Table Of ContentUnconventional carrier-mediated ferromagnetism above room temperature in
ion-implanted (Ga, Mn)P:C
∗
N. Theodoropoulou and A. F. Hebard
Department of Physics, University of Florida,
Gainesville, FL 32611-8440
2
0
0 M.E. Overberg, C.R. Abernathy, and S.J. Pearton
2
Department of Materials Science and Engineering,
n University of Florida, Gainesville, FL 32611-8440
a
J
6 S.N.G. Chu
2 Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974
]
i
c
R.G. Wilson
s
- Consultant, Stevenson Ranch, CA 95131
l
r
t (Dated: February 1, 2008)
m
. Ion implantation of Mn ions into hole-doped GaP has been used to induce ferromagnetic be-
t
a havior above room temperature for optimized Mn concentrations near 3at.%. The magnetism is
m suppressedwhentheMndoseisincreased ordecreased awayfrom the3at.%value,orwhenn-type
GaP substrates are used. At low temperatures the saturated moment is on the order of one Bohr
d- magneton, and the spin wave stiffness inferred from the Bloch-law T3/2 dependence of the magne-
n tization provides an estimate Tc = 385K of the Curie temperature that exceeds the experimental
o value,Tc =270K.Thepresenceofferromagnetic clustersandhysteresistotemperaturesofatleast
c 330K is attributed to disorder and proximity to a metal-insulating transition.
[
PACSnumbers: 75.50.Pp,72.25.Dc
1
v
2
The rather short interval between discovery of ferro- where the dc susceptibility, χ(T), diverges and the “sat-
9
magnetism in the Mn doped III-V semiconductor GaAs urated” moment is observed to have strong temperature
4
1 [1] and the demonstration of unique phenomena such as dependence. These observations suggest a percolation
0 field-effectcontrolofferromagnetism[2],efficientspinin- type of picture in which isolatedferromagnetic magnetic
2 jection to produce circularly polarized light [3, 4], and clusters,immersedinabackgroundofparamagneticmo-
0
spin-dependent resonant tunneling [5], opens a rich and ments, grow in size as the temperature is lowered until,
/
t varied landscape for technological innovation in mag- at T = T , long-range order extends through the whole
a c
m netoelectronics [6]. Although the Curie temperature, system. Thecharacteristics,whichwehaveobservedand
T = 110K, of (Ga,Mn)As is spectacularly high, it is will describe below, thus revealphenomenology that can
- c
d not high enough, and any real breakthrough with re- be used to assess the relevance of distinct yet comple-
n spect to applications will require diluted magnetic semi- mentary theoretical viewpoints [12, 13, 14, 15].
o
conductors (DMS) which exhibit ferromagnetism above
The use of GaP as a host semiconductor for mag-
c
: roomtemperature[6]. Alreadytherearereportsofroom netic dopants is advantageous: it has a large bandgap
v
temperature ferromagnetism in the DMS chalcopyrite
(2.2eV), it is an important component of the AlGaInP
i
X Cd1−xMnxGeP2 [7], (Ga,Mn)N formed by diffusion [8], materials system, which is used for visible light-emitting
r and Co-doped ZnO [9] and TiO2 anatase [10, 11]. diodes and high speed electronics, and it is also nearly
a
In this letter we report on carrier-mediated ferromag- lattice-matched to Si, offering the possibility of combin-
netismin(Ga,Mn)P,dopedp-typewithcarbon(C),that ing DMS with conventional Si circuitry. Epitaxial films
persiststo aboveroomtemperature. HighT behavioris of(Ga,Mn)Phaveshownsingle-phasematerialuptoMn
c
onlyobservedforsamplesthatarepreparedatanoptimal atomicconcentrationsof9%andhystereticmagneticbe-
Mn concentration near 3at.% on heavily p-doped GaP havior that persists up to 250K [16].
substrates. Whenelectron-dopedsubstratesaresimulta- Two types of GaP substrates were used in these ex-
neously implanted with the same Mn concentration, T periments. The first were bulk (100) substrates with
c
is strongly suppressed. Hysteresis in the 3at.% mate- nominally undoped n-type (1016cm−3) background car-
rial extends to temperatures well above the temperature rier densities and the second were 0.4µm-thick epi layers
2
80 taken in different fields and sample orientations.
H=500Oe T
C At low temperatures the saturated moment, M0 =
gµ S, is calculated to be about one Bohr magneton
60 B
n) (µB) per Mn ion, thus implying that for a g-factor of
M
2 the spin S = 1/2. This value is a factor of five
u/g 40 n)80 H=500Oe less than S = 5/2 expected for the half-filled d-band
m M60 of divalent Mn, a discrepancy most likely arising be-
g
M(eFC20 DM(emu/2400 Q cmaourseecolofssetlyronspgacaendtifrearnrodmomaglyneptoicsitcioounpeldinMg namioonnsg[1t2h]e.
The solid line is a three parameter fit to an expression
0 f
0 50 100150200250300350 of the form, M(T) = M0(1−αTη), over the tempera-
Temperature(K)
ture range T ≤ 160K. The observation of a Bloch law
0
0 50 100 150 200 250 300 350 dependence with exponent η = 3/2 is expected for fer-
romagnets and reflects the presence of long-wavelength
Temperature(K)
thermallyexcitedspinwaves. Thedashedlinesdelineate
the 95%confidence band for the best-fit parameters η =
FIG. 1: Temperature dependence of field-cooled (FC) mag- 1.51(6) and α = 1.09(35)×10−4.
netization (per gram Mn) at at an applied field H =500Oe.
At low temperatures a Bloch law (T3/2) dependence (solid The inset of Fig. 1 shows the temperature-dependent
line) was found. The dashed lines are 95% confidencebands. difference ∆M = MFC − MZFC between field-cooled
The dashed line at Tc = 270K marks the field independent (FC) and zero-field-cooled (ZFC) magnetization curves
inflection point (Curie temperature) and the vertical arrows taken at 500Oe. This subtraction advantageously elimi-
inthemainpanelandtheinsetmarktheferromagneticCurie
nates para- and diamagnetic contributions and simulta-
temperature,Θ =236K. Inset: Temperaturedependenceof
f neously indicates the presence of hysteresis if the differ-
the difference ∆M between field cooled (FC) and zero-field
enceisnon-zero. Itshouldbenotedthatthe kneeofthis
cooled (ZFC) magnetizations taken in a field of 500Oe.
curveoccursatalowertemperaturethanthe kneeofthe
FCcurve(mainpanel)andmovestoevenlowertempera-
tureathighermeasuringfields. Thistrendcanbeunder-
stood by realizing that at a giventemperature, ∆M =0
grown by Gas Source Molecular Beam Epitaxy on top
ifthemeasuringfieldishigherthanthemaximumfieldat
of GaP substrates. These layers were heavily C-doped
(p ≈ 1020 cm−3, 10−2Ohm cm) using carbon tetrabro- which hysteresis is observed in M(H) loops acquired at
thesametemperature. Examplesofsuchloopsareshown
mide(CBr4)asthedopantsource[17]. Thesampleswere
implanted with 250keV Mn+ ions to doses of 3-5×1016 in Fig. 2. Hysteresis with a coercive field Hc = 100Oe
cm−2atatemperatureof 350Ctomaximizecrystallinity is quite pronounced at 250K (lower panel) to fields as
high as 600Oe. At lower fields, hysteresis similar to that
[18]. TheseconditionsproducerelativelyflatMnconcen-
tration profiles with peak volume concentrations of 3 or
5at.% over a projected ≈ 0.2µm depth in each sample.
Followingarapidthermalannael,thesampleswerechar-
n) 10 300K
acterized by transmission electron microscopy (TEM), M
5
selected area diffraction pattern analysis (SADP), and g
double crystal x-ray diffraction (DC-XRD) [19]. No ev- u/ 0
m
idence of secondary phase formation of GaMn or MnP e -5 H =50Oe
was found. All magnetic measurements were made with M(-10 C
a Quantum Design SQUID magnetometer with the ap-
plied field parallel to the film surface. Similar results n) 40 250K
M
were also obtained for perpendicular fields. 20
g
ShowninFig.1isaplotofthetemperature-dependent u/ 0
m
magnetization for the 3at.% (Ga0.94Mn0.06P) sample e-20 H =100Oe
cooled in a field of 500Oe. A temperature independent M(-40 C
diamagnetic contribution was subtracted from the back-
-800 -400 0 400 800
ground. In contrastto (Ga,Mn)As [20] the temperature- H(Oe)
dependentmagnetizationhastheclassicconvexoutwards
shapebutwithasubstantialtailextendingtohighertem-
FIG. 2: Magnetization loops showing moderate hysteresis
peratures. The Curie temperature Tc =270K,indicated with coercive field, Hc = 50Oe (upper panel) at 300K and a
by the vertical dashed line, is defined as the inflection stronger hysteresis with H =100Oe (lower panel) at 250K.
c
point and does not shift in position for similar data sets
3
shown at 300K in the upper panel of Fig. 2 extends to and M (T) at each temperature. We identify the fer-
s
330K. romagnetic Curie temperature Θ = 236K (vertical ar-
f
TheFC-ZFCsubtractionisparticularlyeffectivewhen row) by a linear extrapolation of χ−1(T) to zero. The
there are small amounts of ferromagnetic material in hysteresis and accompanying saturated moment extend
thepresenceofalargediamagneticand/orparamagnetic withdiminishing magnitude to substantiallyhighertem-
background. This situation ensues when n-type rather peratures: 330K for the measured hysteresis and 390K
than p-type GaP is implanted with 3at.% Mn (Fig. 3, for a presumptuous linear extrapolation of Ms(T) to
upper panel) orp-type GaPis implanted with 5at.%Mn zero. Using the expression for the Curie constant,
(Fig. 3, lower panel). These data show that the maxi- C = g2µ2BN0S(S +1)/3kB, where N0 is the number of
mum FC-ZFC difference, ∆M, is reduced by a factor of Mn atoms per gram Mn and kB is the Boltzmann con-
15-20andthe T by afactorof≈6,overthe temperature stant, we calculate from the slope of χ−1(T), the result,
c
range300Ktonear50K.Accordingly,anoptimizedT is g2S(S +1) = 410. For g = 2 this implies that S = 10,
c
extremelysensitivetoboththecarrierandMnconcentra- a spin larger by a factor of four than the S = 5/2 of
tions. Furthermore, ferromagnetic impurity phases such isolated Mn++ ions. Such a large S justifies the use of
as MnGa (T > 300K) [20] and MnP (T = 291K) [21] a Langevin(classical)rather than a Brillouin(quantum)
c c
arenotresponsible,since anincreaseofMnleadsto less, functional dependence.
not more, magnetism, and the formation and amount of The data presented here not only show that
suchimpurityphasesshouldalsobeindependentofhole- room-temperature ferromagnetism is obtainable in
doping level. (Ga,Mn)P:C and but also provide a meaningful con-
Although our observations confirm conventional fer- frontationwithpresenttheories[12,13,14,15]describing
romagnetism at low temperatures, the magnetic behav- ferromagnetism in DMS. The ferromagnetism is clearly
ior is considerably more complicated at higher tempera- carriermediated[12,13,14,15]andisoptimizedforhole-
tures as shown by the Fig. 4 plots of the temperature- doped substrates with Mn concentrations near 3at.%.
dependent inverse dc susceptibility χ−1(T) (main panel) Higher concentrations of Mn are deleterious because of
and saturation moment M (T) (inset). The data for increased antiferromagnetic coupling [12] and a possible
s
these plots were obtained at selected temperatures by disorder-inducedself-compensation[15]. AtlowMncon-
recording M (H) to the maximum available field of centrations, RKKY interactions randomize the sign of
ZFC
5T. A temperature-dependent diamagnetic contribution the exchange coupling between Mn spins, and spin-glass
thatvariedlessthan1%overthetemperaturerangestud- rather than ferromagnetic behavior is expected [22].
ied was subtracted from the data. We used a Langevin Our experimental determination of the prefactor α
functional dependence to determine χ = dM /dH discussed above allows a direct calculation of the spin
ZFC
n) 4 n-GaPMn3% 1kOe 160 80
gM 3 mu)140 Mn)6700
mu/ 2 e/e120 mu/g4500
M(e O100 M(es2300
D 1 Mn 80 10
0
g 160 200 240 280 320 360
n) 6 p-GaP:CMn5% 0.5kOe 1( 60 Temperature(K)
M - c
g 4 cd40 Qf=236K
u/
m 20
2
e
M( 0
0 240 260 280 300 320 340
D
Temperature(K)
0 50 100 150 200 250 300
Temperature(K)
FIG. 4: Temperature dependent inverse susceptibility ex-
FIG. 3: Temperature dependence of the difference between trapolated (dashed line) to Θ = 236K. The inset shows
f
field-cooled (FC) andzero-field cooled (ZFC) magnetizations the temperature dependent ”saturated magnetization” de-
(pergramMn)for(upperpanel)n-typeGaPimplantedwith rived from fits of the field-dependent magnetization at each
3at.% Mn and (lower panel) p-type GaP:C implanted with temperaturetoaLangevin function dependenceas described
3at.% Mn. The measuring fields are indicated in the upper inthetext. Forcomparison,thesolidlinereproduces∆M(T)
right hand corners of each panel. at 1000 Oeshown in theinset of Fig. 1.
4
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