We have created a $^{87}$Rb Bose-Einstein condensate in a magnetic trapping
potential produced by a hard disk platter written with a periodic pattern. Cold
atoms were loaded from an optical dipole trap and then cooled to BEC on the
surface with radiofrequency evaporation. Fragmentation of the atomic cloud due
to imperfections in the magnetic structure was observed at distances closer
than 40 $\mu$m from the surface. Attempts to use the disk as an atom mirror
showed dispersive effects after reflection.Comment: 4 pages, 5 figure

A. I. Sidorov

David E. Pritchard

Erik W. Streed

Gretchen K. Campbell

Jongchul Mun

Micah Boyd

Patrick Medley

Wolfgang Ketterle

English

arXiv

We have created a Rb-87 Bose-Einstein condensate in a magnetic trapping potential produced by a hard disk platter written with a periodic pattern. Cold atoms were loaded from an optical dipole trap and then cooled to Bose-Einstein condensation on the surface with radio-frequency evaporation. Fragmentation of the atomic cloud due to imperfections in the magnetic structure was observed at distances closer than 40  孠from the surface. Attempts to use the disk as an atom mirror showed dispersive effects after reflection.Full Tex

Boyd, Micah

W. Streed, Erik

Medley, Patrick

K. Campbell, Gretchen

Mun, Jongchul

Ketterle, Wolfgang

Pritchard, David E.

Griffith Research Online

Atom trapping with a thin magnetic filmMicah Boyd, Erik W. Streed, Patrick Medley, Gretchen K. Campbell, Jongchul Mun, Wolfgang Ketterle, David E. Pritchard∗MIT-Harvard Center for Ultracold Atoms, Research Laboratory of Electronics and Department of Physics,Massachusetts Institute of Technology, Cambridge, MA 02139, USA(Dated: February 6, 2008)We have created a 87Rb Bose-Einstein condensate in a magnetic trapping potential produced by a hard diskplatter written with a periodic pattern. Cold atoms were loaded from an optical dipole trap and then cooled toBEC on the surface with radiofrequency evaporation. Fragmentation of the atomic cloud due to imperfectionsin the magnetic structure was observed at distances closer than 40 µm from the surface. Attempts to use thedisk as an atom mirror showed dispersive effects after ref ection.PACS numbers: 03.75.Be, 03.75.Lm, 75.50.Ss, 75.70.iMicrometer scale magnetic traps for Bose-Einstein conden-sates have been the focus of much experimental work sincetheir f rst demonstration [1, 2]. Their potential uses in atom in-terferometry, precision measurements, and experiment minia-turization have motivated many groups to develop sophisti-cated techniques for manufacturing and controlling these atomchips [3, 4, 5]. However, since the earliest inceptions of atomchips, physical imperfections in the chip surface have lead toperturbations in the trapping potential that prohibit the coher-ent manipulation of atoms close to the surface[6, 7]. Perma-nent magnets offer a possible solution to several problems in-herent to current carrying wire traps. First, the magnets arealmost completely decoupled from the rest of the laboratory,minimizing the effects of environmental electrical noise. Sec-ond, they do not require current to be sourced or sinked, en-abling designs that would be too complicated or impossibleto create using electromagnets. Last, extremely high f eldgradients are possible close to magnetic domain boundaries,whereas traditional atom chip operation is limited by heat dis-sipation from small wires. Although a substrate of magneti-zable material may be made extremely smooth and uniform,imperfections in the process of etching [8] or writing mag-netic structures could lead to some of the same problems aswith wire-based designs. It is not yet known which techniquewill provide the best performance.In this paper we investigate atom trapping with a thin mag-netic f lm; specif cally that of a hard disk platter written witha periodic pattern. This approach offers some advantages overprevious work on neutral atom trapping and BEC creation us-ing permanent magnets [4, 5, 9]. Here we use a thin metal-lic f lm with a large remnant magnetization from a commer-cial product which has already been ref ned to a high degreeand a writing technique more accurate than anything previ-ously demonstrated. Cold 87Rb atoms were f rst loaded intothe magnetic potential formed by the disk and used RF evap-oration to produce BEC. By changing the trapping potential,the atoms could be pushed closer to the surface of the disk toprobe for imperfections in the potential. Finally, the BEC wasdropped onto the disk from a height of 2.7 mm in an attemptto produce a specular ref ection of the atomic cloud.∗The form of the written magnetization on the disk is of crit-ical importance. A surface magnetization of the formM(x) = M0 cos (kx)yˆ, (1)where M0 is the magnetization of the material and k is thewavevector of the sinusoidal pattern produces a magnetic f eldabove the surface(Bx, By, Bz) = B0e−ky (− cos (kx), sin(kx), 0) , (2)which is of uniform coplanar magnitude and decays exponen-tially away from the surface [10]. This is an ideal potential forref ecting weak f eld seeking atoms.The addition of a bias f eld Bx along xˆ (or By along yˆ)produces a series of quadrupole shaped f eld minima abovealternating tracks, depicted in Fig. 1. A second bias f eldBz along zˆ removes the magnetic f eld zero and weak f eldseeking atomsmay be trapped near the surface with radial trapfrequency2ωr = kBx√µBgFmFmBz(3)where m is the atomic mass, and µBgFmF is the Zeemanenergy [5, 10].The magnetic media used in this work was a prototype harddisk with a radius of 65 mm and a thickness of 0.635 mm.The magnetizable material is a dual layer system with a 20nm thick magnetic Co-Cr-Pt based oxide layer and a 200 nmthick “magnetically soft underlayer”, both of which are alsogood conductors. The substrate is glass, the coercivity is 6.8kOe and the magnetic remnant is 0.64 memu/cm2. The easyaxis of magnetization of this prototype “out-of-plane” disk isaligned normal to the surface of the disk, as opposed to mostmodern commercial hard disks which are magnetized in theplane of the surface. This alignment of the magnetic domainsproduces about 6000 G at the surface.The disk was provided to us pre-written using a Guzik spinstand, which is essentially a hard disk read/write head withabsolute positioning capability. This method of writing al-lows for the creation of truly arbitrary pattern impossible tocreate with larger scale magnetic writing devices [5] or physi-cal structures [4], and produces smaller and cleaner structuresthan optical writing techniques [11]. Two patterns were used2FIG. 1: Geometry of the magnetic trap formed by a magnetic f lm.Magnetic f eld lines resulting from the addition of a radial (xˆ) biasf eld to the magnetized surface. The tubes represent the locations ofthe f eld minima where atoms are trapped. The addition of an axialbias f eld of about 1.0 G along zˆprevents atoms loss from Majoranaspin f ips.in this experiment, written on different radial regions of thesame disk. The f rst region had alternating stripes of up anddown magnetization with a period of 2pi/k = λ = 100.0 µmand the second had λ = 1.0 µm. The media has a very squarehysteresis loop which precludes a purely sinusoidal magne-tization. The pattern is instead written as a square wave,and the higher Fourier harmonics should be negligible at theheight where we trap atoms [10]. Figure 2 shows a magneticforce microscopy image of the disk after writing which showsa signif cantly less noisy pattern than magneto-optical thinf lms written with laser beams [12]. Atomic force microscopyshowed a physical roughness of ∼3 nm over a region of 50µm.In our experiment, cold atoms were delivered to the sur-face in a two step process. First, cold 87Rb atoms in theF = 1,mf = −1 state were created in a Ioffe-Pritchardmagnetic trap with T >∼Tc where Tc is the critical temper-ature for Bose-Einstein condensation. The atoms were thentransferred to an optical dipole trap and transported 36 cm toa separate auxiliary chamber (see [13] for more details). Theoptical dipole trap was formed by a λ = 1064 nm laser fo-cused to a 30 µm 1/e2 radius spot, and the focus (with theatoms) was translated into the auxiliary chamber as describedin [14]. By transporting atoms just above Tc, the cloud wasless sensitive to vibrations, and higher laser powers could beused without causing rapid three-body losses.The atoms were loaded onto the surface from the opticaltrap by translating the focus of the optical trap to a positionparallel to and 50 µm above the region with λ = 100 µm. AZ-shaped wire below the disk provided axial conf nement, anda small Bx created radial trapping on the surface. The opti-cal trap was ramped off over 2 seconds, transferring the atomswith almost unity eff ciency. The optical trap enabled load-ing of >90% of the atoms into a single surface trap site. RFEvaporation over 20 seconds from 1.200 MHz to 0.890 MHzproduced a BEC with approximately 50,000 atoms in a trapwith (ωx, ωy, ωz) = (390, 390, 9)× 2pi Hz. The spatial dis-0.0 2.5 5.0 µm0.02.55.0 µmFIG. 2: Magnetic force microscopy image of the disk used in thisexperiment. This scan is over the region with λ = 1.0 µm. The trackedges are of the same uniformity in the λ = 100.0 µm region. Thecharacteristic size of the roughness of the domain boundaries is about30 nm.a) b)FIG. 3: Absorption image of the BEC in trap near the surface. a) Topimaging. The f eld of view is 0.58 mm × 0.58 mm. b) Side imag-ing. The f eld of view is 0.56 mm × 0.56 mm. The double imagecomes from the ref ection of the imaging beam in the platter surface.The trap frequencies were 2pi×(400, 400, 10) Hz. The conf gura-tion of the experiment prevented ballistic expansion, but the bimodaldistribution in trap is still clear.tribution was clearly bimbodal, and condensate fractions of>80% were observed.The atomic cloud was detected with on-resonance absorp-tion imaging (Fig. 3). The platter is a good ref ector (>95%)for 780 nm light, so grazing incidence imaging was used tomeasure the distance from the atoms to the surface. Normalincidence imaging was also used, in which case the imaginglight passed through the atoms twice, bouncing off the disk.The lack of physical structures on the surface resulted in goodimage quality.The axial trap frequency in the surface trap was measuredby imaging oscillations of the atomic cloud, and radial trapfrequencies were measured by parametric heating [Fig. 4].Attempts to release the atoms from the trap and observe bal-listic expansion in time of f ight were hindered by the geome-try of the system and the static nature of the magnetic surface,3543210Trap Frequency (kHz)3.02.01.00.0Applied Bias Field (Gauss)FIG. 4: Radial trap frequency vs applied radial magnetic f eld.Atoms were loaded into a single trap site on the surface and evap-orated to BEC. The trap frequencies were measured with parametricheating, but above 5 kHz other heating effects made it diff cult to re-solve reliably. The deviation from linearity at low applied f elds ismost likely due to a small, off-axis, residual bias f eld.so all of the imaging was done in-trap. The Z-wire trap wasleft on at all times to provide axial trapping, but its effect onthe radial trapping was negligible. Ramping up the currentin external electromagnets increased Bx, and ωr as high as2pi×5 kHz was measured. Using a disk with λ = 10.0 µm, wehave measured ωr as high as 2pi × 16 kHz. High transversetrap frequencies are desired for studies of 1D systems [15],but atom heating and loss at higher trap frequencies preventedsuch studies here.While trapped 40 µm above the surface, the BEC had alifetime of ∼30 s. Previous experiments with atomic cloudsmagnetically trapped near conducting surfaces showed atomloss resulting from spin f ips driven by Johnson noise [7, 16,17]. For this experiment however, the conducting layer wasextremely thin (200 nm), and the atoms were relatively farfrom the surface. These two parameters reduced any spin f iploss below the level of our 1-body losses at a vacuum pressureof ∼ 5× 10−11 Torr.The BEC was also used to probe imperfections in the trap-ping potential. After evaporation, the axial trapping frequencywas reduced from 10 Hz to ∼1 Hz to allow the cloud to ex-pand slightly in one dimension to increase the sensitivity andthe measurement area, and the radial bias f eld (Br) was in-creased to push the atoms closer to the surface. At distancescloser than 40 µm, breakup of the atomic cloud was observed[Fig. 5], and the strength and spatial frequency of the pertur-bations increased as the atoms neared the surface, similar to[6, 18]. The magnitude and size scale of these imperfectionscan be attributed to the sputtering process used to create thef lm. Magnetic grain nucleation occurs randomly, resulting insmall angular misalignments of the axes of anisotropy of indi-vidual magneto-crystalline domains (typically a few degrees),the individual magnetic grains can vary in size by about 25%of their ∼7 nm diameter, and the magnetic moments have adistribution of magnitudes. All of these phenomena createimperfections in atomic trapping potentials.a)b)c)d)e)FIG. 5: Breakup of the atomic cloud as the atoms approached thesurface. The thermal cloud was evaporated to T ≈ Tc, then the axialtrapping frequency was reduced to from 10 Hz to 1 Hz to allow theatoms to expand in one dimension. The axial bias f eld was then in-creased to push the atoms closer to the surface, and the atoms wereimaged in trap. The grazing incidence imaging used here producesa double image of the atoms from the primary and secondary ref ec-tions of the imaging beam off the disk. The f eld of view is 0.29 mm× 2.3 mm, and each frame is an average of f ve distinct images toreduce noise for clarity. At distances smaller than 40 µm, signif cantbreakup was observed. Height for the images shown: a) 35 µm b) 29µm c) 25 µm d) 22 µm e) 19 µm.While the presence of these perturbations does not neces-sarily preclude the creation of neutral atom waveguides usingthis type of substrate, it does limit its usefulness. High trap-ping frequencies and single transverse mode conf nement re-quire close proximity to the surface and will suffer from theobserved imperfections. However, for use as a high reliabil-ity, low noise, and low cost waveguide at distances >50 µm,commercial metallic, magnetic thin f lms are ideal.The magnetized surface was also examined for its useful-ness as an atom mirror, similarly to [8, 19, 20]. In order tominimize residual and time dependent magnetic f elds (frommagnetic trap turnoff), all optical trapping was f rst used tocreate a BEC. The optical dipole trap provided only weak ax-ial conf nement, so a second, 200 mW laser beam with λ =1064 nm and 200 µm waist was added in a crossed conf gura-tion, and allowed more eff cient evaporation. To evaporate toBEC the power was reduced over 2.0 s. An axial bias f eld Bz= 1.2 G was maintained throughout the experiment, and Bxand By were minimized to <20 mG.The region of the disk used as a magnetic mirror was writ-ten with λ = 1.0 µm. This value was chosen to be large com-pared to the magnetic writing precision, but small comparedto the extent of the atomic cloud. The BEC was released fromthe crossed ODT 2.7 mm above the disk. The atoms fell un-der gravity for 23.5 ms and then ref ected off the magneticpotential. The atomic cloud was imaged (in separate runs)from both the top and side after various times of f ight. Sideimaging showed that the ref ection from the disk did not sig-nif cantly effect the axial or vertical velocity distributions ofthe atomic cloud. Top imaging, however, showed signif cantspreading along the vector of the magnetic pattern after re-f ection, analogous to bouncing off a rough mirror [21]. Themagnitude of this dispersion was minimized by f ne tuningBx4andBy , but was impossible to eliminate in our apparatus. Be-fore bouncing off the disk, the xˆ width of the cloud expandedby 1 mm/s, and after bouncing the xˆ width increased by 34mm/s. The ref ection was not performed on the region with λ= 100.0 µm, but experiments using a different disk with λ =10.0 µm showed similar effects.The magnetic properties of the hard disk surface are onepossible source of the observed expansion after ref ection.While the theory presented earlier (Eq. 1) applies to a sinu-soidally magnetized surface, the square hysteresis loop and di-rectional anisotropy of digital recordingmedia prevent record-ing of a pure sine wave, instead forming a square wave ap-proximation of that sine, as discussed in [10]. While thehigher harmonics resulting from this approximation do notnecessarily adversely effect BEC trapping, they create a corru-gation of the planar equipotential that prevented the specularref ection of a macroscopic atomic cloud. Another contribu-tion to the roughness of the ref ection potential is the existenceof small, stray magnetic f elds. Any nonzero component of Bin the xˆyˆ plane creates a regular corrugation in the plane, fur-ther inhibiting specular ref ection.In conclusion, we have demonstrated BEC production onthe surface of a hard disk platter. Small scale imperfections inthe magnetization caused condensate fragmentation close tothe surface, prohibiting its use as a neutral atom waveguide.The disk was also used as an atom mirror, and specular ref ec-tion was observed on two axes. These results are a substantialimprovement over that for early wire based experiments, andthe possibility for more complex structures enables trap ge-ometries impossible for electromagnets. 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Atom trapping with a thin magnetic film

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Atom trapping with a thin magnetic film

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