摘要:

1IntroductionThis year's report is only slightly changed in format from last year's.1Section 1 remains a general introduction in whichhighlights from not only magnetochemistry, but also the rest of magnetismare identified. Section 2 is devoted to a detailed discussion of some specialtopics and Section 3 describes other work classified by compound type.In terms of magnetochemistry, one of the highlights of the year is certainlythe design and synthesis by Ohkoshiet al.2of a magnetic substance with two “compensation” temperatures (inother words the magnetization reverses twice with changing temperature). Thishas been discussed by Kahn3and is describedfurther in Section 2.1.Ferromagnets, both old and new, have also been discussed inNature.In the field of materials science, Mohn has described how the origin of thefamous ferromagnetic “invar effect”, that has puzzled scientistsfor a century, has perhaps finally been solved.4The argument is outlined in Section 2.2. Ceperley has written about a remarkablenew ferromagnet based on calcium boride CaB6.5The doping of half a percent lanthanum ions into this diamagnetic substanceproduces a ferromagnet with an ordering temperature around 600 K; however,further doping destroys this state. The discussion surrounding this unusualsubstance is summarised in Section 2.3.Another exotic type of magnet discussed is “spin ice”. Harris6describes the first measurement of the zero‐pointentropy of a spin ice material, Dy2Ti2O7,by workers at Bell Laboratories. These experiments are described further inSection 2.4 on magnetic frustration.BothNatureandSciencehave reported progress in “high‐Tc”superconductivity.7–9Service8has commented on how “charge stripes”might be the key to understanding high‐Tcor, alternatively,a false lead. The undoped parents of the cuprate highTcmaterials are antiferromagnets. Charge stripes occur when positively chargedholes line up in rows to avoid interacting with the antiferromagnetic spinson neighbouring rows of copper atoms. This unusual phenomenon is just oneof the mysterious links between superconductivity (not just high‐Tc)and magnetism, so it seems appropriate to include a section on this emergingtopic (Section 2.5). However the coverage of high‐Tcwill be very selective: the field is now so complex that the emergence ofa consensus is certainly hampered by the mass of data and sometimes contradictoryarguments. This comment could equally apply to the field of “colossalmagnetoresistance” (CMR). Developments in this area have been reportedinNature.10,11Some selectedstudies of the CMR manganites and other magnetoresistive materials are describedin Section 2.6.This introductory section is concluded with a brief review of some notabledevelopments in other areas of magnetic research including physics, engineering,biology and geophysics.It is common for chemistry undergraduates nowadays to perform or witnessthe levitation of high‐Tcsuperconducting samplesabove a magnet. Such levitation is a characteristic of a diamagnetic material.A ferromagnet will always “flip” as a consequence of Earnshaw'stheorem which derives from the fact that a magnetic field in free space cannotpossess the maximum that is required for stable equilibrium. A diamagnet onthe other hand requires a field minimum for levitation, and this does notviolate Earnshaw's theorem. This year Geim and colleagues12have found a simple way of levitating a ferromagnet that does not violatethe theorem. The trick is to stabilise its equilibrium through the proximityof a diamagnetic substance. The most spectacular demonstration of this isthe levitation of a small but strong NdFeB magnet below a powerful superconductingmagnet, with the stabilisation applied by a human finger and thumb above andbelow the levitating material (in other words the magnet levitates betweenfinger and thumb). Magnetic levitation of the more traditional sort has beenin the news concerning the design of fusion reactors.13Workers at MIT plan to magnetically levitate a 500 kg Nb3Sn superconductingring to produce a “levitated dipole reactor” in which the reactingplasma would be trapped in hot clouds near the magnet. The normal method oftrapping the plasma is in magnetic confinement machines called tokamaks.The creation, manipulation and imaging of magnetic domains are of importancein connection to magnetic recording. Fukumuraet al.14have reported the spontaneous occurrence of “bubble” domains inan inorganic material. These domains (like tiny bubbles of reversed spin)are useful because they are robust against small perturbations and are small;however their creation usually requires a magnetic field. Therefore, the observationby scanning Hall microscopy of close‐packed bubble domains at around70 K in La1.4Sr1.6Mn2O7, a materialwith ferromagnetic layers, is of interest. Hillebrecht15has written about the experiments of Dürret al.16who have imaged magnetic domains by circular dichroism in X‐ray resonantmagnetic scattering. Backet al.17have reported the reversal of domains on an ultrafast time scale. This wasachieved by applying 2 ps magnetic field pulses to reverse the magnetizationof cobalt films. The possible application to ultrafast recording schemes wassuggested. Equally impressive is the experimental determination18of the magnetization of a monodomain nanoparticle by the Foucault method ofLorentz microscopy. This method gives higher spatial resolution than micro‐SQUIDsor magnetic force microscopy. The direction of magnetization in particlesof SmCo5, Fe3O4and carbon‐coatedFe50Co50as small as 5 nm in diameter was determined.Our understanding of magnetic orientation in animals is steadily advancing,but the mechanism of magnetoreception in the higher animals is still a subjectof debate. One idea is that the geomagnetic field interacts with photoreceptors.This year Deutschlanderet al.19have found additional evidence for this. They showed that the magnetic navigationof newts is strongly dependent on the wavelength of light to which they areexposed.The recognition that the earth behaves like a huge magnetic dipole is oneof the oldest established results in science and has been exploited as anaid to navigation for thousands of years. It is now understood that the mainmagnetic field originates from electric currents associated with fluid flowin the liquid iron‐rich outer core, while the solid inner core servesto stabilise the field by preventing fast relaxation. An intriguing fact isthat the field intensity varies on all time scales from seconds to millionsof years. The variation on long time scales can only be established from thegeological record. This year Guyodo and Valet20have provided an authoritative update of paleomagnetic intensity data spanningthe past 800000 years, during which time the field polarity was the same asit is now. Significant dips in the field intensity are found to be correlatedwith directional excursions, which lends support to the hypothesis of Gubbins,21that the excursions occur when the field reversesin the outer core but not the inner core. Langereis22has explained that the new compilation of data also casts considerable doubton the idea that the excursions are anything to do with climate change orthat the field intensity is connected to the earth's orbit.On an even grander scale the sun's magnetic field is also produced by convectiondeep within its interior. It is very weak (about 0.5 mT) but has been reportedby Lockwoodet al.23to have morethan doubled in the past one hundred years. This change coincides with a doublingof the number of sunspots and a brightening of the sun by about 0.1%.The obvious suggestion is that this might account for global warming. However,Parker24stresses that as regards climatechange, “In our present state of ignorance it is not possible to assessthe importance of individual factors”. Elsewhere in the solar system,the global distribution of the magnetization of the crust of the planet Marshas been determined by the Mars Global Surveyor.25

ISSN:0260-1818    

出版商:RSC    

年代:2000

数据来源: RSC