Spintronic devices using thin films of heavy metals: understanding therole of strong spin-orbit coupling and proximity-induced magnetism

C. Bell1, S. Kambe2, R. Springell1, M. Gradhand1
1School of Physics, University of Bristol, UK
2 JAEA, ASRC, Japan

Abstract
Very little is known about the electronic properties of the heavy actinide as thin films mixed
into non-magnetic and magnetic materials. The availability of thin film growth systems withthese materials is very limited: here we use an actinide-dedicated DC magnetron sputteringsystem at the University of Bristol to investigate their possible application in spintronicapplications.

1. Research Objectives
Current spintronics research is focussed on heavy metals as a source of highly spin polarized
currents. They can give rise to large spin Hall effects which can be used for magnetization
switching. Initial investigations of the transport properties of dilute-U/Cu, were begun tocompare and contrast with previous studies ofthe spin-orbit coupling of Bi and Ir atoms in Cu,
as shown in Fig. 1 below [1]. A key question in
the community is what materials are mostsuitable to provide the largest signals. Actinide
materials are of interest in the question, since inthe most naive picture the spin-orbit couplingstrength scales as a function of the atomic
number to the fourth power. The objectives ofthis project are to continue this work and include
Th as an additional type of impurity dopant aswell as U. We will synthesize thin films of copperwith dilute uranium or thorium content,
investigating

severaldifferentTh

concentrations (typically in the range of a few %).

Fig. 1: Spin Hall resistivity in Cu as afunction of induced resistivity due to Ir
impurities. Taken from Ref. [1].

The behavior of actinide materials in proximity toferromagnetic materials should also be understood,
both to allow integration in more complex spintronic
devices, and also for fundamental studies. A key pointis that previous x-ray magnetic circular dichroism
studiesofuranium/ferromagnet

thin

film

superlattices have indicated an induced moment in
the U, which is especially big when the ferromagnet isiron, approaching 0.1 Bohr magnetons per atom justat the interface [2, 3], also see Fig. 2.

Fig.

2:

XMCD

dataofU/Ferromagnet superlattices forthreedifferent

elemental

ferromagnets. Taken from Ref. [3].

2. Research Contents

The main results of this work have not been finalized due a delay in the transfer of the funds toBristol. The contents of the research is focused on actinide materials in proximity to nonmagnetic and ferromagnetic materials. For the former, we will continue the initial investigationof the transport properties of dilute-U/Cu, to compare with previous studies of the spin-orbit
coupling of Bi and Ir atoms in Cu. These measurements are of interest since naively the spinorbit coupling strength scales as a function of the atomic number to the fourth power. For
proximity effects with ferromagnetic materials, previous x-ray magnetic circular dichroism
studies of uranium/ferromagnet superlattices have indicated an induced moment in the U, whichis especially big when the ferromagnet is iron. We aim to understand the strength of the spinorbit interaction in actinide thin films, including the importance of the induced moment, using
magnetization,

magnetoresistance,

andferromagneticresonancemeasurementsin

polycrystalline layers. We can compare to the literature on Pt, where the induced moment has
also been discussed. We will contrast the behavior of different ferromagnets also with Th, whose
electronic structure is less dominated by the 5f electrons: a far weaker induced moment isexpected.
We will synthesize thin films of Cu, Co, Ni and Fe with dilute U or Th, investigating different
concentrations (in the range of a few %). These will be grown in an actinide-dedicated system at
the University of Bristol. The transport properties of these samples, including anomalous Hall
effect and spin Hall effect, will be measured as a function of temperature and applied field. We
aim to compare our experiments to theoretical results using relativistic bandstructure
calculations correctly describing the dilute impurity system. The combination with transport
calculations performed in Bristol will allow for a detailed understanding of the underlying
mechanisms. If time permits we will also exam the ferromagnets Gd (a localized moment system)
and YIG (an insulating material). However preliminary calibration and alloy thin films samples
have been grown for studies of magnetic anisotropy.

3. Research results
Preliminary

samplesofU:Fealloys

have

been

investigated. A key difference between the U:Cu alloys andthe U:Fe samples is the sputtering rate of the magnetic
elements is significantly smaller than the Cu. The samples
are grown by co-sputtering – i.e. the U and Fe are deposited
simultaneously. In order to achieve the lowest U
concentrations a stable plasma at the lowest power must beutilized. The Fe and U rates were separately calibrated via
x-ray reflectivity (XRR) on bilayers of Nb/Fe and Nb/U onglass substrates. Here the Nb layer is to prevent oxidationof the Fe or U. An example XRR plot and fit is shown in Fig.
3. However for the parameter space available we wereFig. 3: XRR calibration of aNb/U bilayer grown on glass atroom temperature.

unable to achieve U concentrations below 20%, which makes it difficult to model the scattering
as from isolated U atoms in the magnetic matrix.
In order to achieve lower U concentrations we have fabricated and are currently testing amask which is placed in front of the U sputter gun to significantly reduce the rate of depositionof U at the substrate position and enable the growth of U concentrations down to 1%.
Despite this setback we have begun preliminary measurements of the effect of U incorporation
into the Fe thin film on the magnetic properties, as well as initial theoretical modeling of U and
Th impurities in Cu using relativistic bandstructure calculations. The theoretical method toquantitatively predict the Spin Hall angles in dilute alloys is based on a relativistic Greens
function method ideally suited for the treatment of impurity systems avoiding artificial
boundary conditions. In Fig. 4 we give a visualization of the asymmetric scattering induced by
the spin-orbit coupling coming from the heavy impurities U and Th in Cu. Preliminary
calculations point to large Hall angles of 7.5% (Cu(U)) and 2.9% (Cu(Th)). Further optimizationof the theoretical work will be undertaken once a systematic set of experimental measurementsof the longitudinal resistivity versus U concentration have been completed.
Experimentally, specifically so far we have looked at the in-plane magnetic anisotropy at room

temperature using a vibrating sample magnetometer. Some typical M(H) data are shown in Fig.
5: here we compare two samples with the same nominal total thickness of 20 nm. The pure Fe
sample shows a clear angular variation of the magnetic anisotropy. The in-plane magnetization
is expected for a film of this thickness, and the magnetic anisotropy is likely the well-known
induced anisotropy due to stray fields in the sputtering chamber during growth. In contrast witha significant concentration of U there are two main observations: the magnetization issignificantly suppressed leading to far more noisy M(H) data, and the angular variation in theanisotropy is far weaker than for the pure Fe sample. This may be associated with the strong
spin-orbit scattering of U [4]. To confirm this issue we need to grow lower concentrations fo U,
as discussed above, and also compare the results to alloys with Pt and other lighter elements.
We also undertook preliminary anisotropic magnetoresistance and anomalous Hall effectmeasurements on these samples after patterning into Hall bars using optical lithography andacid etching, but the signals were relatively weak in the U-alloy samples.

Fig. 4: Visualization of the skew scattering process induced by heavy impurities in Cu. On theFermi surface of Cu the colour code represents the difference in scattering times for spin-up vsspin- down electrons. The breaking of the four-fold rotational symmetry is a direct measure ofasymmetric scattering for spin-up and spin-down electrons via the skew scattering at the heavy
impurities of U (left) and Th (right) in Cu.

In the future we also plan to examine ferromagnetic resonance of these systems and anomalous
Hall effect data as well. We have been unable so far to acquire a Th target, but we hope in thecoming months that we can also obtain Th to compare the results with U as well as Pt.

Fig. 5: M(H) data versus in-plane angle of pure Fe (left) and a U(20 atomic %):Fealloy (right) at room temperature. Film thickness is 20 nm in both cases.

4. Spin-Hall effect measurement by FMR

Fig. 6: Typical FMR spectra around 4kOe (left). The spectra can be well fitted with theLorentzian functions. The linewidth is defined as full width at half maximum (FWHM) of fitted
Lorentzian function. U concentration dependence of FMR linewidth (right). No clear dependence
is observed in the present samples.
We have tried to detect the spin-Hall effect using Ferromagnetic resonance (FMR) method in
diluted-U/Cu thin films prepared by magneto-sputtering method. At first, YIG ferromagnet (1and 3 µm) thin film is developed on substrate, then 0%-2.4%U/Cu thin films are developed on
the YIG thin film. If the spin-Hall effect appears due to the diluted U atoms, FMR linewidth
increases with increasing the U concentration. Figure 6 shows typical FMR spectra and Uconcentration dependence of FMR linewidth obtained by LS fitting based on the Lorentzianfunction. Unfortunately, a significant concentration-dependence of linewidth has not been
observed in the present samples. Although the reason for the absence is not clear at the present,
it seems that a polarized spin injection from YIG to dilute-U/Cu thin film does not work well in
the present case, due to a mismatching at interface between YIG and dilute-U/Cu.
It might be possible that Permalloy (Ni/Fe) ferromagnet thin film is better than YIG for diluteU/Cu thin film, or the thickness of YIG was too large. Indeed, the spin-Hall effect is detected inpure U-pemalloy film by FMR in previous study. We are planning to prepare different thin films
with Permalloy and thinner YIG.
Conclusion
In conclusion, although the project has been delayed we have made significant initial progressin growing and characterizing the magnetic properties of U:Fe alloys. A clean change in magneticanisotropy has been observed for relatively high concentrations of U doped in Fe. Efforts are
currently underway to reduce the sputtering rate of the U to fully explore the most relevant low
density limit in magnetic and non-magnetic systems.

5. References[1] Y. Niimi et al., Phys. Rev. Lett. 106, 126601 (2011) and 109, 156602 (2012).
[2] F. Wilhelm et al., Phys. Rev. B 76, 024425 (2007).

[3] R. Springell et al., Phys. Rev. B 77, 064423 (2008).
[4] P. V. Ong et al., Phys. Rev. B 94, 174404 (2016).