The effects of CoTi co-doping on the magnetic , electrical , and magnetodielectric behaviors of M-type barium hexaferrites

The effects of Co-Ti co-doping on the magnetic, electrical, and magnetodielectric behaviors of M-type barium hexaferrites Yujie Guan,1 Yuanbin Lin,1 Liangying Zou,1 Qing Miao,1 Min Zeng,1 Zhongwu Liu,2 Xingsen Gao,1,a and Junming Liu3 1Institute for Advanced Materials, South China Normal University, Guangzhou 510006, P. R. China 2Department of Metallic Materials Science and Engineering, South China University of Technology, Guangzhou 510640, P. R. China 3Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China


I. INTRODUCTION
Recently there has been an upsurge of research interest in magnetoelectric (ME) effects in hexaferrites.The ME effects have been explored in various hexaferrites, such as Y-type Ba 0.5 Sr 1.5 Zn 2 Fe 12 O 22 , 1 BaSrCoZnFe 11 AlO 22 , 2 Z-type Sr 3 Co 2 Fe 24 O 41 , 3,4 U-type Sr 4 Co 2 Fe 36 O 60 , 5 and M-type BaFe 10.35 Sc 1.6 Mg 0.05 O 19 . 6[8][9] In these spiral magnets, the magnetic order and ferroelectricity are inherently coupled and thus pronounced ME effects could be expected.Such spiral magnetic order induced multiferroicity and ME effects have been extensively reported in transition metal magnates (e.g.TbMnO 3 ), 10 where the ME effects are only available at low temperature and a large magnetic field of several tesla, hampering its further applications.In comparison, hexaferrites enable generating and controlling the ME effects at much higher temperatures and lower magnetic fields, hence being considered as promising candidates for future multiferroic applications.
Among them, M-type barium hexaferrite, BaFe 12 O 19 , denoted as BaM by Braun, 11 is the most-studied compound, which has been widely used as commercially permanent magnets, data storage media and microwave devices.netoanisotropy, stabilized by the strong exchange interactions among the Fe ions. 15By substitution of Fe 3+ with other ions (e.g.7][18][19][20] If proper doped (Co-Ti or Sc-Mg codoping), it may form longitudinal conical cycloidal spin order, producing ferroelectric polarization effects at rather high temperatures. 6,7,21 Hwever, there are very few investigations on the electric or ME effect on BM hexaferrite.Although it has been report that longitudinal conical spin state can be observed in BaFe 10.35 Sc 1.6 Mg 0.05 O 19 , in which spin helicity (vector spin chirality) is robust against magnetic field scans as well as thermal agitations up to room temperature, 6 it suffers from low resistivity, deleteriously affecting its electric as well as ME properties.
In this work, the magnetic, electrical properties and dielectric tunabiltiy of Co-Ti co-doped BaM hexaferrite (BaCo x Ti x Fe 12-2x O 19 , x = 0 ∼ 4) was explored.It was found that Co-Ti co-doping not only significantly changes the magnetic structures of BaM hexaferrite, but greatly enhances (∼3 orders in magnitude) their resistivities.In particular, a magnetic induced dielectric tunability up to ∼−3% can be observed for sample with x = 2 at 1T magnetic field.These demonstrated that the Co-Ti co-doped hexaferrites are possible candidate for room temperature multiferroics.

II. EXPERIMENTAL DETAILS
Co-Ti co-doping BaM (BaCo x Ti x Fe 12-2x O 19 , x = 0 ∼ 4) ceramics were prepared through conventional solid-state reaction.The high purity powders of Fe 2 O 3 , BaCO 3 , Co 2 O 3 and TiO 2 were weighed to the designed ratios, mixed and well ground.The mixture powders were calcined at 1100 • C for 10 h and then pulverized.Finally the powders were pressed into pellets and sintered at 1200 • C for 10 h.The crystalline structures were characterized by X-ray diffraction (XRD), using an X' Pert PRO diffractometer (PANalytical).The magnetism of samples was measured using a vibrating sample magnetometer incorporated into a physics properties measurement system (PPMS).For electrical measurements, the sintered pellets were polished into thin plates with a thickness of 0.5 mm, then silver electrodes were then applied onto both surfaces of the samples, and subsequently the samples were fired at 200 • C for 20 min to ensure the good contact.The resistivities for the as prepared samples were measured at room temperature by two-probe method using a Keithley source current meter (KEITHLEY 6430).The dielectric properties were tested by a high performance impedance analyzer (Alpha-A, Novocontrol Technology) at a frequency range of 10 Hz to 10 MHz.

III. RESULTS AND DISCUSSION
The XRD diffraction patterns of the sintered samples are showed in Fig. 1(a).All the diffraction peaks can be indexed to the magnetoplumbite structure with a space group of P6 3 /mmc, which means that the Co-Ti atoms are incorporated into the lattice matrix.Further detailed analysis found that some diffraction peaks are shifted towards lower angles with increasing doping concentration x, as shown in Fig. 1(b), indicating an increase in lattice parameters.With rising doping concentration, both lattice constants of a-axis and c-axis increase, which is clearly demonstrated in Fig. 1(c).It is noted that the radius of Co 2+ and Ti 4+ ions are 0.72 Å and 0.68 Å, respectively, both of which are slightly larger than that of Fe 3+ ions (0.64 Å).Therefore it is reasonable that the substitution of Fe 3+ ions by Co 2+ and Ti 4+ ions results in the expansion of lattice constant.
The ferromagnetic hysteresis (M-H) loops were shown in Fig. 2(a).It is observed that the saturation magnetizations (M s ) first increases and then decreases with the rise of x, The maximal M s is obtained at x = 0.5.While the coercive field (H c ) declines monotonously, as shown in the inset of Fig. 2(a).In addition, the coercive field (H c ) for sample without doping is of 3.26 kOe, and it starts to reduce remarkably with the increase of x.Finally, it drops down to 0.024 kOe at x = 4.For pure BaFe 12 O 19 , its has a space group of P6 3 /mmc,, in which Fe atoms occupy three octahedral sites (4f 2 , 12k and 2a), one tetrahedral site (4f 1 ), and one trigonal bipyramidal site (2b), 22 for each unit cell (two formula units).Its magnetic structure can be regarded as collinear ferrimagnetic order with 16 Fe in b, a, k sites moment antiparallel with eight Fe spin in f 1 and f 2 per unit cell, producing a net magnetic moment 8μ Fe (40μ B ) at 0 K. 16 The increase of magnetism at initial stage in doped BaFe 12 O 19   ceramics can be attributed to the partial substitution of Fe 3+ ions by low spin Co 2+ (0.8μ B ) on antiparallel f-sites, while replacing of Fe cations in parallel sites (e.g.12k) with nonmagnetic Ti 4+ ions may significantly reduce the local magnetic momentsand consequent to the overall drop of magnetization with further increase of doping content.On the other hand, the monotonously drop of coercivity is mainly attributed to the loss of magnetocrystalline anisotropy by Co-Ti codoping, 19 which can be affected by Co 2+ ions in f 2 -sites.
Furthermore, Co-Ti codoping can also greatly affect the magnetic structures.It is generally accepted that the crystalline structure of BaM hexaferrite is considered as alternative stacking of four block of R, S, R * , and S * , in which RS and R * S * carry same spin moments. 21Without doping, RS and R * S * are collinearly alighted at the same orientation, give rising to the observed ferromagnetism.Once Ti 4+ cations are introduced, they tend to occupy the 12k sites of the unit cell, leading to the decoupling of these two blocks, 21 which promotes the formation of noncollinear magnetic orders.For instance, co-doping may increase the magnetic angle between these RS and R * S * blocks, and form conical spin structures.To verify this, we measured temperature dependent magnetization for the BaCo 3 Ti 3 Fe 6 O 19 ceramics (x = 3), as shown in Fig. 2(b).With temperature increasing from 300 K, the magnetization first increases at lower temperature region, and then gradually decreases at higher temperature region.The maximal magnetization of 1.68 emμ/g is obtained at 410 K, likely correspondent to the transition from collinear ferrimagnetic to conical spin structures as reported in Ref. 6.Our results show that the conical spin structure in BaFe 12 O 19 ceramic can be stabilized up to 410 K in case of Co-Ti co-doping.Further increase of the temperature to 620K, the magnetization dramatically decreased to a value close to zero and remains constant at higher temperature.This is in association with a transition from paramagnetic to ferromagnetic phases occurred at 620 K.
Figure 3 shows the frequency dependence of dielectric constant (ε ) and dielectric loss (tan δ) of BaCo x Ti x Fe 12−2x O 19 (x = 0 ∼ 4) measured at room temperature.For all the samples, both ε and tan δ decrease gradually with increasing frequency.This feature is consistent with previous report. 20t can also be observed that higher content of doping will lead to lower ε and tan δ.The high ε and large tan δ at low frequencies in pure BaFe 12 O 19 ceramic can be attributed to high concentration of charge carriers related to space charge relaxation.For the samples with high doping concentration of x > 2, the ε exhibits rather frequency independent behaviors, and the tan δ reduced by two orders in magnitude as shown in Fig. 3(b).These above results show that Co-Ti co-doping can greatly improve the frequency stability of the dielectric constant and decrease the dielectric loss of BaCo x Ti x Fe 12−2x O 19 ceramics.This is also attributed to the significant enhancement of resistivity in samples with high concentration of doping, as will discuss later.
More interesting result is that the resistivity of the BaM hexaferrite ceramics can be greatly increased by Co-Ti co-doping, as shown in Fig. 4(a).Compared with undoped BaFe 12 O 19 , the resisitivity of Co-Ti co-doping ceramics has been greatly increased.For example, the obtained resistivity of BaCo 4 Ti 4 Fe 4 O 19 ceramic is 4.36 × 10 10 • cm, which shows three orders of magnitude higher than 1.50 × 10 7 • cm for the pure BaFe 12 O 19 .The increase of the resistivity may be attributed to two mechanisms.One is the suppression of electron hopping between Fe 3+ and Fe 2+ , which is main source for high conductive loss in ferrites. 23Substitution of Fe 3+ by Co 2+ and Ti 4+ in octahedral sites can obviously reduce oxygen vacancy concentration due to decreased Fe ions, 24 in responding to the suppression of electron hopping among Fe 3+ /Fe 2+ .The other reason is that the lattice distortion induced by Co-Ti co-doping further confines the electron hopping between Fe 3+ and Fe 2+ . 25The mechanism proposed here is consistent with that of Iqbal's reported for Zr-Cu substituted strontium hexaferrites system. 20o examine the magnetoelectric effect, a dielectric measurement under a magnetic field was performed at room temperature.Here, we define the dielectric tunability as [ε(H ) − ε(0)]/ε(0), where ε(H) and ε(0) are the dielectric constant with and without a magnetic field, respectively.The dielectric tunability as a function of magnetic field is shown in Fig. 4(b).It is found that the dielectric tunability for samples with x < 2 cannot be observed due to the high leakage current.Due to the increase of the resistivity with higher coping concentration, clear dielectric tenability can be observed for high concentration of dopping.For examples, dielectric tunability of ∼−3% and ∼−1.4% have been observed for samples with x = 2 and x = 3 at 1T.The dielectric tenability by magnetic field is believed to be due to the change of electric polarization induced by magnetic field, i.e.ME effects.3][4] Our results demonstrate that the substitution of Fe 3+ by Co 2+ -Ti 4+ for BaM hexaferrites can push forward its applications as new type of room temperature magnetoelectric materials, and more studies are still needed to explore its other room temperature ME properties.

IV. CONCLUSIONS
The single-phase Co-Ti co-doping BaM BaCo x Ti x Fe 12−2x O 19 (x = 0 ∼ 4) have been synthesized successfully by conventional solid-state reaction.It was found that Co-Ti co-doping can greatly change its saturate magnetization, coercivity, as well as induce complicated cycloidal spin orderings.More interestingly, it can also significantly enhance (∼ 3 orders in magnitude) resistivity of the Mtype Barium hexaferrites, which enable further investigation of room temperature electric properties.A dielectric tunability up to ∼−3% was obtained at 1T for sample with x = 2, which demonstrates a clear magnetodielectric effect.These indicate that proper Co-Ti co-doping into BaM hexaferrites is an effective way to make it a room temperature magnetoelectric materials.

FIG. 1 .
FIG. 1.(a) XRD patterns for the BaCo x Ti x Fe 12−2x O 19 (x = 0 ∼ 4) ceramics.(b) Subtle XRD patterns of the peaks in 2θ range 31-38 • .(c) The lattice constants of a-axis and c-axis as a function of doping amount.

FIG. 2 .
FIG. 2. (a) Magnetic hysteresis loops for the BaCo x Ti x Fe 12−2x O 19 (x = 0 ∼ 4) ceramics and the insert shows the saturation magnetization and the coercive field as a function of doping amount.(b) Temperature dependence of the magnetization for BaCo 3 Ti 3 Fe 6 O 19 measured at 0.05 T during field heating process.