arXiv subjects

N. Vlietstra

Publications and source records attributed to N. Vlietstra.

7 recordsLinked to original sources

arXiv2020

All-electrical detection of skyrmion lattice state and chiral surface twists

We study the high-temperature phase diagram of the chiral magnetic insulator Cu$_2$OSeO$_3$ by measuring the spin-Hall magnetoresistance (SMR) in a thin Pt electrode. We find distinct changes in the phase and amplitude of the SMR signal at critical lines separating different magnetic phases of bulk Cu$_2$OSeO$_3$. The skyrmion lattice state appears as a strong dip in the SMR phase. A strong enhancement of the SMR amplitude is observed in the conical spiral state, which we explain by an additional symmetry-allowed contribution to the SMR present in non-collinear magnets. We demonstrate that the SMR can be used as an all-electrical probe of chiral surface twists and skyrmions in magnetic insulators.

cond-mat.mes-hall↗

arXiv2016

Electrical detection of spiral spin structures in Pt|Cu_2OSeO_3 heterostructures

The interaction between the itinerant spins in metals and localized spins in magnetic insulators thus far has only been explored in collinear spin systems, such as garnets. Here, we report the spin-Hall magnetoresistance (SMR) sensitive to the surface magnetization of the spin-spiral material, Cu_2OSeO_3. We experimentally demonstrate that the angular dependence of the SMR changes drastically at the transition between the helical spiral and the conical spiral phases. Furthermore, the sign and magnitude of the SMR in the conical spiral state are controlled by the cone angle. We show that this complex behaviour can be qualitatively explained within the SMR theory initially developed for collinear magnets. In addition, we studied the spin Seebeck effect (SSE), which is sensitive to the bulk magnetization. It originates from the conversion of thermally excited low-energy spin waves in the magnet, known as magnons, into the spin current in the adjacent metal contact (Pt). The SSE displays unconventional behavior where not only the magnitude but also the phase of the SSE vary with the applied magnetic field.

cond-mat.mtrl-sci↗

arXiv2015

Control of spin current by a magnetic YIG substrate in NiFe/Al nonlocal spin valves

We study the effect of a magnetic insulator (Yttrium Iron Garnet - YIG) substrate on the spin transport properties of Ni$_{80}$Fe$_{20}$/Al nonlocal spin valve (NLSV) devices. The NLSV signal on the YIG substrate is about 2 to 3 times lower than that on a non magnetic SiO$_2$ substrate, indicating that a significant fraction of the spin-current is absorbed at the Al/YIG interface. By measuring the NLSV signal for varying injector-to-detector distance and using a three dimensional spin-transport model that takes spin current absorption at the Al/YIG interface into account we obtain an effective spin-mixing conductance $G_{\uparrow\downarrow}\simeq 5 - 8\times 10^{13}~Ω^{-1}$m$^{-2}$. We also observe a small but clear modulation of the NLSV signal when rotating the YIG magnetization direction with respect to the fixed spin polarization of the spin accumulation in the Al. Spin relaxation due to thermal magnons or roughness of the YIG surface may be responsible for the observed small modulation of the NLSV signal.

cond-mat.mes-hall↗

arXiv2014

Simultaneous detection of the spin-Hall magnetoresistance and the spin-Seebeck effect in Platinum and Tantalum on Yttrium Iron Garnet

The spin-Seebeck effect (SSE) in platinum (Pt) and tantalum (Ta) on yttrium iron garnet (YIG) has been investigated by both externally heating the sample (using an on-chip Pt heater on top of the device) as well as by current-induced heating. For SSE measurements, external heating is the most common method to obtain clear signals. Here we show that also by current-induced heating it is possible to directly observe the SSE, separate from the also present spin-Hall magnetoresistance (SMR) signal, by using a lock-in detection technique. Using this measurement technique, the presence of additional 2nd order signals at low applied magnetic fields and high heating currents is revealed. These signals are caused by current-induced magnetic fields (Oersted fields) generated by the used AC-current, resulting in dynamic SMR signals.

cond-mat.mes-hall↗

arXiv2013

Exchange magnetic field torques in YIG/Pt bilayers observed by the spin-Hall magnetoresistance

The effective field torque of an yttrium-iron-garnet film on the spin accumulation in an attached Pt film is measured by the spin-Hall magnetoresistance (SMR). As a result, the magnetization direction of a ferromagnetic insulating layer can be measured electrically. Experimental transverse and longitudinal resistances are well described by the theoretical model of SMR in terms of the direct and inverse spin-Hall effect, for different Pt thicknesses [3, 4, 8 and 35nm]. Adopting a spin-Hall angle of Pt $θ_{SH}=0.08$, we obtain the spin diffusion length of Pt ($λ=1.1\pm0.3$nm) as well as the real ($G_r=(7\pm3)\times10^{14}Ω^{-1}$m$^{-2}$) and imaginary part ($G_i=(5\pm3)\times10^{13}Ω^{-1}$m$^{-2}$) of the spin-mixing conductance and their ratio ($G_r/G_i=16\pm4$).

cond-mat.mes-hall↗

arXiv2013

Spin-Hall Magnetoresistance in Platinum on Yttrium Iron Garnet: Dependence on platinum thickness and in-plane/out-of-plane magnetization

The occurrence of Spin-Hall Magnetoresistance (SMR) in platinum (Pt) on top of yttrium iron garnet (YIG) has been investigated, for both in-plane and out-of-plane applied magnetic fields and for different Pt thicknesses [3, 4, 8 and 35nm]. Our experiments show that the SMR signal directly depends on the in-plane and out-of-plane magnetization directions of the YIG. This confirms the theoretical description, where the SMR occurs due to the interplay of spin-orbit interaction in the Pt and spin-mixing at the YIG/Pt interface. Additionally, the sensitivity of the SMR and spin pumping signals on the YIG/Pt interface conditions is shown by comparing two different deposition techniques (e-beam evaporation and dc sputtering).

cond-mat.mes-hall↗

arXiv2013

YIG thickness and frequency dependence of the spin-charge current conversion in YIG/Pt systems

We report the frequency dependence of the spin current emission in a hybrid ferrimagnetic insulator/normal metal system as function of the insulating layer thickness. The system is based on a yttrium iron garnet (YIG) film [0.2, 1, and 3 μm] grown by liquid-phase-epitaxy coupled with a spin current detector of platinum [6 nm]. A strong YIG thickness dependence of the efficiency of the spin pumping has been observed. The highest conversion factor ΔV/P_{abs} has been demonstrated for the thinner YIG (1.79 and 0.55 mV/mW^{-1} at 2.5 and 10 GHz, respectively) which presents an interest for the realisation of YIG-based devices. A strong YIG thickness dependence of the efficiency of the spin pumping has been also observed and we demonstrate the threshold frequency dependence of the three-magnon splitting process.

cond-mat.mtrl-sci↗