Interest in magnetic nanoparticles has increased in the past few years by virtue of their potential for applications in fields such as ultrahigh-density recording and medicine. Most applications rely on the magnetic order of the nanoparticles being stable with time. However, with decreasing particle size the magnetic anisotropy energy per particle responsible for holding the magnetic moment along certain directions becomes comparable to the thermal energy. When this happens, the thermal fluctuations induce random flipping of the magnetic moment with time, and the nanoparticles lose their stable magnetic order and become superparamagnetic. Thus, the demand for further miniaturization comes into conflict with the superparamagnetism caused by the reduction of the anisotropy energy per particle: this constitutes the so-called 'superparamagnetic limit' in recording media. Here we show that magnetic exchange coupling induced at the interface between ferromagnetic and antiferromagnetic systems can provide an extra source of anisotropy, leading to magnetization stability. We demonstrate this principle for ferromagnetic cobalt nanoparticles of about 4 nm in diameter that are embedded in either a paramagnetic or an antiferromagnetic matrix. Whereas the cobalt cores lose their magnetic moment at 10 K in the first system, they remain ferromagnetic up to about 290 K in the second. This behaviour is ascribed to the specific way ferromagnetic nanoparticles couple to an antiferromagnetic matrix.

Beating the superparamagnetic limit with exchange bias

An insulator does not conduct electricity, and so cannot in general be used to transmit an electrical signal. However, the electrons within an insulator possess spin as well as charge, so it is possible for them to transmit a signal in the form of a spin wave. Kajiwara et al. have now developed a hybrid metal–insulator–metal structure in which an electrical signal in one metal layer is directly converted to a spin wave in the insulating layer. This wave is then transmitted to the second metal layer, where the signal can be directly recovered as an electrical voltage. The observation of voltage transmission in an insulator raises the prospect of insulator-based spintronics and other novel forms of signal delivery. An insulator does not conduct electricity, and so cannot in general be used to transmit an electrical signal. But an insulator's electrons possess spin in addition to charge, and so can transmit a signal in the form of a spin wave. Here a hybrid metal–insulator–metal structure is reported, in which an electrical signal in one metal layer is directly converted to a spin wave in the insulating layer; this wave is then transmitted to the second metal layer, where the signal can be directly recovered as an electrical voltage. The energy bandgap of an insulator is large enough to prevent electron excitation and electrical conduction1. But in addition to charge, an electron also has spin2, and the collective motion of spin can propagate—and so transfer a signal—in some insulators3. This motion is called a spin wave and is usually excited using magnetic fields. Here we show that a spin wave in an insulator can be generated and detected using spin-Hall effects, which enable the direct conversion of an electric signal into a spin wave, and its subsequent transmission through (and recovery from) an insulator over macroscopic distances. First, we show evidence for the transfer of spin angular momentum between an insulator magnet Y3Fe5O12 and a platinum film. This transfer allows direct conversion of an electric current in the platinum film to a spin wave in the Y3Fe5O12 via spin-Hall effects4,5,6,7,8,9,10,11. Second, making use of the transfer in a Pt/Y3Fe5O12/Pt system, we demonstrate that an electric current in one metal film induces voltage in the other, far distant, metal film. Specifically, the applied electric current is converted into spin angular momentum owing to the spin-Hall effect7,8,10,11 in the first platinum film; the angular momentum is then carried by a spin wave in the insulating Y3Fe5O12 layer; at the distant platinum film, the spin angular momentum of the spin wave is converted back to an electric voltage. This effect can be switched on and off using a magnetic field. Weak spin damping3 in Y3Fe5O12 is responsible for its transparency for the transmission of spin angular momentum. This hybrid electrical transmission method potentially offers a means of innovative signal delivery in electrical circuits and devices.

Transmission of electrical signals by spin-wave interconversion in a magnetic insulator

Critical phenomena and renormalization-group theory

This paper provides a detailed overview of developments in transducer materials technology relating to their current and future applications in micro-scale devices. Recent advances in piezoelectric, magnetostrictive and shape-memory alloy systems are discussed and emerging transducer materials such as magnetic nanoparticles, expandable micro-spheres and conductive polymers are introduced. Materials properties, transducer mechanisms and end applications are described and the potential for integration of the materials with ancillary systems components is viewed as an essential consideration. The review concludes with a short discussion of structural polymers that are extending the range of micro-fabrication techniques available to designers and production engineers beyond the limitations of silicon fabrication technology.

/pdf/new-materials-for-micro-scale-sensors-and-actuators-an-21qnaonn0l.pdf

New materials for micro-scale sensors and actuators An engineering review

http://boulph.free.fr/Krzysztof/2003_Recent_and_future_advances.pdf

Recent advances and future directions in magnetic materials

1. Magnetostatic phenomena 2. Magnetic measurements 3. Atomic magnetic moments 4. Macroscopic experimental techniques 5. Magnetic disorder 6. Ferromagnetism 7. Antiferromagnetism and ferrimagnetism 8. Magnetism of metals and alloys 9. Magnetism of ferromagnetic oxides 10. Magnetism of compounds 11. Magnetism of amorphous materials 12. Magnetocrystalline anisotrophy 13. Induced magnetic anisotropy 14. Magnetostriction 15. Observation of domain structures 16. Spin distribution and domain walls 17. Magnetic domain structure 18. Technical magnetization 19. Spin phase transition 20. Dynamic magnetization 21. Various phenomena association with magnetization 22. Engineering applications of magnetic materials

Physics of ferromagnetism

This chapter contains sections titled: Introduction Magnetostriction of Single Crystals Magnetostriction of Polycrystals Physical Origin of Magnetostriction Effect of Stress on Magnetic Properties Effect of Stress on Magnetostriction Applications of Magnetostriction ?>E Effect Magnetoresistance Problems

Magnetostriction and the Effects of Stress

This chapter contains sections titled: Introduction Magnetic Moments of Electrons Magnetic Moments of Atoms Theory of Diamagnetism Diamagnetic Substances Classical Theory of Paramagnetism Quantum Theory of Paramagnetism Paramagnetic Substances Problems

Diamagnetism and Paramagnetism

This chapter contains sections titled: Introduction Magnetic Annealing (Substitutional Solid Solutions) Magnetic Annealing (Interstitial Solid Solutions) Stress Annealing Plastic Deformation (Alloys) Plastic Deformation (Pure Metals) Magnetic Irradiation Summary of Anisotropies ]]>

Induced Magnetic Anisotropy

This chapter contains sections titled: Introduction Single-Domain vs Multi-Domain Behavior Coercivity of Fine Particles Magnetization Reversal by Spin Rotation Magnetization Reversal by Wall Motion Superparamagnetism in Fine Particles Superparamagnetism in Alloys Exchange Anisotropy Preparation and Structure of Thin Films Induced Anisotropy in Films Domain Walls in Films Domains in Films Problems

C. D. Graham

Papers

Physics of ferromagnetism

Magnetostriction and the Effects of Stress

Diamagnetism and Paramagnetism

Induced Magnetic Anisotropy

Fine Particles and Thin Films