Photon Factory Activity Report 2017 #35 (2018)

BL-16A/2016S2-005

Observation of Magnetic Depth Profiles for C Cluster Ion Irradiated FeRh Thin Films
with Depth-resolved X-ray Magnetic Circular Dichroism
Toshiyuki MATSUI1,*, Ryunosuke SOMA1, Akihiro IWASE1, Masako SAKAMAKI2,Kenta AMEMIYA2 and Yuichi SAITOH3
1Osaka Prefecture University, 1-1 Gaken-Cho, Sakai, Osaka 599-8531, Japan
2
Institute of Materials Structure Science, Photon Factory, 1-1 Oho, Tsukuba, 305-0801, Japan
3
National Institutes for Quantum and Radiological Science and Technology, Takasaki, 370-1292, Japan1 Introduction
Equi-atomic FeRh is known to exhibit a first orderphase transition from anti-ferromagnetic to ferromagnetic
around room temperature. It was revealed that ion beamirradiation induced the ferromagnetic state below room
temperature, which was mainly determined by elastic
deposition energy by ion irradiation.[1-3] According toour previous studies on Au cluster ion beam irradiation
for FeRh thin films, however, we revealed that themagnetic moment of the sample irradiated with the Au3cluster ion was larger than that of the sample irradiatedwith the Au single ion especially in the surface region ofthe samples, even when the same number of Au ions wereirradiated.[4] Considering these results, the cluster ion
irradiation is possibly considered to be a method toeffectively deposit the ion irradiation energy at thesurface region, which may accelerate the magnetic phasetransition from AFM to FM. In the present studies, we
examined the effect of carbon cluster ion beam irradiationon the depth profile of magnetization in the surface ofFeRh thin films by depth-resolved technique in the x-ray
magnetic circular dichroism (XMCD).
2 ExperimentIron-Rhodium thin films about 50 nm thick were
fabricated by using ion beam sputtering of Fe50Rh50
target on MgO(100) substrates. Then the samples wereirradiated with 1 MeV C2 cluster ion and 0.5 MeV C1 ion
beam at room temperature by using the ion beam facilities
at Quantum and Radiological Science and Technology(QST)-Takasaki. The ion fluence was determined to be
1.5x1014 C/cm2 and 4.5x1014 C/cm2. When the ion fluence
C is 4.5x1014 C/cm2, the maximum irradiation inducedmagnetization for FeRh is produced for the 0.5 MeV
irradiation sample. The magnetic properties werecharacterized by a superconducting quantum interference
device (SQUID) magnetometer as well as x-ray magnetic
circular dichroism (XMCD) measurements in total
electron yield mode (TEY). In spite that the SQUID
measurement evaluates whole magnetization of the thinfilm samples, the XMCD measurement sensitively
evaluates the surface region (~3 nm) of the thin film
samples. Furthermore, a depth-resolved XMCD
measurement was also performed to evaluate the depth
profile of ion beam irradiation induced ferromagnetism
for the samples. Depth-resolved XMCD techniqueprovides depth-resolved magnetic moments through theXMCD signals with different probing depths bycollecting the emitted electron at various detection angles.
Both TEY and depth-resolved XMCD measurements
were carried out at Beam Line #16A at Photon Factory inHigh Energy Accelerator Research Organization (KEKPF).
3 Results and Discussion
The observed magnetic moment per Fe atom, which
can be calculated from the result of the SQUID andXMCD measurements in TEY mode, is shown in Fig. 1.
The magnetic moment data from the XMCD
measurements were estimated by using sum rule analysis.
As can be seen in the figure, the SQUID measurement
data for the C1 and C2 samples irradiated with 1.5x1014
C/cm2, were almost the same, which suggests that theentire magnetic moment (from surface to 50 nm) of thesamples are almost identical. On the other hand, from theviewpoint of the evaluation of the surface region of thesamples (to about 3 nm), the corresponding magnetic
moment values from the XMCD measurements (TEY) aretotally different. That is, the magnetic moment of the C2irradiation sample is larger than that for the C1 irradiation
sample, which means that the C2 cluster ion irradiationinduced larger surface magnetic moments (~3 nm) rather
than the C1 ion irradiation. With increasing in theFig. 1: Magnetic moment for the unirradiated andirradiated samples with the C1 ion under the ion fluenceof 1.5x1014 C/cm2, C2 cluster ion of 1.5x1014 C/cm2,4.5x1014 C/cm2.

Photon Factory Activity Report 2017 #35 (2018)

Fig. 2: Probing depth dependence of magnetic moment of
Fe atom for irradiated samples under the fluence of
1.5x1014 C/cm2 and 4.5x1014 C/cm2.
irradiation fluence of C2 cluster ion, the magnetic
moment measured by SQUID is found to increase,
whereas that by XMCD is unchanged. These results showthat the cluster ion irradiation can effectively modify themagnetization at the surface region, which is significantly
consistent with the past experimental results on the Au
cluster ion beam irradiation for FeRh thin films.[5]In order to analyze the surface region of the samples
irradiated with C1 ion and C2 cluster ion in detail, we
performed the depth-resolved XMCD measurements.
Figure 2 shows the probing depth dependence of themagnetic moment for the irradiated samples with C1 andC2 cluster ions. The plotted values of the magnetic
moment at each depth can be attributable to the averagemagnetic moment from surface to each depth. As clearlyseen in the figure, the different profiles of the magnetic
moments can be observed for each irradiated sample.
Even from the results of the depth-resolved XMCDmeasurements, the absolute values of the averagemagnetic moment of each sample up to 20 nm in depth,

exhibit quite well consistence with the results of the TEY
mode measurements. However of this, it should be notedthat depth profile of each sample significantly depends onthe cluster ion number as well as the ion fluence. In thecase of the sample irradiated with C1 ions, the magnetic
moment increases with increasing the depth. Hereby themagnetic moment of the C2 irradiated sample shows asteep increase with increasing the depth, then graduallysaturates from 15 nm region in the depth. In contrast tothis, the magnetic moment decreases as the probing depth
increases for the C2 cluster ion irradiated sample withlarger ion fluence (4.5x1014 C/cm2).
In order to evaluate this behavior quantitatively, themodel in which the 8 layers (n=8, in the followingformula) having some amplitude of the magnetization in
the surface region of the samples was considered. Eachlayer is assumed to be 0.2 nm thick. In this model, thedetected XMCD signal at probing depth x, y(x), isrepresented by the following formula:where 'x' and 'a' denote the probing depth and interlayer
distance, respectively. M(top) is a most surface layer ofthe film that is always considered to be 0 μb /atom, since
it is impossible to accurately measure the XMCD signal
at the ultra-surface layer (~0.2 nm), due to the effects ofoxidation, adsorption atoms at surface and so on.
By using this model, we have tried to fit the depth
dependence curves of the magnetization determined by
the XMCD measurements. Here, it is worthwhilereminding that the irradiation induced magnetization ofFeRh is significantly correlated with the depositionenergy through the elastic collisions between the sampleand ion species.[3] According to the knowledge from thepreviousexperiments,
theirradiationinducedFig. 3: Probing depth dependence of the magnetic moment of Fe atom for irradiated samples with (a) C1 ion and (b) C2
cluster ion. The dotted lines represent the simulation result, which details are written in text.

Photon Factory Activity Report 2017 #35 (2018)

Acknowledgement
Cluster ion beam irradiation was performed at QSTTakasaki Ion Accelerators for Advanced Radiation
Application (TIARA).
References[1] M. Fukuzumi et al, Nucl. Instr. and Meth.B, 230, 269(2005).
[2] M. Fukuzumi et al, Nucl. Instr and Meth.B, 245, 161(2006).
[3] N. Fujita et al, J. Appl. Phys. 107, 09E302 (2010).
[4] T. Koide et al, J. Appl. Phys. 115, 17B722 (2014).
Fig. 4: Estimated depth profile of the apparent depositionenergy by elastic collisions.

[5] T. Koide et al., J. Appl. Phys. 117, 17E503 (2015).
[6] J. F. Ziegler, SRIM-2008.

ferromagnetism is well known to be almost linearly
increase with increasing the deposition energy, at theinitial stage of the irradiation. Then further increase in thedeposition energy causes the saturation of the irradiationinduced ferromagnetism, followed by the decrease ofit.[3] It should be mentioned here, that the depositionenergy by ion beam irradiation was calculated by theTRIM simulation using the Monte Carlo method.[6]In case of the C1 ion irradiation, the deposition energy
can be plausibly estimated by using the TRIM simulation.
Since almost uniform energy is calculated to be deposited
regardless of the depth from surface (~ 50 nm), theirradiation induced ferromagnetism at this region (M(j))
can be regarded to be constant values. By using theseparameters, the simulation result shows the upwardsloping probing depth dependence of the magneticmoment. That is, the depth dependence profile of the C1
irradiated sample can be suitably fitted as a broken linethat is shown in Fig. 3(a).
Next, we consider the C2 cluster ion irradiation case. In
order to account for the behavior of the depth profile
indicated in Fig. 2, it is indispensable to assume that themagnetic moment of each layer (M(j)) should have a peaksomewhere. When we consider the previous relation
between the magnetization and the deposition energy, it
can be concluded that the depth dependence of theapparent deposition energy must be the curve as isindicated in Fig. 4. By assuming such curve, the behaviorof the C2 irradiated samples both with small and large
influence, which are indicated in Fig. 3(b), can be fairly
well explained by broken fitting curves in the figure.
Accordingly, it can be concluded that the cluster ion
irradiation effectively causes the ion irradiation induced
ferromagnetism. By using this technique suitably, thesurface magnetic ordering state can designedly modified.
Although it is still unknown that the cluster ion irradiation
can deposit the relatively larger elastic collision energy at
the surface region rather than single ion beam or not, it
can conclude that the cluster ion beam is possible todeposit apparent elastic collision energy effectively. The
detailed mechanism of such behavior as well as theexperimental results on the irradiation effect for C cluster
ion with increased cluster number, will be discussed* matsui@las.osakafu-u.ac.jp