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Magnetic characteristics of polymorphic single crystal compounds DyIr2Si2
Physica B xxx (xxxx) xxx–xxx
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Magnetic characteristics of polymorphic single crystal compounds DyIr2Si2 ⁎
⁎
Kiyoharu Uchimaa, , Toru Shigeokab, , Yoshiya Uwatokoc a b c
General Education, Okinawa Christian Junior College, Nishihara, Okinawa 903-0207, Japan Graduate School of Science and Engineering, Yamaguchi University, Yamaguchi 753-8512, Japan Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
A R T I C L E I N F O
A BS T RAC T
Keywords: DyIr2Si2 Polymorphic compound Antiferromagnetism Metamagnetism Magnetic anisotropy Crystal field effects
We have confirmed that the tetragonal ternary compound DyIr2Si2 shows polymorphism; the ThCr2Si2-type structure as a low temperature phase (I-phase) and the CaBe2Ge2-type one as a high temperature phase (Pphase) exist. A comparative study on magnetic characteristics of the morphs was performed on the I- and Pphase single crystals in order to elucidate how magnetic properties are influenced by crystallographic symmetry. The magnetic behavior changes drastically depending on the structure. The DyIr2Si2(I) shows an antiferromagnetic ordering below TN = 30 K, additional magnetic transitions of T1 = 17 K and T2 = 10 K, and a strong uniaxial magnetic anisotropy with the easy [001] direction. The [001] magnetization shows four metamagnetic transitions at low temperatures. On the other hand, the DyIr2Si2(P) has comparatively low ordering temperature of TN1 = 9.4 K and an additional transition temperature of TN2 = 3.0 K, and exhibits an easy-plane magnetic anisotropy with the easy [110] direction. Two metamagnetic transitions appear in the basal plane magnetization processes. In both the morphs, the χ-T behavior suggests the existence of component-separated magnetic transitions. The ab-component of magnetic moments orders at the higher transition temperature TN1 for the Pphase compound, which is contrast to the I-phase behavior; the c-component orders firstly at TN. The crystalline electric field (CEF) analysis was made, and the difference in magnetic behaviors between both the morphs is explained by the CEF effects.
1. Introduction The tetragonal ternary compounds RIr2Si2 (R = rare earth = rare earth) exhibit polymorphism; they have two different crystallographic structures: the ThCr2Si2-type structure (I4/mmm) of low temperature phase and the CaBe2Ge2-type one (P4/nmm) of high temperature phase [1], which is referred to as the I-phase and P-phase (I and P are quoted from the symbol of corresponding space group.), respectively. The crystal structure of the I-phase has centrosymmetry, while one of the P-phase is non-centrosymmetry. So this RIr2Si2 family is suitable to study how physical properties are influenced by crystallographic symmetry without changing chemical composition. Bazela presented the correlation between the crystal structure and magnetic ordering, and asserted a strong dependence of magnetic ordering on the a/c ratio [2]. The comparative studies with magnetic behaviors of two polymorphs on single crystal compounds are interesting. Comparative studies with physical properties of polymorphs have been performed on some compounds of this family. The high temperature phase (Pphase) compounds of YIr2Si2 and LaIr2Si2 become superconducting at 2.52 K and 1.24 K, respectively, while the low-temperature ones (I-
⁎
phase) are normal down to 1 K [3]. Both CeIr2Si2 polymorphs remain paramagnetic. The I-phase compound behaves as a Fermi-liquid, whereas the P-phase one exhibits non-Fermi-liquid features [4]. In PrIr2Si2, the P-phase compound remains paramagnetic down to 2 K whereas the I-phase one exhibits an antiferromagnetic ordering below 45.5 K. The different types of magnetic anisotropy, an easy axis anisotropy in the I-phase compound and an easy plane one in the Pphase one, has been reported [5]. Magnetism in NdIr2Si2 has been studied only for the I-phase compound. It shows a collinear antiferromagnetic ordering below 32.3 K with Nd magnetic moments along the c-axis [6]. For RIr2Si2 (R = heavy rare earth = heavy rare earth) series, there are a few reports on I-phase polycrystalline compounds; TbIr2Si2, DyIr2Si2 and ErIr2Si2 order antiferromagnetically below 80 K (or 75 K), 40 K and 10 K, respectively [7–10]. We have already reported the magnetic properties of TbIr2Si2 single crystals; The TbIr2Si2(I) shows an antiferromagnetic ordering below TN = 80 K, a strong uniaxial magnetic anisotropy with the easy [001] direction, and isotropic magnetic behavior in the basal plane. On the other hand, the TbIr2Si2 (P) has the comparatively low ordering temperature of TN1 = 11.5 K and an additional transition temperature of TN2 = 5 K, and
Corresponding authors. E-mail addresses: [email protected] (K. Uchima), [email protected] (T. Shigeoka).
http://dx.doi.org/10.1016/j.physb.2017.09.038 Received 27 June 2017; Received in revised form 12 September 2017; Accepted 13 September 2017 0921-4526/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Uchima, K., Physica B (2017), http://dx.doi.org/10.1016/j.physb.2017.09.038
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exhibits an easy-plane magnetic anisotropy with the easy [100] direction. The crystalline electric field effects estimated in the nonKramers TbIr2Si2 compound well explain the magnetic anisotropy [11]. The magnetic behavior of Kramers DyIr2Si2 is also interesting. We succeeded to grow both DyIr2Si2(I) and (P) single crystals, and performed magnetic measurements on the single crystals. In this paper, we present comparative magnetic behaviors and the crystalline electric field (CEF) analyses on both the morphs. 2. Experimental procedure Single-crystals DyIr2Si2 were grown by the Czochralski technique using a tri-arc furnace; that is, polycrystalline samples were prepared by arc-melting a stoichiometric mixture of the pure elements (Dy: purity of 3N, Ir: 3N, and Si: 5N) in an argon atmosphere. Subsequently, a single crystal was grown by the pulling-up method in the tri-arc furnace. The grown crystal was confirmed to be the P-phase and the single phase nature from analysis of X-ray powder diffraction pattern. A single crystal of the I-phase was obtained by annealing a part of the grown single crystal at 800 °C for 7 days. The crystal structure and the single phase nature of the annealed crystal were also confirmed by Xray powder diffraction. The quality of the single crystals was checked, and crystallographic orientations were determined by the back Laue method. The crystal was fixed on a thin plastic plate so that the desired direction is perpendicular to the plate within an experimental angle accuracy of one degree. It was subjected to magnetic measurements. The magnetic susceptibility (which is determined from M/B under B = 0.1 or 0.01 T where magnetization curves are linear.) and low-field magnetization below 5 T and/or 7 T were measured using a MPMS and/or PPMS (Quantum Design). Pulsed high-field magnetization measurements were performed up to 57 T by a pick-up coil method using a non-destructive long-pulse magnet installed at the International MegaGauss Science Laboratory in the Institute for Solid State Physics, the University of Tokyo.
Fig. 2. X-ray diffraction pattern on a DyIr2Si2(P) powder compound at a room temperature. Table 1 Crystallographic data of both the DyIr2Si2 morphs. DyIr2Si2
a (nm)
c (nm)
c/a
I P
0.405 0.410
0.975 0.967
2.41 2.36
[10]. The c/a ratio of the I-phase compound is 1.02 times as large as that of the P-phase one; The I-phase compound, low temperature phase one, elongates along the c-axis comparing with the P-phase one, high temperature phase. Bazela presented the correlation between the crystal structure and magnetic ordering, and asserted a strong dependence of magnetic ordering on the c/a ratio [2]. We can expect that this difference make difference of magnetic anisotropy between two morphs, and change magnetic ordering. 3.2. Magnetic susceptibility
3. Results and discussion
The temperature dependences of magnetic susceptibilities along the main symmetry axes in the tetragonal cell for the I- and P-phase compounds are shown in Figs. 3 and 4, respectively. A precise magnetic anisotropy between the c-axis and directions in the basal plane is evidenced from the figures while an anisotropy within directions in the basal plane is very small. In Fig. 3 (the I-phase compound), the [001] susceptibility shows three anomalies at TN = 30 K, T1 = 17 K and T2 =
3.1. Lattice parameter X-ray diffraction measurements were done to determine to which phase crystals belong. Figs. 1 and 2 show powder diffraction patterns on DyIr2Si2(I) and (P) compounds, respectively at a room temperature. All reflections in Figs. 1 and 2 could be indexed on the tetragonal ThCr2Si2- and the CaBe2Ge2-type structures, respectively, confirming the single phase natures. Then we were able to obtain crystallographic data of both the compounds by least square analysis to the powder pattern, which are shown in the Table 1. The lattice parameters of the I-phase compound are almost in agreement with the previous report
Fig. 3. Temperature dependences of magnetic susceptibilities along the main symmetry axes of a tetragonal cell on a DyIr2Si2(I) single crystal. Broken lines show calculated values using the CEF parameters. The inset is the level scheme deduced from CEF analysis.
Fig. 1. X-ray (Cu-Kα) diffraction pattern on a DyIr2Si2(I) powder compound at a room temperature.
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Fig. 4. Temperature dependences of magnetic susceptibilities along the main symmetry axes of a tetragonal cell on a DyIr2Si2(P) single crystal. Broken lines show calculated values using the CEF parameters. The inset is the level scheme deduced from CEF analysis.
Fig. 5. Magnetization curves along the main symmetry axes at 2 K on the DyIr2Si2(I) single crystal. The inset stands the dM/dB curve on ascending process.
netic Curie temperatures between the c-axis and the c-plane Δθ = θ001 − θab (θab = (θ100 + θ110)/2) is 99.5 K and −8.6 K in the I-phase and Pphase compounds, respectively. These results indicate that magnetic anisotropy and magnetic interactions of the I-phase compound are much larger than those of the P-phase one.
10 K, which is confirmed from specific heat measurements. The temperature of TN is lower than that reported from previous studies, and there is no information on T1 and T2 [10,12]. No anomaly is observed in the basal plane. The transition of TN is an antiferromagnetic ordering one, which is evidenced from the magnetization process mentioned below. This result suggests an appearance of so called “a component-separated magnetic transition”; the c-component of Dy magnetic moments orders independently at TN and the ab-component remains in a paramagnetic state [13,14], and an existence of frustration is expected in this compound. In Fig. 4 (the P-phase compound), two clear cusps are seen at TN1 = 9.4 K and TN2 = 3.0 K along directions in the basal plane whereas one cusp is only seen at TN2 = 3.0 K along the [001] direction. This result also suggests “successive component-separated magnetic transitions”. Here, the ab- and c-component of magnetic moments order independently at TN1 and TN2, respectively. The different types of componentseparated magnetic transitions between the I- and P-phase compounds appear; the c-component of magnetic moments orders firstly in the Iphase compound whereas the ab-component orders firstly at higher transition temperature TN1 in the P-phase one. The ordering temperature of the I-phase compound is considerably higher than that of the Pphase one; it of the I-phase one is 3 times as large as that of the P-phase one, indicating a stronger antiferromagnetic interaction in the I-phase one. The magnetic susceptibilities along the main symmetry directions obey the Curie-Weiss law for the paramagnetic temperature range on both the compounds; the χ−1-T curves become linear for high temperatures. The estimated effective magnetic moments μeff are listed in the Table 2 with other magnetic data. The effective magnetic moments are almost in agreement with the Dy3+ free ion magnetic moment (10.64 μB). This shows that 4f electrons in DyIr2Si2 are well localized and Ir ion is nonmagnetic. In the I-phase compound, paramagnetic Curie temperature along each direction θ001, θ100 and θ110 is 49.7, −43.5 and −56.2 K, respectively. In the P-phase compound, θ001 = −3.0 K, θ100 = 5.3 K and θ110 = 5.9 K which are considerably small compared with those in the I-phase one. The difference in paramag-
3.3. Magnetization process Magnetization curves of both the compounds are shown for low and high temperatures in Figs. 5–8. For the I-phase one (Figs. 5 and 7), it is evidenced that the hard magnetization direction is in the basal plane. Magnetization is almost isotropic within the basal plane, and increases monotonically. There is no transition in the hard magnetization processes. On the other hand, in the easy c-axis process at low temperature (the [001] magnetization process in the Fig. 5), four transitions appear; two slight increases in magnetization at Bc1 = 1.4 T and Bc2 = 1.8 T, and two rapid and large increases at Bc3 = 2.5 T and Bc4 = 5.4 T are seen. The critical field is determined as a peak magnetic field in the dM/dB vs. B curve (the inset of Fig. 5). Hysteresis loops with width of 0.15 T– 0.2 T are observed around Bc3 and Bc4, indicating that these transitions are of the first order. The magnetization saturates above Bc4 at Ms = 9.6 μB/f.u., which almost agrees with the Dy3+ free ion moment (gJ = 10 μB) and that reported from neutron study [10]. The change in magnetization on Bc3, ΔM1, is corresponding to about (6/25) Ms, and accords with the change ΔM2 on Bc4; ΔM2 = (6/25) Ms. The magnetization increases gradually for intermediate magnetic fields of Bc1–Bc3 and Bc3–Bc4. These results suggest that field-induced magnetic phases are long-period incommensurate ones while the simple antiferromagnetic structure, AF-I type with magnetic moments along the c-axis has been reported at B = 0 [10]. From the rapid change with the simple fraction on Bc3 and Bc4, we can speculate the transitions occur by spin-flip; there are 50 Dy magnetic moments in a magnetic unit cell (the magnetic unit cell may be a × a × 25c), and 3 of 50 spins turn over from the opposite direction of magnetic field to the magnetic field one along the c-axis. The field-induced magnetic structures and mechanism of this magnetization process are unknown yet. These metamagnetic transitions are smeared with increasing
Table 2 Magnetic data of both the DyIr2Si2 polymorphs. DyIr2Si2
Tt (K)
θ001 (K)
θ□00 (K)
θ110 (K)
Δθ (Κ)
μeff (μB)
Ms (μB)
Easy axis
I P
30, 17, 9 9.4, 3.0
49.7 −3.0
−43.5 5.3
−56.2 5.9
99.5 −8.6
10.35 10.68
9.6 10.0
[001] [110]
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temperature. The process becomes a two-step metamagnetic one around 10 K, and the metamagnetic transitions disappear above TN; it becomes a paramagnetic one (Fig. 7). High field magnetization processes of the P-phase compound under long-pulsed magnetic fields up to 57 T are shown in Figs. 6 and 8. The easy magnetization direction is the [110] direction in the basal plane. The magnetization increases with a considerably large gradient up to about 3 T, and with a smaller gradient above 3 T. It reaches 10 μB/f.u. (the Dy3+ free ion moment) at 57 T of the maximum field in this experiment. So we believe the magnetization saturates around 57 T. The inset in Fig. 6 shows low field magnetization processes measured by MPMS. Two metamagnetic transitions appear around Bb1 = 0.9 T and Bb2 = 3.4 T in the [110] magnetization process, indicated by arrows in the figure. In the [100] process, two metamagnetic transitions also appear around Ba1 = 1.2 T and Ba2 = 2.0 T. These metamagnetic transitions persist up to near TN for the [110] and [100] directions. In the [001] process, there is no metamagnetic transition. The gradual increase in magnetization on all the symmetry directions in high magnetic fields means that magnetic moments are rotating to the magnetic field direction. For the [100] and [001] directions, magnetization at maximum field is smaller than the saturation moment on the [110] direction. In order to estimate the saturation magnetization for the [100] and [001] directions, we extrapolated B to the infinity in M vs B−2 plot, and determined Ms. The obtained value is 9.8 μB/f.u. for both the directions, which is in agreement with the Dy3+ free ion moment. For high magnetic field region, the magnetization curve along the [001] direction is almost parallel to the [110] one, indicating a large magnetic anisotropy between the [001] and [110] directions. For the paramagnetic temperatures, magnetization curves along the main symmetry directions show no transition; the processes become paramagnetic ones (Fig. 8). The large magnetic anisotrpy still exists.
Fig. 6. Magnetization curves at 1.8 K on the DyIr2Si2(P) single crystal. The inset shows the low field magnetization curves.
3.4. Magnetic phase diagram The B[easy direction]-T magnetic phase diagrams were constructed from the magnetization processes at various temperatures and the temperature dependences of M/B at various magnetic fields for the Iand P-phase compounds, and are shown in Figs. 9 and 10, respectively. There are at least four ordered phases in B001-T phase diagram for the I phase compound. In low temperatures, an additional field-induced phase (indicated by I′) may exist. The phase I for low temperatures and low magnetic fields is the simple antiferromagnetic structure, so called the AF-I type, with magnetic moments along the c-axis as reported
Fig. 7. Magnetization curves at 34 K on the DyIr2Si2(I) single crystal. Broken lines show calculated magnetization using CEF parameters.
Fig. 9. B001-T magnetic phase diagram of the DyIr2Si2(I) single crystal. Open circles are critical fields determined from M(B) curves. Squares are transition temperatures determined from M(T).
Fig. 8. Magnetization curves at 12 K on the DyIr2Si2(P) single crystal. Broken lines show calculated magnetization using CEF parameters.
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487 K, which is comparative with that of the isostructural compound TbIr2Si2(I) [11]. On the other hand, for the P-phase compound, the first and second doublet excited state are at 48 K and 56 K above the ground state, respectively. The overall splitting is 306 K which is smaller than that of the-I phase one, and is also comparative to that of TbIr2Si2(P) [11]. The magnetic susceptibilities (Figs. 3 and 4) and magnetizations (Figs. 7 and 8) for the paramagnetic region are reproduced by the calculated values. The negative sign of B20 in the Iphase compound and positive sign in the P-phase one are consistent with the fact that the easy magnetization direction is the c-axis and the [110] direction in the basal plane, respectively. The deduced values of B20 and B40 for the I-phase compound are in good agreement with those evaluated from Mossbauer study [12]. The value of B20 is on the same order as that of TbIr2Si2(I) and is 3 times as large as the P-phase one. The value of B20 for the P-phase compound is on the same order as those of the isostructural compounds TbIr2Si2(P) and DyCu2Si2 [16] which have the same easy plane-type magnetic anisotropy, and is much smaller than that of the I-phase one. The deduced paramagnetic Curie temperature of the I-phase compound is five times as large as the Pphase one, which is corresponding to the higher ordering temperature of the I-phase one, indicating stronger molecular field interactions of the I-phase one than that of the P-phase one. The multi-step metamagnetic processes in both the ordered states are, of course, not explained by CEF effects only, and other effects such as competing exchange interactions should be required.
Fig. 10. B110-T magnetic phase diagram of the DyIr2Si2(P) single crystal. Open circles are critical fields determined from M(B) curves. Closed one is a transition temperature from M(T).
from the neutron study [10]. We can suggest from magnetization behavior in Fig. 5 that the field-induced phases II and IV are longperiod ones. With respect to the antiferromagnetic structure of the phase III, there is no idea now. In the B110-T phase diagram for the Pphase compound, there are three ordered phases corresponding to the metamagnetic transitions as shown in Fig. 10. The magnetic structures are unknown yet.
3.6. Summary We succeeded the single crystal growth of both the DyIr2Si2 polymorphic compounds; the ThCr2Si2-type structure as a low temperature phase (I-phase) and the CaBe2Ge2-type one as a high temperature phase (P-phase). A comparative study on magnetic characteristics of the morphs were performed on the I- and P-phase single crystals from measurements of magnetic susceptibility and magnetization in order to elucidate how magnetic properties are influenced by crystallographic symmetry. The magnetic behavior changes drastically depending on the structures. The magnetic anisotropy and magnetic interactions of DyIr2Si2(I) are much larger than that of DyIr2Si2(P). It should be noted that the χ-T behavior suggests the existence of “component-separated magnetic transitions” and the types of transitions are different from both the morphs. The abcomponent of magnetic moments orders at the higher transition temperature TN1 for the P-phase compound, which is contrast to the I-phase behavior; the c-component orders firstly at TN. The saturation magnetic moment and effective magnetic moment are in agreement with those of the Dy3+ free ion, which shows that 4f electrons in DyIr2Si2 are well localized and Ir ion is nonmagnetic. The DyIr2Si2(I) shows an antiferromagnetic ordering below TN = 30 K, additional magnetic transitions of T1 = 17 K and T2 = 10 K, and a strong uniaxial magnetic anisotropy with the easy [001] direction. The [001] magnetization shows four metamagnetic transitions at low temperatures. Field-induced long-period magnetic structures are suggested from the behavior in this process. On the other hand, the DyIr2Si2(P) has the comparatively low ordering temperature of TN1 = 9.4 K and an additional transition temperature of TN2 = 3.0 K, and exhibits an easy-plane magnetic anisotropy with the easy [110] direction. Two metamagnetic transitions appear in the basal plane magnetization
3.5. Crystalline electric field The crystalline electric field (CEF) analysis was performed in order to discuss the anisotropic magnetic behaviors on the I- and P-phase. For a site with tetragonal symmetry, the CEF Hamiltonian is given by
HCEF = B20 O20 +B40 O40 +B44 O44+B60 O60 +B64 O64 where Bnm and Onm are the CEF parameters and the Stevens operators, respectively. The total Hamiltonian, Htotal, including the interaction of rare earth moments with the molecular field is used.
Htotal = HCEF + gμB Ji (H + λ M) In this expression, Ji (I = x, y, z) is the component of the angular momentum, λ is the molecular field coefficient. In the calculation, the parameter θP, which is defined as θP = g2μB2λJ(J + 1)/3k, is used instead of λ [15]. The parameters were determined from the best fit of the calculated values to the experimental curves of M(B) at a paramagnetic temperature and χ(T) for the paramagnetic region using a least-squares method. The deduced parameters are listed in Table 3. The calculated magnetic susceptibilities and magnetizations are shown by broken lines in Figs. 3, 4, and 7, 8, respectively. The deduced CEF level schemes for the I- and P-phase compounds are shown in the insets of Figs. 3 and 4, respectively. The CEF level splits into Kramers doublets. For the I-phase compound (in the inset of Fig. 3), the first and second exited doublets exist at 13 K and 66 K above the doublet ground state, respectively, which almost agree with a previous report [12], leading a stable Dy magnetic moment. The overall splitting is
Table 3 Deduced CEF parameters and paramagnetic Curie temperature for both morphs of DyIr2Si2. DyIr2Si2
B20 (K)
B40 (10−3 K)
B44 (10−2 K)
B60 (10−5 K)
B64 (10−4 K)
θp (K)
I P
−2.387 0.791
3.440 2.133
−5.978 −0.974
1.536 2.920
3.011 −5.921
−8.800 −1.870
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processes. The crystalline electric field (CEF) analysis was made. The CEF level splits Kramers doublets. The overall splitting is 487 K and 306 K on I- and P-phase compounds, respectively, which is reasonable comparing with those of isostructural compounds. The negative sign of B20 in the I-phase compound and positive sign in the P-phase one are consistent with the fact that the easy magnetization direction is the caxis and the [110] direction in the basal plane, respectively. The B001-T and B110-T magnetic phase diagrams are obtained for the DyIr2Si2(I) and (P), respectively. There are some magnetic ordered phases whose magnetic structures are unknown yet. Further studies are required.
References [1] H.F. Braun, N. Engel, E. Parthe, Phys. Rev. B 28 (1983) 1389. [2] W. Bazela, J. Alloy. Compd. 442 (2007) 132. [3] L. Valiska, J. Pospisu, J. Prokleska, M. Divis, A. Ludajevova, V. Sechovsky, J. Phys. Soc. Jpn. 81 (2012) 104715. [4] M. Mihalik, M. Divis, V. Sechovsky, Physica B 404 (2009) 3191. [5] M. Mihalik, M. Divis, V. Sechovsky, N. Kozlova, J. Freudenberger, N. Stußer, A. Hoser, Phys. Rev. B 81 (2010) 174431. [6] M. Mihalik, I. Pospisil, A. Hoser, V. Sechovsky, Phys. Rev. B 83 (2011) 134414. [7] M. Slaski, J. Leciejewicz, A. Szytula, J. Magn. Magn. Mater. 39 (1983) 268. [8] A. Szytula, V. Ivanov, L. Vinokurova, Acta Phys. Pol. A 85 (1994) 293. [9] M. Hirjak, B. Chevallier, J. Etourneau, P. Hagenmuller, Mater. Bes. Bull. 19 (1984) 727. [10] J.P. Sanchez, A. Blaise, E. Ressouche, B. Malaman, G. Venturini, K. Tomala, R. Kmiee, J. Magn. Magn. Mater. 128 (1993) 295. [11] T. Shigeoka, Y. Kurata, T. Nakata, T. Fujiwara, K. Matsubayashi, Y. Uwatoko, Phys. Procedia 75 (2015) 837. [12] K. Tomala, A. Blaise, R. Kmiec, J.P. Sanchez, 117, 1992, p. 275. [13] A. Achiwa, J. Phys. Soc. Jpn. 27 (1969) 561. [14] R. Watanuki, G. Sato, K. Suzuki, M. Ishihara, T. Yanagisawa, Y. Nemoto, T. Goto, J. Phys. Soc. Jpn. 78 (2005) 2169. [15] Y. Andoh, J. Phys. Soc. Jpn. 56 (1987) 4075. [16] N.D. Dung, Y. Ota, K. Sugiyama, T.D. Matsuda, Y. Haga, K. Kindo, M. Hagiwara, T. Takeuchi, R. Settai, Y. Onuki, J. Phys. Soc. Jpn. 78 (2009) 024712.
Acknowledgments The high field magnetization measurements under pulsed magnetic fields were performed as a part of the Joint-Research Program in the ISSP, University of Tokyo. The authors warmly thank Professor K. Kindo and Dr. A. Kondo of the ISSP, University of Tokyo, for their help in the high-field magnetization measurements.
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Magnetic characteristics of polymorphic single crystal compounds DyIr2Si2 ⁎
⁎
Kiyoharu Uchimaa, , Toru Shigeokab, , Yoshiya Uwatokoc a b c
General Education, Okinawa Christian Junior College, Nishihara, Okinawa 903-0207, Japan Graduate School of Science and Engineering, Yamaguchi University, Yamaguchi 753-8512, Japan Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
A R T I C L E I N F O
A BS T RAC T
Keywords: DyIr2Si2 Polymorphic compound Antiferromagnetism Metamagnetism Magnetic anisotropy Crystal field effects
We have confirmed that the tetragonal ternary compound DyIr2Si2 shows polymorphism; the ThCr2Si2-type structure as a low temperature phase (I-phase) and the CaBe2Ge2-type one as a high temperature phase (Pphase) exist. A comparative study on magnetic characteristics of the morphs was performed on the I- and Pphase single crystals in order to elucidate how magnetic properties are influenced by crystallographic symmetry. The magnetic behavior changes drastically depending on the structure. The DyIr2Si2(I) shows an antiferromagnetic ordering below TN = 30 K, additional magnetic transitions of T1 = 17 K and T2 = 10 K, and a strong uniaxial magnetic anisotropy with the easy [001] direction. The [001] magnetization shows four metamagnetic transitions at low temperatures. On the other hand, the DyIr2Si2(P) has comparatively low ordering temperature of TN1 = 9.4 K and an additional transition temperature of TN2 = 3.0 K, and exhibits an easy-plane magnetic anisotropy with the easy [110] direction. Two metamagnetic transitions appear in the basal plane magnetization processes. In both the morphs, the χ-T behavior suggests the existence of component-separated magnetic transitions. The ab-component of magnetic moments orders at the higher transition temperature TN1 for the Pphase compound, which is contrast to the I-phase behavior; the c-component orders firstly at TN. The crystalline electric field (CEF) analysis was made, and the difference in magnetic behaviors between both the morphs is explained by the CEF effects.
1. Introduction The tetragonal ternary compounds RIr2Si2 (R = rare earth = rare earth) exhibit polymorphism; they have two different crystallographic structures: the ThCr2Si2-type structure (I4/mmm) of low temperature phase and the CaBe2Ge2-type one (P4/nmm) of high temperature phase [1], which is referred to as the I-phase and P-phase (I and P are quoted from the symbol of corresponding space group.), respectively. The crystal structure of the I-phase has centrosymmetry, while one of the P-phase is non-centrosymmetry. So this RIr2Si2 family is suitable to study how physical properties are influenced by crystallographic symmetry without changing chemical composition. Bazela presented the correlation between the crystal structure and magnetic ordering, and asserted a strong dependence of magnetic ordering on the a/c ratio [2]. The comparative studies with magnetic behaviors of two polymorphs on single crystal compounds are interesting. Comparative studies with physical properties of polymorphs have been performed on some compounds of this family. The high temperature phase (Pphase) compounds of YIr2Si2 and LaIr2Si2 become superconducting at 2.52 K and 1.24 K, respectively, while the low-temperature ones (I-
⁎
phase) are normal down to 1 K [3]. Both CeIr2Si2 polymorphs remain paramagnetic. The I-phase compound behaves as a Fermi-liquid, whereas the P-phase one exhibits non-Fermi-liquid features [4]. In PrIr2Si2, the P-phase compound remains paramagnetic down to 2 K whereas the I-phase one exhibits an antiferromagnetic ordering below 45.5 K. The different types of magnetic anisotropy, an easy axis anisotropy in the I-phase compound and an easy plane one in the Pphase one, has been reported [5]. Magnetism in NdIr2Si2 has been studied only for the I-phase compound. It shows a collinear antiferromagnetic ordering below 32.3 K with Nd magnetic moments along the c-axis [6]. For RIr2Si2 (R = heavy rare earth = heavy rare earth) series, there are a few reports on I-phase polycrystalline compounds; TbIr2Si2, DyIr2Si2 and ErIr2Si2 order antiferromagnetically below 80 K (or 75 K), 40 K and 10 K, respectively [7–10]. We have already reported the magnetic properties of TbIr2Si2 single crystals; The TbIr2Si2(I) shows an antiferromagnetic ordering below TN = 80 K, a strong uniaxial magnetic anisotropy with the easy [001] direction, and isotropic magnetic behavior in the basal plane. On the other hand, the TbIr2Si2 (P) has the comparatively low ordering temperature of TN1 = 11.5 K and an additional transition temperature of TN2 = 5 K, and
Corresponding authors. E-mail addresses: [email protected] (K. Uchima), [email protected] (T. Shigeoka).
http://dx.doi.org/10.1016/j.physb.2017.09.038 Received 27 June 2017; Received in revised form 12 September 2017; Accepted 13 September 2017 0921-4526/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Uchima, K., Physica B (2017), http://dx.doi.org/10.1016/j.physb.2017.09.038
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exhibits an easy-plane magnetic anisotropy with the easy [100] direction. The crystalline electric field effects estimated in the nonKramers TbIr2Si2 compound well explain the magnetic anisotropy [11]. The magnetic behavior of Kramers DyIr2Si2 is also interesting. We succeeded to grow both DyIr2Si2(I) and (P) single crystals, and performed magnetic measurements on the single crystals. In this paper, we present comparative magnetic behaviors and the crystalline electric field (CEF) analyses on both the morphs. 2. Experimental procedure Single-crystals DyIr2Si2 were grown by the Czochralski technique using a tri-arc furnace; that is, polycrystalline samples were prepared by arc-melting a stoichiometric mixture of the pure elements (Dy: purity of 3N, Ir: 3N, and Si: 5N) in an argon atmosphere. Subsequently, a single crystal was grown by the pulling-up method in the tri-arc furnace. The grown crystal was confirmed to be the P-phase and the single phase nature from analysis of X-ray powder diffraction pattern. A single crystal of the I-phase was obtained by annealing a part of the grown single crystal at 800 °C for 7 days. The crystal structure and the single phase nature of the annealed crystal were also confirmed by Xray powder diffraction. The quality of the single crystals was checked, and crystallographic orientations were determined by the back Laue method. The crystal was fixed on a thin plastic plate so that the desired direction is perpendicular to the plate within an experimental angle accuracy of one degree. It was subjected to magnetic measurements. The magnetic susceptibility (which is determined from M/B under B = 0.1 or 0.01 T where magnetization curves are linear.) and low-field magnetization below 5 T and/or 7 T were measured using a MPMS and/or PPMS (Quantum Design). Pulsed high-field magnetization measurements were performed up to 57 T by a pick-up coil method using a non-destructive long-pulse magnet installed at the International MegaGauss Science Laboratory in the Institute for Solid State Physics, the University of Tokyo.
Fig. 2. X-ray diffraction pattern on a DyIr2Si2(P) powder compound at a room temperature. Table 1 Crystallographic data of both the DyIr2Si2 morphs. DyIr2Si2
a (nm)
c (nm)
c/a
I P
0.405 0.410
0.975 0.967
2.41 2.36
[10]. The c/a ratio of the I-phase compound is 1.02 times as large as that of the P-phase one; The I-phase compound, low temperature phase one, elongates along the c-axis comparing with the P-phase one, high temperature phase. Bazela presented the correlation between the crystal structure and magnetic ordering, and asserted a strong dependence of magnetic ordering on the c/a ratio [2]. We can expect that this difference make difference of magnetic anisotropy between two morphs, and change magnetic ordering. 3.2. Magnetic susceptibility
3. Results and discussion
The temperature dependences of magnetic susceptibilities along the main symmetry axes in the tetragonal cell for the I- and P-phase compounds are shown in Figs. 3 and 4, respectively. A precise magnetic anisotropy between the c-axis and directions in the basal plane is evidenced from the figures while an anisotropy within directions in the basal plane is very small. In Fig. 3 (the I-phase compound), the [001] susceptibility shows three anomalies at TN = 30 K, T1 = 17 K and T2 =
3.1. Lattice parameter X-ray diffraction measurements were done to determine to which phase crystals belong. Figs. 1 and 2 show powder diffraction patterns on DyIr2Si2(I) and (P) compounds, respectively at a room temperature. All reflections in Figs. 1 and 2 could be indexed on the tetragonal ThCr2Si2- and the CaBe2Ge2-type structures, respectively, confirming the single phase natures. Then we were able to obtain crystallographic data of both the compounds by least square analysis to the powder pattern, which are shown in the Table 1. The lattice parameters of the I-phase compound are almost in agreement with the previous report
Fig. 3. Temperature dependences of magnetic susceptibilities along the main symmetry axes of a tetragonal cell on a DyIr2Si2(I) single crystal. Broken lines show calculated values using the CEF parameters. The inset is the level scheme deduced from CEF analysis.
Fig. 1. X-ray (Cu-Kα) diffraction pattern on a DyIr2Si2(I) powder compound at a room temperature.
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Fig. 4. Temperature dependences of magnetic susceptibilities along the main symmetry axes of a tetragonal cell on a DyIr2Si2(P) single crystal. Broken lines show calculated values using the CEF parameters. The inset is the level scheme deduced from CEF analysis.
Fig. 5. Magnetization curves along the main symmetry axes at 2 K on the DyIr2Si2(I) single crystal. The inset stands the dM/dB curve on ascending process.
netic Curie temperatures between the c-axis and the c-plane Δθ = θ001 − θab (θab = (θ100 + θ110)/2) is 99.5 K and −8.6 K in the I-phase and Pphase compounds, respectively. These results indicate that magnetic anisotropy and magnetic interactions of the I-phase compound are much larger than those of the P-phase one.
10 K, which is confirmed from specific heat measurements. The temperature of TN is lower than that reported from previous studies, and there is no information on T1 and T2 [10,12]. No anomaly is observed in the basal plane. The transition of TN is an antiferromagnetic ordering one, which is evidenced from the magnetization process mentioned below. This result suggests an appearance of so called “a component-separated magnetic transition”; the c-component of Dy magnetic moments orders independently at TN and the ab-component remains in a paramagnetic state [13,14], and an existence of frustration is expected in this compound. In Fig. 4 (the P-phase compound), two clear cusps are seen at TN1 = 9.4 K and TN2 = 3.0 K along directions in the basal plane whereas one cusp is only seen at TN2 = 3.0 K along the [001] direction. This result also suggests “successive component-separated magnetic transitions”. Here, the ab- and c-component of magnetic moments order independently at TN1 and TN2, respectively. The different types of componentseparated magnetic transitions between the I- and P-phase compounds appear; the c-component of magnetic moments orders firstly in the Iphase compound whereas the ab-component orders firstly at higher transition temperature TN1 in the P-phase one. The ordering temperature of the I-phase compound is considerably higher than that of the Pphase one; it of the I-phase one is 3 times as large as that of the P-phase one, indicating a stronger antiferromagnetic interaction in the I-phase one. The magnetic susceptibilities along the main symmetry directions obey the Curie-Weiss law for the paramagnetic temperature range on both the compounds; the χ−1-T curves become linear for high temperatures. The estimated effective magnetic moments μeff are listed in the Table 2 with other magnetic data. The effective magnetic moments are almost in agreement with the Dy3+ free ion magnetic moment (10.64 μB). This shows that 4f electrons in DyIr2Si2 are well localized and Ir ion is nonmagnetic. In the I-phase compound, paramagnetic Curie temperature along each direction θ001, θ100 and θ110 is 49.7, −43.5 and −56.2 K, respectively. In the P-phase compound, θ001 = −3.0 K, θ100 = 5.3 K and θ110 = 5.9 K which are considerably small compared with those in the I-phase one. The difference in paramag-
3.3. Magnetization process Magnetization curves of both the compounds are shown for low and high temperatures in Figs. 5–8. For the I-phase one (Figs. 5 and 7), it is evidenced that the hard magnetization direction is in the basal plane. Magnetization is almost isotropic within the basal plane, and increases monotonically. There is no transition in the hard magnetization processes. On the other hand, in the easy c-axis process at low temperature (the [001] magnetization process in the Fig. 5), four transitions appear; two slight increases in magnetization at Bc1 = 1.4 T and Bc2 = 1.8 T, and two rapid and large increases at Bc3 = 2.5 T and Bc4 = 5.4 T are seen. The critical field is determined as a peak magnetic field in the dM/dB vs. B curve (the inset of Fig. 5). Hysteresis loops with width of 0.15 T– 0.2 T are observed around Bc3 and Bc4, indicating that these transitions are of the first order. The magnetization saturates above Bc4 at Ms = 9.6 μB/f.u., which almost agrees with the Dy3+ free ion moment (gJ = 10 μB) and that reported from neutron study [10]. The change in magnetization on Bc3, ΔM1, is corresponding to about (6/25) Ms, and accords with the change ΔM2 on Bc4; ΔM2 = (6/25) Ms. The magnetization increases gradually for intermediate magnetic fields of Bc1–Bc3 and Bc3–Bc4. These results suggest that field-induced magnetic phases are long-period incommensurate ones while the simple antiferromagnetic structure, AF-I type with magnetic moments along the c-axis has been reported at B = 0 [10]. From the rapid change with the simple fraction on Bc3 and Bc4, we can speculate the transitions occur by spin-flip; there are 50 Dy magnetic moments in a magnetic unit cell (the magnetic unit cell may be a × a × 25c), and 3 of 50 spins turn over from the opposite direction of magnetic field to the magnetic field one along the c-axis. The field-induced magnetic structures and mechanism of this magnetization process are unknown yet. These metamagnetic transitions are smeared with increasing
Table 2 Magnetic data of both the DyIr2Si2 polymorphs. DyIr2Si2
Tt (K)
θ001 (K)
θ□00 (K)
θ110 (K)
Δθ (Κ)
μeff (μB)
Ms (μB)
Easy axis
I P
30, 17, 9 9.4, 3.0
49.7 −3.0
−43.5 5.3
−56.2 5.9
99.5 −8.6
10.35 10.68
9.6 10.0
[001] [110]
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temperature. The process becomes a two-step metamagnetic one around 10 K, and the metamagnetic transitions disappear above TN; it becomes a paramagnetic one (Fig. 7). High field magnetization processes of the P-phase compound under long-pulsed magnetic fields up to 57 T are shown in Figs. 6 and 8. The easy magnetization direction is the [110] direction in the basal plane. The magnetization increases with a considerably large gradient up to about 3 T, and with a smaller gradient above 3 T. It reaches 10 μB/f.u. (the Dy3+ free ion moment) at 57 T of the maximum field in this experiment. So we believe the magnetization saturates around 57 T. The inset in Fig. 6 shows low field magnetization processes measured by MPMS. Two metamagnetic transitions appear around Bb1 = 0.9 T and Bb2 = 3.4 T in the [110] magnetization process, indicated by arrows in the figure. In the [100] process, two metamagnetic transitions also appear around Ba1 = 1.2 T and Ba2 = 2.0 T. These metamagnetic transitions persist up to near TN for the [110] and [100] directions. In the [001] process, there is no metamagnetic transition. The gradual increase in magnetization on all the symmetry directions in high magnetic fields means that magnetic moments are rotating to the magnetic field direction. For the [100] and [001] directions, magnetization at maximum field is smaller than the saturation moment on the [110] direction. In order to estimate the saturation magnetization for the [100] and [001] directions, we extrapolated B to the infinity in M vs B−2 plot, and determined Ms. The obtained value is 9.8 μB/f.u. for both the directions, which is in agreement with the Dy3+ free ion moment. For high magnetic field region, the magnetization curve along the [001] direction is almost parallel to the [110] one, indicating a large magnetic anisotropy between the [001] and [110] directions. For the paramagnetic temperatures, magnetization curves along the main symmetry directions show no transition; the processes become paramagnetic ones (Fig. 8). The large magnetic anisotrpy still exists.
Fig. 6. Magnetization curves at 1.8 K on the DyIr2Si2(P) single crystal. The inset shows the low field magnetization curves.
3.4. Magnetic phase diagram The B[easy direction]-T magnetic phase diagrams were constructed from the magnetization processes at various temperatures and the temperature dependences of M/B at various magnetic fields for the Iand P-phase compounds, and are shown in Figs. 9 and 10, respectively. There are at least four ordered phases in B001-T phase diagram for the I phase compound. In low temperatures, an additional field-induced phase (indicated by I′) may exist. The phase I for low temperatures and low magnetic fields is the simple antiferromagnetic structure, so called the AF-I type, with magnetic moments along the c-axis as reported
Fig. 7. Magnetization curves at 34 K on the DyIr2Si2(I) single crystal. Broken lines show calculated magnetization using CEF parameters.
Fig. 9. B001-T magnetic phase diagram of the DyIr2Si2(I) single crystal. Open circles are critical fields determined from M(B) curves. Squares are transition temperatures determined from M(T).
Fig. 8. Magnetization curves at 12 K on the DyIr2Si2(P) single crystal. Broken lines show calculated magnetization using CEF parameters.
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487 K, which is comparative with that of the isostructural compound TbIr2Si2(I) [11]. On the other hand, for the P-phase compound, the first and second doublet excited state are at 48 K and 56 K above the ground state, respectively. The overall splitting is 306 K which is smaller than that of the-I phase one, and is also comparative to that of TbIr2Si2(P) [11]. The magnetic susceptibilities (Figs. 3 and 4) and magnetizations (Figs. 7 and 8) for the paramagnetic region are reproduced by the calculated values. The negative sign of B20 in the Iphase compound and positive sign in the P-phase one are consistent with the fact that the easy magnetization direction is the c-axis and the [110] direction in the basal plane, respectively. The deduced values of B20 and B40 for the I-phase compound are in good agreement with those evaluated from Mossbauer study [12]. The value of B20 is on the same order as that of TbIr2Si2(I) and is 3 times as large as the P-phase one. The value of B20 for the P-phase compound is on the same order as those of the isostructural compounds TbIr2Si2(P) and DyCu2Si2 [16] which have the same easy plane-type magnetic anisotropy, and is much smaller than that of the I-phase one. The deduced paramagnetic Curie temperature of the I-phase compound is five times as large as the Pphase one, which is corresponding to the higher ordering temperature of the I-phase one, indicating stronger molecular field interactions of the I-phase one than that of the P-phase one. The multi-step metamagnetic processes in both the ordered states are, of course, not explained by CEF effects only, and other effects such as competing exchange interactions should be required.
Fig. 10. B110-T magnetic phase diagram of the DyIr2Si2(P) single crystal. Open circles are critical fields determined from M(B) curves. Closed one is a transition temperature from M(T).
from the neutron study [10]. We can suggest from magnetization behavior in Fig. 5 that the field-induced phases II and IV are longperiod ones. With respect to the antiferromagnetic structure of the phase III, there is no idea now. In the B110-T phase diagram for the Pphase compound, there are three ordered phases corresponding to the metamagnetic transitions as shown in Fig. 10. The magnetic structures are unknown yet.
3.6. Summary We succeeded the single crystal growth of both the DyIr2Si2 polymorphic compounds; the ThCr2Si2-type structure as a low temperature phase (I-phase) and the CaBe2Ge2-type one as a high temperature phase (P-phase). A comparative study on magnetic characteristics of the morphs were performed on the I- and P-phase single crystals from measurements of magnetic susceptibility and magnetization in order to elucidate how magnetic properties are influenced by crystallographic symmetry. The magnetic behavior changes drastically depending on the structures. The magnetic anisotropy and magnetic interactions of DyIr2Si2(I) are much larger than that of DyIr2Si2(P). It should be noted that the χ-T behavior suggests the existence of “component-separated magnetic transitions” and the types of transitions are different from both the morphs. The abcomponent of magnetic moments orders at the higher transition temperature TN1 for the P-phase compound, which is contrast to the I-phase behavior; the c-component orders firstly at TN. The saturation magnetic moment and effective magnetic moment are in agreement with those of the Dy3+ free ion, which shows that 4f electrons in DyIr2Si2 are well localized and Ir ion is nonmagnetic. The DyIr2Si2(I) shows an antiferromagnetic ordering below TN = 30 K, additional magnetic transitions of T1 = 17 K and T2 = 10 K, and a strong uniaxial magnetic anisotropy with the easy [001] direction. The [001] magnetization shows four metamagnetic transitions at low temperatures. Field-induced long-period magnetic structures are suggested from the behavior in this process. On the other hand, the DyIr2Si2(P) has the comparatively low ordering temperature of TN1 = 9.4 K and an additional transition temperature of TN2 = 3.0 K, and exhibits an easy-plane magnetic anisotropy with the easy [110] direction. Two metamagnetic transitions appear in the basal plane magnetization
3.5. Crystalline electric field The crystalline electric field (CEF) analysis was performed in order to discuss the anisotropic magnetic behaviors on the I- and P-phase. For a site with tetragonal symmetry, the CEF Hamiltonian is given by
HCEF = B20 O20 +B40 O40 +B44 O44+B60 O60 +B64 O64 where Bnm and Onm are the CEF parameters and the Stevens operators, respectively. The total Hamiltonian, Htotal, including the interaction of rare earth moments with the molecular field is used.
Htotal = HCEF + gμB Ji (H + λ M) In this expression, Ji (I = x, y, z) is the component of the angular momentum, λ is the molecular field coefficient. In the calculation, the parameter θP, which is defined as θP = g2μB2λJ(J + 1)/3k, is used instead of λ [15]. The parameters were determined from the best fit of the calculated values to the experimental curves of M(B) at a paramagnetic temperature and χ(T) for the paramagnetic region using a least-squares method. The deduced parameters are listed in Table 3. The calculated magnetic susceptibilities and magnetizations are shown by broken lines in Figs. 3, 4, and 7, 8, respectively. The deduced CEF level schemes for the I- and P-phase compounds are shown in the insets of Figs. 3 and 4, respectively. The CEF level splits into Kramers doublets. For the I-phase compound (in the inset of Fig. 3), the first and second exited doublets exist at 13 K and 66 K above the doublet ground state, respectively, which almost agree with a previous report [12], leading a stable Dy magnetic moment. The overall splitting is
Table 3 Deduced CEF parameters and paramagnetic Curie temperature for both morphs of DyIr2Si2. DyIr2Si2
B20 (K)
B40 (10−3 K)
B44 (10−2 K)
B60 (10−5 K)
B64 (10−4 K)
θp (K)
I P
−2.387 0.791
3.440 2.133
−5.978 −0.974
1.536 2.920
3.011 −5.921
−8.800 −1.870
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processes. The crystalline electric field (CEF) analysis was made. The CEF level splits Kramers doublets. The overall splitting is 487 K and 306 K on I- and P-phase compounds, respectively, which is reasonable comparing with those of isostructural compounds. The negative sign of B20 in the I-phase compound and positive sign in the P-phase one are consistent with the fact that the easy magnetization direction is the caxis and the [110] direction in the basal plane, respectively. The B001-T and B110-T magnetic phase diagrams are obtained for the DyIr2Si2(I) and (P), respectively. There are some magnetic ordered phases whose magnetic structures are unknown yet. Further studies are required.
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Acknowledgments The high field magnetization measurements under pulsed magnetic fields were performed as a part of the Joint-Research Program in the ISSP, University of Tokyo. The authors warmly thank Professor K. Kindo and Dr. A. Kondo of the ISSP, University of Tokyo, for their help in the high-field magnetization measurements.
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