Low-dimensionality-induced tunable ferromagnetism in SrRuO$_3$ ultrathin films

TL;DR

This work addresses how to engineer ferromagnetism by tuning the high density of states (DOS) near the Fermi level in oxide ultrathin films. It combines ARPES/SRPES with alkali-metal dosing and ionic-liquid gating, complemented by DFT+DMFT, to show that a 4-unit-cell SrRuO$_3$ film sits at a magnetic crossover where 2D van Hove and 1D quantum-well DOS contributions enhance itinerant magnetism. As the Fermi level moves away from the high-DOS point via electron doping, the spin-split bands collapse, reducing both the spin polarization and Curie temperature, while transport indicates increased metallicity; these effects are reproduced by theory, linking DOS positioning, electron correlations, and magnetic order. The findings establish a controllable route to tunable quantum phases through DOS engineering in dimensionality-tuned oxide systems, with implications for spintronics and materials design, guided by the Stoner criterion $I N_0 \ge 1$ and its dependence on $E_F$ and occupancy $N$.

Abstract

Quantum materials near electronic or magnetic phase boundaries exhibit enhanced tunability, as their emergent properties become highly sensitive to external perturbations. Here, we demonstrate precise control of ferromagnetism in a SrRuO$_3$ ultrathin film, where a high density of states (DOS), arising from low-dimensional quantum states, places the system at the crossover between a non-magnetic and bulk ferromagnetic state. Using spin- and angle-resolved photoemission spectroscopy (SRPES/ARPES), transport measurements, and theoretical calculations, we systematically tune the Fermi level via electron doping across the high-DOS point. We directly visualize the spin-split band structure and reveal its influence on both magnetic and transport properties. Our findings provide compelling evidence that magnetism can be engineered through DOS control at a phase crossover, establishing a pathway for the rational design of tunable quantum materials.

Figures (4)

• Figure 1: Evolution of the Ru $t_{2g}$ orbital density of states (DOS) with film thickness. (a–c) Schematic DOS for (a) ferromagnetic bulk SrRuO$_3$ (SRO), (b) a 4 unit-cell (uc) SRO film at the magnetic crossover, and (c) a non-magnetic 1 uc SRO film, shown without considering electron correlations. (d,e) $d_{yz(zx)}$ and $d_{xy}$ orbitals, which give rise to one-dimensional quantum well (1D QW) states and a two-dimensional van Hove singularity (2D VHS) in ultrathin SRO films, respectively.
• Figure 2: Angle-resolved photoemission spectroscopy (ARPES) results of a 4 uc SRO thin film. (a) Schematic of the paramagnetic (PM) and ferromagnetic (FM) Fermi surfaces. Red, blue, and green circles correspond to the high symmetry $X$, $\Gamma$, and $M$ points, respectively. (b) Schematic of Fermi surfaces with electron doping. The spin-split gap between the spin-majority and spin-minority bands decreases with electron doping sohn2021sign. (c) Fermi surfaces of the 4 uc SRO as a function of K dosing coverage. Each Fermi surface is obtained by integrating over an energy window of $E_{\rm F}$$\pm$ 10 meV, where $E_{\rm F}$ is the Fermi level. ML indicates a monolayer (see Fig. 3(d) for the definition of ML). The $\gamma$ bands are indicated by white dotted lines. The spin-majority and -minority $\alpha$ bands ($\alpha_{maj}$ and $\alpha_{min}$) are indicated by orange dotted lines, while the spin-minority $\beta$ and its folded bands ($\beta_{min}$ and $\beta'_{min}$) are indicated by blue dotted lines. The navy-colored arrows show that the Fermi momentum, k$_F$, of $\beta_{min}$ band increases with K dosing. (d,e) High-symmetry cuts near $X$ (Cut I) and $M$ (Cut II) with 0 ML (left), 0.6 ML (middle), and 1 ML (right) K coverage, respectively. The upper panel of (e) shows MDCs with fitting results at $E_{\rm F}$. The green inverted triangles indicate the fitted peak positions of the $\alpha_{min}$ and $\alpha_{maj}$ peaks. (f) $\Delta$$k_{\rm F}$ as a function of K thickness. $\Delta k_{\rm F}$ represents the difference in $k_{\rm F}$ between $\alpha_{min}$ and $\alpha_{maj}$. All data were obtained at 6 K.
• Figure 3: Spin-resolved photoemission spectroscopy (SRPES) results of a 4 uc SRO thin film. (a,b) SRPES energy distribution curves for (a) 0 ML and (b) 0.7 ML K-dosed films measured at 10 K and 135 K. Spin polarization vanishes at 135 K. All measurements were taken at the $\Gamma$ point. (c) Temperature-dependent spin polarization measurements at each dosing step. The temperature at which polarization reaches 0 $\%$ is defined as $T_{\rm C}$. The filled triangle indicates the $T_{\rm C}$ of pristine 4 uc SRO, while the open triangle represents the reduced $T_{\rm C}$ for 0.7 ML K coverage. (d) Spin polarization as a function of K coverage. Here, 1 ML of K is defined as the point where the spin polarization begins to saturate, indicated by a black dotted line. The solid lines in (c) and (d) serve as guides to the eye for the temperature- and K-thickness-dependent changes in spin polarization, respectively. Error bars represent the experimental uncertainty of the SRPES measurements (see SM section III for the detail).
• Figure 4: Control of transport properties of SRO via the ionic-liquid gating method. (a) Gate-voltage-dependent resistivity, $\rho_{xx}$, of SRO as a function of temperature. The gate voltage is applied from -3 V to 3 V. Blue triangles mark the kink in the curves, corresponding to the Curie temperature, $T_{\rm C}$. (Inset) Derivative of resistivity near the kink. (b) Gate-voltage-dependent $\rho_{xx}$ at 10 K and $T_{\rm C}$. (c) Schematics of a high DOS at different Fermi levels ($E_{\rm F}$), with the corresponding spin-resolved DOS. The high DOS peak positions for non-magnetic and spin-polarized SRO are marked by a green dot. $\Delta E_{ex}$ denotes the ferromagnetic exchange energy. (d) Schematic illustration of the DOS, electrons, and electronic structure in SRO. All electrons in the schematic are positioned at $E_{\rm F}$. The motion of electrons depends on the relative alignment between the high DOS position and $E_{\rm F}$, leading to changes in both ferromagnetism and conductivity of SRO.