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Applied Physics Letters

Miniature atomic magnetometer integrated with flux concentrators

W. C. Griffith, Ricardo Jiménez-Martínez, Vishal Shah, Svenja A. Knappe, John E. Kitching

Published Jan 12, 2009• 75 citations
Publisher

Abstract

High permeability magnetic flux concentrators are used to enhance the sensitivity of an atomic magnetometer operating in the spin-exchange relaxation-free regime. The magnetometer uses a millimeter scale Rb87 vapor cell and either mu-metal or Mn–Zn ferrite flux concentrators. The measured sensitivity gives excellent agreement with calculations of thermal noise from the concentrator material. The mu-metal concentrators allow a sensitivity of 50fTHz−1∕2, limited by thermal current magnetic noise. The ferrite concentrators are limited by thermal magnetization noise at low frequencies, and reach a sensitivity of 10fTHz−1∕2 for frequencies above 125Hz.

DOI
10.1063/1.3056152

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High permeability magnetic flux concentrators are used to enhance the sensitivity of an atomic magnetometer operating in the spin-exchange relaxation-free regime. The magnetometer uses a millimeter scale Rb87 vapor cell and either mu-metal or Mn–Zn ferrite flux concentrators. The measured sensitivity gives excellent agreement with calculations of thermal noise from the concentrator material. The mu-metal concentrators allow a sensitivity of 50fTHz−1∕2, limited by thermal current magnetic noise. The ferrite concentrators are limited by thermal magnetization noise at low frequencies, and reach a sensitivity of 10fTHz−1∕2 for frequencies above 125Hz.
Miniaturized high-sensitivity atomic magnetometers play a crucial role in biomagnetic measurements. The utilization of multipass cells enhances the sensitivity of atomic magnetometers. This study explores the benefits of employing a multipass cell in spin-exchange relaxation-free (SERF) atomic magnetometers through theoretical and experimental analysis. A signal-to-noise ratio (SNR) model was established by accounting for the optimal detuning frequency of the probe light and low-frequency 1/f noise. This enabled the optimal number of probe light passes through the vapor cell to be determined. Additionally, compact and symmetric reflective optical paths were designed to mitigate polarization gradients in atomic magnetometers with high atomic number density. Experimental results confirmed that the magnetic field sensitivity of the triple-pass configuration (1.8 fT/Hz1/2^{1/2}1/2) was 1.89 times that of the single-pass configuration (3.4 fT/Hz1/2^{1/2}1/2), consistent with the theoretical analysis. This study provides valuable theoretical guidance for analyzing the SNR performance of SERF atomic magnetometers and demonstrates the potential of multipass cells in enhancing their magnetic field sensitivity. The utilization of multipass cells presents a viable approach toward the realization of highly sensitive and portable miniaturized SERF atomic magnetometers.
Alkali-metal magnetometers compete with SQUID detectors as the most sensitive magnetic field sensors. Their sensitivity is limited by relaxation due to spin-exchange collisions. We demonstrate a K magnetometer in which spin-exchange relaxation is completely eliminated by operating at high K density and low magnetic field. Direct measurements of the signal-to-noise ratio give a magnetometer sensitivity of $10\text{ }\mathrm{f}\mathrm{T}\text{ }\mathrm{H}{\mathrm{z}}^{\mathrm{\ensuremath{-}}\mathrm{1}\mathrm{/}\mathrm{2}}$, limited by magnetic noise produced by Johnson currents in the magnetic shields. We extend a previous theoretical analysis of spin exchange in low magnetic fields to arbitrary spin polarizations and estimate the shot-noise limit of the magnetometer to be $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}\text{ }\mathrm{T}\text{ }\mathrm{H}{\mathrm{z}}^{\mathrm{\ensuremath{-}}\mathrm{1}\mathrm{/}\mathrm{2}}$.
We describe a Cs atomic magnetometer operating in the spin-exchange-relaxation-free (SERF) regime. With a vapor cell temperature of $103\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ we achieve intrinsic magnetic resonance widths $\ensuremath{\Delta}B=17\text{ }\ensuremath{\mu}\text{G}$ corresponding to an electron spin-relaxation rate of $300\text{ }{\text{s}}^{\ensuremath{-}1}$ when the spin-exchange rate is ${\ensuremath{\Gamma}}_{\text{SE}}=14\text{ }000\text{ }{\text{s}}^{\ensuremath{-}1}$. We also observe an interesting narrowing effect due to diffusion. Signal-to-noise measurements yield a sensitivity of about 400 pG/Hz400\text{ }\text{pG}/\sqrt{\text{Hz}}400 pG/Hz​. Based on photon shot noise, we project a sensitivity of 40 pG/Hz40\text{ }\text{pG}/\sqrt{\text{Hz}}40 pG/Hz​. A theoretical optimization of the magnetometer indicates sensitivities on the order of 2 pG/Hz2\text{ }\text{pG}/\sqrt{\text{Hz}}2 pG/Hz​ should be achievable in a 1 cm31\text{ }{\text{cm}}^{3}1 cm3 volume. Because Cs has a higher saturated vapor pressure than other alkali metals, SERF magnetometers using Cs atoms are particularly attractive in applications requiring lower temperatures.
The spin-exchange relaxation-free (SERF) atomic magnetometer is one of the ultra-sensitive magnetometers, which must be operated in a low magnetic field and heated more than 100°C. One of the main engineering difficulties in fabricating a SERF atomic magnetometer is to develop a heater that introduces no additional magnetic noise into the measurement system. In this paper, a novel compact current-heating scheme and apparatus for the SERF atomic magnetometer are proposed. The thermal and magnetic simulations of this apparatus are carried out and the results of the simulations demonstrate the feasibility of this heating apparatus: the vapor cell can be heated to a temperature of 103°C and magnetic field introduced in the magnetometer can be limited in 10nT.
The fabrication and performance of a miniature optically pumped atomic magnetometer constructed with microfabricated components are discussed. This device measures the spin precession frequency of Rb87 atoms to determine the magnetic field by use of the Mx technique. It has a demonstrated sensitivity to magnetic fields of 5pT∕Hz1∕2 for a bandwidth from 1to100Hz, nearly an order of magnitude improvement over our previous chip-scale magnetometer. The 3dB bandwidth has also been increased to 1kHz by reconfiguring the miniature vapor cell heater.