Magnetic Properties of the Low-Temperature Modification of Mn7Sn4

Abstract

The high-temperature modification of Mn7Sn4 (HTM; quenched from 760 C) crystallizes in the B82-type of structure with the hexagonal lattice parameters ahex and chex. 1,2) Below the Curie temperature TC 1⁄4 220K, HTM is in a ferrimagnetic state. Re-entrant spin-glass-like (SGL) behaviors appear below a temperature Tf 1⁄4 100K. Two NMR resonance lines for Sn and Sn (just above 320 MHz at 77K) were reported on the B82-type of Mn1:5Sn. 4) The low-temperature modification of Mn7Sn4 (LTM) was obtained by quenching from 400 C. The higher TC 1⁄4 262K and a super-lattice lines indexed with Asuper 1⁄4 3ahex and Csuper 1⁄4 chex were reported on LTM. At 4.2K, several intensity maxima of NMR frequencies between 200– 260MHz appear in LTM, but it does not appear in HTM. In the present paper concerning LTM, we systematically investigate the magnetization and NMR resonance-frequency at external magnetic fields (H). The sample of HTM was gradually cooled from 700 C, annealed at 400 C for 5 days, and then quenched into iced water. All the X-ray powder diffraction peaks of this LTM are indexed with the orthorhombic papameteres of Asuper 1⁄4 2aortho, Bsuper 1⁄4 2bortho, Csuper 1⁄4 4cortho, where aorhto 1⁄4 0:42937 nm, bortho 1⁄4 0:75637 > ð3Þaortho nm and cortho 1⁄4 0:5501 nm (Fig. 1). Zero-field cooling magnetization ( ZFC) and field-cooled one ( FC) are shown in Fig. 2. At H 1⁄4 100Oe, ZFC slowly increases with increasing temperature (T), and shows a peak at a characteristic temperature T 1⁄4 170K. This feature is different from the steep change in ZFC vs T curve 3) around Tf A 2 ZFC vs T curve gives TC 1⁄4 261K for LTM. At 6 and 50K, thermo-remanent magnetization (TRM, rem) was observed on LTM when H 1⁄4 100Oe was switched off [Figs. 3(a) and 3(b)]. It follows the power-law rem 1⁄4 A þ Bt Figure 2 shows that the remanence ð FC ZFCÞ > 0 appears below T These anomalies are characteristics of spin-glass state. The remanence and the peak maximum at T are blurred as H increases, and disappear at ca. 5 kOe. At 6K, hysteresis loops taken between 55 kOe show the remanence values of 6 emu/g. As seen in Fig. 3(c), ZFC vs H curves at 6 and 100K (below T ), and 200K (above T ) show the saturation behavior known on the usual ferromagnetic or ferrimagnetic materials. Magnetizations above 25 kOe give the saturation magnetization through Arrott plot ( 0;A) and law-of-approach-to-saturation ( 0;L) methods. At 6K, 0;A and 0;L give the saturation moment n f 1⁄4 1:57 B/Mn (> n f 1⁄4 1:17 B). The latter method leads the high-field susceptibility ( h) of 24, 26, 65, and 292 in 10 6 cm/g unit for 6, 100, 200, and 300K, respectively. Powder sample of LTM was fixed with paraffin in a pyrex capsule (6mm diameter and 25mm length). Its zero-field NMR frequencies were followed with spin-echo method between 35 and 370MHz at 4.2K. The NMR-line shapes were fitted for Lorentzian type profile. Similar to HTM, the resonance lines were also observed on LTM at 328 and 346MHz. These lines are assigned as the transferred hyperfine field at Sn and Sn nuclei, respectively. This is because the ratio 328=346 1⁄4 0:947 is close to the ratio Fig. 1. X-ray (Cu K 1) diffraction pattern of LTM. The indices based on the orthorhombic fundamental cell with aorhto, bortho and cortho are shown by #, and those based on the super lattice cell with Asuper, Bsuper and Csuper are shown by . No diffraction peak was observed between 10 and 20