Substitution effects on Y-type hexaferrites’ magnetic characteristics

The effect is discussed of the partial replacement of the Me2+ in the Y-type hexaferrite crystal structure with non-magnetic (Mg2+) and magnetic (Ni2+) cations on the observed magnetic characteristics of Ba0.5Sr1.5Me2Fe12O22. Y-type hexaferrite powder material was prepared by citric acid sol-gel self-ignition and sintering at 1170 °C in air to reach Ba0.5Sr1.5MgxNiyFe12O22. To determine the magnetic phase transitions, the ZFC/FC magnetisation was followed in magnetic fields of 50 Oe and 100 Oe between 4.2 K and 300 K; this was complemented with hysteresis loops recorded at the end temperatures. The results show that combining the magnetic Ni2+ with the nonmagnetic Mg2+ in Ba0.5Sr1.5 Me2Fe12O22 can be effective in controlling the magnetic interactions to regulate the temperature of the magnetic transition from helical to ferrimagnetic arrangement above room temperature.


Introduction
Y-type hexaferrites are being extensively researched with a view to clarifying the mechanisms behind the magneto-electric effect.These efforts also led to the emergence of new possibilities for their application in various fields, as evidenced in, e.g., [1][2][3].In this regard, Ba 0.5 Sr 1.5 Me 2 Fe 12 O 22 (Mea divalent cation) is a Y-type hexaferrite attracting strong interest for its exhibiting a magneto-electric effect at room temperature.It is due to the fact that the partial replacement of Ba 2+ with Sr 2+ (smaller ionic radius), causing a distortion in the crystal cell, leads to a re-distribution of Me 2+ and Fe 3+ in the tetrahedral sites and a change in the magnetic spin arrangement to a helical spin ordering.It is well known that Me +2 and Fe +3 cations are arranged in six positions in the unit cell: two tetrahedral (6cIV and 6c * IV) and four octahedral ones (3aVI, 3bVI, 6cVI, and 18hVI) [4].The Me +2 ion parameters (ionic radius, magnetic moment) and their positions in the unit cell can change considerably the properties of the materials discussed here.For example, the Ba 0.5 Sr 1.5 Zn 2 Fe 12 O 22 hexaferrite is a ferrimagnetic insulator with a helical spin order that is stable below its Neel temperature, T N , of 326 K; by varying the temperature one can achieve several magnetic-phase transitions [5,7].In our previous works [8,9] on substituting Zn 2+ with Ni 2+ in Ba 0.5 Sr 1.5 Zn 2-x Ni x Fe 12 O 22 , we found that the half-substitution of the nonmagnetic Zn 2+ with the magnetic Ni 2+ shifts the helical spin ordering towards the high temperatures.As is known, the electrical polarization induced in Ba 2 Mg 2 Fe 12 O 22 by low magnetic fields is due to the magnetic anisotropy being reduced by the presence of Mg 2+ cations [10].In [11], Zhang and co-authors reported a rise in the transition temperature of a helical spin phase to a ferrimagnetic collinear phase in Ba 0.5 Sr 1.5 Zn 2−x Mg x Fe 12 O 22 that resulted from Mg doping.Recently, in [12], we showed that in Ba 0.5 Sr 1.5 Zn 2 Fe 12 O 22 combining the magnetic Ni 2+ with the non-magnetic Mg 2+ , rather than with Zn 2+ , results in shifting the temperature of transition from a screw spin ordering to a ferrimagnetic one above room temperature.Such magnetic frustration sometimes reduces the spin-structure symmetry, which is now believed to affect the charge-distribution symmetry, thus inducing ferroelectricity.In this regard, in the present article we discuss our results on the influence of combining the magnetic cation Ni 2+ with the non-magnetic Mg 2+ on the magnetic-phase transitions in and the properties of the Ba 0.5 Sr 1.5 Mg x Ni y Fe 12 O 22 hexaferrite powder material, where x = 0.5,1.5;y = 1.5, 0.5; and x+y = 2.

Experimental
We prepared the Ba 0.5 Sr 1.5 Ni 1.5 Mg 0.5 Fe 12 O 22 and Ba 0.5 Sr 1.5 Ni 0.5 Mg 1.5 Fe 12 O 22 powder materials by sol-gel self-ignition and burning using citric acid as a chelator.The synthesis procedure followed and the subsequent sintering are described in detail in [12].
The structure and phase content of the sintered pellets were examined by X-ray diffraction (XRD).The magnetic properties were determined; the hysteresis loops were traced at room temperature and at 4.2 K.The behaviour of the magnetisation as a function of the temperature in a magnetic field was explored in ZFC and FC procedures in temperatures ranging from 4.2 K to 300 K.The experimental procedures and the equipment used were presented thoroughly in [12].The hexaferrites of Y type are oxides of complicated structure which often coexist with various other magnetic oxides; such secondary phases appear and persist mostly due to the range of temperatures allowing the Y-phase formation being fairly limited.Figure 1 shows the room-temperature XRD patterns of the prepared Ba 0.5 Sr 1.5 Ni 1.5 Mg 0.5 Fe 12 O 22 and Ba 0.5 Sr 1.5 Ni 0.5 Mg 1.5 Fe 12 O 22 powders; these were indexed in the centrosymmetric R¯3m space group and exhibit peaks attributed to a main phase of Ytype hexaferrite structure.The additional peaks identified as a minor phase belong to the spinel (MgNi)Fe 2 O 4 with an amount of 2.4 wt.%.Concerning the effect of this phase presence, in our earlier works [9,12,13] on various compositions of Y-type hexaferrites we demonstrated that spinel ferrites in such minute amounts do not affect the magnetic phase-transition temperatures.We reported similar behaviour in our previous studies on Ba 0.5 Sr 1.5 NiMgFe 12 O 22 powder [12], as well as in other Y-type hexagonal ferrites with well-pronounced magnetoelectric coupling, as in Ba 0.5 Sr 1.5 Co 2 Fe 12 O 22 [14]; it is assumed that this behaviour is a proof of transitions taking place between different magnetic orders as induced by the varying magnetic field [15,16].This effect is ordinarily associated with the existence of metamagnetic or intermediate phases preceding the establishment of a helical magnetic ordering [16].Three magnetic phases exist as the magnetic field is varied in the case describedone persisting to 2.5 kOe, another persisting in the range 2.5 -5 kOe.Above 5 kOe, the magnetisation exhibits a behaviour typical for ferromagnets.[9] and Ba 0.5 Sr 1.5 NiMgFe 12 O 22 [12].The magnetisation increases smoothly to about 170 K and 200 K at 50 Oe and 100 Oe, respectively (figure 4 (c), (d)).With the further temperature rise towards 300 K, the increase becomes very sharp.For Ba 0.5 Sr 1.5 Ni 0.5 Mg 1.5 Fe 12 O 22 (figure 4 (a), (b)), the ZFC-FC magnetisation reaches a maximum at 49 K and 40 K respectively for 50 Oe and 100 Oe, which has to do with a magnetic-phase transition attributed to the conical spin ordering formed at lower temperatures.This transition in Ba 0.5 Sr 1.5 Ni 0.5 Mg 1.5 Fe 12 O 22 is known to give rise to the onset of a notable magneto-electric coupling.As the temperature rises to about 200 K, the magnetisation decreases and then slowly increases., an increased nickel content, the saturation magnetisation increased.In addition, as the magnesium concentration was increased, the magnetisation vs. temperature curves changed radically indicating a rearrangement of the magnetic structure.Therefore, compared with the Ba 0.5 Sr 1.5 Zn 2 Fe 12 O 22 hexaferrite, where the change from helical to ferrimagnetic order occurs above room temperature, the concurrent presence of the magnetic Ni 2+ and the nonmagnetic Mg 2+ in the divalent Me sites of the Ba 0.5 Sr 1.5 Me 2 Fe 12 O 22 structure opens up an effective way of tuning the magnetic exchange interactions, thus achieving a large magnetoelectric coupling effect even at higher temperatures.

Figure 2
Figure 2 and figure 3 present the initial magnetisation curves of Ba 0.5 Sr 1.5 Ni 0.5 Mg 1.5 Fe 12 O 22 and Ba 0.5 Sr 1.5 Ni 1.5 Mg 0.5 Fe 12 O 22 powders and their hysteresis loops at 300 K and 4.2 K, respectively, taken when the magnetic field was raised to 50 kOe.The magnetisation curves for the two temperatures and powder materials saturate at about 40 kOe, with largest magnetisation values reached of 23 emu/g and 17 emu/g at 4.2 K and 300 K for Ba 0.5 Sr 1.5 Ni 0.5 Mg 1.5 Fe 12 O 22 and 32 emu/g and 24 emu/g at 4.2 K and 300 K for Ba 0.5 Sr 1.5 Ni 1.5 Mg 0.5 Fe 12 O 22 .At 300 K, the magnetisation rises steeply to H ≈ 1.5 kOe and subsequently follows a course typically observed in ferromagnetic materials.A step-wise course is found for the initial magnetisation curve at 4.2 K for the Ba 0.5 Sr 1.5 Ni 0.5 Mg 1.5 Fe 12 O 22 powder (figure 2 (b)).We reported similar behaviour in our previous studies on Ba 0.5 Sr 1.5 NiMgFe 12 O 22 powder[12], as well as in other Y-type hexagonal ferrites with well-pronounced magnetoelectric coupling, as in Ba 0.5 Sr 1.5 Co 2 Fe 12 O 22[14]; it is assumed that this behaviour is a proof of transitions taking place between different magnetic orders as induced by the varying magnetic field[15,16].This effect is ordinarily associated with the existence of metamagnetic or intermediate phases preceding the establishment of a helical magnetic ordering[16].Three magnetic phases exist as the magnetic field is varied in the case describedone persisting to 2.5 kOe, another persisting in the range 2.5 -5 kOe.Above 5 kOe, the magnetisation exhibits a behaviour typical for ferromagnets.

Figure 2 .
Figure 2. Initial magnetisation dependence on the magnetic field (a), (b); hysteresis loops (c) at 4.2 K and 300 K for the Ba 0.5 Sr 1.5 Ni 0.5 Mg 1.5 Fe 12 O 22 powder.The hysteresis loops are very narrow, especially for the Ba 0.5 Sr 1.5 Ni 0.5 Mg 1.5 Fe 12 O 22 material, where the coercive field, H c , and the remanent magnetisation are approximately zero for both temperatures.The sample with a higher Ni 2+ content, Ba 0.5 Sr 1.5 Ni 1.5 Mg 0.5 Fe 12 O 22 , is distinguished by coercivity, H c , of 35 Oe and 66 Oe at 300 K and 4.2 K, respectively.These values are common for hexaferrites of planar magneto-crystalline anisotropy.The remanent magnetisation is very low.

Figure 3 .
Figure 3. Initial magnetisation dependence on the magnetic field (a) and hysteresis loops (b) at 4.2 K and 300 K for Ba 0.5 Sr 1.5 Ni 1.5 Mg 0.5 Fe 12 O 22 .Expanded view of the narrow magnetic hysteresis loops for Ba 0.5 Sr 1.5 Ni 1.5 Mg 0.5 Fe 12 O 22 up to 2 kOe at 4.2 K and 300 K (c).

Figure 4
Figure 4 displays the variation of the ZFC and FC magnetisations with the temperature for Ba 0.5 Sr 1.5 Ni 0.5 Mg 1.5 Fe 12 O 22 (a), (b) and Ba 0.5 Sr 1.5 Ni 1.5 Mg 0.5 Fe 12 O 22 (c), (d) powders in fields of 50 Oe and 100 Oe with the temperature ranging from 4.2 K to 300 K. Their course for these two compositions is completely different.The two curves for Ba 0.5 Sr 1.5 Ni 1.5 Mg 0.5 Fe 12 O 22 behave similarly to these dependences measured in our previous works for Ba 0.5 Sr 1.5 ZnNiFe 12 O 22[9] and Ba 0.5 Sr 1.5 NiMgFe 12 O 22[12].The magnetisation increases smoothly to about 170 K and 200 K at 50 Oe and 100 Oe, respectively (figure4 (c), (d)).With the further temperature rise towards 300 K, the increase becomes very sharp.For Ba 0.5 Sr 1.5 Ni 0.5 Mg 1.5 Fe 12 O 22 (figure4(a), (b)), the ZFC-FC magnetisation reaches a maximum at 49 K and 40 K respectively for 50 Oe and 100 Oe, which has to do with a magnetic-phase transition attributed to the conical spin ordering formed at lower temperatures.This transition in Ba 0.5 Sr 1.5 Ni 0.5 Mg 1.5 Fe 12 O 22 is known to give rise to the onset of a notable magneto-electric coupling.As the temperature rises to about 200 K, the magnetisation decreases and then slowly increases.
Ba 0.5 Sr 1.5 Ni 0.5 Mg 1.5 Fe 12 O 22 and Ba 0.5 Sr 1.5 Ni 1.5 Mg 0.5 Fe 12 O 22 polycrystalline powders were prepared by citric acid sol-gel self-ignition and burning.The magnetisation curve recorded at 4.2 K for Ba 0.5 Sr 1.5 Ni 0.5 Mg 1.5 Fe 12 O 22 exhibited a behaviour indicating the presence of two kinds of ferromagnetic states of different magnetisation values.For Ba 0.5 Sr 1.5 Ni 1.5 Mg 0.5 Fe 12 O 22 , i.e.