Inorganica Chimica Acta 460 (2017) 114–118

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Inorganica Chimica Acta

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Research paper

Role of magnetic concentration in modulating the magnetic propertiesof ultra-small FePt nanoparticles

http://dx.doi.org/10.1016/j.ica.2016.09.0200020-1693/� 2016 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: jrinehart@ucsd.edu (J.D. Rinehart).

Pius O. Adelani, Aaron N. Duke, Benjamin H. Zhou, Jeffrey D. Rinehart ⇑Contribution from the Department of Chemistry and Biochemistry, University of California, San Diego, San Diego, CA 92093, United States

a r t i c l e i n f o

Article history:Received 5 August 2016Received in revised form 8 September 2016Accepted 10 September 2016Available online 12 September 2016

Keywords:FePt nanoparticlesSuperparamagnetismMagnetic blockingPolyol synthesisSurface modification

a b s t r a c t

Magnetic characterization of nanoscale materials is often hindered by the role that sample preparationtechniques play in the determination of interparticle interaction strength. Well-dispersed d = 2.6(4) nmFePt nanoparticles synthesized by a slight modification of a known polyol synthesis were employed tostudy this effect at the extreme lower limit of the nanoscale regime. Suspension in a diamagnetic matrixmaterial at varying concentrations was used to characterize the relationships between average particledistance and representative magnetic properties (zero-field cooled/field-cooled (ZFC/FC) magnetizationcurves, hysteresis M(H) loops at 5 K, and ac-susceptibility). By increasing the interparticle distancethrough diamagnetic dilution, the blocking temperature (TB), anisotropy energy barrier (Ueff), and coer-cive field (Hc) drop continuously until reaching a dilution ratio where the magnetic signals were limitedby the competing diamagnetic contribution from the matrix material. Long-timescale blocking tempera-ture and coercivity are relatively unaffected by particle dilution while the shorter timescale anisotropyenergy barrier (Ueff) and attempt time (s0) are strongly affected. These results demonstrate the needfor well-defined sample preparation conditions when comparing materials properties, especially thosewith applications dependent on superparamagnetism.

� 2016 Elsevier B.V. All rights reserved.

1. Introduction

The continuous demand for higher density magnetic and spin-tronic device architectures has pushed magnetic nanomaterialsdesign to the edge of the molecular regime (1–10 nm) [1,2]. As insemiconductor nanoparticles, size distribution, morphology, andsurface structure can play an outsized role in this size range.Importantly, however, the magnetic characteristics in this regimecan be far more dependent on interparticle interactions, whichcan compete with or even outweigh the effects of intrinsic proper-ties [3]. This means that material characteristics can vary wildlybased on individual preparations of a material – despite havingthe same crystal structure and composition. Properties such ascoercive field (Hc), superparamagnetic blocking temperature (TB),saturation magnetization (Ms) and remnant magnetization (Mr)provide important quality benchmarks for comparing and improv-ing nanoscale magnetic materials; yet, as these properties are thephysical manifestation of intrinsic (innate crystal structure) andextrinsic (surface composition, ligands, morphology, and interpar-ticle spacing) properties, there are frequently large discrepancies

between observations depending on sample preparation [4,5].These discrepancies are not only a matter of magnitude but alsoof sign due to a complex interplay of dipolar interactions, exchangecoupling, and particle clustering effects [6,7]. Certain investiga-tions have demonstrated enhanced coercivity (Hc) and remanentmagnetization (Mr) with increased interparticle distance [8,9].However, observations on other magnetic nanomaterial prepara-tions show no effect on coercivity and a significant drop in blockingtemperature (TB) accompanying diamagnetic dilution [10,11]. Thisdependence on sample preparation can severely limit the abilityfor synthetic chemists to participate in the optimization of materi-als and comparison of findings to those of other researchers.

Most magnetic materials with sub-10 nm diameters are super-paramagnetic as their structures lack the intrinsic anisotropy tosupport magnetic order. One material capable of retaining single-domain ferromagnetism at very small sizes is the binary inter-metallic, FePt. Investigations of the magnetic properties of FePtnanoparticles have increased in recent years due to the ease ofsynthesis in nanoparticle form [2,12–15], and promise for applica-tions in high-density magnetic storage devices (e.g., hard diskdrives), read-write technology, and high-performance permanentmagnets [14–17]. The primary form of technological interest forFePt is the ordered face centered tetragonal (L10) phase (Fig. 1).

Fig. 1. TEM image of the as-synthesized, A1-phase FePt nanoparticles (2.6 ± 0.4 nm)and structure visualization of A1 (disordered, fcc) and L10 (ordered, fct) phases ofFePt.

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This phase has a high Curie temperature (Tc = 750 K) and largemagnetic anisotropy (6.6 MJ/m3), making it desirable as a compo-nent in very small hard magnetic devices. Often the synthetic pre-cursor to formation of L10 FePt is the disordered face centeredcubic (A1) phase [18]. Besides its synthetic accessibility, the A1phase of FePt has been suggested as an important compositionfor use in applications requiring ultra-small superparamagnets[19–21]. We analyze the A1 phase herein because it allows for awider temperature range for characterization around TB, it is lesssusceptible to particle clustering, and it has low temperaturedynamics that facilitate immobilization and dilution. These factorsmake it an ideal system to study the extent to which post-syntheticsample preparation determines the strength of various magneticproperties in sub-3 nm FePt.

2. Synthesis and characterization

2.1. General considerations

The synthesis of FePt nanoparticles was carried out using com-mercially-available reagents without further purification. Platinum(II) acetylacetonate (acetylacetonate = acac) (97%, Aldrich), iron(III) acac (99.9%, Aldrich), oleylamine (70%, Aldrich), dioctyl ether(99%, Aldrich), 1,2-hexadecanediol (98%, TCI), oleic acid (90%, AlfaAesar), n-eicosane (99%, Alfa Aesar), ethanol (99.5%, Fisher Scien-tific) and hexane (98.5%, Fisher Scientific) were used as received.

2.2. FePt nanoparticle synthesis

Nanoparticles of FePt were synthesized using a modification ofa known procedure [22]. A three-necked flask was charged with Pt(acac)2 (196.6 mg, 0.5 mmol), Fe(acac)3 (176.6 mg, 0.5 mmol),1,2-hexadecanediol (520 mg, 2 mmol), oleylamine (170 lL,0.5 mmol), dioctyl ether solution (20 mL) and oleic acid (160 lL,0.5 mmol) under dinitrogen atmosphere. The mixture wasdegassed under reduced pressure (10�3 torr) at 100 �C for 30 minwhile stirring. The solution was then re-exposed to a dinitrogenatmosphere and heated to 286 �C for 30 min to allow nanoparticleformation and growth. The black colloidal suspension wasremoved from heat and allowed to cool to room temperature. Etha-nol was then added and the black product separated by centrifuga-tion. The supernatant was discarded and the black residuedispersed in hexane. This washing procedure was repeated threetimes to remove unreacted starting materials and excess ligand.

The d = 2.6(4) nm fcc FePt nanoparticles were identified by powderX-ray diffraction (Fig. S1) [22] and their size and morphology weredetermined by Transmission Electron Microscopy (TEM, Fig. 1).

2.3. Thermogravimetric analysis and differential scanning calorimetry(TGA/DSC)

Sample measurement was carried out under a flow of nitrogenwith a TGA Q5000 and DSC Q1000 from TA Instruments. A powdersample of FePt (11.3 mg) was heated at 5 �C/min from 25 to 900 �C(Fig. S2).

2.4. Transmission electron microscopy (TEM)

TEM images were obtained using an FEI Spirit operating at120 kV with a 2 k � 2 k Gatan CCD camera. FePt sample for TEManalysis were prepared by air-drying a dilute hexane solution oncarbon-coated copper grids.

2.5. Magnetic measurements

Magnetic measurements were performed using a QuantumDesign MPMS3 3rd Generation SQUID Magnetometer. Samplesfor dilution studies were prepared by diluting d = 2.6(4) nm FePtnanoparticles in n-eicosane. Preparation of the samples describedherein were performed by suspending 11.6 mg (2.2%), 26.3 mg(4.6%), 36.4 mg (6.2%), 47.0 mg (8.0%), 72.1 mg (12%) in 35 mg ofn-eicosane at >70 �C followed by immediate freezing to preventparticle agglomeration and settling. Particles were found to dis-perse well even after multiple heating and freezing cycles withno change in the magnetic data.

The FePt/n-eicosane mixtures were added to 7 mm quartztubes, evacuated, and sealed under static vacuum with a hydro-gen/oxygen flame. Dilution in n-eicosane and sealing was per-formed within 12 h of synthesis to minimize the effects ofsample oxidation.

2.6. Energy-dispersive X-ray spectroscopy (EDS)

EDS measurements were performed on an FEI/Philips XL30Environmental SEM with an Oxford EDS system. The FePt samplewas prepared as a powder dispersed on carbon tape.

3. Results and discussion

Nanoparticles of the A1 phase of FePt were prepared by modifi-cation of a known procedure as described in the experimental sec-tion (Fig. 1) [22]. Oleic acid and oleylamine introduced during thesynthesis promote dispersal in non-polar solvents and prevent par-ticle agglomeration. The small size, low magnetic moment andligation allow dispersal for several weeks without significant set-tling. TEM images of the particles demonstrate an average particlediameter of d = 2.6(4) nm and powder X-ray diffraction displayspeaks consistent with the disordered A1 structure (Fig. S1). Theaverage Fe:Pt ratio was determined by EDS to be 1:2.4(6). This isconsistent with small fcc FePt with a Pt-heavy surface layer.

To study the effects of increasing interparticle separation,diluted samples were prepared for comparison to the concentratedpowder sample. Dilution of the particles was achieved by colloidalsuspension in n-eicosane at different mass concentrations (FePt–n;n = 2.2, 4.6, 6.2, 8.0, and 12%). Since n-eicosane is solid at roomtemperature, this process allows indefinite storage of well-dis-persed, unoxidized samples. To avoid sample variability due toprecipitation, particle growth, or oxidation all dilution sampleswere prepared simultaneously from the same stock of FePt

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nanoparticles. For comparison, an undiluted sample was precipi-tated from the colloidal solution and dried at 298 K under nitrogenatmosphere (FePt–100).

Initial magnetic characterization of the diluted samplesinvolved determining the long-time-scale blocking temperature(TB) through analysis of the temperature dependence of the mag-netization. It should be noted that TB is always defined in the con-text of the timescale of the measurement, which, for the case of thezero-field cooled (ZFC) and field-cooled (FC) measurements dis-cussed here, refers to the time required to collect a dc scan datapoint (sP 10 s). Under ZFC and FC (1000 Oe) conditions (Fig. 2),all FePt–n samples display similar trends indicative of low-temper-ature superparamagnetism [23]. At T � 20 K a stark divergence isseen between the magnetization under FC and ZFC conditionsdue to thermal blocking of magnetization reversal, as expectedfor A1 FePt nanoparticles. The various dilutions display a mono-tonic increase in TB with increasing sample concentration fromFePt–2.2 (TB = 17.4 K) to FePt–12 (TB = 20.1 K) as shown in Table 1.Moreover, the minimal change (DTB � 2.7 K) over all sample con-centrations indicates that dipolar interactions between particlesonly have a small effect on the long-timescale blocking of evenminimally diluted samples. If, however, the sample is undiluted(FePt–100), TB drops to 11.7 K, indicating that some minimal sepa-ration of particles is necessary to see isolated single-domainbehavior. Similar dilution studies on Fe3O4 nanoparticles indicatea much large dependence of TB on concentration, possibly due tomore ordered crystal structures with well-defined anisotropy[10,11].

A significant variation between FePt-n samples can be seen inthe comparison between the peak in ZFC and the divergence pointbetween ZFC and FC datasets. This difference is an indication of thevariation in relaxation times and reaches a maximum in FePt–12and FePt–100 (DT � 6 K). This wide range of magnetic relaxationtimes denotes a heterogenization of the relaxation timescale dueto clustering of some particles while others remain relatively wellisolated.

The trend in the magnitude of the magnetization is also of inter-est, with a factor of two increase across all temperatures withincreasing concentration from FePt–2.2 to FePt–12. Samples wereprepared simultaneously and so the differences cannot be attribu-ted to different levels of oxidation. Thus, cooperative effectsbetween particles must be responsible for the increase. This is

Fig. 2. Magnetization plotted as a function of temperature for all FePt–n samplesmeasured in zero-field cooled (ZFC) and field cooled (FC) modes.

important for optimizing particles for single-particle applicationssuch as magnetic biosensing and single-domain data storage ascollective behavior is not indicative of how the final material willbe utilized. Therefore, proper dilution is necessary to ensure thatthe actual single-particle properties are being measured. In con-trast to the small difference observed for TB, the magnetization isstrongly dependent on achieving a well-diluted sample.

The coercive field (Hc) is another important magnetic measurethat can be strongly influenced by non-intrinsic factors such asparticle surface and interparticle effects. Measurements of magne-tization vs. magnetic field were performed well below TB (T = 5 K)to study the effect of dilution in our FePt samples. Fig. 3 shows thechange in magnetization between the remanent magnetization(Mr) and Hc for all FePt–n samples. Previous studies of FePtnanoparticles have shown a strong size dependence of Hc due tothe dependence of the superparamagnetic energy barrier on parti-cle volume [24]. Many examples of this relationship exist in the lit-erature, but the sample preparation method often consists ofdrying under reducing conditions, thermal annealing, or is simplyleft unspecified. Interestingly, we find that the most concentratedsamples behave roughly like independent particles, with very littlechange in Hc induced by dilution. A small shift of DHc � �500 Oe isseen for the most dilute sample (FePt–2.2) and the most concen-trated sample (FePt–100) indicating that although interparticleinteractions play a minor role on this timescale, they are not com-pletely irrelevant. Likely, FePt–2.2 only exhibits very weak dipolarinteractions, but at slightly higher concentrations the particles canbenefit from cooperative behavior. In the FePt–100 sample, theparticles are very close but not sintered in such a way as toincrease their effective volume. This leads to competition betweendifferent dipolar interaction channels and a consequent decrease inhardness.

A more nuanced understanding of the relaxation properties canbe gained from alternating current (ac) susceptibility measure-ments. The ZFC/FC and M vs. H measurements can only showwhether the magnetization is blocked (long timescale comparedto the measurement technique) or unblocked (short timescalecompared to the measurement technique). With ac magnetic sus-ceptibility we can both quantify the relaxation time and determinethe energy barrier for the magnetization reversal process. The in-phase (v0) and out-of-phase (v00) components of the susceptibilitywere measured at frequencies from 1–1000 Hz and temperaturesfrom 2–30 K (Figs. S4, S5). The data for all samples show a fre-quency dependence indicative of superparamagnetism [25,26].The characteristic relaxation time (s) for each frequency was esti-mated from the peak maxima in v00. This estimation was usedbecause the data could not be fit to a generalized Debye equationdue to the weak frequency dependence and relatively wide distri-bution of relaxation times. These data were found to be linear in aplot of ln(s) vs. 1/T indicating an Arrhenius relationship (Fig. 4).Data for FePt–100 showed no reliable signal due to the wide relax-ation time distribution and thus are not included. For all othersamples, the anisotropy energy barrier (Ueff) and attempt time(s0) were determined by fitting the temperature dependence ofthe experimentally determined relaxation times to an Arrheniusequation (Table 1). For the diluted samples, the value of Ueff

increases with increasing concentration, roughly mirroring thechanges in the long-timescale blocking temperature, TB, observedin the FC/ZFC data. The indication that concentration plays animportant role in the relaxation even at very low particle concen-trations is corroborated by the s0 values displayed by the dilutedparticles. Attempt times for independent superparamagnets gener-ally fall within the range of 10�9–10�11 s, yet we find a range from6.75 � 10�12 s (FePt–2.2) all the way down to 6.00 � 10�13 s (FePt–8.0). This indicates that on the ac susceptibility timescale, the con-centration of particles plays an important role in their properties,

Fig. 3. (Top) Plot of magnetization vs. magnetic field for FePt–100 demonstratingmagnetic hysteresis at 5 K. (Bottom) Enlarged view of the magnetization vs.magnetic field in the second quadrant for all FePt-n samples at T = 5 K showing thevariation in coercivity. The data for FePt-4.6 and FePt-6.2 overlap on the size-scaleshown. Full hysteresis plots for all samples are shown in Fig. S3.

Fig. 4. Plots of the natural log of the relaxation time vs inverse temperature fordiluted samples FePt–2.2 through FePt–12. Solid black lines represent fits to theArrhenius equation. The data for FePt-8 and FePt-12 are overlapping. IndividualArrhenius plots are shown in Fig. S6.

Table 1Magnetic characteristics of FePt nanoparticles.

FePt–n Blocking Temp (TB) Ueff s0 (s) Hc (Oe)

n = 2.2 17.4 230 6.75 � 10�12 14004.6 18.1 239 3.20 � 10�12 19006.2 19.2 242 4.00 � 10�12 19008.0 20.0 276 6.00 � 10�13 190012 20.1 276 5.62 � 10�13 1800100 11.7 – – 1600

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with even highly dilute samples displaying deviations from theexpected properties of the single particle.

4. Conclusions

To efficiently customize and optimize the properties of mag-netic nanoparticles, it is essential to be able to define propertiessuch as Mr, Hc, Ueff, and TB from measurements on bulk quantitiesof material. This means there must be clear indications of howinterparticle interactions are affecting the data before it can becompared to data from similar compounds. Analyzing the proper-ties of 3 nm fcc FePt nanoparticles using ZFC/FC, magnetization vs.magnetic field, and ac-susceptibility data demonstrates that caremust be taken in ensuring proper separation of particles to

suppress interparticle magnetic interactions. Our data show thatfor these ultra-small FePt particles, long-timescale dynamics arerelatively unaffected by particle dilution with changes in the block-ing temperature of only a few Kelvin, however the shorter time-scale dynamics are strongly affected. These differences are vitalfor potential spintronic applications involving high-frequencyswitching of the magnetization, magnetic biosensing applicationswhere averaging over many relaxation decays in a short periodof time is necessary, and magnetic hyperthermia where the mag-netization relaxation time is directly related to the efficiency ofheat production [24]. Future studies will utilize the extremely highsurface-to-volume ratios of these particles to study the ability totune magnetic strength through varying the ligand electronicstructure and introducing shell materials. We will also extendthe research reported here to make ultra-small L10 FePt and studyits concentration dependence to determine the extent to which itmay be miniaturized for high-density data applications.

Acknowledgements

We thank the Hellman Fellows Fund and the University of Cal-ifornia, San Diego for support of this research and Michael DennyJr. for assistance with TGA and DSC data collection.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.ica.2016.09.020.

References

[1] G.I. Bourianoff, P.A. Gargini, D.E. Nikonov, Solid State Electron. 51 (2007) 1426.[2] L. Wu, A. Mendoza-Garcia, Q. Li, S. Sun, Chem. Rev. (2016).[3] S.T. Xu, Y.Q. Ma, G.H. Zheng, Z.X. Dai, Nanoscale 7 (2015) 6520.[4] B. Balamurugan, D.J. Sellmyer, G.C. Hadjipanayis, R. Skomski, Scr. Mater. 67

(2012) 542.[5] R. Skomski, P. Manchanda, P. Kumar, B. Balamurugan, A. Kashyap, D.J. Sellmyer,

IEEE Trans. Magn. 49 (2013) 3215.[6] D. Kechrakos, K.N. Trohidou, J. Magn. Magn. Mater. 262 (2003) 107.[7] J. Bansmann, S.H. Baker, C. Binns, J.A. Blackman, J.P. Bucher, J. Dorantes-Davila,

V. Dupuis, L. Favre, D. Kechrakos, A. Kleibert, K.H. Meiwes-Broer, G.M. Pastor, A.Perez, O. Toulemonde, K.N. Trohidou, J. Tuaillon, Y. Xie, Surf. Sci. Rep. 56 (2005)189.

118 P.O. Adelani et al. / Inorganica Chimica Acta 460 (2017) 114–118

[8] T.Z. Yang, C.M. Shen, Z. Li, H.R. Zhang, C.W. Xiao, S.T. Chen, Z.C. Xu, D.X. Shi, J.Q.Li, H.J. Gao, J. Phys. Chem. B 109 (2005) 23233.

[9] W. Chen, C. Chen, L. Guo, J. Appl. Phys. 108 (2010) 043912.[10] C.J. Bae, Y. Hwang, J. Park, K. An, Y. Lee, J. Lee, T. Hyeon, J.G. Park, J. Magn. Magn.

Mater. 310 (2007) e806.[11] C.J. Bae, S. Angappane, J.G. Park, Y. Lee, J. Lee, K. An, T. Hyeon, Appl. Phys. Lett.

91 (2007) 102502.[12] S.H. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 2000 (1989) 287.[13] S. Zhang, J. Lee, S. Sun, Open Surf. Sci. J. 4 (2012) 26.[14] R.H. Kodama, J. Magn. Magn. Mater. 200 (1999) 359.[15] N.A. Frey, S. Peng, K. Cheng, S.H. Sun, Chem. Soc. Rev. 38 (2009) 2532.[16] M. Estrader, A. Lopez-Ortega, S. Estrade, I.V. Golosovsky, G. Salazar-Alvarez, M.

Vasilakaki, K.N. Trohidou, M. Varela, D.C. Stanley, M. Sinko, M.J. Pechan, D.J.Keavney, F. Peiro, S. Surinach, M.D. Baro, J. Nogues, Nat Commun 4 (2013)2960.

[17] V. Skumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord, J. Nogues,Nature 423 (2003) 850.

[18] J. van Embden, A.S.R. Chesman, J.J. Jasieniak, Chem. Mater. 27 (2015) 2246.[19] S. Maenosono, S. Saita, IEEE Trans. Magn. 42 (2006) 1638.[20] Q.A. Pankhurst, N.T.K. Thanh, S.K. Jones, J. Dobson, J. Phys. D Appl. Phys. 42

(2009) 224001.[21] S. Maenosono, T. Suzuki, S. Saita, J. Magn. Magn. Mater. 320 (2008) L79.[22] C. Liu, X.W. Wu, T. Klemmer, N. Shukla, X.M. Yang, D. Weller, A.G. Roy, M.

Tanase, D. Laughlin, J. Phys. Chem. B 108 (2004) 6121.[23] C.P. Bean, J.D. Livingston, J. Appl. Phys. 30 (1959) S120.[24] M.S. Seehra, V. Singh, P. Dutta, S. Neeleshwar, Y.Y. Chen, C.L. Chen, S.W. Chou,

C.C. Chen, J. Phys. D Appl. Phys. 43 (2010) 145002.[25] J.O. Andersson, C. Djurberg, T. Jonsson, P. Svedlindh, P. Nordblad, Phys. Rev. B

56 (1997) 13983.[26] V. Singh, M.S. Seehra, J. Bonevich, J. Appl. Phys. 105 (2009) 07B518.