Temperature dependence of the Dzyaloshinskii-Moriya constant in ultrathin Pt/Co(Fe)B/Ir films

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It is well known that both the anisotropy K and the exchange stiffness A in magnetic materials decrease as the temperature increases [1]. The temperature dependence of these parameters is often expressed as a power law in the reduced magnetization (m), e.g. calculations for Co indicate that the Callen-Callen law K(m)∼ m3 is a very good approximation for the anisotropy, while for the exchange stiffness, A(m) ∼ m1.8 [1]. Since the domain wall width is proportional to the square root of A/K, it increases with temperature, as does the domain wall energy. However, in thin films with an interfacial Dzyaloshinskii-Moriya interaction (DMI), the DMI constant (D) also enters in the expression for the domain wall energy. As the DMI leads to chiral domain walls and skyrmions, determining the temperature dependence of D and hence the domain wall energy is important from both fundamental and applied points of view. We report on our study of multilayers of Pt/ Co68Fe22B10/Ir and Pt/Co68B32/Ir multilayers, deposited by dc magnetron sputtering onto a silicon oxide substrate. Wide-field Kerr microscopy was used to image the domains in the demagnetised state. We measured the domain period in the temperature range from -264 to 17 °C [Fig.1], enabling a calculation of D following the approach of Woo et al [2] as shown in [Fig.2]. In a second study, we deposited the same multilayers onto BaTiO3 (BTO) (100), which undergoes structural phase transitions close to 0 and -100 °C and thus exerts a strain on the multilayers. Images of domains in a [Pt/CoB/Ir]x10 superlattice were obtained using X-ray magnetic circular dichroism - photoemission electron microscopy (XMCD-PEEM). It was found that the domain period increases as the temperature increases as shown in [Fig.1]. However the DMI for all the samples decreases when the temperature increases as shown in [Fig.1].