ELSEVIER Journal of Magnetism and Magnetic Materials 172 (1997) 193-197

Ferrimagnetic-like behavior of ZnMn,As,

A. Nateprova, I. Tomaka, J. Heimannb, J. Cisowski”, I. Mirebeaud>* a Institute of Applied Physics, Academiei 5, MD 2028 Kishinev, Moldova

b Institute of Physics, Silesian Universi@. Uniwersytecka 4. 40-007 Katowice, Poland ‘Department of Solid State Physics, Polish Academy of Sciences, Wandy 3. 41-800 Zabrze, Poland and Institute of Physics,

Silesian Technical University. Katowice, Poland ’ Laboratoire Lkon Bn’llouin, C.E. Saclay. 91191 Gif-sur-Yvette Cedex, France

Received 13 November 1996

Abstract

The magnetic susceptibility of a single crystal of the magnetic semiconductor ZnMnzAsz has been investigated in the paramagnetic region for the first time. It is shown that this compound behaves as a ferrimagnet with two magnetic sublattices. The molecular field constants and effective moment values are determined for the Mn ions in two different crystallographic positions, corresponding to tetrahedral and octahedral coordinations by the As ions, respectively.

PACS: 75.50.Gg; 75.30.Cr; 75.30.K~

Keywords: Magnetic semiconductors; Ferrimagnetism; Magnetic susceptibility

1. Introduction

ZnMnzASz is a magnetic semiconductor which reveals a large number of interesting phenomena, like the succession of several magnetic transitions, spin-glass like properties, giant magnetoresistivity and anisotropy effects which have attracted a con- siderable interest in the recent years [l].

The crystal structure of ZnMnzAs, has first been studied by means of X-ray scattering [2], showing

that this compound has a layered structure (space group PJml) with the equivalent Mn positions situ- ated at the center of the unit cell.

The detailed investigation of the magnetic prop- erties of ZnMn,Asz was first carried out using a SQUID magnetometer in the temperature range 1.5-320 K, showing strongly anisotropic properties [3]. A transition at high temperature Tc = 320 K was observed, which was considered as a casual transition from a paramagnetic to ferromagnetic state. However, as pointed out in the same work, the saturation magnetic moment (assuming an S = 512 spin value-for localized

* Corresponding author. Fax: + 33-l-6908-6089; e-mail: moments) should be considerably mirehea@bali.cea.saclay.fr. estimated from the experiment.

0304~8853/97/%17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO304-8853(97)00092-9

M;2+ - magnetic higher than that

The neutron diffraction investigations [4] reveal several new features of great importance for the understanding of the magnetic behavior of ZnMn,As,. First of all, the crystal structure of this compound determined by neutron diffraction dif- fers from that proposed in Ref. [2]. For this struc- tural determination, neutrons have a much higher sensitivity than X-rays, since the negative coherent scattering length of Mn yields a very large contrast between Mn and Zn ions. In the real structure of ZnMn,As, shown in Fig. 1, the Mn ions are located at the corners of the unit cell and at the center of the cell. This leads to a Mn monolayer and to a double layer containing both Mn and Zn, where each type of ion occupies in average 50% of the allowed sites in a quasirandom way. Thus one deals with two types of Mn ions in different crystal- lographic positions and having different environ- ments. In particular the Mn ions in the monolayer are octahedrally coordinated by As ions. while in the random layer, they are in tetrahedral coordina- tions with As ions. Secondly, magnetic neutron diffraction reveals the occurrence below T, of two other magnetic transitions, towards an antifer- romagnetic structure then to an helicoidal one. In these two ordered phases, the magnetic moment in the mixed bilayer is smaller than in the full Mn layer. No long range ferromagnetic order is ob- served at T,, but the investigation of the magnetic correlations in the neighborhood of Tc reveals the coexistence of static ferromagnetic correlations co- existing with antiferromagnetic ones.

Zn or Mn ions of random

Mn ions of monolayer

Fig. 1. Crystal structure of ZnMn,As2 from Ref. [4]

All these features allow us to assume the exis- tence of two Mn sites with different magnetic mo- ments. The presence of strong antiferromagnetic couplings evidenced below T,, and the fact that no true ferromagnetic order sets in at T,, also shows that this transition cannot be considered as a casual Curie transition. Therefore we have decided to in- vestigate it in more details. through magnetization measurements.

We have measured the magnetic susceptibility of ZnMn,Asz in the paramagnetic region. The tem- perature dependence of the inverse susceptibility shows a strongly nonlinear behavior, quite different from the CurieeWeiss law followed by ferro or antiferromagnets. Data are analyzed in a similar way as in ferrimagnets, by considering two magnetic sublattices. The first one (sublattice A) is formed by Mn ions of the monolayer and the second (sublattice B) by the Mn ions of the mixed layers (Fig. 1).

2. Experimental results and analysis

The susceptibility measurements were carried out on unoriented single crystals of ZnMnzAsz with an electronic balance by the Faraday method. Single crystals were grown using the Shubnikov- Obraizov technique [S]. Since the upper temper- ature limit of the experiment was about 650 K, an helium atmosphere was used for the sample protection.

The results of the measurements for ZnMn,Asz, presented in the form of the inverse susceptibility l/z as a function of temperature in the range 370-630 K. are shown in Fig. 2. Clearly, the inverse susceptibility above 310 K, namely in the paramag- netic region, follows a nonlinear law. On the other hand, the field dependence of the magnetization in the paramagnetic state is almost linear in the mea- sured fields up to 2 kOe.

In the case of a negative interaction between two sublattices A and B, the magnetic susceptibility, calculated in the molecular field approximation, is known to follow an hyperbolic law. as described by the formula of Refs. [6, 71

1 T-O : _-_A x- c T - 0’ (1)

A. Nateprov et al. /Journal of Magnetism and Magnetic Materials 172 (1997) 193-197 195

where C = CA + CB is the sum of the Curie con- stants of the sublattices A and B, while 8, 8’, and 5 are the hyperbola parameters, which can be ex- pressed via the molecular field constants.

We have performed a least squares fit of the data with Eq. (1). As shown in Fig. 2, the experimental data above 310 K indeed follow closely the solid line which corresponds to the hyperbola of Eq. (l), with the parameters mentioned in the figure cap- tion. We can evaluate the molecular field constants from the hyperbola parameters using the well known set of equations mentioned in Refs. [6,7]:

8’ = vC,C,(2 + a + /q/C)

8 = VC‘&! + vcsp - 8’ )

5 = [W + v~c_,&ap - 1)-J/C ) c = C* + cg.

(2)

200, I I I I

s 150 -

E ‘0 loo- g 3 - so- H=200 Oe

0 I I

200 300 400 500 600 T(K)

Fig. 2. Temperature dependence of the inverse of the magnetic susceptibility of ZnMnzAs,. Squares are the experimental data and the solid line represents a hyperbola given by Eq. (1) with f3 = - 171.6 K, C = 4.37 K emu/mol, 5 = 721.6 K mol/emu and 0’ = 302.6 K.

In these equations, v is the absolute value of the (negative) intersublattice interaction vAB, whereas CI and p are algebraic ratios involving the intrasub- lattice interactions VAA and VBB (a = VAA/V and j? = vaa/v). A positive a (resp. p) value thus corres- ponds to a ferromagnetic interaction within the A (B) sublattice.

The system of Eq. (2) can be simplified if one takes into consideration the geometry of the lattice to estimate the relative magnitudes of the exchange interaction. The A-A interaction is expected to be much smaller than the A-B and B-B ones, since in average the Mn-Mn distances along the c-axis in the sublattice A are twice longer than the Mn-Mn distances in the sublattice B. So we can neglect VAA with respect to VAB and vBB At the same time, the Mn ions in the sublattice B are situated on the same straight line as the As ions. Such arrangement promotes the 180” exchange interaction in the sub- lattice B so that the B-B coupling should be taken into account. Assuming GI = 0, the simplified system of Eq. (2) has then an exact solution. The calculated parameters are given in Table 1 together with the values of the Curie constants and the effective mo- ments on Mn ions. As shown in the table, we find an effective positive interaction in the B sublattice, of the same order of magnitude as the negative interaction vAa. The effective Mn moment in the mixed bilayer is slightly smaller than in the full Mn layer.

We have also studied the influence of Mn con- centration on the susceptibility. As shown in Ref. [S], the ideal stoichiometric composition of ZnMn,As, may be changed, allowing one to sub- stitute up to 50% of the Zn ions by additional Mn ions in the mixed bilayer. We have investigated the magnetic susceptibility of several Zni _xMnz +xA~Z samples with the concentration x ranging from 0 (the stoichiometric compound) to 0.4. We find

Table 1 The Curie constants (C, and C,), effective magnetic moments of Mn ions (~2 and ~5 in the A and B sublattices respectively) and the molecular field constants for ZnMn,AS,

CA (K emu/mol)

CB (K emu/mol)

v.0 a P

2.27 2.10 4.26 4.10 - 107.5 -0 0.58

196 A. Nateprov et al. 1 Journal of Magnetism and Magnetic Materials 172 (1997) 193-197

1.5 -

0.0 0.1 0.2 0.3 0.4 X

Fig. 3. The Curie constant in the B sublattice as a function of Mn concentration in the mixed layer (x = 0 and x = 0.4 corre- spond to ZnMn*As, and Zn0,,Mn2.,As,, respectively).

that all compounds exhibit the same behavior, yielding hyperbolas for the l/x vs. T dependence. The Tc value smoothly increases with increasing Mn concentration and reaches Tc = 324 K for x = 0.4. The molecular field constants determined in the same way as for ZnMnzAsz, exhibit different behaviors with varying Mn concentration. Namely the Curie constant CA of the A sublattice does not change while the Curie constant Cs of the B sublat- tices decreases with increasing Mn concentration (see Fig. 3). The behavior of CA can be readily understood, since the Mn concentration in the magnetic monolayer does not change. The behav- ior of CB call for further explanations. As shown from neutron measurements, all additional Mn ions are located in the mixed Mn layer, so we could a ‘priori’ expect that increasing the Mn concentra- tion in this bilayer should lead to an increase of the exchange Mn interaction. This peculiar behavior of the CB constant is likely related to the anisotropy of the exchange interactions and to the peculiar Mn distribution within the bilayer.

3. Discussion

Clearly, from our measurements, the high tem- perature transition at Tc which was first of all interpreted as a paramagnetic-ferromagnetic transition, is in fact much closer to a ferrimagnetic

one (the value of this transition determined from susceptibility measurements in the paramagnetic region being equal to 309 K instead of 320 K). So our measurement enhance the preponderant role of antiferromagnetic interactions in this system. How- ever, even though the paramagnetic susceptibility may be analyzed in the molecular field approxima- tion, there is no critical behavior at Tc. An invest- igation of this transition by neutron diffraction, to be published later, shows that this transition does not correspond to a true long range order but only to correlations with a finite correlation length.

Below T,, previous magnetization measure- ments have shown the presence of a ferromagnetic moment, much smaller than expected for MnZf ions. The small difference between the moments on the A and B sublattices which we have observed by analyzing the paramagnetic susceptibility (~2 - ~8* = 0.16 p,J is likely related to this low magnetization value. In any case, magnetic mea- surements are sensitive to a ferromagnetic compo- nent which is not an order parameter of the system, as shown by neutron data. The effective moments values found by susceptibility in both sublattices are also significantly higher than the values found by neutrons in the low temperature ordered state (3 and 2.6 pB in the full and mixed layer, respectively, for the stoichiometric composition ZnMn2As2). All these facts show that the molecular field approxi- mation is not valid below Tc

4. Conclusion

Results about the magnetic susceptibility of ZnMn2As2 in the paramagnetic region have been presented for the first time. The temperature de- pendence of the magnetic susceptibility follows an hyperbolic law as in usual ferrimagnets with two different sublattices. This agrees with neutron data which reveal the existence of two Mn sites. We improved the previous magnetic measurements, by showing the strong influence of antiferromagnetic couplings to explain the high temperature behav- ior. We have used a molecular field approximation to analyze our data. This isotropic approach could be valid above T,-, but is clearly insufficient to explain the behavior below T,, due to the layered

A. Nateprov et al. /Journal of Magnetism and Magnetic Materials I72 (1997) 193- I97 191

character of the system. The neutron results, and their analysis below Tc using another (anisotropic) approach will be presented later.

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