Photon Factory Activity Report 2014 #32 (2015) B

BL-16A/2014G677

Magnetic Modification at Sub-Surface of FeRh Bulk by EnergeticIon Beam Irradiation
Toshiyuki Matsui1, Tetsuya Koide1, Akihiro Iwase1, Masako Sakamaki2 and Kenta Amemiya2
1
Osaka Prefecture University, Osaka 599-8531, Japan2
Photon Factory, Tsukuba 305-0801, Japan
1 Introduction
Fe50Rh50 alloy with B2 (CsCl-type) crystal structure
has a unique magnetic behavior of a first orderantiferromagnetic-ferromagnetic phase transition at about
310 K [1,2]. The authors previously reported that themagnetic state (antiferromagnetic, ferromagnetic andparamagnetic) of FeRh could be designedly controlled byion beam irradiation with various energies and ion species[3]. According to those experimental evidences, thechange in magnetization of FeRh by using ion beamirradiation is deductively dominated by the energy
deposition through elastic collisions between the ions andthe samples that introduces the lattice defects in FeRh [4].
The unit energy deposited in the materials through theinteraction with the ion beam can be calculated by using
Transport of Ions in Matter (TRIM) simulation.
Deposition energy through the elastic collisions usuallydepends on a kind of ion and penetration depth, and large
energy is given to the materials just before the penetrate
ion stops.
In the present study, we have tried to modify thesubsurface of FeRh from AF to FM by using ion beamirradiation. As the result of TRIM-code 2008, depositionenergy can be introduced in the sub-surface of FeRh byusing high energy ion beam. So, it must be a reasonableidea to introduce FM layer only in such range bycontrolling the deposition energy. We also previously
reported that we succeeded to fabricate micro scale
magnetic patterns using the high energy ion microbeam
technique [5]. If we succeed to modify in the sub-surfaceof FeRh, it will be possible to control the threedimensional magnetic patterning by combining thesetechniques.
2 Experiment
In order to make such irradiation induced
ferromagnetic layer, FeRh bulk samples were irradiatedwith 2.9 MeV He and 5.9 MeV He ion beams. Since suchion beams can effectively deposit the elastic collisionenergy in the ranging from 3 to 5 µm and 10–12.5 µm
from the surface in accordance with the TRIM simulation,respectively. The magnetic state was evaluated by using
SQUID magnetometer and Fe L-edge XMCD
measurement at BL16A in KEK-PF. Since the penetration
depth of soft X-ray is considered to be about 2 nm,
XMCD observation gives the information of magnetic
moments at the sample surface. We can obtain theinformation whether it is possible to modify the subsurface of FeRh or not when we compare the result ofthese measurements.

3 Results and Discussion
Figure 1 shows the magnetic-hysteresis loops measured
at 5K for the unirradiated and irradiated samples with
2.9MeV and 5.9MeV He ion with the ion fluence of
2x1014 cm-2. As can be seen in the figure, the saturation
magnetization for the unirradiated sample is 0 emu/cc, incontrast that the irradiated samples show some extent.
The saturation magnetization values for the irradiatedsamples were calculated based on the area in which theelastic irradiation energy deposited, that is, simply from 3
to 5 µm depth region for the 2.9MeV He ion irradiatedsamples, and 10–12.5 µm depth region for the 5.9MeV
He ion irradiated samples. Hence, the saturation
magnetization is determined to be 320 emu/cc for the 2.9
MeV sample, and 280 emu/cc for the 5.9MeV samples.
These values seem to be quite low in comparison toliterature values for ferromagnetic FeRh.
The relationship between the deposition energy and thesaturation magnetization for the irradiated FeRh samples
with 30 keV Ga ion beam with various ion fluences isindicated in Fig. 2, which is obtained from our previousstudy. As is discussed in the Experimental Procedure
section, the two samples (2.9MeV and 5.9MeV Heirradiated samples) were irradiated with the same
irradiation fluence of 2x1014 cm-2. However of this, theenergy deposited in the each sample is a little but
different. This is because the energy deposition regions
are different, which cause the difference in the energy
deposited, 9.4x1012 keV/µg (2.9MeV) and 7.7x1012
keV/µg (5.9MeV). As can be seen in the figure, the bothplots seem to quite well fit to the curve drawn from theirradiated sample with 30 keV Ga ion beam. This simply

Fig. 1: M-H curves at 5K for the unirradiated andirradiated samples.

Photon Factory Activity Report 2014 #32 (2015) B

Fig. 2: Relationship between deposition energy andsaturation magnetization of the FeRh samples.

Fig. 3: XMCD spectra for the unirradiated andirradiated samples.

indicates that the ion beam irradiation induced
ferromagnetism in FeRh can be considered in the same
way as 30 keV Ga ion beam case, even when we try tomodify the sub-surface of the samples. Then, theirradiation induced FM in the present samples has not yet
been fully ferromagnetically spin aligned in the present
irradiation condition employed.
Figure 3 shows the XMCD spectra at the Fe L3- and L2edges for the unirradiated sample and the irradiatedsamples with 2.9 and 5.9MeV He ion beam with the ion
fluence of 2x1014 cm-2. The data for the irradiated thinfilm samples with 30 keV Ga ion of the ion fluence of1x1013 cm-2 are also plotted for reference. If the FM layer
is made at the su rface of the irradiated bulk samples, thelarge peaks at the Fe L3-, and L2-absorption edges shouldhave appeared at around 708 eV and 720 eV looking likethe case in the samples irradiated with 30 keV Ga ion. Asfor the Ga ion irradiated thin film samples, uniform
magnetization about 400 emu/cc is induced regardless ofthe depth from the surface. In contrast, the spectra for thesample irradiated with 2.9 and 5.9 MeV He ion beamalmost are unchanged to that for the unirradiated sample:i.e., the XMCD peaks at Fe L3- and L2-edges
corresponding to the presence of the surface
magnetization cannot be observed. These experimental
evidences mean that the FM state at the bulk surface has
not been induced by using high energy He ion beamirradiation. It should be noted here, the results of theXMCD and MCP measurements in the our previous workfor the FM in the FeRh bulk and films by the ion beamirradiation suggested the observed FM to be due to theincrease in spin magnetic moment. Hence, in the present
case, the FM state due to the spin magnetic moment
should be considered to be induced at the sub-surface ofthe samples by the high energy ion beam irradiation.
Considering the experimental evidences indicated inFigs. 1 and 3, the high energy ion beam irradiation like
2.9MeV and 5.9MeV He ion beam did not induced thesurface FM state, but the FM regions at the sub-surface ofFeRh. Since the calculation TRIM simulation reveals thatthe large number of lattice defects are introduced inonly the part in the ranges of 3–5 µm and 10–12.5 µm
from the surface in the FeRh bulk samples for the 2.9 and5.9MeV ion irradiation, respectively, the FM layer arereasonably speculated to be produced in these regions. In
other word, the irradiation induced thin FM layers can bedesignedly produced in any part of the FeRh bulk bycontrolling the ion beam energy and ion species. By
combining these concepts and the two dimensional
magnetic modification technique, the real three
dimensional magnetic modification process can berealized. However, we have not been able to find how tocharacterize this magnetic structure at present.
References[1] M. Fallot et al, Rev. Sci. 77, 498 (1939).
[2] J. S. Kouvel, J. Appl. Phys. 37, 1257 (1966).
[3] N. Fujita et al, Nucl. Instrum. Methods. Phys. Res.,
Sect. B 267, 921 (2009).
[4] T. Koide et al., J. Appl. Phys. 115, 17B722 (2014).
[5] T. Koide et al., Jpn. J. Appl. Phys. 53, 05FC06 (2014).
* t-matsui@21c.osakafu-u.ac.jp