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Penetration of electromagnetic field through La0.75Pb0.25MnO3. Magnetic phase transition studies
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 285 (2005) 118–124 www.elsevier.com/locate/jmmm
Penetration of electromagnetic field through La0.75Pb0.25MnO3. Magnetic phase transition studies A. Rinkevicha, A. Nossova,, V. Vassilievb, E. Vladimirovab a
Electrical Phenomena Lab, Institute of Metal Physics Ural Division of RAS, 18 S.Kovalevskaya St, GSP-170, Ekaterinburg 620219, Russia b Institute of Solid State Chemistry Ural Division of RAS, 91 Pervomaiskaya St, Ekaterinburg 620219, Russia Received 9 April 2004; received in revised form 13 July 2004 Available online 9 August 2004
Abstract Penetration of electromagnetic field into the bulk polycrystalline La0.75Pb0.25MnO3 manganite in a frequency range from 20 kHz to 10 MHz as a function of DC magnetic field in a temperature range near magnetic phase transition was studied experimentally. At low frequencies, variations of penetration coefficient are mostly determined by variations of magnetic permeability while in the megahertz frequency range they are mostly determined by magnetoresistance. At radio frequencies the dynamic magnetic susceptibility can substantially exceed unity even in a temperature range of 10–141 above the Curie temperature. This can be interpreted as confirmation of existence of a local short-range magnetic order above the Curie temperature. r 2004 Published by Elsevier B.V. PACS: 72.30; 07.55 Keywords: Manganites; Radio-frequency properties
1. Introduction Dynamical electromagnetic methods have become a powerful informative tool for investigation of material properties. By varying the frequency of the electromagnetic field, the depth of its penetraCorresponding author. Tel.:+7-343-3783567; fax: +7-343-
3745244 E-mail address: [email protected] (A. Nossov). 0304-8853/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.jmmm.2004.07.024
tion in a conductive medium can be changed and the frequency-dependent and relaxation characteristics of material constants investigated. The value of skin depth and its frequency dependence are determined by magnetic characteristics and conductivity of the medium. Investigation of skin effect is of particular interest for the doped manganite oxides with perovskite structure since ‘‘colossal’’ anomalies of magnetic and transport properties are observed in this class of materials
ARTICLE IN PRESS A. Rinkevich et al. / Journal of Magnetism and Magnetic Materials 285 (2005) 118–124
near a temperature of phase transition. As is experimentally shown in Ref. [1], the value of skin depth in manganites substantially depends on the magnetic field, and experiments with penetration of a radio-frequency electromagnetic field can be very useful for investigations of dynamic magnetic properties of manganites. Radio-frequency dynamic magnetic properties of manganites were studied in Refs. [2–4]. It was shown that the coefficient of penetration of electromagnetic field has a complex dependence on material constants, frequency range, and the thickness of a manganite sample. Three characteristic frequency regions in dynamic response of manganite can be distinguished. The interval of medium frequencies, where the thickness of a sample is smaller than the skin depth, was found to be the most informative. It was shown that the polarization of an electromagnetic field, which passed through a manganite plate, can be changed [4], and the angle of rotation of the plane of polarization of an electromagnetic field is determined by the radiofrequency surface impedance of the plate. An attempt to use the magnetoresistive and dynamic magnetic properties of manganites for the development of combined (magnetic permeability and magnetoresistive) sensors of magnetic fields was made in Refs. [3,5]. Except for the work by Owens [1], all subsequent works, in which the effects of penetration of electromagnetic field through manganites was studied, were carried out at temperatures below the temperature of magnetic phase transition. A manganite sample was in a ferromagnetic state and the influence of the frequency of electromagnetic field and strength of permanent external magnetic field on the coefficient of penetration were investigated in most cases. However, investigations of the magnetic state of manganites are of particular interest in a temperature range including the temperature of the magnetic phase transition since it is accompanied by anomalies of other different physical properties. The purpose of this work was to study the penetration of a radio-frequency electromagnetic field through the lanthanum manganite doped with lead in detail since these manganites have the Curie temperature T C above the room one. We
119
investigated the coefficient of transmission of electromagnetic field through the manganite plate at temperatures both below and above T C at different frequencies and under the application of external DC magnetic field. Thus, the dynamic magnetic characteristics of doped manganite were investigated. The variation of the transmission coefficient in an external magnetic field is mostly determined either by the variation of magnetic permeability or colossal magnetoresistance of the material. As is shown in Refs. [2,3], changes of the transmission coefficient of a radio-frequency electromagnetic field are mostly determined by variation of magnetic permeability.
2. Experimental The variation of the modulus of the coefficient of transmission of electromagnetic field through the plate made of manganite was measured in a frequency range from 20 kHz to 10 MHz. The idea of the experiment is as follows: a plate of manganite is used as a screen through which an electromagnetic field penetrates. The experimental setup is similar to that used in Refs. [2,3]; see Fig. 1. The AC magnetic field H in ; which impinges on the manganite plate, is created by an exciting coil.
Receiver
RF generator
Brass screen
H in
H out
H
Emitting coil
Receiving coil Heater supply Heater
Fig. 1. Experimental setup.
ARTICLE IN PRESS A. Rinkevich et al. / Journal of Magnetism and Magnetic Materials 285 (2005) 118–124
1.0
χ ac , arb.un.
The transmitted magnetic field H out is recorded using a receiving coil. The exciting coil is fed from a radio-frequency generator and the received signal is measured using a selective microvoltmeter. The DC magnetic field H with the strength of up to 10 kOe is applied parallel to the AC magnetic field H in : All measurements were carried out in a temperature range from 273 to 365 K. Bulk polycrystalline single-phase La0.75Pb0.25 MnO3 samples were synthesized by a wet chemical treatment of the precursors [6]. It is known that this method permits to prepare the products with good magnetic and phase homogeneity. Thermal processing of the precursors permits to lower the synthesis temperature to 1223 K and thereby minimize compositional variations of the final product. After sintering, the samples were examined with optical techniques, scanning electron microscopy, and X-ray structural analysis. The dynamic magnetic properties of the La0.75Pb0.25MnO3 manganite were studied using the AC susceptibility (wAC ) measurements at a frequency of f=40 Hz. The temperature dependence of wAC is shown in Fig. 2a. The value of the Curie temperature T C found from this dependence is 351 K. As is seen from Fig. 2a rapid decrease in the wAC (T) curve is observed when approaching T C and the values of wAC are negligibly small above it. Magnetoresistance was measured by the standard four-probe technique in the temperature range 5–300 K and applied magnetic field H up to 50 kOe with current parallel to the magnetic field. In the temperature range above 300 K, magnetoresistance was measured in a magnetic field of up to 10 kOe. Magnetoresistance is defined here as MR(H, T)=[r(0, T)-r(H, T)]/r(0) 100%, where r(0, T) and r(H, T) are the values of resistivity at a temperature of T in zero magnetic field and magnetic field of H, respectively. The MR(10 kOe, T) curve is shown in Fig. 2b. Above T C the value of magnetoresistance substantially decreases. The maximum value of magnetoresistance in the temperature range of 273–380 K is 5.6%. The value of resistivity rð0; 293Þ for the La0.75Pb0.25MnO3 manganite was found to be 0.185 O cm. The value of skin depth d1 at a frequency of 1 MHz for the normal skin effect was estimated assuming the magnetic permeability
0.5
0.0 6.0
- MR(T), %
120
4.0
2.0 280
300
320
340
360
380
T, K Fig. 2. Temperature dependences of MR and AC magnetic susceptibility.
m equal to unity. Under these assumptions, the skin depth, which can be expressed as d1 ¼ ð2r=om0 Þ1=2 ; was found to be equal to 21.4 mm. The condition d1 4d; where d ¼ 1:2 mm is the thickness of the manganite plate, is valid for all frequencies that we used in our studies of penetration of electromagnetic field through the La0.75Pb0.25MnO3 manganite.
3. Results The DC magnetic field dependence of the coefficient of penetration of electromagnetic field through the bulk polycrystalline manganites in the ferromagnetic state was studied in detail earlier [2–4]. As a rule, large variations of the coefficient are observed upon magnetization. These variations have a positive sign, saturate in strong fields,
ARTICLE IN PRESS A. Rinkevich et al. / Journal of Magnetism and Magnetic Materials 285 (2005) 118–124
and can by several times exceed the value of magnetoresistance. These features permit to relate the variations of the penetration coefficient upon application of a DC magnetic field mostly with variations of a dynamic magnetic susceptibility. In this case, we can assume that the value of variation must be substantially lower above the Curie temperature. However, the results obtained in this work permitted to reveal more complicated pictures. The DC magnetic field dependences of the penetration coefficient rm=[A(H, T)A(0, T)]/ A(0,T) 100%, where A(H, T) and A(0, T) are the values of the transmitted signal at a temperature of T in the magnetic field of H and zero magnetic field, respectively, both above and below T C at a frequency of f ¼ 200 kHz are compared in Fig. 3. The shape of the dependences changed substantially at high temperature and a maximum appeared. However, the magnitude of variations of rm in the region of magnetic saturation in both cases does not differ substantially.
121
The results of measurements of the DC magnetic field dependences of the penetration coefficient at different frequencies at a temperature of T ¼ 365 K, i.e., above the Curie temperature, are shown in Fig. 4. The dependence measured at a frequency of 20 kHz increases monotonically. A maximum appears in the dependence measured at a frequency of f ¼ 200 kHz. The values of the rm ðH ! 1Þ parameter in strong magnetic fields are positive and several times greater than the absolute value of magnetoresistance (MR) for the same value of the DC magnetic field. Subsequent increase in frequency results in negative variations of the penetration coefficient in strong magnetic field. The rm ðHÞ dependence is not saturated in the whole interval of magnetic fields of up to 10 kOe. Note that for all measurements below T C ; i.e. in a ferromagnetic region, the rm ðH ! 1Þ parameter was positive at all frequencies. An abrupt decrease above T C is observed in the temperature dependence of rm measured at a frequency of 8 MHz in a 50
40 40
30
20 rm, %
rm , %
20 kH z 200 kH z 1000 kH z 3000 kH z
10 20
289 K 362 K
0
f= 200 kHz
-10
-20
0 0
2
4
6
8
10
H, kOe Fig. 3. Magnetic field dependence of relative variation of penetration coefficient measured at a frequency of 200 kHz.
0
2
4
6
8
10
H, kOe Fig. 4. Frequency dependence of relative variation of penetration coefficient above the Curie temperature.
ARTICLE IN PRESS A. Rinkevich et al. / Journal of Magnetism and Magnetic Materials 285 (2005) 118–124
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f = 8 MHz 15
H = 9 kOe 50
rm , %
rm, %
10
5
40
TC 0
1 kOe 9 kOe f = 200 kHz 30
-5
300 300
320
340
Fig. 5. Temperature dependence of relative variation of penetration coefficient for electromagnetic field at a frequency of 8 MHz in a magnetic field of H=9 kOe.
magnetic field of H ¼ 9 kOe (see Fig. 5). Moreover, in the range above the temperature of magnetic phase transition the value of rm becomes negative. The temperature dependences of rm measured at a frequency of f ¼ 200 kHz for different values of a DC magnetic field are of particular interest. Fig. 6 shows the results of such measurements with an increasing in the strength of a DC magnetic field from 1 to 9 kOe. In both dependences in a ferromagnetic state rm first increases with increasing temperature, then near T C it abruptly decreases. For temperatures T4T C the sign of rm remains positive. These two dependences intersect at a temperature of T 327 K. In the temperature range where rm ðH ¼ 9 kOeÞ4rm ðH ¼ 1 kOeÞ no maximum is observed in the DC magnetic field dependence of rm ; while in the
360
T, K
360
T,K
340
320
Fig. 6. Temperature dependence of relative variation of penetration coefficient for electromagnetic field at a frequency of 200 kHz in a magnetic field of H ¼ 1 and 9 kOe.
temperature range where rm ðH ¼ 9 kOeÞorm ðH ¼ 1 kOeÞ the maximum in the rm ðHÞ curve exists.
4. Discussion Let us consider the penetration of an electromagnetic field through a conductive plate under conditions of normal skin effect and far from conditions of both ferromagnetic resonance and antiresonance. The following expression for the transmission coefficient can be used [7]: D¼
2Z m ; 2Z m chkm d þ Zshkn d
ð1Þ
where km is the wave number in the conductive medium, km ¼ ð1 þ iÞ=d, d ¼ ð2r=omm0 Þ1=2 is the skin depth, m is the relative dynamic differential
ARTICLE IN PRESS A. Rinkevich et al. / Journal of Magnetism and Magnetic Materials 285 (2005) 118–124
magnetic susceptibility, and o ¼ 2pf : The transmission coefficient D depends on the ratio between the impedances of a plate Zm ; free space Z ¼ ðm0 =0 Þ1=2 ; and the ratio between the total thickness of a plate d and the skin depth d. The impedance of a conductive plate is a complex variable and can be calculated according to the formula Z m ¼ ½ð1 þ iÞ=d r:
ð2Þ
For a medium with high conductivity the impedance of a plate is always less than Z, i.e., jZm j5Z: Two possibilities, when either of the summands exceeds the other one in the denominator of (1), are possible. When the thickness of a plate is small and an electromagnetic field of low frequencies is considered, the condition 2Z m ch km dbZ sh km d is satisfied. This case is realized for 2r : ð3Þ Z In this case, the modulus of the penetration coefficient is determined by the expression km d51; d5
jDj 1 ½1=3ðd=dÞ4 ; and the relative variation of the modulus of the penetration coefficient can be found from the formula 1 4 2 2 m2 ðHÞ m2 ð0Þ rm ¼ d o m 0 2 : ð4Þ 12 r ðHÞ r2 ð0Þ Since d5d; the value of rm is small. It increases with increasing frequency. In the other limiting case, when 2Z m ch km d5Z sh km d; for thin plate under the d5d condition the following expression is valid [3]: D¼
2r : Zmd
ð5Þ
Here the penetration coefficient is inversely proportional to the thickness of a plate d. In the following formula, which includes the ratio of penetration coefficients in zero external field and in saturating magnetic field, the initial magnetic permeability m(0) and magnetoresistance Dr/r can be obtained from (5): DðH ! 1Þ ¼ mð0Þ 1 þ Dr : ð6Þ Dð0Þ r
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Moreover, the following formula is valid under the same conditions for an arbitrary value of a magnetic field H: rm ðHÞ ¼
mð0Þ DrðHÞ mð0Þ þ 1 mðHÞ r mðHÞ
ð7Þ
These formulas can be used to analyze the experimental data for the value of penetration coefficient and its variations in a magnetic field. First of all, the temperature dependence of magnetic susceptibility shown in Fig. 2 points to the fact that above the Curie temperature the lowfrequency magnetic susceptibility tends to unity. At such temperatures a long-range magnetic order is absent in the system. However, the data for penetration of radio-frequency electromagnetic field point to the fact that the dynamic highfrequency magnetic susceptibility can differ substantially from unity even at T4T C : Indeed, it follows from formula (7) that when Dr/ro0 (negative magnetoresistance) the positive value of rm is possible only if the initial magnetic susceptibility m(0)41. In particular, for the conditions of Fig. 3 at a temperature of T=362 K4TC, an estimate according to formula (6) gives m(0)E1.4. The fact of existence of the value of the dynamic magnetic susceptibility exceeding unity at T4T C is confirmed by the data shown in Fig. 6 and, partly, in Fig. 4. These data point to a principal possibility of existence of a short-range magnetic order well above the Curie temperature in the manganite sample investigated. This supposition is in accordance with the data available in literature [8,9]. Let us analyze the data shown in Fig. 4 in more detail. Saturation in strong magnetic field with DðH ! 1Þ4Dð0Þ is clearly seen in the dependences measured at low frequencies of 20 and 200 kHz. Variation of penetration at these frequencies is mostly caused by variations of magnetic susceptibility. However different behavior is observed for the frequencies of 1 and 3 MHz: the value of rm becomes negative in strong magnetic fields. According to (7) it is possible only when the value of the initial magnetic susceptibility m(0) is close to unity and variations of the D coefficient are mostly caused by magnetoresistance. This fact is also confirmed by the absence of saturation for
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the dependences with negative sign in strong magnetic fields which are shown in Fig. 4. We suppose that the decrease of the magnetic susceptibility at megahertz frequencies is caused by a frequency dispersion. 5. Summary Penetration of electromagnetic field through the plate made from the La0.75Pb0.25MnO3 manganite near the temperature of a magnetic phase transition was studied experimentally. It is shown that at radio frequencies the dynamic magnetic susceptibility can substantially exceed unity even in a temperature range 10–141 above the Curie temperature. This fact can be interpreted as a confirmation of existence of local short-range magnetic order above T C : It was found that at megahertz frequencies variations of the penetration coefficient in strong DC magnetic field can become negative. In this case they are mostly determined by MR of the material. Acknowledgements This work was partially supported by the program of the Presidium of the Russian Academy of Sciences ‘‘Quantum macrophysics’’, Russian Foundation for Basic Research, and the NSh1380.2003.2 Grant.
References [1] F. Owens, Giant magneto radio frequency absorption in magneto-resistive materials La0.7(Sr,Ca)0.3MnO3, J. Appl. Phys. 82 (6) (1997) 3054–3057. [2] A. Rinkevich, A. Nossov, V. Ustinov, V. Vassiliev, S. Petukhov, Penetration of the electromagnetic waves through doped lanthanum manganites, J. Appl. Phys. 91 (6) (2002) 3693–3697. [3] A. Rinkevich, A. Nossov, M. Rigmant, V. Ustinov, V. Vassiliev, E. Vladimirova, B. Slobodin, Magnetic-fieldassisted giant penetration of a radio-frequency electromagnetic field in lanthanum manganites, IEEE Trans. Magn. 38 (1) (2002) 257–259. [4] A. Rinkevich, A. Nossov, V. Ustinov, V. Vassiliev, E. Mikhaleva, S. Petukhov, Polarization of the radiofrequency electromagnetic field in lanthanum manganites, Phys. Met. Metallogr. 91 (1) (2001) S190–S193. [5] A. Nossov, A. Rinkevich, M. Rigmant, V. Vassiliev, Combined lanthanum manganite magnetoresistive-fluxgate magnetic field sensor, Sensor. Actuator. A 94 (2001) 157–160. [6] E. Vladimirova, V. Vassiliev, A. Nossov, Synthesis of La1xPbxMnO3 colossal magnetoresistive ceramics from coprecipitated oxalate precursors, J. Mater. Sci. 36 (6) (2001) 1481–1486. [7] R.A. Semenov, Technical Electrodynamics, Svyaz, Moscow, vol. 480, 1972 (in Russian). [8] S.E. Lofland, V. Ray, P.H. Kim, S.M. Bhagat, M.A. Manheimer, S.D. Tyagi, Magnetic phase transition in La0.7Sr0.3MnO3: microwave studies, Phys. Rev. B-I. 55 (5) (1997) 2749–2751. [9] A. Schwartz, M. Scheffer, S.M. Anlage, Determination of the magnetization scaling exponent for single-crystal La0.8Sr0.2MnO3 by broadband microwave surface impedance measurements, Phys. Rev. B-II 61 (2) (2000) R870–R873.
Journal of Magnetism and Magnetic Materials 285 (2005) 118–124 www.elsevier.com/locate/jmmm
Penetration of electromagnetic field through La0.75Pb0.25MnO3. Magnetic phase transition studies A. Rinkevicha, A. Nossova,, V. Vassilievb, E. Vladimirovab a
Electrical Phenomena Lab, Institute of Metal Physics Ural Division of RAS, 18 S.Kovalevskaya St, GSP-170, Ekaterinburg 620219, Russia b Institute of Solid State Chemistry Ural Division of RAS, 91 Pervomaiskaya St, Ekaterinburg 620219, Russia Received 9 April 2004; received in revised form 13 July 2004 Available online 9 August 2004
Abstract Penetration of electromagnetic field into the bulk polycrystalline La0.75Pb0.25MnO3 manganite in a frequency range from 20 kHz to 10 MHz as a function of DC magnetic field in a temperature range near magnetic phase transition was studied experimentally. At low frequencies, variations of penetration coefficient are mostly determined by variations of magnetic permeability while in the megahertz frequency range they are mostly determined by magnetoresistance. At radio frequencies the dynamic magnetic susceptibility can substantially exceed unity even in a temperature range of 10–141 above the Curie temperature. This can be interpreted as confirmation of existence of a local short-range magnetic order above the Curie temperature. r 2004 Published by Elsevier B.V. PACS: 72.30; 07.55 Keywords: Manganites; Radio-frequency properties
1. Introduction Dynamical electromagnetic methods have become a powerful informative tool for investigation of material properties. By varying the frequency of the electromagnetic field, the depth of its penetraCorresponding author. Tel.:+7-343-3783567; fax: +7-343-
3745244 E-mail address: [email protected] (A. Nossov). 0304-8853/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.jmmm.2004.07.024
tion in a conductive medium can be changed and the frequency-dependent and relaxation characteristics of material constants investigated. The value of skin depth and its frequency dependence are determined by magnetic characteristics and conductivity of the medium. Investigation of skin effect is of particular interest for the doped manganite oxides with perovskite structure since ‘‘colossal’’ anomalies of magnetic and transport properties are observed in this class of materials
ARTICLE IN PRESS A. Rinkevich et al. / Journal of Magnetism and Magnetic Materials 285 (2005) 118–124
near a temperature of phase transition. As is experimentally shown in Ref. [1], the value of skin depth in manganites substantially depends on the magnetic field, and experiments with penetration of a radio-frequency electromagnetic field can be very useful for investigations of dynamic magnetic properties of manganites. Radio-frequency dynamic magnetic properties of manganites were studied in Refs. [2–4]. It was shown that the coefficient of penetration of electromagnetic field has a complex dependence on material constants, frequency range, and the thickness of a manganite sample. Three characteristic frequency regions in dynamic response of manganite can be distinguished. The interval of medium frequencies, where the thickness of a sample is smaller than the skin depth, was found to be the most informative. It was shown that the polarization of an electromagnetic field, which passed through a manganite plate, can be changed [4], and the angle of rotation of the plane of polarization of an electromagnetic field is determined by the radiofrequency surface impedance of the plate. An attempt to use the magnetoresistive and dynamic magnetic properties of manganites for the development of combined (magnetic permeability and magnetoresistive) sensors of magnetic fields was made in Refs. [3,5]. Except for the work by Owens [1], all subsequent works, in which the effects of penetration of electromagnetic field through manganites was studied, were carried out at temperatures below the temperature of magnetic phase transition. A manganite sample was in a ferromagnetic state and the influence of the frequency of electromagnetic field and strength of permanent external magnetic field on the coefficient of penetration were investigated in most cases. However, investigations of the magnetic state of manganites are of particular interest in a temperature range including the temperature of the magnetic phase transition since it is accompanied by anomalies of other different physical properties. The purpose of this work was to study the penetration of a radio-frequency electromagnetic field through the lanthanum manganite doped with lead in detail since these manganites have the Curie temperature T C above the room one. We
119
investigated the coefficient of transmission of electromagnetic field through the manganite plate at temperatures both below and above T C at different frequencies and under the application of external DC magnetic field. Thus, the dynamic magnetic characteristics of doped manganite were investigated. The variation of the transmission coefficient in an external magnetic field is mostly determined either by the variation of magnetic permeability or colossal magnetoresistance of the material. As is shown in Refs. [2,3], changes of the transmission coefficient of a radio-frequency electromagnetic field are mostly determined by variation of magnetic permeability.
2. Experimental The variation of the modulus of the coefficient of transmission of electromagnetic field through the plate made of manganite was measured in a frequency range from 20 kHz to 10 MHz. The idea of the experiment is as follows: a plate of manganite is used as a screen through which an electromagnetic field penetrates. The experimental setup is similar to that used in Refs. [2,3]; see Fig. 1. The AC magnetic field H in ; which impinges on the manganite plate, is created by an exciting coil.
Receiver
RF generator
Brass screen
H in
H out
H
Emitting coil
Receiving coil Heater supply Heater
Fig. 1. Experimental setup.
ARTICLE IN PRESS A. Rinkevich et al. / Journal of Magnetism and Magnetic Materials 285 (2005) 118–124
1.0
χ ac , arb.un.
The transmitted magnetic field H out is recorded using a receiving coil. The exciting coil is fed from a radio-frequency generator and the received signal is measured using a selective microvoltmeter. The DC magnetic field H with the strength of up to 10 kOe is applied parallel to the AC magnetic field H in : All measurements were carried out in a temperature range from 273 to 365 K. Bulk polycrystalline single-phase La0.75Pb0.25 MnO3 samples were synthesized by a wet chemical treatment of the precursors [6]. It is known that this method permits to prepare the products with good magnetic and phase homogeneity. Thermal processing of the precursors permits to lower the synthesis temperature to 1223 K and thereby minimize compositional variations of the final product. After sintering, the samples were examined with optical techniques, scanning electron microscopy, and X-ray structural analysis. The dynamic magnetic properties of the La0.75Pb0.25MnO3 manganite were studied using the AC susceptibility (wAC ) measurements at a frequency of f=40 Hz. The temperature dependence of wAC is shown in Fig. 2a. The value of the Curie temperature T C found from this dependence is 351 K. As is seen from Fig. 2a rapid decrease in the wAC (T) curve is observed when approaching T C and the values of wAC are negligibly small above it. Magnetoresistance was measured by the standard four-probe technique in the temperature range 5–300 K and applied magnetic field H up to 50 kOe with current parallel to the magnetic field. In the temperature range above 300 K, magnetoresistance was measured in a magnetic field of up to 10 kOe. Magnetoresistance is defined here as MR(H, T)=[r(0, T)-r(H, T)]/r(0) 100%, where r(0, T) and r(H, T) are the values of resistivity at a temperature of T in zero magnetic field and magnetic field of H, respectively. The MR(10 kOe, T) curve is shown in Fig. 2b. Above T C the value of magnetoresistance substantially decreases. The maximum value of magnetoresistance in the temperature range of 273–380 K is 5.6%. The value of resistivity rð0; 293Þ for the La0.75Pb0.25MnO3 manganite was found to be 0.185 O cm. The value of skin depth d1 at a frequency of 1 MHz for the normal skin effect was estimated assuming the magnetic permeability
0.5
0.0 6.0
- MR(T), %
120
4.0
2.0 280
300
320
340
360
380
T, K Fig. 2. Temperature dependences of MR and AC magnetic susceptibility.
m equal to unity. Under these assumptions, the skin depth, which can be expressed as d1 ¼ ð2r=om0 Þ1=2 ; was found to be equal to 21.4 mm. The condition d1 4d; where d ¼ 1:2 mm is the thickness of the manganite plate, is valid for all frequencies that we used in our studies of penetration of electromagnetic field through the La0.75Pb0.25MnO3 manganite.
3. Results The DC magnetic field dependence of the coefficient of penetration of electromagnetic field through the bulk polycrystalline manganites in the ferromagnetic state was studied in detail earlier [2–4]. As a rule, large variations of the coefficient are observed upon magnetization. These variations have a positive sign, saturate in strong fields,
ARTICLE IN PRESS A. Rinkevich et al. / Journal of Magnetism and Magnetic Materials 285 (2005) 118–124
and can by several times exceed the value of magnetoresistance. These features permit to relate the variations of the penetration coefficient upon application of a DC magnetic field mostly with variations of a dynamic magnetic susceptibility. In this case, we can assume that the value of variation must be substantially lower above the Curie temperature. However, the results obtained in this work permitted to reveal more complicated pictures. The DC magnetic field dependences of the penetration coefficient rm=[A(H, T)A(0, T)]/ A(0,T) 100%, where A(H, T) and A(0, T) are the values of the transmitted signal at a temperature of T in the magnetic field of H and zero magnetic field, respectively, both above and below T C at a frequency of f ¼ 200 kHz are compared in Fig. 3. The shape of the dependences changed substantially at high temperature and a maximum appeared. However, the magnitude of variations of rm in the region of magnetic saturation in both cases does not differ substantially.
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The results of measurements of the DC magnetic field dependences of the penetration coefficient at different frequencies at a temperature of T ¼ 365 K, i.e., above the Curie temperature, are shown in Fig. 4. The dependence measured at a frequency of 20 kHz increases monotonically. A maximum appears in the dependence measured at a frequency of f ¼ 200 kHz. The values of the rm ðH ! 1Þ parameter in strong magnetic fields are positive and several times greater than the absolute value of magnetoresistance (MR) for the same value of the DC magnetic field. Subsequent increase in frequency results in negative variations of the penetration coefficient in strong magnetic field. The rm ðHÞ dependence is not saturated in the whole interval of magnetic fields of up to 10 kOe. Note that for all measurements below T C ; i.e. in a ferromagnetic region, the rm ðH ! 1Þ parameter was positive at all frequencies. An abrupt decrease above T C is observed in the temperature dependence of rm measured at a frequency of 8 MHz in a 50
40 40
30
20 rm, %
rm , %
20 kH z 200 kH z 1000 kH z 3000 kH z
10 20
289 K 362 K
0
f= 200 kHz
-10
-20
0 0
2
4
6
8
10
H, kOe Fig. 3. Magnetic field dependence of relative variation of penetration coefficient measured at a frequency of 200 kHz.
0
2
4
6
8
10
H, kOe Fig. 4. Frequency dependence of relative variation of penetration coefficient above the Curie temperature.
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122
f = 8 MHz 15
H = 9 kOe 50
rm , %
rm, %
10
5
40
TC 0
1 kOe 9 kOe f = 200 kHz 30
-5
300 300
320
340
Fig. 5. Temperature dependence of relative variation of penetration coefficient for electromagnetic field at a frequency of 8 MHz in a magnetic field of H=9 kOe.
magnetic field of H ¼ 9 kOe (see Fig. 5). Moreover, in the range above the temperature of magnetic phase transition the value of rm becomes negative. The temperature dependences of rm measured at a frequency of f ¼ 200 kHz for different values of a DC magnetic field are of particular interest. Fig. 6 shows the results of such measurements with an increasing in the strength of a DC magnetic field from 1 to 9 kOe. In both dependences in a ferromagnetic state rm first increases with increasing temperature, then near T C it abruptly decreases. For temperatures T4T C the sign of rm remains positive. These two dependences intersect at a temperature of T 327 K. In the temperature range where rm ðH ¼ 9 kOeÞ4rm ðH ¼ 1 kOeÞ no maximum is observed in the DC magnetic field dependence of rm ; while in the
360
T, K
360
T,K
340
320
Fig. 6. Temperature dependence of relative variation of penetration coefficient for electromagnetic field at a frequency of 200 kHz in a magnetic field of H ¼ 1 and 9 kOe.
temperature range where rm ðH ¼ 9 kOeÞorm ðH ¼ 1 kOeÞ the maximum in the rm ðHÞ curve exists.
4. Discussion Let us consider the penetration of an electromagnetic field through a conductive plate under conditions of normal skin effect and far from conditions of both ferromagnetic resonance and antiresonance. The following expression for the transmission coefficient can be used [7]: D¼
2Z m ; 2Z m chkm d þ Zshkn d
ð1Þ
where km is the wave number in the conductive medium, km ¼ ð1 þ iÞ=d, d ¼ ð2r=omm0 Þ1=2 is the skin depth, m is the relative dynamic differential
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magnetic susceptibility, and o ¼ 2pf : The transmission coefficient D depends on the ratio between the impedances of a plate Zm ; free space Z ¼ ðm0 =0 Þ1=2 ; and the ratio between the total thickness of a plate d and the skin depth d. The impedance of a conductive plate is a complex variable and can be calculated according to the formula Z m ¼ ½ð1 þ iÞ=d r:
ð2Þ
For a medium with high conductivity the impedance of a plate is always less than Z, i.e., jZm j5Z: Two possibilities, when either of the summands exceeds the other one in the denominator of (1), are possible. When the thickness of a plate is small and an electromagnetic field of low frequencies is considered, the condition 2Z m ch km dbZ sh km d is satisfied. This case is realized for 2r : ð3Þ Z In this case, the modulus of the penetration coefficient is determined by the expression km d51; d5
jDj 1 ½1=3ðd=dÞ4 ; and the relative variation of the modulus of the penetration coefficient can be found from the formula 1 4 2 2 m2 ðHÞ m2 ð0Þ rm ¼ d o m 0 2 : ð4Þ 12 r ðHÞ r2 ð0Þ Since d5d; the value of rm is small. It increases with increasing frequency. In the other limiting case, when 2Z m ch km d5Z sh km d; for thin plate under the d5d condition the following expression is valid [3]: D¼
2r : Zmd
ð5Þ
Here the penetration coefficient is inversely proportional to the thickness of a plate d. In the following formula, which includes the ratio of penetration coefficients in zero external field and in saturating magnetic field, the initial magnetic permeability m(0) and magnetoresistance Dr/r can be obtained from (5): DðH ! 1Þ ¼ mð0Þ 1 þ Dr : ð6Þ Dð0Þ r
123
Moreover, the following formula is valid under the same conditions for an arbitrary value of a magnetic field H: rm ðHÞ ¼
mð0Þ DrðHÞ mð0Þ þ 1 mðHÞ r mðHÞ
ð7Þ
These formulas can be used to analyze the experimental data for the value of penetration coefficient and its variations in a magnetic field. First of all, the temperature dependence of magnetic susceptibility shown in Fig. 2 points to the fact that above the Curie temperature the lowfrequency magnetic susceptibility tends to unity. At such temperatures a long-range magnetic order is absent in the system. However, the data for penetration of radio-frequency electromagnetic field point to the fact that the dynamic highfrequency magnetic susceptibility can differ substantially from unity even at T4T C : Indeed, it follows from formula (7) that when Dr/ro0 (negative magnetoresistance) the positive value of rm is possible only if the initial magnetic susceptibility m(0)41. In particular, for the conditions of Fig. 3 at a temperature of T=362 K4TC, an estimate according to formula (6) gives m(0)E1.4. The fact of existence of the value of the dynamic magnetic susceptibility exceeding unity at T4T C is confirmed by the data shown in Fig. 6 and, partly, in Fig. 4. These data point to a principal possibility of existence of a short-range magnetic order well above the Curie temperature in the manganite sample investigated. This supposition is in accordance with the data available in literature [8,9]. Let us analyze the data shown in Fig. 4 in more detail. Saturation in strong magnetic field with DðH ! 1Þ4Dð0Þ is clearly seen in the dependences measured at low frequencies of 20 and 200 kHz. Variation of penetration at these frequencies is mostly caused by variations of magnetic susceptibility. However different behavior is observed for the frequencies of 1 and 3 MHz: the value of rm becomes negative in strong magnetic fields. According to (7) it is possible only when the value of the initial magnetic susceptibility m(0) is close to unity and variations of the D coefficient are mostly caused by magnetoresistance. This fact is also confirmed by the absence of saturation for
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the dependences with negative sign in strong magnetic fields which are shown in Fig. 4. We suppose that the decrease of the magnetic susceptibility at megahertz frequencies is caused by a frequency dispersion. 5. Summary Penetration of electromagnetic field through the plate made from the La0.75Pb0.25MnO3 manganite near the temperature of a magnetic phase transition was studied experimentally. It is shown that at radio frequencies the dynamic magnetic susceptibility can substantially exceed unity even in a temperature range 10–141 above the Curie temperature. This fact can be interpreted as a confirmation of existence of local short-range magnetic order above T C : It was found that at megahertz frequencies variations of the penetration coefficient in strong DC magnetic field can become negative. In this case they are mostly determined by MR of the material. Acknowledgements This work was partially supported by the program of the Presidium of the Russian Academy of Sciences ‘‘Quantum macrophysics’’, Russian Foundation for Basic Research, and the NSh1380.2003.2 Grant.
References [1] F. Owens, Giant magneto radio frequency absorption in magneto-resistive materials La0.7(Sr,Ca)0.3MnO3, J. Appl. Phys. 82 (6) (1997) 3054–3057. [2] A. Rinkevich, A. Nossov, V. Ustinov, V. Vassiliev, S. Petukhov, Penetration of the electromagnetic waves through doped lanthanum manganites, J. Appl. Phys. 91 (6) (2002) 3693–3697. [3] A. Rinkevich, A. Nossov, M. Rigmant, V. Ustinov, V. Vassiliev, E. Vladimirova, B. Slobodin, Magnetic-fieldassisted giant penetration of a radio-frequency electromagnetic field in lanthanum manganites, IEEE Trans. Magn. 38 (1) (2002) 257–259. [4] A. Rinkevich, A. Nossov, V. Ustinov, V. Vassiliev, E. Mikhaleva, S. Petukhov, Polarization of the radiofrequency electromagnetic field in lanthanum manganites, Phys. Met. Metallogr. 91 (1) (2001) S190–S193. [5] A. Nossov, A. Rinkevich, M. Rigmant, V. Vassiliev, Combined lanthanum manganite magnetoresistive-fluxgate magnetic field sensor, Sensor. Actuator. A 94 (2001) 157–160. [6] E. Vladimirova, V. Vassiliev, A. Nossov, Synthesis of La1xPbxMnO3 colossal magnetoresistive ceramics from coprecipitated oxalate precursors, J. Mater. Sci. 36 (6) (2001) 1481–1486. [7] R.A. Semenov, Technical Electrodynamics, Svyaz, Moscow, vol. 480, 1972 (in Russian). [8] S.E. Lofland, V. Ray, P.H. Kim, S.M. Bhagat, M.A. Manheimer, S.D. Tyagi, Magnetic phase transition in La0.7Sr0.3MnO3: microwave studies, Phys. Rev. B-I. 55 (5) (1997) 2749–2751. [9] A. Schwartz, M. Scheffer, S.M. Anlage, Determination of the magnetization scaling exponent for single-crystal La0.8Sr0.2MnO3 by broadband microwave surface impedance measurements, Phys. Rev. B-II 61 (2) (2000) R870–R873.