AngularJs $http Post Requst for JPA OneToOne mapping return String instead JSON data

here is html in AngularJs materials

	<div layout="row">
		<md-input-container class="md-block" flex="100">
        <label>Select Country</label>
			<md-select name="type" ng-model="newDatum.country" >
				<md-option ng-repeat="c in country" value="{{c}}">{{c.name}}</md-option>
			</md-select>
      </md-input-container>
	</div>
	<md-button style="width:100%" type="submit" ng-click="createDatum(newDatum)" class="md-raised md-primary">
		<md-icon class="md-size-3x">arrow_forward</md-icon>   Create
	</md-button>

here is javascript

    $scope.createDatum= function(datum){
			$http({
			method : "POST",
			url : "api/address",
			data : angular.toJson(datum),
			headers : {
				"Content-Type" : "application/json"
			}
		}).then(function(response){
			$mdDialog.show($mdDialog.alert()
				.title('Create \''+datum.contactPersonName+'\' Succesfull!')
				.textContent('Here you have successfully create this \''+datum.contactPersonName.toLowerCase()+'\' configuration!')
				.ok('Ok'));
			$scocontactPersonNamehowProgressBar = false;
			$scope.getAllData();
		}, function(reason){
			$mdDialog.show($mdDialog.alert()
				.title('Create \''+datum.contactPersonName+'\' Error!')
				.textContent('Here you have got error to create this \''+datum.contactPersonName.toLowerCase()+'\' configuration! '+reason.data)
				.ok('Ok'));
			$scope.showProgressBar = false;
			$scope.error_message = reason.data;
		});}

Java entity class mapping

@OneToOne(cascade = CascadeType.MERGE)
@JoinTable(name = "address_country", joinColumns = @JoinColumn(name = "address_id"), inverseJoinColumns = @JoinColumn(name = "country_id"))
private Country country;

Java Compiler error

2020-05-02 15:39:53.691 WARN 5083 --- [nio-8080-exec-5] .w.s.m.s.DefaultHandlerExceptionResolver : Resolved [org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot construct instance of `com.zero.zerodefinition.country.Country` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('{"id":4,"name":"AMERICAN SAMOA","niceName":"American Samoa","countryCode":"AS","phoneCode":"1684","iso3":"ASM","numCode":"16"}'); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `com.zero.zerodefinition.country.Country` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('{"id":4,"name":"AMERICAN SAMOA","niceName":"American Samoa","countryCode":"AS","phoneCode":"1684","iso3":"ASM","numCode":"16"}')
at [Source: (PushbackInputStream); line: 1, column: 227] (through reference chain: com.zero.zerodefinition.address.Address["country"])]

Browser error

{"timestamp":"2020-05-02T09:39:53.692+0000","status":400,"error":"Bad Request","message":"JSON parse error: Cannot construct instance of `com.zero.zerodefinition.country.Country` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('{\"id\":4,\"name\":\"AMERICAN SAMOA\",\"niceName\":\"American Samoa\",\"countryCode\":\"AS\",\"phoneCode\":\"1684\",\"iso3\":\"ASM\",\"numCode\":\"16\"}'); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `com.zero.zerodefinition.country.Country` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('{\"id\":4,\"name\":\"AMERICAN SAMOA\",\"niceName\":\"American Samoa\",\"countryCode\":\"AS\",\"phoneCode\":\"1684\",\"iso3\":\"ASM\",\"numCode\":\"16\"}')\n at [Source: (PushbackInputStream); line: 1, column: 227] (through reference chain: com.zero.zerodefinition.address.Address[\"country\"])","path":"/zerodefinition/api/address"}

how to solve this.! Is there any problem of hibernate mapping! Why this is not converted to json insted string.