Table Of ContentMagnetic Diode at T = 300 K
B. Summers1, A. Dahal1, P. Kampschroeder1, J. Gunasekera1, and D.K. Singh1,∗
1Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211 and
∗email: [email protected]
Wereportthefindingofunidirectionalelectronicproperties,analogoustoasemiconductordiode,
intwo-dimensionalartificialpermalloyhoneycomblatticeofultra-smallbond,withatypicallength
of (cid:39) 12 nm. The unidirectional transport behavior, characterized by the asymmetric colossal en-
hancement in differential conductivity at a modest current application of (cid:39)10-15 µA, persists to
T = 300 K in honeycomb lattice of thickness (cid:39) 6 nm. The asymmetric behavior arises without
the application of magnetic field. A qualitative analysis of experimental data suggests the role of
7
1 magnetic charge or monopoles in the unusual observations with strong implication for spintronics.
0
2
Semiconductor diode and transistor are key elements the length of the element due to magnetostatic inter-
b
in modern electronic devices. The substitution of a action and shape anisotropy.[10, 15] Consequently, two
e
F semiconductor by a magnetic system with resembling types of local spin configurations emerge: all moments
properties, such as an unidirectional biasing of electric either point to or away from a vertex of the honeycomb,
4
current, is expected to enhance the functionalities of also called ’all-in or all-out’ configuration or, two mo-
] the device significantly. It can play the dual role of ments pointing in and one point away from the vertex
l
l the reprogrammable logic and the non-volatile memory (or vice-versa), also called ’two-in and one-out’ (or vice-
a
element.[1–3] Multilayered magnetic devices are known versa)configuration.[10,16]Thelatterspinarrangement
h
to exhibit a very high level of unidirectional spin polar- is also termed as the spin-ice configuration, which re-
-
s ization in applied magnetic field, which can be utilized sults in a net magnetic charge of ±1 unit to a given ver-
e
m to create a magnetic diode with unidirectional current tex. We have created new artificial honeycomb lattice
biasing properties.[2] More recently, synergistic experi- of connecting ultra-small bonds with a typical size of 12
.
t mental and theoretical research efforts on the develop- nm in length and approximately 5 nm in width, see Fig.
a
m ment of a magnetic diode have led to the development 1a-b. Using detailed magnetic, electrical and neutron
of new spintronics devices that includes the design of a scattering measurements on a macroscopic size sample
-
d nanomagnetic logic device, bipolar spin diodes and the of thin (t(cid:39)5 nm) artificial honeycomb lattice, previously
n unipolar spin diode across a magnetic domain wall.[3–8] we demonstrated the temperature dependent evolution
o
However, an application of magnetic field is required in of theoretically predicted novel magnetism that includes
c
[ mostcases. Amagneticsystem,whichcanexhibitunidi- aparamagneticspingasathightemperaturefollowedby
rectionalcurrentbiasingatamodestcurrent(resultingin the development of short-range ordered spin ice state at
2
reasonably small output power) without the application intermediatetemperature,30K<T <220K-250K,and
v
of magnetic field, is still elusive. Additionally, any such finally a tendency to develop the spin solid state, man-
5
5 device must also demonstrate the operational ability at ifested by the distribution of chiral vortex states, below
4 room temperature for practical applications. T = 30 K.[12, 13, 17]
8
0
In this letter, we report the observation of unidi- Measurements of differential conductivity on newly
.
1 rectional current biasing, analogous to a semiconduc- fabricated artificial honeycomb lattice of varying thick-
0 tor diode, in the electronic measurements on artificial nesses have revealed electronic properties that are remi-
7
permalloy honeycomb lattice. The unidirectional elec- niscentofasemiconductordiode. Itisfoundthatthedif-
1
tronic properties arise without the necessity of magnetic ferential conductivity increases by more than two orders
:
v field application. The phenomenological observation of of magnitude for a modest unidirectional current appli-
i
X diode-type function in artificial honeycomb lattice possi- cationof(cid:39)10µA,comparedtothenegligiblevaluenear
bly hints of new and unexplored properties of magnetic zero bias current. Electronic measurement for the oppo-
r
a charges (or monopoles) that are argued to exist in this sitely directed current yields a very small or negligible
system.[9, 10] The design of two-dimensional artificial differential conductivity. The temperature dependence
honeycomb lattice was originally envisaged to comple- of unidirectional biasing strongly depends on the thick-
ment the study of spin ice phenomenon and associated ness of the lattice. For instance, in a very thin lattice,
magnetic monopoles in three dimensional geometrically t(cid:39) 2 nm, the unidirectional biasing is nearly absent. As
frustrated magnets.[9, 11] Since then, it has evolved in a the thickness of the honeycomb lattice increases to t(cid:39) 4
new research arena to explore novel magnetic and elec- nm, this phenomenon starts developing as temperature
tronicproperties.[12–14]Foramoderateaspectratio,de- increasesandbecomesverypronouncedinatemperature
fined by l (length) /t (thickness), of the connecting ele- range of 30 K < T <250 K; incidentally, the same tem-
ment of the honeycomb, the magnetic moment lies along perature regime where the short ranged ordered spin ice
2
usual observations in artificial honeycomb lattice.
(a) (b)
Artificial honeycomb lattice of ultra-small bond was
fabricatedusingacombinationofdiblockcopolymertem-
platesynthesis, reactiveionetchingandmaterialdeposi-
tionatnearparallelangletoachievethetwo-dimensional
characteristic of the system. For the synthesis of diblock
template,weutilizedadiblockcopolymerpolystyrene-b-
poly-4-vinyl pyridine (PS-b-P4VP) of molecular weight
23 K Dalton. It generates a large uniform hexagonal
nanoporous template with the pore dimeter of (cid:39) 12 nm
(c)
and a lattice spacing of (cid:39) 27 nm. Details about the
synthesis of diblock template can be found somewhere
else.[18]Next,thediblocktemplatewasusedastheetch-
ing mask to transfer the hexagonal pattern to the un-
derlying silicon substrate using CF -based reactive ion
4
etching technique. A thin layer of diblock porous tem-
(d)
plate, 4-5 nm thick, still remained present on top of sili-
con substrate. The top layer of the substrate resembled
a honeycomb lattice pattern. This topographical prop-
erty was exploited to create metallic honeycomb lattice
by depositing permalloy, Ni Fe , in near parallel con-
0.8 0.2
figuration (within 1o degree) using a new sample holder,
(e)
specifically designed to limit the deposition to the top
of the substrate only, in an electron-beam evaporator.
The substrate was rotated at a moderate constant speed
aboutitsaxisduringthedepositionprocesstocreateuni-
formity in the film thickness. Atomic force micrographs
of typical artificial honeycomb lattice, fabricated using
(f) technique, are shown in Fig. 1a-b. Electronic measure-
ments were performed using a synchronized combination
of Keithley current source meter 6221 and a nanovolt-
meter 2182A, via a trigger link, on (cid:39) 8 mm × 6 mm
size samples. For the measurement purposes, electrical
contactsweremadeinthefour-probeconfigurationusing
commercially available silver paint. Electronic measure-
mentsinvolvedtheaveragingoftwentydatapointswhere
FIG. 1: Atomic force micrographs of artificial honeycomb
eachdatawasfilteredfor20secondsbeforerecordingthe
lattice and electronic properties measurement data. (a-b)
Atomicforcemicrographsoftypicalartificialhoneycomblat- finalvalue. Sampleswerealwaysstoredinavacuumcon-
tice, after reactive ion etching and material deposition. (c- tainer.
f) Plot of dI/dV as a function of current bias, obtained on In Fig. 1c-f, we plot differential conductivity, dI/dV,
a honeycomb lattice of thickness t (cid:39) 4 nm, at few charac-
results on a honeycomb lattice of thickness t (cid:39) 4 nm as
teristic temperatures. A colossal enhancement in differential
a function of applied current bias at few characteristic
conductivity at I (cid:39) 10 µA compared to the zero bias value,
temperatures. Two features are immediately observed
is observed at T = 30 K. As temperature increases, the en-
hancement in differential conductivity becomes highly asym- in the differential conductivity plot at T = 30 K: first,
metric, suggestingasemiconductordiode-typeunidirectional dI/dV increases by more than two orders of magnitude
electronic properties. (dI/dV) tends to become symmetric withrespecttothezero-biasvalueatthecurrentapplica-
again as T → 300 K, albeit weaker. tionof(cid:39)10µAandsecond,thecolossalgainindifferen-
tial conductivity starts developing asymmetric character
statewasreportedtoexistinahoneycomblatticeofsimi- withrespecttothecurrentbiasdirection. Theasymmet-
larthickness.[17]Atthelatticethicknessoft(cid:39)6nm,the ric behavior becomes prevalent as temperature increases
asymmetric behavior is found to persist to T = 300 K. above T = 30 K. As shown in Fig. 1d, the measurement
For further increase in the thickness of the lattice, uni- at T = 50 K manifest drastically different electronic re-
directional conductance gradually weaken and becomes sponses to oppositely directed current biases. Compared
indistinguishable from the flat background conductivity. to the very small change in dI/dV for negative current
Qualitative analysis of the experimental data suggests bias, a colossal enhancement in differential conductivity
thepossibleroleofinter-elementalenergybehindtheun- isdetectedforpositivecurrentbias. Itindicatesanunidi-
3
FIG.3: Magneticfieldeffectonelectronicproperties. Herewe
FIG.2: Persistenceofdiode-typeelectronicbehaviortoroom
showexperimentalresultsonthehoneycomblatticeofthick-
temperature in t (cid:39) 6 nm thick artificial honeycomb lattice.
nesst(cid:39)4nm,asitexhibitsbothsymmetricandasymmetric
(a) Measurement of differential conductivity in honeycomb
current biasing as a function of temperature. (a) At H = 5
lattice exhibit unidirectional electronic transport to T = 30
T, dI/dV decreases at high current bias at T = 30 K. Yet, it
K.(b)AstemperatureincreasestoT =200K,theasymmet-
maintainsthenear-symmetriccharacter. (b-c)Thefieldeffect
ric behavior weaken. (c) Interestingly enough, the unidirec-
is very pronounced at higher temperatures. A field applica-
tionalelectronicpropertiespersiststoT =300K;albeitwith
tion of H = 5 T strongly suppresses the asymmetric nature
opposite current bias polarity.
of electronic transport and becomes almost symmetric.
rectional electronic transport in the artificial honeycomb on t (cid:39) 6 nm thick artificial honeycomb lattice depict the
lattice of ultra-small bond, which becomes significantly persistenceofstrongunidirectionaltransporttoT =300
pronounced as temperature increases to T = 200 K (see K, for oppositely directed current compared to t (cid:39) 4
Fig. 1e). Such an unidirectional electronic transport nm thick sample. Interestingly, the system tends to pre-
is the hallmark of a nonmagnetic semiconductor diode. serve the unidirectional conductivity enhancement even
Electronicmeasurementswerealsoperformedforreverse at T = 30 K. However, the current bias direction is re-
current sweep to check for possible hysteresis in the con- versed between T = 300 K and 30 K. The changeover
ductivity data. But no such behavior was detected. The occursaroundT =200Kwhereverylittlechangeindif-
asymmetric behavior tend to become symmetric again, ferential conductivity, with respect to the near zero bias
albeit weakly, as T → 300 K. value,isobserved. Measurementswererepeatedmultiple
Increasing the thickness of the honeycomb lattice pro- timesandonmultiplesamplestoverifythereproducibil-
foundly impacts the temperature dependence of the uni- ityofunidirectionalelectronictransportinartificialhon-
directional current biasing. A detailed study was carried eycomb lattice. The asymmetric current biasing gradu-
outinthisregardbysystematicallyvaryingthethickness allydisappearsasthicknessofthelatticeincreasesabove
of the lattice between (cid:39) 2 nm to 9 nm. Differential con- t ≥ 8 nm (see Fig. S1 in the Supplementary Materials).
ductivity data, obtained on honeycomb lattice of vary- Next, we have investigated magnetic field effect on
ing thicknesses, are shown in Fig. S1 in the Supplemen- the electronic properties of artificial honeycomb lattice.
tary Materials. It is found that increasing the thickness Magnetic field was applied in-plane to the sample. In
of the lattice progresses the onset of the unidirectional Fig. 3, we plot the differential conductivity data, ob-
electronic transport to lower temperature in the system. tainedonahoneycombsampleofcharacteristicthickness
Also,noasymmetricenhancementindifferentialconduc- t (cid:39) 4 nm, in applied fields of H = 0 and 5 T. At a field
tivity is observed at any temperature in very thin, (cid:39) 2 application of H = 5 T at T = 30 K, dI/dV decreases
nm, honeycomb lattice. Most surprising behavior, how- significantlyathighercurrentbias,comparedtothezero
ever, is observed in honeycomb sample of intermediate field value. Yet, the symmetric nature of conductivity
thickness. As shown in Fig. 2, electronic measurements biasingismaintained. Amoredramaticfieldeffectisob-
4
served at intermediate temperature, 30 K <T < 250 K, artificial honeycomb lattice.
where the system tends to exhibit strong unidirectional Our qualitative explanation of the unidirectional elec-
electronic transport in zero field. Measurement at H = tronic transport in honeycomb lattice does not explain
5T at T = 150 K show that the asymmetric behavior the persistence of asymmetric biasing to low tempera-
isstronglysuppressedanddifferentialconductivitytends ture in t (cid:39) 6 nm thick honeycomb lattice or, the ab-
to become symmetric. A similar behavior is observed sence of this unusual behavior in very thin, t (cid:39) 2 nm,
in the electronic measurements at T = 200 K. Similar lattice (see Fig. S1). It can be argued that as the hon-
fielddependenceofdifferentialconductivityareobserved eycomb lattice becomes thicker, the gain in the inter-
in other samples. While the field dependence of dI/dV elemental energy prohibits the development of the zero
confirms the role of magnetism in the electronics trans- entropy density spin solid state. Therefore, the system
port measurements, a lower conductivity in applied field remainslockedinthespinicestateevenatlowtempera-
is somewhat surprising. At such a high field, magnetic ture and exhibits the asymmetric current biasing. How-
moment will tend to align along the field direction and ever, thesamesampleexhibitsareversalintheunidirec-
thus creates an effective ferromagnetic (FM) type struc- tionaltransportpropertyastemperaturereducesformT
ture. In general, a ferromagnetic configuration gives rise = 300 K to 30 K. Similarly, a thinner honeycomb lattice
to lower resistance (or higher conductivity) in electronic corresponds to a smaller inter-elemental energy. Hence,
measurements.[19] Further research works, both theoret- the onset temperature to the spin ice phase will be re-
ical and experimental, are desirable to understand the duced in this sample. However, no evidence of unidirec-
field dependence of electronic properties in this system. tional transport is detected at any temperature. Further
By drawing analogy with the current carrying charac- research works are needed to fully understand it.
teristic of a p-n junction semiconductor diode where the In essence, electronic measurements on newly fabri-
currentiscontrolledtoflowinthedesireddirectionbyus- cated artificial honeycomb lattice have revealed interest-
ingp-orn-typedopantsinasemiconductor,itcanbear- ing, yet highly anomalous, properties with strong impli-
gued that magnetic equivalent of electric charge carriers cation for practical applications. It is found that the
(electron and hole) must exist. Previously, we demon- unidirectional colossal enhancement in differential con-
strated the evolution of various magnetic phases as a ductivity can persist to room temperature at the right
functionoftemperatureinanartificialhoneycomblattice thickness of the artificial honeycomb lattice. Most im-
(t(cid:39) 5 nm). Itwas found that the honeycomb latticeun- portantly,theinitiationofunidirectionalelectronictrans-
dergoesatransitionfromparamagneticorspingasphase port does not require magnetic field application. These
to the short-range ordered spin ice phase below T (cid:39) 250 two properties, the persistence of unidirectional trans-
K. At low temperature, T ≤ 30 K, the system tends to port to T = 300 K for a modest current application (re-
developaspinsolidstate, manifestedbythedistribution sulting in small output power) and the absence of the
of chiral vortex states of opposite polarities. The devel- necessity of magnetic field application, makes it highly
opment of spin solid state becomes pronounced as T → attractiveforpracticalapplicationinspintronicsdevices.
0 K.[17] In the spin solid state at low temperature, the We thank G. Vignale for helpful discussion. The re-
vortex circulation of magnetization does not exhibit any search at MU was supported by the U.S. Department of
preferentialdirection. Incidentally,thereisanone-to-one Energy, OfficeofBasicEnergySciencesunderGrantNo.
correspondence between the occurrence of unidirectional de-sc0014461.
electronic transport, in a similar honeycomb lattice (t(cid:39)
4 nm), and the spin ice phase. Since conduction elec-
trons follow the magnetization direction,[20] the nearly
symmetric(orweaklyasymmetric)colossalenhancement
in differential conductivity at low temperature (T ≤ 30 [1] S. A. Wolf et al., Science 294, 1488 (2001)
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[3] M. E. Flatte and G. Vignale, Appl. Phys. Lett. 78, 1273
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(2001).
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[4] I. Zutic, J. Fabian and S. Das Sarma, Rev. Mod. Phys.
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[7] D.Bhowmik,L.YouandS.Salahuddin,Nature Nano 9,
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59 (2014).
artificial honeycomb lattice have demonstrated the exis-
[8] J. Fabian, I. Zutic and S. Das Sarma, Phys. Rev. B 66,
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5
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