Modification of magnetic properties of FeRh intermetallic compounds by energetic ion beam bombardment

Nuclear Instruments and Methods in Physics Research B 267 (2009) 1612–1615

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Modification of magnetic properties of FeRh intermetallic compounds by energetic ion beam bombardment S. Kosugi a, Nao Fujita a, Y. Zushi a, T. Matsui a, N. Ishikawa b, Y. Saito c, A. Iwase a,* a

Department of Materials Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan Japan Atomic Energy Agency (JAEA-Tokai), Tokai-mura, Ibaraki 319-1195, Japan c Japan Atomic Energy Agency (JAEA-Takasaki), 1233 Takasaki, Gumma, 370-1292, Japan b

a r t i c l e

i n f o

Article history: Available online 30 January 2009 PACS: 61.80.Jh 71.20.Lp 75.50.y 75.90.+w

a b s t r a c t Fe–50 at.%Rh bulk alloys were irradiated with 10 MeV 127I, 60 MeV 136Xe and 200 MeV 136Xe ions at room temperature. Effects of the ion irradiation on magnetic properties were measured by using superconducting quantum interference device (SQUID). By comparing the results for higher energy (60 and 200 MeV) ion irradiation with that for lower energy (10 MeV) irradiation, we have obtained the linear relationship between the irradiation-induced magnetization and the deposited energy density though the elastic collisions. This relationship can be used for the quantitative modification of low temperature magnetism of Fe–50 at.%Rh alloy. Ó 2009 Elsevier B.V. All rights reserved.

Keywords: Fe–Rh alloy Anti-ferromagnetic–ferromagnetic transition Energetic ion irradiation Dependence of deposited energy density Modification of magnetic properties

1. Introduction Fe–50 at.%Rh shows a first-order transition from high temperature ferromagnetic (FM) phase to anti-ferromagnetic (AF) phase at about 310 K without any structural change [1,2]. In our previous reports, we have shown that the irradiation with several ions with the energies of 100–200 MeV induces the FM state in Fe–50 at.%Rh alloy even at 20 K [3,4]. Recently, Zushi et al., have concluded from the data of SQUID measurements that the saturated magnetization of Fe–50 at.%Rh samples, which is induced by 120 MeV Ni, 150 MeV Kr, 200 MeV Xe and 200 MeV Au ion irradiations, can be well scaled by the total deposited energy through the elastic collision process [5]. Measurements of the X-ray magnetic circular dichroism (XMCD) using the same samples as those for the SQUID measurements, however, indicate that the irradiation-induced magnetic moment is correlated both with the nuclear stopping power (Sn, the energy deposited through the elastic interaction per unit pass length) and with the electronic stopping power (Se, the energy deposited through the electronic excitation per unit pass length) [6]. One of the differences between the both measure-

ments is that SQUID measures the total magnetic moment of the samples, while XMCD detects the magnetic moment only at the region which X-ray penetrates (the depth of 3–4 lm from the surface). Furthermore, we have so far studied the irradiation effects on Fe–50 at.%Rh by changing the ion mass around the energies of 100–200 MeV. Under such a experimental condition, the nuclear stopping power and the electronic stopping power increase with increasing the ion mass, and it is difficult to distinguish the effects of elastic collisions and that of electronic excitation. In the present study, we have widely varied the energies of irradiating ions (Xe and I which are neighboring elements in the periodic table. Their mass and atomic number are quite similar.) in order to clarify which energy deposition process dominates the irradiation-induced magnetization. This report shows that the low temperature ferromagnetic state in Fe–50 at.%Rh bulk alloy is induced by the energy deposition through the elastic collisions, and that the energetic ion irradiation is a useful tool for the modification of magnetic properties of Fe–50 at.%Rh alloy.

2. Experimental procedure * Corresponding author. Tel./fax: +81 72 254 9810. E-mail address: [email protected] (A. Iwase). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.01.063

Fe–50 at.%Rh bulk samples with a dimension of 5.0  5.0  0.2 mm3 were prepared by cutting from an ingot. These samples

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a

127

I

10

136

Xe Xe

60 200

2.0  1012, 1.0  1013, 2.0  1012, 2.0  1012, 1.0  1013, 1.0  1014

136

5.0  1012 5.0  1013 5.0  1012 5.0  1012 5.0  1013

Se (MeV/ mg/cm2)

Sn (MeV/ mg/cm2)

Projected range (lm)

5.19

1.49

2.5

23.9 39.3

0.443 0.175

5.4 9.7

were annealed at 1373 K for 24 h for processing of homogenization. Some of them were irradiated at room temperature with 10 MeV I ions by using a tandem accelerator at JAEA-Takasaki. Other samples were irradiated with 60 MeV Xe ions and 200 MeV Xe ions by using a tandem accelerator at JAEA-Tokai. Irradiation fluences used in the present experiment are shown in Table 1. The projected range, electron and nuclear stopping powers for each ion, which have been calculate by using the SRIM-2003 code [7] are also listed in Table 1. To investigate the irradiation-induced ferromagnetism, the magnetization of the irradiated and unirradiated samples was measured as a function of applied magnetic field by using a Quantum Design SQUID magnetometer under the following condition; the scanning range of the applied magnetic field was from 6000 to 6000 Oe with increasing and decreasing magnetic field at the rate of 250 Oe/min and the measurement temperature was 20 K.

2

Fluence (cm2)

Magnetic moment [emu/cm ]

Energy (MeV)

0.5

0

-0.5

2x1012/cm2 5x1012/cm2 1x1013/cm2

-1

1x1014/cm2

-1.5 -6000

-4000

-2000

0

2000

4000

6000

Magnetic field [Oe]

b

3. Result and discussion

10MeV I 1.5

1

0.5

0

-0.5 12

2x10 /cm

2

5x1012/cm2

-1

1x1013/cm2 5x1013/cm2

-1.5 -6000

-4000

-2000

0

2000

4000

6000

Magnetic field [Oe]

c

60MeV Xe 1.5

1

2

Magnetic Moment [emu/cm ]

Fig. 1 shows the irradiation-induced magnetic moment at 20 K, which is normalized for the sample with the cross section of 1 cm2, as a function of applied magnetic field for Fe–50 at.%Rh samples irradiated with 200 MeV Xe, 60 MeV Xe and 10 MeV I ions. As can be seen in Table 1, the range of each irradiating ion is much smaller than the sample thickness. Therefore, the irradiation-induced magnetic moment comes only from the near-surface region. The figure clearly shows that for lower ion-fluence, the magnetic moment increases with increasing ion-fluence and the moment is nearly completely saturated at the magnetic field of 6000 Oe. For 200 MeV Xe ions irradiation, however, the magnetic moment decreases after the irradiation to the fluence of 1  1014/cm2 and for 10 MeV I ions, it decreases after the irradiation to the fluence of 5  1013/cm2. From the X-ray diffraction spectra for heavily irradiated samples, we have concluded that the decrease in magnetic moment for higher fluence is attributed to the irradiation-induced destruction of B2 lattice structure [4]. To discuss the effects of the ion irradiation more quantitatively, the value of saturation magnetization is discussed in term of the energy deposited in the sample by the irradiation. As mentioned later, the energy deposited by the irradiation varies as a function of depth from the sample surface, and the irradiation-induced magnetization is also expected to depend on the depth. Therefore, for each irradiated sample we discuss here the average value of saturation magnetization, hMsi, throughout the irradiated region. In Fig. 2(a), the values of hMsi are plotted against the total energy deposited by the electronic excitation process. In Fig. Fig. 2(b), the values of hMsi are plotted against the total energy deposited by the elastic collision process. The values of hMsi cannot be well correlated with the energy deposited through the elastic collision process or that deposited through the electronic excitation process. Fig. 3(a) and (b) shows the depth distributions of, Se and Sn, respectively, for 200 MeV Xe ions and 10 MeV I ions in Fe–

1

5x1013/cm2

2

Ion

200MeV Xe

1.5

Magnetic moment [emu/cm ]

Table 1 Irradiation parameters.

0.5

0

-0.5

-1 2x1012/cm2 5x1012/cm2

-1.5 -6000

-4000

-2000

0

2000

4000

6000

Magnetic field [Oe] Fig. 1. Irradiation induced magnetic moment at 20 K as a function of applied magnetic field for: (a) Fe–50 at.%Rh irradiated with 200 MeV Xe to the ion-fluence of 2  1012, 5  1012, 1  1013, 5  1013 and 1  1014 cm2. (b) Fe–50 at.%Rh irradiated with 10 MeV I to the ion-fluence of 2  1012, 5  1012, 1  1013, 5  1013 cm2. (c) Fe–50 at.%Rh irradiated with 60 MeV Xe to the ion-fluence of 2  1012, 5  1012 cm2.

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a

a

120

4000 200MeV 10MeV

3500

100

Se [eV/Ang./ion]

3000

[emu/g]

80

60

40

2500 2000 1500 1000

20

500 200MeV 10MeV

0

0

0

500

1000

1500

0

2000

2

energy deposited through electronic excitation[10 eV/g]

b

b

6

8

10

8

10

350 200MeV 10MeV

120 300

100

Sn [eV/Ang./ion]

250

80

[emu/g]

4

Depth [µm]

21

60

200 150 100

40 50

20 0

200MeV 10MeV

0

0

2

4

6

Depth [µm] 0

20

40

60

80

100 21

energy deposited through elastic collision [10 eV/g] Fig. 2. Average values of irradiation-induced magnetization as a function of energy deposited (a) through the electronic excitation and (b) through the elastic collisions.

50 at.%Rh alloys. They are calculated by using SRIM-2003 code [7]. The values of Se changes gradually from the surface to the depth of the ion range, while the value of Sn changes largely near the range. The effect of the irradiation on the magnetic properties around the range is also expected to be quite different from that for the nearsurface region, if the energy deposition through the elastic collisions dominates them. To discuss more quantitatively the irradiation-induced magnetization, therefore, we have separated the irradiation effect near the range from that for the region closer to the surface by using the following process; in Fig. 3 we show the depth profiles of Se and Sn for 10 MeV I ion irradiation which are shifted to the region around the range of 200 MeV Xe ions. The shapes of Se and Sn depth profiles for 10 MeV I ions are about the same as those for 200 MeV Xe ion irradiation around the ion range. Then, we can remove the effect of the energy deposition around the ion range from the experimental value of the magnetic moment for 200 MeV Xe ion irradiation by subtracting the value of the magnetic moment for 10 MeV I ion irradiation from that for 200 MeV Xe ion irradiation. From the value of magnetic moment obtained by the above process and the volume of the corresponding region (hatched region of the figure), the average value of the saturated magnetization, hMsi (emu/g) for the region except around the ion range, has been deduced. The same process men-

Fig. 3. Depth distributions of: (a) electronic stopping power, Se, and (b) nuclear stopping power, Sn, for 200 MeV Xe, and 10 MeV I irradiations in the Fe–Rh alloy. The profiles of Se and Sn for 10 MeV I ions, which are shifted the region around the range of 200 MeV Xe ion are also shown.

tioned above has been applied for the combination of the results for 60 MeV Xe irradiation and 10 MeV I irradiation. In Fig. 4(a), the average values of saturated magnetization, hMsi, for the region except around the ion range of 200 MeV and 60 MeV Xe ions are plotted as a function of deposited energy density through the electronic excitation process, while Fig. 4(b) shows the values of hMsi as a function of deposited energy density through the elastic collision process. As can be seen in the figure, the values of hMsi are correlated with the deposited energy density through elastic collisions much better than with that through the electronic excitation. The values of hMsi, the unit of which is emu/g, are proportional to the deposited energy density through the elastic collisions and they are given by

hMsi ffi 1020  ED where ED is the deposited energy density through elastic collision process, and its unit is eV/g. This relationship can be used for the quantitative modification of the near-surface magnetization of Fe–50 at.%Rh bulk alloy at low temperatures. The deposited energy density through the elastic collisions has been converted into the number of atomic displacements or dis-

S. Kosugi et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1612–1615

a

placements per atoms (dpa) of 0.001 cause the magnetization of 10 emu/g. This quantitative relationship between the magnetization and the atomic displacements can be compared with some theoretical calculations, for example the first principle calculation where one vacancy or one anti-site defect is put into the center of a supercell [9]. Finally, it is worth noting here the result of our previous result for 120–200 MeV Ni, Kr, Xe and Au ion irradiation [5], where the average magnetization for the whole region including around the ion range is well correlated with the energy deposited by elastic collisions. We think now that the reason for this good correlation is due to the similar shape of depth profiles of Sn for these irradiations.

100 200MeV-10MeV 60MeV-10MeV

80

[emu/g]

1615

60

40

20

4. Summary 0 0

100

200

300

400

500 21

energy deposited through electronic excitation [10 eV/g]

b

100 200MeV-10MeV 60MeV-10MeV

[emu/g]

80

60

Fe–50 at.%Rh alloys were irradiated with 10 MeV I ions, 60 MeV Xe, ions and 200 MeV Xe ions at room temperature. The saturation magnetizations induced by the irradiation have been analyzed to obtain the relationship between the irradiation-induced magnetization and the deposited energy by the ions. We have found that the magnetization is proportional to the deposited energy through the elastic collisions. By using this linear relationship, we can quantitatively modify the magnetic properties of Fe–Rh alloy. The present study shows that the ion irradiation is a useful tool for the modification of the magnetization of Fe–50 at.%Rh alloy at low temperatures. Acknowledgement

40

The present research has been partially supported by the Reimei Research Promotion project (Japan Atomic Energy agency). 20

References 0

0

2

4

6

8

10 21

energy deposited through elastic collision [10 eV/g] Fig. 4. Average values of saturation magnetization, hMsi, for the region except around the ion range, (a) as a function of energy deposited through electronic excitation and (b) as a function of energy deposited through elastic collisions.

placements per atom (dpa) by using the values of 30 eV and 45 eV as the displacement energies for Fe atom and Rh atom in Fe– 50 at.%Rh, respectively [8]. Then, we have obtained that the dis-

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