The sample is introduced close to the sensor through a 12 mm ID quartz tube that goes through the length of the apparatus. The distance from the center of the sample tube to the center of the sensor cell is about 1 inch. The sample can be heated by electric heaters mounted on the tube or maintained near room temperature by a gentle flow of air through the tube. We use a single layer of radiation shielding, consisting of a gold coating patterned into 0.3 × 0.3 mm² electrically isolated squares to reduce magnetic noise from Johnson currents¹⁶. The radiation transfer power can be much larger in our system than in a typical cryogenic environment, so we use a single-layer radiation shield instead of multi-layer cryogenic insulation. The sample is typically attached to a long rod and rotated at 5 – 10 Hz, thereby introducing a clear modulation of the signal at a frequency above 1/f noise and allowing simultaneous measurements of the two transverse components of the magnetization. Great care must be taken to minimize magnetic contamination of the sample holder. The best results are obtained using a quartz tube etched in acid with the sample held by suction using a vacuum pump attached to the other end of the tube with a rotary connection. We could not find a glue or cement that can hold the sample without introducing some magnetic contamination.

Two examples of sample measurements are shown in [Fig. 2] and [Fig. 3]. Fig. 2 shows measurements of the remanent magnetization of an ancient dolomite rock as a function of temperature. Such thermal demagnetization measurements are very common in paleomagnetic studies¹⁷, but in the past could only be performed with a high sensitivity by repeatedly transferring the sample between a SQUID magnetometer and a magnetically-shielded furnace. The ability to simultaneously heat the sample to as high as 500 °C and measure its magnetization with a sensitivity below 10⁻⁹ emu/Hz^½ is unique to atomic magnetometers. Atomic magnetometers for the measurement of remanent magnetization in paleomagnetic applications are starting to be commercially developed¹⁸.

Fig. 3 shows the magnetic signals that are generated by surface interactions between aluminum and water. Magnetic signatures of corrosion have previously been studied using SQUID magnetometers¹⁹. We have made a few preliminary measurements of this effect using the atomic magnetometer. For this application the sample was held near room temperature by gently blowing air through the access tube. In addition to magnetic signals from corrosive chemicals, such as HCl and NaOH, we have also observed electrochemical magnetic signals using deionized water that are an order of magnitude smaller. We observe a magnetic field that quickly disappears as the water droplet evaporates. The field can be caused by ionic currents due to chemical reaction or possibly due to thermal currents generated by a temperature difference between the evaporating water and aluminum²⁰. No signal is observed if the aluminum surface is coated with a thin polymer coating before the application of the water droplets. The ability to detect weak magnetic signals from the interaction between water and aluminum would be interesting for many non-destructive testing applications, such as the detection of hidden corrosion²¹.

So far we have focused on atomic magnetic field detectors on the centimeter scale, which is a convenient size for measurements on bulk samples. However, in many materials applications one desires to perform magnetic field scans on a much smaller scale. New atomic magnetometers are currently being developed from the millimeter to nanometer scales and may soon become available for materials characterization applications.

The scaling of hot-vapor alkali-metal magnetometers to smaller dimensions is not particularly favorable. To limit the spin relaxation of atoms on cell walls one has to increase the pressure of the buffer gas to slow down the diffusion of atoms. More frequent collisions of alkali-metal atoms with the buffer gas then result in an additional spin relaxation that broadens the magnetic resonance. One can partly compensate for this loss in sensitivity by increasing the density of the alkali-metal atoms in the cell. However, the spin-relaxation cross-sections quickly increase with temperature²² so the higher temperatures required also lead to increased resonance broadening. Currently the most sensitive millimeter sized sensors are being developed at NIST²³ and a sensitivity of 5 fT/Hz^½ has been achieved. Such sensors have been used, for example, for magnetorelaxometry measurements of magnetic nanoparticles²⁴. Another approach that may be promising is the use of anti-relaxation coatings, typically made from long hydrocarbon chains, which largely prevent relaxation of alkali-metal atoms on cell walls. Recent studies of alkene coatings show that they allow up to 10⁶ collisions of Rb atoms with cell walls before spin relaxation²⁵. Such coatings may help to scale down the dimensions of the sensor, although so far they have been shown to operate only at relatively low temperatures.

To improve atomic magnetometer spatial resolution to the sub-millimeter size likely requires a different approach. Measurement with laser-cooled and trapped atoms is a natural technique for this length scale, since cold atomic clouds trapped at a focus of a laser beam or held in a microchip magnetic trap typically have dimensions of 10 – 100 μm. Long spin coherence times have been achieved in optical²⁶ and magnetic traps²⁷, but the number of trapped atoms is typically much smaller than used in vapor cells. The overall sensitivity of cold atom sensors is already quite competitive with SQUID magnetic microscopes. Two different approaches are currently being explored in this active area of research. In one approach the frequency of Larmor spin precession of atoms is measured with a probe laser²⁸, just as in hot atom magnetometers. In another approach, one simply measures the density of the atomic cloud in a shallow trap. The extra energy from interaction of the atomic magnetic moment with the magnetic field ΔE = −μ · B leads to a perturbation of the atomic density that can be directly imaged with a laser²⁹. Atoms with a large magnetic moment, such as Dy, have a larger magnetic interaction energy than alkali-metal atoms and may further increase the sensitivity of such sensors³⁰. One of the challenges of using cold trapped atoms for sample studies is the requirement that they operate in ultra-high-vacuum (UHV); typically 10⁻¹⁰ – 10⁻¹¹ Torr, since any collision with background gas will eject the atoms from the trap. Therefore all samples would have to be introduced through a load-lock system compatible with a UHV environment or placed behind a very thin vacuum window.

Another technique currently being explored for sensitive magnetometry uses nitrogen-vacancy (NV) color centers in diamond³¹. This crystal defect, consisting of a nitrogen substitution next to a carbon vacancy with overall negative charge, is paramagnetic and has very favorable optical and coherence properties. The level diagram of the color center is shown in Fig. 4. The ground state has electron spin S = 1 with a splitting between m_S = 0 and m_S = ±1 states of 2.8 GHz due to magnetic spin-spin interactions with the applied field. In the presence of a magnetic field the m_S = ±1 states are further split by the electron magnetic moment interaction. The color center can be optically excited using a laser at a convenient green wavelength, such as 532 nm, and decays back by emitting florescence at 637 nm. However, the fluorescence properties of m_S = 0 and m_S = ±1 states are different. Whereas color centers excited from the m_S = 0 state are quickly cycled back to the m_S = 0 state, those excited from the m_S = ±1 states have a significant probability of decaying to a metastable singlet state, where they remain for 200 – 500 ns³² before decaying back to the m_S = 0 state. Thus, the initial rate of 637 nm fluorescence depends on the spin state of the color center, whereas after a sufficiently long time all color centers are optically pumped into the m_S = 0. The spin coherence of the ground state is also surprisingly long for a solid state system, reaching T₂ = 2 ms for specially prepared diamond samples depleted in ¹³C isotope³³. Microwaves are used to measure the frequency of the magnetic field-dependent Δm_S = ±1 transitions to determine the magnetic field. A bias field of several mT is applied parallel to the crystal axis of the NV center to split the Δm_S = ±1 transitions, so that they can be resolved individually with microwave spectroscopy. Furthermore, because of local magnetic field fluctuations usually caused by the natural abundance of ¹³C nuclear spins, the highest sensitivity is realized for AC magnetic fields at a frequency of 1 to 100 kHz which are applied synchronously with a microwave spin-echo sequence³⁴. Since the signal is measured by simply detecting the intensity of the fluorescence, a CCD camera can be used to directly image the distribution of the magnetic fields³⁵. So far single NV color centers have reached magnetic field sensitivity of 4 nT/Hz³³ whereas continuous magnetic field mapping can be realized with 20 nT/Hz^½ sensitivity and 250 nm resolution³⁶, close to the sensitivity of LHe-cooled SQUIDs on this length scale. The main advantage of diamond color centers is their ability to operate under ambient conditions and over a wide range of temperatures, without the need for UHV. One of the first examples of such measurements is the detection of the Meissner effect in a high-T_c superconductor with a diamond magnetometer³⁷.

In conclusion, atomic magnetometers have been undergoing rapid development over the last few years. Some of the first atomic sensors suitable for material characterization will soon start to enter the commercial market. They can achieve higher magnetic field sensitivities than SQUID magnetometers and can easily make measurements on samples heated well above room temperature. However, unlike SQUIDs, atomic magnetometers are absolute field sensors and do not easily allow application of large bias fields, in fact, they achieve the highest sensitivity at zero magnetic field. Presently the most sensitive atomic sensors operate on a centimeter length scale. However, active research is ongoing in laser-cooled atom magnetometers and color centers in diamond that should be able to achieve impressive sensitivities on the micron and nanometer scale. Such magnetometers may eventually replace SQUID, Hall, and magnetic force resonance microscopy (MFRM) probes in a variety of magnetic microscopy applications. At the same time, absence of cryogenics should allow atomic magnetic sensors to gain wider acceptance in industrial and geophysical applications.