- https://www.cells.es/en/media/events/alba-public-events/phd-defense-enhancing-the-high-temperature-chiral-magnetic-state-in-ybacufeo5
- PhD Defense "Enhancing the High-temperature Chiral Magnetic State in YBaCuFeO5"
- 2022-07-14T10:00:00+02:00
- 2022-07-14T12:00:00+02:00
- Xiaodong Zhang. Thesis directors: Prof. José Luis García Muñoz (ICMAB-CSIC), Dr. Javier Herrero Martín (ALBA)
- What
- events
- When
- Jul 14, 2022 from 10:00 AM to 12:00 PM (Europe/Madrid / UTC200)
- Where
- Virtual event
- Contact Name
- Javier Herrero Martín
- Web
- Visit external website
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iCal
Abstract
Magnetoelectric multiferroic materials, where magnetic order induces ferroelectricity are attracting high interest because of the importance of controlling magnetism by electric fields and vice versa. Spiral (cycloidal) magnetoelectric (spin-driven) multiferroics are an ideal platform for the pursuit of strong magnetoelectric coupling because spin and ferroelectric orders are coupled “by construction”. Most of the chiral (spiral) magnetoelectric multiferroics investigated in recent years are geometrically frustrated magnets, where the presence of frustrated spin networks produces low spiral transition temperatures TS (typically TS<50 K). This critically limits their potential uses in spintronic and low-power magnetoelectric devices. Exceptionally, the layered perovskite YBaCuFeO5 have been reported to display cycloidal magnetic order at unexpectedly high temperatures. This motivating exception is considered one of the most promising spin-driven multiferroic candidates at high-temperature.
The doctoral Thesis presented here has explored and investigated different strategies to tune and optimize the high-temperature chiral magnetic properties in the YBaBB’O5 family of this layered perovskite, structurally simple in appearance but complex due to the decisive presence of cation disorder. The strategy is based on modifying both structural and physical elements (such as the magnetic or the spin-orbit couplings, the magnetic moments or the single-ion magnetic anisotropy) through cationic substitutions at the divalent Cu (B) and trivalent Fe (B’) sites. Many compounds of the families YBa(Cu,B)FeO5 (B: Co and Mg) and YBaCu(Fe,B’)O5 (B’: Cr and Mn) were prepared as polycrystalline or single crystal samples. They were thoroughly investigated combining magnetometry, X-ray spectroscopies, synchrotron X-ray and neutron diffraction.
Summarizing, (i) we demonstrated an alternative way for increasing Fe/Cu disorder (that controls the level of frustration and TS) not based on the cooling rate. (ii) The T-x magnetic phase diagrams for these B and B’ site substitutions were built up. (iii) A huge increase in the stability of the spiral order (TS) was obtained with divalent Co and Mg dopings preserving a linear TS-qS relationship for the spiral modulation. (iv) A triple point was achieved with a maximum spiral transition temperature TS360 K, well above RT, describing how to increase the spiral stability by acting on the Cu2+ J-T splitting. (v) We studied in detail magnetic phase competition (phase separation) and magnetic anisotropy as a function of temperature and doping in all these families. The spiral orientation may critically determine the ferroelectric and magnetoelectric behavior in these systems. We showed how to induce a systematic reorientation of the spin rotation plane in the spiral phase by doping. Changing from a more helical (k//Q) into a more cycloidal (k⊥Q) spin order, and proving that for selected dopings the most cycloidal spiral found is as well the most thermally stable.
