Banca de QUALIFICAÇÃO: FRANCISCO GOMES DE QUEIROZ

Uma banca de QUALIFICAÇÃO de DOUTORADO foi cadastrada pelo programa.
DISCENTE : FRANCISCO GOMES DE QUEIROZ
DATA : 27/01/2022
HORA: 15:00
LOCAL: GoogleMeet
TÍTULO:

Studies of Magnetic Phases in Holmium Thin Films via Thermal Hysteresis measurements.

PALAVRAS-CHAVES:

Thermal hysteresis. Ho Thin films. Magnetic profiles.

PÁGINAS: 51
GRANDE ÁREA: Ciências Exatas e da Terra
ÁREA: Física
SUBÁREA: Física da Matéria Condensada
ESPECIALIDADE: Materiais Magnéticos e Propriedades Magnéticas
RESUMO:

Rare earth materials (TR's) in their volumetric form without the presence of an external field, only due to thermal effects, present a rich diversity of magnetic configurations for certain specific temperature ranges of each element. Holmium (Ho) and Terbium (Tb), for example, exhibit a helical behavior between Curie (TC) and Neel (TN) temperatures. Below TC, Tb is ferromagnetic, whereas Ho has a conical ferromagnetic component along the c axis and a helical component in the basal plane. Above TN, both are paramagnetic.

Gadolinium (Gd) is characterized by being a simple ferromagnet, presenting an easy magnetization direction along the c axis, between TC and T = 240 K. Below 240 K, the easy magnetization direction moves away from the c axis. The construction of manipulated systems, in which rare earth elements are part of at least one of the constituents, has promoted the discovery of new magnetic patterns. Field and temperature effects, combined with finite size effects, have allowed the control of new magnetic patterns, in addition to promoting new magnetic configurations to the studied system.

In Holmium thin films, with 7, 12 and 24 atomic planes, we found, for the same field strength, H = 100 Oe, a controlled hysteresis width, varying from 56 to 86 K according to the film thickness.

We also observe that it is possible to have two stable magnetic states at the same temperature within the width of the thermal hysteresis. For example, for the 12-layer film we find at T = 80 K, an alternating helical state - HA in the heating branch and a helifan state - HFAN in the cooling branch.

A similar case is observed for the 7-plane film at T = 40 K, with a ferromagnetic state - FM in the heating curve and a state - FAN in the cooling curve. In the 24 atomic layer film, at T = 80 K, within the hysteresis width, we find a SPIN-SLIP-like state in the cooling curve and a FAN state in the heating branch.

We emphasize that in the ZFC-FC process hysteresis was not observed in films with 7, 12 and 24 atomic layers under a field strength H = 100 Oe.

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