Endo, Yasushi

Kitakami, Osamu

Okamoto, Satoshi

Shimada, Yutaka

Applied Physics Letters

English

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Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Determination of first and second magneticanisotropy constants of magnetic recordingmedia著者 遠藤  恭journal orpublication titleApplied Physics Lettersvolume 77number 11page range 1689-1691year 2000URL http://hdl.handle.net/10097/46348doi: 10.1063/1.1310166APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 11 11 SEPTEMBER 2000Determination of first and second magnetic anisotropy constantsof magnetic recording mediaYasushi Endo,a) Osamu Kitakami, Satoshi Okamoto, and Yutaka ShimadaResearch Institute for Scientific Measurements, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577,Japan~Received 1 May 2000; accepted for publication 17 July 2000!We propose a simple method to evaluate the first and second magnetic anisotropy constants ofhigh-density magnetic recording media with their easy axis in the film plane. By measuring themagnetization curve along the film normal, uniaxial anisotropy constants K1 and K2 can bedetermined simultaneously. The present method has been applied to (110) oriented CoCrPtTa harddisk media. The measured anisotropy constants are presented along with their temperaturedependence. We also point out serious shortcomings of the currently used evaluation methods, suchas the rotational hysteresis method and the singular point detection technique, which usuallyoverestimate the magnetic anisotropy constant. © 2000 American Institute of Physics.@S0003-6951~00!03437-9#Magnetic recording is the predominant data storage tech-nology and continues to make rapid advances in data density.In the near future, however, fundamental physical phenom-ena, such as bit instability due to superparamagnetism, maybe encountered that limit the achievable data density. Thesuperparamagnetism is the consequence of random field fluc-tuations due to overcoming the activation energy Kv by ther-mal energy kBT , where K is the anisotropy constant per unitvolume and v is the volume of a reversal unit. In order topredict thermal stability of recording bits, accurate determi-nation of magnetic anisotropy is indispensable.Some systematic studies have been carried out to deter-mine the anisotropy constants of Co–X binary and Co–Cr–X ~X5Pt, Ta, etc! ternary alloys by using epitaxiallygrown single crystal films.1–3 However, it has been very dif-ficult to evaluate the anisotropy constants for the conven-tional polycrystalline thin film media. Most popular methodsnow widely used for polycrystalline samples are the rota-tional hysteresis method,4 the singular point detection tech-nique ~SPD!,5 and utilization of the law of approach tosaturation.6 All these methods give the anisotropy constantKu for samples with random easy axis orientation. In usualdiscussions on thermal stability, this Ku has been regarded asthe first anisotropy constant K1 of the energy expression E5K1 sin2 u1K2 sin4 u1fl . It is obvious that Ku does notcoincide with K1 unless K2 , K3 , . . . 50. For example, avery simple calculation gives Ku5SiKi (i51,2, . . . ) for thecase of the SPD method. If higher-order terms are negligiblysmall with respect to K1 , the above method gives reasonablevalues of K1 . However, this approximation is inappropriatefor pure Co and Co alloys which usually possess somewhatlarge K2 , resulting in over- or underestimation of K1 . There-fore, one must determine both K1 and K2 separately for morequantitative discussions on the thermal stability. In this let-ter, we propose a simple method for the evaluation of high-order anisotropy terms of polycrystalline recording media.The present high-density recording medium usually consistsa!Electronic mail: endo@rism.tohoku.ac.jp1680003-6951/2000/77(11)/1689/3/$17.00Downloaded 14 Feb 2010 to 130.34.135.83. Redistribution subject toof a (110) oriented Co–Cr–Pt ~Ta! recording layer on apolycrystalline Cr~100! underlayer. This means that the easy(c) axis of the recording layer lies in the film plane. In thiscase, it is possible to determine both K1 and K2 simulta-neously, as previously mentioned. Let us take the coordinatesystem shown in Fig. 1. For the (110) oriented Co–Cr–Pt~Ta! recording layer, the easy axis distributes randomly inthe film (x – y) plane. When the film is exposed to an exter-nal field H along z direction, the total energy can be ex-pressed byE5K1 sin2 u1K2 sin4 u12pM s2 sin2 u2M sH sin u ,~1!where K1 and K2 are the first and second anisotropy con-stants, M s is the saturation magnetization, and u is the angleof magnetization with respect to the easy axis in the filmplane. By normalizing all terms of Eq. ~1! by M s2, it can berewritten as«5~k112p!sin2 u1k2 sin4 u2h sin u , ~2!where «5E/M s2, ki5Ki /M s2(i51,2), and h5H/M s . Fromthe equilibrium condition d«/du50, the following relation-ship can be derived:FIG. 1. Coordinate system ~x , y , z! for the magnetic film with the easy axisin the film (x – y) plane. External field Hex is applied along the film normal~z direction!.9 © 2000 American Institute of Physics AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp1690 Appl. Phys. Lett., Vol. 77, No. 11, 11 September 2000 Endo et al.m2514k2hm2k112p2k2, ~3!where m(5sin u) is the normalized magnetization along thefilm normal. By plotting m2 vs h/m , one can determine bothK1 and K2 . At the saturation point (m51), the saturationfield becomes hs52(k112k2)14p , which is the singularpoint detected by the SPD method.5 The same results canalso be obtained from susceptibility measurements by usingthe following expression from Eq. ~3!:x5121~k112p!16k2m2. ~4!We apply this method to typical longitudinal recordingmedia for hard disk drives. The media comprise a Ni–P seedlayer, a CrMo underlayer, the CoCrPtTa magnetic layer, anda carbon overcoat. The magnetic layer has a thickness of 20nm and exhibits good magnetic properties; M s5440 G, Hc~in plane!52750 Oe, M r /M s50.78. Figure 2 shows thex-ray diffraction scan measured using the Cu Ka line. It isclearly noticed that the sample shows excellent (110) ori-entation of hexagonal-close-packed ~hcp!-CoCrPtTa onbody-centered-cubic ~bcc!-CrMo ~100!, meaning that thec-axes lie in the film plane almost perfectly. For magneticFIG. 2. X-ray diffraction scan of CoCrPtTa/CrMo hard disk media. The linesource is Cu Ka line. It is clearly noticed that the sample shows excellent(110) orientation of hcp-CoCrPtTa on bcc-CrMo ~100!.FIG. 3. Magnetization curve of a CoCrPtTa/CrMo hard disk medium mea-sured along the film normal.Downloaded 14 Feb 2010 to 130.34.135.83. Redistribution subject tomeasurements along the film normal, we used both a vibrat-ing sample magnetometer and a superconducting quantuminterference device ~SQUID! with maximum field of 1.5 and5.0 T, respectively. Both equipments gave the same results.The following data shown below are collected using aSQUID. After very careful adjustment of the sample planenormal to the external field, we measured the magnetizationcurves in the temperature range of 250–350 K. Figure 3shows the magnetization curve along the film normal at T5250 K. No remanence is observed, reflecting that the easyaxes are mostly in the film plane as expected from Fig. 2.Using this magnetization curve, m2 is plotted against h/m ,as shown in Fig. 4. From Eq. ~3!, the slope and the intersec-tion with the m2 axis should be equal to 1/4k2 and 2(k112p)/2k2 , respectively. From this relationship we can de-termine the magnetic anisotropy constants K1 and K2 as1.13106 and 0.523106 erg/cc at T5250 K. Figure 5 showsthe temperature dependence of K1 , K2 , and K112K2 . Asthe temperature increases, K1 decreases more rapidly thanK2 . This behavior is very similar to that of hcp-Co.6 Theusefulness of the present method was also confirmed forvarious kinds of recording media; for example, K1 and K2 ofa very low coercivity CoCrPt medium ~Hc51580 Oe,M r /M s50.73! were determined as 0.873106 and 0.583106 erg/cc, respectively. In order to check the accuracy ofthis method, we repeated the same measurement five timesFIG. 4. m2 vs h/m plot for the CoCrPtTa/CrMo hard disk medium shown inFigs. 2 and 3.FIG. 5. Temperature dependence of K1 , K2 , and K112K2 . AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp1691Appl. Phys. Lett., Vol. 77, No. 11, 11 September 2000 Endo et al.for the CoCrPtTa medium previously mentioned. The mea-surement error both for K1 and K2 was less than 10%.In contrast to the present method, the SPD method de-tects Ku5K112K2 , which is about two times larger than K1for the case of the CoCrPtTa medium mentioned above. Inthe case of the rotational hysteresis method, which is themost popular one, the evaluated Ku value is claimed to beseverely influenced by exchange and dipolar interactions be-tween grains.7 Under the assumption that there are no inter-actions between grains, rotational hysteresis would diminishat H52(K112K2)/M s , yielding the same Ku as the SPDmethod. Thus, these two conventional methods have obvi-ously overestimated the first anisotropy constant K1 , be-cause the measured Ku has been treated as the coefficient ofthe sin2 u term in the energy expression for uniaxial magneticanisotropy. This very rough approximation might cause mis-leading discussions on magnetic and thermal behaviors ofrecording media when K2 is not negligibly small comparedwith K1 . One may say that it is also possible to determineboth K1 and K2 from the conventional torque curve mea-sured in the normal plane. In this case, however, a very la-borious procedure is required to correct the unsaturation ef-fect due to the insufficient external field. Moreover, since thetorque imposed on each grain is governed by the angle be-tween the external field and its easy axis, one must know thec axis distribution in the film plane. This difficulty can beDownloaded 14 Feb 2010 to 130.34.135.83. Redistribution subject tocompletely removed in the present method, because the ex-ternal field is always orthogonal to the c axes of all constitu-ent grains.In summary, we have proposed a very simple method forthe evaluation of high-order anisotropy terms of polycrystal-line recording media with the easy axes in the film plane.This enables us to evaluate the magnetic anisotropy con-stants K1 and K2 separately and gives more useful informa-tion for quantitative discussions on the thermal stability ofrecording media.The present work is supported by the Research for theFuture Program of Japan Society for the Promotion of Sci-ence under Grant No. 97R14701, and the Storage ResearchConsortium in Japan. The authors would like to also thankDr. K. Tagami and Dr. T. Ito for their kind help.1 O. Kitakami, N. Kikuchi, S. Okamoto, Y. Shimada, K. Oikawa, Y. Otani,and K. Fukamichi, J. Magn. Magn. Mater. 202, 305 ~1999!.2 N. Kikuchi, O. Kitakami, S. Okamoto, Y. Shimada, A. Sakuma, Y. Otani,and K. Fukamichi, J. Phys.: Condens. Matter 11, L485 ~1999!.3 N. Inaba, Y. Uesaka, and M. Futamoto, IEEE Trans. Magn. 36, 54 ~2000!.4 D. M. Paige, S. R. Hoon, B. K. Tanner, and K. O’Grady, IEEE Trans.Magn. 20, 1852 ~1984!.5 G. Asti and S. Rinaldi, Phys. Rev. Lett. 28, 1584 ~1972!.6 S. Chikazumi, Physics of Ferromagnetism ~Clarendon, Oxford, 1997!, p.503.7 I. A. Beardsley and V. S. Speriosu, IEEE Trans. Magn. 26, 2718 ~1990!. AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Determination of first and second magnetic anisotropy constants of magnetic recording media

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Determination of first and second magnetic anisotropy constants of magnetic recording media

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