key: cord-0814287-twkjdikv
authors: Hou, Weixiao; Yao, Yufei; Li, Yaojin; Peng, Bin; Shi, Keqing; Zhou, Ziyao; Pan, Jingye; Liu, Ming; Hu, Jifan
title: Linearly shifting ferromagnetic resonance response of La(0.7)Sr(0.3)MnO(3) thin film for body temperature sensors
date: 2022-02-24
journal: Front Mater Sci
DOI: 10.1007/s11706-022-0589-5
sha: dd672901e4a8a9bc7fc5bb3d4f53da6ebd632a47
doc_id: 814287
cord_uid: twkjdikv

Human body temperature not only reflects vital signs, but also affects the state of various organs through blood circulation, and even affects lifespan. Here a wireless body temperature detection scheme was presented that the temperature was extracted by investigating the out-of-plane (OP) ferromagnetic resonance (FMR) field of 10.2 nm thick La(0.7)Sr(0.3)MnO(3) (LSMO) film using electron paramagnetic resonance (EPR) technique. Within the range of 34–42 °C, the OP FMR field changes linearly with the increasing or decreasing temperature, and this variation comes from the linear responses of magnetization to the fluctuant temperature. Using this method, a tiny temperature change (< 0.1 °C) of organisms can be detected accurately and sensitively, which shows great potential in body temperature monitoring for humans and mammals.

Body temperature is one of the four essential physiological indices of physical health, and it can provide important information for disease prevention, diagnosis, and treatment [1] [2] [3] . Right now the novel coronavirus is ravaging around the world and the temperature detection is the essential measure for health workers to screen for infected persons. In traditional body temperature detection, the measurement is carried out using a mercury thermometer mostly. Due to its potential mercury pollution, long action time, and the trouble of reset, the mercury thermometer is gradually replaced by invasive electronic thermometers and non-invasive infrared thermometer [4] [5] [6] . However, the invasive measurement is with great expense and complex steps. The working conditions of the infrared thermometer are not good enough, and the distance will also affect the test results. Consequently, it is necessary to develop a comfortable, continuous, accurate, wireless, and highly sensitive measurement method for infants and critically ill patients [7] [8] [9] .

In the electronic thermometer, the probes are made from thermal sensitive materials that its physical properties can be affected significantly by the temperature fluctuation. It is well-known that the magnetism of ferromagnetic materials is closely related to the temperature. According to Curie's law, the magnetization intensity decreases rapidly with increasing temperature when the temperature is approaching its Curie temperature (T C ) [10] [11] . The ferromagnetic perovskite manganite oxide La 0.7 Sr 0.3 MnO 3 (LSMO) is an attractive half-metallic ferromagnetic oxide with low saturation magnetization, colossal magnetoresistance, and high conductive properties [12] [13] [14] . Therefore, it shows great potential in the next generation of electronic devices and magnetic field sensors. The magnetic properties of LSMO is associated with the double exchange effect of itinerant electrons between adjacent Mn 3+ and Mn 4+ via O 2-2p orbitals. The T C of LSMO is about 96°C, so its magnetization decreases dramatically when the temperature increases from room temperature because of the reduction of the double exchange mechanism [15] [16] . If the magnetic parameter of LSMO films and temperature is corresponding one by one within a certain range, the temperature value can be calculated according to its magnetic properties conversely.

In this work, the nano-scale LSMO films were grown on the (0 0 1)-oriented single-crystal SrTiO 3 (STO) substrates, and the electron paramagnetic resonance (EPR) spectrometer was employed to measure the ferromagnetic resonance (FMR) spectra. It is found that the out-ofplane (OP) FMR field (H r ) of 10.2 nm LSMO film changes linearly with the temperature range of 34-42°C, and the response coefficient is over 90 Oe/°C. Conversely, every OP H r changes of 1 Oe corresponded to a tiny temperature fluctuation ( < 0.1°C) under the constant test conditions, and this response is fast, continuous, and precise. Therefore, a wireless temperature detector can be developed based on the temperature-sensitive manganite films, which has great potential for humans and animals body temperature monitoring.

Ferromagnetic LSMO thin films were deposited onto 5 mm  5 mm  0.5 mm (0 0 1)-cut single-crystalline STO substrates by pulsed laser deposition (PLD) method using a commercial LSMO ceramic target. A krypton fluoride (KrF) excimer laser (Lambda Physik COMPEX PRO 205F) was employed with a 248 nm wavelength and the laser energy 250 mJ per pulse laser beam. The optimized deposition condition was achieved at the substrate temperature of 700°C under the O 2 partial pressure of 30 Pa. After the deposition, the as-grown thin films were in situ annealed for 5 min and cooled naturally to room temperature.

The crystallinity and epitaxial behavior of the LSMO film were characterized by high-resolution X-ray diffraction (HRXRD) of PANalytical X'Pert MRD. The film morphology was characterized via atomic force microscopy (AFM; Bruker, Dimension Icon). The FMR spectra were measured in the EPR spectrometer (JES-FA200, JEOL RESONANCE Inc.). The TE 011 mode microwave was 9.2 GHz.

3 Results and discussion Figure 1 presents a HRXRD pattern of LSMO thin film grown on STO (0 0 1) substrates. It is evident that only LSMO (0 0 l) reflections appear along with STO (0 0 l) Bragg peaks. The out-of-plane lattice constant of LSMO films derived from the LSMO (0 0 2) reflection (2θ = 46.99°) is about 3.86 Å, which is smaller than the bulk value of LSMO [17] . Obviously, the relatively larger lattice constant of the STO substrate leads to an in-plane tensile strain of the LSMO film. Then left inset in Fig. 1 shows the AFM image of a representative LSMO film. The rootmean-square (RMS) roughness over a 5 µm  5 µm scanned area is about 0.37 Å. The right inset in Fig. 1 shows the X-ray reflectometry (XRR) pattern of the 10.2 nm thick LSMO thin film. The clear interference patterns of XRR curve demonstrate the smooth surface and interface morphology. This 10.2 nm thick sample is selected to study the influence of temperature on FMR properties.

As we all known, the dynamic magnetism of LSMO films were influenced by the temperature deeply. Here the EPR technology was employed to investigate the FMR spectra, and then the FMR fields can be presented accurately with different temperatures and directions of the external magnetic field [18] [19] [20] . A schematic of the FMR measurements is displayed in Fig. 2 

It means that 1 Oe decrease of OP H r corresponds to 0.0107°C increase in temperature and the linear error is also only 0.21%. The minimum magnetic field that the EPR spectrometer can identify is 0.05 Oe, and this means that it can capture the extremely tiny temperature fluctuation of the LSMO film theoretically, which is far beyond the current temperature detection equipment.

In the process of cooling, the relationship between OP H r of LSMO films and temperature can be fitted as:

ð34:0°C £ t £ 42:0°CÞ

The adjusted determination coefficient is 0.9992, and this indicates that the change of the resonance field still remains linear strictly. The response coefficient of -91. 4 Oe/°C and the absolute value is only 1.8% different from the heating process, demonstrating that the microwave magnetism of the film is stable during the heating and cooling process. The inverse function of Eq. (3) was deduced as:

The response coefficient of temperature to the resonance field was -0.0109°C/Oe, and the absolute value changed only 1.9% compared with the heating process. Therefore, it can be determined that the OP H r of LMSO film changes linearly with the temperature in the range of 34.0-42.0°C, and it is able to respond to the minimal temperature fluctuations sensitively and timely. Importantly, the changing temperature does not damage the magnetic performance of the LSMO film.

In order to explore the fundamental cause of the linearly varying H r mentioned above, the angular dependences of H r for 10.2 nm LSMO film at different temperatures were tested and fitted by the following equation:

where ω is angular frequency for resonance; H 4,? and H 4,// are the OP and IP cubic anisotropy, respectively; γ is the gyromagnetic ratio of 2.8 MHz/Oe; 4πM eff is the effective magnetization of the LSMO film. The OP H r can be expressed by the well-known Kittel equation [21] [22] :

where f is the frequency, and it is fixed at 9.2 GHz in this experiment. The OP H r has a positive linear variation tendency with 4πM eff , while the variation tendency of IP H r is different totally. As shown in Fig. 3(a) , the fitting curves agree very well with the experimental data. The values of M eff were concluded from the theoretical curves and plotted in Fig. 3(b) . It is obvious that the M eff decreases linearly almost as the temperature increases from 34 to 42°C, and the R adj of the linear fitting is 0.9577. The effective magnetization M eff can be given by Refs. [23] [24] :

where K 1 is the anisotropy constant, M s is the saturation magnetization. To investigate temperature-induced M s change, the OP magnetic hysteresis loops of the LSMO film at different temperatures were tested and presented in Fig. 3(c) . When the temperature rises from 34.0 to 42.0°C, the OP M s of the LSMO film increases significantly, and the coercivity almost remains unchanged. The sensitivity and accuracy are the essential indexes of the temperature monitoring systems. For healthy person, their body temperature fluctuates between 36.8 and 37.2°C in general. Therefore, the reversible shift of the OP FMR spectra for 10.2 nm LSMO film was tested. Figure 4 (a) shows temperature induced reversible switching of OP H r between 5207 Oe (36.8°C) and 5158 Oe (37.2°C). With the temperature rise and fall for several times, the variation of OP H r maintains a precise 49 Oe consistently. Figure 4 (b) shows FMR spectra at the first and the tenth change. During this process, the resonance signal presents well shapes, and the linewidth remains unchanged basically. When people get sick, their body temperature fluctuates significantly within a wide temperature range. Furthermore, the body temperature of most mammals (cows, horses, etc.) is different from humans. Therefore the OP FMR spectra under the alternant temperature of 35.0 and 41.0°C for 5 times were tested, and the results were presented in Figs. 4(c) and 4(d) . The OP H r shifts between 

In summary, a wireless body temperature method based on the LSMO film was proposed. The OP H r of 10.2 nm thick LSMO film shifts linearly within the temperature range of 34-42°C. The shift of H r is precise, reversible, and timely. Meanwhile, the repeated changing temperature has no impact on the quality of the signal. Through experimental tests and theoretical simulation, it is found that the linearly shifting H r is caused by the linear change of effective magnetization with temperature. As an alternative, the resonance frequency can illustrate the change of temperature at the fixed magnetic field, which can be studied further in the future.

Disclosure of potential conflicts of interest The authors declare that they have no conflict of interest.

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