NATURE MATERIALS DOL:10.1038/NMAT2406 INSIGHT I REVIEW ARTICLES intergrowth of ferrimagnetic magnetite-rich regions separated by non-magnetic ulvospinel-rich lamellae.The remanent magnetiza- tion in such rocks is used by geophysicists to map the motions of continents and ocean beds resulting from the dynamics of plate tectonics.As an example of the application of electron holography to rock magnetism,Fig.7c,d shows two remanent magnetic states recorded from the same region of the specimen.The induction maps show that individual blocks'of magnetite contain primarily single-domain magnetic states,which have lower energies than vortex states in the presence of strong interactions with neighbour- ing magnetic crystals.They also reveal the magnetostatic inter- action fields between them.Magnetic superstates,in which clusters 2250e 2250e of magnetite blocks act collectively to form vortex and multidomain states that have zero net magnetization,are also visible.The images Afte After illustrate the complexity of the magnetic structure in this system saturating saturating Electron holography has considerable potential for measuring both remanent magnetizations and magnetization reversal mechanisms in rocks,and for understanding mineral magnetism at the nano- metre scale. 200nm Similar magnetic interactions are observed in nature in magnetotactic bacteria,which contain single or multiple chains of ferrimagnetic magnetite (Fe,O)or greigite (FeS)crystals that are typically between 35 and 120 nm in size.In this size range,the crystals are uniformly magnetized single magnetic domains at room 200nm temperature.The arrangement of the crystals in linear chains results in a magnetic moment that orients the bacteria parallel to the geo- magnetic field in an aquatic environment.A magnetic induction map recorded from a single helical bacterial cell containing a chain of equi-dimensional magnetite crystals is shown in Fig.7es.The magnetic phase contours are parallel to each other in the crystals and follow the chain axis.Unlike the more complicated magnetic arrangements seen in Fig.7c,d,the magnetic moment in the linear bacterial chain is maximized,and its remanent magnetic state is almost equal to its saturated state.Although a single chain would appear to be ideal for magnetotaxis,a number of strains of bacteria possess either disorganized multiple arrangements of crystals or Figure 7 Magnetic induction maps of geological and biogenic magnetic large crystals(up to 200 nm in length)that would each be expected particles.a,b,Three-window background-subtracted elemental maps to contain several magnetic domains if they were isolated. acquired from a naturally occurring titanomagnetite sample with a Gatan For a fuller understanding of nanoparticle interactions and imaging filter using the iron L edge (a)and the titanium L edge (b). magnetic response,systematic studies of both continuous ferro- Brighter contrast indicates higher concentrations of iron and titanium in magnetic films4 and lithographically patterned ferromagnetic (a)and (b),respectively.c,d,Magnetic phase contours from the same nanostructures have been carried out using electron holography for region,measured using electron holography.Each image was acquired different film thicknesses and element sizes,thicknesses and shapes, with the specimen in magnetic-field-free conditions.The outlines of illustrating switching variability in nominally identical structuress magnetite-rich regions are marked in white,and the direction of the and prompting consideration of modified shapes such as rings, measured magnetic induction is indicated using arrows and according to slotted disks and slotted rings for applications the colour wheel shown (red,right;yellow,down;green,left;blue,up). The development of electron holography for the characteriza- The image in c was acquired after applying a large(>10,000 Oe)field tion ofelectrostatic fields in materials has a long history,most nota- towards the top left,then the indicated (225 Oe)field towards the bottom bly at the University of Bologna.Although it has been used to right,after which the external magnetic field was removed for hologram characterize the electric fields of microtips,electrically biased car- acquisition.The image in d was acquired after applying identical fields in bon nanotubes"0 and electroceramics,perhaps the most important the opposite directions.(Adapted from ref.77.)e,Contours of 0.064-rad potential application of electron holography in materials science spacing formed from the magnetic contribution to the holographic phase and technology is the prospect that the technique can fulfil the shift acquired from a single bacterial cell (inset)of Magnetospirillum requirement of the semiconductor industry to provide quantitative magnetotacticum strain MS-1,imaged in magnetic-field-free conditions information about electrostatic potentials in doped semiconductors The contours,which spread out at the ends of the chain,are overlaid onto (and in ferroelectric materials")with nanometre spatial resolution. the contribution of the mean inner potential to the phase shift,to allow Figure 8a illustrates the geometry of a semiconductor p-n junc- the positions of the crystals to be correlated with the magnetic contours. tion in a thin TEM sample.The true junction potential(Fig.8b)is (Adapted from ref.81.) assumed to lie within specimen thickness,t which is smaller than the total specimen thickness,t,as a result of the presence of speci- respectively.Similar phase images are now used routinely to charac- men surfaces that are altered electrically as a result of the presence terize electrostatic potentials at source and drain regions in transis- of surface states and specimen preparation for electron microscopy. tors23,and techniques have been developed to measure electrostatic A representative phase image recorded from a silicon p-n junction potentials in reverse-biased semiconductor devices prepared for using electron holography is shown in Fig.8c;p-type and n-type TEM examination using focused-ion-beam milling.However, regions are delineated clearly as areas of darker and lighter contrast, questions still remain about aspects of the interpretation of such NATURE MATERIALS|VOL 8|APRIL 2009 www.nature.com/naturematerials 277 2009 Macmillan Publishers Limited.All rights reservednature materials | VOL 8 | APRIL 2009 | www.nature.com/naturematerials 277 NaTure maTerIals doi: 10.1038/nmat2406 insight | review articles intergrowth of ferrimagnetic magnetite-rich regions separated by non-magnetic ulvöspinel-rich lamellae. The remanent magnetiza￾tion in such rocks is used by geophysicists to map the motions of continents and ocean beds resulting from the dynamics of plate tectonics. As an example of the application of electron holography to rock magnetism, Fig. 7c, d shows two remanent magnetic states recorded from the same region of the specimen. The induction maps show that individual ‘blocks’ of magnetite contain primarily single-domain magnetic states, which have lower energies than vortex states in the presence of strong interactions with neighbour￾ing magnetic crystals79,80. They also reveal the magnetostatic inter￾action fields between them. Magnetic superstates, in which clusters of magnetite blocks act collectively to form vortex and multidomain states that have zero net magnetization, are also visible. The images illustrate the complexity of the magnetic structure in this system. Electron holography has considerable potential for measuring both remanent magnetizations and magnetization reversal mechanisms in rocks, and for understanding mineral magnetism at the nano￾metre scale. Similar magnetic interactions are observed in nature in magnetotactic bacteria, which contain single or multiple chains of ferrimagnetic magnetite (Fe3O4) or greigite (Fe3S4) crystals that are typically between 35 and 120 nm in size. In this size range, the crystals are uniformly magnetized single magnetic domains at room temperature. The arrangement of the crystals in linear chains results in a magnetic moment that orients the bacteria parallel to the geo￾magnetic field in an aquatic environment. A magnetic induction map recorded from a single helical bacterial cell containing a chain of equi-dimensional magnetite crystals is shown in Fig. 7e81. The magnetic phase contours are parallel to each other in the crystals and follow the chain axis. Unlike the more complicated magnetic arrangements seen in Fig. 7c, d, the magnetic moment in the linear bacterial chain is maximized, and its remanent magnetic state is almost equal to its saturated state. Although a single chain would appear to be ideal for magnetotaxis, a number of strains of bacteria possess either disorganized multiple arrangements of crystals82 or large crystals (up to 200 nm in length) that would each be expected to contain several magnetic domains if they were isolated. For a fuller understanding of nanoparticle interactions and magnetic response, systematic studies of both continuous ferro￾magnetic films83,84 and lithographically patterned ferromagnetic nanostructures have been carried out using electron holography for different film thicknesses and element sizes, thicknesses and shapes, illustrating switching variability in nominally identical structures85 and prompting consideration of modified shapes such as rings, slotted disks and slotted rings for applications86. The development of electron holography for the characteriza￾tion of electrostatic fields in materials has a long history, most nota￾bly at the University of Bologna87,88. Although it has been used to characterize the electric fields of microtips89, electrically biased car￾bon nanotubes90 and electroceramics, perhaps the most important potential application of electron holography in materials science and technology is the prospect that the technique can fulfil the requirement of the semiconductor industry to provide quantitative information about electrostatic potentials in doped semiconductors (and in ferroelectric materials91) with nanometre spatial resolution. Figure 8a illustrates the geometry of a semiconductor p–n junc￾tion in a thin TEM sample. The true junction potential (Fig. 8b) is assumed to lie within specimen thickness, tel, which is smaller than the total specimen thickness, t, as a result of the presence of speci￾men surfaces that are altered electrically as a result of the presence of surface states and specimen preparation for electron microscopy. A representative phase image recorded from a silicon p–n junction using electron holography is shown in Fig. 8c; p-type and n-type regions are delineated clearly as areas of darker and lighter contrast, respectively. Similar phase images are now used routinely to charac￾terize electrostatic potentials at source and drain regions in transis￾tors92,93, and techniques have been developed to measure electrostatic potentials in reverse-biased semiconductor devices prepared for TEM examination using focused-ion-beam milling94. However, questions still remain about aspects of the interpretation of such a e b c d 225 Oe After saturating 225 Oe After saturating 200 nm 200 nm Figure 7 | magnetic induction maps of geological and biogenic magnetic particles. a, b, Three-window background-subtracted elemental maps acquired from a naturally occurring titanomagnetite sample with a Gatan imaging filter using the iron L edge (a) and the titanium L edge (b). Brighter contrast indicates higher concentrations of iron and titanium in (a) and (b), respectively. c, d, Magnetic phase contours from the same region, measured using electron holography. Each image was acquired with the specimen in magnetic-field-free conditions. The outlines of magnetite-rich regions are marked in white, and the direction of the measured magnetic induction is indicated using arrows and according to the colour wheel shown (red, right; yellow, down; green, left; blue, up). The image in c was acquired after applying a large (>10,000 Oe) field towards the top left, then the indicated (225 Oe) field towards the bottom right, after which the external magnetic field was removed for hologram acquisition. The image in d was acquired after applying identical fields in the opposite directions. (Adapted from ref. 77.) e, Contours of 0.064-rad spacing formed from the magnetic contribution to the holographic phase shift acquired from a single bacterial cell (inset) of Magnetospirillum magnetotacticum strain MS-1, imaged in magnetic-field-free conditions. The contours, which spread out at the ends of the chain, are overlaid onto the contribution of the mean inner potential to the phase shift, to allow the positions of the crystals to be correlated with the magnetic contours. (Adapted from ref. 81.) nmat_2406_APR09.indd 277 13/3/09 12:08:35 © 2009 Macmillan Publishers Limited. All rights reserved