TOPICAL REVIEW: Supermagnetism

Nickel-doped cobalt ferrite $$[{\text {Co}}_{1- x }\hbox {Ni}_{ x }\hbox {Fe}_{2}\hbox {O}_{4} \,(0\le x\le 1)]$$
 nanoparticles are synthesized by means of hydrothermal method. The structural, morphological and microstructural characterization revealed crystallite size was roughly spherical for lower nickel concentration while for higher ones in diamond shape consisting of nanosized grains. The optical band-gap (
 $$E_{g}$$
 ) values decreased with the $${\text {Ni}}^{2+}$$
 ions (x) concentration being 2.94 and 2.51 eV for $$x=0$$
 and $$x=1$$
 , respectively. The presence of nickel in the cobalt ferrite structure affected the magnetic properties. For instance, the saturation magnetization, $$M_{s}$$
 and remanent magnetization, $$M_{r}$$
 decreased from 369 to 256 emu cm−3 and 131–45 emu cm−3 for $$x=0$$
 and $$x=1$$
 , respectively. The $$M_{s}$$
 data was discussed in term of the three-sublattice of non-collinear spin (canted spin) structure proposed by Yafet and Kittel model. On the other hand the coercivity, $$H_{c}$$
 from 890 Oe to 1590 Oe for $$x=0$$
 and $$x=0.6$$
 , and sharply dropped to 50 Oe for $$x=1.$$
 The enhanced coercivity was discussed in terms of particles size, defects and residual strain which may act as pinning centers. The cubic magnetocrystalline constant, $$K_{1}$$
 determined by using the “law of approach” to saturation decreases with $${\text {Ni}}^{2+}$$
 ions (x) concentration being $$4.9\times 10^{6}$$
 and $$2.8\times 10^{6}$$
 erg cm−3 for $$x=0$$
 and $$x=1$$
 , respectively. These results were discussed in terms of the inter-particle interactions induced by the presence of $${\text {Ni}}^{2+}$$
 ions at the octahedral sites which affected the strength of L–S coupling.

Hydrothermal synthesis of nickel doped cobalt ferrite nanoparticles: optical and magnetic properties

The outlook for improving the energy product of permanent magnets beyond the theoretical limit of 516 kJ/m3 for Nd2Fe14B is discussed. Most promising are nanostructured two-phase magnets consisting of an aligned hard-magnetic skeleton phase possessing exceptionally high uniaxial anisotropy and a soft-magnetic phase which assures a high magnetization. Brown's paradox is discussed for inhomogeneous magnets, and micromagnetic calculations are used to predict energy products as high as 1090 kJ/m3 in suitable nanostructured Sm2Fe17N3/Fe65Co35 composites, such as multilayers or disordered two-phase magnets with common c axis.

/pdf/new-magnets-from-interstitial-intermetallics-2zydocgvz3.pdf

New magnets from interstitial intermetallics

Nanometer-sized Co arrays consisting of 200 nm or 20 nm diameter cylindrical islands, occupying a fractional surface area of 50 to 65%, have been synthesized by electrodeposition through the pores of suitable Al oxide templates. An increase in coercivity for 20 nm compared to 200 nm diameter Co island arrays is explained in terms of a transition to a single domain state. In this size range, demagnetizing fields have a major role in determining the magnetic response of the arrays, which show high saturation fields and a coercivity which is lower than predicted by curling or uniform rotation models. A high effective anisotropy is important in obtaining relatively high values of coercivity (up to 1700 Oe) in highly packed arrays.

Porous aluminum oxide templates for nanometer-sized cobalt arrays

Nucleation in inhomogeneous permanent magnets

Coercivity is the essential property characterizing hard magnetic materials, in particular with respect to normal ferromagnets. It exists when low field domain wall motion does not take place, either the magnetization being already saturated in zero field, or domain walls being pinned at structural heterogeneities. In these two cases, the magnetic configuration of the system is an energy minimum in the field, although it may not be the lowest energy minimum. This energy minimum vanishes at H = He, the coercive field. Then magnetization reversal takes place and the system reaches the lowest energy state.

M. F. Rossignol

Papers

Coercivity in Hard Magnetic Materials