Details of the Magnetization Reversal in Patterned ... - IEEE Xplore

IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014

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Details of the Magnetization Reversal in Patterned Exchange-Biased and Unbiased Thin Films Robert D. Shull1 , Fellow, IEEE, Yuri P. Kabanov2, P. J. Chen1 , Cindi L. Dennis1 , Vladimir S. Gornakov1,2, and Valerian I. Nikitenko1,2 1 National 2 Institute

Institute of Standards and Technology, Gaithersburg, MD 20899-8552 USA of Solid State Physics, Russian Academy of Sciences, Chernogolovka 142432, Russia

New features of the magnetization reversal of a patterned bilayer in the form of a square mesh consisting of 16 µm wide intersecting stripes of Ni79 Fe21 (30 nm) exchange-coupled with IrMn (10 nm) on top were revealed via use of the advanced magneto-optical indicator film technique. A peculiar asymmetry of the remagnetization behavior was observed for the forward and backward branches of the hysteresis loop for fields parallel to the sample’s unidirectional anisotropy. Herein, we compare the magnetization reversal processes in the biased and nonbiased patterned ferromagnetic nano-layers. Index Terms— Domain structure, exchange coupling, magnetization reversal, magnetostatic field.

I. I NTRODUCTION

T

HE magnetization reversal mechanism in exchangebiased systems is one of the most highly debated subjects. Paradoxical disagreements have been revealed (see reviews [1]–[3]) between the experimental data and predictions of the simplest theory [4]. Firstly, a significant discrepancy has been found between the measured value of the hysteresis loop shift and that calculated. Secondly, the simplest theory [4] did not predict a change in coercivity that has also been experimentally observed. To explain these discrepancies, different models have been proposed [1]–[3]. Some of them took into account peculiarities of the antiferromagnet (AFM) spin subsystem in the ground state formed during the preparation of the heterophase bilayer structure and its subsequent cooling down to the temperature of investigation. Reduction in the proportion of uncompensated AFM spins at the interface between layers (due to the existence of atomic steps there, domains in the AFM layer, and so on) would be expected to decrease the exchange shift of a hysteresis loop compared with that calculated by theory [4], even if the reversal process of the ferromagnet (FM) occurred via simple uniform rotation of its spins in the absence of the retarding force of the frozen AFM spins to this rotation. Other models took into account the consequences of inhomogeneities in the forces acting on the spins of quasi-2-D heterophase magnets. In these cases, the magnetization reversal includes an inhomogeneous rotation of spins over the film thickness, thereby leading to the formation of a domain wall (DW) (spin spiral, exchange spring, and so on) parallel to the heterophase interface during magnetization reversal from the ground state. As the magnetic field increases, rotating FM spins increasingly stimulate turns of AFM spins. At high fields, the exchange spring is almost completely localized in the AFM under the interface.

Manuscript received March 7, 2014; revised May 28, 2014; accepted May 30, 2014. Date of current version November 18, 2014. Corresponding author: R. D. Shull (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2014.2328979

An asymmetry in the activity of DW nucleation centers in FMs with unidirectional anisotropy has also been revealed [5]–[7]. For opposite directions of the external magnetic field, domains formed at different places. For fields antiparallel to the unidirectional anisotropy direction (i.e., in the descending branch of the hysteresis loop) domains nucleated at locations in the FM film having a minimum value of the unidirectional anisotropy energy [7]. In the ascending branch of hysteresis loop (i.e., for fields parallel to the unidirectional anisotropy direction), the situation was completely opposite: domains in the FM film formed in the regions having a maximum value of energy of the unidirectional anisotropy spring. In both cases, the reversal of the entire film subsequently occurred via the lateral expansion of nucleated domains. The observed asymmetry of domain formation in the FM was explained qualitatively by the fact that their nucleation at different branches of the hysteresis loop was controlled by different processes. The formation of stable domains during magnetization reversal of the FM film from the ground state occurred due to exchange spring winding in the FM and its penetration into the AFM in areas, where the value of the magnetocrystalline anisotropy energy, K A , was minimal. Domain formation in the switched FM with field reduction was initiated by the unwinding of already existing spin spirals (in the regions of a film where the AFM anisotropy is maximal). Interestingly, it was also discovered [8] that in some cases the decreasing field reversal was accomplished by domain nucleation and growth while the increasing field reversal proceeded via uniform spin rotation. In this paper, we studied the microscopic details of the magnetization reversal of an exchange-biased FM patterned in the form of a square mesh. In such a system, there is a large amount of magnetostatic energy [9]. Since it is well known, the ground state and reversal mechanisms are determined by the competition from different energy sources, such a system will enable a probe of magnetostatic energy influences. II. E XPERIMENTAL P ROCEDURES Two 30 nm thick FM polycrystalline Ni79 Fe21 films were deposited by magnetron sputtering onto Si(100) wafers having

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Fig. 1. Optical micrograph. (a) Top view. (b) and (c) Schematic illustration of the patterned Ni79 Fe21 film and IrMn/Ni79 Fe21 bilayer.

a 250 nm thermal oxide on top. After deposition of the FM films, a saturating magnetic field was applied to one of the films in the plane of the film, and an additional 10 nm thick AFM IrMn film was deposited on top. The films were then microstructured into a rectangular mesh [Fig. 1(a)] by means of ion beam etching through a mesh mask laid on top. Both the FM film [Fig. 1(b)] and the FM/AFM heterostructure [Fig. 1(c)] meshes had a 100 μm spatial period and 16 μm stripe width. The deposition magnetic field H S was directed downward, along a vertical stripe in the FM/AFM pattern of Fig. 1(a), thereby marking the direction of the induced exchange anisotropy. The field used during our magnetization reversal experiments was also applied along the vertical stripes. The hysteresis loops of the patterned FM film [Fig. 2(a)] and FM/AFM structure [Fig. 2(b)] were measured using a SQUID magnetometer. All SQUID and magneto-optical (MO) indicator film (MOIF) measurements were performed below a Neel temperature of IrMn (690 K [1]) at room temperature. Real time visualization of domain structures was provided during the remagnetization process using the MOIF technique [10]. In this technique, a transparent Bi-doped yttrium iron garnet indicator film with an Al mirror bottom surface is placed on top of the sample to image the stray magnetic fields around the film edges and DWs. In the absence of a magnetic field, the garnet magnetization is oriented in-plane, but it is deflected out of plane by perpendicular components of the stray field, H ⊥ , around the sample. Using a polarized light microscope with slightly uncrossed polarizer and analyzer, MO images of the sample’s magnetic structure are obtained in the reflected light due to the double Faraday effect of the indicator film. The black and white colors of the MO image correspond to opposite signs of H ⊥ . III. R ESULT AND D ISCUSSION Fig. 2 shows hysteresis loops of the patterned film and bilayer that were measured when the field was applied along


Fig. 2. Macroscopic hysteresis loops of (a) patterned FM structure and (b) AFM/FM film.

the vertical stripes in the two samples (i.e, perpendicular to the horizontal stripe in the homophase Ni79 Fe21 mesh and along the unidirectional anisotropy axis in the heterophase Ni79 Fe21 /IrMn structure). The former sample has a small coercivity with practically zero magnetocrystalline anisotropy. The latter sample demonstrates a shifted hysteresis loop and an enhanced coercivity due to the unidirectional anisotropy induced by the AFM stripes. First, we examine the reversal behavior of the FM patterned structure, which possesses only shape anisotropy. Fig. 3 shows MO images of a horizontal stripe of the FM mesh and a schematic view of it during its remagnetization from the positive saturated state [Fig. 3(a) and (a1 )] to negative saturation [Fig. 3(e) and (e1 )]. Strong contrast of the MO signal caused by large stray fields is observed on the edges of these stripes, whereas it practically vanishes at the edges of the vertical stripes, where the magnetic field and, consequently, the magnetization are parallel and do not produce any stray field. Decreasing external field gives rise to the decreased MO contrast at the horizontal stripe edge. In addition, a new monopolar (Néel) DW is formed in the middle of the stripe [Fig. 3(b)]. The magnetization in the Néel wall lies in the initial direction, as shown in Fig. 3(b1). On further field decrease, this wall becomes unstable. First, a subdomain with opposite magnetization direction appears in the wall [Fig. 3(b)] forming a cross-tie structure. As it is schematically shown in Fig. 3(b1), this subdomain is bounded by a vortex and an antivortex [11]. The generation of further subdomains continues and reaches an energetically stable density at a zero field [Fig. 3(c)]. In this case, certain sizes of new λ1 and old λ2 subdomains are stabilized [Fig. 3(c1 )]. Furthermore, remagnetization of the horizontal stripe occurs with H directed in the opposite direction. With increasingly negative field, the size of the new subdomain λ1 increases

SHULL et al.: DETAILS OF THE MAGNETIZATION REVERSAL IN PATTERNED EXCHANGE-BIASED AND UNBIASED THIN FILMS

Fig. 3. (a)–(f) MOIF images and (a1 )–(f1 ) related schematics of the domain structure taken during the magnetization reversal of the Ni79 Fe21 film with H perpendicular to the mesh horizontal stripes. (a)–(f) and (a1 )–(f1 ) correspond to the conditions indicated by the circles labeled by the same letters on the hysteresis loop in Fig. 2(a). White and black arrows: directions of the field and magnetization, respectively.

but the old subdomain size λ2 decreases [Fig. 3(d) and (d1 )]. However, it is important to note, the sum (λ1 + λ2 ) and subdomain density remain practically constant up to a certain critical field value H CR . For |H| < |H CR |, the spins in the domains rotate toward H as H becomes increasingly negative as shown in Fig. 3(d1 ). When the H = HCR , unstable old subdomains abruptly annihilate and the horizontal stripe becomes magnetized homogeneously in the negative field direction [Fig. 3(e) and (e1 )]. Note that the cross-tie nucleation fields, density, and critical fields, HCR can be different at different places in the sample. In some cases, in the horizontal stripes double DWs may even form [Fig. 3(f)] with a specific distribution of the magnetization [Fig. 3(f1 )]. However, the magnetization reversal of such strips proceeds in the same way as described above. It is also important to note that the magnetization reversal scenario described above occurs in both hysteresis loop branches, in the case when the magnetic field is oriented perpendicular to the stripe. If the field is deviated from this direction by even 3° to 5°, the reversal of the horizontal stripes occurs through quasi-homogeneous magnetization rotation. Vertical stripes of the mesh are reversed in this field orientation by the formation and motion of DWs, which nucleate very abruptly and run too fast to follow in our apparatus. The unusual magnetization reversal described above for the FM mesh members is dictated by the magnetostatic field in soft patterned magnetic thin films with zero magnetocrystalline anisotropy. Magnetostatic fields, which are formed on the mesh edges during sample magnetization, induce the canted state of the spins exclusively along the horizontal stripe edges. With decreasing field, the spins rotate parallel to the edge in

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Fig. 4. MOIF images of the domain structure of the mesh horizontal stripes taken during the unidirectional-axis remagnetization (a)–(e) from the ground state and (f)–(j) to the ground state of a patterned IrMn/Ni79 Fe21 bilayer. (a)–(j) correspond to the conditions indicated by the circles labeled by the same letters on the hysteresis loop in Fig. 2(b).

Fig. 5. MOIF images of domain structure of the mesh vertical stripes taken sequentially at increasing time while the field was held constant at the appropriate critical field HCR during the unidirectional-axis remagnetization (a)–(c) from the ground state and (d)–(f) to the ground state of a patterned IrMn/Ni79 Fe21 bilayer. The time interval between every neighboring image (frame) is 50 ms.

order to minimize the magnetostatic energy near the edge. Due to the FM exchange interaction, the spins further from the edges also rotate with decreasing field to the same direction as the edge spins. When the spin deviations near the two opposite edges become perpendicular to the interior spins one or two DWs will appear, depending upon whether the spin canting directions near the opposite edges of the horizontal stripes are respectively the same or not. The influence of a unidirectional anisotropy on a magnetization reversal of the same soft FM Ni79 Fe21 films exchange coupled to AFM IrMn is shown in Figs. 4 and 5. In the positive saturation state, the MO contrast [Fig. 4(a)] was the same as in the FM mesh [Fig. 3(a)]. While the field was decreased to zero, the contrast at the horizontal stripe

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edges weakened due to quasi-homogeneous magnetization rotation [Fig. 4(b)]. Subsequent application of an increasingly negative field was accompanied by further magnetization rotation and the formation of a very specific domain structure at the intersections between horizontal and vertical stripes [Fig. 4(c)]. When the negative field reached a first critical value (marked as I on Fig. 2(b)], the magnetization in the vertical stripes quickly reversed its direction [Fig. 5(a)–(c)]. Surprisingly, this abrupt event did not appear to affect the further magnetization rotation in the horizontal stripes [Fig. 4(d) and (e)]; it only affected the MO contrast distribution at the stripe intersections. In fact, it is because of this MO contrast change at the intersections that one knows the vertical stripes have reversed their magnetization direction. Finally, the opposite contrast of Fig. 4(f) is reached when the spin rotation in the horizontal strip reaches alignment with H at saturation in the reversed field. The remagnetization of the exchange-biased sample back to the ground state, however, did not follow the same process described above for the remagnetization from the ground state. This is in contrast to the behavior of the soft unbiased FM mesh. Initially, the remagnetization back to the ground state proceeded by quasi-homogeneous magnetization rotation in the horizontal stripes back toward alignment with the stripe. With decreasing negative field, Néel DWs appeared near the edges. Then, as described above for the FM mesh, a subdomain is nucleated in this DW [Fig. 4(g)]. Furthermore, decrease in the negative field leads to the nucleation of new subdomains [Fig. 4(h)]. The change in intensity of MO contrast along the top edge of the horizontal stripe from black [Fig. 4(h)] to white [Fig. 4(i)] is caused by the growth of new subdomains and the reduction in size of the old ones. When the negative field reached a second critical value [marked as II on Fig. 2(b)], the vertical stripes quickly reversed their magnetization direction [Fig. 5(d)–(f)] similar to the way they reversed during the remagnetization process from the ground state. Furthermore, change of the horizontal stripe remagnetization proceeded by quasi-homogeneous spin rotation [Fig. 4(j)] to the saturated state in a positive field [Fig. 4(a)]. This process is partly accompanied by the growth of new subdomains. These experiments show main differences between the two soft FM samples. For the homophase FM mesh, the stray fields near the horizontal stripe edges play the main role in the remagnetization of these stripes. For the heterophase FM/AFM mesh, the edge spin rotation during remagnetization from the ground state was blocked by the AFM spins coupled with the FM spins. Formation of an AFM spin exchange spring controlled the continued quasi-homogeneous FM spin rotation as H increased to large negative values. During FM/AFM mesh remagnetization back to the ground state, as the negative H value decreases, the edge stray field plays a more significant role in controlling the spin rotation. AFM spin spring unwinding helps now to nucleate DWs along the horizontal stripe edges. As for the FM mesh, this latter process is dependent on how close to perpendicular to the horizontal stripe is the direction of the applied field.


However, unlike the FM mesh case, the substructure of DWs in the FM/AFM mesh disappears slowly, without sharp annihilation, as H approaches saturation. The magnetization reversal of the vertical stripes occurs both from the ground state [Fig. 5(a)–(c)] and to the ground state [Fig. 5(d)–(f)] by the fast movement of unusual DWs, which a different type than those that exist in large unpatterned FM/AFM layered samples. Similar types of DW structure are realized in both remagnetization directions. In saturated stripes [Fig. 5(a) and (d)], there is no stray field along their edges. So, there is no reason for easy nucleation of new domains. The front therefore usually forms near regions, where the mesh connects with the peripheral non patterned film (Fig. 1). For reversal from the ground state, such fronts can also be formed near the intersections. These intersections can also serve as centers for pinning as well. As one can see in Fig. 5, the fronts, which have moved from the bottom and been pinned near an intersection [Fig. 5(b) and (e)], may either transform [Fig. 5(c)] or jump fast to the next intersection [Fig. 5(f)]. However, in the majority of cases, these fronts run through many intersections or through the complete mesh at one time. The complicated structure of the magnetization fronts shown in Fig. 5 is never observed in thin films and heterostructures. The observed kinetics and structure of the revealed front is only inherent to soft FM thin films with a restricted lateral size, where magnetostatic fields play a main role. In our case, any motion of magnetic inhomogeneity along a vertical stripe initiates magnetic charges at the edges, which generate a new magnetic inhomogeneity. This inhomogeneity can then move under an applied field as well. R EFERENCES [1] J. Nogues and I. K. Schuller, “Exchange bias,” J. Magn. Magn. Mater. vol. 192, pp. 203–232, Mar. 1999. [2] R. L. Stamps, “Mechanisms for exchange bias,” J. Phys. D, vol. 33, no. 23, pp. R247–R268, 2000. [3] F. Radu and H. Zabel, “Exchange bias effect of ferro-/antiferromagnetic heterostructures,” Springer Tracts Modern Phys., vol. 227, pp. 97–184, Feb. 2008. [4] W. H. Meiklejohn and C. P. Bean, “New magnetic anisotropy,” Phys. Rev., vol. 105, pp. 904–913, Feb. 1957. [5] V. I. Nikitenko et al., “Asymmetry of domain nucleation and enhanced coercivity in exchange-biased epitaxial NiO/NiFe bilayers,” Phys. Rev. B, vol. 57, pp. R8111–R8114, Apr. 1998. [6] V. I. Nikitenko et al., “Direct experimental study of the magnetization reversal process in epitaxial and polycrystalline films with unidirectional anisotropy,” J. Appl. Phys., vol. 83, no. 11, pp. 6828–6830, 1998. [7] V. I. Nikitenko et al., “Asymmetry in elementary events of magnetization reversal in ferromagnetic/antiferromagnetic bilayers,” Phys. Rev. Lett., vol. 84, pp. 765–768, Jan. 2000. [8] M. R. Fitzsimmons et al., “Asymmetric magnetization reversal in exchange-biased hysteresis loops,” Phys. Rev. Lett, vol. 84, pp. 3986–3989, Apr. 2000. [9] R. Mattheis, K. Ramstock, and J. McCord, “Formation and annihilation of edge walls in thin-film permalloy strips,” IEEE Trans. Magn., vol. 33, no. 5, pp. 3993–3995, Sep. 1997. [10] V. S. Gornakov, Y. P. Kabanov, V. I. Nikitenko, O. A. Tikhomirov, A. J. Shapiro, and R. D. Shull, “Chirality of a forming spin spring and remagnetization features of a bilayer ferromagnetic system,” J. Experim. Theoretical Phys., vol. 126, no. 3, pp. 691–703, 2004. [11] A. Hubert and R. Schafer, “Magnetic films with low anisotropy,” in Magnetic Domains, 3rd Ed. Berlin, Germany: Springer-Verlag, 2008.