IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 3, JUNE 2014 3700305

The Behavior of Nano- and Micro-Magnetic ParticlesUnder a High Magnetic Field Using a

Superconducting MagnetSusumu Tokura, Masakazu Hara, Norihito Kawaguchi, and Naoyuki Amemiya

Abstract—Magnetic force has been used for drug delivery, mag-netic separation, and other kinds of micro- and nano-particlehandling. As fundamental studies of these applications, we haveinvestigated the visualization of the motion of micro-magneticparticles under a dynamic magnetic field. We have also studied thecontactless grasp of a micro-magnetic particle suspended in a fluidby using coil currents. In the study, we have developed a methodfor quantitatively estimating the influencing forces. However, theseexperiments have limitations in the magnetic field strength ofnormal conducting electromagnets or permanent magnets. There-fore, much higher magnetic force is needed for the applicationto drug delivery or cell/DNA manipulation. High magnetic fieldgenerating high magnetic force saturates the magnetization ofmagnetic particles during the particle control, and the magneticsaturation influences the particle behavior. However, such behav-ior of magnetic particles has not been sufficiently studied yet. Inthis paper, the behavior of the particles nearly or fully saturatedunder the high magnetic field of a superconducting magnet wasvisualized successfully, and ferrite and feeble magnetic particleswere introduced in order to gain insight into their behavior.

Index Terms—Feeble magnetic particle, ferrite particle, particlebehavior, superconducting magnet.

I. INTRODUCTION

MAGNETIC forces on micro- or nano-particles such asferrite or feeble magnetic particles in fluid can be used

for drug delivery, cell/DNA manipulation, and other variousapplications [1]–[5]. A biomimetic, microscale system applyinga uniform or cyclic, rotational magnetic field to microswimmershas been reported [4]; cell alignment on a glass plate usingan external static magnetic field has also been reported [6].On the other hand, as fundamental studies of these applica-tions, Kikura et al.[7], [8] reported the flow visualization ofprimary agglomerated magnetic particles considering Brownianmotion. We also have studied the visualization of the motionof micro-magnetic particles under a dynamic magnetic field[9]. Furthermore, we have studied the contactless grasp of amicro-magnetic particle suspended in a fluid at rest or in motionby using coil currents [10]. In the study, we have developed

Manuscript received July 17, 2013; accepted October 7, 2013. Date ofpublication October 30, 2013; date of current version November 15, 2013.

S. Tokura, M. Hara, and N. Kawaguchi are with the IHI Corporation,Yokohama 235-8501, Japan (e-mail: susumu_tokura@ihi.co.jp).

N. Amemiya is with the Department of Electrical Engineering, Kyoto Uni-versity, Kyoto 615-8510, Japan (e-mail: amemiya.naoyuki.6a@kyoto-u.ac.jp).

Digital Object Identifier 10.1109/TASC.2013.2287639

a method for quantitatively estimating the influencing forces,such as magnetic force, gravity, buoyancy, and the force be-tween the magnetized magnetic particles themselves [11], [12].Magnetic torque [13] also causes them to align their magneticmoments to the direction of the applied magnetic field. How-ever, these experiments have limitations in the magnetic fieldstrength of normal conducting electromagnets or permanentmagnets. Therefore much higher magnetic force is needed forthe application to drug delivery or cell/DNA manipulation.

High magnetic field generating high magnetic force saturatesthe magnetization of magnetic particles during the particlecontrol, and the magnetic saturation influences the particlebehavior; some particles will saturate immediately but otherparticles will not saturate due to the difference in magneticproperties. However, such behavior of magnetic particles hasnot been sufficiently studied yet.

This paper reports experimental observations of the behaviorof nano- and micro-magnetic particles nearly or fully saturatedunder the high magnetic field of a superconducting magnetand that of feeble magnetic particles, as an initial step towardsmagnetic manipulation of such particles’ motions.

II. EXPERIMENTAL METHODS

A. Materials

Magnetic ferrite particles (diameter, 300 nm–300 μm) inpure water in a vessel were used. The magnetization of theparticles was measured using a vibrating sample magnetometer(VSM). The magnetization ratio χ was 2.3 (magnetic fieldintensity H = 4× 104 A/m), and the saturated magnetizationwas 1.8× 105 A/m.

In addition, feeble magnetic particles (diameter, 100–1000 nm) in pure water were tested. The saturated magnetiza-tion was 6.8× 103 A/m.

B. Experimental Setup

The experimental setup is shown in Fig. 1. A cryogen-free superconducting magnet having a maximum magnetic fluxdensity of 10 T and maximum excitation speed of 10 T/15 minwas used for magnetic excitation. The diameter and the lengthof the bore were 100 mm and 475 mm. The superconductingmagnet was situated with the bore axis perpendicular to theground. A quartz vessel (10× 10× 50 mm) was set at the

1051-8223 © 2013 IEEE

3700305 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 3, JUNE 2014

Fig. 1. Experimental setup of visualization system in the bore of a super-conducting magnet. Forces affected to the magnetic particles are shown in thefigure.

position in the bore where the particles could be observed, andsilicone tubes (diameter, 6 mm) were connected to the inlet andoutlet of the vessel in order to supply pure water including themagnetic particles. In addition, a magnetic probe was set abovethe quartz vessel in order to measure the magnetic flux densityat the observation position.

The particle tracking velocimetry (PTV) method with greenlaser illumination was applied to visualize the behavior of theparticles in the fluid in the quartz vessel, which was set insidethe bore of the superconducting magnet. The laser illuminatedparticles from the side of the vessel, while the behavior of theparticles was recorded through the borescope from the frontof the vessel. The image analyzer was positioned far from themagnet, while the camera was set directly on top of the bore,and enclosed by a magnetic shield box to avoid electromagneticinterference.

C. Experimental Methods

After readying the apparatus for exciting the superconductingcoil at the temperature of 4.6 K, a pump for circulating theparticles was turned on, and the fluid containing the magneticparticles was supplied to the vessel in the bore of the supercon-ducting magnet for observation at the designated rate of fluidvelocity. Here, the position where the magnetic field gradientwas the highest was 90 mm below the upper face of the bore.The magnetic flux density was 1.7 T (3 T at the center of thebore), and the magnetic field gradient was 19.9 T/m at the

Fig. 2. Typical particle images of ferrite particle behavior in the fluid excitedup to 1.7 T without forced circulation by the pump (a), and with forcedcirculation (b). The image in (b) is 2.5 times as large area as the imagein (a).

position, so that the magnetic particles were able to be fullysaturated.

The magnetic particles were affected by an upward magneticforce in the same direction as buoyancy when the particlesflowed to the underside from the center of the bore due tothe particles being supplied from the bottom end of the bore.On the other hand, the particles were affected by a downwardmagnetic force in the same direction as gravity when theparticles flowed up from the center of the bore. As the positionof the particle observation was upper side from the center ofthe bore, the particles were affected by a downward magneticforce.

The recording of particle images began when the particlesreached the observation area. Excitation by the superconductingcoil was started 20 seconds after the start of recording. Here,the excitation speed was set to 0.17 T/min at the observationposition in order to avoid a rapid increase of the coil temper-ature. The image recording was stopped when the magneticflux density at the observation position reached 1.7 T. Afterchanging the condition of the particle supply as necessary, theparticle image recording was started again and after 20 seconds,the superconducting coil current was decreased by 0.17 T/min.The relation between the magnetic flux density and particlevelocity was analyzed by image processing of the particleimages after the particle recording stopped, when the systemreached 0 T.

III. RESULTS AND DISCUSSIONS

A. Behavior of the Micro- and Nano-Ferromagnetic Particles

The magnetic flux density was increased to 1.7 T and de-creased to 0 T at the speed of 0.17 T/min. Ferromagneticparticles were used, and the recorded data were analyzed todetermine the velocity of individual particles and gain insightinto particle behavior.

Fig. 2 shows the typical particle images of particle behaviorin the fluid excited up to 1.7 T without forced circulation by thepump (a), and with forced circulation (b). As shown in Fig. 2(a),Large chain-clustered particles were observed in 8 seconds

TOKURA et al.: NANO- AND MICRO-MAGNETIC PARTICLES UNDER A HIGH MAGNETIC FIELD 3700305

Fig. 3. Relation between magnetic flux density and particle velocity measuredby the PTV method from 0 T up to 0.25 T with a circulating pump. Symbolcircle shows upward particle velocity under magnetic flux density ranging from0 to 0.1 T, and symbol square shows upward particle velocity under magneticflux density ranging from 0.05 to 0.25 T.

(magnetic flux density: 0.02 T) after starting excitation. InFig. 2(b), the particle velocity in the fluid at 0 T was about270 μm/s upward, and particles could still be observed undera magnetic flux density of 1.4 T due to the forced circulation.Fig. 3 shows the particle velocity measured by a PTV methodfrom 0 T up to 0.25 T with forced circulation. Here, the Y-axisshows the upward particle velocity, and the X-axis shows themagnetic flux density at the observation position. As shown inFig. 3, the velocity of the particles that were carried by theupward viscous force decreased rapidly right after magneticexcitation. On the other hand, the deceleration of the particlevelocity had become low at around 0.1 T.

Here, before the magnetic saturation, the magnetic force Fpb

is given by

Fpb = Vp · (χHa) · ∇(μ0Ha) (1)

where μ0 is the permeability, Vp is the volume of the particle,Ha is the applied magnetic field intensity, and χ is the magne-tization ratio. After the magnetic saturation, the magnetic forceFpa is given by

Fpa = Vp · (Msat) · ∇(μ0Ha) (2)

where Msat is the saturated magnetization which is a constantvalue.

As we expected from expression (1) and (2), it was foundthat the downward magnetic force affecting to the magneticparticles increased in proportion to the square of the magneticflux density when the magnetization of the magnetic particlewas not saturated. On the other hand, the magnetic forceincreased in proportion to the magnetic flux density when themagnetization was saturated. From these results, a noticeabledifference in the particle behavior could be seen in the conditionaround the magnetic saturation.

Fig. 4. Typical images of nano-ferrite particles that were able to be draggedby magnetic force.

Fig. 5. Relation between magnetic flux density and velocity of nano-ferriteparticles that were dragged downward.

Fig. 4 shows images of nano-magnetic particles that did notundergo sedimentation by gravity under no magnetic field andwere dragged after the magnetic excitation process. Here, inorder to extract nano-particles from other micro-particles, theforced circulation by the pump was stopped, and the particleswith diameters more than several μm fell to the bottom withinabout 20 minutes. When the sedimentation of particles bygravity was no longer observed, image recording and excitationof the remaining nano-particles up to 1.7 T were carried out.As shown in Fig. 4, the particles were dragged remarkablydownward by the magnetic force at the magnetic flux densityof 0.5 T. The velocity of these particles that were draggeddownward was measured using a PTV method, and is shownin Fig. 5. From these results, it was found that the nano-magnetic particles could be dragged downward at a velocityof approximately 100 μm/s. Here, the velocity of the particlestended not to increase under the magnetic flux density of morethan 0.4 T. The reason for this tendency is that much the smallersize nano-particles remained while the nano-particles with largediameter decreased according to increase of the magnetic fluxdensity without forced circulation.

3700305 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 3, JUNE 2014

Fig. 6. Typical images of feeble magnetic particles behavior in fluid underdecrease of excitation to 0 T without a circulating pump. The images are feeblemagnetic particle behavior under the magnetic flux density ranging from 1.69to 1.65 T.

Fig. 7. Relation between magnetic flux density and feeble magnetic par-ticles. Symbol circle shows upward particle velocity under magnetic fluxdensity ranging from 0 to 1.7 T. Symbol square and triangle show upwardand downward particle velocity under magnetic flux density ranging from1.7 to 0 T.

B. Behavior of the Feeble Magnetic Particles

Feeble magnetic particles were also tested. The magneticflux density was increased to 1.7 T at the speed of 0.17 T/minunder the initial particle velocity of 200 μm/s upward by forcedcirculation of the pump; after 1.7 T was reached, the pump wasstopped. Then, the magnetic flux density was decreased to 0 Tat −0.17 T/min.

Fig. 6 shows the typical images of the particle behavior withthe decrease in excitation down to 0 T without forced circula-tion. There were almost no large chain-clustered particles underthe magnetic flux density of 1.7 T, and some particles flowedupward by fluid flow and some downward by the magneticforce, opposite to the fluid flow. Fig. 7 shows the relationbetween the magnetic flux density and the velocity of particles.

During the magnetic excitation up to 1.7 T, the particleswere continuously supplied by forced circulation, and theiraverage velocity was approximately 200 μm/s. During thedrop in magnetic excitation down to 0 T, some particles stillflowed upward at the velocity of approximately 20 μm/s. At the

magnetic flux density of 1.7 T, other particles flowed downwardat the velocity of approximately 20 μm/s. On the other hand, atthe magnetic flux density of 0.5 T, the velocity of the particleswhich flowed downward became 0 μm/s; that is to say, therewas no particle that was dragged downward by the magneticforce. Contrary to the case of ferromagnetic particles, there wasno significant change in the particle velocity because the feeblemagnetic particles seemed to have already reached magneticsaturation under the magnetic flux density of 0.5 T.

From these results, feeble magnetic particles with muchlower magnetization than ferrite particles were dragged suc-cessfully under magnetic flux density ranging from 0.5 to 1.7 T.There were almost no large chain-clustered particles at themagnetic flux density of 1.7 T.

IV. CONCLUSION

The magnetic flux density at the observation position in thebore of the superconducting magnet was increased up to 1.7 Tat the rate of 0.17 T/min and then decreased at the reverse rate.Into these conditions, ferrite and feeble magnetic particles wereintroduced in order to gain insight into their behavior, especiallytheir capability of being dragged magnetically in a fluid. Thefollowing results were obtained from this study.

1) The motion of individual particles under high magneticfields was successfully visualized.

2) PTV analysis showed a remarkable difference in thevelocity of ferrite particles before and after magneticsaturation.

3) Feeble magnetic particles with much lower magnetizationthan ferrite particles were dragged successfully undermagnetic flux density ranging from 0.5 to 1.7 T.

4) Less aggregation into short-chain clusters was observedfor feeble magnetic particles than for ferrite particles.

These results constitute useful information for studies on theissues in the handling of micro- or nano-magnetic particles.

ACKNOWLEDGMENT

The authors would like to thank Prof. T. Hikihara,Prof. T. Matsuo, and Prof. T. Nakamura for their detailedcomments and suggestions.

REFERENCES

[1] A. Nacev, C. Beni, O. Bruno, and B. Shapiro, “The behaviors of fer-romagnetic nano-particles in and around blood vessels under appliedmagnetic fields,” J. Magn. Magn. Mater., vol. 323, no. 6, pp. 651–668,Mar. 2011.

[2] C. Wilhelm, F. Gazeau, and J.-C. Bacri, “Rotational magnetic endosomemicrorheology: Viscoelastic architecture inside living cells,” Phys. Rev. E,vol. 67, no. 6, pp. 061908-1–061908-12, Jun. 2003.

[3] E. P. Furlani and K. C. Ng, “Nanoscale magnetic biotransport with ap-plication to magnetofection,” Phys. Rev. E, vol. 77, no. 6, pp. 061914-1–061914-8, Jun. 2008.

[4] U. K. Cheang, D. Roy, J. H. Lee, and M. J. Kim, “Fabrication and mag-netic control of bacteria-inspired robotic microswimmers,” Appl. Phys.Lett., vol. 97, no. 21, pp. 213704-1–213704-3, Nov. 2010.

[5] S. Ostergaard, G. Blankenstein, H. Dirac, and O. Leistiko, “A novel ap-proach to the automation of clinical chemistry by controlled manipulationof magnetic particles,” J. Magn. Magn. Mater., vol. 194, no. 1–3, pp. 156–162, Apr. 1999.

TOKURA et al.: NANO- AND MICRO-MAGNETIC PARTICLES UNDER A HIGH MAGNETIC FIELD 3700305

[6] J. Cai, Y. Li, X. Li, and D. Zhang, “Behavior of micromagnetic particlesin an opposed-poles orientation magnetic field,” J. Magn. Magn. Mater.,vol. 246, pp. 36–39, Apr. 2002.

[7] H. Kikura, J. Matsushita, O. Hirashima, M. Aritomi, and I. Nakatani,“Flow visualization and particle size determination of primary clusters ina water-based magnetic fluid,” J. Magn. Magn. Mater., vol. 289, pp. 392–395, Mar. 2005.

[8] H. Kikura, J. Matsushita, N. Kakuta, M. Aritomi, and Y. Kobayashi,“Cluster formation of ferromagnetic nano-particles in micro-capillaryflow,” J. Mater. Process. Technol., vol. 181, no. 1–3, pp. 93–98, Jan. 2007.

[9] S. Tokura, M. Hara, N. Kawaguchi, J. Izawa, and N. Amemiya, “Vi-sualization of magnetic microparticles in liquid and control of theirmotion using dynamic magnetic field,” J. Appl. Phys., vol. 107, no. 9,pp. 09B521-1–09B521-3, May 2010.

[10] S. Tokura, M. Hara, N. Kawaguchi, J. Izawa, and N. Amemiya, “Con-tactless grasp of a magnetic particle in a fluid and its application toquantifications of forces affecting its behavior,” J. Magn. Magn. Mater.,vol. 353C, pp. 82–89, Nov. 2013.

[11] K. Keshoju, H. Xing, and L. Sun, “Magnetic field driven nanowire rotationin suspension,” Appl. Phys. Lett., vol. 91, no. 12, pp. 123114-1–123114-3,Sep. 2007.

[12] Y. Ido, T. Yamaguchi, and Y. Kiuchi, “Distribution of micrometer-size particles in magnetic fluids in the presence of steady uniformmagnetic field,” J. Magn. Magn. Mater., vol. 323, pp. 1283–1287,2010.

[13] R. Pastor-Satorras and J. M. Rubi, “Dipolar interactions induced orderin assemblies of magnetic particles,” J. Magn. Magn. Mater., vol. 221,pp. 124–131, 2000.