Synthesis of monodisperse iron-platinum (FePt) nanoparticles by reduction of platinum acetylacetonate and decomposition of iron pentacarbonyl in the presence of oleic acid and oleyl amine stabilizers is reported. The FePt particle composition is readily controlled, and the size is tunable from 3- to 10-nanometer diameter with a standard deviation of less than 5%. These nanoparticles self-assemble into three-dimensional superlattices. Thermal annealing converts the internal particle structure from a chemically disordered face-centered cubic phase to the chemically ordered face-centered tetragonal phase and transforms the nanoparticle superlattices into ferromagnetic nanocrystal assemblies. These assemblies are chemically and mechanically robust and can support high-density magnetization reversal transitions.

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Monodisperse FePt Nanoparticles and Ferromagnetic FePt Nanocrystal Superlattices

The recent literature concerning the magnetocaloric effect (MCE) has been reviewed. The MCE properties have been compiled and correlations have been made comparing the behaviours of the different families of magnetic materials which exhibit large or unusual MCE values. These families include: the lanthanide (R) Laves phases (RM2, where M = Al, Co and Ni), Gd5(Si1−xGex)4 ,M n(As1−xSbx), MnFe(P1−xAsx), La(Fe13−xSix) and their hydrides and the manganites (R1−xMxMnO3, where R = lanthanide and M = Ca, Sr and Ba). The potential for use of these materials in magnetic refrigeration is discussed, including a comparison with Gd as a near room temperature active magnetic regenerator material. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0034-4885/68/6/R04/pdf

Recent developments in magnetocaloric materials

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

Much has been written about theory and practice in the law, and the tension between practitioners and theorists. Judges do not cite theoretical articles often; they rarely "apply" theories to particular cases. These arguments are not revisited. Instead the Essay explores the working and interaction of theory and practice, practitioners and theorists. The Essay starts with a story about solving a legal issue using our intellectual tools - theory, practice, and their progenies: experience and "gut." Next the Essay elaborates on the nature of theory, practice, experience and "gut." The third part of the Essay discusses theories that are helpful to practitioners and those that are less helpful. The Essay concludes that practitioners theorize, and theorists practice. They use these intellectual tools differently because the goals and orientations of theorists and practitioners, and the constraints under which they act, differ. Theory, practice, experience and "gut" help us think, remember, decide and create. They complement each other like the two sides of the same coin: distinct but inseparable.

Of Theory and Practice

Interpretation of X-ray Powder Diffraction Patterns . By H. Lipson and H. Steeple. Pp. viii + 335 + 3 plates. (Mac-millan: London; St Martins Press: New York, May 1970.) £4.

X-Ray Diffraction

Erratum: “Direct chemical synthesis of high coercivity SmCo nanoblades” [Appl. Phys. Lett. 93, 032505 (2008)]

A first-order phase transition in mechanically soft, nonstoichiometric Ni2MnGa oriented single crystals was achieved by the application of a magnetic field lower than 1 T on a sample with a forward martensitic transformation temperature TM of 314 K. The observed change in magnetization corresponds to a temperature decrease of 1 K. The entropy variation, corresponding to this partial phase rearrangement, is of the order of −1.5 J/kg K at 1 T, and reaches a value of −18 J/kg K at 5 T.

Magnetic entropy in Ni2MnGa single crystals

Magnetic properties and large coercivity of MnxGa nanostructures

A new method of assessing the magnitude of the magnetic field in a transmission electron microscope using a permanent magnetic material is described. The approach is versatile and simple enough to be implemented for certain scientific or engineering situations in which the exact calibration of the magnetic field in the microscope column using a small Hall probe may be a problem. We have applied this approach to obtain the magnetic field calibration inside a JEM 3000F field emission electron microscope as a function of the objective lens potential. In the course of this in situ calibration it was not necessary to disassemble the microscope or interrupt its operation. The procedure used is versatile and accurate enough to measure magnetic fields up to 20 kOe (μ0H=2 T in SI units) as was confirmed by subsequent Hall-probe field measurement of the same electron microscope. The calibration method described in this article does not require any special Hall-probe holder adaptations and can be applied to any tran...

Magnetic field calibration of a transmission electron microscope using a permanent magnet material

The crystallographic transformation in La{sub 1.88{minus}y}Sr{sub 0.12}Nd{sub y}CuO{sub 4}(y=0, 0.1, 0.2, and 0.4) from the orthorhombic Bmab space group (LTO1 phase) to a low-temperature tetragonal (P4{sub 2}/ncm, LTT phase) phase was examined using high-resolution synchrotron x-ray powder diffraction. For Nd contents y=0.2 and 0.4, the phase transformation to LTT takes place over a significant range of temperature, with the transformation essentially complete at the lowest temperatures attained, {approximately}15 K. For Nd y=0.0 and 0.1, the transformation to LTT is incomplete, with the LTT phase corresponding to about 10{percent} and 20{percent} of the LTO1 intensity at lowest temperatures. The evolution of the LTT phase is discussed in the context of earlier transmission electron microscopy studies. Simulated diffraction profiles based on the observed microstructure are used to explore possible mechanisms for development of the LTT phase. The presence of this minority LTT phase in La{sub 1.88}Sr{sub 0.12}CuO{sub 4} (Nd y=0) helps explain the slightly depressed superconducting transition temperature observed for slightly higher and lower Sr contents in terms of the proposed charge stripe phase. {copyright} {ital 1998} {ital The American Physical Society}

Laura H. Lewis

Papers

Erratum: “Direct chemical synthesis of high coercivity SmCo nanoblades” [Appl. Phys. Lett. 93, 032505 (2008)]

Magnetic entropy in Ni2MnGa single crystals

Magnetic properties and large coercivity of MnxGa nanostructures

Magnetic field calibration of a transmission electron microscope using a permanent magnet material

High-resolution x-ray diffraction study of La{sub 1.88{minus}y}Sr{sub 0.12}Nd{sub y}CuO{sub 4}