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Physica B 356 (2005) 269–275

www.elsevier.com/locate/physb

Future science at next generation neutron sources

Sunil K. Sinhaa,b,�

aDeptartment of Physics, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0319, USAbLos Alamos National Laboratory, Los Alamos, NM 87545, USA

Abstract

We review some of the scientific opportunities which will become available with the advent of increased fluxes of

polarized neutrons and corresponding advances in neutron instrumentation. These range from the study of magnetism

in thin films and novel nanostructures with application in magnetic information storage and retrieval, to the study of

spin-dependent inelastic scattering from highly correlated magnetic systems, frustrated magnetic systems, etc.

Extensions of neutron spin echo techniques offer further opportunities for advances in our knowledge of the dynamics

of soft condensed matter systems.

r 2004 Elsevier B.V. All rights reserved.

Keywords: Future science; Synchrotron X-rays; Spin echo; Emergent phenomena

1. Introduction

We have heard at this meeting about a numberof clever ways to use polarized neutrons. Thus, wehave heard about the development of magneticlenses to focus neutron beams [1], the use ofcomplete three-dimensional polarization analysisto study spin structures [2,3], the exploitation ofdynamical polarization of selected nuclei to gaincontrast, e.g. in biological systems [4], Larmortagging techniques for inelastic neutron scattering,

e front matter r 2004 Elsevier B.V. All rights reserve

ysb.2004.10.089

t of Physics, University of California San

ilman Drive, La Jolla, CA 92093-0319, USA.

4 0173.

ss: ssinha@physics.ucsd.edu (S.K. Sinha).

and surface scattering[5–7]. Considering that nextgeneration neutron sources such as the SNS offerthe possibility of increases in usable flux rangingfrom one to two orders of magnitude for certaininstruments, such techniques will be an importantcomponent of the research carried out at thesesources. Here, I will attempt to discuss moregenerally what kinds of science might be ad-dressed, with or without polarized neutrons. Weshould always bear in mind the caveat that oftenthe most important or popular scientific problemsthat are attacked with any facility or techniqueturn out to be those that were not consideredseriously several years ago! Nevertheless, onecan be confident that some of this new sciencewill include the following: (a) Nanoscience,

d.

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S.K. Sinha / Physica B 356 (2005) 269–275270

particularly the investigation of magnetic nanos-tructures, magnetic thin films, spintronics applica-tions, etc. (b) New subtle kinds of order seen viathe dynamics, e.g. topological order, as in spinglasses or glassy materials or frustrated systems.(c) The study of quantum phase transitions andquantum critical points. (These might requirespecial environmental conditions, such as veryhigh magnetic fields, very low temperatures, highpressures, etc.). (d) Time-dependent scatteringstudies of response functions, slow equilibriumfluctuations, etc. An additional trend, which can beanticipated to grow, is the use of neutron scatteringstudies complemented by synchrotron X-ray studies,in particular, and other techniques such as scanningprobe microscopies. Thus, in the areas of magneticthin films and the equilibrium fluctuations of softcondensed matter systems, both neutrons andsynchrotron X-rays have been brought to bear ondifferent aspects of the same problem, as will be seenin the examples given below.

2. Nanomagnetism

Magnetic scattering of neutrons has been one ofthe mainstays of the application of neutronscattering to condensed matter science. Over thelast decade or so, it has been realized that X-rayscan also be used to study magnetism in solids byutilizing the magnetic cross section for X-rays,which is nevertheless small compared to theordinary charge or Thompson scattering crosssection. The discovery of a large resonant en-hancement of the magnetic cross section for X-rays scattering at the L-edges of transition metaland rare-earth atoms and at the M-edges ofactinide atoms has made X-ray studies of magnet-ism in thin films and multilayers readily feasible atsynchrotron sources, where intense X-ray beamswhich can be tuned to the resonant energies areavailable [8–13].

A current example of a magnetic problem that isbeing simultaneously attacked with both neutronand synchrotron X-ray techniques is that ofexchange bias [14,15]. This relates to the behaviorof a ferromagnetic (FM) film deposited on anantiferromagnet (AF), and is of great technologi-

cal interest to the magnetic recording industry.Below the Neel temperature of the AF, there areso-called uncompensated spins in the latter whichare ‘‘frozen’’, in the sense that they do not respondto low applied magnetic fields, being locked by theexchange interaction with the antiferromagneticspins. By virtue of the exchange coupling of theseuncompensated frozen spins across the FM/AFinterface, the hysteresis curve of the FM film getsshifted along the field axis as if there was ananisotropy field acting on the FM spins (referredto as the exchange bias field). The nature of theseuncompensated spins in the AF is still the subjectof active study, and a quantitative calculation ofthe magnitude of the exchange bias is quitedifficult. Thus, a study of the depth and lateraldistribution of the ferromagnetic spins relative tothe interface as a function of applied fields is ofconsiderable interest.

Fitzsimmons [16] has, in these proceedings,discussed recent neutron reflectivity experimentsinvolving polarization analysis on the systemconsisting of a film of FM Co deposited on anAF single crystal of FeF2. We have also carriedout complementary resonant synchrotron X-rayreflectivity and diffuse scattering studies of thesame system using soft X-rays at the AdvancedLight Source at Berkeley [17].

Some of the unique advantages of resonantmagnetic X-ray scattering are that it allows one tolook at ferromagnetic components element-selec-

tively. Further, because of the high brilliance ofsynchrotron X-ray beams, this includes reflectivityexperiments (where one measures the in-plane-averaged magnetization density profile normal tothe film surface) as well as the off-specularscattering experiments to study the lateral dis-tribution and depth distribution of the magneticdomains on either side of the interfaces. A fullDistorted Wave Born Approximation (DWBA)theory now exists for magnetic resonant X-rayscattering from surfaces and thin films, which onemay use for a quantitative analysis of scatteringdata [18,19]. (For neutrons, a correspondingDWBA theory for magnetic scattering has beenintroduced by Toperverg and coworkers [20–22]).

Fig. 1 shows magnetic hysteresis loops taken asa function of applied magnetic field (applied

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-0.25 0.00 0.25 0.50-0.12

-0.06

0.00

0.06

0.12

I + -

I -

Appl. Field (T)

CoFe x 15

(b)

(a)

Fig. 1. Element specific hysteresis loop at the Co (squares) and

Fe (circles) L3 edge, respectively. Inset shows (a) the exchange

bias HE, and, (b) the coercive field HC, measured as a function

of the incidence angle.

-200 0 200 400 600 800-0.0004

0.0000

0.0004

0.0008

0.0012

0.0016

0 200 400 600 800

0.0

0.3

0.6

0.9

1.2

1.5

1.8

Co Fe

δ ch

arg

e M

agn

etiz

atio

n (

µ B/a

tom

)

Depth (Å)

-0.3-200

(a)

(b)

Fig. 2. Charge (a) and magnetization (b) profiles obtained from

Co L3 edge (black) and Fe L3 edge (red) reflectivity fits. The

blue dashed lines mark the boundaries of the Co and the FeF2

layers.

S.K. Sinha / Physica B 356 (2005) 269–275 271

parallel to the plane of the film) at the Co and FeL-edges, respectively, for the sample of AF FeF2

single crystal (36 nm thick) with a 4 nm film of FMpolycrystalline Co deposited on top. These weremeasured at a fixed value of the scattering angle2y ¼ 61 on the specular reflectivity curve. Theloops correspond to the intensity of the specularreflectivity at the respective edges for circularlypolarized X-rays of a fixed sense of polarization.The difference in the normalized intensity for Cobetween large positive and negative appliedmagnetic fields is to a first approximation propor-tional to a particular Fourier component of the in-plane averaged magnetization density profile ofthe Co film which reverses with applied magneticfield, and the same is true for the intensity at theFe edge. These results show that: (a) there is apositive exchange bias field of about 0.195 T actingon the Co spins (b) there is indeed an uncompen-sated ferromagnetic component to the Fe spins inthe AF FeF2 crystal, which responds to the appliedfield by polarizing in the opposite direction to theFM Co spins, and has the same exchange bias fieldas the Co spins. (This actually refers to theuncompensated, but non-frozen component of theFe spins. Fitzsimmons, in his paper, has discussedthe existence and behavior of the frozen compo-nent, i.e. that part of the uncompensated spins in

FeF2 which stays locked by its interaction to thebulk antiferromagnetism and cannot respond tothe external field). This is presumably due to thestrong antiferromagnetic interaction across theinterface between the Co spins and the uncompen-sated Fe spins. Fig. 2 shows the depth profile ofthe maximum reversible components of the Coand Fe magnetizations obtained from a quantita-tive analysis of the reflectivity profiles measuredover the range of 2y for the experiment. It is seenthat there is a layer of uncompensated Fe spinsnear the interface, which orients antiferromagne-tically to the Co spins. Fig. 3 shows the off-specular scattering as a function of in-plane qx forthe Co magnetization and the Fe magnetization inthe ‘‘saturated’’ portion of the hysteresis loop.What is shown is (I+�I�) or the intensity

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-0.004 0.000 0.004-0.2

0.0

0.2

0.4 Co

Fe

(I+

- I-

)

qx (Å-1)

-0.002 0.002

H = 0.4 T

Fig. 3. Off-specular scattering from a FeF2/Co system at the

Co (black) and Fe (red) edges at 20 K at an applied field of

0.4 T.

S.K. Sinha / Physica B 356 (2005) 269–275272

difference for opposite senses of circular polariza-tion of the X-rays. What is represented isproportional [19,23] to the Fourier transform ofthe in-plane variation of the Fe magnetization,which interferes coherently with the lateral fluc-tuations in the charge density. The broad peaks(and weaker second harmonics) indicate astripe-like in-plane magnetic domain structure ofperiod 1000 nm. The domains exist even in thenominally saturated Co film, and are probably atthe interface, and the Fe domains are identical andoppositely aligned magnetically. What feature ofthe charge variation these domains are correlatedwith, is an intriguing question. While theanalysis of these experimental results is still at apreliminary stage, the combination of resonantmagnetic X-ray and polarized neutron reflectivityis clearly a very powerful one, capable of revealingwhat goes on at the interface in unprecedenteddetail.

Arrays of magnetic dots or ‘‘antidots’’ (i.e.arrays of holes in a magnetic film) are other areas,where both neutrons and synchrotron X-rayscattering can make complementary contributionsto a detailed study of the chemical and magneticstructure (it should be noted that at grazingincidence, the projected coherence length on thesurface of either X-ray or neutron beams canapproach several microns—in at least one direc-

tion, thus allowing diffraction from arrays withspacings of this order). Thus grazing incidence X-ray diffraction has been used to characterize thedetailed three-dimensional shapes of the dots [24],while grazing incidence polarized neutron diffrac-tion has been used to characterize the shape of thedomains in periodic hole arrays in a permalloy film[25] in the residual state.

3. Slow fluctuations in soft condensed matter

Inelastic scattering techniques clearly do nothave adequate energy resolution to study equili-brium fluctuations with time-scales of the order ofnanoseconds, or longer. Thus one must turn tomethods that can measure the scattering function(strictly speaking the so-called intermediate scat-tering function S(Q,t), or the frequency transformof S(Q,o)) as a function of time. For timescales�0.1 ns to hundreds of ns, this can be achievedwith neutron spin echo (NSE), while for timescalesof microseconds to seconds, this can be achievedwith X-ray photon correlation spectroscopy(XPCS). The latter is simply the by-now familiartechnique of dynamical light scattering (where onemeasures the autocorrelation function of theintensity variation in time due to speckle spotsfluctuating in and out of the detector), andrequires the use of coherent beams of X-rays. Suchbeams are available with relatively high flux frombright undulator sources by transmitting the beamthrough micron-sized pinholes 25–40 m from thesource. A recently published example of a systemfor which both surface NSE and grazing incidenceXPCS were used to study the complete range oftime scales is that of a smectic membrane [26], asillustrated in Fig. 4. Here the XPCS data wereconfined to membrane fluctuations with small in-plane wavevectors and long relaxation times, whileNSE was used to study larger q fluctuations.Similar XPCS experiments have recently beencarried out for molten polymer films above theglass transition. [27] Again, such measurements arerestricted to relatively small q, and would benefitfrom complementary grazing incidence NSE mea-surements. The above types of experiments areimportant as probes of viscosity and entanglement

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Fig. 4. Experimental relaxation times for various 8CB samples.

Squares, NSE at wavelength of 0.9 nm; triangles, NSE at

1.5 nm; diamonds, XPCS at wavelength of 0.09 nm for 2

membrane thicknesses; solid line, dispersion curves calculated

for three membrane thicknesses using known parameter for the

surface tension, viscosity, density and elastic moduli (from Ref.

[26]).

S.K. Sinha / Physica B 356 (2005) 269–275 273

effects in very thin films, and as tests of theoreticalmodels of the dynamics.

4. Imaging with neutron beams

Devices capable of focusing synchrotron X-raybeams down to dimensions of �100 nm in the caseof hard X-rays and �30 nm in the case of soft X-rays are now routinely employed at most of thethird-generation synchrotron sources in the world.These devices include Kirkpatrick–Baez focusingmirrors, Fresnel zone-plates and Bragg–Fresnellenses, refractive lenses and thin film waveguides.There is no reason why such devices cannot also beused to produce microfocused neutron beams atthe next generation neutron sources. Such neutronbeams could be used for a variety of imaging andmicroanalysis studies with neutrons, e.g. residualstrain analysis with much smaller mesh sizes [28];microdiffraction from inhomogeneous samplesand neutron three-dimensional microtomography.Phase contrast imaging and quantitative phaseradiography are techniques which do not requiremicrofocused neutron beams, but do require highspatial resolution (�1–5 mm) in the area detectors.Here, simply selectively deuterating portions of the

specimen would provide excellent phase contrast.Preliminary results using such techniques forneutrons have recently been reported [29,30]. Anextension of these methods at future neutronsources might involve polarized neutrons andimaging of magnetic nanostructures using therefractive index contrast provided by the spatialvariation of the magnetization.

5. Other possible growth areas

An area likely to receive increasing attention inthe future is the use of pump–probe methods usingpulsed and polarized neutron beams and pulsedmagnetic fields to study the time-dependentmagnetic response of a system to such fields. Withsuch types of experiments, very high magneticfields (�35 T or greater) could be used. Applica-tions could lie in looking at the development ofnew charge or spin-ordering in very high fields, orthe dynamics of switching in magnetic thin films ormagnetic nanostructures. The latter involve time-scales of �ns or less, and such studies are alreadybeing carried out using X-ray magnetic circulardichroism (XMCD) at current synchrotronsources. The exact spatial and temporal details ofhow mesoscopic and nanoscopic objects reversetheir magnetization is of considerable currentscientific and technological interest. Neutronsources from pulsed neutron sources are currently1–10 ms long. In order to achieve ns time resolu-tion, one would have to use special choppers ortime focusing methods.

A variation of this technique involves ‘‘tickling’’the system with a weak magnetic field which isoscillatory in time with frequency o. The fre-quency-dependent magnetic response at wavevec-tor Q is related to the field by linear response

MðQ;oÞ ¼ wðQ; 0;oÞHðoÞ;

where wðQ; 0;oÞ represents an off-diagonal dyna-mical susceptibility in response to a macroscopicfield. Such components exist for inhomogeneoussystems, e.g. ferromagnetic systems with domainsat small Q values and low frequencies, and containinformation about equilibrium fluctuations of thesystem.

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S.K. Sinha / Physica B 356 (2005) 269–275274

Finally, we can list areas such as the study of‘‘emergent phenomena’’ [31–33], or topological orother subtle kinds of order as challenges whichneutron scatterers will find it difficult to resist withpowerful new neutron sources. As an example ofsuch studies, Fig. 5 illustrates a recent example,namely the use of inelastic neutron scatteringform-factors in reciprocal space to identify spinclusters in a frustrated magnetic system, eventhough the ground state possesses no long-rangemagnetic order [32].

In conclusion, it can be stated that we cancontinue to expect many exciting new discoveriesat future and current neutron sources, particularly

Fig. 5. Wavevector dependence of the inelastic neutron cross

section for ZnCr2O4: (a,b) color images of scattered neutron

intensities from a single crystal of in the (h,k,0) and (h,k,k)

planes, respectively, at T ¼ 15 K for ho ¼ 1 meV; (c,d)

calculated images of the form factor squared for antiferromag-

netic spin clusters on the hexagon, averaged over the four

orientations of the hexagons in a spinel structure. The

agreement with the experiment is excellent (from Ref. [32]).

in the areas of nanoscience, magnetism, and softmatter. Some of this science will require state-of-the-art instrumentation to match the sources andthe experimental challenges encountered, e.g. highor pulsed fields, detectors with high spatialresolution and fast time resolution, etc. The onething that is safe to say is that probably the mostimportant science to emerge will be that, which istotally unexpected at the present time!

Acknowledgements

I would like to acknowledge my collaboratorsinvolved in the magnetic exchange bias systemexperiments briefly reported here: Mike Fitzsim-mons, Sujoy Roy, Michelle Dorn, Ivan Schuller,Oleg Petracic, Jeff Kortright and Karrine Chesnel.I would also like to thank Frank Klose for sendingme information on the capabilities of the SNS. Iwould also like to gratefully acknowledge supportfrom the Office of Basic Energy Science, USDepartment of Energy, through Grant DE-FG02-03ER46084.

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