Imaging, Manipulation, and Spectroscopic Measurements of Nanomagnets by Magnetic Force Microscopy

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Imaging,

Manipulation, and Spectroscopic Measurements of Nanomagnets by Magnetic Force Microscopy Xiaobin Zhu and Peter Grütter

Abstract Magnetic force microscopy (MFM) is a well-established technique for imaging the magnetic structures of small magnetic particles. In cooperation with external magnetic fields, MFM can be used to study the magnetization switching mechanism of submicrometer-sized magnetic particles. Various MFM techniques allow the measurement of a hysteresis curve of an individual particle, which can then be compared to ensemble measurements. The advantage of using MFM-constructed hysteresis loops is that one can in principle understand the origin of dispersion in switching fields. It is also possible to directly observe the correlation between magnetic particles through careful imaging and control of the external magnetic field. In all of these measurements, attention needs to be paid to avoid artifacts that result from the unavoidable magnetic tip stray field. Control can be achieved by optimizing the MFM operation mode as well as the tip parameters. It is even possible to use the tip stray field to locally and reproducibly manipulate the magnetic-moment state of small particles. In this article, we illustrate these concepts and issues by studying various lithographically patterned magnetic nanoparticles, thus demonstrating the versatility of MFM for imaging, manipulation, and spectroscopic measurements of small particles. Keywords: magnetic force microscopy, nanomagnets, scanning probe microscopy.

Introduction Since its invention in 1987,1,2 magnetic force microscopy (MFM) has become a powerful tool for characterizing magnetic structures.3,4 Magnetic storage media and recording heads3,5 as well as magnetic domain structures can be investigated with a routine spatial resolution of 50 nm. More recent, nonstandard developments include magnetic dissipation imaging to investigate

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magnetization dynamics through studying the energy transfer between the cantilever and the magnetic sample,6 low-temperature measurements to investigate magnetic vortices or local variations in the magnetic penetration length in superconductors7–10 and colossal magnetoresistance materials,11 magnetic resonance force microscopy with the aim of measuring single nuclear and

electron spins,12 and microcantilever magnetometery13–15 with exquisite magnetization sensitivity that allows for the direct measurement of de Haas–van Alphen oscillations of the magnetization16 and of hysteresis loops.17 Among these applications, characterizing small magnetic structures is one that is growing in significance.18–21 Small magnetic structures are currently widely studied both from a fundamental research standpoint and for their potential applications in ultrahigh-density storage,19–22 spintronic devices,23 and magnetic logic devices.24–26 This booming interest requires techniques for characterizing these small structures individually. Due to its high spatial resolution and sensitivity, MFM has become one of the most effective tools for characterizing the magnetic structures and magnetization reversal of submicrometersized magnets.18–21,27–30 This article will review the investigation of magnetic nanoparticles using MFM, with special attention given to the minimization of tip stray field effects and the relevant quantitative information which can be obtained.

Imaging Technique MFM is a member of the family of scanning probe microscopy techniques. The basic operation method is very similar to that of other scanning probe microscopies described in this issue. In MFM, a flexible cantilever with a sharp magnetic tip at one end is used as a force sensor. In the commonly used ac mode, the cantilever is driven at its resonance frequency and the interaction gives rise to a change in the cantilever resonance frequency, which can be detected by amplitude-, phase-, or frequencydetection techniques.3,31,32 To obtain a magnetic image, MFM can be operated in the constant-frequency-shift mode,3 tapping/ lift mode, or constant-height mode.33 There are also nonmagnetic interactions between an MFM tip and a magnetic sample (e.g., electrostatic and van der Waals forces). It is therefore very important to separate the magnetic signal from the other interactions, which becomes an essential issue when the sample is not flat. The frequency-shift mode is a contour of constant frequency, which reflects a combination of magnetic, electrostatic, and van der Waals forces; therefore, the obtained images usually have crosstalk with topography. Electrical-field modulation34 can only partially solve this issue, as heterogeneous samples have a lateral variation of the tip–sample contact potential difference, which can lead to variations in tip–sample separation in constant-frequency mode that are not related to topography. The tapping/lift mode of operation can efficiently separate topography contrast and magnetic interaction, although chemical

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contrast from variations in the contact potential can still be a source of crosstalk. In this mode, the image is scanned twice. The sample topography is obtained in the tapping-mode scan using the cantilever oscillation amplitude as a feedback signal. The magnetic contrast is subsequently obtained in the lift-mode scan by monitoring the cantilever’s frequency or phase shift upon rescanning the previously measured topography with a user-controlled height offset. The typical problem associated with a tapping/lift-mode scan is that the MFM tip stray field can produce a substantial distortion of the sample magnetic structures during the tapping scan. This is because the magnetic field (stray field) from the MFM tip is large close to the end of the tip. Irreversible distortion induced by the tip stray field can be substantially reduced by operating the MFM in constant-height mode. In this mode, instead of tracking the sample’s topography, the tip is scanned across the surface at a predetermined constant height while the cantilever frequency shift is monitored. To prevent the MFM tip from crashing into the sample, unavoidable sample tilt can be compensated by tiltcorrection hardware, which allows the MFM tip to fly over the sample surface following a slope obtained through the constant-frequency-shift mode scan.33 The constant-height mode has the best signalto-noise ratio and the potential of increased scan speeds; however, the disadvantage of this mode is that a flat sample is needed, as the minimum tip–sample separation is determined by the sample’s roughness. Tapping/lift mode and constant-height mode are complementary in acquiring good images. For magnetic particles with a large coercivity field, tapping/lift mode can be confidently applied; however, for magnetic particles with small switching fields, constant-height mode should be adopted, since it produces a smaller distortion, which furthermore can easily be detected.35

Versatility of MFM MFM as a Tool for Imaging The most common application of MFM is to characterize a sample’s magnetic domain structure. In the literature, MFM has successfully been used to obtain domain structures in lithographically patterned particles.18,21,27–30 Particle size, magnetocrystalline anisotropy, and shape have a strong influence on the magnetic structure.36 Figure 1 shows several examples of magnetic structures, including single-domain particles, magnetic vortex states, and magnetic structures in magnetic multilayers. Elongated sub-100-nm particles form single-domain structures with a black/white

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Figure 1. (a) Single-domain structure of a Ni particle (length/width/thickness: 200 nm/70 nm/15 nm). (b), (c) Two different vortex states of the same Permalloy (80/20 NiFe) disk with the same scan area: disk size 500 nm, thickness 30 nm. The images show low contrast due to the magnetic moment circulating around the center, resulting in low magnetic stray field from the disk. The dark center in (b) is a vortex core singularity with the magnetic moment pointing into the disk plane; a white center (c) indicates that the magnetic moment is pointing out of the disk plane. Arrows point to the same topography artifact, demonstrating that the central bright spot is magnetic. (d), (e) Mixture of parallel and antiparallel states of NiFe/Cu/Co pseudo-spin-valve particles with a size of 70 nm
550 nm, with NiFe, Cu spacer, and Co layer thicknesses of 6 nm, 3 nm, and 4 nm, respectively. Parallel states show strong contrast, while antiparallel states show weak contrast due to the nearly complete cancellation of the stray field from both magnetic layers. The labels B, C, and D in Figures 1d and 1e represent the magnetization configurations shown in the schematic illustration below the images.

contrast, as shown in Figure 1a: the black spot indicates repulsive tip–sample interactions; the white spot indicates attractive tip–sample interactions. If the magnetic particles are disk-shaped30 or elliptical with a small aspect ratio,29 a magnetic vortex state is formed. Figures 1b and 1c show two vortex states of an identical 500 nm NiFe disk. The circulation of the magnetization is not visible in the images, but different polarities (the dark or light spot in the center) of the vortex core are revealed. Besides characterizing the magnetic structures of single-layer magnetic materials, MFM can also be used to distinguish different magnetic-moment configurations of multilayers, such as pseudo-spin-valve elements, which consist of asymmetric sandwiches containing two magnetic layers with different switching fields: one is magnetically soft and the other is magnetically hard.37 For example, Figures 1d and 1e show the

different magnetic-moment configurations in elongated sub-100-nm-wide pseudo-spinvalve elements. Even though the stray field from the magnetic layers is nearly completely cancelled for the antiparallel configurations, the magnetic contrast difference is still visible (see areas labeled C and D in Figure 1e).

Magnetization Reversal Studied by MFM The fact that MFM measurements are not affected by moderate external magnetic fields21,38 makes MFM an ideal tool not only for identifying the magnetic structure but also for studying magnetization reversal locally.14,21,37 There are two different ways to study the magnetization behavior using MFM: (1) imaging at remanence after the magnetic field is ramped to a certain value, and (2) imaging in the presence of an external magnetic field. Imaging in the

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Imaging, Manipulation, and Spectroscopic Measurements of Nanomagnets by MFM

presence of an external magnetic field is necessary while observing domain-wall movement or reversible magnetization behavior. Imaging at remanence, however, eliminates the combined effect of tip stray field and external magnetic fields,36,39 which can switch magnetic particles during the imaging process. This technique is very suitable for studying magnetic particles with a few distinct states, such as singledomain states, as the magnetic switching field of imaged particles can be obtained precisely.17,33 A hysteresis loop can therefore be constructed by counting the percentage of switched elements as a function of the external magnetic field.40 An example of such a hysteresis loop for single-domain Permalloy (an 80/20 NiFe alloy) particles is shown in Figure 2. Figures 2a and 2b show the magnetic states at remanence after applying magnetic fields of –304 Oe and –510 Oe, respectively. The remanent hysteresis loop is shown in Figure 2c. The hysteresis loop derived from MFM measurements is very consistent with the hysteresis loop obtained by macroscopic magnetometry measurement, as shown in

Figure 2d. The advantage of obtaining a hysteresis loop from MFM images is that it builds a bridge between an individual element and the ensemble, whereas the macroscopic technique only studies the switching behavior of the ensemble.

Local Hysteresis Loop Magnetic properties of particles can be directly measured without imaging by constructing a local hysteresis loop. To perform this measurement, as shown in the inset of Figure 3a,17,37 the MFM tip is located at one end of an element, and the cantileverfrequency shift is monitored while sweeping the external field. The cantilever-frequency shift is proportional to the force gradient between the tip and the sample and is thus a measure of the local sample moment. Typical hysteresis loops for a pseudospin-valve particle37 are shown in Figure 3. When the magnetic field is swept within a small range, such as from –250 Oe to 250 Oe (Figure 3a), only the soft magnetic layer (NiFe) is switched back and forth to form a parallel or antiparallel state: the minor hysteresis loop. However, if the magnetic

Figure 2. Magnetic force microscopy (MFM) image (a) at remanence after applying –304 Oe along the long axis of the elements and (b) after applying –510 Oe. (c) Remanent hysteresis loop constructed from the MFM images. (d) Hysteresis loop obtained by alternating gradient magnetometry on the same array (for comparison). Length/width/thickness of particles: 240 nm/90 nm/10 nm. Tip: 50 nm CoPtCr. The MFM technique used was constant-height imaging in vacuum.

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field spans a larger value, both the hard and soft magnetic layers can be switched. Two additional frequency-shift jumps as a function of the external field appear in the major hysteresis loop, as shown in Figure 3b. The switching behavior of individual particles can be used to interpret the hysteresis loop obtained through macroscopic magnetometry techniques. Therefore, MFM can be used to study individual particle differences.37

Force–Distance Curves Force–distance spectroscopy measurements can provide valuable information about sample’s magnetic properties, such as the local pinning effect.41 In this mode, the cantilever status is monitored as a function of the tip–sample separation. In Figure 4, we show such an example. The inset in Figure 4 shows an MFM image of Ni dots

Figure 3. Hysteresis loops for a single pseudo-spin-valve element from the array shown in Figure 1d. (a) Minor hysteresis loop. Only the soft magnetic layer (NiFe) is switched back and forth, with two individual jumps in the curve. The inset shows the experimental procedure. The disk underneath the tip represents a pseudo-spin-valve element. The cantilever frequency shift is monitored while sweeping the external field. H and the arrows indicate the magnetic field and its direction. (b) Major hysteresis loop. Both the magnetically soft (NiFe) and hard (Co) layers are switched, leading to four jumps in the curve.

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moment needs to be selected to accomplish this. At a large tip–particle separation, MFM can be used to read the magnetic-moment states of the particle, while at a smaller tip–sample separation, the tip stray field can be used to control the particle magneticmoment state, as demonstrated in Figure 5. This local tip-manipulation mode is very useful for writing input to magnetic logic devices and is potentially scalable. The operation of more than a thousand atomic force microscopes in parallel has recently been demonstrated.46 Figure 4. Force–distance curves on two different Ni dots: one has a magneticmoment direction antiparallel to the tip stray field, the other has a moment direction parallel to the MFM tip. By performing force spectroscopy, these different magnetic states can easily be identified. The MFM image of the dots is shown in the inset, with the dots circled for clarity. See text for details.

Interaction and Coupling Magnetic particles will be strongly coupled if they are placed close to each other. It has been demonstrated recently that coupled single-domain magnetic particles can be used to propagate information.24 In a simple picture, assuming a particle with a low coercivity field, a large effective field applied by its neighboring particles can act

as a trigger for the switching of this particle. MFM is a tool that can visualize such coupling.17,47 The switching mechanism of a submicron-sized magnetic disk is a process of vortex nucleation (resulting in a sudden magnetic contrast change from a bipolar single-domain state to a weak magnetic contrast vortex state, where the magnetization is circulating around the disk center), displacement, and annihilation (from a vortex state to a single-domain state).17 For widely spaced disks, the switching field is different from disk to disk, and there is little disk–disk correlation. The switching field difference comes from the differences in individual disks as a result of the sample preparation process. However, for disk chain structures, a strong correlation can be observed, as shown in Figure 6. Initially, all of the disks are in single-domain states, as shown in Figure 6a. As the external field decreases, the edge disks are switched to vortex states, as shown in Figure 6b. This is

50 nm in diameter and 30 nm high.42 One dot shows dark contrast, the other shows bright contrast. Force–distance curves show that the interaction between the tip and the black dot is repulsive, while the interaction between the tip and the white dot is attractive. This result indicates that one dot has a magnetic-moment direction parallel to the MFM tip, while that of the other dot is antiparallel to the tip. Therefore, force– distance curve measurements combined with MFM imaging are very useful in obtaining more quantitative information about a sample’s magnetic structure.

MFM Manipulation It is well known that the stray field from MFM tips can produce distortion of the sample magnetic structure, especially in submicrometer-sized soft magnetic particles.36,40,43,44 One kind of distortion is a direct flip of the particle moment state by the tip. It has been proposed that this effect can be used to locally write a magnetic bit in patterned magnetic disks.19,45 The prerequisite for this process is that the effective tip stray field must be larger than the switching field of the particle.19 The writing and reading process can be performed using two MFM tips—a writing tip with a large magnetic moment and a reading tip with a very small magnetic moment19—or, alternatively, using one tip and the help of an external variable magnetic field.45 In the latter, recording is performed by bringing the MFM tip into contact with the sample while applying an external field parallel to the tip stray field. The canceling process is performed at a large tip–sample separation, while applying an external field antiparallel to the tip stray field. A suitable MFM tip

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Figure 5. MFM manipulation of the magnetization state of a magnetic particle. (a) Schematic diagram of a particle.The MFM tip is located on the left or the right side of the particle, indicated by the dots and labeled 1 or 2. At a large tipsample separation (120 nm), the particle forms a single-domain state, and the magnetic state of the particle can be read, as shown in (b) and (c). At a small tipsample separation (60 nm), the particle moment can be switched, as shown schematically in (d). If the tip is located at Position 1 (the left side of the particle), and the initial magnetic-moment state of the particle is as shown in (b), the approaching tip can switch the magnetization to form a single-domain state (c). If the tip is located at Position 2 (the right side of the particle), and the initial magnetic-moment state is shown in (c), the MFM tip can switch the magnetization to (b) as it approaches the particle. Tip: 50 nm CoPtCr; elliptical NiFe particle: 600 nm long, 150 nm wide, and 30 nm thick.

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domain structures. It is compatible with external magnetic fields, which makes it ideal for studying the magnetization reversal of magnetic particles. It can obtain precise information on the switching fields of particles and can be used to construct hysteresis loops. It also allows direct determination of individual particle differences. MFM is able to study local hysteresis loops by monitoring the cantilever status versus external magnetic field, which can be used to explore the switching mechanisms of individual elements. MFM is potentially ideal for studying the switching field distribution of individual particles, for example, due to thermal switching. Valuable information can be obtained from studying the tip–sample interaction with respect to tip– sample separation. The magnetic field emanating from the tip can be used to apply a localized magnetic field to a sample and locally control the switching of the magnetization of a particle.

Acknowledgments Figure 6. Magnetic force microscopy (MFM) images of two nominally identical Permalloy (80/20 NiFe) disk chains after the application of different external fields. Each disk chain contains 10 disks (diameter, 500 nm; thickness, 25 nm; spacing, 60 nm). MFM tip: 20 nm CoPtCr. The images were obtained at 330 Oe, after applied magnetic fields of (a) 650 Oe, (b) 19 Oe, and (c) 7 Oe. Switching initiated at the edge dots (b), demonstrating strong coupling within each chain. The switching of the disks is strongly correlated. Note that the dots in one disk chain are all switched in (c), while only the edge dots are switched in the chain in the lower part of the image. The MFM technique used was constant-height imaging in vacuum.

because the edge disks have smaller coupling fields generated by neighboring disks, and they are switched earlier. The switched elements form a chainlike structure, as shown in the top row of Figure 6c.47 The reason for the formation of the chain structure is that the switched disks trigger the switching of their neighboring disks. This process acts like information propagation, which has been demonstrated in singledomain nanomagnet chains.24 Potentially, we could use the stray field from MFM tips combined with an external bias magnetic field to trigger the switching of edge disks and to propagate the information through the disk chains.24–26

Summary Magnetic force microscopy is well suited to the characterization of nanomagnets. MFM can be used to characterize magnetic

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The authors are very grateful to V. Metlushko, B. Ilic, J. Beerens, Y. Hao, F.J. Castano, S. Haratani, B. Vogeli, C.A. Ross, H.I. Smith, J. Liang, and J. Xu for fabricating the samples and for their useful discussions. We are also thankful to Y. Hao for the hysteresis loop measurement. This work was generously supported by grants from the Natural Sciences and Engineering Research Council of Canada (NSERC), Le Fonds Québécois de la Recherche sur La Nature et Les Technologies (FCAR), and the Canadian Institute for Advanced Research (CIAR).

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