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. 2016 Aug 25;7:12566.
doi: 10.1038/ncomms12566.

Emergent Nanoscale Superparamagnetism at Oxide Interfaces

Free PMC article

Emergent Nanoscale Superparamagnetism at Oxide Interfaces

Y Anahory et al. Nat Commun. .
Free PMC article


Atomically sharp oxide heterostructures exhibit a range of novel physical phenomena that are absent in the parent compounds. A prominent example is the appearance of highly conducting and superconducting states at the interface between LaAlO3 and SrTiO3. Here we report an emergent phenomenon at the LaMnO3/SrTiO3 interface where an antiferromagnetic Mott insulator abruptly transforms into a nanoscale inhomogeneous magnetic state. Upon increasing the thickness of LaMnO3, our scanning nanoSQUID-on-tip microscopy shows spontaneous formation of isolated magnetic nanoislands, which display thermally activated moment reversals in response to an in-plane magnetic field. The observed superparamagnetic state manifests the emergence of thermodynamic electronic phase separation in which metallic ferromagnetic islands nucleate in an insulating antiferromagnetic matrix. We derive a model that captures the sharp onset and the thickness dependence of the magnetization. Our model suggests that a nearby superparamagnetic-ferromagnetic transition can be gate tuned, holding potential for applications in magnetic storage and spintronics.


Figure 1
Figure 1. ZFC magnetic structure of LMO/STO of different thicknesses.
(af) 1.5 × 1.5 μm2 scanning SOT images of the magnetic field Bz(x,y) of six samples of different thicknesses N=4 to 200 u.c. at ∼100 nm above the surface acquired at 4.2 K. Scale bar is 200 nm. Note that in (a) through (f) the color scale changes by more than three orders of magnitude. (g) Thickness dependence of the r.m.s. value formula image in scanning SOT images (red), global magnetization saturation value Ms (blue), and theoretically calculated Ms (black) which takes into account the SPM and the AFM canting contributions. The inset shows the peak-to-peak field variation Bzptp in SOT images versus N near Nc=5. (h) Schematic scanning SOT microscopy setup.
Figure 2
Figure 2. Field-driven and temporal magnetization reversal dynamics in LMO/STO.
(a,b) Two consecutive 1.5 × 1.5 μm2 images of Bz(x,y) (see Supplementary Movie 2) in an N=12 u.c. sample after a field excursion to formula image (a) and 161 mT (b). (c) Differential image ΔBz(x,y) obtained by direct subtraction of (a) from (b) revealing magnetization reversal events of isolated SPM nanoscale islands (see Supplementary Movie 3). Note an order of magnitude enhanced colour scale. (d) Numerical fit to ΔBz(x,y) in (c) with four in-plane oriented islands using SOT diameter of 104 nm, scan height of 105 nm, and Ne=2 u.c. with resulting magnetic moments m1=1.0 × 105 μB (D1=40 nm), m2=2.6 × 104 μB (D2=20 nm), m3=4.7 × 104 μB (D3=28 nm), and m4=6.8 × 104 μB (D4=33 nm). Scale bar is 200 nm. (e) ΔBz(x,y) image in N=6 u.c. sample showing the thermally activated magnetization reversals of the islands at a constant formula image mT at t=1,140 s after the field ramp (see Supplementary Movie 6). (f) Same as (e) at t=5,320 s. (g) The magnetization relaxation rate dM/dt attained by vectorial summation of the reversal events m in each frame of Supplementary Movie 6. Dotted line is a guide to the eye.
Figure 3
Figure 3. Saturated magnetic state and magnetization reversal process in N=8 unit cells sample.
(ad) Bz(x,y) images of 1.3 × 1.4 μm2 after ZFC (a), after excursion to formula image mT (b), after excursion to formula image mT (c), and in the presence of formula image mT (enhancing instrumental noise) (d). The dark arrows show the formula image direction in each image. Note the strong anti-correlation between (b,c) indicating the full magnetization reversal of the SPM islands. The differences in the intensity in (bd) are due to a drift in the scanning height of the SOT. (eh) Numerical simulations of Bz(x,y) 105 nm above the sample arising from randomly positioned non-overlapping SPM islands at the top and bottom surfaces of the 8 u.c. film using the measured m distribution in (j). (e) ZFC state with random magnetization orientation of SPM islands along x and y easy axes. Scale bar is 200 nm. (fh) Same as (e) with all the moments oriented with equal probability either along −x or −y (f), along +x or +y (g), and only along +x (h) directions. (i) x−y locations of the SPM reversal events with the colour referring to the field formula image at which the reversal occurred (see Supplementary Movie 4). (j) Histogram of the magnetic moments m of the SPM reversals in (i) and the corresponding island diameters D (top axis) using Ne=2 u.c. (k) Cumulative magnetic moment M versus applied field (red) attained by vectorial summation of all the reversal events m after sweeping the field from large negative value. The dashed line shows corresponding schematic reconstruction of M(H). Global M(H) measurement (blue, after subtraction of bare STO M(H)) normalized by the total sample area relative to the imaged area.
Figure 4
Figure 4. Magnetization per Mn atom versus film thickness.
(a) Saturation magnetization derived from global measurements normalized by the number of Mn atoms in the entire thickness N of the films (green), the theoretically calculated average magnetization per Mn atom taking into account the SPM and the AFM canting contributions (red), and the average magnetization assuming a dead layer of Nc=5 u.c. and 0.85 μB per Mn in the rest of NNc live layers (dotted). (b) Saturation magnetization derived from global measurements like in (a) but normalized by the number of Mn atoms in the NNc live layers assuming zero magnetization in the Nc dead layers (blue). The dotted line shows magnetization of 3.5 μB per Mn in the live layers as expected in highly doped LMO.
Figure 5
Figure 5. Theoretical model results and schematics.
(a) Schematics of core Mn3+ spins in A-type AFM configuration (black arrows) in LMO at NNc=5. (b) For high electron concentrations at the LMO/STO interface, a metallic layer of thickness Ne with uniform FM order (red arrows) should be formed at the interface. (c) Theoretically derived phase separated state for N>Nc with SPM islands embedded in AFM matrix. (d) Schematic representation of hole-doped and electron-doped SPM islands in the case of electron transfer from Mn3+ orbitals at the top surface of LMO. (e) Calculated excess charge transfer per unit area q(N)=0.5(1−Nc/N) (blue) and the thickness of electron charge layer Ne (red) versus LMO thickness N. (f) The diameter (blue) and the magnetic moment (red) of the SPM islands versus N in the phase separated state. (g) The areal fraction of the SPM islands versus N.

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