United States

Ensemble magnetization of magnetic nanoparticles (NPs) has a sensitive dependence on the structural arrangement of particles. To better understand the correlation between spatial positions of NPs and ensemble magnetization, an experimental system is needed where particle position can be adjusted while simultaneously probing nanoscale magnetization. Here, we describe experiments using polarization analyzed small angle neutron scattering (PASANS) to study the change in magnetic correlations of ~7 nm iron oxide (Fe&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;) NPs embedded in stretched silicone rubber. PASANS is a powerful tool for mapping both the inter- and intra- particle magnetization of NPs on length scales ranging from 0.1 – 100 nm. The magnetic NP polymer gel, or magnetogel, can be stretched on the neutron beamline during an experiment. The macroscopic stretching is accompanied by nanoscale deformations, where polymer chains move to accommodate a new equilibrium configuration. NPs embedded in the gel change their configuration as a function of applied stress, providing a method to adjust the average center-to-center spacing between particles. The scattering data from the unstretched gel in a 1 T applied magnetic field can be modelled by 7.4 nm iron oxide NPs assembled in a hard sphere packing with 30% volume fraction. When the sample is stretched 3x its original size in a direction parallel to the applied field, the morphology changes dramatically. Specifically, the data can be described using a cluster model where NP dimers have emerged (Figures 1a and 1b). Stretching causes the particles to morph from semi-close packed, but individual, 7.4 nm diameter NPs into dimer-like clusters that are 15 nm long, as viewed perpendicular to the applied field (Figures 2a and 2b). These PASANS experiments clearly demonstrate that stretching the magnetogel modulates the structural characteristics of magnetic NPs in a pronounced, controllable manner.

![](https://assets.underline.io/uploads/markdown_image/1/image/39e948b29d4a9faea0cd8a27867ee2bb.jpg)

Fig. 1: Magnetic scattering from (a) unstretched and (b) 3x stretched gel in a 1 T field. The data set in (a) is fit to a hard sphere-like packing and (b) is fit to a model containing dimers

![](https://assets.underline.io/uploads/markdown_image/1/image/5b350fa63e4727cecd3c135d0fb252f5.jpg)

Fig. 2: Schematic showing rearrangement of NPs in (a) unstretched and (b) 3x stretched configurations.




2022 Joint MMM-INTERMAG

Magnetic Correlations of Iron Oxide Nanoparticles as Probed by PASANS in Stretched Magnetogel Composites

## Dr. Nishikant (Nish) Sonwalkar: Academician, Scientist, Musician

![](https://assets.underline.io/uploads/markdown_image/1/image/0f9b63a0b3deba4a355d98e2c87af68a.png)

Dr. Sonwalkar is regarded as a leading expert in the application of computers in education. He has extensive research and development experience in hypermedia authoring, mobile learning, adaptive learning, and brain-based synaptic learning, advanced scientific visualization, database management systems, and computer simulation.

After receiving doctoral degree from MIT in the area of Molecular Dynamics of Nano-interfaces, he joined MIT as faculty in Mechanical Engineering, but his passion for technology enabled education led him become the founding director of Hypermedia Teaching Facility (Hypermedia Lab) at MIT and later he was appointed as the Principal Educational Architect of MIT.

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NISH 2021

Ensemble magnetization of magnetic nanoparticles (NPs) has a sensitive dependence on the structural arrangement of particles. To better understand the correlation between spatial positions of NPs and ensemble magnetization, an experimental system is needed where particle position can be adjusted while simultaneously probing nanoscale magnetization. Here, we describe experiments using polarization analyzed small angle neutron scattering (PASANS) to study the change in magnetic correlations of ~7 nm iron oxide (Fe3O4) NPs embedded in stretched silicone rubber. PASANS is a powerful tool for mapping both the inter- and intra- particle magnetization of NPs on length scales ranging from 0.1 – 100 nm. The magnetic NP polymer gel, or magnetogel, can be stretched on the neutron beamline during an experiment. The macroscopic stretching is accompanied by nanoscale deformations, where polymer chains move to accommodate a new equilibrium configuration. NPs embedded in the gel change their configuration as a function of applied stress, providing a method to adjust the average center-to-center spacing between particles. The scattering data from the unstretched gel in a 1 T applied magnetic field can be modelled by 7.4 nm iron oxide NPs assembled in a hard sphere packing with 30% volume fraction. When the sample is stretched 3x its original size in a direction parallel to the applied field, the morphology changes dramatically. Specifically, the data can be described using a cluster model where NP dimers have emerged (Figures 1a and 1b). Stretching causes the particles to morph from semi-close packed, but individual, 7.4 nm diameter NPs into dimer-like clusters that are 15 nm long, as viewed perpendicular to the applied field (Figures 2a and 2b). These PASANS experiments clearly demonstrate that stretching the magnetogel modulates the structural characteristics of magnetic NPs in a pronounced, controllable manner.

![](https://assets.underline.io/uploads/markdown_image/1/image/39e948b29d4a9faea0cd8a27867ee2bb.jpg)

Fig. 1: Magnetic scattering from (a) unstretched and (b) 3x stretched gel in a 1 T field. The data set in (a) is fit to a hard sphere-like packing and (b) is fit to a model containing dimers

![](https://assets.underline.io/uploads/markdown_image/1/image/5b350fa63e4727cecd3c135d0fb252f5.jpg)

Fig. 2: Schematic showing rearrangement of NPs in (a) unstretched and (b) 3x stretched configurations.

technical paper

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

The 2022 Joint Conference will offer both in-person and prerecorded on-demand content.

For tickets to the event, please register at the following link: https://magnetism.org/register

Please register for the Joint MMM-Intermag conference to join the event.

This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

Sparked by the limitations of the Von-Neumann architecture, the burgeoning field of neuromorphic computation aims to develop systems that process information both ‘in-memory’ and in a brain-like manner. Neural networks comprise three types of layers – an input layer, hidden layers and an output layer each connected by numeric weights. Training such networks requires continuous updating of each weight requiring both time and energy. Reservoir computing (RC)1 is a subset of neuromorphic computation where training occurs only on the output layer, reducing time and energy cost significantly compared to traditional neural networks, while competing with traditional approaches for time-series classification / prediction task accuracy. Artificial spin networks comprising patterned nanomagnets serve as promising computational substrates for such applications, information can be stored in a single nanomagnet spin (i.e. a nanomagnetic ‘bit’) or the magnetic configuration (microstate) of the entire network. Strong dipolar coupling enables a highly non-linear history-dependent collective response to external stimuli, a key property for RC1. Artificial spin ice (ASI) is one such network where geometric frustration gives rise to a rich microstate space and emergent collective behaviour2 highly promising for RC3,4. However, in these systems individual nanomagnets are constrained to Ising-like states, severely limiting microstate richness. We have previously shown in simulation that combining Ising-like and vortex states allows powerful reconfigurable functionality5. Modifying a subset of bars6,7 in an ASI such that the Ising and vortex states are bistable gives rise to highly non-linear responses to global- and RF-fields allowing multiple reservoir readouts, detectable by the recently developed ‘spectral fingerprinting’ technique8. Here, we explore ASVI-based RC for forecasting and classifying time-series with varying complexity and assess the strengths and limitations of such systems compared against traditional ASI and competing neuromorphic hardware platforms.

Reservoir computing in an artificial spin-vortex ice

Magnetic nanoparticles are increasingly used in biomedical applications such as drug-delivery, gene delivery, hyperthermia, or contrast agents for MRI. Magnetite (Fe3O4) nanoparticles are good candidates for these applications due to their non-toxicity and long-life in the bloodstream. While the structural and magnetic properties of bulk Fe3O4 has been widely studied [1], knowledge is still lacking regarding the magnetic behaviour of collections of Fe3O4 nanoparticles. In particular, when the particles are small and the assembly is superparamagnetic, it is useful to understand the nature of the magnetic correlations within the assemblies. Here we show how inter-particle magnetic correlations can be probed at the nanoscale via x-ray resonant magnetic scattering (XRMS) [2] as illustrated in Fig.1a. By tuning the energy of the x-rays to resonant edges of Fe and comparing opposite polarization helicities, we extract information about the local inter-particle magnetic orders within the nanoparticle assemblies. We show the dependence on particle size, varying from 5 to 11 nm, suggesting an enhancement of magnetic couplings for bigger particles. [3] Additionally, we show model based on chains of nanoparticles, which we use to fit the XRMS data. The data fitting suggests the occurrence of a mix of antiferromagnetic ordering and randomness near remanence at high temperature and near the coercive region at low temperature, for larger particles. [4] Finally we will show preliminary photon correlation (XPCS) (see Fig.1b) and muon sin resonance data obtained on 5 nm Fe3O4 nanoparticles revealing temperature dependence on the dynamics of magnetic fluctuation. [5,6]

![](https://assets.underline.io/uploads/markdown_image/1/image/423b7b0e23ec43cfb7a4335170291ade.jpg)

Nanoscale magnetic correlations and fluctuations in assemblies of Fe3O4 nanoparticles

When temper-annealed around 650 K in the presence of a magnetic-field, off-stoichiometric Heusler alloys in the form Ni50Mn45X5 (X: Al, Ga, In Sn, Sb) decompose and form 2-5 nm full Heusler precipitates which become embedded in a surrounding NiMn neighborhood. The precipitates are cubic and ferromagnetic (FM) whereas the matrix is tetragonal and antiferromagnetic (AF) with a high anisotropy. This spinodal decomposition progresses according to the reaction 5Ni50Mn45X5 → Ni50Mn25X25 + 4Ni50Mn50. The AF/FM interface becomes so strongly pinned in the applied annealing-field direction that a magnetization reversal is not observed in fields up to 20 T. Such materials are relevant to non-volatile magnetic memory applications. In a similar manner, it is possible to temper anneal other Heusler-based materials to obtain precipitation. We adopt this method to prepare spin-polarized FM Co50Ti25Ge25 precipitates in a Pauli paramagnet (PM) Co50Ti50 matrix. The decomposition progresses according to 5Co50Ti45Ge5 → Co50Ti25Ge25 + 4Co50Ti50. The aim is to understand the influence of the spin polarized state on the magnetic and transport properties at reduced dimensions. For this purpose, we carry out x-ray diffraction (XRD), microprobe analysis, magnetization, and resistivity studies. Time-, field-, and temperature-dependent magnetization studies along with XRD show the presence of decomposition into Co50Ti25Ge25 precipitates becoming embedded in a PM matrix. Field-dependent resistivity measurements show asymmetric field-dependence and suggest the preservation of spin-polarization over macroscopic dimensions.

Magnetic and Transport Properties of Shell-ferromagnetically Precipitated Spin-Polarized Co2TiGe in a CoTi matrix

The interface driven exchange bias effect (EB) is a well-studied phenomenon observed in ferromagnetic (FM)/antiferromagnetic (AFM) thin film systems resulting from the interfacial exchange coupling between the F and AF spins [1]. In numerous studies, asymmetrically shaped hysteresis loops have been reported mainly described by coherent rotation of the magnetic domains on the one hysteresis branch and rapid nucleation and domain wall propagation on the other one [2, 3]. Romanens et al. were able to correlate these asymmetries in perpendicular magnetized thin film systems to higher domain nucleation densities when the applied field is aligned antiparallel to the EB field compared to the parallel orientation [4]. Here the statistics of magnetic domain nucleation during the magnetization reversal process in exchange-biased magnetic nano-stripes is investigated by high-resolution optical Kerr microscopy. In such structures the magnetization dynamics are governed by the exchange bias field, the shape anisotropy and stray field interactions between the stripes. The dependency of the nucleation on these parameters is investigated systematically by variation of the lateral dimensions of the stripes and the orientation of the exchange bias fields relative to the stripe axis. Understanding the nucleation behavior of magnetic domains in nanostructures is an important step in the development of spintronic devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/edc0ef18b758f66d641af727d520c25e.jpg)

Investigation of magnetic domain nucleation in exchange-biased nanostructures

Investigation of magnetic domain nucleation in exchange-biased nanostructures

Micrometer diameter bilayers of 20 nm Permalloy/20 nm Cobalt Oxide deposited on nanomechanical resonators were used to investigate exchange bias effects. The mechanical compliances of two resonator axes were enhanced by severing one torsion arm, Figure 1, resulting in a unique three-axis resonator that responds resonantly to torques generated by a three-axis RF field [1]. Our technique permits simultaneous measurement of three orthogonal torque components. Measurements of the anisotropies associated with exchange bias effects have been made. First, at cryogenic temperatures, observations of shifted linear hysteresis loops confirm the presence of a positive exchange bias at the Py/CoO interface. A rotating DC bias field was used to probe in-plane anisotropies through the out-of-plane torque, as shown in Figure 2 (open circles). Training effects in both the linear and rotating hysteresis data were observed and show that most of the training occurs during the first half of the first loop, consistent with an athermal training mechanism [2]. The data from the rotating hysteresis loops were fit with parameters from a macrospin solution to the Landau-Lifshitz-Gilbert equation. In order to reproduce our observations standard anisotropy terms, a spin-flop coupling term [3], and a rotatable anisotropy axis [3,4] are introduced to the model. Each parameter of the exchange bias model accounts for specific features of the rotational loop. A unidirectional anisotropy asymmeterizes the extrema magnitudes. The orientation of the rotatable anisotropy axis with respect to the ferromagnetic magnetization introduces a one-fold rotational symmetry that would otherwise be at least two-fold. The spin-flop term influences the angular positions of the peaks in the torque curve.

![](https://assets.underline.io/uploads/markdown_image/1/image/904fce60d2079057eb98db71f95841b7.jpg)

Figure 1: Electron micrograph of a micromechanical device used for demonstration of the three-axis torque measurement technique. The arrows indicate the three orthogonal torsional displacements.

![](https://assets.underline.io/uploads/markdown_image/1/image/a6f1c83647a790b3299d22bf142cc7ae.jpg)

Figure 2: Thermomechanically calibrated experimental data of the z torque component (resonator at liquid nitrogen temperature). Data were recorded during a rotational hysteresis loop. Fits from macrospin solutions to the LLG equation are shown.



Three-axis torque investigation of exchange bias in a micromagnetic disk

Thin ferromagnetic films with perpendicular anisotropy exhibit nano-sized magnetic domains forming various patterns. When an external magnetic field is applied and cycled, the domains pattern is usually lost. However, if the ferromagnetic layer is coupled with an underlying antiferromagnetic layer, it may show magnetic domain memory (MDM). [1-3] MDM is the ability of the magnetic domains to retain their exact same domain topology upon field cycling. We are studying the effect that illumination by coherent resonant x-rays may have on MDM in a [Co / Pd] / IrMn multilayers. Our x-ray resonant magnetic scattering (XRMS) data suggests that under high dose of x-ray illumination, the material may lose its existing MDM. To investigate this possible effect we have used magneto-transport measurements [4,5] to track the exchange bias while the sample is illuminated by x-rays, using synchrotron radiation (see Fig.1a). Magneto-transport is here used to measure the magnetization loop from which we can extract a bias field. A loss of exchange bias would indicate that the x-rays illumination dose may alter the strength of the exchange couplings and ultimately the amount of MDM. Knowing if a loss of exchange bias has occurred requires collecting magneto-transport data (Fig.1b) as well as XRMS data (Fig.1c) and correlating the observed changes under various dose of x-ray illumination.These measurements include Hall effect signal and as well as magnetoresistance signal, collected both on inner contacts and on outer contacts. For the outer contacts, the spacing between wire bonds is ~1cm, whereas for the inner contacts, the spacing between wire bonds is ~100 . These measurements help us understand the shape of the magneto-transport data collected under x-ray illumination and determine if x-ray illumination has an effect on exchange couplings, and ultimately MDM.

![](https://assets.underline.io/uploads/markdown_image/1/image/766a2108304b1d8f9a1fe3c41b68e2d2.jpg)

a) Instrument used for our x-ray resonant magnetic scattering (XRMS) experiment at the APS, showing the vacuum scattering chamber and the octupolar magnet to apply a field in any direction. b) Magneto-transport measurement taken while [Co/Pd] / IrMn multilayers was illuminated with x-rays. c) XRMS speckle pattern from the [Co/Pd] / IrMn film, measured at the Co L3 edge.

The effect of x-ray illumination on magnetic domain memory in [Co/Pd] / IrMn multilayers

Advances in synthesis and characterization of magnetic nanoparticles (NPs) have yielded great gains in the understanding of their complex magnetic behaviour, with implications for numerous applications. Recent work using Ni NPs as a model soft ferromagnetic (FM) system achieved quantitative understanding of the superparamagnetic blocking temperature-particle diameter relationship1. This hinged, however, on the assumption of an FM NP volume lower than the chemical volume due to a non-FM surface dead shell, indirectly deduced from magnetometry. Here, we completely determine both the chemical and magnetic internal structure of such Ni NPs, via unpolarized, half-polarized, and fully-polarized small-angle neutron scattering (SANS) measurements and analyses, coupled with X-ray diffraction and magnetometry. The postulated magnetic dead shell is not only detected but conclusively identified as a Ni phosphide, derived from the trioctylphosphine commonly used in hot-injection syntheses. The shell thickness (t*) can be controlled by synthesis temperature (Ts), falling to as little as 0.5 nm at 170 °C (Fig. 1). Polarized SANS further reveals bulk-like ferromagnetism in the Ni core, the magnetic scattering length density (SLD) being in ideal agreement with magnetometry (Fig. 2), quantitatively supporting prior modeling of superparamagnetism. These findings advance the understanding of synthesis-structure-property relationships in metallic magnetic NPs, point to a route to ligand-free stabilization, and highlight the power of polarized SANS measurement and analysis capabilities for magnetic NP science and technology. Work supported by the UMN DOE Center for Quantum Materials.

![](https://assets.underline.io/uploads/markdown_image/1/image/d1fcb57216327d31778b53fae71eebc0.jpg)

Fig. 2. Ni core magnetic SLD vs. T (circles) from half-polarized SANS for the NPs at Ts = 170 °C, superimposed on magnetometry field-cooled (FC) (solid line) and zero field-cooled (ZFC) (dashed line) data measured at 50 Oe.


![](https://assets.underline.io/uploads/markdown_image/1/image/9a9b611d4e7684db8891288e9cd064ae.jpg)

Fig. 2. Ni core magnetic SLD vs. T (circles) from half-polarized SANS for the NPs at Ts = 170 °C, superimposed on magnetometry field-cooled (FC) (solid line) and zero field-cooled (ZFC) (dashed line) data measured at 50 Oe.

Chemically-induced Magnetic Dead Shells in Superparamagnetic Ni Nanoparticles from Polarized Small-Angle Neutron Scattering

Cobalt ferrite nanoparticles (NP) are competitive systems in technological and biomedical applications due to a suitable combination of excellent chemical and magnetic characteristics, such as a high chemical stability, surface active sites, and ease of synthesis and functionalization, together with a high anisotropy constant, a high coercivity, and a moderate saturation magnetization. However, in order to control the functional response of Co-ferrite NP, one of the biggest challenges is to quantitatively disentangle the dependence of the overall magnetic response on the composition, structure, or surface chemistry. In this work, we address the key role of the structural defects on the magnetic properties of Co-ferrite NP investigated by element- and site-specific X-ray magnetic circular dichroism (XMCD) combined with high-resolution transmission electron microscopy of individual NP [1]. We studied a series of samples of monodisperse 8 nm NP, prepared by chemical routes [2,3], with similar stoichiometry and cation distribution but with a tunable number of structural defects. Element- specific XAS and XMCD spectra and hysteresis loops reveal that the collinear alignment of Co2+ (Oh) cations is much more affected by the structural disorder than that of the Fe3+ cations. This is because structural defects give rise to effective local anisotropy axes, causing a prevalent Co2+ spin canting through the spin−orbit coupling due to the relatively large value of the partially unquenched moment of these cations. As the structural disorder is further increased, the rest of the cations in the two sublattices are progressively dragged off the ferromagnetic alignment, being Fe3+(Td) cations the least sensitive to defects due to the smaller number of next-nearest neighbors in the Td-sublattice. Our work highlights the importance of combining advanced synthesis with the use of complementary local probes with element, valence, site, and structural sensitivity to enhance the performance of nanoscale magnetic materials.

![](https://assets.underline.io/uploads/markdown_image/1/image/a5aa5eb0e8a24261183ee2e55ee59892.jpg)

HRTEM images for Co-ferriteNP with increasing crystalline quality. Bottom panel: XMCD hysteresis loops measured at 2 K for Co2+ cations in Oh sites for highly-defective (left) and highly-crystalline (right) NP.

Disentangling Atom- and Site- Selective Spin Canting in Co-ferrite Nanoparticles with Tunable Structural Disorder

Developments in the design and fabrication of the high saturation magnetization nanoparticles in the single-domain size regime are highly desirable due to their extensive applications in biomedicine, magnetic resonance imaging, microelectromechanical systems, radio frequency electronics, and soft robotics. In this work, we present for the first time an in-depth investigation of different-sized iron cobalt (Fe65Co35) nanoparticles covering magnetization configurations from flower to vortex to multi-domain states. A finite element micromagnetic model was used to predict magnetic properties of Fe65Co35 nanoparticles ranging in size from 10 to 100 nm. The simulation results reveal that the flower-to-vortex transition of the Fe65Co35 nanoparticles occurs at around 20 nm, corresponding to the particle size with the highest coercivity. A decrease in remanence is expected for larger particle sizes, where simulations showed the most dramatic drop for particles between 40 and 60 nm. Furthermore, the simulated behaviors were verified experimentally. By systematically varying the reaction parameters, Fe65Co35 nanocubes with tunable edge lengths were synthesized. Scanning electron microscope (SEM) images of as-synthesized samples indicate a narrow size distribution. The measured hysteresis loops confirmed the maximum coercive field for cubes of size 17 nm and the sudden decrease in remanence for samples within the range of 40–60 nm. By combining chemical synthesis and micromagnetic calculations, we have experimentally unveiled the size-dependent spin configurations in Fe65Co35 nanoparticles. These results open the door to a more accurate design and control of spin texture, remanence, and coercivity in high magnetization nanoparticles which are vital building blocks of new biomedical and electronic materials.

![](https://assets.underline.io/uploads/markdown_image/1/image/53e8432506b4365fa494f1cd89ca89bd.jpg)

Fig. 1 SEM images of Fe65Co35 nanocubes with various edge lengths, (a) 17 nm, (b) 34 nm, (c) 40 nm, (d) 62 nm, (e) 77 nm, (f) 92 nm, and (g) 97 nm. The scale bars are 200 nm.

![](https://assets.underline.io/uploads/markdown_image/1/image/fe1f235133cb8a4f69fbfa4a62a75798.jpg)

Fig. 2 (a) The simulated spin configuration for a cube of each respective size. (b) Simulated hysteresis loops of a 5×5×5 sphere array with different sizes. (c) Measured hysteresis loops of as-synthesized Fe65Co35 nanocubes at 300 K.

Exploring the Single Domain Limit of High Magnetization Nanoparticles: A Combined Theoretical and Experimental Study

Anthrax is a fatal disease caused by bacteria called bacillus anthracis. The bacteria can be used as a bioterror agent due to its high toxicity and resistance to the external environment of its spore. Dipicolinic acid (DPA) is released when the spore germinates; therefore, sensitively detecting DPA as a biomarker of the spores is focused on by many researchers [1]. Among them, nano-sensors including trivalent lanthanide ions have been used to detect DPA sensitively and promptly because the trivalent lanthanide ion such as europium (Eu3+) and terbium (Tb3+) shows unique fluorescent properties originated from their energy levels, and the intensity can be enhanced by chelating with DPA called antenna effect [2]. However, nanomaterials such as polymers or inorganic nanoparticles containing fluorescent substances usually quench the fluorescence by absorbing the excitation and emission light concurrently [3]. Therefore it is necessary to separate the fluorescent signal from the nanomaterials to improve its sensitivity. Herein, we develop terbium ion-coordinated iron oxide (Fe3O4-Tb) nanoparticles and detect DPA via measuring fluorescent signals from the Tb-DPA complexes [4]. The Fe3O4-Tb nanoparticles have a magnetite (Fe3O4) core and the surface is enveloped by Tb3+ that is coordinated with polyacrylate. Because of the small crystallite size of the Fe3O4-Tb nanoparticle, it has a trivial coercivity; therefore, Fe3O4-Tb nanoparticles do not agglomerate each other and reacts DPA with high reproducibility [5]. When the Fe3O4-Tb nanoparticles contact with DPA, Tb3+ is disassociated from the nanoparticle surface and chelated with DPA. The formed Tb-DPA complexes have a green fluorescence under certain ultraviolet (UV) wavelengths. We separate the iron oxide core via a permanent magnet to prevent quenching of the nanoparticles, then the clear signal shows a lower detection limit (5.1 nM) than the infectious dosage of anthrax (60 μM) [6]. The detecting method also has a high selectivity when compared with other aromatic carboxylic acids, which have a molecular structure similar to DPA.

![](https://assets.underline.io/uploads/markdown_image/1/image/14a2f16e4463e6542107f235846a8f38.jpg)

Fig.1. Detection mechanism for DPA with Fe3O4-Tb nanoparticles and the fluorescent intensity dependent on the concentration of DPA.

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