POLARIZATION EFFECTS IN NEUTRON SCATTERING FROM SPIRAL DOMAINS IN MnP

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POLARIZATION EFFECTS IN NEUTRON

SCATTERING FROM SPIRAL DOMAINS IN MnP

G. Felcher, G. Lander, T. Brun

To cite this version:

G. Felcher, G. Lander, T. Brun. POLARIZATION EFFECTS IN NEUTRON SCATTERING FROM SPIRAL DOMAINS IN MnP. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-577-C1-578.

�10.1051/jphyscol:19711199�. �jpa-00214023�

JOURNAL DE PHYSIQUE Colloque C I , supplkment au no 2-3, Tome 32, Fburier-Mars 1971, page C 1 - 577

POLARIZATION EFFECTS IN NEUTRON SCATTER1 FROM SPIRAL DOMAINS IN MnP (")

G. P. FELCHER and G. H. LANDER Argonne National Laboratory, Argonne, Illinois

T. 0. BRUN

Ames Laboratory, Iowa State University, Ames, Iowa

RBsumb. - Les moments magnCtiques dans le phosphure de manganese sont ordonn6s & basse temperature dans une structure hklicoldale. Par diffraction des neutrons polarisks on a montre que des hClices de sens opposb peuvent occu- per des volumes inkgaux dans un Cchantillon monocristallin. La population des domaines a CtC changke par traitement thermique a 600 O C , mais non par I'application de champs magnktiques. Une comparaison des proprietb de la structure magnetique de MnP (hClicoidale double) et de Ho (h6licoldale simple) a permis d'etablir que dans le premier cas la popu- lation des domaines est plus sensible aux forces appliqukes.

Abstract. - The magnetic moments in MnP order below 50 OK in a spiral arrangement, and a polarized-neutron experiment shows for the first time that spirals of opposite senses may occupy unequal volumes of a single crystal. The domain population was changed by annealing the crystal at 600 OC, but not by the application of magnetic fields. A compa- rison between the properties of the double spiral in MnP and the single spirals, as in holmium, suggests that the domain populations in the former may be more easily influenced by external forces.

I. Introduction. - Several rare-earth and transi- tion-metal compounds have magnetic spiral structures.

A spiral structure can be described by its propagation vector z, which may be parallel to antiparallel to a certain crystallographic direction. Thus two types of domains are possible, corresponding to either a counterclockwise or clockwise rotation of the spiral.

The relative volumes occupied by the two types of domains in the crystal may be determined by neutron diffraction [I]. The neutron diffraction pattern from a simple spiral structure consists of two coherent peaks (satellites + and -) around each nuclear reflection (hkl). Their intensity is given by

where K^ is the unit scattering vector, is an unit vector in the direction of propagation of the spiral, P is the polarization of the incident neutrons. F ; ~ is the structure factor of the satellites, and n is the fraction of the total volume occupied by domains for which 2.; is positive. Using polarized neutrons the ratio of the Bragg intensities a t the satellite reflection for the two different neutron spin states is given by,

The domain population can be measured by this method only if both (P.@ and (8.2) are nonzero.

In a conventional polarized beam experiment P . 2 = 0 for both spin states. The requirement that P.K^ ^{# 0}

may be satisfied either by changing the direction of

the neutron polarjzation or by using the elevated counter technique.

The relative domain population can also be deter- mined by using an unpolarized incident beam and analyzing the spin direction of the scattered neutrons.

The polarization of neutrons scattered by a magnetic satellite is given by

For reflections such that k.; ⁼ 1, eq. (3) shows that a beam of completely polarized neutrons can be obtained if the crystal contains only one type of domain.

11. Experimental. - In the present experiment the spiral domain population has been investigated in a single crystal of MnP. MnP crystallizes in an ortho- rhombic structure (Pnma space group) : below

-

and in zero field the magnetic moments on manganese atoms order in a spiral structure with the propagation vector parallel to the c-axis 12, 31. In the experiment the sample was placed in a variable temperature cryostat with the c-axis vertical. Magnetic fields of up to 15 kOe could be applied parallel to the c-axis.

The incident neutrons were polarized parallel t o the c-axis. Using the elevated-counter technique the pola- rization ratios of a number of satellite and nuclear reflections were measured in zero field at 4.2 OK. The results for the (202) nuclear and satellite reflections are given below :

Index R n

202+ 1.22 _f 0.01 0.580

+

202 nuclear 1.00 ^_f 0.01

202 - 0.840 4 0.004 0.576 0.003

(*) Based on work under the auspices of the Provided other effects do not give rise to R-values U. S. Atomic Energy Commission. different from unity, the value of n indicates that the

39

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711199

C 1 - ^{578 G. P. FELCHER AND G. H. LANDER, T. 0. BRUN}

particular single crystal of MnP had an unequal domain population.

Additional effects that could influence the polariza- tion ratios of the magnetic satellites are the magnetic Schwinger scattering, the magnetic inelastic scattering, and multiple scattering processes. The first effect is far too small to account for the observed ratios. No theory has been formulated on the diffraction effects from spin waves in a spiral structure. However, any polarization dependent cross section should be the same around equivalent reflections. The polarization of the equivalent satellites 202'~ 202+ was measured as a function of the crystal misset from the center of the Bragg position, and was found to vary over the range, but in the opposite way for the two peaks. The spin-wave contributions are, therefore, very small if present at all, and are obscured by multiple scattering effects. The multiple scattering is a more serious problem. In particular the (000)* can readily couple with the (hkl) nuclear reflections so that the doubly diffracted beam appears at the (hkl)' satellite position.

However, if the polarization ratio for both primary reflections is unity, double scattering cannot produce an R different from unity. Tn zero field the nuclear reflections are also contaminated by multiple scattering, ^, but by careful analysis the R-values of the primary reflections were shown to be unity, indicating that the observed polarization effects arise solely from the magnetic satellites.

Several attempts were made to change the domain distribution. These included the application of 15 kOe at 4.2 OK, field cooling from above 50 OK in 15 kOe, and field cooling in 8C kOe, although in the latter case the sample was warmed to room temperature before being transferred to the diffractometer. None of these attempts changed the domain distribution. The crystal was then annealed at 600 OC for three hours, followed

[I] BWME (M.), Phys. Rev., 1963, 130, 1670.

[2] FELCHER G. P.), J. Appl. Phys., 1966, 37, 1056.

[3] FORSYTH

b.

by slow cooling to 400 ^OC. After cooling to 4.2 OK in zero field the volumes occupied by the two domains were found to be almost equal (n ^w 0.52).

111. Discussion. - A previous experiment aimed at determining an unequal spiral domain population was performed with holmium [4]. The domain populations were found to be equal, and could not be modif?ed.

In MnP the spiral structure below 50% may be described as ferromagnetic layers perpendicular to the c-axis, each layer in a unit cell containing one manga- nese atom. The moments on alternate layers form a spiral with a propagation vector parallel to the c-axis.

The crystallographic space group is P 2,111 2,/m 2,/a (No. 62), which is centrosymmetric. The magnetic structure, however, cannot be described in this space group [ 5 ] , and the magnetic space group is a repre- sentation of P n m 2 , , which is non-centrosymmetric.

This loss of inversion symmetry distinguishes MnP from the simple spirals, such as holmium, in which the magnetic structure is a representation of a centro- symmetric space group.

The domain population can be influenced if an external force can be found that raises the energy of one domain with respect to the other. The external forces that can be applied are an electric field, a stress, and a magnetic field. The first would certainly change the relative energy of the two domains if the crystal is non-centrosymmetric. Unfortunately MnP is a good conductor. Considering first-order perturbation only the other two forces cannot differentiate between the energies of the two domains. In the experiment with MnP the crystal was field-cooled in an attempt t o influence the domain population, but no changes were observed. However, the result of annealing at 600 OC indicates that the dislocations play a major role in pinning the antiferromagnetic domain walls.

[4] RENDRICK (H.), Thesis, University of Michigan, 1968 (unpublished).

[51 BERTAUT (E. F.), J. Appl. Phys., 1969, 40, 1592.