Hongzhi Luo

Crystallite size effect on saturation magnetization of fine ferrimagnetic particles

Fine equiaxed γ-CoxFe2-xO3 (x=0, 0.06) particles with diameters of 200–1000 A were prepared by chemical precipitation. The average crystallite sizes of the particles were determined from X-ray line broadening measurements. The saturation magnetization of the particles were measured at 295 and 79 K, respectively. An empirical linear dependence of the specific saturation magnetization σs on the specific surface area Sa of the fine crystallites was obtained in the form of σs(S) = σs(∞)(1 - ASa), where the slope A is different for γ-Fe2O3 and γ-Co 0.06Fe1.94O3 particles. Under the supposition that the fine crystallite consists of two parts, i.e. a surface layer, whose magnetic moment can not be turned entirely along the direction of the applied field, but makes an average canting angle with the field, and an inner part, whose magnetic moment can be aligned along the direction of the applied field, the above empirical formula can be interpreted well.

Effect of site preference of 3d atoms on the electronic structure and half-metallicity of Heusler alloy Mn2YAl

The site preference of 3d atoms Y in Mn2YAl (Y = V, Fe, Co) alloys and its influence on their electronic structures and magnetism have been studied by first-principles calculations. The results prove that elements with more valence electrons than Mn tend to enter the A (0, 0, 0) and C (½, ½, ½) sites and elements with fewer electrons prefer the B (¼, ¼, ¼) site (Wyckoff positions). Meanwhile, it is found that for Mn2VAl and Mn2FeAl, a high spin polarization can be obtained whether the Y atom enters the (A, C) or the B site. In particular, Mn2VAl is half-metallic whether it forms the Cu2MnAl type or the Hg2CuTi type of structure. And a 100% spin polarization can be retained even when a 25% Mn–V antisite disorder occurs. This is quite preferable in practical applications. It is also found that the higher-valent element such as Co at the B (¼, ¼, ¼) site has opposite effects and tends to close the energy gap. Finally, a systemic summarization on the electronic and magnetic properties of Mn2YAl (Y = Ti, V, Cr, Mn, Fe and Co) alloys was made. All of them except for Mn2TiAl are predicted as half-metals. The calculated total spin moment is an integral value and increases from −3µB/f.u. for Mn2TiAl to +2µB/f.u. for Mn2CoAl with increasing number of valence electrons. This agrees with the Slater–Pauling curve quite well. All the Mn2YAl alloys studied here are ferrimagnets.

Influence of size and magnetocrystalline anisotropy on spin canting anomaly in fine ferrimagnetic particles

Fine equiaxed γ‐CoxFe2−xO3 (x=0, 0.06) particles with a diameter ranging from 200 to 1000 A were prepared by chemical precipitation. The average crystallite sizes were determined from x‐ray line broadening measurements. The saturation magnetization and magnetocrystalline anisotropy of the particles were determined by using the approach to saturation. An empirical linear dependence of the specific saturation magnetization σs on the specific surface area Sa of the fine crystallites was obtained in the form of σs(S)=σs(∞)(1−ASa). The slope A which reflects the surface spin canting anomaly is different for γ‐Fe2O3 and γ‐Co0.06Fe1.94O3 particles. Under the supposition of the fine crystallite consisting of two parts, i.e., the surface layer, whose magnetic moment cannot be turned entirely along the direction of the applied field, but makes an average canting angle with the field, and the inner part, whose magnetic moment can be aligned along the direction of the applied field, the above formula can be interpret...

Ferromagnetism in the Mn-based Heusler alloy Mn2NiSb

A Mn-based Heusler alloy Mn2NiSb has been synthesized. First-principles calculations and experimental measurements were employed to investigate its electronic structure and magnetism. Theoretical calculation indicates a ferromagnetic coupling in Mn2NiSb and a total spin moment of 4.21μB/f.u., which agrees quite well with the saturation moment at 5 K. This ferromagnetic coupling arises from the larger lattice constant and more valence electrons compared with other Mn-based Heusler alloys. The partial spin moments for Mn (A), Mn (B), and Ni are 0.6, 3.30, and 0.3μB, respectively. The Curie temperature of Mn2NiSb is 647 K, indicating a strong exchange interaction between Mn atoms.

ON THE RELATIONSHIP BETWEEN COERCIVE FORCE HC AND MAGNETIC VISCOSITY PARAMETER SV IN MAGNETIC-MATERIALS

Abstract Theoretical relationships between the coercive force H c and the magnetic viscosity parameter S v have been obtained which give a very good explanation to the well-known phenomenological Barbier plot. For the strong pinning model, the formula obtained is l g S v = l g H c − l g {( 75 4 )[( 4fb 75 kT ) 2 3 − 1]} , where f is maximum pinning force and b is the range of interaction between domain wall and pin. For the weak pinning model, the obtained theoretical formula is: l g S v = l g H c − l g {25[( 3 1rb 2 25kT ) − 1]} , where r is domain wall energy. For the unpinning (nucleation) process, the obtained formula is: l g S v = l g H c − l g {( 75 2 )[( 4fb 75kT ) − 1] 2 3 } . For uniaxial single-domain particles, the obt ained relationship between H c and S v is: l g S v = l g H c − l g [( 2KV 1 2 p kT )(V 1 2 − V 1 2 p )] , where V p is the critical volume for superparamagnetic behaviour and V is the particle volume. At a given temperature, all the relationships obtained above can be written as: l g S v = const . + l g H c .

On the coercive force and effective activation volume in magnetic materials

Abstract The relationships between lg H c and lg v -1 for the strong domain wall pinning model, the localized weak domain wall pinning model, unpinning (nucleation) of domain walls and uniaxial single domain particles have been deduced, which can be written as, lg H c = lg v -1 + F i (…), where H c is the coercive force, v is the activation volume, F i ( T …) is a complex function which has different forms for different models. Values of F i ( T …) for different models have been calculated. The calculated results showed that F i ( T …) is approximately a constant with the average value -13.91 at room temperature. Therefore lg H c is linearly proportional to lg v -1 for different materials at room temperature, which has been proved by experiments.

Effects of cation distribution on superexchange interaction in cobalt ferrites

The effects of the cation distribution change on the superexchange parameters and magnetic properties of cobalt ferrite powders were investigated. The cation distribution change of cobalt ferrite powders with quenching treatment was confirmed from the Mossbauer experiments. The exchange interaction parameters, i.e. J/sub AA/, J/sub BB/ and J(AB), and Neel temperature of slow-cooled and quenched CoFe/sub 2/O/sub 4/ powders were calculated from the variation of the magnetization with temperature on the basis of the two-sublattice model. With a quenching treatment of CoFe/sub 2/O/sub 4/, the thermal magnetization was changed, and this was attributed to the cation distribution change of cobalt ferrites. The absolute values of J/sub AB/ increased with the quenching treatment of CoFe/sub 2/O/sub 4/, while those of J/sub AA/ and T/sub N/ decreased. The values of J/sub BB/ of cobalt ferrite remained the same with the quenching treatment. This result suggested that the decrease of T/sub N/ of CoFe/sub 2/O/sub 4/ with quenching may be attributed to the decrease of J/sub AA/.

Intermediate phases in rhombohedral Pb(Mg1/3Nb2/3)1-xTixO3 crystal

Phase transformations of (111)-cut Pb(Mg1∕3Nb2∕3)1−xTixO3 (x=0.30) single crystals before and after an electric (E)–field poling have been investigated by means of dielectric permittivity, hysteresis loop, polarization current, and in situ x-ray diffraction as a function of temperature. An R-T-C transition sequence was observed in the unpoled sample upon zero-field heating. R, T, and C are rhombohedral, tetragonal, and cubic phases, respectively. After a prior E-field poling, an extra intermediate monoclinic (possible MA) phase was induced and the crystal underwent an R-MA-T-C phase sequence with significant responses in polarization current. The dielectric dispersion was diminished after poling and reappeared in the cubic state. The dielectric permittivities with and without a prior poling were found to follow the Curie–Weiss equation, e′=C∕(T−T0), above the Burns temperature 505K with the same constants C=1.9×105 K and T0=460K.

Field-induced intermediate orthorhombic phase in (110)-cut Pb(Mg1/3Nb2/3)0.70Ti0.30O3 single crystal

Phase transformations of (110)-cut Pb(Mg1/3Nb2/3)0.70Ti0.30O3 single crystal before and after an electric (E)-field poling have been investigated by means of dielectric permittivity, polarization current, hysteresis loop, and in situ x-ray diffraction as a function of temperature. An R-T-C transition sequence was observed in the unpoled sample upon zero-field heating. R, T, and C are rhombohedral, tetragonal, and cubic phases, respectively. After a prior E-field poling, the crystal underwent an R-O(R)-T-C phase sequence with significant responses in polarization current. O(R) represents that the minor R phase coexists in the orthorhombic matrix. The dielectric permittivities with and without a prior poling were found to follow the Curie–Weiss equation, e′=C/(T−T0), above the Burns temperature 505 K with the same constants C=1.43×105u2002K and T0=462u2002K.

The structural and magnetic properties of Mn2−xFexNiGa Heusler alloys

The effect of Fe substitution on the phase transformation and magnetic properties of Mn2NiGa has been studied. A single bcc phase was obtained in Mn2−xFexNiGa (x=0–0.6). With the substitution of Fe for Mn, the lattice constant decreases gradually. The martensitic transformation can be observed when x=0–0.3. Both the martensitic transformation and austenitic transformation temperatures decrease monotonically with increasing Fe content, which is different from common electron concentration dependence in Ni–Mn–Ga system. The saturation magnetizations Ms of Mn2−xFexNiGa in both austenitic and martensitic phases increase obviously with the doping of Fe, while the variation of TC shows an opposite tendency. Theoretical calculations indicate that both austenitic and martensitic phases are ferrimagnets. The Fe moment varies from positive to negative after the tetragonal distortion, which leads to the decrease of the saturation magnetization.