s. t. bramwell- magnetism

8/3/2019 S. T. Bramwell- Magnetism

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24 Magnetism

S. T. Bramwell

University College London, Department of Chemistry, Christopher Ingold Laboratories, 20 Gordon Street, London, UK WC1H OAJ

Highlights in this years report include new developments in spintronics, a chemicalapproach to premanent magnets, coupled magneto-electro-optical phenomena, andexotic "frustrated" magnets. These reect some of the dominant themes in modernmaterials magnetism.

1 Introduction

This report commences with a brief round up of highlight developments in magnet-ism across diverse subjects particularly those that have aroused some interest in thegeneral science journals.

Among reports of new magnetic substances, one of the most memorable of the yearis that of an organic molecular solid that exhibits simultaneous switching of magnetic,transport and optical properties with a change in temperature.1 This is described as aSelected Topic in section 2. Also described in that section is a new approach tomaking permanent magnet materials that was reported byNature .2,3 Section 2 iscompleted by a discussion of frustrated magnets. In this context,Nature has reported4

on how the application of pressure to a spin liquid can result in its crystallizationor long range magnetic ordering.5The main application of magnetism in chemistry is of course NMR. This year,

Science magazine has reported a new method to gain chemical information fromNMR that uses very low magnetic elds, and thus eliminates the large and expensivemagnets in common use.6 This remarkable breakthrough7 relies on the fact that theNMR line width scales linearly with magnetic eld, so the use of small elds guaran-tees a narrow line width and high signal to noise. At these low elds, the normalchemical shift cannot be resolved butJ couplings between protons and nearby nuclei

remain. This was illustrated by measuring theJ -coupling spectrum of protons intrimethyl phosphate, in which the1H31 P coupling is roughly 10 Hz. The protons werepolarised with a millitesla eld before the NMR was studied in a microtesla eld,where detection was achieved using a SQUID magnetometer.

Science 8 reports on a development in spintronics the fabrication of a micro-scopic NOT gate. In a magnetic nanowire the magnetization lies along the directionof the wire. Domain walls can be made to propagate along the wire by applying a

DOI: 10.1039/b211478j Annu. Rep. Prog. Chem., Sect. A, 2003,99 , 467475 467


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Fig. 1 An organic molecular solid that exhibits a simultaneous magnetic, optical and electronicbistability [after ref. 2]. (a) Basic molecular structure (A) and crystal structure (B). (b) (From topto bottom) Optical transmittance, electrical conductivity and magnetic susceptibility versustemperature. (c) Proposed mechanism for the electronic change near T = 330 K.

Annu. Rep. Prog. Chem., Sect. A, 2003, 99 , 467 475 469


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in the second (demagnetizing) quadrant of the hysteresis loop. This is a measure of themaximum magnetostatic energy that would be stored in free space between the polesof a permanent magnet made from that particular material. The largest known energyproduct is for neodymium iron boride, Nd 2Fe 14 B, at 56.7 MGOe, which owes its pre-eminence to a combination of high magnetization (arising mainly from Fe) and highanisotropy (arising mainly from Nd in a low symmetry environment). However thereare other materials with larger magnetization at room temperature the record isheld by the alloy Fe 65 Co 35 . Such materials are soft (i.e. with low coercivity) andhence have a smaller energy product than Nd 2Fe 14 B, despite a larger magnetization.The challenge is to try and increase the coercivity of soft, large magnetizationmaterials to values comparable with Nd 2Fe 14 B. An idea that has been used for 10 yearsor so is to try and mix two materials a soft and a hard one that are exchangecoupled together (sometimes called an exchange spring magnet). For this couplingto be effective, the dimension of the soft phase should be less than about twice thewidth of the magnetic domain walls in the hard phase. 2

The approach of Zeng et al. 3 was to start with carefully controlled dispersions of nanoparticulate Fe 58 Pt 42 . (size 4 nm) and Fe 3O 4 (size 4 12 nm) in hexane. These weremixed and allowed to self-assemble before being reduced into FePt Fe 3Pt nano-composites by high temperature annealing under an argon hydrogen mixture. Thisprocess transformed the FePt from a disordered face-centred cubic (f.c.c.) to anordered face-centred tetragonal structure which has the strong magnetocrystallineanisotropy required for the hard phase. Meanwhile, the Fe arising from Fe 3O 4 com-bined with FePt to create a new soft f.c.c. phase, Fe

3Pt, with high magnetization. The

mixture of a hard FePt phase and a soft, Fe 3Pt phase was found in the best cases togive an energy product of 20.1 MGOe, a signi cant increase on the value found forsingle phase FePt (13 MGOe). However, as expected, it was found that the ratio of sizes between the particles of the hard and soft phases played an important role. Fig. 2shows the hysteresis loops for two cases, each made from an initial particle mass ratioof Fe 3O 4 : FePt = 1 : 10, but with different size ratios Fe 3O 4 : FePt = 4 nm : 4 nm and12 nm : 4 nm respectively. In the latter case the particles of soft material become toolarge for the exchange coupling to be effective. This means that the two phases do not

switch cooperatively, which is manifest as a kink in the magnetization curve associatedwith the magnetization reversal of the soft phase.The results of Zeng et al. illustrate how chemistry can be used in a controlled and

imaginative way to create new permanent magnet materials.

Geometrically frustrated magnets

The geometrically frustrated pyrochlore antiferromagnet Tb 2Ti 2O 7 furnishes an

example of a spin liquid state, persisting down to millikelvin temperatures.13

Thisyear Mirebeau et al. 5 have studied it under conditions of high pressure (9 GPa) andlow temperature (1 K). They nd that, under applied pressure, the Tb magneticmoments start to magnetically order or crystallize below a Neel temperature of 2.1 K. However, the antiferromagnetic order still coexists with a liquid-like state. Theresults suggest that the transition from disorder to order is rather sudden as a functionof pressure.

470 Annu. Rep. Prog. Chem., Sect. A, 2003, 99 , 467 475


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The spinel material ZnCr 2O 4 has strong antiferromagnetic interactions on thetetrahedral B-site lattice of the spinel structure (identical to the pyrochlore lattice) andis considered a geometrically frustrated antiferromagnet in which quantum mechanicsplays a major role. Lee et al. 14 have presented neutron scattering data at T = 15 K thatsuggests that the scattering is from the uctuations of hexagonal spin clusters, each of

which carries zero average moment. This means that the system behaves not likea system of strongly interacting spins, but like a system of weakly interactingdirectors (i.e. staggered magnetization vectors) of the individual six-spin loops.Such a system was dubbed a protectorate a term which has been used to describestable states emerging in highly correlated many body systems: 15 hence the termdirector protectorate for the behaviour of ZnCr 2O 4.14

SrCu 2(BO 3)2 is an important frustrated quantum magnet in which the Cu 2 ionswith spin one half occupy a planar lattice of orthogonal dimers, with frustratedinteractions between dimers. The ground state is a singlet consisting of a collection of

dimers, with an excitation gap to the rst triplet magnetic states of 3 eV. The magnet-ization curve in very high elds exhibits plateaus at 1/8, 1/4, and 1/3 of saturation,believed to be associated with the crystallisation of triplets onto a superlattice.Evidence from Cu and B NMR 16 has now been shown to be consistent with such asuperlattice from the 1/8 plateau at an applied eld of 27 T. In the plateau state, theitinerent triplets appear to crystallize into a large rhomboid unit cell with 16 spins perlayer.

Fig. 2 Hysteresis loops for nanocomposite Fe 3O 4 : FePt = 1:10, with two different size ratios;(a) Fe 3O 4 : FePt = 4 nm : 4 nm and (b) 12 nm : 4 nm (after ref. 3).



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3 Ionic, covalent and metallic materials

Oxides

Ferroelectromagnets are materials that display simultaneous ferroelectric- and ferro-magnetic-order: the coupling of these phenomena is of potential interest in spin-tronics applications. A report this year 17 has imaged coupled antiferromagnetic andferroelectric domains in the material YMnO 3 using second harmonic generation.The results con rm the coupling of the electrical and magnetic domains in thismaterial.

The perovskite oxide Sr 2FeMoO 6,3 is known to exhibit signi cant room temper-ature magnetoresistance. 18 This year, it has been reported that the insulatingantiferromagnet Sr 2CoMoO 6 with T N = 37 K can be transformed into a room temper-

ature semiconducting ferromagnet with T C =

350 370 K, by chemical removal of oxygen 19 The ferromagnetic phase is found to exhibit 30% magnetoresistance in a 9 Teld at T = 12 K, but at higher temperatures the magnetoresistance becomes veryweak.

Germanates

The magnetic properties of the rare earth iron germanates RFeGe 2O 7 (R=

Y, Pr,Dy, Tm, and Yb) have been investigated by powder neutron diffraction and suscepti-bility measurements. 20 The phases with heavy rare earths or Y are found to havea magnetic transition near to T = 40 K, where the rare earth and iron magneticmoments order antiferromagnetically. The magnetic order in the iron sublattice iscomplete by about 10 K, while the ordered rare earth moment is found to increasemore steeply below this temperature, giving rise to a second anomaly in the suscepti-bility. In the Pr compound (which has a different crystal structure), simultaneousordering of the Pr and Fe moments takes place at the much lower temperature of

T =

3 K.

Borides

Calcium hexaboride (CaB 6) was reported in 1999 21 to be a high temperature weakferromagnet, with the magnetic properties arising from the low density free electrongas, despite the absence of an intrinsically magnetic ion in the material (see the 1999

report). Now it has been claimed22

that the ferromagnetism is simply the result of ironimpurities in the material. The authors of the original report have countered thatalthough it seems likely that foreign substances do give rise to the moment in thematerial, it is not so simple as having alien phases in the sample. 23 Rather, they suggestthat atomic defects interact strongly with the electronic system to give the ferro-magnetism. It seems that more experimental work is required in order to decide who isright.



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Antimonides

The mixed transition metal rare earth antimonide Eu 14 MnSb 11 is isostructural withthe Zintl phases. A 14 MX 11 (A = Ca, Sr, Ba; M = Al, Ga, Mn; X = As, Sb, Bi) 24 It showsferromagnetic ordering of the Mn 3 moments at 92 K, followed by antiferromagneticordering of the Eu 2 moments at 15 K. 25 Large negative magnetoresitsance of about

30% in a eld of 6 T is observed near the 92 K transition, which involves a transitionfrom paramagnetic semiconductor to ferromagnetic metal. Even larger negativemagnetoresistance of about 80% then occurs near the 15 K transition, as the anti-ferromagnetic Eu 2 magnetic moments are aligned by the magnetic eld. 26 This year,Kim et al. have studied the solid solutions Ca 14 xEu xMnSb 11

27,28 and analogousphases of other alkaline earth metals 29 which show a rather sensitive variation of themagnetoreistance on doping. All members of the series continue to show a maxi-mum in the magnetoresistance of about 30% between 50 and 100 K while thelower temperature magnetoresistance is strongly surpressed by doping with thenon-magnetic ions.

4 Coordination complexes and molecular compounds

Extended systems: framework, layer and chain structures

The apparent discovery of ferromagnetism in polymerised fullerenes, 30 was reviewedin last year s Report. This year, Woods et al. 31 have provided further evidence for thisphenomenon. They prepared rhombohedral polymers of C 60 under conditions of high pressure and temperature. These were found to have varying degrees of ferro-magnetism, the strongest being observed in the phases prepared at 800 K, just belowthe temperature of collapse to graphitic carbon. The ferromagnetism was ascribed toradical formation. Blundell 32 has brie y reviewed these developments in the contextof other organic ferromagnets.

A related approach to organic ferromagnetism involves conjugated radicals. In an

organic conjugated di-radical, ferromagnetic exchange coupling may produce a tripletground state. This concept can be extended to conjugated organic polymers thatcontain multiple radical sites. Hence a very large spin value in the ground state (up toS = 5000 33 can be achieved. If this is large enough the magnetization will naturallyblock (i.e. become hard to reorient) which gives the possibility of organics that aremagnetizable at moderate temperatures. Rajca 34 has reviewed the design concepts of such systems, but has stressed the dif culty of obtaining ordering temperaturesmuch higher than 10 K.

Paul et al. 35 have described a layered iron II sulfate complex of formula

[H 3N(CH 2)2NH 2(CH 2)2NH 2(CH 2)2NH 3][Fe 3F 6(SO 4)2], in which the Fe2

ions occupya distorted kagome lattice, well known for its frustrated interactions. The materialshows a large splitting between eld-cooled and zero eld-cooled susceptibility below17 K which might indicate ferrimagnetism or glassy magnetic behaviour. Anotherlayered compound, the organosul de Co(1,2-(O 2C)(S)C 6H 4) has ligand bilayersbound by Co 2 ions. 36 It has been found to undergo a magnetic transition at T = 9 K,apparently to a canted antiferromagnetic state.



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Isolated complexes: zero-dimensional systems

Single molecule magnets (described in previous reports) continue to be of interest.This year, a cyclic compound of nickel with spin S = 12 has been described. 37 Thecompound has formula [Ni 12 (chp) 12 (O 2CMe) 12 (thf ) 6(H 2O) 6] in which the ferro-magnetically coupled Ni 2 ions form a twelve membered ring with overall 3-foldsymmetry. The energy barrier to reorientation of the overall spin ( S = 12) is estimatedto be about 10 K, and the low temperature (0.35 mK) magnetic hysteresis curves showthe stepped form characteristic of relaxation by quantum tunnelling through thequasi-classical barrier. An analogous Co 2 compound with overall spin S = 6 has alsobeen described. 38

A newly discovered complex with ring geometry and antiferromagnetic interactionsis [Cr 8F 8Piv 16 ] where HPiv = pivalic acid (trimethyl acetic acid). 39 In this case the Cr 3

ions form regular octagonal rings and are coupled antiferromagnetically to give anS = 0 ground state. Magnetic susceptibility and torque measurements t the energygap to the rst magnetic state as about 6.6 cm 1. This complex also has a clear EPRspectrum a rare distinction among antiferromagnetic ring systems from whichfurther coupling parameters can be derived.

We nally note a review article by Benelli and Gatteschi 40 that treats the magneticproperties of molecular compounds containing rare-earth ions coupled to transitionmetal ions and organic radicals.

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s. t. bramwell- magnetism

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