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Structure and properties of various fast neutron irradiated magnets
Physica B: Physics of Condensed Matter xxx (2017) 1–5
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Structure and properties of various fast neutron irradiated magnets S. Lee a, V.D. Parkhomenko b, Yu.N. Skryabin b, S.G. Bogdanov b, A.P. Nosov b, c, A.E. Teplykh b, N.V. Kudrevatykh c, A.L. Kholkin c, d, M.A. Semkin b, c, N.V. Urusova c, A.N. Pirogov b, c, * a
Neutron Science Division, HANARO, Korea Atomic Energy Research Institute, 305353 Daejeon, Republic of Korea M.N. Mikheev Institute of Metal Physics, Ural Division of Russian Academy of Science, 620108 Ekaterinburg, Russia c Institute of Natural Sciences and Mathematics, Ural Federal University, 620002 Ekaterinburg, Russia d Department of Physics, CICECO-Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal b
A R T I C L E I N F O
A B S T R A C T
Keywords: Neutron irradiation Diffraction Magnetic transformation
Neutron irradiation allows the materials to transform to a state, in which the properties of material become different from an initial state. This paper presents the results of neutron irradiation of several magnetic materials including intermetallic compounds Nd2Fe14B and Er2Fe14B, multiferroics BiFe0.95Mn0.05O3 and Bi0.85La0.15FeO3, oxides LiMn2O4 and Li0.9FePO4. The fast neutrons (Eeff > 0.1 MeV) have been used in a fluence range from 1 1018 n/cm2 to 2 1020 n/cm2 at 340 K Er2Fe14B alloy becomes amorphous under irradiation, that results in the reduction of Curie temperature to about 200 K. Irradiation destroys charge ordering in LiMn2O4 leading to the transformation from an incommensurate antiferromagnetic to a commensurate ferrimagnetic structure. Neutron irradiation of BiFe0.95Mn0.05O3 oxide is accompanied by decreasing the amount of impurity phases. On the contrary, in Bi0.85La0.15FeO3 fast neutrons result in the appearance of impurity phases. Neutron irradiation distinctly affects the lattice parameters of Li0.9FePO4 compound even with the relatively low fluence.
neutron of the fission spectrum was estimated by the formula [5].
1. Introduction In today's world, a relatively new technological process such as radiation modification is becoming more and more widely used [1–4]. It causes the change in properties of materials under the influence of various types of radiation such as high-energy electrons and ions, gamma and fast neutron irradiation. It is known that fast neutrons (n0) produce radiation damage of solids according to the scheme [4] n0 ⇒ PKA ⇒ ACC,
(1)
where PKA is the primary knocked-out (by fast neutron) atom, which in the process of its deceleration displaces nearest atoms in a crystal; ACC is the atomic-collision cascade, which is the whole group of the knockedout displaced atoms. The process of atomic collisions continues until the full stop of the PKA and displaced atoms. An average geometric size of an individual damaged microregion depends significantly on the mass and energy of PKA as well as on the chemical composition of a target. The ACCs generated by primary knocked atoms in investigated materials can be calculated using “The Stopping and Range of Ions in Matter” program (SRIM_2003.19) [5]. The mean PKA energy upon irradiation with
ЕPKA ¼ 4/М,
(2)
where energy EPKA is in MeV and М is the mass number of the target atom. It should be noted that these calculations are related to a dynamic stage of the cascade (~1012 to 1011 s from the onset of the PKA motion); at the stage of relaxation the topography of the ensemble of defects experiences substantial changes. Nevertheless, these calculations allow one to estimate the scale of disturbances produced by PKAs. The following processes of transformation of irradiated materials can occur at the stage of relaxation of formed radiation defects: some of the atoms, which have appeared at interstices of the crystal lattice recombine with vacancies, some form dislocations, some diffuse into drains (grain boundaries, etc.). In ordered crystalline substances anti-site substitution can take place, which leads to the formation of disordered microregions with chaotic distribution of atoms along the sites of the crystal lattice. Complexes of radiation defects (for example, anti-site substitution and a nearby vacancy) can lead to a substantial chaotic dislocation of atoms from lattice sites and loss of translational symmetry, it means a neutron amorphization. The effect of amorphization takes place only if concentration of defects in the CAC region achieves a certain critical value. The
* Corresponding author. M.N. Mikheev Institute of Metal Physics, Ural Division of Russian Academy of Science, 620108 Ekaterinburg, Russia. E-mail address: [email protected] (A.N. Pirogov). https://doi.org/10.1016/j.physb.2017.11.078 Received 30 August 2017; Received in revised form 22 November 2017; Accepted 27 November 2017 Available online xxxx 0921-4526/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Lee, et al., Structure and properties of various fast neutron irradiated magnets, Physica B: Physics of Condensed Matter (2017), https://doi.org/10.1016/j.physb.2017.11.078
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Physica B: Physics of Condensed Matter xxx (2017) 1–5
BiFe0.95Mn0.05O3 and Bi0.85La0.15FeO3 multiferroics, LiMn2O4 and Li0.9FePO4 oxides.
structure of amorphous materials formed under neutron irradiation differs significantly from the structure of amorphous metals and compounds obtained by quenching from a melt, plasma spraying, etc., which is similar to a close random packing of atoms. In case of neutron amorphization, the radiation defects are represented by the substantial (on the order of 10% of the lattice parameter) displacement of atoms. In the initial state (before irradiation), the atoms of a certain type occupy definitive lattice crystallographic positions in an ordered crystal. Neutron disordering means the formation of microscopic regions, in which the statistical distribution of atoms is realized along the positions of crystal lattice [6]. A structural motive of a crystal is conserved. With such a structural change, it is possible to construct a new atomic environment resulting in new magnetic phase transitions. For example, a transition from an antiferromagnetic structure to a ferromagnetic one [7] or a magnetic ordering occurs in irradiated sample, which was nonmagnetic before neutron irradiation [8,9]. In this paper, we demonstrate the effects of fast neutron irradiation on the properties of several magnetic materials such as Nd2Fe14B and Er2Fe14B intermetallic compounds, BiFe0.95Mn0.05O3 and Bi0.85La0.15FeO3 multiferroics, LiMn2O4, and Li0.9FePO4 oxides. Permanent magnets are produced basing on Nd2Fe14B phase, which has a record high value of magnetic energy product (BH)max > 400 kJ/m3 [10]. However, this value is lower than the theoretical limit (520 kJ/m3). An essential increase of (BH)max can be attained by synthesizing Nd2Fe14B magnets from amorphous state using a subsequent annealing. According to [11], re-crystallization of amorphous sample allows to increase the coercivity from 0.65 T up to 2.9 T. Thus, it is interesting to achieve an amorphous state and to study its magnetic properties. Multiferroic materials have attracted enormous research interest in recent years [12] due to their potential applications and marvelous physical properties. Among them BiFeO3 is the most studied because it exhibits multiferroic properties up to 600 K. However, it is difficult to synthesize a single phase BiFeO3 polycrystalline sample because of the formation of two concomitant phases: Bi25FeO40 and Bi2Fe4O9. To overcome this difficulty, authors of [2] irradiated multiphase BiFeO3 sample with swift heavy Ag ions. They found that the intensities of reflections, originated from Bi25FeO40 and Bi2Fe4O9 phases, disappeared on X-ray diffraction (XRD) patterns at the fluence of 1 1012 ions/cm2. It is known that multi-charged ions and X-ray radiation do not penetrate deep into the material. Therefore, using Ag ions and X-ray diffraction allows obtaining information only on the phase contents in the surface layer of a particle. To get the data on the phase changes in the volume of the sample, neutron irradiation and diffraction are needed. Cathode materials based on LiMn2O4 and LiFePO4 oxides are ones of the most promising materials in lithium-ion batteries [13]. They have a high charge density and become attractive energy-storage systems for portable electronic devices. The LiMn2O4 is also of interest because it is an example of a geometrically frustrated antiferromagnet. Besides, in the LiMn2O4 a charge ordering (Mn3þ-Mn4þ), the Jahn-Teller distortions [14] and a long-range antiferromagnetic ordering are observed at low temperatures. It is interesting to investigate the interrelation between the crystal structure and magnetic ordering in this manganite in both the unirradiated (ordered) and irradiated (disordered) states. In this case, the structural disordering means the redistribution of lithium and manganese cations over octahedral and tetrahedral positions of the spinel lattice. In LiMn2O4 and LiFePO4, the oxygen plays a critical role as it takes place in strongly correlated transition metal oxides. The magnetism in LiFePO4 oxide is quite uncommon because of the lack of the conventional mechanism such as Mott-Habbard, Charge-transfer and Slater effect are applicable to explain the transition to antiferromagnetic ordering [15]. Therefore, for the theory development it is important to get information about the behavior of Fe–O bonds under external influence, for example, neutron irradiation. Therefore, in present paper we try to answer above questions about the effects of fast neutron irradiation on the properties of several magnets such as Nd2Fe14B and Er2Fe14B intermetallic compounds,
2. Material and methods Nd2Fe14B and Er2Fe14B intermetallic compounds were melted in an induction furnace and have been treated via rapid quenching [16]. Bi0.85La0.15FeO3, LiMn2O4 [17], and Li0.9FePO4 were synthesized by a solid state reaction. BiFe0.95Mn0.05O3 powder was synthesized by the citrate-nitrate method. All the samples have been irradiated with fast neutrons (Eeff > 0.1 MeV) at 340 K in the water pool of the IVV-2M reactor (Zarechny, Russia). In an active zone center the neutron flux was 1 1014 cm2 s1. The mean neutron energy was about 2 MeV. The samples were irradiated in the fluence range from Φ ¼ 1 1018 n/cm2 to Φ ¼ 2 1020 n/cm2, that conforms with the irradiation time from 3 h to 23 days. Magnetic measurements have been performed under magnetic fields up to μ0H ¼ 2 T at 293 K using a vibrating sample magnetometer. X-ray diffraction patterns have been recorded with diffractometer DRON-1UM at room temperature. We used Cu Kα radiation. Neutron diffraction experiment has been carried with D2 and D3 diffractometers mounted at horizontal channels of the IVV-2M reactor. Incident neutron beams with the wavelengths of 1.805 Å (D2 diffractometer) and 2.429 Å (D3 diffractometer) were used. The neutron powder diffraction (NPD) patterns were calculated by means of the Fullprof software [18]. 3. Results and discussion Table 1 presents the average energy E and the mean free path L of primary knocked-on atoms in samples under investigation at the fast neutron irradiation. The large difference in values L for light (for example, Li) and heavy (such as Bi) ions is observed. Light ions lose their energy E due to electron ionizations. A heavy ion is absorbed in a crystal causing a disorder. Therefore, mainly the Nd- (and Er-) ions disorder crystals in Nd2Fe14B (Er2Fe14B), whereas the Bi-ions destroys an order in BiFe0.95Mn0.05O3 and Bi0.85La0.15FeO3, the energy of the Mn- and Fe-ions are expended to disorder a crystal in LiMn2O4 and Li0.9FePO4, respectively. We may assume that the effect of neutron irradiation on these samples is large. Fig. 1 shows room temperature NPD patterns of Er2Fe14B intermetallic compound at initial (before irradiation) and irradiated states [16]. Room temperature NPD pattern of Nd2Fe14B differs from the one presented for Er2Fe14B only by intensities of reflections. At an initial state, both samples possess the tetragonal Nd2Fe14B type structure (space group P42/mnm) as the main phase and contain about 7% and 2% of the additional α-Fe phase in Nd2Fe14B and Er2Fe14B, respectively. The magnetic structure of Nd2Fe14B is a collinear ferromagnet with the propagation vector k ¼ 0 and Curie temperature TC ¼ 585 K. The Er2Fe14B compound is ferrimagnet and, as can be seen from the insert on Fig. 1a, its TC ¼ 560 K. As example of refined parameters, we present Table 2 in which crystal and magnetic structure parameters of the
Table 1 Average energy E and mean free path L of primary knocked-on atoms (PKA) for irradiated materials. PKA
Li
E, keV
576
L,Å
2
B
O
P
565
250
125
Nd2Fe14B Er2Fe14B BiFe0.95Mn0.05O3 Bi0.85La0.15FeO3 LiMn2O4 Li0.9FePO4
Mn
Fe
La
Nd
Er
Bi
73
70
29
27
24
19
Nd – 85; Fe – 270; B – 6570 Er – 79; Fe – 273; B – 6659 Bi – 137; Fe – 551; Mn – 592; O – 5131 Bi – 47; La – 190; Fe – 550; O – 5140 Li – 20500; Mn – 570; O – 5610 Li – 23900; Fe – 670; P – 1940; O – 6570
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unirradiated and irradiated states are close to each other which allows to conclude the retention of ferrimagnet character of Er- and Fe-ion moment ordering in the amorphous sample, though the Er- and Fe-sublattices are absent in the crystallographic sense. In the amorphous state, the Nd2Fe14B shows ferromagnetic behavior. But in the irradiated state the TC 450 K, which is by about 135 K lower comparing to the Curie temperature of the crystalline sample. Although magnetization in the fresh and irradiated samples is almost the same, the coercivity decreases noticeably in the amorphous state. The low coercivity in irradiated Nd2Fe14B allows orienting rare-earth- and 3d-transition magnetic moments by external field in the same direction that it is important in stage of re-crystallization of amorphous sample to achieve a high value of (BH)max. The BiFe0.95Mn0.05O3 ferrite was irradiated by fast neutrons up to fluence Φ ¼ 4.6 1019 n/cm2. Fig. 2 presents observed and calculated XRD patterns of this ferrite in initial and irradiated states. The fresh sample contents beside the main BiFe0.95Mn0.05O3 (88%) phase quite big amount of impurity Bi25FeO40 (5%) and Bi2Fe4O9 (7%) phases. Diffraction patterns testify to a distinct decrease of reflection intensity attributed to the impurity phases after neutron irradiation. In the irradiated sample, the content gets 1% and 5% for Bi25FeO40 and Bi2Fe4O9 phases, respectively. Calculations of X-ray and neutron diffraction patterns show that the irradiation leads to the decrease of distortions of the Fe/MnO6 octahedrons due to the equalizing of bond lengths d(Fe/Mn–O). In the initial state two main lengths d(Fe/Mn–O) and angle φ(Fe/Mn–O–Fe/ Mn) were equal to d1(Fe/Mn–O) ¼ 2.125(2) Å, d2(O–Fe/ Mn) ¼ 1.932(2) Å, and φ(Fe/Mn–O–Fe/Mn) ¼ 154.7(1) . After irradiation these parameters are equal to d1(Fe/Mn–O) ¼ 2.090(6) Å, d2(O–Fe/Mn) ¼ 1.963(4) Å, and φ(Fe/Mn–O–Fe/Mn) ¼ 156.1(1) . Calculation of NPD patterns of both sample results in that in range of experimental error antiferromagnetic structure is kept after irradiation; magnetic moment values are equal to 3.6(1) μB in fresh and irradiated samples. However, magnetization curves of unirradiated and irradiated samples are quite different. They are given on inserts in Fig. 2a and b. It can be seen that the magnetization of the fresh sample rises linearly with
Fig. 1. a) Room temperature observed (points) and calculated (line) NPD patterns of Er2Fe14B before irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. The first, second and third rows of tick marks indicate the angle positions of nuclear and magnetic reflections of Er2Fe14B, and nuclear peaks of α-Fe phase, correspondently. Temperature dependence of the susceptibility is shown on the insert. b) Room temperature observed NPD pattern of Er2Fe14B in amorphous state (after irradiation). Temperature dependence of the magnetization at μ0H ¼ 5 mT is given on the insert.
unirradiated Er2Fe14B sample are shown. They are in a good agreement with previous study [10]. The NPD pattern of the irradiated (fluence of Φ ¼ 1.2 1020 n/cm2) Er2Fe14B sample (shown in Fig. 1b) includes narrow half-width peaks at angles 2θ ¼ 53 , 78 and 101 and wide maximum at 2θ ¼ 53 . The narrow peaks are originated from neutron scattering on the α-Fe phase, and the wide maximum is an evidence of existence of the amorphous state in the main phase. In amorphous state, the Er2Fe14B exhibits properties typical for ferrimagnets and spin glass systems. Its TC decreases down to 314(2) K, which is 244 K lower comparing to the crystalline samples (see insets on Fig. 1b). At 5 K, the magnetizations of Table 2 Refined lattice constants a, b, c, unit cell volume V, coordinates of positions (space group P42/mnm), the average Er- and Fe-ion magnetic moments μEr(μB), μFe(μB) in unirradiated sample, contents of the Er2Fe14B and α-Fe phases at room temperature, agreement factor RBragg and χ2. Structural parameter
Er2Fe14B
a, b (Å) c (Å) V (Å3) Er, 4f: x Er, 4g: x Fe, 4e: z Fe, 8j1: x z Fe, 8j2: x z Fe, 16k1: x y z Fe, 16k2: x y z B, 4g: x μEr (μB) μFe (μB) Er2Fe14B (mass%) α-Fe (mass%) RBragg (%) χ2(%)
8.744(1) 11.968(2) 914.3(5) 0.273(1) 0.147(1) 0.112(1) 0.097(1) 0.201(1) 0.318(1) 0.249(1) 0.222(1) 0.567(1) 0.127(1) 0.036(1) 0.360(1) 0.170(1) 0.636(2) 4.1(1) 1.9(1) 98.0(2) 2.0(2) 5.53 4.46
Fig. 2. a) Room temperature observed (points) and calculated (line) XRD patterns of BiFe0.95Mn0.05O3 before irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. The first, second and third rows of tick marks indicate the angle positions of reflections of BiFe0.95Mn0.05O3, Bi25FeO40 and Bi2Fe4O9 phases, correspondently. Magnetization curve measured at T ¼ 293 K is shown on the insert. b) Room temperature observed (points) and calculated (line) XRD patterns of BiFe0.95Mn0.05O3 after irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. The first, second and third rows of tick marks indicate the angle positions of reflections of BiFe0.95Mn0.05O3, Bi25FeO40 and Bi2Fe4O9 phases, correspondently. Magnetization curve measured at T ¼ 293 K is shown on the insert. 3
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increasing external field pointing to antiferromagnetic structure. In the irradiated state, the magnetization increases nonlinearly and its value at μ0H ¼ 2 T is three times bigger than that in the initial state. Therefore, an external magnetic field induces transformations from antiferromagnetic structure to a state like ferromagnetic ordering. Further increase of neutron fluence induces formation of iron oxide phases in the sample. The observed oxides are clearly seen on Fig. 3, which shows NPD patterns of Bi0.85La0.15FeO3 multiferroic in before and after irradiated states. In the initial state, the sample contains the single phase. The irradiation of the sample by the fluence Φ ¼ 5 1020 n/cm2 leads to the formation of Fe3O4 and Bi2O3 oxides. Their concentrations are 11% and 10%, respectively. Crystal structures of the sample in the initial state and the main phase of irradiated sample are described by rhombohedral unit cell (space group R3c). The irradiation induces the expansion of the unit cell of the main phase up to ΔV ¼ 1.1% which can be explained by accumulation of interstitial ions. The observed magnetic structure is antiferromagnetic. The magnetic moment of Fe-ion has components parallel to a- and c-axes, which are 3.3(1) μB and 1.7(1) μB. In both samples the total moment of Fe-ions is equal to 3.7(1) μB. In contrast to Nd2Fe14B and Er2Fe14B, the LiMn2O4 oxide did not experience amorphization even under the fluence Φ ¼ 2 1020 n/cm2. Fig. 4 shows the NPD patterns of LiMn2O4 at initial and irradiated states at 5 K [16]. NPD pattern of the initial state indicates the orthorhombic structure (space group Fddd) with lattice parameters a ¼ 24.663(4) Å, b ¼ 24.891(7) Å, and c ¼ 8.271(3) Å. There are superstructure reflections, the biggest of them is the (4/3 8/3 2) peak located at 2θ 63 and originated from the charge ordering (Mn3þ and Mn4þ). Reflections located on left part of NPD pattern indicate the incommensurate magnetic structure with vector k ¼ 2π/c(0, 0, 0.44). The obtained by us parameters are in good accordance with presented in literature [18,19]. As seen in Fig. 4, there are drastic changes in the shape of NPD pattern after neutron irradiation. There is no (4/3 8/3 2) peak, therefore the charge ordering is demolished. The sample crystallizes in two phases such as cubic (Fd-3m) and tetragonal (I41/amd) structures. Latter structure has lattice periods a ¼ 5.754(1) Å and c ¼ 9.461(1) Å. The irradiation stimulates a considerable redistribution of Mn ions to nonequivalent positions of the cubic structure. Manganese ions (about 17%) moved to non-typical for them 8a positions and the corresponding amount of Li ions occupied the 16d cites. The observed ion redistribution
Fig. 4. a) The 5 K observed (points) and calculated (line) NPD patterns of LiMn2O4 before irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. The tick marks indicate the angle positions of nuclear and magnetic reflections of LiMn2O4. Magnetization curve measured at T ¼ 293 K is shown in the insert. b) The 5 K observed (points) and calculated (line) NPD patterns of LiMn2O4 after irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. The first, second, third and four rows of tick marks indicate the angle positions of nuclear and magnetic reflections of cubic phase LiMn2O4, and nuclear and magnetic peaks of tetragonal phase LiMn2O4, correspondently. Magnetization curve measured at T ¼ 293 K is shown in the insert.
is accompanied by a radical change of the magnetic state of the sample. The incommensurate magnetic structure is transformed to the ferrimagnetic commensurate ordering with propagation vector k ¼ 0 and TC ¼ 80 K. For the LiMn2O4 phase, at 5 K the magnetizations of tetrahedral and octahedral sublattices are equal to 1.0(1) μB and 2.0(1) μB, respectively. Therefore, the total magnetic moment is equal to 1.0(1) μB/ f.u.. As can be seen from inserts in Fig. 4a and b, the magnetization curves of fresh and irradiated samples are noticeably differing. The magnetization curve of the fresh sample is linear, which confirms the neutron diffraction data on the antiferromagnetic ordering. After irradiation, the magnetization increases significantly and its field dependence becomes essentially nonlinear in the region of magnetic fields up to μ0H ~0.5 T. A value of the spontaneous magnetization was determined by means of
Fig. 3. a) Room temperature observed (points) and calculated (line) NPD patterns of Bi0.85La0.15O3 before irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. The first and second rows of tick marks indicate the angle positions of nuclear and magnetic reflections of Bi0.85La0.15O3, correspondently. b) Room temperature observed (points) and calculated (line) NPD patterns of Bi0.85La0.15O3 after irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. The first, second, third and four rows of tick marks indicate the angle positions of nuclear and magnetic reflections of Bi0.85La0.15O3, and nuclear peaks of Fe3O4 and Bi2O3, correspondently.
Fig. 5. a) Room temperature observed (points) and calculated (line) NPD patterns of Li0.9FePO4 before irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. b) Room temperature observed (points) and calculated (line) NPD patterns of Li0.9FePO4 after irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. 4
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extrapolation from high-field region to zero field and equaled to 0.90(5) μB/f.u., being in a good agreement with neutron diffraction data. The large susceptibility of paraprocess should also be noted, as it often points to the noncollinear magnetic structures. An effect of neutron irradiation on the lattice parameters becomes apparent distinctly in case of Li0.9FePO4 multiferroic. Fig. 5 shows the NPD patterns of Li0.9FePO4 before and after irradiated (by fluence Φ ¼ 1 1018 n/cm2) samples. As seen in Fig. 5, the irradiation does not entail obvious change on NPD patterns. Crystal structures of both samples are described by the space group Pnma with close values of coordinate and occupation parameters. However, the irradiation causes the increase of lattice parameters. As can be seen from the inserts in Fig. 5, the a, b and c parameters are increased by ~0.01, 0.03 and 0.02 Å, respectively. Therefore, the neutron irradiation indices are due to increasing of bond Fe–O distances.
References [1] K.-N. Choo, M.-S. Cho, B.-H. Jun, C.-J. Kim, S.-J. Park, Neutron Irradiation of electronic materials at HANARO, J. Appl. Math. Phys. 2 (2014) 837–842, https:// doi.org/10.4236/jamp.2014.29093. [2] B.N. Dash, P. Mallick, P. Dash, R. Biswal, J. Prakash, A. Tripashi, D. Kanjilal, N.C. Mishra, Swift heavy ion irradiation induced modification of structure and surface morphology of BiFeO3 thin film, Bull. Mater. Sci. 36 (5) (2013) 813–818, https://doi.org/10.1007/s12034-013-0550-4. [3] S.F. Dubinin, V.E. Arkhipov, Yu.G. Chukalkin, S.G. Teploukhov, V.D. Parkhomenko, Ya.M. Mukovskiĭ, Magnetic structure and transport properties of atomically disordered crystals of two-dimensional manganites La22xSr1þ2xMn2O7, Phys. Sol. St. 49 (2) (2007) 297–303, https://doi.org/10.1134/S1063783407020205. [4] I. Manika, J. Maniks, R. Zabels, K. Schwartz, R. Grants, A. Dauletbekova, A. Rusakova, M. Zdorovets, Modification of LiF structure by irradiation with swift heavy ions under oblique incidence, IOP Conf. Ser. Mater. Sci. Engin. 49 (2013) 012011–012014, https://doi.org/10.1088/1757-899X/49/1/012011. [5] M.W. Thompson, Defects and Radiation Damage in Metals, Cambridge univ. Press, London and New York, 1969, p. 367. [6] K.E. Sickafus, A.C. Larson, N.Yu. Nastasi, G.W. Hollenberg, F.A. Gamer, R.C. Bradt, Cation disorder in high dose, neutron-irradiated spinel, J. Nucl. Mater. 219 (1995) 128–134, https://doi.org/10.1016/0022-3115(94)00386-6. [7] Yu.G. Chukalkin, Amorphization of oxides under irradiation with fast neutrons, Phys. Sol. St. 55 (8) (2013) 1714–1717, https://doi.org/10.1134/ S1063783413080076. [8] C.L. Prajapat, V. Dube, A.K. Rajarajan, S.V. Thakare, K.C. Jagadeesan, P.K. Mishra, M.R. Singh, S.K. Gupta, G. Ravikumar, Effect of fast neutron irradiation induced defects on the metamagnetic transition in Ce(Fe0.96Ru0.04)2, J. Appl. Phys. 112 (6) (2012) 063922–063925, https://doi.org/10.1063/1.4754577. [9] Y. Liu, G. Wang, S. Wang, J. Yang, L. Chen, X. Qin, B. Song, B. Wang, X. Chen, Defect-induced magnetism in neutron irradiated 6H-SiC single crystals, Phys. Rev. Lett. 106 (2011), https://doi.org/10.1103/PhysRevLett.106.087205, 087205–4. [10] E. Burzo, Permanent magnets based on R-Fe-B and R-Fe-C alloys, Rep. Prog. Phys. 61 (9) (1998) 1099–1266, https://doi.org/10.1088/0034-4885/61/9/001. [11] S. Sab, L. Bessais, C. Djega-Mariadassou, N.H. Dan, N.X. Phuc, Enhancement of coercivity in the nanocomposite R40Fe30Co15Al10B5(R ¼ Nd, Pr), J. Phys. Condens. Matter (2003) 5615–5620, https://doi.org/10.1088/150953-8984/15/32/320. [12] M. Fiebig, T. Lottermoser, D. Meier, M. Trassin, The evolution of multiferroics, Nat. Rev. Mater. 1 (2016), https://doi.org/10.1038/natrevmats.2016.46, 16046–14. [13] H. Xia, Z. Luo, J. Xie, Nanostructured LiMn2O4 and their composites as highperformance cathodes for lithium-ion batteries, Prog. Nat. Sci. Mater. Int. 22 (I) (2012) 572–584, https://doi.org/10.1016/j.pnsc.2012.11.014. [14] Y. Chen, D. Yu, K. An, Stress-induced charge-ordering process in LiMn2O4, Mater. Res. Lett. 5 (2) (2017) 89–94, https://doi.org/10.1080/21663831.2016.1197858. [15] R. Masrour, A. Jabar, A. Benyoussef, M. Hamedoun, E.K. Hlil, Magnetic properties of LiFePO4 compound: a Monte Carlo study, Chem. Phys. Lett. 635 (2015) 268–270, https://doi.org/10.1016/j.cplett.2015.07.007. [16] A.E. Teplykh, Yu.G. Chukalkin, S. Lee, S.G. Bogdanov, N.V. Kudrevatykh, E.V. Rosenfeld, Yu.N. Skryabin, Y. Choi, A.V. Andreev, A.N. Pirogov, Magnetism of ordered and disordered alloys of R2Fe14B (R ¼ Nd, Er) type, J. All. Comp. 581 (2013) 423–430, https://doi.org/10.1016/j.jallcom.2013.07.103. [17] Yu.G. Chukalkin, A.E. Teplykh, A.N. Pirogov, D.G. Kellerman, Magnetism of LiMn2O4 manganite in structurally ordered and disordered states, Phys. Solid State 52 (12) (2010) 2545–2551, https://doi.org/10.1134/s1063783410120164. [18] J. Rodriguez-Carvajal, Recent advances in magnetic structure determination by neutron powder diffraction, Phys. B 192 (1993) 55–69, https://doi.org/10.1016/ 0921-4526(93)90108-i. [19] V. Massarotti, D. Capsoni, M. Bini, P. Scardi, M. Leoni, V. Baron, H. Berg, LiMn2O4 low-temperature phase: synchrotron and neutron diffraction study, J. Appl. Cryst. 32 (1999) 1186–1189, https://doi.org/10.1107/S0021889899011577.
4. Conclusion Intermetallic compounds (Nd2Fe14B and Er2Fe14B), multiferroics (BiFe0.95Mn0.05O3 and Bi0.85La0.15FeO3) and oxides (LiMn2O4 and Li0.9FePO4) have been irradiated by fast neutrons (Eeff > 0.1 MeV). Magnetic measurements and neutron diffraction experiment were carried out on unirradiated and irradiated samples. The Nd2Fe14B and Er2Fe14B intermetallic undergo a transformation from crystalline state to amorphous state after the neutron irradiation with the fluence Φ ¼ 1.2 1020 n/cm2. The value of TC in amorphous state is considerably lower than that in the crystalline state. The neutron irradiation of the BiFe0.95Mn0.05O3 sample by the fluence Φ ¼ 4.6 1019 n/cm2 suppresses the impurity phases Bi25FeO40 and Bi2Fe4O9. The irradiation stimulates the transformation from antiferromagnetic structure to the magnetic state with nonzero spontaneous magnetization. A rise of fluence up to Φ ¼ 5 1020 n/cm2 is accompanied by partial decomposition of the Bi0.85La0.15FeO3 samples. The fast neutron irradiation of the LiMn2O4 stimulates the transformation from antiferromagnetic incommensurate structure to the magnetic commensurate ordering with the spontaneous magnetization. Acknowledgments This research was carried out at IMP Neutron Material Science Complex within the state assignment of FASO of Russia (theme “Flux” No. 01201463334) and supported in part by the State contract (No. 3.6121.2017/8.9) between Ural Federal University and the Ministry of Education and Science of Russian Federation and also supported in part by the project CICECO-Aveiro Institute of Materials (POCI-01-0145FEDER-007679, FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and, when appropriate, co-financed by FEDER under the PT2020 Partnership Agreement.
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Physica B: Physics of Condensed Matter journal homepage: www.elsevier.com/locate/physb
Structure and properties of various fast neutron irradiated magnets S. Lee a, V.D. Parkhomenko b, Yu.N. Skryabin b, S.G. Bogdanov b, A.P. Nosov b, c, A.E. Teplykh b, N.V. Kudrevatykh c, A.L. Kholkin c, d, M.A. Semkin b, c, N.V. Urusova c, A.N. Pirogov b, c, * a
Neutron Science Division, HANARO, Korea Atomic Energy Research Institute, 305353 Daejeon, Republic of Korea M.N. Mikheev Institute of Metal Physics, Ural Division of Russian Academy of Science, 620108 Ekaterinburg, Russia c Institute of Natural Sciences and Mathematics, Ural Federal University, 620002 Ekaterinburg, Russia d Department of Physics, CICECO-Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal b
A R T I C L E I N F O
A B S T R A C T
Keywords: Neutron irradiation Diffraction Magnetic transformation
Neutron irradiation allows the materials to transform to a state, in which the properties of material become different from an initial state. This paper presents the results of neutron irradiation of several magnetic materials including intermetallic compounds Nd2Fe14B and Er2Fe14B, multiferroics BiFe0.95Mn0.05O3 and Bi0.85La0.15FeO3, oxides LiMn2O4 and Li0.9FePO4. The fast neutrons (Eeff > 0.1 MeV) have been used in a fluence range from 1 1018 n/cm2 to 2 1020 n/cm2 at 340 K Er2Fe14B alloy becomes amorphous under irradiation, that results in the reduction of Curie temperature to about 200 K. Irradiation destroys charge ordering in LiMn2O4 leading to the transformation from an incommensurate antiferromagnetic to a commensurate ferrimagnetic structure. Neutron irradiation of BiFe0.95Mn0.05O3 oxide is accompanied by decreasing the amount of impurity phases. On the contrary, in Bi0.85La0.15FeO3 fast neutrons result in the appearance of impurity phases. Neutron irradiation distinctly affects the lattice parameters of Li0.9FePO4 compound even with the relatively low fluence.
neutron of the fission spectrum was estimated by the formula [5].
1. Introduction In today's world, a relatively new technological process such as radiation modification is becoming more and more widely used [1–4]. It causes the change in properties of materials under the influence of various types of radiation such as high-energy electrons and ions, gamma and fast neutron irradiation. It is known that fast neutrons (n0) produce radiation damage of solids according to the scheme [4] n0 ⇒ PKA ⇒ ACC,
(1)
where PKA is the primary knocked-out (by fast neutron) atom, which in the process of its deceleration displaces nearest atoms in a crystal; ACC is the atomic-collision cascade, which is the whole group of the knockedout displaced atoms. The process of atomic collisions continues until the full stop of the PKA and displaced atoms. An average geometric size of an individual damaged microregion depends significantly on the mass and energy of PKA as well as on the chemical composition of a target. The ACCs generated by primary knocked atoms in investigated materials can be calculated using “The Stopping and Range of Ions in Matter” program (SRIM_2003.19) [5]. The mean PKA energy upon irradiation with
ЕPKA ¼ 4/М,
(2)
where energy EPKA is in MeV and М is the mass number of the target atom. It should be noted that these calculations are related to a dynamic stage of the cascade (~1012 to 1011 s from the onset of the PKA motion); at the stage of relaxation the topography of the ensemble of defects experiences substantial changes. Nevertheless, these calculations allow one to estimate the scale of disturbances produced by PKAs. The following processes of transformation of irradiated materials can occur at the stage of relaxation of formed radiation defects: some of the atoms, which have appeared at interstices of the crystal lattice recombine with vacancies, some form dislocations, some diffuse into drains (grain boundaries, etc.). In ordered crystalline substances anti-site substitution can take place, which leads to the formation of disordered microregions with chaotic distribution of atoms along the sites of the crystal lattice. Complexes of radiation defects (for example, anti-site substitution and a nearby vacancy) can lead to a substantial chaotic dislocation of atoms from lattice sites and loss of translational symmetry, it means a neutron amorphization. The effect of amorphization takes place only if concentration of defects in the CAC region achieves a certain critical value. The
* Corresponding author. M.N. Mikheev Institute of Metal Physics, Ural Division of Russian Academy of Science, 620108 Ekaterinburg, Russia. E-mail address: [email protected] (A.N. Pirogov). https://doi.org/10.1016/j.physb.2017.11.078 Received 30 August 2017; Received in revised form 22 November 2017; Accepted 27 November 2017 Available online xxxx 0921-4526/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Lee, et al., Structure and properties of various fast neutron irradiated magnets, Physica B: Physics of Condensed Matter (2017), https://doi.org/10.1016/j.physb.2017.11.078
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BiFe0.95Mn0.05O3 and Bi0.85La0.15FeO3 multiferroics, LiMn2O4 and Li0.9FePO4 oxides.
structure of amorphous materials formed under neutron irradiation differs significantly from the structure of amorphous metals and compounds obtained by quenching from a melt, plasma spraying, etc., which is similar to a close random packing of atoms. In case of neutron amorphization, the radiation defects are represented by the substantial (on the order of 10% of the lattice parameter) displacement of atoms. In the initial state (before irradiation), the atoms of a certain type occupy definitive lattice crystallographic positions in an ordered crystal. Neutron disordering means the formation of microscopic regions, in which the statistical distribution of atoms is realized along the positions of crystal lattice [6]. A structural motive of a crystal is conserved. With such a structural change, it is possible to construct a new atomic environment resulting in new magnetic phase transitions. For example, a transition from an antiferromagnetic structure to a ferromagnetic one [7] or a magnetic ordering occurs in irradiated sample, which was nonmagnetic before neutron irradiation [8,9]. In this paper, we demonstrate the effects of fast neutron irradiation on the properties of several magnetic materials such as Nd2Fe14B and Er2Fe14B intermetallic compounds, BiFe0.95Mn0.05O3 and Bi0.85La0.15FeO3 multiferroics, LiMn2O4, and Li0.9FePO4 oxides. Permanent magnets are produced basing on Nd2Fe14B phase, which has a record high value of magnetic energy product (BH)max > 400 kJ/m3 [10]. However, this value is lower than the theoretical limit (520 kJ/m3). An essential increase of (BH)max can be attained by synthesizing Nd2Fe14B magnets from amorphous state using a subsequent annealing. According to [11], re-crystallization of amorphous sample allows to increase the coercivity from 0.65 T up to 2.9 T. Thus, it is interesting to achieve an amorphous state and to study its magnetic properties. Multiferroic materials have attracted enormous research interest in recent years [12] due to their potential applications and marvelous physical properties. Among them BiFeO3 is the most studied because it exhibits multiferroic properties up to 600 K. However, it is difficult to synthesize a single phase BiFeO3 polycrystalline sample because of the formation of two concomitant phases: Bi25FeO40 and Bi2Fe4O9. To overcome this difficulty, authors of [2] irradiated multiphase BiFeO3 sample with swift heavy Ag ions. They found that the intensities of reflections, originated from Bi25FeO40 and Bi2Fe4O9 phases, disappeared on X-ray diffraction (XRD) patterns at the fluence of 1 1012 ions/cm2. It is known that multi-charged ions and X-ray radiation do not penetrate deep into the material. Therefore, using Ag ions and X-ray diffraction allows obtaining information only on the phase contents in the surface layer of a particle. To get the data on the phase changes in the volume of the sample, neutron irradiation and diffraction are needed. Cathode materials based on LiMn2O4 and LiFePO4 oxides are ones of the most promising materials in lithium-ion batteries [13]. They have a high charge density and become attractive energy-storage systems for portable electronic devices. The LiMn2O4 is also of interest because it is an example of a geometrically frustrated antiferromagnet. Besides, in the LiMn2O4 a charge ordering (Mn3þ-Mn4þ), the Jahn-Teller distortions [14] and a long-range antiferromagnetic ordering are observed at low temperatures. It is interesting to investigate the interrelation between the crystal structure and magnetic ordering in this manganite in both the unirradiated (ordered) and irradiated (disordered) states. In this case, the structural disordering means the redistribution of lithium and manganese cations over octahedral and tetrahedral positions of the spinel lattice. In LiMn2O4 and LiFePO4, the oxygen plays a critical role as it takes place in strongly correlated transition metal oxides. The magnetism in LiFePO4 oxide is quite uncommon because of the lack of the conventional mechanism such as Mott-Habbard, Charge-transfer and Slater effect are applicable to explain the transition to antiferromagnetic ordering [15]. Therefore, for the theory development it is important to get information about the behavior of Fe–O bonds under external influence, for example, neutron irradiation. Therefore, in present paper we try to answer above questions about the effects of fast neutron irradiation on the properties of several magnets such as Nd2Fe14B and Er2Fe14B intermetallic compounds,
2. Material and methods Nd2Fe14B and Er2Fe14B intermetallic compounds were melted in an induction furnace and have been treated via rapid quenching [16]. Bi0.85La0.15FeO3, LiMn2O4 [17], and Li0.9FePO4 were synthesized by a solid state reaction. BiFe0.95Mn0.05O3 powder was synthesized by the citrate-nitrate method. All the samples have been irradiated with fast neutrons (Eeff > 0.1 MeV) at 340 K in the water pool of the IVV-2M reactor (Zarechny, Russia). In an active zone center the neutron flux was 1 1014 cm2 s1. The mean neutron energy was about 2 MeV. The samples were irradiated in the fluence range from Φ ¼ 1 1018 n/cm2 to Φ ¼ 2 1020 n/cm2, that conforms with the irradiation time from 3 h to 23 days. Magnetic measurements have been performed under magnetic fields up to μ0H ¼ 2 T at 293 K using a vibrating sample magnetometer. X-ray diffraction patterns have been recorded with diffractometer DRON-1UM at room temperature. We used Cu Kα radiation. Neutron diffraction experiment has been carried with D2 and D3 diffractometers mounted at horizontal channels of the IVV-2M reactor. Incident neutron beams with the wavelengths of 1.805 Å (D2 diffractometer) and 2.429 Å (D3 diffractometer) were used. The neutron powder diffraction (NPD) patterns were calculated by means of the Fullprof software [18]. 3. Results and discussion Table 1 presents the average energy E and the mean free path L of primary knocked-on atoms in samples under investigation at the fast neutron irradiation. The large difference in values L for light (for example, Li) and heavy (such as Bi) ions is observed. Light ions lose their energy E due to electron ionizations. A heavy ion is absorbed in a crystal causing a disorder. Therefore, mainly the Nd- (and Er-) ions disorder crystals in Nd2Fe14B (Er2Fe14B), whereas the Bi-ions destroys an order in BiFe0.95Mn0.05O3 and Bi0.85La0.15FeO3, the energy of the Mn- and Fe-ions are expended to disorder a crystal in LiMn2O4 and Li0.9FePO4, respectively. We may assume that the effect of neutron irradiation on these samples is large. Fig. 1 shows room temperature NPD patterns of Er2Fe14B intermetallic compound at initial (before irradiation) and irradiated states [16]. Room temperature NPD pattern of Nd2Fe14B differs from the one presented for Er2Fe14B only by intensities of reflections. At an initial state, both samples possess the tetragonal Nd2Fe14B type structure (space group P42/mnm) as the main phase and contain about 7% and 2% of the additional α-Fe phase in Nd2Fe14B and Er2Fe14B, respectively. The magnetic structure of Nd2Fe14B is a collinear ferromagnet with the propagation vector k ¼ 0 and Curie temperature TC ¼ 585 K. The Er2Fe14B compound is ferrimagnet and, as can be seen from the insert on Fig. 1a, its TC ¼ 560 K. As example of refined parameters, we present Table 2 in which crystal and magnetic structure parameters of the
Table 1 Average energy E and mean free path L of primary knocked-on atoms (PKA) for irradiated materials. PKA
Li
E, keV
576
L,Å
2
B
O
P
565
250
125
Nd2Fe14B Er2Fe14B BiFe0.95Mn0.05O3 Bi0.85La0.15FeO3 LiMn2O4 Li0.9FePO4
Mn
Fe
La
Nd
Er
Bi
73
70
29
27
24
19
Nd – 85; Fe – 270; B – 6570 Er – 79; Fe – 273; B – 6659 Bi – 137; Fe – 551; Mn – 592; O – 5131 Bi – 47; La – 190; Fe – 550; O – 5140 Li – 20500; Mn – 570; O – 5610 Li – 23900; Fe – 670; P – 1940; O – 6570
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unirradiated and irradiated states are close to each other which allows to conclude the retention of ferrimagnet character of Er- and Fe-ion moment ordering in the amorphous sample, though the Er- and Fe-sublattices are absent in the crystallographic sense. In the amorphous state, the Nd2Fe14B shows ferromagnetic behavior. But in the irradiated state the TC 450 K, which is by about 135 K lower comparing to the Curie temperature of the crystalline sample. Although magnetization in the fresh and irradiated samples is almost the same, the coercivity decreases noticeably in the amorphous state. The low coercivity in irradiated Nd2Fe14B allows orienting rare-earth- and 3d-transition magnetic moments by external field in the same direction that it is important in stage of re-crystallization of amorphous sample to achieve a high value of (BH)max. The BiFe0.95Mn0.05O3 ferrite was irradiated by fast neutrons up to fluence Φ ¼ 4.6 1019 n/cm2. Fig. 2 presents observed and calculated XRD patterns of this ferrite in initial and irradiated states. The fresh sample contents beside the main BiFe0.95Mn0.05O3 (88%) phase quite big amount of impurity Bi25FeO40 (5%) and Bi2Fe4O9 (7%) phases. Diffraction patterns testify to a distinct decrease of reflection intensity attributed to the impurity phases after neutron irradiation. In the irradiated sample, the content gets 1% and 5% for Bi25FeO40 and Bi2Fe4O9 phases, respectively. Calculations of X-ray and neutron diffraction patterns show that the irradiation leads to the decrease of distortions of the Fe/MnO6 octahedrons due to the equalizing of bond lengths d(Fe/Mn–O). In the initial state two main lengths d(Fe/Mn–O) and angle φ(Fe/Mn–O–Fe/ Mn) were equal to d1(Fe/Mn–O) ¼ 2.125(2) Å, d2(O–Fe/ Mn) ¼ 1.932(2) Å, and φ(Fe/Mn–O–Fe/Mn) ¼ 154.7(1) . After irradiation these parameters are equal to d1(Fe/Mn–O) ¼ 2.090(6) Å, d2(O–Fe/Mn) ¼ 1.963(4) Å, and φ(Fe/Mn–O–Fe/Mn) ¼ 156.1(1) . Calculation of NPD patterns of both sample results in that in range of experimental error antiferromagnetic structure is kept after irradiation; magnetic moment values are equal to 3.6(1) μB in fresh and irradiated samples. However, magnetization curves of unirradiated and irradiated samples are quite different. They are given on inserts in Fig. 2a and b. It can be seen that the magnetization of the fresh sample rises linearly with
Fig. 1. a) Room temperature observed (points) and calculated (line) NPD patterns of Er2Fe14B before irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. The first, second and third rows of tick marks indicate the angle positions of nuclear and magnetic reflections of Er2Fe14B, and nuclear peaks of α-Fe phase, correspondently. Temperature dependence of the susceptibility is shown on the insert. b) Room temperature observed NPD pattern of Er2Fe14B in amorphous state (after irradiation). Temperature dependence of the magnetization at μ0H ¼ 5 mT is given on the insert.
unirradiated Er2Fe14B sample are shown. They are in a good agreement with previous study [10]. The NPD pattern of the irradiated (fluence of Φ ¼ 1.2 1020 n/cm2) Er2Fe14B sample (shown in Fig. 1b) includes narrow half-width peaks at angles 2θ ¼ 53 , 78 and 101 and wide maximum at 2θ ¼ 53 . The narrow peaks are originated from neutron scattering on the α-Fe phase, and the wide maximum is an evidence of existence of the amorphous state in the main phase. In amorphous state, the Er2Fe14B exhibits properties typical for ferrimagnets and spin glass systems. Its TC decreases down to 314(2) K, which is 244 K lower comparing to the crystalline samples (see insets on Fig. 1b). At 5 K, the magnetizations of Table 2 Refined lattice constants a, b, c, unit cell volume V, coordinates of positions (space group P42/mnm), the average Er- and Fe-ion magnetic moments μEr(μB), μFe(μB) in unirradiated sample, contents of the Er2Fe14B and α-Fe phases at room temperature, agreement factor RBragg and χ2. Structural parameter
Er2Fe14B
a, b (Å) c (Å) V (Å3) Er, 4f: x Er, 4g: x Fe, 4e: z Fe, 8j1: x z Fe, 8j2: x z Fe, 16k1: x y z Fe, 16k2: x y z B, 4g: x μEr (μB) μFe (μB) Er2Fe14B (mass%) α-Fe (mass%) RBragg (%) χ2(%)
8.744(1) 11.968(2) 914.3(5) 0.273(1) 0.147(1) 0.112(1) 0.097(1) 0.201(1) 0.318(1) 0.249(1) 0.222(1) 0.567(1) 0.127(1) 0.036(1) 0.360(1) 0.170(1) 0.636(2) 4.1(1) 1.9(1) 98.0(2) 2.0(2) 5.53 4.46
Fig. 2. a) Room temperature observed (points) and calculated (line) XRD patterns of BiFe0.95Mn0.05O3 before irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. The first, second and third rows of tick marks indicate the angle positions of reflections of BiFe0.95Mn0.05O3, Bi25FeO40 and Bi2Fe4O9 phases, correspondently. Magnetization curve measured at T ¼ 293 K is shown on the insert. b) Room temperature observed (points) and calculated (line) XRD patterns of BiFe0.95Mn0.05O3 after irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. The first, second and third rows of tick marks indicate the angle positions of reflections of BiFe0.95Mn0.05O3, Bi25FeO40 and Bi2Fe4O9 phases, correspondently. Magnetization curve measured at T ¼ 293 K is shown on the insert. 3
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increasing external field pointing to antiferromagnetic structure. In the irradiated state, the magnetization increases nonlinearly and its value at μ0H ¼ 2 T is three times bigger than that in the initial state. Therefore, an external magnetic field induces transformations from antiferromagnetic structure to a state like ferromagnetic ordering. Further increase of neutron fluence induces formation of iron oxide phases in the sample. The observed oxides are clearly seen on Fig. 3, which shows NPD patterns of Bi0.85La0.15FeO3 multiferroic in before and after irradiated states. In the initial state, the sample contains the single phase. The irradiation of the sample by the fluence Φ ¼ 5 1020 n/cm2 leads to the formation of Fe3O4 and Bi2O3 oxides. Their concentrations are 11% and 10%, respectively. Crystal structures of the sample in the initial state and the main phase of irradiated sample are described by rhombohedral unit cell (space group R3c). The irradiation induces the expansion of the unit cell of the main phase up to ΔV ¼ 1.1% which can be explained by accumulation of interstitial ions. The observed magnetic structure is antiferromagnetic. The magnetic moment of Fe-ion has components parallel to a- and c-axes, which are 3.3(1) μB and 1.7(1) μB. In both samples the total moment of Fe-ions is equal to 3.7(1) μB. In contrast to Nd2Fe14B and Er2Fe14B, the LiMn2O4 oxide did not experience amorphization even under the fluence Φ ¼ 2 1020 n/cm2. Fig. 4 shows the NPD patterns of LiMn2O4 at initial and irradiated states at 5 K [16]. NPD pattern of the initial state indicates the orthorhombic structure (space group Fddd) with lattice parameters a ¼ 24.663(4) Å, b ¼ 24.891(7) Å, and c ¼ 8.271(3) Å. There are superstructure reflections, the biggest of them is the (4/3 8/3 2) peak located at 2θ 63 and originated from the charge ordering (Mn3þ and Mn4þ). Reflections located on left part of NPD pattern indicate the incommensurate magnetic structure with vector k ¼ 2π/c(0, 0, 0.44). The obtained by us parameters are in good accordance with presented in literature [18,19]. As seen in Fig. 4, there are drastic changes in the shape of NPD pattern after neutron irradiation. There is no (4/3 8/3 2) peak, therefore the charge ordering is demolished. The sample crystallizes in two phases such as cubic (Fd-3m) and tetragonal (I41/amd) structures. Latter structure has lattice periods a ¼ 5.754(1) Å and c ¼ 9.461(1) Å. The irradiation stimulates a considerable redistribution of Mn ions to nonequivalent positions of the cubic structure. Manganese ions (about 17%) moved to non-typical for them 8a positions and the corresponding amount of Li ions occupied the 16d cites. The observed ion redistribution
Fig. 4. a) The 5 K observed (points) and calculated (line) NPD patterns of LiMn2O4 before irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. The tick marks indicate the angle positions of nuclear and magnetic reflections of LiMn2O4. Magnetization curve measured at T ¼ 293 K is shown in the insert. b) The 5 K observed (points) and calculated (line) NPD patterns of LiMn2O4 after irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. The first, second, third and four rows of tick marks indicate the angle positions of nuclear and magnetic reflections of cubic phase LiMn2O4, and nuclear and magnetic peaks of tetragonal phase LiMn2O4, correspondently. Magnetization curve measured at T ¼ 293 K is shown in the insert.
is accompanied by a radical change of the magnetic state of the sample. The incommensurate magnetic structure is transformed to the ferrimagnetic commensurate ordering with propagation vector k ¼ 0 and TC ¼ 80 K. For the LiMn2O4 phase, at 5 K the magnetizations of tetrahedral and octahedral sublattices are equal to 1.0(1) μB and 2.0(1) μB, respectively. Therefore, the total magnetic moment is equal to 1.0(1) μB/ f.u.. As can be seen from inserts in Fig. 4a and b, the magnetization curves of fresh and irradiated samples are noticeably differing. The magnetization curve of the fresh sample is linear, which confirms the neutron diffraction data on the antiferromagnetic ordering. After irradiation, the magnetization increases significantly and its field dependence becomes essentially nonlinear in the region of magnetic fields up to μ0H ~0.5 T. A value of the spontaneous magnetization was determined by means of
Fig. 3. a) Room temperature observed (points) and calculated (line) NPD patterns of Bi0.85La0.15O3 before irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. The first and second rows of tick marks indicate the angle positions of nuclear and magnetic reflections of Bi0.85La0.15O3, correspondently. b) Room temperature observed (points) and calculated (line) NPD patterns of Bi0.85La0.15O3 after irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. The first, second, third and four rows of tick marks indicate the angle positions of nuclear and magnetic reflections of Bi0.85La0.15O3, and nuclear peaks of Fe3O4 and Bi2O3, correspondently.
Fig. 5. a) Room temperature observed (points) and calculated (line) NPD patterns of Li0.9FePO4 before irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. b) Room temperature observed (points) and calculated (line) NPD patterns of Li0.9FePO4 after irradiation. The difference between calculated and observed intensities is shown at the bottom as solid black line. 4
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extrapolation from high-field region to zero field and equaled to 0.90(5) μB/f.u., being in a good agreement with neutron diffraction data. The large susceptibility of paraprocess should also be noted, as it often points to the noncollinear magnetic structures. An effect of neutron irradiation on the lattice parameters becomes apparent distinctly in case of Li0.9FePO4 multiferroic. Fig. 5 shows the NPD patterns of Li0.9FePO4 before and after irradiated (by fluence Φ ¼ 1 1018 n/cm2) samples. As seen in Fig. 5, the irradiation does not entail obvious change on NPD patterns. Crystal structures of both samples are described by the space group Pnma with close values of coordinate and occupation parameters. However, the irradiation causes the increase of lattice parameters. As can be seen from the inserts in Fig. 5, the a, b and c parameters are increased by ~0.01, 0.03 and 0.02 Å, respectively. Therefore, the neutron irradiation indices are due to increasing of bond Fe–O distances.
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4. Conclusion Intermetallic compounds (Nd2Fe14B and Er2Fe14B), multiferroics (BiFe0.95Mn0.05O3 and Bi0.85La0.15FeO3) and oxides (LiMn2O4 and Li0.9FePO4) have been irradiated by fast neutrons (Eeff > 0.1 MeV). Magnetic measurements and neutron diffraction experiment were carried out on unirradiated and irradiated samples. The Nd2Fe14B and Er2Fe14B intermetallic undergo a transformation from crystalline state to amorphous state after the neutron irradiation with the fluence Φ ¼ 1.2 1020 n/cm2. The value of TC in amorphous state is considerably lower than that in the crystalline state. The neutron irradiation of the BiFe0.95Mn0.05O3 sample by the fluence Φ ¼ 4.6 1019 n/cm2 suppresses the impurity phases Bi25FeO40 and Bi2Fe4O9. The irradiation stimulates the transformation from antiferromagnetic structure to the magnetic state with nonzero spontaneous magnetization. A rise of fluence up to Φ ¼ 5 1020 n/cm2 is accompanied by partial decomposition of the Bi0.85La0.15FeO3 samples. The fast neutron irradiation of the LiMn2O4 stimulates the transformation from antiferromagnetic incommensurate structure to the magnetic commensurate ordering with the spontaneous magnetization. Acknowledgments This research was carried out at IMP Neutron Material Science Complex within the state assignment of FASO of Russia (theme “Flux” No. 01201463334) and supported in part by the State contract (No. 3.6121.2017/8.9) between Ural Federal University and the Ministry of Education and Science of Russian Federation and also supported in part by the project CICECO-Aveiro Institute of Materials (POCI-01-0145FEDER-007679, FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and, when appropriate, co-financed by FEDER under the PT2020 Partnership Agreement.
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