Ferromagnetic Resonance Characterization of Nano-FePt by Electron Spin Resonance

1 DST/CSIR-National Centre for Nano-StructuredMaterials (NCNSM),Meiring Naudé Road, Brummeria, Pretoria 0001, South Africa National Laser Centre-Council of Scienti�c and Industrial Research (CSIR), Meiring Naudé Road, Brummeria, Pretoria 0001, South Africa Department of Physics, University of the Witwatersrand, Johannesburg 2050, South Africa Department of Physics, University of Pretoria, Lynwood Street, Pretoria 0001, South Africa Department of Physics, e Polytechnic, University of Malawi, Blantyre, Malawi


Introduction
Magnetic recording media plays a vital role in the development of nonvolatile data storage technologies.Particularly, magnetic hard disk drives are important parts in many devices such as video cameras and computers.e year 1956 marked the generation of �rst magnetic hard disk with recording density of 2 kB/in  that was successfully built by IBM [1].Since then, the areal density (the number of bits/unit area on a disk surface) has successively increased [2].Nowadays, products with an areal density of more than 700 GB/in  are commercially available [3]. e increase in the areal density needs to be continued due to the future demand for information storage that is drastically advancing.Due to advancement in information technology (IT) and computer science, areal density in a level of 1 Tb/in  or more is inevitable.Iron platinum (FePt) nanoparticles (NPs) are actively being pursued as a potential candidate for larger storage capacities on hard-disk drives than any other materials due to its high magnetocrystalline anisotropy (MA) [4].MA is a process when the atomic structure of a crystal of a certain material introduces preferential direction of magnetisation, and in most cases, it will be the easy axis of magnetisation [5].is phenomenon is mostly common in ferromagnetic materials.Traditional magnetic recording materials such as Co/Cr have limitations since their magnetic direction of each recording bit would become unstable at room temperature due to thermal �uctuation [6].FePt does not only possess higher magnetic anisotropy but also possesses better thermal stability [7]; this makes it a better candidate for magnetic recording unlike other materials.
Ferromagnetism is a phenomenon by which a material produces its own magnetic �eld due to the alignments of atomic magnets in a particular direction.is arrangement is energetically preferred, hence, once they get this arrangement they do not get misaligned.is can be removed by demagnetizing �eld, by heating it, by striking.Substances having this property are known as ferromagnetic.Substances are classi�ed as ferromagnetic, antiferromagnetic, nonmagnetic (diamagnetic), or paramagnetic.Paramagnetic substances are ferromagnetic under a certain temperature called Curie temperature.
When a ferromagnetic substance outwardly behaves as a nonmagnetic substance, it can be magnetized by applying a magnetizing �eld upon which it gets magnetized, but when the magnetising �eld is removed, it does not get demagnetized.However, paramagnetic materials do return to a demagnetised state.So, ferromagnetic materials would show magnetization even when the applied �eld is momentarily zero.is phenomenon is due to absorption in the full saturation state and is a direct signature of ferromagnetic state of a material.
An alloy of iron and platinum would show some degree of ferromagnetism from Fe and a proportion of paramagnetism from Pt.In electron spin resonance experiments presented here, FePt possesses what is known as ferromagnetic resonance (FMR) at room temperature as illustrated in Figure 1; this implies that FePt can be ferromagnetic especially when Fe content is elevated.Ferromagnetic materials have a strong attraction to applied magnetic �elds and are able to retain their magnetic properties even aer the �eld has been removed.ey have a large and positive susceptibility.is phenomenon is responsible and used in data mass storage materials.Spin waves, similar to resistance in electricity, is a major drawback to spin alignment in a typical magnetization process.However, ferromagnetic �lms of suitable thickness have been shown to withstand spin waves across the �lm that has been excited by the microwave �eld [8][9][10].ese arise from collective excitations in arrays of spins and can be detected in ferromagnetic resonance experiments [8,11].Spin waves are propagating disturbances in the ordering of magnetic materials.ese intrinsic excitations in magnetic materials are dependent mainly on the chemical composition or structure of the material [8][9][10][11].e Hamiltonian of ESR process takes into account an electron and a nucleus interaction.is interaction can be readily observed with the ESR experiments as hyper�ne structures.In this paper, we present the FMR study for potential high-density magnetic recording and thermal stability of FePt system.e analysis of increasing iron (Fe) content on FePt system and FMR intensity response together with the effect of crystallite size on the FMR intensity is presented.Moreover, the present work will show results on the hyper�ne coupling with narrow width and interestingly of opposite sign to that of FMR line.We believe this phase reversal behaviour is observed for the �rst time in our FePt system.In addition, the study of the angular dependence of the high �eld absorption by FePt gives enough evidence of magnetic anisotropy present in these �lms.
2.2.Sample Characterization.e FMR data were taken at room temperature using an electron spin resonance spectrometer (JEOL-JES-FAS 200) operating at the X-band microwave frequency, with the sample located at the centre of a standard TE 102 microwave resonant cavity and �xed to a goniometer, allowing the study of in-plane and out-ofplane angular dependence of the absorption �eld and linewidth.e FMR spectra were taken using standard phasesensitive detection techniques, applying modulation �elds up to 1.0 mT, modulation frequency of 100 kHz, and microwave power of up to 10 mW.e microwave resonance frequency at   = 0 ∘ for all the samples was 8988.9158 ± 0.8960 MHz.

Results and Discussion
In this paper, measurements were conducted in the usual electron spin resonance (ESR) geometry; the DC static magnetic �eld was parallel to the �lm surface, that is,   = 0 ∘ , and the AC magnetic �eld was also always maintained parallel to the �lm.FMR was observed at fairly high DC static magnetic �eld of ≈295 mT; the FMR spectra were almost symmetric.Upon increasing the iron content in Fe-Pt system, no detectable spin waves modes were identi�ed as evidenced in Figure 1.e FMR spectrum observed at high �eld by Martins et al. [8] and Jin et al. [9] includes spin waves at lower angles giving evidence that FMR can emanate together with spin waves.Our FMR spectra, however, show only the uniform mode with some six features (sextet) seen around 320 mT centre �eld as depicted in the inset Figure 1.We attribute these features to the interaction between electron and nuclear spin angular momenta, a phenomenon known as hyper�ne structure coupling with a spacing between them of 8.482 ± 0.192 mT.ese features have the same intensity as observed in FMR spectra of all other samples.ese features could not be attributed to the Mn marker that is used and are calibration standard inherent in the instrument since the typical  values for these features are totally different from the  value for the Mn 2+ spins.We have also done energy-dispersive X-ray analysis on the samples aer ESR measurements to recheck any Mn trace impurities.Even at higher electron energy, no Mn element was observed.e low magni�cation S�M image in Figure 1 inset shows that these FePt nanoparticles are spherical over a large area.Interestingly, this sextet with narrow line width has phase reversal behaviour in comparison to FMR line.e possible explanation is the transition taking place between converging (or diverging) energy levels [14].is observation was correlated to the phase reversal of the dynamic nuclear polarization (DNP) by Lambe et al. [15] and Abragam and Bleaney [16].is DNP mechanism was �rst realized using the notion of the Overhauser effect [17].is effect relies on stochastic interaction between an electron and a nucleus.While the Overhauser effect is time-dependent electronnucleus interaction, among others it is the solid effect, cross effect, and thermal mixing which are time-independent.us, we would like to view this as an indication that the same DNP-solid effect mechanism is involved in this present experiment.e detailed analysis of this latter phenomenon is beyond the scope of this present work more especially of their angular dependence.�owever, this is visible in �lm FMR spectra, wherein two main peaks at 001 that is shiing to lower angles by adding more iron content and 111 that is not shiing were observed in our XRD data reported elsewhere [13].e absence of spin waves can also be suppressed by patterned media [6,7,18,19].is consists of a regular array of magnetic dots which have uniaxial magnetic anisotropy.is �uali�es the laser solution photolysis techni�ue as model self-assembly method for the preparation of pure materials.Self-assembly is highly regarded as an alternative solution to overcome thermal �uctuation [6,7].ermal �uctuation triggers these spin waves which make magnetic �lms unstable at room temperature; hence, they are unlikely to be excited at lower temperatures.
It is observed that the large, the iron content the larger the spatial size of the nanoparticles (NPs) [4,7,20].is behaviour occurs mostly when the NPs are spherical in nature.Figure 2  Crystalite size (nm) content since an increasing number of magnetic moments from the ferromagnetic Fe interact with microwave magnetic �eld radiation.Moreover, NPs have enhanced surface area and hence increase the chances for effective interaction.Similar FMR spectra without spin waves were also observed by Zaghib et al. [21] at room temperature in lithium iron orthosilicate.However, the observed FMR intensity is extremely less than what we observed in our �lms indicating that the �lms are highly ferromagnetic.Moreover, Sun et al. [4] argued that iron-rich nanocrystal assemblies have the largest coercivity.is is consistent with earlier reports on vacuumdeposited FePt thin �lms [1,22].e increase in FMR intensity with crystallite size as indicated in Figure 3 is in good agreement with [23].e authors claimed that room temperature coercive force decreased with decreasing crystallite size.Such magnetic properties have been probed for a number of samples of circular -Fe 2 O 3 particles of the type used in magnetic recording tapes.A similar work was reported with FePt/Fe 3 O 4 particle size [24].Annealed at 560 ∘ for 30 minutes, 6 nm FePt NPs show a coercivity of 1.2 T, however 3 to 4 nm particles have a coercivity of only 0.5 T [24].
In addition to this present study, the usual Lorentzian distribution equation was used to analyze the resonance �eld with varying iron content.is equation is given as where  0 ,  0 , , and Δ 1/2 represent intensity at resonance magnetic �eld, resonance magnetic �eld, varying magnetic �eld, and the width at half maximum of the �eld, respectively.e derivative of (1), given by is presented by the ESR spectrometer.In this study, either the data were integrated using a trapezium method and (1) was �tted to the resulting data or (2) was simply �tted to data plotted in Figure 1(a).e �uality of �tting of either model to either dataset may be limited due to the multiple nature of the valence state especially of the Fe similar to V compound as well as W and other transition metals [25][26][27].
Either way, a summary of parameters obtained is given in Table 1.It is evident from Table 1 that the resonance �eld (centre �eld) decreases on the platinum rich side and shis �uite signi�cantly to higher �eld on the iron-rich side most likely due to increase in iron content as shown in Figure 4. e widths at half maximum results as a function of iron content are depicted in Table 1.e Δ 1/2 decreases on the Pt-rich side and increases �uite signi�cantly on the iron-rich side� thereaer it decreases upon increasing more iron.It must also be mentioned that the area under ESR traces is proportional to the number of spins available for ESR activity.e area under such traces can be estimated as  0 × Δ 1/2 .In Figure 5, there is a log-normal plot of  0 , Δ 1/2 , and  0 × Δ 1/2 .us, it can be concluded, as expected, that since the area under the Lorentzian trace increases as the Fe content increases, then there are more spins available for FMR activity as more Fe is introduced into the FePt alloy.

Conclusion
We have successfully synthesized NPs of Fe-Pt system using localized surface plasmon resonance.ese NPs were allowed to organise themselves on Si (111) substrates.e higher �eld absorption by the �lms is attributed to the FMR.FMR is a signature of ferromagnetism in our �lms.An FMR spectrum was observed without disturbances arising from spin waves commonly triggered by thermal �uctuations.We �ualitatively attribute such magnetic stability in the �lms to self-assembly of these Fe-Pt system NPs that are spherical.We can be tempted to conclude that such �lm synthesis techni�ue is ideal for good �uality magnetic thin �lm NPs.FMR intensity increased as a function of iron content in the Fe-Pt system making our �lms good candidates for large data storage mediums and spintronics.In addition, an increase in iron content also increased the crystallite size increasing the storage capability as well.

F 1 :
FMR spectrum of Fe 0.78±0.2Pt 0.33 /Si given in (a).e angle   gives the direction of the applied �eld  with respect to the normal �lm.e inset in (i) corresponds to the hyper�ne coupling and a typical S�M image is given (b).

F 3 :
FMR intensity as a function of crystallite size of the thin �lms.

F 4 :
Resonance �eld plotted against Fe content.e plot suggests a higher resonance magnetic �eld as Fe content rises up as expected.
depicts the FMR intensity as plot against the iron content.FMR absorption spectra increase with the Fe FMR intensity (a.u.)