High-temperature solution phase reaction of iron(III) acetylacetonate, Fe(acac)3, with 1,2-hexadecanediol in the presence of oleic acid and oleylamine leads to monodisperse magnetite (Fe3O4) nanoparticles. Similarly, reaction of Fe(acac)3 and Co(acac)2 or Mn(acac)2 with the same diol results in monodisperse CoFe2O4 or MnFe2O4 nanoparticles. Particle diameter can be tuned from 3 to 20 nm by varying reaction conditions or by seed-mediated growth. The as-synthesized iron oxide nanoparticles have a cubic spinel structure as characterized by HRTEM, SAED, and XRD. Further, Fe3O4 can be oxidized to Fe2O3, as evidenced by XRD, NEXAFS spectroscopy, and SQUID magnetometry. The hydrophobic nanoparticles can be transformed into hydrophilic ones by adding bipolar surfactants, and aqueous nanoparticle dispersion is readily made. These iron oxide nanoparticles and their dispersions in various media have great potential in magnetic nanodevice and biomagnetic applications.

Monodisperse MFe2O4 (M = Fe, Co, Mn) Nanoparticles

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

Colossal magnetoresistance—a huge decrease in resistance in response to a magnetic field—has recently been observed in manganese oxides with perovskite structure. This effect is attracting considerable interest from both fundamental and practical points of view1. In the context of using this effect in practical devices, a noteworthy feature of these materials is the high degree of spin polarization of the charge carriers, caused by the half-metallic nature of these materials20,21; this in principle allows spin-dependent carrier scattering processes, and hence the resistance, to be strongly influenced by low magnetic fields. This type of field control has been demonstrated for charge-carrier scattering at tunnelling junctions2,3 and at crystal-twin or ceramic grain boundaries4,5, although the operating temperature of such structures is still too low (⩽150 K) for most applications. Here we report a material—Sr2FeMoO6, an ordered double perovskite6—exhibiting intrinsic tunnelling-type magnetoresistance at room temperature. We explain the origin of this behaviour with electronic-structure calculations that indicate the material to be half-metallic. Our results show promise for the development of ordered perovskite magnetoresistive devices that are operable at room temperature.

Room-temperature magnetoresistance in an oxide material with an ordered double-perovskite structure

We review recent experimental work falling under the broad classification of colossal magnetoresistance (CMR), which is magnetoresistance associated with a ferromagnetic-toparamagnetic phase transition. The prototypical CMR compound is derived from the parent compound, perovskite LaMnO 3. When hole doped at a concentration of 20–40% holes/Mn ion, for instance by Ca or Sr substitution for La, the material displays a transition from a high-temperature paramagnetic insulator to a low-temperature ferromagnetic metal. Near the phase transition temperature, which can exceed room temperature in some compositions, large magnetoresistance is observed and its possible application in magnetic recording has revived interest in these materials. In addition, unusual magneto-elastic effects and charge ordering have focused attention on strong electron–phonon coupling. This coupling, which is a type of dynamic extended-system version of the Jahn–Teller effect, in conjunction with the double-exchange interaction, is also viewed as essential for a microscopic description of CMR in the manganite perovskites. Large magnetoresistance is also seen in other systems, namely Tl 2Mn2O7 and some Cr chalcogenide spinels, compounds which differ greatly from the manganite perovskites. We describe the relevant points of contrast between the various CMR materials.

REVIEW ARTICLE: Colossal magnetoresistance

Half-metallic materials are characterized by the coexistence of metallic behaviour for one electron spin and insulating behaviour for the other. Thus, the electronic density of states is completely spin polarized at the Fermi level, and the conductivity is dominated by these metallic single-spin charge carriers. This exotic physical property could have a significant effect on technological applications related to magnetism and spin electronics. Some ferromagnetic systems, such as Heusler compounds1 and chromium dioxide2, have been predicted theoretically to be half-metallic. However, a half-metallic system has not been demonstrated directly and the predictions are still in doubt3,4. Here we report spin-resolved photoemission measurements of a ferromagnetic manganese perovskite, La0.7Sr0.3MnO3, which directly manifest the half-metallic nature well below the Curie temperature. For the majority spin, the photoemission spectrum clearly shows a metallic Fermi cut-off, whereas for the minority spin, it shows an insulating gap with disappearance of spectral weight at ∼0.6 eV binding energy.

Direct evidence for a half-metallic ferromagnet

The role of grain boundaries in the magnetoresistance (MR) properties of the manganites has been investigated by comparing the properties of epitaxial and polycrystalline films of ${\mathrm{La}}_{067}{D}_{033}\mathrm{Mn}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}} (D=\mathrm{C}\mathrm{a},\phantom{\rule{0ex}{0ex}}\mathrm{S}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{o}\mathrm{r}\phantom{\rule{0ex}{0ex}}\mathrm{v}\mathrm{a}\mathrm{c}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{s})$ While the MR in the epitaxial films is strongly peaked near the ferromagnetic transition temperature and is very small at low temperatures, the polycrystalline films show large MR over a wide temperature range down to 5 K The results are explained in terms of switching of magnetic domains in the grains and disorder-induced canting of Mn spins in the grain-boundary region

Grain-boundary effects on the magnetoresistance properties of perovskite manganite films

We have used a self-aligned lithographic process to fabricate magnetic tunnel junctions of ${\mathrm{La}}_{0.67}$${\mathrm{Sr}}_{0.33}$Mn${\mathrm{O}}_{3}$ down to a few micrometers in size. We have obtained a magnetoresistance ratio as large as 83% at low magnetic fields of a few tens of Oe, which correspond to the coercivities of the magnetic layers. Transmission-electron-microscopy analysis has revealed the heteroepitaxial growth of the trilayer junction structure, ${\mathrm{La}}_{0.67}$${\mathrm{Sr}}_{0.33}$Mn${\mathrm{O}}_{3}$/SrTi${\mathrm{O}}_{3}$/${\mathrm{La}}_{0.67}$ ${\mathrm{Sr}}_{0.33}$Mn${\mathrm{O}}_{3}$. We have observed current-voltage characteristics typical of electron tunneling across an insulating barrier. The large magnetoresistance is likely due to the nearly half-metallic electronic structure of the manganites.

Large magnetotunneling effect at low magnetic fields in micrometer-scale epitaxial la0.67sr0.33mno3 tunnel junctions

The magnetotransport and magnetic properties of epitaxial ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ thin films grown on (001)-oriented MgO substrates by pulsed-laser deposition have been investigated. The magnetoresistance (MR) exhibits a peak around the Verwey transition ${(T}_{\ensuremath{\nu}})$ as has also been reported previously for single crystals. Additionally, we have observed that the MR increases monotonically below 100 K with decreasing temperature. MR values as high as 32% have been observed for a 6600-\AA{}-thick film at 60 K under a 4-T field. The enhanced low-temperature MR is attributed to a magnetic-field-dependent activation energy for electron hopping transport.

Magnetoresistance and magnetic properties of epitaxial magnetite thin films

The transport and magnetic properties of magnetite (Fe3O4) thin films grown epitaxially on single crystal MgO(100) and SrTiO3(100) substrates, and with multiple grain orientations on polycrystalline SrTiO3 substrates, have been investigated. The films are grown using pulsed laser deposition and their epitaxial quality determined using ion channeling measurements. Transport and magnetic studies of Fe3O4 films as a function of thickness and morphology suggest that epitaxial strain and growth defects affect the width and temperature of the Verwey transition. In addition, these factors also significantly influence the magnetic coercivity of the films. The low-field magnetoresistance (MR) behaviors of epitaxial and polycrystalline films as a function of temperature have been compared and they were found to be quite similar, suggesting very small contribution to the MR from grain boundaries.

Transport and magnetic properties of epitaxial and polycrystalline magnetite thin films

We report the observation of a large magnetoresistance (83%) at low magnetic fields of tens of Oe at 4.2 K in the epitaxial trilayer junction structure, La0.67Sr0.33MnO3/SrTiO3/La0.67Sr0.33MnO3. The spin-polarization parameter of the manganite has been determined from the magnetoresistance value. The switching fields of the two magnetic layers were designed by using the magnetic shape anisotropy. By limiting the sweeping field in a low field range (∼100 Oe), we have achieved bistable resistive states at zero field, which is of potential interest for magnetoelectronic applications.

G. Q. Gong

Papers

Grain-boundary effects on the magnetoresistance properties of perovskite manganite films

Large magnetotunneling effect at low magnetic fields in micrometer-scale epitaxial la0.67sr0.33mno3 tunnel junctions

Magnetoresistance and magnetic properties of epitaxial magnetite thin films

Transport and magnetic properties of epitaxial and polycrystalline magnetite thin films

Epitaxial La0.67Sr0.33MnO3 magnetic tunnel junctions