Evidence of Magnetoresistance in the Prussian Blue Lattice during a

J. Phys. Chem. C 2008, 112, 20099–20104

20099

Evidence of Magnetoresistance in the Prussian Blue Lattice during a Voltammetric Scan David Gimenez-Romero,† Jose´ Juan Garcı´a-Jaren˜o,*,†,‡ Jero´nimo Agrisuelas,† and Francisco Vicente† Departament de Quı´mica Fı´sica, UniVersitat de Vale`ncia and Fundació General, U. V., C/ D. Moliner 50, 46100 Burjassot, Vale`ncia, Spain ReceiVed: September 4, 2008; ReVised Manuscript ReceiVed: October 7, 2008

This manuscript reports evidence of magnetoresistance effects in the Prussian Blue lattice during a voltammetric scan at room temperature. Accordingly, the PB is a well-known semiconductor that becomes surprisingly an almost metallic conductor in the presence of an internal magnetic field induced during the voltammetric scan. This offers appealing perspectives for the control of this interesting phenomenon from electrochemical techniques that could be used for the fabrication of the recent phase-change computational memories, which are electronically configurable. Herein, the PB magnetic properties have been monitored in situ by means of resonating magnetic microsensors based on the shift in the resonant frequency of microelectromechanical systems. Introduction The bistability of electronic states in transition-metal complexes represents an important field of research in coordination chemistry. The bistability in these compounds generally arises from strong electron-lattice couplings. Typical examples are spin crossover phenomena1,2 and valence-tautomerism.3 In this latter class, valence-tautomeric hexacyanometallate-like materials with the general formula M′k[M′′(CN)6]l · mH2O (in which M′ (high spin) and M′′ (low spin) are transition metals) have attracted considerable interest owing to their intrinsic magnetic properties, such as high Curie temperatures4,5 or photomagnetic phenomena.6,7 The study of changes in magnetic properties at the molecular level introduces an important route to the development of switches for advanced molecular devices.8 Spin crossover in transition-metal complexes, in which spin-state transformations occur as a result of heat, pressure, or light irradiation, is a key phenomenon for designing these switches.1 Accordingly, the structural and physicochemical properties of the hexacyanometallate-like materials can be tailored by external conditions, such as the magnetic field,9-11 light10,12,13 and by changes in a specific local crystal environment (e.g., voltammetric scans14-16). Thus, the hexacyanometallate-like materials have become the focus for designing molecular switches given that they are relatively easy to synthesize. Magnetoresistance (MR) is manifested as a significant change in the electrical resistance in the presence of a magnetic field.17,18 That is, the value of the electrical resistance increases in the presence of this field when the effect of the magnetoresistance is positive, and it decreases when the effect is negative. The magnetoresistance effect was observed in thin film structures of alternating ferromagnetic and nonmagnetic metal layers. Nevertheless, it has been also observed in nonmultilayer systems such as solid precipitates of a magnetic material in a nonmagnetic matrix (granular GMR effect).19 Recently, studies based on magnetic ferroelectrics have increased the interest in this * E-mail: [email protected]. † Departament de Quı´mica Fı´sica, Univ. Vale`ncia. ‡ Departament de Quı´mica Fı´sica, F.G. Univ. Vale`ncia.

phenomenon since from a technological point of view, the mutual control of electrical and magnetic properties is an attractive possibility in the study of the read heads in modern hard drives and in magnetoresistive random access memories (MRAM). However, the number of candidate materials is limited and the effects are typically too small to be useful in applications.20,21 Accordingly, we report here a striking interplay between electricity and magnetism in the Prussian Blue (PB) lattice that is observed during the voltammetric scan at room-temperature. The PB magnetic properties have been monitored in situ by means of resonating magnetic microsensors based on the shift in the resonant frequency of microelectromechanical systems.20,21 The thickness shear mode resonators have been successfully used as gravimetric sensors by using the Sauerbrey relation between the resonant frequency and added mass22 (often referred to as a quartz crystal microbalance), which reach the subnanogram level.23 However, in addition to the mass of the film coating the resonator, other factors such as its viscoelasticity, solution properties, coercive fields, and temperature contribute to the electroacoustic behavior of the loaded piezoelectric crystal.24 The interactions with the coating or/and solution cause changes in the quartz resonance characteristics which can be studied from the measurement of the resonant frequency of the crystal and completed by means of electroacoustic techniques. Experimental Section The PB structure was electrochemically deposited by immersion in 0.02 M K3Fe(CN)6 (A.R., Panreac), 0.02 M FeCl3 (Sigma), and 0.01 M HCl (A. R., R. P. NORMAPURTM) solution. A controlled cathodic current of 40 µA cm-2 was applied for 150 s to fabrication of PB thin films over 90 nm. After deposition, the PB structure was stabilized by means of cyclic voltammetry in the potential range between -0.20 V and 0.60 V in 0.50 M KCl (A. R., R. P. NORMAPURTM) at 298 K and at 10 mV/s, pH 2.3. Compositional changes were carried out in response to an electrochemical modulation in a typical electrochemical threeelectrode cell. The PB films were deposited on a transparent glass of ITO (indium tin oxide) with a surface area of 1 cm2

10.1021/jp807861q CCC: $40.75  2008 American Chemical Society Published on Web 11/19/2008

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Figure 1. Schematic representation of the PB lattice. Yellow balls correspond to Fe(III) atoms, red balls to Fe(II) atoms, blue balls to nitrogen atoms, and cyan balls to carbon atoms.

for spectroscopy measurements. A platinum plate was used as the counter electrode and the Ag|AgCl|KClsat electrode was used as the reference electrode. The electrolyte used in all the experiments was 0.50 M KCl (A. R., R. P. NORMAPURTM) at 298 K, pH 2.3. The compositional modulation during the spectroscopic measurements was controlled according to the oxidation state of the PB thin films by means of a PalmSens potentiostat (Palm Instrument BV). These measurements were performed by using the spectral range given by a HEλIOS β UV-visible spectroscope (Spectronic Unicam). For electroacoustic measurements, the PB thin films were deposited on a gold electrode of an AT cut quartz crystal (6 MHz, Matel-Fordahl) with a surface of 25 mm2. An Autolab potentiostat-galvanostat (PGSTAT302) was used to modulate the compositional fraction of these materials whereas a RQCM (Maxtek, Inc.) was used at the same time to record the motional resistance and the resonance frequency changes during this modulation. The interaction of the piezoelectric layer with an external magnetic field was realized by means of the approximation of a magnet to an electrode of quartz crystal in air at 1 mm.

Results and Discussion Prussian Blue presents a faced-centered cubic structure at room-temperature.25-27 The PB lattice can be defined as a laminated configuration of high-spin Fe(III) and low-spin Fe(II) layers; see Figure 1. The Fe(III)(NC) sites are ascribed to high-spin metallic atoms (S ) 5/2) whereas the Fe(II)(CN) sites are ascribed to low-spin atoms (S ) 0) where 1/4 of the low-spin Fe(II) sites are missed.28 Thus, the crystalline framework is composed by repetitive units of Fe(II)low-spinCN-Fe(III)high-spin in three spatial directions. The crystalline growth of the PB structure can be electrochemically controlled by galvanostatic procedures.29-31 Indeed, the oxidation state of the iron atoms of the PB structure can be reversibly modulated at room-temperature by electrochemical techniques between the PB (blue form, 0.60 V) and Everitt’s Salt (ES, colorless form, -0.20 V) forms:14,16,32

Fe(II)low-spin-CN-Fe(III)high-spin + 1e- a Fe(II)low-spinCN-Fe(II)high-spin (1) Herein, it is important to emphasize that the ES form and the intermediate states between the PB and ES forms are only

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Figure 2. Evolution of the current during the voltammetric scan of a PB thin film at 10 mV/s and at room-temperature. The electrolyte was 0.50 M KCl at 298 K, pH 2.3.

stable at room temperature at each polarization potential. If a polarization potential is not applied, then the PB form is the stable form. The physical properties of this material change during this scan given that the oxidation state of the high-spin sites is modulated. Two different valence states (II and III) coexist in high-spin sites (Fe(NC)high-spin sites) during the voltammetric scan, and as a result, the coupling between the iron atoms located into these sites may also change given that this one depends on their oxidation states.33 Consequently, the Tc of this material may also change during the voltammetric scan at roomtemperature because Tc ∝ |J|.34 It is important to emphasize that the material remains unchanged before and after the cyclic voltammetric experiment, as eq 1 shows. Electrogravimetric and Acoustic Analysis. The PB films give an electrochemical response in 0.50 M KCl at 298 K, pH 2.3, characterized by a very narrow voltammetric peak, as shown in Figure 2. This behavior may not be explained unless an apparent number of electrons of 1.5 is considered for the singleelectron transfer located in the potential range from 0.15 to 0.28 V.29,30 Therefore, an extraelectric current coexists with the faradaic current associated to this electron transfer during the PB compositional modulation, the current that is responsible for the half-electron. This conclusion is confirmed by means of spectroscopic techniques since the maximum of the derivative of absorbance at 686 nm (the typical spectroscopic transition of PB films35) with respect to the number of exchanged electrons does not coincide with the maximum of current during the PB voltammetric scan. Recently, the analysis of mass changes during electrochemical processes has provided interesting information by means of the mass/electrical charge ratio. The F(dm/dQ) function allows the molecular weight of the species exchanged between the film and the solution during a redox process to be calculated by means of the Faraday’s laws.36 A shift of this function is observed during the voltammetric scan in the potential range 0.15-0.28 V where the extraelectric current detected above is also observed,37,38 zone 1 in Figure 2. At these potentials, the PB single-electron transfer is associated with the potassium ion exchange37 (F(dm/dQ)theor ) -39 g mol-1); however, the F(dm/ dQ) function determined experimentally increases to about -20 g mol-1.38 Therefore, the extraelectric current may be associated to a nonfaradaic process, which contributes to the measured current, but it does not mean a net mass change on the electrode surface, and consequently ∆Q increases whereas ∆m remains unchanged.38

Figure 3. Influence of an external magnetic field on the motional resistance and frequency changes of a quartz resonator in air.

Several resonating magnetic microsensors based on the modification of the resonant frequency of a microelectromechanical system have been developed in the past few years. These microsensors use a quartz crystal piezoelectric material where resonant frequency changes in presence of a magnetic field.39 Electrochemical quartz crystal microbalance (EQCM) is also based in the change in the resonant frequency of a quartz crystal due to changes of the mass deposited on the side in contact with an outer solution. Figure 3 shows how the presence of a magnetic field shifts the frequency of the resonant EQCM as well as the acoustic impedance measurements of this device.40 During the PB electrochemical processes, potassium ions enter into the PB lattice to keep the film electroneutrality, changing largely the resonant frequency of the EQCM device.14 As a result, these changes could hide other shifts in the resonance frequency associated to other phenomena such as the magnetic influence unless an indirect magnitude is evaluated (F(dm/dQ) function). However, the acoustic impedance of PB films does not change largely because the movement of potassium ions and modifications of the acoustic impedance resistance (motional resistance, Rm) associated to the electrochemical scan between the PB (0.60 V) and ES (-0.20 V) forms are smaller; only a small dip in the N-Fe(III)-N angle is observed in a solution with potassium cations.41 As a consequence, changes in the resonating microsensors associated to phenomena that are not related to structure or density changes are better monitored by means of variations of motional resistance than by means of variations in the resonance frequency of the EQCM device. For that, the measurement of the motional resistance is used in this work during the voltammetric scan as a magnetic field sensor where the detectable magnetic field range is similar to the range of the magnetic sensors based on the GMR effect.39 During the PB voltammetric scan at room-temperature, the potential range associated to the nonfaradaic current and to the shift of F(dm/dQ) is also characterized by a sudden increase of the motional resistance (zone 1 in Figure 4) due to the increase of the PB coercive field.40 Thus, the above-mentioned nonfaradaic current could be due to the change of the film coercive field during the voltammetric scan. This is completely in concordance with the recently observed change of the Curie temperature of PB

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Figure 4. Evolution of the absorbance at 1018 nm (forbidden electron charge transition) and the motional resistance of a PB thin film during this modulation.

Gimenez-Romero et al.

Figure 5. Rough estimate of the ratio Tc/J during the cathodic voltammetric scan at 10 mV/s and at room-temperature.

Tc ) analogues during their compositional modulation from electrochemical techniques.42 On the other hand, the increase of the motional resistance in the range between 0.15 V and 0.28 V is coupled with a progressive but significantly smaller increase of this resistance during the whole cathodic scan: the PB lattice is cubic at 0.60 V whereas it is tetragonal distorted cubic at -0.20 V.40 Thus, the crystal structure alterations that occur together with the magnetic phase transitions43 are also detected by means of the motional resistance measurements. Nevertheless, a rapid structural shift could not explain the maximum of the motional resistance between 0.15 V and 0.28 V because this maximum is also observed when the material is completely stabilized at a fixed potential.40 Spectroscopic Analysis. Among others, the reduction of the PB form to the ES form during the voltammetric scan is associated to the decrease of absorbance at 1018 nm. This spectroscopic transition (Figure 4) corresponds to the forbidden electronic charge transfer from the iron(II) atoms surrounded by -CN units (Fe(II)(CN)low-spin with the electronic configuration (t2g)6C) to the iron (III) atoms surrounded by NC units (Fe(III)(NC)high-spin with the electronic configuration (t2g)N3(eg)N2):35 (t2g)6C(t2g)3N(eg)2Nf(t2g)5C(t2g)3N(eg)3N. The decrease of the absorbance at 1018 nm during the whole cathodic voltammetric scan is easily explained by considering that the number of Fe(III)(NC)high-spin sites also decreases during this scan. Nonetheless, it should be noted that this electronic transfer is surprisingly intensified during the cathodic voltammetric scan at roomtemperature in the potential range between 0.15 and 0.28 V (zone 1 in Figure 4). This increase so denotes an increase in the exchange integral at these potentials between the iron atoms located into the high-spin and low-spin sites. Given that spin-spin interactions between high-spin sites strongly implicates the intervening of iron(II) atoms located into low-spin sites,34 this increase in the exchange integral should be reflected by the increase at these potentials of the coupling constant, the parameter that determines the strength of the interaction between neighbor atoms, J, between iron atoms located into high-spin sites. The coupling constant and the exchange integral are related through the equation:44 J ) 2k + 4βS (k, exchange integral; β, resonance integral; S, overlap integral). We can have a rough estimation of the Tc for the PB lattice via the approximate meanfield formula:45,46

√ZMZM′|J|√SM(SM + 1)SM′(SM′ + 1) 3kB

(2)

where SM and SM′ are the local spins on centers M and M′, ZM and ZM′ are the number of nearest neighbors of each type of metal atom, and kB is the Boltzmann constant. The PB magnetic ordering takes place only between highspin iron sites that are separated no less than 10.16 Å.34 During the PB voltammetric scan at room-temperature, highspin Fe(III) (S ) 5/2) are reduced to high-spin Fe(II) (S ) 2) and in consequence, the PB lattice alternates both oxidation states in Fe(NC)high-spin sites. The magnetic ordering during this scan takes place between high-spin Fe(III) atoms (S ) 5/2) and high-spin Fe(II) atoms (S ) 2), the interaction known as A-A′ in the PB-like materials.33 Therefore, the magnetic ordering temperature must increase to a great extent during the voltammetric scan at room-temperature since the coupling constant has a strong dependence on the oxidation state of the two coupled atoms. All complexes with strong coupling correspond to metal pairs in oxidation states III-II (503 > |J| > 122 cm-1).47 Furthermore, anisotropic interactions also represent a very attractive alternative tool to increase the spin reorientation barrier of single-molecule magnets clusters.48 Strains alter the Tc of ferromagnetic and superconducting materials.49 Thus, Haeni et al.21 showed that epitaxial strains can be harnessed to increase Tc by hundreds of degrees and produce room-temperature ferroelectricity in strontium titanate, a material that is not normally ferroelectric at any temperature. Accordingly, PB analogue films show the electrochemical scan led to elongation phenomena,32 as Figure 4 shows via the motional resistance in PB films. Consequently, all these data together with the spectroscopic measurements confirm that the coupling constant of the PB structure increases largely during the PB electrochemical scan at room-temperature in the potential range between 0.15 V and 0.28 V, where there is the maximum number of Fe(II)high-spin atoms surrounded by Fe(III)high-spin atoms. The critical temperature for PB thin films can be controlled by an electrochemical method, as in NiFe Prussian Blue.32 Herein, it is important to emphasize that there is no just one main reason that explains the magnetic behavior since all reasons are related; that is, it cannot be attributed preferably to only one reason. In agreement with the eq 2, Figure 5 shows the evolution of the ratio Tc/J of a PB thin film during the cathodic voltammetric scan at room-temperature. The increase of Tc by hundreds of degrees at these potentials could be explained easily considering

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J. Phys. Chem. C, Vol. 112, No. 50, 2008 20103 References and Notes

Figure 6. Evolution of the motional resistance measured in ref 40 at -1/2 ) F/Ke
τe/dGe steady-state conditions and evolution of the R-1 W τe function calculated in the ref 51 at steady-state conditions. Both magnitudes were calculated at room-temperature. R-1 W is a direct measurement of the film conductivity.53

the value of the maximum of the ratio Tc/J (about 1.2 K cm, Figure 5) and the magnitude of the coupling constant between oxidation states III-II (503 > |J| > 122 cm-1). Considering this, the maximum of the Tc may be between 604 and 146 K in the potential range between 0.15 V and 0.28 V, and as a result, this one could explain the PB magnetic ordering observed during the voltammetric scan at room-temperature. Hence, the nonfaradic current determined experimentally could be explained as the charge/discharge of a magnetocapacitor given that the maximum of Tc/J corresponds to the potential range associated above to the extraelectric current. This is consistent with the fact that these effects are observed better at dynamic conditions (great perturbations, i.e., cyclic voltammetry) than at steadystate conditions (small perturbations, i.e., electrochemical impedance spectroscopy).40 Negative MR effect is manifested as a significant decrease in electrical resistance (conductivity increase) in the presence of an external or internal magnetic field. Accordingly, it has been reported by EIS measurements that the PB is a well-known semiconductor which becomes surprisingly an almost metallic conductor in the potential range associated above to the increase of the ratio Tc/J,29,50,51 270 Ω (PB, 0.60 V) f 3 Ω (intermediate region, 0.20 V) f 4100 Ω (ES, -0.20 V).52 Figure 6 also shows this conductivity increase calculated by means of electrochemi-1/2 ) F/Ke
τe/dGe cal impedance spectroscopy.51 The R-1 W τe function is an estimation of the film conductivity since R-1 W is directly related to this magnitude.53 Furthermore, Figure 6 shows how the decrease of the electrical resistance during the PB voltammetric scan can be associated to the presence of an internal magnetic field at these potentials and therefore to a negative MR effect during this scan at room-temperature. In agreement with this, the PB-laminated configuration showed in Figure 1b has been also observed in materials with MR effect. Conclusions Magnetic properties of Prussian Blue films can be modulated by means of the cyclic voltammetry. This material besides is a novel candidate with negative magnetoresistance effects observed during the voltammetric scan at room-temperature that could be used for the fabrication of the recent electronically configurable molecular-based logic gates employed in chemically assembled electronic nanocomputers. Acknowledgment. This work was supported by FEDERCICyT project CTQ 2007-64005/BQU. D. Gimenez-Romero acknowledges his position to the Generalitat Valenciana.

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