Highly Efficient In-Line Magnetic Domain Wall Injector - American

Jan 13, 2015 - ABSTRACT: We demonstrate a highly efficient and simple scheme for injecting domain walls into magnetic nanowires. The spin transfer tor...
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Highly Efficient In-Line Magnetic Domain Wall Injector Timothy Phung,†,‡ Aakash Pushp,† Luc Thomas,† Charles Rettner,† See-Hun Yang,† Kwang-Su Ryu,† John Baglin,† Brian Hughes,† and Stuart Parkin*,† †

IBM Almaden Research Center, San Jose, California United States Department of Electrical Engineering, Stanford University, Stanford, California United States

‡

S Supporting Information *

ABSTRACT: We demonstrate a highly efficient and simple scheme for injecting domain walls into magnetic nanowires. The spin transfer torque from nanosecond long, unipolar, current pulses that cross a 90° magnetization boundary together with the fringing magnetic fields inherently prevalent at the boundary, allow for the injection of single or a continual stream of domain walls. Remarkably, the currents needed for this “in-line” domain wall injection scheme are at least one hundred times smaller than conventional methods.

KEYWORDS: Spintronics, spin transfer torque, domain wall injection, magnetic nanowires, domain wall dynamics, racetrack memory

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needed for DW creation in the nanowire with PMA and IMA regions can be decreased24,26,27 because the process now simply involves extraction of the 90° magnetization boundary into the PMA region. We show that while this can be carried out conventionally by creating a local magnetic field near the injection site (Figure 1a), DWs can now also be injected by flowing a charge current directly through the nanowire and exploiting the current induced STT obtained at the 90° magnetization boundary (Figure 1b). This is distinct from a previous work23 where the presence of small external assisting magnetic fields was essential to inject a single DW in the presence of a charge current, and where the mechanism of the DW creation and extraction process was unclear. The requirement of external magnetic field limits the functionality of any DW injection scheme as only a single DW of a particular type (either up/down or down/up) can be injected into a PMA nanowire determined by the directionality of the magnetic field. In this work, the requirement of external magnetic field is entirely avoided for DW injection, thereby allowing for the continuous and synchronous injection of a series of up/down and down/up DWs. Furthermore, we identify the underlying key factor, namely the local fringing magnetic fields already prevalent at the 90° magnetization boundary, being critical for the perpetual injection of a series of domain walls upon the application of a current. Figure 1 shows a comparison of the field-based injection (FBI) process of creating DWs with that of the STT based in-

sing tiny magnetic domains to store information is at the heart of hard-disk drive storage as well as several promising memory technologies such as, magnetic random access memory and racetrack memory (RM).1,2 In RM, in particular, information is encoded in tiny domains (Figure 1) separated from each other by domain walls (DWs).3−13 The key to the operation of these devices is the controlled injection14−16 of DWs in nanowires at low power and thus formulating schemes for energy efficient creation of DWs is critical. Conventionally, local magnetic fields obtained from metal contact lines in conjunction with external assisting magnetic fields have been employed to create up/down or down/up DWs in a magnetic nanowire exhibiting perpendicular magnetic anisotropy (PMA), whereas the synchronous motion of a series of DWs along a nanowire is achieved using spin transfer torque17,18 (STT) from charge currents that transport spin angular momentum. We discuss a novel in-line DW injection scheme that entirely avoids the use of any external magnetic field, thereby making this scheme highly advantageous for a stand-alone RM device. This scheme requires the presence of a 90° magnetization boundary in a PMA nanowire, which can be obtained by a number of ways such as ionirradiation,19,20 voltage-controlled anisotropy,21,22 ion-etching,23 and so forth. It is well established that by irradiating small regions in perpendicularly magnetized nanowires19,24 to locally induce in-plane magnetic anisotropy (IMA), a 90° magnetization boundary can be created at the interface between the IMA and PMA regions. (PMA in many metallic magnetic materials is obtained from multilayers of atomically thin ferromagnetic layers, which naturally can be modified by interfacial mixing25 from ion-beam irradiation.) The energy © XXXX American Chemical Society

Received: September 3, 2014 Revised: December 31, 2014

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Figure 1. Schematic comparison of DW injection schemes. (a) Schematic of configuration I and II of the existing FBI scheme. Configuration I requires a total of three current pulsers; two unipolar pulsers for injection of DWs and another unipolar pulser for the shifting of DWs. Configuration II of FBI uses a single bipolar pulser for injection of DWs and a secondary pulser for shifting DWs. (b) A simplified ILI scheme that requires only a single unipolar current pulser. DWs can be sequentially injected and shifted across the RM by a pulse sequence consisting of only injection pulses or a pulse sequence that is composed of alternating injection and shift pulses. The current from the shift pulse is below the critical current needed to inject a DW in the RM. (c) Dense RM architecture made possible by the truly one-dimensional nature of the ILI.

nanowire is fabricated from a thin film PMA magnetic stack comprised of Si(Ox)|100 AlOx|50 Ti60Ni40|3 Co|[ 7 Ni|1.50 Co]6|50 TaN, where the numbers represent the thickness of each layer in Å, by electron beam lithography and Ar+ ion milling. The effective anisotropy of the stack Keff = 1/2HkeffMs ∼ 5 × 105 erg/cm3, where the effective anisotropy field, Hkeff ∼ 2 kOe and Ms ∼ 500 emu/cm3 is the saturation magnetization; Keff > 0 corresponds to the easy axis of the magnetization being out of plane. An 80 nm wide TaN Hall bar (30 nm thick) is then patterned on top of the nanowire. The effective anisotropy at the injection sites, Kinj eff, of a series of nominally identical devices can now be controllably reduced by adjusting the ion irradiation dose in the range from 2 × 1011 to 3 × 1014 ions/ cm2 of a 20 keV Ne+ ion beam (see Supporting Information SI and II), before depositing the gold injection lines and Hall bar contacts. Dosages exceeding ∼1013 ions/cm2 result in the injection site being in-plane magnetized (Kinj eff < 0). inj We first discuss the role of Keff on the nanowire’s magnetization switching under quasi-static external fields (Figure 2b). Interestingly, the switching field, HSW, defined as the minimum global field needed to switch the magnetization of the nanowire in the z-direction (witnessed by an abrupt change in the resistance of the nanowire as a function of 3 magnetic field), is minimized when the Kinj eff ∼ 0 erg/cm . In this case, HSW ∼ 75 Oe, which is more than six times smaller than

line injection (ILI) in a PMA nanowire. For the FBI geometry, two possible configurations are shown in Figure 1a (labeled I and II) that can be used to inject up/down or down/up DWs with local magnetic field and shift them with STT. In configuration I, two unipolar current pulsers on either side of the injection line, and one current pulser to shift the DWs in the racetrack are needed. Alternately, in configuration II one bipolar and one current pulser are required. In principle, configuration I can be modified to only use two pulsers to be connected on either side of the injection line that deliver negative current pulses into the nanowire so as to simultaneously shift the DW with STT upon its creation.13 In any of these scenarios, at least two injection pulsers are required. In contrast, we show that only one unipolar current pulser is needed in the ILI scheme both for injection and shifting of DWs thereby decreasing the complexity of RM. Moreover, the truly one-dimensional nature of the ILI geometry makes the RM architecture much more simplified and streamlined owing to its smaller footprint (Figure 1c) as compared to conventional designs. In order to be able to controllably inject DWs using ILI, it is critical to understand the DW creation process at the 90° magnetization boundary. Figure 2a shows a scanning electron micrograph (SEM) of a typical device along with a schematic of the measurement circuit. First, a 70 nm wide (and ∼8 μm long) B

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Figure 2. Measurements of FBI. (a) SEM image of a typical device and schematic of the circuit used for FBI. (b) Hsw using quasi-static external inj magnetic fields versus Kinj eff of the ion-irradiated site. The red box inset shows the remanant state and DW nucleation process for Keff > 0. The blue inj box inset shows the remanant state for Keff < 0 containing a 90° magnetic boundary already present and switching occurring by DW extraction. (c) Histograms of ΔR after injection for a nanowire with Kinj eff ∼ 0 for increasing injection current; the corresponding Pinj versus Iinj is shown in panel d. (e) Icinj versus Kinj eff of the ion-irradiated site measured on a series of nominally identical nanowires. The right-hand side y-axis represents the equivalent peak magnetic field, H, generated from Icinj. Error bars represent the current range over which the injection probability is between 20 and 80%.

applying HRESET = −1000 Oe. A negative voltage pulse (5 ns long) is then applied along the injection line, implying that current flows toward the pulse generator (Figure 2a). The injection current, Iinj, that flows through the injection line is estimated to be ∼33.3 mA/V. This pulse generates a local magnetic field (peak value of ∼7.1 Oe/mA in the z-direction) which, when sufficiently large, can locally reverse the magnetization underneath the injection line thereby creating two DWs, one of which is injected into the main segment of the nanowire. This procedure is repeated 20 times so as to obtain better statistics. Injection of a DW in the nanowire leads to an increase of its resistance, ΔR, due to a combination of spin dependent scattering at the DW28 and anisotropic magneto-resistance.29 Histograms of ΔR for increasing Iinj are displayed in Figure 2c for a representative device. When the current exceeds a critical value, the peak centered at ΔR ∼ 0 Ω shifts to ΔR ∼ 0.13 Ω indicating the injection of a DW. The injection probability Pinj at each Iinj is shown in Figure 2d. The critical DW injection current, Icinj, defined for 50% injection probability, is shown in Figure 2e as a function of Kinj eff, which closely follows the results from the switching with external field measurements (Figure 2b). Icinj reaches its minimum value of 10.7 mA for Kinj eff ∼ 0, corresponding to a 10-fold reduction compared to the nonirradiated case, thereby making the injection process 100 times more energy efficient. Having understood the mechanism of DW creation from the irradiated regions, we now discuss ILI by using STT rather than magnetic fields to extract DWs. In order to visualize the DW extraction process, we perform polar-MOKE microscopy measurements on microwires (60 μm long and 1 μm wide)

the value for nonirradiated devices, that is in good agreement with results reported in the literature for devices irradiated using focused ion beams.24,27 The dependence of HSW on Kinj eff can be understood by considering that the switching of a nanowire requires both the nucleation of one or several DWs, which takes place when the field exceeds the nucleation field, HN, and the propagation (characterized by the propagation field, HP) of these DWs along the nanowire. This propagation requires overcoming both intrinsic pinning sites and the energy barrier created by the local reduction of the anisotropy caused by ion irradiation (characterized by an extraction field HEX). HSW is determined by the largest of these fields, that is, HSW = max(HN, HP, HEX). In these samples, HP ∼ 70 Oe and is not influenced by the ion irradiation, whereas both HN and HEX inj depend on the irradiation dose. When Keff > 0, the magnetization of the irradiated region remains oriented perpendicular to the plane and switching occurs by nucleation of a reversed domain (as shown in Figure 2b in the red box). In this regime, the switching field varies linearly with the effective anisotropy field because the nucleation is essentially a magnetization rotation process. In contrast, when Kinj eff < 0, the irradiated region is magnetized in-plane (Figure 2b in the blue box). Therefore, a 90° magnetization boundary must exist between the IMA and PMA regions and, consequently, switching only requires the extraction of this boundary beyond the irradiated site. HSW in this case is given by the maximum of HEX and HP, that is, HSW = max(HP, HEX). In all the devices with IMA irradiated sites, we find HSW > HP ∼ 70 Oe, indicating that this extraction process dominates switching. We now turn to the injection of DWs using FBI. The nanowire’s magnetization is first set in the −z direction by C

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Figure 3. Measurements of ILI. (a,b) Optical microscope image of microwire device (60 μm long and 1 μm wide), along with the schematic of the circuit and pulse sequence; polar-MOKE microscopy measurements of DWs injected by ILI in microwires for negative (a) and positive (b) current pulses. (c) oinj for negative (blue symbols) and positive injection pulses (red pulses) as a function of ninj shows stochastic behavior. (d) SEM image of a typical nanowire device and the schematic of the circuit used for electrical detection of ILI, where RH is used to detect the local magnetization of the magnetic nanowire beneath the Hall bar. Graph shows the RH as a function of H with blue/red signals corresponding to downward/upward magnetization states, respectively. (e) H, RH, and Vinj with experiment iteration, where the nanowire is reset with HRESET ∼ 950 Oe and a single injection pulse is applied. Red dots denote reset operation, purple dots signify when field is brought back to zero, and blue dots indicate points during the experiment where pulses are applied along the nanowire. oinj as a function of ninj that shows 100% injection fidelity. (f) H, RH, and Vinj with experiment iteration for the same nanowire showing successive injection of a series of DWs. Multiple pulses are applied to the nanowire sequentially after each reset operation.

injection pulse (12 mA; current density, j ∼ 2.9 × 108 A/cm2, 50 ns long) followed by eight propagation pulses (7.9 mA; j ∼ 1.9 × 108 A/cm2, 25 ns long) are passed along the microwire. Switching the polarity of the pulse waveform (see Supporting Information Movie S1) also switches the side of the central

made from a PMA magnetic stack by standard photolithography. Here the IMA region is created (by a Ga+ focused ion beam) in the center of the microwire rather than beneath the injection line (Figure 3a). Figure 3a,b shows evidence of inline DW injection in the microwire as five repetitions of 1 D

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Figure 4. Mechanism of ILI. (a) A schematic illustration of the magnetization texture (white arrows) near the irradiated IMA region of the magnetic nanowire, showing the 90° magnetization boundary thus formed between IMA and PMA regions. The blue lines indicate the fringing fields from the PMA region as experienced by the IMA region. The red arrows denote the direction of the magnetization rotation due to the adiabatic STT in the x−z plane. The green arrows denote the anticlockwise precession of the magnetization in the x−y plane that is caused by the damping torque originating from the adiabatic STT (here, β = 0). (b) Trajectory of the average magnetization unit vector of the IMA region of the magnetic nanowire. The initial magnetization has a small +z component due to the exchange interaction with the adjacent PMA region. (c) Time-resolved evolution of the magnetization components and the internal energy of the IMA region of the nanowire. (d) Internal energy change in the IMA region of the nanowire for longer times. Yellow bar denotes the section that was plotted in panel c. A DW is injected into the nanowire when the internal energy of the irradiated portion gets maximized. Blue (red) line indicates the event of a down (up) domain creation in panels b and c. (e) Time-resolved micromagnetic simulations showing the continuous injection of DWs observed in narrow nanowires. Arrows indicate the in-plane magnetization component whereas colors indicate the out-of-plane magnetization component. (f) The simulation shows formation of a series of DWs in the nanowire after 20 ns of constant electron current flow. Only the out-of-plane component of the magnetization is shown. (g) Timeresolved micromagnetic simulations showing the incoherent injection observed in wide microwires. Arrows indicate the in-plane magnetization component whereas colors indicate the out-of-plane magnetization component. The irradiated region in panels e and g are to the left of the dashed green lines. E

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current density is varied from 5 × 107 to 5 × 108 A/cm2, which show similar results. Note that the higher the current density is the faster the DW injection rate will be (see Supporting Information SV). Here we choose to present simulation results for a current density of 1 × 108 A/cm2. The relaxed configuration (Figure 4a) shows a gradient in the magnetization at the 90° magnetization boundary due to a combination of the exchange energy and the anisotropies of the two regions. When an electron current is injected from the IMA toward the PMA region, the STT given by −bJm⃗ × (m⃗ × ((∂m⃗ )/(∂x⃗)) (here, bJ is the adiabatic STT coefficient32 and is the unit vector local moment) is experienced in the direction as shown in Figure 4a, which causes the local moments to turn in the anticlockwise direction in the x−z plane. This leads to a damping torque, given by α(m⃗ × ((∂m⃗ )/(∂t)) that causes the magnetization of the IMA region to precess in the x−y plane in the anticlockwise direction. (For simplicity, we do not consider the nonadiabatic STT term, that is, β = 0. We find that the sense of precession of the IMA region is the same for β = 0 as well as for β = α = 0.01 in all of our simulations.) At the same time, the magnetization of the IMA region picks up a component in the −z direction due to the fringing field from the adjacent PMA region (Figure 4a). This is why a transient down/up DW gets formed, which subsequently gets extracted into the PMA segment of the nanowire due to STT. The time evolution of the various components of magnetization of the irradiated IMA region (Figure 4b,c) shows that the DW is extracted after the magnitude of the z-component magnetization and consequently the internal energy of the initially in-plane magnetized irradiated region becomes maximized (Figure 4d). Upon injection of a DW, the PMA region next to the 90° magnetization boundary switches its orientation thereby also changing the direction of its fringing field as experienced by the IMA region. It is this reversal of the field direction along with the reversal of the STT and damping torques that causes the precession of the IMA region to first slow down and then reverse its direction. Hence, for a constant current that exceeds a threshold value, the IMA region keeps switching its rotation direction (Figure 4c−e) after each time injecting a DW into the nanowire. The IMA region being small for such narrow nanowires is strongly exchange coupled and behaves as a single magnetic entity. However, when the dimension of the IMA region is much larger than the magnetic exchange length,33 for example, in a wider microwire (Figure 4g), the in-line DW injection process is not as well controlled because multiple domains are formed in the irradiated region (see Supporting Information Movie S3). This accounts for the stochastic injection of DWs in the wide microwires (Figure 3c) as compared to high fidelity injection in narrow nanowires (Figure 3e). Furthermore, the wire width dependence of the DW injection probability, Pinj, rules out that thermal nucleation could be the origin of ILI; otherwise, we would expect to see stochastic injection for both wide and narrow magnetic wires. (We see similar results in simulations where Kz, Ms, and α are systematically varied: 1.2 × 106 erg/cm2 ≤ Kz ≤ 5 × 106 erg/ cm3, 150 emu/cm3 ≤ Ms ≤ 500 emu/cm3 and 0.001 ≤ α ≤ 0.01). We thus expect the ILI mechanism to be even more reliable for narrower nanowires, which is a prerequisite for denser RM. We have shown that both FBI and ILI schemes of DWs in PMA nanowires can be controlled by engineering the anisotropy at the injection site using a simple self-aligned ion irradiation process. We note that contrary to the FBI method,

injection site where the DWs get injected into the microwire. Direction of the DW injection is consistent with the direction of the electron current, implying that the DWs get created because of the large STT from the electron flow at the 90° magnetization boundary, as discussed earlier. The directionality of DW injection also precludes DW nucleation due to heating of the microwire. Figure 3c summarizes the occurrence of DW injection, oinj (defined as 1 when a DW gets injected and 0 when no DW gets injected) as a function of the injection pulse number ninj. Even though the injection process is rather stochastic in these microwires, it provides a proof-of-concept that DWs can indeed be injected by STT. We now apply this understanding to study ILI in much narrower (70 nm wide) nanowires. Because of the limitation of using visible light, polar-MOKE microscopy measurements cannot be performed on such small dimensions. Instead, the anomalous Hall resistance across the Hall bar, RH, which changes between two distinct values depending upon whether the portion of the nanowire beneath the hall bar is magnetized in +z or in −z direction (Figure 3d), is used to study the switching behavior of the nanowire. Please note that in order to perform RH measurements, a current of −200 μA (j ∼ 6.3 × 107 A/cm2) is continuously passed through the nanowire, which is large enough for a DW, once injected, to creep through the nanowire.30 The key difference in this device is that the irradiated part is not entirely beneath the injection line, lest all the current will short through the thick gold injection line rather than flowing through the thin nanowire and no STT will be obtained at the 90° magnetization boundary. Remarkably, the probability of DW injection in such narrow nanowires (Figure 3e) is much higher (100%) than that in the wide microwires studied under the polar-Kerr microscope (Figure 3c). Furthermore, we experimentally show that consecutive up/down and down/up DWs can be continuously injected by ILI (Figure 3f). In this experiment, the nanowire is first reset by a large field of ∼950 Oe, which is turned off before a series of five unipolar pulses (Vinj = −1.9 V corresponding to a current of −712 μA, j ∼ −2.2 × 108 A/cm2 and pulse width 6 ns) is applied. Remarkably, we find that the DWs can be injected and shifted out of the nanowire repeatedly by applying the same injection pulse without any external magnetic field. This is highlighted in Figure 3f, which shows that the Hall bar resistance toggles between high and low levels as up/down and down/up DWs move past it successively. We have verified that applying a pulse of opposite polarity (+2 V) so that the electron current flows in the opposite direction, that is, from the PMA into the IMA region, does not inject any DW in the nanowire. To confirm that DWs are actually injected at the irradiated site rather than getting spuriously created because of current induced nucleation, we have also verified that no injection occurs when current pulses up to −2 V in amplitude and 100 ns in pulse width are applied to a nonirradiated nanowire. We perform representative micromagnetic simulations31 (which solve the Landau−Lifshitz−Gilbert equation including the conventional STT terms) in Figure 4 (also see Supporting Information Movie S2) to elucidate the mechanism based on STT for DW injection in the ILI scheme. The anisotropy of the magnetic material, Kz = 1.9 x106 erg/cm3, Ms = 400 emu/cm3, and A = 1.2 μerg/cm. We use a polarization of 0.4 and nonadiabatic spin torque parameter β = α = 0.01 (here, α is the Gilbert damping coefficient). The anisotropy is reduced in a small section at the left-hand side of the wire to −7.5 x105 erg/ cm3, which causes its magnetization to point in-plane. The F

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(15) Cowburn, R. P.; Allwood, D. A.; Xiong, G.; Cooke, M. D. J. Appl. Phys. 2002, 91, 6949−6951. (16) Thomas, L.; Rettner, C.; Hayashi, M.; Samant, M. G.; Parkin, S. S. P.; Doran, A.; Scholl, A. Appl. Phys. Lett. 2005, 87, 262501. (17) Slonczewski, J. C. J. Magn. Magn. Mater. 1996, 159, L1−L7. (18) Berger, L. Phys. Rev. B 1996, 54, 9353−9358. (19) Chappert, C.; Bernas, H.; Ferré, J.; Kottler, V.; Jamet, J.-P.; Chen, Y.; Cambril, E.; Devolder, T.; Rousseaux, F.; Mathet, V.; Launois, H. Science 1998, 280, 1919−1922. (20) Rettner, C. T.; Anders, S.; Baglin, J. E. E.; Thomson, T.; Terris, B. D. Appl. Phys. Lett. 2002, 80, 279−281. (21) Shiota, Y.; Nozaki, T.; Bonell, F.; Murakami, S.; Shinjo, T.; Suzuki, Y. Nat. Mater. 2012, 11, 39−43. (22) Wang, W.-G.; Li, M.; Hageman, S.; Chien, C. L. Nat. Mater. 2012, 11, 64−68. (23) Hayashi, M.; Yamanouchi, M.; Fukami, S.; Sinha, J.; Mitani, S.; Ohno, H. Appl. Phys. Lett. 2012, 100, 192411−4. (24) Lavrijsen, R.; Franken, J. H.; Kohlhepp, J. T.; Swagten, H. J. M.; Koopmans, B. Appl. Phys. Lett. 2010, 96, 222502−3. (25) Carcia, P. F.; Shah, S. I.; Zeper, W. B. Appl. Phys. Lett. 1990, 56, 2345−2347. (26) Franken, J. H.; Hoeijmakers, M.; Lavrijsen, R.; Kohlhepp, J. T.; Swagten, H. J. M.; Koopmans, B.; van Veldhoven, E.; Maas, D. J. J. Appl. Phys. 2011, 109, 07D504−3. (27) Franken, J. H.; Hoeijmakers, M.; Lavrijsen, R.; Swagten, H. J. M. J. Phys.: Condens. Matter 2012, 24, 024216. (28) Levy, P. M.; Zhang, S. F. Phys. Rev. Lett. 1997, 79, 5110−5113. (29) McGuire, T. R.; Potter, R. I. IEEE Trans. Magn. 1975, 11, 1018−1038. (30) Metaxas, P. J.; Jamet, J. P.; Mougin, A.; Cormier, M.; Ferré, J.; Baltz, V.; Rodmacq, B.; Dieny, B.; Stamps, R. L. Phys. Rev. Lett. 2007, 99, 217208. (31) Scheinfein, M. R. LLG Micromagnetics Simulator. (32) Zhang, S.; Li, Z. Phys. Rev. Lett. 2004, 93, 127204. (33) Hubert, A.; Schäfer, R. Springer-Verlag: Berlin, 2000. (34) Kaka, S.; Pufall, M. R.; Rippard, W. H.; Silva, T. J.; Russek, S. E.; Katine, J. A. Nature 2005, 437, 389−392.

when the DWs are injected by the ILI scheme, the injection current scales with the cross-sectional area of the nanowire, which is extremely important for device applications. Moreover, the ILI scheme also removes the need of having bipolar injection pulses. We find that the power consumed per DW injection from the ILI scheme in the most conservative estimate is at least 1 order of magnitude smaller than that of the FBI scheme (see Supporting Information SIII). These results can be further extended to the design of high density spin torque nano-oscillators34 based on the STT driven precession of IMA regions about the fringing fields of the neighboring PMA sections of the magnetic nanowire.

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ASSOCIATED CONTENT

* Supporting Information S

Dependence of the anisotropy on the exposure time of the irradiation, the experimental procedure for the creation of lithographically defined anisotropy landscapes in the magnetic nanowires, an energy comparison of injection in the FBI and ILI schemes, and descriptions of the attached movie files. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

(L.T.) TDK-Headway Technologies, Milpitas, California, U.S.A. Author Contributions

T.P. and A.P. contributed equally to the work. Notes

The authors declare no competing financial interest.

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REFERENCES

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