Magnetic Properties of Hexaferrite Nanosized Powders Produced via Mechanoactivation

A study of the relationship between structural parameters and principal magnetic characteristics of nanosized powders of hexagonal ferrimagnetics produced via mechanoactivation has been carried out. The models describing the influence of the size effects on temperatures of magnetic phase transformations, saturation magnetization and magnetic anisotropy of similar materials are discussed..


Introduction
In the last few years the behavior features of the basic magnetic properties of polycrystalline and powdered magnetics with submicron and nanosized crystals and particles has drawn the attention of researchers.Aside the well-known effects of material transition into single-domain and superparamagnetic states, other magnetic effects are observed connected to the increasing role of surface effects in small size magnetic particles, namely, the fluctuations of exchange interactions for atoms belonging to the surface and disturbed in the near-surface layer, the change of the local symmetry of the crystalline field and, therefore, of the magnetic crystallographic anisotropy (MCA).The present paper gives the findings of the investigation into the influence of size effects on the magnetic properties of ultradisperse powders of oxide ferrimagnetics with a hexagonal structure produced via mechanoactivation.The investigation is concerned with hexaferrites of M, W and Y structural types differing by the type of magnetic anisotropy.
The properties of ultradisperse powders of magnetics produced via dispersion in highenergy mills may drastically differ from the same parameters of microparticles manufactured through co-precipitation or synthesis in the glass phase.
During mechanoactivation of powders of multisublattice oxide ferromagnetic compounds dispersion may be followed by some other effects capable of influencing the magnetic properties of the material.In particular, the values of internal elastic stresses are induced and reach very high magnitudes, which significantly increases the role of the magnetoelastic share into the energy of magnetic anisotropy of a magnetic material.The nonequilibrium thermodynamic state of an activated sample appearing as a result of "pumping" a large kinetic energy into the sample may result in the cation redistribution by crystallographically non-equivalent positions and even in cations dislodging from the positions allowed by the crystal symmetry and in the occupation of forbidden positions.This, accordingly, changes the magnetic characteristics.Connected with this, one of the primary research goals is selection of activation modes, under which mainly grinding of particles of the powdered materials takes place, with the other effects playing a small part.W).Barium ferroxdure (BaM) is a uniaxial magnetic material with an anisotropic field of 17.5 kOe at room temperature.This compound contains paramagnetic cations of one kind only (Fe 3+ ) and is therefore an ideal model object to study the size effects influence on the temperature of the transition into the paramagnetic state, because in this case one may not take into account cation redistribution, which is theoretically possible.A state with "light magnetization plane" anisotropy is realized at all temperatures up to the Curie temperature for zinc hexaferrite of the Y structural type.The sequence of spin-orientation "cone-plane-conelight magnetization axis" transitions is observed within the temperature range of 120-300 K in the latter of the above compounds, at room temperature this magnetic is near the "cone-light magnetization axis" phase transition and the effective anisotropy field is conditioned by the existence of two MCA constants at least [1].

Experimental
Dispersion of the powdered samples produced via preliminary grinding in a vibration mill for 1 h (the average crystallite size of 20-50 µm) was carried out in a high-energy planetary mill of MPH type at various ball weight: sample weight ratios: 5:1 ("soft regimes") and 20:1 ("hard regimes"), which corresponds to different values of energy stress during grinding.The dispersion time varied from 0.25 min to 15 min.
The study of the structural characteristics of the activated powders was carried out using an automated polycrystalline difractometer of ADP-1 type (Fe Kα irradiation).To obtain the data on the average crystallites size in the powders and the values of internal stresses we carried out an analysis of physical broadening of the reflexes of hh0 and 00l layer lines using Willets technique [2].
The data on the static magnetic characteristics, namely, saturation magnetization, effective field of magnetic anisotropy and paraprocess susceptibility were obtained by processing the magnetization curves taken in the pulsed fields with the amplitude to 6T using the law of approach of magnetization to saturation.Metrological characteristics of the automated arrangement used to perform the measurements are given in detail in [3].The data on the parameters of magnetic anisotropy of ultradisperse hexaferrites powders were obtained during the examination of ferromagnetic resonance spectra within the frequency range of 12-53 GHz and in the course of measurements of initial magnetic permeability and its temperature dependence.

Investigation results
Figs. 1a,b show the relationships of the average sizes of crystallites in the basal plane and along the C axis, obtained under different activation modes.Under "soft" regimes, the grinding process is practically completed when the treatment period is 180-200 s, and the typical size of hexaferrite particles, La/Lc ≈ 5, remains unchanged.Under "hard" activation regimes the main change of crystalline sizes occurs in 100-120 s, the particle sizes along different crystallographic directions differing a little from each other.Fig. 2 shows the relationships for internal elastic stresses induced by mechanochemical activation in the directions normal to (00l) and (hh0) planes for hexaferrites Co 0.56 Zn 1.44 W. The same figure demonstrates similar data for activated samples subjected to thermal annealing at the temperature of 837 K for 2 h.As is seen, annealing did not practically affect the stresses normal to the basal planes, whereas the values of stresses directed normally to (hh0) planes diminished by a factor of 3.
A study of the temperature dependence of the initial magnetic permeability allowed us to assess the influence of size effects on the temperatures of magnetic phase transformations, including spin-orientation ones in nanosized hexaferrite powders.The most convenient object for such a study is barium ferroxdure (BaM), containing paramagnetic ions of one kind (Fe 3+ ) only in its composition.This fact allows avoiding the effects connected to probable cation redistribution by non-equivalent crystallographic positions.Figs.3a,b give the temperature dependences of the initial magnetic permeability and its derivative for the initial sample of barium ferroxdure, as well as for the sample subjected to mechanoactivation for 180 s.It is seen that an intense activation results in the shift of the temperature of the transition into paramagnetic state into the region of low temperatures and to a marked broadening of the region of the phase transition.Several maximums observed on the magnetic permeability derivative evidence most likely that the activated sample composition is heterogeneous by particle size.b,c show the Curie temperature vs. average particle size along the C axis for all the compounds studied.Anomalous sections of the dependences for Lc < 100 nm are explained by the appearance of the high-temperature magnetic phase resulting from mechanochemical reactions.This proposition is confirmed by the results of the study of ferromagnetic resonance (FMR), whose principal goal was to obtain data on the parameters of magnetic crystallographic anisotropy of ultradisperse hexaferrite powders.
An analysis of field FMR dependences obviously demonstrates that along the initial phase resonance, additional resonances resulting from the appearance of a weakly anisotropic phase appear during prolonged and intense mechanical activation.Figs 5 and 6 represent, as an example, FMR spectra for samples of barium ferroxdure and the Zn 2 Y compound for different activation periods and under different activation conditions.The data on the magnetic anisotropy of activated hexaferrite powders were obtained also on the basis of the analysis of magnetization curves taken in pulsed magnetic fields.

Discussion of research results
Let's consider briefly models, which can be used for interpretation of the received experimental results.It is possible to explain the behavior of the transition temperature in the paramagnetic state during particle size reduction by formation of a perturbated superficial layer with partially broken exchange bonds.Thus, the least characteristic size -size of particles along the hexagonal axis of a crystal has the determining importance.Let's consider, that for surface atoms the number of magnitoactive bonds decreases twice (Zij (s) =Zij /2).Using the formula: Tc(np) = Tc(0) * (1-∆Lc/Lc), and the equation of a molecular field accounting for the transition temperature in a paramagnetic state we have estimated the thickness of the perturbated layer (∆Lc) for particles with various sizes.
In figs.7a,b such dependences are given for BaM and Co 0.56 Zn 1.44 W compounds.It is necessary to note, that the valuesobtained for ∆Lc during particle size reduction come close to C/2.The expression for an effective constant magnetic anisotropy of hexaferrite particles looks like: Keff = (1-∆L'c /Lc) *Kv + (∆L'c /Lc) *Ks+Kc +Km, Where: Kv-constant magnetic crystallographic anisotropy of non-perturbated particle volume; Ks-constant anisotropy of an perturbated layer; Kc =λσ-the contribution of magnitoelastic interactions; Km -contribution causing particle anisotropy.
In fig.8 the dependences of field anisotropy vs. time of mechanical activation received experimentally and results of calculations using the mentioned above ratio are compared.Let's notice that the best agreement is achieved for the value of parameter ∆L'c /Lc= 3C, that is much more thickness of a layer with broken exchange bonds.The reason for this distinction is quite clear.The anisotropy type of a superficial layer is wholly determined by the symmetry of a local crystal field of atoms belonging to the surface, i.e. a field directed in a basic plane.For atoms of a next layer the crystal fields correspond to the non-perturbated particle volume and are directed along hexagonal axes of a crystal.At the same time, atoms of this layer are connected by a very strong exchange interaction with cations of a superficial layer, whose magnetic moments lie in a basal plane.
As a result, for the first subsurface layer cations the magnetic moments will be deviated on some angle φ, whose value can be as a first approximation appreciated from a: sinφ=H a /H ex , ratio where H a and H ex -field of anisotropy and effective field of exchange interaction accordingly.In view of a real parity of a field of anisotropy and the value of molecular fields in barium ferroxdure, the estimation of thickness perturbated of a layer for this compound gives values of about thirty anions of layers, i.e. approximately 3C.The offered ratio for estimation of the influence of dimensional effects on temperatures of magnetic phase transitions and effective fields of anisotropy of nanosized particles of ferrimagnets have enough common character and can be used not only for hexagonal ferrimagnetics, but also for oxide ferrimagnet compounds of other structural types.

Fig. 1
Fig. 1 Dependence of average particle size on the activation time: a -BaM "hard" regime, b -Co 0.56 Zn 1.44 W "soft" regime.

Fig. 2
Fig. 2 Dependence of internal elastic stress on activation time.

Fig. 3
Fig.3 Temperature dependence of the initial magnetic permeability and its derivative for a -the initial sample of barium ferroxdure, b -for the sample subjected to mechanoactivation for 180 s.

Fig. 4
Fig. 4 Dependence of the Curie temperature on average particle size: a -BaM, b -Co 0.56 Zn 1.44 W, c -Zn 2 Y.

Fig. 5
Fig. 5 FMR spectra for the samples of barium ferroxdure.

Fig. 6
Fig. 6 FMR spectra for the samples of Zn 2 Y.

Fig. 7
Fig. 7 Dependences of the thickness of the perturbated layer for: a -BaM, b -Co 0.56 Zn 1.44 W.

Fig. 8
Fig. 8 Dependences of a field anisotropy vs. time of mechanical activation.