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NEUTRON DIFFRACTION STUDY OF MAGNETIC AND CRYSTALLOGRAPHIC PHASE
TRANSFORMATION IN MANGANESE ARSENIDE AS A FUNCTION OF TEMPERATURE AND
PRESSURE
N. Sirota, E. Vasilev, G. Govor
To cite this version:
N. Sirota, E. Vasilev, G. Govor. NEUTRON DIFFRACTION STUDY OF MAGNETIC AND CRYS- TALLOGRAPHIC PHASE TRANSFORMATION IN MANGANESE ARSENIDE AS A FUNCTION OF TEMPERATURE AND PRESSURE. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-987- C1-989. �10.1051/jphyscol:19711351�. �jpa-00214387�
JOURNAL DE PHYSIQUE Colloque C 1, supplment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - 987
NEUTRON DIFFRACTION STUDY OF MAGNETIC AND CRYSTALLOGRAPHIC PHASE TRANSFORMATION IN MANGANESE ARSENIDE AS A FUNCTION OF TEMPERATURE
AND PRESSURE
N. N. SIROTA, E. A. VASILEV and. G. A. GOVOR Institute of Physics of Solids and Semiconductors, Byelorussian
Academy of Sciences, Minsk, Podlesnaya 17, U. S. S. R.
RBsumB. - Le diagramme des phases cristallographiques et magnetiques de l'arskniure de manganese a Bte ktabli par diffraction neutronique dans l'intervalle de tempkratures entre 100 et 400 OKet de pressions entre 0 et 9 000 bars.
Pour la phase ferromagnetique qui existe aux conditions normales de temperature et de pression, le calcul de la densite Clectronique radiale montre un empietement apprkciable des klectrons 3 d de Mn selon l'axe C. La transition ferromagnk- tique-paramagnktique qui a lieu a P = 0 et 318 OK correspond a une diminution, due a la dilatation du rkseau, de l'empik- tement #electrons dans la direction C ; l'apparition d'une deuxikme phase ferromagnktique
a
haute pression et basse tempkrature correspond A une augmentation de l'empietement 3 d dans le plan de base.Abstract. - The magnetic and structure phase diagram of manganese arsenide has been established by neutron diffraction studies over the temperature range 100-400 OK and the pressure range 0-9 000 bars. For the ferromagnetic phase that exists at normal temperature and pressure, calculation of the electron density distribution shows appreciable overlap of the Mn 3 d-electrons along the c axis. The ferromagnetic-to-paramagnetic transition that occurs at P = 0 and 318 OK corresponds to decreased electron overlap in the C direction due to lattice expansion ; the appearance of a second ferromagnetic phase at high pressure and low temperature corresponds to increased 3 d overlap in the basal plane.
There is presently great interest in the nature of the magnetic transformations of MnAs and in its in pecu- liar magnetic and electric prorerties.
The ferroparamagnetic transition (1) observed at 40 OC is a first order transformation accompanied by a discontinuous change of specific volume, electro- conductivity (3), magnetocaloric effect (4), latent heat of transformation ( 3 , compressibility (6), expansion coefficient and other physical properties. A second order transformation is observed at about 400 OK, accompanied, for example, by a change of behaviour of the inverse susceptibility law (1). Goodenough (7) and Menyuk (8) have studied the effect of pressure on the indicated transformations using conductivity and magnetic susceptibility measurements. Using neutron diffraction, Bacon and Street (9) showed that in the ferromagnetic state the magnetic moments of manganese lie in the plane normal to the C axis. This conclusion is confirmed in paper (lo), which finds the magnetic moments of solid solutions MnAs-MnSb to lie in the basal plane.
In this paper we investigate the magnetic and struc- ture phase diagram of MnAs by neutron diffraction for temperatures varying from 100° to 4000K and pressures ranging from 0 to 9 000 bars. The diffracto- meter (1 1) and pressure vessel (12) have been described previously. Carbon disulphide was used as an agent to transfer pressure.
An analysis of the positions and intensities of the reflexions made it possible to plot a phase diagram of the magnetic and structure transformation occurring in MnAs (Fig. 1). Transformations observed on heating and increasing pressure are shown by solid thick lines and are denoted by capital letters. Transfor- mations observed on cooling and decreasing pressure are given by thin lines and are denoted by small letters.
The ferromagnetic-paramagnetic transformation occurs on heating at 318 OK (point A) with a 1.8
%
change of specific volume. Below 318 OK an isothermal rise of pressure produces the same phase transforma- tion, accompanied by a considerable decrease of specific volume which reaches the value of 11,4
%
at 135 OK.
The beginning and the end of the transition are marked by the lines AB and AC respectively. At
'1
JO o 40 40 60. "'
, Pkbor.
FIG. 1. - Phase Diagram.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711351
C 1
-
988 N. N. SIROTA, E. A. VASILEV AND G. A. GOVOR temperatures above 300 OK the transition is a,, -,P,,,
below 300 OK a,, -t
pi,
and below 200 OK a,, -, p".Hence in the region situated between the lines of AB and AC there exist two phases : g, and
P,,,
a,, and or a f m and p". The phases a,,,P,,, P I ,
and p"have nickel-arsenide structure of the B 8 type with different degrees of orthorhombic distortion. Consi- derable hysteresis in the transition from the parama- gnetic phases
P,,, PArn,
p" into the ferromagnetic one a,, is observed on isothermal lowering of pressure.In this case the beginning and the end of the reverse conversion
p,,
-, a,,,Pb,
+ a,, or p" -+ a,, corres- ponds to the l~nes ac and ab respectively.At temperatures between3180 and 380 OK isothermal increase of pressure is accompanied by an insignificant anomalous decrease of the specific volume and some increase of the degree of crystal lattice distortion, the beginning of which is marked on the line KL and the end on the line KM. In the interval of pressure bet- ween the lines KL and KM the transition from phase
Ppm
to phasePLm
takes place. We note that when raising the temperature with P = 0 at point K this transition is not seen, but it becomes noticeable under some pressure P > 0. With decreasing pressure the reverse transitionPk,
-+P,,
is practically not observed because of considerable hysteresis. On isobaric cooling from room temperature at a pressure higher than the value at point d, transition of phasePkm
into/3"
accompanied by considerable decrease of specific volume is ob- served.Beginning and end of transition on cooling are marked by the lines de - and - fe respectively. The reverse transition with increasing temperature is observed without any considerable hysteresis. At values of the pressure exceeding point e (approximately 9 000 bars) this transition is not observed. After completion of the transition
pkm
-, p" further lowering of the tem- perature was accompanied in a narrow range of tem- perature by the appearance of a magnetic component in the reflections.The line gh - characterizes a magnetic transition of a paramagnetic phase P" into a metamagnetic phase
P:,.
In some cases for example, a t the initial measurements, a more intensive transition on this line - gh is observed, which can be characterized as a transition of a para- magnetic phase
/3"
into a ferromagnetic phasePi,.
The changes of the integrated intensity of the nuclear-magnetic reflex (100) and of the lattice para- meter at 120 OK in the ferromagnetic phase of high pressure, in metamagnetic state and in the phase state a,, are given in figure 2.
An isobaric rise of the temperature smoothly dimi- nishes the degree of orthorhombic distortions of the phase p;,, approaching the degree of distortion of the phase a,,.
Figure 3 shows the specific volume change on iso- thermal raising and lowering of the pressure in the vicinity of the a,,
s Ppml
a f , P Pbm and a,, P P G mtransitions. The compressibility of the magnetic phase
Pkm
is considerably smaller than that of the parama- gnetic phase.50 I I I I
$sf
o 40 40 40
40
i ~ oP kbor
FIG. 2. - The change of the integrated intensity of the nuclear-magnetic reflex (100) and of the lattice parameter
at 120 OK :
I@&) -in the ferromagnetic phase of high pressure j;,,
I(&,) in the metamagnetic phase Am,
Z(upm) - 1n the ferromagnetic phase wm,
a(ufm) is the lattice parameter a in the phase acm and a@") is the lattice parameter a in the phases Bhm and Pirn.
FIG. 3. - Change of Specific Volume at Transitions :
One of the peculiarities of the MnAs phase diagram is the presence of the two regions a,, and
P;,
with an ordered spin structure. As was mentioned earlier, raising the temperature at P = 0 leads to discontinuous destruction of magnetic order in a narrow temperature interval. On the other hand, a discontinuous transition from paramagnetic(p)
to ferromagnetic(Pirn
phase)also takes place when lowering the temperature in the
Pirn
region.NEUTRON DIFFRACTION STUDY OF MAGNETIC AND CRYSTALLOGRAPHIC PHASE C 1
-
989The magnetic moment per Mn atom for the ferro- magnetic state P:,, is according to neutron data, equal to 3.24 p, at 6.0 kbar and T = 128 OK. The direction of the moments is still normal to the C-axis.
The formation of a high pressure ferromagnetic phase at low temperatures can be connected with the degree of overlapping of 3 d-electrons at half-distance between Mn atoms. For this purpose the curves of the density distribution of 3 d-electrons for the ferro- magnetic state corresponding to the a,, phase (Fig. 4)
In the structure transition caused by raising the temperature to 45 OC at P = 0, the nearest interatomic distance of Mn atoms increases discontinuously to 2.88
A
resulting in a decrease of the overlap. Using eq. (1) for the radial electron density distribution(where fM is the form-factor value at P = 0 and a is a coefficient) and eq. (2) for the overlap of the density distribution at half the Mn-Mn interatomic-distance
the variation of overlap can be estimated by the coefficient
FIG.$^. - Distribution Functions of 3 d-electron density.
were plotted using the Fourier transformation of the Mn form-factor. From the given distribution, there results at half the distance between Mn atoms a consi- derable overlapping of the 3 d-shells in the C-direction (dMn-,, = 2.85 A) amounting to 7
%
of the maximum value of 3 d-electron radial density. In the basal plane where dMn-Mn = 3.71A,
the d-orbitals can be consi- dered to be non overlapping (localized).Substituting values of AdJd and a equal to 3.1, into the expression (3), the overlap of the density distribu- tion decreases by 20
%
with the transition at 45 OC.It is possible that this change determines the transition from the ferromagnetic to the paramagnetic phase.
Lowering the temperature of the &,-(paramagnetic) phase at high pressure, for example at 6 kbar, reduces considerably the specific volume and increases the orthorhombic distortion of the initial nickel-arsenide structure. This leads to a decrease of the nearest Mn interatomic distance from 3.71
A
for the a,, phase to 2.99A
along the gh line of thePN
-+P;,
phase transition. The result, from eq. (3), is a partial overlap of 3 d-orbitals in the basal plane of about 33%
of the overlap under normal conditions.In the directionof the C-axis, the overlap is about 85
%
of the overlap under normal conditions.This overlap of the 3 d-electron shells of neighbour atoms in the direction of the C-axis and in the basal plane gives rise to the ferromagnetic /3;, phase at high pressures and low temperatures.
References GUILLAUD (C.), J. Phys. Radium, 1951, 12, 223.
WILLIS (B. T. M.) and ROOKSBY (H. P.), Proc. Phys.
SOC. L, 1954, 67, 290.
FISHER (G.) and PEARSON (W. B.), Canad. J. Phys., 1958, 36, 1010.
MEYER (A. J. P.), , , C. R. Acad. Sci. Paris, 1958, 246, 1820.
BATES (L. F.), Proc. Roy. SOC. L., 1928, A 117, 680.
GRAZHDANKINA (N. P.), Uspekhi. Fiz. Nauk., 1968, 291.
GOODENOUGH (J. B.) and KAFALOS, Phys. Rev., 1967, 157, 389.
MENYAK (N.), KAFALAS (J. A.), DWIGHT (K.) and GOODENOUGH (J. B.), Phys. Rev., 1969, 177, 942.
BACON (G. B.) and STREET (R.), Nature, 1955, 175,
C 1 0 2 1 0 .
SIROTA (N. N.) and VASILEV (E. A.), Phys. Stat.
Sol., 1968, 28, K 175.
SIROTA (N. N.) and VASILEY (E. A.), Doklady Akad.
Nauk B. S. S. R., 1968, 12, 786.
SIROTA (N. N.), GOVOR (G. A.), KARAVAI (A. P.), RIZHKOVSKII (V. M.) and SHIPILO (V. B.), Ant.
Document No. 269360.
