VOLUME 77, NUMBER 26 PHYSICAL REVIEW LETTERS 23 DECEMBER 1996
Mechanism of the Giant Magnetoresistance in UNiGa from First-Principles Calculations
V. N. Antonov,* A. Ya. Perlov,* P. M. Oppeneer, A. N. Yaresko,* S. V. Halilov
Max-Planck Research Group “Theory of Complex and Correlated Electron Systems,”
University of Technology, D-01062 Dresden, Germany
(Received 13 August 1996)
The giant magnetoresistance (MR) in UNiGa is investigated from first principles, using density-
functional band-structure theory in the local approximation together with a linear-response ansatz for
the conductivity. The MR is calculated to be very anisotropic, with a large value of 245 6 5% for
current in plane and a giant MR of 264 6 5% for current perpendicular to plane. These values are in
semiquantitative agreement with the measured MR. The basic mechanism of the giant MR is identified
to be a superzone reconstruction of the Fermi surface at the field induced metamagnetic transition.
[S0031-9007(96)01990-4]
PACS numbers: 71.20.Gj, 72.15.Gd
The discovery of the giant magnetoresistance (MR) in
FeCr multilayers [1] has lead to a world-wide interest in
the MR phenomenon as well as to a nearly “avalanchelike”
research activity. After the first report on the giant MR in
FeCr multilayers, giant MRs were discovered in CoCu
multilayers [2], in heterogeneous CoCu and CoAg al-
loys [3], and also in a number of “ordinary” intermetal-
lic compounds [4–7]. One of the essential conditions for
the occurrence of the giant MR in transition-metal (TM)
multilayers is an antiparallel interlayer exchange coupling
across the spacer layer [8]. The giant MR effect appears
when the magnetic moments, which are initially oriented
antiparallel, are aligned parallel in an applied magnetic
field. A similar condition is required for a giant MR in in-
termetallic compounds, which without exception is found
to be intimately related to the occurrence of a ground state
antiferromagnetic (AFM) phase. As compared to the MR
in TM multilayers, the MR in intermetallics is attractive for
theoretical investigations of the MR mechanism for two
reasons: first, in a number of intermetallic compounds a
MR was observed which exceeds by far the giant MR mea-
sured in TM multilayers. Examples are La
12x
Ca
x
MnO
3
[4], and in particular also the uranium based intermetallics,
as, e.g., UNiGa, UNiGe, UPdIn, and UNiSn, where mag-
netic field induced changes of the resistivity up to a factor
of 7 were measured [5,6]. Second, the giant MR in inter-
metallics was in most cases measured on pure single crys-
tals [5–7]. The latter situation is in contrast to that of TM
multilayers, where, depending on the achieved superstruc-
ture quality, incoherent conduction electron scattering off
impurities and interfacial roughness potentials may con-
tribute in a distinct, though quantitatively unknown man-
ner. It is therefore of a fundamental interest for the general
understanding of the MR phenomenon to explain on a first-
principles basis the nature of the giant MR in intermetallic
compounds. In this Letter, we report on such an investi-
gation of the giant MR in UNiGa.
It was discovered recently that in an external field the
resistivity in UNiGa changes enormously [5]. UNiGa
crystallizes in the hexagonal Fe
2
P structure [9,10], which
exhibits a natural layer structure, consisting of planes of
uranium atoms admixed with 25% Ni atoms perpendicu-
lar to the c axis, each of which are separated from one
another by a layer of Ni and Ga atoms. The magnetic mo-
ments of all uranium atoms in one layer are rigidly cou-
pled parallel due to a strong intralayer exchange coupling.
Uniaxial magnetic anisotropy leads to an orientation of the
moments parallel to the c axis [10]. The interuranium
layer exchange coupling is much weaker: With a rela-
tively small magnetic field of less than 1 T it is possible to
flip the magnetic orientation of one complete layer. The
magnetic phase diagram of UNiGa, furthermore, is quite
intricate: there are several AFM phases below the Néel
temperature T
N
39.5 K [10]. Some of these are rather
complex, for instance, with a sequential stacking of the
uranium moments of 1 1 2 1 2 2 12 in the consecu-
tive layers perpendicular to the c direction. In this nota-
tion, “1” means 1c oriented, and “2” means 2c oriented
moments, respectively. The ground state AFM structure
is characterized by a sequential stacking 1 1 2 1 22
of the uranium moments [10]. Recent magnetotransport
measurements on pure single crystals yielded a giant nega-
tive MR of 287% for current perpendicular to plane (CPP)
(i k c k B), and a somewhat smaller MR of 258% for cur-
rent in plane (CIP) (i c k B) [5]. With regard to the nega-
tive sign of the MR, we note that the definition customary
for the MR in uranium intermetallics has been used:
Drr rB 2r0r0 . (1)
This definition differs from the one commonly used for
TM multilayers, in which rB and r0 are interchanged.
Using the multilayer convention, the MR in UNiGa would
be about 650% for CPP and about 140% for CIP. Thus,
the MR in UNiGa closely resembles that of the TM multi-
layers, which are also loosely exchange coupled magnetic
layers, but the MR in UNiGa is considerably larger.
Previously several theories of the giant MR in TM
multilayers were proposed [11 –19]. Initially, these con-
0031-9007 96 77(26) 5253(4)$10.00 © 1996 The American Physical Society 5253