Magnetic Imaging on the Nanometer Scale Using Low-Temperature Scanning Probe Techniques
M. Zech,* C. Boedefeld, F. Otto, and D. Andres attocube systems AG, Koeniginstrasse 11a RGB, Munich, Germany
*
martin.zech@attocube.com
Introduction Multiple techniques now exist for the investigation of
nanoscale magnetic properties, extending from Lorentz microscopy [1] and magneto-optical imaging (MOKE) [2] to scanning probe microscopy approaches (see Figure 1 for an overview). Among the latter, the most widely used techniques offering both high spatial and high magnetic-field resolution are magnetic force microscopy (MFM) [3] and scanning Hall probe microscopy (SHPM) [4]. Both techniques are well known for their versatility and ease of use and can be further adapted for operation in cryogenic conditions. Tis property is crucial for all areas of research where high magnetic fields are required and where the influence of thermal energy/broadening needs to be suppressed. For example, much of today’s fundamental research on superconductivity [5], spintronics [6], and magnetic data storage [7] is taking place at low temperatures. Te MFM and SHPM techniques are complementary in
the sense that SHPM provides the user with non-invasive, quantitative measurements of the local magnetic field, whereas MFM is sensitive to the gradient of the local force but with almost one order of magnitude higher spatial resolution. With its attoMFM and attoSHPM products, attocube systems addresses both of these techniques, allowing the researcher to investigate magnetic properties with high spatial resolution and sensitivity in environments ranging from ultra-low
temperature (down to the mK regime) and high magnetic fields (up to 15 T) to ambient conditions (see Figure 2).
Magnetic Force Microscopy Magnetic force microscopy (MFM) [3] is a technique
derived from atomic force microscopy (AFM) [8], in which an etched silicon cantilever/tip combined with optical deflection detection is used to precisely measure local forces such as those caused by van der Waals or Coulomb interaction. MFM takes advantage of cantilevers with very low spring constant K and tips with magnetic coatings, typically NiCr or cobalt, making them sensitive to the magnetic interaction between tip and sample. Figure 2a shows a schematic of attocube system’s
cantilever-based attoMFM, designed particularly for low tem- perature and high magnetic field applications. Te attoMFM uses a single-mode, fiber-based interferometer [9] to detect tip deflections with noise densities as low as 0.5 pm/Hz1/2 [10]. As with most MFMs, the attoMFM applies an AC modula- tion technique to achieve highest detection sensitivity. In AC mode, the cantilever is mechanically excited at its natural resonance frequency f0 using a piezoelectric material oscil- lating perpendicular to the sample surface. Te magnetic interaction offsets the equilibrium position of the tip, which in most cases is hard to detect and therefore ignored. In addition to the pure DC offset, the natural resonance frequency (as well as amplitude and phase) of the cantilever is also affected by the magnetic interaction. Tis frequency shiſt ∆f = fres − f0 can be easily detected by classical lock-in techniques and is the most relevant physical quantity to measure due to its direct proportionality to the local force derivative [11]: ∂Fz /∂z ~ 2K∆f/f0. Te measurement therefore yields information about the actual local magnetic stray field: ∂Fz /∂z ~ mtip,z∂2Hz /∂z2 (where mtip,z is the magnetization of the tip perpendicular to the sample surface) with very high spatial resolution. In a typical MFM measurement, the cantilever is constantly excited at resonance using a 90° phase-shiſted excitation signal. Using this phase-locked loop (PLL) technique, resonance frequency shiſts as small as 1 µHz can be detected. To separate magnetic information from other influences,
Figure 1: Magnetic field sensitivity as a function of spatial demonstrated for different magnetic imaging techniques [9].
34 resolution,
two techniques are most typically used, referred to as constant height and constant distance mode [12]. In constant height mode, the MFM tip is scanned at a fixed height above the mean sample plane, whereas in constant distance mode the distance between tip and sample is kept precisely constant, compensating any surface corrugation. Constant height mode is typically applied on flat samples, aſter the sample and scan planes have been aligned parallel. Te MFM tip is subsequently retracted by typically 10–100 nm and is then scanned across the surface with scan speeds of up to several 10 µm/s. Any shiſts in resonance frequency or phase are recorded simultaneously. Tis technique is applied to many samples such as hard disks
doi:10.1017/S1551929511001180
www.microscopy-today.com • 2011 November
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