Semiconductive and Photoconductive Properties of the Single Molecule Magnets Mn12-Acetate and Fe8Br8

7 Pages·0.38 MB·English

by  J. M. North

Semiconductive and Photoconductive Properties of the Single Molecule Magnets
Mn -Acetate and Fe Br
12 8 8
J. M. North, D. Zipse, and N. S. Dalal
Department of Chemistry and Biochemistry, and National High Magnetic Field Laboratory,
Florida State University, Tallahassee, Florida 32306-4390, USA
3
0 E. S. Choi, E. Jobiliong, J. S. Brooks, D. L. Eaton
0 Department of Physics, and National High Magnetic Field Laboratory,
2 Florida State University, Tallahassee, Florida 32306-4390, USA

n
a Resistivity measurements are reported for single crystals of Mn12-Acetate and Fe8Br8. Both
J materialsexhibitasemiconductor-like,thermallyactivatedbehavioroverthe200-300Krange. The
7 activation energy, Ea, obtained for Mn12-Acetate was 0.37 ± 0.05 eV, which is to be contrasted
2 with the value of 0.55 eV deduced from the earlier reported absorption edge measurements and
the range of 0.3-1 eV from intramolecular density of states calculations, assuming 2Ea= Eg, the
] optical band gap. For Fe8Br8, Ea was measured as 0.73 ± 0.1 eV, and is discussed in light of the
i availableapproximatebandstructurecalculations. Someplausiblepathwaysareindicatedbasedon
c
s the crystal structures of both lattices. For Mn12-Acetate, we also measured photoconductivity in
- the visible range; the conductivity increased by a factor of about eight on increasing the photon
l
r energy from 632.8 nm (red) to 488 nm (blue). X-ray irradiation increased the resistivity, but Ea
mt was insensitive to exposure.
.
t
a I. INTRODUCTION materials with a unique configuration of large molec-
m
ular units containing transition metal ions and polar-
- izable subunits, nested in a bridging network. One
d The magnetic molecules
might thus expect some sort of semiconducting behav-
n [Mn O (CH COO) (H O) ]·2CH COOH·4H O,
12 12 3 16 2 4 3 2 ior, albeit with high resistivity. Interestingly, informa-
o abbreviated Mn -Ac1, and [(C H N ) Fe (µ -O) (µ -
c OH) ]Br (H O)1B2r·8H O, in sh6ort15Fe3B6r 28, h3ave2bee2n tion about the electrical conductivity is not yet avail-
[ 12 7 2 2 8 8 able for either compound, despite the fact that Mn -Ac
the focus of extensive studies since it was discovered and Fe Br have been studied by dielectric relaxati1o2n16,
1 that they exhibit the rare phenomenon of macroscopic far-infr8ared8 absorption under applied magnetic fields17,
5v quantum tunneling (MQT)3,4,5. As has now been Raman scattering18,19,20, micro-Hall techniques21,22,
1 well established3,4,5,6,7,8,9,10,11,12,13,14,15, both of these micro-SQUIDmagnetometry13,23,EPR7,8,24,25,26,27,28,29,
5 compounds have a net total spin S = 10, and can NMR30,31,32,33,34,35,36, specific heat37,38,39 and magneti-
1 be grown as high quality single crystals1,2,6,7,8. The zation measurements3,4,5,6, neutron scattering40,41,42,43,
0 evidence for MQT consisted of the following obser- and optical absorption44. We note, however, that from
3 vations: (a) below a certain temperature, known as optical absorption measurements Oppenheimer et al.44
0 the blocking temperature, T (2.7 K for Mn -Ac
B 12 havededucedopticalexcitationbandgaps,E ,of1.1and
/ g
at and 1 K for Fe8Br8), their magnetic hysteresis loops 1.75 eV for the minority (inner tetrahedron) and major-
exhibited sharp steps at regular intervals (about 0.46
m ity(crown)spinsystemsinMn -Ac,respectively. These
12
tesla (T) for Mn -Ac, and 0.24 T for Fe Br ), when
12 8 8 values were considered comparable with corresponding
- the field was applied along the easy axes3,4,5,6, and (b)
d theoretical estimates of 0.45 and 2.08 eV by Pederson
n the magnetization relaxation rate became temperature and Khanna45 and of 0.85 and 1.10 eV by Zeng et al.46.
o independent at low temperatures3,4,5,6. Furthermore,
In the present investigation we have carried out electri-
c this quantized hysteretic behavior was found also for
calconductivity measurementsonsingle crystalsof both
v: verydilutesamples,suchasfrozenintoorganicsolvents9. Mn -Ac and Fe Br over the temperature range of 77
12 8 8
i This observation implies that the hysteresis loop is a
X K to 300 K. Confirmatory measurements were made us-
property of every single molecule, rather than that of ing ac dielectric techniques. The results show that both
r a macroscopic domain, hence they have been described
a compounds exhibit fairly clear semiconducting behavior
as single molecule magnets (SMM’s)10. It can thus
(200-300K)withdistinctlydifferenttransportactivation
be expected that these SMM’s hold the potential for
energies. It should be noted that in an intrinsic semi-
becoming an integral part of a molecular-size memory
conductor, the activation energy (or gap) measured via
device9. Mn -Ac has also been proposed as a potential
12 conductivityistobecomparedwith1/2E ,andthusfor
candidate for a quantum computing element11. g
Mn -Ac the agreement with the optical data are satis-
12
To advance our understanding of these materials for factory.
possibleapplications,itisimportanttounderstandtheir We also describe photoconductivity over the visible
electrical conductivity behavior. Although these materi- range and X-ray damage investigations on Mn -Ac to
12
als appear to be insulating, they are single crystalline further probe the nature of electrical transport in these
2
wasmade bypainting twoflatparallelsurfacesofa sam-
ple with conductive (silver or graphite) paste. The ca-
pacitiveanddissipativesignalsweredetectedbyalock-in
amplifierwithanexcitationfrequencyofabout8kHz. In
all cases the measurements were made under vacuum in
a temperature controlled probe.
PhotoconductivitymeasurementsweremadeonMn -
12
AcusingaHe-Nelaserforred(632.8nm)andArgonlaser
for blue (488 nm) and green (514 nm) light. The light
intensity was calibrated before each measurement. Pho-
tocurrent was measured using a lock-in amplifier while
thesamplewasunderdirect current biasandilluminated
by chopped light. A more detailed description of the ex-
periment is published elsewhere48. For the X-ray exper-
iments, Mn -Ac crystals were irradiatedwith 40 kV, 40
12
mA, Cu K radiation at room temperature in order to
α
observe the effects of defects.
III. RESULTS
III.a. Conductivity of Mn12-Ac
ThetemperaturedependenceoftheresistanceR(T)of
aMn -Ac sampleis showninFig. 2(a)fora4-terminal,
12
constantcurrentconfiguration. Theresistivityvaluesare
FIG. 1: (a) Structure of Mn12-Ac with acetate ligand. The on the order of 109 Ω cm at room temperature, and in-
H2O molecules are omitted for clarity1. (b) Structure of crease rapidly in an activated manner upon cooling. Be-
Fe8Br8 showing the 1,4,7 triazacyclononane ligand. The Br low about 200 K, ohmic equilibrium is lost due to the
atoms are omitted for clarity2.
high resistance values. Thus the Arrhenius analysis was
limited to temperatures above this temperature. Over
the 200-300 K range, lnR exhibits a linear dependence
materials. The measured photoconductivity exhibits a
as a function of 1/T as shown in the inset, characteris-
significant wavelength dependence.
tic of a semiconducting system with a well defined band
gap where R(T)≈exp(E /k T), and E is the thermal
a B a
activationenergy. Fromthe slopes of curvesofthe inset,
II. EXPERIMENTAL E is estimated to be 0.38 ± 0.05 eV.
a
Fig. 2(b) shows the temperature dependence of the
Long black rectangular crystals of Mn -Ac were syn- current for the constant voltage (50 V) bias condition.
12
thesized following the procedure of Lis1. The crystals Astheresistanceofthe sampleincreaseswithdecreasing
typically grew to dimensions of about 0.6 x 0.6 x 2.8 temperature, the current rapidly decreases, and is un-
mm3. High quality single crystals of Fe Br were pre- measurable below ∼ 210 K. The corresponding lnR vs
8 8
pared by the method described in the literature2. The 1/T curve is shown in the inset. The linear dependence
Fe Br crystals grew as dark brownorthorhombic plates yields a value of E = 0.36 ± 0.05 eV.
8 8 a
ofabout4.0x6.0x0.5mm3. Thesampleshavebeenrou- Fig. 3 showsthe lnR vs 1/T curve ofa Mn -Ac sam-
12
tinely monitored for quality by NMR, X-ray diffraction, ple obtained by the impedance bridge technique. The
and magnetization measurements. samplewascooledfromroomtemperatureto200K.The
DC resistancemeasurementswereconductedunderei- solid line corresponds to E = 0.36 ± 0.05 eV. The re-
a
ther a constant voltage or a constant current mode us- sistance again shows activated behavior but the linear
ing a conventional four probe technique. A high input relation is not so clear as in the dc resistance case.
impedance(2×1014 Ohm)electrometerwasusedtomea- A significant observation was that the Mn -Ac crys-
12
sure the voltage drop across the sample when constant tals lose solvent upon heating above 300 K. One sample
current was applied. Currents were typically in the 0.1 washeatedto350Kandthencooleddownto200K.The
to 10 nA range, and voltages were generally 100 V or plot of lnR vs 1/T yielded two separate straight lines as
less. The current-voltage characteristics were periodi- can be noted from Fig. 4. The activation energy at the
cally checkedto verify ohmic behavior. AC conductance higher temperature range was 0.35 ± 0.05 eV, while the
measurements were made with a standardac impedance E was 0.18 ± 0.05 eV for the lower temperatures, af-
a
bridge technique. A capacitive electrode configuration ter heating. Thus, care must be taken not to heat the
3
samples above 300 K or so.
T (K)
350 325 300 275 250 225
III.b. Photoconductivity of Mn12-Ac 24
Photoconductivity(PC)wasmeasuredonMn -Acus-
12
ing the ac component of the photocurrent for chopped 23
Ea= 0.36 +/- 0.05 eV
laser light illumination. This was done by biasing the
samplewithdifferentvaluesofdc current48. Fig. 5shows )
thedependenceofthePContheintensity(power)ofthe R ( 22
laser radiation at the three wavelengths used, 632.8 nm n
l
(red), 514 nm (green), and 488 nm (blue). PC is seen to
increasewithphotonenergy. Theincreaseisaboutafac-
21
torofeightwhengoingfrom632.8nmto488nm. Clearly
this enhancement must relate to the creation of charge
carriers by the photons, or to the increase in tempera-
ture due to light absorption, or both. A simple thermal 20
3.6 3.7 3.9
-1
1/T (0.001 K )
25 T (K) FIG.3: lnR(T)vs1/T curveof Mn12-Acmeasured with the
ac impedancebridgetechnique. Thesamplewas cooled from
299 276 256 239 224 211
24 room temperature to 200 K. The solid line is for the curve
20 ) resulting in Ea = 0.36 ± 0.05 eV.
22
WR (
) 15 n 20
WG l
R ( Ea = 0.38 ± 0.05 eV
10 18 T (K)
3.6 4.0 4.4 360 330 300 270 240
1/T (0.001 K-1) 21
5
(a)
0 Ea= 0.18 +/- 0.05 eV
20
200 220 240 260 280 300
T (K)
)
R ( 19
T (K) ln
280 260 240 220 200
6 18
28
) Ea= 0.35 +/- 0.05 eV
nA) 4 Wn R ( 26
I ( l 24 Ea = 0.36 ± 0.05 eV 3.0 3.5 4.0
-1
1/T (0.001 K )
2 3.6 4.0 4.4 4.8
1/T (0.001 K-1)
(b) FIG.4: Mn12-Acsamplewhichwasheatedto350Kandthen
cooled down to 200 K. Straight lines show an Ea = 0.35 ±
0 0.05 eV for the high temperature region, and 0.18 ± 0.05 eV
150 200 250 300 for the low temperature region.
T (K)
FIG. 2: (a) Temperature dependence of resistance R(T) of mechanismisnotsupportedbytheearlierUV-visibleab-
Mn12-Ac measured at a constant dc condition. The arrows sorption data of Oppenheimer et al.44 on Mn12-Ac. The
indicate cooling and warming curves. Inset : lnR vs 1/T spectra show a gradual increase in absorption with pho-
curve yields Ea = 0.38 ± 0.05 eV. (b) Temperature depen- ton energy and the absorption edge was estimated to be
dence of measured current under a constant voltage bias (50 about 1.1 eV. However, over the 632.8 to 488 nm range,
V).Inset : lnR vs 1/T curvewhere Ea = 0.36 ± 0.05 eV the absorption was nearly (within a factor of two) con-
4
stant, while the PC increases by a factor of eight. These
considerationsargueagainstamajorroleofthermalheat-
ing in the mechanism of the observed PC enhancement. T(K)
The effect is thus ascribed to an enhancement of charge 290 280 270 260
carriers due to optical absorption.
18
17
III.c. Effect of X-ray irradiation on Mn12-Ac
16
)
Recently, Hernandez et al.49 observed an increase in R ( Ea =0.73 +/- 0.1 eV
the magnetization tunneling rate of Mn12-Ac caused by n 15
l
defects in the lattice as a result of X-ray irradiation and
heat treatments. In order to probe the possible role of 14
defects in the transport properties of Mn -Ac, we have
12
also carried out an X-ray irradiation study. As the irra- 13
diationdosewasincreasedfrom2hrto20hrs,theoverall
resistivity of the sample increased, but a plot of lnR vs 12
3.4 3.5 3.6 3.7 3.8 3.9
1/T for the different exposure times showed the activa- -1
1/T (1/1000 K )
tion energies remained fairly constant. The radiation-
induceddefectsthusseemtoactastrappingsitesforthe
carriers. This effect is further discussed in Section IV. FIG. 6: Plot of lnR(T) vs. 1/T for Fe8Br8. The solid line
yields a valueof Ea ≈ 0.73 ± 0.1 eV.
III.d. Conductivity of Fe8Br8
0.1 eV, whichis seento be significantly higher than that
of Mn -Ac ( 0.37 ± 0.05 eV). This is discussed later in
Preliminarytemperaturedependentconductivitymea- 12
termsofthebondingofFe Br andMn -Ac. Atpresent
surements have also been carried out on single crystals 8 8 12
no optical data are available for Fe Br for comparison
of Fe Br . Figure 6 shows a typical lnR(T) vs. 1/T 8 8
8 8
plot. The slope of the line yields a value of E = 0.73 ± with the conductivity measurements.
a
Table I summarizes the conductivity results in
comparison to the optical data44 and theoretical
calculations45,46,50.
2
blue
green
IV. DISCUSSION
red
s) 109 Themainresultofthisstudyoftheelectricaltransport
nit 9 in these SMM-type single crystalline materials is that
arb. u 78 tishteicyoefxahigbaiptptehdersmemalilcyoancdtuivcatoterdovceorntdhuecrtiavnigtyeochfa2r0a0c-t3e0r0-
C amplitude ( 456 s (arb. units) 86 blue Kdcmies.isnecPern,hiapsottitnuioocrcneeoangosdefnguteachrpteaipvlcleiyatdyrmrsiemeemraesiatcusroruanenrmdesmupeconetrtnotstrssois.vaeHdlrsiooffimwcsaeuuvnlpeytpr,toootrrhdtdeeterphtsreeoerir--f
P nt 4
3 ocurre 2 grreeden mfuangcntiiotnuadledinepteenmdpenercaetuorfeRa(rTe)n.ecInessoarrdyertotoesutnadbelirsshtatnhde
ot the conduction pathway, we examined the connectivity
2 ph 0 between the neighboring Mn -Ac and Fe Br clusters.
12 8 8
0 2 4 6
Figures7and8showseveraltransportscenariosarepos-
I (nA)
sible. First, as a result of the crystalline lattice, one can
4 5 6 7 89 2 3 4 5 6 7 89
1 10 expect that there will be a band structure with gaps.
Power (mW)
At present, only the electronic structure of the clusters
FIG. 5: Photoconductivity signal of Mn12-Ac as a function hasbeencomputed45,46. Foraband-gappedsemiconduc-
light intensity induced by different wavelengths of light (red: tor, one expects the resistivity to vary as exp(Ea/kBT),
632.8nm,green: 514nmandblue: 488nm). Theinsetshows whereE isrelatedtotheopticalgapE byE =2E 47.
a g g a
theac photocurrentasafunctionofapplied dc currentwhen However, due to the complexity of the crystal structure,
the light intensity is about 1 mW. Data for both increasing it is possible that impurities and/or disorder play signif-
and decreasing direct current bias are shown. icant roles in the charge transport. For example, ther-
5
TABLE I: Comparison of Eg from conductivity and optical
data, and theoretical calculations.
Conductivity Optical Theoretical
Mn12-Ac 0.74 ± 0.1 eV a 1.08 eV b 0.45 eV c
1.75 eV d 2.08 eV e
0.85 eV f
1.10 eV g
Fe8Br8 1.46 ± 0.2 eV a 0.9 eV h
0.9 eV i
aPresentWork,assumingEg =2Ea
bOppenheimeret al.44,minorityspincluster
cPedersonet al.45,minorityspincluster
dOppenheimeret al.44,majorityspincluster
ePedersonet al.45,majorityspincluster
fZenget al.46,minorityspincluster
gZenget al.46,majorityspincluster
hPedersonet al.50,minorityspincluster
iPedersonet al.50,majorityspincluster
FIG. 8: Conduction pathway between two Fe8Br8 molecules.
(R(T) ≈ exp(T /T)γ, with γ = 1/(1+ d), where d is
0
the dimensionality) yielded T ≈ 3×109 K (for d=3).
0
The high sensitivity of the VRH model to defects canbe
tested by introducing them artificially, by ion implanta-
tion, or in the present case, by X-ray irradiation. Since
the T values obtained from the Mott formula indicate
0
anextremelysmalldensityofimpuritysites,i.e. N(E ),
F
a small number of additional defects should decrease T
0
significantly. However, our experimental results on irra-
diated samples (Sec. III.c.) indicate that T and E are
0 a
insensitivetothecreationofdefects. HencetheX-rayin-
vestigation supports the idea of intrinsic semiconductor-
like conduction in Mn -Ac, and by inference, also for
12
Fe Br .
8 8
While we have not been able to arrive at any detailed
picture of the conduction pathways, we offer the fol-
lowing possibilities based on the structure and bonding
characteristics of both lattices. The pathway for Mn -
12
Acisbaseduponsimple Coulombicinteractionsbetween
Mn3+ions,andthepolarmoleculeswhichliebetweentwo
Mn -Ac clusters. A water molecule bound to a Mn3+
12
on the outer crown lies 2.67 A˚away from an unbound
FIG. 7: Schematic ofproposed conduction path between two acetate ligand, which is, in turn, 2.77 A˚ away from a
Mn12-Acmolecules. symmetrically equivalent, unbound acetate ligand adja-
cent to the closest Mn -Ac cluster as seen in Figure 7.
12
The Fe Br conduction pathway is illustrated in Figure
8 8
mally activated hopping between impurity sites can also 8. The proposed pathway between two Fe Br clusters
8 8
give similar temperature dependence. is through an N-H bond in the 1,4,7-triazacyclononane.
If conduction is through a distribution of impurity The conduction pathway thus extends from the N-H to
sites, variable range hopping (VRH) should dominate. a water (2.4 A˚), to a Br− (2.3 A˚), to a water (2.1 A˚),
However, the following considerations argue against a and finally to a hydrogen (2.4 A˚) directly connected to
VRH behavior. Furthermore, the resistivity (∼ 109 Ω the 1,4,7-triazacyclononane on the adjacent Fe Br . It
8 8
cm) is veryhighfor a typicalVRH conductionsystem51. should be noted that the proposed Mn -Ac conduction
12
Moreover, application of the Mott formula for VHR pathway is much more direct than that for Fe Br . This
8 8
6
is consistent with the higher activation energy found for bandgapsforminority(innertetrahedron)spins,andthe
Fe Br . theoretical estimates by Pederson and Khanna45as well
8 8
as Zeng et al.46. Additional optical and theoretical data
areneededforFe Br . Atpresent,calculationsexistonly
8 8
V. SUMMARY for the molecular band-gaps, but not for the entire lat-
tice. Hence, we can only speculate that the inter-cluster
We havefoundthatboth Mn -Ac andFe Br exhibit ligand bridges may play an important role in the con-
12 8 8
a gapped semiconductor-like behavior in their electri- duction mechanism. Further computations on the full
cal transport properties. The limited temperature range crystal band structure are thus desirable.
overwhichtheresistancewasmeasurable,andoverwhich
the materials are stable, restricts a knowledge of the
precise functional form of R(T). Nevertheless, comple-
VI. ACKNOWLEDGEMENTS
mentary photoconductivity and X-ray irradiation stud-
iessupportamodelwherethetransportisgovernedbya
well-definedenergygap. The E ’s havebeendetermined We would like to thank Dr. X. Wei for his assistance
a
to be 0.37 ± 0.05 and 0.73 ± 0.1 eV for Mn -Ac and in the optical measurements. This work is supported
12
Fe Br respectively. Assuming an intrinsic semiconduct- by NSF-DMR 023532, DARPA, and NSF/NIRT-DMR
8 8
ingbehavior,theyleadtoE valuesof0.74±0.10eVfor 0103290. The National High Magnetic Field Laboratory
g
Mn -Acand1.5± 0.2eVfor Fe Br . ForMn -Ac,the is supported through a cooperative agreement between
12 8 8 12
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