Session P19: Magnetic Materials

Observation of Temperature Chaos in Mesoscopic Spin Glasses

Temperature Chaos (TC) results from a change in temperature for spin glasses (SG), polymers, and other glassy materials. When the temperature is changed, TC means that the new state has no memory of the preparation of the initial state. TC was predicted long ago [PRL {\bf 48}, 767 (1982)]. However, ``An experimental measurement of TC is still missing" [EPL {\bf 103}, 67003 (2013)]. One reason for this is the question of length scale. In the thermodynamic limit, even an infinitesimal temperature change, $\Delta T$, will create a chaotic condition. However, by working at the mesoscale, one can establish a length scale sufficiently small to exhibit reversible behavior before crossing over to chaotic behavior as the temperature change increases. Observation of TC is possible because, on reasonable laboratory time scales, the SG correlation length can grow to the size of the thickness of the film, $L$. The lower critical dimension for a SG is $\sim 2.5$, so that the thin film SG crosses over to a glass temperature $T_g=0$. However, there remains quasi-equilibrium SG states with length scales $ [Preview Abstract]

An Investigation of the Spin Glass Properties of the Solid Solution CuAl$_{2-2x}$Ga$_{2x}$O$_{4}$

The complete anti-ferromagnetic oxide spinel solid solution between end members CuAl$_{2}$O$_{4}$ and CuGa$_{2}$O$_{4}$ has been synthesized. The crystallographic and magnetic properties are examined as Ga is replaced with iso-valent Al. Crystallographic results show the solid solution obeys Vegard’s law, and the cation distribution among the tetrahedral and octahedral sites matches well with prior results. The evolution of the magnetic susceptibility is examined as a function of temperature and doping percentage. Evidence is presented that indicates that the majority of the solution displays paramagnetic behaviour at high temperatures and spin glass behaviour below 3K. Specifically, a freezing temperature in the AC susceptibility, irreversibility in the DC magnetization, and relaxation dynamics in the presence of a changing applied field are observed.   

Nonlinear scaling variable at the lower critical dimension: Scaling in the 2D random field Ising model

We systematically analyze the nonlinear invariant scaling variables at bifurcations in the renormalization-group flow, and apply our methods to the two-dimensional random-field Ising model (RFIM). At critical points, the universal scaling functions are usually written in terms of homogeneous invariant combinations of variables, like $ L t^{\nu}$ in the finite-size scaling form for the magnetization $M(T|L)\sim t^{-\beta} M(Lt^{\nu})$, where $t \propto T_c-T$. The renormalization-group flow for the RFIM has a pitchfork bifurcation in two dimensions, where the correlation length has been argued to diverge exponentially, $\xi \propto exp^{1/2At^2}$, leading to the invariant scaling combination $L / \xi \sim L / exp^{1/2At^2}$.  Our analysis, inspired by normal-form theory, suggests that this exponential divergence can take a richer, more general scaling form at a generic pitchfork bifurcation. We explore possible consequences for simulations.   

Response to a field of the $D=3$ Ising spin glass with Janus and JanusII dedicated computers

Using the Janus dedicated computer, and its new generation JanusII, we study the linear response to a field of the Edwards-Anderson model for times that cover twelve orders of magnitude. The fluctuation-dissipation relations are investigated for several values of $t_{\mathrm{w}}$. We observe that the violations of the fluctuation-dissipation theorem can be directly related to the $P(q)$ measured in equilibrium at finite sizes, although a simple statics-dynamics dictionary $L\leftrightarrow \xi(t_{\mathrm{w}})$ is not enough to account for the behavior at large times. We show that the equivalence can be easily restored by taking into account the growth of $\xi(t+t_{\mathrm{w}})$. Interestingly, experimental measurements of the spin glass correlation length rely precisely on the response of a spin glass to a field, although a direct relation between the measured object and the real $\xi$ has never been established. In this work, we mimic the experimental protocol with Janus data, which lets us relate the experimental $\xi$ with the length extracted from the spatial correlation function. These results allow us for the first time to make a quantitative comparison between experiments and simulations, finding a surprising good agreement with measurements in superspin glasses.   

\textbf{Extremely large magnetoresistance and magnetic logic by coupling semiconductor nonlinear transport effect and anomalous Hall Effect}.

Size limitation of silicon FET hinders the further scaling down of silicon based CPU. To solve this problem, spin based magnetic logic devices were proposed but almost all of them could not be realized experimentally except for NOT logic operation. A magnetic field controlled reconfigurable semiconductor logic using InSb was reported. However, InSb is very expensive and not compatible with the silicon technology. Based on our Si based magnetoresistance (MR) device [1], we developed a Si based reconfigurable magnetic logic device [2], which could do all four Boolean logic operations including AND, OR, NOR and NAND. By coupling nonlinear transport effect of semiconductor and anomalous Hall effect of magnetic material, we propose a PMA material based MR device with a remarkable non local MR of \textgreater 20000 {\%} at \textasciitilde 1 mT. Based on this MR device, we further developed a PMA material based magnetic logic device which could do all four Boolean logic operations. This makes it possible that magnetic material does both memory and logic. This may result in a memory-logic integrated system leading to a non von Neumann computer. [1] CH Wan, et al, Nature \textbf{477}, 304, (2011). [2] ZC Luo et al. Adv. Funct. Mater. \textbf{25}, 158, (2015).   

Effects of transverse fields on spin-valve sensor magnetic field measurements

Spin-valve sensors have become a popular magnetic sensor, used in many applications such as magnetic imaging or hard drive heads. Spin-valves are designed to measure only one component of the magnetic field. This component is determined by the composition of the spin-valve layers, the manufacturing conditions, shape anisotropy, and other design decisions. We took magnetic images of L-shaped samples in order to determine if magnetic fields transverse to the direction of measurement affect spin-valve sensors. Specifically, we used a 2 \textmu m by 4 \textmu m Cu-Mn-Ir spin-valve sensor to take image of chips with ``L-shaped'' currents. We find that transverse fields can significantly affect the measurement direction of spin-valve sensors.   

Effect of tantalum on magnetocrystalline anisotropy and tunneling magnetoresistance in MgO/CoFeB junction from ab-initio calculations

Using ab-initio calculations, we demonstrate that boron is energetically attracted toward interface between MgO and CoFeB in MgO/CoFeB junction. We show that both magnetocrystalline anisotropy (MCA) and tunneling magnetoresistance (TMR) decrease when boron is present at the interface. However, when tantalum is used as seeding or capping layer (Ta/CoFeB/MgO), the segregation profile of boron changes. Namely, the most energetically preferable position of boron is inside and near tantalum rather than at MgO/CoFeB interface. Such change of boron segregation profile results in boron diffusion from MgO/CoFeB interface toward tantalum at annealing. The diffusion of boron toward tantalum may explain the experimentally observed effect of tantalum on increase of both MCA and TMR in MgO/CoFeB junction   

Growth and properties of High-quality metal/ yttrium iron garnet/metal sandwich structures

Sandwiched structures of magnetic insulators (e.g. yttrium iron garnet or YIG) between two normal metals are potentially useful for spintronics. In this work, we report our approach of growing a single crystalline YIG thin film on a Pt or Cu thin layer using the combination of sputtering and PLD. First, either 5 nm of Pt or Cu is deposited on (110)-oriented gadolinium~gallium~garnet (GGG) substrate using sputtering and then YIG is grown by PLD at intermediate temperatures followed by rapid thermal annealing at higher temperatures. Surprisingly, YIG films show a well-defined single-crystal reflection high energy electron diffraction (RHEED) pattern, despite that they are grown on polycrystalline Pt or Cu. TEM images show flat, clean and sharp interfaces between YIG/metals and metals/GGG. The magnetic properties show in-plane magnetic anisotropy. However, when thicker metallic layers (20 nm) or amorphous (metals/SiO2) substrate are used, only YIG polycrystalline phase is observed. We will show that by properly controlling the growth conditions the metal/YIG/ metal structures are not only of high structural quality, but also have desired properties for spin current transport.   

Switching behavior of Nb/Exchange spring magnet/Nb Josephson Junctions fabricated by Nanosphere Lithography

Superconductor(S)/ferromagnet(F)/superconductor Josephson junction was fabricated by nanosphere lithography method. Samarium-Cobalt (SmCo)/Permalloy(Py) exchange spring magnet system was used to generate an inhomogeneous magnetic structure in Niobium(Nb)-based Josephson junctions. We introduced nanosphere lithography in our device fabrication in order to decrease the lateral size of junctions and improve the quality of our devices. A bigger size junctions (tens of microns) were fabricated by optical photolithography using a mask.* Materials were deposited through DC magnetron sputtering. Base structure of devices was patterned through photolithography. Modulations of the critical current and IV-curve characteristics of the junction were used to search for direct evidence of the odd-triplet component. In addition, to investigate the switching behavior of S/F/S junction for memory application junction critical current was measured as a function of magnetic field and the angle between an easy axis of ferromagnetic layer and the external magnetic field by rotating the sample under magnetic field. Magnetic switching behavior of the ferromagnetic layers in our junction was also characterized based on this observation. * Junction fabrication in this research by an optical photolithography using a mask was conducted at the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory (CNMS User Project CNMS2014-257).   

How to move domain walls in an antiferromagnet

Domain walls (DWs) in an easy-axis antiferromagnet can be driven by several stimuli: a charge current (in conducting antiferromagnets), a magnon current, and a temperature gradient. In this talk, we discuss the dynamics of a DW induced by two latter external perturbations, which are applicable in both metallic and insulating antiferromagnets. First of all, we study the Brownian dynamics of a DW subjected to a temperature gradient [1]. To this end, we derive the Langevin equation for the DW's center of mass with the aid of the fluctuation-dissipation theorem. A DW behaves as a classical massive particle immersed in a viscous medium. By considering a thermodynamic ensemble of DWs, we obtain the Fokker-Planck equation, from which we extract the average drift velocity of a DW. We briefly address other mechanisms of thermally driven DW motion. Secondly, we analyze the dynamics of a DW driven by circularly polarized magnons [2]. Magnons passing through a DW reverse their spin upon transmission, thereby transferring two quanta of angular momentum to the DW and causing it to precess. A precessing DW partially reflects magnons back to the source. The reflection of magnons creates a previously identified reactive force [3]. We point out a second mechanism of propulsion of the DW, which we term redshift: magnons passing through a precessing DW reduce their linear momentum and transfer the decrease to the DW. We solve the equations of motion for magnons in the background of a uniformly precessing DW with the aid of supersymmetric quantum mechanics and compute the net force and torque applied by magnons to the DW. The theory agrees well with micromagnetic simulations. \\ [4pt] [1] S. K. Kim, O. Tchernyshyov, and Y. Tserkovnyak, Phys. Rev. B \textbf{92}, 020402(R) (2015) \\ [0pt] [2] S. K. Kim, Y. Tserkovnyak, and O. Tchernyshyov, Phys. Rev. B \textbf{90}, 104406(E) (2014) \\ [0pt] [3] E. G. Tveten, A. Qaiumzadeh, and A. Brataas, Phys. Rev. Lett. \textbf{112}, 147204 (2014)   

Spin correlations and spin-wave excitations in Dirac-Weyl semimetals

We study correlations among magnetic dopants in three-dimensional Dirac and Weyl semimetals. Effective field theory for localized magnetic moments is derived by integrating out the itinerant electron degrees of freedom. We find that spin correlation in the spatial direction parallel to local magnetization is more rigid than that in the perpendicular direction, reflecting spin-momentum locking nature of the Dirac Hamiltonian. Such an anisotropy becomes stronger for Fermi level close to the Dirac points, due to Van Vleck paramagnetism triggered by spin-orbit coupling. One can expect topologically nontrivial spin textures under this anisotropy, such as a hedgehog around a single point, or a radial vortex around an axis, as well as a uniform ferromagnetic order. We further investigate the characteristics of spin waves in the ferromagnetic state. Spin-wave dispersion also shows a spatial anisotropy, which is less dispersed in the direction transverse to the magnetization than that in the longitudinal direction. The spin-wave dispersion anisotropy can be traced back to the rigidity and flexibility of spin correlations discussed above.