Broadband magnetotransport in La0.6Sr0.4Mn1−xGaxO3 (0.0 ≤ x ≤ 0.3) at room temperature

Abstract

We report the magnetic field (−2.5 kOe ≤ Hdc ≤ 2.5 kOe) dependence of ac magnetoresistance (ac MR) and magnetoreactance (MX) in bulk samples of La0.6Sr0.4Mn1−xGaxO3 (0 ≤ x ≤ 0.3) carrying radio frequency current (f = 10 MHz–3 GHz) at 300 K. Samples with x ≤ 0.10 are ferromagnetic (FM) and those with x ≥ 0.2 are paramagnetic (PM) at 300 K. While the ac MR in FM samples is negative and shows a single sharp peak at Hdc = 0 at low frequencies, positive double peaks emerge at Hdc = ±Hp for f > 500 MHz, and they shift toward higher Hdc with increasing f. The positive ac MR is larger (∼75% at Hdc = Hp = 800 Oe at 3000 MHz) than 20% negative ac MR for Hdc = 2.5 kOe at 10 MHz. The PM samples also show negative ac MR and single peak for 100 MHz < f ≤ 500 MHz, but the magnitude of ac MR is much reduced compared to the FM samples. Surprisingly, the ac MR increases abruptly at a critical value of the magnetic field for higher f, and this anomaly also shifts upward in Hdc as f increases. The MX shows distinct field dependence in FM and PM samples. The ac MR is dictated by the frequency and field dependence of magnetic permeability. We attribute observed features in the ac MR to ferromagnetic resonance for x = 0.0–0.1 and electron paramagnetic resonance for x ≥ 0.2.