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Colossal Magnetoresistive Oxide Films
Colossal magnetoresistive (CMR) oxides are doped oxide materials that exhibit the largest known low temperature magnetoresistance effect. As a result they are of interest for use in magnetic sensing devices, such as hard disk drive read heads, if their room temperature magnetoresistance can be optimized. The projected applications will probably use the materials in thin film form, necessitating the understanding and optimization of CMR oxide film growth. We have used SPM to study both the growth characteristics and resulting magnetic properties as a function of different experimental parameters.
In optimization for magnetic device applications, an issue that must be addressed is the presence of native and process-induced magnetic structure. In particular, stress-induced magnetic structure will be very important since thin films almost always have residual stress and since these particular materials typically have relatively large magnetostriction constants. Stress in thin films can arise from substrate/film lattice mismatch, defects, and the growth mode. All of these issues are affected by the range of variables involved in thin film growth. These include deposition parameters, post growth cool down and annealing procedures, etc. Our most recent CMR work has focussed on examining the effect of variations in some of these parameters on the magnetic structure of La0.67Sr0.33MnO3 (LSMO) films grown on LaAlO3 (LAO) and SrTiO3 (STO) substrates. We have grown films using pulsed laser deposition and characterized the growth structure with STM and the magnetic structure with magnetic force microscopy (MFM), magnetization, and coercivity measurements.
Full details of the experimental procedures and results can be found in the papers listed in our publications page. General results are presented below.
Figure 1 shows the STM and MFM results from four of the samples. In the STM images (200 nm x 200 NM) in the top panels, the deposition temperature and resulting film thickness are noted. The film number is noted for the three rightmost panels. The STM imaging was done in air at 1.4 V sample bias and 140 pA tunnel current. The MFM images from much larger areas (4 micron x 4 micron) are shown in the lower panels for the corresponding films and the domain widths are noted. All of these films are grown on LAO substrates.
The general trend observed in the STM images is an increase in connectivity, and grain size, for higher growth temperature and a larger number of layers with increased thickness. These characteristics are well understood in terms of variations in diffusion with temperature and strain evolution in thin films. The last film on the right illustrates some of the variation possible in PLD experiments.
The MFM images show maze domains, which would be consistent with compressive in-plane strain in the films due to the film-substrate lattice mismatch. We verified this with x-ray diffraction experiments. Compressive in-plane strain leads to tensile strain in the surface normal direction and an out-of-plane easy magnetization axis. The films also have wider domains as the thickness in increased. This would be consistent with an expected strain lowering in thicker films due to changes in the growth mode. The decreased strain makes the easy axis weaker and fewer wider domains are necessary to provide the necessary energy-lowering demagnetization.
We were also able to probe the field dependence of the magnetic structure with the MFM.
Figure 2 shows MFM images from film #168 as a function of in-plane externally applied magnetic field of a) 31 Oe, b) 100 Oe, c) 261 Oe, d) 404 Oe, e) 603 Oe, and f) 706 Oe. Image areas are 4 micron x 4 micron.
The interpretation of the images in Fig. 2 is straightforward. As the in-plane field is increased, the spins in the film begin to acquire an in-plane component and the out-of-plane magnetization becomes weaker. At slightly larger length scales, the domains begin to change through creation and annihilation of kinks, bubbles, and branches so that they are oriented along the applied field direction as well. Finally, between 600 and 700 Oe, the magnetization is almost entirely in plane. This field value is close to the saturation magnetization field for this film determined in separate experiments with a vibrating sample magnetometer.
Summary
The data shown here just scratch the surface of a very interesting aspect of thin film growth. Understanding how magnetic properties develop as a result of growth structure will benefit disk drive read head research and may also aid in understanding the properties of the storage media on the disks themselves. With scanning probe techniques we have the unique capability to see both structural and magnetic features at once. When coupled with traditional physical property measurements (magnetization, conductivity, etc.) these results can help put together a full picture of the physics and materials science of these complex oxides from nanoscale to mesoscopic regimes.
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Related Links
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Structure/Property Relations, MST-8