Development of a new module for the measurement of the magneto-electric direct and converse effects based on an alternating current susceptometer D. Bueno-Baques, G. Hurtado-Lopez, V. Corral-Flores, S. Gomez, N. R. Diley, and A. Glushchenko Citation: Review of Scientific Instruments 85, 085116 (2014); doi: 10.1063/1.4892863 View online: http://dx.doi.org/10.1063/1.4892863 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Evidence of spin phonon coupling in magnetoelectric NiFe2O4/PMN-PT composite Appl. Phys. Lett. 103, 252902 (2013); 10.1063/1.4850555 Demonstration of magnetoelectric read head of multiferroic heterostructures Appl. Phys. Lett. 92, 152510 (2008); 10.1063/1.2912032 Strong magnetoelectric coupling at microwave frequencies in metallic magnetic film/lead zirconate titanate multiferroic composites Appl. Phys. Lett. 92, 122506 (2008); 10.1063/1.2902316 Correlation between structural deformation and magnetoelectric response in ( 1 − x ) Pb ( Zr 0.52 Ti 0.48 ) O 3 – x Ni Fe 1.9 Mn 0.1 O 4 particulate composites Appl. Phys. Lett. 91, 162905 (2007); 10.1063/1.2799261 Effect of composition on coupled electric, magnetic, and dielectric properties of two phase particulate magnetoelectric composite J. Appl. Phys. 101, 014109 (2007); 10.1063/1.2404773
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 085116 (2014)
Development of a new module for the measurement of the magneto-electric direct and converse effects based on an alternating current susceptometer D. Bueno-Baques,1,2,a) G. Hurtado-Lopez,2 V. Corral-Flores,1,2 S. Gomez,3 N. R. Diley,3 and A. Glushchenko1 1 Department of Physics and Energy Science, University of Colorado at Colorado Springs, 1420 Austin Bluffs Pkwy., Colorado Springs, Colorado 80918, USA 2 Advanced Materials, Research Center for Applied Chemistry, Enrique Reyna H. 140, Saltillo 25294, Mexico 3 Quantum Design, 6325 Lusk Blvd., San Diego, California 92121, USA
(Received 22 April 2014; accepted 31 July 2014; published online 19 August 2014) A new module for the measurement of magneto-electric properties was developed as an add-on for a magnetic AC susceptibility option of a Physical Properties Measurement System (PPMS). The module is capable of recording direct dynamic and static converse magneto-electric effect, i.e., the change in electric polarization due to the application of a small AC magnetic field with a DC magnetic field bias, or the change in the magnetic moment induced by an applied electric field. The versatile module setup supports both measurements in a sequential order without the need of removing or repositioning the sample. Furthermore, AC and DC magnetic susceptibilities can be recorded while performing direct and inverse magneto-electric measurements, respectively, which adds outstanding capabilities to the existing instrument while saving time and resources. Measurements are fully automated and integrated in the PPMS Multivu software platform. Magneto-electric behavior of a BaTiO3 /CoFe2 O4 and BaTiO3 /NiFe2O4 magneto-electric composites, and a Pb(Fe0.5 Nb0.5 )O3 single phase compound were recorded as test measurements. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4892863] I. INTRODUCTION
Multiferroic materials have gained remarkable attention within the last decade after the discovery of new compounds that show a strong coupling.1 The search for these materials has been driven by the possibility of controlling the response of one ferroic order by acting on the other, allowing the development of new multifunctional devices. Much of the work in the field has been devoted to materials that exhibit coupled ferroelectricity and ferromagnetism (single phases or composites), because of the ability to control the polarization by applied magnetic fields and the magnetization by applied voltages.2 In addition to their technical importance, these materials possess a remarkable scientific interest, based on the fact that ferroelectric and ferromagnetic orders are mutually exclusive.3, 4 Furthermore, the simultaneous occurrence of ferromagnetism and ferroelectricity in a material is not conclusive evidence of multiferroic coupling, as the mechanisms leading to each are quite different and do not strongly interrelate.4, 5 Therefore, traditional techniques, including electric polarization, piezoelectricity, magnetization, susceptibility, magnetostriction, and impedance spectroscopy are not capable of proving direct evidence of the magnetoelectric (ME) coupling property, even when the material shows multiple responses. The induced polarization is related to the magnetic field H by P = αH, where α is the second rank magnetoelectric susceptibility tensor. Similarly, magnetic polarization is related to the electric field E by M = αE. First experimental measurements of the ME effect were performed by Astrov,6 a) Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0034-6748/2014/85(8)/085116/5/$30.00
who proved the existence of the ME effect in antiferromagnetic Cr2 O3 , previously theoretically predicted by Landau and Lifshitz.7 Despite that in Astrov setup, the magnetic response was measured in a sample with an electrical field applied, the P(H) approach became more standardized in the scientific community.8–10 The most accepted method to measure the magnetoelectric response consists in determining the voltage across a sample with plated faces as a function of an applied low amplitude AC magnetic field superimposed on a DC bias magnetic field. This voltage is normally measured using a charge amplifier or high impedance amplifier and a lock-in amplifier, taking as a reference a signal proportional in amplitude and phase to the magnetic AC field drive current.11 Magneto-electric coupling encompasses the study of magnetically induced rotation or inversion of ferroelectric domains polarization (direct effect) and electrically induced rotation or inversion of the magnetization (converse effect). In spite of the standardization of the P(H) approach, the characterization of both effects is of capital importance. In magnetostrictive-piezoelectric magneto-electric composites, for instance, it is difficult to predict the behavior of the converse magneto-electric effect by measuring the direct effect, and vice versa.12 Since the driving force of the two orders coupling is the stress imposed to one phase and transferred to the other, the volume change and coupling at the interface play an important role. For example, when applying a magnetic field to the composite to measure the direct effect, the magnetostrictive phase will experience a deformation, and a strain will appear in the piezoelectric phase, which can be measured as a voltage. On the other hand, to measure the converse effect, an electric field is applied to the sample, resulting in a volume change of the piezoelectric phase due to the atom rearrangement in the crystal lattice. This volume change,
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specifically a crystal enlargement in the direction of the applied electric field, generates stress over the magnetostrictive phase, which will suffer a change in magnetization. However, both mechanisms arise from different phenomena and are not the same. The volume change in a piezoelectric perovskite phase is always positive and mainly in the direction of the applied field (shear piezoelectric coefficients can also contribute to the deformation perpendicular to the direction of the applied field), while in the magnetostrictive phase is less noticeable, since the elongation in one direction is compensated by shrinkage in the rest, and the total volume remains almost unchanged. In general, this slightly dissimilarities in the origin of the stress generated at the interface could result in different magneto-electric responses when measured in direct or converse modes. In single phase materials, the magnetic moment degree of freedom is influenced by the electric applied fields, so the study of the magnetization processes is of remarkable importance. The study of switching behavior is imperative for any potential application that is based on the possibility of reversing the magnetization by applying an electric field or vice versa, and it is strongly influenced by the magnetization processes. Additionally, the magneto-electric behavior can be only fully understood if the magnetic point group symmetry is known. This resides in the fact that the magnetoelectric coupling coefficients (α mn ) possess the symmetry of the material.1, 13 Although the measurement of coupling coefficients in the direct mode will establish the possibility of achieving a magneto-electric switching, gathering information regarding the coupling based on electrical stimulus (converse mode) can aid in the determination of magnetic point group symmetries, and thus contribute to the fundamental understanding of the phenomena that originates the coupling properties. In this paper, we report the development of a new platform for the measurement of the direct and converse magnetoelectric effect, as an add-on for a magnetic AC susceptibility option (ACMS) of a Physical Properties Measurement System (PPMS) from Quantum Design.
II. MAGNETO-ELECTRIC MEASUREMENT MODES
As described earlier, both direct and converse effects are of a great importance in modern magneto-electric materials. The measurement of these effects translates in two operation modes for the characterization equipment, namely, the direct and converse modes. The measurement bases for these two modes are described in Secs. II A and II B.
A. Direct effect and mode
In the direct mode, the change in the polarization of a sample is determined as a function of a magnetic field stimulus. The measurement of the change in the polarization implies the detection of charge migration, which strongly depends on the impedance of the measurement system.9, 14 Additionally, while poling and measuring, charges could accumulate at imperfections, grain boundaries and interfaces,
Rev. Sci. Instrum. 85, 085116 (2014)
especially in heterogeneous and polycrystalline materials, which then move towards the electrodes leading to erroneous readings and interpretations. The dynamic direct mode has been established as the most reliable method to measure the direct magneto-electric coefficient, as the noise is dramatically reduced and the charge accumulation problem is mitigated.10 In this method, the effective magneto-electric voltage, related to the change in the polarization, is determined as a function of an AC magnetic field (hAC ) with a DC magnetic field bias (HDC ). This has been the most common mode to date to characterize the coupling properties in magneto-electric multiferroics.11, 13 When exposing a fully poled sample to an external magnetic field, the voltage proportional to the change in the polarization related to the magneto-electric coupling can be considered as VME = f (HT ) with HT = HDC + hAC .
(1)
Assuming that hAC is sufficiently small that any possible change in the total magnetization of the sample is associated with the reversible component, and following the mathematical procedure described in Ref. 11, the direct (dynamic) magneto-electric coefficient could be expressed as α = ε0 εr αME where αME =
AC VME hAC· d
(2)
AC where VME is the amplitude of the magneto-electric voltage measured at the frequency of the applied AC magnetic field and d is the thickness of the sample. The term α ME is commonly referred as the magneto-electric voltage coefficient.
B. Converse effect and mode
Within the converse mode, the change in the magnetization of a sample is determined as a function of an applied DC electric field (static converse mode) or as a function of an AC electric field with a DC electric field bias (dynamic converse mode). The total magnetization (MT ) of a sample at any point of the hysteresis loop could be described as MT = Mrev + Mirr ,
(3)
where Mrev is the reversible component of the magnetization and Mirr is the irreversible component. In order to establish a defined initial magnetization state for the magneto-electric measurement (the magnetic equivalent to an electrically poled state), starting from the demagnetized state a value of Mirr is set by applying a given magnetic field (h1 ) and reducing the field to zero. The resulting minor loop will set a remanent magnetization Mr that will correspond to the value of Mirr for the field h1 . When the value of h1 is equal or higher than the saturation field, the corresponding Mirr will be equal to the maximum remanent magnetization of the sample. In the dynamic converse mode, the change in the reversible magnetization (Mr ) is determined for a given value of the irreversible magnetization (Mirr ) as a function of a DC electric field bias EDC , subjected to a small AC electric field. Within the static converse mode, the electrically induced change in the total
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magnetization is determined starting from a given remanent state set by the value of Mirr . For the dynamic converse effect, the change in the reversible magnetization M∗ induced by the electric field, can be determined as a function of small AC electric field eAC and be expressed as M ∗ = f (ET ) where ET = EDC + eAC .
(4)
Following a similar mathematical procedure as in Ref. 11, the dynamic converse magneto-electric coefficient can be expressed as α=
∗ MAC , eAC
(5)
∗ is the magnetization measured at the frequency of where MAC the AC electric field. The latter implies a measurement of the magnetization at the frequency of an externally applied AC electric field, which is beyond the capabilities of the PPMSACMS option used as a base for implementing the ME measurement system. A realization of the dynamic converse mode have been proposed using a SQUID magnetometer.15 The developed ME measurement system is capable of determining the static converse effect, where the electrically induced change in the total magnetization is measured by extraction magnetometry. In addition to the static converse mode, a pseudo static converse mode was also implemented. Within this mode, the magnetic AC susceptibility is recorded as a function of a DC electric field bias, subjected to a small AC magnetic field. With the system as implemented is possible to study the magnetization processes and magnetization switching behavior under applied electric field.
III. MAGNETO-ELECTRIC MEASUREMENT SYSTEM IMPLEMENTATION
The magneto-electric system was designed in such a way that minimum modifications are required to the base ACMS hardware. A schematic overview of the developed ME system as implemented in the PPMS is shown in Figure 1. The system is capable of performing different measurements without extracting or repositioning the sample, and thus minimizing contact errors and deterioration. Measurements are fully automated and integrated into the options of the PPMS Multivu software platform, so no further special training is required for the end-user. The magneto-electric option electronics are housed in a 2U 19 in. rack unit that can be accommodated into the PPMS model 6000 cabinet (see Figure S1 of the supplementary material16 ). New sample holders were designed to be attached to standard ACMS rods, which in turn were modified to incorporate thin twisted wires to extract and apply signals to the samples (Figure S2 of the supplementary material16 ). Sample holders can accommodate thin films and bulk samples, following the same standards used to setup the contacts in the resistivity pucks. A new ACMS transport cap was designed to incorporate a “spring” wiring to accommodate the displacement sustained by the ACMS rod in both AC susceptibility and extraction modes. The assembly features a low profile detach-
FIG. 1. Schematic overview of the magneto-electric option as implemented in the PPMS with the ACMS option.
able connector to facilitate the rod extraction and installation (Figure S3 of the supplementary material16 ). The direct dynamic mode capability was developed on the base of the AC magnetic susceptibility mode of the ACMS. In this configuration, the AC magnetic field (hAC ) is generated by the AC coil of the susceptometer (from 10 Hz to 10 KHz), while the DC bias magnetic field is applied by the main superconducting magnet of the PPMS. The signal from the sample is conditioned and amplified with a virtual ground high impedance amplifier. Signal from the virtual ground high impedance amplifier is digitalized and synchronically rectified to the reference signal from the AC magnetic field (obtained from the drive reference of the AC coil) using a digital signal processing (DSP)-based dual phase lock-in amplifier, implemented in the DSP/Microcontroller unit (MCU) module. Digital representations of the in-phase and quadrature signals are stored and afterwards converted to analog signals to be acquired by the PPMS through the auxiliary port. The system is able to detect the different AC measurements modes of the AC susceptometer17 and synchronize the output to the operation and measurement sequence of the PPMS-ACMS setup. Finally, the magneto-electric signals can be plotted in real time following the standard procedures implemented in the MultiVu software. The DC measurement capabilities of the ACMS are used for the static converse mode operation of the system. In this configuration, the ACMS operates as an extraction magnetometer. A DC signal reference obtained from the PPMS is amplified and applied to the sample. Similar to the previous mode, the measurement is controlled from the MultiVu software, and can be implemented in fully automated command sequences. In order to establish a fixed operation point, samples are magnetized to a certain remanent state (which will be the magnetic equivalent to an electrically poled state) before running a full M vs. E measurement. The system can apply a maximum positive bias voltage of 200 V DC. In the pseudo static converse mode, the measurement is taken using the regular AC susceptibility configuration of the
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ACMS, with an amplified DC reference signal as DC electric field bias. Within this setup, the system operates in a slightly modified direct operation mode. IV. TEST RESULTS
Magneto-electric behavior of a core-shell (BaTiO3 )0.6 / (CoFe2 O4 )0.4 magneto-electric composite measured in direct mode is presented in Figure 2. Measurement was recorded at room temperature with an AC magnetic field frequency of 10 KHz and an amplitude of 5 Oe. Bias magnetic field was applied along the z axis, while the induced magneto-electric voltage was measured along the y axis, which correspond to the measurement of the transverse (αE ) magneto-electric 31 voltage coefficient. Initial powders preparation details can be found in Ref. 18. Cylindrically shaped pellets were pressed at 600 MPa, achieving more than 55% of the theoretical density of the mixture. The pellets were sintered at 1100 ◦ C for 12 h. Results are consistent with the previously reported magneto-electric data for these kind of composites, measured using a standard direct mode setup.11 The maximum of the ME coefficient is 2.24 mV/cm Oe, which is greater than the expected value according to the tendency showed in Ref. 11 (the order of magnitude is the same, and a value of about 2.0 mV/cm Oe would be expected). These could be related to slightly differences in sample preparation and composition. Due to that the data collection during the measurement is synchronized to the working mode of the ACMS (data could be collected during the ACMS measurement at each detection coil and the calibration coil – 5 points mode17 – and averaged at the end), there is an inherent increase in the sensitivity compared to previous setups. The static converse magneto-electric response M(E) of a core-shell (BaTiO3 )0.8 /(NiFe2 O4 )0.2 magneto-electric composite is shown in Figure 3. Measurement was recorded at room temperature taken at a constant DC magnetic field of 200 Oe and starting from a remanent state set by a field of 10 000 Oe. Sample was prepared in a similar manner to the (BaTiO3 )0.6 /(CoFe2 O4 )0.4 composite described previ-
FIG. 3. Static converse magnetoelectric effect for (BaTiO3 )0.8 /(NiFe2 O4 )0.2 core-shell composite measured at room temperature.
ously. Details of the starting powders preparation could be found in Ref. 19. Here, magnetization was recorded along direction of the applied magnetic field (z axis), perpendicular to electric field (y axis). The magnetization is normalized to the value MmaxE obtained at E = 17 kV/cm. Furthermore, the M(E) curves exhibit a half butterfly-like shape, which indirectly evidences that the strain transferred from the BaTiO3 phase into the NiFe2 O4 phase gives rise to the magnetoelectric coupling in the composites.20 Figure 4 shows the magnetic susceptibility as a function of the temperature, measured for a sample of Pb(Fe0.5 Nb0.5 )O3 at frequencies of 100 Hz and 1000 Hz in the pseudo static converse mode. Sample preparation details are described in Ref. 21. The observed slight deviation in the magnetic susceptibility behavior with the electric field bias applied could suggest a direct coupling between the electric and magnetic ordering in this material. This fact supports the previously observed magneto-dielectric coupling as reported in Ref. 22. Furthermore, this operation mode exposed the magnetic relaxation behavior of the material under the electric bias. The slowdown of the magnetic dynamic response (more prominent at low frequencies) under electric field bias suggests the existence of a magnetic polarization induced by the electric field. This provides a unique experimental data to analyze the coupling mechanisms in this single phase materials and exemplifies the value of the technique.
V. FINAL REMARKS
FIG. 2. Magneto-electric voltage coefficient for (BaTiO3 )0.6 /(CoFe2 O4 )0.4 core-shell composite measured at room temperature.
A new magneto-electric measurement module was developed as an add-on to the popular ACMS option for the Quantum Design PPMS. The module provides the capability to determine the direct and converse magneto-electric effects in a single setup and without extracting or repositioning the sample. Additionally, the module features: (a) full hardware compatibility and minimal modifications to the existing model 6000 and ACMS option; (b) operation fully integrated in Multi-Vu software; (c) full temperature range from 1.9 K to 350 K; and (d) user accessible and customizable (open platform for firmware update).
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FIG. 4. Magnetic susceptibility (real part) for a Pb(Fe0.5 Nb0.5 )O3 sample measured at (a) 100 Hz and (b) 1000 Hz.
ACKNOWLEDGMENTS
Authors acknowledge the financial support of CONACYT through Grant Nos. 174391 and 83813. Authors thank R. Font and O. Raymond for supplying the PFN sample, their technical assistance and valuable comments. 1 W.
Eerenstein, N. D. Mathur, and J. F. Scott, Nature (London) 442, 759 (2006). 2 M. Fiebig, J. Phys. D: Appl. Phys. 38, R123 (2005). 3 N. A. Hill, J. Phys. Chem. B 104, 6694 (2000). 4 R. Ramesh and N. A. Spaldin, Nat. Mater. 6, 21 (2007). 5 S.-W. Cheong and M. Mostovoy, Nat. Mater. 6, 13 (2007). 6 D. N. Astrov, Sov. Phys. JETP 11, 708 (1960). 7 L. D. Landau, Electrodynamics of Continuous Media/by L. D. Landau and E. M. Lifshitz; translated from the Russian by J. B. Skyes and J. S. Bell (Pergamon Press, Reading, MA, 1960). 8 G. T. Rado and V. J. Folen, Phys. Rev. Lett. 7, 310 (1961). 9 J. P. Rivera, Ferroelectrics 161, 165 (1994). 10 M. M. Kumar, A. Srinivas, S. V. Suryanarayana, G. S. Kumar, and T. Bhimasankaram, Bull. Mater. Sci. 21, 251 (1998). 11 G. V. Duong, R. Groessinger, M. Schoenhart, and D. Bueno-Basques, J. Magn. Magn. Mater. 316, 390 (2007).
12 T.
Wu, C.-M. Chang, T.-K. Chung, and G. Carman, IEEE Trans. Magn. 45, 4333 (2009). 13 L. E. Fuentes-Cobas, J. A. Matutes-Aquino, and M. E. Fuentes-Montero, in Handbook of Magnetic Materials, edited by K. H. J. Buschow (Elsevier Science and Technology, 2011), p. 129. 14 H. Bougrine, S. Sergeenkov, M. Ausloos, R. Cloots, and V. V. Gridin, Solid State Commun. 91, 571 (1994). 15 P. Borisov, A. Hochstrat, V. V. Shvartsman, and W. Kleemann, Rev. Sci. Instrum. 78, 106105 (2007). 16 See supplementary material at http://dx.doi.org/10.1063/1.4892863 for further details of components and system parts. 17 Q. Design, Physical Property Measurement System: AC Measurement System (ACMS) User’s Manual (Quantum Design Inc., 2003). 18 V. Corral-Flores, D. Bueno-Baqués, and R. F. Ziolo, Acta Mater. 58, 764 (2010). 19 D. Bueno-Baques, V. Corral-Flores, and R. Ziolo, ICCE-17 (World Journal of Engineering, 2009), p. P99. 20 S. Geprägs, D. Mannix, M. Opel, S. T. B. Goennenwein, and R. Gross, Phys. Rev. B 88, 054412 (2013). 21 O. Raymond, R. Font, G. Alvarez, J. Portelles, G. Srinivasan, and J. M. Siqueiros, MRS Proc. 966, 0966-T03-05 (2006). 22 R. Font, G. Alvarez, O. Raymond, J. Portelles, and J. M. Siqueiros, Appl. Phys. Lett. 93, 172902 (2008).
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 085116 (2014)
Development of a new module for the measurement of the magneto-electric direct and converse effects based on an alternating current susceptometer D. Bueno-Baques,1,2,a) G. Hurtado-Lopez,2 V. Corral-Flores,1,2 S. Gomez,3 N. R. Diley,3 and A. Glushchenko1 1 Department of Physics and Energy Science, University of Colorado at Colorado Springs, 1420 Austin Bluffs Pkwy., Colorado Springs, Colorado 80918, USA 2 Advanced Materials, Research Center for Applied Chemistry, Enrique Reyna H. 140, Saltillo 25294, Mexico 3 Quantum Design, 6325 Lusk Blvd., San Diego, California 92121, USA
(Received 22 April 2014; accepted 31 July 2014; published online 19 August 2014) A new module for the measurement of magneto-electric properties was developed as an add-on for a magnetic AC susceptibility option of a Physical Properties Measurement System (PPMS). The module is capable of recording direct dynamic and static converse magneto-electric effect, i.e., the change in electric polarization due to the application of a small AC magnetic field with a DC magnetic field bias, or the change in the magnetic moment induced by an applied electric field. The versatile module setup supports both measurements in a sequential order without the need of removing or repositioning the sample. Furthermore, AC and DC magnetic susceptibilities can be recorded while performing direct and inverse magneto-electric measurements, respectively, which adds outstanding capabilities to the existing instrument while saving time and resources. Measurements are fully automated and integrated in the PPMS Multivu software platform. Magneto-electric behavior of a BaTiO3 /CoFe2 O4 and BaTiO3 /NiFe2O4 magneto-electric composites, and a Pb(Fe0.5 Nb0.5 )O3 single phase compound were recorded as test measurements. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4892863] I. INTRODUCTION
Multiferroic materials have gained remarkable attention within the last decade after the discovery of new compounds that show a strong coupling.1 The search for these materials has been driven by the possibility of controlling the response of one ferroic order by acting on the other, allowing the development of new multifunctional devices. Much of the work in the field has been devoted to materials that exhibit coupled ferroelectricity and ferromagnetism (single phases or composites), because of the ability to control the polarization by applied magnetic fields and the magnetization by applied voltages.2 In addition to their technical importance, these materials possess a remarkable scientific interest, based on the fact that ferroelectric and ferromagnetic orders are mutually exclusive.3, 4 Furthermore, the simultaneous occurrence of ferromagnetism and ferroelectricity in a material is not conclusive evidence of multiferroic coupling, as the mechanisms leading to each are quite different and do not strongly interrelate.4, 5 Therefore, traditional techniques, including electric polarization, piezoelectricity, magnetization, susceptibility, magnetostriction, and impedance spectroscopy are not capable of proving direct evidence of the magnetoelectric (ME) coupling property, even when the material shows multiple responses. The induced polarization is related to the magnetic field H by P = αH, where α is the second rank magnetoelectric susceptibility tensor. Similarly, magnetic polarization is related to the electric field E by M = αE. First experimental measurements of the ME effect were performed by Astrov,6 a) Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0034-6748/2014/85(8)/085116/5/$30.00
who proved the existence of the ME effect in antiferromagnetic Cr2 O3 , previously theoretically predicted by Landau and Lifshitz.7 Despite that in Astrov setup, the magnetic response was measured in a sample with an electrical field applied, the P(H) approach became more standardized in the scientific community.8–10 The most accepted method to measure the magnetoelectric response consists in determining the voltage across a sample with plated faces as a function of an applied low amplitude AC magnetic field superimposed on a DC bias magnetic field. This voltage is normally measured using a charge amplifier or high impedance amplifier and a lock-in amplifier, taking as a reference a signal proportional in amplitude and phase to the magnetic AC field drive current.11 Magneto-electric coupling encompasses the study of magnetically induced rotation or inversion of ferroelectric domains polarization (direct effect) and electrically induced rotation or inversion of the magnetization (converse effect). In spite of the standardization of the P(H) approach, the characterization of both effects is of capital importance. In magnetostrictive-piezoelectric magneto-electric composites, for instance, it is difficult to predict the behavior of the converse magneto-electric effect by measuring the direct effect, and vice versa.12 Since the driving force of the two orders coupling is the stress imposed to one phase and transferred to the other, the volume change and coupling at the interface play an important role. For example, when applying a magnetic field to the composite to measure the direct effect, the magnetostrictive phase will experience a deformation, and a strain will appear in the piezoelectric phase, which can be measured as a voltage. On the other hand, to measure the converse effect, an electric field is applied to the sample, resulting in a volume change of the piezoelectric phase due to the atom rearrangement in the crystal lattice. This volume change,
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specifically a crystal enlargement in the direction of the applied electric field, generates stress over the magnetostrictive phase, which will suffer a change in magnetization. However, both mechanisms arise from different phenomena and are not the same. The volume change in a piezoelectric perovskite phase is always positive and mainly in the direction of the applied field (shear piezoelectric coefficients can also contribute to the deformation perpendicular to the direction of the applied field), while in the magnetostrictive phase is less noticeable, since the elongation in one direction is compensated by shrinkage in the rest, and the total volume remains almost unchanged. In general, this slightly dissimilarities in the origin of the stress generated at the interface could result in different magneto-electric responses when measured in direct or converse modes. In single phase materials, the magnetic moment degree of freedom is influenced by the electric applied fields, so the study of the magnetization processes is of remarkable importance. The study of switching behavior is imperative for any potential application that is based on the possibility of reversing the magnetization by applying an electric field or vice versa, and it is strongly influenced by the magnetization processes. Additionally, the magneto-electric behavior can be only fully understood if the magnetic point group symmetry is known. This resides in the fact that the magnetoelectric coupling coefficients (α mn ) possess the symmetry of the material.1, 13 Although the measurement of coupling coefficients in the direct mode will establish the possibility of achieving a magneto-electric switching, gathering information regarding the coupling based on electrical stimulus (converse mode) can aid in the determination of magnetic point group symmetries, and thus contribute to the fundamental understanding of the phenomena that originates the coupling properties. In this paper, we report the development of a new platform for the measurement of the direct and converse magnetoelectric effect, as an add-on for a magnetic AC susceptibility option (ACMS) of a Physical Properties Measurement System (PPMS) from Quantum Design.
II. MAGNETO-ELECTRIC MEASUREMENT MODES
As described earlier, both direct and converse effects are of a great importance in modern magneto-electric materials. The measurement of these effects translates in two operation modes for the characterization equipment, namely, the direct and converse modes. The measurement bases for these two modes are described in Secs. II A and II B.
A. Direct effect and mode
In the direct mode, the change in the polarization of a sample is determined as a function of a magnetic field stimulus. The measurement of the change in the polarization implies the detection of charge migration, which strongly depends on the impedance of the measurement system.9, 14 Additionally, while poling and measuring, charges could accumulate at imperfections, grain boundaries and interfaces,
Rev. Sci. Instrum. 85, 085116 (2014)
especially in heterogeneous and polycrystalline materials, which then move towards the electrodes leading to erroneous readings and interpretations. The dynamic direct mode has been established as the most reliable method to measure the direct magneto-electric coefficient, as the noise is dramatically reduced and the charge accumulation problem is mitigated.10 In this method, the effective magneto-electric voltage, related to the change in the polarization, is determined as a function of an AC magnetic field (hAC ) with a DC magnetic field bias (HDC ). This has been the most common mode to date to characterize the coupling properties in magneto-electric multiferroics.11, 13 When exposing a fully poled sample to an external magnetic field, the voltage proportional to the change in the polarization related to the magneto-electric coupling can be considered as VME = f (HT ) with HT = HDC + hAC .
(1)
Assuming that hAC is sufficiently small that any possible change in the total magnetization of the sample is associated with the reversible component, and following the mathematical procedure described in Ref. 11, the direct (dynamic) magneto-electric coefficient could be expressed as α = ε0 εr αME where αME =
AC VME hAC· d
(2)
AC where VME is the amplitude of the magneto-electric voltage measured at the frequency of the applied AC magnetic field and d is the thickness of the sample. The term α ME is commonly referred as the magneto-electric voltage coefficient.
B. Converse effect and mode
Within the converse mode, the change in the magnetization of a sample is determined as a function of an applied DC electric field (static converse mode) or as a function of an AC electric field with a DC electric field bias (dynamic converse mode). The total magnetization (MT ) of a sample at any point of the hysteresis loop could be described as MT = Mrev + Mirr ,
(3)
where Mrev is the reversible component of the magnetization and Mirr is the irreversible component. In order to establish a defined initial magnetization state for the magneto-electric measurement (the magnetic equivalent to an electrically poled state), starting from the demagnetized state a value of Mirr is set by applying a given magnetic field (h1 ) and reducing the field to zero. The resulting minor loop will set a remanent magnetization Mr that will correspond to the value of Mirr for the field h1 . When the value of h1 is equal or higher than the saturation field, the corresponding Mirr will be equal to the maximum remanent magnetization of the sample. In the dynamic converse mode, the change in the reversible magnetization (Mr ) is determined for a given value of the irreversible magnetization (Mirr ) as a function of a DC electric field bias EDC , subjected to a small AC electric field. Within the static converse mode, the electrically induced change in the total
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magnetization is determined starting from a given remanent state set by the value of Mirr . For the dynamic converse effect, the change in the reversible magnetization M∗ induced by the electric field, can be determined as a function of small AC electric field eAC and be expressed as M ∗ = f (ET ) where ET = EDC + eAC .
(4)
Following a similar mathematical procedure as in Ref. 11, the dynamic converse magneto-electric coefficient can be expressed as α=
∗ MAC , eAC
(5)
∗ is the magnetization measured at the frequency of where MAC the AC electric field. The latter implies a measurement of the magnetization at the frequency of an externally applied AC electric field, which is beyond the capabilities of the PPMSACMS option used as a base for implementing the ME measurement system. A realization of the dynamic converse mode have been proposed using a SQUID magnetometer.15 The developed ME measurement system is capable of determining the static converse effect, where the electrically induced change in the total magnetization is measured by extraction magnetometry. In addition to the static converse mode, a pseudo static converse mode was also implemented. Within this mode, the magnetic AC susceptibility is recorded as a function of a DC electric field bias, subjected to a small AC magnetic field. With the system as implemented is possible to study the magnetization processes and magnetization switching behavior under applied electric field.
III. MAGNETO-ELECTRIC MEASUREMENT SYSTEM IMPLEMENTATION
The magneto-electric system was designed in such a way that minimum modifications are required to the base ACMS hardware. A schematic overview of the developed ME system as implemented in the PPMS is shown in Figure 1. The system is capable of performing different measurements without extracting or repositioning the sample, and thus minimizing contact errors and deterioration. Measurements are fully automated and integrated into the options of the PPMS Multivu software platform, so no further special training is required for the end-user. The magneto-electric option electronics are housed in a 2U 19 in. rack unit that can be accommodated into the PPMS model 6000 cabinet (see Figure S1 of the supplementary material16 ). New sample holders were designed to be attached to standard ACMS rods, which in turn were modified to incorporate thin twisted wires to extract and apply signals to the samples (Figure S2 of the supplementary material16 ). Sample holders can accommodate thin films and bulk samples, following the same standards used to setup the contacts in the resistivity pucks. A new ACMS transport cap was designed to incorporate a “spring” wiring to accommodate the displacement sustained by the ACMS rod in both AC susceptibility and extraction modes. The assembly features a low profile detach-
FIG. 1. Schematic overview of the magneto-electric option as implemented in the PPMS with the ACMS option.
able connector to facilitate the rod extraction and installation (Figure S3 of the supplementary material16 ). The direct dynamic mode capability was developed on the base of the AC magnetic susceptibility mode of the ACMS. In this configuration, the AC magnetic field (hAC ) is generated by the AC coil of the susceptometer (from 10 Hz to 10 KHz), while the DC bias magnetic field is applied by the main superconducting magnet of the PPMS. The signal from the sample is conditioned and amplified with a virtual ground high impedance amplifier. Signal from the virtual ground high impedance amplifier is digitalized and synchronically rectified to the reference signal from the AC magnetic field (obtained from the drive reference of the AC coil) using a digital signal processing (DSP)-based dual phase lock-in amplifier, implemented in the DSP/Microcontroller unit (MCU) module. Digital representations of the in-phase and quadrature signals are stored and afterwards converted to analog signals to be acquired by the PPMS through the auxiliary port. The system is able to detect the different AC measurements modes of the AC susceptometer17 and synchronize the output to the operation and measurement sequence of the PPMS-ACMS setup. Finally, the magneto-electric signals can be plotted in real time following the standard procedures implemented in the MultiVu software. The DC measurement capabilities of the ACMS are used for the static converse mode operation of the system. In this configuration, the ACMS operates as an extraction magnetometer. A DC signal reference obtained from the PPMS is amplified and applied to the sample. Similar to the previous mode, the measurement is controlled from the MultiVu software, and can be implemented in fully automated command sequences. In order to establish a fixed operation point, samples are magnetized to a certain remanent state (which will be the magnetic equivalent to an electrically poled state) before running a full M vs. E measurement. The system can apply a maximum positive bias voltage of 200 V DC. In the pseudo static converse mode, the measurement is taken using the regular AC susceptibility configuration of the
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ACMS, with an amplified DC reference signal as DC electric field bias. Within this setup, the system operates in a slightly modified direct operation mode. IV. TEST RESULTS
Magneto-electric behavior of a core-shell (BaTiO3 )0.6 / (CoFe2 O4 )0.4 magneto-electric composite measured in direct mode is presented in Figure 2. Measurement was recorded at room temperature with an AC magnetic field frequency of 10 KHz and an amplitude of 5 Oe. Bias magnetic field was applied along the z axis, while the induced magneto-electric voltage was measured along the y axis, which correspond to the measurement of the transverse (αE ) magneto-electric 31 voltage coefficient. Initial powders preparation details can be found in Ref. 18. Cylindrically shaped pellets were pressed at 600 MPa, achieving more than 55% of the theoretical density of the mixture. The pellets were sintered at 1100 ◦ C for 12 h. Results are consistent with the previously reported magneto-electric data for these kind of composites, measured using a standard direct mode setup.11 The maximum of the ME coefficient is 2.24 mV/cm Oe, which is greater than the expected value according to the tendency showed in Ref. 11 (the order of magnitude is the same, and a value of about 2.0 mV/cm Oe would be expected). These could be related to slightly differences in sample preparation and composition. Due to that the data collection during the measurement is synchronized to the working mode of the ACMS (data could be collected during the ACMS measurement at each detection coil and the calibration coil – 5 points mode17 – and averaged at the end), there is an inherent increase in the sensitivity compared to previous setups. The static converse magneto-electric response M(E) of a core-shell (BaTiO3 )0.8 /(NiFe2 O4 )0.2 magneto-electric composite is shown in Figure 3. Measurement was recorded at room temperature taken at a constant DC magnetic field of 200 Oe and starting from a remanent state set by a field of 10 000 Oe. Sample was prepared in a similar manner to the (BaTiO3 )0.6 /(CoFe2 O4 )0.4 composite described previ-
FIG. 3. Static converse magnetoelectric effect for (BaTiO3 )0.8 /(NiFe2 O4 )0.2 core-shell composite measured at room temperature.
ously. Details of the starting powders preparation could be found in Ref. 19. Here, magnetization was recorded along direction of the applied magnetic field (z axis), perpendicular to electric field (y axis). The magnetization is normalized to the value MmaxE obtained at E = 17 kV/cm. Furthermore, the M(E) curves exhibit a half butterfly-like shape, which indirectly evidences that the strain transferred from the BaTiO3 phase into the NiFe2 O4 phase gives rise to the magnetoelectric coupling in the composites.20 Figure 4 shows the magnetic susceptibility as a function of the temperature, measured for a sample of Pb(Fe0.5 Nb0.5 )O3 at frequencies of 100 Hz and 1000 Hz in the pseudo static converse mode. Sample preparation details are described in Ref. 21. The observed slight deviation in the magnetic susceptibility behavior with the electric field bias applied could suggest a direct coupling between the electric and magnetic ordering in this material. This fact supports the previously observed magneto-dielectric coupling as reported in Ref. 22. Furthermore, this operation mode exposed the magnetic relaxation behavior of the material under the electric bias. The slowdown of the magnetic dynamic response (more prominent at low frequencies) under electric field bias suggests the existence of a magnetic polarization induced by the electric field. This provides a unique experimental data to analyze the coupling mechanisms in this single phase materials and exemplifies the value of the technique.
V. FINAL REMARKS
FIG. 2. Magneto-electric voltage coefficient for (BaTiO3 )0.6 /(CoFe2 O4 )0.4 core-shell composite measured at room temperature.
A new magneto-electric measurement module was developed as an add-on to the popular ACMS option for the Quantum Design PPMS. The module provides the capability to determine the direct and converse magneto-electric effects in a single setup and without extracting or repositioning the sample. Additionally, the module features: (a) full hardware compatibility and minimal modifications to the existing model 6000 and ACMS option; (b) operation fully integrated in Multi-Vu software; (c) full temperature range from 1.9 K to 350 K; and (d) user accessible and customizable (open platform for firmware update).
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FIG. 4. Magnetic susceptibility (real part) for a Pb(Fe0.5 Nb0.5 )O3 sample measured at (a) 100 Hz and (b) 1000 Hz.
ACKNOWLEDGMENTS
Authors acknowledge the financial support of CONACYT through Grant Nos. 174391 and 83813. Authors thank R. Font and O. Raymond for supplying the PFN sample, their technical assistance and valuable comments. 1 W.
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