Detection of electron paramagnetic resonance in La0.6Ca0.4MnO3 using a copper stripcoil

Recently, we had reported the detection of electron paramagnetic resonance (EPR) from magnetoimpedance (MI) measurements in bulk La0.60Ca0.40MnO3 (LCMO) samples using radio frequency (rf) currents [U. Chaudhuri and R. Mahendiran, Appl. Phys. Lett. 115, 092405 (2019)]. Here, we report an alternative method which involves measuring the effective MI changes of a copper stripcoil that encloses the LCMO sample. Magnetoresistance (∆R/R0) and magnetoreactance (∆X/X0) of the sample were measured indirectly via the stripcoil for frequencies of current from f = 0.5 to 2.5 GHz. During the field sweep, ∆R/R0 shows an abrupt increase that is accompanied by a dip in ΔX/X0 at a critical value of dc magnetic field (Hc) when f ≥ 0.9 GHz. Hc increased linearly with frequency (f) of the current in the stripcoil, satisfying the EPR relation fr = (γ/2π)Hdc, where γ is the gyromagnetic ratio and fr is the resonance frequency. The same stripcoil and the sample were also used to measure microwave power absorption using a vector network analyzer. The features observed in both these techniques were strikingly similar to the results obtained from the direct MI measurement in LCMO, which confirms the electrical detection of EPR.Recently, we had reported the detection of electron paramagnetic resonance (EPR) from magnetoimpedance (MI) measurements in bulk La0.60Ca0.40MnO3 (LCMO) samples using radio frequency (rf) currents [U. Chaudhuri and R. Mahendiran, Appl. Phys. Lett. 115, 092405 (2019)]. Here, we report an alternative method which involves measuring the effective MI changes of a copper stripcoil that encloses the LCMO sample. Magnetoresistance (∆R/R0) and magnetoreactance (∆X/X0) of the sample were measured indirectly via the stripcoil for frequencies of current from f = 0.5 to 2.5 GHz. During the field sweep, ∆R/R0 shows an abrupt increase that is accompanied by a dip in ΔX/X0 at a critical value of dc magnetic field (Hc) when f ≥ 0.9 GHz. Hc increased linearly with frequency (f) of the current in the stripcoil, satisfying the EPR relation fr = (γ/2π)Hdc, where γ is the gyromagnetic ratio and fr is the resonance frequency. The same stripcoil and the sample were also used to measure microwave power absorption using a vector ...

Electron paramagnetic resonance (EPR) is a powerful technique to detect unpaired electrons and their interactions with environment in solids. Conventionally, EPR measurement is performed using the microwave cavity resonance method in which a sample placed inside a resonant cavity is irradiated with microwaves of a fixed frequency (f ∼ 9.8 GHz), and sweeping an external dc magnetic field causes maximum absorption of microwave energy when the resonance condition is satisfied. 1 On the other hand, some studies have shown the possibility of detecting EPR, 2 ferromagnetic resonance (FMR), [3][4][5] and nuclear magnetic resonance (NMR) 6 via changes in the photocurrent or dc voltage signals rather than power absorption in a sample exposed to a microwave electromagnetic field. Recently, our group has shown that one can observe EPR features in a bulk sample of the La 1-x CaxMnO 3 (x = 0.35-0.60) series 7 via the magnetoimpedance (MI) method which involves measuring the radio-frequency (rf ) electrical impedance (Z = R + iX, where R is the ac resistance and X is the reactance) of these samples during magnetic field sweep by an rf impedance analyzer. The flow of rf current in the sample induces a circumferential magnetic field in the sample. This rf field (h rf ), when perpendicular to the applied dc magnetic field, excites Zeeman transitions in Mn 3+ and Mn 4+ ions as the dc magnetic field (H dc ) is swept. rf magnetoresistance (MR) and magnetoreactance (MX) display characteristic features at a critical field value which followed EPR relations. However, high-frequency impedance of a conducting sample in a strong skin depth regime is dependent both on the resistivity and the magnetic permeability of the sample (Z = 8 where μ is the dynamic permeability, μ 0 is the vacuum permeability, ρ is the dc resistivity, and f is the frequency of the rf current. Thus, a fundamental question arises as to whether the observed features in the MI measurements observed in Ref. 7 are predominantly affected by the changes in the dynamic permeability (μ) or due to mechanisms related to electrical dc resistivity (or conductivity). Electrical conduction in the paramagnetic state of manganites is due to hopping of eg electrons (S = 1/2) between Mn 3+ and Mn 4+ ions in the background of immobile t 2g core spins (S = 3/2). Strong Hund's coupling between mobile and core electron spins ensures that the eg spin is parallel to t 2g core spins. When the core spin oscillates in response to h rf generated by the rf current in the sample, it can affect the scattering probability of the eg electron and, hence, the resistance. Thus, it is necessary to investigate ARTICLE scitation.org/journal/adv MI via indirect methods, as employed in this article, to understand the mechanisms behind the high frequency MI in manganites. We synthesized a polycrystalline La 0.6 Ca 0.4 MnO 3 (LCMO) sample by the standard solid state reaction method and characterized the sample by room temperature X-ray diffraction and by using the temperature and field dependence of magnetization (M) measurements. The ferromagnetic Curie temperature (TC) of the LCMO sample was 267 K. A sample of size 7 mm × 4 mm × 1 mm in length, width, and thickness, respectively, was cut from a pellet and inserted into a cuboidal copper stripcoil of similar dimensions made from a 0.2 mm thick copper sheet, as shown in Figs. 1(a) and 1(b). The stripcoil method was previously used by us to investigate high frequency magnetization dynamics in the Co 0.6 Zn 0.4 Fe 1.7 Mn 0.3 O 4 bulk sample. 9 One end of the stripcoil was connected to the signal line, and the other end was soldered to the ground of an Sub Miniature version A (SMA) connector. A semirigid rf coaxial cable was used to connect the SMA connector to an rf impedance analyzer (Agilent model E4991A) or a vector network analyzer (VNA) (Agilent model N5230A). E4991A is a single-port instrument [ Fig. 1(c)], while only port 1 of the VNA was used [ Fig. 1(d)]. These instruments were calibrated by performing standard open-short-load calibrations. High frequency current from the signal line flows through the stripcoil and terminates at the ground, creating an axial magnetic field inside the stripcoil, as shown in Fig. 1(b). The stripcoil with the sample was placed at the center of the poles of an electromagnet, and the R and X components of impedance were measured by the impedance analyzer while varying H dc for different frequencies of rf current. MR and MX were calculated using the standard definitions: resistance and reactance at a particular frequency (f ) of rf current and H dc , respectively. At resonance, maximum power is absorbed by the sample from the electromagnetic wave. This absorbed power was also detected by measuring the magnetic field dependence of rf power absorption using the VNA over a broad frequency range (10 MHz-2.5 GHz). The reflection coefficient, S 11 , was measured by the VNA, and the power absorbed by the sample was determined by subtracting the contribution of the empty coil from S 11 values measured with the sample: ΔP(H) = S 11 sample (H) − S 11 empty (H). This was incorporated to verify whether the observed MR and MX features corresponded to a resonance feature. All the measurements were recorded at room temperature since our experimental setups are currently suitable for room temperature measurements only.
The ac MR and −MX, as measured by the stripcoil, are displayed in Figs. 2(a) and 2(b), respectively, as a function of f and H dc . The ac MR is negative in the entire field range for f < 900 MHz, whereas the ac MR for f ≥ 900 MHz initially decreases slowly as the magnetic field increases from the zero value and then shows an abrupt increase at a critical value of the magnetic field (Hc). Hence, the sign of the ac MR changes from negative to positive at Hc. On the other hand, the field dependence of −MX shows a single peak around 0 Oe field for f < 800 MHz but shows two symmetrical peaks about the origin for f ≥ 800 MHz. In the experiment, MX displays a dip, but we have presented as −MX, as shown in Fig. 2(b), for clarity. These features were similar to the direct MI measurements reported earlier in Ref. 7, barring variations in magnitude. In Fig. 2(c), the change in power absorption (−ΔP) as measured by the VNA is also shown for different frequencies of the rf signal while varying the dc magnetic field. ΔP is shown with a negative sign, which indicates that the power was absorbed as the magnetic field was increased. Similar to the ac MR in the strip coil measurement, −ΔP shows symmetrical features about the origin at ±Hc, and the line shapes are strikingly similar to the ac MR line shapes. The sourced rf power (1 dBm for the impedance analyzer and 10 dBm for the VNA) is absorbed at resonance when h rf is perpendicular to H dc . The power absorbed by a sample from the rf magnetic field is related to the imaginary part of the permeability, μ ′′ , according to the relation P abs = 1 2 Vμ ′′ ωh 2 rf , where ω = 2πf, f is the frequency of current, and V is the sample volume. 7 The characteristic double peak feature in all three quantities of MR, −MX, and −ΔP shifts towards higher magnetic field values for higher frequencies of the signal as illustrated in the two-dimensional image plots shown in Figs. 2(e) and 2(f) for MR, MX, and ΔP.
In Fig. 3(a), the ac MR and MX from the MI-stripcoil method are plotted together with −ΔP vs H dc for the frequency f = 2 GHz of the rf current. For comparison, we have also included the ac MR measured by passing an rf current through the sample for the same frequency. The magnitude of ac MR measured using the direct MI method was larger than the signal measured using the stripcoil; hence, the former is presented by dividing it by a factor of 8. The magnetic field dependence of power absorption, −ΔP(H dc ), can be described in terms of the Lorentzian function as 10 where P 0 is the maximum power absorption at H dc = Hr, and ΔH is the linewidth [Full Width at Half Maximum (FWHM)].
Equation (1) is valid for electromagnetic waves in which the electric and magnetic field components are in-phase. However, for electromagnetic waves propagating through a dispersive medium, the electric and magnetic field components become out of phase. In such a case, the power absorption spectrum will contain an additional dispersive term which can be described by an antisymmetric Lorentzian function. 10 Considering the presence of both in-phase (symmetric Lorentzian function) and out-of-phase (antisymmetric Lorentzian function) components, the resultant power absorption by the sample can be expressed as where Asym and Aasym are the coefficients of symmetric and antisymmetric Lorentzian functions, respectively, and C is a constant offset parameter. Similarly, the observed signals in MR and MX were also fitted with symmetric and antisymmetric Lorentzian components. 7 It was found that MR was dominated by the antisymmetric component, while MX was dominated by the symmetric component. The peaks observed in MR coincide with the peaks in the derivative of X (or MX) with respect to magnetic field, as shown in the inset of Fig. 3(b), for f = 2 GHz. From these fits, the resonance field (Hr) was extracted and are presented in Fig. 3 stripcoil-impedance analyzer and stripcoil-VNA methods. The plot of frequency (f ) vs resonance field (Hres) for the sample follows Kittel's equation for ferromagnetic resonance, which can be expressed as 11 where H k is the anisotropy field, Ms is the saturation magnetization, and γ is the gyromagnetic ratio. γ = gμB/ ̵ h, and "g" is the Lande g-factor. We obtain γ/2π = 2.92 ± 0.020 MHz/Oe for the stripcoilimpedance analyzer method and γ/2π = 2.96 ± 0.023 MHz/Oe for the stripcoil-VNA method, whereas 4πMS = 0 and H k = 0 Oe for both the techniques. Hence, the abrupt jumps observed in MR and ΔP are the signatures of EPR. EPR signals in manganites have been attributed to collective precession of the locally exchange coupled Mn 3+ (t 2g 3 eg 1 ; spin S = 2) and Mn 4+ (t 2g 3 ; spin S = 3/2) spin system, [12][13][14] and their intensity is proportional to the total susceptibility of the Mn 4+ and Mn 3+ spins rather than individual Mn 3+ and Mn 4+ spins. In manganites, the eg electron is mobile, whereas the t 2g electrons are localized. The resonance frequency for EPR varies linearly with the dc magnetic field as fr = ( γ 2π )H dc . The analysis of −ΔP vs H dc and the ac MR from direct MI measurement vs H dc gives similar information regarding γ/2π, 4πMS, and H k . Since no current flows through the sample (except for the induced current) when using the stripcoil, one can infer that the EPR signal is predominantly due to dynamic permeability changes caused by the h rf generated along the axis of the stripcoil rather than due to drastic changes in the scattering rate of electrons. Thus, the observed EPR features in direct MI measurement are confirmed to be caused by the field dependence of μ during resonance.
In summary, the magnetoimpedance of the La 0.6 Ca 0.4 MnO 3 sample measured indirectly using a copper stripcoil and an impedance analyzer showed an abrupt increase at a critical value of the applied dc magnetic field, and its position shifted linearly with the frequency of the radio frequency signal in the stripcoil. This feature was attributed to the occurrence of electron paramagnetic resonance which was confirmed by an independent measurement of the magnetic field dependent rf power absorption by the sample using a vector network analyzer. Our present results confirm that the electron paramagnetic resonance signal observed in the direct magnetoimpedance measurements of the sample arose mainly from the magnetic field dependence of the dynamic permeability of the sample and not due to the enhanced scattering of the eg electron.