Preventing Spin Relaxation of Optically Pumped Alkali Metal Atoms

in Magnetometer by Atomically Thin Film Coating

H. Kumagai

, R. Yoshimitsu

, S. Takeda

, E. Ogawa

, T. Kosuge

, H. Ishikawa

, T. Sato

and M. Suzuki

School of Allied Health Sciences, Kitasato University, 1-15-1 Kitasato, Minami, Sagamihara, 252-0373, Japan

School of Science, Kitasato University, 1-15-1 Kitasato, Minami, Sagamihara, 252-0373, Japan

J. A. Woollam Japan Corp, Fuji 2F 5-22-9 Ogikubo, Suginami-ku Tokyo, 167-0051, Japan

Keywords: Atomic Layer Deposition, Optically Pumped Atomic Magnetometer, Spin Polarization.

Abstract: We developed molecular layer deposition method of atomically thin hybrid polymer film for the first time by

developing atomic layer deposition method with sequential surface chemical reactions in order to minimize

the effect of the dipole-dipole interaction between the electron spin of alkali metal atoms and the nuclear spin

of the atoms in the glass of the cell. We controlled film thickness of polymer thin film precisely and finally

aimed at improving the sensitivity of the optically pumped atomic magnetometer. In the presentation, we

report on the relaxation time of spin polarization by atomically thin hybrid polymer film with laser pump-

probe method.

1 INTRODUCTION

The magneto-cardiogram test, which measures a very

small magnetic field generated from the human heart

with a highly sensitive magnetic sensor and performs

two-dimensional mapping analysis, is expected to be

able to evaluate cardiac electrical activity with higher

spatial resolution and higher sensitivity than the

electrocardiogram method in principle. On the other

hand, extremely weak magnetic field measurement of

fT order generated from neurons of the cerebral

cortex is expected to play a very important role in

noninvasively investigating human cranial nerve

activity. At present, a superconducting quantum

interference device (SQUID) having a sensitivity of 1

fT / Hz

1/2

order is used as a magnetic sensor for

magneto-cardiography measurement and magneto-

encephalography measurement. Using SQUID makes

it possible to acquire knowledge about magnetic field

mapping from the heart and brain and basic electrical

activity in vivo, but since SQUID uses

superconducting quantum interference effect, liquid

helium It is necessary to operate it in an extremely

low temperature state, and there is a problem that a

large-sized apparatus becomes expensive

maintenance cost. In recent years, attention has been

paid to an optical pumping atomic magnetometer

which measures a magnetic field by using spin

polarization of alkali-metal atoms through the optical

pumping method. The optical pumping method is a

method in which light is used to create a large

difference in the occupation number of atoms at two

close energy levels and the optically pumped alkali

metal atoms are spin polarized and the magnetic field

applied there is linear. In order to rotate the plane of

polarization of polarized light, we can estimate the

magnetic field at room temperature from this angle of

rotation. It has been energetically studied in the

United States, Europe and the like, and at the National

Institute of Standards and Technology (NIST), it has

become compact to the size of a chip scale small

watch (Schwindt et al., 2004).

In the optically pumped atomic magnetometer, the

rotation angle of the polarization plane of the probe

light becomes more sensitive to the change of the

magnetic field as the relaxation rate of the spin

becomes smaller. Recent reports from Princeton

University reported that sensor sensitivity can reach

sub fT / Hz

1/2

order by using SERF (Spin - Exchange

- Relaxation - Free) state where the relaxation rate of

spin polarization decreases (Kominis et al., 2003).

Optical pumping atomic magnet sensors operating in

the SERF state are expected. Therefore, it is

extremely important to suppress the relaxation rate of

spin polarization to a small value when spin-polarized

atoms are used in an alkali vapor cell in order to

measure with high precision. Although polarized spin

is relaxed by collision of atoms against the inner wall

250

Kumagai, H., Yoshimitsu, R., Takeda, S., Ogawa, E., Kosuge, T., Ishikawa, H., Sato, T. and Suzuki, M.

Preventing Spin Relaxation of Optically Pumped Alkali Metal Atoms in Magnetometer by Atomically Thin Film Coating.

DOI: 10.5220/0009161302500253

In Proceedings of the 13th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2020) - Volume 1: BIODEVICES, pages 250-253

ISBN: 978-989-758-398-8; ISSN: 2184-4305

of the cell, relaxation of spin polarization due to

collision of this atom with the inner wall of the cell

can be suppressed by coating the inner wall of the

cell. This coating is called a spin relaxation

preventing coating, and paraffin (CH

) CH

n> 20) has been widely used so far. The effect of

prevention of spin relaxation by paraffin coating was

first demonstrated by Robinson et al. in 1958

(Robinson et al., 1958). With no relaxation of spin

polarization maximum of about 10,000, it is possible

to collide with the inner wall, and it is expected to be

applied to the field of ultrahigh sensitivity magnetic

sensors and quantum communication. The well-

known paraffin coating on the inner wall surface of

the cell can be said to be an extremely useful means

for obtaining a long spin relaxation time, but the

physical action of its spin relaxation prevention effect

is not well understood at present. According to

Bouchiat et al. (Bouchiat et al., 1966), dipole-dipole

interaction between electron spin of spin-polarized

atom and nuclear spin of coated surface hydrogen

atom, or orbital angular momentum and electron spin

of relative motion between coating surface atoms and

spin-polarized atoms Interaction is said to be the

cause of spin relaxation. However, the result of this

research alone is insufficient to understand the

workings of the coating. For example, an effect of

preventing spin relaxation at a high temperature is

dulled, annealing at 80 °C for several hours in the

presence of alkaline vapor called "aging process"

after coating of paraffin increases spin relaxation

prevention effect (Seltzer et al., 2010), but neither has

been fully understood. Spin relaxation prevention

effect obtained by the same coating material and the

same manufacturing method is greatly different, and

it falls within the skill of coating applicants and

researchers.

One of authors has independently developed an

atomic layer deposition method and an atomic layer

deposition method of oxide by sequential surface

chemical reaction using organometallic gas and water

vapor as a starting material, so that an alkyl group (n

= 1, 2, 3). We found that precise film thickness

control can be realized using "self-limiting

mechanism" appearing in the adsorption process, and

in particular, we have found that it is possible to

realize precise film thickness control using "self-

limiting mechanism", to overcome the extremely

difficult task of making a multilayer film structure

(Kumagai et al., 1997).

2 EXPERIMENTAL SETUP

The experimental setup was comprised of a stainless

steel vacuum chamber with two computer-controlled

leak valves, a capacitor manometer, turbo-molecular

pump (TMP) and quartz crystal unit to allow in situ

measurements during growth of metal oxides. As a

substrate, (100)-oriented Si wafers were used together

with quartz glass cells. The substrate was first

ultrasonically cleaned in conventional organic

solvents, then dipped in 4.7% HF to remove the native

oxide. After rinsing it in overflowing deionized water,

it was loaded into a vacuum chamber. As vapor

sources for the aluminum oxide film, two precursors

were used in Fig.1. high-purity trimethyl-aluminum

(TMA) and ethanol (EtOH) were used as precursor A

and B, respectively.

Figure 1: Atomic layer deposition utilizing two distinct

precursors (A, B) sequentially dosed to the substrate

producing a chemical reaction.

EtOH was prepared by Ethanol JIS special grade,

≥99.5%.These vapors were introduced alternately by

two computer-controlled leak valves into the chamber

which was evacuated by a TMP to a pressure below

-7

Torr. Figure 2 shows input signals applied to the

computer-controlled leak valves for generating each

vapor pulse whose peak vapor pressure reached 1x10

Torr. The duration of supplying each vapor pulse

was 30 s, while the chamber was continuously

exhausted during the growth. The time point exactly

20 s before the first dosing of TMA vapor was defined

as t= 0 s. Therefore, TMA vapor was first introduced

at t = 20 s during the supplying time, and then at t=

110 s, EtOH vapor was first introduced, and then at

t= 200 s, TMA vapor was introduced again. From t =

290 s onwards, these binary vapors were supplied

alternately according to the sequence in Fig. 2.

Atomic layer deposition at room temperature was

carried out by changing the combination of

trimethylaluminum (TMA) and water vapor (H

which was often used in the atomic layer deposition

method, in addition to H

O as ethanol as an oxidizing

Preventing Spin Relaxation of Optically Pumped Alkali Metal Atoms in Magnetometer by Atomically Thin Film Coating

251

agent. Assuming that the introduction cycle period is

6 s, TMA was introduced to the first 0 - 1 s and an

oxidizing agent was introduced to 3 - 4 s. The peak

pressure of the material gas to be introduced was kept

constant at 10

-4

Torr, the total time of introduction of

the source gas was kept constant, the introduction

pulse time and the number of cycles were changed to

seven types, and the deposition characteristics were

investigated. All atomic layer deposition processes

were done at room temperature and a thin film was

deposited on the substrate. The film thickness and the

refractive index were measured with a spectroscopic

ellipsometer.

Figure 2: Input signals applied on the computer-controlled

leak valves for generating each vapor pulse.

3 MOLECULAR LAYER

DEPOSITION OF

ORGANIC-INORGANIC

HYBRID POLYMER THIN

FILM REALIZED

Figure 3 shows variations of vapor pressure in the

vacuum chamber and film thickness with growth time

at room temperature. The vapor pressures in the

vacuum chamber evacuated by a TMP to a pressure

below 10

-7

Torr follows the input signals in Fig. 2.

Duration of each pulse of vapor pressure was 30 s.

The film thickness also shown in Fig. 3 indicates the

thickness from the initial surface (t = 0 s). The

duration of supply of TMA and EtOH vapors which

were introduced in the sequence shown in Fig. 3, was

30 s. Figure 3 shows the case when binary vapors of

TMA and EtOH were supplied by taking into account

the sequence shown in Fig. 2, an increase of around 1

nm is found to occur upon the introduction of TMA.

This is because dosing of TMA to surface -O-H

groups causes chemical reactions by which OH

groups change into -O-Al-CH

groups, whereas

dosing of EtOH to surface Al- CH

, groups causes

chemical reactions in which Al- CH

groups change

into Al-O-H groups. Although actual surface

reactions must be more complicated than the

simpliﬁed picture mentioned here, the picture can

also be supported by infrared spectroscopic studies.

Modiﬁcation of O-H-terminated surfaces to -O-

Al-CH

makes the increase of thickness 0.1 nm

greater than modiﬁcation of Al-CH

-terminated

surfaces to Al-O-H. It was found in Fig. 3 that the ﬁlm

thickness slightly decreased just after the increase,

because high vacuum caused desorption of molecules

which had adsorbed at room temperature. Growth

rates were 0.887 nm/cycle, the same as obtained by

dividing the total thickness of the ﬁlm from an ex situ

variable-angle spectroscopic ellipsometer (LA.

Woollam Co., Inc.) by the number of growth cycles.

This exhibits characteristics of self-limiting nature of

adsorption which are characteristic of the growth

technology in this study.

Figure 3: Variations of vapor pressure in the vacuum

chamber and film thickness with growth time at room

temperature.

4 MEASURE SPIN

POLARIZATION

RELAXATION TIME

To characterize the quality of the film fabricated on

inner-wall of the glass cell, we performed

measurements of the relaxation time of the optical

pumped rubidium atoms with the pump probe method

in Fig.4. The laser frequency of the pump and probe

light beams was same and then tuned to be resonant

to 5S

1/2

(F = 3) -> 5P

3/2

(F' = 2, F' = 3, F' = 4) of

optical transition, until a maximum of fluorescent

intensity in the separate, uncoated cell was obtained.

Then, the pump beam in the cell was supplied by

abrupt opening of a shutter. The atoms on the Rb atom

ground state 5S

1/2

(F = 3) were excited by the

BIODEVICES 2020 - 13th International Conference on Biomedical Electronics and Devices

252

radiation; and the radiation populated 5S

1/2

(F = 2)

ground state through the intermediate atomic upper

states 5P

3/2

(F' = 1, F' = 2, F' = 3, F' = 4). As a result

of the optical pumping process, the amplitude of 5S

1/2

(F = 3) line decreased while the amplitude of 5S

1/2

= 2) line increased.

Figure 4: Schematic of the setup for spin relaxation

measurement.

Figure 5: Temperature dependencies of spin relaxation time

and polarization signal in a Rb cell without coating.

Figure 6: Temperature dependencies of spin relaxation time

and polarization signal in a Rb cell with thin film coated by

sequentially dosing TMA and ethanol vapor.

Figure 5 shows temperature dependencies of spin

relaxation time and polarization signal in a Rb glass

cell without coating. At 35°C, the relaxation time was

estimated as 2.7 ms. In a Rb cell with thin film coated

by sequentially dosing TMA and ethanol vapor,

temperature dependencies of spin relaxation time and

polarization signal are shown in Fig. 6.

The relaxation time in the glass cell with coating

was higher than those without coating. At 35°C, it

was more than 5 ms, twice higher than that without

coating.

5 CONCLUSIONS

We controlled film thickness of hybrid polymer thin

film precisely by developing atomic layer deposition

method with sequential surface chemical reactions

and then could improve the relaxation time of spin

polarization by coating with the thin film. Although

the physical action of its spin relaxation prevention

effect is not well understood at present, it will

definitely improve the sensitivity of the optically

pumped atomic magnetometer.

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