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Extraordinary Hall-effect in colloidal magnetic nanoparticle films
Author’s Accepted Manuscript Extraordinary Hall-effect in colloidal magnetic nanoparticle films Leah Ben Gur, Einat Tirosh, Amir Segal, Gil Markovich, Alexander Gerber www.elsevier.com/locate/jmmm
PII: DOI: Reference:
S0304-8853(16)32248-X http://dx.doi.org/10.1016/j.jmmm.2016.11.106 MAGMA62181
To appear in: Journal of Magnetism and Magnetic Materials Received date: 18 September 2016 Revised date: 17 November 2016 Accepted date: 17 November 2016 Cite this article as: Leah Ben Gur, Einat Tirosh, Amir Segal, Gil Markovich and Alexander Gerber, Extraordinary Hall-effect in colloidal magnetic nanoparticle f i l m s , Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.11.106 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Extraordinary Hall-effect in colloidal magnetic nanoparticle films Leah Ben Gur,a‡ Einat Tirosh,a‡ Amir Segal,b Gil Markovicha* and Alexander Gerberb* a
School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 6997801, Israel. E-mail: [email protected]. b
School of Physics, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 6997801, Israel. E-mail:. ‡ These authors contributed equally. * E-mails: [email protected], [email protected]
Abstract Colloidal nickel nanoparticles (NPs) coated with polyvinylpyrrolidone (PVP) were synthesized. The nanoparticle dispersions were deposited on substrates and dried under mild heating to form conductive films. The films exhibited very small coercivity, nearly metallic conductivity, and a significant extraordinary Hall effect signal. This method could be useful for preparing simple, printed magnetic field sensors with the advantage of relatively high sensitivity around zero magnetic field, in contrast to magnetoresistive sensors, which have maximal field sensitivity away from zero magnetic field.
Keywords: Extraordinary Hall effect, magnetic nanoparticles, magnetic sensor, printed electronics
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1. Introduction The recent development of printed electronics technology is motivated by the promise of low-cost, high volume, high-throughput production of electronic components or devices that are light-weight and small, thin and flexible [1], inexpensive and disposable. Printed electronics is not a substitute for conventional silicon-based electronics with a high integration density and switching speed, but is oriented towards low-cost and high volume market segments where the high performance of conventional electronics is not required [2,3]. Printed electronics also enables the use of advanced nanoscale materials and organic materials for special functionalities and electronic devices [4], such as a various types of sensors [5,6], organic photovoltaics [7], and displays [8]. Printed electronics is very general and includes not only printable interconnects, but also optoelectronics and magnetoelectronics. Printed magnetoelectronics requires development of materials and devices sensitive to magnetic field, similar to the modern silicon-based magnetoelectronics. Recent developments in magnetoelectronics relate to the field of spintronics and, in particular, to the effects of giant magnetoresistance (GMR) and tunnelling magnetoresistance (TMR). Both GMR and TMR phenomena arise due to sensitivity of the spin dependent conductivity in heterogeneous magnetic systems to the relative orientation of the local magnetic moments: high conductivity for parallel moments and low for antiparallel. The specific mechanisms of GMR and TMR are different: GMR is due to spin-dependent scattering in metallic heterogeneous ferromagnets, multilayers and granular mixtures, where non-magnetic normal metal separates between ferromagnetic
2
regions, while TMR is due to the spin dependent tunneling across insulating spacers.
The fabrication of printable ink with GMR properties has been
recently reported [9-11]. In this approach GMR multilayers are fabricated by conventional vacuum deposition, detached from the substrate, ball milled to form micron size flakes and mixed with solvents to form an ink or paste. The dried ink preserves the GMR properties of multilayer flakes and could be used for magnetic field sensing. In this paper, we report on development of printable field sensitive magnetic films operating on a different spin dependent phenomenon: the extraordinary Hall effect (EHE). This effect, well known in ferromagnetic materials, generates a voltage proportional to the magnetization across a current carrying magnetic film. The origin of EHE is spin-orbit scattering that breaks the spatial symmetry of scattered electrons. In the cases of our interest the EHE contribution exceeds significantly the ordinary Hall effect, and the Hall voltage VH can be presented as: (1) where RH, I, t,
EHE,
and M represent the Hall resistance, applied current, film
thickness, extraordinary Hall coefficient and magnetization normal to the film plane, respectively. VH is directly proportional to the magnetization M of the material and as such can be used for sensing magnetic field. Despite being known for more than a century, the effect was not considered seriously for technological development because of its relatively small value in bulk magnetic materials. Recently developed materials demonstrate dramatic enhancement of the EHE magnitude, which makes it attractive for a range of applications [12]. Magnetic field sensitivity of 106 /T, which is three orders 3
of magnitude higher than the best sensitivity achieved in semiconducting Hall materials, has been reported in partially oxidized CoFeB films [13]. An important feature of EHE, particularly relevant for printing technology, is that the EHE signal increases with increasing resistivity. Spin-orbit scattering at the lattice defects, impurities, surfaces, interfaces, phonons and magnons, all contribute to the EHE. Reduction of film thickness or artificial addition of an insulating material into the bulk of ferromagnets increases the EHE resistivity up to four orders of magnitude [14]. Thus, an imperfect structure of magnetic films fabricated by printing techniques is not expected to impede their performance if the magnetic properties of the material are preserved. In particular, granular ferromagnets fabricated by co-deposition of mutually immiscible ferromagnetic metal and an insulator exhibit a dramatic enhancement of the extraordinary Hall effect when the concentration of the metallic component approaches the critical value of the percolation threshold. The phenomenon was called the giant extraordinary Hall effect [14] as the EHE resistivity found in Ni-SiO2 mixtures reached 100 µΩcm, which is a factor up to 104 larger than in pure Ni metal. Similar values were found in other granular ferromagnet-insulator systems, like Co-SiO2 [15], NiFe-SiO2 [16], and Fe-SiO2 [17]. It was also found that the ordinary Hall effect coefficient shows an orders of magnitude growth similar to the EHE. The effect was found not only in magnetic Ni-SiO2 [18] but also in non-magnetic granular systems Cu-SiO2 [19], and Mo-SnO2 [20] on the metallic side of the percolation threshold. Theoretical interpretations of the effect in granular composites were hindered by considerable problems. Classical percolation theory predicts a divergence of the Hall effect at the percolation threshold in three dimensions, but the observed
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values are much greater than the estimate of this theory for finite sample thickness [18]. A local quantum interference theory was suggested by Wan and Sheng [21], in which the presence of small insulating substructures along an infinite metallic cluster leads to profound wave scattering and interference, thus causing significant reduction of the effective carrier density. The model is only valid at low temperatures when quantum corrections (weak localization/ electron-electron interaction) are valid, and cannot explain the "Giant Hall effect" at room temperature. Synthesis methods for the preparation of colloidal magnetic nanoparticles (NPs) has seen a significant progress in the last two decades, in terms of size and composition control [22,23], as well as molecular coating [24], with strong emphasis on biological applications [25,26]. In addition, thin film preparation techniques have received substantial attention [27]. In the present work we have prepared granular Ni films from NP inks which, after drying under controlled conditions, exhibited significant EHE signals. Ni NPs were selected due to the fairly low oxidation of the Ni on exposure to air, relative to other magnetic metals such as Co and Fe, and the relative ease of reduction of the oxide to the metal form. In addition, Ni is a relatively soft magnetic material, which will ensure low coercivity of the NP-based magnetic sensor, increasing its low field sensitivity. 2. Experimental methods 2.1 Ni nanoparticles Preparation Nickel NPs were synthesized by a modified method of borohydride reduction in ethylene glycol [28]. First, the precursor, nickel chloride hexahydrate (4 mmol) and 0.013 mmol of the polymeric surfactant poly(N-vinyl-2-pyrrolidone) of molecular
5
weight of 40,000 g/mol (PVP40) or 3,500 g/mol (PVP3.5) were dissolved in ethylene glycol (16 mL) sequentially, resulting in a green solution. Then, hydrazine hydrate (30 mmol) was added, in order to form a Ni2+-hydrazine complex (a purple solution). The temperature was set to 40C, and after about 40 minutes the solution became bluish-purple. The color of the solution indicates the formation of a [Ni(N2H4)3]+22Cl complex, which is different from the unwanted [Ni(N2H4)6]+22Cl complex. The latter is unstable and decomposes to [Ni(N2H4)4-5]+22Cl [29]. This is why the temperature at this stage is crucial. Finally, the reducing agent, sodium borohydride (13 mmol, dissolved in 700μL of H2O), was added under nitrogen atmosphere, in order to reduce the nickel ions to their metallic state. The solution was stirred for 1 hour until turning black due to formation of nickel NPs. Subsequently, the flask was cooled to room temperature, and the NPs were isolated through centrifugation with ethanol (3,500 rpm for 5 minutes) and re-dispersed in ethylene glycol. The product was kept in a glove box to minimize oxidation. 2.2 Thin film preparation and drying In order to make ~1 m thick Ni NPs thin films (after drying), we have used a bar coating device. The device was built with two spacers that can be replaced, depending on the desired thickness of the wet film. The height of the spacers correlates to the height of the spread solution in a non-trivial manner, depending on the viscosity and surface tension of the NP dispersion as well as the wetting properties of the bar or blade used for the spreading. There is a designated location to place the substrate between the spacers, where vacuum suction keeps the substrate in place. Then, a metal blade was used to spread the concentrated Ni nanoparticle (~30% by weight in ethylene glycol) film on the glass substrate. Since the ink wetted the blade, a residue of ink was left on the blade during the coating process. 6
The thin Ni NP films were dried in a vacuum oven with base pressure of the order of 10 millibars and temperature of ~100C for a period of 15 hours. Typically, a low pressure (about 100 millibar) 5/95% gas mixture of H2/N2 was flown trough the vacuum oven during the first several hours of drying. 2.3 Characterization The synthesized NP dispersions were deposited on carbon coated copper grids and left to dry for transmission electron microscopy (TEM) characterization, using a Technai F20 microscope. Magnetic characterization by a Superconducting Quantum Interference Device (SQUID) magnetometer: The magnetic NPs sample was prepared for magnetization measurements by precipitating the nanoparticles from the ethylene glycol with added ethanol and centrifuging at 5000 rpm for 15 minutes. Then the precipitate was separated from the supernatant and dried to a bulk powder by heating at 100C under a vacuum of ~ 10 mbar. 2.4 EHE measurements For electrical measurements, the Ni NP films deposited on glass slides and dried, were contacted by 4 copper wires using silver paint. The measurements were conducted using a 2T water-cooled electromagnet applying an external magnetic field perpendicular to the substrate. Resistivity, magnetoresistance and Hall effect were measured using the Van der Pauw protocol. The transverse (Hall) signal is antisymmetric with respect to external field, whereas the longitudinal (resistivity) signal is symmetric. Reverse magnetic field reciprocity protocol was used to separate the measured data into symmetric and anti-symmetric parts [30].
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3. Results and discussion The synthesis resulted in Ni NPs in the size range of 3-20 nm (as seen in Figure 1a). Each particle consisted of a single Ni crystal and high resolution TEM images confirmed that the NPs had the bulk FCC Ni structure (see Supplementary material). The nickel NP dispersion in ethylene glycol was spread on glass substrates by a bar coating device and dried under low vacuum and mild heating (see Supplementary material). The dried nickel NP films had a typical thickness of the order of 1 m and roughness (peak values) on the same scale, as measured by profilometry (see Supplementary material).
(a) (b)
20 nm Fig. 1 (a) Transmission electron micrograph of the PVP40 coated Ni NPs. (b) A Ni NP film wired for EHE measurement.
Mild heating (100C) of the film while drying under low vacuum conditions was found important for obtaining metallic conductance, even without a reducing H2 atmosphere, under residual air pressure. XPS measurements of the NP films with PVP40 coating (PVP with molecular weight of 40,000) with and without heating to 100C while drying revealed that the level of oxidation of the NPs did not increase, and even slightly decreased, in the heated film relative to the unheated one (see Supplementary material). Deconvolution of the Ni XPS peaks corresponding to the metallic and NiO species, provided an estimate for the fraction of oxidized nickel in the films of roughly 20-30% out of the total amount of Ni atoms. Such a fraction of 8
oxidized Ni would be responsible for an oxide layer thickness of about 0.350.55 nm for a 10 nm diameter NP. Figure 2 displays the magnetization curve of a PVP40 coated Ni NP powder dried at 100C under vacuum from the ethylene glycol dispersion of the NPs. The magnetization curve was fitted with a Langevin function describing the alignment of an ensemble of paramagnetic (superparamagnetic in the present case) magnetic moments with an external magnetic field H, at temperature T: [
(
)
]
(2)
where x=μ/(kBT), kB is Boltzmann's constant, and 0 is the vacuum permeability. A closer look at the low field part of the magnetization curve did show a small coercivity, probably originating in strongly interacting or even sintered nanoparticles within the film.
Magnetization Fit to Langevin function
30
M (emu/g)
15
0
-15
-150
-30
-100
-50
0
50
100
150
0H (mT)
-2
-1
0
0H (T)
1
2
Fig. 2 Room temperature magnetization of PVP40 coated Ni NP powder as a function of applied magnetic field (symbols). The solid line is a fit of the data by the Langevin function (Eq. 2). The inset is an expanded view around H=0, showing a small coercivity of 10 mT.
The effective NP's magnetic moment ( ) value extracted from the Langevin fit was divided by the average number of Ni atoms per particle (average diameter of 10 nm in this case) and yielded an average magnetic moment of 0.3 B per Ni atom, which is equivalent to 28 emu/g. 9
The bulk saturation magnetization value of nickel is ~0.6 B per atom (55 emu/g). The lower value obtained from the Langevin function fit could be partially attributed to the thin oxide layer around the NPs, as NiO is antiferromagnetic. Remarkably, in spite of the vast differences between
continuous
ferromagnetic films and our NP films, the NP films exhibited a significant EHE signal with a fairly high signal to noise ratio, as shown in Figure 3. About 50 samples of PVP coated Ni NP films were produced and they all exhibited measurable EHE, with Hall resistance (VH/I) values at magnetization saturation of 1-10 m, measured using currents in the range of 1-10 mA. EHE curve saturation varied between ~0.2-0.6 T (depending on average NP size and drying conditions). The slope of the Hall resistance vs. field curves around zero-field is a measure of the sensitivity of the EHE for low magnetic field sensing. The slope values obtained for the various Ni NP films near zero-field were in the range of 10-100 m/T. The EHE vs. field curves were typically linear within a field range of about 50-100 mT.
4
-RH (m
2
0
-2
V I
-4 -2.0
-1.5
-1.0
-0.5
0.0
0.5
H
1.0
I+
1.5
V+ 2.0
0H (T)
Fig. 3 EHE vs. magnetic field measurement of ~10 nm PVP40 coated Ni NP film, performed at room temperature. The inset shows the measurement configuration where 4 contacts are made with silver paint and a Van der Paw technique is used to extract the Hall voltage.
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The NP "paste" based Ni films can be compared favourably with those fabricated by vacuum deposition. Typical sheet resistance values for the Ni NP films were in the range of 10-100 Ω. Assuming that the thickness of our films is 1 m, their resistivity is in the range 103 – 104 µcm, compared with 15-20 µcm in thick polycrystalline Ni films fabricated by e-beam deposition. The EHE resistivity, defined as: ρEHE=VHt/I in magnetically saturated state at high field is in the range of 0.1 – 1.0 µcm, which is equal or higher than ρEHE= 0.1 µcm observed in 100 nm thick vacuum deposited Ni films [31]. The magnitude of the EHE resistivity found in our Ni NP films is significantly lower than the “Giant Hall effect” observed in co-deposited ferromagnet-insulator mixtures [14]. This is in an agreement with predictions of the model developed by Kharitonov and Efetov for tightly packed granular systems [32,33]. The granular material in this model is described as a network of metallic grains below the geometrical percolation threshold with high intragranular conductivity interconnected by tunnel junctions with low intergranular conductivity. The Hall voltage (both ordinary and extraordinary) is assumed to be generated within the grains only and not depending on intergranular connections as long as the intergranular tunneling resistance is lower than the quantum resistance,
. The magnitude of the EHE effect in this
system is expected to be close to that of the bulk, enhanced by a numerical coefficient determined by the shape of the grains and type of granular lattice. On the other hand, the overall resistance of the system is dominated by the inter-granular tunnel junctions. Thus, no correlation between the Hall effect and resistivity is predicted for the granular array. The absence of scaling between the Hall effects and resistivity, predicted by the model, has been experimentally 11
confirmed in granular ferromagnetic Ni-SiO2 mixtures at the metal-insulator transition [34], although the measured values corresponded to the “giant” enhancement regime of the effect. Our system, composed of uniform single grains packed in a dense array, meets explicitly the topology of the KharitonovEfetov model, and the magnitude of the effect is of the bulk order with no “giant Hall” enhancement.
RS (
26.40
26.36
(a)
26.32
26.28
-2.0 2.0
-RH (m
1.5
-1.5
-1.0
-0.5
0.0
0H (T)
0.5
1.0
1.5
2.0
(b)
1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.0
-1.5
-1.0
-0.5
0.0
0H (T)
0.5
1.0
1.5
2.0
Fig. 4 (a) MR curve of ~3 nm PVP40 coated Ni NP film measured with a magnetic field perpendicular to the film. The MR value of ~-0.4% is typical of the 50 samples that were characterized. (b) EHE measurement of the same film.
Figure 4 presents the field dependence of sheet resistance (a) and Hall resistance (b) of a typical NP film measured with field applied perpendicular to the film plane. The magnetoresistance is negative in the entire field range. Linear high field (above 0.5T) magnetoresistance is attributed to spin-magnon scattering. The low field negative magnetoresistance is due to either anisotropic magnetoresistance (AMR), if the film is continuous and macroscopically ferromagnetic, or due to the giant magnetoresistance (GMR) if the film is weakly coupled [35]. The value of (saturation) magnetoresistance, defined as [ ( )
(
)]
( ) 12
(3)
is of the order of 0.1%, which is similar to the one observed in disordered weakly coupled vacuum deposited Ni films [24]. Fig. 4 provides a qualitative illustration to two approaches in fabrication of field sensitive magnetic ink films, where two different parameters are monitored: a) magnetoresistance, and b) Hall effect. Magnetoresistive sensors, suggested by Karnaushenko et al., measure the relative change in resistivity under applied field, which is of the order of 0.1 – 10% in typical disordered materials [9,10]. Such sensors' sensitivity, defined as the slope dR/dH, is zero at zero field, and one needs to apply bias magnetic field to bring the sensor to its sensitive field range. EHE sensors measure an absolute voltage change upon field application, and their response is linear in field with the highest sensitivity occurring at low fields. It is interesting to note that while PVP40 coated NP films had a weak inverse temperature dependence of the resistance, roughly doubling the resistance on reducing the temperature from 300K to 77K, the PVP3.5 (PVP with molecular weight of 3,500) coated NP films had a metallic-like temperature dependence of the resistance. For example, the PVP3.5 based film used for the EHE measurement displayed in Figure 5, had a resistance decrease from 20 to 18 on cooling from 300 to 77K, which indicates a reasonably good electric coupling between NPs. Also in the PVP40 coated NP films the EHE magnitude exhibited basically no temperature dependence. It is reasonable to assume that each of the films had parallel contributions of thermally activated hopping conductance and metallic-like percolating NP networks. In the case of the bulkier PVP40 surfactant the thermally activated mechanism dominated, while in the smaller PVP3.5 the metallic behavior was more dominant. In the films
13
with the PVP3.5 the metal/surfactant ratio is higher, which makes the achievement of metallic percolation easier. The absence of temperature dependence for the EHE effect probably indicates that it primarily arises from the metallic-like percolating networks. In any case, the relatively high resistance of the films indicates that they are probably close to the percolation threshold. 8
300K 77K
6
-RH (m
4 2 0
4 -RH (m
-2 -4
2 0
-2 -4
-6 -0.1
0.0 0H (T)
-8 -2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
0.1
2.0
0H (T)
Fig. 5 EHE vs. magnetic field curves of a film of PVP3.5 coated 4 nm Ni NPs measured both at 300 and 77K. It can be seen that the EHE is weakly dependent on temperature, primarily through the increase in coercivity, which reflects the transition from a superparamagnetic state of the particles into a magnetically blocked stated. The resistance of the device changed from 20 to 18 Ω on cooling from 300 to 77K.
It should also be noted that the films were stable over time and in several cases samples were measured before and after a month's storage in ambient air and the EHE curves were identical. 4. Summary and conclusions It is shown that using a relatively simple process of spreading an ethylene glycol based Ni NP film and drying in vacuum under mild heating it is possible to obtain conducting films which exhibit measurable EHE with a good signalto-noise level and high field sensitivity around zero magnetic field. Consequently, these films could be used as simple magnetic sensors for applications such as contactless switches and linear or rotation motion sensors. 14
The printed device size (with contacts) would typically be of the order of 50100 m. This process is easily transferable to various printing techniques, such as inkjet printing and integrable with 3D printing techniques. The production of ink from such dispersions is fairly straightforward, as vast work has been done on preparation of silver NP inks in a similar manner [3]. The solvent used in the present work, ethylene glycol, belongs to the family of solvents that is typically used for inkjet printing of various types of NPs and the PVP surfactant is often used as a stabilizer to prevent NP agglomeration in such formulations. This would provide a simple, low cost, method to integrate magnetic sensors into 3D printed structures, such as multilayer printed circuit boards. Acknowledgements This work was supported by the Kamin program of the Chief Scientist of the Israeli Ministry of Economy.
References [1] X. Liu, Y. Z. Long, L. Liao, X. Duan and Z. Fan, ACS Nano, 2012, 6, 1888-1900. [2] M. Chason, D. R. Gamota, P. W. Brazis Jr., K. Kalyanasundaram, J. Zhang, K. K. Lian and R. Croswell, MRS Bull., 2006, 31, 471-475. [3] A. Kamyshny and S. Magdassi, Small, 2014, 10, 3515-3535. [4] Z. Fan, J. C. Ho, T. Takahashi, R. Yerushalmi, K. Takei, A. C. Ford, Y. L. Chueh, and A. Javey, Adv. Mater., 2009, 21, 3730-3743. [5] M. Zirkl, A. Sawatdee, U. Helbig, M. Krause, G. Scheipl, E. Kraker, P. A. Ersman, D. Nilsson, D. Platt, P. Bodö, S. Bauer, G. Domann, and B. Stadlober, Adv. Mater., 2011, 23, 2069-2074.
15
[6] D. Makarov, D. Karnaushenko and O. G. Schmidt, ChemPhysChem., 2013, 14, 1771 – 1776. [7] S. Jung, A. Sou, K. Banger, D. H. Ko, P. C. Y. Chow, C. R. McNeill and H. Sirringhaus, Adv. Energy Mater., 2014, 4, 1400432. [8] R. A. Street, W. S. Wong, S. E. Ready, M. L. Chabinyc, A. C. Arias, S. Limb, A. Salleo and R. Lujan, Mater. Today, 2006, 9, 32-37. [9] D. Karnaushenko, D. Makarov, C. Yan, R. Streubel and O. G. Schmidt, Adv. Mater., 2012, 24, 4518-4522. [10] D. Karnaushenko, D. Makarov, M. Stöber, D. D. Karnaushenko, S. Baunack and O. G. Schmidt, Adv. Mater., 2015, 27, 880-885. [11] D. Makarov, M. Melzer, D. Karnaushenko and O. G. Schmidt, Appl. Phys. Rev., 2016, 3, 011101. [12] A. Gerber, J. Mag. Mag. Mater., 2007, 310, 2749-2751. [13] G. Kopnov and A. Gerber, Appl. Phys. Lett., 2016, 109, 022404. [14] A. B. Pakhomov, X. Yan and B. Zhao, Appl. Phys. Lett., 1995, 67, 3497-3499. [15] J. C. Denardin, A. B. Pakhomov, M. Knobel, H. Liu, and X. X. Zhang, J. Phys.: Condens. Matter, 2000, 12, 3397-3399. [16] X. N. Jing, N. Wang, A. B. Pakhomov, K. K. Fung, and X. Yan, Phys. Rev. B 1996, 53, 14032-14035. [17] B. Zhao and X. Yan, J. Appl. Phys. 1997, 81, 4290-4292. [18] A. B. Pakhomov and X. Yan, Solid State Comm., 1996, 99, 139-142. [19] X. X. Zhang, C. Wan, H. Liu, Z. Q. Li, P. Sheng, and J. J. Lin, Phys. Rev. Lett., 2001, 86, 5562-5565. [20] Y. N. Wu, Z. Q. Li and J. J. Lin, Phys. Rev. B, 2010, 82, 092202. [21] C. Wan and P. Sheng, Phys. Rev. B, 2002, 66, 075309. [22] Y. Hou, Z. Xu, C. Rong, J. P. Liu and S. Sun, Adv. Mater., 2007, 19, 3349-3352. [23] S. Zhang, Y. Hao, D. Su, V. V. T. Doan-Nguyen, Y. Wu, J. Li, S. Sun and C. B. Murray, J. Am. Chem. Soc., 2014, 136, 15921-15924. 16
[24] Y. Bao, W. An, C. Heath Turner and K. M. Krishnan, Langmuir, 2010, 26, 478-483. [25] R. Hao, R. Xing, Z. Xu, Y. Hou, S. Gao and S. Sun, Adv. Mater., 2010, 22, 27292742. [26] J. H. Lee, K. Lee, S. Moon, H. Y. Lee, T. G. Park and J. Cheon, Angew. Chem. Int. Ed., 2009, 48, 4174-4179. [27] S. A. Majetich, T. Wen and R. A. Booth, ACS Nano, 2011, 5, 6081-6084. [28] S. H. Wu, D. H. Chen, J. Colloid Interface Sci. 2003, 259, 282. [29] D. Nicholls, R. Swindells, J. Inorg. Nucl. Chem. 1968, 30, 2211. [30] A. Segal, A. Gerber, J. Appl. Phys. 2012, 111, 073902. [31] A. Gerber, A. Milner, L. Goldshmit, M. Karpovski, B. Lemke, H. U. Habermeier and A. Sulpice, Phys. Rev. B, 2002, 65, 054426. [32] M. Y. Kharitonov and K. B. Efetov, Phys. Rev. Lett., 2007, 99, 056803. [33] M. Y. Kharitonov and K. B. Efetov, Phys. Rev. B, 2008, 77, 045116. [34] D. Bartov, A. Segal, M. Karpovski, and A. Gerber, Phys. Rev. B, 2014, 90, 144423. [35] A. Gerber, B. Groisman, A. Milner and M. Karpovsky, Europhys. Lett., 2000, 49, 383-389.
Highlights
Ni nanoparticle ink capable of forming conductive films on drying The Ni nanoparticle films exhibit significant extraordinary Hall effect This system could be used for preparing printed magnetic field sensors integrated in 3D printed structures
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PII: DOI: Reference:
S0304-8853(16)32248-X http://dx.doi.org/10.1016/j.jmmm.2016.11.106 MAGMA62181
To appear in: Journal of Magnetism and Magnetic Materials Received date: 18 September 2016 Revised date: 17 November 2016 Accepted date: 17 November 2016 Cite this article as: Leah Ben Gur, Einat Tirosh, Amir Segal, Gil Markovich and Alexander Gerber, Extraordinary Hall-effect in colloidal magnetic nanoparticle f i l m s , Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.11.106 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Extraordinary Hall-effect in colloidal magnetic nanoparticle films Leah Ben Gur,a‡ Einat Tirosh,a‡ Amir Segal,b Gil Markovicha* and Alexander Gerberb* a
School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 6997801, Israel. E-mail: [email protected]. b
School of Physics, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 6997801, Israel. E-mail:. ‡ These authors contributed equally. * E-mails: [email protected], [email protected]
Abstract Colloidal nickel nanoparticles (NPs) coated with polyvinylpyrrolidone (PVP) were synthesized. The nanoparticle dispersions were deposited on substrates and dried under mild heating to form conductive films. The films exhibited very small coercivity, nearly metallic conductivity, and a significant extraordinary Hall effect signal. This method could be useful for preparing simple, printed magnetic field sensors with the advantage of relatively high sensitivity around zero magnetic field, in contrast to magnetoresistive sensors, which have maximal field sensitivity away from zero magnetic field.
Keywords: Extraordinary Hall effect, magnetic nanoparticles, magnetic sensor, printed electronics
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1. Introduction The recent development of printed electronics technology is motivated by the promise of low-cost, high volume, high-throughput production of electronic components or devices that are light-weight and small, thin and flexible [1], inexpensive and disposable. Printed electronics is not a substitute for conventional silicon-based electronics with a high integration density and switching speed, but is oriented towards low-cost and high volume market segments where the high performance of conventional electronics is not required [2,3]. Printed electronics also enables the use of advanced nanoscale materials and organic materials for special functionalities and electronic devices [4], such as a various types of sensors [5,6], organic photovoltaics [7], and displays [8]. Printed electronics is very general and includes not only printable interconnects, but also optoelectronics and magnetoelectronics. Printed magnetoelectronics requires development of materials and devices sensitive to magnetic field, similar to the modern silicon-based magnetoelectronics. Recent developments in magnetoelectronics relate to the field of spintronics and, in particular, to the effects of giant magnetoresistance (GMR) and tunnelling magnetoresistance (TMR). Both GMR and TMR phenomena arise due to sensitivity of the spin dependent conductivity in heterogeneous magnetic systems to the relative orientation of the local magnetic moments: high conductivity for parallel moments and low for antiparallel. The specific mechanisms of GMR and TMR are different: GMR is due to spin-dependent scattering in metallic heterogeneous ferromagnets, multilayers and granular mixtures, where non-magnetic normal metal separates between ferromagnetic
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regions, while TMR is due to the spin dependent tunneling across insulating spacers.
The fabrication of printable ink with GMR properties has been
recently reported [9-11]. In this approach GMR multilayers are fabricated by conventional vacuum deposition, detached from the substrate, ball milled to form micron size flakes and mixed with solvents to form an ink or paste. The dried ink preserves the GMR properties of multilayer flakes and could be used for magnetic field sensing. In this paper, we report on development of printable field sensitive magnetic films operating on a different spin dependent phenomenon: the extraordinary Hall effect (EHE). This effect, well known in ferromagnetic materials, generates a voltage proportional to the magnetization across a current carrying magnetic film. The origin of EHE is spin-orbit scattering that breaks the spatial symmetry of scattered electrons. In the cases of our interest the EHE contribution exceeds significantly the ordinary Hall effect, and the Hall voltage VH can be presented as: (1) where RH, I, t,
EHE,
and M represent the Hall resistance, applied current, film
thickness, extraordinary Hall coefficient and magnetization normal to the film plane, respectively. VH is directly proportional to the magnetization M of the material and as such can be used for sensing magnetic field. Despite being known for more than a century, the effect was not considered seriously for technological development because of its relatively small value in bulk magnetic materials. Recently developed materials demonstrate dramatic enhancement of the EHE magnitude, which makes it attractive for a range of applications [12]. Magnetic field sensitivity of 106 /T, which is three orders 3
of magnitude higher than the best sensitivity achieved in semiconducting Hall materials, has been reported in partially oxidized CoFeB films [13]. An important feature of EHE, particularly relevant for printing technology, is that the EHE signal increases with increasing resistivity. Spin-orbit scattering at the lattice defects, impurities, surfaces, interfaces, phonons and magnons, all contribute to the EHE. Reduction of film thickness or artificial addition of an insulating material into the bulk of ferromagnets increases the EHE resistivity up to four orders of magnitude [14]. Thus, an imperfect structure of magnetic films fabricated by printing techniques is not expected to impede their performance if the magnetic properties of the material are preserved. In particular, granular ferromagnets fabricated by co-deposition of mutually immiscible ferromagnetic metal and an insulator exhibit a dramatic enhancement of the extraordinary Hall effect when the concentration of the metallic component approaches the critical value of the percolation threshold. The phenomenon was called the giant extraordinary Hall effect [14] as the EHE resistivity found in Ni-SiO2 mixtures reached 100 µΩcm, which is a factor up to 104 larger than in pure Ni metal. Similar values were found in other granular ferromagnet-insulator systems, like Co-SiO2 [15], NiFe-SiO2 [16], and Fe-SiO2 [17]. It was also found that the ordinary Hall effect coefficient shows an orders of magnitude growth similar to the EHE. The effect was found not only in magnetic Ni-SiO2 [18] but also in non-magnetic granular systems Cu-SiO2 [19], and Mo-SnO2 [20] on the metallic side of the percolation threshold. Theoretical interpretations of the effect in granular composites were hindered by considerable problems. Classical percolation theory predicts a divergence of the Hall effect at the percolation threshold in three dimensions, but the observed
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values are much greater than the estimate of this theory for finite sample thickness [18]. A local quantum interference theory was suggested by Wan and Sheng [21], in which the presence of small insulating substructures along an infinite metallic cluster leads to profound wave scattering and interference, thus causing significant reduction of the effective carrier density. The model is only valid at low temperatures when quantum corrections (weak localization/ electron-electron interaction) are valid, and cannot explain the "Giant Hall effect" at room temperature. Synthesis methods for the preparation of colloidal magnetic nanoparticles (NPs) has seen a significant progress in the last two decades, in terms of size and composition control [22,23], as well as molecular coating [24], with strong emphasis on biological applications [25,26]. In addition, thin film preparation techniques have received substantial attention [27]. In the present work we have prepared granular Ni films from NP inks which, after drying under controlled conditions, exhibited significant EHE signals. Ni NPs were selected due to the fairly low oxidation of the Ni on exposure to air, relative to other magnetic metals such as Co and Fe, and the relative ease of reduction of the oxide to the metal form. In addition, Ni is a relatively soft magnetic material, which will ensure low coercivity of the NP-based magnetic sensor, increasing its low field sensitivity. 2. Experimental methods 2.1 Ni nanoparticles Preparation Nickel NPs were synthesized by a modified method of borohydride reduction in ethylene glycol [28]. First, the precursor, nickel chloride hexahydrate (4 mmol) and 0.013 mmol of the polymeric surfactant poly(N-vinyl-2-pyrrolidone) of molecular
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weight of 40,000 g/mol (PVP40) or 3,500 g/mol (PVP3.5) were dissolved in ethylene glycol (16 mL) sequentially, resulting in a green solution. Then, hydrazine hydrate (30 mmol) was added, in order to form a Ni2+-hydrazine complex (a purple solution). The temperature was set to 40C, and after about 40 minutes the solution became bluish-purple. The color of the solution indicates the formation of a [Ni(N2H4)3]+22Cl complex, which is different from the unwanted [Ni(N2H4)6]+22Cl complex. The latter is unstable and decomposes to [Ni(N2H4)4-5]+22Cl [29]. This is why the temperature at this stage is crucial. Finally, the reducing agent, sodium borohydride (13 mmol, dissolved in 700μL of H2O), was added under nitrogen atmosphere, in order to reduce the nickel ions to their metallic state. The solution was stirred for 1 hour until turning black due to formation of nickel NPs. Subsequently, the flask was cooled to room temperature, and the NPs were isolated through centrifugation with ethanol (3,500 rpm for 5 minutes) and re-dispersed in ethylene glycol. The product was kept in a glove box to minimize oxidation. 2.2 Thin film preparation and drying In order to make ~1 m thick Ni NPs thin films (after drying), we have used a bar coating device. The device was built with two spacers that can be replaced, depending on the desired thickness of the wet film. The height of the spacers correlates to the height of the spread solution in a non-trivial manner, depending on the viscosity and surface tension of the NP dispersion as well as the wetting properties of the bar or blade used for the spreading. There is a designated location to place the substrate between the spacers, where vacuum suction keeps the substrate in place. Then, a metal blade was used to spread the concentrated Ni nanoparticle (~30% by weight in ethylene glycol) film on the glass substrate. Since the ink wetted the blade, a residue of ink was left on the blade during the coating process. 6
The thin Ni NP films were dried in a vacuum oven with base pressure of the order of 10 millibars and temperature of ~100C for a period of 15 hours. Typically, a low pressure (about 100 millibar) 5/95% gas mixture of H2/N2 was flown trough the vacuum oven during the first several hours of drying. 2.3 Characterization The synthesized NP dispersions were deposited on carbon coated copper grids and left to dry for transmission electron microscopy (TEM) characterization, using a Technai F20 microscope. Magnetic characterization by a Superconducting Quantum Interference Device (SQUID) magnetometer: The magnetic NPs sample was prepared for magnetization measurements by precipitating the nanoparticles from the ethylene glycol with added ethanol and centrifuging at 5000 rpm for 15 minutes. Then the precipitate was separated from the supernatant and dried to a bulk powder by heating at 100C under a vacuum of ~ 10 mbar. 2.4 EHE measurements For electrical measurements, the Ni NP films deposited on glass slides and dried, were contacted by 4 copper wires using silver paint. The measurements were conducted using a 2T water-cooled electromagnet applying an external magnetic field perpendicular to the substrate. Resistivity, magnetoresistance and Hall effect were measured using the Van der Pauw protocol. The transverse (Hall) signal is antisymmetric with respect to external field, whereas the longitudinal (resistivity) signal is symmetric. Reverse magnetic field reciprocity protocol was used to separate the measured data into symmetric and anti-symmetric parts [30].
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3. Results and discussion The synthesis resulted in Ni NPs in the size range of 3-20 nm (as seen in Figure 1a). Each particle consisted of a single Ni crystal and high resolution TEM images confirmed that the NPs had the bulk FCC Ni structure (see Supplementary material). The nickel NP dispersion in ethylene glycol was spread on glass substrates by a bar coating device and dried under low vacuum and mild heating (see Supplementary material). The dried nickel NP films had a typical thickness of the order of 1 m and roughness (peak values) on the same scale, as measured by profilometry (see Supplementary material).
(a) (b)
20 nm Fig. 1 (a) Transmission electron micrograph of the PVP40 coated Ni NPs. (b) A Ni NP film wired for EHE measurement.
Mild heating (100C) of the film while drying under low vacuum conditions was found important for obtaining metallic conductance, even without a reducing H2 atmosphere, under residual air pressure. XPS measurements of the NP films with PVP40 coating (PVP with molecular weight of 40,000) with and without heating to 100C while drying revealed that the level of oxidation of the NPs did not increase, and even slightly decreased, in the heated film relative to the unheated one (see Supplementary material). Deconvolution of the Ni XPS peaks corresponding to the metallic and NiO species, provided an estimate for the fraction of oxidized nickel in the films of roughly 20-30% out of the total amount of Ni atoms. Such a fraction of 8
oxidized Ni would be responsible for an oxide layer thickness of about 0.350.55 nm for a 10 nm diameter NP. Figure 2 displays the magnetization curve of a PVP40 coated Ni NP powder dried at 100C under vacuum from the ethylene glycol dispersion of the NPs. The magnetization curve was fitted with a Langevin function describing the alignment of an ensemble of paramagnetic (superparamagnetic in the present case) magnetic moments with an external magnetic field H, at temperature T: [
(
)
]
(2)
where x=μ/(kBT), kB is Boltzmann's constant, and 0 is the vacuum permeability. A closer look at the low field part of the magnetization curve did show a small coercivity, probably originating in strongly interacting or even sintered nanoparticles within the film.
Magnetization Fit to Langevin function
30
M (emu/g)
15
0
-15
-150
-30
-100
-50
0
50
100
150
0H (mT)
-2
-1
0
0H (T)
1
2
Fig. 2 Room temperature magnetization of PVP40 coated Ni NP powder as a function of applied magnetic field (symbols). The solid line is a fit of the data by the Langevin function (Eq. 2). The inset is an expanded view around H=0, showing a small coercivity of 10 mT.
The effective NP's magnetic moment ( ) value extracted from the Langevin fit was divided by the average number of Ni atoms per particle (average diameter of 10 nm in this case) and yielded an average magnetic moment of 0.3 B per Ni atom, which is equivalent to 28 emu/g. 9
The bulk saturation magnetization value of nickel is ~0.6 B per atom (55 emu/g). The lower value obtained from the Langevin function fit could be partially attributed to the thin oxide layer around the NPs, as NiO is antiferromagnetic. Remarkably, in spite of the vast differences between
continuous
ferromagnetic films and our NP films, the NP films exhibited a significant EHE signal with a fairly high signal to noise ratio, as shown in Figure 3. About 50 samples of PVP coated Ni NP films were produced and they all exhibited measurable EHE, with Hall resistance (VH/I) values at magnetization saturation of 1-10 m, measured using currents in the range of 1-10 mA. EHE curve saturation varied between ~0.2-0.6 T (depending on average NP size and drying conditions). The slope of the Hall resistance vs. field curves around zero-field is a measure of the sensitivity of the EHE for low magnetic field sensing. The slope values obtained for the various Ni NP films near zero-field were in the range of 10-100 m/T. The EHE vs. field curves were typically linear within a field range of about 50-100 mT.
4
-RH (m
2
0
-2
V I
-4 -2.0
-1.5
-1.0
-0.5
0.0
0.5
H
1.0
I+
1.5
V+ 2.0
0H (T)
Fig. 3 EHE vs. magnetic field measurement of ~10 nm PVP40 coated Ni NP film, performed at room temperature. The inset shows the measurement configuration where 4 contacts are made with silver paint and a Van der Paw technique is used to extract the Hall voltage.
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The NP "paste" based Ni films can be compared favourably with those fabricated by vacuum deposition. Typical sheet resistance values for the Ni NP films were in the range of 10-100 Ω. Assuming that the thickness of our films is 1 m, their resistivity is in the range 103 – 104 µcm, compared with 15-20 µcm in thick polycrystalline Ni films fabricated by e-beam deposition. The EHE resistivity, defined as: ρEHE=VHt/I in magnetically saturated state at high field is in the range of 0.1 – 1.0 µcm, which is equal or higher than ρEHE= 0.1 µcm observed in 100 nm thick vacuum deposited Ni films [31]. The magnitude of the EHE resistivity found in our Ni NP films is significantly lower than the “Giant Hall effect” observed in co-deposited ferromagnet-insulator mixtures [14]. This is in an agreement with predictions of the model developed by Kharitonov and Efetov for tightly packed granular systems [32,33]. The granular material in this model is described as a network of metallic grains below the geometrical percolation threshold with high intragranular conductivity interconnected by tunnel junctions with low intergranular conductivity. The Hall voltage (both ordinary and extraordinary) is assumed to be generated within the grains only and not depending on intergranular connections as long as the intergranular tunneling resistance is lower than the quantum resistance,
. The magnitude of the EHE effect in this
system is expected to be close to that of the bulk, enhanced by a numerical coefficient determined by the shape of the grains and type of granular lattice. On the other hand, the overall resistance of the system is dominated by the inter-granular tunnel junctions. Thus, no correlation between the Hall effect and resistivity is predicted for the granular array. The absence of scaling between the Hall effects and resistivity, predicted by the model, has been experimentally 11
confirmed in granular ferromagnetic Ni-SiO2 mixtures at the metal-insulator transition [34], although the measured values corresponded to the “giant” enhancement regime of the effect. Our system, composed of uniform single grains packed in a dense array, meets explicitly the topology of the KharitonovEfetov model, and the magnitude of the effect is of the bulk order with no “giant Hall” enhancement.
RS (
26.40
26.36
(a)
26.32
26.28
-2.0 2.0
-RH (m
1.5
-1.5
-1.0
-0.5
0.0
0H (T)
0.5
1.0
1.5
2.0
(b)
1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.0
-1.5
-1.0
-0.5
0.0
0H (T)
0.5
1.0
1.5
2.0
Fig. 4 (a) MR curve of ~3 nm PVP40 coated Ni NP film measured with a magnetic field perpendicular to the film. The MR value of ~-0.4% is typical of the 50 samples that were characterized. (b) EHE measurement of the same film.
Figure 4 presents the field dependence of sheet resistance (a) and Hall resistance (b) of a typical NP film measured with field applied perpendicular to the film plane. The magnetoresistance is negative in the entire field range. Linear high field (above 0.5T) magnetoresistance is attributed to spin-magnon scattering. The low field negative magnetoresistance is due to either anisotropic magnetoresistance (AMR), if the film is continuous and macroscopically ferromagnetic, or due to the giant magnetoresistance (GMR) if the film is weakly coupled [35]. The value of (saturation) magnetoresistance, defined as [ ( )
(
)]
( ) 12
(3)
is of the order of 0.1%, which is similar to the one observed in disordered weakly coupled vacuum deposited Ni films [24]. Fig. 4 provides a qualitative illustration to two approaches in fabrication of field sensitive magnetic ink films, where two different parameters are monitored: a) magnetoresistance, and b) Hall effect. Magnetoresistive sensors, suggested by Karnaushenko et al., measure the relative change in resistivity under applied field, which is of the order of 0.1 – 10% in typical disordered materials [9,10]. Such sensors' sensitivity, defined as the slope dR/dH, is zero at zero field, and one needs to apply bias magnetic field to bring the sensor to its sensitive field range. EHE sensors measure an absolute voltage change upon field application, and their response is linear in field with the highest sensitivity occurring at low fields. It is interesting to note that while PVP40 coated NP films had a weak inverse temperature dependence of the resistance, roughly doubling the resistance on reducing the temperature from 300K to 77K, the PVP3.5 (PVP with molecular weight of 3,500) coated NP films had a metallic-like temperature dependence of the resistance. For example, the PVP3.5 based film used for the EHE measurement displayed in Figure 5, had a resistance decrease from 20 to 18 on cooling from 300 to 77K, which indicates a reasonably good electric coupling between NPs. Also in the PVP40 coated NP films the EHE magnitude exhibited basically no temperature dependence. It is reasonable to assume that each of the films had parallel contributions of thermally activated hopping conductance and metallic-like percolating NP networks. In the case of the bulkier PVP40 surfactant the thermally activated mechanism dominated, while in the smaller PVP3.5 the metallic behavior was more dominant. In the films
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with the PVP3.5 the metal/surfactant ratio is higher, which makes the achievement of metallic percolation easier. The absence of temperature dependence for the EHE effect probably indicates that it primarily arises from the metallic-like percolating networks. In any case, the relatively high resistance of the films indicates that they are probably close to the percolation threshold. 8
300K 77K
6
-RH (m
4 2 0
4 -RH (m
-2 -4
2 0
-2 -4
-6 -0.1
0.0 0H (T)
-8 -2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
0.1
2.0
0H (T)
Fig. 5 EHE vs. magnetic field curves of a film of PVP3.5 coated 4 nm Ni NPs measured both at 300 and 77K. It can be seen that the EHE is weakly dependent on temperature, primarily through the increase in coercivity, which reflects the transition from a superparamagnetic state of the particles into a magnetically blocked stated. The resistance of the device changed from 20 to 18 Ω on cooling from 300 to 77K.
It should also be noted that the films were stable over time and in several cases samples were measured before and after a month's storage in ambient air and the EHE curves were identical. 4. Summary and conclusions It is shown that using a relatively simple process of spreading an ethylene glycol based Ni NP film and drying in vacuum under mild heating it is possible to obtain conducting films which exhibit measurable EHE with a good signalto-noise level and high field sensitivity around zero magnetic field. Consequently, these films could be used as simple magnetic sensors for applications such as contactless switches and linear or rotation motion sensors. 14
The printed device size (with contacts) would typically be of the order of 50100 m. This process is easily transferable to various printing techniques, such as inkjet printing and integrable with 3D printing techniques. The production of ink from such dispersions is fairly straightforward, as vast work has been done on preparation of silver NP inks in a similar manner [3]. The solvent used in the present work, ethylene glycol, belongs to the family of solvents that is typically used for inkjet printing of various types of NPs and the PVP surfactant is often used as a stabilizer to prevent NP agglomeration in such formulations. This would provide a simple, low cost, method to integrate magnetic sensors into 3D printed structures, such as multilayer printed circuit boards. Acknowledgements This work was supported by the Kamin program of the Chief Scientist of the Israeli Ministry of Economy.
References [1] X. Liu, Y. Z. Long, L. Liao, X. Duan and Z. Fan, ACS Nano, 2012, 6, 1888-1900. [2] M. Chason, D. R. Gamota, P. W. Brazis Jr., K. Kalyanasundaram, J. Zhang, K. K. Lian and R. Croswell, MRS Bull., 2006, 31, 471-475. [3] A. Kamyshny and S. Magdassi, Small, 2014, 10, 3515-3535. [4] Z. Fan, J. C. Ho, T. Takahashi, R. Yerushalmi, K. Takei, A. C. Ford, Y. L. Chueh, and A. Javey, Adv. Mater., 2009, 21, 3730-3743. [5] M. Zirkl, A. Sawatdee, U. Helbig, M. Krause, G. Scheipl, E. Kraker, P. A. Ersman, D. Nilsson, D. Platt, P. Bodö, S. Bauer, G. Domann, and B. Stadlober, Adv. Mater., 2011, 23, 2069-2074.
15
[6] D. Makarov, D. Karnaushenko and O. G. Schmidt, ChemPhysChem., 2013, 14, 1771 – 1776. [7] S. Jung, A. Sou, K. Banger, D. H. Ko, P. C. Y. Chow, C. R. McNeill and H. Sirringhaus, Adv. Energy Mater., 2014, 4, 1400432. [8] R. A. Street, W. S. Wong, S. E. Ready, M. L. Chabinyc, A. C. Arias, S. Limb, A. Salleo and R. Lujan, Mater. Today, 2006, 9, 32-37. [9] D. Karnaushenko, D. Makarov, C. Yan, R. Streubel and O. G. Schmidt, Adv. Mater., 2012, 24, 4518-4522. [10] D. Karnaushenko, D. Makarov, M. Stöber, D. D. Karnaushenko, S. Baunack and O. G. Schmidt, Adv. Mater., 2015, 27, 880-885. [11] D. Makarov, M. Melzer, D. Karnaushenko and O. G. Schmidt, Appl. Phys. Rev., 2016, 3, 011101. [12] A. Gerber, J. Mag. Mag. Mater., 2007, 310, 2749-2751. [13] G. Kopnov and A. Gerber, Appl. Phys. Lett., 2016, 109, 022404. [14] A. B. Pakhomov, X. Yan and B. Zhao, Appl. Phys. Lett., 1995, 67, 3497-3499. [15] J. C. Denardin, A. B. Pakhomov, M. Knobel, H. Liu, and X. X. Zhang, J. Phys.: Condens. Matter, 2000, 12, 3397-3399. [16] X. N. Jing, N. Wang, A. B. Pakhomov, K. K. Fung, and X. Yan, Phys. Rev. B 1996, 53, 14032-14035. [17] B. Zhao and X. Yan, J. Appl. Phys. 1997, 81, 4290-4292. [18] A. B. Pakhomov and X. Yan, Solid State Comm., 1996, 99, 139-142. [19] X. X. Zhang, C. Wan, H. Liu, Z. Q. Li, P. Sheng, and J. J. Lin, Phys. Rev. Lett., 2001, 86, 5562-5565. [20] Y. N. Wu, Z. Q. Li and J. J. Lin, Phys. Rev. B, 2010, 82, 092202. [21] C. Wan and P. Sheng, Phys. Rev. B, 2002, 66, 075309. [22] Y. Hou, Z. Xu, C. Rong, J. P. Liu and S. Sun, Adv. Mater., 2007, 19, 3349-3352. [23] S. Zhang, Y. Hao, D. Su, V. V. T. Doan-Nguyen, Y. Wu, J. Li, S. Sun and C. B. Murray, J. Am. Chem. Soc., 2014, 136, 15921-15924. 16
[24] Y. Bao, W. An, C. Heath Turner and K. M. Krishnan, Langmuir, 2010, 26, 478-483. [25] R. Hao, R. Xing, Z. Xu, Y. Hou, S. Gao and S. Sun, Adv. Mater., 2010, 22, 27292742. [26] J. H. Lee, K. Lee, S. Moon, H. Y. Lee, T. G. Park and J. Cheon, Angew. Chem. Int. Ed., 2009, 48, 4174-4179. [27] S. A. Majetich, T. Wen and R. A. Booth, ACS Nano, 2011, 5, 6081-6084. [28] S. H. Wu, D. H. Chen, J. Colloid Interface Sci. 2003, 259, 282. [29] D. Nicholls, R. Swindells, J. Inorg. Nucl. Chem. 1968, 30, 2211. [30] A. Segal, A. Gerber, J. Appl. Phys. 2012, 111, 073902. [31] A. Gerber, A. Milner, L. Goldshmit, M. Karpovski, B. Lemke, H. U. Habermeier and A. Sulpice, Phys. Rev. B, 2002, 65, 054426. [32] M. Y. Kharitonov and K. B. Efetov, Phys. Rev. Lett., 2007, 99, 056803. [33] M. Y. Kharitonov and K. B. Efetov, Phys. Rev. B, 2008, 77, 045116. [34] D. Bartov, A. Segal, M. Karpovski, and A. Gerber, Phys. Rev. B, 2014, 90, 144423. [35] A. Gerber, B. Groisman, A. Milner and M. Karpovsky, Europhys. Lett., 2000, 49, 383-389.
Highlights
Ni nanoparticle ink capable of forming conductive films on drying The Ni nanoparticle films exhibit significant extraordinary Hall effect This system could be used for preparing printed magnetic field sensors integrated in 3D printed structures
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