JOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 10 15 MAY 2002 Magnetic force microscope study of antiferromagnet­ferromagnet exchange coupled films Taras Pokhil,a) Dian Song, and Eric Linville Seagate, 7801 Computer Avenue S., Minneapolis, Minnesota 55435 Magnetic microstructure in micron and submicron size elements made of bilayer antiferromagnet­ ferromagnet AFM/FM AFM: NiMn, PtMn, and IrMn; FM: NiFe and CoFe exchange coupled polycrystalline films have been studied using a magnetic force microscope. AFM/FM elements with various thickness of FM layer 50­500 A have been examined and compared with nonexchange biased FM elements of the same size, shape, and thickness. Micromagnetic structures observed in AFM/FM elements with thick 200 A FM layer indicated that, in addition to unidirectional anisotropy, the AFM layer induces uniaxial anisotropy in a FM layer. Bilayers with a NiMn or PtMn AFM layer exhibited higher induced uniaxial anisotropy than ones with IrMn. In the elements with a thin 100 A FM layer and NiMn or PtMn as an AFM layer, a local switching of the magnetization direction under an external applied field has been observed. The size of the ``switched'' areas depends on the material and thickness of the FM and AFM layers. No local switching, just slight rippling of magnetization in the FM, was observed in the samples with an IrMn AFM layer. The results can be explained using either the model of thermally activated switching of AFM grains or the model of induced uniaxial anisotropy at the AFM/FM interface suggesting local variations of induced uniaxial anisotropy. In both models, the in-plane exchange in the FM layer has to be taken into account. © 2002 American Institute of Physics. DOI: 10.1063/1.1452225 The phenomenon of exchange anisotropy, related to the Micromagnetic measurements were done using magnetic exchange coupling at the interface between an antiferromag- force microscopy MFM on samples patterned into small net AFM and a ferromagnet FM was discovered by elements by means of photolithography. Patterning was done Meiklejohn and Bean over 40 years ago.1 The technological after the samples had been annealed in the field. The image applications stimulated extensive theoretical and experimen- of magnetic charges at the element edges allows simple de- tal studies of this effect in thin films. Few theoretical models termination of the average magnetization direction in the el- have been suggested for the explanation of the exchange ement. Elements studied in this work were elongated in the anisotropy.2­5 During the last years, a few studies of micro- direction perpendicular to exchange induced anisotropy, magnetic behavior of AFM/FM systems have been carried making induced anisotropy compete with shape anisotropy. out which show, for instance, a difference in micromagnetic Micromagnetic structures observed in studied AFM/FM structure formed during magnetization of AFM/FM in the elements with thick 200 A FM layer indicated that in directions parallel and antiparallel to the exchange field.6 Mi- addition to unidirectional anisotropy, the AFM layer induces cromagnetic structure depends on the materials and thickness uniaxial anisotropy in the FM layer.7 Figure 1 shows a se- of AFM and FM layers. Especially complex structures can be quence of MFM images of a 2 5 m NiMn 280 A / observed in polycrystalline AFM/FM films. NiFe 200 A element obtained under external field progres- In the present article, we report the results of a system- sively increased in the direction opposite to the pinning field atic study of magnetic microstructure with high spatial reso- and then decreased back to zero. A seven-domain closure lution 50 nm in micron size elements made of bilayer structure forms in the element when the applied field ap- AFM/FM AFM: NiMn, PtMn, and IrMn; FM: NiFe and proximately compensates the pinning field. In this state, the CoFe exchange coupled polycrystalline films with various magnetization in most of the element is oriented parallel to thickness of the layers. the exchange induced anisotropy axis and at 90° to the shape Samples of AFM/FM bilayer films were deposited on anisotropy axis. In the sample, the exchange induced oxidized Si 001 substrates using dc magnetron sputtering. uniaxial anisotropy energy per unit area of the film surface is In most of the studied samples, the FM layer was deposited about 1.5 10 1 erg/cm2.8 It overcomes the shape anisot- on top of the AFM layer. NiMn, PtMn, and IrMn were used ropy of the 200 A thick 2 5 m NiFe element. The effect of as the AFM layer materials, while NiFe and CoFe were used induced uniaxial anisotropy is stronger in the samples with as the FM layers. All samples in this study were annealed in NiMn and PtMn AFM layers, in which structural grains are a magnetic field. Anneal conditions temperature, time, and characterized by uniaxial magnetocrystalline anisotropy, than magnetic field were different for the samples with different in the samples with IrMn, in which the grains, most likely, AFM layer material and thickness. The conditions were cho- have cubic magnetocrystalline anisotropy. A four-domain sen to maximize the exchange field for a particular sample Landau­Lifshitz structure forms in the 2 5 m NiFe 100 structure. A /IrMn 70 A element when the external field compensates the pinning field 75 Oe Fig. 2 c and 2 f . This indicates a Electronic mail: taras that shape anisotropy of the 100 A thick element overcomes ­g­pokhil@seagate.com 0021-8979/2002/91(10)/6887/3/$19.00 6887 © 2002 American Institute of Physics Downloaded 07 Jun 2002 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp 6888 J. Appl. Phys., Vol. 91, No. 10, 15 May 2002 Pokhil, Song, and Linville FIG. 1. MFM images of a 2 5 m NiMn 280 A /NiFe 200 A element obtained under external field progressively increased in the direction oppo- site to pinning field and then decreased: 0 Oe a , 50 Oe b , 100 Oe c , 250 Oe d , 50 Oe e , 0 Oe f , and 75 Oe g . Before patterning, the film was annealed at 270 °C for 2 h under 250 Oe field. FIG. 3. MFM images of a 2 5 m NiMn 280 A /NiFe 100 A element exchange induced uniaxial anisotropy. The induced uniaxial obtained under external field progressively increased in the direction oppo- anisotropy manifests itself results in the delay of the for- site to pinning field and then decreased: 0 Oe a , 200 Oe b , 300 Oe c , 570 Oe d , 150 Oe e , 50 Oe f , 0 Oe g , and 0 Oe h . Image mation of four-domain structure when the field is increased h was collected 10 min after the collection of image g . Isolated areas from 0 to 75 Oe or decreased from 300 Oe to 75 Oe Figs. with switched magnetization direction are clearly seen in f . Image g 2 a ­2 c or Figs. 2 d ­2 f . Under the experimental condi- shows reversal of magnetization in the switched areas back to pinning di- rection during collection of the image. Before patterning, the film was an- nealed at 270 °C for 2 h under the applied field. Inset shows hysteresis loop of the parent film before patterning. tions, the element remained in a frustrated state, in which magnetization is switching between various metastable states, for tens of minutes before it comes to the stable four- domain state. In the elements with a thin 100 A FM layer and NiMn or PtMn as the AFM layer a local switching of mag- netization direction under external applied field has been ob- served. Sequences of MFM images of 2 5 m NiMn 280 A /NiFe 100 A and PtMn 250 A /NiFe 100 A elements ob- tained under external field progressively increased in the di- rection opposite to pinning field and then decreased back to zero are presented in Figs. 3 and 4, respectively. Magnetiza- tion in the switched areas is 180° or close to 180° twisted relative to the magnetization in adjacent areas. In the MFM images, switched areas show up as closely spaced pairs of black and white spots with a direction from ``black'' to ``white'' opposite to the ``black-to-white'' direction of the image of magnetic charges at the edges of the element. Iso- lated switched areas are clearly seen in Fig. 3 f . A single, FIG. 2. MFM images of a 2 5 m NiFe 100 A /IrMn 70 A element ob- isolated switched area in the PtMn 250 A /NiFe 100 A ele- tained under external field progressively increased in the direction opposite ment is also shown in Fig. 5. The size of the switched areas to pinning field and then decreased: 0 Oe a , 75 Oe b , 75 Oe c , 300 Oe d , 75 Oe e , 75 Oe f , 50 Oe g , and 0 Oe h . Image c in the described samples is 0.1­0.2 m, which is five to ten was collected 20 min after the collection of image b , f was collected 10 times greater than the size of structural grains in the films min after e . Images b and e show unstable micromagnetic structure, 20­30 nm . The areas switch in a viscous manner. Pairs of which changes during the collection of the image. Before patterning, the film was annealed at 250 °C for 2 h under applied field. Inset shows hyster- successive MFM images taken under constant applied field esis loop of the parent film before patterning. Figs. 3 g , 3 h and Figs. 4 f , 4 g show switching of the Downloaded 07 Jun 2002 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp J. Appl. Phys., Vol. 91, No. 10, 15 May 2002 Pokhil, Song, and Linville 6889 FIG. 6. MFM images of a 2 5 m PtMn 100 A /CoFe 100 A element obtained under external field progressively increased in the direction oppo- site to pinning field and then decreased: 0 Oe a , 300 Oe b , 550 Oe c , 50 Oe d , 0 Oe e , and 100 Oe f . Before patterning, the film was annealed at 300 °C for 4 h under an applied field. Inset shows hysteresis FIG. 4. MFM images of a 2 5 m PtMn 250 A /NiFe 100 A element loop of the parent film before patterning. obtained under external field progressively increased in the direction oppo- site to pinning field and then decreased: 0 Oe a , 200 Oe b , 300 Oe local switching was observed in the samples with IrMn AFM c , 570 Oe d , 200 Oe e , 100 Oe f , 100 Oe g , and 0 Oe h . layer. Exchange with IrMn induces slight rippling of magne- Image g was collected 20 min after the collection of image f . Image f tization in the FM layer Fig. 2 . Unlike NiMn or PtMn, shows reversal of magnetization in the switched areas back to pinning di- rection during collection of the image. Before patterning, the film was an- whose structural grains have uniaxial magnetocrystalline an- nealed at 260 °C for 8 h under applied field. Inset shows the hysteresis loop isotropy, IrMn grains are likely characterized by cubic mag- of the parent film before patterning. netocrystalline anisotropy. This is the most likely reason for the lower observed perturbation of the FM layer spin struc- ture in the samples with an IrMn AFM layer than in the magnetization direction in the areas over time as a result of samples with NiMn or PtMn AFM. thermal excitation and excitation by the MFM tip field. Simi- Observed local magnetization switching can be ex- lar micromagnetic behavior was observed in NiMn/CoFe and plained using either the model of thermally activated switch- PtMn/CoFe bilayers Fig. 6 . However, the size of switching ing of AFM grains9,10 or the model of induced uniaxial an- areas was significantly larger. Most likely it is due to the isotropy at the AFM/FM interface7 suggesting local greater in-plane exchange length in CoFe than in NiFe. No variations of induced uniaxial anisotropy. In both models, the in-plane exchange in the FM layer has to be taken into ac- count to explain the effect of the FM layer material and thickness. An increase of the in-plane exchange in the FM layer or its thickness increases the weight of the FM ex- change energy in the total energy of the system and sup- presses perturbation of the FM layer spin structure caused by the exchange with the AFM layer. 1 W. H. Meiklejohn and C. P. Bean, Phys. Rev. 102, 1413 1956 ; 105, 904 1957 . 2 D. Mauri, H. C. Seigmann, P. S. Bagus, and E. Kay, J. Appl. Phys. 62, 3047 1987 . 3 A. P. Malozemoff, J. Appl. Phys. 63, 3874 1988 . 4 N. C. Koon, Phys. Rev. Lett. 78, 4865 1997 . 5 FIG. 5. MFM images of a part of a 2 5 m PtMn 250 A /NiFe 100 A K. Takano, R. H. Kodama, A. E. Berkowitz, W. Cao, and G. Thomas, element. The unidirectional exchange field and induced anisotropy axis are Phys. Rev. Lett. 79, 1130 1997 . 6 perpendicular to the long side of the element. Images show the micromag- V. I. Nikitenkio, V. S. Gornakov, L. M. Dedukh, Y. P. Kabanov, A. F. netic state of the element after applying a 600 Oe field opposite to the Khapikov, A. J. Shapiro, R. D. Shull, A. Chaiken, and R. P. Michel, J. exchange field and then reducing it consequently to 50 Oe a and 0 Oe b , Appl. Phys. 83, 6828 1998 . 7 c b -first image taken after reducing field to zero, c -second taken T. C. Schulthess and W. H. Butler, J. Appl. Phys. 85, 5510 1999 . 8 image . Area with magnetization opposite to the magnetization in the rest of T. Pokhil, S. Mao, and A. Mack, J. Appl. Phys. 85, 4916 1999 . 9 the element is seen in the top left-hand side corner of the element. Magne- E. Fulcomer and S. H. Charap, J. Appl. Phys. 43, 4190 1972 . 10 tization in the area switches in the direction of exchange field during scan- C. Hou, H. Fujiwara, and F. Ueda, J. Magn. Magn. Mater. 198, 450 ning of the image b . 1999 . Downloaded 07 Jun 2002 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp