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Giant magnetoimpedance and colossal ac magnetoresistance of a Cu coil wound on La0.67Sr0.33MnO3
Solid State Communications 151 (2011) 47–50
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Giant magnetoimpedance and colossal ac magnetoresistance of a Cu coil wound on La0.67 Sr0.33 MnO3 Jifan Hu a,∗ , Yifei Wang a , Juan Chen a,b , Hongwei Qin a , Bo Li c a
School of Physics, State Key Laboratory for Crystal Materials, Shandong University, Jinan 250100, China
b
Group of Nanomagnetics and Applications, School of Medicine, Shandong University, Jinan 250012, China
c
Department of Functional Materials, Central Iron and Steel Research Institute, Beijing 100081, China
article
info
Article history: Received 1 March 2010 Received in revised form 18 September 2010 Accepted 18 October 2010 by A. Pinczuk Available online 26 October 2010 Keywords: A. Magnetically ordered materials D. Electronic transport
abstract We observed the giant magnetoimpedance and colossal ac magnetoresistance for a Cu coil wound on La0.67 Sr0.33 MnO3 under low dc magnetic fields. Even though the dc magnetoresistance 1R/R0 for La0.67 Sr0.33 MnO3 plate is only −2.4% under H = 12 kOe, a Cu coil wound on La0.67 Sr0.33 MnO3 plate exhibits a colossal ac magnetoresistance 1R/R0 of −93% at 10 MHz and a giant magnetoimpedance 1Z /Z0 of −59% in a wide frequency range of 500 kHz–10 MHz under a longitudinal field H = 600 Oe. The transverse magnetoimpedance is weaker than the longitudinal one. The giant magnetoimpedance and colossal ac magnetoresistance for a Cu coil wound on La0.67 Sr0.33 MnO3 are connected with the variation of permeability induced by dc magnetic field. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Perovskite rare earth manganites, La1−x Ax MnO3 (A = Ca, Ba, Sr) have attracted much attention due to the colossal magnetoresistance (CMR) and the strong interplay among lattice, electronic transport and magnetic properties [1,2]. There is a transition from insulator (or semiconductor) to metal near Curie temperature TC . The CMR effects are pronounced in a limited range of temperature near TC . One problem of the CMR for application is the requirement for a large applied magnetic field about several teslas. For La–Sr–Mn–O (such as La0.7 Sr0.3 MnO3 and La0.67 Sr0.33 MnO3 ) sintered manganites, values of magnetoresistance are only −3% under magnetic field H = 1.2 T at room temperature [3,4]. The CMR phenomenon can be explained by the double-exchange interaction [5] and the strong Jahn–Teller distortion with the electron–phonon coupling [6]. In addition to the CMR effect, La1−x Ax MnO3 manganites could exhibit colossal magnetoabsorption at microwave high frequencies (∼GHz) [7–9] and large magnetoimpedance at radio frequencies (∼kHz or MHz) [10–22] under low fields. Giant magnetoimpedance effects have been found in soft magnetic metallic materials, such as Co–Fe–Si–B amorphous and Fe based nanocrystalline wires and ribbons [23–28]. Similar to the cases for conventional metallic materials, skin effect should occur at high
∗
Corresponding author. Tel.: +86 531 88566143; fax: +86 531 88377031. E-mail address: [email protected] (J. Hu).
0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2010.10.026
frequencies when an ac current passes through La1−x Ax MnO3 ferromagnetic metals. Application of external dc magnetic field induces a change of permeability and brings about a variation of penetration depth, resulting in a large change of impedance in La1−x Ax MnO3 ferromagnetic metals. A giant magnetoimpedance was also observed by Ghatak et al. [17] in Ag-doped La0.7 Sr0.3 MnO3 nanocrystalline samples (prepared by a wet chemical method) at room temperature, where the signals of impedance were collected from a coil wound on the manganite. They found ac magnetoresistance 1R/R0 of about −64% under H = 600 Oe and 1R/R0 = −70% under H = 4 kOe at ω = 15 MHz for La0.7 Sr0.25 Ag0.05 MnO3 . In the present work, we reported the magnetoimpedance effect for an enamelled Cu coil wound on La0.67 Sr0.33 MnO3 prepared with conventional solid-state sintering method. We observed a colossal ac magnetoresistance 1R/R0 of −93% at 10 MHz and a giant magnetoimpedance 1Z /Z0 of −59.3% at 2 MHz under longitudinal H = 600 Oe. The observed giant magnetoimpedance and colossal ac magnetoresistance are connected with the variation of permeability induced by dc magnetic field. 2. Experiments The sintered samples of La0.67 Sr0.33 MnO3 were prepared using the conventional solid-state reaction method with a sintering temperature of 1230 °C. X-ray diffraction analysis indicated that samples were of single phase with rhombohedral perovskite structure. The Curie temperature TC was determined as 362 K
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J. Hu et al. / Solid State Communications 151 (2011) 47–50
the dc magnetic fields were applied in the direction parallel and perpendicular to the direction of the length of the plate, respectively. 3. Results and discussion
Fig. 1. The ac frequency dependence of resistance R, reactance X and impedance Z for the Cu coil alone and the Cu coil wound on La0.67 Sr0.33 MnO3 at room temperature without dc magnetic field, respectively. The inset is the frequency dependence of resistance R for the Cu coil alone.
from thermal magnetic curve measured using a thermal magnetic analyzer at 200 Oe. The metal–insulator transition temperature TMI was about 357 K determined from the temperature dependence of the resistivity. The dc magnetoresistance 1R/R0 under H = 12 kOe was measured as about −2.4%. The magnetoimpedance measurement was carried out using an impedance analyzer at room temperature. The La0.67 Sr0.33 MnO3 plate with 15 mm length, 2.5 mm thickness and 2.8 mm width was used for the measurement, where the impedance signals were collected from an enamelled Cu coil of 9 turns wound on the La0.67 Sr0.33 MnO3 plate. The amplitude of ac currents was maintained as 20 mA. The ac field Hac produced by the Cu coil is about 12 A/m (about 0.15 Oe). The longitudinal and the transverse magnetoimpedance of the Cu coil wound on La0.67 Sr0.33 MnO3 were measured, where
Fig. 1 shows the ac frequency dependence of resistance R, reactance X and impedance Z for the Cu coil alone and the Cu coil wound on La0.67 Sr0.33 MnO3 without dc magnetic field at room temperature, respectively. Values of R, X , Z increase with an increase of frequency for the above two systems. The increase of Z with frequency for the Cu coil is due to skin effect. All values of R, X , Z for the system of Cu coil wound on La0.67 Sr0.33 MnO3 are larger than those for the Cu coil alone. In fact, the impedance Z collected from a coil wound on a magnetic material is usually used for the measurement of permeability, where the shape of the magnetic material is toroidal [29]. For the present work, a plate of La0.67 Sr0.33 MnO3 was used, and there is a leakage of magnetic flux. Nevertheless, we may approximately have the following expressions for a coil wound on the magnetic plate X ≈ b1 ωµ0 µ′
(1)
R ≈ Rcoil + b2 ωµ0 µ
′′
(2)
where µ and µ are real and imaginary parts of permeability for the magnetic material, µ0 is the vacuum permeability, Rcoil is the loss resistance of the coil itself, b1 and b2 are the coefficients. The effective inductance L for the Cu coil wound on La0.67 Sr0.33 MnO3 is larger than that (Lcoil ) for the Cu coil alone. The longitudinal dc field dependence of magnetoresistance 1R/R0 = [R(H )− R0 ]/R0 for the Cu coil wound on La0.67 Sr0.33 MnO3 at room temperature is shown in Fig. 2(a). Even though the dc ′
′′
a
d
b
e
c
f
Fig. 2. The longitudinal dc field dependence of magnetoresistance, magnetoreactance and magnetoimpedance for the Cu coil wound on La0.67 Sr0.33 MnO3 at room temperature, respectively.
J. Hu et al. / Solid State Communications 151 (2011) 47–50
49
a
b
Fig. 3. The ac frequency dependence of magnetoimpedance 1Z /Z0 , magnetoresistance 1R/R0 and magnetoreactance 1X /X0 for the Cu coil wound on La0.67 Sr0.33 MnO3 under a longitudinal dc field H = 600 Oe at room temperature.
magnetoresistance 1R/R0 is only −2.4% for La0.67 Sr0.33 MnO3 plate under H = 12 kOe, a colossal ac magnetoresistance effect occurs for the Cu coil wound on La0.67 Sr0.33 MnO3 plate. As shown in Fig. 2(a), the ac magnetoresistance 1R/R0 can reach −45.2% under longitudinal field H = 50 Oe, −71.3% under longitudinal field H = 100 Oe, −93% under longitudinal field H = 600 Oe at 10 MHz for the Cu coil wound on La0.67 Sr0.33 MnO3 plate. The magnetoresistance 1R/R0 shows a strong dependence on frequency. At low frequency such as 500 kHz, the magnetoresistance 1R/R0 almost reaches its saturation value (about −32%) under a smaller field H = 200 Oe. As shown in Fig. 3, the magnetoresistance 1R/R0 increases with an increase of ac frequency up to f = 10 MHz. The longitudinal dc field dependence of the magnetoreactance 1X /X0 = [X (H ) − X0 ]/X0 and magnetoimpedance 1Z /Z0 = [Z (H ) − Z0 ]/Z0 for the Cu coil wound on La0.67 Sr0.33 MnO3 at room temperature are also plotted in Fig. 2(b) and (c), respectively. The impedance Z and reactance X decrease with an increase of dc magnetic field, showing giant negative magnetoimpedance effect. A magnetoreactance 1X /X0 of −59.3% and a magnetoimpedance 1Z /Z0 of −59.4% occur at 2 MHz under 600 Oe. The maximum slope sensitivity of 1Z /Z0 is about 0.63%/Oe. In addition, the field dependence of magnetoimpedance 1Z /Z0 and magnetoreactance 1X /X0 are very similar. The definition of magnetoimpedance ratio, [Z (H ) − Z (Hmax )]/Z (Hmax ), was also frequently used for labeling the giant magnetoimpedance in Co-rich amorphous and Fe nanocrystalline wires and ribbons, where the value of Hmax is usually the external magnetic field sufficient to saturate the impedance, or the field available for the given experimental equipment in practice [26]. For comparison, the field dependence of [R(H ) − R(Hmax )]/ R(Hmax ), [X (0)− X (Hmax )]/X (Hmax ) and [Z (H )− Z (Hmax )]/Z (Hmax ) under Hmax = 600 Oe for the Cu coil wound on La0.67 Sr0.33 MnO3 were also plotted in Fig. 2(d)–(f). Values of [Z (H ) − Z (Hmax )]/ Z (Hmax ) = 146% at 2 MHz and [R(0) − R(Hmax )]/R(Hmax ) = 1315% at 10 MHz could be obtained. The maximum sensitivity of [Z (H ) − Z (Hmax )]/Z (Hmax ) could reach 16%/Oe. It has been found that the (Co94 Fe6 )75 Si10 B15 amorphous microwire exhibits a [Z (H ) − Z (Hmax )]/Z (Hmax ) of 125% with Hmax = 10 Oe at 3.22 MHz and the maximum sensitivity of 50%/Oe [27]. The Fe73.5 Si13.5 B9 Cu1 Nb3 wire annealed at 600 °C for 1 h shows a [Z (H ) − Z (Hmax )]/Z (Hmax ) of 200% with Hmax = 100 Oe at 0.5 MHz [28]. In the frequency range from 100 kHz to 10 MHz, values of magnetoimpedance 1Z /Z0 are mainly controlled by those of magnetoreactance 1X /X0 (see Fig. 3), since the values of R are much smaller than those of X, and the values of Z are controlled by those of X (see Fig. 1). The frequency dependence of magnetoimpedance 1Z /Z0 and magnetoreactance 1X /X0 under a certain longitudinal H = 600 Oe are not pronounced. The value of magnetoreactance 1X /X0 becomes somewhat smaller at
c
Fig. 4. The transverse dc field dependence of magnetoresistance 1R/R0 , magnetoreactance 1X /X0 and magnetoimpedance 1Z /Z0 for the Cu coil wound on La0.67 Sr0.33 MnO3 at room temperature, respectively.
high frequency such as 10 MHz. The magnetoimpedance 1Z /Z increases with frequency slightly at first, undergoes a peak value −59.3% at about 2–3 MHz, and finally decreases (see Fig. 3). For colossal dc magnetoresistance, the dc magnetic fields tend to align the local spins and enhance the transfer integral tij , leading to the decrease of the dc resistance through interplay between the double-exchange effect and electron–phonon coupling [30]. For giant magnetoimpedance when high frequency currents pass through ferromagnetic metallic manganites La1−x Ax MnO3 , the dc magnetic field influences the impedance by affecting the penetration depth δ via the permeability µ [10–12]. For the high frequency limit kd ≫ 1, the impedance Z of ferromagnetic metallic La1−x Ax MnO3 is proportional to the root of permeability µ, frequency ω and resistivity ρ [31]. For a Cu coil wound on ferromagnetic manganites, according to Eqs. (1) and (2), the dc magnetic field induces the changes of resistance R and reactance X directly through the variation of permeability for manganites La1−x Ax MnO3 . Fig. 4 shows the transverse dc magnetic field dependence of magnetoresistance 1R/R0 , magnetoreactance 1X /X0 and magnetoimpedance 1Z /Z0 for the Cu coil wound on La0.67 Sr0.33 MnO3 at room temperature, where the dc fields are applied perpendicular to the direction of plate length. Different from the longitudinal case, 1R/R0 does not reach its saturation value at f = 500 kHz. Fig. 5 shows the ac frequency dependence of magnetoimpedance 1Z /Z0 , magnetoresistance 1R/R0 and magnetoreactance 1X /X0 for the Cu coil wound on La0.67 Sr0.33 MnO3 under a transverse dc field H = 600 Oe at room temperature. With an increase of frequency, the transverse magnetoresistance 1R/R0 has a maximum value (−70.1%) at 8 MHz. The transverse magnetoimpedance 1Z /Z0 experiences a maximum value (about −37.2%) at 900 kHz. Comparing Fig. 3 with Fig. 5, one can clearly see that the transverse magnetoimpedance is weaker than the longitudinal one. The La0.67 Sr0.33 MnO3 plate sintered at a high temperature (such as 1230 °C) is almost magnetically isotropic itself. When an ac
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J. Hu et al. / Solid State Communications 151 (2011) 47–50
Fig. 5. The ac frequency dependence of magnetoimpedance 1Z /Z0 , magnetoresistance 1R/R0 and magnetoreactance 1X /X0 for the Cu coil wound on La0.67 Sr0.33 MnO3 under a transverse dc field H = 600 Oe at room temperature.
reaches −59% in a wide frequency range of 500 kHz–10 MHz. The maximum sensitivity of longitudinal 1Z /Z0 for the Cu coil wound on La0.67 Sr0.33 MnO3 is about 0.63%/Oe, which is larger than that of the Hall sensor (about 0.15%/Oe). The head length of sensor based on the Cu coil wound on La0.67 Sr0.33 MnO3 is much longer than that of the Hall sensor (10–100 µm), and the cost (including associated electronics) of the former is more than that of the latter. Even though there are hundreds of papers on giant magnetoimpedance, investigation about the noise characteristics has not received much attention and the related reports are very few. The noise issue for the Cu coil wound on La0.67 Sr0.33 MnO3 is not very clear now, and will be studied in the coming later. Usually, the ferromagnetic material with high permeability could be chosen as the rod twisted by the Cu coil. However, the strong magnetoimpedance could also occur for such a material, in which the field induced change of permeability is very large, even though the permeability value itself is very small. The initial circular permeability of a La2/3 Sr1/3 MnO3 (sintered at 1300 °C) is about 33 at room temperature, however, the circular permeability changes of 1µ′ /µ′ (0) = −24.9% and 1µ′′ /µ′′ (0) = −49.8% can be obtained when a small dc field of 300 Oe is applied [22]. Near Curie temperature TC , the exchange interaction becomes weak and a low dc field might induce a large change of permeability. The room temperature magnetoimpedance performance of manganites could be optimized by enhancing the permeability change, through shifting the TC to near room temperature. It is expected that an appropriate substitution of Mn in La0.67 Sr0.33 MnO3 could bring about an enhancement of room temperature magnetoimpedance. Acknowledgement
Fig. 6. Curves of magnetization (M) versus magnetic field (H) measured at room temperature for La0.67 Sr0.33 MnO3 plate under dc magnetic fields parallel or perpendicular to the sample length. The inset shows the longitudinal and transverse M–H curves in the low field range H ≤ 600 Oe.
current passes through the Cu coil wound on La0.67 Sr0.33 MnO3 plate, an ac field Hac is produced along the length direction of the sample, parallel/antiparallel to the longitudinal dc field H but perpendicular to the transverse one. For an ac current of 20 mA, the amplitude of Hac is about 12 A/m (about 0.15 Oe). Fig. 6 shows the curves of the magnetization (M) versus the magnetic field (H) measured at room temperature for La0.67 Sr0.33 MnO3 plate under external magnetic fields parallel or perpendicular to the length direction of the sample. The longitudinal and transverse M–H curves in the low field range H ≤ 600 Oe are also plotted in the inset of Fig. 6, respectively. The difference between longitudinal and transverse M–H curves comes from the different demagnetization fields. The value of demagnetization factor for transverse direction is higher than that for longitudinal one, due to the shape of the sample and direction of the applied fields. The difference between the longitudinal and the transverse magnetoimpedance for the Cu coil wound on La0.67 Sr0.33 MnO3 plate mainly originates from the effect of a demagnetization field. 4. Conclusions In the present work, the magnetoimpedance in a Cu coil wound on La0.67 Sr0.33 MnO3 under low dc magnetic fields was investigated. The observed giant magnetoimpedance and colossal ac magnetoresistance for the Cu coil wound on La0.67 Sr0.33 MnO3 are connected with the variation of permeability induced by the dc magnetic field. The transverse magnetoimpedance is smaller than the longitudinal one, mainly due to the effect of demagnetization field. The ac magnetoresistance 1R/R0 can reach −93% under longitudinal field H = 600 Oe at 10 MHz, meanwhile the magnetoimpedance 1Z /Z0
This work is supported by National Natural Science Foundation of China (Grant Nos. 50872074, 50872069 and 51072103). References [1] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [2] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [3] J.M.D. Coey, J. Appl. Phys. 85 (1999) 5576. [4] J.F. Hu, H.W. Qin, J. Chen, R.K. Zheng, J. Appl. Phys. 91 (2002) 8912. [5] C. Zener, Phys. Rev. 82 (1951) 403. [6] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. [7] V.V. Srinivasu, S.E. Lofland, S.M. Bhagat, K. Ghosh, S.D. Tyagi, J. Appl. Phys. 86 (1999) 1067. [8] S.D. Tyagi, S.E. Lofland, M. Dominguez, S.M. Bhagat, C. Kwon, M.C. Robson, R. Ramesh, T. Venkatesan, Appl. Phys. Lett. 68 (1996) 2893. [9] V.V. Srinivasu, S.E. Lofland, S.M. Bhagat, J. Appl. Phys. 83 (1998) 2866. [10] J.F. Hu, H.W. Qin, Solid State Commun. 116 (2000) 159. [11] J.F. Hu, H.W. Qin, J. Magn. Magn. Mater. 234 (2001) 419. [12] H.W. Qin, J.F. Hu, J. Chen, Y.Z. Wang, Z.X. Wang, J. Appl. Phys. 91 (2002) 10003. [13] A. Rinkevich, A. Nossov, V. Ustinov, V. Vassiliev, S. Petukhov, J. Appl. Phys. 91 (2002) 3693. [14] J.F. Hu, H.W. Qin, H.D. Niu, L.M. Zhu, J. Chen, W.W. Xiao, Y. Pei, J. Magn. Magn. Mater. 261 (2003) 105. [15] G.M.B. Castro, A.R. Rodrigues, F.L.A. Machado, A.E.P. de Araujo, R.F. Jardim, A.K. Nigam, J. Alloys Compd. 369 (2004) 108. [16] M. Nadeem, M.J. Akhtar, A.Y. Khan, Solid State Commun. 134 (2005) 431. [17] S.K. Ghatak, B. Kaviraj, T.K. Dey, J. Appl. Phys. 101 (2007) 023910. [18] P. Dutta, P. Dey, T.K. Nath, J. Appl. Phys. 102 (2007) 073906. [19] H.W. Qin, J.F. Hu, Y.M. Hao, J. Chen, M.H. Jiang, J. Magn. Magn. Mater. 320 (2008) 2770. [20] V.B. Naik, R. Mahendiran, Appl. Phys. Lett. 94 (2009) 142505. [21] A. Rebello, C.L. Tan, R. Mahendiran, Solid State Commun. 149 (2009) 1204. [22] J.F. Hu, H.W. Qin, Y.F. Wang, B. Li, J. Magn. Magn. Mater. 322 (2010) 3245. [23] L.V. Panina, K. Mohri, K. Bushida, M. Noda, J. Appl. Phys. 76 (1994) 6198. [24] R.S. Beach, A.E. Berkowitz, Appl. Phys. Lett. 64 (1994) 3652. [25] J. Velázquez, M. Vázquez, D.X. Chen, A. Hemando, Phys. Rev. B 50 (1994) 16737. [26] M.H. Phan, H.X. Peng, Prog. Mater. Sci. 53 (2008) 323. [27] M. Vázquez, J.M. Garcia-Beneytez, J.P. Sinnecker, A.P. Zhukov, J. Appl. Phys. 88 (2000) 6501. [28] M. Knobel, M.L. Sanchez, C. Gomez-Polo, P. Marin, M. Vázquez, A. Hernando, J. Appl. Phys. 79 (1996) 1646. [29] X.Y. Xia, Z.A. Liu, Y.C. Ma, Magnetic Materials and Measuring Devices, Electronics Industry Press, Beijing, 1984. [30] J.M.D. Coey, M. Viret, S. von Molnár, Adv. Phys. 48 (1999) 167. [31] J.F. Hu, H.W. Qin, Mater. Sci. Eng. B 100 (2003) 304.
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Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Giant magnetoimpedance and colossal ac magnetoresistance of a Cu coil wound on La0.67 Sr0.33 MnO3 Jifan Hu a,∗ , Yifei Wang a , Juan Chen a,b , Hongwei Qin a , Bo Li c a
School of Physics, State Key Laboratory for Crystal Materials, Shandong University, Jinan 250100, China
b
Group of Nanomagnetics and Applications, School of Medicine, Shandong University, Jinan 250012, China
c
Department of Functional Materials, Central Iron and Steel Research Institute, Beijing 100081, China
article
info
Article history: Received 1 March 2010 Received in revised form 18 September 2010 Accepted 18 October 2010 by A. Pinczuk Available online 26 October 2010 Keywords: A. Magnetically ordered materials D. Electronic transport
abstract We observed the giant magnetoimpedance and colossal ac magnetoresistance for a Cu coil wound on La0.67 Sr0.33 MnO3 under low dc magnetic fields. Even though the dc magnetoresistance 1R/R0 for La0.67 Sr0.33 MnO3 plate is only −2.4% under H = 12 kOe, a Cu coil wound on La0.67 Sr0.33 MnO3 plate exhibits a colossal ac magnetoresistance 1R/R0 of −93% at 10 MHz and a giant magnetoimpedance 1Z /Z0 of −59% in a wide frequency range of 500 kHz–10 MHz under a longitudinal field H = 600 Oe. The transverse magnetoimpedance is weaker than the longitudinal one. The giant magnetoimpedance and colossal ac magnetoresistance for a Cu coil wound on La0.67 Sr0.33 MnO3 are connected with the variation of permeability induced by dc magnetic field. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Perovskite rare earth manganites, La1−x Ax MnO3 (A = Ca, Ba, Sr) have attracted much attention due to the colossal magnetoresistance (CMR) and the strong interplay among lattice, electronic transport and magnetic properties [1,2]. There is a transition from insulator (or semiconductor) to metal near Curie temperature TC . The CMR effects are pronounced in a limited range of temperature near TC . One problem of the CMR for application is the requirement for a large applied magnetic field about several teslas. For La–Sr–Mn–O (such as La0.7 Sr0.3 MnO3 and La0.67 Sr0.33 MnO3 ) sintered manganites, values of magnetoresistance are only −3% under magnetic field H = 1.2 T at room temperature [3,4]. The CMR phenomenon can be explained by the double-exchange interaction [5] and the strong Jahn–Teller distortion with the electron–phonon coupling [6]. In addition to the CMR effect, La1−x Ax MnO3 manganites could exhibit colossal magnetoabsorption at microwave high frequencies (∼GHz) [7–9] and large magnetoimpedance at radio frequencies (∼kHz or MHz) [10–22] under low fields. Giant magnetoimpedance effects have been found in soft magnetic metallic materials, such as Co–Fe–Si–B amorphous and Fe based nanocrystalline wires and ribbons [23–28]. Similar to the cases for conventional metallic materials, skin effect should occur at high
∗
Corresponding author. Tel.: +86 531 88566143; fax: +86 531 88377031. E-mail address: [email protected] (J. Hu).
0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2010.10.026
frequencies when an ac current passes through La1−x Ax MnO3 ferromagnetic metals. Application of external dc magnetic field induces a change of permeability and brings about a variation of penetration depth, resulting in a large change of impedance in La1−x Ax MnO3 ferromagnetic metals. A giant magnetoimpedance was also observed by Ghatak et al. [17] in Ag-doped La0.7 Sr0.3 MnO3 nanocrystalline samples (prepared by a wet chemical method) at room temperature, where the signals of impedance were collected from a coil wound on the manganite. They found ac magnetoresistance 1R/R0 of about −64% under H = 600 Oe and 1R/R0 = −70% under H = 4 kOe at ω = 15 MHz for La0.7 Sr0.25 Ag0.05 MnO3 . In the present work, we reported the magnetoimpedance effect for an enamelled Cu coil wound on La0.67 Sr0.33 MnO3 prepared with conventional solid-state sintering method. We observed a colossal ac magnetoresistance 1R/R0 of −93% at 10 MHz and a giant magnetoimpedance 1Z /Z0 of −59.3% at 2 MHz under longitudinal H = 600 Oe. The observed giant magnetoimpedance and colossal ac magnetoresistance are connected with the variation of permeability induced by dc magnetic field. 2. Experiments The sintered samples of La0.67 Sr0.33 MnO3 were prepared using the conventional solid-state reaction method with a sintering temperature of 1230 °C. X-ray diffraction analysis indicated that samples were of single phase with rhombohedral perovskite structure. The Curie temperature TC was determined as 362 K
48
J. Hu et al. / Solid State Communications 151 (2011) 47–50
the dc magnetic fields were applied in the direction parallel and perpendicular to the direction of the length of the plate, respectively. 3. Results and discussion
Fig. 1. The ac frequency dependence of resistance R, reactance X and impedance Z for the Cu coil alone and the Cu coil wound on La0.67 Sr0.33 MnO3 at room temperature without dc magnetic field, respectively. The inset is the frequency dependence of resistance R for the Cu coil alone.
from thermal magnetic curve measured using a thermal magnetic analyzer at 200 Oe. The metal–insulator transition temperature TMI was about 357 K determined from the temperature dependence of the resistivity. The dc magnetoresistance 1R/R0 under H = 12 kOe was measured as about −2.4%. The magnetoimpedance measurement was carried out using an impedance analyzer at room temperature. The La0.67 Sr0.33 MnO3 plate with 15 mm length, 2.5 mm thickness and 2.8 mm width was used for the measurement, where the impedance signals were collected from an enamelled Cu coil of 9 turns wound on the La0.67 Sr0.33 MnO3 plate. The amplitude of ac currents was maintained as 20 mA. The ac field Hac produced by the Cu coil is about 12 A/m (about 0.15 Oe). The longitudinal and the transverse magnetoimpedance of the Cu coil wound on La0.67 Sr0.33 MnO3 were measured, where
Fig. 1 shows the ac frequency dependence of resistance R, reactance X and impedance Z for the Cu coil alone and the Cu coil wound on La0.67 Sr0.33 MnO3 without dc magnetic field at room temperature, respectively. Values of R, X , Z increase with an increase of frequency for the above two systems. The increase of Z with frequency for the Cu coil is due to skin effect. All values of R, X , Z for the system of Cu coil wound on La0.67 Sr0.33 MnO3 are larger than those for the Cu coil alone. In fact, the impedance Z collected from a coil wound on a magnetic material is usually used for the measurement of permeability, where the shape of the magnetic material is toroidal [29]. For the present work, a plate of La0.67 Sr0.33 MnO3 was used, and there is a leakage of magnetic flux. Nevertheless, we may approximately have the following expressions for a coil wound on the magnetic plate X ≈ b1 ωµ0 µ′
(1)
R ≈ Rcoil + b2 ωµ0 µ
′′
(2)
where µ and µ are real and imaginary parts of permeability for the magnetic material, µ0 is the vacuum permeability, Rcoil is the loss resistance of the coil itself, b1 and b2 are the coefficients. The effective inductance L for the Cu coil wound on La0.67 Sr0.33 MnO3 is larger than that (Lcoil ) for the Cu coil alone. The longitudinal dc field dependence of magnetoresistance 1R/R0 = [R(H )− R0 ]/R0 for the Cu coil wound on La0.67 Sr0.33 MnO3 at room temperature is shown in Fig. 2(a). Even though the dc ′
′′
a
d
b
e
c
f
Fig. 2. The longitudinal dc field dependence of magnetoresistance, magnetoreactance and magnetoimpedance for the Cu coil wound on La0.67 Sr0.33 MnO3 at room temperature, respectively.
J. Hu et al. / Solid State Communications 151 (2011) 47–50
49
a
b
Fig. 3. The ac frequency dependence of magnetoimpedance 1Z /Z0 , magnetoresistance 1R/R0 and magnetoreactance 1X /X0 for the Cu coil wound on La0.67 Sr0.33 MnO3 under a longitudinal dc field H = 600 Oe at room temperature.
magnetoresistance 1R/R0 is only −2.4% for La0.67 Sr0.33 MnO3 plate under H = 12 kOe, a colossal ac magnetoresistance effect occurs for the Cu coil wound on La0.67 Sr0.33 MnO3 plate. As shown in Fig. 2(a), the ac magnetoresistance 1R/R0 can reach −45.2% under longitudinal field H = 50 Oe, −71.3% under longitudinal field H = 100 Oe, −93% under longitudinal field H = 600 Oe at 10 MHz for the Cu coil wound on La0.67 Sr0.33 MnO3 plate. The magnetoresistance 1R/R0 shows a strong dependence on frequency. At low frequency such as 500 kHz, the magnetoresistance 1R/R0 almost reaches its saturation value (about −32%) under a smaller field H = 200 Oe. As shown in Fig. 3, the magnetoresistance 1R/R0 increases with an increase of ac frequency up to f = 10 MHz. The longitudinal dc field dependence of the magnetoreactance 1X /X0 = [X (H ) − X0 ]/X0 and magnetoimpedance 1Z /Z0 = [Z (H ) − Z0 ]/Z0 for the Cu coil wound on La0.67 Sr0.33 MnO3 at room temperature are also plotted in Fig. 2(b) and (c), respectively. The impedance Z and reactance X decrease with an increase of dc magnetic field, showing giant negative magnetoimpedance effect. A magnetoreactance 1X /X0 of −59.3% and a magnetoimpedance 1Z /Z0 of −59.4% occur at 2 MHz under 600 Oe. The maximum slope sensitivity of 1Z /Z0 is about 0.63%/Oe. In addition, the field dependence of magnetoimpedance 1Z /Z0 and magnetoreactance 1X /X0 are very similar. The definition of magnetoimpedance ratio, [Z (H ) − Z (Hmax )]/Z (Hmax ), was also frequently used for labeling the giant magnetoimpedance in Co-rich amorphous and Fe nanocrystalline wires and ribbons, where the value of Hmax is usually the external magnetic field sufficient to saturate the impedance, or the field available for the given experimental equipment in practice [26]. For comparison, the field dependence of [R(H ) − R(Hmax )]/ R(Hmax ), [X (0)− X (Hmax )]/X (Hmax ) and [Z (H )− Z (Hmax )]/Z (Hmax ) under Hmax = 600 Oe for the Cu coil wound on La0.67 Sr0.33 MnO3 were also plotted in Fig. 2(d)–(f). Values of [Z (H ) − Z (Hmax )]/ Z (Hmax ) = 146% at 2 MHz and [R(0) − R(Hmax )]/R(Hmax ) = 1315% at 10 MHz could be obtained. The maximum sensitivity of [Z (H ) − Z (Hmax )]/Z (Hmax ) could reach 16%/Oe. It has been found that the (Co94 Fe6 )75 Si10 B15 amorphous microwire exhibits a [Z (H ) − Z (Hmax )]/Z (Hmax ) of 125% with Hmax = 10 Oe at 3.22 MHz and the maximum sensitivity of 50%/Oe [27]. The Fe73.5 Si13.5 B9 Cu1 Nb3 wire annealed at 600 °C for 1 h shows a [Z (H ) − Z (Hmax )]/Z (Hmax ) of 200% with Hmax = 100 Oe at 0.5 MHz [28]. In the frequency range from 100 kHz to 10 MHz, values of magnetoimpedance 1Z /Z0 are mainly controlled by those of magnetoreactance 1X /X0 (see Fig. 3), since the values of R are much smaller than those of X, and the values of Z are controlled by those of X (see Fig. 1). The frequency dependence of magnetoimpedance 1Z /Z0 and magnetoreactance 1X /X0 under a certain longitudinal H = 600 Oe are not pronounced. The value of magnetoreactance 1X /X0 becomes somewhat smaller at
c
Fig. 4. The transverse dc field dependence of magnetoresistance 1R/R0 , magnetoreactance 1X /X0 and magnetoimpedance 1Z /Z0 for the Cu coil wound on La0.67 Sr0.33 MnO3 at room temperature, respectively.
high frequency such as 10 MHz. The magnetoimpedance 1Z /Z increases with frequency slightly at first, undergoes a peak value −59.3% at about 2–3 MHz, and finally decreases (see Fig. 3). For colossal dc magnetoresistance, the dc magnetic fields tend to align the local spins and enhance the transfer integral tij , leading to the decrease of the dc resistance through interplay between the double-exchange effect and electron–phonon coupling [30]. For giant magnetoimpedance when high frequency currents pass through ferromagnetic metallic manganites La1−x Ax MnO3 , the dc magnetic field influences the impedance by affecting the penetration depth δ via the permeability µ [10–12]. For the high frequency limit kd ≫ 1, the impedance Z of ferromagnetic metallic La1−x Ax MnO3 is proportional to the root of permeability µ, frequency ω and resistivity ρ [31]. For a Cu coil wound on ferromagnetic manganites, according to Eqs. (1) and (2), the dc magnetic field induces the changes of resistance R and reactance X directly through the variation of permeability for manganites La1−x Ax MnO3 . Fig. 4 shows the transverse dc magnetic field dependence of magnetoresistance 1R/R0 , magnetoreactance 1X /X0 and magnetoimpedance 1Z /Z0 for the Cu coil wound on La0.67 Sr0.33 MnO3 at room temperature, where the dc fields are applied perpendicular to the direction of plate length. Different from the longitudinal case, 1R/R0 does not reach its saturation value at f = 500 kHz. Fig. 5 shows the ac frequency dependence of magnetoimpedance 1Z /Z0 , magnetoresistance 1R/R0 and magnetoreactance 1X /X0 for the Cu coil wound on La0.67 Sr0.33 MnO3 under a transverse dc field H = 600 Oe at room temperature. With an increase of frequency, the transverse magnetoresistance 1R/R0 has a maximum value (−70.1%) at 8 MHz. The transverse magnetoimpedance 1Z /Z0 experiences a maximum value (about −37.2%) at 900 kHz. Comparing Fig. 3 with Fig. 5, one can clearly see that the transverse magnetoimpedance is weaker than the longitudinal one. The La0.67 Sr0.33 MnO3 plate sintered at a high temperature (such as 1230 °C) is almost magnetically isotropic itself. When an ac
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J. Hu et al. / Solid State Communications 151 (2011) 47–50
Fig. 5. The ac frequency dependence of magnetoimpedance 1Z /Z0 , magnetoresistance 1R/R0 and magnetoreactance 1X /X0 for the Cu coil wound on La0.67 Sr0.33 MnO3 under a transverse dc field H = 600 Oe at room temperature.
reaches −59% in a wide frequency range of 500 kHz–10 MHz. The maximum sensitivity of longitudinal 1Z /Z0 for the Cu coil wound on La0.67 Sr0.33 MnO3 is about 0.63%/Oe, which is larger than that of the Hall sensor (about 0.15%/Oe). The head length of sensor based on the Cu coil wound on La0.67 Sr0.33 MnO3 is much longer than that of the Hall sensor (10–100 µm), and the cost (including associated electronics) of the former is more than that of the latter. Even though there are hundreds of papers on giant magnetoimpedance, investigation about the noise characteristics has not received much attention and the related reports are very few. The noise issue for the Cu coil wound on La0.67 Sr0.33 MnO3 is not very clear now, and will be studied in the coming later. Usually, the ferromagnetic material with high permeability could be chosen as the rod twisted by the Cu coil. However, the strong magnetoimpedance could also occur for such a material, in which the field induced change of permeability is very large, even though the permeability value itself is very small. The initial circular permeability of a La2/3 Sr1/3 MnO3 (sintered at 1300 °C) is about 33 at room temperature, however, the circular permeability changes of 1µ′ /µ′ (0) = −24.9% and 1µ′′ /µ′′ (0) = −49.8% can be obtained when a small dc field of 300 Oe is applied [22]. Near Curie temperature TC , the exchange interaction becomes weak and a low dc field might induce a large change of permeability. The room temperature magnetoimpedance performance of manganites could be optimized by enhancing the permeability change, through shifting the TC to near room temperature. It is expected that an appropriate substitution of Mn in La0.67 Sr0.33 MnO3 could bring about an enhancement of room temperature magnetoimpedance. Acknowledgement
Fig. 6. Curves of magnetization (M) versus magnetic field (H) measured at room temperature for La0.67 Sr0.33 MnO3 plate under dc magnetic fields parallel or perpendicular to the sample length. The inset shows the longitudinal and transverse M–H curves in the low field range H ≤ 600 Oe.
current passes through the Cu coil wound on La0.67 Sr0.33 MnO3 plate, an ac field Hac is produced along the length direction of the sample, parallel/antiparallel to the longitudinal dc field H but perpendicular to the transverse one. For an ac current of 20 mA, the amplitude of Hac is about 12 A/m (about 0.15 Oe). Fig. 6 shows the curves of the magnetization (M) versus the magnetic field (H) measured at room temperature for La0.67 Sr0.33 MnO3 plate under external magnetic fields parallel or perpendicular to the length direction of the sample. The longitudinal and transverse M–H curves in the low field range H ≤ 600 Oe are also plotted in the inset of Fig. 6, respectively. The difference between longitudinal and transverse M–H curves comes from the different demagnetization fields. The value of demagnetization factor for transverse direction is higher than that for longitudinal one, due to the shape of the sample and direction of the applied fields. The difference between the longitudinal and the transverse magnetoimpedance for the Cu coil wound on La0.67 Sr0.33 MnO3 plate mainly originates from the effect of a demagnetization field. 4. Conclusions In the present work, the magnetoimpedance in a Cu coil wound on La0.67 Sr0.33 MnO3 under low dc magnetic fields was investigated. The observed giant magnetoimpedance and colossal ac magnetoresistance for the Cu coil wound on La0.67 Sr0.33 MnO3 are connected with the variation of permeability induced by the dc magnetic field. The transverse magnetoimpedance is smaller than the longitudinal one, mainly due to the effect of demagnetization field. The ac magnetoresistance 1R/R0 can reach −93% under longitudinal field H = 600 Oe at 10 MHz, meanwhile the magnetoimpedance 1Z /Z0
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