UC Berkeley

This work covers the characterization and manipulation of two temperature-driven phenomena: the meta-magnetic transition in FeRh thin films and the ultrafast magnetization reversal in amorphous Gd-Tb-Co thin films.

For FeRh, the separate effects of strain and film thickness were determined on the antiferromagnetic-to-ferromagnetic phase transition temperature of FeRh thin films by both experiment and density functional calculations. Strain was introduced by epitaxial growth onto MgO, SrTiO$_3$ and KTaO$_3$ substrates. Film thicknesses below 15 nm substantially suppress the transition temperature, T$^*$, to below room temperature in unstrained films. For strained films, tensile/compressive strain decrease/increase T$^*$ respectively. KTaO$_3$ (001) substrates produce sufficient compressive strain to increase the transition temperature of 10 nm FeRh films above room temperature, useful for many proposed applications previously limited by the stabilization of the ferromagnetic state at small thicknesses. These results demonstrate that a judicious use of film thickness and substrate can be used to manipulate FeRh's transition temperature over a $\sim$200 K range.

Amorphous (\textit{a}-) ferrimagnetic Gd$_{22-x}$Tb$_x$Co$_{78}$ thin films with perpendicular magnetic anisotropy show ultrafast helicity-independent all-optical switching (HI-AOS) from x = 0 to x = 18. Increasing Tb content causes increasing values of the magnetic anisotropy constant, slower remagnetization rates and higher critical fluences. Magnetic anisotropy and saturation magnetization do not correlate with magnetization reversal upon irradiation. The ultrafast magnetization measurements show that the dynamics of reversal are fastest in pure \textit{a}-Gd$_{22}$Co$_{78}$ and progressively slow down as Tb at.\% increases until only demagnetization occurs in \textit{a}-Tb$_{22}$Co$_{78}$. Annealing reduces the anisotropy, increases the damping and causes slower remagnetization rates. Atomistic spin dynamics reproduces the experimental dynamics as well as the increased critical fluence required for switching. Increased damping from greater spin-orbit coupling arising from Tb's $L = 3$ value is responsible for the slower dynamics and greater critical fluence and explains why switching has not been observed in Tb-Co alloys. The high anisotropy in \textit{a}-Gd$_{22-x}$Tb$_x$Co$_{78}$ thin films for x $\geq$ 12 makes them excellent candidates for new high-density memory devices taking advantage of ultrafast magnetization reversal mechanisms.

Time resolved X-ray magnetic circular dichroism (TR-XMCD) measurements of \textit{a}-Gd$_{10}$Tb$_{12}$Co$_{78}$ revealed the element-specific ultrafast magnetization reversal dynamics. It was found that Co demagnetizes fastest with a time constant of 320 fs, followed by Tb and Gd with time constants of 410 fs and 630 fs respectively. This is consistent with previous work showing that Tb's greater spin lattice coupling allows for thermal energy to be more easily transferred to the lattice, allowing it to demagnetize faster compared to Gd.

Structural characterization of the amorphous structure of Tb-Co alloys performed via fluctuation electron microscopy indicates that not all amorphous structures are the same as evidenced by differences in the mid range order (MRO) as a function of growth temperature. A distinction between MRO along the growth direction and along the in-plane direction suggests that the perpendicular magnetic anisotropy in these amorphous alloys is positively correlated with the degree of MRO. This technique enables structural characterization of amorphous materials by describing the differences between amorphous configurations as a function of MRO and tilt angle.

Amorphous, ferrimagnetic Tb-Co thin films prepared with a thin Ta underlayer and either a Ta or Pt overlayer show evidence of both a soft and a hard magnetic phase despite no sign of this at room temperature. Low temperature magnetometry measurements reveal the decoupling of the two magnetic phases with decreasing temperature due to increased anisotropy energy at lower temperatures. Decreasing the film thickness to 2 nm, slightly above the superparamagnetic limit found at 1 nm, a soft, low density phase was isolated and found to be present in all the films as confirmed with x-ray reflectivity (XRR) and Rutherford backscattering spectrometry (RBS) measurements. For greater thicknesses, the bottom layer retains its soft magnetic nature, while the remainder of the film is denser and has strong perpendicular magnetic anisotropy, leading to exchange-spring behavior when the anisotropy becomes large, either at low temperatures or via a Pt overlayer which adds a strong interfacial anisotropy to the layer. Micromagnetic simulations reproduced the experimental hysteretic behavior by incorporating the experimentally-determined anisotropy and magnetization parameters into a soft/hard bilayer model. The Pt capping layer produces a slightly larger anisotropy constant than the Ta capping.

A membrane-based X-ray transparent heater for application of large temperature gradients was designed -- with guidance from a heat-transfer simulation -- and fabricated for synchrotron-based imaging of domain wall motion driven by the spin Seebeck effect.