Probing ice-rule-breaking transition in $\rm{Dy_2Ti_2O_7}$ thin film by proximitized transport and magnetic torque

TL;DR

The study addresses whether the spin-ice state in Dy2Ti2O7 persists in thin-film form and demonstrates a proximitized transport approach using a conductive Bi2Ir2O7 cap to sense ice-rule-breaking transitions. An ~18 nm DTO film on YSZ with a ~3 nm BIO cap is grown epitaxially, with CTM providing a bulk-sensitive comparison to the BIO resistivity measurements. From the phase behavior and field-angle dependence, the authors extract an effective nearest-neighbor interaction $J_{eff}=1.054$ K and a spin-axis distortion parameter $\b\epsilon=+0.051$, indicating a tilt of the Ising axes and a modest modification of interactions due to strain and lattice expansion. The results validate proximitized transport as a robust probe for insulating frustrated magnets in thin films and suggest the spin-ice manifold can be preserved under epitaxial constraints, enabling thin-film engineering of spin-ice physics.

Abstract

While the spin ice state of bulk pyrochlores such as $\rm{Dy_2Ti_2O_7}$ and $\rm{Ho_2Ti_2O_7}$ has been extensively studied in the last several decades due to its unique degenerate ground state and emergent monopole excitation, whether it survives in the thin-film form remains a mystery. The limited volume of thin-film sample makes it challenging to study the intrinsic magnetic properties. Here, we synthesized 18nm-thick $\rm{Dy_2Ti_2O_7}$ thin film on YSZ (Yttria-stabilized Zirconia with 9.5 mol% $\rm{Y_2O_3}$) substrate and capped it by a thin conductive $\rm{Bi_2Ir_2O_7}$ layer, and performed the proximitized magnetoresistance measurements. Our study found that the ice-rule-breaking phase transition survives but with a modified effective nearest-neighbor interaction ($\rm{J_{eff}}=$ 1.054 K) and distorted Ising spin axes ($\rmε=+0.051)$ compared to the bulk crystal. The results are supported by the simultaneously measured capacitive torque magnetometry. Our study demonstrates that proximitized transport is an effective tool for thin films of insulating frustrated magnets.

Figures (3)

• Figure 1: (a): Schematic diagram of the BIO/DTO bilayer on the YSZ substrate and the proximitized transport measurement configuration. The current is along [1-10] axis, and the magnetic field is in (1-10) plane and always perpendicular to the current. (b): The measured BIO resistivity as a function of the field angle in (1-10) plane at different applied magnetic field at 0.03 K.
• Figure 2: (a): The measured capacitance change $\rm{C(B)-C(0~T)}$ and BIO resistivity as a function of angle in (1-10) plane with a 10 T applied magnetic field at 0.03 K (blue line is capacitance change and green line is resistivity). The filled colors highlight the different phases. The phase boundary is defined at the position of the resistivity peaks. (b): The field dependence results at 0.03 K with the magnetic field along different directions around [111] axis in the (1-10) plane. The $\rm{\theta_c}$ values are denoted in the middle panel. Top panel: Effective magnetic moment vs. magnetic field. Middle panel: Derivative of effective magnetic moment vs. magnetic field. Bottom panel: Magnetoresistance (MR) vs. magnetic field (vertical offsets are applied on the curves for clarity). The black curve is the results when the magnetic field is applied along [111] axis. The vertical lines represent peaks/dips of effective magnetic moment derivatives for comparison with the MR anomaly.
• Figure 3: (a): Green dots represent the critical angles derived from the peak of angle-dependent BIO resistivity (Fig. \ref{['fig1']}(b)). Blue dots represent the critical field extracted from the MR anomaly in Fig. \ref{['fig3']}(b) (bottom) by fitting exponentially modified Gaussian distribution. Examples of the fitting are illustrated in Fig. S1 supplemental and Fig. S2 supplemental. The brown dots represent the critical field which is extracted from the peaks/dips in the derivative of effective magnetic moment in Fig. \ref{['fig3']}(b) (middle). The red line is a guide to the eyes for illustrating the phase boundary. (b): Green dots represent the critical angles derived from the peak of angle-dependent BIO resistivity at different fields (Fig. \ref{['fig1']}(b)). Orange line represents the theoretical phase boundary of the DTO bulk by using Eq. \ref{['eq1']} with parameters $\rm{J_{eff}}=$ 1.01 K and $\mathrm{B_{dem}}=$ 0.88 T. Brown line is obtained by increasing $\rm{J_{eff}}$ by a factor of 2. Black curve represents the calculated phase boundary by using Eq. \ref{['eq2']} with parameters $\rm{J_{eff}}=$ 1.054 K, $\rm{\epsilon}=$ +0.051 and $\mathrm{B_{dem}}=$ 0.88 T. (c): Schematic that shows the off-center of the Ising spin axes. The coordinate of the tetrahedron corners are: $\rm{A_0(-1,-1,-1), A_1(1,-1,1), A_2(-1,1,1), A_3(1,1,-1)}$.