RAPID COMMUNICATIONS PHYSICAL REVIEW B VOLUME 57, NUMBER 22 1 JUNE 1998-II Temperature-dependent crossover from ferro- to antiferromagnetic interlayer alignment due to magnetic anisotropy energy P. Poulopoulos, U. Bovensiepen, M. Farle,* and K. Baberschke Institut fušr Experimentalphysik, Freie Universitašt Berlin, Arnimallee 14, D-14195 Berlin-Dahlem, Germany Received 23 February 1998 The temperature dependence of the interlayer coupling of perpendicularly magnetized Ni/Cu/Ni 001 trilay- ers is studied. A crossover in the alignment of the sublayer magnetizations from parallel to antiparallel is observed with increasing temperature, although the interlayer exchange is measured to be antiferromagnetic for all temperatures. The crossover is the result of the competition between the antiferromagnetic interaction and the magnetic anisotropy energy. This Ni/Cu/Ni trilayer with moderate exchange coupling serves as a model for magnetic multilayers in which the interlayer coupling can be switched with temperature. S0163-1829 98 53122-0 The oscillatory interlayer exchange coupling of ferromag- trilayer deposited on a Cu 001 single crystal the remanent netic transition metal layers through nonmagnetic spacers sublayer magnetizations change from FM to AFM ordering has been intensely studied over the last decade. Its depen- with increasing temperature. Despite this change in ordering dence on the thickness of the spacer and the ferromagnetic we find that the interlayer exchange coupling is antiferro- layers and on the crystallographic orientation in tri- and mul- magnetic at all temperatures and decreases linearly with tem- tilayers has been experimentally determined and theoreti- perature in the expected way.7,11 The origin of this apparent cally described.1­7 In comparison, relatively little work was crossover from FM to AFM originates from orbital magne- devoted to the temperature dependence of the exchange cou- tism and the resulting MAE. At low temperature MAE domi- pling. Exchange coupling a priori is temperature indepen- nates and fixes the sublayer magnetizations at remanence in a dent. In multilayers, however, the interlayer coupling follows a monotonic temperature dependence,8­11 which is attributed parallel orientation after application of an external field H to the softening of the density of states at the Fermi edge.7 100 Oe. With increasing temperature the MAE decreases In this model a change of sign of the coupling constant J more rapidly than the AFM interlayer exchange and the rem- is not expected. Experimentally, however, a crossover from anent state of the trilayer flips to an AFM alignment. From ferromagnetic FM to antiferromagnetic AFM orientation these results based on a careful analysis of minor and major of the layer magnetizations was observed with decreasing8 or hysteresis loops we conclude: a The determination of the increasing9,10 temperature. This was attributed to sample im- exchange-coupling constant sign from the remanent magne- perfections leading to a competition between AFM and FM tization state measured, for example, in electron coupled regions. Here we would like to draw attention to a spectroscopies1,8,10 may give the wrong sign of J in weakly fundamental mechanism which has not been discussed ap- coupled multilayers. b The consequences of orbital mo- propriately in the literature of exchange-coupled multilayers, mentum, that is spin-orbit coupling and MAE as a function that is, the importance of orbital momentum and magnetic of temperature need to be included in the theory of coupling anisotropy energy MAE for the ordering of the layer mag- phenomena. c Manipulation of the MAE may provide a netizations in weakly coupled multilayers. In limiting cases useful tool to tune the FM-to-AFM crossover temperature for MAE has been included in the discussion of the reversal technological application. process of the magnetization in coupled layers resulting in The thickness and temperature dependencies of the MAE simple models where MAE or J could be used as fitting of Ni/Cu 001 ultrathin films have been thoroughly parameters only.12 The temperature dependence of both studied.13­15,18 Up to seven monolayers ML , the easy axis quantities was not considered. Here we focus explicitly on of the magnetization lies in the film plane and an unusual the temperature dependence of J and MAE. It is well known spin-reorientation phase transition to the out-of-plane direc- that most thin film structures are distorted, for example, te- tion occurs by increasing the film thickness.18 The tempera- tragonal Ni/Cu 001 ,13­17 which lifts the quenching of the ture dependence of the magnetization and its specific features orbital momentum and gives rise to a by-orders-of- near the Curie temperature (TC) for perpendicularly magne- magnitude-increased MAE. The latter one becomes of simi- tized Ni/Cu 001 films are also available.14 In the present lar magnitude as the exchange constant, and it is temperature work, a 9-ML-thick Ni film was prepared on a Cu 001 sub- dependent. We suggest that MAE should be taken into ac- strate under ultrahigh vacuum UHV as described count and that not only J but also MAE determines the ef- elsewhere.13­16 Ni on Cu 001 grows layer by layer in a te- fective interlayer coupling, and it serves to manipulate the tragonal face-centered symmetry with very small interface coupling phenomena for multilayer application as we will roughness after annealing to 450 K.16,17 On the top of the show in this paper. first Ni layer, a 5 ML Cu layer with a thickness gradient of We demonstrate experimentally that in a perpendicularly 0.1 ML/mm was grown. This Cu thickness is close to the magnetized 12 ML Ni 001 /5 ML Cu 001 /9 ML Ni 001 crossover between ferro- and antiferromagnetic coupling for 0163-1829/98/57 22 /14036 4 /$15.00 57 R14 036 © 1998 The American Physical Society RAPID COMMUNICATIONS 57 TEMPERATURE-DEPENDENT CROSSOVER FROM FERRO- . . . R14 037 FIG. 1. Kerr ellipticity at remanence , , at 50 Oe and at 100 Oe for 5 ML Cu/9 ML Ni/Cu 001 a and for the trilayer b as a function of temperature. The orientations of the magnetization of the bottom short and the top layer long arrow are indicated. various transition metals Fe, Co, Ni separated by Cu 001 .19 Finally, a 12 ML Ni layer was grown on the top of the bilayer. FIG. 2. Hysteresis loops at RT a - c and 396 K d - f as The magnetic properties of the trilayers at temperatures explained in the text. Arrows in a and d indicate the orientations higher than room temperature RT were studied in situ via of the two sublayer magnetizations. The arrows in e show the path of the bottom layer magnetization. the polar magneto-optic Kerr effect MOKE . Measurements were performed along the easy axis of magnetization. Temperature-dependent magnetization curves were recorded H 50 Oe an antiparallel configuration of the bottom and in a way described earlier.14 Over many heating cycles up to top layer magnetization is established. At about 75 Oe also 450 K no changes in the magnetic response were observed. Mtop starts to reverse towards the external field, and the pro- In Fig. 1 a the perpendicular Kerr ellipticity at rema- cess is completed just below the field of 100 Oe. From the nence }r and at an external field (H 50 Oe) is shown for hysteresis loop of Fig. 2 a it is easy to separate the contri- the 5 ML Cu/9 ML Ni/Cu 001 bilayer. Similar behavior butions of M near the Curie temperature was observed in our previous bot Fig. 2 b and M top Fig. 2 c . The experi- mental hysteresis loops and the extracted bottom and top studies on perpendicularly magnetized 8­10 ML Ni/Cu 001 layer loops at T 396 K are shown in Figs. 2 d -2 f . The ultrathin films.14 The r and (H 100 Oe) data for the main difference between 303 K and 396 K is the orientation trilayer 12 ML Ni/5 ML Cu/9 ML Ni/Cu 001 between RT of the sublayer magnetizations M and 450 K are shown in Fig. 1 b . The total Kerr ellipticity bot at remanence Figs. 2 a signal is strongly increased due to the contribution of the and 2 d . The extracted minor loop Fig. 2 e shows that top Ni layer. Up to 360 K Mbot follows an inverted path above 360 K. Near remanence r is equal to (H 100 Oe) and the sublayer magnetizations are parallel. Above 360 K Mbot at 396 K is oriented opposite to the external field at 396 r is abruptly reduced and the sublayer magnetizations are anti- K while at 303 K the regular behavior is found. To show parallel at H 0 Oe and parallel at H 100 Oe as the arrows more clearly the existence of AFM interlayer exchange cou- indicate. pling at both temperatures we traced minor-hysteresis loops r coincides again with (H 100 Oe) at higher temperatures close to 450 K , where the magnetization of in the following way: The orientation of Mtop was kept fixed the bottom Ni layer has vanished after entering the paramag- since its coercivity was not exceeded when recording the netic phase. This behavior is observed reversibly over the minor loops shown in Fig. 3. The asymmetry of the top and full temperature interval when heating and cooling several bottom layer made it possible to trace the loop of Mbot only, times. The signal at 450 K originates exclusively from the see, for example, Ref. 20. The results at RT open squares top Ni layer. In a simple analysis one could conclude that the and 396 K closed squares are shown in Fig. 3. In both cases exchange coupling has reversed from FM to AFM near a the minor loops are exchange shifted towards the same di- crossover temperature Tx 370 K. However, the results pre- rection and the negative sign of Hexch indicates an antiferro- sented below unambiguously show that also in the FM magnetic interaction. The Hexch values of 17 Oe at 396 K and coupled region (T 370 K) the exchange is AFM! 32 Oe at RT correspond to values of the interlayer exchange For the trilayer shown in Fig. 1 hysteresis loops were coupling constant J 10 3 erg/cm2 0.4 eV/atom .6 These recorded at various temperatures. In Fig. 2 a the RT hyster- values are more than two orders of magnitude smaller than esis loop is plotted. The field of 100 Oe forces both sublayer for most tri- or multilayers. magnetizations Mbot for the bottom and Mtop for the top Ni The temperature dependence of the exchange field Hexch layer to be parallel to each other. At H 30 Oe the bottom is plotted in Fig. 4. A linear dependence of Hexch closed layer magnetization smaller arrow starts to reverse and at squares as a function of temperature is observed which is in RAPID COMMUNICATIONS R14 038 POULOPOULOS, BOVENSIEPEN, FARLE, AND BABERSCHKE 57 FIG. 3. Minor-hysteresis loops showing the magnetization of the FIG. 4. Hexch closed squares , Hc closed triangles , Hc,bot bottom layer at 303 K open squares and 396 K solid squares . open squares or (Hexch Hc) closed diamonds as a function of Mtop was biased along the negative field direction. The negative temperature. The dashed line is a guide to the eye. exchange field Hexch indicates the antiferromagnetic interaction. The magnetization has been calibrated in Gauss as explained in Ref. 14. ample by Heinrich et al.22 but only at a single temperature. They observed a parallel sublayer remanent magnetization while the interlayer coupling was found to be antiferromag- agreement with reports on other exchange-coupled netic. Also recent magnetoresistance data showed the effect multilayers.7,11 No change of the sign of Hexch and thus of of MAE.23 However, the temperature dependence of MAE the interlayer exchange with temperature is observed. The was not experimentally determined, and the anisotropy con- values of the coercivities of the minor loops Hc Fig. 3 are stants were used as fit parameters only. also included in Fig. 4 closed triangles , together with the In this work we have measured a crossover from ferro- values of Hc,bot Figs. 2 b and 2 e open squares . One magnetic to antiferromagnetic interlayer order as a function should note that the temperature dependence of Hc of the of temperature in weakly coupled trilayers. We show that it minor loop is related to the one of MAE Ref. 21 which has is an apparent crossover of the coupling due to the been measured previously.13,15,18 It decreases more strongly temperature-dependent competition of the antiferromagnetic than the exchange field, and the competition between Hc , interlayer exchange coupling and the magnetic anisotropy that is MAE, and Hexch , that is exchange coupling, deter- energy. The exchange coupling is antiferromagnetic at all mines the orientation of the magnetizations. This is even temperatures. The large MAE dominates at low temperature more evident if one regards the sum of Hexch Hc dia- and fixes the sublayer magnetizations in a parallel configu- monds which is identical with the coercivity Hc,bot of the ration at remanence. At higher temperature the MAE is bottom layer Fig. 4 . Hence, we conclude that the change of strongly reduced and the AFM exchange coupling dominates the sign of Hc,bot , that is the crossover to AFM coupling, at yielding the experimentally observed AFM configuration at 360­370 K is the result of this competition. Thus the effects remanence. It is concluded that orbital momentum and MAE of the antiferromagnetic exchange coupling favoring an an- which are strongly enhanced in noncubic multilayer struc- tiparallel sublayer magnetization alignment become more tures should be included in the analysis of coupling cross- prominent at higher temperatures, and a coupling crossover over phenomena. Last, but not least, we mention that our is recorded. However, the interlayer exchange is AFM at all crossover temperature T temperatures. This is an important result since it shows that x of 370 K is in a perfect range for technological application. Small manipulation in metallurgy the balance of MAE and AFM exchange as a function of and film preparation may bring T temperature determines the reorientation of the bottom layer. x to slightly above ambient temperature which is best for practical use. The problem to determine the exchange coupling sign from the remanent magnetization was also discussed for ex- This work was supported by the DFG, Sfb 290. *Corresponding author. Fax: 4930 838-3646; electronic ad- 5 N. K. Flevaris, Ph.D. thesis, Northwestern University, 1983; N. dress: babgroup@physik.fu-berlin.de K. Flevaris, in Magnetism and Structure in Systems of Reduced 1 J. Unguris, R. J. Celotta, and D. T. Pierce, Phys. Rev. Lett. 79, Dimension, edited by R. F. C. Farrow et al. Plenum, New York, 2734 1997 . 1993 , pp. 425­438. 2 P. Grušnberg, R. Schreiber, Y. Pang, M. B. Brodsky, and H. Sow- 6 A. Fert, P. Grušnberg, A. Barthelemy, F. Petroff, and W. Zinn, J. ers, Phys. Rev. Lett. 57, 2442 1986 . Magn. Magn. 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