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Negative magnetoresistance in organic ionic semiconductor:TTFCOONH3Ph
Solid State Communications 165 (2013) 27–32
Contents lists available at SciVerse ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Negative magnetoresistance in organic ionic semiconductor: TTFCOONH3Ph Yuka Kobayashi n, Satoshi Sumi, Takeshi Terauchi, Hideo Iwai National Institute for Materials Science (NIMS), Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 24 January 2013 Received in revised form 4 April 2013 Accepted 9 April 2013 by Z. Tang Available online 19 April 2013
TTFCOONH3Ph is a recently synthesized open-shell ionic semiconductor, the electronic state of which differs from that of typical organic closed-shell semiconductors. Magnetotransport properties were examined using a single-crystal sample, and found to exhibit small negative magnetoresistance (∼0.2%) for 9 T at room temperature (rt). The magnetization curve verifies the existence of a ferromagnetic (35%) and a paramagnetic (65%) component at rt, which is very similar to that of diluted magnetic semiconductors, despite the absence of any ferromagnetic metal elements. Electron spin resonance reveals weak localization of paramagnetic molecular spins, and moreover, ferromagnetic resonance confirms the existence of magnetically ordered spins in addition to the paramagnetic ones. The origin of the spin-polarized transport is discussed. & 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Organic ionic semiconductor D. Ferromagnetism D. Magnetotransport
1. Introduction Magnetotransport attracts a lot of attention not only from an academic viewpoint, but also because of its potential spintronics applications [1]. Negative magnetoresistance is observed in specific materials with ferromagnetic components, for example, Fe–Ni based permalloys [2], magnetic transition metal oxides [3], and III–V semiconductors with magnetic impurity, the so-called diluted magnetic semiconductors (DMS) [4] etc., where electron (hole) transport couples with the ferromagnetic properties of coexisting magnetic components. Recently, some nonmagnetic organic light-emitting diode (OLED) materials have been reported to exhibit fairly large negative magnetoresistance (MR) up to 10% for small magnetic fields of less than 100 mT at room temperature (rt) [5]. The mechanisms for this effect are believed to differ from the abovementioned inorganic magnetic systems: magnetic-field-dependent transport may be caused by a mixing of singlet and triplet excitons [5], by bipolarons [6], or by generation of secondary charge carriers [7], etc. Anilinium tetrathiafulvalene-2-carboxylate (TTFCOONH3Ph) is an organic ionic semiconductor composed purely of light elements, and its electronic state is more similar to doped inorganic semiconductors such as III–V semiconductors or doped transition metal oxides than to that of OLED-type closed-shell semiconductors [8]. It is simply stated, a band insulator with a 1.31 eV optical bandgap, the absolute value of which is comparable to that of GaAs [9], but it is doped by a partial inclusion of an open-shell TTF+COO−NH2Ph component in a closed-
shell TTFCOO−NH3+Ph component, providing an acceptor (impurity) level for the emergence of semiconducting properties [8] (Fig. 1(a)). The intrinsic dopant of this system is spontaneously included a protonic defect in an ammonium ion, namely, the PhNH2 species, as schematically shown in Fig. 1(b). The direct current (DC) conductivity of the single crystal exhibits semiconducting behavior with 0.16 S/cm at rt with an activation energy (Ea) of 0.11 eV [8]. The charge transport of the material is quite unique because the hopping movement of hole carriers is likely associated with dielectric relaxation; the ionic transformation, PhNH2↔PhNH3+ is inevitable in the hole-conduction process TTF+COO−↔TTFCOO− because of the suppression of electrostatic repulsion. Single-crystal X-ray crystallographic analysis has revealed that the conducting TTFCOO− and TTF+COO− moieties are self-assembled in one dimension and are in supramolecular association with a salt bridge (charged hydrogen bond). Herein we report on the magnetotransport of single crystal TTFCOONH3Ph. Examinations on the constitution and properties of the molecular spin reveal that the origin of the spin-polarized hole transport is related to the anomalous magnetic properties of this material. The isotope-substituted TTFCOOND3C6D5 provides a clear insight into a possible mechanism of the magnetotransport properties in these systems.
2. Experimental methods 2.1. Sample preparation and characterization
n
Corresponding author. Tel.: +81 29 859 2154; fax: +81 29 859 2101. E-mail address: [email protected] (Y. Kobayashi).
0038-1098/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2013.04.014
A single crystal of TTFCOONH3Ph (H) was grown by recrystallization from n-hexane/ethyl acetate, following a procedure reported
28
Y. Kobayashi et al. / Solid State Communications 165 (2013) 27–32
Fig. 1. Schematic diagram of the organic ionic semiconductor, TTFCOONH3Ph. (a) Band structure with an acceptor level, (b) molecular arrangement self-assembled by one-dimensional salt-bridge network, where the protonic defects are partly included.
previously [8]. The isotope-substituted TTFCOOND3C6D5 (D) was synthesized using commercially available deuterated aniline, C6D5ND2, and its single crystal was grown as H. Inductively coupled plasma mass spectrometry (ICP-MS) measurements were carried out for single crystals of 20 mg using Agilent 4500 (Agilent Technologies). Time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements were carried out for single crystals set on a carbon sheet using PHI TRIFT V nanoTOF (ULVAC-PHI). 2.2. Physical measurements Magnetotransport measurements for a single crystal were carried out with the four-probe method using a liquid helium cryostat in the temperature range 120–350 K and up to 79 T with a physical properties measurement system (Quantum Design PPMS-9). The magnetic field was vertically applied to a single crystal with four probes in a line. The dimensions of the single crystal are very small, ca. 0.02 0.2 0.9 mm3, with a magnetic moment that is too small to be detected as a sole crystal even with the high-resolution magnetometer. Therefore, single crystals of 0.4 mg were roughly oriented on a nonmagnetic polymer sheet by hand for the magnetic measurements. Magnetic susceptibility was measured with 0.1 T of the applied field as a function of temperature from 5 to 320 K. These measurements were performed in the temperature range 5–320 K using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-7) and a field up to 75 T. Diamagnetic correction was done by Pascal's constant and the subtraction of a linear diamagnetic component. Electron spin resonance (ESR) and ferromagnetic resonance (FMR) measurements were performed using the oriented single crystals at 300 K and 123 K using an electron spin resonance spectrometer (JEOL JES-FA100).
3. Results and discussion 3.1. Magnetotransport properties Fig. 2(a) shows the MR of the H single crystal, where {[(ρM/ρ0) −1]n100} (%) (ρΜ: resistivity for an applied field, ρ0: resistivity for 0 T) values are plotted for the magnetic field from −9 T to 9 T at 300 K. The MR effect is observed at 300 K, but the value is less than 0.2% for 9 T, which is much smaller than the values of OLED semiconductors reaching 10% for low field at the mT scale [6]. Moreover, the negative MR increases almost linearly with the applied magnetic field, the behavior of which is totally different from that of OLED semiconductors, which is represented by an
empirical law: MR∝B2/(|B|+B0)2 or B2/(B2+B02), where B is a magnetic field and B0≈5 mT [6]. We further measured the MR of the D sample in addition to that of the H sample at 350, 300 and 250 K as shown in Fig. 2(b), and found that it increases with decreasing temperature in both cases and exhibits a small isotope effect; MR of D is relatively smaller than that of H at all temperatures. The temperature and field dependence of MR in H and D are quite similar to those of DMS systems doped with magnetic ions; for example, a single crystal Mn-doped GaN (with Mn concentrations: 1–9%) [10]. Although temperature dependence of MR in H down to 120 K was further measured to understand the magnetotransport properties as shown in an electronic supplementary information (ESI), quite large errors occur in the measurement, probably due to coexisting non-negligible ionic transformation, PhNH2↔PhNH3+ at lower temperatures [8]. Anomalous Hall effect of H was also measured at 300 and 160 K (ESI). It seems to respond to the applied field, but the behavior could not be explained completely by the experiment. The H and D do not exhibit remarkable difference in their absolute values to positive and negative fields in either MR or anomalous Hall effect. The weak asymmetric behavior to the fields in the magnetotransport may correlate with the magnetic properties of this system. 3.2. Magnetic properties The magnetic susceptibility of the roughly oriented single crystals of H and D is shown in Fig. 3(a). The absolute value of the susceptibility is 6.0 10−3 emu/mol for H and 8.5 10−3 emu/mol for D at rt, and 8.5 10−3 emu/mol for both samples at 5 K, although these values include errors caused by incompleteness of the alignment of the single crystals. They exhibit weak temperature dependence, which is not similar to the characteristic magnetic properties superimposed paramagnetic and ferromagnetic (impurity Curie component) behavior generally observed in organic radical (semi) conductors [11]. We point out that the absolute value at rt was one order of magnitude higher than that of strongly correlated organic conductors [12] and at the highest level in all organic electroactive materials as far as we know. Another possibility for describing the temperature dependence is ferromagetism with a higher Tc above rt. The magnetization curve of H and D for applied magnetic field (MH curve) from −5 to 5 T at 300, 10 and 5 K, respectively, is shown in Fig. 2(b,c). A hysteresis loop is observed in the inset which is zoomed in 7100 mT, as is predicted by the weak temperature dependence. We point out that the behavior is, however, originally anomalous because this material is a pure organic compound without any ferromagnetic metal elements. The shape of the loop further reveals that ferromagnetic spins possess small coercive force, which
Y. Kobayashi et al. / Solid State Communications 165 (2013) 27–32
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Fig. 2. Magnetotransport properties. (a) MR (%) of H measured for the field from −9 to 9 T at 300 K. (b) MR (%) of H and D measured for the field from 0 to 9 T at 350, 300 and 250 K, respectively.
Fig. 3. (Color online) Magnetic properties of H (red) and D (blue). (a) The magnetic susceptibility as a function of temperature from 5 to 320 K, (b) magnetization curve (MH curve) for applied magnetic field from −5 to 5 T of H at 300, 10 and 5 K and (c) MH curve of D at 300, 10 and 5 K. Inset indicates the plots exhibiting a hysteresis loop observed in the field from −100 to 100 mT.
reasonably explains the behavior of MR (Fig. 2a) and anomalous Hall effect (ESI) exhibiting a weak asymmetric shape for positive and negative fields. There exist two different magnetic components in the loop: a ferromagnetic component immediately saturated for low field, 7100 mT, and a slowly saturating component from 7100 mT to 75 T. The latter component is significantly observed in H at 5 K. The mixed behavior is very similar to that of a (Ga, Mn)As system, where the latter is considered to be from a paramagnetic contribution and can be fitted by a modified Brillouin function with T and TAF [13]. Here we adopted the modified Brillouin function for the slowly saturating magnetic component, as shown in the solid curves in Fig. 3(b,c), and found that the behavior of the latter component is well fitted by the function. The fitting parameter TAF, which indicates average strength of antiferromagnetic coupling between noncontributing spins to ferromagnetic component, was determined to be 1.5 at 5 K, 10 at 10 K, and 300 at 300 K, in H and D, respectively. The ferromagnetic spin was set as S¼½, assuming a molecular spin. The fitting estimates that the ferromagnetic component (Mferro) is in 31% (5 K and 10 K) and 35% (300 K), and the paramagnetic component (Mpara) is in 69% (5 K and10 K) and 65% (300 K) with saturation magnetization (Ms) 26 emu/mol for H, and on the other hand, for D, Mferro is in 71% (5 K), 64% (10 K and 300 K), and Mpara is in 29% (5 K) and 36% (10 K and 300 K) with 14 emu/mol of Ms. This fitting revealed that the amount of ferromagnetic spins is almost the same in H and D, but the paramagnetic component induces a large difference reaching almost double the saturation magnetization magnitude, Ms. It is of great interest that the ionic part plays a critical role in controlling magnetic properties, particularly in the paramagnetic component, as was shown in the H/D isotope effect of Ms, which provides an insight into a possible mechanism of spinpolarized transport.
3.3. Impurity analysis The magnetic properties of the present semiconductors were obviously anomalous, exhibiting very similar behavior with the (Ga, Mn)As system [13], despite the absence of ferromagnetic metal elements. Of primary concern is the possibility of contamination of magnetic impurity in the sample, although the single crystals were recrystallized from organic solvents and inorganic impurities are normally excluded from the resultant recrystallized single crystal in this procedure. ICP-MS analysis was carried out for detecting a small amount of metallic impurity because of a common procedure for this purpose. An amount of 20 mg of the single crystals was used with ICP-MS for the presence of the ferromagnetic elements Fe, Ni, and Co, which could be possibly included in any compounds synthesized in an usual organic laboratory, and these elements were found in 3.6, 0.09, and below 0.03 ppm, respectively. It should be noted that the magnetic moment from the trace metallic impurity is two orders of magnitude lower than that of Ms in H and D. We further confirmed the purity of the sole single crystal by TOF-SIMS (Fig. 4). All molecular fragments were successfully observed with high intensity in the spectrum (Fig. 4(b)), and the magnetic ions were not confirmed (Fig. 4(d–f)). These analyses verifies the purity of the single crystals, and indicates whether the ferromagnetic component originates from purely organic elements. 3.4. ESR and FMR analyses The properties of molecular spins were examined by ESR and FMR using the roughly oriented single crystals used for the magnetic measurements (Fig. 3). Fig. 5(a) shows the ESR signals for H and D at 300 K and 123 K, and Fig. 5(b,c) displays their FMR signals at 300 K
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Y. Kobayashi et al. / Solid State Communications 165 (2013) 27–32
Fig. 4. (Color online) TOF-SIMS analysis of H. (a) Pictures for the measured sample and (b–f) TOF-SIMS spectra with mass number: (b) from 0 to 350, (c) from 350 to 1200, (d) from 53 to 59, which covers Fe mass number, (e) from 56 to 60, which covers Co mass number, (f) from 58 to 64, which covers Ni mass number. A light blue bar indicates a predicted peak for Fe, Co, and Ni, in (d), (e), and (f).
Fig. 5. (Color online) Properties of molecular spins in H (red) and D (blue). (a) ESR spectra of H in red and D in blue at 300 K (bold line) and 123 K (thin line), respectively. FMR spectra (b) at 300 K and (c) at 123 K.
and 123 K, respectively. The major signal is attributed to the TTF+COO− species with 2.00761 for H [8,14] and with 2.00739 for D at rt, and shows a difference in a normalized intensity between H and D (Fig. 5 (a)). The intensity of H is larger than that of D at each temperature, indicating a more localized character of the molecular spins in H. Moreover, in each sample, the intensity at the lower temperature is much larger than that at the higher temperature, showing an itinerancy of molecular spins with a finite activation energy. Fig. 5(b,c) displays the signals from 230 to 360 mT, where two components are observed: paramagnetic spins (major one) and ferromagnetic spins
(minor one). The minor signal seems to be a part of spin-wave resonance with a larger g-value than 2.00761, which is derived from magnetically ordered spins. The ratio between the major and minor signal cannot be estimated, because most of the spin wave is deduced to be incorporated in the major signal. The amount of ferromagnetic component is almost the same at 300 K and 123 K, which is also consistent with the magnetic properties. It shoud be noted here that FMR signal is not observed above Tc in DMS [15]. Therefore, Tc of this organic ferromagnetic system is over 300 K at least, which is consistent with the negative MR observed even at 350 K (Fig. 2(b)).
Y. Kobayashi et al. / Solid State Communications 165 (2013) 27–32
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Fig. 6. (Color Online) The transport properties of H (red) and D (blue). (a) DC conductivity from 120 K to 320 K measured with the four-probe method and (b) schematic diagram of the impurity band in the band gap between the valence band (VB) and the conduction band (CB).
3.5. Origin of the spin-polarized transport The MR behavior, MH curve, and FMR signal clearly demonstrate that the magnetotransport properties are very similar to those of DMS systems, though no magnetic ion is included in the materials. These results strongly suggest that the spin-polarized transport correlates with a magnetic-exchange interaction between free carriers and ferromagnetic spins considered in DMS [16]. It is reported that MR is larger at a low-doped insulating state than that at a heavily doped metallic state in (Ga,Mn)As system [13]. The impurity band is relatively localized with larger Ea at the former state, where strong electronic correlations are essential because the long-range Coulomb potentials contributing to the binding energy of an isolated impurity are dominant [9]. Therefore, the electronic state deduced from Ea is an important factor in understanding the magnetotransport in DMS. The DC conductivity and Ea of H and D are shown in Fig. 6(a). Although they are quite similar in absolute value; 0.16 S/cm (Ea ¼0.11 eV) for H and 0.18 S/cm (Ea ¼0.10 eV) for D, H provides a more localized feature for the impurity band as illustrated in Fig. 6(b), which is consistent with the isotope effect in MR (Fig. 2(b)) and ESR (Fig. 5(a)). We point out that the Ea of H is exactly the same as that of the (Ga,Mn) As system [9], which implies that the component composing the impurity band plays the same role as the Mn ion in the (Ga,Mn)As system, that is to say, leading to a spin-polarized transport. The lowest occupied molecular orbital (LUMO) providing the impurity level, which is the pair orbital of the singly occupied molecular orbital (SOMO) consisting of TTF+COO−NH2Ph as the dopant, may play the key role here. Thermally-activated one-electron excitation from the top of the valance band composed of closed-shell TTFCOO-NH3+Ph to the impurity band induces hole transport, which likely accompanies the ionic transformation as mentioned in the introduction section. The coupled transport between molecular spins and ions is a possible origin of the isotope effect in magnetotransport and magnetic properties, especially Ms, strongly suggesting carrier-mediated ferromagnetism in this system.
4. Conclusions The magnetotransport and magnetic properties of the single crystal organic ionic semiconductor, TTFCOONH3Ph, are very similar to those of DMS, despite the absence of ferromagnetic metal elements. Negative magnetoresistance (∼0.2%) is observed at rt and it increases with decreasing temperature. The MH curve with the hysteresis loop clarifies the ferromagnetic behavior with 26 emu/mol of saturation magnetization, composed of a ferromagnetic (35%) and a paramagnetic (65%) component at rt. ICP-MS
and TOF-SIMS analyses verify the purity of the single crystal. ESR reveals weak localization of paramagnetic molecular spins, and moreover, FMR confirms the existence of magnetically ordered spins in addition to the paramagnetic ones. The isotope effect observed in MR, Ea and Ms suggests a carrier-mediated mechanism for the ferromagnetism in this system.
Acknowledgments We acknowledge Prof. S. Maekawa (JAEA) and Prof. M. Mori (JAEA) for essential discussions on magnetic properties. We are also grateful for Dr. T. Fujii (The Univ. of Tokyo), Prof. T. Nakamura (IMS), and Dr. H. Kino (NIMS) for fruitful discussions on transport measurements, ESR measurements and theoretical aspects, respectively. We acknowledge Prof. I. Terasaki (Nagoya Univ.) and Prof. H. Fukuyama (Tokyo Science Univ.) for fruitful discussions in the early stage of this study. We thank Ms. M. Takahashi for an assistance of physical measurements. ESR measurements were carried out using facilities provided by Center of Materials Research for Low Carbon Emission in NIMS of “Low-Carbon Research Network”. This study was partially supported by ‘NEXT’ Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and Nano-Integration Foundry in NIMS. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ssc.2013.04.014. References [1] (a) A. Brataas, Nature 452 (2008) 419; (b) D.D. Awschalom, M.E. Flatte, Nature Phys 3 (2007) 153. [2] J. Heremans, J. Phys. D: App. Phys 26 (1993) 1149. [3] (a) T. Kimura, Y. Tokura, Ann. Rev., Mat. Sci. 30 (2000) 451; (b) A.J. Millis, Nature 392 (1998) 147; (c) A.P Ramirez, J. Phys. Condens. Matter 9 (1997) 8171. [4] (a) T. Dietl, Nature Mat. 9 (2010) 965; (b) T. Jungwirth, J. Sinova, J. Masek, J. Kucera, A.H. MacDonald, Rev. Mod. Phys. 78 (2006) 809; (c) R. Janisch, P. Gopal, N.A Spaldin, J. Phys. Condens. Matter 17 (2005) R657. [5] J.L. Martina, J.D. Bergeson, V.N. Prigodin, A.J. Epstein, Syn. Met. 160 (2010) 291. [6] P.A. Bobbert, T.D. Nguyen, F.W.A. van Oost, B. Koopmans, M. Wohlgenannt, Phys. Rev. Lett. 99 (2007) 216801. [7] B. Hu, Y. Wu, Nature Mat. 6 (2007) 985. [8] Y. Kobayashi, S. Sumi, T. Terauchi, D. Hashizume, Dalton Trans. 42 (2013) 3821. [9] T. Jungwirth, J. Sinova, A.H. MacDonald, B.L. Gallagher, V. Novák, K.W. Edmonds, A.W. Rushforth, R.P. Campion, C.T. Foxon, L. Eaves, E. Olejník, J. Mašek, S.-R. Eric Yang, J. Wunderlich, C. Gould, L.W. Molenkamp, T. Dietl, H. Ohno, Phys. Rev. B 76 (2007) 125206.
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[10] H.J. Choi, H.K. Seong, J. Chang, K.I. Lee, Y.J. Park, J.J. Kim, S.K. Lee, R. He, T. Kuykendall, P .Yang, Adv. Mater. 17 (2005) 1351. [11] (a) A. El-Ghayoury, C. Mézière, S. Simonov, L. Zorina, M. Cobián, E. Canadell, C. Rovira, B. Náfrádi, B. Sipos, L. Forró, P. Batail, Chem. Eur. 16 (2010) 14051; (b) S.K. Pal, M.E. Itkis, F.S. Tham, R.W. Reed, R.T. Oakley, R.C. Haddon, Science 309 (2005) 281. [12] K. Kanoda, J. Phys. Soc. Jpn. 75 (2006) 051007.
[13] A. Oiwa, S. Katsumoto, A. Endo, M. Hirasawa, Y. Iye, H. Ohno, F. Matsukura, A. Shen, Y. Sugawara, Solid State Commun. 103 (1997) 209. [14] K. Furukawa, T. Nakamura, Y. Kobayashi, T. Ogura, J. Phys. Soc. Jpn. 79 (2010) 053701. [15] R. Mark, A. Hanbicki, P. Lubitz, M. Osofsky, J.J. Krebs, B. Jonker, J. Mag. Mag. Mat. 250 (2002) 164. [16] H. Akai, Phys. Rev. Lett. 81 (1998) 3002.
Contents lists available at SciVerse ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Negative magnetoresistance in organic ionic semiconductor: TTFCOONH3Ph Yuka Kobayashi n, Satoshi Sumi, Takeshi Terauchi, Hideo Iwai National Institute for Materials Science (NIMS), Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 24 January 2013 Received in revised form 4 April 2013 Accepted 9 April 2013 by Z. Tang Available online 19 April 2013
TTFCOONH3Ph is a recently synthesized open-shell ionic semiconductor, the electronic state of which differs from that of typical organic closed-shell semiconductors. Magnetotransport properties were examined using a single-crystal sample, and found to exhibit small negative magnetoresistance (∼0.2%) for 9 T at room temperature (rt). The magnetization curve verifies the existence of a ferromagnetic (35%) and a paramagnetic (65%) component at rt, which is very similar to that of diluted magnetic semiconductors, despite the absence of any ferromagnetic metal elements. Electron spin resonance reveals weak localization of paramagnetic molecular spins, and moreover, ferromagnetic resonance confirms the existence of magnetically ordered spins in addition to the paramagnetic ones. The origin of the spin-polarized transport is discussed. & 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Organic ionic semiconductor D. Ferromagnetism D. Magnetotransport
1. Introduction Magnetotransport attracts a lot of attention not only from an academic viewpoint, but also because of its potential spintronics applications [1]. Negative magnetoresistance is observed in specific materials with ferromagnetic components, for example, Fe–Ni based permalloys [2], magnetic transition metal oxides [3], and III–V semiconductors with magnetic impurity, the so-called diluted magnetic semiconductors (DMS) [4] etc., where electron (hole) transport couples with the ferromagnetic properties of coexisting magnetic components. Recently, some nonmagnetic organic light-emitting diode (OLED) materials have been reported to exhibit fairly large negative magnetoresistance (MR) up to 10% for small magnetic fields of less than 100 mT at room temperature (rt) [5]. The mechanisms for this effect are believed to differ from the abovementioned inorganic magnetic systems: magnetic-field-dependent transport may be caused by a mixing of singlet and triplet excitons [5], by bipolarons [6], or by generation of secondary charge carriers [7], etc. Anilinium tetrathiafulvalene-2-carboxylate (TTFCOONH3Ph) is an organic ionic semiconductor composed purely of light elements, and its electronic state is more similar to doped inorganic semiconductors such as III–V semiconductors or doped transition metal oxides than to that of OLED-type closed-shell semiconductors [8]. It is simply stated, a band insulator with a 1.31 eV optical bandgap, the absolute value of which is comparable to that of GaAs [9], but it is doped by a partial inclusion of an open-shell TTF+COO−NH2Ph component in a closed-
shell TTFCOO−NH3+Ph component, providing an acceptor (impurity) level for the emergence of semiconducting properties [8] (Fig. 1(a)). The intrinsic dopant of this system is spontaneously included a protonic defect in an ammonium ion, namely, the PhNH2 species, as schematically shown in Fig. 1(b). The direct current (DC) conductivity of the single crystal exhibits semiconducting behavior with 0.16 S/cm at rt with an activation energy (Ea) of 0.11 eV [8]. The charge transport of the material is quite unique because the hopping movement of hole carriers is likely associated with dielectric relaxation; the ionic transformation, PhNH2↔PhNH3+ is inevitable in the hole-conduction process TTF+COO−↔TTFCOO− because of the suppression of electrostatic repulsion. Single-crystal X-ray crystallographic analysis has revealed that the conducting TTFCOO− and TTF+COO− moieties are self-assembled in one dimension and are in supramolecular association with a salt bridge (charged hydrogen bond). Herein we report on the magnetotransport of single crystal TTFCOONH3Ph. Examinations on the constitution and properties of the molecular spin reveal that the origin of the spin-polarized hole transport is related to the anomalous magnetic properties of this material. The isotope-substituted TTFCOOND3C6D5 provides a clear insight into a possible mechanism of the magnetotransport properties in these systems.
2. Experimental methods 2.1. Sample preparation and characterization
n
Corresponding author. Tel.: +81 29 859 2154; fax: +81 29 859 2101. E-mail address: [email protected] (Y. Kobayashi).
0038-1098/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2013.04.014
A single crystal of TTFCOONH3Ph (H) was grown by recrystallization from n-hexane/ethyl acetate, following a procedure reported
28
Y. Kobayashi et al. / Solid State Communications 165 (2013) 27–32
Fig. 1. Schematic diagram of the organic ionic semiconductor, TTFCOONH3Ph. (a) Band structure with an acceptor level, (b) molecular arrangement self-assembled by one-dimensional salt-bridge network, where the protonic defects are partly included.
previously [8]. The isotope-substituted TTFCOOND3C6D5 (D) was synthesized using commercially available deuterated aniline, C6D5ND2, and its single crystal was grown as H. Inductively coupled plasma mass spectrometry (ICP-MS) measurements were carried out for single crystals of 20 mg using Agilent 4500 (Agilent Technologies). Time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements were carried out for single crystals set on a carbon sheet using PHI TRIFT V nanoTOF (ULVAC-PHI). 2.2. Physical measurements Magnetotransport measurements for a single crystal were carried out with the four-probe method using a liquid helium cryostat in the temperature range 120–350 K and up to 79 T with a physical properties measurement system (Quantum Design PPMS-9). The magnetic field was vertically applied to a single crystal with four probes in a line. The dimensions of the single crystal are very small, ca. 0.02 0.2 0.9 mm3, with a magnetic moment that is too small to be detected as a sole crystal even with the high-resolution magnetometer. Therefore, single crystals of 0.4 mg were roughly oriented on a nonmagnetic polymer sheet by hand for the magnetic measurements. Magnetic susceptibility was measured with 0.1 T of the applied field as a function of temperature from 5 to 320 K. These measurements were performed in the temperature range 5–320 K using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-7) and a field up to 75 T. Diamagnetic correction was done by Pascal's constant and the subtraction of a linear diamagnetic component. Electron spin resonance (ESR) and ferromagnetic resonance (FMR) measurements were performed using the oriented single crystals at 300 K and 123 K using an electron spin resonance spectrometer (JEOL JES-FA100).
3. Results and discussion 3.1. Magnetotransport properties Fig. 2(a) shows the MR of the H single crystal, where {[(ρM/ρ0) −1]n100} (%) (ρΜ: resistivity for an applied field, ρ0: resistivity for 0 T) values are plotted for the magnetic field from −9 T to 9 T at 300 K. The MR effect is observed at 300 K, but the value is less than 0.2% for 9 T, which is much smaller than the values of OLED semiconductors reaching 10% for low field at the mT scale [6]. Moreover, the negative MR increases almost linearly with the applied magnetic field, the behavior of which is totally different from that of OLED semiconductors, which is represented by an
empirical law: MR∝B2/(|B|+B0)2 or B2/(B2+B02), where B is a magnetic field and B0≈5 mT [6]. We further measured the MR of the D sample in addition to that of the H sample at 350, 300 and 250 K as shown in Fig. 2(b), and found that it increases with decreasing temperature in both cases and exhibits a small isotope effect; MR of D is relatively smaller than that of H at all temperatures. The temperature and field dependence of MR in H and D are quite similar to those of DMS systems doped with magnetic ions; for example, a single crystal Mn-doped GaN (with Mn concentrations: 1–9%) [10]. Although temperature dependence of MR in H down to 120 K was further measured to understand the magnetotransport properties as shown in an electronic supplementary information (ESI), quite large errors occur in the measurement, probably due to coexisting non-negligible ionic transformation, PhNH2↔PhNH3+ at lower temperatures [8]. Anomalous Hall effect of H was also measured at 300 and 160 K (ESI). It seems to respond to the applied field, but the behavior could not be explained completely by the experiment. The H and D do not exhibit remarkable difference in their absolute values to positive and negative fields in either MR or anomalous Hall effect. The weak asymmetric behavior to the fields in the magnetotransport may correlate with the magnetic properties of this system. 3.2. Magnetic properties The magnetic susceptibility of the roughly oriented single crystals of H and D is shown in Fig. 3(a). The absolute value of the susceptibility is 6.0 10−3 emu/mol for H and 8.5 10−3 emu/mol for D at rt, and 8.5 10−3 emu/mol for both samples at 5 K, although these values include errors caused by incompleteness of the alignment of the single crystals. They exhibit weak temperature dependence, which is not similar to the characteristic magnetic properties superimposed paramagnetic and ferromagnetic (impurity Curie component) behavior generally observed in organic radical (semi) conductors [11]. We point out that the absolute value at rt was one order of magnitude higher than that of strongly correlated organic conductors [12] and at the highest level in all organic electroactive materials as far as we know. Another possibility for describing the temperature dependence is ferromagetism with a higher Tc above rt. The magnetization curve of H and D for applied magnetic field (MH curve) from −5 to 5 T at 300, 10 and 5 K, respectively, is shown in Fig. 2(b,c). A hysteresis loop is observed in the inset which is zoomed in 7100 mT, as is predicted by the weak temperature dependence. We point out that the behavior is, however, originally anomalous because this material is a pure organic compound without any ferromagnetic metal elements. The shape of the loop further reveals that ferromagnetic spins possess small coercive force, which
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Fig. 2. Magnetotransport properties. (a) MR (%) of H measured for the field from −9 to 9 T at 300 K. (b) MR (%) of H and D measured for the field from 0 to 9 T at 350, 300 and 250 K, respectively.
Fig. 3. (Color online) Magnetic properties of H (red) and D (blue). (a) The magnetic susceptibility as a function of temperature from 5 to 320 K, (b) magnetization curve (MH curve) for applied magnetic field from −5 to 5 T of H at 300, 10 and 5 K and (c) MH curve of D at 300, 10 and 5 K. Inset indicates the plots exhibiting a hysteresis loop observed in the field from −100 to 100 mT.
reasonably explains the behavior of MR (Fig. 2a) and anomalous Hall effect (ESI) exhibiting a weak asymmetric shape for positive and negative fields. There exist two different magnetic components in the loop: a ferromagnetic component immediately saturated for low field, 7100 mT, and a slowly saturating component from 7100 mT to 75 T. The latter component is significantly observed in H at 5 K. The mixed behavior is very similar to that of a (Ga, Mn)As system, where the latter is considered to be from a paramagnetic contribution and can be fitted by a modified Brillouin function with T and TAF [13]. Here we adopted the modified Brillouin function for the slowly saturating magnetic component, as shown in the solid curves in Fig. 3(b,c), and found that the behavior of the latter component is well fitted by the function. The fitting parameter TAF, which indicates average strength of antiferromagnetic coupling between noncontributing spins to ferromagnetic component, was determined to be 1.5 at 5 K, 10 at 10 K, and 300 at 300 K, in H and D, respectively. The ferromagnetic spin was set as S¼½, assuming a molecular spin. The fitting estimates that the ferromagnetic component (Mferro) is in 31% (5 K and 10 K) and 35% (300 K), and the paramagnetic component (Mpara) is in 69% (5 K and10 K) and 65% (300 K) with saturation magnetization (Ms) 26 emu/mol for H, and on the other hand, for D, Mferro is in 71% (5 K), 64% (10 K and 300 K), and Mpara is in 29% (5 K) and 36% (10 K and 300 K) with 14 emu/mol of Ms. This fitting revealed that the amount of ferromagnetic spins is almost the same in H and D, but the paramagnetic component induces a large difference reaching almost double the saturation magnetization magnitude, Ms. It is of great interest that the ionic part plays a critical role in controlling magnetic properties, particularly in the paramagnetic component, as was shown in the H/D isotope effect of Ms, which provides an insight into a possible mechanism of spinpolarized transport.
3.3. Impurity analysis The magnetic properties of the present semiconductors were obviously anomalous, exhibiting very similar behavior with the (Ga, Mn)As system [13], despite the absence of ferromagnetic metal elements. Of primary concern is the possibility of contamination of magnetic impurity in the sample, although the single crystals were recrystallized from organic solvents and inorganic impurities are normally excluded from the resultant recrystallized single crystal in this procedure. ICP-MS analysis was carried out for detecting a small amount of metallic impurity because of a common procedure for this purpose. An amount of 20 mg of the single crystals was used with ICP-MS for the presence of the ferromagnetic elements Fe, Ni, and Co, which could be possibly included in any compounds synthesized in an usual organic laboratory, and these elements were found in 3.6, 0.09, and below 0.03 ppm, respectively. It should be noted that the magnetic moment from the trace metallic impurity is two orders of magnitude lower than that of Ms in H and D. We further confirmed the purity of the sole single crystal by TOF-SIMS (Fig. 4). All molecular fragments were successfully observed with high intensity in the spectrum (Fig. 4(b)), and the magnetic ions were not confirmed (Fig. 4(d–f)). These analyses verifies the purity of the single crystals, and indicates whether the ferromagnetic component originates from purely organic elements. 3.4. ESR and FMR analyses The properties of molecular spins were examined by ESR and FMR using the roughly oriented single crystals used for the magnetic measurements (Fig. 3). Fig. 5(a) shows the ESR signals for H and D at 300 K and 123 K, and Fig. 5(b,c) displays their FMR signals at 300 K
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Fig. 4. (Color online) TOF-SIMS analysis of H. (a) Pictures for the measured sample and (b–f) TOF-SIMS spectra with mass number: (b) from 0 to 350, (c) from 350 to 1200, (d) from 53 to 59, which covers Fe mass number, (e) from 56 to 60, which covers Co mass number, (f) from 58 to 64, which covers Ni mass number. A light blue bar indicates a predicted peak for Fe, Co, and Ni, in (d), (e), and (f).
Fig. 5. (Color online) Properties of molecular spins in H (red) and D (blue). (a) ESR spectra of H in red and D in blue at 300 K (bold line) and 123 K (thin line), respectively. FMR spectra (b) at 300 K and (c) at 123 K.
and 123 K, respectively. The major signal is attributed to the TTF+COO− species with 2.00761 for H [8,14] and with 2.00739 for D at rt, and shows a difference in a normalized intensity between H and D (Fig. 5 (a)). The intensity of H is larger than that of D at each temperature, indicating a more localized character of the molecular spins in H. Moreover, in each sample, the intensity at the lower temperature is much larger than that at the higher temperature, showing an itinerancy of molecular spins with a finite activation energy. Fig. 5(b,c) displays the signals from 230 to 360 mT, where two components are observed: paramagnetic spins (major one) and ferromagnetic spins
(minor one). The minor signal seems to be a part of spin-wave resonance with a larger g-value than 2.00761, which is derived from magnetically ordered spins. The ratio between the major and minor signal cannot be estimated, because most of the spin wave is deduced to be incorporated in the major signal. The amount of ferromagnetic component is almost the same at 300 K and 123 K, which is also consistent with the magnetic properties. It shoud be noted here that FMR signal is not observed above Tc in DMS [15]. Therefore, Tc of this organic ferromagnetic system is over 300 K at least, which is consistent with the negative MR observed even at 350 K (Fig. 2(b)).
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Fig. 6. (Color Online) The transport properties of H (red) and D (blue). (a) DC conductivity from 120 K to 320 K measured with the four-probe method and (b) schematic diagram of the impurity band in the band gap between the valence band (VB) and the conduction band (CB).
3.5. Origin of the spin-polarized transport The MR behavior, MH curve, and FMR signal clearly demonstrate that the magnetotransport properties are very similar to those of DMS systems, though no magnetic ion is included in the materials. These results strongly suggest that the spin-polarized transport correlates with a magnetic-exchange interaction between free carriers and ferromagnetic spins considered in DMS [16]. It is reported that MR is larger at a low-doped insulating state than that at a heavily doped metallic state in (Ga,Mn)As system [13]. The impurity band is relatively localized with larger Ea at the former state, where strong electronic correlations are essential because the long-range Coulomb potentials contributing to the binding energy of an isolated impurity are dominant [9]. Therefore, the electronic state deduced from Ea is an important factor in understanding the magnetotransport in DMS. The DC conductivity and Ea of H and D are shown in Fig. 6(a). Although they are quite similar in absolute value; 0.16 S/cm (Ea ¼0.11 eV) for H and 0.18 S/cm (Ea ¼0.10 eV) for D, H provides a more localized feature for the impurity band as illustrated in Fig. 6(b), which is consistent with the isotope effect in MR (Fig. 2(b)) and ESR (Fig. 5(a)). We point out that the Ea of H is exactly the same as that of the (Ga,Mn) As system [9], which implies that the component composing the impurity band plays the same role as the Mn ion in the (Ga,Mn)As system, that is to say, leading to a spin-polarized transport. The lowest occupied molecular orbital (LUMO) providing the impurity level, which is the pair orbital of the singly occupied molecular orbital (SOMO) consisting of TTF+COO−NH2Ph as the dopant, may play the key role here. Thermally-activated one-electron excitation from the top of the valance band composed of closed-shell TTFCOO-NH3+Ph to the impurity band induces hole transport, which likely accompanies the ionic transformation as mentioned in the introduction section. The coupled transport between molecular spins and ions is a possible origin of the isotope effect in magnetotransport and magnetic properties, especially Ms, strongly suggesting carrier-mediated ferromagnetism in this system.
4. Conclusions The magnetotransport and magnetic properties of the single crystal organic ionic semiconductor, TTFCOONH3Ph, are very similar to those of DMS, despite the absence of ferromagnetic metal elements. Negative magnetoresistance (∼0.2%) is observed at rt and it increases with decreasing temperature. The MH curve with the hysteresis loop clarifies the ferromagnetic behavior with 26 emu/mol of saturation magnetization, composed of a ferromagnetic (35%) and a paramagnetic (65%) component at rt. ICP-MS
and TOF-SIMS analyses verify the purity of the single crystal. ESR reveals weak localization of paramagnetic molecular spins, and moreover, FMR confirms the existence of magnetically ordered spins in addition to the paramagnetic ones. The isotope effect observed in MR, Ea and Ms suggests a carrier-mediated mechanism for the ferromagnetism in this system.
Acknowledgments We acknowledge Prof. S. Maekawa (JAEA) and Prof. M. Mori (JAEA) for essential discussions on magnetic properties. We are also grateful for Dr. T. Fujii (The Univ. of Tokyo), Prof. T. Nakamura (IMS), and Dr. H. Kino (NIMS) for fruitful discussions on transport measurements, ESR measurements and theoretical aspects, respectively. We acknowledge Prof. I. Terasaki (Nagoya Univ.) and Prof. H. Fukuyama (Tokyo Science Univ.) for fruitful discussions in the early stage of this study. We thank Ms. M. Takahashi for an assistance of physical measurements. ESR measurements were carried out using facilities provided by Center of Materials Research for Low Carbon Emission in NIMS of “Low-Carbon Research Network”. This study was partially supported by ‘NEXT’ Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and Nano-Integration Foundry in NIMS. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ssc.2013.04.014. References [1] (a) A. Brataas, Nature 452 (2008) 419; (b) D.D. Awschalom, M.E. Flatte, Nature Phys 3 (2007) 153. [2] J. Heremans, J. Phys. D: App. Phys 26 (1993) 1149. [3] (a) T. Kimura, Y. Tokura, Ann. Rev., Mat. Sci. 30 (2000) 451; (b) A.J. Millis, Nature 392 (1998) 147; (c) A.P Ramirez, J. Phys. Condens. Matter 9 (1997) 8171. [4] (a) T. Dietl, Nature Mat. 9 (2010) 965; (b) T. Jungwirth, J. Sinova, J. Masek, J. Kucera, A.H. MacDonald, Rev. Mod. Phys. 78 (2006) 809; (c) R. Janisch, P. Gopal, N.A Spaldin, J. Phys. Condens. Matter 17 (2005) R657. [5] J.L. Martina, J.D. Bergeson, V.N. Prigodin, A.J. Epstein, Syn. Met. 160 (2010) 291. [6] P.A. Bobbert, T.D. Nguyen, F.W.A. van Oost, B. Koopmans, M. Wohlgenannt, Phys. Rev. Lett. 99 (2007) 216801. [7] B. Hu, Y. Wu, Nature Mat. 6 (2007) 985. [8] Y. Kobayashi, S. Sumi, T. Terauchi, D. Hashizume, Dalton Trans. 42 (2013) 3821. [9] T. Jungwirth, J. Sinova, A.H. MacDonald, B.L. Gallagher, V. Novák, K.W. Edmonds, A.W. Rushforth, R.P. Campion, C.T. Foxon, L. Eaves, E. Olejník, J. Mašek, S.-R. Eric Yang, J. Wunderlich, C. Gould, L.W. Molenkamp, T. Dietl, H. Ohno, Phys. Rev. B 76 (2007) 125206.
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