Positive giant magnetoresistance in ferrimagnetic/Cu/ferrimagnetic films

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  • Positive giant magnetoresistance in ferrimagnetic/Cu/ferrimagnetic filmsChih-Huang Lai, Chao-Cheng Lin, B. M. Chen, Han-Ping D. Shieh, and Ching-Ray Chang

    Citation: Journal of Applied Physics 89, 7124 (2001); doi: 10.1063/1.1357112 View online: http://dx.doi.org/10.1063/1.1357112 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/89/11?ver=pdfcov Published by the AIP Publishing

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    [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:72.39.209.255 On: Sun, 30 Mar 2014 01:46:51

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    JOURNAL OF APPLIED PHYSICS VOLUME 89, NUMBER 11 1 JUNE 2001

    [This aduction electrons were only scattered by Co or Fe moments.In this work, we fabricated a structure of spin valves com-posed of ferrimagnetic/spacer/ferrimagnetic layers with themagnetization perpendicular to the film planes. To under-stand the GMR effect of this perpendicular spin valve, theeffects on GMR ratio of spacer thickness and of ferrimag-netic composition were investigated. In these ferrimagneticspin valves, by changing the composition of ferrimagnets orby inserting a Co layer at the interface, the sign of GMR canbe manipulated. The mechanism of positive GMR in theseferrimagnetic spin valves will be discussed.

    Spin valves with the structure SiN/200 TbFeCo/t Cu/100 GdFeCo/SiN were prepared by dc magnetron sput-tering onto Si wafers at room temperature. The spin valvewas sandwiched by SiN protective layers to prevent TbFeCoand GdFeCo oxidation. Ferrimagnetic layers were cosputtedusing a transition metal (TM:Fe80Co20) and a rare-earth tar-get ~RE: Tb or Gd!. The composition of ferrimagnetic filmswas controlled by the relative sputtering power of TM and

    rich!/17 Cu/100 Gd22.4~Fe80Co20!77.6 ~TM rich! areshown in Fig. 1. @The notation TM rich represents that themagnitude of the TM moments is larger than that of REmoments. The compensation compositions of these ferro-magnets are Tb22.7~Fe80Co20!77.3 and Gd23~Fe80Co20!77.# Thehysteresis loops @Fig. 1~a!# clearly demonstrate the charac-teristic of perpendicular magnetization with the coercivity ofGd22.4~Fe80Co20!77.6 and Tb20.5~Fe80Co20!79.5 of 110 Oe and11.5 kOe, respectively. The minor loop and the correspond-ing R H curve are shown in Figs. 1~b! and 1~c!. The inter-layer coupling is 270 Oe, and the MR ratio is 21%. FromFigs. 1~b! and 1~c!, the parallel state corresponds to a low-resistance state, representing a negative MR effect. Figure 2shows the dependence of absolute MR ratio and interlayercoupling on Cu thickness ~all MR ratios in Fig. 2 are nega-tive!. As the Cu thickness increased, MR ratio and interlayercoupling both decreased. Notice that the interlayer couplingis significantly higher than the ferromagnetic spin valves.The coercivity of GdFeCo and TbFeCo oscillates with Cuthickness, as depicted in Fig. 3.a!Electronic mail: chlai@mse.nthu.edu.twPositive giant magnetoresistance in feChih-Huang Laia) and Chao-Cheng LinDepartment of Materials Science and Engineering, NationRepublic of ChinaB. M. Chen and Han-Ping D. ShiehInstitute of Electro-Optical Engineering, National Chiao TRepublic of ChinaChing-Ray ChangDepartment of Physics, National Taiwan University, TaipeSpin valves composed of ferrimagnetic/Cu/ferrimmagnetization perpendicular to the film planes. Bylayers, both negative and positive giant magnetoresistspin valves. For samples consisting of both transitnegative GMR values were obtained. Due to the highthe shunting effect of Cu led to relatively small MR rnm Cu. For spin valves consisting of rare-earth ~RE-GMR values were observed. A thin layer of Co wasmanipulate the sign of GMR. 2001 American Ins

    Typical spin valves consist of ferromagnetic/spacer/ferromagnetic/antiferromagnetic layers with the magnetiza-tion lying in the film plane. Differential spin scattering hasbeen adapted for explaining the giant magnetoresistance~GMR! effect in spin valves.1 Multilayers consist of rareearth metals and transition metals show no GMR,2 while(Co/Dy/Co/Cu/Co/Cu)n multilayers show a positive GMReffect3 ~that is, smaller resistance in zero field than in thesaturation field! at 4.2 K. The positive GMR effect wasattributed to the antiparallel alignment of adjacent Comoments in a high field due to the antiferromagnetic cou-pling between Dy and Co. Trilayers made ofCo65Fe35 /Ag/~Co65Fe35!50Gd50 also showed a large resis-tance in a high field.4 Authors have concluded that the con-7120021-8979/2001/89(11)/7124/3/$18.00rticle is copyrighted as indicated in the article. Reuse of AIP content is sub

    72.39.209.255 On: Sun, rimagneticCuferrimagnetic films

    Tsing Hua University, HsinChu, 300, Taiwan,

    g University, HsinChu, 300, Taiwan,

    Taiwan, Republic of Chinagnetic layers were fabricated with thehanging the composition of ferrimagneticce ~GMR! can be observed in ferrimagnetic-metal ~TM-! rich TbFeCo and GdFeCo,

    esistivity of amorphous ferrimagnetic films,io. The negative MR effect was 1% for 1.7ich TbFeCo and TM-rich GdFeCo, positiveserted between RE-rich TbFeCo and Cu totute of Physics. @DOI: 10.1063/1.1357112#

    RE, and was calibrated by Rutherford backscattering spec-trometry. After depositions, the samples were magnetized inthe field of 14 kOe to saturate TbFeCo and GdFeCo. Thehysteresis loops were measured using a vibrating samplemagnetometer or perpendicular magneto-optical Kerr effect.Magnetoresistance was measured using a four-point probewith the applied field perpendicular to the film plane, whichmakes current direction perpendicular to the magnetization.Four points were aligned in a straight line. The MR ratio isdefined as (R02Rsat)/R0 , where R0 and Rsat are the resis-tance at the zero field and the saturation field, respectively.Therefore, the positive MR means that the resistance at thezero field is higher than that at the saturation field. The hys-teresis loop and MR curve of 200 Tb20.5~Fe80Co20!79.5 ~TM4 2001 American Institute of Physicsject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:30 Mar 2014 01:46:51

  • [This aFor samples consisting of both TM-rich TbFeCo andGdFeCo, negative GMR values were obtained, as shown inFig. 1. In fact, spin valves consisting of both RE-richTbFeCo and GdFeCo also showed negative GMR.On the other hand, for samples consisting of RE-richTbFeCo @Tb27.6~Fe80Co20!72.4# and TM-rich GdFeCo@Gd22.4~Fe80Co20!77.6# ~or TM-rich TbFeCo and RE-richGdFeCo!, positive GMR values were observed, that is, theresistance is high at the zero field. A thin layer of Co wasinserted between RE-rich TbFeCo and Cu to manipulate the

    FIG. 1. ~a! Hysteresis loop, ~b! minor loop of ~a!, and ~c! R H curve ofSiN/200 Tb20.5~Fe80Co20!79.5 ~TM rich!/ 17 Cu/100 Gd22.4~Fe80Co20!77.6 ~TM rich!/SiN. The arrows represent the sequence ofthe applied field.

    FIG. 2. Dependence of the absolute MR ratio and the interlayer coupling on

    J. Appl. Phys., Vol. 89, No. 11, 1 June 2001the Cu thickness for SiN/200 Tb20.5~Fe80Co20!79.5 ~TM rich!/t Cu/100 Gd22.4~Fe80Co20!77.6 ~TM rich!/SiN. ~All MR values are negative!.

    rticle is copyrighted as indicated in the article. Reuse of AIP content is sub72.39.209.255 On: Sun, sign of GMR. The positive GMR ratio increased with in-creasing Co thickness, shown in Fig. 4, for thickness lessthan 1 nm. When the thickness of Co reached 1.5 nm, thenegative GMR appeared.

    In the spin valves composed of ferrimagnetic layers, thecoercivity of ferrimagnetic layers can be manipulated by thecomposition. The hysteresis loop, shown in Fig. 1~a!,showed a distinguished difference in coercivity between fer-rimagnets, so the antiparallel state of the net magnetizationbetween ferrimagnetic layers can be achieved. High resis-tance was observed in the R H curve corresponding to theantiparallel state @shown in Fig. 1~c!#, which suggested thatthe origin of the observed magnetoresistance be the GMReffect. Since the measuring current is in the film plane, per-pendicular to the magnetization, anisotropic magnetoresis-tance can be excluded. The four-point probes with a linearconfiguration for current and voltage probes exclude theprobability of Hall effect contribution on resistance change.The resistivity of amorphous ferrimagnets (r;200 mV cm2) is about 50 times larger than that of Cu,which leads to significant current shunting effect and to asmall MR ratio. Figure 2 shows that the MR ratio decreaseswith increasing Cu thickness, consistent with the shuntingeffect of Cu. It should be noticed that the interlayer coupling

    FIG. 3. Variation of coercivity of Gd22.4~Fe80Co20!77.6 andTb20.5~Fe80Co20!79.5 with Cu thickness for SiN/200 Tb20.5~Fe80Co20!79.5~TM rich!/t Cu/100 Gd22.4~Fe80Co20!77.6 ~TM rich!/SiN.

    FIG. 4. Dependence of MR ratio on the Co thickness for 200

    7125Lai et al.Tb27.6~Fe80Co20!72.4 ~RE rich!/t Co/20 Cu/100 Gd22.4~Fe80Co20!77.6~TM rich!.

    ject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:30 Mar 2014 01:46:51

  • ferrimagnets, the sublattice magnetizations of transition met-als are antiparallel. Since 4 f electrons have little contributionto the differential spin scattering,2,4 the GMR effect in ferri-magnetic spin valves is mainly contributed from the 3d sub-lattice. When the magnetization of GdFeCo is switched, that

    7126 J. Appl. Phys., Vol. 89, No. 11, 1 June 2001 Lai et al.

    [This asignificantly increases as Cu thickness decreases. In ferro-magnetic spin valves, maximum GMR occurs at the opti-mum Cu thickness.5 For spin valves with thin Cu, the anti-parallel state is difficult to obtain due to the strong interlayercoupling; for spin valves with thick Cu, current shuntingeffect occurs so MR decreases. In ferrimagnetic spin valves,large interlayer coupling limited the minimum Cu thickness.

    The oscillation of coercivity with Cu thickness was ob-served, as shown in Fig. 3. To explain the above-mentionedexperimental findings, we propose an indirect exchange cou-pling model, which was well known in a layered systemcontaining two ferromagnetic layers and a nonmagneticspacer,6,7 to relate the oscillatory coercivity to the oscillatoryinterlayer coupling. As in a FM/spacer/FM system, the cou-pling between the ferrimagnetic layers will oscillate as afunction of the spacer thickness, due to the quantum welleffect for conduction electrons in the spacer.7 For Cu thick-ness with ferromagnetic coupling, the coercivity of GdFeCois enhanced for the ferromagnetic coupling from the TbFeColayer, while the coercivity of TbFeCo decreases since thereversal of magnetic moment within GdFeCo induces theinstability of the magnetic moment of TbFeCo. For the anti-ferromagnetic coupling region, starting from the saturationstate, the low coercivity GdFeCo will become unstable eas-ily, but after it reverses the TbFeCo will remain more stablethan usual. In other words, the oscillatory coercivities forTbFeCo and GdFeCo should be out of phase and that isshown in Fig. 3. In addition, since the magnetization in oursystem is perpendicular to the film, the surface magneticpoles are large, which may enhance the long-range dipolarinteraction. This strong long-range interaction may enhancethe interlayer coupling and results in a strong dependence ofinterlayer field on Cu thickness, as shown in Fig. 2.

    For the samples consisting of TM-rich GdFeCo and RE-rich TbFeCo, when the net magnetizations are parallel in

    FIG. 5. Schematic diagrams for the direction of moments in TbFeCo ~RErich!/t Co/Cu/GdFeCo ~TM rich!. ~a! t

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