, measured in CaMnO3.
0 0
100
200
300 T (K)
400
500
Fig. 2. Expected behavior for DH(T) vs. T (see text).
600
ARTICLE IN PRESS M.T. Causa et al. / Physica B 398 (2007) 464–467
466 4
a x = 1 .0 0.5 T (K) 0.0
2
(arb. units)
0.10
N
ESR
T 1.0
10
x = 1 .0 0
I
3
0
200
400
600
IESR (emu / mol)
ΔH (kGauss)
1.5
TN
0 0
100
200
300 T (K)
400
500
0.05 x = 0.90 x = 0.95
1 Y1-xCaxMnO3
x = 0.95
0.00 100
0
150
200
250
300
b 3
χdc
2
x = 0.2 x = 0.3 x = 0.33 x = 0.35
0
100
0 0
100
200
300
200
300 T (K)
400
0
χ
0.02
400
100
200
300
400
500
600
T (K)
(emu/mol)
2000
0.00
500
x = 0.75
0.01
ESR
x = 0.75
4000
IESR
x = 0.45
100
x = 0.5
0.02
I
1
ΔH (Gauss)
6000
emu / mol
x = 0.50
IESR (arb. units)
ΔH (kGauss)
x = 0.90
0.00
500
100
0
T (K)
0
200 300 T (K)
400
100
200
Fig. 3. DH vs. T for the indicated x values.
500
500 O -PM
300
3. Line intensity
ESR not observed
400
200
The intensity (IESR), or area under the absorption curve, was obtained by means of numerical integration. If all the magnetic species contribute to the resonance, IESR(T) should be proportional to w(T) and this usually occurs in the PM-phase. Below Tm, large anisotropy and exchange fields are present, the spins equilibrium state changes, and the PM resonance modes cannot be excited. The AFM resonance in polycrystalline samples is hard to observe and Tm is usually determined by the disappearance of the ESR spectrum [12]. This is the case for x in the range (0.9–1.0) shown in Fig. 4a, where IESR(T) is compared with w(T). Note that IESR(T)ow(T) below 150 K indicating an
400
Fig. 4. IESR vs. T for the indicated x values.
T (K)
are consistent with TETOO0 in both cases. This behavior resembles that found in the LaMnO3+d series [12] where similar anomalies in DH(T) coincide with the Jahn–Teller transition. A complete analysis of the DH(T) behavior for concentrations in the neighborhoods of x ¼ 0.75 and 0.50 is in progress. Finally, lines from Y-rich samples (see inset in Fig. 3b) are very large and approximately constant in our experimental T-range. Notice that, in spite of the different T-dependences, the lines enlarge when x decreases, supporting our assumptions (see Fig. 2).
300 T (K)
O´-PM
100 O´- Magnetic state 0 0.0 AFM / FIM
0.2
0.4
0.6 x
0.8
1.0 AFM + Pol.
Fig. 5. Phase diagram T vs. x. Circles, triangles and squares are results from ESR, susceptibility and structural experiments, respectively. Lines are eyes guide.
increasing fraction of spins becomes non-resonant maybe due to the localization of the, already mentioned, magnetic polarons. Finally, in Fig. 4b, we show IESR(T) for doping
ARTICLE IN PRESS M.T. Causa et al. / Physica B 398 (2007) 464–467
concentrations where structural O–O0 transitions are expected. In these cases, besides the maximum at TETm, a second peak appears at TETOO0 4Tm. This peak marks the transition between two magnetic regimes: FM-like in the O-phase and AFM-like in the O0 -phase [7]. We resume all the experimental facts in the phase diagram T vs. x shown in Fig. 5. Here, we show also Tm and TOO0 derived from magnetic and structural studies. As broad O–O0 transitions are observed in X-ray diffraction studies [3], we have taken TOO0 as a characteristic T where discontinuities in the cell-volume derivative (dV/dT) take place. Regarding the magnetic phases, the magnetic ordering type reached at intermediate concentrations is not known yet and detailed studies at low temperatures are in progress. In summary, we have studied the series Y1xCaxMnO3 that have recently attracted the interest of the researchers due to its relation with multiferroic materials. We have carried out our investigation through measurements of ESR parameters as function of x and T. ESR is a powerful technique that has allowed us to progress in the description of the YCa manganites providing some details that are hidden in ordinary magnetic and structural experiments.
467
Acknowledgments Argentinean institutions ANPCyT (PICT 20770-53 and 25317) and CONICET (PIP 5250 and DT Ph.D.-fellowship) have partially supported this work. E.W. and M.T. are CONICET researchers. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
D. Vega, et al., J. Solid State Chem. 156 (2001) 458. O. Agu¨ero, et al., Physica B 320 (2002) 47. D. Tobı´ a, Graduate Thesis, U. Buenos Aires, November 2005. D. Vega, et al., Physica B 320 (2002) 37. H. Aliaga, et al., J. Phys.: Condens. Matter 15 (2003) 249. H. Aliaga, et al., J. Magn. Magn. Mater. 226–230 (2001) 791. M.T. Causa, et al., J. Magn. Magn. Mater. 272–276 (2004) 81. H. Aliaga, et al., Physica B 320 (2002) 75. D. Tobı´ a, et al., Physica B 384 (2006) 41. D.L. Huber, Phys. Rev. B 12 (1975) 31. M. Tovar, et al., Phys. Rev. B 66 (2002) 024409. D.L. Huber, et al., Phys. Rev. B 60 (1999) 12155. M.T. Causa, et al., Phys. Rev. B 58 (1998) 3233. M.S. Seehra, D.L. Huber, in: C.D. Graham Jr., H. Lander, J.J. Rhyne (Eds.), AIP Conference Proceedings No. 24 on Magnetism and Magnetic Materials, New York, 1975, p. 241.