Influence of phosphorus substitution on Mn0.63Cr0.37As

Journal of Magnetism and Magnetic Materials 94 (1991) 347-354 North-Holland

347

Influence of phosphorus substitution on Mno.63Cro.37As A.F. Andresen a, K. Barner b, H. Fjellvhg c, A. Kjekshus c, H. Rager d, U. Sondermann and S. Stolen c a Institute for Energy Technology, N-2007 Kjeller, Norway b IV. Physikalisches lnstitut der Universitiit Gi~ttingen, Fachbereich Physik, W-3400 Gi~ttingen, Germany c Department of Chemistry, University of Oslo, N-0315 Oslo 3, Norway d lnstitut flir Mineralogie, Petrologie und Kristallographie der Philipps-Universitgit, W-3550 Marburg Germany Received 12 January 1990; in revised form 16 April 1990

Mn0.63Cr0.37AsI _xPx has been investigated for 0.000 < x < 0.100 by X-ray and neutron diffraction, magnetometric, DTA, DSC and ESR measurements. The results show that there is complete solid solubility throughout the studied composition range with random distribution of Mn, Cr and As, P over the metal and non-metal sublattice, respectively. At and below room temperature the crystal structure is of the MnP type. Phosphorus acts as a positive chemical pressure generator. The characteristic magnetic transitions of Mn0.63Cr0.37As on decreasing temperature, para (P) to hetimagnetic (He) followed by H c to Ha, are only maintained at a very low substitution level, x < 0.008. Further increase of the phosphorus content suppresses H c and favours ferromagnetism (F). For x > 0.05 the F mode becomes the only ordered magnetic state. A tentative magnetic phase diagram for Mn0.63Cr0.37AsI _xPx (0.00 < x < 0.10) is advanced. Interesting features of ESR data are presented, and connections to the magnetic phase diagram are proposed.

1. Introduction Over the last decade, considerable efforts have been made to clarify and connect the structural and magnetic properties of the M n l - t Cr/As phase; cf. refs. [1-7] and references therein. For compositions close to t = 0.40, two antiferromagnetic spin structures (i.e. spiral structures denoted H a and He) are in equilibrium under certain thermodynamic conditions of temperature, pressure and composition [2,4,6-10]. Since the transition between these phases, which crystallographically both belong to the MnP type structure, is of the first order, a small but significant two-phase region develops in relation to the H a to He transition in the Mnl_tCrtAs solid solution phase [4,6,7,9,10]. In the composition interval close to the coexistence range of the MnP,H a and MnP,H~ type phases, both stable and metastable phases have been observed and characterized [5,6,8]. The metastable phases are likely to be connected with structural and compositional defects, and have

been found to undergo rather peculiar diffusionless phase separations [5,8]. In connection with the phase transitions and the phase separation, some questions, e.g. the physical interpretation of the observed magnetic specific heat [6-8,11] remain open. A tempting approach to a better understanding of these behaviours is to investigate what happens to the phase transitions and separations in Mnl_/CrtAs when the phase is subjected to chemical pressure, e.g. on introducing a positive pressure substituent like P instead of As [8,12]. In the isostructural MnAs-MnP system, such substitution is demonstrated to favour ferromagnetism [13-15]. In this work, various structural and physical properties of M n l _ t C r t A s l _ x P x are discussed, and results from electron spin resonance (ESR) measurements are considered in relation to a proposed phase diagram.

2. Experimental Small scale samples of Mnt_tCrtAsl_xPx, t = 0.37 and 0.00 < x < 0.10 were made by mixing

0304-8853/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)

348

A.F. Andresen et aL / Phosphorus substitution in Mno.o3Cro37As

pre-prepared Mn0.63Cr0.37As and Mn0.63Cr0.37P samples in the appropriate ratios, followed by repeated heat-treatment at 9 0 0 ° C in evacuated, scaled silica glass ampoules, interrupted by intermediate crushings. The Mn0.63Cr0.37As and Mn0.63Cr0.37P samples were similarly synthesized from MnAs, CrAs, MnP and CrP, and the binary phases were in turn prepared from the elements according to the procedures described in refs. [13,16]. Powder X-ray diffraction (PXD) data were collected at room-temperature with a Guinier camera (Cu K a 1 radiation, Si as internal standard) and at temperatures between 100 and 900 K with an E n r a f - N o n i u s Guinier Simon camera. In addition, powder diffraction data, and in particular intensities, were recorded between 100 and 300 K with a vertical powder diffractometer equipped with a special cryostat. Powder neutron diffraction ( P N D ) data were collected between 10 and 300 K by the OPUS I I I two-axis diffractometer at the J E E P II reactor, Kjeller, using monochromatized neutrons of wavelength 187.7 pm. Magnetic susceptibility data were recorded by means of a Faraday balance. D T A and heat capacity (DSC) data were collected with a Mettler T A 3000 system for samples contained in A1 cans. ESR spectra of powder samples were recorded by a Varian spectrometer at a frequency ~0 = 9.244 GHz.

3. Result and discussion

3.1. Structural properties All samples of M n l _ t C r t A s l _ x P x, 0.00 < x < 0.10, take the orthorhombic MnP type structure at ambient conditions. In the diffraction patterns (PXD, PND), only reflections indexable according to space group Pnma were observed. This indicates that the Mn and Cr atoms are, on the long range, randomly distributed over the metal sites (4c; x --- 0.005, z = 0.20) and the As and P atoms over the non-metal sites (4c; x = 0.20, z ~ 0.58). This was further substantiated by Rietveld refinements of P N D data for x = 0.000, 0.008 and 0.050 (at 295 K, R n - - 0 . 0 4 8 - 0 . 0 6 2 , Rp = 0.118-0.138). Furthermore, all the diffraction peaks exhibited

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normal experimental half-widths which rules out large scale c o n c e n t r a t i o n i n h o m o g e n e i t i e s throughout the samples. Nevertheless, on a smaller volume scale, a certain clustering may occur. Note that the properties of M n ] _ t C r t A s samples are strongly affected by inhomogeneities [5]. However, for the presently studied samples, such effects are minimized and the results refer to the equilibrium samples (type C) of ref. [5]. The variation of the unit cell dimensions with x is shown in fig. 1, and all axes are seen to decrease upon increasing P content. In this way, substitution of As by P simulates an external pressure, see also ref. [14]. The volume change brought about by the substitution varies linearly with x in the examined composition interval 0.00 < x < 0.10, with A V / A x = - 4 5 . 5 × 1 0 6 p m 3. A linear behaviour is actually also found for, e.g., the F e A s FeP system [17] where A V / A x = - 17.1 × 106 p m 3 applies to the whole solid solution range. In the C r A s - C r P system, the variation is also linear except for a small composition range close to CrAs, the average A V / A x being - 2 2 . 6 × 1 0 6 p m 3 [18].

A.F. Andresen et aL / Phosphorus substitution in Mno.6~Cro.3zAs

The extraordinary high value of AV/Ax for the M n l _ , C r t A s t _ x P x solid solution phase is thus remarkable. However, for M n A s - M n P an even larger (but non-linear) volume contraction occurs. AV/Ax = --105.9 × 10 6 p m 3 is obtained by linear extrapolation between MnAs and MnAs0.82PoJ 8 [13]. In the latter system, the volume reduction is probably a composite effect, the major component of which being related to changes in the electronic band structure, which in a simple model, may be considered as a conversion of high spin manganese to low spin manganese [13,15,19-24]. At high temperatures, the structure turns to the hexagonal NiAs type. For x --- 0.000, the continuous MnP ~ NiAs type transition occurs at T o = (675 + 5) K. The substitution of As by P, causes an increase in the transition temperature, and for x = 0.080 T D = (720 + 5) K. This agrees with the fact that the distorted MnP type structure is more favoured among the phosphides compared with the arsenides. As can be seen from the variation of b and c~ v/3 in fig. 1, the orthorhombic distortion increases at room temperature with increasing P content, and the deviation of the c/b axial ratio (not shown) from the orthohexagonal value of v/Jcan here be taken as a measure of the degree of the distortion. At temperatures below ambient, powder X-ray diffraction studies indicate that first as well as second order transitions occur in different temperature and composition intervals. Using Mn0.63Cr0.37As as a reference material, the structural changes (as evident from the temperature dependence of the unit cell dimensions of Mn0.63Cr0.37As0.995P0.005 in fig. 2) identify the para (P) to H C and the H Cto H a transitions at T N and T s, respectively. Whereas the unit cell dimensions vary continuously with temperature through the T N transition, a discontinuous change in all dimensions occurs at T s. The H C and H a phases coexist in a temperature range around T s. In this range, the relative amounts of the Hc and H a phases are continuously altered upon changing the temperature. This can be illustrated by considering the temperature dependence of the integrated intensities of various sets of Bragg reflections which, respectively, are characteristic of o n e or the other of the two phases (see fig. 4 in ref. [8]).

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The low temperature X-ray diffraction data for x = 0.010 (fig. 3), show a very different behaviour from that discussed for x = 0.005. Neither magnetostrictive expansion of the b-axis (indicative of the onset of H C magnetic order), nor discontinuous changes in any of the dimensions are observed. However, at about 190 K, a strong, possibly magnetostrictive, shortening of the b-axis takes place on further lowering of the temperature. This is indicative of a second order transition, but the exact transition temperature cannot be evaluated from the presented results. Similar data were obtained for x = 0.050 where the shortening of the b-axis takes place more smoothly over a larger temperature interval. A large scale sample with composition Mn0.raCro.37Aso.992P0.008 w a s studied with the neutron diffraction technique at temperatures between 10 and 300 K. The sample was prepared with an desired composition of x = 0.005. However, as judged from its unit cell dimensions (PXD), the effective

A.F. Andresen et al. / Phosphorussubstitution in Mno.63Cro.37As

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A similar neutron diffraction study for x = 0.050 gave rather different results. The structure still remains of the MnP type, and in fact it becomes even more distorted in terms of the atomic displacements of the Mn,Cr and As,P atoms as well as in terms of the c / b ratio. However, no satellite reflections indicating the existence of incommensurate magnetic structures could be detected. Instead ferromagnetic intensity contributions to various fundamental (nuclear) reflections were observed. According to Rietveld [25] refinements, using the Hewat version [26] of the programme, the magnetic moments at T < Tc = (217 + 3) K are aligned parallel to the b-axis. At 10 K the magnetic moment is # = (1.6 + 0.1)# B.

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(real) phosphorus content proved to be somewhat larger, and the composition was on this basis re-labelled as x = 0.008. (This undesirable situation with slight changes in composition, is hard to prevent for larger scale solid solution samples (up-scaling by more than a factor 20) when a c o m p o s i t i o n resolution of Ax < 0.005 for Mn I _,C, As is aimed at.) By using neutron instead of X-ray diffraction, the magnetically ordered state of the sample can be evaluated together with the structural state. The study confirms the MnP type crystal structure. At T < T N = (192 _ 4) K magnetic order of the H a type prevails. The temperature dependence of the integrated intensity of the magnetic 000 ± satellite reflection is shown in fig. 4. N o detectable hysteresis was found for the H a to P transition, nor were any indications found for the occurrence of a H c phase at temperatures above the upper stability temperature of the H a phase. It should be noted that for "unsubstituted" Mn0.63Cr0.37As both H c and H a phases occur, the former at the higher temperatures.

The diffraction studies, together with measurements of physical and thermal properties, provide a means to advance a tentative phase diagram for Mn0.63Cr0.37ASl_xP x. Accurate Cp data for x = 0.000 are already at hand [6], and Cp curves for x = 0 . 0 1 0 and 0.100 are shown in fig. 5. For x = 0.010 the observed peak corresponds to the H a to P transition, and that for x = 0.100 to the F to P spin disordering process. Included in fig. 5 is also Cp data for Mn0.60Cr0.a0As which provides an example of the continuous H c to P phase transition. Qualitatively seen, the various Cp curves look

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A.F. Andresen et a L / Phosphorus substitution in Mno.63CrojzAs I

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netism, and the value of dTc/dp = + 11 K / k b a r reported for t = 0.39 [10] is extremely large. The present D T A data (measured upon heating) show that T c increases almost linearly with x in the c o m p o s i t i o n range 0.040 < x < 0.100, with d T c / d x = 490 K. On the basis of these linear relationships, and assuming the applicability of the chemical pressure concept, the proportionality parameter c = Ap/Ax can be derived. The obtained value ( = 44.5 kbar is of the same order as found for various positive pressure substituents in MnAs. However, if one instead compares the unit cell dimensions of Mn0.63Cr0.37As 1_xPxwith those of Mn0.63Cr0.37As under pressure, the higher value ( = 87 kbar is obtained. For Mn0.645Cr0.355As under external pressure [10], the Tsl (first order magnetic) transition line between H a and F phases is very steep, and for the pressure range 1.3 to 2 kbar the slope ATst/Ap is about 105 K / k b a r . Using the correspondence between chemical and external pressure as stated above, ATsl/Ax = 4700 K is derived, which in turn implies that Tsl would decrease from say 200 to 0 K within a narrow composition interval of Ax = 0.04. This value is helpful when estimating the H a to F phase boundary in fig. 6. For M n l _ t C r t A s an applied magnetic field can convert the antiferromagnetic phases into a ferromagnetic phase [7]. On increasing the Cr content of the M n l _ t C r t A s solid solution phase towards t = 0.38, the critical magnetic field necessary to induce ferromagnetism from the H a state is continuously reduced. For t = 0.37 this critical field amounts to about 6 kOe. A further reduction of the critical field is expected when turning to the quaternary phase, and this must be considered when evaluating the experimental data collected for samples subjected to magnetic fields.

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Microwave absorption spectra of the metallic, NiAs type phases MnAs and MnSb, have already been reported, and these ESR data were related to the ferromagnetic couplings which occur in the phases [27,28]. The observed line-widths are broad compared with those usually found for paramagnetic ions in insulating compounds. This is

A.F. Andresen et aL / Phosphorus substitution in Mno.63Cro,37As

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due to a coupling within the spin system. Despite the large line widths, relevant information can be deduced from the shifts and splittings of the resonance lines. In figs. 7, 8 and 9 derivatives of the absorption of the microwave susceptibility for samples of Mno.63Cr0.37AS]_xPx are shown. F r o m magnetization data one can infer that x = 0.03 is a ferromagnet (spontaneous or field induced) with a Curie temperature of 214 K. Fig. 7a shows the resonance at 220 K, i.e. in the paramagnetic range just above

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T c [8]. The line is broad and asymmetric. At 200 K (fig. 7b), i.e. just below Tc, there is a marked shift of the zero crossing of the derivative of the resonance line - hereafter referred to as the magnetic resonance field B 0. At the same time a doublet structure in the spectrum occurs. Since these changes occur close to Tc it is tempting to propose that the shift in B 0 towards lower values are due to demagnetization effects. For a rotational ellipsoidal particle, one has the resonance condition [29]

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A.F. Andresen et al. / Phosphorus substitution in Mno.63Cro.3zAs I

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eq. (2). The decrease in B 0 for T < T c fits well, although no saturation of B 0 could be reached for the experimentally chosen w. The additional structure of the observed lines below T c (fig. 7b) m a y reflect a splitting of the shape distribution or imply that resonances of different subsystems are involved. A tentative curve fitting using symmetric Lorentzian lines, gives a m i n i m u m n u m b e r of two contributions for T > T c and three for T < T o This suggests that the microwave absorption in these phases might yield information which can not be obtained by other methods. Samples with x = 0.000 and 0.005 (cf. the phase diagram in fig. 6) are according to neutron diffraction studies and magnetization measurements helimagnetically ordered at low temperatures. The H a and H , phases show a quite different magnetostrictive behaviour, and whereas high magnetic fields are required to obtain a spin aligned state for the H c phase, the H a phase can be converted into a ferromagnet by weak or moderate fields [7,8]. These features are also reflected in the B0(T ) curves. F o r x = 0.000 and 0.005 no change in B0 is observed at T N which suggests a low magnetic anisotropy for the H c phase. O n the other hand, a large change is observed at T s, see figs. 8 and 10, thus suggesting a large anisotropy for the H a

353

phase. However, one c a n n o t rule out that instead of spin precession a r o u n d the anisotropy field, b r o a d e n e d exchange resonances m a y be involved. F o r x = 0.000, no demagnetization effects are seen, whereas for x = 0.005 a drop in B0(T) occurs for T = 140 K. The drop suggests a (partial or complete) field induced H a to F transition (cf. fig. 6), which is in accordance with the behaviour of Mn-rich samples of M n l _ t C r t A s in magnetic fields. It should be noted that the given transition temperature is deduced f r o m measurements performed in an external magnetic field. Fits of the E S R spectra using symmetrical lines, require a m i n i m u m n u m b e r of two lines in order to account for the experimental spectra for x = 0.005, cf. fig. 9. This m a y reflect the anisotropic distribution of the grains or result from para- and antiferromagnetic resonance of different subsystems. In the paramagnetic range for x = 0.005, the resonance is broad, but nearly symmetric. This suggests that for the antiferromagnetic regime of this sample, it should be feasible to analyze the subsystems using symmetric Lorentzian lines.

Acknowledgements J.-W. Schiinemann is thanked for helpful discussions. This work has received financial support from the N o r w e g i a n Council for Science and the Humanities.

References [1] N. Kazama and H. Watanabe, J. Phys. Soc. Japan 30 (1971) 1319. [2] K. Selte, A. Kjekshus, P.G. Peterzrns and A.F. Andresen, Acta Chem. Scand. A 32 (1978) 653. [3] R. W~hl, H.J. Krokoszinski and K. B~irner, J. Magn. Magn. Mat. 13 (1979) 119. [4] H. Fjellvfig and A. Kjekshus, Acta Chem. Scand. A 39 (1985) 671. [5] A.F. Andresen, K. BMner, H. Fjellvfig, K. Heinemann, A. Kjekshus and U. Sondermann, J. Magn. Magn. Mat. 58 (1986) 287. [61 N. Komada, E.F. Westrum, H. Fjellvfig and A. Kjekshus, J. Magn. Magn. Mat. 65 (1987) 37. [7] A. Zi~ba, H. Fjellvfig and A. Kjekshus, J. Magn. Magn. Mat. 68 (1987) 115.

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A.F. Andresen et al. / Phosphorus substitution in Mno.63Cro.3zAs

[8] J.-W. Schiinemann, K. Biirner, H. Fjellv~ig, A. Kjekshus, J. Eigemann, E. Gmelin and U. Sondermann, Phys. Stat. Sol. (a) 110 (1988) 141. [9] A.F. Andresen, H. Fjellvhg, A. Kjekshus and B. Lebech, J. Magn. Magn. Mat. 62 (1986) 247. [10] A. Zi~ba, R. Zach, H. Fjellv/tg and A. Kjekshus, J. de Phys. 49 (1988) C8-203. [11] K. Barrier, C. Santandrea, U. Neitzel and E. Gmelin, Phys. Stat. Sol. (b) 123 (1984) 541. [12] H. Fjellv~g, A. Kjekshus, S. Stalen and A.F. Andresen, Acta Chem. Scand. A 42 (1988) 214. [13] H. Fjellvhg, A.F. Andresen and K. B~ner, J. Magn. Magn. Mat. 46 (1984) 29. [14] A. Zigba, R. Zach, H. Fjellvhg and A. Kjekshus, J. Phys. Chem. Solids 48 (1987) 79. [15] S. Stolen, H. Fjellv~ig, F. Gronvold and A. Kjekshus, Proc. Second Asian Thermophys. Prop. Conf. (1989) p. 279. [16] H. Fjellv[tg and A. Kjekshus, Acta Chem. Scand. A 38 (1984) 1. [17] K. Selte, A. Kjekshus, T.A. Oftedal and A.F. Andresen, Acta Chem. Scand. A 28 (1974) 957. [18] K. Selte, H. Hjersing, A. Kjekshus, A.F. Andresen and P. Fischer, Acta Chem. Stand. A 29 (1975) 695.

[19] H.-J. Krokoszinski, C. Santandrea, E. Gmelin and K. B~irner, Phys. Stat. Sol. (a) 113 (1982) 185. [20] K. B~irner and E. Gmelin, Phys. Stat. Sol. (b) 132 (1985) 440. [21] H. Fjellv~tg, A. Kjekshus and S. St¢len, J. Solid State Chem. 64 (1986) 123. [22] A.F. Andresen, H. Fjellv~g, O. Steinsvoll, A. Kjekshus, S. Stolen and K. B~irner, J. Magn. Magn. Mat. 62 (1986) 241. [23] H. Fjellv/lg, F. Gronvold, A. Kjekshus and S. Stalen, J. Phys. C 20 (1987) 3005. [24] A.K. Labban, E.F. Westrum, H. Fjellv[tg, F. Gronvold, A. Kjekshus and S. Stalen, J. Solid State Chem. 70 (1987) 185. [25] H.M. Rietveld, J. Appl. Crystallogr. 2 (1969) 65. [26] A.W. Hewat, UKAERE Harwell Report RRL 73/897, Harwell (1973). [27] G.D. Adam and K.J. Standley, Proc. Phys. Soc. A 66 (1953) 823. [28] M. Baribaud, J.C. Caerou, G. Fourcaudot and J. Mercier, Phys. Stat. Sol. (a) 60 (1980) 201. [29] C. Kittel, Phys. Rev. 71 (1947) 270.