The magnetic field influence on magnetostructural phase transition in Ni$_{2.19}$Mn$_{0.81}$Ga
D.~A.~Filippov
Magnetic properties of a polycrystalline alloy Ni$_{2.19}$Mn$_{0.81}$Ga, which undergoes a first-order magnetostructural phase transition from cubic paramagnetic to tetragonal ferromagnetic phase, are studied. Hysteretic behavior of isothermal magnetization $M(H)$ has been observed in a temperature interval of the magnetostructural transition in magnetic fields from 20 to 100~kOe. Temperature dependencies of magnetization $M$, measured in magnetic fields $H = 400$ and 60~kOe, indicate that the temperature of the magnetostructural transition increases with increasing magnetic field.
JMMM-Filippov.tex








\documentclass[letterpaper,aps,prb,superscriptaddress,twocolumn]{revtex4}

\usepackage{graphicx}

\begin{document}



\title{The magnetic field influence on magnetostructural phase
transition in Ni$_{2.19}$Mn$_{0.81}$Ga}



\author{D.~A.~Filippov}

\affiliation{Physical Faculty, Moscow State University, Moscow
119899, Russia}


\author{V.~V.~Khovailo}

\affiliation{Institute of Fluid Science, Tohoku University, Sendai
980--8577, Japan}


\author{V.~V.~Koledov}

\affiliation{Institute of Radioengineering and Electronics,
Russian Academy of Sciences, Moscow 101999, Russia}


\author{E.~P.~Krasnoperov}

\affiliation{RRC, Kurchatov Institute, Moscow 123182, Russia}


\author{R.~Z.~Levitin}

\affiliation{Physical Faculty, Moscow State University, Moscow
119899, Russia}


\author{V.~G.~Shavrov}

\affiliation{Institute of Radioengineering and Electronics,
Russian Academy of Sciences, Moscow 101999, Russia}


\author{T.~Takagi}

\affiliation{Institute of Fluid Science, Tohoku University, Sendai
980--8577, Japan}




\begin{abstract}
Magnetic properties of a polycrystalline alloy
Ni$_{2.19}$Mn$_{0.81}$Ga, which undergoes a first-order
magnetostructural phase transition from cubic paramagnetic to
tetragonal ferromagnetic phase, are studied. Hysteretic behavior
of isothermal magnetization $M(H)$ has been observed in a
temperature interval of the magnetostructural transition in
magnetic fields from 20 to 100~kOe. Temperature dependencies of
magnetization $M$, measured in magnetic fields $H = 400$ and
60~kOe, indicate that the temperature of the magnetostructural
transition increases with increasing magnetic field.
\end{abstract}


%\pacs{64.70.Kb, 75.50.Cc}

%\date{\today}


\maketitle





Mn-containing Heusler alloys Ni$_{2+x}$Mn$_{1-x}$Ga have been
intensively studied during last years owing to their unique
combination of ferromagnetism and a structural (martensitic) phase
transition. Since martensitic transformation in Ni--Mn--Ga is of a
thermoelastic type, the materials exhibit a well-defined one- and
two-way shape memory effect~[1,2]. For the stoichiometric
Ni$_2$MnGa composition, martensitic transition temperature $T_m =
202$~K and Curie temperature $T_C = 376$~K~[3]. A phase diagram of
magnetic and structural transitions in Ni$_{2+x}$Mn$_{1-x}$Ga
alloys studied, theoretically and experimentally, in Refs.~[4,5].
It has been found, that a partial substitution of Mn for Ni leads
to a decrease of Curie temperature $T_C$ and an increase of the
martensitic transition temperature $T_m$ until they merge in a
compositional interval $x = 0.18 - 0.20$ (Fig.~1). The application
of an external magnetic field results in an increase of the
martensitic transition temperature [6--8]. If applied magnetic
field is strong enough, a reversible magnetostructural transition
can be realized in the magnetic field at constant temperature and
pressure~[9]. This means that the structural state of a sample can
be switched reversibly and that magnetic shape memory effect can
be realized by means of magnetic-field-induced martensitic
transition~[10,~11]. Although Ni$_{2+x}$Mn$_{1-x}$Ga alloys were
studied by a variety of methods, a systematic study of their
magnetic properties in the vicinity of the magnetostructural
transition is lacking. Magnetic properties of
Ni$_{2.18}$Mn$_{0.82}$Ga were studied in Ref.~[12], where
hysteretic behavior of temperature dependencies of magnetization
$M(T)$ has been observed. However, the anomalous field dependence
of magnetization $M$, reported in Ref.~[12], was found to be
sample dependent and, therefore needs further investigation. The
aim of this work is to investigate magnetic properties of
polycrystalline Ni$_{2.19}$Mn$_{0.81}$Ga alloy in strong magnetic
fields in the vicinity of the magnetostructural transition.


Polycrystalline Ni$_{2.19}$Mn$_{0.81}$Ga ingots were prepared by a
conventional arc-melting method in argon atmosphere. To attain a
good compositional homogeneity, the ingots were remelted three
times. Since weight loss during melting was approximately 0.1\%,
the composition of the ingots was assumed to be the nominal one.
The ingots were annealed in vacuum at 1050~K for 3~days. The
sample for measurements was cut from the middle part of the ingot.
Temperature and magnetic field dependencies of magnetization were
measured by a magnetometer equipped with a Hall sensor. Magnetic
fields up to 100~kOe were generated by a Bitter magnet.
Measurements of $M(H)$ were done in the following way. Initially
the sample was heated to $T = 380$~K, well above the temperature
of the transition. After that the sample was cooled down to a
desired temperature, which was controlled by a temperature
controller. In these measurements the magnetic field was changed
with a step of 10~kOe and was kept for some time in order to
insure that processes of structural transformation for a given
strength of the magnetic field had completely been finished.


\begin{figure}[h]
\begin{center}
\includegraphics[width=7cm]{fig-1.eps}
\caption{The composition dependence of Curie temperature $T_C$ and
the martensitic transition temperature $T_m$ in
Ni$_{2+x}$Mn$_{1-x}$Ga ($x = 0 - 0.20$).}
\end{center}
\end{figure}





Typical temperature dependencies of magnetization $M$ in the
vicinity of the magnetostructural transition are shown in Fig.~2.
It is evident that as in the case of weak magnetic field $H =
400$~Oe (Fig.~2a), in a strong magnetic field $H = 60$~kOe
(Fig.~2b), the magnetization has a prominent temperature
hysteresis. Therefore, the magnetic transition in the
Ni$_{2.19}$Mn$_{0.81}$Ga alloy possesses characteristics typical
of a first-order phase transition. Under the action of the
magnetic field the characteristic temperatures of the
magnetostructural transition shift upward with a rate of $\approx
0.1$~K/kOe. This value agrees well with the data, published in
Refs.~[7,9,12].


\begin{figure}[t]
\begin{center}
\includegraphics[width=\columnwidth]{fig-2.eps}
\caption{Temperature dependencies of the magnetization of
Ni$_{2.19}$Mn$_{0.81}$Ga, measured in $H = 400$~Oe (a) and $H =
60$~kOe (b).}
\end{center}
\end{figure}


\begin{figure}[b]
\begin{center}
\includegraphics[width=\columnwidth]{fig-3.eps}
\caption{Isothermal magnetization curves of
Ni$_{2.19}$Mn$_{0.81}$Ga measured in the vicinity of the
magnetostructural transition.}
\end{center}
\end{figure}



Isothermal magnetization curves taken at various temperatures in
the vicinity of the magnetostructural transition are shown in
Fig.~3. The $M(H)$ curves, measured at $T = 327$ (Fig.~3a) and
345~K (Fig.~3d) have field dependencies of magnetization typical
of a ferromagnet and a paramagnet, respectively. Contrary to this,
the $M(H)$ curves, measured in the temperature interval of the
mixed (martensite and austenite) state (Fig.~3b and 3c)
demonstrate hysteretic anomalies of magnetization in magnetic
field up to 100~kOe. As is seen from Fig.~3b, in the magnetic
field 0--20~kOe the behavior of the magnetization is similar to
the paramagnetic state. Further increase of the magnetic field
results in an anomalous growth of the magnetization, which
reaches, in a magnetic field of 100~kOe, 90\% of the magnetization
of the ferromagnetic state at $T = 327$~K (Fig.~3a). Upon
subsequent removal of the magnetic field, the magnetization
remains constant down to 20~kOe, where a drastic decrease, typical
for technical magnetization curve of a ferromagnet, is observed.
Based on the behavior of $M(H)$, measured at $T = 339$~K, it can
be suggested that induced by the magnetic field, low-temperature
(martensitic) ferromagnetic state of the sample remains stable
upon removal of the magnetic field.


The $M(H)$ dependencies, measured during application and removal
of magnetic field at $T = 341$~K are shown in Fig.~3c. At this
temperature the behavior of the magnetization is similar to the
paramagnetic state in magnetic fields up to 30~kOe. When the
magnetic field increases from 30 to 100~kOe, the magnetization
anomalously grows and reaches a value of 60\% of the magnetization
of the ferromagnetic state at $T = 327$~K. Upon subsequent
decrease of the magnetic field from 100 to 30~kOe, the attained
value of the magnetization is preserved. A rapid change of $M$
occurs at $H = 30$~kOe, where the magnetization drops to a value
of 110\% of the magnetization of the paramagnetic state at $T =
345$~K. During further decrease of the magnetic field from 30 to
0~kOe, the behavior of the magnetization is typical of the
paramagnetic state. The anomalous $M(H)$ dependencies measured at
$T = 341$~K can be explained as due to a reversible structural
transformation from paramagnetic austenite to ferromagnetic
martensite, induced by a magnetic field at a constant temperature.


The presented results of the magnetic measurements are in
accordance with experimental results, reported by Dikshtein
\textit{et al}.~[9], where a reversible magnetic-field-induced
structural phase transition was observed by an optical method. As
is seen from Fig.~3, induction of a structural transition by a
magnetic field is accompanied by unusual behavior of the
isothermal magnetization $M(H)$ in the temperature interval of the
coexistence of ferromagnetic martensite and paramagnetic
austenite. Actually, similar anomalies on $M(H)$ dependencies were
observed in Gd$_5$(Si$_x$Ge$_{4-x}$)~[13], which is another
representative of the intermetallic compounds with coupled
magnetostructural transition. Since such systems, with a close
relation between crystallographic structure and magnetism, are of
a great technological significance, further studies of
magnetostructural transitions in Ni$_{2+x}$Mn$_{1-x}$Ga~[12],
Co--Ni--Ga, and Co--Ni--Al~[14] alloys are of considerable
interest.

\bigskip

We are grateful to Professor A.~N.~Vasil�ev for helpful
discussions. This work was partially supported by the Grant of
RFBR--BRFBR 02--02--81030 Bel2002a, by the Grant of RFBR
02--02--16636a, and by the Grant-in-Aid for Scientific Research
(B) No.~11695038 from the Japan Society for the Promotion of
Science, and Izumi Science Technology Foundation.







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\end{thebibliography}





\end{document}

cond-mat0303187.extracted_bib
\begin{thebibliography}{40}

\bibitem{1-k} V.~V.~Kokorin and V.~A.~Chernenko, Phys. Met. Metal. \textbf{68}, 111
(1989).

\bibitem{2-i} S.~Isokawa, M.~Suzuki, M.~Ohtsuka, M.~Matsumoto, and K.~Itagaki,
Mater. Trans. \textbf{42}, 1886 (2001).

\bibitem{3-w} P.~J.~Webster, K.~R.~A.~Ziebeck, S.~L.~Town,
and M.~S.~Peak, Philos. Mag. B \textbf{49}, 295 (1984).


\bibitem{4-b} A.~D.~Bozhko, A.~N.~Vasil�ev, V.~V.~Khovailo, V.~D.~Buchel�nikov,
I.~E.~Dikshtein, S.~M.~Seletskii, and V.~G.~Shavrov, JETP Lett.
\textbf{67}, 227 (1998).

\bibitem{5-v} A.~N.~Vasil'ev, A.~D.~Bozhko, V.~V.~Khovailo,
I.~E.~Dikshtein, V.~G.~Shavrov, V.~D.~Buchelnikov, M.~Matsumoto,
S.~Suzuki, T.~Takagi, and J.~Tani, Phys. Rev. B \textbf{59}, 1113
(1999).

\bibitem{6-d} I.~Dikshtein, V.~Koledov, V.~Shavrov, A.~Tulaikova,
A.~Cherechukin, V.~Buchel�nikov, V.~Khovailo, M.~Matsumoto,
T.~Takagi, and J.~Tani, IEEE Trans. Magn. \textbf{35}, 3811
(1999).

\bibitem{7-b} A.~D.~Bozhko, A.~N.~Vasil'ev, V.~V.~Khovailo,
I.~E.~Dikshtein, V.~V.~Koledov, S.~M.~Seletskii, A.~A.~Tulaikova,
A.~A.~Cherechukin, V.~G.~Shavrov, and V.~D.~Buchelnikov, Sov. JETP
\textbf{88}, 954 (1999).

\bibitem{8-i} K.~Inoue, K.~Enami, Y.~Yamaguchi, K.~Ohoyama, Y.~Morii,
Y.~Matsuoka, and K.~Inoue, J. Phys. Soc. Jpn. \textbf{69}, 3485
(2000).

\bibitem{9-d} I.~E.~Dikshtein, D.~I.~Ermakov, V.~V.~Koledov, L.~V.~Koledov,
T.~Takagi, A.~A.~Tulaikova, A.~A.~Cherechukin, and V.~G.~Shavrov,
JETP Lett. \textbf{72}, 373 (2000).

\bibitem{10-s} V.~G.~Shavrov, A.~V.~Glebov, I.~E.~Dikshtein, V.~V.~Koledov,
D.~V.~Kosolapov, E.~P.~Krasnoperov, T.~Takagi, A.~A.~Tulaikova,
and A.~A.~Cherechukin, J. Radioelectron. \textbf{5} (2001)
(http://jre.cplire.ru/jre/may01/1/text.html).

\bibitem{11-c} A.~A.~Cherechukin, I.~E.~Dikshtein, D.~I.~Ermakov,
A.~V.~Glebov, V.~V.~Koledov, D.~A.~Kosolapov, V.~G.~Shavrov,
A.~A.~Tulaikova, E.~P.~Krasnoperov, and T.~Takagi, Phys. Lett. A
\textbf{291}, 175 (2001).

\bibitem{12-k} V.~V.~Khovailo, T.~Takagi, J.~Tani, R.~Z.~Levitin,
A.~A.~Cherechukin, M.~Matsumoto, and R.~Note, Phys. Rev. B
\textbf{65}, 092410 (2002).

\bibitem{13-l} E.~M.~Levin, V.~K.~Pecharsky, and K.~A.~Gschneidner Jr.,
Phys. Rev. B \textbf{62}, R14625 (2000).

\bibitem{14-o} K.~Oikawa, T.~Ota, F.~Gejima, T.~Ohmori, R.~Kainuma, and
K.~Ishida, Mater. Trans. \textbf{42}, 2472 (2001).



\end{thebibliography}