Title




On the coupling of magnetic moments to superconducting quantum interference devices
 
Author




 
Abstract




We investigate the coupling factor phi( mu) that quantifies the magnetic flux phi per magnetic moment mu of a pointlike magnetic dipole that couples to a superconducting quantum interference device (SQUID). Representing the dipole by a tiny currentcarrying (Amperian) loop, the reciprocity of mutual inductances of SQUID and Amperian loop provides an elegant way of calculating phi(mu)(r,e(mu)) vs. position r and orientation e(mu) of the dipole anywhere in space from the magnetic field BJ(r) produced by a supercurrent circulating in the SQUID loop. We use numerical simulations based on London and GinzburgLandau theory to calculate phi (mu) from the supercurrent density distributions in various superconducting loop geometries. We treat the farfield regime ( r greater than or similar to a= inner size of the SQUID loop) with the dipole placed on (oriented along) the symmetry axis of circular or square shaped loops. We compare expressions for phi (mu) from simple filamentary loop models with simulation results for loops with finite width w (outer size A > alpha), thickness d and London penetration depth lambda(L )and show that for thin ( d << alpha ) and narrow (w < alpha) loops the introduction of an effective loop size a(eff) in the filamentary loopmodel expressions results in good agreement with simulations. For a dipole placed right in the center of the loop, simulations provide an expression phi(mu)(a,A,d,lambda(L)) that covers a wide parameter range. In the nearfield regime (dipole centered at small distance z above one SQUID arm) only coupling to a single strip representing the SQUID arm has to be considered. For this case, we compare simulations with an analytical expression derived for a homogeneous current density distribution, which yields excellent agreement for lambda(L)>w,d . Moreover, we analyze the improvement of phi(mu) provided by the introduction of a narrow constriction in the SQUID arm below the magnetic dipole. 
 
Language




English
 
Source (journal)




Superconductor science and technology.  Bristol
 
Publication




Bristol
:
2024
 
ISSN




09532048
 
DOI




10.1088/13616668/AD1AE9
 
Volume/pages




37
:2
(2024)
, p. 112
 
Article Reference




025010
 
ISI




001145725500001
 
Full text (Publisher's DOI)




 
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