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Local spin reversal and associated magnetic responses in Ga-substituted Pb-hexaferrites
Journal of Magnetism and Magnetic Materials 324 (2012) 2866–2870
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Local spin reversal and associated magnetic responses in Ga-substituted Pb-hexaferrites Eun Hye Na a, Jung-Hoon Lee a, Suk-Jin Ahn a, Kun-Pyo Hong b, Yang Mo Koo c, Hyun Myung Jang a,n a
Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea Neutron Science Division, Department of Basic Science and Technology, Korea Atomic Energy Research Institute (KAERI), Daejon 305-353, Republic of Korea c Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 February 2012 Available online 4 May 2012
We have studied magnetic structure and properties of Ga-substituted Pb-hexaferrites having the stoichiometry of PbFe12 xGaxO19 with x ¼ 6 (i.e., Fe:Ga ¼ 1:1). According to the neutron diffraction results, this compound is characterized by a collinear spin structure below its Curie temperature ( 325 K). Analysis of the neutron diffraction patterns further indicates that the magnetic-moment direction of Fe3 þ ions located at the octahedral 2a sublattice is downward while that of the unsubstituted PbFe12O19 is upward at room temperature. With decreasing temperature, the Fe3 þ magnetic moment at the octahedral 2a sublattice undergoes a reorientation to the upward direction while that of the unsubstituted PbFe12O19 remains upward down to 5 K. This selective local spin reversal at the 2a sublattice of PbFe6Ga6O19 was attributed to the weakening of the superexchange interaction between the octahedral 2a site and the tetrahedral 4fIV site upon the preferential substitution of Ga ions for Fe ions at these two neighboring sites. Comparison of the neutron diffraction results with dc magnetization responses and ac susceptibilities further indicates that the paramagnetic–ferrimagnetic transition at 325 K (Tc) is followed by the local spin reversal at lower temperatures. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ferrimagnetic Spin arrangement Neutron diffraction Magnetic susceptibility
1. Introduction Hexagonal PbFe12O19 is a ferrimagnetic material with its Fe-magnetic moment aligned parallel to the crystallographic c-axis[1]and does belong to well-known M-type hexaferrites having the common stoichiometry of MFe12O19, where M¼ Ba, Pb, and Sr. There have been extensive research efforts to partially replace Fe ions by some other cations to improve magnetic and dielectric properties of M-type hexaferrites [2,3]. The influence of the cationic distribution and the consequent sublattice magnetization of M-type hexaferrites have been studied by neutron ¨ diffraction and Mossbauer spectroscopy measurements[2,4,5]. Among numerous studies done on the partial replacement of Fe ions by diamagnetic or paramagnetic ions, the Ga-substitution is expected to play a unique role. This is because Ga ion is trivalent, with its ionic radius comparable to that of Fe3 þ . Thus, the trivalent Ga ions are expected to minimize the formation of oxygen vacancy defects and consequently to improve the electrical resistivity and the dielectric loss. Furthermore, since Ga ions are nonmagnetic, the Ga-ion substitution for Fe ions would
n
Corresponding author. E-mail address: [email protected] (H.M. Jang).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.04.031
modulate the degree of the superexchange interaction between neighboring Fe ions and thus alter magnetic properties [6]. Although the Ga-substituted M-type hexaferrite had been ¨ studied by Mossbauer spectroscopy, the variation of its magnetic order caused by the Ga-substitution is not clearly understood and is still an open question [7–9]. Maryˇsko and co-workers [9] reported the results of magnetic measurements on single crystals of PbFe12 xGaxO19 for x o8. Their study elucidated a peculiar phenomenon of the crystallographic direction-dependent Curie temperature (Tc) for x43 and hypothesized a non-collinearity in the magnetic order. Albanese et al. [8]also proposed a noncollinear magnetic ordering of PbFe12 xGaxO19 for x43 based on their measurements of the composition-dependent saturation magnetization (Ms). However, they were not able to find any ¨ evidence of the non-collinear magnetic ordering by Mossbauer spectroscopy measurements since the spectra are too complicated to be resolved. Accordingly, the main purpose of the present study is elucidate the evolution of the magnetic structure of the single-crystalline hexagonal PbFe6Ga6O19 (i.e., Fe:Ga ¼1:1) by performing the temperature-dependent neutron diffraction measurements and subsequently carrying out the Rietveld analysis of these diffraction patterns. More specifically, we first determined the Ga-ion occupancies for various sites by the Rietveld refinement of the
E.H. Na et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2866–2870
high-temperature diffraction data obtained at a temperature above Tc (325 K). Then, the magnetic moments of various sublattices below Tc (5, 100, and 300 K) are separately determined. From the neutron diffraction analysis, we are able to correlate the observed anomalous behavior of the ac magnetic susceptibility, wac(T), with the local spin reversal at the octahedral 2a sublattice.
2. Experimental Methods PbFe12 xGaxO19 single crystals (x ¼6) used in the present study were grown by the PbO flux method with PbO, Fe2O3, and Ga2O3 as the starting ingredients. For this purpose, the following mole ratio of the flux was adopted: Pb:Fe:Ga¼4:6:6. Thus, the amount of the PbO flux used is three times of the stoichiometic amount of the PbO ingredient. This starting powder mixture was then placed in a Pt crucible and dissolved at 1300 1C for 12 h. The flux melt was first cooled down to 800 K with a cooling rate of 1 1C/h and subsequently cooled to room temperature with a faster
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cooling rate of 6 1C/h. For diffraction and magnetic measurements, the flux-grown single crystals were suitably grinded to obtain single-crystalline powder samples. The composition of Ga in PbFe12 xGaxO19, as determined by inductively coupled plasmaoptical emission spectroscopy (ICP-OES), is x¼6. Magnetic susceptibility of the single-crystalline PbFe6Ga6O19 powder sample was measured using a magnetic property measurement system (MPMS, Quantum Design). Both the temperature-dependent magnetization and the ac magnetic susceptibility reveal that the para-to-ferrimagnetic transition temperature of PbFe6Ga6O19 is 325 K (Tc). Neutron diffraction data were collected over the scattering angle between 10o and 160o with a 2ystep of 0.05o using a high-resolution powder diffractometer (with ˚ at the Hanaro Center of the Korea l of neutron beam ¼1.8349 A) Atomic Energy Research Institute (KAERI). The Rietveld method was used to refine the atomic and magnetic structures of the hexagonal PbFe6Ga6O19. For this purpose, we adopted a wellknown software package called ‘FULLPROF Program Suite’.
3. Results and discussion 3.1. Refined crystal structure and preferential substitution of Ga ions
Fig. 1. A schematically represented refined crystal structure of PbFe6Ga6O19 having the hexagonal P63/mmc symmetry. The Rietveld refinement is based on the neutron diffraction data obtained at 500 K.
In order to examine the crystal and magnetic structures of PbFe6Ga6O19, neutron powder diffraction experiments were carried out at several selected temperatures below and above Tc ( 325 K) in the absence of any bias magnetic field. The diffraction pattern obtained at 500 K was first analyzed to obtain: (i) the crystal structure parameters (i.e., atomic positions and lattice parameters, etc.) and (ii) the Ga-ion distribution among different sublattices as the diffraction data at 500 K (4 Tc) do not reflect any magnetic component but include crystal-structure components only. The refined crystal structure and the crystal parameters obtained using the Rietveld refinement of the diffraction data are presented in Fig. 1 and Table 1, respectively. According to the structure model of the M-type hexaferrite (MFe12O19) first proposed by Gorter [7,10], 12 Fe ions per formula unit are distributed among five distinct sublattices: (i) six in the octahedral sublattice k (spin-up), (ii) one in the octahedral sublattice a (spin-up), (iii) one in the five-fold sublattice b (spin-up), (iv) two in the tetrahedral sublattice fIV (spin-down), and (v) two in the octahedral sublattice fVI (spin-down). According to the result presented in Table 1, the Ga-ion substitution occurs at all five sublattices but with a marked preference for the octahedral 2a and tetrahedral 4fIV (4f1) sites. On the other hand, a limited value of the Ga-occupancy is found in the five-fold 2b and octahedral 4fVI (4f2) sites. This conclusion on the Ga-ion distribution does not accord well with the previously proposed model [8] in which Ga
Table 1 Details of the refined crystal structure of PbFe6Ga6O19 having the hexagonal P63/mmc symmetry, where X, Y, and Z denote the fractional atomic coordinates. Atom Pb Fe1/Ga1 Fe2/Ga2 Fe3/Ga3 Fe4/Ga4 Fe5/Ga5 O1 O2 O3 O4 O5
Coordination
Octahedral Five-fold Tetrahedral Octahedral Octahedral
Sublattice
X
Y
Z
Occupancy (Fe/Ga)
2d 2a 2b 4f1 4f2 12k 4e 4f 6h 12k 12k
0.66666 0 0 0.33333 0.33333 0.1674 0 0.3333 0.18147 0.15623 0.50266
0.33333 0 0 0.66666 0.66666 0.33479 0 0.66666 0.36285 0.31244 1.00522
0.25 0 0.25 0.02678 0.18938 0.10844 0.15004 0.05577 0.2501 0.05166 0.14913
1 0.282/0.718 0.861/0.139 0.369/0.691 0.761/0.239 0.422/0.578 1 1 1 1 1
˚ The goodness of fit (w2) was 6.64, and the following values were obtained for the two representative The refined lattice parameters are: a¼ 5.8640 A˚ and c ¼23.1178 A. reliability factors: RB (Bragg reliability factor) ¼ 4.91 and Rf ¼3.46.
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ions are distributed rather uniformly over the five distinct sites for x r5 but with a slightly diminished occupancy at the 2b site beyond this value (i.e., x45). 3.2. Collinear spin structure and selective spin reversal Fig. 2 presents neutron diffraction patterns of PbFe6Ga6O19 obtained at four selected temperatures, 5 K, 100 K, 300 K, and 500 K. For all four patterns, no reflection appears at a scattering angle which is different from the Bragg reflection positions corresponding to the space group P63Xmmc. This indicates that P63/mmc centrosymmetric structure is maintained over a wide temperature range between 5 K and 500 K. As shown in Fig. 2, the intensity of the [00L] reflection at 2y ¼18.31 is nearly independent of temperature over a wide temperature range. Recall that magnetic scattering occurs only when the magnetic moment vector has a non-zero component projected on the scattering plane considered [11]. The observation that the [00L]-peak intensity is nearly independent of temperature thus indicates that there does not exist any magnetic-moment component which projects on [00L] planes (ab in-planes) for a wide temperature range. This, in turn, indicates that the magnetic moment is aligned parallel to the c-axis with the absence of any ab-plane component. Besides, the degree of the long-range ordering is considered to be essentially unchanged since the magnetic [10L] reflection neither shows any peak broadening nor has a tendency of the peak-intensity decrease as temperature goes down to 5 K [12]. Then, the magnetic structures at 5, 100, and 300 K were computed using the collinear ferrimagnetic model of Gorter [10]. More specifically, the refinements at 5, 100, and 300 K were done on the basis of the cation distribution obtained from the high-temperature (500 K) measurements and all cations were taken into consideration. Firstly, the values of the Ga-occupancy factor for various sites were fixed at 500 K values during the initial refinement and the magnetic moments and crystal parameters were subsequently estimated. In the second run, these occupancy factors were then given as free parameters to obtain the magnetic and crystal parameters. They all showed some variations but the discrepancies in the occupancy factor between these two runs are within the error limit, confirming the reliability of the present estimate of the Ga-occupancy factors.
w2 ¼ 7.19, RB ¼2.30, Rf ¼ 1.30, and Rmag (reliability factor for the magnetic refinement)¼ 2.56 at 5 K. (ii) w2 ¼7.07, RB ¼ 2.30, Rf ¼ 1.36, and Rmag ¼2.76 at 100 K. (iii) w2 ¼ 6.70, RB ¼ 2.42, Rf ¼ 1.51, and Rmag ¼4.49 at 300 K. These R values
Fig. 2. Observed (red-colored dots) and calculated (black line) neutron diffraction patterns of PbFe6Ga6O19 at 5 K. The green-colored vertical bars at the first row below the diffraction pattern indicate the positions of the nuclear Bragg reflections while the vertical bars at the second row depict the locations of the magnetic Bragg peaks. The blue line at the bottom shows the difference between the observed and calculated diffraction patterns. The inset shows the low-angle diffraction patterns (2y ¼ 15–401) obtained at various temperatures, 5, 100, 300 and 500 K. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3 presents the refined magnetic structure of PbFe6Ga6O19 at 5 K and 300 K. As shown in Table 2, the estimated magnetic moment of Fe3 þ ions at the octahedral 2a sublattice is 1.384 mB at 300 K with a downward spin configuration. Contrary to this, the magneticmoment direction at the octahedral 2a site of the pristine (unsubstituted) PbFe12O19 is known to be upward [8]. The Fe3 þ magnetic moment at the octahedral 2a sublattice of PbFe6Ga6O19 undergoes a reorientation to the upward direction beginning at a temperature higher than 100 K (Table 2) while that of the unsubstituted PbFe12O19 remains unchanged (upward) down to 5 K. It is known that the local weakening of the octahedral– tetrahedral superexchange interaction which is caused by the substitution of non-magnetic ions for Fe3 þ ions in ferrimagnetic oxides can give rise to either a random canting of the magnetic moments or a local spin reversal [13–15]. In addition, it is known that the magnitude and the sign of the octahedral–tetrahedral sublattice coupling is susceptible to the variation of temperature [16]. Thus, the observed selective spin reversal with a gradual variation in the magnitude of the spin moment at the octahedral 2a site of PbFe6Ga6O19 (Table 2) can be attributed to the weakening of the temperature-sensitive superexchange interaction between the octahedral 2a sublattice and the tetrahedral 4fIV
Fig. 3. Schematic representations of the local magnetic moments at various sites of the hexagonal PbFe6Ga6O19, showing the selective spin-moment reversal occurring at the octahedral 2a sublattice.
Table 2 Local magnetic moments for various sublattices of the M-type hexaferrite, PbFe6Ga6O19, at three selected temperatures below Tc. Coordinates
Octahedral Five-fold Tetrahedral Octahedral Octahedral Total Mz (mB/f.u) Ms (mB/f.u)
Wyckoff position
2a 2b 4fIV 4fVI 12k
Magnetic moment (mB/Fe) T¼ 5 K
T ¼100 K
T¼ 300 K
1.448 3.386 4.002 3.621 4.438 6.096 6.22
0.978 3.008 3.903 3.479 4.008 4.838 4.93
1.384 1.596 2.402 1.714 2.028 1.737 1.76
Temperature-dependent representative reliability factors are as follows: (i)
ensure the reliability of the present Rietveld refinement.
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(4f1) sublattice, which is caused by the preferential substitution of Ga ions (for Fe ions) at these two neighboring sites (Table 1). One can theoretically estimate the net magnetization per formula unit (Mz) by the summation of various sublattice magnetization values using the following formula [17]: M z ¼ n2a s2a þ n2b s2b þ n2a s2a þ 2n4f IV s4f IV þ 2n4f VI s4f VI þ 6n12k s12k
ð1Þ
where ni and si, respectively, denote the Fe-ion occupancy factor and the local magnetic moment at the sublattice i. We have compared the computed net magnetization value with the saturation magnetization (Ms) value obtained from the experimental magnetization-field (M–H) hysteresis curve shown in Fig. 4. As presented in Table 2, the two values agree with each other for all three temperatures examined. 3.3. Temperature-dependent dc magnetization and ac susceptibility According to the temperature-dependent dc magnetization curve, M(T), presented in Fig. 5(a), the onset of a long-range magnetic ordering is approximately 325 K (Tc). As presented, the out-of-plane (H//c-axis) magnetization is much bigger than the in-plane (H//ab-plane) magnetization, which coincides with the uniaxial ferrimagnetism along the c-axis, as deduced from the neutron diffraction data (Section 3.2). According to Fig. 5(a), ZFC (zero-field cooling) and FC (field cooling) curves bifurcate beginning at 320 K and another magnetic anomaly appears at 290 K. Below 70 K, the ZFC magnetization decreases while the FC magnetization even increases with decreasing temperature. Fig. 5(b) shows the temperature-dependent ac magnetic susceptibility obtained using an ac amplitude of 10 Oe (in the absence of any bias-field). Both w0 and w00 show their peaks at 325 K which coincides with the long-range magnetic ordering temperature (Tc) deduced from the M(T) curve (Fig. 5(a)). Another prominent feature of the ac susceptibility is the appearance of a broad peak over a wide temperature range between 70 K and 290 K, and the dc magnetization is represented by a plateau-like response in this temperature interval. Fig. 6 presents the frequency-dependent magnetic susceptibility for a wide range of temperature. The most prominent feature of the real part of the ac susceptibility (w0 ) is that an intense peak at Tc ( 325 K) is followed by the appearance of a broad lower-temperature peak beginning at 290 K. The imaginary part of the ac susceptibility (w00 ) also shows essentially the same feature, which is nearly independent of the ac frequency. In addition to this, there
Fig. 4. Magnetization-field (M–H) hysteresis curves of PbFe6Ga6O19 having the hexagonal P63/mmc symmetry at various temperatures below Tc.
Fig. 5. Temperature-dependent (a) dc magnetization and (b) ac susceptibility curves of the hexagonal PbFe6Ga6O19, showing a large discrepancy between ZFC and FC curves beginning at 70 K.
appears a large discrepancy between the ZFC and FC curves beginning at 70 K (Fig. 5(a)). All these observations suggest that the appearance of the broad susceptibility response beginning at 290 K is possibly caused by a transition to a reentrant spin-glass state [16,18]. However, as shown in Fig. 6, the peak temperature decreases with increasing ac frequency, which is in direct disagreement with the ac-frequency-dependent peak temperature observed in many spin-glass systems [19]. As discussed previously, the degree of the long-range ordering remains essentially unchanged since the magnetic (10 L) reflection neither shows any peak broadening nor has a tendency of the peak-intensity reduction as temperature decreases down to 5 K [12]. This again excludes the possibility of the transition to a reentrant spin-glass state with decreasing temperature. Besides, it is known that the coercive field does increase substantially when a system undergoes the transition to a spin-glass state [19,20]. As shown in Fig. 4, the coercive field remains at a small value down to 5 K (a few tens of Oe). This observation also indicates that a reentrant spin-glass state is not relevant to the hexagonal PbFe6Ga6O19 system down to 5 K. A broad nature of the ac susceptibility peak appeared between 70 and 290 K suggests that a single magnetic transition is not related to this broad ac response [21]. Considering this, we suggest that a domain-wall pinning mechanism which is initiated by the local spin reversal at the octahedral 2a site is primarily responsible for the appearance of this broad ac susceptibility response. If the spin reversal at the 2a site invokes spin-lattice coupling, the resulting lattice distortion would lead to the formation of a defect structure. Then, this defect structure not only becomes a new pinning center for the domain-wall motion but also expedites a rearrangement of the domain structure [22]. The domain-wall pinning then explains the observed large
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Acknowledgments This work was supported by the Brain Korea 21 project 2011 and by the WCU (World Class University) program through the National Research Foundation funded by the Ministry of Education, Science and Technology of Korea (R32-10147). References
Fig. 6. Temperature-dependent real and imaginary parts of the ac magnetic susceptibility of the hexagonal PbFe6Ga6O19 measured at various ac frequencies between 10 and 1000 Hz.
discrepancy between the ZFC and FC curves below 70 K [19,23]. Indeed, the domain-structure variation induced by the spinphonon coupling had been proposed to account for the discrepancy between the ZFC and FC curves observed in ferromagnetic FeCr2S4 [22,24]. According to the temperature-dependent local magnetic moment (Table 2), the selective spin reversal at the octahedral 2a site occurs at a temperature between 100 K and 300 K. As shown in Fig. 6, the ac susceptibility is characterized by a broad response between 70 and 290 K with the peak temperature at 160 K. These two observations suggest that the local spin reversal occurs at a temperature above 100 K but is characterized by a continuous, gradual transition over a wide temperature range. The peak temperature of the ac susceptibility then can presumably be identified with the statistically most probable temperature for the local spin reversal.
4. Conclusions In conclusion, analysis of the neutron diffraction patterns indicates that the Fe3 þ magnetic moment at the octahedral 2a sublattice of PbFe6Ga6O19 undergoes a switching in its direction beginning at a temperature higher than 100 K. This selective spin reversal with a gradual variation in the magnitude of the local magnetic moment at the octahedral 2a site is explained in terms of the weakening of the temperature-sensitive superexchange interaction between the octahedral 2a sublattice and the tetrahedral 4fIV sublattice, which is caused by the preferential substitution of Ga ions at these two neighboring sites. The selective local spin reversal is then correlated with the broad ac susceptibility response between 70 and 290 K.
[1] G.K. Thompson, B.J. Evans, The structure–property relationships in Mtype hexaferrites: hyperfine interactions and bulk magnetic properties, Journal of Applied Physics 73 (1993) 6295–6297. ˜ o´n, F. Leccabue, B.E. Watts, Mossbauer ¨ [2] G. Albanese, S. Dı´az-Castan and magnetic investigation of scandium and indium substituted PbFe12O19 hexagonal ferrite, Journal of Materials Science 35 (2000) 4415–4420. ˜ o´n, F. Leccabue, B.E. Watts, G. Albanese, Magnetic and [3] S. Dı´az-Castan ¨ Mossbauer investigation of Sc- and In-substituted PbFe12O19 hexaferrites, Journal of Magnetism and Magnetic Materials 196 (1999) 458–460. [4] Z. Somogyva´ri, E. Sva´b, K. Krezhov, L.F. Kiss, D. Kapta´s, I. Vincze, E. Beregi, F. Boure e, Non-collinear magnetic order in a Sc-substituted barium hexaferrite, Journal of Magnetism and Magnetic Materials 304 (2006) e775–e777. [5] A. Collomb, P. Wolfers, X. Obradors, Neutron diffraction studies of some hexagonal ferrites: BaFe12O19, BaMg2–W and BaCo2–W, Journal of Magnetism and Magnetic Materials 62 (1986) 57–67. [6] M.J. Iqbal, M.N. Ashiq, P.H. Gomez, J.M. Munoz, Synthesis, physical, magnetic and electrical properties of Al–Ga substituted co-precipitated nanocrystalline strontium hexaferrite, Journal of Magnetism and Magnetic Materials 320 (2008) 881–886. [7] H. Stepankova, J. Englich, NMR study of Ga and Al substituted hexagonal ferrites with magnetoplumbite structure, IEEE Transactions on Magnetics 30 (1994) 988–990. [8] G. Albanese, F. Leccabue, B.E. Wattes, S. Diaz-castanon, Magnetic and ¨ Mossbauer investigation of PbFe12 xGaxO19 hexagonal ferrites, Journal of Materials Science 37 (2002) 3759–3763. [9] M. Marysko, Z. Frait, S. Krupicka, FMR and static magnetic properties of gallium substituted magnetoplumbite, Journal de Physique IV France 07 (1997) 347–348. [10] E.W. Gorter, Saturation magnetization of some ferrimagnetic oxides with hexagonal crystal structures, Proceedings of the IEEE 104B (1957) 255–260. [11] Y. Takada, T. Nakagawa, M. Tokunaga, Y. Fukuta, T. Tanaka, T.A. Yamamoto, T. Tachibana, S.I. Kawano, Y. Ishii, N. Igawa, Crystal and magnetic structures and their temperature dependence of Co2Z-type hexaferrite (Ba,Sr)3Co2Fe24O41 by high-temperature neutron diffraction, Journal of Applied Physics 100 (2006) 043904. [12] V. Kiryukhin, S. Lee, W. Ratcliff II, Q. Huang, H.T. Yi, Y.J. Choi, S.W. Cheong, Order by static disorder in the ising chain magnet Ca3Co2-xMnxO6, Physical Review Letters 102 (2009) 187202. [13] G. Albanese, M. Carbucicchio, G. Asti, Spin-order and magnetic properties of BaZn2 Fe16 O27 (Zn2–W) hexagonal ferrite, Applied Physics 11 (1976) 81–88. [14] D. Vladikoava, L. Ilkov, Influence of the superexchange interactions on the saturation magnetization and Curie temperature in substituted yttrium iron garnets, Physica Status Solidi A 156 (1996) 461. [15] B.X. Gu, Magnetic properties of Ba2(ZnxFe1 x)2Fe28O46 xtype hexaferrites, Journal of Applied Physics 75 (1994) 4114–4116. [16] S. Srinath, P. Poddar, H. Srikanth, B.C. Sales, D. Mandrus, Observation of a new magnetic anomaly below the ferromagnetic Curie temperature in Yb14MnSb11, Physical Review Letters 95 (2005) 227205. ¨ [17] P. Wartewig, M.K. Krause, P. Esquinazi, S. Rosler, R. Sonntag, Magnetic properties of Zn- and Ti-substituted barium hexaferrite, Journal of Magnetism and Magnetic Materials 192 (1999) 83–99. [18] N. Hanasaki, K. Watanabe, T. Ohtsuka, I. Ke´zsma´rki, S. Iguchi, S. Miyasaka, Y. Tokura, Nature of the transition between a ferromagnetic metal and a spinglass insulator in pyrochlore molybdates, Physical Review Letters 99 (2007) 086401. [19] J.H. Dho, W.S. Kim, N.H. Hur, Reentrant spin glass behavior in Cr-doped perovskite manganite, Physical Review Letters 89 (2002) 027202. [20] I.A. Campbell, S. Senoussi, F. Varret, J. Teillet, A. Hamzic´, Competing ferromagnetic and spin-glass order in a AuFe alloy, Physical Review Letters 50 (1983) 1615–1618. [21] N. Mohapatra, S.N. Jammalamadaka, S.D. Das, E.V. Sampathkumaran, Magnetic anomalies in Nd6Co1.67Si3: a first-order transition in the low-temperature isothermal magnetization behavior, Physical Review B 78 (2008) 054442. [22] M. Mertinat, V. Tsurkan, D. Samusi, R. Tidecks, F. Haider, Low-temperature structural transition in FeCr2S4, Physical Review B 71 (2005) 100408. [23] X.X. Zhang, J.M. Herna ndez, J. Tejada, R. Sole´, X. Ruiz, Magnetic properties and domain-wall motion in single-crystal BaFe10.2Sn0.74Co0.66O19, Physical Review B 53 (1996) 3336–3340. [24] V. Tsurkan, J. Hemberger, M. Klemm, S. Klimm, A. Loidl, S. Horn, R. Tidecks, AC susceptibility studies of ferrimagnetic FeCr2S4 single crystals, Journal of Applied Physics 90 (2001) 4639.
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Local spin reversal and associated magnetic responses in Ga-substituted Pb-hexaferrites Eun Hye Na a, Jung-Hoon Lee a, Suk-Jin Ahn a, Kun-Pyo Hong b, Yang Mo Koo c, Hyun Myung Jang a,n a
Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea Neutron Science Division, Department of Basic Science and Technology, Korea Atomic Energy Research Institute (KAERI), Daejon 305-353, Republic of Korea c Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 February 2012 Available online 4 May 2012
We have studied magnetic structure and properties of Ga-substituted Pb-hexaferrites having the stoichiometry of PbFe12 xGaxO19 with x ¼ 6 (i.e., Fe:Ga ¼ 1:1). According to the neutron diffraction results, this compound is characterized by a collinear spin structure below its Curie temperature ( 325 K). Analysis of the neutron diffraction patterns further indicates that the magnetic-moment direction of Fe3 þ ions located at the octahedral 2a sublattice is downward while that of the unsubstituted PbFe12O19 is upward at room temperature. With decreasing temperature, the Fe3 þ magnetic moment at the octahedral 2a sublattice undergoes a reorientation to the upward direction while that of the unsubstituted PbFe12O19 remains upward down to 5 K. This selective local spin reversal at the 2a sublattice of PbFe6Ga6O19 was attributed to the weakening of the superexchange interaction between the octahedral 2a site and the tetrahedral 4fIV site upon the preferential substitution of Ga ions for Fe ions at these two neighboring sites. Comparison of the neutron diffraction results with dc magnetization responses and ac susceptibilities further indicates that the paramagnetic–ferrimagnetic transition at 325 K (Tc) is followed by the local spin reversal at lower temperatures. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ferrimagnetic Spin arrangement Neutron diffraction Magnetic susceptibility
1. Introduction Hexagonal PbFe12O19 is a ferrimagnetic material with its Fe-magnetic moment aligned parallel to the crystallographic c-axis[1]and does belong to well-known M-type hexaferrites having the common stoichiometry of MFe12O19, where M¼ Ba, Pb, and Sr. There have been extensive research efforts to partially replace Fe ions by some other cations to improve magnetic and dielectric properties of M-type hexaferrites [2,3]. The influence of the cationic distribution and the consequent sublattice magnetization of M-type hexaferrites have been studied by neutron ¨ diffraction and Mossbauer spectroscopy measurements[2,4,5]. Among numerous studies done on the partial replacement of Fe ions by diamagnetic or paramagnetic ions, the Ga-substitution is expected to play a unique role. This is because Ga ion is trivalent, with its ionic radius comparable to that of Fe3 þ . Thus, the trivalent Ga ions are expected to minimize the formation of oxygen vacancy defects and consequently to improve the electrical resistivity and the dielectric loss. Furthermore, since Ga ions are nonmagnetic, the Ga-ion substitution for Fe ions would
n
Corresponding author. E-mail address: [email protected] (H.M. Jang).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.04.031
modulate the degree of the superexchange interaction between neighboring Fe ions and thus alter magnetic properties [6]. Although the Ga-substituted M-type hexaferrite had been ¨ studied by Mossbauer spectroscopy, the variation of its magnetic order caused by the Ga-substitution is not clearly understood and is still an open question [7–9]. Maryˇsko and co-workers [9] reported the results of magnetic measurements on single crystals of PbFe12 xGaxO19 for x o8. Their study elucidated a peculiar phenomenon of the crystallographic direction-dependent Curie temperature (Tc) for x43 and hypothesized a non-collinearity in the magnetic order. Albanese et al. [8]also proposed a noncollinear magnetic ordering of PbFe12 xGaxO19 for x43 based on their measurements of the composition-dependent saturation magnetization (Ms). However, they were not able to find any ¨ evidence of the non-collinear magnetic ordering by Mossbauer spectroscopy measurements since the spectra are too complicated to be resolved. Accordingly, the main purpose of the present study is elucidate the evolution of the magnetic structure of the single-crystalline hexagonal PbFe6Ga6O19 (i.e., Fe:Ga ¼1:1) by performing the temperature-dependent neutron diffraction measurements and subsequently carrying out the Rietveld analysis of these diffraction patterns. More specifically, we first determined the Ga-ion occupancies for various sites by the Rietveld refinement of the
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high-temperature diffraction data obtained at a temperature above Tc (325 K). Then, the magnetic moments of various sublattices below Tc (5, 100, and 300 K) are separately determined. From the neutron diffraction analysis, we are able to correlate the observed anomalous behavior of the ac magnetic susceptibility, wac(T), with the local spin reversal at the octahedral 2a sublattice.
2. Experimental Methods PbFe12 xGaxO19 single crystals (x ¼6) used in the present study were grown by the PbO flux method with PbO, Fe2O3, and Ga2O3 as the starting ingredients. For this purpose, the following mole ratio of the flux was adopted: Pb:Fe:Ga¼4:6:6. Thus, the amount of the PbO flux used is three times of the stoichiometic amount of the PbO ingredient. This starting powder mixture was then placed in a Pt crucible and dissolved at 1300 1C for 12 h. The flux melt was first cooled down to 800 K with a cooling rate of 1 1C/h and subsequently cooled to room temperature with a faster
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cooling rate of 6 1C/h. For diffraction and magnetic measurements, the flux-grown single crystals were suitably grinded to obtain single-crystalline powder samples. The composition of Ga in PbFe12 xGaxO19, as determined by inductively coupled plasmaoptical emission spectroscopy (ICP-OES), is x¼6. Magnetic susceptibility of the single-crystalline PbFe6Ga6O19 powder sample was measured using a magnetic property measurement system (MPMS, Quantum Design). Both the temperature-dependent magnetization and the ac magnetic susceptibility reveal that the para-to-ferrimagnetic transition temperature of PbFe6Ga6O19 is 325 K (Tc). Neutron diffraction data were collected over the scattering angle between 10o and 160o with a 2ystep of 0.05o using a high-resolution powder diffractometer (with ˚ at the Hanaro Center of the Korea l of neutron beam ¼1.8349 A) Atomic Energy Research Institute (KAERI). The Rietveld method was used to refine the atomic and magnetic structures of the hexagonal PbFe6Ga6O19. For this purpose, we adopted a wellknown software package called ‘FULLPROF Program Suite’.
3. Results and discussion 3.1. Refined crystal structure and preferential substitution of Ga ions
Fig. 1. A schematically represented refined crystal structure of PbFe6Ga6O19 having the hexagonal P63/mmc symmetry. The Rietveld refinement is based on the neutron diffraction data obtained at 500 K.
In order to examine the crystal and magnetic structures of PbFe6Ga6O19, neutron powder diffraction experiments were carried out at several selected temperatures below and above Tc ( 325 K) in the absence of any bias magnetic field. The diffraction pattern obtained at 500 K was first analyzed to obtain: (i) the crystal structure parameters (i.e., atomic positions and lattice parameters, etc.) and (ii) the Ga-ion distribution among different sublattices as the diffraction data at 500 K (4 Tc) do not reflect any magnetic component but include crystal-structure components only. The refined crystal structure and the crystal parameters obtained using the Rietveld refinement of the diffraction data are presented in Fig. 1 and Table 1, respectively. According to the structure model of the M-type hexaferrite (MFe12O19) first proposed by Gorter [7,10], 12 Fe ions per formula unit are distributed among five distinct sublattices: (i) six in the octahedral sublattice k (spin-up), (ii) one in the octahedral sublattice a (spin-up), (iii) one in the five-fold sublattice b (spin-up), (iv) two in the tetrahedral sublattice fIV (spin-down), and (v) two in the octahedral sublattice fVI (spin-down). According to the result presented in Table 1, the Ga-ion substitution occurs at all five sublattices but with a marked preference for the octahedral 2a and tetrahedral 4fIV (4f1) sites. On the other hand, a limited value of the Ga-occupancy is found in the five-fold 2b and octahedral 4fVI (4f2) sites. This conclusion on the Ga-ion distribution does not accord well with the previously proposed model [8] in which Ga
Table 1 Details of the refined crystal structure of PbFe6Ga6O19 having the hexagonal P63/mmc symmetry, where X, Y, and Z denote the fractional atomic coordinates. Atom Pb Fe1/Ga1 Fe2/Ga2 Fe3/Ga3 Fe4/Ga4 Fe5/Ga5 O1 O2 O3 O4 O5
Coordination
Octahedral Five-fold Tetrahedral Octahedral Octahedral
Sublattice
X
Y
Z
Occupancy (Fe/Ga)
2d 2a 2b 4f1 4f2 12k 4e 4f 6h 12k 12k
0.66666 0 0 0.33333 0.33333 0.1674 0 0.3333 0.18147 0.15623 0.50266
0.33333 0 0 0.66666 0.66666 0.33479 0 0.66666 0.36285 0.31244 1.00522
0.25 0 0.25 0.02678 0.18938 0.10844 0.15004 0.05577 0.2501 0.05166 0.14913
1 0.282/0.718 0.861/0.139 0.369/0.691 0.761/0.239 0.422/0.578 1 1 1 1 1
˚ The goodness of fit (w2) was 6.64, and the following values were obtained for the two representative The refined lattice parameters are: a¼ 5.8640 A˚ and c ¼23.1178 A. reliability factors: RB (Bragg reliability factor) ¼ 4.91 and Rf ¼3.46.
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ions are distributed rather uniformly over the five distinct sites for x r5 but with a slightly diminished occupancy at the 2b site beyond this value (i.e., x45). 3.2. Collinear spin structure and selective spin reversal Fig. 2 presents neutron diffraction patterns of PbFe6Ga6O19 obtained at four selected temperatures, 5 K, 100 K, 300 K, and 500 K. For all four patterns, no reflection appears at a scattering angle which is different from the Bragg reflection positions corresponding to the space group P63Xmmc. This indicates that P63/mmc centrosymmetric structure is maintained over a wide temperature range between 5 K and 500 K. As shown in Fig. 2, the intensity of the [00L] reflection at 2y ¼18.31 is nearly independent of temperature over a wide temperature range. Recall that magnetic scattering occurs only when the magnetic moment vector has a non-zero component projected on the scattering plane considered [11]. The observation that the [00L]-peak intensity is nearly independent of temperature thus indicates that there does not exist any magnetic-moment component which projects on [00L] planes (ab in-planes) for a wide temperature range. This, in turn, indicates that the magnetic moment is aligned parallel to the c-axis with the absence of any ab-plane component. Besides, the degree of the long-range ordering is considered to be essentially unchanged since the magnetic [10L] reflection neither shows any peak broadening nor has a tendency of the peak-intensity decrease as temperature goes down to 5 K [12]. Then, the magnetic structures at 5, 100, and 300 K were computed using the collinear ferrimagnetic model of Gorter [10]. More specifically, the refinements at 5, 100, and 300 K were done on the basis of the cation distribution obtained from the high-temperature (500 K) measurements and all cations were taken into consideration. Firstly, the values of the Ga-occupancy factor for various sites were fixed at 500 K values during the initial refinement and the magnetic moments and crystal parameters were subsequently estimated. In the second run, these occupancy factors were then given as free parameters to obtain the magnetic and crystal parameters. They all showed some variations but the discrepancies in the occupancy factor between these two runs are within the error limit, confirming the reliability of the present estimate of the Ga-occupancy factors.
w2 ¼ 7.19, RB ¼2.30, Rf ¼ 1.30, and Rmag (reliability factor for the magnetic refinement)¼ 2.56 at 5 K. (ii) w2 ¼7.07, RB ¼ 2.30, Rf ¼ 1.36, and Rmag ¼2.76 at 100 K. (iii) w2 ¼ 6.70, RB ¼ 2.42, Rf ¼ 1.51, and Rmag ¼4.49 at 300 K. These R values
Fig. 2. Observed (red-colored dots) and calculated (black line) neutron diffraction patterns of PbFe6Ga6O19 at 5 K. The green-colored vertical bars at the first row below the diffraction pattern indicate the positions of the nuclear Bragg reflections while the vertical bars at the second row depict the locations of the magnetic Bragg peaks. The blue line at the bottom shows the difference between the observed and calculated diffraction patterns. The inset shows the low-angle diffraction patterns (2y ¼ 15–401) obtained at various temperatures, 5, 100, 300 and 500 K. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3 presents the refined magnetic structure of PbFe6Ga6O19 at 5 K and 300 K. As shown in Table 2, the estimated magnetic moment of Fe3 þ ions at the octahedral 2a sublattice is 1.384 mB at 300 K with a downward spin configuration. Contrary to this, the magneticmoment direction at the octahedral 2a site of the pristine (unsubstituted) PbFe12O19 is known to be upward [8]. The Fe3 þ magnetic moment at the octahedral 2a sublattice of PbFe6Ga6O19 undergoes a reorientation to the upward direction beginning at a temperature higher than 100 K (Table 2) while that of the unsubstituted PbFe12O19 remains unchanged (upward) down to 5 K. It is known that the local weakening of the octahedral– tetrahedral superexchange interaction which is caused by the substitution of non-magnetic ions for Fe3 þ ions in ferrimagnetic oxides can give rise to either a random canting of the magnetic moments or a local spin reversal [13–15]. In addition, it is known that the magnitude and the sign of the octahedral–tetrahedral sublattice coupling is susceptible to the variation of temperature [16]. Thus, the observed selective spin reversal with a gradual variation in the magnitude of the spin moment at the octahedral 2a site of PbFe6Ga6O19 (Table 2) can be attributed to the weakening of the temperature-sensitive superexchange interaction between the octahedral 2a sublattice and the tetrahedral 4fIV
Fig. 3. Schematic representations of the local magnetic moments at various sites of the hexagonal PbFe6Ga6O19, showing the selective spin-moment reversal occurring at the octahedral 2a sublattice.
Table 2 Local magnetic moments for various sublattices of the M-type hexaferrite, PbFe6Ga6O19, at three selected temperatures below Tc. Coordinates
Octahedral Five-fold Tetrahedral Octahedral Octahedral Total Mz (mB/f.u) Ms (mB/f.u)
Wyckoff position
2a 2b 4fIV 4fVI 12k
Magnetic moment (mB/Fe) T¼ 5 K
T ¼100 K
T¼ 300 K
1.448 3.386 4.002 3.621 4.438 6.096 6.22
0.978 3.008 3.903 3.479 4.008 4.838 4.93
1.384 1.596 2.402 1.714 2.028 1.737 1.76
Temperature-dependent representative reliability factors are as follows: (i)
ensure the reliability of the present Rietveld refinement.
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(4f1) sublattice, which is caused by the preferential substitution of Ga ions (for Fe ions) at these two neighboring sites (Table 1). One can theoretically estimate the net magnetization per formula unit (Mz) by the summation of various sublattice magnetization values using the following formula [17]: M z ¼ n2a s2a þ n2b s2b þ n2a s2a þ 2n4f IV s4f IV þ 2n4f VI s4f VI þ 6n12k s12k
ð1Þ
where ni and si, respectively, denote the Fe-ion occupancy factor and the local magnetic moment at the sublattice i. We have compared the computed net magnetization value with the saturation magnetization (Ms) value obtained from the experimental magnetization-field (M–H) hysteresis curve shown in Fig. 4. As presented in Table 2, the two values agree with each other for all three temperatures examined. 3.3. Temperature-dependent dc magnetization and ac susceptibility According to the temperature-dependent dc magnetization curve, M(T), presented in Fig. 5(a), the onset of a long-range magnetic ordering is approximately 325 K (Tc). As presented, the out-of-plane (H//c-axis) magnetization is much bigger than the in-plane (H//ab-plane) magnetization, which coincides with the uniaxial ferrimagnetism along the c-axis, as deduced from the neutron diffraction data (Section 3.2). According to Fig. 5(a), ZFC (zero-field cooling) and FC (field cooling) curves bifurcate beginning at 320 K and another magnetic anomaly appears at 290 K. Below 70 K, the ZFC magnetization decreases while the FC magnetization even increases with decreasing temperature. Fig. 5(b) shows the temperature-dependent ac magnetic susceptibility obtained using an ac amplitude of 10 Oe (in the absence of any bias-field). Both w0 and w00 show their peaks at 325 K which coincides with the long-range magnetic ordering temperature (Tc) deduced from the M(T) curve (Fig. 5(a)). Another prominent feature of the ac susceptibility is the appearance of a broad peak over a wide temperature range between 70 K and 290 K, and the dc magnetization is represented by a plateau-like response in this temperature interval. Fig. 6 presents the frequency-dependent magnetic susceptibility for a wide range of temperature. The most prominent feature of the real part of the ac susceptibility (w0 ) is that an intense peak at Tc ( 325 K) is followed by the appearance of a broad lower-temperature peak beginning at 290 K. The imaginary part of the ac susceptibility (w00 ) also shows essentially the same feature, which is nearly independent of the ac frequency. In addition to this, there
Fig. 4. Magnetization-field (M–H) hysteresis curves of PbFe6Ga6O19 having the hexagonal P63/mmc symmetry at various temperatures below Tc.
Fig. 5. Temperature-dependent (a) dc magnetization and (b) ac susceptibility curves of the hexagonal PbFe6Ga6O19, showing a large discrepancy between ZFC and FC curves beginning at 70 K.
appears a large discrepancy between the ZFC and FC curves beginning at 70 K (Fig. 5(a)). All these observations suggest that the appearance of the broad susceptibility response beginning at 290 K is possibly caused by a transition to a reentrant spin-glass state [16,18]. However, as shown in Fig. 6, the peak temperature decreases with increasing ac frequency, which is in direct disagreement with the ac-frequency-dependent peak temperature observed in many spin-glass systems [19]. As discussed previously, the degree of the long-range ordering remains essentially unchanged since the magnetic (10 L) reflection neither shows any peak broadening nor has a tendency of the peak-intensity reduction as temperature decreases down to 5 K [12]. This again excludes the possibility of the transition to a reentrant spin-glass state with decreasing temperature. Besides, it is known that the coercive field does increase substantially when a system undergoes the transition to a spin-glass state [19,20]. As shown in Fig. 4, the coercive field remains at a small value down to 5 K (a few tens of Oe). This observation also indicates that a reentrant spin-glass state is not relevant to the hexagonal PbFe6Ga6O19 system down to 5 K. A broad nature of the ac susceptibility peak appeared between 70 and 290 K suggests that a single magnetic transition is not related to this broad ac response [21]. Considering this, we suggest that a domain-wall pinning mechanism which is initiated by the local spin reversal at the octahedral 2a site is primarily responsible for the appearance of this broad ac susceptibility response. If the spin reversal at the 2a site invokes spin-lattice coupling, the resulting lattice distortion would lead to the formation of a defect structure. Then, this defect structure not only becomes a new pinning center for the domain-wall motion but also expedites a rearrangement of the domain structure [22]. The domain-wall pinning then explains the observed large
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Acknowledgments This work was supported by the Brain Korea 21 project 2011 and by the WCU (World Class University) program through the National Research Foundation funded by the Ministry of Education, Science and Technology of Korea (R32-10147). References
Fig. 6. Temperature-dependent real and imaginary parts of the ac magnetic susceptibility of the hexagonal PbFe6Ga6O19 measured at various ac frequencies between 10 and 1000 Hz.
discrepancy between the ZFC and FC curves below 70 K [19,23]. Indeed, the domain-structure variation induced by the spinphonon coupling had been proposed to account for the discrepancy between the ZFC and FC curves observed in ferromagnetic FeCr2S4 [22,24]. According to the temperature-dependent local magnetic moment (Table 2), the selective spin reversal at the octahedral 2a site occurs at a temperature between 100 K and 300 K. As shown in Fig. 6, the ac susceptibility is characterized by a broad response between 70 and 290 K with the peak temperature at 160 K. These two observations suggest that the local spin reversal occurs at a temperature above 100 K but is characterized by a continuous, gradual transition over a wide temperature range. The peak temperature of the ac susceptibility then can presumably be identified with the statistically most probable temperature for the local spin reversal.
4. Conclusions In conclusion, analysis of the neutron diffraction patterns indicates that the Fe3 þ magnetic moment at the octahedral 2a sublattice of PbFe6Ga6O19 undergoes a switching in its direction beginning at a temperature higher than 100 K. This selective spin reversal with a gradual variation in the magnitude of the local magnetic moment at the octahedral 2a site is explained in terms of the weakening of the temperature-sensitive superexchange interaction between the octahedral 2a sublattice and the tetrahedral 4fIV sublattice, which is caused by the preferential substitution of Ga ions at these two neighboring sites. The selective local spin reversal is then correlated with the broad ac susceptibility response between 70 and 290 K.
[1] G.K. Thompson, B.J. Evans, The structure–property relationships in Mtype hexaferrites: hyperfine interactions and bulk magnetic properties, Journal of Applied Physics 73 (1993) 6295–6297. ˜ o´n, F. Leccabue, B.E. Watts, Mossbauer ¨ [2] G. Albanese, S. Dı´az-Castan and magnetic investigation of scandium and indium substituted PbFe12O19 hexagonal ferrite, Journal of Materials Science 35 (2000) 4415–4420. ˜ o´n, F. Leccabue, B.E. Watts, G. Albanese, Magnetic and [3] S. Dı´az-Castan ¨ Mossbauer investigation of Sc- and In-substituted PbFe12O19 hexaferrites, Journal of Magnetism and Magnetic Materials 196 (1999) 458–460. [4] Z. Somogyva´ri, E. Sva´b, K. Krezhov, L.F. Kiss, D. Kapta´s, I. Vincze, E. Beregi, F. Boure e, Non-collinear magnetic order in a Sc-substituted barium hexaferrite, Journal of Magnetism and Magnetic Materials 304 (2006) e775–e777. [5] A. Collomb, P. Wolfers, X. Obradors, Neutron diffraction studies of some hexagonal ferrites: BaFe12O19, BaMg2–W and BaCo2–W, Journal of Magnetism and Magnetic Materials 62 (1986) 57–67. [6] M.J. Iqbal, M.N. Ashiq, P.H. Gomez, J.M. Munoz, Synthesis, physical, magnetic and electrical properties of Al–Ga substituted co-precipitated nanocrystalline strontium hexaferrite, Journal of Magnetism and Magnetic Materials 320 (2008) 881–886. [7] H. Stepankova, J. Englich, NMR study of Ga and Al substituted hexagonal ferrites with magnetoplumbite structure, IEEE Transactions on Magnetics 30 (1994) 988–990. [8] G. Albanese, F. Leccabue, B.E. Wattes, S. Diaz-castanon, Magnetic and ¨ Mossbauer investigation of PbFe12 xGaxO19 hexagonal ferrites, Journal of Materials Science 37 (2002) 3759–3763. [9] M. Marysko, Z. Frait, S. Krupicka, FMR and static magnetic properties of gallium substituted magnetoplumbite, Journal de Physique IV France 07 (1997) 347–348. [10] E.W. Gorter, Saturation magnetization of some ferrimagnetic oxides with hexagonal crystal structures, Proceedings of the IEEE 104B (1957) 255–260. [11] Y. Takada, T. Nakagawa, M. Tokunaga, Y. Fukuta, T. Tanaka, T.A. Yamamoto, T. Tachibana, S.I. Kawano, Y. Ishii, N. Igawa, Crystal and magnetic structures and their temperature dependence of Co2Z-type hexaferrite (Ba,Sr)3Co2Fe24O41 by high-temperature neutron diffraction, Journal of Applied Physics 100 (2006) 043904. [12] V. Kiryukhin, S. Lee, W. Ratcliff II, Q. Huang, H.T. Yi, Y.J. Choi, S.W. Cheong, Order by static disorder in the ising chain magnet Ca3Co2-xMnxO6, Physical Review Letters 102 (2009) 187202. [13] G. Albanese, M. Carbucicchio, G. Asti, Spin-order and magnetic properties of BaZn2 Fe16 O27 (Zn2–W) hexagonal ferrite, Applied Physics 11 (1976) 81–88. [14] D. Vladikoava, L. Ilkov, Influence of the superexchange interactions on the saturation magnetization and Curie temperature in substituted yttrium iron garnets, Physica Status Solidi A 156 (1996) 461. [15] B.X. Gu, Magnetic properties of Ba2(ZnxFe1 x)2Fe28O46 xtype hexaferrites, Journal of Applied Physics 75 (1994) 4114–4116. [16] S. Srinath, P. Poddar, H. Srikanth, B.C. Sales, D. Mandrus, Observation of a new magnetic anomaly below the ferromagnetic Curie temperature in Yb14MnSb11, Physical Review Letters 95 (2005) 227205. ¨ [17] P. Wartewig, M.K. Krause, P. Esquinazi, S. Rosler, R. Sonntag, Magnetic properties of Zn- and Ti-substituted barium hexaferrite, Journal of Magnetism and Magnetic Materials 192 (1999) 83–99. [18] N. Hanasaki, K. Watanabe, T. Ohtsuka, I. Ke´zsma´rki, S. Iguchi, S. Miyasaka, Y. Tokura, Nature of the transition between a ferromagnetic metal and a spinglass insulator in pyrochlore molybdates, Physical Review Letters 99 (2007) 086401. [19] J.H. Dho, W.S. Kim, N.H. Hur, Reentrant spin glass behavior in Cr-doped perovskite manganite, Physical Review Letters 89 (2002) 027202. [20] I.A. Campbell, S. Senoussi, F. Varret, J. Teillet, A. Hamzic´, Competing ferromagnetic and spin-glass order in a AuFe alloy, Physical Review Letters 50 (1983) 1615–1618. [21] N. Mohapatra, S.N. Jammalamadaka, S.D. Das, E.V. Sampathkumaran, Magnetic anomalies in Nd6Co1.67Si3: a first-order transition in the low-temperature isothermal magnetization behavior, Physical Review B 78 (2008) 054442. [22] M. Mertinat, V. Tsurkan, D. Samusi, R. Tidecks, F. Haider, Low-temperature structural transition in FeCr2S4, Physical Review B 71 (2005) 100408. [23] X.X. Zhang, J.M. Herna ndez, J. Tejada, R. Sole´, X. Ruiz, Magnetic properties and domain-wall motion in single-crystal BaFe10.2Sn0.74Co0.66O19, Physical Review B 53 (1996) 3336–3340. [24] V. Tsurkan, J. Hemberger, M. Klemm, S. Klimm, A. Loidl, S. Horn, R. Tidecks, AC susceptibility studies of ferrimagnetic FeCr2S4 single crystals, Journal of Applied Physics 90 (2001) 4639.