Investigation of the effects of structural properties on the magnetic properties of gadolinium

nanocrystals Tenzin Sangpo1, Jonathan M. Logan2, Ian McNulty2

1Division of Physical Sciences, Department of Physics, Reed College

2Center for Nanoscale Materials, Nanoscience & Technology Division, Argonne National Laboratory (Dated: July 20, 2016)

Elemental gadolinium (Gd) is one of the earliest known and most intensively studied magnetocaloric materials, yet little is known about the structural, magnetic, and thermal properties of Gd nanocrystals because of their tendency to oxidize. Here, we report on characterization of stable Gd nanocrystal samples grown by a sputter-coating, dewetting, and encapsulation process on various crystal substrates using x-ray diffraction, x-ray microscopy, electron microscopy, atomic force microscopy, and SQUID magnetometry. We found that the shape, structure and orientation of the Gd nanocrystals is highly dependent on the substrate on which they are grown. We pre-selected the most promising samples for based the quality of their x-ray diffraction data then measured their entropic response with a SQUID magnetometer. The entropy data are consistent with published data from thin-film Gd. In future we aim to characterize the influence of local lattice strain on the magnetization of the Gd nanocrystals by dichroic x-ray coherent diffractive imaging.

Introduction:

Magnetic anisotropy in ferromagnetic and crystalline materials such as Gd has been well documented for some time. The absence of spherical symmetry in the electronic orbitals in materials creates preferential axes of magnetization. Factors such as sample shape, strain and crystal structure impact the total energy of a magnetic system. Although these factors have been studied in bulk and thin film Gd samples, their effects on nanocrystals have yet to be seen. This is due primarily to the fact that Gd oxidizes quickly at the nanoscale, and high quality, oxide-free Gd nanocrystals have only just been developed [Logan 2016].

We selected the best samples for the various substrate types based on diffraction data obtained with a laboratory x-ray diffractometer. We used scanning electron microscopy (SEM) and atomic force microscopy (AFM) to evaluate the overall shape and size of Gd nanocrystals. We characterized the magnetization properties of the samples using a superconducting quantum interference device (SQUID) magnetometer.

We determined the sample quality on the basis of

the height and fineness of our Gd peaks in the diffraction patterns. We selected the best specimen among all the samples for a given substrate. The diffractometer data also allowed us to determine the crystal orientation. We determined the shape and structure of Gd nanocrystals using SEM and atomic force microscopy. While the former excels primarily at two-dimensional (2D) imaging, the latter gives us a quantitative measure crystal height.

Nanofluorescence and nanodiffraction maps verify the presence of Gd in the form of nanocrystals. We obtained magnetization information using the SQUID magnetometer.

Data Analysis & Discussion:

For the MgO substrate, we see prominent Gd peaks between 32.50 and 33.50 degrees for our samples. So we know Gd crystals on MgO have (011)-orientation and the tungsten coating has (002) orientation. However, we saw large variations in the Gd crystal quality even for the same substrates. Judging the plots on the basis of the Gd peak height and fineness, samples ‘c’ and ‘e’ stood out as the worst and the best respectively.

FIG 1.a: X-ray diffraction plots for Gd on MgO.

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FIG. 1.b: SEM images of samples ‘c’ and ‘e’ – the worst and the best – respectively from XRD data. The samples were both tilted at 52 deg0. We see the sample ‘e’ contains larger, well-separated Gd nano-islands, which may explain the better XRD data from that sample. We also collected diffraction data for Gd nanocrystals grown on other substrates such as sapphire (Al2O3), tungsten (W) and silica. Again, significant variations in between crystal quality for the same substrate types are evident from the plots. Using SEM we imaged the best and worst samples of a given substrate to understand why, and to compare with the MgO results as above. An influence of sample position during sputter deposition of the substrate is also expected.

FIG. 1.c: X-ray diffraction plots for Gd on Al2O3.

For the Al2O3 substrate (Fig. 1.c), samples 'a' or 'd' are equally worthy of further study. Sample 'b' does show a peak though understandably not as large since this sample had been diced to ¼ the area of the other samples. We are unable to account for the peak for 'c' based on the expected crystal reflections for Gd. We speculate that this sample has become oxidized. The Gd crystals on Al2O3 are completely (002)-oriented along surface normal and the tungsten encapsulation has (011)-orientation on the Al2O3 substrate.

FIG. 1.d: X-ray diffraction plots for Gd on silica.

Again with silica, samples 'a' and 'd' (especially the latter) are worth further study. Sample 'a' does seem to have a taller second peak though. Sample 'b' peak seems like a minor bump in comparison. The silica substrate has a mixture of (002) and (011) oriented Gd nanocrystals. The tungsten encapsulation has (011)-orientation on silica. As a general rule, sample ‘b’ of all substrates seems to have shorter and broader Gd peaks relative to other samples. Since sample ‘b’ has been diced and is a quarter the size of the other samples, it seems plausible to sum the contributions from the individual Gd nanocrystals.

FIG. 1.e: X-ray diffraction plots for Gd on tungsten.

The diffraction data (Fig. 1.e) for the Gd nanocrystals grown on the tungsten substrate are too noisy to differentiate the samples based solely on crystal quality. We do see two peaks, one slightly more than 29 deg0, and the faint one slightly less than 32 deg. Broadening the

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range of the x-axis only makes the data more inconclusive.

FIG. 1.f: X-ray diffraction plots for Gd on tungsten.

We are currently gathering magnetization data from the selected samples and their respective substrate types using a Quantum Design MPMS SQUID magnetometer. The magnetization data for Gd on Al2O3 is shown in Fig. 5. From the magnetization vs. temperature plot (fig. 5) for the tungsten-encapsulated Gd nanocrystals (W/Gd/W), we conclude that the Curie temperature (Tc) of the Gd nanocrystals on Al2O3 substrate is around 290 K. Above this temperature, the sample becomes paramagnetic, with a much smaller magnetization. We are in the process of collecting magnetization data for the samples grown on other substrates to see the effects of substrate choice on magnetic properties.

FIG. 5: Magnetization vs. Temperature for W/Gd/W on Al2O3

substrate.

SEM images of the Gd nanocrystals grown on their respective substrates are presented in Fig. 6.

Fig. 6 shows that the Gd nanocrystals grown on substrates MgO and tungsten (W) have a well-defined structure that almost appears like little boxes. On the other hand, those grown on Al2O3 and silica appear to be less well defined.

Among the given substrates, we had previously been able to compare the SEM images of the sample that had Gd nanocrystals grown on Al2O3 and its x-ray diffraction patterns with its nanofluorescence and nanodiffraction maps. We sought to determine the extent of correspondence in between the two results and did indeed find striking parallels in their outcomes.

FIG. 7: (a) SEM image of Gd nanoislands grown on Al203 substrate. (b) X-ray diffraction data for the same sample.

We can say that the Gd nano-crystals have a preference to orient themselves along the (002) direction parallel to the surface normal. The samples seem stable in air with the Gd not getting oxidized. The preliminary plots of nanofluorescence and nanodiffraction maps of Gd nanocrystals grown on Al2O3 substrate are shown in Fig.8.

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FIG. 8: (a) Gd nanofluorescence map and (b) nanodiffraction map.

These were performed at the CNM/APS Hard X-ray Nanopobe at beamline 26-ID-C. The sample and detector were aligned to record the Gd (002) Bragg peak while simultaneously collecting Gd fluorescence data. As is apparent in the patterns above, we expect the nanofluorescence maps to show Gd segregated into nanocrystals (regions with higher Gd concentration are brighter). The nanodiffraction map confirms that the Gd nanocrystals prefer to orient themselves with c-axis parallel to the surface normal.

We are currently developing a synchrotron x-ray technique to measure the 3-D strain and magnetization of individual nanocrystals. This technique is based on the Bragg Coherent Diffraction Imaging (BCDI) technique. In BCDI, Fourier transformations on the reciprocal space diffraction pattern from the beam scattered by the sample are used to reconstruct its real 3-D space image. To also simultaneously obtain magnetization information, we need to obtain X-ray Magnetic Circular Dichroism (XMCD) contrast in the diffraction geometry.

FIG. 1: Scanning nanodiffraction experiment setup

The relationship between x-ray cross-section σ, its degree of circular polarization PC, and the sample magnetization M is given by the expression

𝜎 ∝ 𝜎 0 + M PC cos𝜃, (1)

where σ0 is absorption not related to magnetization and 𝜃 is the angle between magnetic moment orientation of the absorbing atom and the helicity of the incident ray.

FIG. 2: Dichroic - BCDI experimental setup

In measuring the absorption cross-section for x-rays with opposite helicities, we are able to obtain the flipping ratio i.e.

∆!!= ! ↑↑ !!(↑↓)

! ↑↑ !!(↑↓) (2)

where 𝐼 ↑↑ is the intensity for the photons with spin angular momentum parallel to spin direction of magnetized electrons, and 𝐼(↑↓) is for the antiparallel case. It allows us to overcome the unfavorable ratio of magnetic signal i.e. 𝐼 ↑↑ − 𝐼(↑↓), to charge signal i.e. 𝐼 ↑↑ + 𝐼(↑↓). A direct measure for the local magnetic moment’s orientation angle 𝜃 with respect to the helicity of the incident x-ray can be attained from the flipping ratio. We are currently able to produce circularly polarized x-rays with a nano-focus of 50 nm by using a diamond XPR installed at the APS Beamline 26-ID-C. Work is currently underway at the APS Beamline 34-ID-B for the BCDI method. The procedures for the imaging techniques should not significantly differ from those of their more conventional counterparts respectively that use linearly coherent x-rays.

Fig. 3: Diamond XPR setup at APS Beamline 34-ID-B.

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Conclusion:

The x-ray diffraction data and SEM images of the best and worst samples grown on MgO indicate that the height and narrowness of the Gd diffraction peaks are very strongly related to the fineness of the Gd nanocrystal structure. MgO ‘e’ not only has better defined structure but also offers a sharper and taller Gd peak whereas MgO ‘c’ fails in both cases. It is yet to be verified if this is also the case with how easily the samples are magnetized, among other magnetization characteristics. ___________________________________ References: Casey W. Miller, Dustin D. Belyea, Brian J. Kirby. C. W. Miller, D. D. Belyea, and B. J. Kirby. Magnetocaloric effect in nanoscale thin films and heterostructures. J. Vac. Sci. Technol., A 32, 040802 (2014). Huldt, G., and Maia, F. Tutorial in Diffraction Imaging. Molecular Biophysics Course 2007, Uppsala University. Retrieved from xray.bmc.uu.se/~ekeberg/molbiofys/tutor ia l .pdf J. M. Logan, D. Rosenmann, T. Sangpo, and I. McNulty. The effect of substrate choice on the shapes, crystalline orientations, and magnetic properties of oxide-free, tungsten-encapsulated gadolinium nanoislands (in preparation). J. M. Logan, R. Harder, L. Li, D. Haskel, P. Chen, R. Winarski, P. Fuesz, D. Schlagel, D. Vine, C. Benson, and I. McNulty. Hard x-ray polarizer to enable simultaneous three-dimensional nanoscale imaging of magnetic structure and lattice strain. J. Synchrotron Radiat. 23, (2016).

Wang, G. –C., Lu, T, -M., RHEED Transmission Mode and Pole Figures: Thin Film and Nanostructure Texture Analysis, Springer-Verlag, New York, 227p.

Winarski, R. P. Magnetic Contrast Nanotomography. Presented at the 12th International Conference on X-ray Microscopy, Melbourne, (2014).