In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Exchange-spring magnets are nanocomposites that are composed of magnetically hard and soft phases that interact by magnetic exchange coupling. Such systems are promising for advanced permanent magnetic applications, as they have a large energy product--the combination of permanent magnet field and magnetization--compared to traditional, single-phase materials. Conventional techniques, including melt-spinning, mechanical milling and sputtering, have been explored to prepare exchange-spring magnets. However, the requirement that both the hard and soft phases are controlled at the nanometre scale, to ensure efficient exchange coupling, has posed significant preparation challenges. Here we report the fabrication of exchange-coupled nanocomposites using nanoparticle self-assembly. In this approach, both FePt and Fe3O4 particles are incorporated as nanometre-scale building blocks into binary assemblies. Subsequent annealing converts the assembly into FePt-Fe3Pt nanocomposites, where FePt is a magnetically hard phase and Fe3Pt a soft phase. An optimum exchange coupling, and therefore an optimum energy product, can be obtained by independently tuning the size and composition of the individual building blocks. We have produced exchange-coupled isotropic FePt-Fe3Pt nanocomposites with an energy product of 20.1 MG Oe, which exceeds the theoretical limit of 13 MG Oe for non-exchange-coupled isotropic FePt by over 50 per cent.

Exchange-coupled nanocomposite magnets by nanoparticle self-assembly

TOPICAL REVIEW: Nanomagnetics

The magnetic refrigeration technique based on the magnetocaloric effect (MCE) has attracted increasing interest because of its high efficiency and environment friendliness. in this article, our recent progress in exploring effective MCE materials is reviewed with emphasis on the MCE in the LaFe13-xSix-based alloys discovered by us. These alloys show large entropy changes over a wide temperature range near room temperature. The effects of magnetic rare-earth doping, interstitial atoms and high pressure on the MCE have been systematically studied. Special issues, such as appropriate approaches to determining the MCE associated with the first-order magnetic transition, the depression of magnetic and thermal hysteresis, and the key factors determining the magnetic exchange in alloys of this kind, are discussed. The applicability of giant MCE materials to magnetic refrigeration near ambient temperature is evaluated. A brief review of other materials with significant MCE is also presented.

Recent Progress in Exploring Magnetocaloric Materials

The principles of operation of permanent magnets are summarized, and their development is reviewed. The key figure of merit, the energy product, improved exponentially over much of the 20th century, doubling roughly every 12 years. Yet it has not improved significantly in the last 20 years. Constraints on further development are explained, together with the limits of 1/4 μ0/Ms2 on energy product and 2K1/μ0Ms on coercivity, where K1 is the uniaxial anisotropy constant and Ms is the spontaneous magnetization. The challenge of making rare-earth free magnets with a large energy product is discussed, as well as nanocomposite megajoule magnets and the development of new magnetically hard thin-films with perpendicular anisotropy which are potentially interesting for spin electronics or magnetic recording.

Hard Magnetic Materials: A Perspective

An oxide bilayer junction has been fabricated by growing a La0.32Pr0.35Ca0.33MnO3 film on 0.5 wt % Nb-doped SrTiO3 crystal, and its behavior under magnetic field is experimentally studied. It is found that external field greatly affected the rectifying property and the resistance of the junction, causing an extremely large magnetoresistance. The most striking observation of the present work is that the magnetoresistance of the junction can be either positive or negative, depending on temperature and applied current, and is asymmetric with respect to the direction of the bias current. These results reveal the great potential of the manganites in configuring artificial devices.

Effects of magnetic field on the manganite-based bilayer junction

In a real magnet, the relation between isothermal remanence Jr(H) and dc demagnetization remanence Jd(H) is expressed as δm(H)=[Jd(H)−Jr(∞)+2Jr(H)]/J(∞). It is believed that nonzero δm is due to the interactions between particles in the magnet. Using Pr2Fe14B as a sample, the relation is examined by the micromagnetic finite element method. The positive value of δm is primarily caused by intergrain exchange coupling. The decrease of intergrain exchange coupling results in the drop of the maximum value of δm. However, the variation of anisotropy in grain boundaries produces no change in the maximum value of δm. A Henkel plot is suggested to be effective for checking intergrain exchange coupling in magnets.

Investigation on intergrain exchange coupling of nanocrystalline permanent magnets by Henkel plot

The magnetic properties of $\mathrm{La}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{)}_{11.4}{\mathrm{Si}}_{1.6}$ $(0l~xl~0.08)$ compounds were investigated. A reentrant spin glass behavior for the sample with $x=0.06$ and a paramagnetic-to-spin glass transition for the sample with $x=0.08$ were observed. The spin-glass behavior for the sample with $x=0.08$ was systematically studied by the dc and ac measurements. $\ensuremath{\Delta}{T}_{f}(\ensuremath{\omega})/[{T}_{f}(\ensuremath{\omega})\ensuremath{\Delta}{\mathrm{log}}_{10}\ensuremath{\omega}],$ a possible distinguishing criterion to assert the presence of a spin-glass phase was 0.02 for the sample with $x=0.08.$ The frequency-dependent data turned out to be well described by the conventional critical slowing down law $\ensuremath{\tau}/{\ensuremath{\tau}}_{0}={\ensuremath{\varepsilon}}^{\ensuremath{-}\mathrm{zv}}.$ The fit to this critical slowing down law yielded the values ${\ensuremath{\tau}}_{0}{=10}^{\ensuremath{-}12}\mathrm{s},$ $zv=8.28,$ and the transition temperature ${T}_{\mathrm{SG}}=45.24\mathrm{K}.$ This spin-glass behavior in $\mathrm{La}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{)}_{11.4}{\mathrm{Si}}_{1.6}$ compounds is mainly ascribed to the competing interaction between the Fe-Fe ferromagnetic ordering and the Fe-Mn, Mn-Mn antiferromagnetic ordering.

Spin-glass behavior in La ( Fe 1 − x Mn x ) 11.4 Si 1.6 compounds

The magnetization reversal has been examined by the temperature dependence of the coercivity, the initial magnetization curve, minor hysteresis loops, and thermal activation in isotropic nanocrystalline Pr-Fe-B ribbons. The coercivity mechanism is found to vary with temperature. At 20 K, the coercivity is mainly determined by strong pinning (by random inhomogeneities); while at room temperature it is mainly controlled by the nucleation of domain and localized pinning at grain boundaries. The influence of the grain-boundary character on magnetic hardening and the temperature dependence of intergrain exchange coupling and anisotropy have been investigated to discuss the coercivity mechanism.

Coercivity of isotropic nanocrystalline Pr12Fe82B6 ribbons

Magnetic and transport properties of ${\mathrm{YMn}}_{6}{\mathrm{Sn}}_{6\ensuremath{-}x}{\mathrm{Ga}}_{x} (0l~xl~1.8)$ compounds with the ${\mathrm{HfFe}}_{6}{\mathrm{Ge}}_{6}$-type structure were investigated. It was found that Ga substitution leads to a contraction of the unit-cell volume. A transition from an antiferromagnetic to a ferromagnetic (or ferrimagnetic) state can be observed for $0.1l~xl~0.2$ with increasing temperature. The antiferro--ferromagnetic transition for samples with $xl~0.2$ can also be induced by an external field. The metamagnetic transition field is very low, and decreases with increasing Ga concentration. Higher Ga concentration $(xg0.2)$ leads to the samples being ferromagnetic in the whole temperature range below the Curie temperature. Ga substitution weakens the interlayer magnetic coupling between the Mn spins. Corresponding to the metamagnetic transition, a magnetoresistance as large as 32% in a field of 5 T was observed at 5 K for the sample with x=0.2. Results from the ${}^{119}\mathrm{Sn}$ M\"ossbauer spectra indicate that the Mn--Mn coupling through the ``Mn--Sn--Sn--Mn'' slab change from antiferromagnetic in ${\mathrm{YMn}}_{6}{\mathrm{Sn}}_{6}$ to ferromagnetic by the substitution of Ga for Sn.

Shao-ying Zhang

Papers

Effects of magnetic field on the manganite-based bilayer junction

Investigation on intergrain exchange coupling of nanocrystalline permanent magnets by Henkel plot

Spin-glass behavior in La ( Fe 1 − x Mn x ) 11.4 Si 1.6 compounds

Coercivity of isotropic nanocrystalline Pr12Fe82B6 ribbons

Magnetism and giant magnetoresistance of YMn 6 Sn 6-x Ga x (x=0-1.8) compounds