Comparison of a near-field ferromagnetic resonance probe with pump-probecharacterization of CoCrPt mediaT. W. Clinton, Nadjib Benatmane, J. Hohlfeld, and Erol Girt

Citation: Journal of Applied Physics 103, 07F546 (2008); doi: 10.1063/1.2838160 View online: http://dx.doi.org/10.1063/1.2838160 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/103/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Ferromagnetic resonance properties of granular Co-Cr-Pt films measured by micro-fabricated coplanarwaveguides J. Appl. Phys. 111, 07B919 (2012); 10.1063/1.3679411 Magnetization suppression in Co/Pd and CoCrPt by nitrogen ion implantation for bit patterned media fabrication J. Appl. Phys. 107, 123910 (2010); 10.1063/1.3431529 Origins of the damping in perpendicular media: Three component ferromagnetic resonance linewidth inCo–Cr–Pt alloy films Appl. Phys. Lett. 92, 022506 (2008); 10.1063/1.2834835 High field ferromagnetic resonance measurements of the anisotropy field of longitudinal recording thin-film media J. Appl. Phys. 91, 1417 (2002); 10.1063/1.1428804 Influence of stress on nucleation field of CoCrPt perpendicular media J. Appl. Phys. 91, 772 (2002); 10.1063/1.1423395

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Comparison of a near-field ferromagnetic resonance probe with pump-probe characterization of CoCrPt media

T. W. Clinton,1,a� Nadjib Benatmane,1,2 J. Hohlfeld,1 and Erol Girt31Seagate Research, Pittsburgh, Pennsylvania 15222, USA2Department of Physics, Georgetown University, Washington, DC 20057, USA3Recording Media Organization, Seagate Technology, Freemont, California 94538, USA

�Presented on 8 November 2007; received 26 September 2007; accepted 24 November 2007;published online 25 March 2008�

A near-field microwave technique is used to locally probe ferromagnetic resonance �FMR� in aseries of CoCrPt alloys with varying perpendicular anisotropy �5 kOe�Hk�15 kOe�. We observebroad FMR linewidths �f on the order of a gigahertz and higher, indicating significant damping andsample inhomogeneities. These results are compared to time-domain measurements of the samesamples using a pump-probe technique, where we observe relaxation times �, considerably shorterthan 500 ps, using a damped sinusoid to model the magnetization dynamics. We find the local FMRmeasurements to yield time scales consistent with the pump-probe data, using the Fourierrelationship, �f =1 /��. Thus, this near-field technique is capable of quantitative characterization ofhigh-anisotropy and highly damped magnetic systems, something that has not been demonstratedbefore with a local FMR technique. © 2008 American Institute of Physics.�DOI: 10.1063/1.2838160�

The continuous advance in operating speeds for compu-tational and data-intensive systems has accelerated the needfor reliable high frequency characterization measurements.For magnetic recording systems, in particular, the switchingspeeds of a magnetic device or a magnetic bit are influencedby their own intrinsic resonances and damping mechanisms.In fact, data rates �gigahertz� are already bumping up againstthe ferromagnetic resonance �FMR� of many materials ofinterest, hence, the need to properly understand the magne-tization dynamics present in such systems. There is particularneed for measurement methods that are local and put norestriction on the sample’s size, shape, or geometry, so struc-tured devices or bulk materials can be characterized in theirworking environment.

We have demonstrated a local near-field microwavetechnique that is nondestructive and broadband.1,2 The proberelies on a thin-film current path between the inner and outerconductors of a microcoax to generate an oscillating field hrf,as depicted in Fig. 1. The rf field can then couple to a mag-netic sample, exciting FMR, and the system’s response ismonitored using a vector network analyzer �VNA� connectedto the probe. The VNA also acts as the source for the incom-ing microwave signal. Ultimately, the complex reflection co-efficient �S11� is measured by the VNA, from which surfaceimpedance �Zs� and magnetic permeability ��� are extracted,as they are directly related to one another.3 The probe sensi-tivity is good enough to make quantitative measurements ofFMR frequency �fFMR�, anisotropy field �Hk�, saturationmagnetization �Ms�, linewidth ��f�, and intrinsic ��� andextrinsic damping, where results are consistent with boththeory and independent measurements.1–5

In this report, we discuss our results on a series of rela-tively high-anisotropy magnetic alloys of CoCrPt, all ofwhich have perpendicular anisotropy but in-plane magneti-zation, and compare our data with that of time-domain mea-surements on this same series using pump-probe techniques,along with that of resonant cavity FMR measurements.

We have developed a fabrication method that has en-hanced the probe’s sensitivity6 over previous experiments1,2

and enabled the measurements presented in this report. Afocused ion beam �FIB�-deposited buffer layer of SiO2 �orhigh-resistivity, “dirty” Pt� bridges the inner and outer con-ductors of the coax. The nonmagnetic coax is made by Pico-Probe and has a 100 �m separation between the inner andouter conductors, which is, thus, our maximum bridgelength. The use of the FIB allows for the deposition of asmooth surface on nearly a nanometer scale, as the FIBdeposition is very forgiving to rough base surfaces, as is thecase with the Teflon dielectric that the buffer spans. Once thebuffer is deposited and patterned, a thin Cu layer is evapo-

a�Electronic mail: thomas.w.clinton@seagate.com. FIG. 1. Schematic of FMR probe with Cu /SiO2 microbridge.

JOURNAL OF APPLIED PHYSICS 103, 07F546 �2008�

0021-8979/2008/103�7�/07F546/3/$23.00 © 2008 American Institute of Physics103, 07F546-1

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rated over the coax cross section, after which the Cu aroundthe structure is etched away, so the buffer determines thecurrent path’s shape and dimensions.

At angular frequency �=2�f , the probe and sample aremodeled as a lumped-element circuit1 and the total imped-ance at the terminals of the equivalent transformer, ZT, canbe written as

ZT�f� � i�L0�1 − k2� + ZS�f�k2L0

Lx, �1�

where L0 is the probe/loop inductance, ZS is the complexsurface impedance of the sample, k �0�k�1� describes theprobe-to-sample coupling, k=�M2 / �LxL0�, M is the mutualinductance between the probe and sample, and Lx is the in-ductance of the probe’s image in the sample �Lx→L0 for aperfect image�. In the thin-film limit �t0�skin depth, �, thesurface impedance has the form ZS= i�t0�o�r, and t0 is thesample thickness, �o is the free-space permeability, and �r

=�1− i�2 is the complex magnetic permeability of thesample.3

The total impedance is extracted from the reflection co-efficient S11 using the relation ZT�f�= �1+S11� / �1−S11�Z0,where Z0 is the characteristic impedance of the coax. Themeasurement sequence consists of a dc field applied perpen-dicular to the rf magnetic field hrf, where the reflection coef-ficient S11

FMR is then measured with the VNA and convertedinto load impedance ZT

FMR. A background measurement ofS11

noFMR is taken with no dc field applied, and again convertedto impedance, ZT

noFMR. The signal is then isolated with thebackground subtraction �from Eq. �1��:

�Z�f ,Hdc� = ZTFMR − ZT

noFMR = k2L0

LxZS

FMR�f ,Hdc� . �2�

The permeability is extracted from the measurement usingrelations that follow Eq. �2�,

�r �1

k2

Lx

L0

1

�t0�0�Im��Z� − i Re��Z�� . �3�

The imaginary part of the permeability Im��� is evalu-ated �according to Eq. �3�� and plotted in the top half of Fig.2 for a 1000 Oe dc field applied in plane. The bottom half ofFig. 2 is time-domain pump-probe7 data taken on the sameseries of samples with a dc field of 2315 Oe applied at 62°from the plane of the films. Presently, the dc field of theFMR experiment is limited to an in-plane orientation, whilethat of the pump-probe is limited to angles out of plane.Although the orientation of the field is different in the twomeasurements, their results are consistent, which we discussbelow.

The CoCrPt samples are continuous media all havingout-of-plane magnetic anisotropy. The anisotropy is differentfor the four samples, as is the saturation magnetization, bothof which are controlled by the different content of Cr and Pt.For all the samples, the magnetization lies in plane becausethe effective anisotropy Hkeff=Hk−4�MS is negative, as thedemagnetization field drives the magnetization in plane.These materials are of interest because of their application asmagnetic storage media in hard disk drives, and the charac-

terization of this particular parameter space can yield insightinto the effects of anisotropy on the magnetization dynamics.Columns I-III of Table I are data extracted from VSM char-acterization of these samples.

We have measured the FMR frequency over a broadrange of in-plane dc fields Hdc and the results are plotted inFig. 3 for the entire CoCrPt series. Here, we plot the FMRfrequency versus the external dc field. The data is fit usingthe expression for the FMR frequency appropriate for out-of-plane anisotropy, an in-plane field, and the magnetization inplane,8

fFMR =

2�Hdc�1 −

Hkef f

Hdc, �4�

where is the gyromagnetic ratio ��2.8 MHz /Oe� and Hkeff

is the fitting parameter. We also analyzed the data using themagnetization orientation as an additional fitting parameter,where it could vary out of plane. However, in all cases the

FIG. 2. �Top� Im��� vs frequency for the CoCrPt series with an externalfield of 1000 Oe applied in plane. �Bottom� Pump-probe measurements�Kerr signal vs pump-probe delay� on the same series of samples with2315 Oe applied 62° out of plane.

TABLE I. Columns I-III are VSM measurements on CoCrPt series. Nega-tive values of Hkeff imply in-plane magnetization, as demagnetization over-comes the out-of-plane anisotropy. Hkeff values of column IV were measuredwith the FMR probe.

Hk

�Oe�4�MS

�G�Hkeff

�Oe�FMR: Hkeff

�Oe�

Co90Cr10 6700 12 553 −5853 −5735Co85Cr15 8110 11 110 −3000 −3447Co92Cr8 6700 15 683 −8983 −7316Co87Cr5Pt8 12 200 14 125 −1925 −4676

07F546-2 Clinton et al. J. Appl. Phys. 103, 07F546 �2008�

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best fits are for the magnetization in plane, which is consis-tent with HKeff being negative, so Eq. �4� describes the datawell. The values of HKeff we extract are included in Table I,where we see consistent and reasonably good agreement withthose of the VSM. Some of the discrepancy may be due tothe limited magnitude of our dc field source ��2000 Oe�,which is smaller than HKeff, so we cannot adequately saturatethe magnetization in any of our measurement sequences.

We can extract the frequency linewidths �full width athalf maximum� and the FMR frequency from permeabilitydata like that in Fig. 2, while the FMR frequency can beshifted by varying the dc field, as demonstrated in Fig. 3. Theresults are plotted in Fig. 4, where the solid symbols aremeasured by the FMR probe. We also include equivalentlinewidths measured with the pump probe �open symbols�,obtained from fits to the time-domain data of Fig. 2 using adamped sinusoid, and fitting parameters of frequency andrelaxation time �. The relaxation times are converted to line-widths using the Fourier relation �f =1 /��. The black linesare guides to the eye, where we see the pump-probe datafollow the trends observed from the FMR data for the respec-tive samples.

Although we do not yet have a theoretical understandingof the behavior in Fig. 4, there are some clear trends in thedata. The frequency dependence of �f cannot be explainedby a simple Landau–Lifschitz–Gilbert �LLG� form, �, fordamping.1,9 Instead, the source of the linewidths is morelikely described by �intrinsic and extrinsic� damping andsample inhomogeneities. In addition to the intrinsic LLGdamping �confluent processes such as magnon-electron scat-tering�, there also is extrinsic damping via two-magnon pro-cesses, as a result of scattering from grains, grain boundaries,etc. Both of these processes lead to losses in the system. Anadditional source of the linewidths is sample inhomogene-ities �not a real loss�, which typically result in a distributionof material properties, such as anisotropy, �Hk, that will,indeed, broaden the FMR signals.1,9

The two samples with Pt content, Co92Pt8 �solid andopen triangles in Figs. 3 and 4� and Co87Cr5Pt8 �solid andopen squares�, have the broadest linewidths and, equiva-lently, shortest relaxation times. The samples without Pt,Co85Cr15, and Co90Cr10, reveal a decrease in linewidths �in-crease in �� with decreasing Cr content. Lastly, other than theCo87Cr5Pt8 sample, the linewidths are decreasing at the high-est frequencies. This is consistent with other measurementson samples with significant inhomogeneities and extrinsicdamping.1,5

We have demonstrated the high utility of a near-fieldmicrowave probe by characterizing a series of CoCrPt alloyswith varying perpendicular anisotropy �5 kOe�Hk

�15 kOe�. These are relatively complex magnetic systemswith dynamics that require high sensitivity and high band-width to characterize. The frequency-domain measurementsare consistent with the time-domain measurements of thesame samples using a pump-probe technique, which furtherestablishes the value of the FMR probe. It is capable ofquantitative characterization of high-anisotropy and highlydamped magnetic systems, something that has not been dem-onstrated before with a local FMR technique.

We thank Carl Patton for valuable discussions, NilsGokemeijer and Anthony Langzettel for technical assistance,and Tim Cornell and Werner Scholz for critical readings ofthe manuscript.

1D. I. Mircea and T. W. Clinton, Appl. Phys. Lett. 90, 142504 �2007�.2T. W. Clinton, D. I. Mircea, N. Benatmane, N. J. Gokemeijer, S. Wu, andS. D. Harkness IV, IEEE Trans. Magn. 43, 2349 �2007�.

3A. L. Sukstanskii and V. Korenivski, J. Phys. D 34, 3337 �2001�.4C. Kittel, Phys. Rev. 73, 155 �1948�.5S. S. Kalarickal, P. Krivosik, M. Wu, C. E. Patton, M. L. Schneider, P.Kabos, T. J. Silva, and J. P. Nibarger, J. Appl. Phys. 99, 093909 �2006�.

6N. Benatmane and T. W. Clinton, “The effects of a micro-bridge bufferlayer on the sensitivity of a local ferromagnetic resonance probe,” J. Appl.Phys. �in press�.

7For a review of the pump-probe technique, see, for example, R. J. Hicken,A. Barman, V. V. Kruglyak, and S. Ladak, J. Phys. D 36, 2183 �2003�.

8M. J. Hurben and C. E. Patton, J. Appl. Phys. 83, 4344 �1998�.9N. Mo, J. Hohlfeld, M. u. Islam, C. S. Brown, E. Girt, P. Krivosik, W.Tong, A. Rebei, and C. E. Patton, “Origins of the damping in perpendicu-lar media: Three component ferromagnetic resonance linewidth inCo-Cr-Pt alloy films,” Appl. Phys. Lett. 92, 022506 �2008�.

FIG. 3. Field dependence of fFMR and fits to Eq. �4� �solid black traces� forthe CoCrPt series.

FIG. 4. Linewidth vs frequency for the CoCrPt series. FMR data �solidsymbols� and pump-probe data �open symbols� are both included in thefigure. The black traces are guides to the eye.

07F546-3 Clinton et al. J. Appl. Phys. 103, 07F546 �2008�

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